Patent Application
20240284805
The invention relates to a method of producing a solid-state component, in particular for a quantum component, preferably for a qubit, comprising one or more thin films, the one or more thin films comprising a first material and each said film having a thickness selected between a monolayer and 100 nm and being deposited onto a substrate surface of a substrate, wherein the production process is carried out in a reaction chamber sealed with respect to the ambient atmosphere. Further, the invention relates to a solid-state component, in particular for a quantum component, preferably for a qubit, comprising one or more thin films, one of the one or more thin films comprising a first material with a thickness between a monolayer and 100 nm and being deposited onto a substrate surface of a substrate. In addition, the invention relates to a quantum component comprising such a solid-state component according to the present invention and to an apparatus for producing such a solid-state component according to the present invention.
One of the most challenging topics of modern technology is the production of quantum components. Such quantum components can be used in quantum computers, in particular for handling and/or transmission of quantum information, as both processes are based on storing and unitary processing of quantum states in said quantum components. An example of quantum components are qubits. In current technology, qubits are implemented for instance as ion traps, semi-conductor components, topological components and super conducting components. In principle all, but in particular the last three of these examples can be constructed based on solid-state components.
For effective quantum computing, aforementioned quantum states respectively the storage of aforementioned quantum states in a quantum component need to be stable over a sufficient long time period. For the current generation of quantum computing devices, this minimum value is around 100 μs. Interactions, in particular inelastic interactions, with the environment of the quantum component storing respective quantum state, for instance with phonons and in particular with charged or chargeable or magnetic defects of the quantum component, destroy the quantum state in most of the cases. Hence, the durability of quantum states, represented by the so-called coherence time, is limited in present quantum components based on solid-state components. For instance with respect to super conducting qubits of the state of the art, the maximum reachable coherence time is in the order of 1 ms. However, for effective quantum computing, this value is not sufficient.
As mentioned above, the coherence time of quantum state depends on the environment in which the quantum state is stored in the quantum component. In contrast to many forms of external interference, like for instance electromagnetic radiation, which can be countered by shielding measures, and phonons in the quantum component, which can be suppressed by running the respective quantum component at temperatures in the vicinity of OK, defects in the structure of the quantum component itself like missing or additional atoms or any discontinuity of a symmetry, in particular of a lattice symmetry, are intrinsically limiting the maximal possible value of the coherence time.
In view of the above, it is an object of the present invention to provide an improved method of producing a solid-state component, an improved solid-state component, an improved quantum component and an improved apparatus for producing a solid-state-component which do not have the aforementioned drawbacks of the state-of-the-art. In particular it is an object of the present invention to provide method of producing a solid-state component, a solid-state component, a quantum component and an apparatus for producing a solid-state-component, whereby the produced, implemented or presented solid-state component comprises a reduced number of defects and hence in a quantum component based on said solid-state component an increased coherence time can be achieved.
This object is satisfied by the respective independent patent claims. In particular, this object is satisfied by a method of producing a solid-state component according to claim 1, by a solid-state component according to each of the claims 26 and 27, by a quantum component according to claim 28 and by an apparatus for producing a solid-state component according to claim 33. The dependent claims describe preferred embodiments of the invention. Details and advantages described with respect to a method according to the first aspect of the invention also refer to a solid-state component according to the second and third aspect of the invention, to a quantum component according the fourth aspect of the invention and to an apparatus for producing a solid-state component according to the fifth aspect of the invention and vice versa, if of technical sense.
According to the first aspect of the invention, the object is satisfied by a method of producing a solid-state component, in particular for a quantum component, preferably for a qubit, comprising one or more thin films, the one or more thin films comprising a first material and each said film having a thickness selected between a monolayer and 100 nm and being deposited onto a substrate surface of a substrate, wherein the production process is carried out in a reaction chamber sealed with respect to the ambient atmosphere.
The method according to the first aspect of the present invention is characterized by the following steps of
The method according to the first aspect of the present invention is adapted for producing a solid-state component with a reduced number of defects. In particular, the solid-state components produced with a method according to the first aspect of the present invention ideally comprise a number of defects per cm2 and layer such that it allows the fabrication of qubit structures with qubit relaxation times and qubit coherence times above 100 μs, preferably above 1000 μs, even more preferably above 10 ms.
By that, such solid-state components produced with a method according to the first aspect of the present invention are splendidly suitable for a usage as basis of a quantum component, in particular for a qubit. The measures taken for achieving such a low number of defects are described in the following.
In its most simple embodiment, the solid-state component produced with a method according to the first aspect of the present invention comprises one thin film with a thickness selected between a monolayer and 100 nm. However, also two or more such thin films stacked on top of each other are possible. Said thin film comprises a first material.
The lowermost thin film is deposited onto a substrate surface of a substrate. Preferably but not exclusively, the substrate material can be provided as a single crystal. If the solid-state component comprises two or more thin films, preferably each of the two or more thin films is successively deposited onto the preceding thin film.
In the following, the steps of the method according to the first aspect of the present invention are described. In particular, the contribution of each step with respect to a decrease of possible defects of the produced solid-state component will be illustrated.
All steps of the method according to the first aspect of the present invention forming the production process of the solid-state component are carried out in a reaction chamber. Said reaction chamber is sealed with respect to the ambient atmosphere and can thereby contain a reaction atmosphere different to the ambient atmosphere. In other words, within the reaction chamber, any reaction atmosphere suitable for the current step of the method according to the first aspect of the present invention can be contained. In particular, the reaction atmosphere can even be changed for the different steps of the method according to the first aspect of the present invention, if suitable and/or necessary. The reaction chamber can comprise a single reaction volume, but also embodiments of the reaction chamber with two or more reaction volumes sealable against each other are possible.
The starting condition of the method is a substrate already placed in the reaction chamber. A previously performed preparation of the substrate surface of the substrate, for instance a chemical cleaning or degassing in vacuum, in advance of the arrangement of the substrate in the reaction chamber is possible. The reaction chamber is sealed with respect to the ambient atmosphere.
In the first step a) of the method according to the first aspect of the present invention the substrate surface of the substrate is prepared. For this, the reaction chamber is filled with a suitable first reaction atmosphere for the planned preparation of the substrate surface, for instance a vacuum or a reaction atmosphere containing oxygen. Further, a first electromagnetic radiation is coupled into the reaction chamber by suitable coupling means, for instance by a chamber window in a chamber wall of the reaction chamber.
The first electromagnetic radiation impinges onto the substrate. Preferably, it impinges onto a surface of the substrate opposed to the substrate surface, namely in most of the cases onto the backside of the substrate. Alternatively, or additionally, also a direction of the first electromagnetic radiation directly impinging onto the substrate surface of the substrate is possible. The first electromagnetic radiation can be provided as a single beam or as two or more separated beams, and additionally be pulsed or continuous.
By at least partly absorbing the energy of the impinging first electromagnetic radiation, the substrate is heated. By this, impurities on the substrate surface can be evaporated and also be oxidized, if a first reaction atmosphere containing oxygen is used.
In addition, the heating of the substrate can also lead to annealing processes. In other words, the number of missing or additional atoms on the substrate surface can be reduced and also discontinuities of a symmetry present on the substrate surface or even within its bulk can be healed.
Using a first electromagnetic radiation for the aforementioned healing also provides the advantage that no additional heater or other means for connecting the substrate to such a heater, like conductive silver, is necessary within the reaction chamber. Impurities and defects caused by the presence of additional elements arranged within the reaction chamber can thereby be avoided.
In summary, after step a) of the method according to the first aspect of the present invention the substrate is provided in the reaction chamber with a substrate surface well prepared for the following deposition of the one or more thin films. In particular, already the substrate surface of the substrate comprises preferably no or at least a very low and limited number of defects.
In the second step b) of the method according to the first aspect of the present invention, one or more thin films comprising a first material are deposited onto the substrate surface prepared in step a) of the method according to the first aspect of the present invention. The first material can be chosen suitable for the purpose of the produced solid-state component, in particular for instance with respect to conductivity and/or super conducting properties.
The first material is evaporated and/or sublimated within the reaction chamber. For this, a source element comprising the first material is arranged in the reaction chamber. Likewise to the heating of the substrate in step a), also in step b) of the method according to the first aspect of the present invention a second electromagnetic radiation is used, again coupled into the reaction chamber by suitable coupling means, for the evaporation and/or sublimation of the first material. All advantages described above with respect to the usage of the first electromagnetic radiation, in particular the possibility to avoid additional elements within the reaction chamber, can thereby also be provided in step b). The evaporation and/or sublimation of the first material can thereby be provided in an exceptionally clean way with high purity of the evaporated and/or sublimated first material.
In addition, an evaporation and/or sublimation rate can be adjusted easily by accordingly choosing the intensity of the second electromagnetic radiation. Hence the evaporation and/or sublimation rate most suitable for a steady and even deposition of the first material onto the substrate surface, in other words for a steady and even growth rate of the respective thin film, can be used. As a result, the deposited one or more thin films comprises preferably no or at least a very low and limited number of defects.
As already described above, also the substrate surface comprises preferably no or at least a very low and limited number of defects. As defects of the substrate surface would be inherited by the deposited one or more thin films, the aforementioned feature of no or at least a very low and limited number of defects of the deposited one or more thin films is further improved by the already defect free or at least extremely defect depleted substrate surface.
In addition, the usage of the second electromagnetic radiation allows the usage of a wide range of first materials. In particular, all pure chemical elements providable in solid or fluid form can be easily used as first material in the method according to the first aspect of the present invention. With a source element suitably constructed as a gas cell, the remaining gaseous chemical elements can also be used as first material in the method according to the first aspect of the present invention. In summary, a deposition of thin films onto the substrate surface with an extremely high purity with respect to foreign atoms and crystal impurities can be provided.
However, the method according to the first aspect of the present invention is not restricted to chemically pure materials. In particular also chemical compounds, mixtures or alloys can be used as first material.
Further, for step b) the reaction chamber is filled with a second reaction atmosphere suitably chosen for the intended deposition of the one or more thin films. As an example, for a deposition of only the evaporated and/or sublimated first material with high purity, a high or ultrahigh vacuum can be chosen as second reaction atmosphere. On the other hand, for instance a second reaction atmosphere containing oxygen allows an oxidation of the evaporated and/or sublimated first material and hence the deposition of a thin film of the respective oxide.
In summary, the one or more thin films deposited in step b) can be chosen from a vast variety of possible compositions. The different possibilities have in common that the deposited thin film is of high purity. Further, the deposited one or more thin films comprises no or at least a very low number of defects.
In the last step c) of the method according to the first aspect of the present invention the solid-state component is formed. For this, a third electromagnetic radiation is coupled into the reaction chamber, again by suitable coupling means. The third electromagnetic radiation is used for illuminating the one or more thin films deposited in step b) and/or the substrate and can be chosen accordingly to one or more of the purposes described in the following. Again, the usage of an electromagnetic radiation for these purposes provides all advantages described above with respect to the thereby rendered possible absence of additional elements within the reaction chamber.
First of all, the solid-state component can be tempered. For this, the electromagnetic radiation heats the solid-state component whereby the heating triggers annealing processes. In other words, the already low number of missing or additional atoms on the surface of the solid-state component can be reduced further and also discontinuities of a symmetry present on the surface and/or in the bulk of the solid-state component can be healed.
In addition, also a controlled cooling of the solid-state component can be provided. For this, the heating of the solid-state component is gradually reduced, in particular with a smaller rate than an unaffected cooling of the solid-state component, normally dominated by radiation cooling, would take place. This is especially advantageous if the substrate and the formed solid-state component comprise different thermal expansions. Internal tension caused by said different thermal expansions during a rapid cooling of the solid-state component, which would again cause defects in the solid-state component, can therefore be avoided.
Further, for step c) the reaction chamber is filled with a third reaction atmosphere suitable chosen for the intended processes, in particular for tempering and/or controlled cooling of the formed solid-state component.
In summary, in step c) of the method according to the first aspect of the present invention the preferred lack or at least the very low number of defects in the formed solid-state component is at least preserved or yet even lowered further.
As described above, all steps a) to c) of the method according to the first aspect of the present invention provide measures for lowering the number of defects in the produced solid-state component. Nevertheless, an exposure to external influences, in particular an exposure to the ambient atmosphere, would lessen or even completely destroy all positive effects provided by the measures taken according to steps a) to c).
Hence it is crucial for the method according to the first aspect of the present invention that during the steps a) to c) the reaction chamber stays sealed with respect to the ambient atmosphere and both the substrate and the subsequent solid-state component, respectively, continuously stay in the reaction chamber. In other words, all steps a) to c) are successively carried out within the same reaction chamber, even though maybe in different reaction volumes of the reaction chamber, and the sealing against the ambient atmosphere stays intact during the whole process. Hence, all steps a) to c) of the method according to the first aspect of the present invention are carried out in-situ. External influence on the substrate and the subsequent formed solid-state component, in particular a contact to the ambient atmosphere, can thereby avoided.
In other words, together with the requirement of the continuous arrangement of the substrate and the subsequent formed solid-state component in the reaction chamber the measures of each of the steps a) to c) for lowering the number of defects in the formed solid-state component sum up and even reinforce each other.
In summary, the solid-state components produced with a method according to the first aspect of the present invention comprises no or at least a very low number of defects per cm2 and layer. By that, such solid-state components produced with a method according to the first aspect of the present invention are splendidly suitable for a usage as basis of a quantum component, in particular for a qubit. Especially, the absence or at least very low number of defects allow the fabrication of qubit structures with qubit relaxation times and qubit coherence times above 100 μs, preferably above 1000 μs, even more preferably above 10 ms.
In addition, the method according to the first aspect of the present invention can comprise that laser light, in particular laser light with a wavelength between 10 nm to 100 μm, preferably with a wavelength selected in the visual or infrared range, especially with a wavelength between 350 nm and 20 μm, is used as first electromagnetic radiation and/or second electromagnetic radiation and/or third electromagnetic radiation. Laser light comprises the advantage that it is coherent and can be provided over a wide range of wavelengths and intensities. For each specific purpose of the respective electromagnetic radiation in steps a) to c), an appropriate laser light can be chosen. For instance for heating of a wide variety of oxide substrates, an infrared laser, in particular a CO2 laser, with a wavelength of around 10 μm can be used. On the other hand, for evaporating and/or sublimating a metallic first material, lasers with wavelengths of around 1 μm or around 0.5 μm are suited better. The laser light can be provided in pulses, or more preferably in a continuous way. A very homogeneous heating of the substrate surface, evaporation and/or sublimation of the first material and illumination of the one or more thin films and/or the substrate, can thereby be provided.
In addition, the method according to the first aspect of the present invention can be improved by that for the first electromagnetic radiation and the second electromagnetic radiation, and/or for the second electromagnetic radiation and the third electromagnetic radiation, and/or for the first electromagnetic radiation and the third electromagnetic radiation, laser light with the same wavelength is used. By using laser light of the same wavelength as at least two of the three used electromagnetic radiations, the number of laser sources necessary for the method according to the first aspect of the present invention, and subsequently the complexity of the apparatus for carrying out this method, can be reduced. Initial cost and also maintenance cost can also be lowered.
Further, the method according to the first aspect of the present invention can comprise that the first reaction atmosphere and/or the second reaction atmosphere and/or the third reaction atmosphere is chosen from the following list:
This list is not closed, and if suitable also other atmospheres can be chosen. The gaseous atmospheres listed above can be provided with a pressure of 10−8 hPa to ambient pressure, respectively up to 1 hPa. The oxygen variants O2 and O3 are can preferably be provided in a ratio of approximately 9:1 as produced by an inline glow discharge ozone generator.
The method according to the first aspect of the present invention can also be characterized in that the first reaction atmosphere and/or the second reaction atmosphere and/or the third reaction atmosphere is at least partly ionized, in particular ionized by plasma ionization. Ionized atoms or molecules of a reaction atmosphere can provide an enhancement in reactivity of the respective reaction atmosphere. Hence, if a reaction of the respective reaction atmosphere is desired, for instance for a deposition of a thin film containing a reaction product of the evaporated and/or sublimated first material and elements of the respective second reaction atmosphere, ionizing the atoms and/or molecules of the respective second reaction atmosphere can be of advantage.
Furthermore, the method according to the first aspect of the present invention can comprise that the first reaction atmosphere and the second reaction atmosphere and the third reaction atmosphere are identical. In other words, the reaction atmosphere stays the same during all of the steps a) to c). Changing the current reaction atmosphere after finishing one of the steps a) or b) and/or moving the substrate or the subsequent solid-state component, respectively, into another reaction volume, is not necessary. In summary, the method according to the first aspect of the present invention can be simplified.
In an alternative embodiment of the method according to the first aspect of the present invention, the first reaction atmosphere and the second reaction atmosphere are different and are exchanged between step a) and step b) and/or the second reaction atmosphere and the third reaction atmosphere are different and are exchanged between step b) and step c). This is the case, if the solid-state component to be produced demands different reaction atmospheres in at least two of the steps a) to c) of the method according to the first aspect of the present invention.
As already mentioned above, by choosing a reaction atmosphere most suitable for the specific purpose, the quality of the resulting solid-state component with respect to defects can be optimized. In practice, different reaction atmospheres for different steps of the method according to the first aspect of the present invention can preferably be provided with a reaction chamber, which already comprises different reaction volumes which are sealable against each other and can therefore contain different reaction atmospheres. Between the steps of the method according to the first aspect of the present invention, the substrate or the subsequent solid-state device can thereby simply internally moved within the reaction chamber without breaking the sealing with respect to the ambient atmosphere.
Further, the method according to the first aspect of the present invention can be characterized in that a substrate is used with a material chosen from the following list:
This list is not closed, and if suitable also other materials can be chosen for the respective substrate. In this context, the respective substrate is preferably provided as a single crystal.
In particular, the method according to the first aspect of the present invention can be enhanced by that a substrate is used which is similar to the thin film in one or more of the following aspects, preferably in all of the following aspects:
Similar to the thin film in the scope of the present invention means that a value of the respective aspect of the substrate deviates less than 10%, preferably less than 5%, form the value of the respective aspect of the thin film. By choosing a substrate similar to the thin film an abrupt transition from the substrate to the thin film, which again might cause defects, can be avoided. The quality of the produced solid-state component can thereby be improved further.
Further, the method according to the first aspect of the present invention can comprise that in step a) at least the substrate surface is heated to a temperature between 900° C. and 3000° C., in particular 1000° C. to 2000° C. These temperatures are found to be most suitable for effectively healing defects in a wide variety of substrate materials. Preferably, the heating of the substrate surface can be applied indirectly via heating the backside of the substrate, as such backside heating allows an especially compact setup and in particular is also suitable for illuminating the substrate in step c) of the method according to the first aspect of the present invention.
In addition, the method according to the first aspect of the present invention can be characterized in that step a) includes providing a flux of a termination material directed onto the substrate surface. Preferably, the termination material is one of the materials of the substrate. Thereby the flux of termination fills defects in the substrate surface of the substrate. As the heating of the substrate also included in step a) can lead to an evaporation and/or sublimation of substrate material, the flux of termination material can be used to balance this effect. Ideally, an equilibrium between atoms leaving the substrate surface and atoms added to the substrate surface will be established. This equilibrium can be adjusted by adapting the temperature of the substrate and/or the flux of the termination material. In summary, the quality of the substrate surface with respect to defects can be improved further.
In another embodiment of the method according to the first aspect of the present invention, a substrate holder is used for holding the substrate, the substrate holder comprising a smaller absorption with respect to the first electromagnetic radiation and/or the third electromagnetic radiation in comparison to the substrate. In other words, even if the substrate holder is accidentally illuminated in steps a) and/or c) of the method according to the first aspect of the present invention by the first and third, respectively, electromagnetic radiation, it will absorb less energy and therefor will be heated less. Thereby it can be ensured that the substrate, except for the evaporation sources, is the element with the highest temperature within the reaction chamber, which effectively prevents an absorption of impurities onto the substrate out of the reaction chamber.
Further, the method according to the first aspect of the present invention can comprise that in step b) the first material comprises two or more different material components and the source element accordingly comprises two or more distinct component sections, whereby each component section provides one of the two or more material components, and whereby the second electromagnetic radiation accordingly comprises two or more component beams, each of the two or more component beams adapted for the evaporation and/or sublimation of one of the two or more material components.
In other words, the first material is not limited to a single source, but can also be composed of two or more components, wherein each component can be provided in a separate source. In other word, as nevertheless each of the material components is provided separately and in high purity, in particular even as pure chemical elements, by the method according to the first aspect of the present invention also thin films consisting of chemical compounds and/or alloys can be provided with high chemical purity and low number of defects.
Essentially, the two or more components are evaporated and/or sublimated simultaneously. The two or more different material components can bond to each other already within the reaction chamber and/or after the respective deposition onto the substrate surface. The source element can comprise a common holding structure for the two or more distinct component sections. Alternatively, also separate entities of the source element, each providing one or more component sections, are possible. In the following, the expression “source” is used in the meaning of both, a component section of a source element, and a separate source element, respectively. In summary, the variety of providable first materials for the one or more thin films can be enlarged.
Preferably, the method according to the first aspect of the present invention can be characterized in that the evaporation and/or sublimation of step b) is carried out below the plasma threshold of the first material. Thereby it is ensured that only evaporation and/or sublimation of the first material takes place. In addition, the evaporated and/or sublimated first material is provided in an electrical neutral state and hence disturbing charging effects within the reaction chamber can be avoided.
In addition, the method according to the first aspect of the present invention can comprise that for the first material a metal is used, preferably copper aluminum, tantalum and/or niobium, and/or a superconducting material, in particular a metal, is used, which is superconductive at temperatures >˜4K, preferably >˜77K, preferably tantalum or niobium or aluminum or granular aluminum or NbN or NbTiN or TiN. Metals and/or in particular superconducting materials are most suitable for solid-state devices, which are intended to be used as qubits. In particular superconducting materials, which are superconductive at temperatures >77K allow a cooling already with liquid nitrogen, which is very convenient.
According to another preferred embodiment of the method according to the first aspect of the present invention, the first material used for the evaporation and/or sublimation in step b) is self-supporting and can thereby be provided crucible free. Hence no other material is present near to the spot where the second electromagnetic radiation hits the surface of the first material and evaporates and/or sublimates the first material. Impurities caused by evaporation and/or sublimation and/or incorporation into the melt and subsequent coevaporation from the melt of a holding structure holding the first material, such as for instance a crucible, can thereby be avoided.
In addition, the method according to the first aspect of the present invention can comprise that the material of the thin film deposited in step b) is a reaction product of the evaporated and/or sublimated first material and a component of the second reaction atmosphere. For instance, if the second reaction volume comprises oxygen, an oxide of the first material can be provided as material for the thin film deposited onto the substrate surface. Also other reaction products, such as for instance nitrides or halides, are possible. In summary, the range of possible materials for the one or more thin films deposited onto the substrate surface can thereby be enlarged.
Further, the method according to the first aspect of the present invention can be characterized in that step c) comprises two or more separated tempering iterations. In each tempering iteration some of the still present defects in the formed solid-state device are healed. By providing two or more tempering iterations, the finally resulting number of defects can be lowered further.
In a preferred enhancement of the method according to the first aspect of the present invention, step c) comprises a cooling controlled by the third electromagnetic radiation after each of the one or more tempering iterations. As mentioned above, a rapid cooling of the solid-state component may cause new defects, in particular if the substrate and the formed solid-state component comprise different thermal expansions. This can be avoided by inserting a distinct controlled cooling step between each of the two or more tempering iterations. In particular, the third electromagnetic radiation is used for heating the substrate and/or the solid-state component, whereby the amount of heating is gradually reduced and thereby a slow and controlled cooling is achieved.
In yet another embodiment of the method according to the first aspect of the present invention, step b) is repeated one or more times for providing a multi-layer structure for the thin film. Hereby the first material used in the different iterations of step b) can be identical or different. Also repetitions of step b) with iteration patterns with respect to the used first materials in the respective step b) are possible. In other words, possible layer sequences for such multi-layer structures can be for instance but are not limited to AAAAAA, ABABABA, ABCABC, ABACAD, ABBACC with different first materials A, B, C, D. In particular, also more than four different first materials and especially also a number of layers different to six can be employed. A vast variety of multi-layer structures for the solid-state component can thereby be provided.
Additionally, the respective iterations of step b) can preferably be not carried out seamlessly back-to-back but separated with a break in between. By that it can be ensured that the conditions within the reaction chamber, for example with respect to the temperature and state of matter of the surface of the respective source element illuminated by the first electromagnetic radiation, return to their initial values as provided before the first iteration of step b). Hence each of the repetitions of step b) is carried with the same environmental conditions and therefore an especially pure and even multi-layer structure can be provided.
Preferably, the method according to the first aspect of the present invention can be enhanced by that after each repetition of step b), an iteration of step c) carried out. In other words, after each repetition of step b) the layers already deposited onto the substrate are tempered and/or cooled in a controlled way. All advantages described above provided by carrying out any of the varieties of step c) can therefore be provided for each of the layers of the multi-layer structure which forms the solid-state component. As a result, the number of defects in the finally formed solid-state component can be lowered further.
In an enhanced embodiment of the method according to the first aspect of the present invention, each step b) and each step c) are identically carried out with respect to the used electromagnetic radiations and the used reaction atmospheres and the first material. Hence the solid-state component produced by this embodiment of the method according to the first aspect of the present invention comprises a multi-layer structure with two or more identical layers.
In an alternative enhancement of the method according to the first aspect of the present invention, for one or more of the one or more repetitions one or more of the following parameters are changed:
In other words, for instance different first materials can be used for the different layers of the multi-layer structure.
As another example, by changing the second reaction atmosphere between vacuum and oxygen while keeping a metal as first material, a multi-layer structure with alternating layers of the pure metal and its oxide can be provided. In summary, the possible variations of providable multi-layer structures are more or less not limited, but share the common feature that each variation can be provided with no or at least a very low number of defects.
Preferably, the method according to the first aspect of the present invention can further comprise that as final procedure of step a) one or more buffer layers comprising a buffer material are deposited onto the substrate surface, whereby the buffer material is evaporated and/or sublimated by a fourth electromagnetic radiation coupled into the reaction chamber while the reaction chamber contains a fourth reaction atmosphere, whereby preferably the fourth electromagnetic radiation and the fourth reaction atmosphere are identical to a respective one used in one of the steps a), b) or c).
As mentioned above, preferably a substrate is used which is similar to the thin film to be deposited onto the substrate surface of the substrate. However, this preferred way of carrying out the method according to the first aspect of the present invention is not always feasible. By adding a buffer layer, differences between the substrate and the thin film with respect to lattice symmetry, lattice parameter, surface reconstruction and/or surface termination can be bridged, or the quality of the substrate surface can be improved if a buffer layer of the same material as the bulk substrate can be grown with a higher structural quality than the bulk substrate itself.
Preferably, by using more than one buffer layer, the buffer materials and the respective fourth reaction atmospheres for the deposition of the respective buffer layers can be chosen differently in such that the resulting buffer layer closest to the substrate is similar in the sense of the present invention to the substrate, whereby in each added buffer layer the similarity to the substrate is reduced and simultaneously a similarity to the thin film is enhanced, resulting in a topmost buffer layer, which is similar in the sense of the present invention to the thin film. In consequence, a smooth deposition of the thin film onto the buffer layers, in particular onto the topmost buffer layer, can be provided without seeds for defects caused by the differences between the substrate and the one or more thin films.
A typical buffer material for such a buffer layer is for instance aluminum.
The usage of electromagnetic radiation for evaporation and/or sublimation of the buffer material can provide the same advantages as described above with respect to the usage of the second electromagnetic radiation. In particular, no further elements or components are needed within the reaction chamber for the evaporation and/or sublimation of the buffer material and hence impurities caused by such further elements and components can be avoided.
In addition, the method according to the first aspect of the present invention can comprise that after the carrying out of the last step b) one or more cover layers comprising a cover material are deposited onto the one or more thin films, whereby the cover material is evaporated and/or sublimated by a fifth electromagnetic radiation coupled into the reaction chamber while the reaction chamber contains a fifth reaction atmosphere, whereby preferably the fifth electromagnetic radiation and the fifth reaction atmosphere are identical to a respective one used in one of the steps a), b) or c).
Such a cover layer, also known as cap layer, shields the one or more thin films against the environment and hence provides protection of the solid-state component. An endurance of the low number of defects present in the solid-state component can therefore be enlarged. In particular, an undesired deposition of further material onto the topmost surface of the one or more thin films can therefore be provided.
The usage of electromagnetic radiation for evaporation and/or sublimation of the cover material can provide the same advantages as described above with respect to the usage of the second electromagnetic radiation. In particular, no further elements or components are needed within the reaction chamber for the evaporation and/or sublimation of the cover material and hence impurities caused by such further elements and components can be avoided.
According to a second aspect of the present invention, the object is satisfied by a solid-state component, in particular for a quantum component, preferably for a qubit, comprising one or more thin films, one of the one or more thin films comprises a first material with a thickness between a monolayer and 100 nm and is deposited onto a substrate surface of a substrate.
The solid-state component according to the second aspect of the present invention is characterized in that the solid-state component is obtainable by a method according to one of the preceding claims. By that the solid-state component according to the second aspect of the present invention can provide all advantages described above with respect to the method according to the first aspect of the present invention.
According to a third aspect of the present invention, the object is satisfied by a solid-state component, in particular for a quantum component, preferably for a qubit, comprising one or more thin films, one of the one or more thin films comprising a first material with a thickness between a monolayer and 100 nm and is deposited onto a substrate surface of a substrate.
The solid-state component according to the third aspect of the present invention is characterized in that one of the one or more thin films, preferably all of the one or more thin films, each have qubit relaxation times and qubit coherence times above 100 μs, preferably above 1000 μs, even more preferably above 10 ms. Such thin films have very few and preferably no defects and enable the use of such devices as qubits. Preferably the solid-state component according to the third aspect of the present invention is obtainable by a method according to the first aspect of the present invention.
According to a fourth aspect of the present invention, the object can be satisfied by a quantum component, preferably a qubit, comprising solid-state component. The quantum component according to the fourth aspect of the invention can be characterized in that the solid-state component is a solid-state component according to the second or third aspect of the present invention, respectively. By that the quantum component according to the fourth aspect of the present invention can provide all advantages described above with respect to the solid-state component according to the second or third aspect of the present invention.
Further, the quantum component according to the fourth aspect of the present invention can comprise that the quantum component is a superconducting qubit, in particular a charge qubit or a flux qubit or a phase qubit. Superconducting qubits are based on currents which flow with no resistance due to their superconductivity. By that said currents are robust with respect to external disturbances and hence are able to maintain the quantum state represented by the currents over long time periods. Coherence times above 100 μs, preferably above 1000 μs, even more preferably above 10 ms, can be reached.
In addition, the quantum component according to the fourth aspect of the present invention can be enhanced in that superconducting qubit comprises thin films with a multi-layer structure comprising one or more superconducting layers and one or more isolating layers. In particular, the respective layers of the multi-layer structure of the thin films are deposited using a method according to the first aspect of the present invention and hence comprise all advantages described above with respect to the method according to the first aspect of the present invention. In particular, the respective layers comprise no defects or at least a very low number of defects. Hence the reachable coherence times are increased further.
In another embodiment, the quantum component according to the fourth aspect of the present invention can comprise that one or more of the one or more superconducting layers consists of one of the following materials:
Both lists are not closed, and also other suitable materials can be used both for the superconducting layers and the isolating layers, respectively.
In addition, the quantum component according to the fourth aspect of the present invention can be characterized in that the one or more superconducting layers and/or the one or more isolating layers comprise a thickness between 1 nm and 300 nm, preferably a thickness between 10 nm and 200 nm. In particular, superconducting layers can preferably comprise a thickness of between 5 nm and 300 nm, whereas isolating layers can preferably comprise a thickness of between 20 nm and 300 nm. Further, isolating barriers between superconducting elements, both of them preferably built by layers or at least parts of the layers of the multi-layer structure, can preferably comprise a thickness between 1 nm and 10 nm. By choosing thicknesses within the above-mentioned ranges, an especially good performance of the quantum component according to the fourth aspect of the present invention can be provided.
According to a fifth aspect of the present invention, the object can be satisfied by an apparatus for producing a solid-state component according to the second and/or third aspect of the present invention and/or for carrying out a method according to the first aspect of the present invention, at least comprising:
Preferably, the apparatus according to the fifth aspect of the present invention is a TLE (thermal laser evaporation) apparatus. Further, by producing a solid-state component according to the second or third aspect of the present invention, and especially by carrying out a method according to the first aspect of the present invention, the apparatus according to the fifth aspect of the present invention can provide all advantages described above with respect to the solid-state component according to the second or third aspect of the present invention and/or described with respect to the method according to the first aspect of the present invention.
In a preferred embodiment, the apparatus according to the fifth aspect of the present invention can comprise that the reaction chamber comprises at least two separated reaction volumes, whereby the at least two reaction volumes are sealable against each other and whereby the substrate arrangement can be moved between the at least two reaction volumes within the reaction chamber continuously sealed with respect to the ambient atmosphere. By that a change of the reaction atmosphere during the production of the solid-state component can be simplified.
Without two or more separate reaction volumes, changing the reaction atmosphere can be difficult and cumbersome. For instance, for changing the reaction atmosphere from gaseous substance A to gaseous substance B, as first step it has to be ensured that A is no longer present in the reaction chamber, i.e. a vacuum, in particular an ultra high vacuum, has to be established within the reaction chamber, which can be time consuming. Only afterwards, B can be filled into the reaction chamber.
In contrast to that, with two or more reaction volumes, which are sealed with respect of the ambient atmosphere and also sealable against each other, each of the reaction volumes can contain a different reaction atmosphere. For changing the reaction atmospheres, for instance after carrying out step a) and before the begin of step b), the substrate arrangement is simply moved from one of the reaction volumes into the other. Preferably, suitable valves and materials locks are arranged between the reaction volumes. In summary, this simplifies the production of the solid-state component and an enormous saving of time can be provided.
The invention will be explained in detail in the following by means of embodiments and with reference to the drawing in which is shown:
If required, a process gas G can be introduced into the vacuum chamber 12 from a gas supply 20 along the arrow pointing into said vacuum chamber 12. The process gas G, also known as reaction gas can be selected from such gases such as oxygen, ozone, plasma-activated oxygen, nitrogen, plasma-activated nitrogen, hydrogen, F, Cl, Br, I, P, S, Se, and Hg, or compounds such as NH3, SF6, N2O, CH4. The pressure of the process gas G can be selected in the range of 10−8 hPa to ambient pressure, respectively for pure ideal conditions in the range of 10−8 hPa to 1 hPa.
The vacuum pump 18 optionally together with the gas supply 20 provides a respective reaction atmosphere in the reaction chamber 10, i.e. a vacuum optionally combined with a pre-defined gas atmosphere.
The reaction chamber comprises a substrate arrangement 22 at which a substrate 24 can be arranged. In practice it is possible to provide a plurality of substrate arrangements 22 and/or to arrange a plurality of substrates 24 on one or more substrate arrangements 22.
The substrate 24 that is used can typically be a single crystal wafer, with a material of the wafer typically being selected from the group of members consisting of: SiC, AlN, GaN, Al2O3, MgO, NdGaO3, DyScO3, TbScO3, TiO2, (LaAlO3)0.3(Sr2TaAlO6)0.35 (LSAT), Ga2O3, SrLaAlO4, Y:ZrO2 (YSZ) and SrTiO3. Such single crystal wafers are typically used in the production of solid state components, and are interesting candidates for the production of quantum components, such as qubits.
During coating and pre-treatment of the substrate 24, which can be present in the form of a single crystal wafer, the substrate 24 is heated using a substrate heating laser 26.
The substrate heating laser 26 is typically an infrared laser that operates with a wavelength in the infrared region, specifically with a wavelength selected in the range of ca. 1 to 20 μm, especially of around 8 to 12 μm. Such wavelengths can e.g. be made available via a CO2 laser 26.
The substrate heating laser 26 typically heats a substrate surface 48 of the substrate 24, i.e. a frontside of the substrate 24, via indirect heating via a backside 50 of the substrate 24. Thereby the substrate surface 48 can be heated to a temperature between 900° C. and 3000° C., in particular 1000° C. to 2000° C. Consequently, the intensity of the substrate heating laser 26 is varied to achieve the various desired temperatures in dependence on the sublimation rate respectively sublimation temperature of the substrate constituent having the highest sublimation rate.
Typically the intensity of the substrate heating laser 26 can be varied within the range of 4 W to 1 KW for substrate sizes of 5×5 mm2 or 10×10 mm2. To be able to reach the required preparation temperatures, 100 W are required for a 10×10 mm2 sapphire substrate to reach 2000° C., 500 W are required for a 10×10 mm2 SrTiO3 substrate to reach 1400° C. The required temperature varies significantly. According to Planck's radiation law, the emitted power per area depends on the emittance of the material, which is a material property, and upon temperature as T4, which means that the required power increases dramatically with temperature.
To cover the range of temperatures for the preparation of epitaxial templates according to the invention, we find a necessary maximum power density on the substrate of 1 kW/cm2, with significantly smaller values such as e.g. around 100 W/cm2 for sapphire at 2000° C.
Due to the dramatic T4 dependence on temperature, the substrate heating laser at the same time requires a high dynamic range with the ability to maintain stable low power levels for materials that require lower temperatures for substrate preparation, and in particular for the deposition of epitaxial layers on the substrate template at lower temperatures.
It should also be noted that the substrate 24 may be heated from the front, the side or in a different manner. Depending on the heating means, it should simply be ensured that the temperature of the substrate surface 48 can be heated to within a range of 900° C. to 3000° C., in order to be able to ensure that one of the substrate constituents, i.e. one of the elements forming the substrate, can be moved along the substrate surface 48 during the heating step and may desorb or sublimate from the substrate surface 48 for generation of a desired epitaxial template 60 (see e.g.
The temperature of the substrate surface 48 can be measured using a pyrometer or the like (not shown).
As indicated by the double headed arrow 28, the substrate arrangement 22 can be transferred into and out of the vacuum chamber 12 using a suitable apparatus (not shown).
In order to coat the substrate 24 with one or more layers of thin films 62 (see
In this context it should be noted that a material of the respective source 30, 32 can be selected from any element of the periodic table, provided it is solid at the temperatures and pressures selected within the respective vacuum chamber 12 used for the deposition of the thin film 62.
In this connection it should be noted that preferred materials for the respective source 30, 32 are Sc, Ti, V, Cr, Mn, Fe, Co, Ni, Cu, Zn, Zr, Nb, Mo, Ru, Hf, Al, Mg, Ca, Sr, Ba, Y, Rh, Ta, W, Re, Ir, Ga, In, Si, Ge, Sn, Eu, Ce, Pd, Ag, Pt, and Au, if the above elements are deposited in an oxygen/ozone mixture as a reaction atmosphere with approximately 10% to deposit binary oxides as thin films 62. In order to deposit single crystal thin films 62, a vacuum atmosphere is typically used.
First and second source heating lasers 36, 38 that are respectively directed at the first and second source elements 30, 32 are also provided. The first and second source heating lasers 36, 38 make available different evaporation and/or sublimation temperatures at the first and second source elements 30, 32.
The first and second source heating lasers 36, 38 typically make available laser light at the first and second source elements 30, 32 with a wavelength selected between 280 nm and 20 μm. For metallic sources, it is preferred if the source heating lasers 36 and 38 make available light in the wavelength range selected between 350 nm and 800 nm due to the increasing absorptivity of metals at shorter wavelengths. Although high-power lasers with short wavelengths below 515 nm are not yet commercially viable, the highest absorptivities according to low-power measurements can be expected at 300 nm. Should lasers with this wavelength become available, the preferred wavelength for the source heating lasers would be 300 nm±20 nm.
In this context it should further be noted that the lasers 26, 36, 38 can be operated in pulsed modes, but are preferably used as continuous sources of radiation. A continuous laser 26, 36, 38 introduces less energy per unit time than a pulsed source which could lead to a damaged source 30, 32
In order to sublimate and/or evaporate elements from the first and second source elements 30, 32 to ensure that these arrive at the substrate surface 48 for coating of the substrate 24, a suitable intensity of the first and second source heating lasers 36, 38 has to be selected. This intensity depends on the distance of the first and second source elements 30, 32 from the substrate surface 48. For a given flux density at the substrate surface, the intensity increases and/or decreases as the first and second source elements 30, 32 are moved away from and/or towards the substrate surface 48.
In the present examples, the substrate surface 48 is placed 60 mm away from the respective first and second source elements 30, 32. The intensity of the laser is correlated approximately to the square of the distance between the first and second source elements 30, 32 and the substrate surface 48. Hence for an increase of a factor two in the distance between the first and second source elements 30, 32 and the substrate surface 48, the intensity of the laser has to be increased by approximately a factor of four.
Hence, the intensities specified in the following are for a distance of 60 mm between the first and second source elements 30, 32 and the substrate surface 48. If a larger distance is selected then the intensity of the respective first and second source heating lasers 36, 38 has to be increased and vice versa, if the distance is reduced.
Generally speaking the substrate heating laser 26, the first and second source heating lasers 36, 38 make available laser light, in particular laser light with a wavelength between 10 nm to 100 μm, preferably with a wavelength selected in the visual or infrared range, especially with a wavelength between 280 nm and 1.2 μm. These lasers 26, 36, 38 make available first electromagnetic radiation and/or second electromagnetic radiation and/or third electromagnetic radiation, and/or further types of electromagnetic radiation.
The first and second source heating lasers 36, 38 are provided to evaporate and/or sublimate first and second materials from the first and second source elements 30, 32 by heating the first and second source elements 30, 32 to a temperature below the plasma threshold of the first material and/or of the second material.
A shielding aperture 40 is schematically illustrated in the vacuum chamber 12 that functions as a shield to prevent the sublimated and/or evaporated source material to deposit on an entrance window 52 of the chamber. If such a layer of material is deposited on the window 52, then the intensity of the respective laser 26, 36, 38 has to be adapted over time to compensate for this material absorbed on the window.
Moreover, the shielding aperture 40 can also act as a shield to prevent reflected laser light of one of the lasers 26, 36, 38 from being focused back into one of the lasers 26, 36, 38 which could destroy the respective laser 26, 36, 38.
The shielding aperture 40 can also form a part of a beam shaping system of one or more of the respective lasers 26, 36, 38 and can hence be used as a coupling means for coupling the respective electromagnetic radiation from the first and second source heating lasers 36, 38 into the reaction chamber 10 and onto the first and second source elements 30, 32.
Generally speaking a respective window 52 is arranged between each one of the lasers 26, 36, 38 and the reaction chamber 10 in order to couple the respective laser light into the reaction chamber 10 as further coupling means.
This means that the coupling means can comprise any kind of optical element or laser light beam shaping element that can be used to couple the light from one of the lasers 26, 36, 38 into the reaction chamber, i.e. on to the substrate 24 respectively onto one or more of the first and second source elements 30, 32 for its intended use.
It should be noted in this context that the reaction chamber 10 may also only comprise a single source element 30, or more than two source elements 30, 32, with the further source elements either making available further materials of the same or different kind that can be deposited onto one or more substrates 24 in the reaction chamber 10.
In this context it should be noted that if two or more source elements 30, 32 are made available in the vacuum chamber 12 that laser light from one of the first and second source heating lasers 36, 38 can be directed at one source element 30, 32 for the sublimation and/or evaporation of a thin film 62 comprising the material of the respective source element 30, 32, but not of the other source element 32, 30.
This process can be repeated for each source element provided in the vacuum chamber 12 in order to form multiple different layers and multi-layer and alloy or composite structures on the substrate 24.
Similarly both source elements 30, 32, and if provided, further source elements can have laser light from one of the first and second source heating lasers 36, 38, and if provided from a third source heating laser directed thereat in order to simultaneously sublimate and/or evaporate source material from a plurality of source elements 30, 32 in order to deposit a thin film 62 on the surface 48 of the substrate 24 for the deposition of a compound on the surface 48 of the substrate 24.
Hence the material of the thin film 62 or layer deposited on the substrate 24 is a reaction product of the evaporated and/or sublimated material and a component of the reaction atmosphere, i.e. if provided a compound reacted with the process gas G or a single material thin film 62 if the sublimation and/or evaporation is carried out in vacuum.
Regardless of how many source elements 30, 32 are provided in the vacuum chamber 12 and impinged with laser light at any one given time, a process gas may be introduced into the vacuum chamber and bring about a reaction of the evaporated and/or sublimated source material with the process gas, in order to generate thin films formed of compounds of the source material and of the process gas, such as oxides, as will also be discussed in the following.
It should further be noted that a material of the first and/or second source elements 30, 32 that is used for the evaporation and/or sublimation can be self-supporting and can thereby be provided crucible free, e.g. a Ta source element 30, 32 can be provided that has no crucible associated therewith.
Such reaction chambers 10 may be beneficially selected in the formation of multi-layered films (see
In this way the reaction chamber 10 comprises at least two separated reaction volumes 14, 16, whereby the at least two reaction volumes 14, 16 are sealable against each other, e.g. via the gate valve 44 and whereby the substrate arrangement can be moved between the at least two reaction volumes 14, 16 within the reaction chamber 10 continuously sealed with respect to the ambient atmosphere.
In this context it should be noted that the first reaction atmosphere and the second reaction atmosphere and, if provided, the third or further reaction atmospheres may be identical.
Alternatively, the first reaction atmosphere and the second reaction atmosphere and/or the third reaction atmosphere are different and are exchanged between different reaction volumes 14, 16 or within the first volume 14 and/or reaction volume 16, and/or the second reaction atmosphere and the third reaction atmosphere are different and are exchanged between different reaction volumes 14, 16 or within the first volume 14 and/or reaction volume 16.
In this context it should further be noted that the first reaction atmosphere and/or the second reaction atmosphere and/or the third or further reaction atmospheres are at least partly ionized or excited, in particular ionized by plasma ionization and/or excitation. Excitation describes the transition of one or more electrons within an atom or molecule to energetically higher levels. The relaxation from such higher levels may provide additional energy to enable or improve the chemical reaction between the evaporated atoms or molecules and the activated or ionized reaction gas.
Also for the preparation of the substrate surface 48, for the deposition of the one or more thin films, and for the terminal tempering and/or cooling, respectively, different reaction atmospheres might be suitable. Hence, the availability of different reaction volumes 14, 16 can be of further advantage.
In this context it should be noted that if a solid-state device, in particular a quantum device, preferably for a qubit, comprising one or more thin films 62 should be produced, with the one or more thin films 62 comprising a first material and each said film 62 having a thickness selected between a monolayer and 100 nm and being deposited onto a front surface of a substrate, then the production process can be carried out in a reaction chamber 10 as shown in
Such a method comprises the steps of:
In this context it should be noted that a possible method of preparing the front surface 48 of the substrate 24 can be made available in accordance with the following teaching. It should however, be noted that for less pure layer structures on the substrate 24 also conventional cleaning and purification steps can be carried out.
A specific method of preparing a surface 48 of a single crystal wafer 24 as an epitaxial template 60, the surface 48 comprising surface atoms and/or surface molecules, the single crystal wafer 24 comprising a single crystal composed of two or more elements and/or two or more molecules as substrate constituents, each element and molecule respectively having a sublimation rate, the method comprising the steps of:
Optionally, the surface 48 of the substrate 24 can be irradiated with a continuous flux of the same species to obtain a defined flux equilibrium between atoms or molecules leaving the surface and reaching the surface (chemical potential). This step usually leads to a surface reconstruction which may have energetically equivalent in-plane orientations.
Thereby one can cause a symmetry breaking of the atoms and/or molecules present at the substrate surface 48 due to the step orientation which forces the surface 48 to form only one of the different in-plane orientations.
Crystalline layers that have an orientation uniquely defined with respect to the crystal orientation of the substrate 24 (epitaxial layers) may grow in different in-plane orientations if the surface has different orientations of the surface reconstruction. This leads to defects in the epitaxial layers. This is avoided if using the method of preparing the substrate as disclosed herein by providing only one single orientation of a surface 24 reconstructed using this method.
In this context it should be noted that the sublimation rates of the two or more elements and/or two or more molecules at a given temperature usually differ from one another.
The step of heating the single crystal wafer 24 comprises two heating components: a first component of heating the single crystal wafer 24 at a surface disposed remote from the surface 48 to be treated and a second component of heating is provided by irradiating the surface 48 to be treated with thermal black-body radiation generated by the hot evaporation sources 32, 34.
The flux introduces a pressure on the surface 48 which competes with the desorption flux from the surface, thereby establishing an equilibrium which defines the chemical potential of the flux species at the surface.
Heating the substrate surface and irradiating it with a balancing flux of the volatile component causes several processes to become active.
The first one is the definition of a specific termination (‘black’ or ‘white’, schematically), referring to
The second one is the mobilization of atoms along the surface such that the lowest energy surface in terms of the step structure is adopted, which is the lowest number of steps given by the step height of the first step and the miscut angle.
The third one is the formation of a specific surface reconstruction determined mainly by the substrate temperature and the chemical potential of the volatile flux as controlled by setting the volatile flux.
The fourth one is the selection between different energetically equivalent orientations of the surface unit cell by the choice of miscut direction as shown schematically in
The flux of material, e.g. oxygen for a sapphire substrate 24 fills defects in the surface 48 and aids in providing a surplus of atoms to obtain an equilibrium between atoms leaving and adding atoms to the surface 48. This can be varied by adapting the pressure exerted by the flux, i.e. the amount of oxygen impinged onto the substrate.
By way of example it should be noted that the sublimation temperature is typically a temperature greater than 950° C., around 1700° C. for sapphire and around 1300° C. for SrTiO3.
The two or more elements and/or two or more molecules of the crystal forming the single crystal wafer 24 can be selected from the group of members consisting of: Si, C, Ge, As, Al, O, N, O, Mg, Nd, Ga, Ti, La, Sr, Ta and combinations of the foregoing, by way of example, the single crystal wafers 24 can be made from one of the following compounds SiC, AlN, GaN, Al2O3, MgO, NdGaO3, DyScO3, TbScO3, TiO2, (LaAlO3)0.3(Sr2TaAlO6)0.35 (LSAT), Ga2O3, SrLaAlO4, Y:ZrO2 (YSZ) and SrTiO3.
The step of heating is carried out by the substrate heating laser 26 optionally in combination with one of the first and second source heating lasers 36, 38 provided the respective source comprises a material of the single crystal wafer 24 that has the highest sublimation rate and that should be continuously supplied towards the substrate.
The step of heating during the preparation of the substrate 24 is typically carried out in a vacuum atmosphere selected in the range of 10−8 to 10−12 hPa if no equilibrium between the desorbing flux and a compensating stabilization flux is desired.
With a stabilizing flux, the step of heating during the preparation of the substrate 24 is typically carried out in a vacuum atmosphere selected in the range of 10−6 to 103 hPa.
Thereby an epitaxial template 60 can be formed as shown schematically e.g. in
Generally speaking the substrate 24 is selected such that the substrate matches the layer structure that is to be grown/deposited thereon. Generally speaking a substrate 24 is used which is the same as the thin film 62 grown thereon or deviates by at most 10% from the thin film 62 in one or more of the following aspects, preferably in all of the following aspects: lattice symmetry, lattice parameter, surface reconstruction, and surface termination.
In order to facilitate this it may be necessary or beneficial to deposit a buffer layer on the surface 48 prior to depositing a thin film 62 thereon.
The invention describes a solution to the problem of providing an essentially single crystalline template for subsequent epitaxy or other applications in which a uniform atomic arrangement both normal to the surface 48 and in-plane is advantageous.
For the typical case shown, the step height of the terrace 58 structure does not match the lattice constant of the epitaxial layer 60. This causes the formation of stacking offsets at step edges 66, where the unit cells of the epitaxial layer 60 become shifted with respect to each other. For clarity, in
Using the method of preparation described herein allows to prepare a surface 48 as an epitaxial template 60 that offers both a uniform surface chemistry on all terrace 58 surfaces and a single in-plane orientation of the (usually reconstructed) surface atomic arrangement. The situation shown in
The situations shown in
In practice, a complete coverage can only be achieved with the less volatile element or formula unit covering the surface 48, since such chemical equilibria typically require many orders of magnitude of pressure difference between the different constituents to reach a nearly complete dominance of one element or formula unit. Notably, also the intrinsic volatility difference between the two usually amounts by itself to several orders of magnitude.
The method of preparation therefore consists of heating the substrate crystal 24 to a temperature at which at least the most volatile constituent of the crystal sublimates from the surface 48. It may be necessary to even irradiate the surface 48 with a flux of the volatile species at higher temperatures to avoid the decomposition of the crystal 24 into different, unwanted compounds. Using a sufficiently high temperature such that
In practice, the surface 48 does not switch between bulk-terminated surface layers, but instead forms surface reconstructions, in which the surface atoms rearrange into positions different from the bulk, often even with different stoichiometries, such that the surface energy is minimized. This is illustrated in
Depending on the pressures of the impinging species and the surface temperature, often different surface reconstructions are possible for a given termination, e.g. on sapphire, where there are at least two different Al-rich surface reconstructions.
A surface reconstruction usually involves the formation of a surface supercell spanning several unit cells of the underlying bulk crystal. An arbitrary illustrative example is shown in
This is an undesired configuration, since it leads to faulty boundaries where the domains meet. When used as a template for epitaxial growth, such different surface reconstruction domains may also cause different orientations of the epitaxial film 62 grown on top of it, thereby transferring the in-plane surface reconstruction domain boundaries into the epitaxial film 62 as three-dimensional planar domain boundaries between crystallites of different orientation. This problem may be solved by breaking the symmetry of the surface 48, and thereby favoring one surface unit cell orientation over the other by making them energetically inequivalent.
The proposed way to achieve this according to the invention is the orientation and slope of the surface miscut. When cutting the substrate discs (‘wafers’ 24) from the bulk single crystal, the cutting plane may be directed slightly away from the crystal plane. Depending on this viscinal miscut angle, the prepared surface 48 will have terrace widths and terrace orientations that depend on the direction of the cut and can therefore be controlled at will. Looking at one possible example of a cubic in-plane crystal structure, three different resulting terrace structures are shown schematically in
This miscut, just as any other way of breaking the symmetry of the system, may now be used to favour one of the two different surface unit cells as indicated in
While the in-plane orientation of the step edges, corresponding to the azimuthal component of the miscut angle, selects one surface unit cell orientation over the other, the absolute value of the miscut angle, its polar component, is also important in stabilizing the single orientation structure. At high temperatures, entropy introduces statistical disorder into any system. In this case, as the in-plane surface unit cell orientation is established at an edge and then propagated from unit cell to unit cell, this may lead to faults with again oppositely oriented unit cells at certain average distances on each terrace. With a sufficiently high absolute value of the miscut angle, e.g. 0.05°, the stabilizing steps that imprint one orientation over the other occur at such short distance that this deviation, and thereby increase of defect density, can be avoided.
In the first step a) of the method, shown on the left of
In addition, the first reaction atmosphere 116 can be chosen such that also a composition of the substrate surface 48 is maintained, i.e. a suitable reaction or process gas G can be used, e.g. oxygen in the case of Al2O3 to avoid oxygen depletion and the formation of oxygen vacancies. Further, also a flux of termination material T can be directed onto the substrate surface 48. Preferably, the termination material T comprises, especially consists of, an element of the material of the substrate 24. By this, the termination material T can fill defects on the substrate surface 48 caused by missing atoms or molecules and/or can provide a pressure on the substrate surface 48, preventing atoms or molecules to evaporate from the substrate surface 48.
As an overall result, after step a) the substrate surface 48 is preferably free or at least depleted of defects with respect to the lattice structure of the substrate 24, whereas in addition also defects with respect to surface reconstruction and sur-face termination can be drastically reduced, preferably down to zero.
In the following step b), shown in the middle of
In this connection it should be noted that a thin film 62 as described herein is a layer of atoms or molecules of the same kind, or a formula unit as a closed film, having a thickness between a monolayer and 100 nm.
As shown in “B” of
During the deposition process, the reaction chamber 10 can be filled with a second reaction atmosphere 118. In addition to a high vacuum as second reaction atmosphere 118, as preferably used for high purity thin films 62 consisting of the first material 126, also a suitable process gas G can be used as second reaction atmosphere 118. By this, evaporated and/or sublimated first material 126 (depicted as arrow 126 in “B” of
In summary, after step b) one or more thin films 62 are deposited onto the substrate surface 48. By using a second electromagnetic radiation 106, a wide range of first materials 126 can be used, where-by the range of possible compositions of materials of the one or more thin films 62 is further enlarged by choosing a suitable second reaction atmosphere 118. Further, an especially pure evaporation and/or sublimation of the first material 126 can be ensured. Hence, also building on the preferably defect free substrate sur-face 48, the one or more thin films 62 are preferably also free or at least depleted of substrate-induced defects.
In the last step c) of the method, depicted in
The third electromagnetic radiation 108 can serve two purposes. First of all, the applied heat can be used to temper the solid-state component 100. A further reduction of the already low number of defects of the solid-state component 100 can thereby be provided.
Secondly, also a controlled cooling of the solid-state component 100 can be provided by a suitable variation, in particular reduction, of the intensity of the third electromagnetic radiation 108. Defects caused by different thermal expansions of the substrate 24 and the one or more thin films 62 can thereby be avoided.
Both the tempering and the controlled cooling, respectively, can be supported by filling the reaction chamber 10 with a suitable third reaction atmosphere 120.
In summary, the solid-state components 100 produced with a method shown in a very basic version in
The evaporated and/or sublimated buffer material 132 (see the respective arrow 132 in
A snap-shot of a possible embodiment of step b) of the method is shown in
In the depicted embodiment, the second electromagnetic radiation 106 comprises two component beams 114, one of them directed onto the first source 30 comprising the first material 126, the other directed onto the second source 32 comprising the second material 128. The respective component beam 114 is adoptedly chosen for the evaporation and/or sublimation of the respective material 126, 128.
The evaporated and/or sublimated first and second materials 126, 128, see the respective arrows 126, 128, are deposited together and form one thin film 62. For instance, both materials 126, 128 can be metal elements, and the thin film 62 is formed by an alloy of these metals.
Please note that the thin films 62 depicted in
The evaporated and/or sublimated cover material 136 (see the respective arrow 136 in
In
The solid-state components 100 have in common that they comprise a low enough number of defects per cm2 and layer that have qubit relaxation times and qubit coherence times above 100 μs, preferably above 1000 μs, even more preferably above 10 ms and/or is produced by the method according to the present invention. The low number of defects of the solid-state component 100 provides long coherence times for the quantum component 102.
The quantum component 102 shown in
In contrast to that,
In addition, the quantum component 102 comprises a buffer layer 134 consisting of a buffer material 132 between the substrate 24 and the downmost layer of the thin films 62. As already described with respect to
Further, the quantum component 102 comprises a cover layer 138 consisting of a cover material 136 covering and protecting the thin films 62. As already described with respect to
As described in the foregoing a plurality of thin films 62 can be deposited on the substrate surface 48, the various thin films 62 can be made of different materials in order to form mulit-layerd and multi-material films 62 on the substrate 24.
An element, such as a metal is used for the first material and/or the second material of the first and second source elements 30, 32 in order to form the thin film 62.
To exemplify the technical feasibility of the present invention,
The many spots exemplify a highly ordered two-dimensional crystal surface. The mirror-symmetric pattern of diagonal lines shows that the RHEED beam is aligned along one of the principal crystal axes of the substrate. In this case, the surface reconstruction is rotated by +9° with respect to the bulk lattice. This becomes clear in
The symmetric pattern of concentric circles without any other observable spots evidences a single surface reconstruction with a single rotation of +9° on the entire substrate surface. The −9° orientation is entirely absent, confirming the feasibility of the method to select one out of several energetically equivalent surface reconstructions according to the present invention.
By changing the pressure of the oxygen process gas to 0.75×10−1 hPa, the chemical potential for oxygen atoms to leave the surface 48 is shifted and the minimum-energy configuration of the surface 48 is no longer the single-rotation reconstruction observed for the lower pressure.
The single crystalline nature of the domains is evident from the single atomic steps visible on the surface, and the alignment of the step and domain edges along the axes of the underlying epitaxial template with a sixfold (every 60°) hexagonal symmetry.
The X-ray scan shown in
Finally,
It is also possible to grow layers of compounds as thin films 62. For this purpose a method of forming a layer 62 of a compound having a thickness selected in the range of a monolayer to several μm on a substrate is carried out. As described in the foregoing the substrate 24 could be a single crystal wafer. The substrate 24 is arranged in a process chamber, such as the reaction chamber 10 disclosed in
In this context it should be noted that the laser light from the first and second source heating lasers 36, 38 is directed at the surface of the source directly facing the substrate 24.
The reaction chamber pressure is typically selected in the range of 10−6 to 101 hPa. On carrying out the method of forming a compound the step of providing a reaction atmosphere usually comprises an evacuation of the process chamber 10 to a first pressure and then introducing the process gas G to obtain a second pressure, the reaction chamber pressure in the reaction chamber 10.
The first pressure is typically lower than the second pressure and the second pressure is selected in the range of 10−11 to 10−2 hPa.
A temperature of at least the shroud and/or of an inner wall of the reaction chamber 10 is temperature controlled to a temperature selected in the range of 77 K to 500 K.
The source material is selected from the group of members consisting of Sc, Ti, V, Cr, Mn, Fe, Co, Ni, Cu, Zn, Zr, Nb, Mo, Ru, Hf, Al, Mg, Ca, Sr, Ba, Y, Rh, Ta, W, Re, Ir, Ga, In, Si, Ge, Sn, Eu, Ce, Pd, Ag, Pt, Au, alloys of the foregoing and combinations of the foregoing.
The laser light irradiating the one or more sources 30, 32 with laser light in order to sublimate and/or evaporate atoms and/or molecules of the source material is focused at the one or more sources 30, 32 with an intensity selected in the range of 1 to 2000 W for a spot size of 1 mm2 and a distance between the one or more sources and the substrate selected in the range of 50 to 120 mm.
The laser light irradiating the one or more sources 30, 32 with laser light having a wavelength in the range of 280 nm to 20 μm, especially in the range of 450 nm to 1.2 μm.
The compound deposited on the substrate can be one of an oxide, a nitride, a hydride, a fluoride, a chloride, a bromide, an iodide, a phosphide, a sulphide, a selenide or a mercury compound.
At higher pressures of the process gas G, the evaporated atoms or molecules suffer more collisions with the gas atoms, leading to a randomization of their direction and kinetic energies. This results in a much smaller fraction of the evaporated atoms or molecules reaching the substrate 24, which, however, may still be useful for forming a layer 62 in some cases, in particular for short working distances and large substrates. The formation of the compound or oxide layer 62 on the substrate 24 under these conditions may take place under several conditions:
Of particular interest is a transport reaction in which the source material 126 reacts with the gas G to form a metastable compound with a higher evaporation/sublimation rate than the source material 126 itself. This material further reacts in the gas phase and deposits as the final compound as a thin film 62, or deposits on the substrate 24 and reacts with further gas G to form the final, stable compound as a thin film 62.
TiO2: for TiO2 the source material is Ti, the compound deposited on the substrate is predominantly anatase or rutile TiO2, the laser light has a wavelength selected in the range of 515 to 1070 nm, in particular in the range of 1000 to 1070 nm, with an intensity in the range of 1 to 2000 W corresponding to a power density of 0.001 to 2 KW/mm2 on the source surface, in particular in the range of 100 to 200 W corresponding to a power density of 0.1 to 0.2 KW/mm2, a process gas being a mixture of O2 and O3, in particular with an O3 content of 5 to 10 weight %, with a reaction chamber pressure of 10−11 to 1 hPa, in particular of 10−6 to 10−2 hPa, and a compound layer thickness selected in the range of 0 to 1 μm obtainable within a time period of 0 to 180 min, especially 700 nm within a time period of 15 to 30 min with a working distance of 10 mm to 1 m, in particular 40 to 80 mm and a substrate diameter of 5 to 300 mm, in particular 51 mm.
NiO: for NiO the source material is Ni, the compound deposited on the substrate is predominantly NiO, the laser light has a wavelength selected in the range of 515 to 1070 nm, in particular in the range of 1000 to 1070 nm, with an intensity in the range of 1 to 2000 W corresponding to a power density of 0.001 to 2 KW/mm2 on the source surface, in particular in the range of 100 to 350 W corresponding to a power density of 0.1 to 0.35 KW/mm2, a process gas being a mixture of O2 and O3, in particular with an O3 content of 5 to 10 weight %, with a reaction chamber pressure of 10-11 to 1 hPa, in particular of 10−6 to 10−2 hPa, and a compound layer thickness selected in the range of 0 to 1 μm obtainable within a time period of 0 to 50 min, especially 500 nm within a time period of 10 to 20 min with a working distance of 10 mm to 1 m, in particular 40 to 80 mm and a substrate diameter of 5 to 300 mm, in particular 51 mm.
Co3O4: for Co3O4 the source material is Co, the compound deposited on the substrate is predominantly Co3O4, the laser light has a wavelength selected in the range of 515 to 1070 nm, in particular in the range of 1000 to 1070 nm, with an intensity in the range of 1 to 2000 W corresponding to a power density of 0.001 to 2 KW/mm2 on the source surface, in particular in the range of 100 to 200 W corresponding to a power density of 0.1 to 0.2 KW/mm2, a process gas being a mixture of O2 and O3, in particular with an O3 content of 5 to 10 weight %, with a reaction chamber pressure of 10-11 to 1 hPa, in particular of 10−6 to 10−2 hPa, and a compound layer thickness selected in the range of 0 to 1 μm obtainable within a time period of 0 to 90 min, especially 200 nm within a time period of 10 to 20 min with a working distance of 10 mm to 1 m, in particular 40 to 80 mm and a substrate diameter of 5 to 300 mm, in particular 51 mm.
Fe3O4: For Fe3O4 the source material is Fe, the compound deposited on the substrate is predominantly Fe3O4, the laser light has a wavelength selected in the range of 515 to 1070 nm, in particular in the range of 1000 to 1070 nm, with an intensity in the range of 1 to 2000 W corresponding to a power density of 0.001 to 2 KW/mm2 on the source surface, in particular in the range of 100 to 200 W corresponding to a power density of 0.1 to 0.2 KW/mm2, a process gas being a mixture of O2 and O3, in particular with an O3 content of 5 to 10 weight %, with a reaction chamber pressure of 10−11 to 1 hPa, in particular of 10−6 to 10−2 hPa, and a compound layer thickness selected in the range of 0 to 10 μm obtainable within a time period of 0 to 30 min, especially of 5 μm within a time period of 10 to 20 min with a working distance of 10 mm to 1 m, in particular 40 to 80 mm and a substrate diameter of 5 to 300 mm, in particular 51 mm.
CuO: For CuO the source material is Cu, the compound deposited on the substrate is predominantly CuO, the laser light has a wavelength selected in the range of 500 to 1070 nm, in particular in the range of 500 to 550 nm, with an intensity in the range of 1 to 900 W corresponding to a power density of 0.001 to 0.9 kW/mm2 on the source surface, in particular in the range of 200 to 400 W corresponding to a power density of 0.2 to 0.4 KW/mm2, a process gas being a mixture of O2 and O3, in particular with an O3 content of 5 to 10 weight %, with a reaction chamber pressure of 10−11 to 1 hPa, in particular of 10−6 to 10−2 hPa, and a compound layer thickness selected in the range of 0 to 1 μm obtainable within a time period of 0 to 100 min, especially of 0.15 μm within a time period of 15 to 30 min with a working distance of 10 mm to 1 m, in particular 40 to 80 mm and a substrate diameter of 5 to 300 mm, in particular 51 mm.
Vanadium Oxide: For Vanadium Oxide the source material is V, the compound deposited on the substrate is predominantly V2O3, VO2 or V2O5, the laser light has a wavelength selected in the range of 515 to 1100 nm, in particular in the range of 1000 to 1100 nm, with an intensity in the range of 1 to 2000 W corresponding to a power density of 0.001 to 2 kW/mm2 on the source surface, in particular in the range of 60 to 120 W corresponding to a power density of 0.06 to 0.12 KW/mm2, a process gas being a mixture of O2 and O3, in particular with an O3 content of 5 to 10 weight %, with a reaction chamber pressure of 10-11 to 1 hPa, in particular of 10−6 to 10−2 hPa, and a compound layer thickness selected in the range of 0 to 1 μm obtainable within a time period of 0 to 60 min, especially of 0.3 μm within a time period of 10 to 20 min with a working distance of 10 mm to 1 m, in particular 40 to 80 mm and a substrate diameter of 5 to 300 mm, in particular 51 mm.
Nb2O5: For Nb2O5 the source material is Nb, the compound deposited on the substrate is predominantly Nb2O5, the laser light has a wavelength selected in the range of 515 to 1100 nm, in particular in the range of 1000 to 1100 nm, with an intensity in the range of 1 to 2000 W corresponding to a power density of 0.001 to 2 KW/mm2 on the source surface, in particular in the range of 200 to 400 W corresponding to a power density of 0.2 to 0.4 KW/mm2, a process gas being a mixture of O2 and O3, in particular with an O3 content of 5 to 10 weight %, with a reaction chamber pressure of 10-11 to 1 hPa, in particular of 10−6 to 10−2 hPa, and a compound layer thickness selected in the range of 0 to 2 μm obtainable within a time period of 0 to 20 min, especially of 1.4 μm within a time period of 10 to 20 min with a working distance of 10 mm to 1 m, in particular 40 to 80 mm and a substrate diameter of 5 to 300 mm, in particular 51 mm.
Cr2O3: For Cr2O3 the source material is Cr, the compound deposited on the substrate is predominantly Cr2O3, the laser light has a wavelength selected in the range of 515 to 1100 nm, in particular in the range of 1000 to 1100 nm, with an intensity in the range of 1 to 2000 W corresponding to a power density of 0.001 to 2 kW/mm2 on the source surface, in particular in the range of 20 to 80 W corresponding to a power density of 0.02 to 0.08 KW/mm2, a process gas being a mixture of O2 and O3, in particular with an O3 content of 5 to 10 weight %, with a reaction chamber pressure of 10−11 to 1 hPa, in particular of 10−6 to 10−2 hPa, and a compound layer thickness selected in the range of 0 to 1 μm obtainable within a time period of 0 to 30 min, especially of 0.5 μm within a time period of 10 to 20 min with a working distance of 10 mm to 1 m, in particular 40 to 80 mm and a substrate diameter of 5 to 300 mm, in particular 51 mm.
RuO2: For RuO2 the source material is Ru, the compound deposited on the substrate is predominantly RuO2, the laser light has a wavelength selected in the range of 515 to 1100 nm, in particular in the range of 1000 to 1100 nm, with an intensity in the range of 1 to 2000 W corresponding to a power density of 0.001 to 2 KW/mm2 on the source surface, in particular in the range of 200 to 600 W corresponding to a power density of 0.2 to 0.6 kW/mm2, a process gas being a mixture of O2 and O3, in particular with an O3 content of 5 to 10 weight %, with a reaction chamber pressure of 10−11 to 1 hPa, in particular of 10−6 to 10−2 hPa, and a compound layer thickness selected in the range of 0 to 1 μm obtainable within a time period of 0 to 300 min, especially of 0.06 μm within a time period of 10 to 20 min with a working distance of 10 mm to 1 m, in particular 40 to 80 mm and a substrate diameter of 5 to 300 mm, in particular 51 mm.
ZnO: For ZnO the source material is Zn, the compound deposited on the substrate is predominantly ZnO, the laser light has a wavelength selected in the range of 515 to 1100 nm, in particular in the range of 1000 to 1100 nm, with an intensity in the range of 1 to 2000 W corresponding to a power density of 0.001 to 2 kW/mm2 on the source surface, in particular in the range of 5 to 10 W corresponding to a power density of 0.005 to 0.010 KW/mm2, a process gas being a mixture of O2 and O3, in particular with an O3 content of 5 to 10 weight %, with a reaction chamber pressure of 10−11 to 1 hPa, in particular of 10−6 to 10−2 hPa, and a compound layer thickness selected in the range of 0 to 1 μm obtainable within a time period of 0 to 20 min, especially of 1.4 μm within a time period of 10 to 20 min with a working distance of 10 mm to 1 m, in particular 40 to 80 mm and a substrate diameter of 5 to 300 mm, in particular 51 mm.
MnO: For MnO the source material is Mn, the compound deposited on the substrate is predominantly MnO, the laser light has a wavelength selected in the range of 515 to 1100 nm, in particular in the range of 1000 to 1100 nm, with an intensity in the range of 1 to 2000 W corresponding to a power density of 0.001 to 2 KW/mm2 on the source surface, in particular in the range of 5 to 10 W corresponding to a power density of 0.005 to 0.010 KW/mm2, a process gas being a mixture of O2 and O3, in particular with an O3 content of 5 to 10 weight %, with a reaction chamber pressure of 10−11 to 1 hPa, in particular of 10−6 to 10−2 hPa, and a compound layer thickness selected in the range of 0 to 1 μm obtainable within a time period of 0 to 20 min, especially of 1.4 μm within a time period of 10 to 20 min with a working distance of 10 mm to 1 m, in particular 40 to 80 mm and a substrate diameter of 5 to 300 mm, in particular 51 mm.
SC2O3: For SC2O3 the source material is Sc, the compound deposited on the substrate is predominantly Sc2O3, the laser light has a wavelength selected in the range of 515 to 1100 nm, in particular in the range of 1000 to 1100 nm, with an intensity in the range of 1 to 2000 W corresponding to a power density of 0.001 to 2 KW/mm2 on the source surface, in particular in the range of 20 to 50 W corresponding to a power density of 0.02 to 0.05 KW/mm2, a process gas being a mixture of O2 and O3, in particular with an O3 content of 5 to 10 weight %, with a reaction chamber pressure of 10−11 to 1 hPa, in particular of 10−6 to 10−2 hPa, and a compound layer thickness selected in the range of 0 to 1 μm obtainable within a time period of 0 to 20 min, especially of 1.3 μm within a time period of 10 to 20 min with a working distance of 10 mm to 1 m, in particular 40 to 80 mm and a substrate diameter of 5 to 300 mm, in particular 51 mm.
Mo4O11 or MoO3: For Mo4O11 or MoO3 the source material is Mo, the compound deposited on the substrate is predominantly Mo4O11 or MoO3, the laser light has a wavelength selected in the range of 515 to 1100 nm, in particular in the range of 1000 to 1100 nm, with an intensity in the range of 1 to 2000 W corresponding to a power density of 0.001 to 2 kW/mm2 on the source surface, in particular in the range of 400 to 800 W corresponding to a power density of 0.4 to 0.8 KW/mm2, a process gas being a mixture of O2 and O3, in particular with an O3 content of 5 to 10 weight %, with a reaction chamber pressure of 10−11 to 1 hPa, in particular of 10−6 to 10−2 hPa, and a compound layer thickness selected in the range of 0 to 4 μm obtainable within a time period of 0 to 30 min, especially of 4.0 μm within a time period of 10 to 20 min with a working distance of 10 mm to 1 m, in particular 40 to 80 mm and a substrate diameter of 5 to 300 mm, in particular 51 mm.
ZrO2: For ZrO2 the source material is Zr, the compound deposited on the substrate is predominantly ZrO2, the laser light has a wavelength selected in the range of 515 to 1100 nm, in particular in the range of 1000 to 1100 nm, with an intensity in the range of 1 to 2000 W corresponding to a power density of 0.001 to 2 KW/mm2 on the source surface, in particular in the range of 300 to 500 W corresponding to a power density of 0.3 to 0.5 kW/mm2, a process gas being a mixture of O2 and O3, in particular with an O3 content of 5 to 10 weight %, with a reaction chamber pressure of 10-11 to 1 hPa, in particular of 10−6 to 10−2 hPa, and a compound layer thickness selected in the range of 0 to 1 μm obtainable within a time period of 0 to 100 min, especially of 0.2 μm within a time period of 15 to 25 min with a working distance of 10 mm to 1 m, in particular 40 to 80 mm and a substrate diameter of 5 to 300 mm, in particular 51 mm.
HfO2: For HfO2 the source material is Hf, the compound deposited on the substrate is predominantly HfO2, the laser light has a wavelength selected in the range of 515 to 1100 nm, in particular in the range of 1000 to 1100 nm, with an intensity in the range of 1 to 2000 W corresponding to a power density of 0.001 to 2 KW/mm2 on the source surface, in particular in the range of 250 to 400 W corresponding to a power density of 0.25 to 0.4 KW/mm2, a process gas being a mixture of O2 and O3, in particular with an O3 content of 5 to 10 weight %, with a reaction chamber pressure of 10−11 to 1 hPa, in particular of 10−6 to 10−2 hPa, and a compound layer thickness selected in the range of 0 to 1 μm obtainable within a time period of 0 to 40 min, especially of 0.6 μm within a time period of 15 to 25 min with a working distance of 10 mm to 1 m, in particular 40 to 80 mm and a substrate diameter of 5 to 300 mm, in particular 51 mm.
Al2O3: For Al2O3 the source material is Al, the compound deposited on the substrate is predominantly Al2O3, the laser light has a wavelength selected in the range of 515 to 1100 nm, in particular in the range of 1000 to 1100 nm, with an intensity in the range of 1 to 2000 W corresponding to a power density of 0.001 to 2 KW/mm2 on the source surface, in particular in the range of 200 to 400 W corresponding to a power density of 0.2 to 0.4 KW/mm2, a process gas being a mixture of O2 and O3, in particular with an O3 content of 5 to 10 weight %, with a reaction chamber pressure of 10−11 to 1 hPa, in particular of 10−6 to 10−2 hPa, and a compound layer thickness selected in the range of 0 to 1 μm obtainable within a time period of 0 to 20 min, especially of 1.0 μm within a time period of 15 to 25 min with a working distance of 10 mm to 1 m, in particular 40 to 80 mm and a substrate diameter of 5 to 300 mm, in particular 51 mm. For Al, higher growth rates of more than 1 μm per minute are achievable due to growth mode 4 with laser powers of 300 to 500 W.
Thermal laser evaporation (TLE) is a particularly promising technique for the growth of metal films. Here, we demonstrate that thermal laser evaporation is also suitable for the growth of amorphous and polycrystalline oxide films. We report on a spectrum of binary oxide films that have been deposited by laser-induced evaporation of elemental metal sources in oxygen-ozone atmospheres. The oxide deposition by TLE is accompanied by an oxidation of the elemental metal source, which systematically affects the source molecular flux. Fifteen elemental metals were successfully used as sources for oxide films grown on unheated substrates, employing one and the same laser optic. The source materials ranged from refractory metals with low vapor pressures, such as Hf, Mo, and Ru, to Zn, which readily sublimates at low temperatures. These results reveal that TLE is well suited for the growth of ultraclean oxide films.
Oxide films 62 are of great interest for realizing new functionalities due to their broad spectrum of intriguing and useful properties. Virtually all deposition techniques are used for the growth of oxide films, including electron-beam evaporation (EBE), molecular beam epitaxy (MBE), pulsed laser deposition (PLD), sputtering, and atomic layer deposition (ALD). Thermal laser evaporation (TLE) has recently been demonstrated to be a promising technique for growing ultraclean metal films because it combines the advantages of MBE, PLD, and EBE by thermally evaporating metallic sources with a laser beam.
By utilizing the adsorption-controlled growth mode, MBE is particularly suited for growing films of superior structural quality. In MBE, molecular fluxes of source materials are generated by evaporating the source materials. However, ohmic heaters, which are preferred for this purpose, limit the use of reactive background gases. This restriction can be critical for the growth of complex metal oxides. Furthermore, elements with low vapor pressure, such as B, C, Ru, Ir, and W, cannot be evaporated by external ohmic heating. To evaporate those elements requires EBE, but that technique is not optimal for achieving precise and stable evaporation rates. PLD transfers a source material onto a substrate via short-period, high-power laser pulses. Although PLD can operate with a high background pressure of reactive gases, the precise control of the material composition is challenging, in particular if the film composition is to be varied smoothly.
Laser-assisted evaporation had been proposed and attempted for the thin film deposition after the invention of laser. However, the evaporation by continuous-wave (cw) laser was abandoned due to the formation of nonstoichiometric films, while the evaporation by high-power density pulsed laser led to the invention of PLD. Along with the development of cw laser technology, TLE has been recently rediscovered as a candidate for epitaxial growth of complex materials, which can combine the advantages of MBE, PLD, and EBE while eliminating their respective weaknesses. Lasers 36, 38 placed outside the vacuum chamber 12 evaporate pure metal sources 30, 32 by local heating, which requires only a simple setup and allows the precise evaporation control of each source element, high purity of the source materials, and the almost unlimited choice of background gas G composition and pressure. In many cases, the locally molten source 30, 32 forms its own crucible. By avoiding impurity incorporation from the crucible, the source 30, 32 is guaranteed to remain highly pure. The potential of TLE to deposit elemental metallic and semiconducting films 62 has been realized by the deposition of a wide range of elements as films 62, ranging from high-vapor-pressure elements such as Bi and Zn to low-vapor-pressure elements such as W and Ta.
Whereas using TLE to grow oxide films 62 and heterostructures may also be highly advantageous, it is not obvious that it is possible in an oxidizing atmosphere. Oxidation of the heat source (filament), which plagues MBE and EBE, is trivial to avoid in TLE. However, the metal sources 30, 32 themselves are prone to oxidation when heated by a laser beam in an oxidizing atmosphere. If the source oxidizes, the laser radiation is no longer absorbed only by the original source material but also by its oxide. Indeed, the entire source or the surface of the source may oxidize, or the oxide may form a partial layer floating on a melt pool. In addition, the molecular fluxes of the source materials may be generated by both the metallic part of source and by the source material oxide. To do so, we performed a series of evaporation experiments in which elemental metal sources 30, 32 having high or low vapor pressures were evaporated by laser irradiation in a variety of oxygen-ozone atmospheres. For simplicity in exploring the evaporation process, we used substrates 24 of unheated Si (100) wafers coated by their native oxide. We readily succeeded in growing oxide films 62, using the same laser optics and laser wavelengths of 1030-1070 nm for every element explored as the first and second source heating lasers 36, 38. Our experiments reveal that the evaporation of elemental sources in strongly oxidizing atmospheres is applicable for oxide film growth despite the oxidation of the source 30, 32 during the process. We also find that different oxide phases are obtainable in a given atmosphere by tuning the oxidizing atmosphere. The deposition process is furthermore found to display a characteristic variation as a function of the oxygen-ozone pressure.
A schematic of the TLE chamber 10 used in this study is shown in
A flowing oxygen-ozone mixture 20 and a cascaded pumping system 18 comprising two turbomolecular pumps and a diaphragm pump connected in series was employed for the precise control of the chamber pressure Pox, which was varied between <10−8 and 10−2 hPa. Ozone accounted for approximately 10 wt % of the total flow provided by the glow-discharge continuous-flow ozone generator (not shown). The setting of the valve controlling this gas flow was held constant during each deposition to provide a constant flow. During the evaporation process, Pox and the temperatures of source 30, 32 and substrate 24 were monitored by pressure gauges and the thermocouples (not shown). Using the same deposition geometry, we used TLE to evaporate fifteen different metal elements to deposit oxide films 62. Each element was evaporated in several runs using the same laser power and laser optics but different values of Pox ranging from 10−8 to 1031 2 hPa.
Scanning electron microscopy (SEM) was employed to measure the film thickness and to study its microstructure. The crystal structures of the deposited films 62 were identified by x-ray diffraction. Photoemission spectroscopy was performed to reveal the oxidation states of the TLE-grown TiO2 films 62. If a film 62 was found to be amorphous, it was later subjected to an additional two-hour Ar anneal at 500° C. for crystallization.
Owing to the consumption of the oxygen-ozone gas mixture caused by oxidation of the source 30, 32 and the evaporated material, Pox frequently decreased during deposition, as illustrated by
However, the decrease of the background pressure is not observed in all instances. The pressure change is small or even absent in two cases: first, if the source 30, 32 has already been fully oxidized at the beginning of the process; second, if the oxidation of the source 30, 32 is intrinsically unfavorable. The thermal laser evaporation of Ni in the oxidizing atmosphere is an example of the first case. A decrease of Pox is observed only for Pox<10−4 hPa. At higher pressure, the Ni source 30, 32 becomes covered by its oxide. Further oxidation is therefore suppressed, and the decrease of Pox disappears. The predominant vapor species obtained by heating Ni under strongly oxidizing conditions is therefore provided by NiO. The thermal laser evaporation of Cu is an example of the second case, as the oxidation of Cu is relatively unfavorable. Above 1000° C. and in an oxygen pressure range of 10−4-10−2 hPa, metallic Cu is more stable than its oxides. In the experiment, the source temperature in the irradiated area exceeds 1085° C., as is evident from the fact that the Cu is locally molten. At this temperature, liquid Cu is the thermodynamically stable phase, and elemental Cu is expected to provide the dominant vapor species. Indeed, no significant change of the chamber pressure occurs during the evaporation of Cu as shown in
We have tested fifteen metallic elements as sources for the TLE growth of oxide films (Table 1).
To investigate the structure of the films 62 in more detail, we performed cross-sectional SEM. As shown in
As the source 30, 32 is heated locally, it behaves like a flat, small-area evaporation source 30, 32, providing as function of emission angle a cosine-type flux distribution. Indeed, SEM measurements show that the films 62 are thinner towards the wafer edge. With the evaporation parameters we used, the reduction of the film thickness towards the edge equals ˜20% in most cases, slightly higher than the theoretically expected value of ˜15%. We attribute this effect to the notable pitting of the source during evaporation, which concentrates the molecular flux.
Our studies show that, as expected, the phase of the deposited oxide is a function of the oxidizing gas pressure. This behavior is illustrated for Ti and Ni films 62 in
The oxidation states of the TLE-grown TiO2 films 62 were analyzed by XPS and compared to TiO2 films grown by EBE. Whereas the as-deposited EBE sample comprises a significant amount of Ti3+, TLE samples contain mostly Ti4+. We attribute this phenomenon to the oxygen-ozone background, which suppresses the thermal dissociation of TiO2, TiO2(s)→TiO(g)+½O2(g), and oxidizes the deposited material.
Interestingly, we have found that the oxidation behavior of TLE-grown Ni oxide films 62 differs markedly from that of Ti oxide films 62. Under UHV conditions, a cubic phase is found also for metallic Ni (
Pox also affects the deposition rate of TLE-grown oxide films 62.
Why does Pox alter the deposition rate of oxide films grown by TLE in these two rather characteristic ways? We suggest that this behavior is controlled by the vapor pressure of the source's 30, 32 oxidized surface layer. The deposition rate increases with Pox if the vapor pressure of the oxide formed at the source surface exceeds that of the metal. This corresponds to the Ti-like deposition rate behavior. Formation of TiO2 gas vapor, Ti(s)+O2(g)→TiO2(g), is an exothermic reaction leading to effective generation of oxide vapor from the source. As the metal oxidation rate increases with a power of Pox (oxidation rate∝Pox), the deposition rate will increase correspondingly with Pox, as observed for Fe and Nb. In contrast, the Ni-like scenario is found if the vapor pressure of the metal exceeds that of the oxide. As the vapor pressure of NiO is about one order of magnitude smaller than that of Ni, a NiO coverage of a source reduces the deposition rate by the same factor. This understanding is supported by the observation that the abrupt decrease of the deposition rate of Ni occurs at 10−3 hPa, the same pressure at which the pressure drop in the chamber disappears, revealing that the source is passivated by a NiO layer 62 at this Pox.
The growth of polycrystalline oxide films 62 by TLE has thus been demonstrated. The films 62 having tunable oxidation states and a crystal structure can be grown by evaporating pure metal sources in oxygen-ozone pressures of up to 101 hPa, irrespective of possible oxidation of the sources 30, 32. From a wide range of metal sources comprising low and high-vapor-pressure elements, polycrystalline films 62 in various oxidation states were deposited with growth rates of several Å/s on unheated Si (100) substrates 24. Determining the degree of source oxidation, the pressure of the oxidizing gas strongly affects the deposition rate as well as the composition and phase of the resulting oxide films 32. Our work paves the way to TLE growth of epitaxial oxide heterostructures of ultrahigh purity for diverse compounds.
| Filing Document | Filing Date | Country | Kind |
|---|---|---|---|
| PCT/EP2021/068244 | 7/1/2021 | WO |
