Patent Application
20230010039
This application claims priority based on European Patent Application no. 21184454.3, filed Jul. 8, 2021, which is incorporated by reference in its entirety.
The present disclosure relates to a semiconductor structure and a method for forming the semiconductor structure.
There is an increasing interest to integrate III-V semiconductor devices with further devices, e.g., further semiconductor devices, typically incorporating a non-III-V material such as Si, on the same base substrate. Combining the III-V semiconductor devices and further devices in a hybrid semiconductor structure may combine the benefits of both types of devices.
III-V semiconductor devices in radio-frequency (RF) integrated circuits typically have higher power, linearity, and efficiency characteristics when compared to, for instance, Si-based semiconductor devices. This can be especially true for telecommunication standards at millimeter wave frequencies, such as those used in the 5G and 6G telecommunication standards. Furthermore, FinFET-type III-V semiconductor devices are suitable for scaling above 100 GHz and typically have good linearity performance.
On the other hand, further devices made of different materials than III-V materials may provide other benefits. In particular Si-based semiconductor devices, such as used in Si-CMOS, are relatively low in price, and can provide a high transistor density, a mature back-end-of-the-line (BEOL), and at present the best technology option for digital baseband.
The combined benefits of III-V semiconductor devices and further devices are best exploited when said devices are separated by distances smaller than a micron. Thereby, for example, a high density and good communication between the III-V semiconductor devices and further semiconductor devices may be achieved. Therefore, the III-V semiconductor devices and further devices can be provided, i.e., integrated, on one and the same base substrate, e.g., on a single Si wafer.
This integration is, however, far from straightforward. Firstly, III-V-based RF device stacks are typically grown as thick layers on high-resistivity Si(111). On the other hand, Si devices are typically fabricated on doped Si(100). III-V materials grow as polycrystalline layers on Si(100). Such polycrystalline layers are not fit for devices.
The formation of III-V materials on Si(111) is, however, also problematic. This is mainly due to lattice and thermal mismatches. To overcome such mismatches, a complicated, thick strain relaxing buffer layer may be provided. Even then, however, the crystalline defect density (such as threading dislocations and anti-phase boundaries) is still relatively high in the top active region of the III-V material.
Furthermore, for example, Ga (which is a III-type material often used in III-V semiconductor devices) forms a eutectic alloy with Si (which is typically used as substrate) at elevated temperatures. This may result in the etching of the Si substrate during the growth of GaN. Although an AlN nucleation layer may be grown on Si(111), on which the GaN may be grown, Al, Ga, or N may still diffuse into the insulating Si. Thereby leading to, for example, possible unwanted conducting channels being formed in Si. Furthermore, AlN can be very problematic to grow selectively due to low surface diffusivity and desorption.
In addition, due to the high growth temperatures required for growing III-V materials, the III-V material are generally grown before Si-CMOS is fabricated. Finally, thick buffer layers of III-V can be problematic. Exposing the Si for the subsequent CMOS fabrication would require the etching of deep trenches. However, highly scaled Si-CMOS devices cannot be fabricated in deep trenches.
There is, therefore, a need in the art for a semiconductor structure combining a III-V semiconductor device and a further device on the same base substrate, and a method for forming said structure, that overcomes at least partially one or more of the above problems.
It is an object of the present disclosure to provide a good method for manufacturing a semiconductor structure comprising a III-V semiconductor device and at least part of a further device on a base substrate, and a semiconductor structure manufactured thereby.
The above objective can be accomplished by a method and device according to the present disclosure.
It is a benefit of embodiments of the method of the present disclosure that the growth of a III-V semiconductor material is independent of the type of base substrate used. It is a benefit of embodiments of the present disclosure that the III-V semiconductor device may be combined with a Si-type further semiconductor device on the same base substrate. It is a benefit of embodiments of the present disclosure that there may be no need for any wafer bonding or silicon on insulator substrates for the integration.
The III-V semiconductor material can be formed on a first two-dimensional layered crystal material over the base substrate. It is a benefit of embodiments of the present disclosure that, thereby, the III-V semiconductor can be grown via van der Waals epitaxy. It is a benefit of embodiments of the present disclosure that, thereby, restrictions due to lattice and thermal mismatch are reduced. It is a benefit of embodiments of the present disclosure that simple and economic growth procedures may be achieved.
It is a benefit of embodiments of the present disclosure that III-V semiconductor devices and further devices are combined in a semiconductor structure. It is a benefit of embodiments of the present disclosure that many degrees of freedom are present for the semiconductor structure. The semiconductor structure may, therefore, be provided in the desired form and in a desired density.
It is a benefit of embodiments of the present disclosure that the III-V semiconductor device may have a small thickness since a thick buffer layer may not be required. It is a benefit of embodiments of the present disclosure that, thereby, the III-V semiconductor device may be combined with further devices, e.g., further semiconductor devices, such as with Si-based CMOS devices.
It is a benefit of embodiments of the present disclosure that two-dimensional layered crystal materials typically do not have out-of-plane dangling bonds. Thereby, good diffusion of adatoms may be possible on the two-dimensional layered crystal materials. It is a benefit of embodiments of the present disclosure that high-quality nucleation layers for the III-V semiconductor material may be formed even at low temperatures. On substrates having dangling bonds, low temperature growth may be typically inhibited by lack of surface mobility of adatoms due to the dangling bonds.
It is a benefit of embodiments of the present disclosure that the III-V semiconductor material can be grown at low temperatures as thin layers with high quality. It is a benefit of embodiments of the present disclosure that the III-V semiconductor material is not fixed by covalent bonds. Thereby, a thermal expansion mismatch between a base substrate and the III-V semiconductor material during cool down after growth of the III-V semiconductor material typically do not result in bowing or cracking of the base substrate. Bowing and cracking may be detrimental for the performance of a III-V semiconductor device and for further processing for forming Si CMOS structures.
It is a benefit of embodiments of the present disclosure that due to the selective growth, the III-V semiconductor material has exposed sidewalls. Thereby, if there is still strain present during cool down despite the lack of dangling bonds of the two-dimensional layered crystal material, this strain may relax towards the exposed sidewalls of the selectively grown III-V semiconductor material. It is a benefit of embodiments of the present disclosure that, even if threading dislocations or other crystal defects form during the growth (e.g., at low temperatures), they can be removed by post-growth annealing.
In a first aspect, the present disclosure relates to a method for manufacturing a semiconductor structure comprising a III-V semiconductor device in a first region of a base substrate and at least part of a further device, e.g., a further semiconductor device, in a second region of the base substrate, the method comprising: a. obtaining a base substrate comprising the first region and the second region, different from the first region, b. providing a buffer layer over a surface of the base substrate at least in the first region, wherein the buffer layer comprises at least one monolayer of a first two-dimensional layered crystal material, c. forming, over the buffer layer in the first region, and not in the second region, a III-V semiconductor material, and d. forming, in the second region, at least part of the further device.
In a second aspect, the present disclosure relates to a semiconductor structure comprising: a base substrate comprising a first region and a second region, a buffer layer over a surface of the base substrate at least in the first region, wherein the buffer layer comprises at least one monolayer of a first two-dimensional layered crystal material, a III-V semiconductor material over the buffer layer in the first region, and at least part of a further device in the second region. The semiconductor structure of the second aspect is typically obtainable by the method of the first aspect.
Particular aspects of the disclosure are set out in the accompanying independent and dependent claims. Features from the dependent claims may be combined with features of the independent claims and with features of other dependent claims as appropriate and not merely as explicitly set out in the claims.
Although there has been constant improvement, change and evolution of devices in this field, the present concepts are believed to represent substantial new and novel improvements, including departures from prior practices, resulting in the provision of more efficient, stable and reliable devices of this nature.
The above and other characteristics, features and benefits of the present disclosure will become apparent from the following detailed description, taken in conjunction with the accompanying drawings, which illustrate, by way of example, the principles of the disclosure. This description is given for the sake of example only, without limiting the scope of the disclosure. The reference figures quoted below refer to the attached drawings.
In the different figures, the same reference signs refer to the same or analogous elements.
The present disclosure will be described with respect to particular embodiments and with reference to certain drawings but the disclosure is not limited thereto but only by the claims. The drawings described are only schematic and are non-limiting. In the drawings, the size of some of the elements may be exaggerated and not drawn on scale for illustrative purposes. The dimensions and the relative dimensions do not correspond to actual reductions to practice of the disclosure.
Furthermore, the terms first, second, third, and the like in the description and in the claims, are used for distinguishing between similar elements and not necessarily for describing a sequence, either temporally, spatially, in ranking, or in any other manner. It is to be understood that the terms so used are interchangeable under appropriate circumstances and that the embodiments of the disclosure described herein are capable of operation in other sequences than described or illustrated herein.
Moreover, the terms top, bottom, over, under, and the like in the description and the claims are used for descriptive purposes and not necessarily for describing relative positions. It is understood that the terms so used are interchangeable under appropriate circumstances and that the embodiments of the disclosure described herein are capable of operation in other orientations than described or illustrated herein.
The term “comprising”, used in the claims, should not be interpreted as being restricted to the means listed thereafter; it does not exclude other elements or steps. It is thus to be interpreted as specifying the presence of the stated features, integers, steps, or components as referred to, but does not preclude the presence or addition of one or more other features, integers, steps or components, or groups thereof. The term “comprising” therefore covers the situation where only the stated features are present and the situation where these features and one or more other features are present. The word “comprising” according to the disclosure therefore also includes as one embodiment that no further components are present. Thus, the scope of the expression “a device comprising means A and B” should not be interpreted as being limited to devices consisting only of components A and B. It means that with respect to the present disclosure, the only relevant components of the device are A and B.
Similarly, the term “coupled”, also used herein, should not be interpreted as being restricted to direct connections only. The terms “coupled” and “connected”, along with their derivatives, may be used. It should be understood that these terms are not intended as synonyms for each other. Thus, the scope of the expression “a device A coupled to a device B” should not be limited to devices or systems wherein an output of device A is directly connected to an input of device B. It means that there exists a path between an output of A and an input of B which may be a path including other devices or means. “Coupled” may mean that two or more elements are either in direct physical or electrical contact, or that two or more elements are not in direct contact with each other but yet still co-operate or interact with each other.
Reference throughout this specification to “one embodiment” or “an embodiment” means that a particular feature, structure, or characteristic described in connection with the embodiment is included in at least one embodiment of the present disclosure. Thus, appearances of the phrases “in one embodiment” or “in an embodiment” in various places throughout this specification are not necessarily all referring to the same embodiment, but may. Furthermore, the particular features, structures, or characteristics may be combined in any suitable manner, as would be apparent to one of ordinary skill in the art from this disclosure, in one or more embodiments.
Similarly, it should be appreciated that in the description of exemplary embodiments of the disclosure, various features of the disclosure are sometimes grouped together in a single embodiment, figure, or description thereof for the purpose of streamlining the disclosure and aiding in the understanding of one or more of the various disclosure aspects. This method of disclosure, however, is not to be interpreted as reflecting an intention that the claimed disclosure requires more features than are expressly recited in each claim. Rather, as the following claims reflect, the disclosure aspects lie in less than all features of a single foregoing disclosed embodiment. Thus, the claims following the detailed description are hereby expressly incorporated into this detailed description, with each claim standing on its own as a separate embodiment of this disclosure.
Furthermore, while some embodiments described herein include some but not other features included in other embodiments, combinations of features of different embodiments are meant to be within the scope of the disclosure, and form different embodiments, as would be understood by those in the art. For example, in the following claims, any of the claimed embodiments can be used in any combination.
Furthermore, some of the embodiments are described herein as a method or combination of elements of a method that can be implemented by a processor of a computer system or by other means of carrying out the function. Thus, a processor with the necessary instructions for carrying out such a method or element of a method forms a means for carrying out the method or element of a method. Furthermore, an element described herein of an apparatus embodiment is an example of a means for carrying out the function performed by the element for the purpose of carrying out the disclosure.
In the description provided herein, numerous specific details are set forth. However, it is understood that embodiments of the disclosure may be practiced without these specific details. In other instances, well-known methods, structures, and techniques have not been shown in detail in order not to obscure an understanding of this description.
Reference will be made to transistors. These are devices having a first main electrode such as a drain, a second main electrode such as a source, and a control electrode such as a gate for controlling the flow of electrical charges between the first and second main electrodes.
Reference is made to two-dimensional layered crystal materials. Two-dimensional layered crystal materials are known in the art as layered materials, as two-dimensional materials, and as single-layer materials. Two-dimensional layered crystal materials are solid materials. The material comprises one or more two-dimensional layers, i.e., monolayers. Herein, the term “two-dimensional” refers to the fact that two dimensions of each layer are much larger than a third dimension. Each monolayer is typically crystalline. Along the third dimension, each layer (i.e., each monolayer) of the material comprises typically up to three atomic layers. Examples of two-dimensional layered crystal materials are metal dichalcogenides, wherein each layer of the material comprises three atomic layers along the third dimension. That is, metal dichalcogenides comprise a layer of metal atoms between two chalcogenide layers. Other examples are graphene and hexagonal boron nitride, wherein each layer of the material comprises one atomic layer along the third dimension. Typically, covalent bonds exist between atoms within each layer of the layered material. Coupling between adjacent layers in a two-dimensional layered crystal material is typically due to van der Waals bonding, and, typically, no covalent bond is present between adjacent layers. Typically, surfaces of two-dimensional layered crystal materials do not have dangling, i.e., free, bonds.
In a first aspect, the present disclosure relates to a method for manufacturing a semiconductor structure comprising a III-V semiconductor device in a first region of a base substrate and at least part of a further device, e.g., a further semiconductor device, in a second region of the base substrate, the method comprising: a. obtaining a base substrate comprising the first region and the second region, different from the first region, b. providing a buffer layer over, e.g., on (i.e., in physical contact with), a surface of the base substrate at least in the first region, wherein the buffer layer comprises at least one monolayer of a first two-dimensional layered crystal material, c. forming, over, e.g., on, the buffer layer in the first region, and not in the second region, a III-V semiconductor material, and d. forming, in the second region, at least part of the further device.
The first region may have any shape suitable for forming the III-V semiconductor device thereover. The second region may have any shape suitable for forming the further device thereover. In embodiments, the first region has a first extent, e.g., a length, of from 0.1 to 500 micrometre in a first direction, and a second extent, e.g., a width, of from 0.1 to 500 micrometre in a second direction, perpendicular to the first direction. In embodiments, the second region has a first extent, e.g., a length, of from 0.1 to 500 micrometre in a first direction, and a second extent, e.g., a width, of from 0.1 to 500 micrometre in a second direction, perpendicular to the first direction. In embodiments, a distance between the first and second region can be smaller than 2 μm, generally from 10 nm to 1 μm, such as from 50 nm to 200 nm. In embodiments, a distance between the III-V semiconductor device and the further device may be smaller than 2 μm, generally from 10 nm to 1 μm, such as from 50 nm to 200 nm. It is a benefit of embodiments of the present disclosure that a good communicative coupling between the III-V semiconductor device and the further device, e.g., the further semiconductor device, may be achieved.
The III-V semiconductor device may be any device that uses a III-V semiconductor material for it to function, such as a diode or a transistor. In embodiments, the III-V semiconductor device is a transistor, such as a field-effect transistor. In embodiments, a channel of the transistor can be formed in the III-V semiconductor material. Typically, the III-V semiconductor material comprises at least two layers: a III-V buffer layer (e.g. GaN) and a III-V active layer (e.g. GaN), over and generally on the III-V buffer layer. Typically, the buffer layer can be grown until a single-crystalline and coalesced layer is formed as its top surface. In embodiments, the active layer may comprise a III-V channel layer and a barrier layer having a larger bandgap than the III-V channel layer. In embodiments, the III-V semiconductor material may comprise a nucleation layer (e.g. AlN) at the base of the III-V buffer layer. In embodiments, the III-V semiconductor material may comprise one or more III-N semiconductor materials, generally one or more of GaN, AlN, InN, AlGaN, InGaN, InAlN, and GaInAlN. In some embodiments, the III-V semiconductor material may comprise a ternary or quaternary compound. The ternary or quaternary compound is typically used as a barrier layer. In some embodiments, it may comprise one of AlGaN, InGaN, InAlN, and GaInAlN. It is a benefit of embodiments of the present disclosure that using ternary or quaternary compounds may yield good III-V semiconductor devices. For example, if the ternary or quaternary compound is used as barrier layer, strain with respect to the buffer layer may be well controlled, while providing a good bandgap offset with respect to the buffer layer.
The further device may be a further semiconductor device, but the present disclosure is not limited thereto. For example, the further device may be a capacitor structure. In the case of a capacitor structure, the further device comprises two conducting layers, e.g., graphene, separated by an electrically insulating layer, e.g., hexagonal boron nitride. In some embodiments, the further device can be a further semiconductor device. The further semiconductor device may be any device that uses a semiconductor material, other than a III-V material, for it to function, such as a diode or a transistor. In embodiments, the further semiconductor device can be a transistor, such as a field-effect transistor. The further semiconductor device typically comprises a semiconductor material that is different from a III-V semiconductor material, such as a group IV semiconductor material (typically silicon or germanium) or a second two-dimensional layered crystal material.
In embodiments wherein the further device is the further semiconductor device, the at least part of the further semiconductor device comprises a silicon semiconductor material or at least one monolayer, e.g., from 1 to 100 monolayers, of a second two-dimensional layered crystal material. The second two-dimensional layered crystal material may be independently selected from the materials mentioned with respect to the first two-dimensional layered crystal material.
In embodiments, step d comprises forming the second two-dimensional layered crystal material in the second region. In embodiments, the at least part of the further device may be a contact, e.g., gate contact, of the further device. In embodiments, such as when the second two-dimensional layered crystal material is a conductor, e.g., graphene, the second two-dimensional layered crystal material may form a gate contact of the further device. In embodiments, such as when the second two-dimensional layered crystal material is an electrical insulator, e.g., hexagonal boron nitride, the second two-dimensional layered crystal material may form a gate dielectric of the further device. In embodiments, such as when the second two-dimensional layered crystal material is a semiconductor, e.g., a transition metal dichalcogenide, the second two-dimensional layered crystal material may form a channel layer of the further device. In particular embodiments, the further device is a transistor comprising a stack of three different two-dimensional layered crystal materials, comprising a two-dimensional layered crystal material that is a semiconductor, a two-dimensional layered crystal material that is an electrical insulator over the semiconductor, and a two-dimensional layered crystal material that is a conductor over the electrical insulator. In embodiments, the at least part of the further device comprises at least part of the buffer layer in the second region, i.e., the second two-dimensional layered crystal material comprises at least part of the buffer layer in the second region. In these embodiments, the second two-dimensional layered crystal material and the first two-dimensional layered crystal material comprise a same material. In these embodiments, the forming step d may comprise removing part of the first two-dimensional layered crystal material so as to form the at least part of the further device in the second region. In other words, step d may comprise removing part of the first two-dimensional layered crystal material so as to separate the first two-dimensional layered crystal material in two spatially separated two-dimensional layered crystal materials: the first two-dimensional layered crystal material and the second two-dimensional layered crystal material. Herein, the removing may be performed on part of the first two-dimensional layered crystal material that is in the second region or not in the second region. It is a benefit of embodiments of the present disclosure that no additional steps of depositing or transferring the second two-dimensional layered crystal material are required. In embodiments wherein the further device is a further semiconductor device, step d of forming the at least part of the further semiconductor device may comprise any technique known to the skilled person with respect to Si CMOS fabrication. For example, step d may comprise forming a Si-based P-type metal oxide transistor. Techniques that may be used for forming the Si-based P-type metal oxide transistor are well-known to the skilled person. In some embodiments, step d may be performed after step c. Thereby, the forming of the III-V semiconductor material, which may require high temperatures, may not damage the at least part of the further device.
As the III-V semiconductor material is grown over the buffer layer, the base substrate may not influence the growth of the III-V material. Therefore, the base substrate may comprise any type of material. In embodiments, the base substrate comprises, e.g., consists of, a germanium or silicon wafer. In some embodiments, the base substrate comprises crystalline silicon, such as Si(111) or Si(100), usually Si(100). In embodiments, the base substrate comprises silicon oxide or silicon nitride covering the crystalline silicon, e.g. Si(100). In these embodiments, the buffer layer may be formed over, e.g., on, the silicon oxide or silicon nitride. In embodiments, the base substrate comprises doped silicon. This is usually used for the formation of a Si-based device (i.e. a device having a Si active layer) for the further, semiconductor, device, for which, typically, doped silicon may be used. The surface of the base substrate may be typically flat, but the disclosure is not limited thereto.
In embodiments, providing the buffer layer comprises providing the buffer layer over a surface of the base substrate in the first region and in the second region. In these embodiments, the buffer layer may also be provided over a surface of the base substrate outside of the first region and the second region, for example over a surface separating the first region and the second region. In embodiments, the buffer layer may be a continuous, i.e., non-intermittent, layer. It is benefit of embodiments of the present disclosure that no selective growth of the buffer layer may be required. It is a benefit of embodiments of the present disclosure that the buffer layer in the second region may function as part of the further device.
In embodiments, the method comprises a step a′ after step a, and before step c of: a′. forming a dielectric mask layer over the surface of the base substrate in the second region and not in the first region. In embodiments, the dielectric mask layer has a thickness of from 5 nm to 1000 nm. In embodiments, the dielectric mask layer may be formed over a surface of the base substrate separating the first region and the second region. The dielectric mask layer may act as a mask for subsequent growth of the III-V semiconductor material. That is, the III-V semiconductor material only grows on exposed surfaces of the base substrate, uncovered by the dielectric mask layer. In embodiments, the dielectric mask layer comprises a silicon nitride or a silicon oxide. In embodiments, the dielectric mask layer may be formed on, i.e., in physical contact with, the surface of the base substrate. In these embodiments, step a′ may be typically performed before step b. It is a benefit of embodiments of the present disclosure that the dielectric mask layer may act as a mask for the formation, e.g., the growth, of the buffer layer on surfaces of the base substrate uncovered by the dielectric mask layer. In different embodiments, the dielectric mask layer may be formed on, i.e., in contact with, the buffer layer. In embodiments, the method comprises a step c′″, usually before step d, usually after step c, of removing the dielectric mask layer.
In embodiments, the method comprises a step a″ after step a, and before step b, of: a″ depositing the dielectric mask layer in the first region and in the second region. In these embodiments, in step b, the buffer layer may be provided over, e.g., on, the dielectric mask layer, usually in the first region and not in the second region, usually not in a region separating the first region from the second region. It is a benefit of embodiments of the present disclosure that, in step c, the III-V semiconductor material is only formed over, e.g., on, the buffer layer, and not on the dielectric mask layer not covered by the buffer layer.
In embodiments where the buffer layer is provided over a surface of the base substrate in the first region and in the second region, the method may comprise a step b′ after step b and before step c of: b′. introducing crystallographic defects in the buffer layer in the first region and not in the second region. The crystallographic defects may be introduced, for example, by application of a laser beam, an electron beam, an ion beam, e.g., with He, Ga, or Xe ions, or a plasma. Generally, a plasma may be applied. Alternatively, the defects may be introduced by NH3 and H2 treatment. The crystallographic defects may function as nucleation sites for growth of the III-V semiconductor material. Thereby, the III-V semiconductor material may grow at a higher rate on part of the buffer layer comprising the crystallographic defects than on part of the buffer layer not comprising the crystallographic defects, e.g., on pristine buffer layer. As a result, the III-V semiconductor material layer selectively grows on part of the buffer layer comprising the crystallographic defects.
In embodiments, the method comprises a step c′ after step c of: c′. reducing an interface area between the surface of the base substrate in the first region and a surface of the buffer layer in the first region. Typically, the interface area may be defined by a top surface of the base substrate contacting a bottom surface of the buffer layer. In embodiments, the interface area may be reduced by removing at least part of the base substrate in the first region contacting the buffer layer. In embodiments, the base substrate may be selectively removed, selectively with respect to the buffer layer, and usually selectively with respect to the III-V semiconductor structure. In embodiments, the base substrate may be selectively removed by selective etching. In embodiments, the selective etching comprises wet etching, for example, using HF as etchant. HF may selectively etch the base substrate, e.g., SiO2, with respect to Si, graphene, and GaN. In embodiments, the selective etching comprises plasma etching, using, for example, a fluorine containing plasma. The fluorine containing plasma may remove SiN selectively with respect to SiO2 or Si. It is a benefit of embodiments of the present disclosure that, by reducing the interface area, an energy loss from the III-V semiconductor device to the base substrate may be reduced. This may be particularly beneficial when the buffer layer comprises graphene. For example, an insulating buffer layer, such as hexagonal boron nitride, may be used to limit the energy loss. In embodiments wherein the buffer layer is formed on and above a surface of the dielectric mask layer, the interface area may, instead, be an interface area between the surface of the dielectric mask layer and a surface of the buffer layer. In embodiments, the interface area may be reduced, in step c′, by at least 1%, such as from 5 to 100%, such as from 5% to 80%. The reducing of the interface area may result in a gap between the top surface of the base substrate (or of the dielectric mask layer if the buffer layer is formed on a surface of the dielectric mask layer) and the bottom surface of part of the buffer layer, which is typically free of solid material. The gap may have a length ranging from 10 nm to 5000 nm.
In embodiments, the method comprises a step c″ after step c and usually before step d of: c″. forming a protective layer over the III-V semiconductor material. In embodiments, the protective layer covers at least a top surface of the III-V semiconductor material, generally also side surfaces of the III-V semiconductor material. The protective layer generally comprises a silicon nitride or a silicon oxide. In embodiments, the protective layer may be formed by chemical vapor deposition. In embodiments, step c′″ of removing the dielectric mask layer can be performed after step c″. In embodiments, the protective layer has a thickness of from 20 to 1000 nm. In embodiments, the protective layer can be thicker than the dielectric mask layer. This may protect the III-V semiconductor material from being etched during the removing of the dielectric mask layer. In embodiments, the dielectric mask layer can be a different material than the protective layer. For example, one of the dielectric mask layer and the protective layer may comprise, e.g., consist of, silicon nitride and the other may comprise, e.g., consist of, silicon oxide. This may enable selective etching of the dielectric mask layer with respect to the protective layer. It is a benefit of embodiments of the present disclosure that the protective layer may protect the III-V semiconductor material from damage during the removal of the dielectric mask layer. It is a benefit of embodiments of the present disclosure that the protective layer may protect the III-V semiconductor material from damage due to step d of forming, in the second region, at least part of the further device.
In embodiments, the buffer layer comprises from 1 to 100 monolayers of the first two-dimensional layered crystal material. It is a benefit of embodiments of the present disclosure that the buffer layer may be thin, yet prevent the base substrate from influencing the growth of the III-V semiconductor material. Due to the absence of dangling bonds on the surface of the buffer layer, growth of the III-V semiconductor material may proceed according to the structure, e.g., crystal structure, of the III-V semiconductor material. It is a benefit of embodiments of the present disclosure that the growth of the III-V semiconductor material on the buffer layer is independent of the crystal structure of the first two-dimensional layered crystal material. In embodiments, the monolayer of the first two-dimensional layered crystal material comprises at least one of, usually consists of at least one of, graphene, silicene, borophene, germanene, black phosphorus, hexagonal boron nitride, or a transition metal dichalcogenide monolayer. Generally, the monolayer of the first two-dimensional layered crystal material comprises at least one of, usually consists of at least one of, graphene, hexagonal boron nitride, or a transition metal dichalcogenide monolayer. In embodiments, the transition metal dichalcogenide monolayer may be at least one of WTe2, MoSe2, WSe2, MoTe2, WS2, or MoS2, GaSe, GaTe, InSe, Bi2Se3, NbSe2, NbS2, TaS2TiS2. NiSe2. In some embodiments, the metal dichalcogenide monolayer may be at least one of WS2 or MoS2.
In embodiments, the buffer layer may be formed over the base substrate via transfer of the buffer layer. In these embodiments, the buffer layer can be obtained from an external supply material, for example via mechanical exfoliation. That is, the at least one monolayer may be exfoliated from the external supply material, and transferred over the base substrate. Alternatively, the at least one monolayer can be grown on a metal substrate different from the base substrate, thereby forming the external supply material. Subsequently, the at least one monolayer may be transferred via a wet chemical transfer or dry transfer technique. In different embodiments, the buffer layer can be grown over the base substrate. For example, the buffer layer may be formed by chemical vapor deposition. The buffer layer may be formed on the base substrate, e.g., Si. Alternatively, the buffer layer may be formed on a metal layer deposited on the base substrate, e.g., Si. Herein, the metal layer may, subsequently, be removed selectively. In embodiments, the buffer layer may be formed using a chemical vapor deposition technique such as metal-organic vapour-phase epitaxy, molecular beam epitaxy, atomic layer deposition, or pulsed laser deposition.
In embodiments, step c comprises a step of depositing the III-V semiconductor material. In embodiments wherein the III-V semiconductor material is deposited on the buffer layer, step c may comprise a step, before depositing the III-V semiconductor material, of applying a plasma or an NH3 and H2 pre-treatment, to a top surface of the buffer layer. It is a benefit of embodiments of the present disclosure that homogeneous nucleation of the III-V semiconductor material may be achieved. Furthermore, the surface of the buffer layer may be cleaned in H2, e.g., to remove any contaminations introduced during transfer or formation of the buffer layer. In embodiments, step c of forming the III-V semiconductor material comprises forming an active III-V material on the buffer layer, such as at a temperature ranging from 1000° C. to 1300° C., usually ranging from 1200° C. to 1300° C. In embodiments, step c of forming the III-V semiconductor material comprises the steps of: c1. Depositing a buffer III-V material at a temperature ranging from 1000 to 1200° C., and c2. depositing an active III-V material on the buffer III-V material at a temperature ranging from 1000° C. to 1300° C., usually ranging from 1210 to 1300° C. In these embodiments, step c2 is typically performed at a higher temperature than step c1. As step c1 may be performed at a low temperature, adatoms on the buffer layer may have a low desorption rate. Therefore, a high nucleation density on the buffer layer may be achieved. Thereby, a homogeneous, e.g., continuous, buffer III-V material may be obtained. Furthermore, a faster formation of the buffer III-V material may be achieved. Step c2 may have a high growth rate for, and yield a low amount of defects in, the active III-V material. However, the disclosure is not limited thereto, and instead, only the active III-V material may be deposited. In embodiments, step c1 may be performed, instead, at a temperature below 1000° C., such as below 700° C., usually below 500° C. In embodiments, step c2 may be performed, instead, at a temperature below 1000° C., such as below 700° C., usually below 500° C. For example, particular precursors, such as hydrazine or tri-ethyl gallium, may enable lower temperature formation, i.e., epitaxial growth, of III-V material, e.g., buffer III-V material of active III-V material. It is a benefit of embodiments of the present disclosure, that low-temperature processes may induce less damage to other structures. Generally, the buffer III-V material has a monocrystalline structure. It is a benefit of embodiments of the present disclosure that forming the buffer layer may result in the growth of a low defect, e.g., monocrystalline, buffer III-V material. The buffer III-V material may be semiconducting. The buffer III-V material may have a larger defect density than the active III-V material. That may be as the buffer III-V material is formed on the buffer layer, and at a lower temperature. However, usually, the defect density may be the same for the buffer III-V material and the active III-V material. The active III-V material typically has a monocrystalline structure, such as a diamond lattice or a hexagonal lattice, e.g., a wurtzite lattice. In embodiments, in step c1, less than 1000 nm, less than 200 nm, usually less than 50 nm, of the buffer III-V material may be deposited. In embodiments, in step c1, at least 1 nm, such as at least 10 nm, of the buffer III-V material may be deposited. It is a benefit of embodiments of the present disclosure that the amount of buffer III-V material that has to be deposited is limited. Thereby, a distance between the active III-V material and the further device may be limited. In embodiments, in step c2, from 10 nm to 1000 nm of active III-V material may be deposited.
In particular embodiments, the buffer III-V material may be formed on the surface of the base substrate in the first region. In these embodiments, the buffer layer may be formed on the buffer III-V material. In these embodiments, the active III-V semiconductor material, e.g., a monocrystalline III-V semiconductor material, may be deposited on the buffer layer. In these embodiments, the buffer layer may function as a back gate contact of a III-V semiconductor device. In alternative embodiments, a III-V buffer material, instead of the active III-V semiconductor material, may be deposited on top of the buffer layer. In these embodiments, the buffer layer may form a device, embedded in III-V material, wherein an active III-V semiconductor material may be also grown on top to form a III-V semiconductor device. The resulting structure would have a buffer III-V material on top of a base substrate, a buffer layer on top of the buffer III-V material, a buffer III-V material on top of the buffer layer, and an active III-V semiconductor material on top of the buffer III-V material.
In embodiments, step c comprises a step c3 of: c3. forming a mask comprising a pattern over the III-V semiconductor material, and transferring the pattern into the III-V semiconductor material and, usually, into the buffer material underneath the III-V semiconductor material. In embodiments, the transferring of the pattern comprises performing etching techniques, such as dry or wet etching techniques, usually a dry etching technique. In embodiments, the dry etching comprises reactive ion etching. In these embodiments, wet etching may be performed after the reactive ion etching, e.g., KOH when the III-V semiconductor material is GaN. This may remove any surface damage introduced in sidewalls of the III-V semiconductor material. In embodiments, the transferring of the pattern comprises performing lithography, such as extreme-UV lithography or electron beam lithography. In embodiments comprising steps c1 and c2, step c3 may be performed after c1 and c2. In embodiments, c3 comprises removing the buffer material in the second region, and possibly in a region separating the first region from the second region.
In embodiments, when the first semiconductor device is a transistor, the method may comprise during step c2 a step of forming a channel layer and forming a barrier layer over (e.g., on) the channel layer. Further steps in the fabrication of the transistor may be a step of forming a source, a drain, and a gate.
Any features of any embodiment of the first aspect may be independently as correspondingly described for any embodiment of the second aspect of the present disclosure.
In a second aspect, the present disclosure relates to a semiconductor structure comprising: a base substrate comprising a first region and a second region, a buffer layer over a surface of the base substrate at least in the first region, wherein the buffer layer comprises at least one monolayer of a first two-dimensional layered crystal material, a III-V semiconductor material over the buffer layer in the first region, and at least part of a further device in the second region. The semiconductor structure of the second aspect may be typically obtainable by the method of the first aspect.
In embodiments, the III-V semiconductor material comprises a buffer III-V material over, e.g., on, the buffer layer. In embodiments, the III-V semiconductor material comprises an active III-V material over, e.g., on, the buffer III-V material. In embodiments, the semiconductor structure comprises a protective layer over the III-V semiconductor material. In embodiments, the semiconductor structure comprises a III-V semiconductor device comprising a channel comprising the III-V semiconductor material.
In embodiments, the further device comprises a second two-dimensional layered crystal material. In embodiments, the second two-dimensional layered crystal material may comprise a different material than the first two-dimensional layered crystal material. In embodiments, the further device comprises at least part of the buffer layer in the second region. In these embodiments, the second two-dimensional layered crystal material comprises the same material as the first two-dimensional layered crystal material.
In embodiments, the semiconductor structure comprises a dielectric mask layer over, e.g., on, the surface of the base substrate in the second region. In these embodiments, the dielectric mask layer may be over, e.g., on the buffer layer, when present, in the second region. In particular embodiments, the semiconductor structure comprises the dielectric mask layer over, e.g., on, the surface of the base substrate in the first region. In these embodiments, the buffer layer in the second region may be over, e.g., on the dielectric mask layer in the first region.
Any features of any embodiment of the second aspect may be independently as correspondingly described for any embodiment of the first aspect of the present disclosure.
The disclosure will now be described by a detailed description of several embodiments of the disclosure. It is clear that other embodiments of the disclosure can be configured according to the knowledge of persons skilled in the art without departing from the technical teaching of the disclosure, the disclosure being limited only by the terms of the appended claims.
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In particular embodiments, at least part of the first two-dimensional layered crystal material in the second region 12 can be retained. Thereby, at least part of the first two-dimensional layered crystal material in the second region 12 may form, i.e., is, a second two-dimensional layered crystal material 7 that is part of a further device in the second region 12.
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It is to be understood that although embodiments, specific constructions, and configurations, as well as materials, have been discussed herein for devices according to the present disclosure, various changes or modifications in form and detail may be made without departing from the scope of this disclosure. Steps may be added or deleted to methods described within the scope of the present disclosure.
| Number | Date | Country | Kind |
|---|---|---|---|
| 21184454.3 | Jul 2021 | EP | regional |
