Patent Application
20220228293
The present invention relates to forming semiconductor thin films, using epitaxial deposition processes.
Research into new materials for power electronic devices has emerged as an inseparable part of sustainable development and efficient handling of electrical energy during the past three decades. Such power devices may convert DC power generated by solar cells and fuel cells to AC power, thereby making it usable by consumers. Alternatively, such power devices may convert AC power supplied by a provider to DC power, thereby making it usable in charging the battery of an electric car or a portable electronic device.
Wide bandgap semiconductors such as GaN (gallium nitride) and SiC (silicon carbide) have been considered as candidate materials for power devices to overcome the limitations of their traditional predecessors (such as Si (silicon) and GaAs (gallium arsenide)) in meeting the growing needs and the stringent requirements of the high energy demand society today.
Thanks to nearly three decades of research on GaN electronics, strategies for GaN heteroepitaxy on common substrates (such as sapphire, Si, SiC, diamond, and even β-Ga2O3) as well as n- and p-type doping of GaN have been successfully developed and are being used in commercial power conversion devices today (see below References no. 10-12). Wurtzite GaN (w-GaN) with a hexagonal crystal structure (belonging to the space group P63mc) is the widely used polymorph in GaN electronic devices.
During the past few years, Ga2O3(gallium oxide) has been proposed as an alternative material for such semiconductor devices promising to offer higher efficiency in power handling than the materials in use today and expected to compete with and complement the outstanding properties of GaN as the frontrunner material for power electronic devices (see References no. 1-5). In addition to power applications, Ga2O3 expands the wavelength span of optoelectronic devices to the deep UV (see below Reference no 3).
The properties of Ga2O3 depend on its crystal structure (see below Reference no. 3). Most attention has been devoted to monoclinic β-Ga2O3 as the most stable polymorph (belonging to the space group C2/m). With the recent availability of β-Ga2O3 bulk wafers (grown from the melt at high temperatures, ca. 1800° C.) (see below References no. 1, and 3-5) homoepitaxial thin films of the β-Ga2O3 polymorph can be deposited on its native substrate (see below References no. 1, 3, and 4). However, there are currently a number of challenges that limit the development of β-Ga2O3 electronic devices including inherent complexities in its crystal structure, limited success in heteroepitaxial growth of β-Ga2O3 on foreign substrates (see below Reference no. 6), and the lack of successful p-type doping of β-Ga2O3 (see below Reference no. 7).
On the other hand, the rhombohedral α-Ga2O3 polymorph (belonging to the space group R
In order to implement Ga2O3 in next-generation electronic devices, there is a need in the art for forming a thin film comprising α-Ga2O3, preferably with relatively low amounts of other Ga2O3 polymorphs (such as β-Ga2O3) and impurities, and using an energy-efficient fabrication process.
The present invention relates to a deposition strategy for obtaining high quality α-Ga2O3 on GaN-compatible substrates with atomic level control over the crystal structure. Without restriction to a theory, it is believed that sustained hexagonal scaffolding at the atomic scale, as a result of using a GaN-mediated Ga2O3 deposition approach described herein, enabled as an example by plasma-enhanced ALD, plays a unique role in steering the atoms to form the crystal structure of α-Ga2O3 at a low thermal budget. This approach minimizes the formation of β-Ga2O3 domains and hinders formation of a mixed-phase material. It also makes integration of GaN and Ga2O3 components on a monolithic substrate possible and facilitates fast-track exploitation of GaN technology advancements (such as thermal management and doping) for development of Ga2O3 electronics.
In one aspect, the present invention comprises a method for forming a thin film comprising alpha-gallium oxide (α-Ga2O3) on a GaN-compatible substrate in a reaction chamber, the method using an epitaxial deposition process comprising the steps of:
In embodiments, the layer of w-GaN is a single monolayer of w-GaN, and the layer of α-Ga2O3 is a single monolayer of α-Ga2O3.
In embodiments, the epitaxial deposition process may comprise an atomic layer deposition (ALD) process comprising the sequential steps of:
In embodiments, the layer of gallium precursor is a single monolayer of gallium precursor, the layer of w-GaN is a single monolayer of w-GaN, and each of the layers of α-Ga2O3 is a single monolayer of α-Ga2O3.
In embodiments, the GaN-compatible substrate is a non-native substrate, which may be sapphire, and more particularly, c-plane sapphire.
In embodiments, the thin film comprises less than 10% β-Ga2O3, by ratio of mass of β-Ga2O3 to mass of α-Ga2O3 and β-Ga2O3, collectively.
In embodiments, the method may be performed at a temperature of less than about 500° C., and preferably less than about 300° C., such as 277° C., which is relatively low in the context of crystalline material growth. The w-GaN deposition process, which in some embodiments may be achieved by using atomic layer deposition, forms a sacrificial w-GaN layer. Within the w-GaN deposition process, which in some embodiments may be plasma-enhanced, a highly symmetric atomic scale scaffold of gallium atoms is created by taking advantage of the sacrificial w-GaN layer as an intermediate step during α-Ga2O3 growth. Establishing the scaffold together with the use of highly reactive plasma species allow this GaN-mediated α-Ga2O3 deposition process to be performed at low thermal budget while resulting in a highly oriented α-Ga2O3 thin film with vanishing amounts of nitrogen and carbon impurities as well as a larger bandgap and larger refractive index compared to the conventionally deposited Ga2O3.
In another aspect, the invention comprises a thin film of α-Ga2O3 formed by atomic layer deposition, and specifically a thin film formed by a method described herein. In some embodiments, the thin film is formed at a process temperature of less than about 500° C., and preferably less than about 300° C.
Embodiments of the invention may be useful for development of high performance and energy-efficient α-Ga2O3 electronics, advancing n-type and p-type doping of α-Ga2O3, and integrating complementary GaN and α-Ga2O3 semiconducting components on a single monolithic substrate to achieve the superior functionalities needed to meet the emerging requirements of modem power handling circuitries (including decreased energy loss, size, weight and cost).
A low temperature GaN-compatible deposition technology for Ga2O3 may be a key enabling technology for wide bandgap semiconductors, leading to energy-efficient electronic devices, not only in performance but also an energy-efficient fabrication process.
In the drawings, like elements may be assigned like reference numerals. The drawings are not necessarily to scale, with the emphasis instead placed upon the principles of the present invention. Additionally, each of the embodiments depicted are but one of a number of possible arrangements utilizing the fundamental concepts of the present invention.
The invention relates to formation of semiconductor thin films using an epitaxial deposition process. Any term or expression not expressly defined herein shall have its commonly accepted definition understood by a person skilled in the art. As used herein, the following terms have the following meanings.
“Atomic layer deposition” or “ALD” is a subclass of chemical vapor deposition, used to deposit thin films onto a substrate. ALD typically involves the sequential use of gas phase reactants, and/or plasma phase reactants, and surface chemical processes.
“Epitaxial deposition process”, as used herein, refers to a process that involves placing a substrate in a reaction chamber, and introducing one or more precursor (reactant) materials into the reaction chamber, such that the precursor(s) or their reaction product(s), deposit on the substrate to form a non-amorphous, crystalline layer having defined crystallographic orientation(s) relative to the underlying layer(s). In non-limiting embodiments, the epitaxial deposition process may comprise chemical vapor deposition (CVD) processes, physical vapor deposition (PVD) processes, or any other suitable deposition techniques as are known to a person skilled in the art of forming thin films. Chemical vapor deposition (CVD) processes may be performed using a variety of techniques known to a person skilled in the art, with non-limiting embodiments including metal-organic CVD (MOCVD), mist CVD, low pressure CVD, atmospheric CVD, plasma-assisted CVD (also referred to as plasma-enhanced CVD), photo-assisted CVD, molecular layer deposition (MLD), and atomic layer deposition (ALD) including spatial ALD, thermal ALD, plasma-assisted ALD (also referred to as plasma-enhanced ALD), and photo-assisted ALD. Metal-organic vapor phase epitaxy (MOVPE), halide vapor phase epitaxy (HVPE) and liquid phase epitaxy (LPE) may also be used. Physical vapor deposition (PVD) processes may be performed using a variety of sputtering techniques known to a person skilled in the art, with non-limiting embodiments including ion beam deposition, reactive sputtering, magnetron sputtering, and RF diode sputtering. Physical vapor deposition (PVD) processes may also be performed using a variety of evaporation techniques known to a person skilled in the art, with non-limiting embodiments including thermal evaporation, c-beam evaporation, pulsed laser deposition (PLD), and molecular beam epitaxy (MBE) including reactive MBE.
“Gallium precursor”, as used herein, refers to a substance comprising gallium atoms, which is suitable for use as reactant in an epitaxial deposition process. In non-limiting embodiments, including embodiments where the epitaxial deposition process is an atomic layer deposition process, the gallium precursor may comprise one or a combination of the substances shown in the table of
“Monolayer”, as used herein, refers to a single layer of atoms, or molecules.
“Nitrogen precursor”, as used herein, refers to a substance comprising nitrogen atoms, which is suitable for use as a reactant in an epitaxial deposition process. In non-limiting embodiments, including embodiments where the epitaxial deposition process is an atomic layer deposition process, the nitrogen precursor may comprise one or a combination of nitrogen (N2) gas or plasma, ammonia (NH3) gas or plasma, or a N2/H2 forming gas or plasma.
“GaN-compatible substrate”, as used herein, refers to a substrate (e.g., a wafer, a membrane, a multilayer, or a laminated structure) comprising a material other than α-Ga2O3 (i.e., a “non-native substrate”) and/or α-Ga2O3 (i.e., a “native substrate”). In embodiments, the non-native substrate may comprise sapphire, Si, SiC, or diamond, or any other suitable substrate known in the art.
“N2/H2 forming gas plasma”, as used herein, refers to a plasma formed from a mixture of nitrogen gas (Nz) and hydrogen gas (H2). In non-limiting embodiments, the N2/H2 forming gas plasma is formed from a mixture of 95% N2 gas and 5% Hz gas, by volume. In other embodiments, the N2/H2 forming gas plasma may be formed from a mixture of N2 gas and H2 gas having a different volumetric ratio of N2 gas and H2 gas. It is within the skill of a person skilled in the art of thin film deposition to select a suitable volumetric ratio of N2 gas and H2 gas to react with a gallium precursor to form w-GaN. Usually, the amount of H2 gas is selected to be less than about 5.7% by volume to avoid the risk of spontaneous or hazardous combustion of H2 gas.
“Oxygen precursor”, as used herein, refers to a substance comprising oxygen atoms, which is suitable for use as reactant to react with wurtzite gallium nitride (w-GaN) to form α-Ga2O3 in an epitaxial deposition process. In non-limiting embodiments, including embodiments where the epitaxial deposition process is an atomic layer deposition process, the oxygen precursor may comprise one or a combination of the substances shown in the table of
Embodiments of the invention comprise a novel, self-regulated process, using an epitaxial deposition process, for controlling Ga2O3 crystallinity to achieve α-Ga2O3 through stepwise in-situ oxidation of w-GaN. In particular embodiments, the epitaxial deposition process is an atomic layer deposition process, involving in-situ plasma-enhanced oxidation of w-GaN.
Step (704) is directed to reacting the layer of w-GaN on the substrate with an oxygen precursor to form a layer (e.g., a monolayer) of α-Ga2O3 on the substrate. In this embodiment, the epitaxial deposition process is used to deposit an oxygen precursor on the first layer of w-GaN, and react therewith, to form a first layer of α-Ga2O3 on the substrate.
Steps (706) to (710) are a repetition of steps (700) to (704), performed in respect to the substrate with the first layer of α-Ga2O3 formed thereon as a result of step (704). These steps are directed to forming an additional layer (e.g., a monolayer) of α-Ga2O3 on the substrate. As indicated by step (712), steps (706) to (710) may be repeated as many times as desired to create additional layers (e.g., monolayers) of α-Ga2O3 on the substrate, with each repetition forming one such layer.
In one embodiment, α-Ga2O3 is formed on a GaN-compatible substrate in a reaction chamber, using ALD in consecutive cycles each consisting of an optimized sequence as follows:
The first four steps (i) to (iv) of this sequence result in a coherent monolayer of w-GaN through which Ga atoms form a stable and highly symmetric scaffold (i.e., possessing 6-fold symmetry). The scaffold steers the oxygen atoms into forming the crystal structure of α-Ga2O3 upon oxygen plasma exposure in the remaining two steps (v) and (vi) of the sequence. The cycles are repeated until the desired thickness of α-Ga2O3 material is deposited.
The entire deposition is optimized to achieve crystallinity at the low temperature of the substrate 277° C., thereby establishing an energy-efficient fabrication process for growing crystalline Ga2O3 films on GaN-compatible substrates on which about one monolayer of heteroepitaxial w-GaN can be initially grown to serve as the template. Once such template is available, the deposition process proceeds in cycles described above to achieve α-Ga2O3 through stepwise construction of an atomic scale hexagonal scaffold of Ga atoms while taking advantage of plasma species to transform nitride to oxide at a low thermal budget. Additionally, this GaN-mediated deposition strategy provides a new platform for direct deployment of GaN dopant candidates to Ga2O3 during growth and moving toward realization of bipolar Ga2O3 electronic devices. Fabrication of Ga2O3 devices on GaN-compatible substrates using this deposition strategy also allows for the transfer of pertinent thermal management technologies that are already established for GaN electronics (see below Reference no. 13) which will mitigate the low thermal conductivity of Ga2O3 and make devices available that are able to concurrently handle higher power, higher voltage, and higher operating temperatures.
The role of ALD in formation of no more than one monolayer of w-GaN in each cycle is a positive enabling factor in establishing a scaffold that is compatible with the position of Ga atoms in α-Ga2O3; this indicates the crucial role of controlling the number of deposited monolayers in the success of this GaN-mediated deposition process. Such control may be achieved by a number of epitaxial deposition processes including, but not limited to, different variations of molecular beam epitaxy (MBE), chemical vapor deposition (CVD), pulsed laser deposition (PLD), and atomic layer deposition (ALD). Accordingly, for each cycle, step, or sub-step of the epitaxial deposition process, it may be desirable for each gallium precursor layer to be formed as a single monolayer (i.e., no more than one monolayer), for each w-GaN layer to be formed as a single monolayer (i.e., no more than one monolayer), and for each α-Ga2O3 to be formed as a single monolayer. (i.e., no more than one monolayer). It will be understood, however, that practical limits of controlling material deposition may mean that each layer may be an incomplete monolayer (e.g., in ALD where precursor molecules can shadow the surface and do not allow a complete monolayer to form), or may include slightly more than one monolayer (e.g., some variations of MBE and CVD may result in deposition of slightly more than one monolayer depending on the precursors, the reaction conditions, etc.). The present invention is intended to include cases of such slight deviations from a single monolayer and to include adjustments made on the number of repetitions of cycles, steps, or sub-steps to account for those deviations and/or to form up to a theoretically less or more packed layer of material.
As can be seen by comparing
Sapphire, and more particularly c-plane sapphire, may be selected as the non-native substrate due to availability of abundant data for achieving high quality w-GaN on this substrate (see below References no. 17 and 18). As shown in
As a reference, depositing Ga2O3 directly on sapphire (i.e., without any GaN layers involved) at the same deposition temperature resulted in a mixture of α-Ga2O3 and β-Ga2O3 in the film; this is shown in
These results demonstrate that if the Ga atoms establish a hexagonal arrangement (i.e., an arrangement with a higher degree of symmetry) by forming a monolayer of w-GaN, subsequent exposure to oxygen plasma can successfully interchange the anions while preserving the Ga scaffold thereby leading to formation of a high quality α-Ga2O3 layer. This self-regulated crystallization process is favored further by considering that the stacking sequence of atoms in both w-GaN and α-Ga2O3 is of the hexagonal closest packing (hcp) type with both N anions in w-GaN and O anions in α-Ga2O3 being surrounded by 4 Ga atoms (i.e., both anions have a coordination number of 4), while β-Ga2O3 has the stacking sequence of a distorted cubic closest packing (ccp) type with coordination number of 6 for two of the O anions and 4 for one of the O anions. Therefore, even though β-Ga2O3 domains are demonstrated to form in the absence of structural restrictions to atomic diffusion in the reference Ga2O3 film (see
Using such a hexagonal scaffold at the atomic scale also offers the potential to provide a means by which metal dopant atoms (especially those that are known to be compatible with w-GaN) can be incorporated into the GaN layer in situ (see, for example, below References no. 19 and 20 for methods to incorporate dopant atoms into ALD films in situ), subsequently oxidized, and thereby be embedded in the α-Ga2O3 structure during the deposition.
To investigate the structure of the α-Ga2O3 film deposited by the GaN-mediated approach further, cross-section TEM analysis was performed, and representative results are shown in
In addition to crystal structure, investigating optical properties of thin films can provide insights into the quality and performance of the material. To that end, in-situ ellipsometry measurements were performed on the GaN-mediated in-situ oxidized Ga2O3 film, as well as a reference Ga2O3 film with no GaN layers involved during its deposition, and a reference GaN film. In all cases, the substrate was c-plane sapphire, and the measurements were performed after 450 doses of TEG (which resulted in an ˜22 nm α-Ga2O3 film deposited by using the GaN-mediated approach, as well as an ˜26 nm reference α-Ga2O3/β-Ga2O3 mixed-phase film, and an ˜22 nm reference GaN film, respectively—the difference in thickness of the two Ga2O3 films is consistent with the fact that β-Ga2O3 has a larger molar volume than α-Ga2O3 (see below References no. 8 and 9); thus, inclusion of β-Ga2O3 domains in the film results in a thicker film for a constant number of TEG doses). As shown in
Depositions were done at 277° C. on single-side polished (Ra<0.3 nm) prime quality c-plane sapphire wafers (see below Reference no. 6 for detailed specifications of the wafers) by using a Kurt J. Lesker ALD 150-LX™ system equipped with a remote inductively coupled plasma (ICP) source and a load lock. The error in determining the actual deposition temperatures was ±3° C. The pressure of the reactor was ˜1.1 Torr with ˜1000 sccm continuous flow of argon. In addition, 60 sccm oxygen or N2/H2 forming gas was introduced to the reactor during plasma exposures with ˜600 W forward power. This setup is also explained in detail elsewhere (see below References no. 6 and 23). Triethylgallium, TEG, (Strem Chemicals, Inc.) was electronic grade (99.9999% Ga) in a stainless steel Swagelok™ cylinder assembly which was not heated during the depositions; all other gases (argon, oxygen, and forming gas) were of ultrahigh purity (99.999%, Praxair Canada, Inc.). Substrates were exposed to 60 s plasma to remove contamination and pretreat the surface prior to deposition. Reference GaN depositions were done by using a recipe consisting of 0.1 s TEG dose, 3 s argon purge, 15 s N2/H2 forming gas plasma dose, and 2 s argon purge. Reference Ga2O3 depositions were done by using a recipe consisting of 0.1 s TEG dose, 20 s argon purge, 10 s oxygen plasma dose, and 12 s argon purge (reducing the two purge times down to 3 s and 2 s, respectively, did not change the deposition results for the reference Ga2O3). GaN-mediated Ga2O3 depositions were done by using a recipe consisting of 0.1 s TEG dose, 6 s argon purge, 15 s N2/H2 forming gas plasma dose, 13 s argon purge, 1.5 s oxygen plasma dose, and 10 s argon purge (the N2/H2 forming gas plasma dose time was chosen to ensure completion of GaN formation reactions while the oxygen plasma dose time was chosen to ensure complete conversion of nitride to oxide using GaN and Ga2O3 enthalpies of formation (see below Reference no. 24) as guides).
Ellipsometry measurements were done by using a J. A. Woollam M-2000DI™ spectroscopic ellipsometer, permanently mounted on the reactor at an incident angle of 70°, in the spectral range of 0.73-6.40 eV (equivalent to 190-1700 nm) at intervals less than 0.05 eV. Ellipsometry data analysis was done by using CompleteEASE™ software. Thickness and optical constants of the films were obtained based on Tauc-Lorentz modelling of the ellipsometry data (see below Reference no. 6 for detailed explanation of the modelling procedure).
Out-of-plane coupled 1D XRD scans were performed by using a Rigaku Ultima-IV™ diffractometer equipped with a cobalt source, a D/Tex™ ultrahigh-speed position sensitive detector, and a K-β filter at a scan rate of 2°/min and 0.02° steps (which is equivalent to 0.6 s/step exposure). The patterns were converted to copper wavelength for easier comparison with the literature.
Cross-section TEM lamella was prepared by low-energy ion polishing (to minimize damage) by using a ThermoFisher Helios Hydra DualBeam™ plasma-focused-ion-beam (PFIB) system. HRTEM images were obtained by using a Titan 80-300™ HRTEM instrument. Atomic resolution STEM analyses (including STEM images, EDS maps, and nano-beam diffraction patterns) were performed by using a Thermo Scientific Themis Z™ S/TEM instrument equipped with 4 windowless EDS detectors arranged symmetrically around the sample to allow EDS mapping of light elements such as C, N, and O.
Interpretation.
The corresponding structures, materials, acts, and equivalents of all means or steps plus function elements in the claims appended to this specification are intended to include any structure, material, or act for performing the function in combination with other claimed elements as specifically claimed.
References in the specification to “one embodiment”, “an embodiment”, etc., indicate that the embodiment described may include a particular aspect, feature, structure, or characteristic, but not every embodiment necessarily includes that aspect, feature, structure, or characteristic. Moreover, such phrases may, but do not necessarily, refer to the same embodiment referred to in other portions of the specification. Further, when a particular aspect, feature, structure, or characteristic is described in connection with an embodiment, it is within the knowledge of one skilled in the art to affect or connect such module, aspect, feature, structure, or characteristic with other embodiments, whether or not explicitly described. In other words, any module, element or feature may be combined with any other element or feature in different embodiments, unless there is an obvious or inherent incompatibility, or it is specifically excluded.
It is further noted that the claims may be drafted to exclude any optional element. As such, this statement is intended to serve as antecedent basis for the use of exclusive terminology, such as “solely,” “only,” and the like, in connection with the recitation of claim elements or use of a “negative” limitation. The terms “preferably,” “preferred,” “prefer,” “optionally,” “may,” and similar terms are used to indicate that an item, condition or step being referred to is an optional (not required) feature of the invention.
The singular forms “a,” “an,” and “the” include the plural reference unless the context clearly dictates otherwise. The term “and/or” means any one of the items, any combination of the items, or all of the items with which this term is associated. The phrase “one or more” is readily understood by one of skill in the art, particularly when read in context of its usage.
The term “about” or “˜” can refer to a variation of 5%, f 10%, f 20%, or 25% of the value specified. For example, “about 50” percent can in some embodiments carry a variation from 45 to 55 percent. For integer ranges, the term “about” or “˜” can include one or two integers greater than and/or less than a recited integer at each end of the range. Unless indicated otherwise herein, the term “about” or “˜” is intended to include values and ranges proximate to the recited range that are equivalent in terms of the functionality of the composition, or the embodiment.
As will be understood by one skilled in the art, for any and all purposes, particularly in terms of providing a written description, all ranges recited herein also encompass any and all possible sub-ranges and combinations of sub-ranges thereof, as well as the individual values making up the range, particularly integer values. A recited range includes each specific value, integer, decimal, or identity within the range. Any listed range can be easily recognized as sufficiently describing and enabling the same range being broken down into at least equal halves, thirds, quarters, fifths, or tenths. As a non-limiting example, each range discussed herein can be readily broken down into a lower third, middle third and upper third, etc.
As will also be understood by one skilled in the art, all language such as “up to”, “at least”, “greater than”, “less than”, “more than”, “or more”, and the like, include the number recited and such terms refer to ranges that can be subsequently broken down into sub-ranges as discussed above. In the same manner, all ratios recited herein also include all sub-ratios falling within the broader ratio.
The following publications cited herein are indicative of the level of one skilled in the art and are incorporated herein by reference in their entireties, except for any subject matter disclaimers or disavowals, and except to the extent that the incorporated material is inconsistent with the express disclosure herein, in which case the language in this disclosure controls.
This application claims priority to U.S. Provisional Patent Application No. 63/137,874, filed on Jan. 15, 2021, the entire disclosure of which is incorporated by reference herein in its entirety for all purposes.
| Number | Date | Country | |
|---|---|---|---|
| 63137874 | Jan 2021 | US |
