Crystalline gallium oxide grown by a combination of solid-phase epitaxy and overlayer growth

Abstract

Methods of forming gallium oxide with high crystal quality using a combination of solid-phase epitaxy (SPE) and a second deposition method are provided. In the methods, a crystalline Ga2O3 base layer comprising or consisting of β-phase Ga2O3 is formed on the surface of a substrate via SPE, whereby an amorphous thin film of gallium oxide is sputtered onto the substrate surface and the amorphous thin film is then converted into a crystalline layer of Ga2O3 via a thermal anneal. This crystalline layer is characterized by a very low surface roughness and, therefore, provides a high-quality base layer for the subsequent homoepitaxial growth of a high-crystal-quality overlayer of Ga2O3 using a second growth method.

Description

Atty. Dkt. No.00300-0423-PCT CRYSTALLINE GALLIUM OXIDE GROWN BY A COMBINATION OF SOLID- PHASE EPITAXY AND OVERLAYER GROWTH CROSS-REFERENCE TO RELATED APPLICATIONS

[0001] The present application claims priority to U.S. provisional patent application number 63 / 611,365 that was filed December 18, 2023, the entire contents of which are incorporated herein by reference. BACKGROUND

[0002] Gallium oxide (Ga2O3) is a good semiconductor material for high-power devices, due to its bandgap, high dielectric constant, and high breakdown field. However, the thermal conductivity of Ga2O3is very low, which dramatically reduces the lifetimes and current carrying capabilities of gallium oxide-based electronic devices. Current methods of growing Ga2O3films hetero-epitaxially produce either rough films or multi-phase or multi-orientation films that are not of high electrical quality. Therefore, there is a need for methods of gallium oxide growth that produce smooth, single-crystalline, and high-electrical quality films. SUMMARY

[0003] Structures that include layers of high-crystal-quality Ga2O3 are provided. Methods of growing crystalline Ga2O3layers on a substrate surface are also provided.

[0004] One embodiment of a structure includes: a substrate; and a layer of crystalline Ga2O3 comprising or consisting of β-phase Ga2O3 on a surface of the substrate, the layer of crystalline Ga2O3having a layer thickness of 1 µm or less and a surface with an average surface roughness (RA) of less than 7 nm over a surface area of at least 1 µm2. In some embodiments of the structures, the layer of crystalline Ga2O3has a layer thickness of 50 nm or less. The structure can, optionally, include an overlayer of crystalline Ga2O3 on the surface of the layer of crystalline Ga2O3, wherein layer of crystalline Ga2O3and the overlayer of crystalline Ga2O3 form a distinguishable interface.

[0005] One embodiment of a method of growing crystalline Ga2O3 includes the steps of: depositing an amorphous gallium oxide on a surface of the substrate via radiofrequency sputtering to form an amorphous film of gallium oxide; simultaneously or sequentially annealing the amorphous film of gallium oxide to convert the amorphous film of gallium oxide into a crystalline layer of Ga2O3 comprising β-phase Ga2O3 having and a layerAtty. Dkt. No.00300-0423-PCT thickness of no greater than 1 µm and a surface with an average surface roughness (RA) of less than 3 nm over a surface area of at least 1 µm2; and growing an overlayer of crystalline gallium oxide on crystalline layer of Ga2O3 by a second method.

[0006] Other principal features and advantages of the invention will become apparent to those skilled in the art upon review of the following drawings, the detailed description, and the appended claims. BRIEF DESCRIPTION OF THE DRAWINGS

[0007] Illustrative embodiments of the invention will hereafter be described with reference to the accompanying drawings.

[0008] FIG.1 shows X-ray reflectivity plots of simulated and measured amorphous gallium oxide films with a thickness of 25.4 nm on a sapphire substrate.

[0009] FIG.2 shows an atomic force microscopy (AFM) scan of an as-deposited amorphous gallium oxide film with an average roughness (RA) of 0.076 nm.

[0010] FIG.3A-3D show AFM images of layers comprising crystalline β-phase Ga2O3on sapphire substrates formed using: (a) a vertical annealing position (FIGS.3A and 3B) and (b) a flat annealing condition (FIGS.3C and 3D). FIGS.3A and 3C cover a surface area of 1 µm by 1 µm; FIGS.3B and 3D cover a surface area of 5 µm by 5 µm. The RA for the surfaces of the layers is provided under each image.

[0011] FIG.4 shows an X-ray diffraction (XRD) omega-2theta scan of a layer comprising β-phase Ga2O3 on a sapphire substrate showing that the Ga2O3 has a (-201) crystal orientation.

[0012] FIGS.5A and 5B show AFM images of the layer comprising crystalline β-phase Ga2O3 on miscut sapphire substrates. The miscut angle is 4º. FIG.5A covers a surface area of 1 µm by 1 µm; FIG.5B covers a surface area of 5 µm by 5 µm. The RAfor the surfaces of the layers is provided under each image.

[0013] FIG.6 shows an XRD omega-2-theta scan of the layer comprising crystalline β- phase Ga2O3on a miscut sapphire substrate (miscut angle of 4º) showing that the Ga2O3has a (-201) crystal orientation.

[0014] FIGS.7A – 7H show AFM images of the surfaces of bare planar and miscut sapphire substrates and MOCVD-grown layers comprising β-phase Ga2O3 on the planar andAtty. Dkt. No.00300-0423-PCT miscut sapphire substrates. FIGS.7A and 7B are the AFM images for the MOCVD-grown Ga2O3on bare planar sapphire and on the SPE-grown Ga2O3base layer on planar sapphire, respectively, over a surface area of 1 µm by 1 µm.. FIGS.7C and 7D are the AFM images for the MOCVD-grown Ga2O3on bare planar sapphire and on the SPE-grown Ga2O3base layer on planar sapphire, respectively, over a surface area of 5 µm by 5 µm. FIGS.7E and 7F are the AFM images for the MOCVD-grown Ga2O3on bare miscut sapphire and on the SPE- grown Ga2O3 base layer on miscut sapphire, respectively, over a surface area of 1 µm by 1 µm. FIGS.7G and 7H are the AFM images for the MOCVD-grown Ga2O3on bare planar sapphire and on the SPE-grown Ga2O3 base layer on miscut sapphire, respectively, over a surface area of 5 µm by 5 µm.

[0015] FIGS.8A and 8B show XRD omega-2-theta scans for a bare planar sapphire substrate, an Ga2O3 overlayer grown on an SPE-grown Ga2O3 base layer on planar sapphire, and the SPE-grown Ga2O3base layer on planar sapphire (FIG.8A) and XRD omega-2-theta scans for a bare miscut sapphire substrate, an Ga2O3 overlayer grown on an SPE-grown Ga2O3base layer on miscut sapphire, the SPE-grown Ga2O3base layer on miscut sapphire and the SPE-grown Ga2O3 base layer on planar sapphire (FIG.8B).

[0016] FIGS.9A – 9H show AFM images of the surfaces of base layers comprising β- phase Ga2O3on SiC substrates. FIGS.9A, 9B, 9E, and 9F are images of the crystalline Ga2O3on grown on planar (i.e., non-miscut) SiC. FIGS.9C, 9D, 9G, and 9H are images of the crystalline Ga2O3on grown on miscut SiC. FIGS.9A, 9C, 9E, and 9G cover a surface area of 1 µm by 1 µm; FIGS.9B, 9D, 9F, and 9H cover a surface area of 5 µm by 5 µm. The RA for the surfaces of the layers is provided on each image.

[0017] FIG.10 shows an XRD omega-2-theta scan of a layer comprising crystalline β- phase Ga2O3 on a SiC substrate showing that the Ga2O3 has a (111) crystal orientation. DETAILED DESCRIPTION

[0018] Methods of forming gallium oxide with high crystal quality using a combination of solid-phase epitaxy (SPE) and a second gallium oxide growth technique are provided. In some embodiments the second gallium oxide growth technique is metal-organic chemical vapor deposition (MOCVD). However other growth techniques, including other epitaxial growth techniques can be used as the second growth technique. Other growth techniquesAtty. Dkt. No.00300-0423-PCT include, molecular-beam epitaxy (MBE), Hydride Vapor Phase Epitaxy (HVPE), and Pulsed Laser Deposition (PLD).

[0019] In the methods, a thin crystalline base layer of gallium oxide is formed on the surface of a substrate via SPE, whereby an amorphous thin film of gallium oxide is sputtered onto the substrate surface and the amorphous thin film is then converted into a layer of crystalline Ga2O3 comprising or consisting of β-phase Ga2O3 via a thermal anneal. This crystalline layer of Ga2O3is characterized by a very low surface roughness and, therefore, provides a high-quality base layer for the subsequent homoepitaxial growth of a high-crystal- quality overlayer of gallium oxide via another growth technique, such as MOCVD, MBE, HVPE, or PLD.

[0020] By way of illustration the crystalline Ga2O3 base layers having a surface roughness of 7.0 nm or lower, as measured by average surface roughness (RA) over a surface area of 1.0 µm2, can be formed. This includes crystalline Ga2O3 base layers having a surface roughness of 6 nm or lower, 5 nm or lower, 3 nm or lower, 2 nm or lower, 1 nm or lower, 0.5 nm or lower, 0.4 nm or lower, and 0.3 nm or lower, as measured by average surface roughness (RA) over a surface area of 1.0 µm2. The Ra of a surface can be measured using AFM images, as illustrated in the Example below.

[0021] The Ga2O3 base layers can be very thin to minimize the effect of their low thermal conductivity on the performance of electronic devices formed thereon. By way of illustration, crystalline Ga2O3 base layers having a thickness of 500 nm or lower, a thickness of 250 nm or lower, a thickness of 200 nm or lower, a thickness of 100 nm or lower, a thickness of 50 nm or lower, a thickness of 40 nm or lower, a thickness of 30 nm or lower, or a thickness of 20 nm or lower can be formed. By way of illustration only, embodiments of the base layers have a thickness in the range from 5 nm to 40 nm, including base layers having a thickness in the range from 10 nm to 30 nm.

[0022] Single-crystalline β-phase-pure layers of the Ga2O3can be formed using the methods described herein. However, the Ga2O3 need not be absolutely single-crystalline or β- phase-pure for many applications, and Ga2O3layers that are nearly single-crystalline and / or nearly β-phase-pure can also be formed. Thus, the Ga2O3 layer can include phases other than the β-phase, but such phases a typically present in concentrations of 10% or lower, 5% or lower, 1% or lower, or 0.1% or lower. The phase purity of the Ga2O3 can be determined by using x-ray diffraction based on the area of the peak for the phase, as measured against theAtty. Dkt. No.00300-0423-PCT peaks for a calibration sample of that phase. Similarly, while it is desirable for the layer to be single-crystalline, relatively small deviations from single-crystallinity are tolerable for many applications. Thus, Ga2O3 layers may contain small non-uniformities in the crystal orientations, but such uniformities typically make up a small portion of the Ga2O3layers. By way of illustration, in the Ga2O3 layers the degree of a single crystal orientation, as measured by volume, is typically 90% or better, 95% or better, 99% or better, or 99.9% or better. The degree of single-crystallinity can be measured by mapping the x-ray diffraction rocking curve for the Ga2O3. The single-crystal, or nearly single-crystal nature and the phase purity, or near phase purity, of the crystalline Ga2O3 is reflected in the low roughness of the crystalline Ga2O3layers. Methods for measuring the degree of single-crystallinity can be found in Harrington, G. F.; Santiso, J. Back-to-Basics Tutorial: X-Ray Diffraction of Thin Films. J Electroceram 2021, 47 (4), 141–163; and Pandey, A.; Dalal, S.; Dutta, S.; Dixit, A. Structural Characterization of Polycrystalline Thin Films by X-Ray Diffraction Techniques. J Mater Sci: Mater Electron 2021, 32 (2), 1341–1368.

[0023] The crystalline Ga2O3may be formed on a variety of substrates. However, the methods are particularly useful for the deposition of Ga2O3 on substrates having a high thermal conductivity because such substrates provide a heat sink for gallium oxide-based electronic devices, offsetting the low thermal conductivity of the gallium oxide itself. Suitable substrates include, but are not limited to, silicon carbide (SiC), diamond, and aluminum nitride (AlN). However, lower thermal conductivity substrates, such as sapphire (Al2O3) and gallium nitride (GaN) substrates can also be used. The substrate may itself be Ga2O3, such as a Ga2O3wafer, including r-plane, m-plane, and a-plane Ga2O3. The substrate can be a bulk substrate of a single material or can be a multilayered substrate comprising an underlying bulk substrate with a second substrate material overlayer on a surface of the bulk substrate. Examples include AlN-on-sapphire, AlN-on-SiC, AlN-on-diamond, and GaN-on- SiC. The surfaces of the substrates upon which the Ga2O3is formed can be planar or miscut. By way of illustration only, the surfaces can have a miscut angle, Φ, of 0 < Φ ≤ 10.

[0024] In the sputter deposition process, a gallium oxide sputtering target is bombarded by energetic ions (e.g., a gaseous plasma) causing the material of the gallium oxide sputtering target to be ejected from the target and deposited on the surface of the substrate as a gallium oxide film. Sputter deposition can be carried out using, for example, radiofrequency (RF) sputtering, including RF magnetron sputtering. In RF sputtering, radiowaves are passed through a gas (referred to herein as a sputtering gas) to form energetic ions of the sputteringAtty. Dkt. No.00300-0423-PCT gas. In RF magnetron sputtering, magnets are used to apply a magnetic field around the sputtering target, whereby the resulting negative voltage induces the energetic ions to strike the sputtering target. The sputtering gas may a pure gas or a mixture of gases, such as a mixture of an inert gas and oxygen. In some embodiments of the sputtering methods, the sputtering gas is a mixture of argon and O2.

[0025] The sputtering can be conducted in a sputtering chamber that houses the substrate and the sputtering target. The sputtering chamber can be evacuated through a vacuum port using one or more pumps. The pressure in chamber can be controlled by pumps and flow valves which can be controlled with a pressure controller. The substrate may be mounted to a heater that is configured to heat the substrate during sputter deposition. For RF magnetron sputtering, one or more magnets are mounted to the sputtering target and an RF power supply is connected to the magnets and configured to apply a negative voltage to the sputtering targets.

[0026] During sputtering, the power, sputtering gas pressure, sputtering gas flow, temperature, and deposition time can be adjusted to deposit an amorphous gallium oxide thin film having the desired thickness. To control the film thickness, it is desirable to use a very low film growth rate. For example, film growth rates of 100 nm / hour or slower, including film growth rates of 50 nm / hour or slower and film growth rates of 15 nm / hour or slower may be used. For purposes of illustration, the Example below describes suitable conditions for the sputter deposition of an amorphous gallium oxide thin film. The sputter deposited film is amorphous and, therefore, is non-crystalline and lacks long range order.

[0027] The amorphous sputtered gallium oxide film is then annealed at an elevated temperature for a time sufficient to convert the amorphous gallium oxide into crystalline Ga2O3 via solid phase epitaxial re-growth of the amorphous gallium oxide. In the crystalline Ga2O3, the entire layer (or nearly the entire layer) has a single orientation. As illustrated in the Examples, the crystal orientation can be (-201) or (111), and other crystal orientations, such as (002), can be achieved, depending upon the substrate. It should be understood that the XRD peaks corresponding to these crystal orientations may belong to a group that includes additional peaks that are multiples of the first peak. For example, a crystalline material having a (-201) orientation will also display peaks at (-402) and (-603); this crystal orientation can, therefore, be referred to as the (-201) group orientation. The temperature and time will depend upon the composition of the substrate and the thickness of the gallium oxide film. SuitableAtty. Dkt. No.00300-0423-PCT annealing temperatures include, for example, 600 ºC to 1500 ºC. Suitable annealing times include, for example, 30 minutes to 10 hours. If a rapid thermal anneal (RTA) is used, the time may be from a few (e.g., 3) seconds to 30 minutes. For purposes of illustration, the Example below describes suitable conditions for the annealing of an amorphous gallium oxide thin film.

[0028] Optionally, the sputter and anneal cycle can be carried out multiple (two or more) times to grow a low surface roughness base layer to a desired thickness. The sputtering and / or annealing conditions can be the same or different in different cycles.

[0029] Once the crystalline base layer of Ga2O3 has been formed via SPE, an overlayer of gallium oxide is grown on the base layer via MOCVD or another growth method. Because the base layer is of such high crystal quality, the crystalline gallium oxide overlayer will also have a high crystal quality, as reflected by a low surface roughness and low dislocation density. By way of illustration, Ga2O3 overlayers with RA surface roughnesses of 5 nm or less, 1 nm or less, or 0.5 nm or less can be achieved. To mitigate the effect of gallium oxide’s low thermal conductivity the gallium oxide overlayer may also be thin. For example, the overlayer may have a thickness in the range from 100 nm to a few (e.g., 3 µm). However, thicker or thinner overlayers can be grown. The interface between the gallium oxide base layer and the homoepitaxially grown overlayer is distinguishable, that is - it can be identified / observed using such techniques as scanning electron microscope (SEM), transmission electron microscopes (TEM), or secondary ion mass spectrometry (SIMS). Thus, the base layer is a discrete layer that is separated from the overlayer by a distinguishable interface; it is not merely an arbitrarily defined sub-layer within a larger homogeneous layer of single-crystal Ga2O3.

[0030] During gallium oxide overlayer growth using MOCVD, a solid film of gallium oxide (Ga2O3) is formed on the SPE-grown gallium oxide base film by the thermal decomposition of gallium precursors and oxygen precursors on the surface. The precursor molecules adsorb onto the surface where they undergo heterogeneous and / or decomposition reactions. In the earliest phase of MOCVD, the Ga and O adatoms are mobile and migrate across the surface of the substrate until they reach a high-energy surface site or trap site on which the adatom becomes immobilized. The immobilized atoms provide the starting point for the nucleation phase of the MOCVD process. During the nucleation phase, gallium oxide nuclei (“islands”) grow in the vertical and lateral directions on the substrate surface via theAtty. Dkt. No.00300-0423-PCT incorporation of mobile gallium and oxygen adatoms. In this process, the SPE-grown base layer effectively acts as a homoepitaxy-type substrate.

[0031] As they grow, the gallium oxide nuclei begin to coalesce into a coherent film layer. This begins the phase of the MOCVD known as coalescence. As coalescence continues, complete coalescence of the nuclei is eventually achieved and sustained vertical growth of gallium oxide from the coalesced film commences and is continued until a layer having a desired thickness is formed.

[0032] The gallium and oxygen precursors are introduced into a MOCVD reactor chamber, along with a carrier gas, and are transported to the substrate surface via fluid transport and / or diffusion. The gallium precursors are compounds that contain gallium atoms. Examples of suitable Ga-containing precursors include organometallic molecules, such as, but not limited to, trimethyl gallium (TMGa), or triethylgallium (TEGa). The oxygen precursors are molecules or compounds that contain oxygen atoms. Examples of suitable oxygen precursors include O2(g), H2O, NO, or N2O.

[0033] During gallium oxide overlayer growth using MBE, a solid film of gallium oxide (Ga2O3) is formed on the SPE-grown gallium oxide base film by deposition from a molecular beam. MBE can be carried out in a vacuum chamber equipped with a gallium metal source, such as an effusion cell equipped with a crucible and a heater. The gallium metal source evaporates solid gallium and directs its vapor, as a beam, at the SPE-grown gallium oxide base film. The vacuum chamber is also equipped with a reactive oxygen source that generates ozone or oxygen radicals. The oxygen and gallium beam react to deposit Ga2O3on the base film via MBE growth.

[0034] More information regarding epitaxial growth techniques for Ga2O3, including HVPE and PLD techniques, can be found in the literature. See, for example, Rahaman, Imteaz, et al. "Epitaxial Growth of Ga2O3: A Review." Materials 17.17 (2024): 426; K. Sasaki, M. Higashiwaki, A. Kuramata, T. Masui, and S. Yamakoshi, “MBE grown Ga2O3and its power device applications,” Journal of Crystal Growth 378, 591–595 (2013); and M.-Y. Tsai, O. Bierwagen, M.E. White, and J.S. Speck, “β-Ga2O3growth by plasma-assisted molecular beam epitaxy),” Journal of Vacuum Science & Technology A 28(2), 354–359 (2010). EXAMPLES

[0035] Example 1: Crystalline β-phase Ga2O3on Sapphire.Atty. Dkt. No.00300-0423-PCT

[0036] This example illustrates the growth of a base layer comprising crystalline β-phase Ga2O3grown on a planar sapphire substrate and on a 4º miscut sapphire substrate via SPE. The base layers were used as growth substrates for the subsequent homoepitaxial growth of high-quality crystalline gallium oxide overlayers via MOCVD.

[0037] Gallium oxide on Planar Sapphire: An amorphous Ga2O3film was deposited on an Al2O3 substrate by RF sputtering using a Denton Discovery 24 sputter deposition system. The Al2O3underwent a high-temperature anneal prior to the Ga2O3film deposition. The sputtering conditions included a power of 100 W, 15 mtorr chamber pressure, 20 sccm Ar flow, 2 sccm O2flow, and 200 °C deposition temperature. The growth rate was 12.5 nm / hour. Then, the film was transferred to a high-temperature furnace and annealed at 850 °C for 3 hours in air under atmospheric pressure. RAroughness measurements were obtained on both amorphous and crystalline Ga2O3 on sapphire samples through Atomic Force Microscopes (AFM) using a Bruker Icon Atomic Force Microscope. Low surface roughnesses were obtained: 0.076 nm for a 1 × 1 µm2scan area for the amorphous film; and 0.27 nm for a 5 × 5 µm2scan area for the crystalline film. Film thickness was measured by X-ray reflectivity (XRR) by using an XRD Panalytical Empyrean instrument before and after crystallization with a thickness of 25.4 nm. XRD was performed on the annealed Ga2O3on sapphire by using the XRD Panalytical Empyrean instrument. Omega 2 theta plots are obtained, which demonstrated beta phase with -201, -402, and -603 orientations.

[0038] FIG.1 shows the XRR plots of simulated and measured amorphous gallium oxide films with a thickness of 25.4 nm on a sapphire substrate. AMASS software was used for the simulation and the simulated plot was generated by fitting the experimental plot with known material parameters.

[0039] FIG.2 shows an AFM scan of an as-deposited amorphous gallium oxide film with an average roughness (RA) of 0.076 nm.

[0040] FIG.3A-3D are AFM images of crystalline layers comprising β-phase Ga2O3on sapphire substrates formed using: (a) a vertical annealing position (FIGS.3A and 3B); and (b) a flat annealing condition (FIGS.3C and 3D). FIGS.3A and 3C cover a surface area of 1 µm by 1 µm; FIGS.3B and 3D cover a surface area of 5 µm by 5 µm. The Ra for the surfaces of the layers is provided under each image.

[0041] FIG.4 shows an XRD omega-2-theta scan of crystalline layers comprising β- phase Ga2O3 on a sapphire substrate showing that the Ga2O3 has a (-201) crystal orientation.Atty. Dkt. No.00300-0423-PCT

[0042] Gallium oxide on Miscut Sapphire: An amorphous Ga2O3 film was deposited on a miscut Al2O3substrate by RF sputtering using a Denton Discovery 24 sputter deposition system. The sputtering conditions included a power of 100 W, 15 mtorr chamber pressure, 20 sccm Ar flow, 2 sccm O2flow, and 200 °C deposition temperature. Then, the film was transferred to a high-temperature furnace and annealed at 850 °C for 4 hours in air under atmospheric pressure. RAsurface roughness measurements were obtained on both amorphous and crystalline Ga2O3 on miscut sapphire substrates through AFM using a Bruker Icon Atomic Force Microscope. Low roughnesses were obtained: 0.196 nm for a 5 × 5 µm2scan area for the amorphous sample; and 0.6 nm for a 1 × 1 µm2scan area, and 0.624 nm for a 5 × 5 µm2scan area after crystallization. Layer thickness was measured by XRR using an XRD Panalytical Empyrean instrument before and after crystallization with a thickness of 25.7 nm. XRD was performed on annealed Ga2O3on miscut sapphire using the XRD Panalytical Empyrean instrument. Omega 2 theta plots were obtained, which demonstrated beta phase with -201, -402, and -603 orientations.

[0043] FIGS.5A and 5B show AFM images of the layer comprising crystalline β-phase Ga2O3 on miscut sapphire substrates. The miscut angle is 4º. FIG.5A covers a surface area of 1 µm by 1 µm; FIG.5B covers a surface area of 5 µm by 5 µm. The RAfor the surfaces of the layers is provided under each image.

[0044] FIG.6 shows an XRD omega-2-theta scan of the layer comprising crystalline β- phase Ga2O3on a miscut sapphire substrate (miscut angle of 4º) showing that the Ga2O3has a (-201) crystal orientation.

[0045] MOCVD Heteroepitaxy:

[0046] Four substrates were utilized to grow Ga2O3 buffer layers on the Ga2O3 base layers via MOCVD: a planar sapphire substrate; the SPE-grown (crystalline) Ga2O3layer on the planar sapphire substrate; a miscut sapphire substrate; and the SPE-grown (crystalline) Ga2O3layer on the miscut sapphire substrate. Ga2O3was deposited with a thickness of 200 nm on each of the four samples at a growth temperature of 820 °C. After MOCVD growth, the surface morphology was characterized through AFM on a Bruker Icon AFM instrument. The 200 nm-thick Ga2O3 overlayers on the sapphire substrate, the SPE-grown Ga2O3 on sapphire substrate, the miscut sapphire substrate, and the SPE-grown Ga2O3on miscut sapphire substrate, had roughnesses of 5 nm, 4.61 nm, 5.7 nm, and 4.12 nm for a 1 × 1 µm2scan area, respectively. For a 5 × 5 µm2scan area, the roughnesses for the four samples wereAtty. Dkt. No.00300-0423-PCT 7.84 nm, 5.5 nm, 8.93 nm, and 5.52 nm, respectively. The roughness measurement data is presented in Table 1.

[0047] Table 1. Average Surface Roughness for MOCVD-Grown Ga2O3 Overlayers on Bare Sapphire and on Crystalline Ga2O3 Base Layers on Sapphire. Nuclea Roughness Roughness Substrate Thickness tion thickness 1*1 µm25 *5 µm2

[0048] FIGS.7A – 7H show AFM images of the surfaces of bare planar and miscut sapphire substrates and MOCVD-grown layers comprising β-phase Ga2O3on the planar and miscut sapphire substrates. FIGS.7A and 7B are the AFM images for the MOCVD-grown Ga2O3on bare planar sapphire and on the SPE-grown Ga2O3base layer on planar sapphire, respectively, over a surface area of 1 µm by 1 µm.. FIGS.7C and 7D are the AFM images for the MOCVD-grown Ga2O3on bare planar sapphire and on the SPE-grown Ga2O3base layer on planar sapphire, respectively, over a surface area of 5 µm by 5 µm. FIGS.7E and 7F are the AFM images for the MOCVD-grown Ga2O3on bare miscut sapphire and on the SPE- grown Ga2O3 base layer on miscut sapphire, respectively, over a surface area of 1 µm by 1 µm. FIGS.7G and 7H are the AFM images for the MOCVD-grown Ga2O3 on bare planar sapphire and on the SPE-grown Ga2O3 base layer on miscut sapphire, respectively, over a surface area of 5 µm by 5 µm.

[0049] XRD omega-2-theta scans were obtained for all four samples. The Ga2O3overlayers grown on the SPE-grown Ga2O3 base layers had predominately beta-phase -201, - 402, and -603 peaks, while Ga2O3grown on the bare planar sapphire or bare miscut sapphire showed parasitic peaks and had higher surface roughnesses. The XRD omega-2-theta scans for a bare planar sapphire substrate, an Ga2O3 overlayer grown on an SPE-grown Ga2O3 base layer on planar sapphire, and the SPE-grown Ga2O3base layer on planar sapphire are shown in FIG.8A. The XRD omega-2-theta scans for a bare miscut sapphire substrate, an Ga2O3 overlayer grown on an SPE-grown Ga2O3base layer on miscut sapphire, the SPE-grownAtty. Dkt. No.00300-0423-PCT Ga2O3 base layer on miscut sapphire and the SPE-grown Ga2O3 base layer on planar sapphire are shown in FIG.8B.

[0050] Example 2: Crystalline β-phase Ga2O3 on Silicon Carbide.

[0051] This example illustrates the growth of a base layer of crystalline gallium oxide grown on SiC via SPE, which can serve as a base film for the subsequent homoepitaxial growth of a high-quality single-crystal gallium oxide layer via a more rapid deposition technique.

[0052] An amorphous Ga2O3film was deposited on thermally annealed SiC (planar and miscut) substrates by RF sputtering using a Denton Discovery 24 sputter deposition system. The sputtering conditions included a power of 100 W, 15 mtorr chamber pressure, 20 sccm Ar flow, 2 sccm O2 flow, and 200 °C deposition temperature. Then, the film was transferred to a high-temperature furnace and annealed at 850-925 °C for 7 hours in air under atmospheric pressure. RA surface roughness was measured for both the amorphous and crystalline Ga2O3 films through AFM using a Bruker Icon Atomic Force Microscope. Low roughnesses were obtained: 0.735 nm over a 1 × 1 µm2scan area for the amorphous Ga2O3on planar SiC; and 0.793 nm for a 1 × 1 µm2scan area for the amorphous Ga2O3 on miscut SiC. After crystallization with different annealing conditions the samples had average surface roughnesses ranging from as low as 0.695 nm to high of 1.83 nm for a 1 × 1 µm2scan area, and 0.705 nm to 6.6 nm for a 5 × 5 µm2scan area. Layer thicknesses were measured by XRR using an XRD Panalytical Empyrean instrument. The two thicknesses were 25 nm ± 0.5 nm or 50 nm ± 0.5 nm, which did not show significant thickness change before and after crystallization. XRD was performed on the annealed Ga2O3 on planar and miscut SiC using the XRD Panalytical Empyrean instrument. Omega-2-theta plots were obtained on both substrates, which demonstrated beta phase with 111 orientations.

[0053] FIGS.9A – 9H show AFM images of the surfaces of base layers comprising β- phase Ga2O3on SiC substrates. FIGS.9A, 9B, 9E, and 9F are images of the crystalline Ga2O3on grown on planar (i.e., non-miscut) SiC. FIGS.9C, 9D, 9G, and 9H are images of the crystalline Ga2O3on grown on miscut SiC. FIGS.9A, 9C, 9E, and 9G cover a surface area of 1 µm by 1 µm; FIGS.9B, 9D, 9F, and 9H cover a surface area of 5 µm by 5 µm. The RA for the surfaces of the layers is provided on each image.

[0054] FIG.10 shows an XRD omega-2-theta scan of a layer comprising crystalline β- phase Ga2O3 on a SiC substrate showing that the Ga2O3 has a (111) crystal orientation.Atty. Dkt. No.00300-0423-PCT

[0055] Unless otherwise indicated, measurable quantitative values described herein that are temperature and / or pressure dependent refer to the values as measured at room temperature (23 ºC) and standard atmospheric pressure (1 atm).

[0056] The word "illustrative" is used herein to mean serving as an example, instance, or illustration. Any aspect or design described herein as "illustrative" is not necessarily to be construed as preferred or advantageous over other aspects or designs.

[0057] The foregoing description of illustrative embodiments of the invention has been presented for purposes of illustration and of description. It is not intended to be exhaustive or to limit the invention to the precise form disclosed, and modifications and variations are possible in light of the above teachings or may be acquired from practice of the invention. The embodiments were chosen and described in order to explain the principles of the invention and as practical applications of the invention to enable one skilled in the art to utilize the invention in various embodiments and with various modifications as suited to the particular use contemplated. It is intended that the scope of the invention be defined by the claims appended hereto and their equivalents.

Claims

Atty. Dkt. No.00300-0423-PCT WHAT IS CLAIMED IS:

1. A structure comprising: a substrate; and a layer of crystalline Ga2O3 comprising β-phase Ga2O3 on a surface of the substrate, the layer of crystalline Ga2O3having a layer thickness of 250 nm or less and a surface with an average surface roughness (RA) of less than 7 nm over a surface area of at least 1 µm2.

2. The structure of claim 1, wherein the layer of crystalline Ga2O3has a layer thickness of 50 nm or less and further comprises an overlayer of crystalline Ga2O3 on the surface of the layer of crystalline Ga2O3, wherein the layer of crystalline Ga2O3and the overlayer of crystalline Ga2O3 form a distinguishable interface.

3. The structure of claim 2, wherein the layer of crystalline Ga2O3 has a layer thickness of 30 nm or less and an average surface roughness (RA) of less than 1 nm over a surface area of at least 1 µm2.

4. The structure of claim 1, wherein the substrate is a SiC substrate.

5. The structure of claim 4, wherein the Ga2O3of the layer of crystalline Ga2O3has a (111) crystal orientation.

6. The structure of claim 5, wherein the layer of crystalline Ga2O3has an average surface roughness (RA) of less than 1 nm over a surface area of at least 1 µm2.

7. The structure of claim 1, wherein the substrate is a sapphire substrate.

8. The structure of claim 7, wherein the Ga2O3of the layer of crystalline Ga2O3has a (-201) group crystal orientation.

9. The structure of claim 8, wherein the layer of crystalline Ga2O3has an average surface roughness (RA) of less than 1 nm over a surface area of at least 1 µm2.

10. The structure of claim 9, wherein the surface of the sapphire substrate is miscut.

11. The structure of claim 1, wherein the substrate is a diamond substrate.Atty. Dkt. No.00300-0423-PCT 12. The structure of claim 1, wherein the substrate is an AlN substrate, a GaN substrate, or a bulk Ga2O3substrate.

13. The structure of claim 2, wherein non-β-phase Ga2O3makes up less than 10% of the Ga2O3 in the layer of crystalline Ga2O3.

14. The structure of claim 13, wherein at least 90% of the Ga2O3in the layer of crystalline Ga2O3 has a single crystal orientation.

15. A method of growing crystalline Ga2O3, the method comprising: (a) depositing amorphous gallium oxide on a surface of a substrate via radiofrequency sputtering to form an amorphous film of gallium oxide; (b) simultaneously or sequentially annealing the amorphous film of gallium oxide to convert the amorphous film of gallium oxide into a crystalline layer of Ga2O3 comprising β- phase Ga2O3 having a layer thickness of no greater than 250 nm and a surface with an average surface roughness (RA) of less than 3 nm over a surface area of at least 1 µm2; (c) optionally, carrying out one or more additional crystalline Ga2O3 growth cycles on the crystalline layer of Ga2O3, each additional crystalline Ga2O3growth cycles comprising: (i) depositing amorphous gallium oxide on the previously formed crystalline layer of Ga2O3via radiofrequency sputtering to form a new amorphous film of gallium oxide; and (ii) simultaneously or sequentially annealing the new amorphous film of gallium oxide to convert the new amorphous film of gallium oxide into a crystalline layer of Ga2O3comprising β-phase Ga2O3having a layer thickness of no greater than 250 nm and a surface with an average surface roughness (RA) of less than 3 nm over a surface area of at least 1 µm2; and (d) growing an overlayer of crystalline gallium oxide on the crystalline layer of Ga2O3by a second method.

16. The method of claim 15, wherein the second method is metal-organic chemical vapor deposition.Atty. Dkt. No.00300-0423-PCT 17. The method of claim 15, wherein the second method is molecular beam epitaxy, hydride vapor phase epitaxy, or pulsed laser deposition.

18. The method of claim 15, wherein the substrate is a SiC substrate, a diamond substrate, or an AlN substrate.

19. Crystalline Ga2O3made according to the method of claim 15.

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