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• • C—CHEMISTRY; METALLURGY
  • C30—CRYSTAL GROWTH
  • C30B—SINGLE-CRYSTAL GROWTH; UNIDIRECTIONAL SOLIDIFICATION OF EUTECTIC MATERIAL OR UNIDIRECTIONAL DEMIXING OF EUTECTOID MATERIAL; REFINING BY ZONE-MELTING OF MATERIAL; PRODUCTION OF A HOMOGENEOUS POLYCRYSTALLINE MATERIAL WITH DEFINED STRUCTURE; SINGLE CRYSTALS OR HOMOGENEOUS POLYCRYSTALLINE MATERIAL WITH DEFINED STRUCTURE; AFTER-TREATMENT OF SINGLE CRYSTALS OR A HOMOGENEOUS POLYCRYSTALLINE MATERIAL WITH DEFINED STRUCTURE; APPARATUS THEREFOR
  • C30B25/00—Single-crystal growth by chemical reaction of reactive gases, e.g. chemical vapour-deposition growth
  • C30B25/02—Epitaxial-layer growth
  • C30B25/18—Epitaxial-layer growth characterised by the substrate
  • C30B25/20—Epitaxial-layer growth characterised by the substrate the substrate being of the same materials as the epitaxial layer
  • C30B25/205—Epitaxial-layer growth characterised by the substrate the substrate being of the same materials as the epitaxial layer the substrate being of insulating material
• • C—CHEMISTRY; METALLURGY
  • C30—CRYSTAL GROWTH
  • C30B—SINGLE-CRYSTAL GROWTH; UNIDIRECTIONAL SOLIDIFICATION OF EUTECTIC MATERIAL OR UNIDIRECTIONAL DEMIXING OF EUTECTOID MATERIAL; REFINING BY ZONE-MELTING OF MATERIAL; PRODUCTION OF A HOMOGENEOUS POLYCRYSTALLINE MATERIAL WITH DEFINED STRUCTURE; SINGLE CRYSTALS OR HOMOGENEOUS POLYCRYSTALLINE MATERIAL WITH DEFINED STRUCTURE; AFTER-TREATMENT OF SINGLE CRYSTALS OR A HOMOGENEOUS POLYCRYSTALLINE MATERIAL WITH DEFINED STRUCTURE; APPARATUS THEREFOR
  • C30B25/00—Single-crystal growth by chemical reaction of reactive gases, e.g. chemical vapour-deposition growth
  • C30B25/02—Epitaxial-layer growth
  • C30B25/16—Controlling or regulating
• • C—CHEMISTRY; METALLURGY
  • C30—CRYSTAL GROWTH
  • C30B—SINGLE-CRYSTAL GROWTH; UNIDIRECTIONAL SOLIDIFICATION OF EUTECTIC MATERIAL OR UNIDIRECTIONAL DEMIXING OF EUTECTOID MATERIAL; REFINING BY ZONE-MELTING OF MATERIAL; PRODUCTION OF A HOMOGENEOUS POLYCRYSTALLINE MATERIAL WITH DEFINED STRUCTURE; SINGLE CRYSTALS OR HOMOGENEOUS POLYCRYSTALLINE MATERIAL WITH DEFINED STRUCTURE; AFTER-TREATMENT OF SINGLE CRYSTALS OR A HOMOGENEOUS POLYCRYSTALLINE MATERIAL WITH DEFINED STRUCTURE; APPARATUS THEREFOR
  • C30B25/00—Single-crystal growth by chemical reaction of reactive gases, e.g. chemical vapour-deposition growth
  • C30B25/02—Epitaxial-layer growth
  • C30B25/18—Epitaxial-layer growth characterised by the substrate
  • C30B25/183—Epitaxial-layer growth characterised by the substrate being provided with a buffer layer, e.g. a lattice matching layer
• • C—CHEMISTRY; METALLURGY
  • C30—CRYSTAL GROWTH
  • C30B—SINGLE-CRYSTAL GROWTH; UNIDIRECTIONAL SOLIDIFICATION OF EUTECTIC MATERIAL OR UNIDIRECTIONAL DEMIXING OF EUTECTOID MATERIAL; REFINING BY ZONE-MELTING OF MATERIAL; PRODUCTION OF A HOMOGENEOUS POLYCRYSTALLINE MATERIAL WITH DEFINED STRUCTURE; SINGLE CRYSTALS OR HOMOGENEOUS POLYCRYSTALLINE MATERIAL WITH DEFINED STRUCTURE; AFTER-TREATMENT OF SINGLE CRYSTALS OR A HOMOGENEOUS POLYCRYSTALLINE MATERIAL WITH DEFINED STRUCTURE; APPARATUS THEREFOR
  • C30B29/00—Single crystals or homogeneous polycrystalline material with defined structure characterised by the material or by their shape
  • C30B29/10—Inorganic compounds or compositions
  • C30B29/40—AIIIBV compounds wherein A is B, Al, Ga, In or Tl and B is N, P, As, Sb or Bi
  • C30B29/403—AIII-nitrides
  • C30B29/406—Gallium nitride
• • C—CHEMISTRY; METALLURGY
  • C30—CRYSTAL GROWTH
  • C30B—SINGLE-CRYSTAL GROWTH; UNIDIRECTIONAL SOLIDIFICATION OF EUTECTIC MATERIAL OR UNIDIRECTIONAL DEMIXING OF EUTECTOID MATERIAL; REFINING BY ZONE-MELTING OF MATERIAL; PRODUCTION OF A HOMOGENEOUS POLYCRYSTALLINE MATERIAL WITH DEFINED STRUCTURE; SINGLE CRYSTALS OR HOMOGENEOUS POLYCRYSTALLINE MATERIAL WITH DEFINED STRUCTURE; AFTER-TREATMENT OF SINGLE CRYSTALS OR A HOMOGENEOUS POLYCRYSTALLINE MATERIAL WITH DEFINED STRUCTURE; APPARATUS THEREFOR
  • C30B7/00—Single-crystal growth from solutions using solvents which are liquid at normal temperature, e.g. aqueous solutions
  • C30B7/10—Single-crystal growth from solutions using solvents which are liquid at normal temperature, e.g. aqueous solutions by application of pressure, e.g. hydrothermal processes
  • C30B7/105—Single-crystal growth from solutions using solvents which are liquid at normal temperature, e.g. aqueous solutions by application of pressure, e.g. hydrothermal processes using ammonia as solvent, i.e. ammonothermal processes

Definitions

• GROUP III NITRIDE CRYSTALS THEIR FABRICATION METHOD, AND METHOD OF FABRICATING BULK GROUP III NITRIDE CRYSTALS IN SUPERCRITICAL AMMONIA
• the invention relates to a substrate or a bulk crystal of semiconductor material used to produce semiconductor wafers for various devices including optoelectronic devices such as light emitting diodes (LEDs) and laser diodes (LDs), and electronic devices such as transistors. More specifically, the invention provides crystals of group III nitride such as gallium nitride. The invention also provides various methods of making these crystals.
• Gallium nitride (GaN) and its related group III nitride alloys are the key material for various optoelectronic and electronic devices such as LEDs, LDs, microwave power transistors, and solar-blind photo detectors.
• LEDs are widely used in displays, indicators, general illuminations, and LDs are used in data storage disk drives.
• heterogeneous substrates such as sapphire and silicon carbide because GaN substrates are extremely expensive compared to these heteroepitaxial substrates.
• group III nitride causes highly defected or even cracked films, which hinder the realization of high-end optical and electronic devices, such as high-brightness LEDs for general lighting or high-power microwave transistors.
• HVPE hydride vapor phase epitaxy
• GaN substrates having dislocation density less than 10 5 cm 2 can be obtained by ammonothermal growth. Since the ammonothermal method can produce a true bulk crystal, one can grow one or more thick crystals and slice them to produce GaN wafers. In the ammonothermal growth, bulk crystals of GaN are grown on seed crystals. However, since GaN or other group III nitride crystals do not exist in nature, one must fabricate GaN seed crystal with other method.
• ammonothermal bulk growth Most methods today rely on seeds taken from a crystal formed by ammonothermal growth. Seeds thick enough for use in ammonothermal growth that are produced by e.g. vapor phase epitaxy typically crack, especially on a nitrogen-polar face of a crystal (such as the c-plane face). Consequently, while people may have tried obtaining seeds for ammonothermal crystal growth by forming the seeds in a faster-growth method, people have met with limited success in producing seeds via a method in which crystals grow faster than in an ammonothermal process.
• This invention discloses group III nitride crystal which may be used for seed crystals in the ammonothermal bulk growth. In addition, this invention discloses methods of fabricating group III nitride crystals, which may be used for seed crystals in the
• this invention discloses a method of growing bulk crystals of group III nitride in supercritical ammonia using the group III nitride crystals as seeds.
• the invention provides a wafer or other substrate of group III nitride having a first, nitrogen-polar c-plane side and a second side opposite to the first side.
• the first side has an exposed nitrogen-polar face of single crystalline or highly oriented polycrystalline group III nitride.
• the second side has either a group III polar, c-plane face of polycrystalline phase or amorphous phase of group III nitride.
• the structural quality of the group III nitride is highest on the first side and gradually degrades towards the second side. The structural degradation from the first side to the second side can therefore be gradual and continual. With this structure, the first side is free of crystal cracks.
• a wafer or crystal as disclosed herein has a first, nitrogen- polar face that is single-crystal group III nitride and a second face that is oriented polycrystalline group III nitride, unoriented polycrystalline group III nitride, amorphous group III nitride, or a mixture of these.
• a wafer or crystal as disclosed herein has a first, nitrogen-polar face that is oriented polycrystalline group III nitride and a second face that is unoriented polycrystalline group III nitride, amorphous group III nitride, or a mixture of these.
• the second face has poorer structural quality than the first face of the wafer or crystal so that the wafer or crystal has sufficiently low stress within it that the resulting wafer or crystal does not crack on its first, nitrogen- polar face.
• the invention also provides methods of fabricating group III nitride crystal explained above. Using an epitaxial growth method such as HVPE, group III nitride crystal is grown on a substrate with group III polar face exposed. By changing growth condition such as temperature and/or ambient oxygen concentration during the growth, the structural quality is degraded gradually and preferably continually through growth.
• the grown group III nitride crystal on the substrate may split into two wafers upon cooling inside the epitaxial growth reactor, after cooling inside the epitaxial growth reactor, or outside of the epitaxial growth reactor.
• One of the split wafers has a first side exposing nitrogen polar c-plane and a second side exposing group III polar c-plane, polycrystalline phase or amorphous phase of group III nitride.
• the invention provides methods of growing bulk crystal of group III nitride such as gallium nitride using the group III nitride crystal explained above in supercritical ammonia.
• FIG. 1 is a schematic drawing of the group III nitride crystal.
• FIG. 2 is a schematic drawing of group III nitride and substrate depicted at steps
• the group III nitride crystal of the present invention is typically used as a seed crystal for ammonothermal bulk growth.
• the group III nitride is typically GaN although it can be any solid solution of group III nitride expressed as Ga x AlyIni-x- y N (0 ⁇ x ⁇ l, 0 ⁇ x+y ⁇ l).
• the group III nitride crystal has a first side exposing nitrogen polar surface of c-plane and a second side exposing either group III polar (e.g. Ga polar in the case of GaN) surface of c- plane, polycrystalline phase, or amorphous phase of group III nitride.
• group III nitride crystal has a high structural-quality nitrogen polar first face and poorer structural-quality second face. With this structure, one can eliminate cracks exposed on the first side of the crystal.
• the structural quality of the first side is higher than that of the second side.
• "Structural quality” means how perfect the atomic arrangement is within the bulk crystal (the overall uniformity of the crystal lattice's unit cell across a plane of the bulk crystal, where the plane is parallel to the surface of the substrate on which the bulk crystal is grown), which can be evaluated with X-ray rocking curve or other analytical methods. If it is evaluated with X-ray rocking curve, the FWHM of the rocking curve of 002 reflection is smaller for the first side than the second side.
• the characteristics of the group III nitride crystal explained here may be suitable for usage as seed crystals in the ammonothermal bulk growth.
• epitaxial growth method such as HVPE.
• Other methods like metalorganic chemical vapor deposition (MOCVD), molecular beam epitaxy (MBE), flux method, high-pressure solution growth, sputtering can also be used as long as the method is compatible with a heterogeneous substrate such as sapphire, silicon carbide, silicon and gallium arsenide.
• MOCVD metalorganic chemical vapor deposition
• MBE molecular beam epitaxy
• flux method high-pressure solution growth
• sputtering can also be used as long as the method is compatible with a heterogeneous substrate such as sapphire, silicon carbide, silicon and gallium arsenide.
• the structural quality is degraded along the growth direction by changing growth conditions as more group III nitride is deposited on a substrate.
• the growth conditions are changed to degrade structural quality gradually (i.e. the unit cell is made less uniform or perfect in new growth as growth proceeds as shown by e.g. XRD results) so that the group III nitride has high-quality in a plane parallel to the surface of the substrate but poorer quality in a plane farther away from the substrate.
• the structural quality may be continually degraded from the high quality plane to the plane farther from the substrate.
• the change may be e.g. linear, exponential, or other continuous function.
• the structural quality may alternatively be progressively changed, step by step, preferably by small amounts.
• a sufficient amount of the group III nitride is deposited on the substrate to form a wafer of group III nitride that can be used in subsequent ammonothermal growth using that wafer.
• the quality is degraded at a rate that provides high-quality group III nitride at or near a separation point from the substrate (where the first face of the separated wafer will be) but sufficiently poor quality at the second face that the bulk group III nitride of the wafer has low stress.
• the low amount of stress prevents the first, nitrogen-polar face from cracking when the wafer is separated from the substrate on which the wafer was grown but still provides a wafer which is suitable for use in an ammonothermal method of growing high- quality group III nitride on its first, nitrogen-polar face.
• the resultant group III nitride crystal is therefore one that has a high structural quality, nitrogen-polar first face and a poorer structural quality second face with lower stress than a comparative crystal formed by maintaining crystal growth conditions constant during crystal growth or adjusting conditions to improve structural quality of new growth.
• the resultant crystal is also thick enough for use as a seed in a subsequent ammonothermal growth of group III nitride on its first, nitrogen-polar face.
• Growth temperature and/or concentration of impurity such as oxygen in the ambient gas can be changed during growth to degrade crystal quality. For instance, growth temperature can be decreased during group III nitride growth to reduce crystal quality of the group III polar face.
• oxygen can be changed during group III nitride growth to reduce crystal quality of the group III polar face.
• the growth conditions can be changed continually to degrade crystal quality farther from the substrate.
• the change may be e.g. linear, exponential, or other continuous function, such as by decreasing the temperature linearly or increasing oxygen concentration in the ambient linearly.
• the change in growth conditions may alternatively be progressively changed, step by step, preferably by small amounts.
• the resulting group III nitride substrate can therefore have good crystal quality on a nitrogen-polar face, lower stress in the group III nitride, and poorer crystal quality at a group III polar face of the substrate.
• the grown group III nitride crystal on the substrate may sometimes split into two pieces (we call this as self-separation) possibly due to the graded structure, difference in thermal expansion between the substrate and group III nitride, and/or difference in lattice constant between the substrate and group III nitride.
• Self-separation often occurs in new group III nitride that was deposited on group III nitride layer 7 of Fig. 2 when a heterogeneous substrate is used to make a crystal or wafer, such that group III nitride layer 7 is thicker than group III nitride layer 3 on substrate 2.
• Layer 7 in this instance may include group III nitride layer 3 that was originally deposited on substrate 2 as well as a portion of new group III nitride grown on layer 3. If self-separation does not occur one can use a conventional removal method such as grinding or laser lift-off to remove the substrate.
• the group III crystal explained above is suitable as a seed for ammonothermal bulk GaN growth.
• an ammonothermal method such as one disclosed in the United States Utility Patent Application Serial No. 61/058,910 (now U.S. Pat. No. 8,236,237), bulk crystal of group III nitride may be grown on the group III crystal. Due to crack- free surface and higher structural quality on the first side, bulk crystal grown on such seeds show good crystal quality.
• the schematic of the group III nitride crystal in this invention is presented in FIG. 1.
• the group III nitride crystal has a first side (1A) exposing nitrogen polar c-plane of group III nitride with a miscut angle less than +/- 5 degree.
• the crystal has a second side (IB) opposite to the first side which exposes either group III polar c-plane, polycrystalline phase or amorphous phase of group III nitride.
• the first side is single crystalline or highly oriented polycrystalline group III nitride and the second side is single crystalline, polycrystalline, or amorphous group III nitride.
• the structural quality on the first side is higher than that on the second side.
• the structural quality means perfection of atomic arrangement in the crystal and is typically characterized with X-ray diffraction or other analytical methods.
• X-ray diffraction FWHM of X-ray rocking curve from 002 reflection is recorded from both sides.
• the FWHM from the first side is smaller than that from the second side.
• the FWHM from the first side is typically smaller than 1000 arcsec, preferably less than 500 arcsec, and more preferably less than 200 arcsec.
• the FWHM from the second side is typically more than 500 arcsec, preferably 1000 arcsec. If the second surface is either polycrystalline phase or amorphous phase, one cannot detect 002 peak in the X-ray. This mean that the structural quality is poor. Even if other analytical methods are used, the crystal quality on the first side shows higher than that on the second side.
• the change in the structural quality from the first side to the second side is preferably gradual and may also be progressive or continual.
• the non-uniform structural quality along the c-axis direction helps to eliminate cracks on the first side because the low-quality crystal underneath the first side acts as a buffer to reduce residual stress in the crystal.
• the thickness of the group III nitride crystal is typically more than 0.1 mm to maintain its shape, preferably more than 0.3 mm, and more preferably between 0.4 to 1 mm.
• the group III nitride crystal can be fabricated with an epitaxial growth method like HVPE.
• Other methods such as MOCVD, MBE, a flux method, high-pressure solution growth or sputtering can be used as long as these methods are compatible with heterogeneous substrates such as sapphire, silicon carbide, silicon and gallium arsenide.
• High structural quality and crack-free characteristic on the first side is beneficial to the ammonothermal bulk growth because a bulk group III nitride crystal is typically grown on the nitrogen polar c-plane in this method.
• the first surface is preferably lapped and polished for high level of flatness and appropriate atomic arrangement on the surface.
• the first side is optionally polished with chemical mechanical polishing to obtain atomically flat surface and remove subsurface damage caused by the prior process.
• an epitaxial growth of group III nitride is conducted on a substrate preferably with HVPE.
• the substrate may be heterogeneous substrate such as sapphire, silicon carbide, silicon or gallium arsenide, or homogeneous substrate such as GaN, A1N, InN or their solid solutions.
• Other epitaxial growth method such as MOCVD, MBE, a flux method, high-pressure solution growth or sputtering can be used as long as these methods are compatible with heterogeneous substrates such as sapphire, silicon carbide, silicon and gallium arsenide.
• FIG 2 shows a particular sequence of process steps in a method according to this invention.
• FIG2A shows a substrate 2 before growing group III nitride.
• an epitaxial growth reactor such as HVPE reactor
• single crystalline or highly oriented polycrystalline group III nitride layer 3 is grown.
• the growth temperature is typically between 950 and 1150 °C.
• the growth condition such as temperature is gradually decreased and/or concentration of impurity in ambient gas is gradually increased to deteriorate the structural quality gradually to form the group III nitride crystal layer 4.
• the gradual change in structural quality is depicted by the crystal color darkening in the direction of growth.
• the group III nitride crystal on the substrate is cooled.
• the group III nitride crystal may crack along a horizontal line 5, resulting in splitting into two wafers.
• One wafer 6 contains the substrate and the other wafer is the group III nitride crystal in this invention. We call this splitting as self-separation.
• This group III nitride crystal can be used as a seed crystal in the ammonothermal growth of bulk GaN.
• seed crystals mineralizer such as sodium metal
• flow restricting devices such as baffles
• gallium containing nutrient such as polycrystalline GaN and ammonia
• the inside of the high-pressure reactor is divided into at least two regions namely a seed region and a nutrient region.
• ammonobasic condition i.e. using alkali metal or alkali earth metal as a mineralizer
• the seed region is located below the nutrient region.
• the baffles separate these two regions.
• the high-pressure reactor is heated so that the appropriate temperature difference is made to grow group III nitride.
• the group III nitride crystal in this invention is beneficial to the ammonothermal growth.
• the group III polar side of the crystal we can grow high-quality GaN crystal selectively on the nitrogen polar side.
• There are a few ways of masking the group III polar face One way is to attach two seeds together on group III polar sides to make one hybrid seed exposing nitrogen polar surface on the both sides.
• the other way is to mount the seed crystal on a metal plate such as vanadium, nickel, silver, and nickel-chromium alloys with nitrogen polar facing up.
• a metal plate such as vanadium, nickel, silver, and nickel-chromium alloys with nitrogen polar facing up.
• GaN crystal was grown by HVPE. 2" c-plane sapphire substrate having GaN layer grown by MOCVD was loaded in an HVPE reactor. After ramping the substrate temperature to about 1000 °C under constant flow of ammonia and nitrogen, gallium chloride gas was introduced to grow single crystalline GaN. After three hours of growth, the growth temperature was gradually reduced over 13 hours. The temperature was reduced linearly by 100°C over 13 hours, resulting in a temperature reduction rate of 100°C per 13 hours. After growing total 16 hours (3 hours of constant temperature and 13 hours of graded temperature), the supply of gallium chloride was stopped and the furnace was turned off. At about 800 °C, the ammonia supply was stopped. The GaN crystal was cooled in the reactor until the temperature reaches about 300 °C. When the crystal was taken out of the reactor, the GaN crystal was self-separated from the substrate portion.
• the substrate portion has a layer of GaN, the self-separation occurred somewhere inside the GaN crystal.
• the thickness of c-plane sapphire was 0.45 mm, the thickness of the portion containing the sapphire was 0.89 mm, the thickness of the GaN crystal separated from the substrate was 1.78 mm.
• the first side of the GaN crystal showed clear color whereas the second side of the GaN crystal showed grayish/blackish color.
• the clear GaN crystal contains oxygen of less than about 10 17 cm "3 whereas the grayish/blackish GaN contains oxygen of more than about 10 19 cm 3 .
• the X-ray measurement showed 002 peak from the first side (nitrogen polar side) but no peak from the second side. This means that the second side surface is covered with either polycrystalline or amorphous GaN.
• the first side was free of cracks.
• the miscut angle measured with X-ray rocking curve was within +/- 5 degree.
• Both sides of the GaN crystal were ground with a diamond grinder to obtain a GaN wafer having a thickness of 1.1 mm.
• the FWHM of X-ray rocking curve from the first side was 1382 arcsec whereas the second side did not show a 002 peak.
• both sides of the GaN crystal wafer were further ground and lapped with diamond slurry.
• the total thickness became 0.85 mm with Ra roughness on the nitrogen side of 0.5-0.8 nm and Ra roughness on the gallium side of 0.8-1.2 nm.
• the FWHM of the X-ray rocking curve from the first side improved to 1253 arcsec. The first side did not have any crack.
• the GaN crystal wafer obtained in Example 2 was used as a seed crystal for ammonothermal bulk growth.
• a high-pressure reactor was filled with the seed, sodium metal, baffles, polycrystalline GaN nutrient and ammonia. Then, the high-pressure reactor was tightly sealed and heated to about 550 °C. After 11 days of growth, a bulk GaN crystal having thickness of about 2.07 mm was obtained. The FWHM of the X-ray rocking curve from the first side improved to 1048 arcsec. The crystal also did not have crack.
• a GaN crystal was grown by HVPE. 2"
• a c-plane sapphire substrate having a GaN layer grown by MOCVD was loaded in a HVPE reactor. Although the growth conditions and duration were the same as Example 1 , the substrate and new crystal did not completely separate.
• the thickness excluding the sapphire substrate portion was 2.63 mm.
• the FWHM of the X-ray rocking curve from 002 reflection on the first side was about 925 arcsec, whereas the FWHM on the second side was 1580 arcsec.
• the Ga polar side had low-quality crystalline GaN (i.e. highly oriented polycrystalline) on the exposed second face.
• the residual sapphire substrate was removed with a diamond grinder, and then both sides of the GaN crystal were also ground to obtain a wafer of 0.44 mm thickness.
• the group III nitride crystal of this invention has higher structural quality on the nitrogen polar surface and is free of cracks. Such crystal is suitable for a seed crystal in the ammonothermal bulk growth.
• a method of fabricating the group III nitride crystal in this invention uses epitaxial growth of group III nitride on a substrate, followed by self- separation or removal of the substrate. By changing growth conditions gradually during crystal growth, the structural quality is deteriorated gradually, thus avoiding crack generation on the first side of the crystal. By using such crystals for ammonothermal bulk growth, one can obtain high-quality bulk crystals of group III nitride such as GaN. Possible modifications
• HVPE as an epitaxial growth method
• other methods such as MOCVD, MBE, a flux method, high-pressure solution growth or sputtering can be used as long as they are compatible with heterogeneous substrates.
• RBS Rutherford backscattering
• RHEED reflection high- energy electron diffraction
• TEM transmission electron microscopy

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