WO2018217973A1

WO2018217973A1 - Nitrogen-polar and semipolar gan layers and devices formed on sapphire with a high-temperature a1n buffer

Info

Publication number: WO2018217973A1
Application number: PCT/US2018/034308
Authority: WO
Inventors: Jie Song; Jung Han
Original assignee: Yale University
Priority date: 2017-05-26
Filing date: 2018-05-24
Publication date: 2018-11-29

Abstract

Methods and structures for forming epitaxial layers of nitrogen-polar and nitrogen-polar semipolar Ill-nitride materials on unpatterned and patterned sapphire substrates are described. Nitrogen-polar semipolar GaN can be grown from inclined c-plane facets of sapphire and coalesced to form a continuous layer of (2021) GaN over the sapphire substrate. Nitridation of the sapphire and a high-temperature AIN buffer layer can be used to form nitrogen-polar and nitrogen-polar semipolar GaN.

Description

NITROGEN-POLAR AND SEMIPOLAR GaN LAYERS AND DEVICES FORMED ON SAPPHIRE WITH A HIGH-TEMPERATURE A1N BUFFER

BACKGROUND

Technical Field

The technology relates to methods and structures for forming nitrogen-polar and nitrogen-semipolar Ill-nitride layers and devices on sapphire substrates.

Discussion of the Related Art

Gallium nitride (GaN) and other Ill-nitride materials are widely recognized as desirable materials for fabrication of integrated devices. These materials typically have wider band gaps than silicon-based semiconductors and can be used to make electro- optical devices (e.g., LEDs and diode lasers) that emit radiation in the green and blue regions of the visible spectrum. Also, because of their wide band-gap, Ill-nitride materials can exhibit higher breakdown voltages when used for fabricating integrated transistors, making these materials attractive for high-power electronics.

Like silicon, Ill-nitride materials may be grown as high-purity, crystalline material. Unlike silicon, Ill-nitride materials are typically more difficult and expensive to grow than silicon, so that bulk substrates of Ill-nitride materials are not currently as commercially feasible as bulk silicon substrates. As a result, researchers have developed, and continue to develop, methods for epitaxially growing integrated-circuit- grade Ill-nitride layers on silicon or other crystalline substrates. Once grown, integrated devices may be fabricated in the Ill-nitride epitaxial layers using planar mi crofabri cation techniques.

SUMMARY

Methods and structures associated with forming nitrogen-polar and nitrogen- polar semipolar gallium-nitride material layers on patterned sapphire substrates (PSS) are described. According to some embodiments, nearly purely nitrogen-polar GaN can be formed using a process that includes high-temperature deposition of an aluminum-nitride buffer layer. Nitrogen-polar semipolar gallium-nitride (GaN) layers can be formed by topographically patterning selectively-cut sapphire substrates to form inclined c-plane surfaces. The techniques may be applied to various Ill-nitride materials (e.g., InGaN, InAIN, InGaAIN, AIN, InN, etc.) and are not limited to only gallium-nitride materials.

Some embodiments relate to a wafer comprising a sapphire substrate, a buffer layer, and nitrogen-polar or nitrogen-polar semipolar gallium nitride material formed on the buffer layer, wherein an x-ray diffraction rocking curve for a (002) crystallographic orientation of the gallium nitride material exhibits a full-width-half-maximum value that is less than or equal to 0.25 degrees.

Some embodiments relate to a method for forming a nitrogen-polar or nitrogen- polar semipolar gallium nitride material. The method can comprise acts of nitridizing a growth surface of a sapphire substrate; forming an AIN buffer layer at a temperature of at least 850 °C on the nitridized growth surface; and epitaxially growing the nitrogen-polar or nitrogen-polar semipolar gallium nitride material on the AIN buffer layer.

Some embodiments relate to a method for forming a nitrogen-polar gallium nitride material epitaxial layer on a sapphire substrate. The method can include acts of performing a nitridation process on a growth surface of a bare sapphire substrate, and subsequently forming an AIN buffer layer on the growth surface. The AIN buffer layer can be formed at a temperature of at least 850 °C. During formation of the AIN buffer layer, the V/III ratio can be between 200 and 4000. The method can further include forming a GaN epitaxial layer on the AIN buffer layer. The GaN epitaxial layer can be formed at a temperature of at least 900 °C.

Some embodiments relate to a method for forming a nitrogen-polar semipolar gallium nitride material epitaxial layer on a sapphire substrate. The method can include an act of patterning trenches in a sapphire substrate or receiving a sapphire substrate having trenches patterned therein. The method can further include performing a nitridation process on an inclined growth surface in a trench of the patterned sapphire substrate, and subsequently forming an AIN buffer layer on the growth surface. The AIN buffer layer can be formed at a temperature of at least 850 °C. During formation of the AIN buffer layer, the V/III ratio may be between 200 and 4000. The method can further include forming a GaN epitaxial layer on the AIN buffer layer, and continuing growth of the GaN epitaxial layer until a coalesced layer of GaN is formed over the sapphire. The GaN epitaxial layer can be formed at a temperature of at least 900 °C. The method can also include polishing the surface of the GaN epitaxial layer to form a flat surface.

The foregoing apparatus and method embodiments may be implemented with any suitable combination of aspects, features, and acts described above or in further detail below. The foregoing and other aspects, embodiments, and features of the present teachings can be more fully understood from the following description in conjunction with the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

The skilled artisan will understand that the figures, described herein, are for illustration purposes only. It is to be understood that in some instances various aspects of the embodiments may be shown exaggerated or enlarged to facilitate an understanding of the embodiments. The drawings are not necessarily to scale, emphasis instead being placed upon illustrating the principles of the teachings. In the drawings, like reference characters generally refer to like features, functionally similar and/or structurally similar elements throughout the various figures. Where the drawings relate to microfabrication, only one device and/or portion of a substrate or wafer may be shown to simplify the drawings. In practice, a large plurality of devices or structures can be fabricated in parallel across a large area of a wafer or entire wafer. The drawings are not intended to limit the scope of the present teachings in any way.

FIG. 1A shows an optical microscope image (top, before etching in potassium hydroxide) and scanning-electron microscope (SEM) image (bottom, after etching in KOH) of an epilayer of gallium nitride grown on a buffer layer of aluminum nitride, where the buffer layer was grown at 500 °C.

FIG. IB shows an optical microscope image (top, before etching in potassium hydroxide) and SEM image (bottom, after etching in KOH) of an epilayer of GaN grown on a buffer layer of A1N, where the buffer layer was grown at 600 °C.

FIG. 1C shows an optical microscope image (top, before etching in potassium hydroxide) and SEM image (bottom, after etching in KOH) of an epilayer of GaN grown on a buffer layer of A1N, where the buffer layer was grown at 750 °C. FIG. ID shows an optical microscope image (top, before etching in potassium hydroxide) and SEM image (bottom, after etching in KOH) of an epilayer of GaN grown on a buffer layer of A1N, where the buffer layer was grown at 850 °C.

FIG. 2 graphs a dependence of the area ratio of nitrogen-polar (N-polar) GaN on the growth temperature of the A1N buffer, according to some embodiments.

FIG. 3 graphs x-ray-rocking-curve full-width-half-maximum values of N-polar GaN with an A1N buffer grown at different temperatures, according to some

embodiments.

FIG. 4A is a plot of in-situ optical reflectance of a growing epilayer of N-polar Ga-N on an A1N buffer layer, where the buffer layer is deposited at a temperature of 850 °C, according to some embodiments.

FIG. 4B; is a plot of in-situ optical reflectance of a growing epilayer of N-polar Ga-N on an A1N buffer layer, where the buffer layer is deposited at a temperature of 950 °C, according to some embodiments.

FIG. 4C is a plot of in-situ optical reflectance of a growing epilayer of N-polar

Ga-N on an A1N buffer layer, where the buffer layer is deposited at a temperature of 1150 °C, according to some embodiments.

FIG. 5A shows an atomic force microscope (AFM) image of an N-polar GaN epilayer (5 microns by 5 microns sample image area) epitaxially grown on an A1N buffer layer, where the buffer layer is deposited at a temperature of 850 °C, according to some embodiments.

FIG. 5B shows an AFM image of an N-polar GaN epilayer (5 microns by 5 microns sample area) epitaxially grown on an A1N buffer layer, where the buffer layer is deposited at a temperature of 950 °C, according to some embodiments.

FIG. 5C shows an AFM image of an N-polar GaN epilayer (5 microns by

5 microns sample area) epitaxially grown on an A1N buffer layer, where the buffer layer is deposited at a temperature of 1150 °C, according to some embodiments.

FIG. 6A shows a two-dimensional AFM image (top) and three-dimensional AFM image (bottom) of an A1N buffer layer (3 microns by 3 microns sample image areas) formed on sapphire and grown at a temperature of 600 °C, according to some

embodiments. FIG. 6B shows a two-dimensional AFM image (top) and three-dimensional AFM image (bottom) of an A1N buffer layer (3 microns by 3 microns sample image areas) formed on sapphire and grown at a temperature of 950 °C, according to some

embodiments.

FIG. 6C shows a two-dimensional AFM image (top) and three-dimensional AFM image (bottom) of an A1N buffer layer (3 microns by 3 microns sample image areas) formed on sapphire and grown at a temperature of 1150 °C, according to some embodiments.

FIG. 7A is a SEM image under low magnification showing a cross-sectional view of (2021) GaN stripes formed on topographically-patterned sapphire, according to some embodiments. GaN crystal orientations are indicated by the arrows.

FIG. 7B is a SEM image under higher magnification showing a cross-sectional view of a (2021) GaN stripe formed on topographically-patterned sapphire, according to some embodiments. GaN crystal orientations are indicated by the arrows.

FIG. 7C is a SEM image under low magnification showing an approximately 45° view of two (2021) GaN stripes formed on topographically-patterned sapphire before etching with KOH.

FIG. 7D is a SEM image under low magnification showing an approximately 45° view of two (2021) GaN stripes formed on topographically-patterned sapphire after etching with KOH. The roughened facets after etching indicate a uniform nitrogen- polarity.

FIG. 8A plots an x-ray diffraction (XRD) 2θ/ω scan showing single orientation GaN has been achieved, with the (2021) plane parallel to the plane in which the sapphire substrate lies.

FIG. 8B plots XRD rocking curves of the on-axis (2021) plane with rocking axis perpendicular (inner curve) and parallel (outer curve) to patterned stripes of GaN.

FIG. 9A is a cross-sectional SEM image of a 10-micron-thick (2021) GaN epitaxial layer grown on a patterned sapphire substrate.

FIG. 9B is a cross-sectional transmission-electron microscope (TEM) image under two-beam condition taken along a diffraction vector of g = (10Ϊ0).

FIG. 10A is a Nomarski optical microscope image of (2021) GaN grown on a patterned sapphire substrate before chemical-mechanical polishing (CMP). FIG. 10B is a Nomarski optical microscope image of (2021) GaN grown on a patterned sapphire substrate after chemical-mechanical polishing (CMP).

FIG. IOC is a photograph of a two-inch diameter wafer of (2021) GaN grown on a patterned sapphire substrate after chemical-mechanical polishing (CMP).

FIG. 11 shows and AFM image of a 10-micron-by-lO-micron area of the polished (2021) GaN epilayer, according to some embodiments. The root-mean-square surface roughness is less than 2 nanometers.

FIG. 12 depicts a light-emitting diode (LED) structure formed on a CMP- processed (2021) GaN-on-sapphire wafer, according to some embodiments.

FIG. 13 plots an XRD 2θ/ω radial scan of a (202Ϊ) LED formed with three pairs of InGaN/GaN multiple quantum wells in the active region of the diode, according to some embodiments.

FIG. 14 is a current-voltage (I-V) curve plotted for a (202Ϊ) GaN LED die. The inset is a photograph of the die showing electroluminescence, according to some embodiments.

FIG. 15 plots several electroluminescent (EL) spectra with different injection currents under pulsed operation of a (2021) GaN LED, according to some embodiments.

FIG. 16A plots a dependence of integrated EL intensity as a function of injection current for a (2021) GaN LED, according to some embodiments.

FIG. 16B plots two dependences of peak emission wavelength on injection current for two (2021) GaN LEDs, according to some embodiments. The upper trace (diamonds) shows a reduced dependence for LEDs formed according to the present embodiments.

When referring to the drawings in the following detailed description, spatial references "top," "bottom," "upper," "lower," "vertical," "horizontal," and the like may be used. For example, "vertical" may be used to refer to a direction normal to the substrate surface, and "horizontal" may be used to refer to a direction parallel to the substrate surface when referring to the drawings. "Upper," "top," or "above" may be used to refer to a vertical direction away from the substrate, whereas "lower," "bottom," or "below" may be used to refer to a vertical direction toward the substrate. Such references are used for teaching purposes, and are not intended as absolute references for embodied devices. An embodied device may be oriented spatially in any suitable manner that may be different from the orientations shown in the drawings.

The features and advantages of the embodiments will become more apparent from the detailed description set forth below when taken in conjunction with the drawings.

DETAILED DESCRIPTION

Because of their wide band-gap values, Ill-nitride materials, such as gallium nitride materials and in particular GaN, are desirable materials for fabricating green- wavelength or blue-wavelength light-emitting devices and for making high-power or high-voltage integrated circuit devices. The inventors have recognized and appreciated that some crystal orientations of Ill-nitride materials can provide improved device performance over other crystal orientations. Accordingly, the inventors have developed methods and structures for selectively forming nitrogen-polar, nitrogen semipolar (also referred to as nitrogen-polar semipolar), and other desired orientations of GaN and gallium nitride materials on sapphire substrates. Gallium nitride materials include GaN and its alloys (e.g., GalnN, GaAIN, GaAlInN, etc.).

For some applications, nitrogen-polar (000Ϊ) (N-polar) GaN may be a desirable material, because of its opposite spontaneous polarization field direction compared to gallium-polar (Ga-polar) structures. The reversal of the spontaneous polarization can provide a beneficial change of heteroj unction band profiles and device characteristics, leading to improved device performance. For example, N-polar c-plane light-emitting diodes (LEDs) can exhibit a reduced efficiency droop and a lower threshold current density, and improve incorporation of indium into InGaN multiple quantum wells (MQWs) for long wavelength visible LEDs. With regard to field-effect transistors (FETs), the location of a two-dimensional electron gas near an AlGaN/GaN

heteroj unction can be altered by switching from Ga- to N-polar orientation to achieve different device applications and device performance.

Nonpolar and semipolar orientations of gallium nitride (GaN) may also address long-standing problems due to high polarization fields in light-emitting diodes (LEDs). These high fields are associated with several deleterious effects in the LEDs, such as a reduced recombination efficiency, an increased barrier for hole injection, increased electron leakage, and Auger recombination due to increased carrier densities. Nitrogen- semipolar GaN (2021) can be particularly beneficial for high-efficiency LEDs and laser diodes (LDs), because of a favorable direction of the internal polarization field and surface atomic configuration.

Although N-polar GaN has been formed on bulk substrates of limited size and cross-sliced to obtain other orientations, the bulk substrates are expensive and generally of sizes that are not compatible with current mass-production facilities. There currently exists no reliable method for producing N-polar semipolar GaN of a selected orientation on large-area substrates (e.g., substrates having a diameter of 2 inches or larger).

According to some embodiments, nearly purely N-polar GaN can be epitaxially grown on an unpatterned sapphire substrate using a high-temperature aluminum-nitride (AIN) buffer layer. The inventors have found that the temperature at which the AIN buffer layer is formed plays an important role in the subsequent formation of N-polar GaN. In some implementations of forming N-polar GaN, a c-plane (0001) sapphire substrate (of any suitable diameter) can be used as a base substrate on which a high- temperature AIN buffer layer and N-polar GaN layer are formed. The sapphire substrate can have a 2° off-cut towards the A-axis.

The growth surface of the sapphire substrate can be smooth (e.g., chemical - mechanically polished) and cleaned prior to growing layers. The growth surface may further be subj ected to a nitridation process. The nitridation process can be carried out with the substrate at a temperature between 900 °C and 1000 °C, during which the growth surface is exposed to a mixture of N₂ and NH₃ gases. The N₂ flow rate may be between approximately 3 and approximately 7 slm. The NH₃ flow rate may be between approximately 1 and approximately 5 slm. The duration of nitridation may be between approximately 0.5 and approximately 5 minutes, though other durations may be used in some cases.

After nitridation, a high-temperature AIN buffer layer can be grown on the substrate' s growth surface using metal-organic chemical -vapor deposition (MOCVD), for example. The thickness of the AIN layer can be between 10 nm and 100 nm. In some implementations, the thickness of the AIN buffer layer is between 10 nm and 30 nm. The temperature at which the AIN buffer layer is formed is preferably at least 750 °C, and in some cases, at least 850 °C. The growth temperature may be as high as 1200 °C. The inventors have found that the growth temperature for forming the AIN buffer is critical in determining whether the subsequent epitaxial GaN layer will be N-polar, Ga- polar, or mixed polar, as described further below. For growth temperatures of the AIN layer above 800 °C, the subsequent epitaxial GaN layer will be greater than 90% N- polar.

Additional growth conditions for the AIN buffer layer may include a V/III molecular ratio of at least 200. The V/III ratio may be as high as 8000 in some cases. In some implementations, the V/III ratio may be no higher than 4000, and the V/III ratio may be maintained at a ratio between 200 and 4000. Trimethylaluminum (TMA) and ammonia (NH₃) gases can be used to epitaxially grow the AIN buffer layer. The flow rate of TMA may be between 10 μιηοΐ/min to 200 μιηοΐ/min, and the flow rate of NH₃ may be between 0.2 slm and 3 slm . The chamber pressure may be between 50 mbar and 300 mbar.

Growth of N-polar GaN can be carried out on the AIN buffer using MOCVD, for example, though the invention is not limited to growth by MOCVD. In some embodiments, growth of thin buffer and III-V epitaxy layers may be performed using, at least in part, atomic layer deposition (ALD) techniques. According to some

implementations, growth of the GaN epitaxial layer may be performed by MOCVD using NH₃ and triethylgallium (TEG) or trimethylgallium (TMGa) gases. The flow rate for NH₃ may be between 0.5 and 4 slm, and the flow rate for the gallium gas carrier may be between 40 μιηοΐ/ιηίη and 240 μιηοΐ/ιηίη. The chamber pressure may be between 50 mbar and 300 mbar, and the temperature during growth may be between 950 °C and 1100 °C. According to some embodiments, the growth rate of the III-V epitaxial layer may be between 0.5 micron/hour and 2.5 microns/hr. In some embodiments, the growth rate is controlled to be between 1.0 micron/hr and 1.5 microns/hr.

Polarity of the epitaxial GaN can be ascertained by wet etching in potassium hydroxide solution (KOH, 4.5 M) at room temperature for approximately 5 min, according to some embodiments. Surface morphology can be examined by Nomarski optical microscopy, scanning electron microscopy, and atomic force microscopy, as described further below. Crystalline quality can be characterized by X-ray diffraction and/or transmission electron microscopy.

The influence of growth temperature of an AIN buffer layer (on nitridized sapphire) on the properties of GaN epilayers are illustrated in FIG. 1A-FIG. ID. The images show that the growth temperature of the AIN buffer layer plays a critical role in achieving essentially purely N-polar GaN with a smooth surface and high crystalline quality. The top image in each figure shows an optical microscope image of an as-grown GaN epilayer (approximately 500 nm thick). The bottom image in each figure shows an SEM image of the GaN surface after KOH etching. Ga-polar terminated GaN is very inert to KOH etching, whereas N-polar terminated GaN will be etched in KOH solution and the surface of N-polar GaN will become very rough. Growth temperatures of the AIN buffer layer for each case are as follows: FIG. 1A 500 °C; FIG. IB 600 °C; FIG. 1C 750 °C; and FIG. ID 850 °C.

As seen in FIG. 1 A, the surface of epitaxial GaN with an AIN buffer grown on sapphire at 500 °C is featureless after KOH etching, indicating that the GaN is purely Ga-polar. Increasing the AIN buffer growth temperature to 600 °C shows that the majority of the GaN surface area is still Ga-polar. However, regions of N-polar GaN begin to form, resulting in etched regions of the surface, as can be seen in the SEM image of FIG. IB. Further increases in temperature result in a larger portion of the epitaxial GaN layer forming as N-polar material. By increasing the growth temperature of the AIN buffer layer to 850 °C, the GaN epilayer becomes very rough when etched by KOH. Sub-micron pyramidal structures form over the entire wafer surface after KOH etching, as can be seen in the SEM image of FIG. ID. These results indicate that the GaN epilayer forms as essentially all N-polar when an AIN buffer layer is grown on nitridized sapphire at a temperature above 750 °C.

The formation of N-polar GaN as a function of growth temperature of the AIN buffer can be summarized more quantitatively by calculating an area ratio of N-polar GaN regions to total observed area for each AIN growth temperature. FIG. 2 plots the area ratio as a function of AIN growth temperature. FIG. 2 indicates that if the AIN buffer is grown at a temperature below 600 °C, Ga-polar GaN will be formed even though an aggressive nitridation is conducted on sapphire substrate. When the growth temperature of the AIN buffer is around 750 °C, GaN with mixed-polarity will be formed. At 850 °C, nearly purely N-polar GaN is formed by epitaxial growth. Epitaxial GaN layers grown at 950 °C and 1150 °C were also found to be essentially purely N- polar. Although purely N-polar GaN is of interest, the region of mixed polarity indicated in FIG. 2 may be of interest for potential applications in non-linear optics and acoustics, lateral polarity semiconductor junctions, and polarity-selective patterning of GaN.

Growth of N-polar GaN can be due to several effects. According to some embodiments, the nitridation process may convert the sapphire (A1₂0₃) surface to a spinel structure of aluminum-oxynitride (Al_xO_yN_z) by a continuous substitution of oxygen atoms in sapphire by nitrogen, providing a structure on which to stepwise form N-polar AIN. The inventors have appreciated that an Al-0 bond is much stronger than an Al-N bond, and believe that a layer of Al_xO_yN_z is regenerated at low growth temperature. This layer of Al_xO_yN_z may lead to a metal-polar AIN buffer and subsequently a Ga-polar GaN epilayer grown on metal-polar AIN buffer. When an AIN buffer layer is grown at high temperature, Al_xO_yN_z may dissociate at high temperature and convert to pure Al-N bonds with N atoms terminated on the surface under N-rich ambient, due to a higher decomposition rate of NH₃ at high temperatures. An N-polar AIN buffer may then form on the N-terminated substrate surface, and thus N-polar GaN is generated.

Crystalline quality of the epitaxial GaN layers formed on AIN buffer layers grown at different temperatures can be characterized by x-ray diffraction (XRD).

Rocking curves produced by XRD typically produce one or more peaks, each of which can be characterized by a full-width-half-maximum (FWHM) value. The FWHM value indicates quality of the crystalline structure, where smaller values indicate better quality.

FIG. 3 plots FWHM values from XRD rocking curves for two crystallographic orientations (002) and (102) of six different GaN epitaxial layers. The six GaN epilayers were formed under the same growth conditions, but on AIN buffer layers formed at six different temperatures. The crystalline quality of a GaN epilayer grown on an AIN buffer grown at temperature of below 600 °C is very poor with (002) and (102) FWHM values of about 0.5° and 0.45°, respectively. Increasing the growth temperature of the AIN buffer from 600 °C to 850 °C significantly improves the crystalline quality of the GaN epilayer as more of the GaN is formed as N-polar. At temperatures above 950 °C, there is comparatively less improvement in crystallinity, since essentially all of the GaN is N-polar. According to some embodiments, nitrogen-polar GaN and nitrogen-polar semipolar GaN grown according to the present embodiments can have XRD rocking curves for (002) crystallographic orientations that exhibit FWHM values less than or equal to 0.25 degrees.

Even though nearly pure N-polar GaN can be achieved with A1N buffers grown at temperatures of about 850 °C and above, the inventors observed a significant difference in the optical reflectance curves that monitor the growth evolution of the epitaxial GaN layer. For these measurements, the wavelength of the light source used for in-situ reflectance measurements was about 550 nm. FIG. 4A-FIG. 4C shows optical reflectance curves generated during epitaxial growth of three GaN layers formed on three different substrates. For the first substrate (FIG. 4A), the A1N buffer layer was grown at 850 °C. For the second substrate (FIG. 4B), the A1N buffer layer was grown at 950 °C. For the third substrate (FIG. 4C), the A1N buffer layer was grown at 1150 °C. The final thicknesses of the GaN epilayers is about 500 nm.

A full amplitude oscillation in the optical reflectance curve may imply that a quasi-2D growth mode occurs and indicate that these three GaN epilayers with A1N buffer layers grown at different temperatures above 850 °C are all N-polar. The optical reflectance curves may also indicate other aspects of the GaN epilayers. For example, the reflectance curve of N-polar GaN with an A1N buffer grown at 850 °C decays very dramatically in the oscillation intensity, and can indicate a substantial and accumulated surface roughening. The reflectance curve for GaN with an A1N buffer grown at 950 °C decays much less quickly than for the ΑΓΝ buffer grown at 850 °C, and indicates an improvement in the surface morphology of GaN with an A1N buffer grown at 950 °C. The reflectance curve for the GaN epilayer formed on an A1N buffer grown at 1150 °C exhibits a gradual increase initially for several oscillations, suggesting further

improvement to the quality of the GaN epilayer.

The surface morphology of N-polar GaN epilayers formed on A1N grown at different temperatures can be quantified further with an atomic-force microscope (AFM). FIG. 5A - FIG. 5B show AFM images of GaN epilayers grown over A1N that was grown at 850 °C (FIG. 5A); 950 °C (FIG. 5B); and 1150 °C (FIG. 5C). The scanning probe measured surface topology over an area that was 5 microns by 5 microns in size. The root mean square (RMS) surface roughness of the three GaN epilayers, as measured by the AFM, are shown in the images and are 5.6, 4.7, and 2.5 nm, respectively, for the GaN epilayer with an AIN buffer grown at 850 °C, 950 °C, and 1150 °C. The AFM results indicate that the surface of the GaN epilayer becomes smoother by increasing the growth temperature of the AIN buffer, consistent with the results shown in FIG. 4A- FIG. 4C. The surface roughness was found to be approximately uniform across a two- inch diameter wafer. Large-area substrates (e.g., 100-mm-diameter, 150-mm-diameter, 250-mm-diameter, etc.) can be used in some embodiments to form large wafers that provide a Ill-nitride layer (such as GaN), where the wafers can be accommodated by an MOCVD reactor.

Further measurements can be made to evaluate the influence of AIN buffer layers on the crystalline quality of subsequently-grown GaN epilayers. For example, AIN buffer layers may be grown at different temperatures, and then examined directly with an AFM to evaluate surface morphology. According to some embodiments, the AIN buffer layers are not subjected to an anneal step after growth. FIG. 6A-FIG. 6C show surface morphologies of AIN buffer layers grown on nitridized sapphire at temperatures of approximately 600 °C, 950 °C, and 1150 °C, respectively. The upper AFM image in each figure is a plan view, two-dimensional image where shading indicates height of the surface. The lower AFM image is a computer-generated three-dimensional perspective view produced from the same data acquired for the upper AFM image. Growth temperatures for the AIN layers are shown in each figure.

Surface roughness values (RMS) measured by the AFM are 1.4, 1.9, and 2.0 nm, respectively, and are not significantly different. However, the density of large and medium-size AIN grains differs between the three growth conditions. For an AIN buffer layer grown at 600 °C, a low density of large grains (white spots in top AFM image corresponding to grain sizes larger than about 50 nm). At low temperatures, there is a high density of small grain sizes (e.g., less than about 25 nm). The density of large grain sizes is less than about 3/μπι² (3 >< 10⁸ cm^"2).

When the growth temperature of AIN is increased to 900 °C, the density of large size grains increases appreciably, to about 17/μπι². Also, the density of small grains reduces. A further increase in temperature to 1150 °C appears to increase the density of medium size grains, reduce the density of small grains, and maintain the density of large grains between about 15/μπι² and about 25/μπι². At 1150 °C, the density of large size grains increases to about 19/μπι². A higher density of the large-size grains in the AIN buffer layer may promote the lateral growth and coalescence of initial GaN islands grown on AIN grains. This may lead to an increased volume of defect-free columnar domains, and thereby improve the crystal quality of the GaN epilayer. The increase and stabilization of large grain density at 900 °C and above may explain the trend in FWHM values of the (002) and (102) XRC peaks plotted in FIG. 3. For example, above 900 °C there is a small change in large grain density, so that the initial growth conditions for the GaN epilayer improves by a small amount. Accordingly, the XRC FWFDVI values measured for the GaN epilayers reduce slightly for AIN growth temperatures above about 900 °C.

In view of the foregoing, it is possible to epitaxially grow high quality N-polar GaN on large-area, unpatterned, sapphire substrates using suitable growth conditions for an AIN buffer layer. The growth conditions for the buffer layer may comprise forming an AIN buffer layer at temperatures over 900 °C and at high V/III ratios (e.g., between 200 and 4000). The GaN epilayer may have a surface roughness (RMS) as low as 2.5 nm. In some cases, the surface roughness (RMS) may be between about 2.5 nm and about 5.0 nm. To improve surface quality of the GaN, a wafer can be subjected to chemical-mechanical polishing after growth, according to some embodiments.

Other orientations (e.g., nitrogen polar semipolar orientations) of GaN may be formed by topographically patterning the sapphire substrate, as will now be described.

According to some embodiments, sapphire substrates can be selected or cut such that the [0001] sapphire direction with respect to the sapphire substrate's surface points in essentially a same direction as the [0001] GaN direction, such that the (2021) GaN surface is essentially parallel to the sapphire substrate's surface (e.g., the c-plane offcut 75.09° toward the a-direction [sapphire (2243) offcut 0.45°]). Trenches can then be formed in the sapphire substrate' s surface using photolithography processes to produce a patterned sapphire substrate (PSS), as described in international patent application PCT/US201/025907, for example, which is incorporated herein by reference. An example of a portion of a PSS 705 is shown in the SEM image of FIG. 7A. The trenches can be patterned across the substrate by photolithography to have a spatial period of about 6 μιη, with about 3 μιη wide stripes, though other periods and widths can be used in other embodiments. The trenches can be etched via reactive-ion etching (RIE), for example, to a depth of 1 μιτι, though other etch depths can be used. In some implementations, a finer period can be used. For example, the spatial periodicity of the trenches may have a periodicity between 200 nm and 5 microns. The width of the trenches may be between 30% and 90% of the spatial periodicity, and the depth of the trenches may be between 10% and 60% of the width of the trenches.

Except for the desired c-plane sapphire sidewall (oriented so that its normal points in the (000Ϊ) direction (see FIG. 7A and FIG. 7B), all of the exposed facets of the sapphire substrate can be masked with a dielectric thin film (e.g., Si0₂) to prevent epitaxial growth on the covered facets. Deposition of the dielectric can be performed using a technique of shadow evaporation at properly inclined angles, as described further in international patent application PCT/US201/025907, which is incorporated herein by reference.

After the undesirable growth facets are masked, the PSS can be loaded into a MOCVD reactor chamber for growing (2021) GaN. During the MOCVD growth, trimethylgallium (TMG), trimethylaluminum (TMA), and ammonia ( H3) can be used as Ga, Al, and N sources, respectively. Growth conditions for an A1N buffer layer and GaN epitaxial layer can be the same as those described above, according to some embodiments. For example, the above-described nitridation process and A1N buffer layer growth at high temperature can be performed.

Subsequently, GaN can be grown on the A1N buffer layer at approximately 1030 °C temperature, approximately 100 mbar pressure, approximately 1 10 nmol/min TMG flow rate, and approximately 1 slm NH₃ flow rate. In some embodiments, temperature during GaN growth may be between 900 °C and 1 100 °C. In some embodiments, the pressure may be between 50 mbar and 300 mbar. In some embodiments, the flow rate of TMG may be between 40 μιηοΐ/ιηίη and 240 μιηοΐ/ιηίη. In some embodiments, the flow rate of NH₃ may be between 0.5 slm and 4 slm. As noted above, the growth temperature of the A1N buffer plays a very important role in achieving N-polarity of the GaN epilayer.

After growth of the GaN epilayer, the samples can be characterized using a Nomarski optical microscope, a scanning electron microscope (SEM), an atomic force microscope (AFM), X-ray diffraction (XRD), and/or a transmission electron microscope (TEM). The polarity of (2021) GaN can be examined by wet-etching with potassium hydroxide (KOH) solution (e.g., 4.5 M) at room temperature for about 5 min, though shorter or longer etch times can be used.

FIG. 7A shows a cross-sectional SEM image of early-stage growth of GaN from c-plane (0001) sidewalls of a patterned sapphire substrate, according to some

embodiments. A high-temperature AIN buffer layer, as described above, was formed on the sidewalls prior to GaN growth. The observed cross-sectional profiles of GaN in FIG. 7A and FIG. 7B (magnified further) is different from a triangular profile that occurs in the early stage of (2021) GaN growth, which is usually bounded by two (1011) planes, an optional (10Ϊ0) plane, and a N-polar (000Ϊ) facet. In FIG. 7A and FIG. 7B, the GaN stripes are bounded by a (000Ϊ) facet, two (10ΪΪ) planes, a (10Ϊ0) plane, a (1011) plane, and a Ga-polar (0001) plane, as marked in FIG. 7B.

The GaN stripes were etched in a KOH solution (4.5 M) at room temperature to confirm the polarity of the GaN layer, because KOH is found to be effective in etching N-polar GaN but does not etch Ga-polar GaN. FIG. 7C shows a 45° tilted-view SEM image of the as-grown GaN stripes with smooth facets, before etching. The facets 730 are believed to be N-polar (000Ϊ) facets. After etching in KOH, these facets become rough with microscale pyramids appearing. The roughened facets can be seen in FIG. 7D. The roughening etch in KOH confirms that the facets are N-polar (000Ϊ) and that selective, uniform growth of N-polar GaN from inclined c-plane sapphire can be achieved.

According to crystallography, when the sidewall of the GaN stripes points into the [000Ϊ] direction at the selected off-cut angle, a direction that is normal to the surface of the sapphire substrate and the forming layer of GaN will be the [2021] direction, as depicted in FIG. 7A. An XRD 2θ/ω scan can be conducted with a scan range from 30° to 90° to confirm the postulated crystal orientation. Results of such a scan are shown in FIG. 8A. Only two peaks corresponding to GaN (2021) and sapphire (2243) diffraction are visible, indicating that single (2021)-orientation GaN has been achieved with the direction parallel to the sapphire (2243) orientation. Performing multiple XRD samplings from the center to the edge of a 2-inch-diameter sapphire substrate on which the GaN was grown, confirmed that essentially polarity-pure (2021) GaN can be formed from the selective sidewall growth over the entire wafer, and that the invention is not limited by wafer size. Larger sapphire wafers can be used to form N-polar, essentially polarity-pure (2021) GaN layers over larger areas, provided the wafers can be accommodated by an epitaxial growth chamber.

After confirming the polarity of the GaN epilayer crystal orientation, epitaxial growth of the GaN can be continued to form a thick coalesced layer of GaN over the PSS. The crystalline quality of a thick GaN layer (e.g., more than 2 microns thick) can be evaluated using XRD. On-axis XRD rocking curves with (2021) diffraction are shown in FIG. 8B, with the rocking axis perpendicular (inner curve 810, "|-" symbol) and parallel (outer curve 820, "||" symbol) to the patterned stripes in the sapphire. The measured full-width-at-half-maximum (FWHM) values of (2021) GaN grown according to some embodiments, with the rocking axis perpendicular and parallel to patterned stripes, are 352 and 504 arcsec, respectively. A complete diffraction analysis, including many off-axis diffractions such as (0002«), (ηθηθ), and (1122(«-1)) for n = 1-3, shows that all of these yield rocking curve linewidths below 700 arcsec. The comprehensive XRD characterizations with linewidths around 0.1-0.2° indicate that semipolar (2021) GaN, with a microstructural quality comparable to that of c-plane GaN directly grown on sapphire, can be achieved. The combination of flexible slicing of sapphire substrates, patterning of the sapphire substrates, and good control of the polarity of GaN selectively grown on the c-plane sapphire sidewalls can enable producing device-quality, large- area semipolar GaN in arbitrary surface orientations.

FIG. 9A shows the cross-sectional SEM image of a ΙΟ-μιη-thick (2021) GaN layer grown on a patterned sapphire substrate, according to some embodiments. Since the (2021) plane is a higher-index plane that is likely to have a higher surface energy, the resultant growth surface after the stripe coalescence grown under H₂ carrier gas may be stabilized by two low-index facets: (10Ϊ0) and (1011). Alternative conditions may exist that can help to stabilize the (2021) plane.

The microstructural quality of thick epilayers of (2021) GaN can examined using a TEM. FIG. 9B shows a cross-sectional TEM image under two-beam condition taken along a diffraction vector of g = (10Ϊ0). The majority of the N-polar growth region above the trenches in sapphire exhibits a low density of defects. However, the Ga-polar growth direction above the sapphire terrace has a high density of straight dark contrasting lines inclined at an angle of ~ 15° with respect to a surface normal to the sapphire terrace. The contrast is attributed to the presence of basal plane stacking faults (SFs). Usually, basal plane stacking faults are generated in the N-polar GaN region over a dielectric mask during heteroepitaxy. However, the majority of SFs in the (2021) GaN epilayer grown on the patterned sapphire substrate is generated in the Ga-polar GaN region. The specific mechanism for the generation of these SFs is a subject of further study.

As can be seen in FIG. 9A, the "surface" of a (2021)-oriented GaN epilayer formed on a PSS is composed of (10Ϊ0) and (1011) facets. These facets can be eliminated using chemical -mechanical polishing (CMP) to produce a planar GaN (2021) surface. Such a surface can be accessed, for example, to grow AlGalnN heterostructures at atomic scale on the (2021) plane. According to some embodiments, the zigzagged or undulating surface of an as-grown, thick GaN epilayer of other orientations can also be planarized by performing a direct CMP.

FIG. 10A and FIG. 10B show Nomarski optical microscope images of the GaN epilayer surface before and after a CMP process, respectively. The CMP process can remove between 2 μπι and 6 μπι of GaN, according to some embodiments, though more or less GaN can be removed in some cases. After CMP (FIG. 10B), the surface of the GaN epilayer becomes optically flat. For example, a specularly reflective 2-inch- diameter (2021) GaN epilayer on a PSS can be obtained, as shown in FIG. IOC. For the sample shown in FIG. IOC, subsequent cross-sectional SEM imaging of cleaved samples was used to estimate the remaining thickness of GaN after CMP. These images showed that the remaining thickness is in a range of approximately 5-6 μπι across the 2-inch- diameter wafer.

Surface morphology after the CMP process can also be examined using an atomic force microscope (AFM). A result from AFM measurements of a sample fabricated according to the present embodiments is shown in FIG. 11. A root-mean-square (RMS) roughness of about 1.4 nm is measured for a scan area of 10 μπι x 10 μπι. It is observed from several measurements across the wafer that both the RMS roughness and the atomic morphology for the (2021) GaN epilayer remain indistinguishable between the center and edge of the 2-inch-diameter wafer. In embodiments, the RMS surface roughness of a wafer having an epitaxial gallium nitride material layer can be less than 2 nm across a large area wafer. After producing an atomically smooth (2021) GaN epilayer on sapphire, integrated electronic or optoelectronic devices can be formed using the GaN epilayer. For example, InGaN LEDs can be formed on the (2021) GaN epilayer. An example structure for an LED formed using the GaN epilayer is shown schematically in FIG. 12, though other structures or other devices can be used in other embodiments.

The growth of sample LEDs can comprise forming about 2 μιη of Si-doped GaN regrown on the CMP -processed (2021) GaN-on-sapphire epilayer. Subsequently, three pairs of undoped InGaN (3 nm)/ GaN (8 nm) multiple quantum wells (MQWs) can be formed on the Si-doped GaN. After that, approximately 200-nm-thick p-type GaN was grown with a Mg doping level of about 5 ^χ 10¹⁹ cm^-3. According to some embodiments, N₂ carrier gas can be used during the regrowth of Si-doped GaN and InGaN MQWs in order to retain a smooth, facet-free surface morphology of the GaN surface after the CMP process. In some embodiments, all layers of the LEDs' structures can be grown in the N₂ carrier gas. According to some implementations, LED devices can be fabricated using photolithography techniques with Cl-based inductively-coupled plasma etching. For the sample LEDs, each die area measured about 600 μιη ^χ 600, though other areas may be used. Ni (20 nm)/Au (50 nm) bilayers can be used as both n- and p-type contacts, according to some embodiments.

According to some embodiments, an LED sample according to the structure shown in FIG. 12 can be characterized using XRD. A result of a 2θ/ω radial XRD scan for an LED sample is shown in the graph of FIG. 13. In the graph, the 0, -1, -2, and -3 order satellite peaks are clearly defined and labeled. According to the space between satellite peaks, the +1 order satellite peak is located at about 70.37°, which is overlapped with the GaN peak at 70.41°. The presence of clear and high-order satellite peaks from InGaN/GaN MQWs indicates abrupt quantum well interfaces with microfacet-free, planar wells over large length scales. According to the 2θ/ω scan, the quantum well period was extracted to be 10.7 nm, and quantum well/barrier widths were deduced to be approximately 2.9 nm/7.8 nm from the growth information, which agrees well with the designed period of MQWs. In some embodiments, InGaN MQWs were coherently grown on a GaN template according to the XRD reciprocal space mapping result (not shown here) on asymmetric facets. The indium composition in InGaN quantum wells was determined to be about 26.1% from the peak separation in the 2θ/ω scan, using the method of Vickers et al. (Vickers, M. E.; Hollander, J. L.; McAleese, C; Kappers, M. J.; Moram, M. A.; Humphreys, C. J. Determination of the Composition and Thickness of Semi-Polar and Non-Polar Ill-Nitride Films and Quantum Wells Using X-Ray

Scattering. J. Appl. Phys. 2012, 111 (4), 43502-1-43502-12).

An I-V curve is shown in FIG. 14 for an LED having the structure depicted in

FIG. 12. The LED is formed on a smooth (2021) GaN epilayer that was produced according to the present embodiments. A photo of probes attached to the (2021) GaN LED that is operating at 30 mA is shown in the inset of FIG. 14. The LED exhibits a turn-on voltage of about 2.1 V. The low turn-on voltage can be due to several effects. First, a high density of defects (including both SFs and threading dislocations) in the semipolar (2021) GaN may result in a high leakage current. Second, back diffusion of Mg into the MQWs region may degrade the quality of the p-n junction.

Room-temperature electroluminescence (EL) measurements may be performed under pulsed conditions with a duty cycle of about 2% to prevent self-heating, for example, with a current pulse width and repetition rate of 10 and 2 kHz, respectively. Electroluminescent spectra for the sample LED are shown in FIG. 15. The different spectra correspond to different current injection levels from 20 mA (lowest trace) to 100 mA (highest trace) under pulsed conditions at room temperature. The line width of the EL spectrum at an injection current of 100 mA is about 47 nm, which is broader than a typical c-plane LED. The broader linewidth of the EL spectrum may be attributed in part to the presence of a high density of defects (SFs and threading dislocations). The broader linewidth may also be due to indium composition fluctuations inside the LEDs quantum wells, which in turn may be due to nanofacet formation during the growth of (2021) or (202Ϊ) InGaN MQWs.

FIG. 16A plots integrated EL intensity as a function of injection current for the sample LED. The sublinear light emission suggests that the parameters related to either active layer design, doping profile, or current leakage pathways caused by a high density of one or both of SFs and threading dislocations may be further improved upon. For example, stacking faults and/or dislocations may be reduced in some embodiments by using a finer periodicity of trenches patterned in the sapphire substrate. Although stacking faults and dislocations may occur near the surface of the sapphire, they may terminate part way through a thick epitaxial layer, so that a surface region of a thick epitaxial layer has a lower defect density.

The dependence of the peak position of EL on the injection current is plotted in FIG. 16B (diamonds) for the sample LED. When the injection current is increased from 10 mA to 100 mA, the peak position is blue-shifted by about 3.4 nm. As a comparison, a c-plane LED with 5 pairs of InGaN/GaN MQWs grown at similar conditions is also subjected to the EL measurement, and its dependence of peak wavelength on injection current is plotted in FIG. 16B (squares). The blue-shift for the c-plane LED is about 8.8 nm, which is significantly larger than that for the (2021) LED. The (2021) LED exhibits a decreased quantum-confined Stark effect (QCSE) due to the reduced polarization field in the nitrogen-polar semipolar (2021) GaN. It is also noted that the EL intensity of the semipolar (2021) LED is about 4 times lower than that of the c-plane LED under an injection current of 100 mA because of the reduced crystalline quality of (2021) GaN and a nonoptimized structure for the (2021) LED.

A wafer, device, or structure comprising gallium nitride material formed on a sapphire substrate can be embodied in different configurations. Example configurations include combinations of configurations (1) through (8) as described below.

(1) A wafer comprising a sapphire substrate; a buffer layer; and nitrogen-polar or nitrogen-polar semipolar gallium nitride material formed on the buffer layer, wherein an x-ray diffraction rocking curve for a (002) crystallographic orientation of the gallium nitride material exhibits a full-width-half-maximum value that is less than or equal to 0.25 degrees.

(2) The wafer of configuration (1), wherein the sapphire substrate is patterned and includes trenches.

(3) The wafer of configuration (1) or (2), wherein a root-mean-square surface roughness of the gallium nitride material is less than 2 nanometers.

(4) The wafer of any one of configurations (1) through (3), wherein a crystal growth surface of the sapphire substrate is nitridized.

(5) The wafer of any one of configurations (1) through (4), wherein the buffer layer comprises aluminum nitride. (6) The wafer of any one of configurations (1) through (5), wherein the gallium nitride material is nitrogen-polar semipolar gallium nitride (GaN) for which a (20(21) ) facet of the GaN is essentially parallel to a plane in which the sapphire substrate lies.

(7) The wafer of any one of configurations (1) through (6), further comprising at least one integrated electronic or optoelectronic device formed on the nitrogen-polar or nitrogen-polar semipolar gallium nitride material.

(8) The wafer of any one of configurations (1) through (7), wherein the at least one integrated electronic or optoelectronic device includes multiple quantum wells.

Methods for making a wafer comprising gallium nitride material formed on a sapphire substrate can include various processes. Example methods include

combinations of processes (9) through (20) as described below. These processes may be used, at least in part, to make wafers, devices, or structures of the configurations listed above.

(9) A process for forming a nitrogen-polar or nitrogen-polar semipolar gallium nitride material, the method comprising nitridizing a growth surface of a sapphire substrate; forming an A1N buffer layer at a temperature of at least 850 °C on the nitridized growth surface; and epitaxially growing the nitrogen-polar or nitrogen-polar semipolar gallium nitride material on the A1N buffer layer.

(10) The process of (9), wherein nitridizing a growth surface comprises exposing the growth surface to NH₃ and N₂ gases with the substrate at a temperature between 900 °C and 1000 °C.

(11) The process of (9) or (10), further comprising, during nitridizing, flowing the NH₃ gas at a flow rate between approximately 1 slm and approximately 5 slm; and flowing the N₂ gas at a flow rate between approximately 3 slm and approximately 7 slm.

(12) The process of any one of (9) through (11), wherein a duration of the nitridizing is between approximately 0.5 minutes and approximately 5 minutes.

(13) The process of any one of (9) through (12), wherein during formation of the A1N buffer layer, a V/III ratio of the Al and N is maintained between 200 and 4000.

(14) The process of any one of (9) through (13), wherein the A1N buffer layer is not annealed following its formation.

(15) The process of any one of (9) through (14), wherein the A1N buffer layer is formed at a chamber pressure between 50 millibar and 300 millibar. (16) The process of any one of (9) through (15), wherein forming the A1N buffer layer comprises flowing trimethylaluminum gas at a flow rate between 10

micromole/min and 200 micromole/min; and flowing ammonia gas at a flow rate between 0.2 slm and 3 slm.

(17) The process of any one of (9) through (16), wherein epitaxially growing the gallium nitride material comprises growing the gallium nitride material at a temperature between 950 °C and 1100 °C.

(18) The process of any one of (9) through (17), further comprising patterning the sapphire substrate to form a plurality of trenches in a planar surface of the sapphire substrate, wherein the growth surface comprises one inclined wall of a trench of one of the plurality of trenches.

(19) The process of any one of any one of (9) through (18), further comprising performing a chemical mechanical polishing process to planarize a surface of the nitrogen-polar semipolar gallium nitride material.

(20) The process of any one of (9) through (19), further comprising including N₂ carrier gas during regrowth of the nitrogen-polar semipolar gallium nitride material on the planarized surface.

CONCLUSION

Virtually any N-polar semipolar or Ga-polar semipolar GaN orientation in an epitaxial layer can be possible by using a correctly sliced and patterned sapphire substrate, according to some embodiments. Also, essentially purely N-polar GaN can be formed on sapphire, in some implementations. Although epitaxial growth of GaN is described, nitrogen-semipolar and nitrogen-polar orientations of other Ill-nitride materials (e.g., (Al, In, Ga)N and their alloys (AlGaN, AlGalnN, InGaN, etc.) may be grown according to embodiments described above in which one or more chemical species of gases are added to or substituted for a gas that carries gallium during epitaxial growth. In some implementations, a portion or all of an epitaxially-grown layer can be doped to have n-type or p-type conductivity.

The terms "approximately" and "about" may be used to mean within ±20% of a target parameter (e.g., dimension, temperature, orientation, pressure, etc.) in some embodiments, within ±10% of a target parameter in some embodiments, or within ±5% of a target parameter in some embodiments. The term "essentially" may be used to mean within ±3% of a target parameter in some embodiments. The terms "approximately" and "about" may include the target parameter.

Selective etching, as used herein, comprises subjecting a substrate or wafer to an etchant that preferentially etches at least one material at a faster rate than a second material. In some cases, the second material may be formed as a hard mask (e.g., an inorganic material such as an oxide, nitride, metal, or the like) or soft mask (e.g., a photoresist or polymer). In some embodiments, the second material may be part of a device structure that has different material characteristics than the first material (e.g., doping density, material composition, or crystal structure). The etch may be a dry etch or wet etch.

The technology described herein may be embodied as a method, of which at least one example has been provided. The acts performed as part of the method may be ordered in any suitable way. Accordingly, embodiments may be constructed in which acts are performed in an order different than illustrated, which may include performing some acts simultaneously, even though shown as sequential acts in illustrative embodiments. Additionally, a method may include more acts than those illustrated, in some embodiments, and fewer acts than those illustrated in other embodiments.

Although the drawings typically depict a small portion of an epitaxially-grown GaN layers, it will be appreciated that a large area or entire wafer may be covered with such an epitaxially-grown layer. Further, the epitaxial layer may be planarized (e.g., by chemical -mechanical polishing) and integrated-circuit devices (e.g., transistors, diodes, thyristors, light-emitting diodes, laser diodes, photodiodes and the like) may be fabricated using the epitaxially-grown material. In some embodiments, the integrated- circuit devices may be used in consumer electronic devices such as smart phones, tablets, PDA's, computers, televisions, sensors, lighting, displays, as well as application-specific integrated circuits.

Having thus described at least one illustrative embodiment of the invention, various alterations, modifications, and improvements will readily occur to those skilled in the art. Such alterations, modifications, and improvements are intended to be within the spirit and scope of the invention. Accordingly, the foregoing description is by way of example only and is not intended as limiting.

Claims

CLAIMS What is claimed is:

1. A wafer comprising:

a sapphire substrate;

a buffer layer; and

nitrogen-polar or nitrogen-polar semipolar gallium nitride material formed on the buffer layer, wherein an x-ray diffraction rocking curve for a (002) crystallographic orientation of the gallium nitride material exhibits a full-width-half-maximum value that is less than or equal to 0.25 degrees.

2. The wafer of claim 1, wherein the sapphire substrate is patterned and includes trenches.

3. The wafer of claim 1, wherein a root-mean-square surface roughness of the gallium nitride material is less than 2 nanometers.

4. The wafer of claim 1, wherein a crystal growth surface of the sapphire substrate is nitridized.

5. The wafer of any one of claims 1 through 4, wherein the buffer layer comprises aluminum nitride.

6. The wafer of claim 1, wherein the gallium nitride material is nitrogen-polar semipolar gallium nitride (GaN) for which a (2021) facet of the GaN is essentially parallel to a plane in which the sapphire substrate lies.

7. The wafer of any one of claims 1 through 4 and 6, further comprising at least one integrated electronic or optoelectronic device formed on the nitrogen-polar or nitrogen- polar semipolar gallium nitride material.

8. The wafer of claim 7, wherein the at least one integrated electronic or

optoelectronic device includes multiple quantum wells.

9. A method for forming a nitrogen-polar or nitrogen-polar semipolar gallium nitride material, the method comprising:

nitridizing a growth surface of a sapphire substrate;

forming an AIN buffer layer at a temperature of at least 850 °C on the nitridized growth surface; and

epitaxially growing the nitrogen-polar or nitrogen-polar semipolar gallium nitride material on the AIN buffer layer.

10. The method of claim 9, wherein nitridizing a growth surface comprises exposing the growth surface to NH₃ and N₂ gases with the substrate at a temperature between 900 °C and 1000 °C.

11. The method of claim 10, further comprising during nitridizing:

flowing the NH₃ gas at a flow rate between approximately 1 slm and

approximately 5 slm; and

flowing the N₂ gas at a flow rate between approximately 3 slm and approximately 7 slm.

12. The method of claim 10, wherein a duration of the nitridizing is between approximately 0.5 minutes and approximately 5 minutes.

13. The method of any one of claims 9 through 12, wherein during formation of the AIN buffer layer, a V/III ratio of the Al and N is maintained between 200 and 4000.

14. The method of claim 9, wherein the AIN buffer layer is not annealed following its formation.

15. The method of claim 9, wherein the AIN buffer layer is formed at a chamber pressure between 50 millibar and 300 millibar.

16. The method of claim 9, wherein forming the AIN buffer layer comprises:

flowing trimethylaluminum gas at a flow rate between 10 micromole/min and

200 micromole/min; and

flowing ammonia gas at a flow rate between 0.2 slm and 3 slm.

17. The method of any one of claims 9 through 12 and 14 through 16, wherein epitaxially growing the gallium nitride material comprises growing the gallium nitride material at a temperature between 950 °C and 1100 °C.

18. The method of claim 9, further comprising patterning the sapphire substrate to form a plurality of trenches in a planar surface of the sapphire substrate, wherein the growth surface comprises one inclined wall of a trench of one of the plurality of trenches.

19. The method of any one of claims 9 through 12, 14 through 16, and 18, further comprising performing a chemical mechanical polishing process to planarize a surface of the nitrogen -polar semipolar gallium nitride material.

20. The method of claim 19, further comprising including N₂ carrier gas during regrowth of the nitrogen-polar semipolar gallium nitride material on the planarized surface.