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In this paper, the growth of semi-polar GaN thick films was investigated on an m-plane sapphire by hydride vapor phase epitaxy (HVPE). By optimizing parameters of the HVPE process, both (11−22) and (10−13) semi-polar GaN films have been successfully obtained with a mirror surface and excellent crystal quality on different GaN templates grown by metal organic chemical vapor deposition (MOCVD), respectively. The influence of MO-GaN templates on the HVPE growth was further studied. The results revealed that the same plane of a semi-polar HVPE-GaN thick layer was grown on a single-phase template, such as GaN (11−22)HVPE on (11−22)MO-template and GaN (10−13)HVPE on (10−13)MO-template. However, on mixed-phase MO-templates, a single-phase (11−22) GaN thick layer was found to grow even if the original templates contain both (11−22) and (10−13) plane components. Meanwhile, in contrast with the difficult growth of GaN (10−13) with a smooth surface, it is easier to acquire GaN (11−22) thick layers by a HVPE method. A wider window for the growth of the (11−22) plane could partly explain the reason why the pure GaN (11−22) plane could be grown on mixed-phase ((11−22) & (10−13)) MO-templates. In order to explain this phenomenon, both formation energy (Ef) and migration barrier (Em) of these semi-polar faces were calculated. Our results indicated that the Ef of GaN (10−13) (−0.56 eV) is higher than that of (11−22) (−10.73 eV), and the Em of (10−13) (0.30 eV) is lower than that of (11−22) (1.89 eV). So the (11−22)-plane growth was more likely to be prevalent in competition between two phases.

A thin GaN transition layer (GTL) technology is proposed to control stress and reduce the dislocation density in the initial growth of GaN on an AlN nucleation layer on Si (111) substrates. The GTL has a thickness of 50 nm and is the thinnest transition layer reported for GaN growth on Si substrates. It is found that the density and aspect ratio of GaN islands markedly decrease and the size of GaN islands observably increases under GTL growth conditions. Therefore, the compressive stress relaxation in the initial GaN growth caused by island deformation and island coalescence can be significantly reduced and more compressive stress is built up in the GaN epilayer. Meanwhile, the dislocations originating from the AlN nucleation layer are inclined at an average angle as large as 37° and therefore, most of the dislocations interact and annihilate in the GTL. In this way, efficient stress control and dislocation reduction in the initial GaN growth are simultaneously realized. Moreover, thanks to great reduction of the dislocation density in the GTL, the compressive stress relaxation due to dislocation inclination in subsequent GaN growth is also significantly decreased. As a result, a 1.5 μm crack-free GaN epilayer, whose full width at half maximum (FWHM) values of the (002) and (102) X-ray rocking curves are 270 and 521 arcsec, respectively, is obtained. With a simple growth process, the quality of the GaN epilayer directly grown on the AlN nucleation layer is greatly improved, which demonstrates great application potential for the growth of a fully vertical device structure on Si substrates.

Stress-induced self-separation is one of the most efficient process for preparing native GaN substrate. The control of GaN film thickness is the key point for GaN film separating from substrate completely. Considering the bowing of bilayer, we studied the radial stress in GaN film before separation. A shrunken circular delamination front model was proposed to derive the unrelaxed stress after separation, and the energy release rate for GaN/sapphire systems of different thicknesses was investigated during the whole separation process. A critical thickness about 500–700 μm was determined for separating 2-inch (5.08 cm) GaN film from a sapphire substrate. By precisely controlling the GaN film thickness around such critical thickness, the complete separation rate could be increased greatly to 74%, which is of great importance in realizing the industrialization of GaN substrate. Keywords: GaN, Stress-induced self-separation, Bilayer thick film, Residual stress, Fracture

The anisotropic extraction dependence of polarized light on propagation path in AlGaN-based deep-ultraviolet (DUV) light-emitting diodes (LEDs) is investigated by simulations and photoluminescence (PL) measurements. Theoretical calculations based on k⋅p approximation and Monte Carol ray tracing indicate that there are two kinds of polarized sources with different angular distributions in ~280 nm AlGaN-based LEDs, s-polarized (spherical-shaped) and p-polarized (dumbbell-shaped) sources, which have different extraction behaviors. It is found that the total light extraction intensities are improved with decreasing the propagation path, and the lateral surface extraction gradually becomes dominant. Moreover, the extraction intensity of s-polarized light improves more than that of p-polarized light when the propagation path decreases, leading to a greater polarization degree. Polarization-resolved PL measurements show that the polarization degree of extracted light from lateral facet of the AlGaN multiple quantum well sample can be enhanced from 1% to 17% as the average propagation path reduces by 0.6 mm, which is consistent with the simulation results of the anisotropic dependence of light extraction on propagation path. Our results are significant for understanding and modulating the anisotropic extraction behavior of polarized light to realize high efficiency AlGaN-based DUV LEDs.

The growth mechanism of exceptionally long N-polar GaN sub-micrometer (sub-μm) rods has not been revealed to date. In this work, we investigated the effects of the V/III molar ratio and HCl (diluted in carrier gas) on the lengthening of a MOCVD-GaN rod template by HVPE. It is found that a low V/III molar ratio and HCl (diluted in carrier gas) help form vertical sidewalls and suppress the lateral growth on the top part of exceptionally long GaN sub-μm rods. Simulation results revealed that the low V/III molar ratio leads to reactive species distributing almost exclusively on the top part of GaN rods, which can effectively prevent crystal growth on the bottom of GaN rods. Chlorine ions support the growth by etching the sidewalls. After growth, the diameter and length of each GaN rod are more than 1.5 μm and 70–80 μm, respectively. Finally, an empirical growth model was developed to account for the exceptionally long GaN sub-μm rods under HVPE growth.

In this paper, crack-free 2-inch bulk GaN wafer with the thickness up to 3 mm was obtained by HVPE homoepitaxy. A new method of acid wet etching was used to pre-treat GaN substrate before re-growth. The formation of the mesh-like subsurface crack and interface layer were found to be suppressed between the re-growth layer and as-grown GaN substrate. EDS and time varied contact angle measurement proved that chemical etching would decrease the oxygen related surface adsorption and increase atoms diffusion length during HVPE homoepitaxial growth. Moreover, Morphology, Low temperature photoluminescence measurements indicated a reduction in stress of wet etching treated as-grown GaN substrate due to etching effect on its N face. High quality bulk GaN with the dislocation density of 1×10 6 cm −2 was achieved by using wet etching and HVPE multiple re-growth. It would offer a simple method to obtain bulk GaN with thicker layer and high quality.

The uniformity of flow field inner the reactor plays a crucial role for hydride vapor phase epitaxy (HVPE) crystal growth and its more important for large scale substrate. A new nozzle structure was designed by adding a push and dilution (PD) gas pipe in the center of gas channels for a 4-inch HVPE (PD-HVPE) system. Experimental results showed that the thickness inhomogeneity of 46 μm 4-inch GaN layer could reach ±1.8% by optimizing PD gas, greatly improved from ±14% grown with conventional nozzle. The simulations of the internal flow field were consistent with our experiment, and the enhancement in uniformity should be attributed to the redistribution of GaCl and NH 3 upon the wafer induced by PD pipe. The full width at half maximum (FWHM) of X-ray diffraction rocking curves for the 4-inch GaN film were about 224 and 200 arcsec for (002) and (102) reflection. The dislocation density of as-grown GaN was about 6.4×10 7 cm −2 .

A magnetron sputtered AlN buffer layer is applied for preparing 2 in. free-standing (FS) GaN on the GaAs substrate by hydride vapor phase epitaxy (HVPE). The AlN buffer layer with excellent compactness can prevent As atoms from diffusing to the GaN growth surface. Thus it serves as an effective protective layer for GaAs substrate and also a seed layer for the deposition of GaN in the early stage of the high-temperature GaN growth. Benefiting from the decomposition property of GaAs at temperature higher than 900 °C, thick GaN film self-separated completely from GaAs substrate during growth. By this way, a 2 in. FS-GaN layer with the thickness of 1.3 mm is finally obtained after the long growth time about 20 h.

The wide-bandgap materials Al-rich AlGaN materials, and their applications on the deep UV LEDs, photodetectors, and power devices are reviewed. The physical properties including energy bandgap, optical properties, polarization properties, etc. are also discussed.

In this letter, a new simple method is introduced for obtaining freestanding GaN. The method combines hydride vapor phase epitaxy and heat-treatment of sapphire, which has been implemented to improve mechanical properties before the growth of GaN template by metalorganic chemical vapor deposition. After hydride vapor phase epitaxy on the GaN template, spontaneous separation took place inside the GaN layer at room temperature, in which process, a new phenomenon called delamination inside GaN layer was observed. The delamination was believed to be driven by the large thermal mismatch stress and the interface shear stress at the wafer edge. Two-inch-diameter freestanding GaN with high quality was thus obtained. The full width at half maximum for X-ray rocking curve were about 108 and 81 arcs for (0 0 2) and (1 0 2) reflections. The dislocation density of the freestanding GaN was about 3–7×106 cm−2.

In order to solve the problems of GaN heteroepitaxy on sapphire substrate, some techniques were explored. Freestanding GaN substrates have been made by hydride vapor phase epitaxy (HVPE), laser lift-off (LLO), and chemical mechanical polishing techniques. Wafer bending and cracking in the HVPE growth were partly settled by pulsed flow modulation method. High-crystal quality was established for 1.2 mm thick GaN substrate by X-ray diffraction measurement, in which the full width of half maximum values were 72, 110 arcsec for (102), (002) peaks. A novel micro-size patterned sapphire substrate (PSS) and a nano PSS were also fabricated. High-power vertical structure light emitting diodes (VSLEDs) have been developed by Au–Sn eutectic wafer bonding, homemade micro-area LLO, and light extraction structure preparation. The high-injection-level active region with low temperature GaN sandwiched layers was used for low-efficiency droop. The light output power of VSLED was achieved as 400 mW driven at 350 mA, and the dominant wavelength is about 460 nm. The structures and properties of strain modulated superlattices (SLs) and quantum wells as well as advanced simulation of carriers transport across the electron blocking layer were investigated in laser diodes. The hole concentration was achieved as high as 1.6 × 1018 cm−3 in AlGaN/GaN SLs:Mg by inserting an AlN layer. High-quality AlGaN epilayers and structures were grown by MOCVD. Some device structures of UV LEDs and detectors were demonstrated. The emission wavelength of 262 nm UV LED has been successfully fabricated. At last, high-quality InN and InGaN materials for solar cell were grown by boundary-temperature-controlled epitaxy and growth-temperature-controlled epitaxy. Hall-effect measurement showed a recorded electron mobility of 3,280 cm2/(V s) and a residual electron concentration of 1.47 × 1017 cm−3 at 300 K.

A new nozzle structure was developed in an improved multi-wafer hybrid vapor phase epitaxy (IHVPE) system by adding an inner dilution gas (ID) pipe between V and III groups gas channels. Experimental results showed that the thickness distribution of 2-inch GaN layer depended strongly on the flow rate of ID gas. The uniformity of film can arrive at ±3–4% by optimizing ID gas, which was better than that of ±30% grown in the old conventional multi-wafer hybrid vapor phase epitaxy (CHVPE) system. Meanwhile, the crystal quality and surface morphology were also greatly improved for GaN film by using the new reactor structure. The FWHM values of (002) and (102) were reduced from 342″ and 806″ to 207″ and 254″, respectively. AFM result of surface roughness (RMS, 10 μm×10 μm) of GaN layer was also lowered from 1.226 nm to 0.798 nm. It was partly because of the suppression of parasitic polycrystalline deposition due to the ID gas. This simple and economic method could provide an effective solution to simultaneously fabricate multiple GaN wafer with good thickness uniformity, high crystal quality and low cost.

This study demonstrates improvement of crystal quality of GaN film by growing it on a micro-faceted GaN template fabricated by in situ etching with an HCl/N 2 etching gas mixture at 1050 °C in hydride vapor phase epitaxy (HVPE) reactor. Cathodoluminescence images of the cross-sectional structure showed that there were six sublays in the GaN film grown with in situ etching for five times. Etch pit density (EPD) and high-resolution X-ray diffraction (HR-XRD) measurements showed that the crystal quality of GaN thick film grown on micro-faceted GaN template was better than that of the as-grown GaN. A large number of columnar structures were formed on the micro-faceted GaN template at the initial stage of regrowth process, the coalescence of which played an important role in the reduction of threading dislocation density during the GaN growth. A model has been developed to explain the mechanism of forming process of the columnar structures.

Hexagonal boron nitride (h-BN) is known as promising 2D material with a wide band-gap (~6 eV). However, the growth size of h-BN film is strongly limited by the size of reaction chamber. Here, we demonstrate the large-roll synthesis of monolayer and controllable sub-monolayer h-BN film on wound Cu foil by low pressure chemical vapor deposition (LPCVD) method. By winding the Cu foil substrate into mainspring shape supported by a multi-prong quartz fork, the reactor size limit could be overcome by extending the substrate area to a continuous 2D curl of plane inward. An extremely large-size monolayer h-BN film has been achieved over 25 inches in a 1.2” tube. The optical band gap of h-BN monolayer was determined to be 6.0 eV. The h-BN film was uniformly transferred onto 2” GaN or 4” Si wafer surfaces as a release buffer layer. By HVPE method, overgrowth of thick GaN wafer over 200 μm has been achieved free of residual strain, which could provide high quality homo-epitaxial substrate.

About 1.2 mm thick GaN bulk crystals were obtained by combining a pulsed NH3-flow modulation (PFM) method and a self-separation method of short-shutting NH3 flow when using hydride vapor phase epitaxy (HVPE). High crystal quality of bulk GaN was evaluated by X-ray rocking curves (XRC) and the full width at half maximum (FWHM) values were 110, 72 and 83 arcsec for (002), (102) and (100) reflection planes, respectively. The PFM method is proved to be effective in reducing cracks and keeping the surface smooth. And the method of short-shutting NH3 flow can lead to GaN thick layer separate from sapphire substrate when cooling from the high growth temperature. Growth and separation mechanisms were investigated. Two states were found in PFM method. With PFM method modulating between high quality state and low stress state, 300 μm thick GaN layers without cracks were obtained. Study of spontaneous separation mechanism revealed that the separation attributed to formation of voids inside the GaN layer.

Several non-polar a-plane GaN films had been grown by hydride vapor phase epitaxy (HVPE) on different designed metal organic chemical deposition (MOCVD) GaN templates, which exhibited various ridge-like sidewall facets surface morphologies. The templates induced a lateral growth at the early stage of the HVPE growth, and resulted in a kind of maskless epitaxy lateral overgrown (ELO) process. It is found that the dislocation reduced differently along [1 0 0 0] and [ 1 1 ¯ 0 0 ] directions in these HVPE a-plane GaN layers. In [0 0 0 1] direction, the dislocation reduction resulted from the optimal surface roughness value of the template. In [ 1 1 ¯ 0 0 ] direction, the inclined facet might be a main factor for the dislocation reduction in HVPE-GaN films. The maskless ELO process had a significant influence on decreasing the dislocation density.

The effect of the off-cut angle of an r -plane sapphire substrate has been investigated on the growth of a -plane AlN thick layer by low-pressure hydride vapor phase epitaxy (LP-HVPE). The off-cut angle ( θ ) was changed from +5.0° (close to c -axis) to −5.0° (close to m -axis). Results show that the crystalline quality and surface morphology are very sensitive to the sign of θ off-angle. The plus θ off-angle is found to be dramatically reduce the full-widths at half-maximum (FWHM) of X-ray rocking curves (XRC), compared with the minus θ off-angle. In-plane FWHM anisotropic feature marked as M- or W-shape dependence on azimuth angle was observed for a -plane AlN. The shape and degree of anisotropy depend on the sign of θ off-angle, while the plus of θ off-angle will leads to the W-shape and the decreased anisotropy. The minimum crystal tilts and twists of the films are observed for the vicinal sapphires with the plus off-angles of +0.2° to +1.0°.

We studied the influence of the growth temperature and HCl flow rate on the morphological evolution of crack-free thick GaN films by using a home-made horizontal hydride vapor phase epitaxy on sapphire substrates. Optical difference microscopy, scanning electron microscopy (SEM), atomic force microscopy (AFM), and cathodoluminescence (CL) were carried out to reveal the surface property of the GaN epilayer. It was found that the higher growth temperature is a key factor to obtain mirror, colorless and flat GaN surface. However, this key effect of temperature was modulated by HCl flow rate (HCl > 15 sccm). The surface RMS roughness was reduced from 206 to 2.51 nm for 10 μm × 10 μm scan area when GaN was grown at 1070 °C with HCl flow rate up to 30 sccm. These samples also reduced their (0 0 0 2) FWHM result from 1000 to 300 arcsec and showed a strong near-band-edge peak in CL spectra. Results indicated that growth temperature influence growth velocities on different crystalline planes, which will lead to the different morphologies obtained. High growth temperature can improve the lateral growth rate of vertical {1 1 − 2 0} facets and reduce the vertical growth rate of top {0 0 0 1} facet combined with higher HCl flow rate, which leads to completely coalescence of surface.

High-quality thick GaN films without cracks were achieved by using a new nozzle structure in the reactor grown by the hydride vapor phase epitaxy on sapphire substrates. Optical contrast microscopy, scanning electron microscopy (SEM), atomic force microscopy (AFM), X-ray double diffraction (XRD) and cathodoluminescence (CL) were carried out to reveal the surface, crystal and optical properties of the GaN epilayer. It was found that the nozzle structure in the reactor has a large effect on the growth rate, surface flat, crystal quality, and the uniformity of the growth. Compared with the old one, the new nozzle structure (denoted as multi-layers nozzle) can improve dramatically the properties of thick GaN. Mirror, colorless and flat GaN thick film was obtained and its (0 0 0 2) FWHM results were reduced from 1000 to 300 arcsec when the new nozzle was used. AFM result revealed a step flow growth mode for GaN layer with the new nozzle. Room-temperature CL spectra on the GaN films showed a strong near-band-edge peak for the new nozzle, but there is only weak emitting peak for the old nozzle. New nozzle structure can improve the uniform of flow field near the surface of substrates compared with the old one, which leads to the improvement of properties of GaN thick film by hydride vapor phase epitaxy (HVPE).

In this letter, Raman spectra for a series of (Ga, Mn)N samples with different Mn concentration grown by MOCVD have been investigated. A new vibrational mode at 577 cm−1 obtained is assigned to the local vibrational mode (LVM) of Mn substituting the Ga site. Another mode at 667 cm−1 arises from vibrational mode of nitrogen vacancies-related defects. It is found that Mn doping increases the lattice disorder and other built-in defects such as the nitrogen vacancies. The room temperature ferromagnetism is not only dependent on the substitutional Mn content but also strongly related to these lattice disorder and negative defects in (Ga, Mn)N films.

The effects of In doped low-temperature (LT) AlGaN interlayer on the properties of GaN/Si(1 1 1) by MOCVD have been investigated. Using In doping LT-interlayer can decrease the stress sufficiently for avoiding crack formation in a thick (2.0 μm) GaN layer. Significant improvement in the crystal and optical properties of GaN layer is also achieved. In doping is observed to reduce the stress in AlGaN interlayer measured by high-resolution X-ray diffraction (HRXRD). It can provide more compressive stress to counteract tensile stress and reduce crack density in subsequent GaN layer. Moreover, as a surfactant, indium is observed to cause an enhanced PL intensity and the narrowed linewidths of PL and XRD spectra for the LT-interlayer. Additionally, the crystal quality of GaN layer is found to be dependent on the growth parameters of underneath In-doped LT-AlGaN interlayer. The optimal parameters, such as TMIn flow rate, TMAl flow rates and thickness, are achieved to obtain nearly 2.0 μm thick crack free GaN film with advanced optical and crystal properties.

The temperature dependence of the formation of nano-scale indium clusters in InAlGaN quaternary alloys, which are grown by metalorganic chemical vapour deposition on GaN/Si(111) epilayers, is investigated. Firm evidence is provided to support the existence of phase separation, or nano-scale In-rich clusters, by the combined results of high-resolution transmission electron microscopy (HRTEM), high-resolution x-ray diffraction (HRXRD) and micro-Raman spectra. The results of HRXRD and Raman spectra indicate that the degree of phase separation is strong and the number of In clusters in the InAlGaN layers on silicon substrate is higher at lower growth temperatures than that at higher growth temperatures, which limits the In and Al incorporated into the InAlGaN quaternary alloys. The detailed mechanism of luminescence in this system is studied by low temperature photoluminescence (LT-PL). We conclude that the ultraviolet (UV) emission observed in the quaternary InAlGaN alloys arises from the matrix of a random alloy, and the second emission peak in the blue-green region results from the nano-scale indium clusters.

The subband structure and inter-subband transition as a function of gate voltage are determined by solving the Schrodinger and Poisson equations self-consistently in an AlxGa1-xN/GaN heterostructure. Different aluminum mole fraction and thickness of AlxGa1-xN barrier are considered. Calculation results show that energy difference between the first and second subband covers a wide range (from several tens to hundreds milli-electron volt) by applying different gate voltage, which corresponds to the midinfrared and long-wave infrared wavelength scope. Furthermore, such a modulation on the subband transition energy is much more pronounced for the structure with thin barrier. When the applied positive gate voltage is increased, the triangle well formed at the interface turns to be deeper and narrower, which enhances the confinement for electrons. As a result, the overlap between electron wave function at two subbands increases, and thus the optical intersubband transition also enhances its intensity. This tendency is in good agreement with the available data in the literature. (c) 2005 Elsevier B.V. All rights reserved.

A new method is demonstrated to be effective in reducing mismatch-induced tensile stress and suppressing the formation of cracks by inserting InAlGaN interlayers during the growth of GaN upon Si (1 1 1) substrate. Compared with GaN film without quaternary interlayer, GaN layer grown on InAlGaN compliant layers shows a five times brighter integrated PL intensity and a (0 0 0 2) High-resolution X-ray diffraction (HRXRD) curve width of 18 arcmin. Its chi(min), derived from Rutherford backscattering spectrometry (RBS), is about 2.0%, which means that the crystalline quality of this layer is very good. Quaternary InAlGaN layers, which are used as buffer layers firstly, can play a compliant role to endure the large mismatch-induced stress and reduce cracks during the growth of GaN epitaxy. The mechanisms leading to crack density reduction are investigated and results show that the phase immiscibility and the weak In-N bond make interlayer to offer tenability in the lattice parameters and release the thermal stress. (c) 2005 Elsevier B.V. All rights reserved.

InGaN/GaN quantum dots were grown on the sapphire (0 0 0 1) substrate in a metalorganic chemical vapor deposition system. The morphologies of QDs deposited on different modified underlayer (GaN) surfaces, including naturally as grown, Ga-mediated, In-mediated, and air-passivated ones, were investigated by atomic force microscopy (AFM). Photo luminescence (PL) method is used to evaluate optical properties. It is shown that InGaN QDs can form directly on the natural GaN layer. However, both the size and distribution show obvious inhomogeneities. Such a heavy fluctuation in size leads to double peaks for QDs with short growth time, and broad peaks for QDs with long growth time in their low-temperature PL spectra. QDs grown on the Ga-mediated GaN underlayer tends to coalesce. Distinct transform takes place from 3D to 2D growth on the In-mediated ones, and thus the formation of QDs is prohibited. Those results clarify Ga and In's surfactant behavior. When the GaN underlayer is passivated in the air, and together with an additional low-temperature-grown seeding layer, however, the island growth mode is enhanced. Subsequently, grown InGaN QDs are characterized by a relatively high density and an improved Gaussian-like distribution in size. Short surface diffusion length at low growth temperature accounts for that result. It is concluded that reduced temperature favors QD's 3D growth and surface passivation can provide another promising way to obtain high-density QDs that especially suits MOCVD system. (c) 2004 Elsevier Ltd. All rights reserved.

We performed atomistic modeling and simulations on (a + c)-type dislocations of various core configurations in wurtzite GaN. The atomic-scale stress distribution around the dislocation cores in GaN was for the first time computed using a large-scale atomic/molecular massively parallel simulator. We confirmed from a theoretical viewpoint that the double 5/6-atom ring core structure is stable, whereas the 5/7-atom ring core naturally dissociates. The dissociation of (a + c)-type dislocations with the 5/7-atom ring core structure intrinsically originates from the anisotropy of local atom stress instead of impurities around dislocation cores.

The wurtzite structure of GaN determines the anisotropy of its mechanical properties, which is significant in the processes of material preparation and application. Experimental results reveal that the fracture toughness along nonpolar m ‐ and a ‐planes of wurtzite GaN is similar, 1.54 MPa · m 1/2 and 1.59 MPa · m 1/2 , respectively, but obviously smaller than the value along c ‐plane, about 2.11 MPa · m 1/2 .

Erosion behavior of in-situ 15 vol% TiC particle reinforced Al–5Cu based composite and the monolithic Al–5Cu alloy was studied at oblique and normal impacts. The erosion rate of the composite was higher than that of the monolithic alloy. There existed low-angle peaks in erosion rate vs. impact angle curves for the two materials, which indicated ductile erosion behavior although the composite had a low tensile ductility. Theoretical model for ductile materials cannot predict the erosion rate of the composite. The presence of TiC particles in the composite increased, rather than decreased the erosion rate even though the particles obviously increased the hardness of the composite. Erosion process involved the displacement of the target material and the removal of the displaced material. By comparison to the monolithic alloy, the composite exhibited higher resistance to the displacement of the target material due to higher hardness, while lower resistance to the removal of the displaced material due to poor ductility. The combined effect from these two aspects resulted in a higher erosion rate of the composite than that of the monolithic alloy.

The curvature and interface shear stress of GaN-sapphire system are studied by establishing the mechanical equations based on two main assumptions: (a) the thickness of GaN film can be compared to the thickness of sapphire substrate, and (b) the thickness of GaN film is non-uniform. Our results show that the curvature of GaN-sapphire system is a variable within the whole circular system. The interface shear stress changes direction around at the middle of radius for the circular system, and the curvature have an important effect on the interface shear stress due to the consideration of non-uniform thickness for GaN film.

AlN was grown on r - and c -plane sapphire substrates by low-pressure hydride vapor phase epitaxy (LP-HVPE). Nitridation and buffer methods were used and compared. Results show that the buffer method is appropriate for the growth of a-plane AlN. In-plane stresses were measured and found to be different in the two in-plane directions parallel and perpendicular to the AlN c-axis. In-plane stress anisotropy is reduced at high temperature leading to a smoother surface, partly owing to a decreased difference in the growth rates between two in-plane directions. However, too high a temperature decreases the crystal quality of a-plane AlN. Thus, there exists an optimal temperature range for the growth of a-plane AlN, in which improved crystal and surface qualities can both be obtained. (© 2009 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

c-plane (0001) AlN layers were grown on sapphire (11-20) and (0001) substrates by hydride vapor phase epitaxy (HVPE) and metal-organic vapor phase epitaxy (MOVPE), respectively. The growth temperatures were adjusted from 1430-1500 °C and the reactor pressure was kept constant at 30 Torr. Mirror and flat c-plane AlN were obtained grown on both a-plane and c-plane sapphire. Crystalline quality and surface roughness are improved with increasing growth temperature, detected by high resolution X-ray diffraction (HRXRD) and atomic force microscopy (AFM). The Full widths at half maximum (FWHM) values of (10-12) diffraction are 519 and 1219 arcsec for c-plane AlN grown on a-plane sapphire and c-plane sapphire, respectively. It indicates that a-plane sapphire substrate benefits to decrease dislocations density.

We present a scheme for in situ self-release of thick GaN 2-in. wafer from sapphire substrate by engineering the gradient of misfit strains. Release energies of a-, m-, and c-planes of wurtzite GaN are systematically calculated under different biaxial strains by using first-principles method. The results reveal that the c-plane separation will take place under graded strains around −2.8%, where a drastic transition interface of release energy may strongly reduce the strength of c-plane bonding. Based on this mechanism, uniform thick GaN epilayer (>450 μm) is grown on (0001) sapphire substrate by hydride vapor phase epitaxy and subjected to a graded compressive strain field by bowing, fulfilling the c-plane separation condition. As a result, high quality free-standing GaN wafer (350 μm) can be achieved by self-release simply during the cooling process.

The a- and a+c-type threading dislocation (TD) inclinations were observed in Mg-doped tensile-strained GaN/Al0.15Ga0.85N superlattice (SL) with a high-temperature (HT) AlN interlayer (IL). Most of the inclined dislocations are a-type, toward the equivalent [100] line direction, following dislocation climb model, while a+c-type dislocations gliding on (101) second slip plane inclined toward the [110] line direction. The analysis by comparing samples gave no evidence of Mg dopants to affect the TD inclination. The stress evolution, specifically, the stress transformation of the tensile-stress-relieved AlGaN barrier to the compressive-strain-enhanced GaN well caused by the HT-AlN IL, dominantly affected the a- and a+c-type TD inclinations.

Wet etching was performed on N polar GaN, which was fabricated by laser lift-off from a sapphire substrate. Dodecagonal pyramids appeared on the N-polar GaN surface after immersion into hot H3PO4 solution even if it had been etched previously with hot KOH solution. According to the symmetry of the space group of C6v4-P6mc, the oblique angle and crystallographic plane indices of the pyramid facets were obtained. It was observed that the oblique angles of the etched facets decreased from the tip to the base of the pyramids. The etching rate was fast when the etching temperature was above 130 °C, and the oblique angle at the base was reduced. The enhancement of light output with increasing etching temperature has been confirmed. The polarization charges on the different facets were assigned to a kinetics-limited process by the special behavior of the hot H3PO4 etching.

We investigated the direct growth of c-plane AlN on sapphire (11.0) and (00.1) substrates by low-pressure hydride vapor phase epitaxy (LP-HVPE) without using any buffer, interlayer, or pulsed source supply. The a-plane sapphire was confirmed to be effective for obtaining high-quality AlN with an rms surface roughness of 0.156 nm for 2×2 µm2 scan and a large crack-free thickness of 3.6 µm. This was partly attributed to the hexagonal c-axis of a-sapphire substrate being perpendicular to the hexagonal c-axis of AlN, leading to the reduced twist degree of AlN. X-ray rocking curves yielded full widths at half maximum (FWHM) of 487 and 636 arcsec for (10.2) and (10.0) reflections, respectively, indicating a low edge dislocation density (reduced twist degree) in the film.