Yuanbin Dai

Improving the Quality of GaN Crystals by Using Graphene or Hexagonal Boron Nitride Nanosheets Substrate

The progress in nitrides technology is widely believed to be limited and hampered by the lack of high-quality gallium nitride wafers. Though various epitaxial techniques like epitaxial lateral overgrowth and its derivatives have been used to reduce defect density, there is still plenty of room for the improvement of gallium nitride crystal. Here, we report graphene or hexagonal boron nitride nanosheets can be used to improve the quality of GaN crystal using hydride vapor phase epitaxy methods. These nanosheets were directly deposited on the substrate that is used for the epitaxial growth of GaN crystal. Systematic characterizations of the as-obtained crystal show that quality of GaN crystal is greatly improved. The fabricated light-emitting diodes using the as-obtained GaN crystals emit strong electroluminescence under room illumination. This simple yet effective technique is believed to be applicable in metal-organic chemical vapor deposition systems and will find wide applications on other crystal growth.

Influence of stress in GaN crystals grown by HVPE on MOCVD-GaN/6H-SiC substrate

GaN crystals without cracks were successfully grown on a MOCVD-GaN/6H-SiC (MGS) substrate with a low V/III ratio of 20 at initial growth. With a high V/III ratio of 80 at initial growth, opaque GaN polycrystals were obtained. The structural analysis and optical characterization reveal that stress has a great influence on the growth of the epitaxial films. An atomic level model is used to explain these phenomena during crystal growth. It is found that atomic mobility is retarded by compressive stress and enhanced by tensile stress.

Direct growth of freestanding GaN on C-face SiC by HVPE.

In this work, high quality GaN crystal was successfully grown on C-face 6H-SiC by HVPE using a two steps growth process. Due to the small interaction stress between the GaN and the SiC substrate, the GaN was self-separated from the SiC substrate even with a small thickness of about 100 μm. Moreover, the SiC substrate was excellent without damage after the whole process so that it can be repeatedly used in the GaN growth. Hot phosphoric acid etching (at 240 °C for 30 min) was employed to identify the polarity of the GaN layer. According to the etching results, the obtained layer was Ga-polar GaN. High-resolution X-ray diffraction (HRXRD) and electron backscatter diffraction (EBSD) were done to characterize the quality of the freestanding GaN. The Raman measurements showed that the freestanding GaN film grown on the C-face 6H-SiC was stress-free. The optical properties of the freestanding GaN layer were determined by photoluminescence (PL) spectra.

Growth of high quality GaN on a novel designed bonding-thinned template by HVPE

In this paper, GaN films were grown on a novel designed template via hydride vapour phase epitaxy (HVPE). The template was fabricated by bonding a thinned MOCVD-GaN/sapphire template to a Si wafer (BTMG). Both the lattice deformation and thermal mismatch stress in the film can be decreased. The GaN film grown on the BTMG template displayed a smaller full width at half maximum (FWHM) for the (002) and (102) reflections than the film grown on the MOCVD-GaN/sapphire template (MG), characterized by high-resolution X-ray diffraction (HRXRD). The Raman measurements showed that the residual stress of the HVPE GaN film grown on the BTMG template was greatly released. The better optical quality of the GaN film grown on the BTMG template was detected in comparison to the film grown on the MG template by photoluminescence (PL) spectra. These results indicated that high quality GaN films can be obtained by using the new template.

Influence of V/III ratio on stress control in GaN grown on different templates by hydride vapour phase epitaxy

GaN crystals were grown on MOCVD-GaN/Al2O3 templates (MGA) and MOCVD-GaN/6H–SiC templates (MGS) in a hydride vapour phase epitaxy (HVPE) process where the V/III ratio was controlled. The tensile stress that exists in MGS was controlled by an increasing V/III ratio. The compressive stress that appears in MGA was controlled by a decreasing V/III ratio. The mechanism of stress control using the V/III ratio is discussed in terms of the interrelation of the stress, the V/III ratio and the crystal growth. The stress in these two kinds of substrates causes differences in atomic mobility which may be compensated by varying the V/III ratio. It is found that a larger V/III ratio results in a higher atomic mobility. Thus, atomic mobility is retarded by compressive stress and increased by tensile stress. This method of stress control has been shown to provide worthwhile guidance for GaN growth on different templates, and under different conditions in other investigations.

Characterization of dislocations in MOCVD-grown GaN using a high temperature annealing method

In this paper, a high temperature annealing (HTA) method is proposed to characterize dislocations in GaN prepared by metal–organic chemical vapor deposition (MOCVD) on c-plane (0001) sapphire. The templates were each annealed at different temperatures for 5 min. Molten KOH–NaOH eutectic (E) etching was also applied (at 500 °C for 30 min) to test and verify this new method. A comparison between the HTA template and the E etching template was made. The morphology and distribution of HTA pits and E etching pits were examined using scanning electron microscopy (SEM). Atomic force microscopy (AFM) was employed to study the shape of the HTA pits. Transmission electron microscopy (TEM) and cathodoluminescence (CL) investigation were used to further confirm the origin and density of the dislocations.

Epitaxial growth of a self-separated GaN crystal by using a novel high temperature annealing porous template

In this paper, a MOCVD-GaN/Al2O3 (MGA) template was annealed under appropriate annealing conditions and N2 flow to form a porous template. The porous template was used for growing GaN by HVPE. The GaN crystal was easily separated from the porous template with the assistance of the microporous structure. The self-separated GaN crystal grown on the porous template showed a smaller full width at half maximum (FWHM) for (002) and (102) reflections in the HRXRD measurement than that grown on the MGA template. The PL results indicate that the optical quality of the GaN crystal on the porous template was improved and the dislocation density decreased. The Raman results showed that the stress in the GaN crystal grown on the porous template is much smaller than that on the MGA template. These results show that the crystalline quality of GaN crystals was improved by using the porous template.

EBSD crystallographic orientation research on strain distribution in hydride vapor phase epitaxy GaN grown on patterned substrate

A GaN crystal was grown by hydride vapor phase epitaxy (HVPE) on a substrate with patterned SiO2 masks. The growth mode of HVPE-GaN was analysed, and the crystallographic orientation relationship between GaN and the sapphire substrate was identified by electron backscatter diffraction (EBSD) Kikuchi diffraction patterns and pole figures. The lattice mismatch was calculated from the crystallographic orientation relationship of the heteroepitaxial structure and the lattice parameters. EBSD mapping was employed to investigate the disorientation distribution of GaN along the [001] direction. The GaN strain in the heteroepitaxial structure was identified from the crystallographic orientation information. Raman spectrum results demonstrated that the residual stress in GaN was beneficially reduced in this growth mode.

Large Area Stress Distribution in Crystalline Materials Calculated from Lattice Deformation Identified by Electron Backscatter Diffraction

We report a method to obtain the stress of crystalline materials directly from lattice deformation by Hookes law. The lattice deformation was calculated using the crystallographic orientations obtained from electron backscatter diffraction (EBSD) technology. The stress distribution over a large area was obtained efficiently and accurately using this method. Wurtzite structure gallium nitride (GaN) crystal was used as the example of a hexagonal crystal system. With this method, the stress distribution of a GaN crystal was obtained. Raman spectroscopy was used to verify the stress distribution. The cause of the stress distribution found in the GaN crystal was discussed from theoretical analysis and EBSD data. Other properties related to lattice deformation, such as piezoelectricity, can also be analyzed by this novel approach based on EBSD data.

A novel porous substrate for the growth of high quality GaN crystals by HVPE

An original high temperature annealing (HTA) technology has been established to fabricate a porous substrate with a layer of inverted pyramid structures. The details about the formation of the porous substrate and the related mechanism were discussed. It was proved that the porous structures were formed through an etching-like mechanism assisted by SiO2 patterned masks. High-quality GaN crystals have been prepared using the as-prepared porous substrate. The growth experiments demonstrated that such porous substrates were mechanically fragile. The porous structures are suitable to be used as a release layer for the growth of GaN crystals with low stress and defect density.