Katsushi Nishino

DIRECT EVIDENCE THAT DISLOCATIONS ARE NON-RADIATIVE RECOMBINATION CENTERS IN GAN

Plan-view transmission electron microscopy (TEM) and cathodoluminescence (CL) images were taken for the same sample at exactly the same location in n-type GaN grown on sapphire substrate by metalorganic chemical vapor deposition (MOCVD). There was a clear one to one correspondence between the dark spots observed in CL images and the dislocations in TEM foils, indicating that the dislocations are non-radiative recombination centers. The hole diffusion length in n-type GaN was estimated to be neighboring 50 nm by comparing the diameters of the dark spots in thick samples used for CL and samples that were thinned for TEM observation. The efficiency of light emission is high as long as the minority carrier diffusion length is shorter than the dislocation spacing.

Reduction of Double Positioning Twinning in 3C-SiC Grown on α-SiC Substrates

Crystal growth of cubic silicon carbide (3C-SiC) on α-SiC (6H- and 15R-SiC) substrates was carried out by chemical vapor deposition. 3C-SiC (111) can be epitaxially grown on 6H- and 15R-SiC (0001) substrates. Due to the peculiar stacking sequence of α-SiC, double positioning boundaries (DPBs) appear in the 3C-SiC (111) layers. The layer on 15R-SiC has far fewer DPBs than that on 6H-SiC. Successive etching of a thick grown layer and successive observation of a growing surface revealed that the DPBs decreased anisotropically as crystal growth proceeded. Facets formed along DPBs were analyzed by atomic force microscopy. The angles between the facets and the grown surface (111) varied with the crystallographic orientation of DPBs. DPBs may decrease due to the lateral growth from the facets. The difference in the velocities of the anisotropic decrease in DPBs was discussed on the basis of the number of dangling bonds on the facets.

CONFIGURATION OF DISLOCATIONS IN LATERAL OVERGROWTH GAN FILMS

The dislocation distribution and emission profile of sublimation lateral overgrowth GaN and metalorganic chemical vapor deposition films have been studied using transmission electron microscopy and cathodoluminescence. A close relationship between the emission profile and the dislocation distribution has been observed. The results show that the dislocations not only affect the band edge emission, but also the yellow emission. It is observed that the dislocations propagate laterally in the overgrowth region. The mechanism of the change in the propagation direction of dislocations has been discussed.

Photoluminescence of Homoepitaxial 3C-SiC on Sublimation-Grown 3C-SiC Substrates

Bulk 3C-SiC growth by a sublimation method was carried out. The grown layer was used as a substrate for chemical vapor deposition, that is, homoepitaxial growth of 3C-SiC was carried out for the first time. The substrate exhibited some impurity-related photoluminescence peaks and no exciton-related peak was observed. The epilayer had some sharp exciton-related peaks near the band edge, which showed high crystallinity of the epilayer. Au Schottky barrier diodes were fabricated on the epilayer, and the diode showed good characteristics. The quality of 3C-SiC was improved by homoepitaxial growth.

Fabrication of High-Output-Power AlGaN/GaN-Based UV-Light-Emitting Diode Using a Ga Droplet Layer

We report a new method of increasing the output power of an ultraviolet light-emitting diode (UV-LED) based on an AlGaN/GaN single quantum well (SQW). In this method, a thin Ga droplet layer is intentionally grown before the growth of an AlGaN/GaN SQW active layer. The Ga droplet layer causes a spatial and compositional fluctuation on the SQW active layer, which induces exciton localization in the potential minima. The LEDs fabricated with the Ga droplet layer show an emission peak of 353 nm and a higher optical output power than those of the same structure but without the Ga droplet layer.

Nucleation Control in the Growth of Bulk GaN by Sublimation Method

Nucleation of GaN crystallites by sublimation method on sapphire (0001), scratched sapphire (0001), SiO2, Si (111), and metalorganic chemical vapor deposition(MOCVD)-GaN/sapphire (0001) was investigated. The density of the nuclei on SiO2 and Si (111) was estimated to be 6 × 103/cm2. On the other hand, a continuous film rather than discrete crystallites was obtained on both the MOCVD-GaN and the scratched sapphire. Growth nucleation control was performed by partly covering the MOCVD-GaN or scratched sapphire (0001) with SiO2. As a result, hexagonal columns about 200 µm in diameter and 200 µm high were selectively and uniformly grown at the window sites. The diameter of each hexagonal column was much larger than the width of each window. This method can be used for device processing utilizing the crystallites.

Effect of Oxygen on the Activation of Mg Acceptor in GaN Epilayers Grown by Metalorganic Chemical Vapor Deposition

The effect of oxygen mixed in nitrogen on p-type activation in Mg-doped GaN epilayers grown by metalorganic chemical vapor deposition (MOCVD) was investigated. The samples annealed in N2/O2 (1%) ambient exhibited the best electrical properties with respect to hole concentration. SIMS data suggested that oxygen reacted with hydrogen present in the Mg-doped GaN samples during the thermal annealing process, thereby enhancing the activation of Mg acceptors.

Characterization of bulk GaN grown by sublimation technique

We have characterized a bulk GaN grown by sublimation technique by means of the high resolution X-ray diffraction measurements, the secondary ion mass spectrometry measurements, the Raman spectroscopy measurements and the resistivity measurements. The results indicated that a bulk GaN was of relatively high purity with a very high crystal perfection.

Surface Pretreatment of Bulk GaN for Homoepitaxial Growth by Metalorganic Chemical Vapor Deposition

Homoepitaxial growth of GaN on bulk GaN prepared by the sublimation method was performed by metalorganic chemical vapor deposition (MOCVD). Two kinds of bulk substrates were used for homoepitaxial growth: (i) a polished bulk GaN C-face, selectively grown on MOCVD-GaN on a sapphire (0001) substrate which was partly covered by SiO2 (?quasi-bulk? GaN) and (ii) a bulk GaN spontaneously nucleated on the source powder used in the sublimation method (?free-standing? bulk GaN). A smooth surface and strong CL band-edge emission from the homoepitaxial layer were obtained when proper surface pretreatment of the substrate, such as H3PO4 etching, RIE and photoassisted wet chemical etching (PAW), was adapted before loading into the MOCVD reactor. The CL intensity of the band-edge emission was stronger and the CL image more uniform for homoepitaxial GaN compared to that of heteroepitaxial GaN on sapphire. The dislocation density of the homoepitaxial layer was also decreased after proper surface treatment of the substrate.

REACTIVE ION ETCHING OF GAN AND ALXGA1-XN USING CL2/CH4/AR PLASMA

Reactive ion etching (RIE) of p-GaN and p-AlxGa1-xN has been investigated using Cl2/CH4/Ar plasmas in the conventional RIE technique. It has been found that variation of CH4 percentage in gas mixtures leads to changes in the etch rate of both p-GaN and p-Al0.15Ga0.85N. The maximum etch rate for p-GaN has been found to be 257 nm/min in Cl2/Ar plasma containing 2.5% CH4 and the value is 170 nm/min for p-Al0.15Ga0.85N in Cl2/Ar plasma containing 10% CH4. With increasing rf power, the etch rates for p-GaN and p-Al0.15Ga0.85N reach as high as 433 nm/min and 255 nm/min respectively. Under optimum conditions of gas composition, radiofrequency (rf) power, and temperature, an anisotropic and smooth etch profile is obtained. The etched surface exhibits small roughness.