Haruo Sunakawa

Thick GaN Epitaxial Growth with Low Dislocation Density by Hydride Vapor Phase Epitaxy

Thick GaN layers were grown by hydride vapor phase epitaxy (HVPE) with the aim of using these layers as a homoepitaxial substrate to improve device quality of laser diodes or light emitting diodes. HVPE is very useful for thick layer growth since the growth rate can reach from several ten up to one hundred micron per hour. In this experiment, the growth began as selective growth through openings formed in a SiO2 mask. Facets consisting of {1101} planes were formed in the early stage and a continuous film developed from the coalescence of these facets on the SiO2 mask. As a result, GaN layers with a dislocation density as low as 6×107 cm-2 were grown on 2-inch-diameter sapphire wafers. These GaN layers were crack-free and had mirror-like surface.

DEFECT STRUCTURE IN SELECTIVELY GROWN GAN FILMS WITH LOW THREADING DISLOCATION DENSITY

We have characterized by transmission electron microscopy (TEM) defect structures in GaN films grown selectively in hydride vapor-phase epitaxy (HVPE). In this experiment, growth was achieved on SiO2-stripe-patterned GaN layers that had been grown by metalorganic vapor-phase epitaxy (MOVPE) on sapphire substrates. Cross-sectional TEM revealed unambiguously that most of the dislocations, which originated from threading dislocations vertically aligned in the MOVPE-grown layer, propagated laterally around the SiO2 mask in the HVPE-grown film before the film thickness amounted to about 5 μm. This change of the propagation direction prevented the dislocations from crossing the film to the surface region and thus principally led to a drastic reduction in the threading dislocation density in thicker films.

Transmission electron microscopy of defects in GaN films formed by epitaxial lateral overgrowth

We have investigated by transmission electron microscopy (TEM) defect morphology and structure in GaN films formed using an epitaxial lateral overgrowth (ELO) technique on SiO2-mask/window-stripe-patterned GaN layers in hydride vapor-phase epitaxy. In this experiment, the regions overgrown on the SiO2 masks were thoroughly examined. Cross-sectional TEM clearly revealed characteristic defects along the [0001] direction in the overgrown region, which consisted of arrays of dislocations running along the mask stripe direction. These defects caused crystallographic tilting in that region near the mask with respect to the other region grown from the window area. We also observed, at the coalesced site on the mask, vertical repropagation of dislocations that had propagated laterally during ELO. The origin of the observed defects and their influence on the residual dislocation distribution near the film surface are discussed.

Preparation of freestanding GaN wafers by hydride vapor phase epitaxy with void-assisted separation

We have developed a novel technique for preparing large-scale freestanding GaN wafers. Hydride vapor phase epitaxy (HVPE) growth of thick GaN layer was performed on a GaN template with a thin TiN film on the top. After the cooling process of the HVPE growth, the thick GaN layer was easily separated from the template by the assistance of many voids generated around the TiN film. As a result, a freestanding GaN wafer was obtained. The wafer obtained had a diameter of 45 mm, and a mirror-like surface. The-full-width-at-half-maximum (FWHM) of (0002) and (1010) peaks in the X-ray rocking curve profile were 60 and 92 arcsec, respectively. The dislocation density was evaluated at 5×106 cm-3 by etch pit density measurement.

GaAs Atomic Layer Epitaxy by Hydride VPE

GaAs atomic layer epitaxy is demonstrated in hydride vapor phase epitaxy by a dual-grown-chamber method. GaAs substrate is alternatively exposed to GaCl and As4 gases by transferring the substrate between two chambers. The growth rate was examined for differing growth conditions and was found to depend only on substrate transfer cycles. Furthermore, the present method can be applied to selective growth.

Effects of oxygen and water vapour introduction during MOCVD growth of GaAlAs

Abstract The effects of oxygen (O 2 ) and water vapour (H 2 O) introduction on the properties of MOCVD grown GaAlAs have been investigated. It was found that the role of O 2 in GaAlAs MOCVD is clearly different from that of H 2 O. O 2 introduction reduces the carrier concentration and the photoluminescence efficiency, and degrades surface morphology, while the Al composition in the grown layer and the growth rate are kept constant. On the other hand, H 2 O introduction causes reduction of the Al composition and the growth rate, while the other properties are little influenced. The results could be interpreted by the different reactions of trimethyl aluminum with the oxygen species of O 2 and H 2 O in the vapour phase prior to the growth region.

Room-Temperature Continuous-Wave Operation of InGaN Multi-Quantum-Well Laser Diodes Grown on an n-GaN Substrate with a Backside n-Contact

Continuous-wave operation at room-temperature has been demonstrated for InGaN multi-quantum-well (MQW) laser diodes (LDs) grown on low-dislocation-density n-GaN substrates with a backside n-contact. The current, current density and voltage at the lasing threshold were 144 mA, 10.9 kA/cm2 and 10.5 V, respectively, for a 3 µm wide ridge-geometry diode with high-reflection dielectric coated mirrors. Single-transverse-mode emission was observed in the far-field pattern of the LDs and the beam full width at half power in the parallel and perpendicular directions was 6° and 25°, respectively.

High-quality InGaN MQW on low-dislocation-density GaN substrate grown by hydride vapor-phase epitaxy

Abstract A low-dislocation-density thick GaN layer was successfully grown using selective-area HVPE growth combined with epitaxial lateral overgrowth. The InGaN MQWs fabricated on this thick GaN layer showed superior optical properties compared with that on a sapphire substrate. Mg diffusion, induced by threading dislocations, was greatly suppressed for the LEDs on the HVPE-grown GaN layer. The results clearly indicate that the HVPE-grown GaN substrate will be useful for achieving high-performance light-emitting devices.

Self-organized propagation of dislocations in GaN films during epitaxial lateral overgrowth

Dislocation propagation and defect evolution in GaN films formed by epitaxial lateral overgrowth (ELO) are examined by transmission electron microscopy. A novel effect that induces self-organized propagation of preexisting dislocations in ELO films is evaluated. This propagation forms dislocations into bundle structures along the stripes of masks used for ELO. The dislocation bundling gives rise to crystallographic tilting in the overgrown region on the mask and leads to a total reduction of threading dislocation density in the film.

Fabrication of Freestanding GaN Wafers by Hydride Vapor-Phase Epitaxy with Void-Assisted Separation

A novel technique for fabricating large-diameter freestanding GaN wafers has been developed. This technique uses a porous GaN template with a TiN nano-net on top as the starting substrate for hydride vapor-phase epitaxy (HVPE) growth. A mechanically weak layer containing numerous small voids is formed between the template and the thick GaN layer during HVPE growth. The thermal stress due to the difference in thermal expansion coefficients between the GaN and sapphire results in self-separation during the cooling process after HVPE growth. As a result, freestanding GaN wafers with a diameter of 45 mm are obtained. These wafers are crack-free and have a mirror-like surface. The full widths at half maximum of GaN (0002) and (1010) peaks in the X-ray rocking curve profile are narrow, i.e. 60 and 92 arcsec, respectively. The dislocation density is low, i.e. 5 × 10 6 cm -2 .