Yasushi Iyechika

Recent Progress in Selective Area Growth and Epitaxial Lateral Overgrowth of III-Nitrides: Effects of Reactor Pressure in MOVPE Growth

Effects of reactor pressure on the epitaxial lateral overgrowth (ELO) via low pressure MOVPE have been studied in relation to the growth temperature. For the ELO GaN on SiO2 stripes along the 〈11-00〉 direction of the underlying GaN, by decreasing reactor pressures from 500 to 40 Torr or by increasing growth temperatures from 950 to 1050 °C, the (0001) surfaces become broad and the side walls are varied from inclined {112-2} surfaces to vertical {112-0} surfaces. For stripes along the 〈112-0〉 direction, the shapes of ELO GaN are independent of the reactor pressures and the growth temperatures. The mechanism of the morphological change is discussed based on the stability of the surface atoms. Typical ELO GaN layers with two-step growth are demonstrated and characterized in their crystalline properties.

Transmission Electron Microscopy Investigation of Dislocations in GaN Layer Grown by Facet-Controlled Epitaxial Lateral Overgrowth

We investigated the propagation of the threading dislocations in the GaN layer grown by facet-controlled epitaxial lateral overgrowth (FACELO). The mixed-type dislocations were bent toward the mask areas and they were terminated at the voids on the SiO2 masks. On the other hand, the pure edge dislocations were bent in the direction of the mask stripe. No dislocations originating from the GaN/sapphire interface propagated to the surface. As a result, it was confirmed that a large reduction of dislocation density was achieved. Therefore, FACELO seems to be a promising technique for the realization of a GaN wafer of low dislocation density.

Buried Tungsten Metal Structure Fabricated by Epitaxial-Lateral-Overgrown GaN via Low-Pressure Metalorganic Vapor Phase Epitaxy

A buried tungsten (W) structure with GaN is successfully obtained by epitaxial lateral overgrowth (ELO) technique via low-pressure metalorganic vapor phase epitaxy (LP-MOVPE). The selectivity of GaN growth on the window regions is excellent. GaN with a striped W metal pattern is easily decomposed above the low temperature of 500°C by the catalytic effect of W. It is difficult to bury the W mask because severe damage occurs in the GaN epilayer under the mask. It is found that employing an underlying AlGaN/GaN heterostructure with a narrow W stripe mask width (L/S=2/2 µm) leads the epilayer to be free from damage, resulting in a good W buried structure.

Carrier-gas dependence of ELO GaN grown by hydride VPE

The morphological change of epitaxial lateral overgrowth (ELO) GaN during the hydride vapor phase epitaxy process has been investigated from the viewpoint of the effect of carrier-gas species. The lateral growth rates for using the carrier gas of only N 2 are higher than those for using the H 2 + N 2 mixed carrier gas. In the case of SiO 2 stripe masks along the direction of the underlying GaN, {1 1 2 2} and {3 3 6 2} facets are observed in the sidewalls of ELO GaN for using the H 2 + N 2 mixed carrier gas. Results of cathodoluminescence and X-ray diffraction measurements suggest the crystal quality of the ELO GaN grown in the H 2 + N 2 mixed carrier gas is better than that in the N 2 carrier gas.

Crystal Orientation Fluctuation of Epitaxial-Lateral-Overgrown GaN with W Mask and SiO2 Mask Observed by Transmission Electron Diffraction and X-Ray Rocking Curves

Fluctuation of the crystal orientation of epitaxial-lateral-overgrown (ELO) GaN with a tungsten (W) mask is compared to that with a SiO2 mask by means of selected-area transmission electron diffraction (TED) and X-ray rocking curves (XRCs). XRCs of (0004) reflection exhibited three peaks for ELO-GaN on a SiO2 mask, and were confirmed to originate from domains of different c-axis orientation by TED. On the other hand, fluctuation of the c-axis was not observed for ELO-GaN with a W mask. These results indicate that the crystalline quality of ELO-GaN with a W mask is better than that with a SiO2 mask.

High Quality GaN Grown by Facet‐Controlled ELO (FACELO) Technique

We fabricated high-quality GaN by low-pressure metalorganic vapour phase epitaxy (LP-MOVPE) using Facet Controlled Epitaxial Lateral Overgrowth (FACELO) technique. Density and distribution of threading dislocations (TDs) in the GaN epitaxial layer strongly depended on the ELO mask and window widths, and were related to facet structures during the ELO process. It was found that tilt and twist of c-axis in the FACELO GaN were very small. Low temperature cathodoluminescence (CL) spectra of the FACELO GaN with low TD density exhibited excellent crystallographic quality.

Investigation of Thermal Annealing Process of GaN Layer on Sapphire by Molecular Dynamics

Molecular dynamics (MD) calculations have been performed to simulate the heteroepitaxial growth process of GaN on sapphire (0001) substrates and the thermal stability of the low-temperature GaN layer during the annealing process has been investigated. The MD simulations indicated that the surface morphology and the crystallinity of GaN thin layers grown on sapphire at low temperature of 800 K was amorphouslike and very rough. The surface atomic morphology of the low-temperature GaN layer was improved after the thermal annealing MD simulations at 1100 and 1200 K and showed atomically hexagonal structures. The crystallinity of the GaN epitaxial layer on the sapphire (0001) substrate is found to be strongly dependent on the temperature of annealing.

High Quality GaN Grown by Raised-Pressure HVPE

Effects of reactor pressure on growth of GaN by hydride vapour phase epitaxy (HVPE) have been investigated. For growth at 0.4 atm for 1 h, a large number of cracks were observed in GaN layers with the thickness about 30 μm and the FWHM values in (0004) and (10-10) X-ray rocking curves (XRCs) were larger than 350 arcsec. For growth at 1.2 atm for 1 h, GaN layers with the thickness about 60 μm were free from cracks and the FWHMs in both (0004) and (10-10) XRC were dramatically reduced to narrower than 200 arcsec.

Distribution of Threading Dislocations in Epitaxial Lateral Overgrowth GaN by Hydride Vapor-Phase Epitaxy Using Mixed Carrier Gas of H2 and N2

Distribution of threading dislocations (TDs) in epitaxial lateral overgrowth (ELO) GaN by hydride vapor-phase epitaxy (HVPE) has been investigated. Two types of carrier gas, i.e., N2 and H2+N2, were used in the HVPE in order to change the facet structure of the ELO layer. Results of cathodoluminescence (CL) mapping and HCl vapor-phase etching revealed lower TD density in the ELO layer grown using the H2+N2 mixed carrier gas than that using the N2 carrier gas.

Computational Studies on GaN Surface Polarity and InN/GaN Heterostructures by Density Functional Theory and Molecular Dynamics

We have performed periodic density functional calculations and molecular dynamics (MD) simulations to investigate the reconstructions of the GaN (0001) surface and the heteroepitaxial growth process of InN thin films on the GaN surface. Grown GaN planes have a polar configuration. Surface energy calculations predict that the reconstruction of the N-terminated GaN (0001) surface is energetically more favorable than that of the Ga-terminated surface. MD results suggest that the growth of InN thin films on Ga- and N-terminated surfaces is different. On the N-terminated surface, the surface morphology of the grown InN layer is three-dimensional and rough. On the other hand, on the Ga-terminated surface, it is observed that the InN molecules have adequate migration mobility for growth and this suggests that the growth follows the two-dimensional growth mode.