AlInN alloys are promising as cladding layers in GaN-based visible laser diodes (LDs) because they show a large index contrast to GaN or InGaN in whole visible wavelengths at an alloy composition lattice-matched to GaN. To consider the application to cladding layers in LDs, a sufficiently-thick film with a smooth surface is necessary. In this study, therefore, we grew 300-nm-thick AlInN films with various alloy compositions on a c-plane GaN/sapphire template or a free-standing (FS) GaN substrate by metalorganic chemical vapor deposition (MOCVD). The results showed that no lattice relaxation occurred for samples with InN molar fractions from 0.144 to 0.197, and the InAlN films with low InN molar fractions showed a relative smooth surface. However, it turned into a granular surface morphology resulting from a columnar polycrystalline structure when the InN molar fraction exceeded a compositional boundary of in-plane lattice matching. Eventually, it was confirmed that epitaxial AlInN films with a good crystal quality and smooth surface roughness were grown at alloy compositions almost perfectly lattice-matched to GaN/sapphire and FS-GaN. As for the smooth-surface AlInN single-layers, the optical constants as well as energy bandgaps were determined. On the day in the conference, we would like to present new proposals for applications of the high-quality AlInN films to components in devices other than cladding layers in LDs.
An AlGaN/GaN HEMT on Si has received significant attention due to the availability of large sized Si substrate at low cost. The limiting factors for high quality GaN/Si are large lattice and thermal expansion-coefficient mismatches between GaN and Si, which lead to high dislocation densities, wafer bowing and crack formation. Therefore, it is imperative to grow high quality GaN/Si with minimum wafer bowing and without crack in order to improve the device performances.
The AlGaN/GaN HEMTs were grown on 8-inch Si substrates using MOCVD technique. The HEMT structure consisted of the high-temperature-grown AlN nucleation layer (HT-AlN NL), the HT-Al0.3Ga0.7N intermediate layer (HT-AlGaN IL), the AlGaN/AlN strained layer superlattice (SLS), the GaN layer and the Al0.26Ga0.74N barrier layer. The HT-AlN NL was effective in avoiding the reaction between Ga and Si, which resulted in the specular surface morphology. The characteristic of the HT-AlN NL affected the vertical breakdown characteristics. The wafer b owing can be minimized by use of SLS and GaN because of counter-balance of thermal and lattice mismatches between SLS and GaN.
The AlGaN/GaN HEMT exhibited a Hall mobility of 1730 cm2/vs, a sheet carrier density of 7.4x1012 cm-2 and the wafer bowing value of 42 um. The vertical voltage at 1 uA/mm was between 950 V and 1000 V across the wafer. The normally-off devices were fabricated by using gate-recess and MOS technology. The devices exhibited good dc characteristics with drain current maximum of 300 mA/mm, threshold voltage of +2.4 V and 3-terminal off-breakdown voltage of 1650 V.
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