The epitaxial growth of an AlN layer on a Si(111) substrate at room temperature by DC magnetron sputtering was investigated. The predeposition of a 5-nm-thick Al layer on the Si substrate before the AlN deposition was found to be crucial for the epitaxial growth of the AlN layer. The orientation relationships of AlN/Al/Si were observed to be AlN ∥ Al ∥ Si and AlN ∥ Al[011] ∥ Si, indicating the epitaxial growth of the AlN layer on the Si(111) substrate. This epitaxial growth of the AlN layer was attributed to the smaller lattice mismatches between AlN and Al and AlN and Al[011] than that between AlN and Si.

We demonstrated AlGaN-based deep ultraviolet light-emitting diodes (DUV LEDs) with periodic air-voids-incorporated nanoscale patterns enabled by nanosphere lithography and epitaxial lateral overgrowth (ELO) on a 4-in. sapphire substrate. The nanoscale ELO improved the crystal quality of overgrown epitaxial layers at a relatively low growth temperature of 1050 °C and at small coalescence thickness less than 2 μm. The light output power of the DUV LED was enhanced significantly by 67% at an injection current of 20 mA. We attribute such a remarkable enhancement to the formation of embedded periodic air voids which cause simultaneous improvements in the crystal quality of epitaxial layers by ELO and light extraction efficiency enabled by breaking the predominant in-plane guided propagation of DUV photons.

An ultra-thin (26nm) sapphire (Al2O3) membrane was used as a compliant substrate for the growth of high quality GaN. The density of misfit dislocations per unit length at the interface between the GaN layer and the sapphire membrane was reduced by 28% compared to GaN on the conventional sapphire substrate. Threading dislocation density in GaN on the sapphire membrane was measured to be 2.4×108/cm2, which is lower than that for GaN on the conventional sapphire substrate (3.2×108/cm2). XRD and micro-Raman results verifed that the residual stress in GaN on the sapphire membrane was as low as 0.02GPa due to stress absorption by the ultra-thin compliant sapphire membrane.

Nanoheteroepitaxy (NHE) of GaN on an AlN/Si(111) nanorod structure was investigated by metal-organic chemical vapor deposition. Silica nanosphere lithography was employed to fabricate a periodic hexagonal nanorod array with a narrow gap of 30nm between the nanorods. We were successful in obtaining a fully coalesced GaN film on the AlN/Si(111) nanorod structure. Transmission electron microscopy revealed that threading dislocation (TD) bending and termination by stacking faults occurred near the interface between GaN and the AlN/Si(111) nanorods, resulting in the reduction of TD density for the NHE GaN layer. The full width at half-maximum of the X-ray rocking curve for (102) plane of the NHE GaN was found to decrease down to 728arcsec from 1005arcsec for the GaN layer on a planar AlN/Si(111) substrate, indicating that the crystalline quality of the NHE GaN was improved. Also, micro-Raman measurement showed that tensile stress in the NHE GaN layer was reduced significantly as much as 70% by introducing air voids between the nanorods.

A new method of growing a semi-insulating GaN layer for high electron mobility transistor (HEMT) structure by eliminating a degenerate layer located at the GaN/sapphire interface is proposed. In the process, during the temperature ramp-up after the growth of a low temperature GaN buffer layer, tri-methylgallium (TMGa) was flowed into the reactor together with ammonia, leading to the growth of an additional GaN layer of high carbon concentration. We confirmed that the introduction of TMGa induced high carbon concentration in the degenerate layer. The incorporated carbon formed deep acceptors, C-N, compensating donors such as O-N, resulting in the elimination of the degenerate layer. A HEMT device made on the sample grown by the new growth process shows a good pinch-off characteristic and high off-state breakdown voltage over 800 V at a gate voltage of -4 V, indicating the new scheme is effective to grow a high quality semi-insulating GaN layer. (C) 2015 Elsevier B.V. All rights reserved.

In this study, a nano-columnar low-temperature (LT) GaN buffer was used to reduce the wafer bowing of a GaN layer grown on a sapphire substrate. A significant reduction in the extent of wafer bowing was observed for the GaN layer for the preserved nano-columnar LT GaN layer when compared with the conventional GaN layer. These results suggest that the preserved nano-columnar structure helped relax the GaN layer strain energy associated with thermal expansion mismatch. The flow of TMGa during the temperature ramp-up from LT to high-temperature was found to be an important process parameter to preserving the nano-columnar structure of LT GaN, resulting in less bowed GaN on sapphire.

Low-temperature (LT) GaN layers with a columnar microstructure were grown on InN layers with various thicknesses. Subsequently, thick, high-temperature GaN layers were grown on the columnar LT GaN/InN/sapphire template. The microstructure of the InN interlayer and the LT GaN columnar structure were investigated by transmission electron microscopy. From the X-ray diffraction analysis, it was shown that the InN layer was decomposed at the initial stage of LT GaN growth. The depth profile of Ga and In atoms from the GaN/InN/sapphire structure obtained by secondary ion mass spectrometry showed that the distribution of In atoms was confined only within the columnar LT GaN and InN/GaN layer interface. Wafer bowing was measured by optical methods and a significant reduction in the extent of wafer bowing was observed for the samples with the columnar LT GaN layer compared with the GaN layer grown on conventional LT GaN buffer layers. It is suggested that the epitaxial growth of a columnar LT GaN layer induced by a polycrystalline InN interlayer helped relax the GaN layer strain energy associated with thermal expansion mismatch, resulting in a less bowed GaN epitaxial layer on the sapphire substrate.

A new growth method has been proposed and verified to be effective for improving the crystalline quality of GaN epilayers grown by radio-frequency plasma-assisted molecular beam epitaxy (RF-MBE) at low temperature. In this new method, an indium (In) layer is predeposited before the main growth of low-temperature (LT) GaN. The improved quality for LT GaN probably comes from a selective growth process between In and gallium (Ga) in predeposited In coverage, which is similar to the process of several-monolayer-level liquid-phase epitaxy. This method may enable the extension of the application field of nitride semiconductors, for example, to solar cells.