Reduction of Defects in AlGaN Grown on Nanoscale-Patterned Sapphire Substrates by Hydride Vapor Phase Epitaxy

Abstract

In this study, a 3-μm-thick AlGaN film with an Al mole fraction of 10% was grown on a nanoscale-patterned sapphire substrate (NPSS) using hydride vapor phase epitaxy (HVPE). The growth mechanism, crystallization, and surface morphology of the epilayers were examined using X-ray diffraction, transmission electron microscopy (TEM), and scanning electron microscopy at various times in the growth process. The screw threading dislocation (TD) density of AlGaN-on-NPSS can improve to 1–2 × 109 cm−2, which is significantly lower than that of the sample grown on a conventional planar sapphire substrate (7 × 109 cm−2). TEM analysis indicated that these TDs do not subsequently propagate to the surface of the overgrown AlGaN layer, but bend or change directions in the region above the voids within the side faces of the patterned substrates, possibly because of the internal stress-relaxed morphologies of the AlGaN film. Hence, the laterally overgrown AlGaN films were obtained by HVPE, which can serve as a template for the growth of ultraviolet III-nitride optoelectronic devices.

Structural and Stress Properties of AlGaN Epilayers Grown on AlN-Nanopatterned Sapphire Templates by Hydride Vapor Phase Epitaxy

“…Figure 3 a–c shows the top-view SEM images of CSS, AlN/NPSS, and NPSS [ 17 ], respectively. It can be seen that the prepared NPSS with hole patterns in this work, and the fabrication process is described in the method section.…”

“…It can be seen that the prepared NPSS with hole patterns in this work, and the fabrication process is described in the method section. Figure 4 a–c shows the top-view SEM images of the AlGaN layer grown on CSS, AlN/NPSS, and NPSS templates [ 17 ], respectively. Because of the lattice mismatch between the AlGaN and the CSS’s rough surface, incomplete 3D island coalescence with a hexagonal structure was formed ( Figure 4 a).…”

“…The corresponding surface roughness of these AlGaN samples was examined by AFM using a scan area of 10 μm 2 . As shown in Figure 5 , the root mean square (RMS) values of the AlGaN/CSS, AlGaN/AlN/NPSS, and NPSS [ 17 ] were 79.1, 6.66, and 14.9, respectively. The large RMS value for the surface roughness of the AlGaN film grown on CSS (i.e., AlGaN/CSS) might be due to the large lattice mismatch between the film and the substrate.…”

“… Top-view SEM images of the surface morphologies of the AlGaN epilayers grown on the ( a ) CSS; ( b ) AlN/NPSS; and ( c ) NPSS [ 17 ]. …”

“…Epitaxial lateral overgrowth (ELOG) techniques on microstripe (or honeycomb) shape-patterned sapphires have shown a promising result in reducing the defect density of the AlGaN layer [ 14 , 15 , 16 ]. In addition, the uses of nanopatterned sapphire substrates (NPSSs) improve the crystalline quality of the AlGaN layer by ELOG [ 17 ]. Published research using in situ AlN buffer layer below the grown Al 0.45 Ga 0.55 N layer showed that it could not only enhance the crystallinity but also affect the surface morphology due to the misorientated crystallites [ 18 ].…”