Control of Initial Nucleation by Reducing the V/III Ratio during the Early Stages of GaN Growth

Abstract

We investigated, by atomic force microscopy (AFM), the effect of the V/III ratio on the surface morphologies of GaN epilayers during the early stage of high-temperature (HT) GaN growth. The epilayers were grown on sapphire substrates by low-pressure metal-organic chemical vapor deposition (LP-MOCVD). We observed that initial HT-GaN islands in the early stage of HT-GaN growth were larger when a lower V/III ratio was used during the low-temperature (LT) GaN-buffer-layer growth or during the early stage of HT-GaN growth (the HT-GaN-buffer growth). These larger islands were attributed to a reduced nuclei density when a lower V/III ratio was used during the LT-GaN-buffer-layer growth, or to an increased growth aspect ratio (the ratio of vertical and lateral growth rates) when a lower V/III ratio was used in the early stage of HT-GaN growth. The larger islands led to fewer defects when they coalesced, which demonstrated that the quality of GaN epilayers grown on sapphire substrates by LP-MOCVD can be improved by using a low V/III ratio during the LT or HT GaN-buffer-layer growth.Introduction A high density of threading dislocations, on the order of 10 10 cm ± ±2 , remains in device-quality GaN epilayers grown on sapphire substrates by metal-organic chemical vapor deposition (MOCVD) because of the large differences between the lattice parameters and thermal expansion coefficients of the GaN and the sapphire substrate [1]. The dislocation density must be reduced to improve the optical and electrical properties of GaN-based materials and, thus, improve the device performance. Several groups have recently demonstrated that the use of a three-dimensional (3D) growth mode in the early stage of GaN growth plays an important role in the reduction of the defect density [2 to 4]. Furthermore, a very high V/III ratio is generally required to grow high-quality GaN epilayers on sapphire substrates by MOCVD [5,6]. However, we found that the crystalline quality of GaN epilayers grown on sapphire substrates by low-pressure MOCVD (LP-MOCVD) can be effectively improved by using a low V/III ratio during the LT-GaN-buffer growth or the initial stage of HT-GaN growth (the HTGaN-buffer-layer growth) [7,8]. We attributed this improvement to enhanced island growth (increased island size) in the early stage of the HT-GaN growth; this enhanced growth was due to the lower V/III ratio and led to fewer crystalline defects [2 to 4]. Our in situ optical reflectance monitoring supported this finding: we observed that the transition from the three-dimensional (3D) to the 2D growth mode (the recovery of the reflectance) was slower with a lower V/III ratio [7,8]. To gain further insights into the effect of the V/III ratio on the GaN growth process, we have since investigated the morphology of the GaN epilayers in the first stages of HT-GaN growth since the morphology is closely related to the reflectance behavior.

Understanding GaN nucleation layer evolution on sapphire

“…Optical reflectance has previously been used to investigate GaN film evolution [30,[38][39][40][41][42][43][44]. In these optical reflectance studies, particular attention has been paid to the NL evolution [30,38,41,44] and the 3D roughening to 2D smoothing transition at high T [38][39][40]42,43].…”

“…In these optical reflectance studies, particular attention has been paid to the NL evolution [30,38,41,44] and the 3D roughening to 2D smoothing transition at high T [38][39][40]42,43]. From these studies and accompanying AFM images, correlations have been established between the reflectance signal and the GaN morphology.…”

Realising epitaxial growth of GaN on (001) diamond

“…Alteration of the V/III ratio is reported to give good control on the early stages of GaN growth 24 and has been shown to greatly influence the characteristics of the grown epilayer. 25 Because of the variation in lateral growth speed with V/III ratio, it is also commonly employed as a steering parameter in lateral epitaxial overgrowth.…”

Microstructural evolution in m-plane GaN growth on m-plane SiC

“…Such a morphological variation is related to a gradient in V/III ratio due to the gas-phase diffusion of Ga source. 10 Two consequences of such a decreasing V/III ratio toward the edge are ͑i͒ a reduction in nucleation probability toward the edge 11 and ͑ii͒ a change in the shape of the island. 12 When properly interpreted, the spatial variation of the morphology in fact renders a panoramic record regarding the coalescence dynamics.…”