The In-Plane-Two-Folders Symmetric a-Plane AlN Epitaxy on r-Plane Sapphire Substrate

Fabi Zhang,Zhiwen Liang,Wei Luo,Tai Li,Junjie Kang,Lijie Huang,Qi Wang,Jiakang Cao,Shangfeng Liu,Chenhui Zhang,Ye Yuan,Jin Zhang

doi:10.3390/sym14030573

Abstract

In the present work, a single-crystalline epitaxial nonpolar a-plane AlN film with in-plane two-folder symmetries was successfully achieved on an r-plane sapphire substrate, by combining physical vapor deposition and a high-temperature annealing technique. Moreover, by varying the AlN thickness, the evolution of crystalline quality and structure were systematically investigated using X-ray diffraction, Raman spectroscopy, and atomic force microscopy. The crystalline quality was much improved by the annealing treatment. Most importantly, when the thickness of AlN was increased up to 1000 nm, the AlN lattice was found to endure strong distortion along the out-of-plane direction, and the lattice showed an obvious expansion. The change of the surface morphology induced by high-temperature annealing was also tracked, and the morphology displayed structural anisotropy along the [11¯00] direction. Our results act as a crucial platform to better understand and employ the nonpolar AlN template; in particular, it is of importance for subsequent device fabrication.

Highlights

In the past decades, the development of GaN based light-emitting diode (LED) and laser devices has thoroughly revolutionized human luminescence history [1–5]
One of the significant reasons is the strong polarization induced by the quantum confined stark effect (QCSE), which causes the deviation of carrier wave function along the c-axis [6,7]: the spontaneous polarization along the [0001] direction in a conventional LED sets up an electric field that undesirably impedes the carrier recombination in the quantum well region [6–8]
Such a phenomenon gradually dominates upon increasing the c-axis polarization contribution, e.g., it is obviously enhanced when aluminum concentration is increased in an AlGaN compound [9–11]. This is detrimental to luminance devices with a high Al concentration, e.g., an LED or laser in the ultraviolet-C region (UVC band, wavelength < 280 nm) [10]

Summary

Introduction

The development of GaN based light-emitting diode (LED) and laser devices has thoroughly revolutionized human luminescence history [1–5]. One of the significant reasons is the strong polarization induced by the quantum confined stark effect (QCSE), which causes the deviation of carrier wave function along the c-axis [6,7]: the spontaneous polarization along the [0001] direction in a conventional LED sets up an electric field that undesirably impedes the carrier recombination in the quantum well region [6–8] Such a phenomenon gradually dominates upon increasing the c-axis polarization contribution, e.g., it is obviously enhanced when aluminum concentration is increased in an AlGaN compound [9–11]. By fully combining physical vapor deposition and a high temperature annealing technique, we successfully achieve a single-crystalline non-polarized a-plane AlN on a semi-polarized r-plane sapphire substrate. Afterwards, the as-grown AlN templates were annealed in a tube furnace at 1700 ◦C for 5 h, and the annealing ambience was nitrogen

X-ray Diffraction (XRD) Characterization

Findings

Result and Discussion