Distorted wurtzite unit cells: Determination of lattice parameters of non-polar a-plane AlGaN and estimation of solid phase Al content

TL;DR

This paper presents a method to accurately determine lattice parameters and solid phase Al content in non-polar a-plane AlGaN, accounting for anisotropic strain-induced distortions in the wurtzite unit cell.

Contribution

It introduces a simple procedure to measure lattice parameters from high-resolution X-ray diffraction and derives an expression for solid phase Al content considering anisotropic strain effects.

Findings

Lattice distortion in non-polar nitrides is orthorhombic.

The proposed method accurately measures lattice parameters.

The approach estimates solid phase Al content in a-plane AlGaN.

Abstract

Unlike c-plane nitrides, ``non-polar" nitrides grown in e.g. the a-plane or m-plane orientation encounter anisotropic in-plane strain due to the anisotropy in the lattice and thermal mismatch with the substrate or buffer layer. Such anisotropic strain results in a distortion of the wurtzite unit cell and creates difficulty in accurate determination of lattice parameters and solid phase group-III content (x_solid) in ternary alloys. In this paper we show that the lattice distortion is orthorhombic, and outline a relatively simple procedure for measurement of lattice parameters of non-polar group III-nitrides epilayers from high resolution x-ray diffraction measurements. We derive an approximate expression for x_solid taking into account the anisotropic strain. We illustrate this using data for a-plane AlGaN, where we measure the lattice parameters and estimate the solid phase Al content, and also show that this method is applicable for m-plane structures as well.