Composition Inhomogeneity in Nonpolar (101̅0) and Semipolar (202̅1) InAlN Layers Grown by Plasma-Assisted Molecular Beam Epitaxy

Abstract

In this study, we report the indium incorporation efficiency and structural quality of nonpolar (101̅ 0) and semipolar (202̅ 1) InAlN layers grown by plasma-assisted molecular beam epitaxy. The indium content is found to depend on surface orientation, and it is 5.7% for the nonpolar m-plane, 16.2% for semipolar, and 17.4% for c-plane (0001); this results in the indium incorporation efficiency trend (101̅ 0) ≪ (202̅ 1) < (0001). One-dimensional strain relaxation is observed for the m-plane InAlN layer, while the semipolar and c-plane InAlN layers are strained to GaN. The structural quality of the m-plane and semipolar InAlN layers is assessed by scanning transmission electron microscopy (STEM) with particular focus on the composition homogeneity. Nonpolar and semipolar InAlN cross sections observed by STEM along the [12̅ 10] direction exhibited a columnar structure along the growth direction. No composition modulation is visible along the [0001] and [1̅ 014] directions for the m-plane and semipolar InAlN layers, respectively. The low indium content of the m-plane InAlN layer is attributed to the strain-induced compositional pulling effect predicted theoretically for this surface orientation. This effect is believed to be also responsible for the increase of incorporation efficiency in the top part of the layer after its relaxation by cracking.