Strain-stress study of AlxGa1-xN/AlN heterostructures on c-plane sapphire and related optical properties

TL;DR

This study systematically investigates the strain, stress, and optical properties of AlxGa1-xN/AlN heterostructures grown on sapphire, revealing how composition affects crystal quality, strain distribution, and bandgap, with implications for optoelectronic applications.

Contribution

It provides a detailed analysis of strain and optical properties across AlxGa1-xN compositions using advanced X-ray and Raman techniques, highlighting the dependence on Al content.

Findings

Compressive strain on c-plane and tensile strain on a-plane vary with Al composition.

Lattice mismatch decreases as Al content increases, affecting crystal quality.

Bandgap is slightly lower than predicted due to tensile strain effects.

Abstract

This work presents a systematic study of stress and strain of AlxGa1-xN/AlN with composition ranging from GaN to AlN, grown on a c-plane sapphire by metal-organic chemical vapor deposition, using synchrotron radiation high-resolution X-ray diffraction and reciprocal space mapping. The c-plane of the AlxGa1-xN epitaxial layers exhibits compressive strain, while the a-plane exhibits tensile strain. The biaxial stress and strain are found to increase with increasing Al composition, although the lattice mismatch between the AlxGa1-xN and the buffer layer AlN gets smaller. A reduction in the lateral coherence lengths and an increase in the edge and screw dislocations are seen as the AlxGa1-xN composition is varied from GaN to AlN, exhibiting a clear dependence of the crystal properties of AlxGa1-xN on the Al content. The bandgap of the epitaxial layers is slightly lower than predicted value due to a larger tensile strain effect on the a-axis compared to the compressive strain on the c-axis. Raman characteristics of the AlxGa1-xN samples exhibit a shift in the phonon peaks with the Al composition. The effect of strain is also discussed on the optical phonon energies of the epitaxial layers. The techniques discussed here can be used to study other similar materials.