Stress dependence of the near-band-gap cathodoluminescence spectrum of GaN determined by spatially resolved indentation method

Abstract

A microscopic procedure has been proposed for evaluating the stress dependence of the (room-temperature) cathodoluminescence (CL) excitonic band emitted from the (0001) crystallographic plane of GaN in a field-emission-gun scanning electron microscope. The room-temperature near-band-gap emission (generally referred to as the excitonic band) mainly consisted of a band arising from free exciton (FX). However, an asymmetric morphology was found for the band, which thus needed to be deconvoluted into the main FX band and a shoulder. The spectral location at intensity maximum of the overall excitonic band under stress-free conditions was observed at room temperature at around 365nm. Experimentally measured spectral shifts were precisely retrieved nearby the tip of a Vickers indentation microcrack, while CL intensity probe response functions were collected at different acceleration voltages at a sharp interface between a GaN film and its sapphire substrate. Based on these assessments, the magnitude of the piezospectroscopic coefficient (i.e., the spectral shift rate versus the trace of a biaxial stress tensor) Π=1.35±0.01nm∕GPa of the excitonic (cumulative) band of GaN could be evaluated. This study not only emphasizes the importance of microscopic piezospectroscopic calibration procedures for precise residual stress assessments in GaN-based devices, but also the need of deconvoluting the electron probe for minimizing the error involved with its finite size.