Raman tensor elements for wurtzitic GaN and their application to assess crystallographic orientation at film/substrate interfaces

Abstract

This study is aimed at establishing a method of polarized/confocal Raman spectroscopy capable of quantitatively assessing crystallographic orientation in wurtzitic GaN with a micron-scale resolution. First, Raman selection rules are explicitly put forward from a theoretical viewpoint in their complete form; then, experimentally retrieved intensities of the Raman signal as a function of Euler angles are fitted to the obtained theoretical dependencies in order to quantify a set of Raman tensor elements using experiments on known crystallographic planes of a wurtzitic GaN single-crystal. According to the above two procedures, a spectroscopic algorithm, incorporating the use of Raman tensor elements and Euler angles in tandem, becomes available for estimating unknown crystallographic orientations. As an application of the developed method, a confocal Raman probe was used to non-destructively unfold the relative orientation of a wurtzitic GaN epilayer with respect to (0001)-oriented sapphire substrate. The microscopic distribution of tilt/twist angles in space for the GaN lattice could be measured in a bulk region of the sample with no cut or other sample manipulations being necessary. Polarized/confocal Raman spectroscopy provides a practical and convenient tool for characterizing on the micron scale the interfacial disorder that affects energy transport phenomena at GaN/substrate interfaces.