Characterization of stress in thin-film wurtzite GaN grown on c-plane sapphire by molecular beam epitaxy.

The stress distribution in wurtzite gallium nitride (GaN) grown on c-plane sapphire substrates by molecular beam epitaxy is characterized. Micro ($\mu )$-Raman spectroscopy is particularly useful for stress characterization because of its ability to measure the spectral shifts in Raman peaks in a material, and correlate those shifts with stress and strain. The phonon deformation potential is determined by applying pressure to the material using a four-point strain fixture while simultaneously monitoring the applied pressure using a strain gauge and recording the Raman spectrum. The deformation potentials are then used to determine stress distribution; the spectral positions of the E$_{2}$ Raman mode ($\nu $ = 569 cm-1) in GaN and A$_{1g}$ Raman mode ($\nu $ = 418 cm$^{-1})$ in sapphire are recorded at each spatial position in a raster map. The $\mu $-Raman spectroscopy is performed using a Renishaw InVia Raman spectrometer with argon ion ($\lambda $ = 514.5 nm, h$\nu $ = 2.41 eV) and helium-neon ($\lambda $ = 633 nm, h$\nu $ = 1.96 eV) excitation sources, and the data is collected across the samples with 5- to 10-$\mu $m spatial resolution. Inherent stress and evidence of significant damage in the GaN layer due to MEMS processing will be discussed.