Comparative optical studies of <i>p</i>-type and unintentionally doped GaN: The influence of annealing

Abstract

We present Raman studies of p-type and unintentionally doped GaN epitaxial layers grown by metalorganic vapor phase epitaxy onto c-plane sapphire substrates. The E2 (high) Raman mode from a series of thermally annealed p-type samples shows that a compressive lattice distortion is induced with increasing annealing temperature. This is further corroborated by our photoluminescence measurements which show that the blue luminescence at 2.8 eV undergoes a redshift upon increasing the annealing temperatures beyond 650 °C. In comparing the Raman and photoluminescence spectra from the various samples we discuss the importance of two possible mechanisms: local distortion and longitudinal optical phonon–plasmon coupling.