Comparative optical studies of p-type and unintentionally doped GaN: The influence of annealing

We present Raman studies of p-type and unintentionally doped GaN epitaxial layers grown by metalorganic vapor phase epitaxy onto c-plane sapphire subs trates. The E-2 (high) Raman mode from a series of thermally annealed p-typ e samples shows that a compressive lattice distortion is induced with incre asing annealing temperature. This is further corroborated by our photolumin escence measurements which show that the blue luminescence at 2.8 eV underg oes a redshift upon increasing the annealing temperatures beyond 650 degree sC. In comparing the Raman and photoluminescence spectra from the various s amples we discuss the importance of two possible mechanisms: local distorti on and longitudinal optical phonon-plasmon coupling. (C) 2001 American Inst itute of Physics.