SI IN GAN - ON THE NATURE OF THE BACKGROUND DONOR

A characterization of the Si impurity in GaN is performed by Raman spe ctroscopy. Applying hydrostatic pressure up to 25 GPa we study the beh avior of the LO phonon-plasmon mode in a series of high mobility Si do ped GaN films. In contrast to earlier results on unintentionally doped bulk GaN crystals no freeze out of the free carriers could be observe d in Si doped samples. We find that Si is a shallow hydrogenic donor t hroughout the pressure range studied. This result positively excludes Si incorporation as a dominant source of free electrons in previously studied bulk GaN samples.