INTERPLAY OF ELECTRONS AND PHONONS IN HEAVILY-DOPED GAN EPILAYERS

Raman spectroscopy is used to analyze the effect of electrons on the l attice dynamics of unintentionally heavily doped GaN. The deposition t emperature of GaN buffer layers on sapphire substrates is found to hav e an important influence on the presence of free carriers in GaN layer s, evidenced by plasmon coupling to the A(1)(LO) phonon. Data from inf rared measurements are used to calculate the Raman line shape of q= 0 coupled A(1)(LO)-plasmon modes in a dielectric approach and give a goo d fit of the L-(q= 0) component observed in Raman spectra. In particul ar, the fitting procedure applied to spatially resolved micro-Raman me asurements reveals an inhomogeneous concentration of electrons on the scale of hexagonal microcrystallites. Partial screening of phonons wit h wave vectors differing from the q=0 transfer of incident and scatter ed photons is invoked to explain LO-like scattering over the whole spe ctral range of optical phonons, attributed to charge density fluctuati ons on account of its polarization properties. (C) 1997 American Insti tute of Physics.