Disorder-activated infrared modes and surface depletion layer in highly Si-doped hexagonal GaN

Three infrared-active low-polar modes are reported for highly Si-doped hexagonal (alpha-) GaN. The 0.8-1.6 mum thick films, grown by metal organic vapor phase epitaxy or molecular beam epitaxy on (0001) sapphire substrates, were studied by infrared spectroscopic ellipsometry. For GaN epilayers with free-electron concentration N greater than or equal to 8x10(18) cm(-3) we observe, besides the usual GaN transverse-optical lattice modes and coupled longitudinal-optical phonon-plasmon modes, a band of additional modes at 567.4 +/-2.5, 752.5 +/-0.9, and 855.0 +/-0.9 cm(-1). We tentatively assign the first one to the disorder-activated high E-2 GaN mode and the third mode to an acoustic-optical combination band, whereas the origin of the second mode remains unclear. Furthermore, the ellipsometric spectra of highly n-conductive Si-doped GaN reveal thin carrier-depleted regions at the sample surface. (C) 2001 American Institute of Physics.