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 (α-) GaN. The 0.8-1.6 μm 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≥8×10¹⁸ 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 E₂ 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.