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 ${\text{N⩾8×10}}^{18} {\mathrm{cm}}^{\text{−3}}$ 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.