Anisotropic phonon properties and effective electron mass in α-Ga2O3

The ordinary and extraordinary infrared dielectric functions of α-Ga2O3 thin films with the corundum structure were investigated. The films were grown by mist chemical vapor epitaxy on (101¯0) sapphire substrates in (101¯0) surface orientation. They were doped by the donor tin with the resulting free-electron concentrations between 1017 and 1019 cm−3. Structural characterization revealed the absence of strain and the high crystallographic quality of the samples. It allows one to resolve all 7 Raman modes. 5 out of 6 infrared active transverse optical phonon modes lie in the investigated wave number range >250 cm−1, and are unambiguously identified by spectroscopic ellipsometry. For the highest doped sample, the free-carrier contribution (plasmon) to the infrared spectra allowed accurate determination of effective electron masses. They are m⊥*=(0.297±0.010)m0 (perpendicular to [0001]) and m||*=(0.316±0.007)m0 (parallel to [0001]) for an electron concentration of n = 1.1 × 1019 cm−3.