Raman scattering in heavily donor doped β -Ga2O3

β-Ga2O3 crystals doped with the donor impurities Si or Sn are investigated by Raman spectroscopy. In addition to the well-known intrinsic Raman allowed phonon modes, we discover several spectral features when the doping concentration exceeds the Mott criterion for the metal-insulator transition (∼ 3 × 1018 cm−3). The most prominent extra Raman peak at 255 cm−1 is an asymmetrically broadened one. It is due to single-particle, electronic excitations involving the impurity band formed by effective-mass-like (hydrogenic) shallow donors. A similar type of excitation is attributed to a Raman line at 675 cm−1 that appears below room temperature in Si doped samples and might be due to a non-hydrogenic donor. Furthermore, we observe four longitudinal phonon-plasmon coupled modes at 215 cm−1, 280 cm−1, 400 cm−1, and 560 cm−1 associated with infrared active phonon modes. They arise from inelastic light scattering via charge-density fluctuations.