Raman scattering in heavily donor doped β-Ga2O3

beta-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 (similar to 3x10(18)cm(-3)). The most prominent extra Raman peak at 255cm(-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 675cm(-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 215cm(-1), 280cm(-1), 400cm(-1), and 560cm(-1) associated with infrared active phonon modes. They arise from inelastic light scattering via charge-density fluctuations.