Raman scattering in heavily donor doped β-Ga_2O_3

摘要

β-Ga_2O_3 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 × 10~(18) 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.