Identification of Combination Phonon Modes in Pure and Doped GaSe Crystals by THz Spectroscopy

Terahertz transmission of (i) pure gallium selenide, (ii) 500-ppm indium-doped gallium selenide, and (iii) 500-ppm chromium-doped gallium selenide is taken in the range 0.2-3.0 THz (6.67-100 cm^-1 ) to identify the effect of doping on phonon modes. Fourier transform of time domain THz transmission data is fitted with a modified Drude-Smith model to derive phonon mode oscillator strength, phonon relaxation rate, plasma frequency, and momentum relaxation time of free carriers. It is observed that the effect of doping on defect states and carrier concentration rightly accounts the measured plasma frequency and momentum relaxation time. The THz absorption lines are appropriately attributed to the combination of phonon modes obtained by Raman spectroscopy.