Conductivity in sulfur doped gallium selenide crystals measured by terahertz time-domain spectroscopy

In this paper, the conductivity of intrinsic GaSe, S doped 2.5 mass% GaSe, and S doped 7 mass% GaSe crystals, in the range of 0.3-2.5 THz, was measured by transmission terahertz time-domain spectroscopy, and fitted well with Drude-Smith-Lorentz model which was introduced lattice vibration effect. It was found that the real part of conductivity decreased with S doping, which was caused by the gradual shift of the Fermi energy level of GaSe crystals to the charge neutrality level due to the generation of substitution impurities and gap impurities by S doping, resulting in the reduction of carrier concentration. The intrinsic GaSe and S doping 2.5 mass% GaSe had a clear lattice vibration peak at about 0.56 THz, while GaSe: S 7% had no lattice vibration peak near 0.56 THz, which was mainly due to the S doping increased the structural hardness of the crystal and reduced the interlayer rigidity vibration of the crystal. All three samples had obvious narrow lattice vibration peaks at about 1.81 THz, and the intensity first decreased and then increased with S doping, which mainly due to a small amount of S doping reduced the local structural defects of GaSe and weakened the intensity of the narrow lattice vibration peak, while excessive S doping generated β-type GaS crystals, increased the local structural defects of the crystals and the intensity of the narrow lattice vibration peak. With the increase of S doping, the intensity of the broad lattice vibration peak of GaSe crystal weakened or even disappeared at about 1.07 THz and 2.28 THz, mainly due to the S doping resulting in S substitution impurities and GaS gap impurities, which reducing the fundamental frequency phonon vibration intensity, thereby weakening the lattice vibration caused by the second-order phonon difference mode of the crystal. The results show that the appropriate concentration of S doping can effectively suppress the lattice vibration of GaSe crystal, reduce the conductivity and power loss in the THz band. This study provides important data support and theoretical basis for the design and fabrication of low loss THz devices.