III–VI Chalcogenide Semiconductor Crystals for Broadband Tunable THz Sources and Sensors

The layered chalcogenide semiconductor GaSe has been grown under various crystal growth conditions for optimum performance for tunable terahertz (THz) wave generation and broadband THz detection. Low-temperature photoluminescence (PL), Raman spectroscopy, optical absorption/transmission, electrical charge transport property measurements, and THz time-domain spectroscopy (TDS) have been used to characterize the grown crystals. It is observed that indium doping enhances hardness of the grown GaSe crystals, which is very useful for processing and fabricating large-area devices. GaSe crystals have demonstrated promising characteristics with good optical quality (absorption coefficient $\le 0.1\,{\rm cm}^{ - 1}$ in the spectral range of 0.62–18 μm), high dark resistivity ( $\ge$$10^9 \,\Omega \,{\rm cm}$), wide bandgap (2.01 eV at 300 K), good anisotropic ($\,{\rm and}\, \bot$) electrical transport properties ( $\mu _{\hbox{e/h}}, \,\tau _{\hbox{e/h}}, \,{\rm and}\,\mu \tau _{\hbox{e/h}}$ ) and long-term stability. The THz emission measurements have shown that the GaSe crystals are highly efficient for broadband tunable THz sources (up to 40 THz), and sensors (up to 100 THz). Additionally, new THz frequencies (0.1–3 THz) have been observed for the first time from an anisotropic binary and a ternary semiconductor crystal. Details of characterizations as well as optimum crystal growth conditions including simulation and computer modeling are described in this paper.