Synthesis of In2S3 and Ga2S3 crystals for oxygen sensing and UV photodetection

• Growth of high-quality In2S3 and Ga2S3 bulk crystals for O2 and UV sensing.
• In2S3 crystallizes in a tetragonal structure while Ga2S3 is monoclinic.
• Oxygen sensing is based on formation of surface oxide (In2O3) on In2S3.
• Ga2S3 is a superior blue to UV photodetector material owing to its wide band gap.

Crystal growth, characterization, and sensing application of In2S3 and Ga2S3 are demonstrated herein. Single crystals of In2S3 and Ga2S3 were grown by chemical vapor transport method using ICl3 as the transport agent. The as-grown In2S3 crystals essentially show dark red and Ga2S3 displays transparent and light-yellow colored. X-ray diffraction measurements show tetragonal phase of the as-grown In2S3 crystals while Ga2S3 crystallizes in the monoclinic structure. The optical band edges of In2S3 and Ga2S3 were characterized by absorption and thermoreflectance measurements. The optical band gaps of In2S3 and Ga2S3 are 1.935 eV and 3.052 eV, respectively. The direct gaps of In2S3 and Ga2S3 lie in between red to violet visible region. Photoluminescence (PL) experiments characterize defect and surface-state emissions of the In2S3 and Ga2S3 crystals. The In2S3 can easily form surface oxide in environmental air with an In2O3 growth rate of ∼100 nm/day (i.e. for oxygen sensing). The PL intensity of the In2O3 formed on In2S3 may be a sensing index for oxygen content detection. Besides, Ga2S3 may emit a broadened visible white light (near band edge) due to the existence of intrinsic defects inside the crystal. The Ga2S3 crystal also displays a high-sensitivity photoconductivity change illuminated by a 405 nm laser. According to the experimental results, In2S3 can be an oxygen sensor with the formation of surface oxide (i.e. In2O3), while Ga2S3 may possess potential capability in fabrication of blue to UV photodetector.