Optical behavior and structural property of CuAlS2 and AgAlS2 wide-bandgap chalcopyrites

Single crystals of CuAlS2 and AgAlS2 were grown by chemical vapor transport method using ICl3 as the transport. The as-grown CuAlS2 crystals reveal transparent and light-green color. Most of them possess a well-defined (112) surface. The AgAlS2 crystals essentially show transparent and white color in vacuum. As the AgAlS2 was put into the atmosphere, the crystal surface gradually darkened and became brownish because of the surface reaction with humidity or hydrogen gas. After a long-term chemical reaction process, the AgAlS2 will transform into a AgAlO2 oxide with yellow color. From x-ray diffraction measurements, both CuAlS2 and AgAlS2 as-grown crystals show single-phase and isostructural to a chalcopyrite structure. The (112) face is more preferable for the formation of the chalcopyrite crystals. The energies of interband transitions of the CuAlS2 and AgAlS2 were determined accurately by thermoreflectance measurements in a wide energy range of 2–6 eV. The valence-band electronic structures of CuAlS2 and AgAlS2 have been detailed and characterized using polarized-thermoreflectance measurements in the temperature range between 30 and 300 K. The band-edge transitions belonging to the E∥ and E⊥ polarizations have been, respectively, identified. The band edge of AgAlS2 is near 3.2 eV while that of AgAlS2 is about 3.5 eV. On the basis of the experimental analyses, optical and sensing behaviors of the chalcopyrite crystals have been realized.