Optical behavior and structural property of CuAlS_2 and AgAlS_2 wide-bandgap chalcopyrites

摘要

Single crystals of CuAlS_2 and AgAlS_2 were grown by chemical vapor transport method using ICl_3 as the transport. The as-grown CuAlS_2 crystals reveal transparent and light-green color. Most of them possess a well-defined (112) surface. The AgAlS_2 crystals essentially show transparent and white color in vacuum. As the AgAlS_2 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 AgAlS_2 will transform into a AgAlO_2 oxide with yellow color. From x-ray diffraction measurements, both CuAlS_2 and AgAlS_2 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 CuAlS_2 and AgAlS_2 were determined accurately by thermoreflectance measurements in a wide energy range of 2-6 eV. The valence-band electronic structures of CuAlS_2 and AgAlS_2 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 AgAlS_2 is near 3.2 eV while that of AgAlS_2 is about 3.5 eV. On the basis of the experimental analyses, optical and sensing behaviors of the chalcopyrite crystals have been realized.