Structural property and optical band edge of Ag(In_0.5Al_0.5)S₂

Single crystals of Ag(In_0.5Al_0.5)S₂ were grown by chemical vapor transport using ICl₃ as a transport agent. The as-grown Ag(In_0.5Al_0.5)S₂ essentially shows dark red and its absorption edge close to red-to-green energy portion. The stoichiometric content of the Ag(In_0.5Al_0.5)S₂ was evaluated by energy dispersion X-ray spectroscopy. The X-ray diffraction measurement confirmed that the crystal is single phase and crystallized in the chalcopyrite structure. The lattice constants of the crystals were determined to be a = 5.763 ± 0.08 Å and c = 10.623 ± 0.016 Å, respectively. Raman spectroscopy revealed a prominent A₁ (282 cm⁻¹) peak (i.e. main characteristic mode), which verified the chalcopyrite structure of the Ag(In_0.5Al_0.5)S₂. The absorption-edge anisotropy of the Ag(In_0.5Al_0.5)S₂ has been studied by polarization-dependent transmission measurements with the linearly polarized lights along \({\text{E}}\parallel \,{ < }11\bar{1}{ > }\) (denoted as E_‖) and along \({\text{E}} \bot \,{ < }11\bar{1}{ > }\) (denoted as E_⊥) onto the {112} plane of the crystal. The unpolarized band gap determined by transmission measurement was about 2.31 ± 0.01 eV, which is lying in between the lower E_‖ (~2.24 ± 0.01 eV) and higher E_⊥ (~2.40 ± 0.01 eV) polarized gaps. The absorption-edge anisotropy by the E_‖ and E_⊥ polarizations has been clearly demonstrated herein. On the basis of the experimental results, the structure and optical band edge of the Ag(In_0.5Al_0.5)S₂ is thus realized.