Influence of cation-vacancy defects on the properties of CuInSe₂–ZnIn₂Se₄ solid solutions

Highlights: • CuInSe₂–ZnIn₂Se₄n-type semiconductor crystals were grown by the horizontal variant of the Bridgman method. • Their band gaps increased from 1 eV up to 1.08 eV with increasing amount of ZnIn₂Se₄ in the range of 5–20 mol%. • The solid solutions containing 5–10 mol% ZnIn₂Se₄ had high electron density, evidencing their nearly degenerate state. • Different charge transport mechanisms were identified, depending on the ZnIn₂Se₄ content and temperature. • Amount of Zn atoms controls the concentration of deep acceptors V_Cu, compensating shallow donors In_Cu. - Abstract: CuInSe₂–ZnIn₂Se₄n-type semiconductor crystals were grown by the horizontal variant of the Bridgman method. Their band gap was estimated from the position of the intrinsic absorption band edge. The obtained values of the band gap vary monotonically with the composition of the compounds: at 300 K they increased from 1 eV up to 1.08 eV with increasing amount of ZnIn₂Se₄ in the range of 5–20 mol%. The solid solutions containing 5–10 mol% ZnIn₂Se₄ had demonstrated weak temperature dependence of the electrical conductivity, high electron density and low photoconductivity. This evidences their nearly degenerate state. In the crystals with 15 and 20 mol% ZnIn₂Se₄ a variable range hopping conduction mechanism was found to dominate in the range ∼30–110 K and a thermally activated conductivity at T ⩾ 130 K was observed. The determined values of the donor activation energy for the samples with 15 and 20 mol% ZnIn₂Se₄ were 0.018 eV and 0.04 eV, respectively. The characteristic feature of the spectral distribution of their photoconductivity at 30–70 K is presence of a narrow peak at 1160–1190 nm