Band offsets and optical bowings of chalcopyrites and Zn-based II-VI alloys

Abstract

Using first-principles band-structure theory we have systematically calculated the (i) alloy bowing coefficients, (ii) alloy mixing enthalpies, and (iii) interfacial valence- and conduction-band offsets for three mixed-anion (CuInX2, X=S, Se, Te) and three mixed-cation (CuMSe2, M=Al, Ga, In) chalcopyrite systems. The random chalcopyrite alloys are represented by special quasirandom structures (SQS). The calculated bowing coefficients are in good agreement with the most recent experimental data for stoichiometric alloys. Results for the mixing enthalpies and the band offsets are provided as predictions to be tested experimentally. Comparing our calculated bowing and band offsets for the mixed-anion chalcopyrite alloys with those of the corresponding Zn chalcogenide alloys (ZnX, X=S, Se, Te), we find that the larger p−d coupling in chalcopyrite alloys reduces their band offsets and optical bowing. Bowing parameters for ordered, Zn-based II-VI alloys in the CuAu, CuPt, and chalcopyrite structures are presented: we find that ordered Zn2SeTe has bowing coefficients of 1.44 and 3.15 eV in the CuAu and CuPt structures, while the random ZnSexTe1−x alloy has a bowing of 1.14 eV. The band alignment between CuInSe2 and CuInSe2-derived ordered vacancy compounds are also presented.