Atomic-scale structure, cation distribution, and bandgap bowing in Cu(In,Ga)S2 and Cu(In,Ga)Se2

Abstract

Mixed chalcopyrite semiconductors like Cu(In,Ga)S2 and Cu(In,Ga)Se2 are characterized by the coexistence of different local atomic arrangements around the S or Se anion. The resulting anion displacement strongly influences the material bandgap. We studied the atomic-scale structure of Cu(In,Ga)S2 as a function of composition using x-ray absorption spectroscopy and valence force field simulations. Applying a specially developed model for not fully random cation distributions, we find that structural relaxation of the anion with respect to In and Ga contributes significantly more to the bandgap bowing observed for Cu(In,Ga)S2 and Cu(In,Ga)Se2 than relaxation with respect to Cu and group-III atoms.

Local versus global electronic properties of chalcopyrite alloys: X-ray absorption spectroscopy and ab initio calculations

“…EX-AFS measurements of both CIGS and CIGSe have shown that the element-specific bond lengths are nearly constant over the whole compositional range despite the change in lattice constants. 5,12 This behavior closely resembles the findings for other mixed semiconductor systems as first reported for (In,Ga)As 27 and later confirmed for many other III-V and II-VI ternary compounds.…”

“…A further validation of our model structures comes from the com- parison with EXAFS measurements of CIGS alloys which yield the element-specific atomic-scale structure, in particular the Cu-S, Ga-S and In-S distances. 12 The atomic positions obtained with the HSE functional always yield bond lengths in excellent agreement with experimental data (see Table 1 of the supplemental material 4 ). In contrast, PBE and, to a smaller extent, PBE+U overestimates Ga-S and In-S bond lengths, while Cu-S bond lengths remain quite close to HSE and experimental values.…”

“…28,30 In mixed cation systems, this is achieved by a displacement of the anion from its ideal lattice site 31 which has been shown to influence the band gap of the material for both III-V alloys 32 and mixed chalcopyrites. 5,12 This demonstrates again the strong similarity between these tetrahedrally coordinated mixed semiconductors and highlights the correlation between atomic-scale structure and electronic properties. However, due to the increased complexity of the chalcopyrites compared to the III-V or II-VI compounds, two different displacement mechanisms must be distinguished for CIGS and CIGSe as discussed in detail in Refs.…”

“…12 In contrast, both the S edge position and the average group-III-S bond length vary with changing In/Ga ratio. This strongly suggests that the absorption edge position, i.e.…”

“…However, the electronic states of the CIGS compounds are known to be very sensitive to modifications of the internal structural parameters. 11,12 This fact necessitates a proper description of the microscopic atomic structure of CIGS alloys, and it motivates our choice to work with supercells, despite the increased computational cost.…”