The Effect of Copper Vacancies on the Optical Bowing of Chalcopyrite Cu(In,Ga)Se2 Alloys

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Abstract
The optical bowing behavior of polycrystalline thin film Culn1-yGaySe2 alloys is dependent upon the Cu stoichiometry. The variation in optical band gap, Eg, for alloys in which Cu is near stoichiometric (25 at.%) is parabolic and follows the relationship: Eg(y) = 1.011 + 0.421 y + 0.244 y2 (eV), where y is the alloy parameter, [at.% Ga] / [at.%Ga + at.%In]. Contrary to this, films with Cu-poor stoichiometries (∼19 at.% Cu) exhibit little alloy bowing: Eg(y) = 1.01 + 0.733 y - 0.046 y2 (eV). The increase in Eg with Cu deficiency appears to be the result of both a structural effect associated with tetragonal lattice shrinkage, ΔV = Vstoichiometric - VCu-poor (resulting from the presence of Cu vacancies) and a chemical effect, possibly associated with a counterbalance between p-d repulsion and anion displacement effects.