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.