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Optical energy gap values and deformation potentials in four Cu-III-VI2 chalcopyrite compounds

Values of the optical energy gap E0 as a function of temperature T in the range 10 to 300 K were obtained by optical absorption and modulated reflectance measurements on samples of CuInSe2, CuGaSe2, CuInS2 and CuGaS2. The variations of E0 with T were fitted well by a Manoogian-Leclerc equation of the form E0(0, 0)-E0(T, 0)=UT+V phi (coth phi /2T-1). Values of (dE0/dT)1, the electron-phonon interaction contribution to the variation of the energy gap with temperature and (dE0/dT)s the static component, were obtained from the V phi and U terms respectively. Comparison with values from the pressure coefficient dE0/dP indicated that in addition to the lattice-dilation term (dE0/dT)2, (dE0/dT)s contains a further contribution, labelled (dE0/dT)3, attributed to a change with temperature of the position coordinate u of the anions. From the values of (dE0/dT)1 and (dE0/dT)2, values were determined for the acoustic deformation potentials of the conduction band Ce and of the valence band Ch.