Dependence of optical properties on structural and compositional parameters in CuGaSe2

The dielectric ic constant of CuGaSe2 as a function of stoichiometric devia tions has been obtained from photomodulated spectral al ellipsometry measur ements. Optical gaps have been computed by fitting experimental and differe ntiated data of the imaginary part of the dielectric constant to function e psilon (2) Of the Lorentz model and to the second derivative of epsilon (2) . In each sample, three transition energy values have been found in the 1.6 5-1.72, 1.83-1.95, and 2.97-3.14 eV ranges. The presence of point defects I educes the energy gap values. For nonstoichiometric samples, changes in th e first energy gap values have been analyzed as a function of the displacem ent of the position of the anion in the unit cell. The shirts in the valenc e band structure have been analyzed and it is concluded that the difference between the first and second transition energies, (E-g2-E-g1), is also aff ected by stoichiometric deviations, so that the Gamma ((2))(5v) level in sa mples with point defects is closer to Gamma ((2))(4v) level than in stoichi ometric ones. Changes in (E-g3-E-g2) and (E-g3-E-g1) are related to Se posi tion. (C) 2000 American Vacuum Society. [S0734-2101(00)03206-1].