Optical modulation techniques applied in the analysis of chalcopyrite semiconductor heterostructures

Abstract

The structural properties of chalcopyrite single crystals and monocrystalline epitaxial layers of ternary (CuGaSe2, CuInSe2, CuGaS2, CuInS2) and quaternary (CuIn1−xGaxSe2) chalcopyrite absorbers with applications in solar-cell device technology are analysed by optical modulation techniques. Photoreflectance (PR) Spectroscopy is applied at room and low temperatures to quantify elastic strain effects. With respect to bulk chalcopyrites, epitaxially grown layers exhibit band energy shifts due to mismatch and thermal strain evolving in semiconductor heterostructures. In the uppermost 100 nm of the 500 nm thick layers, the magnitude of the respective stress measures 100 MPa, at 300 K, and 400 MPa, at 20 K, and is reduced by up to 50% compared with the stress at the chalcopyrite/GaAs-substrate interface. The overall strain calculated from the energy shift of the PR spectra is compared with the strain calculated in terms of elasticity theory. The coexistence of biaxial and hydrostatic strain due to partial anion/cation substitution in ternary to form quaternary chalcopyrite layers is also discussed. Based on the evaluation of effective strain as a result of both hydrostatic and biaxial strain in quaternary chalcopyrite layers CuIn1−xGaxSe2 and the evaluation of the PR spectra of the layers, the band energies of the respective non-strained quaternary alloy with x = 0.19 are determined to be Ea = 1.09 eV and Eb = 1.22 eV.