Temperature-dependent study of the band-edge excitonic transitions of Cu 2ZnSiS 4 single crystals by polarization-dependent piezoreflectance

Abstract

The temperature dependence of the band-edge excitonic transitions of Cu 2ZnSiS 4 single crystals were characterized by using polarization-dependent piezoreflectance (PzR) in the temperature range of 10–300 K. The PzR measurements were carried out on the as-grown basal plane with the normal along [2 1 0] and the c axis parallel to the long edge of the crystal platelet. The PzR spectra revealed polarization-dependent E ⊥ ex and E | | ex features for E⊥ c and Е||c polarization, respectively. Both E ⊥ ex and E | | ex features are associated with the interband excitonic transitions at Γ point and can be explained by crystal-field splitting of valence band. From a detailed lineshape fit to the PzR spectra, the temperature dependence of the transition energies and broadening parameters of the band-edge excitons were determined accurately. The temperature dependence of near band-edge excitonic transition energies were analyzed using Varshni and Bose–Einstein expressions. The temperature dependence of the broadening parameter of excitonic features also has been studied in terms of a Bose–Einstein equation that contains the electron (exciton)–longitudinal optical phonon-coupling constant. The parameters that describe the temperature variation of the excitonic transition energies and broadening parameters were evaluated and discussed.