Optical properties and lineshape analysis of II-VI compounds obtained by spectroscopic ellipsometry

Abstract

Developments in the use of wide-gap II-VI semiconductor materials have recently led to the demonstration of blue-green diode laser action in ZnSe-based heterostructures.1,2 We have used spectroscopic ellipsometry to investigate the optical constants of several II-VI compounds involved in the realisation of such laser devices. Ellipsometric modelling has allowed access to the above and below bandgap optical constants of the films involved. Thin films of ZnSe, ZnSSe, ZnCdSe, and ZnMgSSe grown by molecular beam epitaxy (MBE) on GaAs were compared. The alloy composition (S, Cd, and/or Mg content) of the compound materials has been varied and variations in the energy position of the interband transitions with alloy composition were observed. Lineshape studies were performed and data for the optical interband transitions in the films mentioned were obtained. The position and presence of the optical interband transitions E0, E0 + A0, E1 and E1 + Δ1 in the measured data for the ZnSe films is in good agreement with data available on bulk material and with recent data for thin films.3 Our results for the optical behaviour of the ZnSe films are in reasonable agreement with data obtained from a simplified model of interband transitions for single crystalline non-doped ZnSe bulk material available in recent literature.3 This model allows a further parametrisation of optical response for the films. Lineshapes of MBE-grown ternary and quaternary II-VI compound films were studied for the first time. Figure 1 shows the lineshape results for a ZnS0.15Se0.85 film on GaAs. Best fit results were obtained using an excitonic lineshape.4 Figure 2 shows an evaluation of the E0, E0 + Δ0, and E1 E1 + Δ1 interband transitions with change in alloy composition for the ZnSSe films examined. Similar results were obtained for ZnCdSe and for ZnMgSSe thin films.