Characterisation of Cd1-xMgxSe solid solutions by spectroscopic ellipsometry

Optical properties of bulk Cd1-xMgxSe mixed crystals, grown from a CdSe, Mg , and Se melt by the Bridgman method, are presented. The alloys have a wurt zite type structure in the investigated range of composition, i.e. for 0 le ss than or equal to x less than or equal to 0.4. Measurements of the comple x pseudo-dielectric function, (epsilon(omega)) = (epsilon (1)(omega) + i ep silon (2)(omega)), were performed at room temperature in the photon energy range of 1.25-4.5 eV, using a computer-controlled variable angle of inciden ce ellipsometer. The spectral dependence of the effective refractive index and the absorption coefficient were also derived. The structure observed in the dielectric functions, attributed to the interband transitions E-0, E-0 + Delta (0) and E-1, have been analysed by fitting the (epsilon(omega)) an d d(2)(epsilon(omega))/d omega (2) spectra to the damped harmonic oscillato r and critical-point line shapes, respectively. For all critical points, th e energy increases with increasing Mg concentration. The ellipsometric resu lts of a fundamental band-gap shift due to Mg content are compared with the rate of increase of the excitonic energy, derived from photoluminescence s pectra. (C) 2001 Elsevier Science Ltd. All rights reserved.