Near band gap photoreflectance studies in CdTe, CdTe:V and CdTe:Ge crystals

The contactless modulation spectroscopy technique of photoreflectance (PR) has been used to study the near band edge transitions in CdTe, CdTe:V and CdTe:Ge bulk crystals in the range of 14 and 400 K for the first time. The lineshape of the PR spectra for the crystals is seen to follow the third derivative functional form (TDFF) of electrorefiectance (ER) in the low field limit. Using the line shape analysis of the spectra at different temperatures, the variation of band gap (E_0), phase factor (β) and energy broadening parameter (I?) with temperature are studied. The temperature variation of band gap for these crystals is seen to follow the Varshni relation with coefficient values α = 4.357 x 10^(-4) eV K^(-l) , β= 183.4 K for undoped, α = 4.635 x 1O^(-4) eV K^(- I), β = 184.5 K for vanadium-doped and α = 4.508 x 10^(-4) eV K^(- 1) β = 230.5 K for Ge doped crystals. The Varshni relation is found to be valid for the whole range of temperature studied for undoped and Ge-doped crystals, where as for V-doped crystals, Varshni relation is valid upto about 250 K. Effects of vanadium and germanium doping on the energy broadening parameter in CdTe are discussed