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, Cd Te:V and CdTe:Ge bulk crystals in the range of 14 and 400 K for the fi rst time. The lineshape of the PR spectra for the crystals is seen to follow the third derivative functional form (TDFF) of electroreflectan ce (ER) in the low held limit. Using the line shape analysis of the sp ectra at different temperatures, the variation of band gap (E(0)), pha se factor (theta) and energy broadening parameter (Gamma) with tempera ture are studied. The temperature variation of band gap for these crys tals is seen to follow the Varshni relation with coefficient values al pha = 4.357 x 10(-4) eV K-1, beta = 183.4 K for undoped, alpha = 4.635 x 10(-4) eV K-1, beta = 184.5 K for vanadium-doped and alpha = 4.508 x 10(-4) eV K-1, beta = 230.5 K for Ge doped crystals. The Varshni rel ation is found to be valid for the whole range of temperature studied for undoped and Ge-doped crystals, where as for V-doped crystals, Vars hni relation is valid upto about 250 K. Effects of vanadium and german ium doping on the energy broadening parameter in CdTe are discussed.