Study of the effect of thermal annealing on the optical and electrical properties of vacuum evaporated amorphous thin films in the system Ge20Te80-xBix

We systematically studied the effect of thermal annealing on the optical and electrical properties of amorphous semiconducting thin films in the system Ge(20)Te(80-x)Bi(x()x = 0, 0.19, 2.93, 7.35) prepared by flash evaporation in a vacuum of 1 x 10(-6) Torr. The films are characterized by x-ray diffraction (XRD) and electron probe micro analysis. The annealing temperature is kept at 150 degreesC, 180 degreesC and 220 degreesC. No crystallization of the thin films is achieved on annealing up to the temperature of 150 degreesC. At a higher temperature of annealing, microcrystals of Te, Bi2Te3, Ge-Te, etc, are observed along with an amorphous phase as indicated by XRD analysis. The fundamental optical absorption edge and reflection spectra of as-prepared and annealed films are determined. Optical interband transitions are observed for various films (as-prepared and annealed). The presence of crystalline Bi2Te3 in films annealed at 220 degreesC is also supported by the reflection spectrum. The optical energy gap (E-g), the slope parameter (Delta) of the absorption edge and the tailing parameter (B-1) of the energy band tails are computed from the optical data. The dc electrical conductivity (sigma(dc)) of various films is studied in the temperature range of 150-450 K. We observe that two types of conduction take place: conduction through extended states in the higher temperature region, and conduction through localized states in the band tails and at the Fermi level by the hopping process assisted by phonons at lower temperatures. The data at higher temperatures have been fitted with the expression sigma(dc) = sigma(0) exp(-DeltaE/kT) and the electrical parameters, DeltaE and sigma(0)(-), are also determined. It is observed that the bismuth concentration and annealing temperature dependences of the optical and electrical parameters are different in the two regions of compositions, x less than or equal to 2.93 and x > 2.93, indicating structural differences in the two sets of compositions. It is pointed out that the bulk form of these amorphous semiconductors exhibits a carrier sign reversal at a bismuth concentration of about 3.5 at%. However, the thin-film form of these amorphous semiconductors does not show such a carrier sign reversal in the electrical transport.