KINETICS OF ELECTRICAL-CONDUCTIVITY ENHANCEMENT IN BISMUTH SULFIDE THIN-FILMS .2. OPTOELECTRONIC PROPERTIES (FILM) AND PHASE-TRANSFORMATIONS (POWDER) UNDER OXYGEN ANNEALING

The optoelectronic properties of oxygen annealed bismuth sulphide thin films have been determined and correlated to the physico-chemical tra nsformations of the powder studied by Differential Scanning Calorimetr y (DSC). A maximum in photoconductivity of 0.54 Ohm(-1) cm(-1) was obt ained at 170 degrees C with short annealing times. The temperature dep endence of the saturation rate indicates the adsorption of oxygen up t o 230 degrees C. At higher temperatures oxygen gets chemisorbed and st rongly diminishes the overall mobility of the film. The physico-chemic al transformations observed by DSC on bismuth sulphide powder show two first order irreversible transitions in the temperature range of 200- 300 degrees C. The first has been identified with crystallization of b ismuthinite (Bi2S3) and BiS2. The second transition is quite broad and uncovers the decomposition of BiS2 into Bi and S and the incipient fo rmation of bismuth oxides and sulphates. At these temperatures, the co nversion of bismuth crystals into sulphates and oxides leaves bismuthi nite as the only conductive medium, explaining the decrease on conduct ivity at higher temperatures and duration of annealing. Oxidation of b ismuthinite takes place in two stages between 400-500 degrees C, a hig h activation energy followed by a non-activated and fast oxidation pro cess.