KINETICS OF ELECTRICAL-CONDUCTIVITY ENHANCEMENT IN BISMUTH SULFIDE THIN-FILMS .2. OPTOELECTRONIC PROPERTIES (FILM) AND PHASE-TRANSFORMATIONS (POWDER) UNDER OXYGEN ANNEALING
Me. Rincon et al., KINETICS OF ELECTRICAL-CONDUCTIVITY ENHANCEMENT IN BISMUTH SULFIDE THIN-FILMS .2. OPTOELECTRONIC PROPERTIES (FILM) AND PHASE-TRANSFORMATIONS (POWDER) UNDER OXYGEN ANNEALING, Journal of physics and chemistry of solids, 57(12), 1996, pp. 1947-1955
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.