LOW-TEMPERATURE VAPOR-PHASE DEPOSITION OF BISMUTH SULFIDE FILMS

A nearly stoichiometric bismuth sulphide precipitate obtained as a by- product of the chemical deposition technique is used as a source in lo w-temperature vapour phase deposition of Bi2S3 thin films. At relative ly low deposition temperatures (250 degrees C < T < 380 degrees C) hig hly polycrystalline Bi2S3 films are formed on glass substrates, while very thin films are obtained at lower temperatures (95-250 degrees C). The highest photocurrent (I-ph similar to 10(-4) A) is found in the f irst group while the highest photosensitivity (S = 56) is found in the amorphous or too thin films. XRD patterns of bismuth sulphide powder before and after 5 h deposition show its enrichment in Si due to the e nrichment in S and sulphur-rich bismuth sulphide species (BiS2)(n) of the vapour phase. For films deposited at temperatures in the range of 290-380 degrees C, the change in composition of the source causes diff erent deposition rates and preferential growth of some of the bismuthi nite planes (2 theta < 25 degrees for as-deposited powder and 2 theta > 25 degrees for bismuth-rich sources), compositional changes in the f ilms (stratification) does not seem to be relevant at these temperatur es. The bandgap (E-g) of the films decreases as the deposition tempera ture increases from E-g = 2 eV at T-dep = 95-205 degrees C to 1-1.3 eV at T-dep = 290-380 degrees C. Similar results were obtained with diff erent sources, which indicates the stronger dependence of E-g with res pect to substrate temperature than with respect to the type of source.