CADMIUM TELLURIDE FILMS PREPARED BY PULSED ELECTRODEPOSITION

Cadmium telluride polycrystalline films were deposited on various transparent semiconductors on glass using periodic pulse electrolysis from an aqueous solution of Cd2+ and HTeO2+ ions. Substrates included fluorine doped tin oxide/glass, tin oxide/indium tin oxide/glass and on those substrates with an electrodeposited cadmium sulphide film on the oxide. The properties of the deposited films were determined as a function of variables, viz. initial cathodic voltage V1, on-time t1, second cathodic voltage V2, on-time t2, solution concentration and type of substrate. Film quality was judged by adherence, continuity, optical quality, composition and morphology. The preferred deposition conditions (versus SCE) were V1 = -0.76 V, t1 = 1s, V2 = - 0.60 V, t2 = 0.1 s using a stirred 90-degrees-C solution with composition 2.5 M Cd2+, 160 ppm HTeO2+ and pH of 1.7. Films deposited under those conditions were cubic polycrystals. X-ray diffraction spectra showed reflections from the (111) (220) and (311) planes with the most intense being the (111) reflection. As the concentration of species in the solution decreased, the reflection intensities from the (220) and (311) planes decreased relative to the (111) reflection. After annealing under conditions to type convert the n-CdTe to p-CdTe, the crystallinity improved and if a CdS layer was present, the (220) and (311) planes were further developed relative to the (111) plane. Resistivity through the film was (3.0 +/- 0.8) x 10(8) OMEGA cm but reduced to (1.0 +/- 0.3) x 10(3) OMEGA cm after annealing. The band gap was 1.48 +/- 0.03 eV for both deposited and annealed films. There was a limited range over which the deposition variables could be altered. The pulse duration for the more cathodic phase needed to be longer than the less cathodic phase for adherent films. Better adherence was achieved when pulse durations were greater than 0.1 s, especially for the more cathodic phase. The magnitude of the pulse duration and potential in each phase of a particular cycle determined whether the deposited film was rich or deficient in cadmium and whether the film adhered.