Electrodeposition and characterization of cadmium telluride and lead telluride thin films

In chapter 2 we report the growth of CdTe/PbTe superlattice on polycrystalline gold and indium tin oxide (ITO). These systems have been characterized by cyclic voltammetry, Auger electron spectroscopy, X-ray photoelectron spectroscopy, X-ray diffraction, UV-vis-NIR absorption spectroscopy and time-resolved transient absorption spectroscopy. Our data, the steady state optical spectra in particular, demonstrate unambiguously the formation of a stable CdTe/PbTe quantum well under these conditions. Preliminary femtosecond time-resolved measurements, in which the dynamics of the system appear to be dominated by carrier trapping dynamics, suggest that the quality of epitaxy is lower on ITO than on Au surfaces. Strategies for improving epitaxy are discussed. In chapter 3 we present a study of the underpotential deposition (UPD) of Te monolayers onto Au electrodes from perchloric acid solution using electrochemical quartz microgravimetry (EQCM) and chronocoulometry. We find that tellurium(IV) oxide (as either TeO ₂ or HTeO₂+) is adsorbed on Au at potentials positive of the UPD region. EQCM experiments, in which an Au electrode poised at + 0.7 V vs. Ag/AgCl in 0.1 M HClO₄ electrolyte was monitored as function time before and after the addition of TeO₂, showed a slow adsorption of a submonolayer of TeO₂. Energy dispersive X-ray (EDX) measurements confirm the presence of a Te-containing species on the surface. Two distinct reductive UPD features at approximately +0.400 V and −0.100 V vs. AgjAgCl, respectively, were observed by cyclic voltammetry. EQCM measurements indicate that there is a small, reproducible mass decrease accompanying the first UPD wave, consistent with the loss of H2O from the surface as an adsorbed layer of HTeO₂+ is reduced to a submonolayer of Te atoms. Chronocoulometry indicates a charge density of 160 μC/cm2 for the first UPD wave, which, when taken together with the mass change data, is consistent with the 4-electron reduction of adsorbed HTeO₂+ to Te. In addition, analysis of the current-time data indicates that this process occurs by a two-dimensional instantaneous nucleation and growth mechanism resulting in an ordered overlayer with a fractional coverage of ca. 0.41. The second UPD peak results in the formation of a dense monolayer by deposition of HTeO₂+ from solution. This process also appears to occur by a direct 4-electron reduction and exhibits simple Langmuir adsorption behavior as evidenced by exponentially decaying current-time transients. The saturation coverage is approximately 0.9 in comparison to the number of gold atoms on the surface.; Growth of CdTe and PbTe thin films is reported in chapter four. CdTe and PbTe have a wide range of applications including thermoelectric generators, solar cells and infra-red detectors. In this chapter we present optimized conditions for electrocodeposition of thin films of CdTe and PbTe. A study by EDX and XPS helped in determining concentrations needed for stoichiometric growth of CdTe and PbTe thin films. EQCM gave insights into the deposition mechanism and preliminary XRD data showed that both CdTe and PbTe have preferential orientation growth. Conditions for optimal film growth are described. Film formation was studied by EQCM and electrochemistry and XRD.