SEMICONDUCTOR PARTICULATE FILMS ON SOLID SUPPORTS

CdS and ZnS semiconductor particulate films, prepared in negatively charged monolayer interfaces and transferred to solid supports, have been characterized by reflectivity, absorption spectrophotometry, transmission electron microscopy, scanning tunneling microscopy, and electrical measurements. Incident-angle-dependent reflectivity measurements established the optical thicknesses of the semiconductor particulate films on solid supports to be only 5% smaller than those determined at the monolayer interfaces prior to their transfer. Absorption spectra of CdS and ZnS particulate films on quartz supports showed maxima around 239 and 200 nm and shoulders around 475 and 315 nm, respectively. Plots of absorbances at a given wavelength against thickness were linear for CdS and ZnS particulate films. Direct bandgaps for 63-, 125-, 163-, 204-, 263-, and 298-angstrom-thick CdS particulate films were evaluated to be 2.54, 2.48, 2.46, 2.44, 2.43, and 2.42 eV. Similarly, a direct bandgap of 3.75 eV was assessed for the 458-angstrom-thick ZnS particulate film. Heating of the CdS and ZnS films to 300-500-degrees-C shifted the direct bandgaps to those corresponding to bulk semiconductors. Transmission electron micrographs of CdS films revealed the presence of CdS particles in a narrow size distribution with average diameters of 47 angstrom. The presence of 20-30-angstrom-thick, 40-50-angstrom-diameter CdS and 10-25-angstrom-diameter ZnS particles in CdS and ZnS films were discerned by scanning tunneling microscopy. CdS films had dark resistivities of (3-6) x 10(7) OMEGA cm, which decreased upon illumination; they also developed photovoltages upon illumination. The action spectrum of the photoconductivity corresponded to the absorption spectrum of the CdS particulate film, indicating its origin to be the conduction band electrons and valence band holes produced in bandgap excitation.