Dependence of resistivity in evaporated cadmium selenide thin films on preparation conditions and temperature

摘要

The lateral resistivity of evaporated CdSe thin films has been measured using the van der Pauw technique. Measurements were performed as a function of the film thickness, the deposition rate and the substrate temperature during deposition. Such measurements are useful in the context of the development of inexpensive solar cells based on this material. In contrast to some other cadmium compounds the mean resistivity appeared to increase with increasing thickness, prior to a rapid decrease at a thickness above about 1 micrometer. This effect is thought to be related to the varying composition of the evaporating CdSe charge during the course of the evaporation process. Films deposited at a substrate temperature of 200 degrees Celsius (473 K) showed a rapid increase in resistivity from below 10$+2$/ approximately ega m to above 1 approximately ega m as the deposition rate increased up to approximately 0.5 nm s$+$MIN@1$/, while for rates above this value and up to 3 nm s$+$MIN@1$/ the resistivity remained essentially constant. This behavior is thought to be related to a decrease in mobility and/or free carrier concentration resulting respectively from increasing grain boundary scattering and trapping effects, as a result of a decrease in the mean grain size with increasing deposition rate. Resistivity was strongly dependent on the substrate temperature during deposition, showing a moderate increase with increasing temperature up to about 75 degrees Celsius (approximately equal to 350 K), followed by a very rapid increase of typically three decades up to 100 degrees Celsius (373 K), above which the value stabilized at an essentially constant value. Previous work has identified the origin of lower resistivity CdSe films as being the result of an excess of Cd ions while stoichiometric films are normally of higher resistivity. The present results indicate that a substrate temperature in excess of 100 degrees Celsius (approximately equal to 400 K) is necessary for the deposition of stoichiometric films. As expected conductivity was thermally activated and the resistivity decreased with increasing temperature. Samples prepared at a substrate temperature of 200 degrees Celsius (473 K) showed activation energies of approximately 0.02 eV and 0.14 eV at lower and higher temperatures, respectively. The low temperature behavior is consistent with conduction via a hopping mechanism, while the latter is appropriate for thermal excitation over inter-crystalline potential barriers as proposed by Petritz and previously observed in CdS films.