Evidence for Negative Electron Affinity in Laser Irradiated ZnTe Thin Films

Local transport properties of laser irradiated ZnTe thin films are reported. By rastering the laser beam (of 532 nm wavelength) appropriately, ZnTe decomposes into n-type ZnTe and Te and a grating like structure with micro-stripes of ZnTe separated by grooves of Te is obtained. Conductive atomic force microscopy studies and local I-V measurements made on the stripes and grooves show that the film properties are mainly determined by chemical composition, rather than by the topography of the film. When the tip is positively biased, the current images closely match the topography images. In contrast, when the tip is negatively biased the current and topography images are very different and a large negative current was also observed in the grooves. This is attributed to the variation in charge separation (interface capacitance) caused by the rough surface. It is shown that Te forms ohmic contact with Au tip, but the junction exhibits Schottky diode behavior under low biasing voltages. The large current at both high positive and negative tip biasing may arise due to semi-metallic properties of Te. The I-V characteristics measurement reveals formation of Schottky barrier between ZnTe-Au junctions with a very low value (32.4 – 78.3 meV) of barrier which indicates the presence of negative electron affinity.