Electronic transport properties in evaporated cadmium selenide thin films

Abstract

Cadmium selenide (CdSe) films prepared by vacuum vapour deposition onto glass sub strates maintained at various temperatures have been studied in relation to their struc tural, optical and electrical properties. Such films have potential applications in solar cells and gas detectors. Structural studies using X-ray diffraction revealed that the films are polycrystalline with a preferred orientation in the [002] direction for films deposited at room temperature. Optical measurements suggested a direct optical band gap of approximately 1.66 eV and spin-orbit splitting of the valence band. Lateral resistivity measurements, performed in the dark, showed values increasing to approximately 5 n m in the thickness range 0.1 - 1.0 uii», followed by a rapid decrease for greater thicknesses. Other factors that affected resistivity were rate of deposition and substrate temperature. Activation energy values were typically 0.02 eV in the low tem perature region and 0.14' eV in the high temperature region. DC conduction processes were found to depend on the type of metallic electrodes and the strength of the electric field applied. Samples having two aluminium electrodes exhibited ohmic conduction followed by space-charge-limited conductivity (SCLC). Mobility was also measured in such films, yielding a value of typically 7.6 x 10-5 m2 V-I s-1 at a voltage level of 0.3 V. Schottky barrier heights of approximately 1V were determined at the AI/CdSe interface. Measurements were also performed on samples having combinations of Au and either AI, Ag, Cu and In. In most cases ohmic conductivity and SCLC were ob served, with Poole-Frenkel type conduction frequently observed under certain polarity conditions. Possible reasons for the appearance of the different types of conductivity are advanced for each structure, which include the establishment of interfacial barrier layers and highly doped regions close to the electrodes. AC measurements suggest that conductivity increases monotonically with frequency at low temperatures. Conductiv ity was sensitive to temperature in the low frequency range, but less sensitive in the v high frequency range. The activation energies determined suggested hopping conduc tion at low temperatures, giving way to free band conduction at high temperatures. Measurements of conductivity, capacitance and loss tangent were in reasonable accord with existing theory