Optoelectronic Properties of ZnSe, ITO, TiO2 and ZnO Thin Films

Zinc selenide (ZnSe) a II-VI compound semiconductor with cubic zinc blende structure and a direct bandgap of 2.7 eV is found to be a very promising material for optoelectronic devices (Venkatachalam et al., 2007a). Semiconductor heterostructures employing zinc selenide and related alloys are an option for the production of optoelectronic devices emitting in the blue – green spectral range (Haase et al., 1991; Jeon et al., 1991). Nowadays there is a concentrated effort to produce high quality p-type zinc selenide based blue laser diodes (Drechsler et al., 1997; Fung et al., 1997). Particularly, Schottky photodiodes with fast response in the ultraviolet-visible range is more focused. Because Schottky barriers result in both very fast switching times and low forward voltage drop. The silicon photodiodes give more response in the infrared range; the main reason for this is the band gap value. The reported band gap value of Si is 1.1 eV. By providing an overlayer on the silicon surface, the silicon Schottky diodes give more response in the ultraviolet-visible range. Due to their direct energy gap in the visible range, zinc selenide would be perfectly suitable for this. The lattice mismatch, the difference in the thermal expansion coefficients, as well as the different chemical properties of zinc selenide and silicon (Si) are some of the sources of crystal defects generated at the interface between zinc selenide and silicon heterostructures. Zinc selenide has either a sphalerite structure with lattice parameter a = 5.668 Å or wurtzite structure with lattice parameters a = 3.820 Å and c = 6.626 Å. The lattice constant value of cubic silicon is reported as 5.6576 (JCPDS, 1990, card number 37). Usually, zinc selenide films were deposited onto gallium arsenide substrate, because of the high lattice match between zinc selenide and gallium arsenide (0.27 %). However, the production cost of gallium arsenidebased device is much higher than that of silicon. The lattice mismatch between zinc selenide and silicon is quite large (4.4 %) compared with gallium arsenide (0.27 %). However, in practice, the substrate strongly influences the nucleation and the mobility of the elements deposited on the substrate. In fact, the higher the substrate temperature, the higher the mobility of the deposited elements; therefore, the stoichiometric composition of zinc (Zn) and selenium (Se) in the zinc selenide film occurs at a sufficiently high substrate temperature (Chrisey & Hubler, 1994). A new PIN – like (Si (p)/ZnSe (n-)/ZnSe (n+)) visible photodiode was fabricated in 1996 using vapor phase epitaxy (Lour & Chang, 1996). They