OPTICAL AND ELECTRICAL CHARACTERIZATION OF ZnO THIN FILM

Abstract

The optical and electrical studies on ZnO thin film are reported. Thin film of ZnO is deposited on glass substrate by physical vapor deposition method. In this method, Zn powder is evaporated at a temperature of 400°C in the presence of oxygen and argon gases, and the resulting ZnO is deposited on the glass substrate which is kept at liquid nitrogen temperature. The crystallinity of this ZnO film is studied using XRD technique. The XRD pattern suggests that the nature of this film is polycrystalline. The prominent peaks observed at 31.78, 34.34, 36.18, and 56.32 correspond to the (100), (002), (101), and (110) planes, confirming the formation of hexagonal zinc oxide phase (JCPD 36-1451 for wurtzite zinc oxide). The XRD spectrum very clearly demonstrates that the film deposited in oxygen atmosphere has a dominant (101) orientation. The d hkl values are estimated for this as-deposited ZnO thin-film. It is observed that these calculated values in close agreement with the reported d hkl values for ZnO . Debye–Scherrer equation is used to estimate the size of these nanoparticles. It is found that size estimated by Debye–Scherrer equation agrees well with the size observed by TEM images. It is clear from the transmission electron microscope (TEM) images that the film contains nanoparticles of ZnO and the diameter of these nanoparticles varies from 5–20 nm. In optical properties, the UV visible spectrum of these nanoparticles is recorded in the wavelength range (300–900 nm). The absorption coefficient increases exponentially with the increase in photon energy. The direct optical band gap is calculated which comes out to be 3.54 eV. The value of Urbach energy (E U ) is also calculated using the slope of the plot ln α versus photon energy and it comes out to be 805.8 meV. For electrical properties, the DC conductivity of ZnO film deposited on glass substrate is measured in the temperature range of 450–300 K. On the basis of temperature dependence of DC conductivity of ZnO film, it is suggested that the conduction takes place via thermally activated process.