Optical, Electrical, and Adhesive Properties of ZnO Thin Films

ZnO films were sputtered onto glass substrates to thicknesses from 100 A to 500 A under the following conditions; (a) as-deposited films were maintained at room temperature (RT); (b) films were post-annealed at 150 °C for 1 h, and (c) films were post-annealed at 250 °C for 1 h. X-ray diffraction (XRD) result thus obtained demonstrate that ZnO has a wurtzite structure with a (002) texture diffraction peak with a 2θ of 34° range. The intensity of the ZnO (002) peak increased with film thickness and upon post-annealing. As the ZnO thin film thickness increased and post-annealing was carried out, the grains became larger. A spectral analyzer was utilized to measure transmittance for various thicknesses. Post-annealing treatment promoted the growth of grains, yielding a large mean grain size and, therefore, low transmittance. The as-deposited ZnO thin film with a thickness of 100 Å had a transmittance maximum of approximately 88% and a reflectance minimum of around 12%. Additionally, the four-point probe measurements revealed that p decreased as the ZnO thickness increased and with post-annealing treatment because grain boundaries and the surface of thin films scatter electrons, so thinner films have a greater resistance. ZnO with a thickness of 500 Å that underwent post-annealing treatment at 250 °C had a minimum resistivity of 7.6 x 10⁻³ Ω · cm. Adhesion critically influences the surface energy of films. The surface energy of as-deposited ZnO films was higher than that following post-annealing treatments, revealing that the adhesion of the as-deposited ZnO films was stronger than that following post-annealing treatment because the degree of crystallinity was lower. Accordingly, the thickness and crystallinity of ZnO importantly affects its optical, electrical, and adhesive characteristics. Finally, thinner as-deposited ZnO films exhibited better optical and adhesive properties.