Effect of Thickness on the Structure and Properties of ZnO Thin Films Prepared by Pulsed Laser Deposition

Abstract

ZnO thin films with various thicknesses were prepared onto glass substrates by pulsed laser deposition. The crystallinity, microstructure and surface morphology of ZnO thin films were investigated by X-ray diffraction (XRD), scanning electron microscopy (SEM) and scanning probe microscopy (SPM). As the thickness of the films increased, the grain size and the surface roughness increased as well the tensile stress decreased. Also, to study the optical-electrical properties of the films, we tested the carrier concentration, mobility, resistivity, impedance spectroscopy and optical transmission spectra. The results showed that the carrier concentration gradually increased as the thickness increased from 20 to 106 nm, thereafter decreased as the thickness further increased to 216 nm. The main contribution to the conduction of the films with thickness from 20 to 106 nm gradually converted from grain boundaries to grains, and converted into grain boundaries again for the films with a thickness of 216 nm. As the thickness of the films decreased, it was found that the blue shift of absorption edge of optical transmission spectra. Effects of the thicknesses on the optical and electrical properties for ZnO films were related to their grain size, stress and carrier concentration.

Highly transparent and reproducible nanocrystalline ZnO and AZO thin films grown by room temperature pulsed-laser deposition on flexible Zeonor plastic substrates

“…These [51] in which RT glass substrates were used, but differ from those of Zhu et al [37] and Novotny et al [52] in which tensile stress values of ~ 0.3 and 0.9 GPa were found for ZnO deposited on glass. Deposition temperatures of ~ 350 ºC were used in these last two works as well as a 750 ºC oxygen anneal in [52].…”

“…These effects will be reflected in the variations of the film properties with film thickness. For example, Zhu et al [37] studied the change of the crystallinity, microstructure and surface morphology of ZnO thin films of various thicknesses prepared by PLD on glass; while Guillen and Herrero [38] conducted a similar study for AZO films of various thicknesses deposited on glass at room temperature by DC sputtering. Finally, all growth parameters being equal, comparison of the properties of ZnO and AZO films will tell the specific effects of doping by aluminium substitution on the zinc lattice sites [10].…”

Correlation between the stress in ZnO thin films and the Urbach band tail width

“…1) [11,12,14] are given in Table 1. The values of ∆c and σ are calculated using to the following equation for σ [19]:…”

“…where c film is the lattice parameter of c axis for the ZnO films, and c bulk -is the lattice parameter of ZnO in bulk, namely the unstrained lattice parameter (c bulk is 0.5206 nm [19]. The value of σ is negative, demonstrating that the stresses in the deposited films are tensile in a direction of the c-axis.…”

Heteroepitaxial ZnO nano hexagons on p-type SiC

“…Indeed, a thin layer was observed by XTEM (Figure 2b). Table 1 presents data on interplanar space, lattice constant, strain and stress along c-axis obtained by Bragg's equation [33,34] and biaxial strain model [35]. Using the interplanar spacing the stress along the c-axis of the ZnO film can be expressed as [36]: as obtained from the ASTM card for bulk ZnO [33][34][35][36].…”