Room Temperature Synthesis and Optical Properties of Nanostructured Ba-Doped ZnO Thin Films

Abstract

Zinc oxide films with and without barium-doping have been prepared by the SILAR technique with different percentages (1 M%, 3 M%, 5 M% and 7 M%) of Ba content on microscopic glass slides. The effects of the concentration of barium doped on zinc oxide (BaZO) were characterized by X-ray diffraction (XRD), UV-Vis Spectroscopy, Scanning Electron Microscopy (SEM) and Energy Dispersive X ray (EDX).The XRD studies confirmed that the structural growth of ZnO films were polycrystalline in nature. The Ba doped ZnO films deposited were found to have strong orientation at (002) lattice plane which exhibited polycrystalline hexagonal wurtzite structure. The SEM showed that the grain size, thus the roughness, increased as a result of increase in the level of barium doping. Optical bandgaps obtained decreased from 3.41 eV to 3.19 eV with an increase in Ba concentration.

Keywords: Ba-Zinc Oxide; SILAR Technique; Thin films; optical properties; SEM; XRD.

1. Introduction

For many years, the synthesis and characterization of ZnO thin films have been an interesting area of research. The wurtzite structure of ZnO forms an n-type semiconductor with direct band gap

of about 3.37eV at a temperature of 300K [1], and high electronic mobility. As a wide and direct band gap semiconductor, ZnO thin films have attracted more and more attention in optoelectronic devices [2–5], photo-thermal conversion systems, heat mirrors, blue/UV light electrode devices, solid state detectors, crystal clear electrodes, heterojunction solar cells etc [6–10]. Undoped ZnO thin films have definite restrictions in their usage. In order to broaden the potential areas where ZnO thin films can be used, dopant ions have to be integrated into them to obtain definite desired properties like lower or higher melting point, broader or narrower band gap, higher optical absorbance, etc. by applying different techniques.

Different techniques have been applied to obtain ZnO nanostructured thin films such as RF magnetron sputtering [11], chemical vapour deposition [12], chemical bath deposition [13], pulse laser deposition [14]. Others include spray pyrolysis [15], sol-gel process [16], chemical co-precipitation [17], hydrothermal [18], ultrasonic irradiation assisted solution route [19], solid-state reaction [20], successive ionic layer adsorption and reaction (SILAR) [21,22],etc. Among these techniques, SILAR technique has lately attracted appreciable attention because of its low cost and simplicity. Furthermore, this technique requires low temperatures and facilitates large-area deposition.

 In recent work, an attempt has been made to synthesize Ba doped ZnO thin films by SILAR technique, since it involves adsorption of a layer of complex ion on the substrate after that reaction of the adsorbed ion layer [23]. Ba doped ZnO is an important technological material used for example in varistors and liquid sensors’ guiding layers[24]. Precisely, doping ZnO with Ba procures to ZnO films a rougher surface, a stronger internal stress, and a low density crystalline structure [25].This paper investigates the effect of varying percentage of barium (Ba) as a dopant on zinc oxide (ZnO) thin films synthesized by SILAR method with ZnSO₄.2H₂O and BaCl₂ as precursor and dopant materials respectively.

2. Experimental Details