Growth of ZnO thin films synthesized by chemical routes for optoelectronic applications

Abstract

Over past decade, thin films have been important component in device fabrication of many technological applications such optical transmitters, gas sensors, conducting films, solar cells [1]. In recent years, a new category of thin films technology is based on luminescent materials which are mainly used in applications as white light emitting diodes (LED) and field emission displays (FED). Recently, luminescent thin films have been proposed to efficiency enhancement of solar cells [2]. Zinc oxide (ZnO) is currently one of the key functional materials in advanced optoelectronic and photonic applications, including photovoltaics, due to its high transparency across the solar spectrum, excellent electrical properties, and the possibility to synthesize a rich variety of nanostructures [3].In this study, the ZnO thin films were synthetized using a typical chemical bath process. Before the chemical bath, a ZnO layer was deposited on glass substrates by sol-gel method using dip-coating technique. The sol solution was prepared by mixing zinc acetate dihydrate (Zn(CH3COO)2·2H2O), diethylamine (DEA), and adequate volume of deionized water, the mixture was added to 25 ml ethyl alcohol. The cleaned glass was dipped in the sol-gel by a controlled withdrawal speed of 200 mm/min. The dip-coating process was repeated 1 and 3 times to get seed ZnO layers (ZnO-1C and ZnO-3C, respectively). The seed ZnO layers were then placed in the heated (80 ºC) aqueous solution containing 0.025M zinc nitrate hexahydrate (Zn(NO3)2·6H2O) and hexamethylenetetramine (HMT) for 30 min.The morphological information about the ZnO thin films, prior and after coating with the chemical bath process, was obtained by SEM. Fig. 1a) shows the surface imagen of ZnO seed layer deposited by solgel method. It can be observed the formation of nanoparticles which are spherical in shape with a diameter around 30-50 nm. Figure 1b) and 1c) present the nanorods growth using seed ZnO layers with 1 and 3 cycles of deposit, respectively. Comparing the two figures, it can be seen that nanorods diameter slightly increases from 80 to 120 nm as the seed layer growth cycle is increased. However, for ZnO-3C thin film, it is evident that the nanorods grew disorderly which could be explained due to an inhomogeneous deposit of ZnO seed layer on the surface of glass substrate. In order to analyze the structural properties of obtained ZnO thin films, they were characterized by Raman spectroscopy. Both Raman spectra shown four fundamental bands located at 98, 380, 436, and 580 cm -1 , which correspond to the E2(low), A1(TO), E2(high), and E1(LO) vibrational modes of ZnO in hexagonal phase (Fig. 2a)). The Raman bands around 200 and 330 cm -1 are associated to the second order 2E2(low) and multi-phonon E2(high)-E2(low) modes, respectively. As can be observed, there is a decrease in the intensity of all the Raman bands for ZnO-3C film respect to the ZnO-1C film. This indicated that the film had a low degree of crystallinity, possibly due to the formation of structural defects induced by...