Growth of ZnO thin film on SiO2 substrates by the RF magnetron sputtering method

Abstract

In recent years, ZnO thin films have been extensively studied for surface acoustic wave (SAW) devices, optical wave-guides, and transparent conducting coatings [1]. Since ZnO has a wide band gap of 3.37 eV, low power threshold for optical pumping at room temperature, and UV emission resulting from a large exciton binding energy of 60 meV, it can be used in light emitting diodes (LED), photodetectors, electroluminescence devices and the next generation of UV lasers. ZnO films have been grown by various deposition methods, such as sputtering [2], sol-gel process [3], spray pyrolysis [4, 5], pulsed laser deposition [6, 7], ion beam deposition [8], plasma enhanced chemical vapor deposition (PECVD) [9], atomic layer deposition (ALD) [10], filtered cathodic vacuum arc technique [11], evaporation [12], metal-organic chemical vapor deposition (MOCVD) [13], and molecular beam epitaxy (MBE) [14]. Among them, one of the most commonly used techniques is a sputtering method due to its simplicity and the possibility of obtaining good orientation and uniform films even on amorphous substrate or at low growth temperature. Since amorphous substrates such as SiO2 and glass substrate have obvious technological advantages and potential applications [15], we have grown ZnO film on SiO2 substrate using a RF magnetron sputtering system. Although many researchers have grown ZnO films on sapphire, Si or glass substrates, there are few reports on growing ZnO thin films on SiO2 using a RF sputtering technique. The ZnO film was deposited on SiO2 substrates. The SiO2 layer had been thermally grown with a thickness of 60 nm. Before loading into the reactor, the substrate was cleaned in acetone for 10 min then rinsed in deionized water for one minute. In this experiment, we used a ZnO (99.99% purity) target with a diameter and a thickness of 75 mm and 60 mm, respectively. Fig. 1 shows a schematic diagram of the RF sputtering system used in our experiments. RF sputtering was carried out in an Ar (99.99% purity) gas atmosphere by supplying RF power of 300 W at a frequency of 13.56 MHz. The flow rate of the Ar gas was set to 30 sccm. The distance between target and substrate was about 80 mm. The ZnO film was grown at 200 ◦C temperature at a pressure of 5.0 × 10−2 Torr and deposition was carried out for 30 min. Before deposition, the pressure of the RF sputtering system was about 6 × 10−6 Torr. The structural characteristics of the films were ana-