Structure And Optical Properties Of ZnO Thin Films

Zinc oxide (ZnO) is a wide band gap (~3.3 eV) semiconductor with a hexagonal wurtzite structure. Its exciton binding energy of 60 meV is much larger than that of ZnSe (20 meV) or GaN (21 meV). Moreover, the epitaxial growth of ZnO thin films can be realized at lower temperatures, as compared with GaN, SiC and wide band gap II-VI semiconductors. These characteristics make ZnO a potential material for room-temperature ultraviolet emission devices. Up to now a large number of investigations have been carried out on the microstructure and optical properties of ZnO thin films. In this dissertation, pulsed-laser deposition (PLD) was used to prepare highly c-axis oriented ZnO thin films. The primary research centers on the influence of the oxygen vacancy, A1N buffer layer and Mg doping on the structure and optical properties of ZnO thin films. The main results are as follows:1. Highly c-axis oriented ZnO thin films have been obtained by laser ablating the Zn target in the oxygen atmosphere at room temperature. Changes of the c-axis lattice constant and optical energy gap caused by oxygen vacancies were studied via the control of the oxygen vacancy concentration afforded by changing the oxygen pressure during the deposition process. The results show that ZnO thin films prepared at room temperature have a preferred c-axis growth orientation. With increasing the oxygen pressure, the lattice constant c decreases and the optical energy gap shifts towards higher energy values. Besides, it is also found that there exists a 10-20 meV energy difference between the ultraviolet absorption edge and intrinsic emission peak.2. The A1N thin film with a preferred c-axis growth orientation was grown on Si(lOO) by PLD. Then, with the A1N buffer layer the quasi-epitaxial growth of the ZnO thin film was successfully realized on the Si substrate. By comparing ZnO/AlN/Si with ZnO/Si, the effect of the A1N buffer layer on the structure and optical properties of ZnO thin films was investigated. The results indicate that the highly c-axis oriented ZnO thin film has been obtained on Si(100) with the A1N buffer layer. The full width at half maximum (FWHM) of ZnO(002) diffraction peak is 0.216°, suggesting a good crystalline quality. Further, the quasi-epitaxial growth of the ZnO/AlN/Si thin film is confirmed by atomic force microscopy (AFM). As for the optical properties, the photoluminescence spectra show that the fluorescence intensity of the ZnO/AlN/Si thin film is nearly an order of magnitude higher than that of the...