Synthesis And Characteristics Of ZnO Thin Films By Thermal Oxidation And Pulsed Laser Deposition Techniques

Zinc oxide (ZnO) is a versatile II-VI semiconductor material with a direct-wide band-gap of 3.37eV at room temperature (RT). In the past decade, the growth techniques and optoelectric properties of ZnO thin films were extensively studied due to its potential device applications in the blue and ultraviolet (UV) region. In this paper, ZnO thin films were fabricated on Si (111) substrates by two different kinds of techniques, one is the thermal oxidation of metallic Zn, and the other is the pulsed laser deposition (PLD). The structural, morphology, and optical properties of the samples were investigated by X-ray diffraction (XRD), scanning electron microscopy (SEM), reflection high-energy electron diffraction (RHEED), and photoluminescence (PL) spectra measurements, respectively. For the ZnO films prepared by thermal oxidation method, XRD studies indicate that the film prepared at 500℃ has the best crystalline quality with the smallest full width at half maximum (FWHM) of 0.24° from (002) diffraction peak. The biaxial built-in stress in the films changes from the compressive to the tensile by increasing the oxidation temperature from 350 to 800℃, and the conversion temperature is in the range of 450-5 00℃. In RT photoluminescence (PL) spectra, the film oxidized at 500℃ exhibits the most intense UV emission with the smallest FWHM of 94.8meV, and the corresponding intensity ratio (R\) of UV emission to deep-level (DL) emission is as high as 162. The deep level emission becomes dominant in the PL spectra when the oxidation temperature exceeds 700℃, and the possible origin of this phenomenon is discussed. For the ZnO thin films prepared by PLD technique. In situ RHEED observations reveal that ZnO thin films directly deposited on Si have a polycrystalline structure, and the crystallinity is deteriorated with an increase of the substrate temperature from 550 to 700℃. By employing a homo-buffer layer deposited at 500℃ in no-oxygen ambient, epitaxial ZnO thin films are obtained which all exhibit aligned spotty RHEED patterns. Among these epitaxial ZnO thin films, the film grown at 650℃ shows the best structural quality with a (002) XRD peak FWHM of 0.185°. Although the samples synthesized on both Si and ZnO/Si at 650℃ show the best optical properties, the ZnO polycrystalline film directly deposited on Si exhibits the more intense UV emission with a RI of 122 which is much lager than the value (38) for the epitaxial thin film with a homo-buffer. This result may be caused by the ZnO buffer layer including a lotof oxygen vacancies, and reasonable preventive methods are suggested to obtain highly optical quality films.