White-light Emitting ZnO-SiO 2 Nanocomposite Thin Films Prepared by Sputtering Method
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White-light Emitting ZnO-SiO2 Nanocomposite Thin Films Prepared by Sputtering Method Yu-Yun Peng1,2, Tsung-Eong Hsieh1 and Chia-Hung Hsu2 1
Department of Materials Science and Engineering, National Chiao-Tung University, Hsinchu, Taiwan 300, ROC 2
Research Division, National Synchrotron Radiation Research Center (NSRRC), Hsinchu, Taiwan 300, ROC
ABSTRACT ZnO-SiO2 nanocomposite thin films were prepared using RF sputtering method without substrate heating. The photoluminance (PL) measurements showed that the ZnO-SiO2 nanocomposite thin films are able to emit white light consisting of violet, blue, and green-yellow band emissions. The blue emission was attributed to the large amount of ZnO/SiO2 interfaces, which enlarges the depletion layer width and then enhances the related transition. Further analyses indicated that the defect structure of samples could be manipulated by the amount and distribution of ZnO nanoparticles in SiO2 matrix to yield distinct luminescence property. INTRODUCTION Zinc oxide (ZnO) is a wide-bandgap semiconductor (Eg = 3.25 - 3.5 eV) with many desirable physical properties In addition to emission in UV region, the ZnO also emits a broad luminescence emission in the green-yellow region. Its large exciton binding energy (59 meV) gives rise to the high efficiency exciton emission at room temperature. Many studies proposed that the nano-sized ZnO with an enormous surface-to-volume ratio exhibits unique luminescence property different from that of the bulk ZnO [1-3]. In this work, white-light emitting ZnO-SiO2 nanocomposite thin films were prepared using RF sputtering without substrate heating. The emission property and the microstructure of the samples are also presented. EXPERIMENTAL ZnO chips were placed on a 3” quartz target during RF sputtering to fabricate ZnO-SiO2 nanocomposite thin films. The number of chips was adjusted to control the ZnO content. Sputtering was performed at the condition of 100 W RF power at 5 mtorr Ar pressure. Glasses are used as substrates and all the deposited layers are approximately 140 nm thick. No substrate heating upon deposition or post-growth annealing was carried out. The microstructure of the samples was characterized by x-ray diffraction (XRD, MacScience M18XHF-SRA, with λ = 0.154 nm) and transmission electron microscopy (TEM, Philips TECNAI 20). The composition
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was examined by x-ray photoemission spectroscopy (XPS) with an Mg-K source (American Physical Electronics ESCA PHI 1600). The PL spectra were measured at room temperature using a 325 nm He-Cd laser. RESULTS AND DISCUSSION Table I shows the contents of ZnO in the thin film samples. Nearly white-light emission was observed for samples with low ZnO content (see Fig. 1). As ZnO content increases, the lights emitted gradually change to yellow and the shapes of the emission spectra change as well. TEM images show that ZnO forms nano-sized crystals embedded in the amorphous SiO2 matrix (see Fig. 2). The sizes of the nanoparticles remains about 3-6 nm regardless of the ZnO
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