Visible luminescence from highly textured Tb3+ doped RF sputtered zinc oxide films

Abstract

Highly transparent, luminescent, c-axis oriented Tb3+ doped ZnO films are prepared by RF magnetron sputtering technique. The structural, morphological, optical and luminescence properties of these films are investigated as a function of Tb3+ doping concentration by X-ray diffraction (XRD), micro-Raman spectroscopy, atomic force microscopy (AFM), field emission scanning electron microscopy (FESEM), spectroscopic ellipsometry, UV-Visible spectroscopy and photoluminescence spectroscopy. The as-deposited films are found to be highly crystalline with wurtzite hexagonal phase of ZnO. The characteristic features of hexagonal wurtzite structure of ZnO, particularly the appearance of non-polar E2 modes are easily identified from the Raman spectra of the films. The surface morphology of the films revealed by FESEM and AFM images present a dense distribution of grains. The elemental analysis carried out using energy dispersive X-ray (EDX) spectra confirms the incorporation of Tb3+ ions in the ZnO lattice. The films are highly transparent in the visible region. Using ellipsometric analysis, the variation of refractive index, dielectric constant and thickness of the films are studied as a function of Tb3+ doping concentration. The photoluminescence spectra of the Tb3+ doped ZnO films recorded using an excitation radiation of wavelength 325nm from a He-Cd laser exhibit visible luminescence ~430, 490, 516 and 542nm. The origin of visible emissions ~490 and 542nm in the doped films can be attributed to 5D4→7F6 and 5D4→7F5 transition of Tb3+ ion respectively. The intensity of the emission at 542nm is found to be decreasing at higher doping concentration due to concentration quenching effect. The blue emission in the films can be attributed to the electron transition from shallow donor level formed by interstitial Zn atoms to the top of the valence band. The origin of the visible emission ~516nm is attributed to antisite oxygen defects in ZnO.