Stokes shift, blue shift and red shift of ZnO-based thin films deposited by pulsed-laser deposition

Abstract

Undoped ZnO and X-doped (Mg and Cd) ZnO thin films (5.0 at % dopant) were deposited on glass substrates at 400 °C using pulsed-laser deposition. An X-ray diffractometer (XRD) was used to investigate the structural properties of the thin films. XRD measurements indicated that all of the thin films had a preferred (0 0 2) orientation. A scanning probe microscope in AFM mode was used to investigate the surface morphologies of the thin films. The X-doped ZnO thin films had rougher surfaces and larger grain sizes compared to the undoped one. A spectrophotometer was used to measure the transmissions of the thin films. The transmittances of the X-doped ZnO thin films were obviously smaller than that of undoped one. The absorption edge was found to shift depending on the dopant materials. The absorption edge of Mg-doped ZnO was found to be blue shifted. However, the absorption edge was found to be red shifted for Cd-doped ZnO thin film. To calculate the band-gap energies of the thin films, the relative absorption coefficients were calculated. By using linear fittings of the absorption edges, the band-gap energies of thin films were derived as 3.271, 3.434, and 3.216 eV for undoped, Mg-doped, and Cd-doped ZnO thin films, respectively. In photoluminescence (PL) spectra, near band edge (NBE) emissions were observed in all of the films. By comparing the results from absorption spectra and PL spectra, the Stokes shifts were observed. The peaks of the NBE emissions were found to be located at 3.260, 3.395, and 3.205 eV for undoped, Mg-doped, and Cd-doped ZnO thin films, respectively. The Stokes shifts were calculated to be 11, 39, and 11 meV, respectively, for undoped, Mg-doped, and Cd-doped ZnO thin films.