Structural, optical and luminescence characterization of Co-doped ZnO thin films

Abstract

Co inclusion in ZnO induced changes of structural and optical properties was studied using X-ray diffraction (XRD), Raman spectroscopy, ATR-FTIR spectroscopy, UV–Visible spectroscopy and photoluminescence spectroscopy (PL) respectively. As a function of Co molar ratio x in the precursor solution, XRD patterns of corresponding samples revealed a change of crystalline parameters of ZnO from x = 0.06. The appearance of Co₃O₄ and Zn(OH)₂ secondary phases, for x = 0.10, suggested a slowdown of the reaction of the formation of ZnO induced by Co inclusion. These results were evidenced by Raman spectroscopy and ATR-FTIR spectroscopy. This last also revealed the enhancement of ZnO surface adsorption of CO₂ and H₂O after Co inclusion. As a function of Co molar ratio, the mean transmittance in the visible light region decreased from 70.33 % to 23.25 % and the band gap energy E_g decreased from 3.45 to 2.67 eV for x = 0.08. The increase of the band gap energy for x = 0.10 may be caused by the decrease of sp-d exchange interactions induced by the formation of Co₃O₄ and Zn(OH)₂ phases. Except for x = 0.10, all PL spectra, presented an ultraviolet emission peak around 375 nm and a near infrared emission peak around 754 nm. The intensity of these two emission peaks decreased, as a function of Co molar ratio, to completely disappear, for x = 0.10, leading the place to a red emission peak around 697 nm, attributed to Co-based impurities, and other peaks, between 400 and 562 nm, attributed to deep level defects in ZnO. These results make this material suitable for use in the spintronics domain, in gas sensing devices, light emitting diodes (LED) and solar cells.