Strong near band edge emission of (Ce, Yb) co-doped ZnO thin films after high temperature annealing

Abstract

We studied the photoluminescence (PL) properties of (Ce + Yb) co-doped ZnO thin films as a function of high temperature annealing. The films were fabricated by magnetron sputtering. After 1000-1100°C annealing, the near band edge (NBE) emissions of the films were dozens to a hundred times stronger than that of undoped ZnO, while the Yb 3+ emission (~980 nm) was quite weak, indicating that energy transfers from the ZnO host to Yb 3+ ions in the films were not efficient. X-ray diffraction analysis and scanning electron microscopy observations demonstrated that the (Ce + Yb) co-doping had a large effect on the morphology and crystallinity of the films. The crystallinity enhancement of the films is considered to be the main reason for the strong NBE enhancements of the co-doped ZnO films.

Effect of Eu doping on the near band edge emission of Eu doped ZnO thin films after high temperature annealing

“…The most striking feature is that there are small nanoscale voids within the film. Similar phenomena were reported in our previous work on Ce and Yb doping [11,13], which could result from diffusion processes (Kirkendall-voids). The change of the O 2peak before and after the Ar etching, could be mainly due to change of chemisorbed oxygen rather than the variation of defects.…”

“…The FWHM can also have contributions from strain, so the Scherrer formula gives a minimum size, but the fact that the FWHM decreases confirms that the crystallinity of the sample films has improved by the high temperature annealing. That is similar to what we have observed for ZnO films sputtered similarly to S0-S7 but doped with Yb and/or Ce [11][12][13]. The inset of Fig.…”

“…of 450-700 nm (2.76-1.77 eV) for each PL spectra (the ratio for S6 is not shown to minimize clutter). The ratios increase with increasing anneal temperature, which has been reported to be correlated with improved crystal quality of the film for some RE doped ZnO films [11][12][13]17]. We will in the following see that this correlation also holds for these films.…”

“…In this work, we focus on the enhancement of UV emission by high temperature annealing and the effect of Eu doping on the UV emission. We here test the hypothesis, deduced from our previous observations [11][12][13], that Eu additions to ZnO sputtered films will have similar effects as we have observed for Ce and Yb doping.…”

“…Among the oxide hosts that have been investigated are ZnO-SiO 2 based systems which appear attractive. Inspired by that we have previously studied ZnO films doped with cerium (Ce) and/or ytterbium (Yb) reacting with a Si substrate [11][12][13]. These films were sputter-grown from RE oxides and ZnO composite targets with an O 2 partial pressure yielding oxygen rich films.…”