Microstructures and optical properties of Cu-doped ZnO films prepared by radio frequency reactive magnetron sputtering
Highlights
• The effect of Cu doping, substrate temperatures and annealing temperatures on the crystallization behavior and optical properties of the films were studied. • ZnO films show a stronger preferred orientation toward the c-axis and uniform grain size after Cu doping. • The (0 0 2) orientation of ZnO:Cu film was improved by appropriate substrate temperature. • Intense blue–green luminescence is obtained from the sample deposited at higher substrate temperature. • The crystal quality ZnO:Cu film and the intensity of emission were markedly improved after annealing.
Introduction
As one promising metal–oxide material in the semiconductor field due to its direct wide-band gap (3.37 eV) and large exciton binding energy (60 meV) at room temperature [1], ZnO has received considerable attention for its potential optoelectronic applications in UV region, such as UV light emitting diodes (LED), blue luminescent devices, laser diodes (LD) [2], [3], [4], [5] and so on. As is well known, doping in ZnO film with suitable elements offers an effective method to adjust their electrical and optical properties [6]. Recently, many elements such as Al [7], [8], Mg [9], [10], Co [11], Ga [12], [13], [14], Sn [13], S [15], and Cu [16], [17], [18] have been doped or alloyed into ZnO film and good properties have been obtained. Among the various types of doped ZnO thin films, Cu is a prominent luminescence activator in II–VI compounds, which is considered that Cu dopant could modify the luminescence of ZnO crystals through creating localized impurity levels [16]. In addition, Cu has many physical and chemical properties that are similar to those of Zn. Cu doping has been reported to be able to change the microstructure and optical properties of ZnO thin films [18]. The effect of substrate temperature and annealing temperature on structural properties and optical properties of ZnO:Cu film or ZnO film have been extensively reported and the PL spectra of ZnO:Cu film has been reported [19], [20]. Therefore, substrate temperature is also one of the key factor affecting the quality of the ZnO:Cu film. It is necessary to clarify the effects of dopant and the different conditions of growth in order to develop ZnO:Cu film with high quality and good performance.
In this paper, pure and ZnO:Cu films were prepared on glass substrate by radio frequency (RF) reactive magnetron sputtering technique at different substrate temperatures. The effect of substrate temperature on the structural and optical properties of ZnO:Cu thin films were systematically investigated by scanning electronic microscopy (SEM), X-ray diffraction (XRD) and a fluorescence spectrophotometer. Finally, we discussed the influence of annealing temperature on the structural and optical properties of ZnO:Cu films.
Section snippets
Experiment
Pure and ZnO:Cu films were deposited on glass substrates using radio frequency reactive magnetron sputtering. A high-purity Zn target (99.9999% purity, 60 mm in diameter) and glass (Corning 7105) substrate were used in the experiments. The distance between target and substrate was 50 mm. Before putting into the deposition chamber, the glass substrates were ultrasonically cleaned in acetone and rinsed in deionized water. To conduct Cu doping, Cu foils (purity ∼99.9%) were pasted to Zn target, with
Effect of Cu dopant on the microstructure and morphology of ZnO films
Fig. 1 shows the XRD patterns of (a) pure ZnO film and (b) ZnO:Cu film grown at 150 °C. Both films (samples a and b) were polycrystalline with a structure that belonged to the ZnO hexagonal wurtzite type; no phases corresponding to other oxides were detected, which indicated that Cu ions substituting Zn ions did not change the hexagonal wurtzite structure in ZnO:Cu film. Both samples had a preferential orientation along the c-axis. As shown in Fig. 1, the intensity of the (0 0 2) diffraction peak
Conclusion
Pure and ZnO:Cu films are deposited using RF reactive magnetron sputtering at different substrate temperatures and annealing temperatures. XRD measurements revealed that ZnO films showed a stronger preferred orientation toward the c-axis and the FWHM of the ZnO (0 0 2) peaks decreased after Cu-doping. As for ZnO:Cu films, the strongest ZnO:Cu (0 0 2) peak and the minimum FWHM could be obtained and the residual compressive stress of the ZnO:Cu films decreased with the substrate temperature
Acknowledgments
This work was supported by the National Natural Science Foundation of China (grant no. 10874140) and Natural Science Foundation of Gansu Province (grant no. 0710RJZA105).
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