Fabrication of p-type ZnMgO films via pulsed laser deposition method by using Li as dopant source
Introduction
ZnO has now attracted much attention as a promising material for short-wavelength optoelectronic devices, such as light emitting diodes and laser diodes, because of its wide band gap of 3.37 eV and large exciton binding energy of 60 meV at room temperature [1], [2]. Meanwhile, fabrication of ZnMgO is necessary for widening the band gap of ZnO and improving the efficiency of quantum confinement structures from the viewpoint of band gap engineering [3]. However, the realization of p-type ZnO and ZnMgO is difficult due to the asymmetric doping limitations. Due to the considerable worldwide efforts, many groups have successfully achieved p-type ZnO by doping with N [4], [5], [6], P [7], As [8] and Li [9], as well as codoping with N and Al [10], N and In [11], N and Ga [12]. The investigations on p-type ZnMgO films, however, are currently very limited. Only P-doped ZnMgO fabricated by pulsed laser deposition (PLD) [13] and Al–N co-doped ZnMgO fabricated by dc reactive magnetron sputtering [14] have been reported. On the other hand, Group-I species substituting for Zn, such as LiZn, theoretically possess shallow acceptor levels [15]. Experimentally, contradictory results have been obtained. According to previous literatures [16], [17], [18], Li doping typically increases the resistivity of n-type ZnO, leading to semi-insulating samples. Recently, Zeng et al. reported on Li-doped p-type ZnO by magnetron sputtering [9], [19]. In this regards, we investigate p-type behavior in Li-doped Zn0.9Mg0.1O thin films grown by PLD. The characteristics of the films are readily presented.
Section snippets
Experiments
Li-doped Zn0.9Mg0.1O thin films were deposited on glass substrates by PLD. The target was a high-purity ZnO–MgO–Li2O ceramic disk with Mg content of 10 at.% and Li content of 0.4 at.%. A KrF excimer laser (Compex102, 248 nm, 25 ns) was employed as the ablation source. The vacuum chamber was pumped to a base pressure of 4 × 10−4 Pa. The substrates were first cleaned with alcohol and then rinsed in de-ionized water before being loaded into the chamber. The growth temperature varied from 440 to 610 °C.
Results and discussion
The average thickness of the Li-doped Zn0.9Mg0.1O thin films is approximately 300 nm measured by cross-sectional scanning electron microscopy (FEI Sirion 200 FEG SEM). Fig. 1 shows XRD patterns of the Li-doped Zn0.9Mg0.1O thin films grown at different temperatures. Only one peak corresponding to ZnO (0 0 0 2) plane is observed, and no other phase, such as MgO, is detected in the patterns. It is suggested that all the Li-doped Zn0.9Mg0.1O thin films are of highly (0 0 0 2) preferential orientation.
Conclusions
In summary, we have presented Li monodoping method to realize p-type Zn0.9Mg0.1O thin films via PLD. The optimal p-type conduction is achieved at the growth temperature of 550 °C with a resistivity of 65 Ω cm, a carrier concentration of 2.2 × 1017 cm−3, and a Hall mobility of 0.44 cm2 V−1 s−1. The as-grown films show acceptable crystallinity with (0 0 0 2) orientation and high transmittance in the visible region. More detailed investigations on Li-doped ZnMgO are in progress. This approach is expected as
Acknowledgements
This work was supported by National Basic Research Program of China under Grant No. 2006CB604906, National Natural Science Foundation of China under Grant No. 50532060, No. 60340460439 and Zhejiang Provincial Natural Science Foundation of China under Grant No. Y405126.
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