Dependence of properties of N–Al codoped p-type ZnO thin films on growth temperature

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Abstract

N–Al codoped p-type ZnO thin films have been realized by dc reactive magnetron sputtering in N2O ambient. Hall measurement, X-ray photoelectron spectroscopy, X-ray diffraction and optical transmission were carried out to investigate the effect of growth temperature on the properties of codoped films. Results indicated that N–Al codoped p-type ZnO films with good structural, electrical and optical properties can only be obtained at an intermediate temperature region (e.g., 500 °C). The codoped p-type ZnO had the lowest resistivity of 57.3 Ω cm, and a carrier concentration up to 2.52 × 1017 cm−3. In addition, the N–Al codoped p-type ZnO film deposited at 500 °C was of good crystallinity with a (0 0 2) preferential orientation, and high transmittance about 90% in the visible region.

Introduction

Zinc oxide is a novel II–VI compound semiconductor with various electrical, optical, acoustic, and chemical properties because of its wide direct band gap of 3.37 eV at room temperature. It has attracted considerably attention as a promising material for short-wavelength optoelectronic applications, such as light emitting diodes (LEDs) and laser diodes (LDs) [1], [2]. Recently, significant efforts have been made to realize p-type ZnO [3], [4], [5], [6]. However, due to asymmetric doping limitations, ZnO can easily be doped n-type, but is rather difficult to dope p-type [7]. Among possible acceptor dopants, nitrogen is a good candidate for producing a shallow acceptor level in ZnO [7]. Yamamoto and Yoshida [8], based upon first-principle calculations, proposed a donor–acceptor codoping method, simultaneous doping with acceptors (e.g., N) and reactive donors (e.g., Al, Ga, or In), which can be expected to decrease the Madelung energy due to the formation of suchlike 2N–Ga complex in ZnO and enhance the incorporation of N, making p-type doping of ZnO with good properties possible. A few reports have emerged indicating some success with the codoping approach using nitrogen and gallium (N–Ga) by pulsed laser deposition [5] or molecular beam epitaxy [6].

Recently, further theoretical calculations suggested that the codoping with nitrogen and aluminum (N–Al) should be a better candidate among various codoping techniques [9]. Moreover, compared with Ga or In, Al has lots of advantages, such as low cost, environment friendly, and so forth. In this paper, we reported p-type ZnO films prepared using the N–Al codoping method by dc reactive magnetron sputtering in N2O ambient. The effect of growth temperature on the properties of codoped films was systematically examined.

Section snippets

Experiments

N–Al codoped ZnO films, as well as N-doped ZnO films, were prepared on glass or Si (1 0 0) substrates by dc reactive magnetron sputtering. The target for codoped films was a disc of Zn metal (99.99% purity) mixed with 0.15 wt.% Al (99.99% purity), and for N-doped films was a pure Zn metal. High purity N2O (99.99%), acting as both oxygen source and nitrogen doping source, was used as the sputtering gas. A constant total pressure was maintained at a value of 5 Pa. Before the deposition, the target

Results and discussion

The electrical properties of ZnO films, prepared on glass substrates, were examined by Hall measurements at room temperature. The results are summarized in Table 1. To examine the reliability and repeatability of the conduction of the films, the electrical measurements were performed several times, and much the same results were obtained as expected. All the N–Al codoped films show p-type conduction, indicating successful realization of p-type ZnO by codoping of nitrogen and aluminum. As the

Conclusions

In summary, we have presented a promising N–Al codoped method to realize p-type ZnO films. N2O acts both oxygen source and nitrogen doping source. Results showed that the codoping with N and Al induces the formation N–Al bands in ZnO. The properties of codoped films are dependent greatly on the growth temperature. The codoped film obtained at 500 °C possesses good p-type conduction at room temperature, as well as good crystallinity with (0 0 2) orientation and high transmittance in the visible

Acknowledgements

This work was supported by the National Natural Science Foundation of China under Contract No. 90201038. The authors would like to thank Professor P.M. He (the Physics Department, Zhejiang University) for his help in XPS analysis, and Professor Z.G. Ji (State Key Lab of Silicon Materials, Zhejiang University) for his help in the XRD analysis.

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