Optical and electrical properties of ZnO:Al thin films synthesized by low-pressure pulsed laser deposition

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Abstract

ZnO:Al thin films were prepared at a low oxygen pressure between 0.02 and 0.1 Pa by pulsed laser deposition (PLD). The structure as well as their optical and electrical properties was investigated by X-ray diffraction, optical transmittance spectroscopy, and Hall measurements. The ZnO:Al films possess resistivity of the order of 10−4 Ω cm and the optical transmittance exceeds 80% in the visible range. The highest electron concentration (1.18×1021 cm−3) is achieved at a deposition pressure of 0.02 Pa and it decreases slightly with increasing oxygen pressure. The band gap is found to depend on the electron concentration.

Introduction

Zinc oxide (ZnO), with a band gap (∼3.37 eV) and large exciton binding energy (∼60 meV), has received broad attention for the potential applications in the ultraviolet (UV) light emitting devices (LED) and laser diode (LD) [1], [2], [3], [4]. Besides, ZnO is an excellent transparent and conducting oxide (TCO) material by doping B, Al, Ga, or In [5], [6], [7], [8]. The resistivity of 1×10−4 Ω cm has been achieved in n-type ZnO films. Thus, ZnO is a promising alternative to indium tin oxide (ITO) or SnO2 in TCO applications. Recently, Al or Ga doped ZnO (AZO, GZO) films have been widely applied as transparent electrodes of the solar cells [9], [10], [11]. Nevertheless, heavily doped ZnO films always exhibit low transmittance and high reflectance behavior in the infrared (IR) region, which affects the applications in thin-film solar cells. Thus, it is necessary to improve the carrier mobility for increasing the transparency in the near IR band while maintaining low resistivity [12]. It is known that the deposition pressure is an important parameter affecting the performance of TCO films. Low pressure is propitious to increase the energy of the particles reaching the substrate, and improve the crystallization of AZO films. Pulsed laser deposition (PLD) is an important method for growing ZnO films of good crystal quality. The deposition pressure could be varied in a rather large range using this method.

In this study, highly transparent and conducting AZO thin films were prepared under low-pressure O2 ambient of 0.02–0.1 Pa by PLD. The effects of oxygen pressure on the structural, electrical, and optical properties of the films were investigated and discussed in detail. The low resistivity of 3.0–4.0×10−4 Ω cm was achieved in the AZO films. It was found that the optical properties were sensitive to the carrier concentration.

Section snippets

Experimental

AZO thin films were prepared on glass substrates at 300 °C by pulsed laser deposition, using a ceramic target. The target was fabricated using high-purity ZnO (99.99%) and Al2O3 (99.99%), with an atomic ratio of 98:2. The deposition was carried out in high-purity O2 ambient (99.9995%) in the range 0.02–0.1 Pa. Prior to deposition, the deposition chamber was evacuated to a pressure of 10−4 Pa to ensure a high background vacuum. A KrF laser operating at 248 nm and 5 Hz was used as excitation source.

Results and discussion

Fig. 1 shows the XRD patterns of AZO films deposited under various pressures. The deposition pressure varied in the range of 0.02–0.1 Pa. A low pressure could reduce collision of the particles with O2 and enhance the mobility of the deposition atoms, which could decrease the defects in AZO films and improve the quality of films. As seen, only (0 0 2) peak was observed in the patterns of all the samples, showing a polycrystalline wurtzite structure with c-axis perpendicular to the substrate. No

Conclusions

In summary, the AZO films have been deposited on glass substrates by pulsed laser deposition. The AZO films exhibited a low resistivity of 3.0×10−4 Ω cm and a high transmittance over 90% in the visible band. As the oxygen pressure increased, the films showed an increase in resistivity and a reduction in electron density. It was found that the optical band gaps were dependent on the electron density. We hope that this study will shed light on understanding the relationship between optical

Acknowledgments

The work is supported by the National Natural Science Foundation of China no. 51072181 and the Doctoral Fund of Ministry of Education of China under Grant no. 20090101110044.

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