Electrical and optical properties of Al doped cadmium oxide thin films deposited by radio frequency magnetron sputtering

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Abstract

Cadmium oxide thin films with different percentages of aluminum doping have been synthesized via radio frequency magnetron sputtering technique. Thin films were deposited on glass and silicon substrates with different percentages of aluminum at a substrate temperature of 573 K and pressure of 0.1 mbar in Ar+O2 atmosphere. The deposited films were characterized by studying their structural, electrical and optical properties. The X-ray diffraction pattern revealed good crystallinity with preferred (1 1 1) orientation in the films. Aluminum doping in CdO thin films were confirmed by X-ray photoelectron spectroscopic studies and actual doping percentages were also measured from it. The optical band gap was found to decrease first and then increase with increasing percentages of aluminum concentrations. The electrical conductivity was found to increase with increase of aluminum doping concentration up to 5% but for higher doping concentration (>5%) the conductivity was found to decrease.

Introduction

Transparent conducting oxides (TCOs) have wide applications in the field of optoelectronic devices. Thin films of n-type transparent conducting oxides (n-TCOs) such as zinc oxide (ZnO) [1], cadmium oxide (CdO) [2], [3], [4], indium oxide (InO) [5], tin oxide (SnO2) [6], indium tin oxide (ITO) [7] and p- type transparent conducting oxides (p-TCOs) such as CuAlO2 [8], [9], SrCu2O2 [10], tin–gallium oxide [11], etc. have received considerable attention mainly due to their important potential applications, which include photovoltaic solar cells, gas sensors, transparent electrodes and other optoelectronic devices. High electrical conductivity (even without doping) and high optical transmittance in the visible region of the solar spectrum along with a moderate refractive index (r.i.) make CdO thin film useful for various applications such as solar cells, phototransistors, photodiodes, etc. Different physical and chemical deposition techniques such as spray pyrolysis [12], [13], [14], sol–gel [15], [16], [17], [18], DC magnetron sputtering [19], [20], [21], radio frequency (RF) sputtering [22] chemical bath deposition [23], [24], [25], [26], etc. have been employed to prepare CdO thin films. In this work we have employed RF sputtering technique to prepare Al-doped CdO because RF sputtering is one of the most versatile techniques [27] for thin film deposition. Doping of CdO thin films incorporating various elements such as Sn [28], In [4] and F [29] have already been studied. Literature survey reveals that there is no report on Al doped CdO thin films, except our previous work, where we reported CdO:Al thin films by sol–gel method [16]. Along with the technological interest in CdO thin films doped with different elements there is considerable interest in understanding the influence of degenerate doping on the optical properties, particularly on the band gap of the host semiconductor [30]. In this work we have studied in detail the effect of Al doping on the optical and electrical properties of CdO thin films synthesized by RF magnetron sputtering technique.

Section snippets

Target preparation and film deposition

Targets for the sputtering were fabricated by taking appropriate amount of commercially available polycrystalline CdO powder (99.9%, Aldrich) and different amounts of aluminum powder (99.99%) so that Al atomic percentages in the mixtures were 2%, 5%, 7% and 10%. The powders were mixed in a mortar thoroughly for about 1 h and it was then placed in a grooved aluminum holder (5 cm in diameter) and pressed into pellet by hydrostatic pressure of approximately 100 kgf/cm2 and used as target for RF

Structural and morphological characterization

The XRD patterns of the films are shown in Fig. 1. Different peaks in the XRD pattern appeared at 2θ values 33.09°, 38.54° and 55.28° due to reflections from (1 1 1), (2 0 0) and (2 2 0) planes, respectively, of CdO. The relatively stronger intensity of the peak at 2θ=33.09° indicates preferential (1 1 1) orientation of the films [31]. XRD pattern also reveals that CdO has face centered cubic (FCC) structure with a lattice parameter 4.695 Å. The information on the strain ε as well as the crystallite

Conclusions

CdO thin films with Al doping were prepared by RF magnetron sputtering on glass substrates at a fixed temperature of 573 K and pressure of 0.1 mbar in argon+oxygen atmosphere. Good crystallinity with proper cubic structure was revealed from the XRD study. The AFM image indicated presence of nearly spherical grains with average size ∼40 nm and with average rms roughness of 6.7 nm. XPS study for the compositional analysis shows nonstoichiometric CdO films with O/Cd ratio of 0.75. The electrical

Acknowledgments

One of us (BS) wishes to thank the University Grants Commission (UGC), Government of India, for awarding him a junior research fellowship (JRF) during the work. The authors also wish to acknowledge the financial support by the Department of Science and Technology (DST) and the University Grants Commission (UGC), Government of India, during the execution of the work.

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