Electrical properties of Li-doped NiO films

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Abstract

Lithium-doped NiO films were deposited by radio frequency magnetron sputtering on Corning 1737 glass substrates. The Li concentrations in the films varied from 0 to 16.29 at.%, as determined by wavelength-dispersive X-ray analysis and inductively coupled plasma mass spectrometry. The effects of Li content on properties such as microstructure, resistivity, and electrical stability were studied. The results show that the doped Li ions tend to occupy crystal defect sites such as vacancies or segregate on the film surface. Initially, doped Li occupied the nickel vacancies in the film, decreasing the electrical conductivity. When the Li concentration was further increased, some Li segregated on the film surface and formed bulges at high Li concentrations. These Li-rich oxides covering the film surface served as partitions between the film and moisture from the atmosphere. As a result, the Li-doped NiO films show a relatively high arrestment to electrical resistance aging.

Introduction

NiO is a promising candidate for p-type semi-transparent conducting films with band gap energy from 3.6 to 4.0 eV.1 Hence NiO film is transparent in visible light. Defect in NiO film formed Ni3+ ions which make the film black. The transmittance of the as-deposited NiO film is about 40% in the visible range, it can be as high as 80% after heat-treated at 300 °C.1 NiO films have a wide range of applications, such as the anode material in organic light emitting diodes (OLED),2 electrochromic display devices,3 and gas sensors.4 Although stoichiometric NiO is an insulator with a resistivity of 1013 Ω cm at room temperature, its resistivity can be lowered by an increase of Ni3+ ions, which results from the creation of nickel vacancies or by the incorporation of monovalent atoms such as Li in NiO crystallities.5

According to the literature, NiO films have been prepared by sputtering,1 chemical vapor deposition,6 and the spray pyrolysis process.7 Among these methods, sputtering is the most widely used. Due to the non-equilibrium nature of sputtering, the defect concentration is higher than that for films prepared by other methods. The resistivity of sputtered NiO film can be as low as 1.4 × 10−1 Ω cm.1 Li doping can also be used to increase the electrical conductivity of NiO films. Li-doped NiO films have been prepared using the spray pyrolysis technique8 and the combinatorial pulse laser deposition technique.9 However, the resistivity of Li-doped NiO films prepared by the above methods is ∼1 Ω cm, which is almost one order of magnitude higher than that of un-doped NiO films prepared by the sputtering process.

Hence, if Ni3+ ions contributed from nickel vacancies created by the sputtering process and the incorporation of Li atoms can coexist in the NiO films, the conductivity is expected to increase further. Stabilizing the electrical properties10, 11 is another important issue for applications of NiO films. Therefore, the effects of Li doping on the electrical stability of sputtered NiO films are also investigated.

Section snippets

Experimental methods

NiO films were deposited on Corning 1737 glass substrates by RF magnetron sputtering. The base pressure achieved was 6 × 10−3 Pa. Pure oxygen was used as the working gas (99.99%) during sputtering and the working pressure was kept at 1.33 Pa. The target material was ceramic NiO (99.99%). The target power was fixed at 200 W and the target area was about 26 cm2. The concentrations of Li in the thin films were adjusted by placing 0–15 Li2O disks on the target surface. The thickness and diameter of the Li

Composition of sputtered thin films

The properties of non-stoichiometric oxide semiconductors strongly depend on trace amounts of impurities and the atmosphere. Reaction parameters such as the substrate temperature, atmosphere, and fabrication techniques have a considerable effect on electrical properties. We thus fixed all other parameters and focused on studying the effect of dopant concentration on the electrical properties of NiO films.

The compositions of the doped NiO films were determined by WDS and ICP-MS. The O/Ni atomic

Conclusions

Pure NiO and Li-doped NiO films were deposited on Corning 1737 substrates by RF reactive magnetron sputtering using Li2O pieces as the doping source. Film composition, resistivity, electrical stability, and microstructure were examined. The results can be summarized as follows.

Increasing the number of Li2O disks was found to effectively increase the Li content in the NiO films. The O/Ni ratio and the electrical conductivity slightly decreased with a small amount of Li in the NiO films because

Acknowledgments

The authors would like to thank the National Science Council of Taiwan for financially supporting this research under grants NSC 95-2221-E-006-081-MY3, NSC 96-2622-E-024-003-CC3, and NSC 96-2221-E-024-020-MY3.

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