Reactive magnetron sputtering of transparent conductive oxide thin films: Role of energetic particle (ion) bombardment

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ransparent conductive oxides (TCOs) are degenerately doped compound semiconductors with wide band gaps (Eg . 3 eV), which are used as transparent electrodes in optoelectronic devices. Reports on the influence of negative ions on the electrical properties of TCO films are reviewed and compared with our results. It was reported that the radial resistivity distributions depend (i) on the excitation mode of the magnetron (direct current or radio frequency), (ii) on the erosion state of the sputtering target, and (iii) on the density of the ceramic targets. This can be explained by the fact that the negative ions in magnetron discharges (in our case O) are generated at the target surface and accelerated toward the growing films. Their energy and their radial distribution depend on the discharge voltage and the shape of the emitting surface, i.e., of the erosion groove. Ways for reducing the effect of negative ion bombardment are discussed.

I. INTRODUCTION

The material class of transparent conductive oxides, which are wide-band gap semiconductors (Eg . 3 eV), is both of scientific and technical interest since these oxides can be used as transparent electrodes.1 Their development started in the 1950s with CdO and indium tin oxide (ITO).2,3 Today, mostly tin-doped indium oxide, zinc oxide (ZnO) doped with group III elements, and tin oxide (SnO2) doped with group V elements are used as n-type transparent conductive oxide (TCO) materials.4 Because of the required transparency (at least in the visible spectral range) and the low resistivity, these compound semiconductors have to be doped degenerately, i.e., up to very high charge carrier (electron) concentrations (N ≫ 1019 cm3). The high carrier densities reaching N  1021 cm3 lead to a significant absorption at free carriers (electrons) which sometimes limits its use, for instance in thin film solar cells. One goal is therefore to achieve the low resistivity q 5 (eNl)1 by maximizing the mobility l of the electrons. Furthermore, it is required that the defect density in the band gap is low to reduce the absorption in the TCO layer. The electrical transport in TCO materials is governed by various scattering processes, which are typical for semiconductors: (i) intrinsic scattering by lattice vibrations (acoustic, optical) and (ii) extrinsic scattering due to intentional and unintentional dopants and defects in the films (ionized impurities, defect clusters, point defects, dislocations, stacking faults, grain boundaries, i.e., crysa)

Address all correspondence to this author. e-mail: [email protected] DOI: 10.1557/jmr.2011.428 J. Mater. Res., Vol. 27, No. 5, Mar 14, 2012

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tallographic defects).5 Since TCO layers for transparent electrodes are highly doped polycrystalline films, extrinsic scattering is the dominant process. Although the scattering at ionized impurities, i.e., at the dopants, cannot be avoided (at least in homogeneously doped films), the density of crystallographic defects can be influenced and controlled