Charge transport through localized states in sputtered amorphous silicon suboxides

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Charge transport through localized states in sputtered amorphous silicon suboxides J.J. van Hapert, N. Tomozeiu, E.E. van Faassen, A.M. Vredenberg, F.H.P.M. Habraken Debye institute, Utrecht University, P.O.Box 80.000, 3508 TA Utrecht, The Netherlands

ABSTRACT Using an RF magnetron sputtering technique, thin layers (~500 nm) of amorphous silicon suboxides (a-SiOx) were deposited, with oxygen/silicon ratios x ranging from 0 to 1.8. These layers contain a large density (1020-1021 cm-3) of, mostly silicon dangling bond, defect states. The level of conduction decreases several orders of magnitude with increasing x. The temperature dependence of the DC conductivity showed that the variable range hopping conduction mechanism is dominant for all x, over the temperature range 30- 330 K. In this mechanism the extent of localization and density of states around the Fermi level determine the conductance. We conclude that the decrease in conductance with increasing oxygen content must, for a large part, be due to a variation in the localization, since Electron Spin Resonance (ESR) measurements showed no decrease in defect density with increasing x. We performed DC conduction measurements at both low and high electric field strengths, showing phenomena, which are consistently desribed within the variable range hopping (VRH) model. These measurements allow the extraction of quantitative information, concerning both the localization and the density of the states involved in the hopping process.

INTRODUCTION The technological importance of the amorphous silicon suboxide system a-SiOx, as Si/SiO2 interface layer or as semi-conducting/insulating layer with tunable resistivity, makes the study of the structural and electrical properties of this system of commercial as well as fundamental interest. Non-stoichiometric hydrogenated a-SiOx:Hy is studied extensively in the past to understand the influence of a small percentage of oxygen (1-10%) on various properties of amorphous silicon [1,2]. More recently, a-SiOx(:Hy) has gained new interest as a wide band gap material for applications in (photo)electronic devices. Studies have reported on both sputtered [3] and chemical vapor deposited [4] layers a-SiOx(:Hy). The mechanisms of electrical conduction differ between the materials deposited with different techniques, and are not all well understood. This study presents the characterization of RF magnetron sputtered a-SiOx. These materials appear to have an unusually high density of localized defect states, resulting in a hopping conduction mechanism even up to room temperature. This provides a good opportunity to investigate the properties of the localized defect states, i.e. the localization parameter of these states, which is of importance in all hopping and scattering mechanisms. In this study we model the conductance of a-SiOx in terms of a (variable range) hopping process, for all x. To understand the dependence of the conductance on oxygen concentration we investigate both temperature and electric field dependencies of the conductan

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