Electron Spin Resonance and Electronic Conductivity in Moderately Doped n-type Microcrystalline Silicon as a Probe for t
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Electron spin resonance and electronic conductivity in moderately doped n-type microcrystalline silicon as a probe for the density of gap states T. Dylla, R. Carius, F. Finger Institut für Photovoltaik, Forschungszentrum Jülich, 52425 Jülich, Germany ABSTRACT Electron spin resonance accompanied by conductivity measurements in n-type microcrystalline silicon with different doping concentrations and different structure compositions has been applied for the study of the density of gap states and the influence of these states on charge carrier density. We studied doping concentrations close to the defect density where the doping induced Fermi level (EF) shift is determined by compensation of gap states. We found a correlation between the EF shift, the intrinsic defect density and structural changes. INTRODUCTION Microcrystalline silicon (µc-Si:H) prepared by plasma enhanced chemical vapor deposition (PECVD) is a successfully established material as an absorber layer in thin film solar cells [1–4]. In spite of the technological success and the intensive material investigations, the relationship between the material’s electronic properties and device performance are not well understood and investigations of this relationship are of current interest [3, 5]. In this context study of the nature and density of defects and the influence of these defects on transport properties is of importance. A popular method to do this is electron spin resonance (ESR) in which, however, it always has to be confirmed how the spin density (NS) can be related to the density of relevant defects. In most cases in undoped material the spin density NS will be equivalent to the dangling bond density, NDB. Recently numerous reports on ESR properties of highly crystalline doped-Si:H [6,7,8] and on intrinsic material with a systematic variation of the material structure near the microcrystalline– amorphous transition have been presented [9]. Highly crystalline n-doped material shows a nearly linear dependence of the dark conductivity (σdark) on the doping concentration for PC=[PH3]/([PH3]+[SiH4]) higher than 10ppm. For lower doping the conductivity and presumably the Fermi level (EF) shift deviate from this linear dependence [10]. We assume that in this range the doping induced EF shift is determined by compensation of gap states, and a study of material with low doping levels could be used as a probe for the density of gap states. These investigations should be performed on material with different defect densities of the “intrinsic” undoped state. Such defect density variation is observed upon variation of the process gas mixture used for preparation in PECVD. It was found that the spin density decreases from NS=7x1016 to 3x1015cm-3 with increase of the silane concentration SC=[SiH4]/([SiH4]+[H2]) from 2% to 8%, which results in material with structural compositions all the way from highly crystalline to amorphous [9]. In the present work we investigate material prepared in the SC vs. PC matrix, using doping concentrations close to the intrinsic spin
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