Microscopic Model for Creation And Removal of Metastable Dangling Bonds in a-Si:H
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MICROSCOPIC MODEL FOR CREATION AND REMOVAL OF METASTABLE DANGLING BONDS IN a-Si:H M.J. Powell, S.C. Deane and R.B. Wehrspohn1 Philips Research Laboratories, Redhill, Surrey, RH1 5HA, United Kingdom 1 Max-Planck-Institute of Microstructure Physics, Weinberg 2, 06120 Halle, Germany ABSTRACT We present a new microscopic model for metastable Si dangling bond defect creation in hydrogenated amorphous silicon, which is applicable to both light-induced and carrier-induced defect creation. The key feature of our model is that hydrogen is always in the tedrahedral-like site, which is strongly bound in amorphous silicon, and never in the bond-centered site. Breaking of Si-Si bonds and successive stabilisation by bond-switching of nearby hydrogen from doubly hydrogenated Si-Si bonds (SiHHSi) results in two hydrogen-stabilised dangling bonds (SiHD). Since hydrogen is in the Td site in all configurations, this defect creation reaction is consistent with ESR, NMR, hydrogenation experiments and the mechanical stress dependence of defect creation. INTRODUCTION Hydrogenated amorphous silicon is one of the most studied amorphous semiconductors. It is predominantly used in thin film transistors and solar cells. One of the major limitations of amorphous silicon today is the formation of dangling bond defects during light illumination or carrier accumulation. In spite of 20 years of research there is no consensus for a microscopic model that can explain all the experimental results. It is well established that dangling bonds are created. The key problem revolves around the role of hydrogen. It is generally considered that H motion probably plays some role in stabilising the defects, for example, by inserting a hydrogen atom into a broken Si-Si bond to form a SiHD defect (an intimate Si dangling bond and Si-H bond). However a H atom placed at the bond-centered position, would imply that the dangling bond and H atom are separated by about 2 Å, which would lead to a measurable hyperfine broadening of the ESR signal, which has never been observed [1]. In the following, we discuss key experimental results on defect creation divided in local and global techniques. Local techniques like ESR and NMR, give direct information about the microscopic environment of hydrogen and dangling bonds. Global techniques like hydrogenation experiments or mechanical stress experiments give indirect evidence about the role of hydrogen during defect creation. KEY EXPERIMENTAL RESULTS Local techniques There are few measurement techniques, which yield direct information about the local environment of dangling bonds in amorphous silicon. These are mainly ESR, NMR and related techniques. The first important observation is that there is no difference in the ESR signal for stable and metastable defects in amorphous silicon [2]. The next question arises from the local A11.1.1 Downloaded from https://www.cambridge.org/core. Teachers College Library - Columbia University, on 22 Apr 2018 at 12:13:15, subject to the Cambridge Core terms of use, available at https://www.cam
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