Defect and Tail States in Microcrystalline Silicon investigated by pulsed ESR
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Defect and Tail States in Microcrystalline Silicon investigated by pulsed ESR P. Kanschat*, H. Mell**, K. Lips*, and W. Fuhs * Hahn-Meitner-Institut, Abt. Silicium-Photovoltaik, Kekuléstr. 5, D-12489 Berlin, Germany ** Philipps-Universität Marburg, Fachbereich Physik, Renthof 5, D-35032 Marburg, Germany ABSTRACT We report on a detailed analysis of paramagnetic states in a doping series of microcrystalline silicon, µc-Si:H, by pulsed electron spin resonance. We identify two dangling bond like structures at g = 2.0052 (db1) and g = 2.0043 (db2). Whereas db1 is evenly distributed in the gap, the db2 state is found to be localized in the lower part of the gap. The CE resonance at g ≈ 1.998 is assigned to electrons in conduction band tail states. In p-doped samples, we observe a broad structure CH at g ≈ 2.08 which we identify with holes trapped in valence band tail states. It is shown that the CH state behaves very similar on illumination as the CE resonance. In n-type samples a pair of hyperfine split lines (A ≈ 11 mT) is found which apparently does not originate from 31P-donor states. On the basis of our results we propose a qualitative model for paramagnetic states in µc-Si:H. INTRODUCTION The electronic properties of disordered semiconductors like microcrystalline silicon (µc-Si:H) may be largely affected by localized states. Providing that these states are paramagnetic, ESR can serve as a powerful tool for their microscopic characterization. A variety of ESR studies on µc-Si:H has been carried out in the past and several signals have been identified [1-4]. The first is a dangling bond like structure around g = 2.005 which has been proposed to consist of two peaks at g = 2.0052 and g = 2.0043 [4]. We refer to these resonances as db1 and db2, respectively. The second structure is the CE line with g ≈ 1.998. It has been shown for P-doped samples that the spin density, NCE, measured at T < 40 K is equal to the doping concentration, nP, and the carrier concentration obtained from Hall effect measurements at T = 300 K [4]. Nevertheless, the nature of these states it is still under discussion. From g-value and spinrelaxation time measurements alone it cannot be decided whether the resonance originates from neutral P-donor states, shallow intrinsic or extrinsic states and/or electrons in the conduction band [4-6]. Whereas the ESR spectrum of c-Si:P (nP < 5⋅1017 cm-3, T < 30 K) is dominated by a pair of hyperfine split lines (A = 4.2 mT) attributed to neutral 31P-donors, the µc-Si:H spectrum is dominated by the CE-line, although a pair of low intensity hyperfine lines (hf) with A ≈ 11 mT has been found as well [4]. In B-doped µc-Si:H a very broad structure at g ≈ 2.1 has been observed, which increases with B-doping [3]. This line is speculated to originate from Bacceptor or hole tail states [7]. In this work we study the influence of the position of the Fermi level on the population of paramagnetic states in µc-Si:H by pulsed ESR aiming at their identification and density of states distribution, DOS. Pulsed ESR has a few co
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