The Role of Hydrogen for Disordered Silicon
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The Role of Hydrogen for Disordered Silicon N. H. Nickel Hahn-Meitner-Institut Berlin, Kekuléstr. 5, D-12489 Berlin, Germany.
ABSTRACT The properties of hydrogen in disordered silicon are of great interest due to its beneficial and deleterious aspects. This paper highlights some of the manifold properties of hydrogen in silicon. The influence of dopants on hydrogen diffusion in poly-Si, laser-induced dehydrogenation and crystallization of amorphous silicon, and hydrogen bonding in a-Si:H, poly-Si, and c-Si are reviewed.
1. INTRODUCTION The properties of hydrogen in disordered silicon have been investigated in great detail. Without the ability of H to eliminate preexisting defects such as Si dangling bonds, amorphous (a-Si:H), microcrystalline (µc-Si), and polycrystalline silicon (poly-Si) based devices would not be possible. However, the presence of H also gives rise to deleterious effects such as lightinduced defect generation which has been observed in a-Si:H [1] and in H passivated poly-Si [2]. Although the exact microscopic mechanism of this effect remains unsolved, the participation of H was demonstrated experimentally [2]. In addition to the light-induced defect generation mechanism metastable behavior of the temperature dependence of the electrical dark conductivity was observed in hydrogenated polySi [3] and µc-Si [4]. In poly-Si and µc-Si rapid thermal quenching results in an enhancement of the electrical dark conductivity, σD, by up to eight and two orders of magnitude, respectively, with an equilibrium temperature of 268 K. The effect originates from the formation and dissociation of bond-center hydrogen [H(BC)] [5]. This complex consists of a single H atom residing in a Si-Si bond-center site, which has been identified as a donor complex in single crystal silicon [6,7]. One approach to stabilize a-Si:H is to control the hydrogen content in the samples. Higher deposition temperatures have been suggested and the increased stability was attributed to a significant reduction in the hydrogen content from 10 at.% to 4 at.% [8]. On the other hand, when amorphous silicon films are deposited at high temperatures (e.g.: 400 °C) in a remote CVD system a hydrogen content of 10 at.% is maintained. These samples also exhibit an improved thermal stability [9]. These observations clearly show that metastability is not simply connected to the amount of H in amorphous silicon. The key to understand metastability seems to be hydrogen bonding and migration in the amorphous network. Some of the most powerful characterization methods that have been applied are infrared (IR) absorption and Raman spectroscopy. The incorporation of H in disordered silicon gives rise to local vibrational modes (LVM’s). The Si−H stretching vibrations occur at around 2000 to 2100 cm-1 whiles the wagging modes are located near 640 cm-1. A detailed analysis showed that these LVM’s shift with increasing H concentration to higher frequencies. A thorough review of these results can be found in Ref. [10]. A1.5.1 Downloaded from https://www.cambridge.or
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