Amorphous semiconductors studied by first-principles simulations: structure and electronic properties
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1153-A04-03
Amorphous semiconductors studied by first-principles simulations: structure and electronic properties Karol Jarolimek1,2 and Robert A. de Groot2 and Gilles A. de Wijs2 and Miro Zeman1 1 Electrical Energy Conversion Unit/DIMES, Delft University of Technology, P.O. Box 5053, 2600 GB Delft, The Netherlands 2 Electronic Structure of Materials, Institute for Molecules and Materials, Radboud University Nijmegen, Heyendaalseweg 135 , 6525 AJ Nijmegen, The Netherlands
ABSTRACT A simulation study of the electronic structure of two amorphous semiconductors with great technological importance, namely hydrogenated amorphous silicon (a-Si:H) and hydrogenated amorphous silicon nitride (a-SiN:H), is presented. The simulations were based on density functional theory (DFT) which provides accurate description of interactions between the atoms for a wide range of chemical environments. Our model structures were prepared by a coolingfrom-liquid approach. We found that the cooling rate during the thermalization process has a considerable impact on the quality of the resulting models. A rate of 0.023 K/fs proved to be sufficient to prepare models with low defect concentrations. To our knowledge we present for the first time calculations that are entirely based on the first-principles and produce defect-free models of both a-Si:H and a-SiN:H. Although creating models without any defects is important, on the other hand a small number of defects present in the models can give valuable information about the structure and electronic properties of defects in a-Si:H and a-SiN:H. The presence of both the dangling bond and the floating bond was observed. Structural defects were related to electronic defect states within the band gap. In a-SiN:H the Si-Si bonds induce states at the valence and conduction band edges, thus decreasing the band gap energy. This finding is in agreement with measurements of the optical band gap, where increasing the nitrogen content increases the band gap. INTRODUCTION Amorphous semiconductors are materials of a great technological interest. Hydrogenated amorphous silicon (a-Si:H) deposited by the plasma-enhanced chemical vapor deposition (PECVD) technique is widely used in large area electronic devices such as solar cells and liquidcrystal displays [1]. Thin films of hydrogenated amorphous silicon nitride (a-SiN:H) grown by PECVD are routinely used as insulation in integrated circuits [2,3]. Other applications include optical waveguides [4], nonvolatile memory devices [5] and passivation layers [6]. There have been numerous theoretical studies on amorphous semiconductors in the past. The models of their atomic structures have been prepared with methods ranging from classical potentials to the state-of-the-art ab initio methods. Atomic structures are often prepared by cooling from the liquid using a molecular dynamics (MD) simulation [7-12]. This approach was criticized for generating a-Si:H models with a high defect concentration that was unrealistic [13]. A device quality a-Si:H material has a defect con
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