Correlations between higher-order rings and microvoids in hydrogenated amorphous silicon
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Correlations between higher-order rings and microvoids in hydrogenated amorphous silicon Parthapratim Biswas1,2 and David Alan Drabold2 1 Department of Physics and Astronomy, The University of Southern Mississippi, Hattiesburg, MS 39406, U.S.A. 2 Department of Physics and Astronomy, Ohio University, Athens, OH 45701, U.S.A. ABSTRACT In this paper we report the structure of voids in several thousand atom models of hydrogenated amorphous silicon. The models are produced by jointly employing experimental information from Smets and coworkers [1] and first principles simulations [2]. We demonstrate the existence of a useful correlation between the presence of large irreducible rings and the voids in hydrogenated amorphous silicon networks. Molecular hydrogen is observed in the models, and discussed. INTRODUCTION Hydrogenated amorphous silicon a-Si:H is an important material having applications in solar cells [3], thin film transistors [4], position detection sensors [5], light emitting diodes [6], memory switching devices and other uses [7]. The network structure of the material is characterized by the presence of various defects that play a decisive role to determine its electronic and optical properties of the material [7]. While coordination defects are important, so too are extended inhomogeneities, such as voids, which constitute an important part of the network morphology at high concentration of hydrogen. The voids introduce density fluctuations that may be detected experimentally via small-angle X-ray scattering (SAXS) [8] or similar experiments that can probe electronic density fluctuations in the sample. Likewise, the concentration and the evolution of hydrogen in the network can be obtained via secondary-ion mass spectrometry (SIMS) [9]. Experimental methods, such as, calorimetry [10], multiplequantum nuclear magnetic resonance [11], and infrared absorption spectroscopy [1, 12, 13] provide evidence of the presence of microvoids with diameter in the range of few nanometers (1.0-4.0 nm) depending on the hydrogen content, the history and the method of sample preparation, and the resolution of the measurements. While these measurements provide evidence of the presence of voids and their approximate dimension, the nature and structural features associated with the voids are quite unclear. SAXS data are generally interpreted within the Guinier approximation [14], and is valid for inhomogeneous systems in the dilute limit of the scatterers with a nanometer resolution. Thus, it is instructive to address the problem via direct simulations of voids by developing models involving several nanometers in size using accurate density-functional calculations. In this short paper, we summarize the results from our recent work on computational modeling of voids at high concentration of hydrogen. To this end, we develop a new hydrogenation scheme by using experimental infrared absorption data on film mass density from Ref. [1], which is then coupled with ab initio interactions to generate realistic configurations of
a-Si:H at d
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