Correlation of a-Si:H properties with hydrogen distribution

Optoelectronic Properties of Amorphous Silicon the Role of Hydrogen: from Experiment to Modeling

“…Furthermore, in order to further verify the validity of the model, the authors have also studied the special case of metastable Si-H-Si bonds, observed experimentally by Darwich et al (1995), and have confirmed Darwich's claim within experimental error. Gaspari et al (2009) indicate that the decrease in the vibrational frequency with respect to that of a stable mono-hydride bond is due to the sharing of the hydrogen electron density between two Si atoms. This decreases the Si-H bond strength, increases the bond length and results in reduction of the vibrational frequency.…”

“…However, hydrogen migration and other complex properties must be investigated within a realistic structure to obtain useful information. Kupchak et al (2008) and Gaspari et al (2009) have shown that the commonly used radial distribution function (RDF) is not a sufficient validation parameter, and that the recreation of physically consistent vibrational spectra does, on the other hand, represent a sound validation protocol. The authors have used AIMD to model bulk a-Si:H under various conditions.…”

“…In terms of computing operations, these calculations involve mainly matrix operations and fast Fourier transforms (FFT). Details of this approach can be found in (Gaspari et al, 2009;Kupchak et al, 2008). This approach has led to reproducing all the fundamental vibrational signals in a-Si:H. Computer visualization was used to demonstrate some of the most important features, like the existence of poly-hydride bonding.…”

“…The vibrational spectra obtained for the "good" samples were consistent with experimental data and showed also time stability, as shown in Figures 10 and 11. As previously mentioned, Gaspari et al (2009) have examined the appropriateness of using RDF as a validation parameter for assessing the realism of a simulated structure. The authors compared a variety of a-Si:H samples with H content from 0 to 20%, and have found that the RDF practically does not depend on the amount of hydrogen in the sample.…”

“…According to the authors, the relative ratio of these phases and their distribution has an important role in determining the properties of a-Si:H. This is a fundamental fact that needs to be taken into account, if a model has to be used to simulate processes connected with hydrogen dynamics, including the testing of the models outlined above that have come to prominence as explanations of the Staebler-Wronski effect. Gaspari et al (2010) have examined the hydrogen distribution in simulated samples, obtained by ab-initio Molecular Dynamics (AIMD), by examining the H-H radial distribution function. It was noted that the H-structure and its distribution within the underlying silicon network is crucial in determining the properties of a-Si:H and for finding whether the sample possesses high quality characteristics for photovoltaic or micro-electronic applications.…”