I. M. Kupchak

Temperature dependent vibrational spectra and bond dynamics in hydrogenated amorphous silicon

We present the results of extensive modeling of hydrogenated amorphous silicon (a-Si:H) by combined ab initio molecular dynamics, an improved signal processing technique, and computer visualization, focusing on vibrational properties of a-Si:H. By comparing the theoretical and experimental vibrational spectra we correlate the hydrogen and silicon dynamics with the structural stability, bonding geometry, and diffusion in the a-Si:H material. Microscopic characteristics that cannot be obtained by other techniques, namely, hydrogen migration, bond switching, and silicon dangling bond passivation, are extracted from the atomic dynamics. We also demonstrate that this method offers the possibility of accessing other important macroscopic characteristics of a-Si:H and its stability in general. The approach we have developed can be used as well to model all aspects of a-Si:H dynamics, including the detrimental Staebler–Wronski effect.We present the results of extensive modeling of hydrogenated amorphous silicon (a-Si:H) by combined ab initio molecular dynamics, an improved signal processing technique, and computer visualization, focusing on vibrational properties of a-Si:H. By comparing the theoretical and experimental vibrational spectra we correlate the hydrogen and silicon dynamics with the structural stability, bonding geometry, and diffusion in the a-Si:H material. Microscopic characteristics that cannot be obtained by other techniques, namely, hydrogen migration, bond switching, and silicon dangling bond passivation, are extracted from the atomic dynamics. We also demonstrate that this method offers the possibility of accessing other important macroscopic characteristics of a-Si:H and its stability in general. The approach we have developed can be used as well to model all aspects of a-Si:H dynamics, including the detrimental Staebler–Wronski effect.

Dependence of A-Si:H density of states on hydrogen content: Modeling and experiment.

The dependence of the density of states (DOS) in the gap of hydrogenated amorphous silicon (a-Si:H) on hydrogen content is analyzed theoretically by ab-initio Molecular Dynamics (AIMD) and Density Functional Theory (DFT). Initial results indicate that we can follow the changes in the DOS with different bonding configuration. Experimental verification of this analysis was commenced using the Constant Photocurrent Method (CPM) and Isothermal Capacitance Transient Spectroscopy (ICTS) on samples grown by the Saddle Field Glow Discharge technique, which allows us to control the nature of the Hydrogen bonding in the amorphous silicon matrix.

Hydrogenated amorphous silicon (a-Si:H) based solar cell: Material characterization and optimization

We present first-principles finite temperature molecular dynamics (MD) results of extensively simulated hydrogen bonding and diffusion inside an hydrogenated amorphous silicon (a-Si:H) network. The motivation comes from the necessity of fabricating and characterizing a-Si:H films for solar cells applications grown for an OCE/CMM sponsored project: the “Sonus PV Photovoltaic Highway Traffic Noise Barrier”. The main goal of the project is to achieve the integration of fiberglass noise reduction barriers with solar cells by optimizing the films and p-i-n junction parameters and the solar cell design/geometry (top contact grid parameters, in particular) for maximum performance.

Graphene-Boron Nitride 2D Heterosystems Functionalized with Hydrogen: Structure, Vibrations, Optical Response, Electron Band Engineering and Bonding

We characterise from first principles the structure and bonding in 2D heterosystems made of bilayers or trilayers of graphene and graphene-like-materials (GLMs), stacked on top of each other, and functionalized using hydrogen. The effects of electron band gap opening and tuning, as well as formation of strongly bonded multilayers have been predicted. The linear and nonlinear optical and vibrational spectra were modelled for hydrogenated alternating graphene monolayers with insulating hexagonal boron nitride (h-BN) films. Here we focus mostly on the structural aspect of the 2D heterosystems. The simulated atomic and related electron structures indicate that submonolayer hydrogenation of the outer surfaces of multilayer systems induces covalent interlayer bonds and enables electron gap engineering in otherwise gapless graphene or wide-band gap h-BN. Calculated structural, vibrational, electronic and optical properties of the systems of interest aim to enabling in-situ noninvasive characterization of graphene based multilayers.

Computational experiment in non-crystalline materials from first-principles: A case study of vibrations in hydrogenated amorphous silicon

Non-crystalline materials are important systems that can be investigated by Molecular Dynamics (MD); the recent advances in High Performance Computing (HPC) make it possible to successfully employ massively parallel MD simulations for the derivation of fundamental macroscopic properties from micro-dynamics. Although vibrational spectra are one of the most informative and easily measurable experimental signatures of non-crystalline systems, their inherent structural disorder, related instability and diffusion makes the task of computational spectral analysis very challenging. To solve this problem we developed a novel approach that extracts vibrational spectra of disordered systems from parameter-free modeling of the structural, dynamical and electronic properties of the materials. The combination of abinitio Molecular Dynamics (AIMD), improved signal processing technique and computer visualization allowed us to correlate the microscopic dynamics with a variety of macroscopic and experimentally measured pr...

Dynamics and bonding of bond-centered hydrogen in amorphous hydrogenated Si: vibrational and optical signatures

It is well accepted that the so called bond-centered hydrogen (BCH) is an important and frequently occurring structural complex for both amorphous and crystalline semiconductors [1]. BCH defects play a significant role in crystalline silicon and especially in amorphous hydrogenated silicon (a-Si:H) due to its application in photovoltaics and microelectronics. Vibrational and optical spectra of amorphous hydrogenated silicon (a-Si:H) contain essential information about the microscopic properties of the hydrogen atoms, including stability of the hydrogen bond responsible for a-Si:H quality for photovoltaic application. To decode this information from the experimental spectra, we developed a computational approach to comprehensively track hydrogen behaviour in both ordered (crystalline) and disordered (non-crystalline) materials, and applied it to a-Si:H [2,3]. Our focus is on different hydrogen complexes, responsible for stability of the a-Si:H material and its degradation. Since the bond-centered hydrogen is a typical complex in both crystalline silicon (c-Si) and amorphous silicon [1], we present a parameter free comparative vibrational and optical simulation of BCH in c-Si and a-Si:H. Vibrational spectra, electron density of states (DOS) and optical response were calculated for BCH and related systems. We have identified vibrational signatures of hydrogen instability in the amorphous Si network and c-Si. Bond-centered-hydrogen complexes observed have been characterized vibrationally and optically.

Correlation of a-Si:H properties with hydrogen distribution

Hydrogenated amorphous silicon (a-Si:H) has been the subject of considerable studies in the past 30 years, due to its growing application in large area optoelectronic devices, and especially in solar cells (see, for instance, [1–2]). In particular, the microscopic details of disordering, hydrogen migration and bonding within the amorphous silicon network are crucial for the understanding of a-Si:H, including the detrimental Staebler-Wronski (SW) effect [3], and for the improvement of the overall quality of the material.

Lattice and Hydrogen Dynamics in Hydrogenated Amorphous Silicon: First‐Principles Molecular Dynamics versus Experiment

We present the results of extensive ab‐initio Molecular Dynamics (AIMD) simulation of the structural, electronic and vibrational properties of hydrogenated amorphous silicon (a‐Si:H) in a wide range of hydrogen concentration and preparation conditions. We focus mainly on vibrational spectra as important and unique signatures of a variety of a‐Si:H properties. A comparison with experiment allowed us to correlate processes at microscopic atomic level, such as vibrations, chemical bonding and diffusion with macroscopic properties of the amorphous material.

First-principles Modeling of Structure, Vibrations, Electronic Properties and Bond Dynamics in Hydrogenated Amorphous Silicon: Theory versus Experiment

We carried out extensive first-principles modeling of microscopic structural, vibrational, electronic properties and chemical bonding in hydrogenated amorphous silicon (a-Si:H) in a wide range of hydrogen concentration and preparation conditions. The theory has been compared with experimental results to comprehensively characterize this semiconductor material. The computer modeling includes ab-initio Molecular Dynamics (MD), atomic structure optimization, advanced signal processing and computer visualization of dynamics. We extracted parameters of hydrogen and silicon bonding, electron charge density and calculated electron density of states (EDOS) and hydrogen diffusion. A good agreement of the theory with various experiments allowed us to correlate microscopic processes at the atomic level with macroscopic properties. Here we focus on correlation of the amorphous structure of the material, atom dynamics and electronic properties. These results are of increasing interest due to extensive application of a-Si:H in modern research and technology and to the significance of detailed understanding of the material structure, bonding, disordering mechanisms and stability.