Thermally stimulated H emission and diffusion in hydrogenated amorphous silicon (original) (raw)
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Dynamics of hydrogen in hydrogenated amorphous silicon
Pramana, 2003
The problem of hydrogen diffusion in hydrogenated amorphous silicon (a-Si:H) is studied semiclassically. It is found that the local hydrogen concentration fluctuations-induced extra potential wells, if intense enough, lead to the localized electronic states in a-Si:H. These localized states are metastable. The trapping of electrons and holes in these states leads to the electrical degradation of the material. These states also act as recombination centers for photo-generated carriers (electrons and holes) which in turn may excite a hydrogen atom from a nearby Si-H bond and breaks the weak (strained) Si-Si bond thereby apparently enhancing the hydrogen diffusion and increasing the light-induced dangling bonds.
2010
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.
Hydrogen dynamics and light-induced structural changes in hydrogenated amorphous silicon
Physical Review B, 2006
A direct ab initio calculation of network dynamics and diffusion both for the ground state and light excited state for a-Si:H was performed. In the light excited state there was observed enhanced hydrogen diffused and formation of new silicon dihydride configurations: (H-Si Si-H) 2 (H-Si Si-H) and SiH 2 . For the first time, we show the detailed dynamic pathways that arise from light induced occupation changes, and provide one explicit example of defect creation and paired H formation.
Network structure and dynamics of hydrogenated amorphous silicon
Journal of Non-Crystalline Solids, 2008
In this paper we discuss the application of current ab initio computer simulation techniques to hydrogenated amorphous silicon (a-Si:H). We begin by discussing thermal fluctuation in the number of coordination defects in the material, and its temperature dependence. We connect this to the "fluctuating bond center detachment" mechanism for liberating H bonded to Si atoms. Next, from extended thermal MD simulation, we illustrate various mechanisms of H motion. The dynamics of the lattice is then linked to the electrons, and we point out that the squared electronlattice coupling (and the thermally-induced mean square variation in electron energy eigenvalues) is robustly proportional to the localization of the conjugate state, if localization is measured with inverse participation ratio. Finally we discuss the Staebler-Wronski effect using these methods, and argue that a sophisticated local heating picture (based upon reasonable calculations of the electron-lattice coupling and molecular dynamic simulation) explains significant aspects of the phenomenon.
Electronic and transport properties of hydrogenated amorphous silicon
Physical review. B, Condensed matter, 1985
We have extended previous coherent-potential-approximation calculations of the electronic and transport properties of hydrogenated amorphous silicon (a-Si), in order to examine the effects of fully dispersed hydrogen in a-Si. The present calculation replaces random vacancies in the Si matrix by single H atoms instead of the four-H-atom clusters previously considered. In addition, to eliminate dangling-bond states in the gap we have introduced an ad hoc reconstruction of the lattice around the vacancy by effectively saturating the dangling orbitals with other Si atoms. Our results reinforce previous claims that an understanding of various experiments in a-Si:H can be obtained from first-principles calculations which neglect topological disorder and the precise configuration of the hydrogen atoms. The present calculations lead to an improved agreement with the photoemission and optical absorption data.
Physical Review B, 2006
Differential scanning calorimetry ͑DSC͒ was used to study the dehydrogenation processes that take place in three hydrogenated amorphous silicon materials: nanoparticles, polymorphous silicon, and conventional device-quality amorphous silicon. Comparison of DSC thermograms with evolved gas analysis ͑EGA͒ has led to the identification of four dehydrogenation processes arising from polymeric chains ͑A͒, SiH groups at the surfaces of internal voids ͑AЈ͒, SiH groups at interfaces ͑B͒, and in the bulk ͑C͒. All of them are slightly exothermic with enthalpies below 50 meV/͑H atoms͒, indicating that, after dissociation of any SiH group, most dangling bonds recombine. The kinetics of the three low-temperature processes ͓with DSC peak temperatures at around 320 ͑A͒, 360 ͑AЈ͒, and 430°C ͑B͔͒ exhibit a kinetic-compensation effect characterized by a linear relationship between the activation entropy and enthalpy, which constitutes their signature. Their Siu H bond-dissociation energies have been determined to be E͑Siu H͒ 0 = 3.14 ͑A͒, 3.19 ͑AЈ͒, and 3.28 eV ͑B͒. In these cases it was possible to extract the formation energy E͑DB͒ of the dangling bonds that recombine after Siu H bond breaking ͓0.97 ͑A͒, 1.05 ͑AЈ͒, and 1.12 ͑B͔͒. It is concluded that E͑DB͒ increases with the degree of confinement and that E͑DB͒ Ͼ 1.10 eV for the isolated dangling bond in the bulk. After Siu H dissociation and for the low-temperature processes, hydrogen is transported in molecular form and a low relaxation of the silicon network is promoted. This is in contrast to the high-temperature process for which the diffusion of H in atomic form induces a substantial lattice relaxation that, for the conventional amorphous sample, releases energy of around 600 meV per H atom. It is argued that the density of sites in the Si network for H trapping diminishes during atomic diffusion.
The Journal of Physical Chemistry C, 2013
The optical absorption properties of hydrogenated amorphous silicon (a-Si:H) are important in solar applications and from the perspective of fundamental materials science. However, there has been a long standing question from experiment of the dependence of the optical gap on the hydrogen content in a-Si:H. To reconcile this debate, we present density functional theory simulations of models of hydrogenated a-Si:H, with different hydrogen concentrations, up to and including full hydrogen saturation. We discuss the dependence of the optical and mobility gaps in fully saturated and undersaturated a-Si:H. Oversaturation with hydrogen results in a dramatic change in the properties of a-Si:H and is beyond the scope of this paper. For undersaturated hydrogen contents, both gaps increase with increasing hydrogen concentration until hydrogen saturation is achieved. Our key finding is that at saturation, the optical and mobility gaps converge to a value independent of the hydrogen content. Our analysis thus resolves the contradiction between experimental data examining the effect of hydrogen content up to saturation and interpretations based on conventional expectations regarding the hydrogen dependence of the optical and mobility gaps up to saturation and provides new insight on the materials properties of hydrogenated amorphous silicon that can be used for sample preparation.
International Journal of Theoretical and Applied Nanotechnology, 2021
In order to investigate various properties of hydrogenated amorphous silicon (a-Si:H) for improvement of low conversion efficiency and stability of solar cells, a series of quantum simulations based on the density functional theory combined with the tight binding model were performed for a-Si:H with various hydrogen concentrations and cooling rates. The radial distribution function (RDF) for Si-Si pairs indicates that samples with higher H concentration (20% and 25%) give a structure in better agreement with experiments, but the RDF of Si-H pairs suggests that samples with lower H concentration (14%) may give more appropriate structure. The coordination number () analysis indicates that more defects (dangling bonds and floating bonds) exist in 20% and 25% H concentration samples. Overall, a-Si:H with 14% H concentration gives most preferable structure. The cooling rate has also much effect on the structure. Sample with the slowest cooling rate is slightly more structured based on Si-Si pair RDF and. The electron transport of a-Si and a-Si:H were evaluated and the superiority of a-Si:H was confirmed.
Molecular hydrogen diffusion in nanostructured amorphous silicon thin films
Physical Review B, 2009
We study hydrogen stability and its evolution during thermal annealing in nanostructured amorphous silicon thin films. From the simultaneous measurement of heat and hydrogen desorption, we obtain the experimental evidence of molecular diffusion in these materials. In addition, we introduce a simple diffusion model which shows good agreement with the experimental data.