Direct Experimental Evidence for Molecular Hydrogen in Amorphous Si: H (original) (raw)
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On the hydrogen in hydrogen-containing amorphous silicon
Journal of non-crystalline solids, 2007
The present contribution reports on refined and constrained analyses of the complex proton-NMR free-induction-decay (FID) signal of hydrogen-containing amorphous silicon (a-Si:H), produced by thermal decomposition of pressurized monosilane. The spectra comprise clustered and diluted protons and the mole fractions of these are described by an algebraic sum of two exponential terms of the type I AE exp(ÀkP) where I and k are constants and P the initial monosilane pressure in the decomposition reactor. The NMR spin-spin relaxation time of the clustered protons is found to be approximately constant (T 2G % 15 ls) and independent of P. For the diluted protons, the spin-spin relaxation time are found to increase monotonically with increasing P (T 2L ranging from 85to85 to 85to140 ls) and could be approximated by a second-order polynomial in P. The number of diluted protons and their average nearest-neighbor separation are found to increase with increasing P from 7A˚at0.28MPato7 Å at 0.28 MPa to 7A˚at0.28MPato8.5 Å at 6.61 MPa. These findings show that the amorphous raw-silicon exhibits an inhomogeneous distribution of diluted protons. The distribution becomes more homogeneous with increasing P. The simplest description of the clustered configurations is that the number of protons and their average nearest-neighbor distance remain constant and independent of P, and that only the number of the clusters increases with increasing P. The amount of diluted and clustered protons, complies with the total amount of hydrogen found by chemical analysis and by FT-IR spectroscopy.
Hydrogen in crystalline and amorphous silicon
Journal of Non-Crystalline Solids, 1991
... Trieste, ltalv `Laboratorio Tecnologie Avanzate Superfici a Catalisi (TASC) del Consorzio lnteruniversitario Nazionale ai Fisica della ... to a-Si until very recently, and one had to rely largely on phenomenological models. ... I 2 3 4 1000/T(K) An analysis of the diffusive path on the ...
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.
Theoretical study of hydrogen microstructure in models of hydrogenated amorphous silicon
Physica Status Solidi A-applications and Materials Science, 2010
We study the distribution of hydrogen and various hydride configurations in realistic models of a-Si:H for two different concentration generated via experimentally constrained molecular relaxation approach (ECMR) [1]. The microstructure corresponding to low ( < 10%) and high (> 20%) concentration of H atoms are addressed and are compared to the experimental results with particular emphasis on the size of H clusters and local environment of H atoms.
Electronic states and total energies in hydrogenated amorphous silicon
Physical Review B Condensed Matter, 1982
The effects of bulklike and surfacelike surroundings on the electronic density of states of a variety of Si-H bonding conformations in hydrogenated amorphous silicon are examined using the cluster Bethe-lattice approach. Firstly, we discover that two fundamentally different bonding patterns, with different consequences for the doping mechanism, are consistent with ultraviolet photoemission spectroscopy (UPS) data. These are (1) H atoms bonded in microcrystalline regions and (2) clusters of monohydrides (SiH) in a continuous random network. Our results suggest an experiment in which x-ray photoemission spectroscopy and UPS taken together should distinguish between (1) and (2) and hence contribute toward understanding doping. Secondly, by using the calculated densities of states, the energies of a number of conformations and dehydrogenation reactions are calculated with the use of an empirical bond-strength total-energy scheme. Our results agree with results from annealing experiments. We introduce an improvement in the Bethe-lattice method which permits efficient solution of a second-neighbor tight-binding Hamiltonian, and which is valid for ¹h-neighbor interactions. We also estimate the Hubbard U, Stokes shifts, and electronic states associated with neutral and charged dangling bonds.
An attempt is made to highlight the importance of inhomogeneities in hydrogenated amorphous silicon (a-Si:H), in controlling its electronic properties. We note that hydrogen increases the gap in a-Si:H and that hydrogen is distributed inhomogeneously in it. This gives rise to long-range potential fluctuations, which are mostly uncorrelated and usually ignored. These and other such considerations have not only enabled us to gain new insights into the behaviour of a-Si:H in general, but have also allowed us to resolve several unsolved puzzles. Among these are questions like why undoped a-Si:H is n-type, why the creation of dangling bonds upon light soaking (LS) so inefficient, why a-Si:H degrades more upon LS when it is doped, why the reciprocity fails for light-induced degradation, why presence of nanocrystalline silicon improves stability and so on. We provide evidence to support some of our ideas and make suggestions for verifying the others.
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.
Metadynamical approach to the generation of amorphous structures: The case ofa-Si:H
Physical Review B, 2016
We present a dynamical approach to generate defect-free continuous-random-network (CRN) models of hydrogenated amorphous silicon (a-Si:H). Using the atomic coordination number of silicon as a collective variable and few configurational constraints, we have shown that classical metadynamics can be used to construct CRN models of a-Si with arbitrary concentrations of dangling-bond coordination defects. These defective networks have been subsequently hydrogenated to produce high-quality models of a-Si:H using ab initio total-energy calculations to generate hydrogen (H) microstructures for H concentrations from 7 to 22 at. %. The structural, electronic, optical, and vibrational properties of the models are examined, and the microstructure of the hydrogen distribution is analyzed and compared with experimental data from neutron scattering, spectroscopic ellipsometry, infrared spectroscopy, and nuclear magnetic resonance studies. The results obtained from the models are found to be in excellent agreement with the experimental data.