We calculate from first principles the optical spectra of the hydrides in the sodium alanate hydrogen storage system: NaH, NaAlH4, and Na 3AlH6. In particular we study the effects of systematic improvements of the theoretical description. To benchmark the calculations we also measure the optical response of a thin film of NaH. The simplest calculated dielectric functions are based upon independent electrons and holes, whose spectrum is obtained at the G0W0 level. Successive improvements consist of including partial self-consistency (so-called GW 0) and account for excitonic effects, using the Bethe-Salpeter equation (BSE). Each improvement gives a sizable blue shift or red shift of the dielectric functions, but conserves the trend in the optical gap among different materials. Whereas these shifts partially cancel at the highest (GW 0-BSE) level of approximation, the shape of the dielectric functions is strongly modified by excitonic effects. Calculations at the GW 0-BSE level give a good agreement with the dielectric function of NaH extracted from the measurements. It demonstrates that the approach can be used for a quantitative interpretation of spectra in novel hydrogen storage materials obtained via, e.g., hydrogenography.@en

We present a theoretical study of hydrogenated amorphous silicon (a-Si:H) with a device quality hydrogen concentration of 11 at%. We used a first principle, parameters-free method. The interaction between the atoms was treated quantum mechanically within the density functional theory approximation. Amorphous structures were prepared by cooling from the liquid phase. When using a cooling rate of 0.02¿K/fs defect-free structures were prepared. All silicon atoms were fourfold coordinated and there were no defect states in the band gap. The calculated short range order showed a good agreement with available neutron scattering measurements. We further calculated the formation energy of dangling bonds (DBs; threefold coordinated Si atom) in all three charge states (negative, neutral, and positive) as a function of the Fermi energy. Interestingly, the DB correlation energies can have both positive and negative values.@en

We present a theoretical study of hydrogenated amorphous silicon nitride (a-SiNx:H), with equal concentrations of Si and N atoms (x=1), for two considerably different densities (2.0 and 3.0¿g/cm3). Densities and hydrogen concentration were chosen according to experimental data. Using first-principles molecular-dynamics within density-functional theory the models were generated by cooling from the liquid. Where both models have a short-range order resembling that of crystalline Si3N4 because of their different densities and hydrogen concentrations they show marked differences at longer length scales. The low-density nitride forms a percolating network of voids with the internal surfaces passivated by hydrogen. Although some voids are still present for the high-density nitride, this material has a much denser and uniform space filling. The structure factors reveal some tendency for the nonstoichiometric high-density nitride to phase separate into nitrogen rich and poor areas. For our slowest cooling rate (0.023 K/fs) we obtain models with a modest number of defect states, where the low (high) density nitride favors undercoordinated (overcoordinated) defects. Analysis of the structural defects and electronic density of states shows that there is no direct one-to-one correspondence between the structural defects and states in the gap. There are several structural defects that do not contribute to in-gap states and there are in-gap states that do only have little to no contributions from (atoms in) structural defects. Finally an estimation of the size and cooling rate effects on the amorphous network is reported.@en

We present a theoretical study of hydrogenated amorphous silicon (a-Si:H) with a device quality hydrogen concentration of 11 at%. We used a first principle, parameters-free method. The interaction between the atoms was treated quantum mechanically within the density functional theory approximation. Amorphous structures were prepared by cooling from the liquid phase. When using a cooling rate of 0.02¿K/fs defect-free structures were prepared. All silicon atoms were fourfold coordinated and there were no defect states in the band gap. The calculated short range order showed a good agreement with available neutron scattering measurements. We further calculated the formation energy of dangling bonds (DBs; threefold coordinated Si atom) in all three charge states (negative, neutral, and positive) as a function of the Fermi energy. Interestingly, the DB correlation energies can have both positive and negative values.@en

Abstract: We use a molecular-dynamics simulation within density-functional theory to prepare realistic structures of hydrogenated amorphous silicon. The procedure consists of heating a crystalline structure of Si64H8 to 2370 K, creating a liquid and subsequently cooling it down to room temperature. The effect of the cooling rate is examined. We prepared a total of five structures which compare well to experimental data obtained by neutron-scattering experiments. Two structures do not contain any structural nor electronic defects. The other samples contain a small number of defects which are identified as dangling and floating bonds. Calculations on a bigger sample (Si216H27) show similar properties (radial distribution functions, band gap, and tail states) compared to the Si64H8 sample. Finally the vibrational density of states is calculated and compared to inelastic neutron-scattering measurements.@en

Atomistic models of amorphous solids enable us to study properties that are difficult to address with experimental methods. We present a study of two amorphous semiconductors with a great technological importance, namely a- Si:H and a-SiN:H. We use first-principles density functional theory (DFT), i.e. the interatomic forces are derived from basic quantum mechanics, as only that provides accurate interactions between the atoms for a wide range of chemical environments (e.g. brought about by composition changes). This type of precision is necessary for obtaining the correct short range order. Our amorphous samples are prepared by a cooling from liquid approach. As DFT calculations are very demanding, typically only short simulations can be carried out. Therefore most studies suffer from a substantial amount of defects, making them less useful for modeling purposes. We varied the cooling rate during the thermalization process and found it has a considerable impact on the quality of the resulting structure. A rate of 0.02 K/fs proves to be sufficient to prepare realistic samples with low defect concentrations. To our knowledge these are the first calculations that are entirely based on first-principles and at the same time are able to produce defect-free samples. Because of the high computational load also the size of the systems has to remain modest. The samples of a-Si:H and a-SiN:H contain 72 and 110 atoms, respectively. To examine the convergence with cells size, we utilize a large cell of a-Si:H with a total of 243 atoms. As we obtain essentially the same structure as with the smaller sample, we conclude that the use of smaller cells is justified. Although creating structures without any defects is important, on the other hand a small number of defects can give valuable information about the structure and electronic properties of defects in a-Si:H and a-SiN:H. In our samples we observe the presence of both the dangling bond (undercoordinated atom) and the floating bond (over-coordinated atom). We relate structural defects to electronic defect states within the band gap. In a-SiN:H the silicon-silicon bonds induce states at the valence and conduction band edges, thus decreasing the band gap energy. This finding is in agreement with measurements of the optical band gap, where increasing the nitrogen content increases the band gap.@en