An electron diffraction study of amorphous silicon oxide films

Amorphous silicon oxide films have been examined by high energy electron diffraction using the sector-microphotometer method of data collection common to gas phase electron diffraction. This data was analyzed with a least-squares procedure that is designed to minimize extraneous detail in the radial distribution function obtained by the Fourier sine transform of the interference function. The results of this analysis for thin film SiO₂ show that the overall bonding topology of the thin film agress well with that of bulk (vitreous) SiO₂ examined by X-ray diffraction. The experimental short distance parameters for the films whose composition was determined to be ～SiO_1.3, SiO, and SiO_0.8 are found to be consistent with those expected for a mixture of tetrahedrally bonded amorphous Si and SiO₂ phases in which the scale of the Si-like and SiO₂-like regions is of the order of a few basic tetrahedral units. This result is in agreement with previous examinations of SiO powder by X-rays and a previous examination of thin silicon oxide films by electron diffraction.     

Inelastic neutron scattering from amorphous solids

Utilization of the inelastic neutron scattering technique for the study of the dynamics of amorphous solids is discussed. Excitation energies of interest range from a fraction of a milli-electron-volt (meV) to several hundred meV. For coherent scatterers, the scattering-vector dependence of the cross section gives information on the correlation between the motions of different atoms. Progress in the application of the technique to several classes of materials is reviewed, and the possibilities offered by the new generation of pulsed-spallation neutron sources are considered. Existing instrumentation on presently operating sources should already be able to measure wide-ranging scattering functions for comparison to model predictions for several types of amorphous solids. Higher flux sources, now at the proposal stage, would allow greater detail in the scattering to be observed and thus aid in discriminating between models.     

Structural characterization of silicon nanocrystals from amorphous silicon oxide materials

Silicon nanocrystals have been produced by disproportionation reaction of bulk silicon monoxide at temperatures higher than 1073 K. More specific, samples annealed at 1123, 1173, 1223 and 1323 K as well as the starting material SiO have been examined. X-ray diffraction, high-resolution transmission electron microscopy and infrared spectroscopy have been employed in order to investigate the structure of the produced silicon nanocrystals. Photoluminescence measurements reveal a three band emission with maxima positioned at 1.33, 1.52 and 1.67 eV. The intensity of the photoluminescence emission increases with the annealing temperature exponentially. This fact can be directly correlated with the disproportionation reaction which results in bigger amounts of silicon nanocrystals by increasing the annealing temperature and is discussed here. Also a possible explanation is given for the origin of each emission band.     

Dielectric function of amorphous silicon films,its oxidation and voids,studied by electron spectroscopy

The energy loss spectra of extremely pure and of oxidized amorphous silicon films were measured with 30 keV electrons in the energy loss range 2–25 eV with a resolution of 0.1 eV. The Si films were prepared by electron beam evaporation at a pressure below 10^?9 Torr during the evaporation process. The influence of contact to the atmosphere and to water on the energy loss spectrum was also investigated. The energy loss spectra can be described well by means of the dielectric theory extended to a multilayer system. The results of the measurements lead to the conclusion that the density of voids in the pure a-Si films deposited at room temperature is less than 5% of the bulk. This should be true as long as the effect of the voids can be described by a dielectric function.     

Microstructural analysis of paracrystalline atomistic models of amorphous silicon

A detailed microstructural analysis of amorphous silicon is performed by means of a numerical modeling technique. Paracrystalline models of amorphous silicon, first proposed by Treacy, Gibson and Keblinski, have been generated. Nanocrystallites of various sizes and concentrations have been introduced into a continuous random network that was generated with a vacancy model. Using the conjugate gradient method, the structures have been relaxed by minimizing their total strain energy described by the anharmonic Keating model. The computed pair correlation functions of these structural models bring to the fore a unique behavior of the paracrystalline networks in the context of diffraction experiments; they appear amorphous as the continuous random network model. The paracrystalline model remains denser than the crystalline phase, contrary to experimental observations. However, the former is found to be less homogenous than the CRN model, thus giving a satisfactory explanation of the nanoscale fluctuation electron microscopy data reported recently by Treacy and coworkers.     

Modification of amorphous hydrogenated silicon by co-sputtered aluminum

The electronic and optical properties of a-Si_1?xH_x have been modified by the incorporation of aluminum. Samples were prepared by rf sputtering in a hydrogenated atmosphere from a composite silicon-aluminum target. This paper reports on several modified material parameters including the optical band gap, electrical conductivity, and thermal activation energy. Aluminum concentrations up to 10.6% in the target have been investigated. It is observed that the optical band gap remains constant at 1.83 eV for Al concentrations up to 2.7%. For higher concentrations there is a marked decrease in optical gap. The conductivity initially decreases with small Al concentration and the activation energy increases, characteristic of compensation of the inherently n-type material. For higher Al concentrations the conductivity increases by seven orders of magnitude and the activation energy decreases to a minimum of about 0.2 eV. The increase in conductivity can be explained by both the movement of the Fermi level and the shrinking band gap. Microprobe analyses have also been performed to determine the amount of Al actually incorporated into the films. Finally, implications of these results are discussed and compared to previously reported results on gas phase doping and ion implantation.     

Microstructural analysis of nanoporous paracrystalline atomistic models of amorphous silicon

A detailed microstructural analysis of amorphous silicon is performed via numerical modeling technique. Nanoporous paracrystalline models have been proposed. Intermixed nanocrystallites and nanovoids of various sizes and concentrations have been introduced into a continuous random network that was generated with a vacancy model. Using the conjugate gradient method, the structures have been relaxed by minimizing their total strain energy described by the anharmonic Keating model. The obtained nanoporous structures are energetically competitive with the voidless paracrystalline networks. Nanoporous models with large voids are energetically more favorable than those with small voids. The nanoporous paracrystalline model is less dense than the crystalline phase, contrary to the paracrystalline model. This density decreases with increasing the void size for a fixed void volume fraction. Nanoporous paracrystalline structures reproduce the experimental structure factor better than the paracrystalline network. They account for, in particular, the intense small-angle scattering observed for some a-Si samples. The paracrystallites form amorphous zones but with local and topological ordering neatly better than the surrounding matrix. Such structural heterogeneity gives so a satisfactory explanation of the nanoscale fluctuation electron microscopy data reported recently by Treacy and Gibson.     

Diffraction intensities and the structure of amorphous carbon

A model structure of amorphous carbon is investigated incorporating layered domains connected by means of a random network, the relative proportions of the two regions being a variable of the model. It is shown that for a certain relative proportion of the two regions there is good agreement between the predicted and experimentally observed electron and X-ray scattering intensities. The effects of covalent bonding are also studied and shown to be significant for small k.     

Photoluminescence of hydrogenated amorphous silicon

Photoluminescence has beeb proven to be a powerful technique in the characterization of a-Si:H. In particular, it has contributed to the elucidation of some aspects of the electronic structure. However, there is a set of controversial topics still under discussion including the idenntity of the luminescent transition and the origin of broadening of the emission spectrum. In this paper we study these problems and show that the specified width has its origin in both disorder and electron-phonon interaction.Luminescent decay at low temperature has been studied and lifetimes from 10^?8 to 10^?2 s have been confirmed.Photoconductivity and photoluminescence are shown to behave in a complementary way and activation energies for both processes are obtained. Also, the photoluminescence quenching and photoconductivity enhanced under an applied electric field have been measured an interpreted.     

Polycrystalline silicon with large disk-shaped grains by Ni-mediated crystallization of doped amorphous silicon

Silicide mediated crystallization (SMC) of p-doped amorphous silicon (a-Si) has been studied. There are the different grain-shapes of crystallization of doped and non-doped a-Si. Non-doped a-Si is crystallized with needle-shaped grains, while it is observed the disk-shaped grains are formed in crystallized doped a-Si. The crystallization of slightly doped a-Si exhibits larger grain size compared with non-doped and heavily doped films. The p-dopant in a-Si suppresses the formation of the NiSi₂ precipitate which act as a crystallization nucleus, causing continuous grain growth and the formation of disk-shaped grains.     

X-ray scattering by porous silicon modulated structures

A multilayered porous structure formed as a result of the anodization of a Si(111)(Sb) substrate in an HF:C₂H₅OH (1: 2) solution with a periodically alternating current has been investigated by high-resolution X-ray diffraction. It is established that, despite 50% porosity, a thickness of 30 μm, and significant strain (4 × 10⁻³⁾, the porous silicon structure consists mainly of coherent crystallites. A model of coherent scattering from a multilayered periodic porous structure is proposed within the dynamic theory of diffraction. It is shown that the presence of gradient strains of 5 × 10⁻⁴ or higher leads to phase loss upon scattering from porous superlattices and the suppression of characteristic satellites in rocking curves.     

Structural studies of amorphous solids by small-angle neutron scattering

Sophisticated new methods of preparation developed over the last decade have considerably extended the range and nature of amorphous materials, many of which have a microstructure on a scale of 10–1000 Å. Whilst in some cases this is due to the composition, in others it is a direct result of the preparation technique. This paper reviews the present state of small-angle scattering as a tool to study the microstructure in the four main classes of amorphous materials: bulk glass, metal ribbons, thin films and inorganic gels. The techniques specific to neutron scattering, which have been developed to separate the various components of the scattering curves, are reviewed.     

Dangling bonds in amorphous silicon investigated by multifrequency EPR

Paramagnetic coordination defects in undoped hydrogenated amorphous silicon (a-Si:H) are studied using multifrequency pulsed electron-paramagnetic resonance (EPR) spectroscopy at S-, X-, Q- and W-band microwave frequencies (3.6, 9.7, 34, and 94 GHz, respectively). The improved spectral information extractable from a multifrequency fitting procedure allows us to conclude that the g tensor exhibits a rhombic splitting instead of axial symmetry. Our methods allow for precise and accurate determination of the g tensor principal values g_x = 2.0079(2), g_y = 2.0061(2) and g_z = 2.0034(2) and their distribution parameters (g strain).     

Improvement of amorphous silicon n-i-p solar cells by incorporating double-layer hydrogenated nanocrystalline silicon structure

We develop a double-layer p-type hydrogenated nanocrystalline silicon (p-nc-Si:H) structure consisting of a low hydrogen diluted i/p buffer layer and a high hydrogen diluted p-layer to improve the hydrogenated amorphous silicon (a-Si:H) n-i-p solar cells. The electrical, optical and structural properties of p-nc-Si:H films with different hydrogen dilution ratio (R_H) are investigated. High conductivity, low activation energy and wide band gap are achieved for the thin films. Raman spectroscopy and high-resolution transmission electron microscopy (HRTEM) analyses indicate that the thin films contain nanocrystallites with grain size around 3-5 nm embedded in the amorphous silicon matrix. By inserting a p-nc-Si:H buffer layer at the i/p interface, the overall performance of the solar cell is improved significantly compared to the bufferless cell. The improvement is correlated with the reduction of the density of defect states at the i/p interface.     

Properties of hydrogenated amorphous silicon prepared by chemical vapor deposition

Properties of hydrogenated amorphous silicon (a-Si:H) prepared by chemical vapor deposition (CVD) are reported and compared to corresponding properties of glow discharge a-Si:H. The CVD material was produced from mixtures of silane, disilane, trisilane and higher polysilanes in hydrogen carrier gas at one atmosphere total pressure, at substrate temperatures from 420 to 530 °C. The photovoltaic properties of our present CVD a-Si:H are somewhat inferior to those of the best glow discharge a-Si:H. However, as discussed below, there are some indications that higher quality CVD a-Si:H may be possible.     

Structural characterization of amorphous silicon nitride by molecular dynamics simulation

Computer simulation, using the molecular dynamics (MD) technique, has been carried out on amorphous silicon nitride (a-Si₃N₄) with simple Busing-type potentials. From the MD simulation, the following points have been deduced. (1) The average Si---N bond length obtained from MD results is r_Si---N=1.74 Å, and its coordination number, N_Si---N, is 3.95. The bond angles around a Si and a N atom, N---Si---N and Si---N---Si, are found to be 109.8° ± 12.36° and 127.08° ± 16.63°, respectively. The N---Si---N value obtained is in very good agreement with the tetrahedral bond angle (= 109.47°). Hence, the short-range structural arrangement of a-Si₃N₄ comprises tetrahedral SiN₄ units. The MD results presented in this study also indicate that there exist only a small number of defects such as dangling bonds. (2) These MD results are in good agreement with the reported X-ray and neutron data. The a-Si₃N₄ structure can be reproduced by the MD simulation given in this study.