Pure Amorphous Silicon Research Articles

High Resolution Radial Distribution Function of Pure Amorphous Silicon

The structure factor $S(Q)$ of high purity amorphous Si membranes prepared by ion implantation was measured over an extended $Q$ range ( $0.03\char21{}55{\AA{}}^{\ensuremath{-}1}$). Calculation of the first neighbor shell coordination ( ${C}_{1}$) as a function of maximum $Q$ indicates that measurement of $S(Q)$ out to at least $40{\AA{}}^{\ensuremath{-}1}$ is required to reliably determine the radial distribution function (RDF). A $2%$ change in ${C}_{1}$ and subtle changes in the rest of the RDF were observed upon annealing, consistent with point defect removal. After annealing at 600 \ifmmode^\circ\else\textdegree\fi{}C, ${C}_{1}\phantom{\rule{0ex}{0ex}}=\phantom{\rule{0ex}{0ex}}3.88$, which would explain why amorphous Si is less dense than crystalline Si.

Approximateab initiocalculation of vibrational properties of hydrogenated amorphous silicon with inner voids

We have performed an approximate ab initio calculation of vibrational properties of hydrogenated amorphous silicon $(a\ensuremath{-}\mathrm{S}\mathrm{i}:\mathrm{H})$ using a molecular dynamics method. A 216-atom model for pure amorphous silicon $(a\ensuremath{-}\mathrm{Si})$ has been employed as a starting point for our $a\ensuremath{-}\mathrm{S}\mathrm{i}:\mathrm{H}$ models with voids that were made by removing a cluster of silicon atoms out of the bulk and terminating the resulting dangling bonds with hydrogens. Our calculation shows that the presence of voids leads to localized low-energy $(30--50 {\mathrm{cm}}^{\ensuremath{-}1})$ states in the vibrational spectrum of the system. The nature and localization properties of these states are analyzed by various visualization techniques.

Relaxation and crystallization of amorphous silicon carbide probed by optical measurements

Abstract Optical spectroscopy in the visible (300–1100 nm) and in the infrared (400–4000cm−1) regions was used to monitor the relaxation and crystallization processes of pure amorphous silicon carbide (a-SiC) thin films upon annealing at temperatures between 200 and 1000°C. These films were obtained by ion implantation of crystalline material with 200keVkr+ at a fluence of 2 × 10 ions cm−2. The refractive index n and the absorption index k were calculated from the ultraviolet-visible transmittance and reflectance, and information on the vibration modes of the Si-C bonds was detected from infrared transmittance. Thermal treatment changes the optical properties of a-SiC; in particular, annealing at temperatures lower than 800°C resulted in a continuous variation in both the refractive index and the absorption index and in a decrease in the infrared silicon-carbon peak width. Annealing at higher temperatures produces sudden variations in the shape of the refractive index and in the infrared silicon-carbon pe...

Influence of Mo on the Epitaxial Crystallization of Silicon

ABSTRACTThe influence of Mo atoms on the solid phase epitaxial crystallization of amorphous silicon layers on (100) and (111) Si substrates has been studied by RBS/channeling and cross-sectional TEM. For this purpose, Mo doped amorphous surface layers were produced by low temperature 180 keV Mo+ ion implantation with different doses. Mo is observed to cause enhanced crystallization rates for both (100) and (111) substrates, compared to literature data on pure amorphous silicon. Similar to pure Si, recrystallization in <100> directions is much faster than in <111> directions, where two different velocities are found. For (111) substrates, the formation of thick, uniformly twinned layers is observed. Annealing for several hours at 550° C does not lead to detectable changes of the Mo depth distribution, but for high doses the formation of hexagonal MoSi2 precipitates is observed.

Ion implantation into amorphous solids

We describe the effects of implantation of silicon into pure amorphous silicon and hydrogenated amorphous silicon and of erbium into soda-lime silicate glass and hydrogenated amorphous silicon. The former implantations and subsequent annealing treatments are used as a means of controlled defect creation. The experiments lead to new insights into the relation between defect density and bond-angle variation in both types of amorphous silicon. Erbium implantation is used to produce planar optical waveguide materials. It is demonstrated that erbium implantation into glass and amorphous silicon leads to efficient photoluminescence. It appears that these materials are attractive candidates for Er-doped waveguide amplifiers.

Optical and strucutral properties of hydrogen implanted silicon carbon alloys

The hydrogen effects on the optical and structural properties of amorphous silicon carbon alloys are investigated for different hydrogen concentration (0–20 at.%) and a fixed silicon to carbon concentration ratio ( χ Si χ c = 1 ). Hydrogen was introduced into a preamorphized SiC sample by ion implantation of H ions with energies in the range 5–20 keV and fluences in the range 10 17−5 × 10 17 ions/cm 2 obtaining an uniform concentration in the amorphous layer (130 nm thick). Raman spectra evidences the presence of both a-Si and a-C structures even in the unhydrogenated samples and the optical energy gap increases with hydrogen concentration. Thermal annealing at 350°C induces a further increase of the optical energy gap, in particular from 1.45 to 1.65 eV in the 20 at.% hydrogen containing a-SiC:H. These and other results, compared to those obtained in pure amorphous silicon and carbon layers, suggest a similarity between the optical and structural behaviour of silicon carbon alloys and that of amorphous carbon.

Chemical and physical sputtering of fluorinated silicon

Molecular dynamics simulations were performed on low-energy argon-ion bombardment (200, 50, and 20 eV) of silicon layers with varying amounts of fluorine incorporated. At low fluorine incorporation in the layers (F/Si<0.5), only physical sputtering was observed, although the physical sputtering yield increased compared to pure amorphous silicon. At higher levels of fluorine incorporation into the silicon layer, ion impact resulted in the formation of weakly bound SiFx (x=1–3) species in the layer. This phenomenon appears to be similar to chemical sputtering as defined by Winters and Coburn [H. F. Winters and J. W. Coburn, Surf. Sci. Rep. 14, 164 (1992)]. The overall yield, due to both physical and chemical sputtering, was found to follow a square-root dependence on ion energy. The threshold ion impact energy for the formation of weakly bound species in heavily fluorinated silicon layers extrapolated to ≤4 eV, and for physical sputtering to about 20 eV. The simulations imply that the source of the ion-neutral synergism in ion-assisted etching occurs on the collision cascade time scale (∼10−12 s) with the creation of these weakly bound species. The overall rate determining step for ion-assisted etching, however, is often a much slower process involving thermal desorption, chemical reaction, or diffusion. This difference between the source of the ion-neutral synergy (creation of weakly bound species in 1 ps or less) and the rate determining step (often on much longer time scales) has probably contributed to the confusion that has surrounded discussions of the mechanisms of ion-assisted etching.

Investigations on the structure of evaporated pure amorphous silicon

Some search on the Cambridge Structural Database and Reverse Monte Carlo calculations were performed for pure amorphous Si. On the basis of diffraction data three dimensional models were constructed for determining the disordered structure of a-Si. The pair correlation function was derived directly from the large scale models.

Electron localization in models of hydrogenated amorphous silicon and pure amorphous silicon

Presents the first calculations of the electronic density of states, the electrical conductivity and the inverse participation ratio as a function of energy for a large model of hydrogenated amorphous silicon. The model contains 1555 Si atoms and 400 H atoms and all Si atoms are fourfold coordinated. This model has a clear gap in the density of electronic states. The 'mobility gap' (estimated from the conductivity versus energy curves) is about 0.6 eV wider than the gap in the electronic density of states. The authors also present for comparison the same calculations on a defected model of amorphous Si with 1728 atoms which contains states within the gap arising mainly from departure from tetrahedral geometry.

High-spatial-resolution analysis of Ge layers in Si

This paper describes the results of preliminary experiments to characterize the focused-probe properties of a new 200 kV field-emission TEM/STEM. Probe currents are measured using a Faraday stage. Probe size is measured by high-angle annular-dark-field STEM line scans across a Ge monolayer embedded coherently in single-crystal silicon. These measurements show the the instrument is capable of producing probes with a full-width at half-maximum of order 3nm and less with at least a few tenths of a nanoampere of current. This is sufficient to execute X-ray microanalysis measurements with good signal-to-noise at nanometer lengths scales. As part of the same set of experiments, this work shows experimentally that the HAADF scattering from pure amorphous silicon is stronger than that from pure crystalline silicon, possibly due to the increased Debye-Waller factor and the absence of chanelling effects in the amorphous structure.

Activation-energy spectrum and structural relaxation dynamics of amorphous silicon.

The structural relaxation dynamics in pure (unhydrogenated) amorphous silicon (a-Si) following ion-irradiation-induced defect injection is investigated by measuring the changes in electrical conductivity. It is found that the conductivity of a-Si decreases over three orders of magnitude upon relaxation by defect annihilation. Subsequent ion irradiation reverses the conductivity changes, confirming that the observed conductivity change is solely due to structural relaxation. Through a combined analysis of both in situ and ex situ measurements of dynamics of change in conductivity upon structural relaxation, the density of relaxation states with activation energy Q associated with localized electron states near the Fermi level is measured. It is found to be a continuous, monotonically decreasing function extending from Q0.9 eV to Q>2.5 eV, and in situ measurement of conductivity suggests a lower limit of Q0.23 eV. Knowledge of this quantity in turn enables a quantitative estimate of the activation energy spectrum for structural relaxation of a-Si. Results are compared with previous investigations of structural relaxation and solid-phase epitaxy of a-Si, and the relevance of defect injection and structural relaxation in a-Si to irradiation-enhanced nucleation of crystal silicon is discussed.

Structure of evaporated pure amorphous silicon: Neutron-diffraction and reverse Monte Carlo investigations.

A neutron-diffraction measurement in the 0--23 A${\mathrm{\r{}}}^{\mathrm{\ensuremath{-}}1}$ inverse space interval was performed on pure amorphous Si. With the structure factor obtained experimentally three-dimensional models were constructed by reverse Monte Carlo simulation for the determination of the atomic structure of a-Si. The radial distribution function was calculated directly from the large-scale models and was derived traditionally from these wide range spectra, as well.

Model of hydrogenated amorphous silicon and its electronic structure.

Using previously generated large models of pure amorphous silicon [J. M. Holender and G. J. Morgan, J. Phys. Condens. Matter 3, 7241 (1991)] we have now constructed structural models of hydrogenated silicon. They are obtained by ``hydrogenation'' of our models of amorphous silicon, i.e., by addition of the hydrogen atoms into a model of a-Si containing undercoordinated and overcoordinated atoms followed by relaxation using molecular dynamics. The electronic structure of the models is calculated using the phenomenological tight-binding model. It is shown that addition of hydrogen reduces drastically the density of electronic states associated with defects, producing a clearly defined gap.

Improved electrical properties of non-hydrogenated a-Si densified by IBAD

Some effects of ion bombardment on the growth of non-hydrogenated amorphous silicon films were studied using samples prepared with low-energy ion-beam-assisted deposition (IBAD). Inert gas ions were used to determine the effects of the ion beam on pure amorphous silicon without the complication of simultaneous chemical effects. The ion bombardment was found to increase the film density, improve film stability, increase the effective conductivity activation energies and to move the position of the infrared absorption peak assigned to silicon-carbon bonds to larger wavenumbers. The largest effective conductivity activation energies and the lowest conductivities were obtained from films made with Ar+ ions and bombarding energies around 100 eV. However, these 'best' films still do not have a sufficiently low defect density to be useful semiconductors. The improvements are consistent with the thesis that densification of the growing film is the primary effect of the beam. However, for the conditions used, the densification appears to saturate, leaving a relatively large number of dangling bonds in the films. These may result from the presence of Ar known to be in the films, although the results are consistent with the topological model which suggests that the best conditions for forming covalently bonded solids are obtained when the number of bonding constraints is equal to the number of degrees of freedom.

Defects in amorphous silicon probed by subpicosecond photocarrier dynamics

The photocarrier dynamics in pure nonhydrogenated amorphous silicon (a-Si) have been studied with subpicosecond resolution using pump-probe reflectivity measurements. The photocarrier lifetime increases with the annealing temperature from 1 ps for as-implanted a-Si to 11 ps for a-Si annealed at 500 °C. The lifetime in annealed a-Si can be returned to the as-implanted level by ion irradiation. These observations indicate that a-Si can accommodate a variable number of defect-related trapping and recombination centers. The saturated defect density in as-implanted a-Si is estimated to be ≊1.6 at. %. Comparison with Raman spectroscopy suggests that various kinds of structural defects are present in a-Si.

Quantum Simulation of Amorphous Silicon: Preparation, Structure and Properties

ABSTRACTWithin a tight-binding molecular dynamics scheme we investigate pure amorphous silicon (a-Si) obtained by direct quenching from the melt. Using different rates for the cooling process, we demonstrate that both structural and electronic properties of a-Si depend on the sample preparation. Possible size-effects are also investigated using 64- and 216-atom supercells. Finally, we discuss the reliability and transferability of the present scheme for large scale simulations of covalent materials.

Amorphous silicon studied by ab initio molecular dynamics: Preparation, structure, and properties.

We present a first-principles molecular-dynamics study of pure amorphous silicon obtained by simulated quench from the melt. A cooling rate of ${10}^{14}$ K/s is sufficient to recover a tetrahedral network starting from a well-equilibrated metallic liquid having average coordination larger than 6. Dramatic changes in physical properties are observed upon cooling. In particular, a gap forms in the electronic spectrum, indicating a metal-to-semiconductor transition. The as-quenched structure has average coordination very close to 4, but contains several coordination defects as well as a large fraction of distorted bonds. Subsequent annealing reduces the amount of strain and the number of defects present in our system. The average structural, dynamical, and electronic properties of our sample are in good agreement with the available experimental data. We report a detailed analysis of the structural relaxation processes accompanying annealing and compare our findings with recent experiments.

Large picosecond energy fluctuations of single atoms of a-Si observed in molecular-dynamics studies.

Molecular-dynamics simulations on samples of pure and fluorine-doped amorphous silicon in various temperature ranges are described. These reveal large transient coordinate fluctuations of single atoms in solids. The observation of these fluctuations supports kinetic theories based on the existence of short-lived large energy fluctuations (SLEF's) of single atoms in solids at finite temperatures. The characteristic energies, time intervals, and frequencies of the observed SLEF's are in good qualitative agreement with the SLEF theory predictions.

Structural relaxation and defect annihilation in pure amorphous silicon.

Thick amorphous Si layers have been prepared by MeV self-ion-implantation and the thermodynamic and structural properties examined by calorimetry, Raman-spectroscopy, and x-ray-diffraction techniques. Defects have been introduced into well-annealed amorphous and single-crystal Si by He, C, Si, and Ge bombardment. The defect structures are examined by these techniques and by transmission electron microscopy. The structure of amorphous Si in intermediate states of relaxation or annealing have been determined. It is shown that amorphous Si formed by either implantation or deposition contains a large population of point defects and point-defect clusters. Amorphous Si formed by laser quenching cannot be distinguished from well-annealed amorphous Si. Structural relaxation, also known as short-range ordering, can be understood as annihilation of a large fraction of these defects. Both structural relaxation in amorphous Si and defect annihilation in crystalline Si obey bimolecular reaction kinetics. The defect-formation and -annihilation processes are similar in amorphous and crystalline Si. Defect saturation occurs in amorphous Si at estimated defect concentrations of about 1 at. %. These formation and annihilation properties are intrinsic to pure amorphous Si. For hydrogenated amorphous Si, it is pointed out that the metastable-defect-creation and -annealing processes are essentially different from the annihilation processes in pure amorphous Si.

Dynamical models of hydrogenated amorphous silicon.

The results of our molecular-dynamics simulation of bulk hydrogenated amorphous silicon using empirical potentials are presented. More specifically, we discuss a dynamical procedure for incorporating hydrogen into a pure amorphous silicon matrix, which is derived from the concept of floating bonds put forward by Pantelides [Phys. Rev. Lett. 57, 2979 (1986)]. The structures resulting from this model are compared with those obtained with use of a static approach recently developed by us. This method exhibits considerable improvement over the previous one and, in particular, unambiguously reveals the strain-relieving role of hydrogen. While the former model leads to substantial overcoordination, the present one results in almost perfect tetrahedral bonding, with an average coordination number Z=4.03, the lowest value ever achieved using a Stillinger-Weber potential. The simulations are also used to calculate the vibrational densities of states, which are found to be in good accord with corresponding neutron-scattering measurements.