Relaxation In Amorphous Silicon Research Articles

Hybrid Reverse Monte Carlo and electron phase contrast image simulations of amorphous silicon with and without paracrystals

Atomic networks of as-implanted and relaxed amorphous silicon solids were simulated using a Hybrid Reverse Monte Carlo algorithm constrained by high-resolution electron diffraction data. No significant structural distinction was observed between the two forms of amorphous silicon. A nanometer-sized crystallite was inserted into the as-implanted structure, to model medium-range order due to paracrystals, and the atomic network was energetically relaxed whilst maintaining consistency with experiment. Experimental pair–pair correlations were then simulated using a stochastic generalised Debye sum of fourth order. The idealised pair–pair correlation calculations were not able to readily distinguish between models with and without paracrystals. On the other hand, wave mechanical simulations surprisingly showed that paracrystals could be experimentally imaged using phase contrast transmission electron microscopy and/or nanoscale electron diffraction on a contemporary aberration-corrected microscope.

High-Resolution Radial Distribution Function of Pure Ion-Implanted Amorphous Silicon Measured Using Tilted-Illumination Selected-Area Electron Diffraction

High-resolution radial distribution functions of as-implanted and thermally relaxed amorphous silicon created by ion implantation were measured using tilted-illumination selected area electron diffraction at room temperature. The diffracted intensities were measured out to a maximum scattering vector 2 sin(θ)/λ of 3.3-3.7 Å-1. The volume-averaged pair-correlation statistics of as-implanted and relaxed ion-implanted amorphous silicon are virtually indistinguishable with coordination numbers of 3.7 ± 0.3 and 3.9 ± 0.3 (for neighbors closer than 3 Å) and average bond angles of 109 ± 0.5° and 110 ± 0.6°, respectively. The atomic rearrangements in ion-implanted amorphous silicon due to a low temperature anneal are subtle.

Measurement of the specific heat and determination of the thermodynamic functions of relaxed amorphous silicon

The specific heat, cp, of two amorphous silicon (a-Si) samples has been measured by differential scanning calorimetry in the 100–900 K temperature range. When the hydrogen content is reduced by thermal annealing, cp approaches the value of crystalline Si (c-Si). Within experimental accuracy, we conclude that cp of relaxed pure a-Si coincides with that of c-Si. This result is used to determine the enthalpy, entropy, and Gibbs free energy of defect-free relaxed a-Si. Finally, the contribution of structural defects on these quantities is calculated and the melting point of several states of a-Si is predicted.

Junction capacitance study of an oxygen impurity defect exhibiting configuration relaxation in amorphous silicon–germanium alloys deposited by hot-wire CVD

We report the observation of light induced electron capture in oxygen contaminated (∼5×1020cm−3) hydrogenated amorphous silicon–germanium alloys grown by hot-wire chemical vapor deposition (HWCVD). By examining the time evolution of dark capacitance after 1.2eV photoexcitation, we are able to estimate the free energy barrier (⩾0.8eV) for the release of electrons into the conduction band. Such a large thermal barrier, for a defect whose optical threshold is centered (∼1.35eV) so close to the band-gap (1.5eV), indicates significant configurational relaxation once the oxygen impurity state is occupied with photoexcited electrons.

Nanoindentation-induced phase transformation in crystalline silicon and relaxed amorphous silicon

Nanoindentation-induced phase transformation in both crystalline silicon (c-Si) and relaxed amorphous silicon (a-Si) have been studied. A series of nanoindentations were made with a sharp diamond Berkovich tip. During nanoindentations, maximum loads were applied from 1000 to 6000μN, with a 1000μN/s loading rate. A slow unloading rate at 100μN/s was chosen to favor the formation of the high-pressure polycrystalline phases (Si-III and Si-XII). A fast unloading rate within 1s was used to obtain a-Si end phase. The nanoindentation behavior and the structure of deformation regions were examined by load–depth characteristics curves and Raman. Large differences were observed between the transformation behavior in c-Si and that in relaxed a-Si. Indentation curves in c-Si present plastic deformation curves with elbow (no pop-out) on the unloading curves, even for loads up to 9000μN. On the other hand, indentations in relaxed a-Si give rise to the same plastic deformation as c-Si at low loads (1000–2000μN), whereas show clear pop-outs at high loads (above 3000μN). Raman results suggest that high-pressure phases (HPPs) can occur more easily within a relaxed a-Si matrix than in a c-Si matrix. The results suggest a significantly different indentation behavior and phase transformation sequence in c-Si and relaxed a-Si at the nanoscale.

Response to “Comment on ‘Phase transformations induced in relaxed amorphous silicon by indentation at room temperature’ ” [Appl. Phys. Lett. 87, 016102 (2005)

Abstract

Comment on “Phase transformations induced in relaxed amorphous silicon by indentation at room temperature” [Appl. Phys. Lett. 85, 5559 (2004)

Comment on “Phase transformations induced in relaxed amorphous silicon by indentation at room temperature” [Appl. Phys. Lett. 85, 5559 (2004)

Phase transformations induced in relaxed amorphous silicon by indentation at room temperature

The deformation behavior of self-ion-implanted amorphous-Si (a-Si) has been studied using spherical nanoindentation in both relaxed (annealed) and unrelaxed (as-implanted) a-Si. Interestingly, phase transformations were clearly observed in the relaxed state, with the load–unload curves from these samples displaying characteristic discontinuities and cross-sectional transmission electron microscopy images indicating the presence of high-pressure crystalline phases Si-III and Si-XII following pressure release. Thus, an amorphous to crystalline phase transformation has been induced by indentation at room temperature. In contrast, no evidence of a phase transformation was observed in unrelaxed a-Si, which appeared to deform via plastic flow of the amorphous phase. Furthermore, in situ electrical measurements clearly indicate the presence of a metallic Si phase during loading of relaxed a-Si but no such behavior was observed for unrelaxed a-Si

Energy landscape of relaxed amorphous silicon

We analyze the structure of the energy landscape of a well-relaxed 1000-atom model of amorphous silicon using the activation-relaxation technique (ART nouveau). Generating more than 40,000 events starting from a single minimum, we find that activated mechanisms are local in nature, that they are distributed uniformly throughout the model and that the activation energy is limited by the cost of breaking one bond, independently of the complexity of the mechanism. The overall shape of the activation-energy-barrier distribution is also insensitive to the exact details of the configuration, indicating that well-relaxed configurations see essentially the same environment. These results underscore the localized nature of relaxation in this material.

GISAXS study of structural relaxation in amorphous silicon

The structural changes induced in single crystal silicon implanted with silicon ions above the amorphisation threshold were studied by Grazing Incidence Small Angle X-ray Scattering technique. Silicon samples were implanted with silicon ions at 30 keV to the dose of 5×10 15 atoms/cm 2. A well-defined layer of amorphous silicon, thick about 40 nm was formed below the surface. As implanted samples were subsequently relaxed by thermal annealing at 350 °C. The analysis have shown that the amorphous layer exhibits a granular structure that develops with annealing. A model will be presented for the film structure changes obtained by data evaluation based on the distorted wave Born approximation.

Size Distribution Of Embedded Nano-Crystallites In Polymorphous Silicon Studied By Raman Spectroscopy And Photoluminescence

AbstractPolymorphous Silicon (pm-Si:H) deposited by Plasma Enhanced Chemical Vapour Deposition (PECVD) has emerged as an alternative material to amorphous silicon (a-Si:H). Deposition parameters of pm-Si:H are such that small crystallites get embedded in a relaxed amorphous silicon matrix, thus improving the optical and electrical properties. We study the size of crystallites and degree of order in pm-Si:H using Raman and photoluminescence (PL) spectra of pm-Si:H and a-Si:H. Raman Spectra of a variety of hydrogenated nanostructured silicon (pmSi:H) and amorphous Silicon (a-Si:H) samples grown at different pressures were analyzed. Deconvolution of observed multiple peaks in photoluminescence spectra and fitting to Gaussian size distribution also yields particle size to be in the range of 2.3 to 3.5nm in agreement with Transmission Electron Microscopy and Raman results.

Low temperature relaxation in amorphous silicon made by ion implantation

A 3 μm thick surface layer of small crystalline Si discs has been made amorphous by ion implantation at liquid nitrogen temperature. Upon heating the samples to room temperature for the first time, an irreversible heat release is observed, which is attributed to low-temperature structural relaxation of the amorphous Si. The time constant at –50°C is about 3–4 times longer than those at 200°C and 500°C. The instantaneous heat release during the heating phase is larger than above room temperature, but not as large as would be expected if the heat release were proportional to the number of states at the Fermi level that is removed.

Ion beam irradiation of relaxed amorphous silicon carbide

In-situ transmittance measurements at λ=633 nm are used during ion irradiation to probe the defect generation in relaxed amorphous silicon carbide (SiC). The optical constants of amorphous SiC are strongly correlated to the thermal history of the material and the transmittance of ion implanted amorphous SiC (unrelaxed amorphous) increases after annealing in the temperature range 100–700°C. The transmittance of annealed amorphous SiC (relaxed) during subsequent implantation decreases and saturates to the value of unrelaxed amorphous. In-situ transmittance measurements allow to follow directly the defect generation and to measure the fluence at which the transmittance saturates (derelaxation fluence). The effect of different ions (He and Ar) on these phenomena is explored. The obtained results are compared and discussed with similar measurements performed on amorphous silicon.

Spectro-ellipsometric Studies of Amorphization and Thermal Annealing in Ion-implanted Silicon

The damage profiles in the P+, BF2+, As+ and B+ ion-implanted silicon specimens are investigated using the nondestructive spectroscopic ellipsometry (SE) technique. The effective dielectric functions of the damaged layers are calculated using the Bruggeman's effective medium approximation, assuming that the damaged layer can be optically represented by the mixture of crystalline and amorphous silicon. By selectively using either the dielectric function of implanted amorphous silicon, or that of the relaxed amorphous silicon as the reference data for the amorphous silicon, we have improved the accuracy in modeling. The model parameters regarding the damaged layer thickness and the degree of amorphization are found to depend on implanting ion species, implantation energies, and total doses. Also, the implantation induced damage profiles are computer simulated and compared with the SE results. When annealed at lower temperatures, implanted amorphous silicon turns into the relaxed amorphous silicon and starts to recrystallize from the interface of the c-Si side due to solid-phase epitaxial growth. At annealing temperatures higher than 600°C, relaxed amorphous layers become crystalline silicon.

Generalized defect annihilation kinetics for structural relaxation in amorphous silicon

Abstract A general approach to the dynamics of structural relaxation in amorphous silicon is developed. A form is chosen for the recombination kinetics of defects which removes the ad-hoc assumption made in previous approaches, namely that defects recombine only with others of identical activation energy. The generalized approach is tested quantitatively by modelling the structural relaxation of amorphous silicon and comparing the results with the experimental data. It is found that the generalized recombination kinetics formalism is necessary in order to describe accurately the time-resolved relaxation data.

Monte Carlo Simulations of Defect Relaxation in Amorphous Silicon

ABSTRACTThe structural memory model of slow defect relaxation in a-Si:H is extended to the limit of long defect filling times. The model was proposed in order to explain unusual, defect filling-time dependent capacitance transients that were observed for short defect filling times. For long defect filling pulses however, the experiments show normal charge emission transients that saturate into filling-time independent transients. We present two possibilities for explaining the approach to saturation within the structural memory model. Results of Monte Carlo simulations of the models are discussed.

Transition from relaxed to derelaxed amorphous silicon: Optical characterization

Optical constants of relaxed, derelaxed, and partially relaxed amorphous silicon (a-Si) in the range 0.4–0.9 μm are reported. The thermodynamical state of amorphous silicon (a-Si) has been changed either by thermal treatments or low dose ion implantation. Ellipsometry has been used to evaluate the complex refractive index for several amorphous states with enthalpy content between that of the fully relaxed and fully derelaxed a-Si. We observed a strong correlation between the electronic structure as probed by our optical measurements and the topological short-range order as probed by Raman scattering.

Structural relaxation in amorphous silicon prepared by ion implantation

Structural relaxation processes in amorphous silicon (a-Si) prepared by Si +, Ge + and P + implantation into Si at room temperature were investigated by Raman spectroscopy, electron spin resonance (ESR), and positron annihilation. Raman measurements indicated that the average bond angle deviation Δθ in a-Si networks first decreased with low temperature (200–450°C) annealing, but slightly increased with the annealing time. Based upon ESR and positron annihilation measurements, the density of dangling bonds and the free volume (e.g. microvoids) in a-Si layers were found to behave in a similar manner as Δθ during thermal treatments. We propose the following model for the results described above. Microvoids stored in the a-Si layer are annihilated and/or dissociated, which results in an increase in Δθ or dangling bond densities.

Generalized Activation Energy Spectrum Theory: A New Approach for modeling Structural Relaxation in Amorphous Solids

ABSTRACTA general approach to the dynamics of structural relaxation in amorphous solids is developed. A form of the recombination kinetics of defects is chosen which removes the ad hoc assumption made in previous theories that defects recombine only with others of identical activation energy. The generalized theory is tested quantitatively by modelling the structural relaxation of amorphous silicon, and comparing the results with the experimental data on structural relaxation. It is found that the generalized theory is necessary in order to accurately describe the time-resolved relaxation data. The generalized theory is also applied to estimate the effect of irradiation on the nucleation kinetics of crystal silicon, and is found to agree well with experimental data.

Structural relaxation in amorphous silicon and the role of network defects

Structural relaxation of amorphous Si (a-Si) can be viewed as either a general network rearrangement (short range ordering) or as the analogue in a-Si of defect annihilation processes in crystal Si (c-Si). Some recent experimental results are discussed in the light of these two alternatives. The annealing kinetics of ion bombardment damage in a-Si and c-Si are remarkably similar. The solubility of Cu in a-Si decreases with annealing, whereas the diffusivity increases which suggests removal of trapping sites (defects) from the material as relaxation proceeds. These phenomena can be understood when it is assumed that structural relaxation of a-Si is fully controlled by annihilation of point defects. The defect density in fully unrelaxed a-Si is estimated to correspond to several at.% displaced atoms. The absence of a measurable densification upon structural relaxation of a-Si suggests mutual annihilation of low-and high-density defects.