Defects In Amorphous Silicon Research Articles

Understanding Defects in Amorphous Silicon with Million-Atom Simulations and Machine Learning.

The structure of amorphous silicon (a-Si) is widely thought of as a fourfold-connected random network, and yet it is defective atoms, with fewer or more than four bonds, that make it particularly interesting. Despite many attempts to explain such "dangling-bond" and "floating-bond" defects, respectively, a unified understanding is still missing. Here, we use advanced computational chemistry methods to reveal the complex structural and energetic landscape of defects in a-Si. We study an ultra-large-scale, quantum-accurate structural model containing a million atoms, and thousands of individual defects, allowing reliable defect-related statistics to be obtained. We combine structural descriptors and machine-learned atomic energies to develop a classification of the different types of defects in a-Si. The results suggest a revision of the established floating-bond model by showing that fivefold-bonded atoms in a-Si exhibit a wide range of local environments-analogous to fivefold centers in coordination chemistry. Furthermore, it is shown that fivefold (but not threefold) coordination defects tend to cluster together. Our study provides new insights into one of the most widely studied amorphous solids, and has general implications for understanding defects in disordered materials beyond silicon alone.

Understanding Defects in Amorphous Silicon with Million‐Atom Simulations and Machine Learning

AbstractThe structure of amorphous silicon (a‐Si) is widely thought of as a fourfold‐connected random network, and yet it is defective atoms, with fewer or more than four bonds, that make it particularly interesting. Despite many attempts to explain such “dangling‐bond” and “floating‐bond” defects, respectively, a unified understanding is still missing. Here, we use advanced computational chemistry methods to reveal the complex structural and energetic landscape of defects in a‐Si. We study an ultra‐large‐scale, quantum‐accurate structural model containing a million atoms, and thousands of individual defects, allowing reliable defect‐related statistics to be obtained. We combine structural descriptors and machine‐learned atomic energies to develop a classification of the different types of defects in a‐Si. The results suggest a revision of the established floating‐bond model by showing that fivefold‐bonded atoms in a‐Si exhibit a wide range of local environments–analogous to fivefold centers in coordination chemistry. Furthermore, it is shown that fivefold (but not threefold) coordination defects tend to cluster together. Our study provides new insights into one of the most widely studied amorphous solids, and has general implications for understanding defects in disordered materials beyond silicon alone.

Electronic transport properties of a-Si:H

To investigate the electron transport properties of hydrogenated amorphous silicon (a-Si:H), a series of quantum simulations and electron transport analyses were performed. The target system is a nano-scale junction of a-Si:H with various hydrogen concentrations sandwiched between two metal electrodes. The density functional based tight binding simulation was conducted to obtain the electronic structure, and the non-equilibrium Green’s function method was adopted to evaluate the electron transmission coefficient and the electric current under a bias field. It is confirmed that the hydrogen atoms passivate a part of defects in amorphous silicon, but the remaining defects realize the energy states in the bandgap; the p orbitals of silicon atoms mainly contribute to the electron transmission. The transport behavior is greatly affected by the hydrogen concentration. The interface between a-Si:H and the metal electrodes also influences the transport behavior through changing the spatial charge density inside the a-Si:H.

Plasma-Induced Electronic Defects: Generation and Annihilation Kinetics in Hydrogenated Amorphous Silicon

Electronic defects in semiconductor devices play important roles in device performance and reliability. These defects are created during fabrication using plasma-based processing technology. The authors investigate the generation and annihilation of defects in amorphous silicon by measuring the photocurrent during processing with hydrogen and argon plasmas. Unfavorable species for defect generation and favorable species for defect recovery are distinguished. These results are beneficial for improving semiconductor plasma processing, which is used to make state-of-the-art devices such as FinFETs, computer memory, and solar cells.

Dominant structural defects in amorphous silicon

The nature of disorder in amorphous silicon (a-Si) is explored by investigating the spatial arrangement and energies of coordination defects in a numerical model. Spatial correlations between structural defects are examined on the basis of a parameter that quantifies the probability for two sites to share a bond. Pentacoordinated atoms are found to be the dominant coordination defects. They show a tendency to cluster, and about 17% of them are linked through three-membered rings. As for tricoordinated sites, they are less numerous, and tend to be distant by at least two bond lengths. Typical local geometries associated to under and overcoordinated atoms are extracted from the model and described using partial bond angle distributions. An estimate of the formation energies of structural defects is provided. Using molecular-dynamics calculations, we simulate the implantation of high-energy atoms in the initial structure in order to study the effect of relaxation on the coordination defects and their environments.

Ab initio modelling of boron related defects in amorphous silicon

AbstractWe have modeled boron related point defects in amorphous silicon, using an ab initio method, the Density functional theory‐pseudopotential code Aimpro. The boron atoms were embedded in 64 atom amorphous silicon cubic supercells. The calculations were performed using boron defects in 15 different supercells. These supercells were developed using a modified Wooten‐Winer‐Weaire bond switching mechanism. In average, the properties of the 15 supercells agree with the observed radial and bond angle distributions, as well the electronic and vibrational density of states and Raman spectra. In amorphous silicon it has been very hard to find real self‐interstitials, since for almost all the tested configurations, the amorphous lattice relaxes overall. We found that substitutional boron prefers to be 4‐fold coordinated. We find also an intrinsic hole‐trap in the non‐doped amorphous lattice, which may explain the low efficiency of boron doping. The local vibrational modes are, in average, higher than the correspondent crystalline values (© 2012 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Absorption coefficient modeling of microcrystalline silicon thin film using Maxwell-Garnett effective medium theory

Considering the Mott-Davis density of state model and Rayleigh scattering effect, we present an approach to model the absorption profile of microcrystalline silicon thin films in this paper. Maxwell-Garnett effective medium theory was applied to analyze the absorption curves. To validate the model, several experimental profiles have been established and compared with those results from the model. With the assistance of the genetic algorithm, our results show that the absorption curves from the model are in good agreement with the experiments. Our findings also indicate that, as the crystal volume fraction increases, not only do the defects in amorphous silicon reduce, but the bulk scattering effect is gradually enhanced as well.

The dangling-bond defect in amorphous silicon: Statistical random versus kinetically driven defect geometries

Amorphous and micro-crystalline silicon (a-Si:H, μc-Si) are key materials for resource-saving thin-film solar cells. However, the efficiency of such devices is severely limited by light-induced Si dangling-bond defects, which can be detected by electron paramagnetic resonance (EPR). We report density-functional theory calculations on a set of random dangling bonds created in supercell models of a-Si:H and compare calculated hyperfine and g-tensor distributions to the ones obtained from a recent multi-frequency EPR spectral analysis. Our results show that the g-tensor does not exhibit axial symmetry as has been previously assumed, but is clearly rhombic. The hyperfine coupling to the undercoordinated Si atom, on the other hand, is almost perfectly axial. This apparent discrepancy in the symmetry properties is shown to be a consequence of the underlying coupling mechanisms and how these are influenced by structural disorder.However, the hyperfine distribution calculated from our random models underestimates the experimentally observed 30% red-shift when going from c-Si to a-Si:H. We suggest that only a subset of possible dangling-bond configurations is observed in experiment. We discuss plausible mechanisms that would give rise to such a selection, and new experiments to test these hypotheses.

Thermally induced defect photoluminescence in hydrogenated amorphous silicon

The intrinsic defect photoluminescence of hydrogenated amorphous silicon (a-Si:H) films has been investigated at high intensities of optical pumping that lead to heating of the film. It has been revealed that, for short heating times, the intensity of the defect photoluminescence increases exponentially with an increase in the temperature with an activation energy of 0.85 eV, which is considerably higher than the activation energy (∼0.2 eV) determined from experiments on classical annealing. This and other experimental results on the temperature dependence of the intensity and kinetics of the defect photoluminescence have been explained in terms of the “hydrogen glass” model by thermally induced generation of intrinsic defects in amorphous silicon. The results of the calculations are in good agreement with the experimental data on the defect photoluminescence that reflects the formation and annihilation of defects for short heating times under optical excitation.

Photoluminescence spectral dispersion as a probe of structural inhomogeneity insilica

We perform time-resolved photoluminescence measurements on point defects inamorphous silicon dioxide (silica). In particular, we report data on the decaykinetics of the emission signals of extrinsic oxygen deficient centres of thesecond type from singlet and directly excited triplet states, and we use themas a probe of structural inhomogeneity. Luminescence activity in sapphire(α-Al2O3) is studied as well and used as a model system to compare the optical properties ofdefects in silica with those of defects embedded in a crystalline matrix. Only fordefects in silica did we observe a variation of the decay lifetimes with emissionenergy and a time dependence of the first moment of the emission bands. Thesefeatures are analysed within a theoretical model, previously proposed in D’Amico et al (2008 Phys. Rev. B 78 014203), with an explicit hypothesis about the effect introduced bythe disorder of vitreous systems. Separate estimations of the homogeneous andinhomogeneous contributions to the measured emission linewidth are obtained.It is found that inhomogeneous effects strongly condition both the triplet andsinglet luminescence activities of oxygen deficient centres in silica, although thedegree of inhomogeneity of the triplet emission turns out to be lower than that ofthe singlet emission. Inhomogeneous effects appear to be negligible in sapphire.

Interstitial-based boron diffusion dynamics in amorphous silicon

Using density-functional theory calculations we identified an interstitial-based fast boron diffusion mechanism in amorphous silicon. We found that interstitial-like point defects, omnipresent in as-implanted silicon, to be very stable in an amorphous network and can form highly mobile pair with Boron atoms. The transient existence of such point defects in amorphous silicon is suggested to play an important role in boron diffusion. We found the activation energy for this pathway to be 2.73 eV, in good agreement with experimental results. In addition, this mechanism is consistent with the experimentally reported transient and concentration-dependent features of boron diffusion in amorphous silicon.

Use of Tritium in the Study of Defects in Amorphous Silicon

Hydrogen is known to strongly affect the physical properties of amorphous semiconductors. Indeed hydrogen is introduced during the growth of amorphous silicon films, used in active matrix displays and solar cells, to passivate silicon dangling bonds and to relax the lattice thereby reducing the density of states in the energy gap by several orders of magnitude and giving rise to device grade material. Ideally, hydrogenated amorphous silicon (a-Si:H) is a continuous covalently bonded random network of silicon-silicon and silicon-hydrogen atoms, with the predominant nearest neighbour environment similar to that of crystalline silicon. a-Si:H typically contains about 10 atomic percent hydrogen.Tritium can readily substitute for hydrogen in a-Si:H without altering the physicochemical properties of the material. Tritium decay leads to a change in the local bond structure of the material as helium detaches from bonds leaving behind dangling bonds. The decay rate of tritium and therefore the rate of dangling bond formation is determined by the half-life of tritium. Hence, tritium provides a unique avenue to dynamically study the effect of dangling bonds on the density of states in the energy gap and therefore on the optoelectronic properties of a-Si:H. Tritiated hydrogenated amorphous silicon (a-Si:H:T) was deposited using mixtures of tritium and silane gases in a dc saddle-field glow-discharge deposition system. The amount of tritium in the films was controlled by adjusting the relative flow of tritium and silane gases into the deposition chamber.Photoluminescence, isothermal capacitive transient spectroscopy and constant photocurrent spectroscopy were used to measure defect concentration as a function of time in the films. The defect concentration was found to increase between 1 and 2 orders of magnitude, in about 300 hours. Thermal annealing decreased the defect concentration. It was found that tritium permits a study of the change in the density of defect states due to dangling bond formation in a-Si:H without the uncertainties introduced by the use of multiple samples.

Tritium Induced Defects in Amorphous Silicon

ABSTRACTWe report the growth of tritium induced defects in tritium doped hydrogenated amorphous silicon (a-Si:H,T) as measured by electron spin resonance (ESR) and photothermal deflection spectroscopy (PDS). The measurements allow one to examine the accumulation of defects in a-Si:H,T where the defect production mechanism is known. Defects produced by tritium decay are found to be much less numerous than the number of decayed tritium atoms and they are metastable like Staebler-Wronski defects. These results provide new insight into the metastable defect creation and the role of hydrogen motion.

Wannier functions characterization of floating bonds in a-Si

We investigate the electronic structure of over-coordinated defects in amorphous silicon via density-functional total-energy calculations, with the aim of understanding the relationship between topological and electronic properties on a microscopic scale. Maximally localized Wannier functions (MLWF)are computed in order to characterize the bonding and the electronic properties of these defects. The five-fold coordination defects give rise to delocalized states extending over several nearest neighbors (NNs), and therefore to very polarizable bonds and anomalously high Born effective charges for the defective atoms.

Coordination defects in amorphous silicon and hydrogenated amorphous silicon: A characterization from first-principles calculations

Abstract We study by means of the first-principles pseudopotential method the coordination defects in amorphous silicon and hydrogenated amorphous silicon, also in their formation and their evolution upon hydrogen interaction. An accurate analysis of the valence charge distribution and of the ‘electron localization function’ allows us to resolve possible ambiguities in the bonding configuration, and in particular to identify clearly threefold (T3) and fivefold (T5) coordinated defects. We found that electronic states in the gap can be associated with both kinds of defect and that in both cases the interaction with hydrogen can reduce the density of states in the gap.

Direct simulation of ion-beam-induced stressing and amorphization of silicon

Using molecular dynamics (MD) simulation, we investigate the mechanical response of silicon to high dose ion-irradiation. We employ a realistic and efficient model to directly simulate ion beam induced amorphization. Structural properties of the amorphized sample are compared with experimental data and results of other simulation studies. We find the behavior of the irradiated material is related to the rate at which it can relax. Depending upon the ability to deform, we observe either the generation of a high compressive stress and subsequent expansion of the material, or generation of tensile stress and densification. We note that statistical material properties, such as radial distribution functions are not sufficient to differentiate between different densities of amorphous samples. For any reasonable deformation rate, we observe an expansion of the target upon amorphization in agreement with experimental observations. This is in contrast to simulations of quenching which usually result in denser structures relative to crystalline Si. We conclude that although there is substantial agreement between experimental measurements and most simulation results, the amorphous structures being investigated may have fundamental differences; the difference in density can be attributed to local defects within the amorphous network. Finally we show that annealing simulations of our amorphized samples can lead to a reduction of high energy local defects without a large scale rearrangement of the amorphous network. This supports the proposal that defects in amorphous silicon are analogous to those in crystalline silicon.

Formation of optically active centers in films of erbium-doped amorphous hydrated silicon

We have observed photoluminescence at 1.54 µm from a-Si:H films doped with erbium of various degrees of purity. It is shown that the additional introduction of oxygen activates the Er ions. The effect of silicides and defects in amorphous silicon a-Si:H films and in crystalline silicon c-Si is investigated.

Modeling and realization of a high-gain homojunction a-Si:H bulk barrier phototransistor

We report an extensive analysis of amorphous silicon bulk-barrier phototransistors. Starting from a new analytical model for the device, we take into account the peculiarity of the material and, in particular, of the p-doped base, which we find to critically affect device operation. We demonstrate the possibility of obtaining lightly p-doped material suitable for high-gain devices, and we use the equilibrium model of defects in amorphous silicon as a guideline for our work. Finally, measurements on a number of high-gain devices verify our theoretical predictions.

The relationship between hydrogen and electronic defects in amorphous silicon

Two important models for understanding of amorphous silicon are the defect pool model and the hydrogen density of states. The defect pool model explains the electronic density of states and has significantly improved the understanding and modelling of amorphous silicon devices. The hydrogen density of states has been useful to understand hydrogen diffusion, hydrogenation—dehydrogenation experiments and optimal growth conditions. Previously these models were separate and inconsistent with each other. A single model has been developed which encompasses both of these models, and can be used to explain a large number of phenomena in a unified and consistent manner. Here the central concepts underlying the model are explained, and some examples are given of the wide variety of data which it fits.

Charge-defect thermodynamic equilibrium and ‘‘metastable’’ defects in amorphous silicon

A thermodynamic equilibrium description of the high-temperature (150–250 °C) steady-state behavior of light-induced defects in amorphous silicon is presented. The entropy and enthalpy of dangling-bond formation are quantified. In contrast to the behavior of vacancies in single-crystalline silicon the creation of the dangling-bond defect in amorphous silicon produces negative entropy and enthalpy changes indicating that lattice relaxations contribute to the free-energy changes. Over the temperature range examined, the creation of dangling bonds lowers the free energy due to the relatively large negative enthalpy change. Practical issues such as the estimation of the saturated dangling-bond density resulting from given illumination level at temperatures too low to experimentally observe true saturation are also considered.