Silicon Self-interstitials Research Articles

Thermal stability of undoped polycrystalline silicon layers on antimony and boron-doped substrates

The structure of as-deposited and annealed polycrystalline silicon layers has been investigated by scanning electron microscopy and x-ray diffraction. The structure of intentionally undoped layers prepared by low pressure chemical vapor deposition at a temperature of 640 °C was found to be stable upon annealing at temperatures lower than about 900 °C. On the other hand, primary recrystallization of the layers has been observed during annealing at temperatures in the range of 900 to 1150 °C. Isochronal annealing revealed the activation energy for the primary recrystallization of undoped layers as 0.6 eV. The activation energy for diffusion of silicon self-interstitials along the grain boundaries was calculated to be 2.2 eV. The difference in grain-growth process was observed for the undoped layers grown either (i) on lightly boron-doped substrate or (ii) on the substrate heavily doped with antimony. The different grain-growth mechanism was found to be a consequence of antimony diffusion into the polycrystalline layer.

DLTS Studies of Carbon Related Complexes in Irradiated N- and P-Silicon

DLTS studies of transformation kinetics of different carbon–related complexes in electron irradiated n- and p-type silicon have been performed. It has been found that silicon self-interstitials have very low mobility even at room temperature in p-Si, but become extremely mobile under elec-tron injection. It is shown that upon annealing of interstitial carbon in p-Si a metastable state for interstitial carbon-interstitial oxygen complex is formed. This state has an energy level of about Еv+0.36 eV. The formation of the stable and metastable states takes place concurrently. The observed features of the carbon-related complexes formation are likely related to the existence of different crystallographic orientation of the equiprobable pathways through which the interstitial carbon and oxygen atoms can approach each other.

Probing the Behaviors of Point Defects in Silicon and Germanium Using Isotope Superlattices

In order to probe the fundamental behaviors of point defects in silicon and germanium, we studied the self-diffusion using isotope superlattices. In ion-implanted germanium, vacancies are in thermal equilibrium and transient enhanced diffusion is not present under the experimental conditions employed in this study. In contrast, silicon self-interstitials are supersaturated in ion-implanted silicon and the self-interstitial concentration is going down to the thermal equilibrium value toward the surface.

Behaviors of neutral and charged silicon self-interstitials during transient enhanced diffusion in silicon investigated by isotope superlattices

We investigated the contributions of neutral and charged silicon self-interstitials to self- and boron diffusion during transient enhanced diffusion in silicon. We simultaneously observed self- and boron diffusion in silicon using Snati/S28i isotope superlattices. A calculation based on diffusion equations involving {311} defects and boron-interstitial cluster models was employed to reproduce the diffusion profiles in silicon-implanted (intrinsic) and boron-implanted (extrinsic) silicon isotope superlattices, followed by annealing. To investigate the diffusion processes, the time evolution of the silicon self-interstitial profiles during the transient diffusion was simulated. The results directly demonstrate that excess neutral self-interstitials dominantly enhance the self-diffusion during the transient process in the intrinsic conditions, while doubly positively charged self-interstitials dominate the self-diffusion in the extrinsic conditions.

Gigantic uphill drift of vacancies and self-interstitials in silicon

Point defect transport in a growing crystal includes a drift along the temperature gradient, G, at a velocity αG. It was not clear if the drift is negligible or strong in silicon crystal growth. It is now found that reported microdefect patterns in crystals grown with a temporarily halt provide a clear evidence in favour of a strong (even gigantic) drift of both kinds of intrinsic point defects. The drift coefficients α V (for vacancies) and α I (for self-interstitials) are deduced by fitting the simulating defect profiles to the observed location of halt-induced interstitial region immersed into a vacancy-type crystal.

Vacancy engineering by optimized laser irradiation in boron-implanted, preamorphized silicon substrate

In this letter, the effect of vacancies generated by preirradiated laser on dopant diffusion and activation in preamorphized silicon substrate has been studied. Laser-induced melting in silicon was used to generate excess vacancies near the maximum melt depth before silicon substrate amorphization and subsequent boron implantation. We demonstrate that by matching the preirradiated laser melt depth with the implant amorphize depth, it can effectively reduce the silicon self-interstitials released from the end-of-range defect band. The results show great suppression in boron transient enhanced diffusion and significant removal of end-of-range defects. This is attributed to the recombination of laser-generated excess vacancies with preamorphizing induced free silicon interstitials at the end-of-range region.

Extended 'Five-Stream' Model for Diffusion of Implanted Dopants in Silicon during Ultra-Shallow Junction Formation in VLSI Circuits

Ion implantation of different dopants (donors and acceptors) into crystalline silicon with subsequent thermal annealing is used for the formation of ultra-shallow p-n junctions in VLSI technology. The experimentally observed phenomenon of transient enhanced diffusion (TED) during annealing hinders further downscaling of advanced VLSI circuits. However, modern mathematical models of dopant diffusion, which are based on the so-called “five-stream” approach, and software packages such as SUPREM4 encounter difficulties in describing TED. In this work, an extended five-stream model for diffusion in silicon is developed, which takes into account all the possible charge states of point defects (vacancies and silicon self-interstitials) and diffusing pairs “dopant atom–vacancy” and “dopant atom–silicon self-interstitial”. The model includes diffusion and drift of differently charged point defects and pairs in the internal electric field and the kinetics of interaction between unlike species. The expressions for diffusion fluxes and sink/source terms that appear in the non-linear, non-steady-state reaction-diffusion equations are derived for both donor and acceptor dopants accounting for multiple charge states of the diffusing species.

Effect of Hydrostatic Pressure on Self-Interstitial Diffusion in Si, Ge, Si<Ge> Crystals: Quantum-Chemical Simulations

A theoretical modeling of the diffusion of self-interstitials in silicon and germanium crystals both at normal and high hydrostatic pressure has been carried out using molecular mechanics, semiempirical (PM3, PM5) and ab-initio (SIESTA) methods. According to the simulation for the Si and Ge neutral interstitials (I0) both in silicon and germanium crystals more stable configuration is <110> split interstitial. T is the stable configuration for the double positive interstitial I++, but the interstitial is displaced from the high-symmetry site. Stability of <110> splitinterstitial is not changed under hydrostatic pressure. The activation barriers for the diffusion of interstitials were determined and equal to ΔEa(Si)(<110> -> T1)=0.69 eV; ΔEa (Ge)(<110> -> T1)=1.1 eV. For mixed interstitials the calculated activation barriers equal Si Emix = 1.06 eV, Ge Emix = 0.86 eV. Hydrostatic pressure decreases the activation barriers ΔEa(Si), ΔEa (Ge).

Properties of vacancies and self-interstitials in silicon deduced from crystal growth, wafer processing, self-diffusion and metal diffusion

Vacancies and self-interstitials in silicon are involved, in a straightforward way, in the formation of grown-in microdefects, diffusion of metals (Au, Zn), self-diffusion and installation of vacancy depth profiles in thin quenched wafers. The diffusivities and equilibrium concentrations of the intrinsic point defects, in dependence of temperature, could be deduced by analyzing these phenomena. The defect diffusivities are high while the equilibrium concentrations are remarkably low.

Parameters of Intrinsic Point Defects in Silicon Based on Crystal Growth, Wafer Processing, Self- and Metal- Diffusion

The vacancies and self-interstitials in silicon are involved, in a straightforward way, in various phenomena such as formation of grown-in microdefects, diffusion of metals (Au, Zn), self-diffusion and installation of vacancy depth profiles into wafers by Rapid Thermal Annealing. The available data is sufficient to deduce the diffusivities and equilibrium concentrations of the intrinsic point defects leaving only one parameter (the migration energy of self-interstitials) not specified. The diffusivities are high while the equilibrium concentrations are remarkably low.

Intrinsic Point Defects in Silicon: a Unified View from Crystal Growth, Wafer Processing and Metal Diffusion

There are several phenomena where the properties of vacancies and self-interstitials in silicon are manifested in straightforward ways. These include the formation of grown-in microdefects, the diffusion of metals (such as Au, Zn), self-diffusion and the installation of vacancy depth profiles in wafers by Rapid Thermal Annealing. Combining features extracted from the analysis of these phenomena, it is possible to define the diffusivities and equilibrium concentrations of the intrinsic point defects. Their diffusivities are remarkably high, and have weak temperature dependence. Their equilibrium concentrations are very low, and have strong temperature dependence.

The Role of Silicon Interstitials in the Formation of Boron-Oxygen Defects in Crystalline Silicon

Oxygen-rich crystalline silicon materials doped with boron are plagued by the presence of a well-known carrier-induced defect, usually triggered by illumination. Despite its importance in photovoltaic materials, the chemical make-up of the defect remains unclear. In this paper we examine whether the presence of excess silicon self-interstitials, introduced by ion-implantation, affects the formation of the defects under illumination. The results reveal that there is no discernible change in the carrier-induced defect concentration, although there is evidence for other defects caused by interactions between interstitials and oxygen. The insensitivity of the carrier-induced defect formation to the presence of silicon interstitials suggests that neither interstitials themselves, nor species heavily affected by their presence (such as interstitial boron), are likely to be involved in the defect structure, consistent with recent theoretical modelling.

Impact of oxygen on carbon precipitation in polycrystalline ribbon silicon

This article reports experimental evidence for the effect of oxygen on carbon precipitation in polycrystalline ribbon silicon. Four sets of wafers subject to various heat treatments have been examined by infrared spectroscopy. It is found that carbon precipitation in an oxygen-containing wafer consists of two distinct steps, namely, an initial rapid oxygen–carbon coprecipitation in the very first hour annealing, followed by slow precipitation during subsequent prolonged annealing. A high oxygen content enhances carbon precipitation throughout the two steps. It is shown that the formation of interstitial carbon in the presence of excess silicon self-interstitials generated during oxygen precipitation plays an important role in increasing the carbon precipitation rate in the first hour annealing. Because of the absence of interstitial injection during the following slow precipitation process, the enhancement effect of oxygen can only arise from an increase in precipitation sites. It is proposed that the oxygen–carbon coprecipitates formed in the very first hour annealing provide sites for continuous carbon precipitation. This explains why carbon impurities precipitate faster in a high oxygen-containing wafer, even after removal of all the interstitial oxygen from the silicon matrix.

Electrical and optical properties of rod-like defects in silicon

Self-interstitials in silicon can aggregate to form rod-like defects (RLDs) having both electrical and optical activity. We carry out local density functional calculations for both {113} and {111} RLDs to determine their structures and electrical activity. We find that small {113} RLDs are more stable than {111} RLDs but this reverses for larger defects. We attribute the electrical activity of {113} RLDs found in deep level transient spectroscopy studies with the bounding dislocations and the 0.903 eV photoluminescence to vacancy point defects lying on the habit plane.

Anomalous solubility of implanted nitrogen in heavily boron-doped silicon

It is established that an anomalously high electron concentration can be attained in heavily boron-doped silicon using ion implantation followed by implantation with nitrogen at an elevated temperature; the concentration of electrons can exceed that of boron. A model of this phenomenon is suggested that is based on the reaction of displacing boron atoms from the lattice sites by silicon self-interstitials with subsequent filling of the vacancies with nitrogen atoms (so that donor centers are formed).

Defect energetics of β-SiC using a new tight-binding molecular dynamics model

We present the calibration of a semi-empirical and orthogonal tight-binding total energy model for defect energetics in β-SiC, as based on a state-of-the-art ab initio data base for the formation energies of carbon and silicon vacancies, antisites, and self-interstitials. The present total energy model is further applied within a molecular dynamics framework to investigate the silicon and carbon interstitial defect contribution to the self-diffusion in β-SiC. We provide a fully atomistic model for both migration path and diffusivity, giving also a quantitative estimation of energy migration barriers.

Carbon-mediated aggregation of self-interstitials in silicon: A large-scale molecular dynamics study

The carbon-mediated aggregation of silicon self-interstitials is investigated with large-scale parallel molecular dynamics. The presence of carbon in the silicon matrix is shown to lead to concentration-dependent self-interstitial cluster pinning, dramatically reducing cluster coalescence and thereby inhibiting the nucleation process. The extent of cluster pinning increases with cluster size for the range of cluster sizes observed in the simulation. The direct effect of carbon on single self-interstitials is shown to be of secondary importance, and the concentration of single self-interstitials as a function of time is essentially unchanged in the presence of carbon. A quasi-single-component mean-field interpretation of the atomistic simulation results is proposed and further confirms these conclusions. Based on these results, it is suggested that the experimentally observed effect of carbon on transient-enhanced diffusion of boron could be due to the direct interaction between carbon atoms and self-interstitial clusters.

Role of fluorine in suppressing boron transient enhanced diffusion in preamorphized Si

We have explained the role of fluorine in the reduction of the self-interstitial population in a preamorphized Si layer under thermal treatment. For this purpose, we have employed a B spike layer grown by molecular-beam epitaxy as a marker for the self-interstitial local concentration. The amorphized samples were implanted with 7×1012, 7×1013, or 4×1014 F/cm2 at 100 keV, and afterwards recrystallized by solid phase epitaxy. Thermal anneals at 750 or 850 °C were performed in order to induce the release of self-interstitials from the end-of-range (EOR) defects and thus provoke the transient enhanced diffusion of B atoms. We have shown that the incorporation of F reduces the B enhanced diffusion in a controlled way, up to its complete suppression. It is seen that no direct interaction between B and F occurs, whereas the suppression of B enhanced diffusion is related to the F ability in reducing the excess of silicon self-interstitials emitted by the EOR source. These results are reported and discussed.

Self-interstitials in 3C-SiC

We report results from density-functional plane-wave pseudopotential calculations forcarbon and silicon self-interstitials in cubic silicon carbide (3C-SiC). Several initial ionicconfigurations are used in the search for the global total-energy minimum includingtetragonal, split [100] and split [110] geometries. Neutral carbon interstitials are found tohave several nearly degenerate total-energy minima configurations in split-interstitialgeometries, with formation energies ranging—besides higher metastable ones—from 6.3 to6.7 eV in stoichiometric SiC. By contrast, the neutral silicon interstitials have a clear singleminimum total-energy configuration at the tetrahedral configuration with carbon nearestneighbours, exhibiting a formation energy of 6.0 eV. The split interstitial in the [110]direction at the silicon site and the tetrahedral configuration with silicon nearestneighbours are metastable and have significantly higher formation energies. The presentcalculations indicate that the carbon interstitial introduces deep levels in the band gapwhile the silicon interstitial at the tetrahedral site behaves like a shallow donor.

Boron ripening during solid-phase epitaxy of amorphous silicon

By means of large-scale molecular dynamics simulations we provide a thorough atomic-scale picture of boron incorporation in crystalline silicon upon solid-phase epitaxy. The present results show that boron can either be incorporated as a substitutional dopant or form clusters with a low content of silicon self-interstitials. A full characterization of the formation process of boron-interstitial clusters and their stoichiometry is presented. The present results are consistent with available experimental information and also provide a deep physical insight into B-doped silicon solid-phase epitaxy.