Defects In Crystalline Silicon Research Articles

Opportunities for new materials synthesis by hot wire chemical vapor process

Over the last 20 years the hot wire chemical vapor deposition (HWCVD) technique is being explored as an effective alternative to the conventional plasma enhanced chemical vapor deposition (PECVD) for silicon based thin film devices and is claimed to have various advantages. An important point to be appreciated is the mixed nature of the HWCVD process. On one hand it offers all the benefits of being a chemical vapor process and on the other it has the flavor of physical vapor deposition due to the generation of precursors at a line/plane like source (wire array) far away from the substrate albeit with subsequent transport accompanied by secondary gas phase reactions. The possible control over the secondary gas phase reactions gives a unique feature to the HWCVD process. Apart from employing HWCVD for the preparation of a-Si:H and μc-Si:H with high deposition rates we have extended the applicability of the HWCVD to the synthesis of piezoresistive microcrystalline silicon, diffusion barriers of a-SiC:H, silicon nanowires, boron carbide for thermal neutron detectors, stress free a-SiN:H thin films for MEMS devices and metal nano-templates for semiconductor nanowire synthesis. We also established the applicability of HWCVD in surface nano-engineering to incorporate different functionalities (without actually depositing any film) i.e. nano-engineering or nano-modification of the surface to avoid electromigration on low-k dielectric layers and reduce surface defects in crystalline silicon and also bring about nano-crystallization of metallic thin films. Hence I would like to coin a more general nomenclature for this technique and refer to it as hot wire chemical vapor process (HWCVP). This paper discusses the results and outcomes of some of the case studies that we have carried out employing the HWCVP.

Temperature Induced Evolution of Bond-Centered Hydrogen (BCH) Defects in Crystalline Silicon: Dynamical, Electronic, Vibrational and Optical Signatures

ABSTRACTThe thermal stability, evolution and structure of the bond-centered-hydrogen (BCH) defect in crystalline silicon, its temperature induced dissociation, and the new H complexes formed have been studied in the temperature range from 50 K to 650 K by first-principles molecular dynamics (MD). We demonstrate that BCH is stable at 60 K, but decays at and above 310 K in agreement with experimental results. The dissolved BCH forms new complexes: transitional interstitials, stable monohydride-like and monohydride/dihydride-like complexes. The calculated asymmetric vibrational frequency of H in the BCH complex is 2000 cm-1, very close to the experimental values. Calculated vibrational frequencies, electron charge densities, electron densities of states (DOS), and optical spectra demonstrate noticeable differences for the different geometries with the BCH, interstitial and monohydride-like complexes, especially in the vicinity of the energy gap. The BCH complex is found to induce characteristic donor states below the conduction band, and raises the Fermi level to above the donor state energies.

Integrated atomistic modelling of interstitial defect growth in silicon

A new theoretical approach that combines Metropolis Monte Carlo, tight-binding molecular dynamics, and density functional theory calculations is introduced as an efficient technique to determine the structure and stability of native defects in crystalline silicon. Based on this combined approach, the growth behaviour of self-interstitial defects in crystalline Si is presented. New stable structures for small interstitial clusters (I n , 5 ≤ n ≤ 16) are determined and show that the compact geometry appears favoured when the cluster size is smaller than 10 atoms (n < 10). The fourfold-coordinated dodeca-interstitial (I12) structure with C 2h symmetry is identified as an effective nucleation centre for larger extended defects. This work provides the first theoretical support for earlier experiments that suggest a shape transition from compact to elongated structures around n = 10. We also provide some theoretical evidence that suggests that {3 1 1} extended defects grow slowly along ⟨2 3 3⟩ and relatively faster along ⟨1 1 0⟩, which is consistent with typical defect aspect ratios observed through transmission electron microscopy.

Localisation and identification of recombination‐active extended defects in crystalline silicon by means of focused ion‐beam preparation and transmission electron microscopy

AbstractThis paper reports on the localisation of extended defects related to the co‐precipitation of transition metal impurities in silicon and their preparation for subsequent analysis using techniques of transmission electron microscopy (TEM). Two approaches are described, i.e. (i) localisation of recombination‐active defects by light‐beam induced current followed by preferential chemical etching and TEM sample preparation using focused‐ion beam (FIB), and (ii) the replacement of the chemical etching by in situ Ga+ irradiation in the FIB system. The former technique is successfully applied to copper‐rich precipitate colonies in silicon whereas the latter proofs to be the superior approach for nickel‐rich particles. Structural and chemical analyses using TEM techniques are described for both cases (© 2009 WILEY‐VCH Verlag GmbH &amp; Co. KGaA, Weinheim)

Structure, chemistry and electrical properties of extended defects in crystalline silicon for photovoltaics

AbstractThe electronic properties of present‐day multicrystalline silicon (mc‐Si) materials for photovoltaic applications are strongly influenced by point defects, their mutual interaction and their interaction with dislocations and grain boundaries. This paper presents results from fundamental investigations of metal impurity interaction with extended defects, namely a small‐angle grain boundary and bulk microdefects. It is shown that the distribution of copper silicide precipitates closely follows the density of bulk microdefects indicating the underlying physics of ‘good’ and ‘bad’ grains frequently observed in mc‐Si. Co‐precipitation of copper and nickel in the same samples leads to virtually the same distribution of multimetal silicide precipitates which according to light‐beam induced current measurements show the same recombination activity as single‐metal silicide particles. Transmission electron microscopy is used to show that for copper‐rich and nickel‐rich conditions two types of silicides co‐exist, i.e. Cu3Si precipitates containing a small amount of nickel and NiSi2 precipitates containing some copper. Finally, phosphorus‐diffusion gettering (PDG) is discussed as the main gettering process used in presentday silicon photovoltaics. Special emphasis is put on the effect of extended defects and their interaction with metal impurities on PDG kinetics. It is shown that different limiting processes will be simultaneously operative in mc‐Si as a result of inhomogeneous bulk defect distributions (© 2009 WILEY‐VCH Verlag GmbH &amp; Co. KGaA, Weinheim)

Interaction of metal impurities with extended defects in crystalline silicon and its implications for gettering techniques used in photovoltaics

Multicrystalline silicon materials for photovoltaic applications inherently contain extended defects like grain boundaries, dislocations, microdefects and in some cases also second phase precipitates due to high concentrations of light elements (carbon, nitrogen or oxygen) and transition metal impurities. The latter are known to reduce the minority carrier lifetime and hence should be removed by gettering during solar cell processing. This paper discusses the influence of extended defects on the spatial distribution of copper- and nickel-related silicide precipitates for a model system containing a small angle grain boundary and in one part silicon oxide pecipitates partly associated with punched-out dislocations. Phosphorus-diffusion gettering under conditions of mostly precipitated metal impurities is discussed in terms of quantitative simulations. It is shown that two regimes can be distinguished where gettering kinetics are either limited by precipitate dissolution or phosphorus in-diffusion. Finally, binding of metal impurities to dislocations is considered and its effect on gettering kinetics is illustrated in terms of gettering simulations.

Hydrogen/silicon complexes in silicon from computational searches

Defects in crystalline silicon consisting of a silicon self-interstitial atom and one, two, three, or four hydrogen atoms are studied within density-functional theory (DFT). We search for low-energy defects by starting from an ensemble of structures in which the atomic positions in the defect region have been randomized. We then relax each structure to a minimum in the energy. We find a new defect consisting of a self-interstitial and one hydrogen atom (denoted by {I,H}) which has a higher symmetry and a lower energy than previously reported structures. We recover the {I,H_2} defect found in previous studies and confirm that it is the most stable such defect. Our best {I,H_3} defect has a slightly different structure and lower energy than the one previously reported, and our lowest energy {I,H_4} defect is different to those of previous studies.

The Temperature Evolution of the Hydrogen Plasma Induced Structural Defects in Crystalline Silicon

Hydrogenated n and p doped Czochralski Si substrates have been studied by means of atomic force microscopy, scanning and transmission electron microscopy, Raman spectroscopy and microwave photoconductivity decay techniques. The measurements show that the surface is roughest in ndoped samples which are plasma treated at high frequency. The cone density was found to be highest on p-doped samples, which correlates well to the higher density of defects observed in pdoped samples. The surface cones were found to consist of nanograins, twins and stacking faults with random orientations, several hydrogen induced defects and bubbles. The size, density and formation depth of the subsurface defects were seen to depend on doping type, doping level, plasma frequency and hydrogenation time. Raman spectroscopy shows formation of nearly free hydrogen molecules, which are presumed to be located in nano-voids or platelets. These molecules dissolved at temperatures around 600°C. By means of the &amp;-PCD measurements, it is demonstrated that hydrogen-initiated structural defects act as active recombination centres, which are responsible for the degradation of the minority carrier lifetime.

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.

Planar defects in crystalline silicon caused by self-irradiation

We have analysed by computer simulation the evolution of defects caused by self-irradiation of crystalline silicon (c-Si) at high temperatures. A classical molecular dynamics simulation (MD) was followed by defect analysis using the pixel mapping (PM) method. The incident Si ion energy was 5keV and the target temperature was set to 1000K. In the present simulation, we aimed to reproduce experimentally observed {311} planer defects. So far we did not observe long chain structures towards the 〈110〉 direction, nor remarkable platelet {311} planar defects. Nevertheless we observed a significant increase of 〈110〉-oriented self-interstitial dimers and a small fraction of linear trimers, which will be the initial stages of 〈110〉-rod formation.

DLTS properties of iron defects in crystalline silicon used in solar cells

Crystalline silicon used in solar cells has been investigated using deep level transient spectroscopy (DLTS). In majority-carrier pulse sequence an interstitial iron deep level was observed. However, the investigation of this deep level peak with different filling pulsewidths shows that this level consists of two superimposed levels. The activation energies of these levels are 375 meV ( F 1 ) and 480 meV ( F 2 ). The capture cross section of the level ( F 1 ) with the lower activation energy is nearly two orders of magnitude larger than the capture cross section of defect F 2 . Both capture cross sections show, over a wide range, no temperature dependence indicating that nonradiative recombination mechanisms other than multiphonon emission are involved. The concentration ratio between both defects is nearly 1:2.

DLTS properties of iron defects in crystalline silicon used in solar cells

Crystalline silicon used in solar cells has been investigated using deep level transient spectroscopy (DLTS). In majority-carrier pulse sequence an interstitial iron deep level was observed. However, the investigation of this deep level peak with different filling pulsewidths shows that this level consists of two superimposed levels. The activation energies of these levels are 375 meV ( F 1) and 480 meV ( F 2). The capture cross section of the level ( F 1) with the lower activation energy is nearly two orders of magnitude larger than the capture cross section of defect F 2. Both capture cross sections show, over a wide range, no temperature dependence indicating that nonradiative recombination mechanisms other than multiphonon emission are involved. The concentration ratio between both defects is nearly 1:2.

Ion beam induced defects in crystalline silicon

A short review of the current understanding and modelling of the formation of ion beam induced defects in crystalline silicon is given in the first part of this article. Some recent experiments on the evolution of {1 1 3} defects formed after Si + implants in CVD grown wafers and on the formation of small clusters in ultra-low energy high-dose B implanted Si are then presented. It is found that, independently of the experimental conditions, {1 1 3} defects evolve in all cases following a non-conservative Ostwald Ripening mechanism. In some cases, {1 1 3} defects have been found to transform into dislocation loops, while in other cases they completely dissolve during annealing. After RTA annealing of ultra-low energy high-dose B + implanted Si, small two-dimensional {1 0 0} loops are formed. Both Si and B atoms are contained in the defects. These results indicate that boron interstitial clusters can be imaged by TEM and thus can be much bigger than generally assumed.

Global Parameterization of Multiple Point-Defect Dynamics Models in Silicon

The task of determining globally robust estimates for the thermophysical properties of intrinsic point defects in crystalline silicon remains challenging. Previous attempts at point-defect model regression have focused on the use of a single type of experimental data but as of yet no single parameter set has produced predictive models for a variety of point-defect related phenomena. A stochastic optimization technique known as simulated annealing is used to perform simultaneous regression of multiple models. Specifically, zinc diffusion in Si wafers and the dynamics of the so-called interstitial-vacancy boundary during Czochralski crystal growth are used to systematically probe point-defect properties. A fully transient model for point-defect dynamics during crystal growth is presented which employs a sophisticated adaptive mesh refinement algorithm to minimize the computational expense associated with each optimization. The resulting framework leads to a quantitatively coherent picture for both experimental systems, which are modeled with a single set of point-defect thermophysical properties. The results are shown to be entirely consistent with other recent model-fitting estimates and indicate that as the number of experiments considered simultaneously within this framework increases it should be possible to systematically specify these properties to higher precision. © 2003 The Electrochemical Society. All rights reserved.

Anisotropic magnetic centers and conduction electrons in hydrogenated microcrystalline silicon

Electron spin resonance of anisotropic magnetic centers (dangling bonds) and conduction electrons in hydrogenated microcrystalline silicon (μc-Si:H) prepared by plasma-enhanced chemical vapor deposition (PECVD) has been observed at room temperature. The anisotropic g-shifts of dangling bonds in μc-Si:H are discussed in terms of tight-binding approaches and in comparison with those of dangling bonds in μc-Si:H prepared by hot-wire CVD and hydrogenated amorphous silicon (a-Si:H), P b centers at the Si–SiO 2 interface and defects in crystalline silicon (c-Si). The spin densities of anisotropic magnetic centers and conduction electrons were measured as a function of gas-dilution ratio of SiH 4 into H 2 in mixture gas used in PECVD and are discussed.

Stability of defects in crystalline silicon and their role in amorphization

Using molecular-dynamics simulation techniques, we have investigated the role that point defects and interstitial-vacancy complexes have on the silicon amorphization process. We have observed that accumulation of interstitial-vacancy complexes in concentrations of 25% and above lead to homogeneous amorphization. However, we have determined the basic properties of the interstitial-vacancy complex, and showed that it is not as stable at room temperature as previously reported by other authors. From our simulations we have identified more stable defect structures, consisting of the combination of the complex and Si self-interstitials. These defects form when there is an excess of interstitials or by incomplete interstitial-vacancy recombination in a highly damaged lattice. Unlike the interstitial-vacancy complex, these defects could survive long enough at room temperature to act as embryos for the formation of extended amorphous zones and/or point defect clusters.

C s–H 2* defect in crystalline silicon

An infrared absorption study has been carried out on hydrogen-doped Si crystals with high carbon content. Hydrogen (deuterium) was introduced into the crystals by high-temperature (1250–1350°C) in-diffusion from H 2 (D 2) gas ambient. After hydrogenation several local vibrational mode lines were observed in the ranges of 660–690, 790–820, 1920–1960, 2120–2230, and 2680–2950 cm −1. Two Si–H (Si–D) lines with frequencies at 792.0 (571.6) and 1921.8 (1401.2) cm −1 were found to have the same dependence on hydrogenation temperature and the same annealing behaviour, therefore they were assigned to a single defect. Measurements on samples co-doped with hydrogen and deuterium revealed the appearance of new lines at 793.2, 1400.3, and 1922.7 cm −1 indicating that the defect of interest incorporates two weakly interacting hydrogen atoms. Uniaxial stress measurements showed that the 792.0 cm −1 line represents an E mode (two-fold degenerate) of a trigonal centre. A correlation was found in annealing behaviour of the lines at 792.0 and 1921.8 cm −1 with that of a line at 2752.3 cm −1. The lines at 792.0 and 1921.8 cm −1 were assigned to Si–H bend and stretch modes, respectively, and the line at 2752.3 cm −1 to a C–H stretch mode of a CH 2* complex with C 3v symmetry. One hydrogen atom of the complex is bound to a C atom and locates on C 3v axis close to bond-centre site, and the other is bound to a Si atom and locates at an antibonding site.

Structure and Dynamics of Point Defects in Crystalline Silicon

Ab-initio theoretical methods are increasingly being used to study the properties of defects in covalent materials such as silicon. Static properties include potential energy surfaces, binding energies, and electronic structures. The dynamics involve diffusion, vibrational properties, and defect reactions. This paper describes a powerful combination of methods used to study the interactions involving vacancies, self-interstitials, and impurities in Si. The methods are Hartree-Fock in molecular clusters and density-functional based molecular-dynamics simulations in periodic supercells. Three examples of applications are discussed: 1. the dissociation of interstitial H 2 molecules by vacancies and self-interstitials. and the formation of H 2 * , 2. the aggregation of vacancies leading to the formation of the ring-hexavacancy (V 6 ), and 3. the trapping of interstitial copper at V 6 and the origin of the electrical activity of copper precipitates.

Monte Carlo Analysis of the Evolution from Point to Extended Interstitial Type Defects in Crystalline Silicon

AbstractWe present a Lattice Kinetic Monte Carlo study of the atomic evolution leading to {311} defects formation upon annealing of a damaged Si-crystal. Self-interstitial (I) agglomeration is modeled by using local interaction and considering the energetic cost to under/over coordinate the Si atoms belonging to an I-complex. The static properties of the I aggregates as derived by molecular dynamics calculations, in the two extreme regimes of very small and very large clusters, have been mapped in the model. The typical evolution of an excess of Si ions is characterized by three distinct stages: (1) the formation of clusters consisting of few interstitials in a over-coordination state, (2) their redistribution in larger agglomerates containing a few of these small I clusters all preserving their original structure, (3) a transition leading to Is rearrangement along the ≪110&gt; chains, which are the structural units of {311} defects. The duration of the preliminary stages critically depends on temperature and density of the added atoms.

Structural modifications in silicon irradiated successively by N + and He + or Ar 8+ and He + ions

The behaviour of He-ion produced radiation defects in crystalline silicon near the interface with an amorphous layer formed by ions of chemically active (N) and inert (Ar) implants was studied. The TEM-investigation of the structural modifications in Si along the ion beam direction was carried out by a novel technique. The boundary of an amorphous layer formed by chemically active nitrogen ions was found to work as a useful getter for radiation defects; a property not found after noble-gas implantation.