Defects In Crystalline Silicon Research Articles

Athermal annealing of pre-existing defects in crystalline silicon

Systematic investigations of electronic energy loss (Se) effects on pre-existing defects in crystalline silicon (Si) are crucial to provide reliance on the use of ionizing irradiation to anneal pre-existing defects, leading to successful implementation of this technology in the fabrication of Si-based devices. In this regard, the Se effects on nonequilibrium defect evolution in pre-damaged Si single crystals at 300 K has been investigated using intermediate-energy ions (12 MeV O and Si ions) that interact with the pre-damaged surface layers of Si mainly by ionization, except at the end of their range where the nuclear energy loss (Sn) is no longer negligible. Under these irradiation conditions, experimental results and molecular dynamics simulations have revealed that pre-existing disorder in Si can be almost fully annealed by subsequent irradiation with intermediate-energy incident ions with Se values as low as 1.5–3.0 keV/nm. Selective annealing of pre-existing defect levels in Si at room temperature can be considered as an effective strategy to mediate the transient enhanced diffusion of dopants in Si. This approach is more desirable than the regular thermal annealing, which is not compatible with the processing requirements that fall below the typical thermal budget.

Hydrogen-induced defects in crystalline silicon during growth of an ultrathin a-Si:H layer

Hydrogen (H) atom-induced defects in crystalline silicon (c-Si) have been experimentally studied during growth of hydrogenated amorphous silicon (a-Si:H) by H2 diluted silane (SiH4) discharge. The H atom-induced defects are effectively generated at the initial growth of a-Si:H, where H atoms penetrate into c-Si through an ultrathin a-Si:H layer (≲4.0 nm). The generation of such defects is enhanced under the high H-flux condition, i.e. the high H2 dilution. However, the defect generation is strongly suppressed once a relatively-thick a-Si:H layer (≳4.0 nm) is grown over the c-Si surface. Interestingly, Si-related precursors such as SimHn radicals are not efficient to generate such defects. A pure SiH4 discharge is thus beneficial for the suppression of the H atom-induced defects in c-Si during the initial growth of an a-Si:H layer.

Characterization of Mono-Crystalline and Multi-Crystalline Silicon by the Extended Lateral Photovoltage Scanning and Scanning Photoluminescence

Lateral Photovoltage Scanning (LPS) and Scanning Photoluminescence (SPL) methods are used simultaneously to reveal spatial distribution of electrically active defects in crystalline silicon. Defect information can be gained fast by measuring the phase shift between laser modulation and the LPS/SPL signals detected as the real and imaginary parts of the measurement signals. The laser power was varied from 10 μW to 100 mW at laser wavelengths of 660 nm and 830 nm. Simultaneously detected LPS and SPL signals from one and the same excited spot were used to determine minority charge carrier lifetimes. The signal gathering by two different methods shows a huge benefit to investigate the nature of recombination active defects. The minority carrier lifetime determined by SPL depends on the injection level while the minority lifetime analysed by LPS does not show a similar behaviour. Defect-related charge minority carrier lifetime maps of SPL scans show high similarity to those measured by MDP but have a higher local resolution. To investigate the smoothing effect, LPS measurements were made on mc-Si and FZ-Si samples.

Formation of electronic defects in crystalline silicon during hydrogen plasma treatment

Electronic defects in crystalline silicon induced by hydrogen plasma treatments are studied, based on in-situ photocurrent measurements and real-time spectroscopic ellipsometry. The electronic defects are generated by the plasma treatments, and annihilated partially by postannealing. The generation and annihilation of defects strongly depends on both the treatment time and the annealing temperature. A long-time plasma treatment results in the formation of the residual defects in the silicon bulk. The density of these defects is estimated to be of the order of 1013 cm−2. Interestingly, the electronic defects are formed even before a strong modification of the surface structure, i.e., the formation of a nanometer-scale disordered surface layer.

Interaction Between Hydrogen and Vacancy Defects in Crystalline Silicon

Hydrogen is one of the most important impurities in silicon. It is a mobile and highly reactive species that can passivate dangling bonds at dislocations, surfaces, and interfaces, which has been widely used in the microelectronics and solar cell industry for improving device performance. Vacancy defects are elementary complexes containing dangling bonds, and the study of their interaction with hydrogen is of significant importance. In this work, the interactions of hydrogen with the vacancy‐oxygen complex (VO) and the divacancy (V2) are discussed, which are the most dominant and fundamental vacancy defects stable at room temperature. It is shown that VO and V2 can interact with both atomic and diatomic hydrogen species. This complicates the interpretation of experimental data and results in different reaction paths in differently prepared samples. Besides, some of important electronic properties, particularly electronic levels for V2Hn with n > 1, are not experimentally established.

On the Implantation of Protons into Silicon Plates in the Case of a Mechanically Stressed Surface Layer

The effect of mechanical stresses of different signs on the formation of radiation defects in crystalline silicon is studied alongside the effect of strain on the accumulation of radiation defects in KEF-4.5 silicon after the implantation of protons with an energy of 140 and 500 keV and a dose of 2.5 × 1015 cm–2. The stress-strain state of a thin circular plate with a strained surface layer is calculated. It is shown that the applied mechanical stresses generally have an effect on the thickness of the damaged layer, whose thickness decreases with an increase in compressive stresses near the surface in comparison with its thickness in the undamaged sample and grows in the opposite case.

Identification of Extended Defect Atomic Configurations in Silicon Through Transmission Electron Microscopy Image Simulation

We used atomistic simulation tools to correlate experimental transmission electron microscopy images of extended defects in crystalline silicon with their structures at an atomic level. Reliable atomic configurations of extended defects were generated using classical molecular dynamics simulations. Simulated high-resolution transmission electron microscopy (HRTEM) images of obtained defects were compared to experimental images reported in the literature. We validated the developed procedure with the configurations proposed in the literature for {113} and {111} rod-like defects. We also proposed from our procedure configurations for {111} and {001} dislocation loops with simulated HRTEM images in excellent agreement with experimental images.

Review of New Technology for Preparing Crystalline Silicon Solar Cell Materials by Metallurgical Method

The goals of greatly reducing the photovoltaic power cost and making it less than that of thermal power to realize photovoltaic power grid parity without state subsidies are focused on in this paper. The research status, key technologies and development of the new technology for preparing crystalline silicon solar cell materials by metallurgical method at home and abroad are reviewed. The important effects of impurities and defects in crystalline silicon on its properties are analysed. The importance of new technology on reducing production costs and improving its quality to increase the cell conversion efficiency are emphasized. The previous research results show that the raw materials of crystalline silicon are extremely abundant. The product of crystalline silicon can meet the quality requirements of solar cell materials: Si ≥ 6 N, P < 0.1 ppm, B < 0.08 ppm, Fe < 0.1 ppm, resistivity > 1 Ω cm, minority carrier life > 25 μs; cell conversion efficiency of about 19.3%, the product costs < 10 dollars / kg, the product energy consumption < 30 kwh / kg. The existing problems are pointed out. The prospect of the new metallurgical process with low cost, low energy consumption, low carbon and sustainable development are prospected.

On the equilibrium concentration of boron-oxygen defects in crystalline silicon

We determine the equilibrium concentration of the BO defect in boron-doped Czochralski-grown silicon after prolonged (up to 150h) annealing at relatively low temperatures between 200 and 300°C. We show that after sample processing, the BO concentration has not necessarily reached the equilibrium state. The actually reached state depends on the detailed temperature profile of the last temperature treatment before the light-induced degradation (LID) is performed. For the investigated Cz-Si materials with base resistivities ranging between 0.5 and 2.5Ωcm, we observe that an annealing step at 200°C for 50h establishes the equilibrium, independent of the base resistivity. Experiments performed at different temperatures reveal that the equilibrium defect concentration decreases with increasing annealing temperature. This observation can be understood, assuming a mobile species which is distributed between at least two different sinks. A possible defect model is discussed.

X-ray photoelectron spectroscopy analysis of boron defects in silicon crystal: A first-principles study

We carried out a comprehensive study on the B 1s core-level X-ray photoelectron spectroscopy (XPS) binding energies and formation energies for boron defects in crystalline silicon by first-principles calculation with careful evaluation of the local potential boundary condition for the model system using the supercell corresponding to 1000 Si atoms. It is reconfirmed that the cubo-octahedral B12 cluster in silicon crystal is unstable and exists at the saddle point decaying to the icosahedral and S4 B12 clusters. The electrically active clusters without any postannealing of ion-implanted Si are identified as icosahedral B12 clusters. The experimentally proposed threefold coordinated B is also identified as a ⟨001⟩B-Si defect. For an as-doped sample prepared by plasma doping, the calculated XPS spectra for complexes consisting of vacancies and substitutional B atoms are consistent with the experimental spectra. It is proposed that, assuming that the XPS peak at 187.1 eV is due to substitutional B (Bs), the experimental XPS peaks at 187.9 and 186.7 eV correspond to interstitial B at the H-site and ⟨001⟩B-Si defects, respectively. In the annealed samples, the complex of Bs and interstitial Si near the T-site is proposed as a candidate for the experimental XPS peak at 188.3 eV.

Corona discharge initiation for TV control of defects in crystalline silicon

The article presents the results of experimental studies to control of defects of monocrystalline silicon wafers. Monocrystalline silicon is one of the major solar energy materials. Therefore, control of its defects is an important task that leads to saving of production resources and reduce the cost of photovoltaic solar cells put into operation. Defects in the wafer of monocrystalline silicon are observed under the excitation of negative pulsed corona discharge in the air gap over them. This method of corona discharge excitation in the planar electrode system with a dielectric insulating wafer allows adjusting the voltage and current of the discharge. A feature of the proposed method is to produce a corona discharge at a relatively low voltage of 1-1,5 kV. Using a transparent electrode makes it possible to observe defects and measure their brightness and geometrical parameters of the corona via TV information and measuring system. In particular, the authors have observed the luminescence of the defects in the crystalline silicon wafer under the excitation of negative pulsed corona discharge in the air. The results can be used to control the silicon wafers in the manufacture of photovoltaic solar cells.

Diffusion of point defects in crystalline silicon using the kinetic activation-relaxation technique method

We study point-defect diffusion in crystalline silicon using the kinetic activation-relaxation technique (k-ART), an off-lattice kinetic Monte Carlo method with on-the-fly catalog building capabilities based on the activation-relaxation technique (ART nouveau), coupled to the standard Stillinger-Weber potential. We focus more particularly on the evolution of crystalline cells with one to four vacancies and one to four interstitials in order to provide a detailed picture of both the atomistic diffusion mechanisms and overall kinetics. We show formation energies, activation barriers for the ground state of all eight systems, and migration barriers for those systems that diffuse. Additionally, we characterize diffusion paths and special configurations such as dumbbell complex, di-interstitial (IV-pair+2I) superdiffuser, tetrahedral vacancy complex, and more. This study points to an unsuspected dynamical richness even for this apparently simple system that can only be uncovered by exhaustive and systematic approaches such as the kinetic activation-relaxation technique.

A unified approach to modelling the charge state of monatomic hydrogen and other defects in crystalline silicon

There are a number of existing models for estimating the charge states of defects in silicon. In order of increasing complexity, these are (a) the Fermi-Dirac distribution, (b) the Shockley-Last model, (c) the Shockley-Read-Hall model, and (d) the Sah-Shockley model. In this work, we demonstrate their consistency with the general occupancy ratio α, and show that this parameter can be universally applied to predict the charge states of both monovalent and multivalent deep levels, under either thermal equilibrium or steady-state conditions with carrier injection. The capture cross section ratio is shown to play an important role in determining the charge state under non-equilibrium conditions. The application of the general occupancy ratio is compared with the quasi-Fermi levels, which are sometimes used to predict the charge states in the literature, and the conditions where the latter can be a good approximation are identified. The general approach is then applied to the prediction of the temperature- and injection level-dependent charge states for the technologically important case of multivalent monatomic hydrogen, and several other key monovalent deep levels including Fe, Cr, and the boron-oxygen complex in silicon solar cells. For the case of hydrogen, we adapt the model of Herring et al., which describes the charge states of hydrogen in thermal equilibrium, and generalize it for non-equilibrium conditions via the inclusion of the general occupancy ratio, while retaining the pre-factors which make the model more complete. Based on these results, the impact of temperature and injection on the hydrogenation of the key monovalent defects, and other pairing reactions, are discussed, demonstrating that the presented model provides a rigorous methodology for understanding the impact of charge states.

Charge carrier passivating nitrogen–phosphorus defects in crystalline silicon

In this work, the geometric and electronic structure of the neutral and charged nitrogen–phosphorus defects were for the first time rigorously investigated by density functional theory. The ground state structures were located by screening all possible geometric configurations of the defects. It is shown that the modified self-interstitial nitrogen–phosphorus defect passivate the free carrier states of isolated substitutional phosphorus, known to be an excellent dopant in crystalline silicon. Furthermore, the band gap is shown to be similar in magnitude to bulk silicon, but direct. However, this study indicate that the nitrogen–phosphorus defect is possibly less stable than the self-interstitial nitrogen dimer at high nitrogen defect concentrations. Finally, the vibrational spectra were analyzed by means of linear response theory and phonon calculations. The resulting vibrational spectra yield a peak split of the modified self-interstitial nitrogen mode of 4THz compared to the isolated self-interstitial nitrogen defect.

Imaging As-Grown Interstitial Iron Concentration on Boron-Doped Silicon Bricks via Spectral Photoluminescence

The interstitial iron concentration measured directly on the side face of a silicon brick after crystallization and brick squaring can give important early and fast feedback regarding its material quality. Interstitial iron is an important defect in crystalline silicon, particularly in directionally solidified ingots. Spectral photoluminescence intensity ratio imaging has recently been demonstrated to independently provide high-resolution bulk lifetime images and is therefore ideally suited to assess spatially variable multicrystalline silicon bricks. Here, we demonstrate this technique to enable imaging of the interstitial iron concentration on boron-doped silicon bricks and thick silicon slabs. We present iron concentration studies for two directionally solidified silicon bricks of which one is a standard multicrystalline and the other is a seeded-growth ingot. This lifetime-based measurement technique is highly sensitive to interstitial iron with detection limits down to concentrations of about 1 × 10 10 cm -3 . Its accuracy is enhanced, as the injection level remains below 2 × 10 12 cm -3 during the measurement and, hence, avoids the influence of injection level dependences on the conversion factor, although it remains dependent on the knowledge of the electron capture cross section of interstitial iron in silicon. Access to both bulk lifetime and dissolved iron concentration provides a valuable parameter set of as-grown crystal quality and the relative recombination fraction of interstitial iron via Shockley-Read-Hall (SRH) analysis. Simulated interstitial iron concentration profiles support the presented experimental data.

Germanium-doped crystalline silicon: Effects of germanium doping on boron-related defects

Recently it has been recognized that germanium (Ge) doping can be used for microelectronics and photovoltaic devices. This article reviews the recent results about the effects of Ge doping on boron-related defects in crystalline silicon. Behavior of Ge interacting with the acceptor dopants is also discussed therein. In addition, the article provides a comprehensive review on the effect of Ge doping to the formation of iron–boron pairs and boron–oxygen defects that is responsible for the light induced degradation (LID) of the carrier lifetime. The improvement silicon-based solar cells application from Ge doping is discussed as well, including the increment of cell efficiency and the power output of corresponding modules under sunlight illumination.

Influence of hydrogen effusion from hydrogenated silicon nitride layers on the regeneration of boron-oxygen related defects in crystalline silicon

The degradation effect boron doped and oxygen-rich crystalline silicon materials suffer from under illumination can be neutralized in hydrogenated silicon by the application of a regeneration process consisting of a combination of slightly elevated temperature and carrier injection. In this paper, the influence of variations in short high temperature steps on the kinetics of the regeneration process is investigated. It is found that hotter and longer firing steps allowing an effective hydrogenation from a hydrogen-rich silicon nitride passivation layer result in an acceleration of the regeneration process. Additionally, a fast cool down from high temperature to around 550 °C seems to be crucial for a fast regeneration process. It is suggested that high cooling rates suppress hydrogen effusion from the silicon bulk in a temperature range where the hydrogenated passivation layer cannot release hydrogen in considerable amounts. Thus, the hydrogen content of the silicon bulk after the complete high temperature step can be increased resulting in a faster regeneration process. Hence, the data presented here back up the theory that the regeneration process might be a hydrogen passivation of boron-oxygen related defects.

Micromagnetic technology for detection of carbon impurity in crystalline silicon

This paper proposes a method of lossless micromagnetic detection in the geomagnetic field for detecting traces of carbon impurity defects in crystalline silicon. The magnetization tests show that crystalline silicon is a diamagnetic substance with a stronger relative permeability than carbon. Micromagnetic decay theory is gained according to the energy decay. When the geomagnetic field penetrates through the materials, the apparent magnetic susceptibility can be calculated and subsequently used to project the images. The resulting image clearly showed the location of the defects. Test results are proved by the metallographic phase and spectral analysis. New method and ideas are provided for effective detection of trace carbon impurity defects in the crystalline silicon.

Influence of hydrogen on the regeneration of boron-oxygen related defects in crystalline silicon

When exposed to light, boron doped monocrystalline Czochralski grown silicon suffers from degradation of the minority carrier lifetime due to the formation of recombination active boron-oxygen related defects. The so called regeneration procedure is able to convert these recombination active defects into a new less recombination active state characterized by a higher minority charge carrier lifetime and stability under illumination. However, the exact working principle on microscopic scale is still unknown even though some influencing factors were identified. The role of hydrogen in the regeneration process is investigated in this work. We find that the characteristic regeneration time constant is subject to variation depending on the process parameters of a Plasma Enhanced Chemical Vapor Deposition a-SiNx:H deposition, namely the applied gas flows, as well as on the thermal history of the sample prior to applying the regeneration procedure. The positive effect of a short high temperature (800–900 °C) step leads to the idea that the presence of atomic hydrogen in the silicon bulk is crucial for the regeneration effect to occur. The different regeneration behavior of samples with variable thickness of a hydrogen diffusion barrier, namely an Al2O3 layer capped by SiNx:H, supports those results. Finally, the importance of hydrogen for regeneration is directly shown on samples having different hydrogen bulk concentrations due to direct hydrogenation in a Microwave Induced Remote Hydrogen Plasma reactor. A new model to explain the effect of the regeneration of boron-oxygen related defect centers based on the possible role of atomic hydrogen is presented.

Lithiation of silicon via lithium Zintl-defect complexes from first principles

An extensive search for low-energy lithium defects in crystalline silicon using density-functional-theory methods and the ab initio random structure searching (AIRSS) method shows that the four-lithium-atom substitutional point defect is exceptionally stable. This defect consists of four lithium atoms with strong ionic bonds to the four under-coordinated atoms of a silicon vacancy defect, similar to the bonding of metal ions in Zintl phases. This complex is stable over a range of silicon environments, indicating that it may aid amorphization of crystalline silicon and form upon delithiation of the silicon anode of a Li-ion rechargeable battery.