Complexes In Crystalline Silicon Research Articles

Ring Defects Associated with Boron–Oxygen‐Related Degradation in p‐Type Silicon Heterojunction Solar Cells

Silicon heterojunction (SHJ) cell architectures, which have dominated silicon single‐junction efficiency records for the past 10 years, are processed at relatively low temperatures, on the order of ≈250 °C. Recombination‐active oxygen complexes in crystalline silicon, formed from interstitial oxygen (Oi), typically require temperatures higher than this to form. Therefore, it is typically assumed that SHJ cells are immune to such defects. This contrasts with the high‐temperature passivated emitter and rear cell (PERC) and tunneling oxide passivating contact (TOPCon) architectures, which can suffer from oxygen precipitates that are recombination active and difficult to predict. Herein, ring‐like defects are observed in boron‐doped p‐type SHJ solar cells, which leads to a degradation of open‐circuit voltage. It is shown that the spatial variation of this recombination activity is related to the boron–oxygen defect, the variation of which is likely due to the radial Oi distribution. Although boron‐doped p‐type wafers are no longer the industry standard, the defect engineering of wafers for SHJ production, using high‐temperature processing, is gaining significant interest. Such wafers can have an increased susceptibility to ring‐like defects. Therefore, spatially inhomogeneous defects causing recombination may become increasingly relevant for SHJ cells.

Electronic Properties and Structure of Boron–Hydrogen Complexes in Crystalline Silicon

The subject of hydrogen–boron interactions in crystalline silicon is revisited with reference to light and elevated temperature‐induced degradation (LeTID) in boron‐doped solar silicon. Ab initio modeling of structure, binding energy, and electronic properties of complexes incorporating a substitutional boron and one or two hydrogen atoms is performed. From the calculations, it is confirmed that a BH pair is electrically inert. It is found that boron can bind two H atoms. The resulting BH2 complex is a donor with a transition level estimated at E c–0.24 eV. Experimentally, the electrically active defects in n‐type Czochralski‐grown Si crystals co‐doped with phosphorus and boron, into which hydrogen is introduced by different methods, are investigated using junction capacitance techniques. In the deep‐level transient spectroscopy (DLTS) spectra of hydrogenated Si:P + B crystals subjected to heat‐treatments at 100 °C under reverse bias, an electron emission signal with an activation energy of ≈0.175 eV is detected. The trap is a donor with electronic properties close to those predicted for boron–dihydrogen. The donor character of BH2 suggests that it can be a very efficient recombination center of minority carriers in B‐doped p‐type Si crystals. A sequence of boron–hydrogen reactions, which can be related to the LeTID effect in Si:B is proposed.

Kinetics of the permanent deactivation of the boron-oxygen complex in crystalline silicon as a function of illumination intensity

Based on contactless carrier lifetime measurements performed on p-type boron-doped Czochralski-grown silicon (Cz-Si) wafers, we examine the rate constant Rde of the permanent deactivation process of the boron-oxygen-related defect center as a function of the illumination intensity I at 170°C. While at low illumination intensities, a linear increase of Rde on I is measured, at high illumination intensities, Rde seems to saturate. We are able to explain the saturation by assuming that Rde increases proportionally with the excess carrier concentration Δn and take the fact into account that at sufficiently high illumination intensities, the carrier lifetime decreases with increasing Δn and hence the slope of Δn(I) decreases, leading to an apparent saturation. Importantly, on low-lifetime Cz-Si samples no saturation of the deactivation rate constant is observed for the same illumination intensities, proving that the deactivation is stimulated by the presence of excess electrons and not directly by the photons.

Supercell-size convergence of formation energies and gap levels of vacancy complexes in crystalline silicon in density functional theory calculations

Results for supercell-size convergence of formation energies and charge transition levels of vacancy complexes ${V}_{n}\phantom{\rule{0.28em}{0ex}}(1\ensuremath{\le}n\ensuremath{\le}11)$ in crystalline Si are reported for the ab initio density functional theory (DFT) with generalized gradient approximation (GGA) pseudopotentials. When extrapolated to the dilute limit, the formation energy of an uncharged vacancy becomes $3.74\ifmmode\pm\else\textpm\fi{}0.1\phantom{\rule{0.16em}{0ex}}\mathrm{eV}$, and the binding energy of an uncharged divacancy becomes $1.9\ifmmode\pm\else\textpm\fi{}0.2\phantom{\rule{0.16em}{0ex}}\mathrm{eV}$. Stable ${V}_{n}$ clusters are built on the basis of sixfold rings $(n\ensuremath{\le}6)$ and of octahedral voids $(n\ensuremath{\ge}7)$. If the well-known underestimate of the band gap by the DFT and the accuracy of extrapolations are taken into account, the extrapolated levels are in good agreement with experiment. We discuss the implications for simulation of vacancy clustering during thermal quenching, for interpretation of deep-level spectroscopy and electron paramagnetic resonance in irradiated Si, and for cell and Brillouin-zone sampling choice when DFT-related methods beyond local-density approximation or GGA are used.

Ab-initio calculation study on the formation mechanism of boron-oxygen complexes in c-Si

Boron-oxygen (B-O) complex in crystalline silicon (c-Si) solar cells is responsible for the light-induced efficiency degradation of solar cell. However, the formation mechanism of B-O complex is not clear yet. By Ab-initio calculation, it is found that the stagger-type oxygen dimer (O2ist) should be the component of B-O complex, whose movement occurs through its structure reconfiguration at low temperature, instead of its long-distance diffusion. The O2ist can form two stable “latent centers” with the Bs, which are recombination-inactive. The latent centers can be evolved into the metastable recombination centers via their structure transformation in the presence of excess carriers. These results can well explain the formation behaviors of B-O complexes in c-Si.

Energetics of hydrogen/lithium complexes in silicon analyzed using the Maxwell construction

We have studied hydrogen/lithium complexes in crystalline silicon using density-functional-theory methods and the ab initio random structure searching (AIRSS) method for predicting structures. A method based on the Maxwell construction and convex hull diagrams is introduced, which gives a graphical representation of the relative stabilities of point defects in a crystal and enables visualization of the changes in stability when the chemical potentials are altered. We have used this approach to study lithium and hydrogen impurities in silicon, which models aspects of the anode material in the recently suggested lithium-ion batteries. We show that hydrogen may play a role in these anodes, finding that hydrogen atoms bind to three-atom lithium clusters in silicon, forming stable ${$H,3Li$}$ and ${$2H,3Li$}$ complexes, while the ${$H,2Li$}$ complex is almost stable.

Hydrogen/nitrogen/oxygen defect complexes in silicon from computational searches

Point defect complexes in crystalline silicon composed of hydrogen, nitrogen, and oxygen atoms are studied within density-functional theory (DFT). Ab initio Random Structure Searching (AIRSS) is used to find low-energy defect structures. We find new lowest-energy structures for several defects: the triple-oxygen defect, {3O}, triple oxygen with a nitrogen atom, {N, 3O}, triple nitrogen with an oxygen atom, {3N,O}, double hydrogen and an oxygen atom, {2H,O}, double hydrogen and oxygen atoms, {2H,2O} and four hydrogen/nitrogen/oxygen complexes, {H,N,O}, {2H,N,O}, {H,2N,O} and {H,N,2O}. We find that some defects form analogous structures when an oxygen atom is replaced by a NH group, for example, {H,N,2O} and {3O}, and {H,N} and {O}. We compare defect formation energies obtained using different oxygen chemical potentials and investigate the relative abundances of the defects.

First-principles studies of small arsenic interstitial complexes in crystalline silicon

We present a first-principles study of the structure and dynamics of small As-interstitial complexes (${\text{AsI}}_{2}$, ${\text{As}}_{2}{\text{I}}_{2}$, ${\text{AsI}}_{3}$, and ${\text{As}}_{2}{\text{I}}_{3}$) in crystalline Si. These complexes can be important components of stable As-interstitial clusters or play a key role in interstitial-mediated formation of As-vacancy clusters. Neutral ${\text{AsI}}_{2}$ and ${\text{As}}_{2}{\text{I}}_{2}$ are identified as fast-diffusing species that contribute to As transient enhanced diffusion. We demonstrate that the extended defect configuration ${\text{As}}_{2}{\text{I}}_{3}^{\text{ext}}$ is a stable configuration with a binding energy of 2.64 eV. ${\text{As}}_{2}{\text{I}}_{3}^{\text{ext}}$ can serve as a nucleation site and facilitate the formation of larger As-interstitial clusters in presence of excess Si interstitials and high As concentration. We also discuss the implications of our findings on As transient enhanced diffusion and clustering and highlight the role of small As-interstitial complexes in ultrashallow junction formation.

Investigations on the long time behavior of the metastable boron–oxygen complex in crystalline silicon

AbstractBoron and oxygen contamination in Czochralski‐grown (Cz) silicon leads to a degradation of the minority charge carrier lifetime within short times due to the formation of recombination active complexes. The formation of these complexes is investigated for longer times showing a further development of the defect. This development called ‘regeneration’ is triggered by illumination or applied forward voltages and leads to a new state of the defect. This new state of the defect is proven to be less recombination active allowing higher stable minority carrier lifetimes and conversion efficiencies of solar cells. The influences of temperature and light intensity are discussed. Copyright © 2007 John Wiley & Sons, Ltd.

Kinetics of the electronically stimulated formation of a boron-oxygen complex in crystalline silicon

We present experimental data relating to the slow stage of the illumination-induced or electron-injection-induced generation, in crystalline $p$-type silicon, of the carrier-recombination center believed to be the defect complex ${({\mathrm{B}}_{s}{\mathrm{O}}_{2i})}^{+}$ formed by diffusion of oxygen interstitial dimers $\mathrm{O}_{2i}{}^{++}$ to substitutional boron atoms $\mathrm{B}_{s}{}^{\ensuremath{-}}$ and, taking account of those data, we consider a detailed theoretical model for the kinetics of the diffusion reaction. The model proposes that the generation rate of the ${({\mathrm{B}}_{s}{\mathrm{O}}_{2i})}^{+}$ defects is controlled by the capture of a majority-carrier hole by the dimer following the capture of a minority-carrier electron and by the Coulomb attraction of the $\mathrm{O}_{2i}{}^{++}$ to the $\mathrm{B}_{s}{}^{\ensuremath{-}}$ atom, and leads to predictions for the defect generation rate that are in excellent quantitative agreement with experiment.

Structure and dynamics of the diarsenic complex in crystalline silicon

Based on first-principles density functional theory (DFT) calculations within the generalized gradient approximation (GGA), we propose a structural model for a diarsenic $({\mathrm{As}}_{s}\ensuremath{-}{\mathrm{As}}_{i})$ complex in Si and a mechanism for its reorientation, diffusion and dissociation. We find that the lowest-energy structure of ${\mathrm{As}}_{s}\ensuremath{-}{\mathrm{As}}_{i}$ consists of two As atoms bonded together in a single lattice site with a bond axis which is tilted slightly from the [110] direction. Our study demonstrates the ${\mathrm{As}}_{s}\ensuremath{-}{\mathrm{As}}_{i}$ pair may undergo reorientations within a lattice site and diffusion, with energy barriers of 1.05 and $1.33\phantom{\rule{0.3em}{0ex}}\mathrm{eV}$, respectively. Dissociation of an ${\mathrm{As}}_{s}\ensuremath{-}{\mathrm{As}}_{i}$ pair is likely to occur predominantly by the liberation of an $\mathrm{As}\ensuremath{-}{\mathrm{Si}}_{i}$ pair to leave behind a substititional As atom. The dissociation barrier is predicted to be $1.33\phantom{\rule{0.3em}{0ex}}\mathrm{eV}$, and the ${\mathrm{As}}_{s}\ensuremath{-}{\mathrm{As}}_{i}$ binding energy is calculated to be $1.00\phantom{\rule{0.3em}{0ex}}\mathrm{eV}$ relative to dissociation products ${\mathrm{As}}^{0}$ and $\mathrm{As}\mathrm{Si}_{i}{}^{0}$. We also discuss the role of ${\mathrm{As}}_{s}\ensuremath{-}{\mathrm{As}}_{i}$ pairs in As transient enhanced diffusion and clustering.

Interaction between interstitials and arsenic-vacancy complexes in crystalline silicon

Using density functional theory calculations, we have examined the interaction of interstitials with various arsenic-vacancy complexes and compared these events to interstitial-vacancy (I-V) interactions. We find that the vacancy of AsmV(m=1–4) and AsmV2(m=2,3) complexes is easily annihilated by I-V recombination, with barriers of 0–0.08 eV and 0.16–0.21 eV, respectively, for the mono- and the di-vacancy associated complexes. The energy gain from the I-V recombination turns out to be significant (>1.3eV), implying that As would remain more favorably as Asn (or AsmIn) complexes, rather than as AsmVn in the presence of a large amount of excess interstitials. This suggests that interstitials may play an important role in As transient enhanced diffusion and agglomeration, especially at the early stage of postimplantation thermal annealing. We present the reaction paths and bonding mechanisms for the vacancy annihilation of Vn and AsmVn clusters by I-V recombination.

Recombination-enhanced formation of the metastable boron–oxygen complex in crystalline silicon

The formation process of the boron- and oxygen-related defect complex in crystalline silicon, responsible for the performance degradation of solar cells made on boron-doped Czochralski silicon (Cz–Si), is investigated on Cz–Si solar cells as a function of the applied voltage in the dark at temperatures ranging from 298 to 373 K. We show that the defect formation is not only a consequence of illumination or the application of a forward bias voltage but also occurs under equilibrium conditions at elevated temperatures in the dark. It can be partly suppressed by applying a reverse voltage. Our findings provide clear experimental evidence that a recombination-enhanced mechanism correlated with the total recombination rate is the driving force of the formation of the metastable defect.

Self-interstitial trapping by carbon complexes in crystalline silicon

By combining model-potential molecular-dynamics simulations and ab initio calculations we investigate the microscopic mechanism of silicon trapping by carbon substitutional defects ${(C}_{S}).$ We find that, upon silicon trapping, carbon is converted into an interstitial mobile complex ${(C}_{I})$ by an efficient exothermic reaction. Interstitial carbon ${C}_{I}$ may further interact either with another ${C}_{S},$ forming the well-known ${C}_{I}{C}_{S}$ dicarbon complex, or with extra silicon and carbon interstitials. In particular, we identify and characterize two structures, namely, ${C}_{I}I$ and ${C}_{I}{C}_{I}.$ They are found energetically stable so that they could play a crucial role in the process of carbon aggregation. According to our calculations ${C}_{I}{C}_{I}$ may be formed by the interaction of one I with a ${C}_{I}{C}_{S},$ proving that the latter is not a deactivated trap for interstitials. Our results further suggest that ${C}_{I}I$ and ${C}_{I}{C}_{I}$ are seeds for further carbon aggregation.

Simulation of the kinetics of oxygen complexes in crystalline silicon

The formation kinetics of thermal double donors (TDD's) is studied by a general kinetic model with parameters based on accurate ab initio total-energy calculations. The kinetic model includes all relevant association, dissociation, and restructuring processes. The simulated kinetics agrees qualitatively and in most cases quantitatively with the experimentally found consecutive kinetics of TDD's. It also supports our earlier assignments of the ring-type oxygen chains to TDD's [Pesola et al., Phys. Rev. Lett. $84,$ 5343 (2000)]. We demonstrate with the kinetic model that the most common assumption that only the ${\mathrm{O}}_{2}$ dimer acts as a fast diffusing species would lead to an unrealistic steady increase of the concentration of ${\mathrm{O}}_{3}.$ The neglect of restructuring processes leads to an anomalous increase of oxygen dimers and negligible concentrations of TDD's. The capture of interstitial oxygens by diffusing oxygen chains and the escaping of interstitial oxygens from the chains fully dominate the formation kinetics.

Equilibrium structure of erbium-oxygen complexes in crystalline silicon

The equilibrium structure of ${\mathrm{ErO}}_{n}$ $(nl~6)$ complexes in crystalline silicon has been investigated by density-functional computations. Two different geometries have been considered, corresponding to the substitutional and tetrahedral interstitial site for erbium. All atomic coordinates have been optimized by Car-Parrinello molecular dynamics. The resulting structures have low symmetry, with E-O distances of \ensuremath{\sim}2.35 \AA{}. The substitutional site is the most stable one for $nl~2,$ while the tetrahedral interstitial is favored for $ng2.$

First-principles study of electric-field gradients at the Cd site for neutral hydrogen-cadmium complexes in crystalline silicon

In the present work, a first-principles investigation of electric-field gradients (EFG's) at the Cd site for possible neutral complexes of Cd-H in crystalline silicon has been performed. This was motivated by experimental results observed by perturbed angular correlation spectroscopy, and with the aim of trying to establish the validity of the configurational models proposed in the literature. We use the full-potential linear-muffin-tin-orbital method, within the local-density functional theory, and the supercell approach. We have allowed local relaxations of the Cd and Si nearest and next-nearest neighbors for different positions of H in the high and low electron-density regions. We have found that H in an intrabond site is unstable (saddle point), and that a bridgelike configuration is the energetically favorable one, where the Cd atom and the nearest silicon suffer relaxations along the [111] direction. The antibond positions of low electronic density regions were also studied, and have been found to be also local minima. The calculated EFG's in these configurations that locally minimize the total energy of the supercell (H at bridgelike and H in antibond sites to Cd and ${\mathrm{Si}}_{1}$) give quadrupolar coupling constants which are in agreement with the experimental results.

Donor-hydrogen complexes in crystalline silicon

Experimental results are presented on the study of Sb-H complexes in crystalline silicon, employing 119Sb→119Sn source Mossbauer spectroscopy and a low-energy H implantation technique. In addition to a visible component, we observe a large decrease of the Mossbauer intensity associated with the trapping of hydrogen, even at low temperatures. This is interpreted as the formation of a component with a negligible recoilless fraction. The different Mossbauer components were studied as a function of H dose, H-implantation temperature and annealing temperature. The data show that the visible component is associated with the well-known SbH complex, whereas the invisible component is associated with the formation of SbHn (n≥2) complexes. We show that these complexes are in thermal equilibrium with a larger hydrogen reservoir (H2*), which governs their thermal stability. No Sb-H complexes are observed in p-type Si after H-implantation, in agreement with the current belief that hydrogen has a deep donor level in the gap. The microscopic structure of the various Sb-H and Sn-H complexes was studied with first-principles calculations using the pseudopotentialdensity-functional approach. The structure of the Sb-H complex is found to be similar to the P-H complex, with the H in an antibonding site of a Si atom neighbouring the Sb impurity. For SbH2 three configurations are found with energies differing by less than ≈ 0.1 eV. We find that the reaction SbH+H≠SbH2 is exothermic. We argue that the SbH2 complexes are shallow donors, irrespective of the structure. Therefore, the formation of SbH2 may depassivate the sample.

Evidence for the formation of tellurium-hydrogen complexes in crystalline silicon.

Evidence for the formation of tellurium-hydrogen complexes in crystalline silicon.

Semiempirical electronic-structure calculations of hydrogen-phosphorus-vacancy complexes in crystalline silicon

The author reports on self-consistent calculations for the electronic structure of three hydrogen-phosphorus-vacancy complexes in crystalline silicon, based on a semiempirical tight-binding theory. These complexes, denoted by V-1H-3P, V-2H-2P, and V-3H-1P, consist of a vacancy and substitutional phosphorus impurities on the nearest-neighbour sites of the vacancy. The remaining silicon dangling bonds in the complexes are all saturated by hydrogen atoms. Each of the complexes is described by a large repeated supercell and the authors use the recursion method for computing local densities of states and orbital electron occupancies. They find that the three complexes do not introduce energy levels into the fundamental band gap, revealing that the electrical activity of the silicon dangling bonds can be well passivated by hydrogen atoms at bonding positions through strong orbital interactions and by phosphorus atoms at substitutional positions through Coulomb attractions. However, the complexes can have defect resonant states outside of the band gap. The authors show that these resonant states can be well described in terms of the symmetric combinations of the phosphorus orbitals and the symmetric combinations of the hydrogen and the hydrogen-saturated silicon orbitals.