Kinetics of large B clusters in crystalline and preamorphized silicon

Producción CientíficaWe present an extended model for B clustering in crystalline or in preamorphized Si and with validity under conditions below and above the equilibrium solid solubility limit of B in Si. This model includes boron-interstitial clusters (BICs) with BnIm configurations—complexes with n B atoms and m Si interstitials—larger (n > 4), and eventually more stable, than those included in previous models. In crystalline Si, the formation and dissolution pathways into large BICs configurations require high B concentration and depend on the flux of Si interstitials. In the presence of high Si interstitial flux, large BICs with a relatively large number of interstitials (m ≥ n) are formed, dissolving under relatively low thermal budgets. On the contrary, for low Si interstitial flux large BICs with few interstitials (m ≪ n) can form, which are more stable than small BICs, and whose complete dissolution requires very intense thermal budgets. We have also investigated the kinetics of large BICs in preamorphized Si, both experimentally and theoretically. B was implanted at a high-dose into preamorphized Si, and the B precipitation was studied by transmission electron microscopy and by sheet resistance and Hall measurement techniques. A simplified model for B clustering and redistribution in amorphous Si is proposed, including the experimental value for the B diffusivity in amorphous Si and the energetics of BICs. Our model suggests that B2, B3I, B4I and B4I2 clusters are the most energetically favored configurations, with relative abundance depending on B concentration. After recrystallization, thermal anneals up to 1100 °C evidence that BICs evolve under very low flux of Si interstitials under the particular experimental conditions considered. Simulations indicate that for very high B concentrations and low Si interstitial flux a significant fraction of the initial small BICs evolves into larger and very stable BIC configurations that survive even after intense thermal budgets, as confirmed by energy filtered transmission electron microscopy analyses. The correlation between simulations and Hall measurements on these samples suggest that hole mobility is significantly degraded by the presence of a high concentration of BICs.Ministerio de Economía, Industria y Competitividad (Project TEC2008-06069)Junta de Castilla y León (programa de apoyo a proyectos de investigación - Ref. VA011A09

Insights on the atomistic origin of X and W photoluminescence lines in c-Si from ab initio simulations

Producción CientíficaWe have used atomistic simulations to identify and characterize interstitial defect cluster configurations candidate for W and X photoluminescence centers in crystalline Si. The configurational landscape of small self-interstitial defect clusters has been explored through nanosecond annealing and implantation recoil simulations using classical molecular dynamics. Among the large collection of defect configurations obtained, we have selected those defects with the trigonal symmetry of the W center, and the tetrahedral and tetragonal symmetry of the X center. These defect configurations have been characterized using ab initio simulations in terms of their donor levels, their local vibrational modes, the defect induced modifications of the electronic band structure, and the transition amplitudes at band edges. We have found that the so-called I3-V is the most likely candidate for the W PL center. It has a donor level and local vibrational modes in better agreement with experiments, a lower formation energy, and stronger transition amplitudes than the so-called I3-I, which was previously proposed as the W center. With respect to defect candidates for the X PL center, our calculations have shown that none of the analyzed defect candidates match all of the experimental characteristics of the X center. Although the Arai tetra-interstitial configuration previously proposed as the X center cannot be excluded, the other defect candidates for the X center found, I3-C and I3-X, cannot be discarded either.Ministerio de Economía, Industria y Competitividad (project TEC2011-27701 and TEC2014-60694-P)Junta de Castilla y León (programa de apoyo a proyectos de investigación – Ref. VA331U14)Red Española de Supercomputación (project QCM-2014-3-0034

Improved physical models for advanced silicon device processing

Producción CientíficaWe review atomistic modeling approaches for issues related to ion implantation and annealing in advanced device processing. We describe how models have been upgraded to capture physical mechanisms in more detail as a response to the accuracy demanded in modern process and device modeling. Implantation and damage models based on the binary collision approximation have been improved to describe the direct formation of amorphous pockets for heavy or molecular ions. The use of amorphizing implants followed by solid phase epitaxial regrowth has motivated the development of detailed models that account for amorphization and recrystallization, considering the influence of crystal orientation and stress conditions. We apply simulations to describe the role of implant parameters to minimize residual damage, and we address doping issues that arise in non-planar structures such as FinFETs.Ministerio de Ciencia e Innovación - FEDER (Proyect TEC2014-60694-P)Junta de Castilla y León (programa de apoyo a proyectos de investigación – Ref. VA331U14

Molecular dynamics simulations of damage production by thermal spikes in Ge

Producción CientíficaMolecular dynamics simulation techniques are used to analyze damage production in Ge by the thermal spike process and to compare the results to those obtained for Si. As simulation results are sensitive to the choice of the inter-atomic potential, several potentials are compared in terms of material properties relevant for damage generation, and the most suitable potentials for this kind of analysis are identified. A simplified simulation scheme is used to characterize, in a controlled way, the damage generation through the local melting of regions in which energy is deposited. Our results show the outstanding role of thermal spikes in Ge, since the lower melting temperature and thermal conductivity of Ge make this process much more efficient in terms of damage generation than in Si. The study is extended to the modeling of full implant cascades, in which both collision events and thermal spikes coexist. Our simulations reveal the existence of bigger damaged or amorphous regions in Ge than in Si, which may be formed by the melting and successive quenching induced by thermal spikes. In the particular case of heavy ion implantation, defect structures in Ge are not only bigger, but they also present a larger net content in vacancies than in Si, which may act as precursors for the growth of voids and the subsequent formation of honeycomb-like structures.Ministerio de Economía, Industria y Competitividad (Project TEC2008-06069)Junta de Castilla y León (programa de apoyo a proyectos de investigación - Ref. VA011A09

W and X Photoluminescence Centers in Crystalline Si: Chasing Candidates at Atomic Level Through Multiscale Simulations

Producción CientíficaSeveral atomistic techniques have been combined to identify the structure of defects responsible for X and W photoluminescence lines in crystalline Si. We used kinetic Monte Carlo simulations to reproduce irradiation and annealing conditions used in photoluminescence experiments. We found that W and X radiative centers are related to small Si self-interstitial clusters but coexist with larger Si self-interstitials clusters that can act as nonradiative centers. We used molecular dynamics simulations to explore the many different configurations of small Si self-interstitial clusters, and selected those having symmetry compatible with W and X photoluminescence centers. Using ab initio simulations, we calculated their formation energy, donor levels, and energy of local vibrational modes. On the basis of photoluminescence experiments and our multiscale theoretical calculations, we discuss the possible atomic configurations responsible for W and X photoluminescence centers in Si. Our simulations also reveal that the intensity of photoluminescence lines is the result of competition between radiative centers and nonradiative competitors, which can explain the experimental quenching of W and X lines even in the presence of the photoluminescence centers.Ministerio de Ciencia e Innovación (Proyect TEC2014-60694-P)Junta de Castilla y León (programa de apoyo a proyectos de investigación - Ref. VA331U14)Spanish Supercomputing Network (RES) Project No. QCM-2014-3-003

Molecular dynamics simulation of the early stages of self-interstitial clustering in silicon

Producción CientíficaWe have studied the early stages of self-interstitial clustering in silicon using molecular dynamics simulation techniques. We have generated silicon samples of over 200,000 atoms where we introduced a 0.5% extra concentration of self-interstitials. Then samples were annealed at several temperatures. During the simulations we observed the formation of interstitial clusters with different atomic structures, ranging from spherical and amorphous-like clusters, to highly ordered extended configurations such as (110) chains, {111} rod-like defects and dislocation loops, and {100} planar defects. This last type of defects, while common in germanium, have not been observed in silicon until very recently, in ultra-fast laser annealing experiments. The particular morphology of formed interstitial clusters is found to be related to the annealing temperature, as it is observed in the experiments. From the molecular dynamics simulations we have analyzed the atomic mechanisms leading to the formation and growth of interstitial clusters, with special attention to the newly found {100} planar defectsMinisterio de Ciencia e Innovación (Proyect TEC2011-27701)Junta de Castilla y León (programa de apoyo a proyectos de investigación - Ref. VA331U14

Atomistic modeling of ion implantation technologies in silicon

Producción CientíficaRequirements for the manufacturing of electronic devices at the nanometric scale are becoming more and more demanding on each new technology node, driving the need for the fabrication of ultra-shallow junctions and finFET structures. Main implantation strategies, cluster and cold implants, are aimed to reduce the amount of end-of-range defects through substrate amorphization. During finFET doping the device body gets amorphized, and its regrowth is more problematic than in the case of conventional planar devices. Consequently, there is a renewed interest on the modeling of amorphization and recrystallization in the front-end processing of Si. We present multi-scale simulation schemes to model amorphization and recrystallization in Si from an atomistic perspective. Models are able to correctly predict damage formation, accumulation and regrowth, both in the ballistic and thermal-spike regimes, in very good agreement with conventional molecular dynamics techniques but at a much lower computational cost.Ministerio de Ciencia e Innovación (Proyect EC2011-27701

A detailed approach for the classification and statistical analysis of irradiation induced defects

Producción CientíficaNew criteria are presented for the classification and statistical analysis of defects from irradiation cascades that allow a more detailed description of the diversity of damage, especially amorphous regions. Classical molecular dynamics simulations are used to analyze the damage produced by 2 keV Si recoils annealed at 1000 K for 1 ns. Based on a density grouping criterion of elementary defects (displaced atoms and empty lattice sites) the non-uniformity of local defect density within damage regions is revealed. The density criterion is able to distinguish dense damage regions which evolve independently upon annealing (although they are connected by some defects), while keeping small and compact regions unaltered. Damage regions are classified according to the size, net number of defects and compactness, calculated by averaging the distance among all defects, parameters that have a direct impact on their stability.Ministerio de Ciencia e Innovación (Proyect TEC2011-27701

Generation of amorphous Si structurally compatible with experimental samples through the quenching process: A systematic molecular dynamics simulation study

Producción CientíficaThe construction of realistic atomistic models for amorphous solids is complicated by the fact that they do not have a unique structure. Among the different computational procedures available for this purpose, the melting and rapid quenching process using molecular dynamics simulations is commonly employed as it is simple and physically based. Nevertheless, the cooling rate used during quenching strongly affects the reliability of generated samples, as fast cooling rates result in unrealistic atomistic models. In this study, we have determined the conditions to be fulfilled when simulating the quenching process with molecular dynamics for obtaining amorphous Si (a-Si) atomistic models structurally compatible with experimental samples. We have analyzed the structure of samples generated with cooling rates ranging from 3.3 1010 to 8.5 1014 K/s. The obtained results were compared with experimental data available in the literature, and with samples generated by other state-of-the-art and more sophisticated computational procedures. For cooling rates below 1011 K/s, a-Si samples generated had structural parameters within the range of experimental samples, and comparable to those obtained from other refined modeling procedures. These computationally slow cooling rates are of the same order of magnitude than those experimentally achieved with pulsed energy melting techniques. Samples obtained with faster cooling rates can be further relaxed with annealing simulations, resulting in structural parameters within the range of experimental samples. Nevertheless, the required annealing times are on the order of microseconds, which makes this annealing step non practical from a computational point of view.Ministerio de Ciencia e Innovación (Proyect TEC2014-60694-P and TEC2017-86150-P)Junta de Castilla y León (programa de apoyo a proyectos de investigación - Ref. VA097P17 and VA119G18

Temperature effect on damage generation mechanisms during ion implantation in Si. A classical molecular dynamics study

Producción CientíficaWe have studied the temperature effect on the damage generation mechanisms in silicon, suppressing the influence of dynamic annealing. We have done dedicated classical molecular dynamics simulations to determine how the ballistic mechanism and the thermal spikes are affected by temperature. We have quantified the minimum energy required to permanently displace an atom from its lattice position by a ballistic collision. We have found that the displacement energy threshold does not change appreciably with temperature. However, when subthreshold energy is simultaneously deposited in several neighboring particles in a finite volume, i.e. when thermal spikes occur, there is an enhancement of the generation of damage with increasing temperature. In high energy recoils both mechanisms are combined, and it results in an increase of the generated damage with temperature.Ministerio de Ciencia e Innovación (Proyect TEC2011-27701