https://uvadoc.uva.es/handle/10324/28025 Electrónica 2026-04-20T05:55:22Z https://uvadoc.uva.es/handle/10324/77892 We performed classical molecular dynamics simulations to investigate, from an atomistic point of view, the formation of dislocations during the epitaxial growth of Ge on Si. We show that simulations at 900 and 1000 K with deposition rates of 10 monolayers per second provide a good compromise between computational cost and accuracy. In these conditions, the ratio between the Ge deposition rate and the ad-atom jump rate is analogous to that of out-of-equilibrium experiments. In addition, the main features of the grown film (intermixing, critical film thickness, dislocation typology, and surface morphology) are well described. Our simulations reveal that dislocations originate in low-density amorphous regions that form under valleys of the rough Ge film surface. Atoms are squeezed out of these regions to the surface, releasing the stress accumulated in the film and smoothing its roughness. Amorphous regions grow until atoms begin to rearrange in dislocation half-loops that propagate throughout the Ge film. The threading arm ends of the dislocation half-loops move along the surface following valleys and avoiding islands. The film surface morphology affects the propagation path of the dislocation half-loops and the resulting dislocation network. 2026-01-01T00:00:00Z https://uvadoc.uva.es/handle/10324/77889 A Deep Learning approach is devised to estimate the elastic energy density at the free surface of an undulated stressed film. About 190000 arbitrary surface profiles are randomly generated by Perlin noise and paired with the corresponding elastic energy density profiles , computed by a semi-analytical Green’s function approximation, suitable for small-slope morphologies. The resulting dataset and smaller subsets of it are used for the training of a Fully Convolutional Neural Network. The trained models are shown to return quantitative predictions of , not only in terms of convergence of the loss function during training, but also in validation and testing, with better results in the case of the larger dataset. Extensive tests are performed to assess the generalization capability of the Neural Network model when applied to profiles with localized features or assigned geometries not included in the original dataset. Moreover, its possible exploitation on domain sizes beyond the one used in the training is also analyzed in-depth. The conditions providing a one-to-one reproduction of the “ground-truth” profiles computed by the Green’s approximation are highlighted along with critical cases. The accuracy and robustness of the deep-learned are further demonstrated in the time-integration of surface evolution problems described by simple partial differential equations of evaporation/condensation and surface diffusion. 2025-01-01T00:00:00Z https://uvadoc.uva.es/handle/10324/66550 We studied epitaxial growth of Ge films on Si(001) 2×1 at different temperatures using classical molecular dynamics simulations. Ge-Si intermixing contributes to strain accommodation mostly in the original Si substrate surface and first grown Ge layer. Stress accumulation is further released by the generation of dislocations whose amount and type depend on temperature. At high temperatures, a larger amount and more variety of dislocations are formed, thus affecting the surface morphology and consequently the size of 3D islands. 2023-01-01T00:00:00Z https://uvadoc.uva.es/handle/10324/66548 We used ab initio calculations to characterize PnVm(n=1−6,m=1,2) clusters in crystalline Si by calculating their formation energy, dipole moment and local vibrational modes. This information served us to discuss which PnVm complexes might be more relevant in doping during epitaxial growth or by ion implantation, and their possible behavior under microwave annealing treatments that was recently demonstrated as a promising process in technological nodes beyond 3 nm. 2023-01-01T00:00:00Z https://uvadoc.uva.es/handle/10324/58974 The surface diffusion and intermixing of Ge ad-atoms over Si (001) 2 × 1 substrates using classical molecular dynamics (CMD) simulations are characterized here. Several interatomic potentials, parametrizations, and parameter mixing rules are contemplated. A novel simulation scheme is devised to characterize the effective frequency of surface diffusion and intermixing events overcoming the inherent difficulties related to their interdependency in heteroepitaxial systems. The effective energy barriers of these events encompass different atomistic mechanisms weighted by their occurrence probabilities. The overall description of surface diffusion and intermixing based on Stillinger–Weber (SW) potential is in agreement with ab initio calculations and experimental observations, though some atomistic details differ. This study is extended to Si(001) substrates with stressed Ge monolayers grown on top. It is found that Ge ad-atom dynamics is accelerated with respect to the case of the pure Si substrate and that diffusion across dimer rows is mainly mediated by the atomic exchange of the Ge ad-atom with a Ge atom on the surface. 2023-01-01T00:00:00Z https://uvadoc.uva.es/handle/10324/55603 The improved radiation hardness of p-type Si detectors is hindered by the radiation-induced acceptor removal process, which is not fully understood yet. Through atomistic modeling of displacement damage and dopant interactions, we analyze the acceptor removal under neutron irradiation, providing physical insight into its microscopic origin. Our results show that the fast decay of the effective dopant concentration (Neff) at low irradiation fluences is due to B deactivation caused by Si self-interstitials. The intriguing increase of the acceptor removal parameter with the initial dopant concentration (Neff,0) is explained by the limited number of mobile Si self-interstitials that survive annihilation and clustering processes. The sublinear dependence of the removal parameter on Neff,0 is associated to the inhomogeneity of damage for low Neff,0 and the formation of B-interstitial clusters with several B atoms for high Neff,0. The presence of O and C modifies B deactivation mechanisms due to the key role of BiO defects and the trapping of vacancies and Si self-interstitials, but for the impurity concentrations analyzed in this work ([O] >> [C]) it has little effect on the overall amount of removed acceptors. At high irradiation fluences, the reported increase of Neff is attributed to the formation of defect-related deep acceptors. From the analysis of the defect concentrations resulting from neutron irradiation and the occupancy of small clusters with acceptor levels reported in literature, we point out the tetra-vacancy cluster as one of the main contributors to Neff with negative space charge. 2022-01-01T00:00:00Z https://uvadoc.uva.es/handle/10324/55159 Intra-die device variation due to pattern layout effects associated with the development of ultra-fast annealing processes is one of the major scaling challenges for advanced CMOS devices. In this paper, we show that an excellent and universal correlation can be established between on-die device variation and a new reflectance characterization technique with sufficient resolution. This approach has the potential to be universally applicable to virtually any structure pattern. In addition, we conducted simulations of the thermal annealing effect on 2D doping profiles by considering the effects of temperature sensitivity, reflectivity, and active dopant fraction. Our results show that the observed on-die variation was caused mainly by using a rapid thermal annealing (RTA) process rather than by flash annealing (FLA). We further concluded that pattern-induced device variation is mainly due to the redistribution of the dopants, instead of from dopant activation. To mitigate the pattern loading effect from thermal annealing, we employed a light absorbing layer to eliminate the within-die reflectivity variation. We found that we could successfully reduce electrical on-die variation by 50%. 2022-01-01T00:00:00Z https://uvadoc.uva.es/handle/10324/54389 The relentless scaling of semiconductor devices pushes the doping level far above the equilibrium solubility, yet the doped material must be sufficiently stable for subsequent device fabrication and operation. For example, in epitaxial silicon doped above the solubility of phosphorus, most phosphorus dopants are compensated by vacancies, and some of the phosphorus-vacancy clusters can become mobile around 700 °C to further cluster with isolated phosphorus ions. For efficient and stable doping, we use microwave annealing to selectively activate metastable phosphorus-vacancy clusters by interacting with their dipole moments, while keeping lattice heating below 700 °C. In a 30-nm-thick Si nanosheet doped with 3 × 1021 cm−3 phosphorus, a microwave power of 12 kW at 2.45 GHz for 6 min resulted in a free-electron concentration of 4 × 1020 cm−3 and a junction more abrupt than 4 decades/nm. The doping profile is stable with less than 4% variation upon thermal annealing around 700 °C for 5 min. Thus, microwave annealing can result in not only efficient activation and abrupt profile in epitaxial silicon but also thermal stability. In comparison, conventional rapid thermal annealing can generate a junction as abrupt as microwave annealing but 25% higher sheet resistance and six times higher instability at 700 °C. 2022-01-01T00:00:00Z https://uvadoc.uva.es/handle/10324/51137 Emergent alternative Si processes and devices have promoted applications outside the usual processing temperature window and the failure of traditional defect kinetics models. These models are based on Ostwald ripening mechanisms, assume pre-established defect configurations and neglect entropic contributions. We performed molecular dynamics simulations of self-interstitial clustering in Si with no assumptions on preferential defect configurations. Relevant identified defects were characterized by their formation enthalpy and vibrational entropy calculated from their local vibrational modes. Our calculations show that entropic terms are key to understand defect kinetics at high temperature. We also show that for each cluster size, defect configurations may appear in different crystallographic orientations and transformations among these configurations are often hampered by energy barriers. This induces the presence of non-expected small-size defect cluster configurations that could be associated to optical signals in low temperature processes. At high temperatures, defect dynamics entails mobility and ripening through a coalescence mechanism. 2022-01-01T00:00:00Z https://uvadoc.uva.es/handle/10324/51136 The effective dopant concentration in p-type Si detectors reduces with irradiation fluence at low fluences due to the acceptor removal process, which degrades detector performance and shortens its lifetime. This effect has been experimentally characterized and parametrized, but its microscopic origin is still unknown. We use atomistic simulations to gain insight into acceptor removal in neutron irradiation by modeling damage generation and defect-dopant interactions. We analyze the effect on dopant deactivation of the Si di- and tri-interstitial diffusion, the inhomogeneity of irradiation damage and the wafer temperature rise during irradiation. We characterize defect generation rates and identify the relevant defect-dopant interactions. Acceptor removal occurs mainly through the formation of Bi pairs and small boron-interstitial clusters, and it is limited by the availability of mobile Si interstitials. The presence of impurities (O, C) modifies B-complexes favoring the formation of BiO, but has a limited effect on the amount of removed acceptors. 2022-01-01T00:00:00Z https://uvadoc.uva.es/handle/10324/46388 Due to the intrinsic features of laser annealing treatments in semiconductors, i.e., localized irradiation with a space- and time-dependent thermal field that leads to far-from-equilibrium conditions, experimental analysis can reveal only specimen postirradiation characteristics. In order to understand how the laser-irradiated system evolves and reaches its final state, a theoretical work based on full process simulation is required. Traditional models for laser processing simulation were based on continuum techniques. However, they prove to be insufficient at the nanoscale. Atomistic simulation techniques, in turn, are handy for studying detailed interactions, at both electronic and atomic levels, including out-of-equilibrium situations as those present in laser processing. In this chapter, we review the main techniques for atomistic simulation to be applied for laser modeling. We also present some recent results on atomistic modeling of laser-related phenomena in Si, in particular melting and regrowth processes, defect and dopant kinetics, dopant segregation, and the anomalous formation of extended defects. 2021-01-01T00:00:00Z https://uvadoc.uva.es/handle/10324/45124 Junction stability and donor deactivation in silicon at high doping limit has been a long-standing issue in advanced semiconductor devices. Recently, heavily doped epitaxial Si:P layer with phosphorus concentrations as high as 3 × 1021 at./cm3 has been employed in nanowire field-effect transistor (FET) devices for sub-3 nm technology node as low resistance source-drain and channel stressor. In such highly doped Si:P, the actual dopant activation is much less than nominal phosphorus concentration due to inactive phosphorus atoms arising from dopant-vacancy defects (PnV) clustering phenomenon. Even with state-of-the-art high temperature millisecond annealing, this epitaxial film is thermally unstable upon subsequent thermal treatments. To overcome this limitation, we demonstrate a selective dopant activation scheme which results from the dipole moments of inactive PnV structures within the crystal lattice and their direct energy coupling with the external electric field. It's found that superior stability in dopant activation can be achieved through microwave annealing when a specific temperature and field conditions are met using a triple-parallel-susceptor setup in the microwave cavity. Based on experimental results and ab-initio calculation, we proposed a model, whereas the microwave-PnV interactions result in a specific distribution of dopant defect dominated by thermally stable P4V clusters through elimination of unstable low order PnV, leading to the suppression of donor deactivation and achieving thermally stable junction. 2021-01-01T00:00:00Z https://uvadoc.uva.es/handle/10324/33903 We used classical molecular dynamics simulations to reproduce basic properties of Si, Ge and SiGe using different empirical potentials available in the literature. The empirical potential that offered the better compromise with experimental data was used to study the surface stability of these materials. We considered the (100), (100)2×1 and (111) surfaces, and we found the processing temperature range to avoid the structural degradation of studied surfaces. 2019-01-01T00:00:00Z https://uvadoc.uva.es/handle/10324/33892 Electronic devices operating in harsh radiation environments must withstand high radiation levels with minimal performance degradation. Recent experiments on the radiation hardness of a new vertical p-type JFET power switch have shown a significant reduction of forward drain current under non-ionizing conditions. In this work, atomistic simulations are used to study the impact of irradiation-induced displacement damage on forward characteristics. Damage models have been updated to produce a better description of damage-dopant interactions at RT. Our results show that excess self-interstitials produced by irradiation deactivate a significant amount of B atoms, thus reducing the effective dopant concentration. 2019-01-01T00:00:00Z https://uvadoc.uva.es/handle/10324/33729 By using classical molecular dynamics simulations and a novel technique to identify defects based on the calculation of atomic strain, we have elucidated the detailed mechanisms leading to the anomalous generation and growth of {001} loops found after ultra-fast laser annealing of ion-implanted Si. We show that the building block of the {001} loops is the very stable Arai tetra-interstitial [N. Arai, S. Takeda, M. Kohyama, Phys. Rev. Lett. 78, 4265 (1997)], but their growth is kinetically prevented within conventional Ostwald ripening mechanisms under standard processing conditions. However, our simulations predict that at temperatures close to the Si melting point, Arai tetra-interstitials directly nucleate at the boundaries of fast diffusing self-interstitial agglomerates, which merge by a coalescence mechanism reaching large sizes in the nanosecond timescale. We demonstrate that the crystallization of such agglomerates into {001} loops and their subsequent growth is mediated by the tensile and compressive strain fields that develop concurrently around the loops. We also show that further annealing produces the unfaulting of {001} loops into perfect dislocations. Besides, from the simulations we have fully characterized the {001} loops, determining their atomic structure, interstitial density and formation energy. 2019-01-01T00:00:00Z https://uvadoc.uva.es/handle/10324/32405 We combine focused experiments with molecular dynamics simulations to investigate in detail the formation of {001} loops in nanosecond laser-annealed silicon. We demonstrate that at temperatures close to the melting point, self-interstitial rich silicon is driven into dense liquid-like droplets that are highly mobile within the solid crystalline matrix. These liquid droplets grow by a coalescence mechanism and eventually transform into {0 0 1} loops through a liquid-to-solid phase transition in the nanosecond timescale. 2018-01-01T00:00:00Z