Iván Santos

Improved atomistic damage generation model for binary collision simulations

We have carried out a classical molecular dynamics study to quantify the conditions under which damage is generated by ion implantation in silicon at energies below the displacement threshold. The obtained results have been used to construct a general framework for damage generation which captures the transition from ballistic (high above the displacement threshold) to thermal (around and below the displacement threshold) regime. The model, implemented in a binary collision code, has been successfully used to simulate monatomic and especially molecular implantations, where nonlinear effects occur. It reproduces the amount and morphology of generated damage at atomic level in good agreement with classical molecular dynamics simulations but with a computational gain factor of ∼103 to ∼104. The incorporation of this damage model to process simulators will improve the prediction of amorphization conditions and provide a convenient tool for simulating molecular implants not available to date. Although this wor...

Atomistic analysis of the annealing behavior of amorphous regions in silicon

We have analyzed the features of recrystallization of amorphous regions, using an atomistic amorphization-recrystallization model that considers the Si interstitial-vacancy pair as the building block for the amorphous phase. Both small amorphous pockets and large continuous amorphous layers are modeled as an accumulation of Si interstitial-vacancy pairs. In our model recrystallization is envisioned as a local rearrangement of atoms, the recrystallization rate of Si interstitial-vacancy pairs being determined by their local coordination. This feature explains the differences in the annealing behavior of amorphous regions with different topologies, the faster regrowth velocity of the damage tail compared with the continuous amorphous layer, and the independence of the regrowth velocity on the amorphous layer depth.

Atomistic process modeling based on Kinetic Monte Carlo and Molecular Dynamics for optimization of advanced devices

Combined Molecular Dynamics and Kinetic Monte Carlo simulations are used in hierarchical models to gain physical understanding for process optimization in advanced devices. Thermal budget for the removal of defects in advanced millisecond anneals is evaluated. Alternatives to overcome the imperfect regrowth of narrow Si structures are proposed. The compromise between implant and anneal parameters for doping of FinFETs are presented, considering lateral diffusion and activation.

Atomistic modeling of impurity ion implantation in ultra-thin-body Si devices

Source/drain formation in ultra-thin body devices by conventional ion implantation is analyzed using atomistic simulation. Dopant retention is dramatically reduced by backscattering for low-energy and low-tilt angles, and by transmission for high angles. For the first time, molecular dynamics and kinetic Monte Carlo simulations, encompassing the entire Si body, are applied in order to predict damage during implant and subsequent recovery during anneal. These show that amorphization should be avoided as recrystallization in ultra-thin-body Si leads to twin boundary defects and poly-crystalline Si formation, despite the presence of a mono-crystalline Si seed. Rapid dissolution of end-of range defects in thin-body Si, caused by surface proximity, does not significantly reduce diffusion lengths. The conclusions of the atomistic modeling are verified by a novel characterization methodology and electrical analysis.

Molecular dynamics simulation of the regrowth of nanometric multigate Si devices

We use molecular dynamics (MD) simulation techniques to study the regrowth of nanometric multigate Si devices, such as fins and nanowires, surrounded by free surfaces and interfaces with amorphous material. Our results indicate that atoms in amorphous regions close to lateral free surfaces or interfaces rearrange at a slower rate compared to those in bulk due to the discontinuity of the lateral crystalline template. Consequently, the recrystallization front which advances faster in the device center than at the interfaces adopts new orientations. Regrowth then proceeds depending on the particular orientation of the new amorphous/crystal interfaces. In the particular case of 〈110〉 oriented fins, the new amorphous/crystal interfaces are aligned along the 〈111〉 direction, which produces frequent twining during further regrowth. Based on our simulation results, we propose alternatives to overcome this defected recrystallization in multigate structures: device orientation along 〈100〉 to prevent the formation o...

Molecular dynamics simulations of damage production by thermal spikes in Ge

Molecular 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 ma...

Elucidating the atomistic mechanisms driving self-diffusion of amorphous Si during annealing

We have analyzed the atomic rearrangements underlying self-diffusion in amorphous Si during annealing using tight-binding molecular dynamics simulations. Two types of amorphous samples with different structural features were used to analyze the influence of coordination defects. We have identified several types of atomic rearrangement mechanisms, and we have obtained an effective migration energy of around 1 eV. We found similar migration energies for both types of samples, but higher diffusivities in the one with a higher initial percentage of coordination defects.

Molecular dynamics study of damage generation mechanisms in silicon at the low energy regime

We have used classical molecular dynamics simulations to study the damage generation mechanisms in silicon for energy transfers below the atomic displacement energy. These low energy interactions, usually ignored in binary collision based models, establish the difference in damage morphology for different ions. Our work is focused on determining the conditions under which amorphous pockets are formed using a molecular dynamics simulation scheme. We have incorporated the effect of low energy interactions in a binary collision model using our simulation results. This improved model is able to reproduce the damage structures obtained with molecular dynamics but with a much lower computational cost.

Atomistic study of the structural and electronic properties of a-Si:H/c-Si interfaces

We investigate the structural and electronic properties of the interface between hydrogenated amorphous silicon (a-Si:H) and crystalline silicon (c-Si) by combining tight-binding molecular dynamics and DFT ab initio electronic structure calculations. We focus on the c-Si(100)(1×1)/a-Si:H, c-Si(100)(2×1)/a-Si:H and c-Si(111)/a-Si:H interfaces, due to their technological relevance. The analysis of atomic rearrangements induced at the interface by the interaction between H and Si allowed us to identify the relevant steps that lead to the transformation from c-Si(100)(1×1)/a-Si:H to c-Si(100)(2×1)/a-Si:H. The interface electronic structure is found to be characterized by spatially localized mid-gap states. Through them we have identified the relevant atomic structures responsible for the interface defect states, namely: dangling-bonds, H bridges, and strained bonds. Our analysis contributes to a better understanding of the role of such defects in c-Si/a-Si:H interfaces.

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

We 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 I 3-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 I 3-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, I 3-C and I 3-X, cannot be discarded either.