Luis A. Marqués

Ion-beam-induced amorphization and recrystallization in silicon

Ion-beam-induced amorphization in Si has attracted significant interest since the beginning of the use of ion implantation for the fabrication of Si devices. A number of theoretical calculations and experiments were designed to provide a better understanding of the mechanisms behind the crystal-to-amorphous transition in Si. Nowadays, a renewed interest in the modeling of amorphization mechanisms at atomic level has arisen due to the use of preamorphizing implants and high dopant implantation doses for the fabrication of nanometric-scale Si devices. In this paper we will describe the most significant experimental observations related to the ion-beam-induced amorphization in Si and the models that have been developed to describe the process. Amorphous Si formation by ion implantation is the result of a critical balance between the damage generation and its annihilation. Implantation cascades generate different damage configurations going from isolated point defects and point defect clusters in essentially ...

Atomistic modeling of amorphization and recrystallization in silicon

We propose an atomistic model to describe the evolution of the damage generated by irradiation in Si, going from isolated point defects to the formation of continuous amorphous layers. The elementary units used to reproduce the defective zones are Si interstitials, vacancies and the bond defect, which is a local distortion of the Si lattice without any excess or deficit of atoms. More complex defect structures can be formed as these elementary units cluster. The amorphous pockets are treated as agglomerates of bond defects characterized by their local coordination. The model is able to reproduce the abrupt regime in the crystal-amorphous transition in Si and the epitaxial recrystallization upon annealing as observed in the experiments. The model extends the atomistic kinetic Monte Carlo simulation technique to high implant doses, adequately describing the amorphization and regrowth in Si.

Ion beam induced recrystallization of amorphous silicon: A molecular dynamics study

We use molecular dynamics techniques to study the ion beam induced enhancement in the growth rate of microcrystals embedded in an amorphous silicon matrix. The influence of the ion beam on the amorphous‐to‐crystal transformation was separated into thermal annealing effects and defect production effects. Thermal effects were simulated by heating the sample above the amorphous melting point, and damage induced effects by introducing several low energy recoils in the amorphous matrix directed at the crystalline grain. In both cases, the growth rate of the microcrystals is enhanced several orders of magnitude with respect to the pure thermal process, in agreement with experimental results. The dynamics of the crystallization process and the defect structures generated during the growth were analyzed and will be discussed.

Atomistic analysis of the evolution of boron activation during annealing in crystalline and preamorphized silicon

We use kinetic nonlattice Monte Carlo atomistic simulations to investigate the physical mechanisms for boron cluster formation and dissolution in complementary metal-oxide semiconductor (MOS) processing, and the role of Si interstitials in the different processes. For this purpose, B implants in crystalline Si as well as B implants in preamorphized Si are analyzed. For subamorphizing B implants, a high concentration of Si interstitials overlaps with the B profile and this causes a very quick B deactivation for both low- and high-dose B implants. For B implants in preamorphized silicon, B is activated during the regrowth of the amorphous layer if the B concentration is lower than 1020cm−3 and remains active upon annealing. However, if B concentrations higher than 1020cm−3 are present, as occurs in the formation of extensions in p-channel MOS transistors, B atoms are not completely activated during the regrowth. Moreover, the injection of Si interstitials from the end-of-range defects leads to additional B ...

Atomistic modeling of deactivation and reactivation mechanisms in high-concentration boron profiles

We use kinetic nonlattice Monte Carlo atomistic simulations to investigate the physical mechanisms for boron cluster formation and dissolution at very high B concentrations, and the role of Si interstitials in these processes. For this purpose, high-dose, low-energy B implants and theoretical structures with fully active box shaped B profiles were analyzed. Along with the theoretical B profile, different Si interstitial profiles were included. These structures could be simplifications of the situation resulting from the regrowth of preamorphized or laser annealed B implants. While for B concentrations lower than 1020 cm−3, B clusters are not formed unless a high Si interstitial concentration overlaps the B profile, our simulation results show that for higher B concentrations, B clusters can be formed even in the presence of only the equilibrium Si interstitial concentration. The existence of a residual concentration of Si interstitials along with the B boxes makes the deactivation faster and more severe.

Atomistic analysis of defect evolution and transient enhanced diffusion in silicon

Kinetic Monte Carlo simulations are used to analyze the ripening and dissolution of small Si interstitial clusters and {113} defects, and its influence on transient enhanced diffusion of dopants in silicon. The evolution of Si interstitial defects is studied in terms of the probabilities of emitted Si interstitials being recaptured by other defects or in turn being annihilated at the surface. These two probabilities are related to the average distance among defects and their distance to the surface, respectively. During the initial stages of the defect ripening, when the defect concentration is high enough and the distance among them is small, Si interstitials are mostly exchanged among defects with a minimal loss of them to the surface. Only when defects grow to large sizes and their concentration decreases, the loss of Si interstitials through diffusion to the surface prevails, causing their dissolution. The presence of large and stable defects near the surface is also possible when the implant energy i...

The curious case of thin-body Ge crystallization

The authors investigate the templated crystallization of thin-body Ge fin structures with high aspect ratios. Experimental variables include fin thickness and thermal treatments, with fin structures oriented in the 〈110〉 direction. Transmission electron microscopy determined that various crystal defects form during crystallization of amorphous Ge regions, most notably {111} stacking faults, twin boundaries, and small crystallites. In all cases, the nature of the defects is dependent on the fin thickness and thermal treatments applied. Using a standard 600 °C rapid-thermal-anneal, Ge structures with high aspect ratios crystallize with better crystal quality and fewer uncured defects than the equivalent Si case, which is a cause for optimism for thin-film Ge devices.

Physical insight into boron activation and redistribution during annealing after low-temperature solid phase epitaxial regrowth

Kinetic Monte Carlo simulations of B diffusion and activation in preamorphized Si during annealing after solid phase epitaxial regrowth have been used to provide insight into the mechanisms that drive these phenomena. Simulations show that the presence of an initially high active B concentration along with a Si interstitial supersaturation set by end of range defects leads to simultaneous B deactivation and uphill diffusion through the capture of mobile interstitial B in the high concentration region during subsequent anneal treatments. Once the Si interstitial supersaturation decays close to equilibrium values, B clusters dissolve and emitted B diffuses downhill, following the B concentration gradient. The active B concentration at the minimum state of activation becomes higher as the annealing temperature increases as a consequence of a faster increase of the B cluster dissolution rate compared with the formation rate.

Role of silicon interstitials in boron cluster dissolution

We present kinetic nonlattice Monte Carlo atomistic simulations to investigate the role of Si interstitials in B cluster dissolution. We show that the presence of Si interstitials from an oxidizing anneal stabilize B clusters and slow down B cluster dissolution, compared to anneal in inert ambient. We have also analyzed the influence of injected Si interstitials from end of range defects, due to preamorphizing implants, on B deactivation and reactivation processes. We have observed that the B cluster evolution can be clearly correlated to the evolution of Si interstitial defects at the end of range. The minimum level of activation occurs when the Si interstitial supersaturation is low because the end of range defects have dissolved or reach very stable configurations, such as dislocation loops.

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...