A computer code for the simulation of ion beam irradiation of nanostructures has been developed. The code simulates the transport of energetic ions through matter by means of a Monte Carlo algorithm, similar to the often-used TRIM code (Ziegler et al. (1985) [1]). New effects occur compared to bulk, when irradiating nanostructures, which are of the same size as the ion range or the damage cascade. To account for these effects, the target in our code does not consist of layers like in TRIM but can be defined as an arbitrary 3-dimensional structure. This allows to obtain more accurate 3D distributions of implanted ions and implantation damage for nanostructures, which cannot be described by a stack of layers. We demonstrate the functionality of the code by comparing simulations with ion beam implantation into nanowires.

Ion beam irradiation of nanostructures – A 3D Monte Carlo simulation code

Abstract This paper describes the fundamental principles and experimental status of gas cluster ion beam (GCIB) processing as a new technique with promise for practical industrial applications. A review is presented of the theoretical and experimental characteristics of new gas cluster ion bombardment processes and of related equipment development. The impacts of accelerated cluster ions upon substrate surfaces impart very high-energy densities in the impact regions of individual clusters and produce non-linear processes that are not present in the impacts of individual atomic ions. These unique bombardment characteristics are expected to facilitate new industrial applications that would not be possible by traditional ion beam processing. Among these are shallow ion implantation, high rate sputtering, surface cleaning and smoothing, and low-temperature thin film formation.

Nano processing with gas cluster ion beams

A computational study of organic thin-film growth using a combination of ab initio based energy calculations and kinetic Monte Carlo (KMC) simulations is provided. A lattice-based KMC model is used in which binding energies determine the relative rates of diffusion of the molecules. This KMC approach is used to present "landscapes" or "maps" that illustrate the possible structural outcomes of growing a thin film of small organic molecules, represented as a two-site dimer, on a substrate in which the strength of organic-substrate interactions is allowed to vary. KMC provides a mesoscopic-scale view of sub-monolayer deposition of organic thin films on model substrates, mapped out as a function of the flux of depositing molecules and the temperature of the substrate. The morphology of the crystalline thin films is shown to be a strong function of the molecule-molecule and molecule-substrate interactions. A rich variety of maps is shown to occur in which the small organic molecules either stand up or lie down in a variety of different patterns depending on the nature of the binding to the surface. In this way, it is possible to suggest how to tailor the substrate or the small organic molecule in order to create a desired growth habit. In order to demonstrate how this set of allowable maps is reduced in the case where the set of energy barriers between substrate and organic molecule are reliably known, we have used Gaussian 98 calculations to establish binding energies for the weak van der Waals interactions between a)-pairs of pentacene molecules as a function of orientation and b) pentacene and two substrates, silicon surfaces passivated with cyclopentene molecules and a crystalline model of silicon dioxide. The critical nucleation size and the mode of diffusion of this idealized two-site dimer model for pentacene molecules are found to be in good agreement with experimental data.

A computational study of the sub-monolayer growth of pentacene

An interatomic potential has been developed to describe interactions in silicon, carbon and silicon carbide, based on the environment-dependent interatomic potential (EDIP) (Bazant et al 1997 Phys. Rev. B 56 8542). The functional form of the original EDIP has been generalized and two sets of parameters have been proposed. Tests with these two potentials have been performed for many properties of SiC, including bulk properties, high-pressure phases, point and extended defects, and amorphous structures. One parameter set allows us to keep the original EDIP formulation for silicon, and is shown to be well suited for modelling irradiation-induced effects in silicon carbide, with a very good description of point defects and of the disordered phase. The other set, including a new parametrization for silicon, has been shown to be efficient for modelling point and extended defects, as well as high-pressure phases.

http://iopscience.iop.org/article/10.1088/0953-8984/22/3/035802/pdf

An environment-dependent interatomic potential for silicon carbide: calculation of bulk properties, high-pressure phases, point and extended defects, and amorphous structures.

Sputtering yields of crystalline silicon carbide and silicon have been determined experimentally for bombardment by Ne+, Ar+ and Xe+ ions in the energy range between 0.5 and 5 keV under 60° sputtering with respect to the surface normal. Sputter crater measurements on SiC and Si and Auger depth profiles of SiC on Si have been carried out in order to determine the sputtering yields. The measurements are compared with Monte Carlo simulations which have been computed by the simulation static codes, TRIM and TRIRS and by the dynamic codes DYTRIRS and T-DYN as well as with the sputter theory. The simulation results depend strongly on the input parameters which are not well known especially for SiC. The TRIM simulation fits the experimental results very well and the differences between the results of the simulation programs are sometimes greater than their difference from experimentally measured sputtering yields.

The estimation of sputtering yields for SiC and Si

Growth of three-dimensional SiC clusters on Si modelled by KMC

The novel binary collision approximation Monte Carlo (BCA-MC) computer codes TRIRS and DYTRIRS for simulating ion sputtering of polyatomic nonuniform amorphous targets are presented. TRIRS simulates the collision cascade in a target and related secondary processes, including sputtering, damage generation etc., being more realistic than similar MC-BCA codes in modeling low-energy interatomic collisions. These improvements ensure better simulation of low-energy atomic collision processes in nonuniform targets, like sputtering, and ultra-low energy ion implantation. DYTRIRS is the extension of TRIRS which simulates the dynamics of the ballistic stage of ion-induced modification and sputtering for a target under high-fluence ion irradiation. The efficiency of DYTRIRS is verified by comparing the simulation of sputtering and secondary-ion mass-spectrometry in-depth compositional profiling of molecular-beam epitaxy grown two-dimensional (Al,Ga)AsGaAs (001) heterostructures, including structures with silicon and aluminum marker layers.

Computer simulation of ion sputtering of polyatomic multilayered targets

Individual Si and C adatoms, as well as SiC clusters, on a Si surface are simulated by the molecular dynamics method in the course of investigation of the initial stages of formation of a SiC layer on silicon with the help of molecular beam epitaxy. The potential energy surfaces for Si and C adatoms on the (2 × 1) reconstructed Si(001) surface and on the nonreconstructed Si(111) surface, as well as on the Si(111) surface with a SiC cluster, are calculated and analyzed. The values of migration barriers for adatoms on these surfaces are calculated. The effect of the SiC cluster on deformation of the surface region of Si(111) and on the migration of adatoms is investigated. The deep minima observed on the potential energy surfaces immediately above a cluster and at its boundaries can trap diffusing adatoms. The distributions of stresses and strains in the silicon lattice under a cluster on the surface are studied and described.

Study of Si and C adatoms and SiC clusters on the silicon surface by the molecular dynamics method

Using the sputtering version of the Monte-Carlo (MC) computer code TRIRS (TRansport of Ions and Recoils in Solid) we studied the collision sputtering processes under ion bombardment of a structureless target. Basically, the TRIRS calculation procedure is similar to the one used in the well known TRIM code. The important feature of the TRIRS with respect to other similar MC codes is the elaborate design of the particles trajectories, i.e., the scattering angle is calculated exactly using a scattering integral; the asymptotic trajectories of particles are computed taking into account a `time integral' for the specific interatomic potential used in calculation; during low energy events not only binary collisions, but also many-bode interaction may be taken into account. Comparing the calculation results and experimental data, we can see that TRIRS code is more realistic than TRIM in modeling the low-energy interatomic collisions, which dominate in sputtering processes. The special dynamic version of TRIRS code (DYTRIRS) was used to study the sputtering in the course of high-fluence irradiation when the target may be modified during irradiation. The high-fluence effects in preferential sputtering have also been discussed.

Monte Carlo computer simulation of ion sputtering

The new approach for the determination of displacement threshold energies (E d ) of impurity atoms in multicomponent targets has been proposed. The approach combines an experimental SIMS-profiling technique and a computer simulation by a dynamic DYTRIRS code. The developed approach was applied for the determination of E d the for Al and In impurity atoms in GaAs targets.

Evgeni E. Zhurkin

Papers

Growth of three-dimensional SiC clusters on Si modelled by KMC

Computer simulation of ion sputtering of polyatomic multilayered targets

Study of Si and C adatoms and SiC clusters on the silicon surface by the molecular dynamics method

Monte Carlo computer simulation of ion sputtering

Displacement threshold energies of impurity atoms in GaAs heterostructures