-

We propose an atomistic model to describe the damage generation during ion irradiation in Si and its evolution upon anneal. We have included new features to the classical models used to describe damage in crystalline Si, that allow us to extend the atomistic approach to the modeling of continuous amorphous layers. The elementary units to describe the defective lattice are Si interstitials, vacancies and the bond defect, which is a local distortion of the lattice without any excess or deficit of atoms. More complex defect structures can be formed by the coalescence of these elementary units. The competition between the damage generation and its annihilation determines the damage accumulation that eventually may lead to amorphous layers. The same model is used for amorphizing and non-amorphizing implants, and the amorphization is the result of the simulation itself and not established as an input parameter.

We have studied the influence of the so called “bond defect” in the silicon amorphization process using molecular dynamics simulation techniques. The bond defect consists in a local distortion of the silicon lattice with no excess or deficit of atoms, and it can be formed during ion-beam irradiation. Even though the bond defect lifetime is too short to justify damage accumulation at usual implantation temperatures, we have observed however that the interaction between close bond defects can generate more stable structures which behave as the amorphous pockets created by ion irradiation. We have seen as well that the recombination of a given amount of damage created by bond defect accumulation depends of its spatial distribution.

Conventional simulation tools for microelectronic technological processes will be soon obsolete since the scaling-down of semiconductor devices requires atomic scale design. In this context only the concurrent use of different complementary methodologies can satisfy the demands of accurate and efficient modelling. We have applied a series of approaches to a noteworthy case: the B-type doping of Si. The choice of appropriate methodology depends on the peculiar problem we must address. Statics and migration mechanism are studied by quantum mechanical calculation. These calculations validate semiempirical approaches based on atomic particle–particle potential which are applied to the system evolution simulation. These last methodologies are useful when the kinetic evolution occurring during the processes is characterized by rearrangements in different structural identities. Moreover, using these simulations, we can set the parameters ruling the complex dissolution rates in the models which can be applied to the large system simulation.

The effect of deposition temperature on the formation of hard amorphous carbon (a-C) films from low and medium energy ions was studied by atomic scale simulation. Applying classical molecular dynamics with a modified hydrocarbon potential of Brenner, ion-beam deposition of carbon films with a thickness of ∼3–10 nm was simulated for ion energies E=10–80 eV, and for substrate temperatures ranging from 100 to 900 K. The dependence of the sp3 content and intrinsic stress in the simulated films on substrate temperature was found to agree qualitatively with experiment. At low ion energies and low substrate temperatures, the sp3 fraction increases with ion energy, giving at E>30 eV a highly sp3-bonded tetrahedral amorphous carbon (ta-C) with a high compressive stress. This trend also remains at room temperature, however with lower values of the sp3 content and stress, whereas above 500 K simulations with the Brenner potential predict a sharp transition from ta-C to graphitic carbon with dominating sp2 bonding.

Calculations have in the present study been carried out for N, P and S impurities in diamond using the DFT and Hartree–Fock levels of theory. The chemisorption of a specific gaseous impurity species (NHx, PHx, SHx), followed by the substitutional incorporation of N (or P or S) into the upper surface atomic layer, and finally the incorporation of N (or P or S) into a bulk position, have thermodynamically been studied in a preliminary series of calculations with the aim to investigate the n-type doping occurring during CVD growth of diamond. The main purpose was to look for the possibility to achieve a kinetic trapping and/or good solubility by using extremes in surface chemistry and design of precursors. The effect of choice of n-type dopant (N, P, S) on materials properties like (i) thermodynamics, (ii) solubilities, (iii) geometrical structures and (iv) electronic structures, have then been of a special interest to study more thoroughly.

Structural relaxation processes in tetrahedral amorphous carbon (ta-C) are examined at the atomic scale using computer simulation techniques and Brenner’s bond-order potential. The amorphous carbon networks generated by ion-beam deposition simulation are employed as structural models for as-prepared ta-C. The models possess high intrinsic compressive stresses (∼10 GPa) typical of as-grown ta-C films. Simulating annealing by the molecular-dynamics method, structural changes due to the relaxation of the ta-C networks were observed. In agreement with the experiment, it is shown that low-temperature structural relaxation in ta-C is accompanied by a considerable stress reduction with only minor changes in the structural disorder and density. A complete stress relief is found to occur at Ta∼1000 K. The stress relief mechanism discussed on the basis of the molecular-dynamics simulations includes diffusionless structural transformations within the ta-C networks and does not require oriented clustering of sp2-bonded atoms.

In this paper we address the issue of C and Si vacancy diffusion in SiC by means of first principles electronic structure calculations. Besides a thorough analysis of the stable configurations involved, we characterize the minimum energy paths and the corresponding barriers of all the relevant diffusion mechanisms.

The Stillinger–Weber empirical potential was modified and its parameters were determined to achieve a realistic description of the microscopic structure and the energetics of different planar defects and their interactions in wurtzite GaN. The formulation was based on the adjustment of the parameters in order to represent the Ga–Ga, N–N and Ga–N bonds. The input data comprises of the different crystalline phases of gallium, nitrogen and GaN. A satisfactory agreement on the values of the energy versus atomic volume per atom was obtained compared to those derived by ab initio calculations and experimental data for all the cases studied. By employing the modified Stillinger–Weber potential the energy of translation domain boundaries, which have been observed experimentally in GaN thin films, was calculated providing results comparable with ab initio calculations.

Some insights into an experimental approach to the growth of defect-free epitaxial films of group-III nitrides were provided by an investigation based on ab initio molecular orbital methods. From the theoretical point of view, use of a molecular beam of dialkyl group-III nitrene characterized not only by two unpaired electrons almost localized at the nitrogen atom but also by the covalently bound pair of one group-III and one nitrogen atoms is advantageous to the low-temperature growth of stoichiometric group-III nitride. A previous result that the beam of dimethylaluminumnitrene ((CH3)2AlN) is obtainable by the method of photo-dissociation of energetic compound beams (PDECB) using dimethylaluminum azide ((CH3)2AlN3) as source material was confirmed by examinations of the decomposition mechanisms of dialkyl group-III azides. Furthermore, dimethylgallium azide ((CH3)2GaN3) possessing the nature of unimolecular metastable dye was found to be a promising PDECB source compound appropriate for photo-assisted chemical beam epitaxy of gallium nitride.