A computational study of surface topography arising from energetic particle interactions

A computational investigation into the development of surface topographies subjected to energetic particle bombardment has been undertaken. Classical Molecular Dynamics (MD) and on-the- y kinetic Monte Carlo (otfKMC) techniques were employed and di erent bombardment conditions were considered. Surface topography development is of interest due to applications such as ion etching, which can be used in the manufacture of semiconductor devices. Crater formation on a HfO2-MgO interface system was investigated using a variety of methods. Initially single atom and cluster bombardments were performed, highlighting the radiation tolerance of the interface system. Subsequently, swift heavy ion bombardment of the interface was considered using a MD thermal spike model and an electron stripping with recombination model. Both models gave similar results to those seen experimentally: hillocks forming on the surfaces over the impact points of the ions; and ion tracks forming around the paths of the ions in the material. Hillock heights and sputtering yields were shown to increase linearly with the electronic stopping force of the bombarding ion, for the range of systems we considered. Bullet impacts on armour plating (SiC) have been simulated using MD. The bullet was modelled by a hard sphere that was forced into the substrate to the target depth. Both 4H and 6H SiC polytypes were considered with di erent bullet sizes and speeds. The 4H system resulted in the displacement of less atoms and also a much lower sputtering yield than for the similar 6H system. However dislocations were seen to propagate through the 4H system but not the 6H one. A large amount of sputtering was observed in the higher speed 6H simulations, with the ejection of many big clusters of atoms. These clusters generally had a high temperature (around 1,500 K) with speeds typically in excess of 1,000 m/s. Surface topography development by way of multiple impacts on Au was investigated using two di erent methodologies. Initially a traditional, MD based, methodology was used to model Au self bombardment of the high index f3 11 0g surface, which has been shown to produce interesting features. The disadvantage of this type of method is that MD cannot simulate time scales long enough to allow di usion between impacts. The MD method was shown to lead to a build up of defects in the systems: a result of the artificially high dose rate. An improved method was then used to model Ar and Au bombardment of both f0 1 0g and f3 11 0g Au surfaces. This hybrid MD-otfKMC technique enabled realistic time scales to be achieved. MD was used to model the ballistic phase while otfKMC was used to model di usion between events. The erosion rate of the surface was shown to be almost linear with time while the roughness of the surface was shown to oscillate: indicative of the healing process that occurs between bombardment events

Modelling the sputtering of Au surfaces using a multi time-scale technique

We present results from an atomistic computer simulation model of the sputtering of gold crystal surfaces under 500 eV ion bombardment by Au and Ar ions for doses up to 1014 ions cm−2. The multi time-scale technique uses molecular dynamics to calculate the fast ballistic collision processes in the early stages of the cascade, whereas an on-the fly kinetic Monte Carlo technique is used to model the relaxation and diffusion processes between successive ion impacts when the defect motion has begun to be dominated by rare events. The results indicate a large amount of crystalline recovery between impacts, some facetting of the crystal surfaces but no large sub-surface defect accumulation. Because of this recovery process, sputtering yields and energy distributions are in good agreement with those obtained assuming a perfect crystal surface and also with those experimentally measured

Modelling of dissolved H in Ga stabilised δ-Pu

The behaviour of hydrogen in Ga stabilised δ-Pu has been investigated using atomistic computer simulation techniques. We have considered only the solid solution of H in Pu-Ga. H diffusivity in the undamaged material was calculated and was shown to depend on the Ga concentration of the Pu-Ga alloy. Furthermore, localised regions of high Ga concentration within the material were shown to block H diffusion pathways. These are important findings and could allow for the possibility to control H diffusion if it were possible to control the Ga configuration within the system. The interaction of H with simple point defects was also investigated and suggests that H will behave differently in cascade damaged systems compared to undamaged systems. Vacancies were observed to trap any H interstitials that enter their vicinity, while the likelihood of dissociation was very low, effectively reducing the H diffusion coefficient to zero. On the other hand, binding energy calculations show that it is energetically unfavourable for a H interstitial to be close to a Pu interstitial. No long range interaction between H and the single point defects was observed. Crown Copyright © 2013 Published by Elsevier B.V. All rights reserved

Reaction pathways in atomistic models of thin film growth

The atomistic processes that form the basis of thin film growth often involve complex multi-atom movements of atoms or groups of atoms on or close to the surface of a substrate. These transitions and their pathways are often difficult to predict in advance. By using an adaptive kinetic Monte Carlo (AKMC) approach many complex mechanisms can be identified so that the growth processes can be understood and ultimately controlled. Here the AKMC technique is briefly described along with some special adaptions that can speed up the simulations when, for example, the transition barriers are small. Examples are given of such complex processes that occur in different material systems especially for the growth of metals and metallic oxides

Prediction of irradiation spectrum effects in pyrochlores

The formation energy of cation antisites in pyrochlores (A2B2O7) has been correlated with the susceptibility to amorphize under irradiation, and thus, density functional theory calculations of antisite energetics can provide insights into the radiation tolerance of pyrochlores. Here, we show that the formation energy of antisite pairs in titanate pyrochlores, as opposed to other families of pyrochlores (B = Zr, Hf, or Sn), exhibits a strong dependence on the separation distance between the antisites. Classical molecular dynamics simulations of collision cascades in Er2Ti2O7 show that the average separation of antisite pairs is a function of the primary knock-on atom energy that creates the collision cascades. Together, these results suggest that the radiation tolerance of titanate pyrochlores may be sensitive to the irradiation conditions and might be controllable via the appropriate selection of ion beam parameters

Atomistic surface erosion and thin film growth modelled over realistic time scales

We present results of atomistic modelling of surface growth and sputtering using a multi-time scale molecular dynamics–on-the-fly kinetic Monte Carlo scheme which allows simulations to be carried out over realistic experimental times. The method uses molecular dynamics to model the fast processes and then calculates the diffusion barriers for the slow processes on-the-fly, without any preconceptions about what transitions might occur. The method is applied to the growth of metal and oxide materials at impact energies typical for both vapour deposition and magnetron sputtering. The method can be used to explain growth processes, such as the filling of vacancies and the formation of stacking faults. By tuning the variable experimental parameters on the computer, a parameter set for optimum crystalline growth can be determined. The method can also be used to model sputtering where the particle interactions with the surface occur at a higher energy. It is shown how a steady state can arise in which interstitial clusters are continuously being formed below the surface during an atom impact event which also recombine or diffuse to the surface between impact events. For fcc metals the near surface region remains basically crystalline during the erosion process with a pitted topography which soon attains a steady state roughness