We study a one-dimensional fixed-energy version ͑that is, with no input or loss of particles͒ of Manna's stochastic sandpile model. The system has a continuous transition to an absorbing state at a critical value of the particle density, and exhibits the hallmarks of an absorbing-state phase transition, including finite-size scaling. Critical exponents are obtained from extensive simulations, which treat stationary and transient properties, and an associated interface representation. These exponents characterize the universality class of an absorbing-state phase transition with a static conserved density in one dimension; they differ from those expected at a linear-interface depinning transition in a medium with point disorder, and from those of directed percolation.

Defect formation in compound semiconductors such as GaAs under ion irradiation is not as well understood as in Si and Ge. We show how a combination of ion range calculations and molecular dynamics computer simulations can be used to predict the atomic-level damage structures produced by MeV ions. The results show that the majority of damage produced in GaAs both by low-energy self-recoils and 6 MeV He ions is in clusters, and that a clear majority of the isolated defects are interstitials. Implications of the results for suggested applications are also discussed.

Predicting range profiles of low-energy ͑0.1-10 keV/amu͒ ions implanted in materials is a long-standing problem of considerable theoretical and practical interest. We combine here the best available method for treating the nuclear slowing down, namely a molecular-dynamics range calculation method, with a method based on density-functional theory to calculate electronic slowing down for each ion-target atom pair separately. Calculation of range profiles of technologically important dopants in Si shows that the method is of comparable accuracy to previous methods for B, P, and As implantation of Si, and clearly more accurate for Al implantation of Si.Calculating the force which slows down energetic ions traversing in materials ͑the stopping power͒ is a longstanding problem of considerable theoretical and practical interest. 1-3 While the stopping power caused by collisions between an ion and atoms can now be predicted very accurately, 4-6 there is still uncertainty in how the stopping caused by collisions between an ion and electrons ͑electronic stopping͒ should be calculated for ion velocities below the Bohr velocity ͑namely at energies of the order of 0.1-10 keV/amu͒. This is an especially pressing problem for obtaining range profiles for dopants implanted in crystal channel directions in semiconductors, since on one hand the uncertainties are particularly large in this case and on the other hand this case is important for the microelectronics industry.The most common approach for obtaining stopping powers is to first derive a stopping power for a proton in a material, and then use a scaling law to obtain the stopping power of heavier ions. To obtain the stopping power of the proton, the most popular approach is to use models 7,8 based on the scattering phase shifts for Fermi-surface electrons. The phase shifts are determined within the density-functional theory ͑DFT͒ ͑Ref. 9͒ for a proton embedded in a homogeneous electron gas. 10,11 This approach has proven successful for some technologically important ion-target combinations, such as B-Si, P-Si, and As-Si, 12,5,13 but leads into severe difficulties ͑a physically unjustified parameter value͒ for the case of Al-Si. 5 However, the original model with self-consistently determined phaseshifts offers another, frequently overlooked, approach to obtain stopping powers for heavy ions. Instead of using scaling laws, it is possible to explicitly calculate phase shift factors for any given ion-target atom combinations. The phase shift factors can be calculated directly from DFT, so this approach does not use any empirical or fitted input factors.Another noteworthy approach to predicting the electronic stopping is the local plasma approximation. However, this model does not presently treat channeling anisotropies in the electron distribution, 14 and hence is not applicable here.In this paper, we combine the best available simulations methods for calculating ion range profiles with electronic stopping powers derived from realistic three-dimensional charge distributions of t...

Recent experiments have shown that when gold atom clusters bombard copper with an energy of 10 keV/atom, the mean range of the gold atoms is independent of the cluster size, but the straggling ͑broadening͒ of the depth distribution is an increasing function of the cluster size. The same set of experiments did not show this effect when the target was amorphous Si. Using molecular dynamics computer simulations we have studied this effect by simulating Au cluster bombardment of Cu and Si with energies 1-10 keV/atom. We found that in Cu, the mean range is not fully independent of the cluster size, but the dependence on cluster size is so weak it is hard to observe experimentally. On the other hand, we found a strong enhancement of the straggling in Cu, but not in Si, in agreement with the experiments. By following the time dependence of the straggling we show that this is due to the massive heat spike effects which are present in Cu but not Si.