Ivan Chakarov

2D Monte Carlo simulation of proton implantation of superconducting YBa2Cu3O7−δ thin films through high aspect ratio Nb masks

Abstract Perturbation of proton beam damage profile due to sidewall interactions in very high aspect ratio implant masks has been studied using Monte Carlo simulations. The model structure is composed of amorphous Nb metal mask, crystalline high temperature superconducting YBa 2 Cu 3 O 7− δ (YBCO) thin film, and amorphous LaAlO 3 substrate. The simulation results reveal the existence of enhanced proton beam penetration in target materials due to sidewall interactions.

The computer simulation of energetic particle-solid interactions

Abstract Over the last decade the use of computer simulation in predicting physical phenomena associated with ion beam processing of materials has increased both in use and reliability. This is partly due to the dramatic increase in computer power and decrease in computer cost, but is also being achieved due to an increased understanding of the physical processes occurring. With the increase in computer power has come not just the ability to perform more complex calculations but also the methods for complex data representation in animated form. By animating the results it is much easier to observe collective effects such as acoustic wave propagation at a surface due to, for example, molecule impact.

CRYSTAL-BINARY COLLISION SIMULATION OF ATOMIC-COLLISIONS AND DYNAMIC DAMAGE BUILDUP IN CRYSTALLINE SILICON

Abstract Channeling profiles of 5–200 keVB+, 100–500 keV P+ and 40–300 keV As+ implanted near and axes in crystal silicon are calculated using the binary collision approximation code CRYSTAL. Calculated profiles are compared to carefully chosen experimental ones. Also, high dose phosphorus profiles with damage accumulation are compared to published experimental data.

High quality YBa/sub 2/Cu/sub 3/O/sub 7-/spl delta// Josephson junctions and junction arrays fabricated by masked proton beam irradiation damage

High quality single Josephson junctions and junction arrays with 10 junctions in series have been fabricated using masked proton beam irradiation damage technology. Monte Carlo simulation of the irradiation damage profile underneath the metal mask has been carried out systematically to guide the metal mask structure design. A high resolution and high aspect ratio metal mask opening was fabricated by focused 30 keV Ga ion beam milling. Various nonconducting oxide buffer layers have been investigated for a Ga contamination free mask fabrication. A contamination free irradiation process and subsequent removal of metal mask after ion irradiation are the keys for the improved properties of junctions fabricated with YBCO.

AN INVESTIGATION OF COLLISION PROPAGATION IN ENERGETIC ION INITIATED CASCADES IN COPPER

Abstract Using simple Binary Collision simulations of energetic ion initiated collision cascades, particles are considered to undergo a series of binary collisions with their surroundings. In Molecular Dynamics simulation it is difficult to even define what is meant by a collision as the interaction potentials are infinite in nature and consequently all particles are considered to interact with all other particles. By making a suitable definition of a collision for Molecular Dynamics we are able to compare the temporal behaviour of the number of collisions occurring during the propagation of a collision cascade between the two different calculation schemes. An investigation is made of the number of collisions as a function of time occurring in collision cascades. We compare these results to the time ordered version of MARLOWE. By making further definitions about what makes a many body collision, we further investigate the numbers of many body collisions occurring during a number of collision cascades.

Monte Carlo simulations of energetic proton beam irradiation damage defect productions in YBCO thin films with Au masks

Abstract Recent experimental progress in high Tc Josephson junction fabrication using masked ion beam irradiation damage technique demands a further study on the irradiation damage defect accumulation and distribution inside YBCO thin films under different Au mask structures. Both primary energy depositions and irradiation produced vacancy concentrations have been estimated using Monte Carlo simulation codes CRYSTAL and TRIM-CASCADE, respectively. The properties of the irradiation damaged barrier layer are very closely associated with the effective production and distribution of lattice vacancies on different sublattice sites. With a 50 keV proton beam irradiation, a uniform and effective production of vacancies is expected with 100 nm YBCO thin film on LaAlO3 substrate covered with 150 nm Au mask. A similar vacancy distribution is expected with a thinner Au mask and a lower energy proton beam irradiation.

COMPUTER-SIMULATION OF CAICISS OF ALAS(001)

Abstract Recent coaxial impact-collision ion scattering spectroscopy (CAICISS) experiments studying growth kinetics have indicated that 2 keV He ions can be backscattered through 180° from deep - 56 A - within the target without losing more than a few hundred eV above the kinematic energy loss for 180° scattering. Using simple average stopping data it is difficult to satisfactorily explain how particles can penetrate this deep and come back retaining so much of their initial energy. We use the MARLOWE binary collision code to show that it is indeed possible for particles to be scattered from this deep in the solid and still have sufficient energy to appear in the surface peak.

Trajectory separation of channeled ions in crystalline materials

Abstract Spatial distributions of ions implanted into crystals can be of a very complex shape with “lobes” due to ions penetrating through open channels in several directions. This paper suggests an analytical model which represents such a distribution as a linear combination of “random” distribution and one or more “channeled” distributions. This study is focused on the algorithm of the separation of ion trajectories into several distributions. The first distribution includes those ions which have undergone predominantly random collisions. The other distributions include those ions which have undergone mainly “weak” collisions and traveled mostly along the main channeling directions. Our binary collision approximation (BCA) simulator is used for generating and analyzing ion trajectories. The spatial moments can be extracted from each separated distribution. It is shown that 2D analytical distributions obtained as a linear combination of distributions derived from these moments and aligned along corresponding channeling direction are in a very good agreement with direct BCA calculations.

Optimisation of masked ion irradiation damage profiles in YBCO thin films by Monte Carlo simulation

Systematic Monte Carlo simulation studies have been carried out to search for a possible optimal experimental condition on the irradiation damage profile production with a given mask structure. The results suggest that minimum ion scattering broadening tails could be achieved with projectile ranges in Nb mask about half of the mask thickness. Provided the projectile range to mask thickness is maintained, similar irradiation damage profiles could be created by different ions, including ions as heavy as Cu þ . 2002 Elsevier Science B.V. All rights reserved.

Computationally Efficient Model for 2D Ion Implantation Simulation

A computationally efficient method for ion implantation simulation is presented. The method allows two-dimensional ion implantation profiles in arbitrary shaped structures to be calculated and is valid for both amorphous and crystalline materials. It uses an extension of the one-dimensional dual Pearson approximation into the second dimension.