Atomistic simulation of damage production by atomic and molecular ion irradiation in GaN

Abstract

Gallium nitride (GaN) has emerged as one of the most important semiconductors in modern technology. GaN-based device technology was mainly pushed forward by invention of p-type doping and the successful fabrication of light emitting diodes (LEDs) and laser diodes (LDs). Intensive studies in the last 20 years on GaN have significantly advanced the understanding of the properties and have expanded the range of practical applications. Beside basic lighting, current applications of GaN include high-power and high temperature electronics, microwave, optoelectronic devices, and so on.The successful production of optical devices demands efficient tuning of charge carrier lifetime where defect engineering plays a vital role. During growth, varying the level of recombination centers is difficult, whereas ion irradiation can do this job efficiently on a final product. On the other hand, during doping, undesirable defects can also be produced and epitaxial GaN is known to have a highly defective structure. Thus, having both positive and negative aspects, it is very important to have a detailed understanding of irradiation-induced defects.To explain experimental findings, atomic level understanding is necessary, but it is not always possible to have an atomistic view of defect dynamics in experiments. Some damage build-up studies by single ions have been reported in the literature, but not many by molecular ions. In this thesis, the irradiation of GaN by single and molecular ions by the means of atomistic simulations was studied. The irradiation response of both bulk and nano-structured GaN system were studied. For bulk studies, all projectiles were irradiated having the same energy per mass. The damage by molecular ions showed strong dynamic annealing. No non-linearity had been observed in the total number of point defects between single and molecular ions. On the other hand, molecular ions produce larger clusters of point defects than single ions. These large defect clusters can be one of the mechanisms of the experimentally observed faster carrier decay time for molecular projectiles. Defects were mostly concentrated at the surface and near-surface regions, which is also evident from experiments.ii Comparison between a similar mass single ion and a molecular ion show that a single ion produced more defect clusters than molecular ions. This suggests that heavy ions are even more efficient than similar mass cluster ions to quench the carrier lifetime.

Nonlinear effects in defect production by atomic and molecular ion implantation

“…37 Similar elastic waves have been observed in the present simulations for Al 3 and Al 4 clusters at MD simulation times of $3 ps (not illustrated here). No shift was observed in time of occurrence of the elastic wave peak with larger box sizes.…”

“…37 Such a high value for the ratio is due to the efficient intra-cascade recombination in the material. 37 In the present case, it can also be observed that despite creating more damage, lower ratios are exhibited by aluminum molecular ion implantations. This could be due to the formation of defect clusters during the quenching phase.…”

Single and molecular ion irradiation-induced effects in GaN: experiment and cumulative MD simulations

“…Both these effects occur throughout the entire range of silver ions. Although Ag and PF4 have comparable mass, the atoms comprising a molecule separate in the first surface layers, as it was shown in [40,49]. That is why in the bulk we do not observe any significant difference in point defect production between a molecule and its constituents.…”

“…The simulation cell containing a total of 5.5 million atoms was equilibrated at 300 K using Berendsen temperature and pressure control [43] before the irradiation runs. Details of the simulation procedure are given elsewhere [33,40]. Irradiation simulations were done by F, P, PF2, PF4 and Ag ions (see Table 1).…”

A multiscale and multiphysics framework to simulate radiation damage in nano-crystalline materials

“…This issue makes modeling techniques, especially molecular dynamics (MD) simulations, the preferred method to understand the fundamental aspects of radiation damage in materials. Several MD studies have helped to understand the atomistic mechanisms during the cascade damage process in many materials, including metals and multicomponent alloys [8,9], semiconductors [10], and others materials [11]. MD simulations have also been used as the primary tool to understand damage mechanisms in radiation environments.…”