Multiscale Simulations Of Irradiation Effects On MOS Devices And Graphene

With the developing of semiconductor technology,many new devices emerge.When the devices working in complex radiation environments,such as nuclear explosion,galactic cosmic rays and so on,the reliability of devices becomes a key factor.Currently,there are many numerical simulation methods which are adapted to the physical processes at different time and space scales.To radiation damage in devices,a comprehensive understanding of the physical mechanism is the basic research.But now,the whole physical image of radiation damage is far from established.In this dissertation,by means of numerical simulations,the processes of radiation damage in devices and materials are described,and the author's major contributions are outlined as follows:1.Computer simulations of the radiation damage created in silicon-on-insulator?SOI?devices irradiated by neutrons?10Me V-20MeV?using the Monte Carlo simulation are carried out.Based on the GEANT4 program and the cross-section biasing method,the transportations of neutrons are simulated,while the specific parameters of secondary particles are obtained.To investigate the damage by secondary particles,their energy deposits in the devices are calculated by means of the SRIM program.Results show that the defects induced by incident neutrons are evenly distributed in the devices,but the defects produced by secondary particles are not.In the gate dielectric layer,the primary knock-on atoms?PKAs?play a very important role in producing defects.In the buried oxide layer,all secondary particles contribute to the damage through the increasing of energy deposit with depth.Through the combination of GEANT4 and SRIM,a detailed physical mechanism of radiation damage caused by neutrons in the SOI devices is obtained.Simulations of displacement damage effects created in nanoscale MOSFET irradiated by galactic cosmic rays using the Monte Carlo simulation and molecular dynamics method are carried out.Based on the Geant4 package,the transportations of high energetic protons are simulated,while the specific parameters of PKAs are obtained.To investigate the defects induced by PKAs and secondary knock-on atoms?SKAs?,their energy depositions in the devices are calculated by means of the SRIM package.In addition,the microscopic process of collision cascade provoked by PKA is simulated using LAMMPS package.The results show that displacement atoms directly induced by incident protons are evenly distributed in the gate oxide.The oxygen PKAs with a far larger number than the silicon PKAs play an important role in energy depositions.Vacancies are mainly induced by SKAs,although PKAs lose much energy to dislodge atoms from their normal locations.In gate oxide,the numbers of silicon and oxygen vacancies are nearly equal and grow with the depth increasing.Through the combination of Monte Carlo simulation and molecular dynamics method,a multiscale physical picture of displacement damage caused by galactic cosmic rays in the nanoscale MOSFET is obtained.2.Molecular dynamics simulation with the aid of LAMMPS code is carried out to simulate the process of radiation collision cascade in silicon established in atomic scale to give a predictive description of initial damage states in the aim of elucidating the mechanisms of micro-structure evolution process.Meanwhile,some significant relevant factors taken into consideration are homologous temperature,the energy and projectile direction of PKAs and type of the projectile particles that may have influence on the radiation collision cascade.The results show that the influences of homologous temperature and the projectile direction of PKAs are not obvious,while the energy of PKAs and the type of projectile particles have obvious influence on the radiation collision cascade.In addition,a comparison between SRIM and LAMMPS is carried out.3.The damage production induced by swift heavy ion irradiation in bilayer graphene?BLG?is investigated by molecular dynamics method.By given energy to a cylindrical region,the carbon chains even nanoholes can be produced,which depends on the electronic energy loss?dE/dx?.For BLG,the minimum value needed to generate defects lies in 5-7 keV/nm.A low density core and a high density shell structure can be seen while the radii of tracks are obtained.With increasing the values of dE/dx,the track radius is first increasing and then saturates.The analysis of defects indicates that only a small part of the defects can be recombined and the radiation damage in BLG is less severe than in single layer grapheme?SLG?.The damage production induced by swift heavy ion irradiation in amorphous nano-SiO₂ is investigated by molecular dynamics method.By given energy to a cylindrical region,the central nanoholes can be produced,which depends on the electronic energy loss?dE/dx?.For the material used in this work,the minimum value needed to generate defects lies in 3.6-7.2 ke V/nm.A low density core and a high density shell structure can be seen while the radii of tracks are obtained.With increasing the values of dE/dx,the track radius is first increasing and then saturates.Based on the thermal spike model,the process of energy deposited into the material due to electron-phonon coupling is analyzed.By given energy to a cylindrical region,the stress accumulates in this area.And then,a pressure wave emanates outwards,which leads to the atoms in the thermal spike region escaping from the upper and lower surface in the form of clusters.As a consequence,the nanoholes in the track center finally form.4.The swift heavy ion?SHI?irradiation effect on SLG is investigated.The radial dose distributions of delta rays produced by bismuth ions are calculated by Monte Carlo method.The radial dose rapidly decreases with increasing the distance from the track center.The energy deposited into the lattice due to electron-phonon coupling is simulated by molecular dynamics method.By given energy to a cylindrical region,the stress accumulates in this area.And then,a pressure wave emanates outwards,which leads to the atoms in the thermal spike region escaping from the upper and lower surface in the form of clusters.As a consequence,the defect configuration is composed of a central pore and surrounding carbon chain loops.In order to make a comparison,the SHI irradiation effects on BLG and trilayer graphene are investigated.