Molecular Dynamics Simulation Of Radiation Effects In Silica And Barium Titanate Ferroelectric Gate

With the continuous progress of science and technology,the exploring of outer space has attracted much attention in recent years.As we all know,the outer space is filled with a mass of radiation,which would cause serious radiation damage in the electronic device of spacecraft.In traditional Flash memory based on Si semiconductors,amorphous SiO₂,which is as an insulating gate layer in the Flash memory,is capable of storing tunneling charges and controlling the on and off states of the transistor.Therefore,the insulating intensity of the gate layer is important for the reability of information storage,such as the retention property.However,the insulating layer is vulnerable to radiation damage under the irradiation environment,leading to the loss of tunneling charges and data storage ability.Ferroelectric memory based on the polarization states of ferroelectric thin film has an excellent resistance ability to the radiation,which is essentially different from the traditional Flash memory.In outer space,therefore,the electronic device based on the ferroelectric memory can greatly reduce the radiation damage effects.However,irradiation resistance of ferroelectric memory is not enough.Though the flux is low,the powerful fast heavy ion in the outer space may bring serious radiation damage to ferroelectric memory,which results in failure of ferroelectric memory.In the heavy ions radiation processes,the existence of recoil atoms plays an important role in radiation effects.In order to investigate the radiation effects induced by heavy ions in gate oxide layer and ferroelectric gate and the deep mechanism of the radiation damage,the cascade collision processes of recoil atoms produced by heavy ions in gate oxide layer and ferroelectric gate were studied by using molecular dynamics method,and the effects of defects formation on the performace in gate oxide layer and ferroelectric gate were analyzed.In this paper,the radiation process of high energy heavy ion in the SiO₂ oxide layer and BaTiO₃ ferroelectric thin film was studied.The radiation effect in the two materals induced by the primary knock-on atoms?PKA?in the cascade collision processes were studied.Firstly,the Monte-Carlo method was used to simulate the Au ion bombardment in SiO₂ thin film,which found that the energies of recoil atoms is less than 200 eV.The energies and quantities of the recoil atoms were investigated.Using the molecular dynamics method,the cascade collision processes of recoil atoms of PKA_Si and PKA_O in the SiO₂ were simulated,and the types and quantites of induced defects were analyzed.The defect of Si₃,Si₅ and O₁ are found to be the main defects.It is found that the number of defects is independent of the type of PKA.The number of defects induced by two PKA was similar.In the process cascade collision,22 defects were generated by PKA_Si or PKA_O with the energies of 200 eV.The percentage of Si₃,Si₅ and O₁ defects is 32%,23%and 32%,respectively.The ab initio molecular dynamics method was used to simulate the process of cascade collision induced by PKA with the energies of 200 eV,and the total number of defects is similar to that of molecular dynamics method.The results show that the defects density is up to 2.431 nm^-3.In addition,the size of the damage area was calculated.The radius and the depth of the damage area generated by 200 eV PKA reaches 1.4nm and 2.0 nm,respectively.In order to analyze the effects of the defects produced by PKA with 200 eV energies on the density of electronic states,the perfect system with192 atoms and the system with three O vacancies or two Si vacancies were computed.The results show that the conductivity of SiO₂ is enhanced This may result in the leakage current in the effect of external electric field,even make the film breakdown,and damage the memory.Second,using the ab initio molecular dynamics method,the process of cascade collision induced by three types of PKA was simulated.The number of O,Ba and Ti vacancies versus the relaxation time was analyzed.It is found that the main defect is O vacancy.The number of O vacancies is 4,and the number of Ti or Ba vacancies is 3.The defects density in BaTiO₃ films is about 0.496 nm^-3.It can be seen that the quantities of defects are few and the defects producing process in BaTiO₃ is difficult compared with that in SiO₂.Furthermore,the presence of defects does not have a serious effect on the polarization of BaTiO₃ thin film.The O?vacancy makes the ferroelectric hysteresis loop shift to the negative direction of the electric field,and reduces the remanent polarization by about 19%.However,the ferroelectric hysteresis loop is mostly intact.The coercive field is also reduced about 22%,which is benefit to polarization switching and low power consumption.In a word,it's benefit to the polarization switching.The O?vacancy reduces the coercive field and the remanent polarization by about 10%,which has the least negative effects on polarization switching.The Ti vacancy increases the coercive field by about 32%and reduces the remanent polarization by about 15%.The existence of the Ti vacancy in the film,however,can still make the polarization switch.And the quantities of the Ti vacancy is small,therefore,the Ti vacancy has a little effect on the polarization switching.The existence of Ti vacancy reduces the coercive field by about 33%,and the remanent polarization is not changed,which will be benefit to the polarization switching.In addition,the effects of four types of vacancy on the density of electronic states of BaTiO₃ thin film were analyzed.The calculation shows that the defect of four vacancies would enhance the conductivity of BaTiO₃ thin film,making it easy to generate leakage current under applied electric field.According to the simulation,it can be seen that the quantities and the density of defects generated in the gate oxide layer of SiO₂ thin film is much more than that generated in the ferroelectric gate of Ba Ti O₃ thin film.Compared with SiO₂ thin film,the defects in BaTiO₃ thin film are more likely to be eliminated,resulting in less stable defects and lower defects density.The defects in the SiO₂ thin film and BaTiO₃thin film would enhance the conductivity in each system,which may produce leakage current under the applied electric field.The traditional Flash memory stores the data by storing the charge in the gate oxide layer.The enhancement of the conductivity makes it easy to lose the charge,thus making the data faded,and finally would seriously affecting the ability of data storage in Flash memory.The storage of data in ferroelectric memory depends on the bistable states of polarization in ferroelectric thin film.And the defects in ferroelectric thin film induced by high energy heavy ions does not seriously affects on the polarization switching.However,the existence of defects may cause the ferroelectric thin film to generate leakage current,reducing the ability of polarization.It can be seen that the ferroelectric memory still has the excellent ability of data storage in the radiation environment of high energy heavy ions compared with traditional Flash memory.