| In recent years,with the continuous emergence of semiconductor materials,the second generation semiconductor Ga As and the third generation semiconductor Ga N have gradually become the research hotspot.They have direct and wide band gaps,high voltage withstand abilities,high electron mobilities,stable structures and other excellent properties.Therefore,they have become the converter materials of nuclear batteries.For nuclear batteries used in extreme environments,e.g.deep space and polar,it is very important to consider the radiation damage caused by cosmic high-energy rays and internal radiation sources.The interaction between irradiated particles and semiconductor materials will cause ionization damage and displacement damage,and the permanent defects caused by displacement damage will introduce defect energy levels in semiconductor,change the energy band structure and electronic density of states of materials,and affect the transport properties of carriers.Besides,the defects will become the recombination center of carriers in the material,which will reduce the lifetime of carriers.These micro radiation damage will affect the macro electrical performances of the battery,such as the maximum electrical output power,energy conversion efficiency,the stability of the converter,the battery lifetime and so on.Therefore,it is very important to explore the micro mechanism and the evolution process of radiation damage,the micro along with the influence of micro damage area on material properties.In this paper,the primary knock-on atom(PKA)generated by 10 Me V protons in the material is analyzed by LAMMPS,and 10-30 ke V PKA is selected for radiation damage simulation.The average threshold energies of dissociation for Ga As and Ga N materials are 17.9 e V and 83.3 e V,respectively.For the irradiated Ga As material system,six kinds of mesoscopic defect states are given.And then,the energy band structure diagram of mesoscopic defect states is obtained by first principle analysis.Compared with the perfect crystal structure,the changes of Fermi level and band gap can be observed.The effective mass of carriers in different directions under different defect states are obtained by calculating the quadratic curvature of the bottom of conduction band and the top of valence band.Generally speaking,the defects decrease the effective mass of electrons and increase the effective mass of holes.With the increase of the band gap,the carrier concentration decreases rapidly,and the carrier concentration varies with the defect states.The carrier concentration of gap defect states of Ga vacancy or As interstitial is higher than that of other defect states,and the gap defect states of Ga interstitial or As vacancy lead to the rapid decrease of carrier concentration.Finally,the calculation method of carrier mobility of polar semiconductor is provided.By calculating the mobility of different defect states,the mobility of holes in Ga As is from 486 cm~2/V·s to 978 cm~2/V·s,and the electron mobility lies from9304 cm~2/V·s to 13241 cm~2/V·s.The electrical output power degradation of nuclear cells after irradiation is also calculated by considering the changes of carrier lifetime and minority carrier diffusion length,and under different defect states,it is in between 12%and 43%.In general,this study analyzes and explores the changes of energy band structure in the micro damage area,and evaluates the degradation of the macro electrical output performance of the converter after irradiation damage.It has guiding significance for selecting suitable semiconductor materials,improving the electrical output performance of nuclear battery and radiation hardening.Besides,the analysis of the electronic structure of mesoscopic defect states based on first principles is of great significance to the study of the influence of mesoscopic defect states on the properties of materials. |
PDF Full Text Request