| Compared with traditional semiconductors such as Si and Ga As,the third generation of semiconductor Si C has large band gap,high thermal conductivity,high carrier saturation transfer speed,high critical breakdown electric field intensity,and excellent physical-chemical stability.It leads to the application of Si C-based electronic devices more suitable and superior in high power,high frequency,high temperature and other demanding environments.However,due to the large gap width and complex surface state of Si C,it is difficult for Si C to form good Ohmic contact with metal electrode,which greatly affects the working efficiency of Si C-based devices.Therefore,the preparation of traditional Si C-based devices requires the injection of heavy doping on the Si C surface and the formation of good Si C-metal Ohmic contact with the metal by high temperature annealing.However,the Si C heavy doping process is difficult and expensive,and the high-temperature treatment is easy to generate defects on the Si C surface/interface,which can easily affect the stability of Si C-metal Ohmic contact.Due to the instantaneous high energy density output and ultra-fast energy injection effect of ultraviolet laser,and the high absorption rate of ultraviolet wavelength laser on Si C,researches on surface modification of Si C using ultraviolet laser irradiation have been studied in recent years.However,up to now,it is still not able to properly regulate Si C surface states to reach device application level,and the law and mechanism of laser action on the Si C surface state and its defect behavior still need to be systematically studied and clarified,so as to obtain effective information to improve the performance of Si C based devices by purposefully regulating the electrical contact properties of Si C surface.Therefore,the study of laser-induced Si C surface state change and its regulation law and mechanism has important scientific significance and application value,and is the basic starting point of this paper.According to the high energy photon(6.5 e V)characteristics of Ar F excimer laser(wavelength 193 nm),this thesis conducts the research on the regulation of laser irradiation on the surface induced defect state of 4h-sic and the electric contact modification.The Ar F excimer laser irradiation in air and vacuum induced O,N,C atomic defects on 4H-Si C was systematically studied using transmission electron microscope(TEM),conductive atomic force microscopy(C-AFM).The distribution characteristics and formation mechanism of O,N,C atom defects under different environmental conditions were analyzed using X-ray photoelectron spectroscopy(XPS),Fourier transform infrared spectroscopy(FTIR).Aiming at the limitation of metal/Si C Ohmic contact at present,the influence of O,N and C defect states on metal/4H-Si C barrier was further studied,which significantly improved the electrical contact characteristics of 4H-Si C surface.The Ohmic contact on the surface realized the improved without the traditional annealing process.Based on density functional theory(DFT),the contribution mechanism of laser-induced O,N,C and other atomic defect states to surface electrical contact modification is explained,and the theoretical basis for the tunable preparation of high-performance Si C-based devices is enriched.It is found that the Schottky barrier of 4H-Si C surface irradiated by Ar F excimer laser can be regulated between 0.38±0.05 e V and 1.82±0.1 e V.Since the photon energy of the Ar F excimer laser is 6.5 e V,which is much higher than the bond energy of the Si-C bond(3.21 e V),the photochemical reaction caused by the bond breaking will occurs when the accumulated energy absorbed by the 4H-Si C surface exceeds the energy required for the Si-C bond and the vibration energy required for the fracture.In the air,most Si atoms in the plasma generated by Ar F excimer laser irradiation will be deposited on the surface of 4H-Si C,forming a metastable non-chemical ratio and Si O_x/Si texture with O vacancy.The induced C atoms bind to the N atoms in the air then the graphene layer doped with Pyridinic-N and Pyrrolic-N was generated.The N-doped graphene's Fermi level conduction band is offset by about 0.4 e V,which reduces the barrier height of the metal/4H-Si C contact.Therefore,it was confirmed that the combined effects of Si O_x/Si generated by laser irradiation,resulting in the thinning of contact barrier width,and N-doped multilayer graphene,resulting in the reduction of contact barrier height,led to the modification of 4H-Si C surface electrical contact induced by Ar F excimer laser irradiation in the air.For the laser irradiation experiment in vacuum,the generation and distribution characteristics of single vacancy(SV),double vacancy(DV),multi-vacancy(MV)and various C atom defects induced by laser irradiation,as well as the lattice distortion of stone-wales(SW)and other C defect states were systematically analyzed.The formation energy of SW(55-77)is about 5 e V,which belongs to the C atomic defect with small formation energy.The photon energy(6.5 e V)of the 193 nm wavelength Ar F laser can directly meet the energy required for defect formation.However,the formation energies of C atomic defects such as SV(5-9),DV(5-8-5),MV(7-55-7)were all greater than6.5ev.At this time,the multi-pulse irradiation(>1000 times)of Ar F excimer laser on the 4H-Si C surface formed a non-equilibrium process of energy injection,and the existence of such non-equilibrium state was conducive to the emergence of vacancy or amorphous.C atomic defects such as SV(5-9),DV(5-8-5)and MV(7-55-7)were induced by 193 nm deep ultraviolet laser.To explore the formation mechanism of 4H-Si C surface C atomic defect state induced by Ar F excimer laser,the 4H-Si C surface defect state can be purposefully optimized to improve the surface function.Further study shows that different C atomic defect states have different effects on4H-Si C electrical contact modification.For the defects of SV(5-9)and MV(7-55-7),the loss of C atom leads to the increase of dangling bonds,which increases the local density of states of electrons near the Fermi energy level.Thus,the probability of electron-hole recombination increases,resulting in the weakening of the electrical transmission performance between the metal and 4H-Si C in the concentration area of these defect states.SW(55-77)or DV(5-8-5)defects are caused by displacement of C atoms lattice distortion and the length of C-C bond become shorter,resulting higher charge density difference between C atoms.Therefore,compared to the SV(5-9)and MV(7-55-7)defect,SW(55-77)or DV(5-8-5)defect mode between clusters of metal and 4H-Si C electrical transmission performance is better.To clarify the regulation mechanism of the influence of different C atomic defect states on the 4H-Si C electrical transmission characteristics is of guiding significance for the study of laser irradiation on the interface contact characteristics of metal/Si C.In this thesis,through the systematical study of Ar F excimer laser irradiation in air and vacuum for the tuning of 4H-Si C surface state,the distribution characteristics,forming and tuning mechanism of laser induced O,N,C atomic defects on 4H-Si C in different atmosphere condition were identified,and change progress of surface electric contact on 4H-Si C caused by the defects was tested and analyzed,which realizes the tunable electric contact 4H-Si C surface modification.The study of the physical process and mechanism provides a meaningful experimental basis and theoretical reference for further research on Si C modification by laser irradiation and development of Si C-based high-performance optoelectronic devices. |
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