Studies Of Irradiation Effects On 6H-SiC

In this thesis, irradiation-induced defects in 6H-SiC samples after neutron andelectron irradiation having different energies have been studied using lowtemperature photoluminescence (LTPL) and deep level transient spectroscopy (DLTS)techniques. The studies include electron energy dependence of formation of defectscreated and their annealing behaviors. Based on their annealing process, we cananalyze the evolvements of the defects such as transfer, disaggregation andrecombination behaviors during annealing. Furthermore, their influence on thecarrier's transport has been discussed.Silicon carbide (SIC) has unique physical properties for fabricatinghigh-frequency, high-power, high-temperature and irradiation resistant electrondevices, which are expected to operate in the radiation environment of military andastronautic domains. Further more, they can be used to fabricate optoelectricaldevices. Ion implantation is the only available method to realize selective doping dueto the low diffusivity of impurities at room temperature, after that, high temperatureannealing is needed to electrically active the dopants. However, it will inevitablyproduce unwanted defects such as radiation-induced defects remaining in theoperational area of the device even after high temperature annealing procedure.Further, secondary defects can be introduced during the other heat treatments of SiCor SiC devices' operating in the high-radiation environments. We can use an electronbeam to simulate the radiation effect ofγ-ray in 6H-SiC, and that electron irradiation will generate some simple defects such as monovacancy, divacancy, vacancycomplex with other impurities, and so on. We can find the mechanisms of the defectgeneration and dissociation by studying the electron irradiation-induced defects,which are very useful to fabricate high performance irradiation resistant SiC devices.In this thesis, we have solved the following problems by studying neutron/electronwith different energies irradiation-induced defects in the epitaxial 6H-SiC:Photoluminescence studies on the irradiated n-type 6H-SiC:1. In the previous reported studies, only the D₁-center was observed sinceelectron or ion-implantation usually accompanied by high temperature annealing inthe SiC materials, and it was considered to be a kind of primary defect, but we thinkD₁-center possibly exists primary since its low temperature annealing features areunknown. The irradiation-induced primary defects are very important since they willinfluence the electrical properties of SiC device operating in radiation environments.So we focused on the low temperature annealing properties of the irradiation-induceddefects thus identifying whether Dl-center had primary defect.Three new sharp lines at 478.6 nm/483.3 nm/486.1nm were observed in theas-irradiated n-type 6H-SiC samples after neutron, 0.5MeV and 1.7MeV electronsirradiations, respectively. After annealing at 350℃, the defect lines S₁/S₂/S₃ becameweak and completely disappeared at 500℃. These lines were not seen in theun-irradiated sample in this range, so it indicated they were a kind ofirradiation-induced primary defects.2. The well-known D₁-center(L₁/L₂/L₃) located at 472.4/476.9/482.5nmemerged after a higher temperature 700℃annealing, which strongly indicated itbelong to secondary defect, heat treatments showed that its intensity increased withtemperature in the range 700℃-1100℃, and also it can withstand high temperatureannealing at 1600℃. It was observed in all polytypes after various kinds of particlesbombardment and act as carrier traps. Maybe there exist complicated transition statesin the formation of D₁ from its primary defects, so it is very important to study thegeneration and annealing behaviors of D₁-center, which has potential technologyvalue, for instance, design high temperature annealing parameter of ion implantation and to the greatest extent to reduce the production of D₁-center.3. In the reported studies, D₁-center was considered to be the same defects withthe E₁/E2 observed in deep level transient spectroscopy (DLTS), according to ourexperimental results, the generation behaviors of D₁-center is different from that ofE₁/E₂, so it implied that they are not originated from the same defects: D₁-center is akind of secondary defect but E₁/E₂ are irradiation-induced primary defects. What'smore, the primary defects of D₁-center and E₁/E₂ have different annealing behaviorsand they are not related to the same defects.Deep level transient spectroscopy studies on the electron-irradiated p-type6H-SiC:1. In our previous paper, DLTS measurement was performed to studyelectron-irradiation-induced defects in p-type 6H-SiC, two deep levels, H1 and H2were observed in 1.7MeV as-irradiated sample, but the detailed microstructure of H1and H2 was not given. In this work, P-type 6H-SiC samples were irradiated withelectrons having energies of 0.3MeV, 0.4MeV, 0.5MeV and 1.7MeV, deep leveltransient spectroscopy (DLTS) measurements were employed to study the defectscreated. For E_e≥0.3MeV, deep level H1 was detected, while E_e≥0.4MeV, defect H2appeared. Also, they have different thermal stability,～500℃for H1 and～250℃forH2, respectively. By considering the minimum energy required to displace the atomsin SiC lattice, analyzing the generation of H1 and H2, and studying their annealingbehavior, it was suggested H1 and H2 were related to different carbon vacancydefect.2. Thermal annealing behavior studies of electron-irradiated p-type 6H-SiC wereperformed in the range of 350℃-1600℃, and secondary defects can be observed. Tothe 0.4MeV and 0.5MeV sample, the secondary defects G generated at 500℃wereannealed at 900℃annealing, after 700℃annealing, the secondary defects I and Kwere observed and they were disappeared after 1400℃annealing, and newsecondary M appeared after that. But to the 1.7MeV sample, two secondary defectsK and C1 were generated after annealing at 700℃and 900℃, respectively. Fromabove, the annealing of p-type 6H-SiC is much different from that of n-type 6H-SiC. This is the first time to systematically study the annealing behaviors of theirradiation-induced defects irradiated with different electron energies in p-type6H-SiC, and experimentally illuminate the complexity of secondary defects'generation. In the process of fabrication n-channel SiC MOSFET devices, ionimplantation is required to create source and drain area, after that, high temperatureannealing will be performed to electrically active the dopants implanted, so far onlyless studies on this field were reported, so it is necessary to study the irradiatedp-type SiC in the point of view that the design of ion implantation technology in thefabrication of CMOS devices. Further more, the impurities in the p-type 6H-SiCintroduce relative deep level in the band gap (AI: E_A=0.21～0.27eV; B: E_A=0.30～0.35eV), the electrically active rate is consequently very low. So the studies on theannealing behaviors of irradiation-induced defects in p-type 6H-SiC provideimportant information on ionimplantation.