Evaluation of Optical and Electrical Properties on Neutron-Irradiated SiC Semiconductors

利用光谱学和电学测量方法，研究了反应堆中子辐照下碳化硅（4H和6H-SiC）单晶的光学和电学性质。分别在京都大学反应堆（kurl - ltl）的低温辐照环路设施和东京大学快中子反应堆（YAYOI）的室温下进行辐照，在辐照样品中观察到780mm处有较宽的吸收带。780mm带的起源归因于硅空位型缺陷（V_<si>型中心），因为它与由硅空位和捕获电子组成的顺磁缺陷中心具有相似的退火行为。在不同温度下辐照的SiC晶体中，V_<si>型中心的生产效率在200K左右达到最大值。V_<si>型中心的辐照温度依赖性特征与氧化物或金属在20K ~ 400K具有负陡梯度的辐照温度依赖性特征明显不同。另一方面，观察了中子辐照SiC样品的电阻率变化。辐照样品的电阻率随快中子通量的增大而增大。电阻率的增大可以用载流子密度的减小来解释。从高温退火行为可以看出，该材料的电阻率和载流子迁移率有两个退火阶段（350K和500K）。800K退火后，两者均恢复约40%，载流子密度未见变化。此外，还研究了中子嬗变掺杂SiC的影响。结果表明，由^<29>Si(n, γ)^<30>Si反应产生的^<30>P的一部分可以有效地充当电子供体。

Optical and electrical properties of silicon carbide (4H and 6H-SiC) single crystals irradiated with reactor neutrons have been evaluated by the optical spectroscopy and electric measurement methods. The irradiations were carried out at some temperatures in the low temperature irradiation loop facility of the Kyoto University Reactor (KUR-LTL) and at room temperature in the fast neutron reactor of the Tokyo university (YAYOI), In the irradiated samples, a broad absorption band at 780mm was observed. The origin of the 780mm band was attributed to the defect of silicon vacancy type (V_<si>-type center) because it has similar annealing behavior with the paramagnetic defect center consisting of a Si vacancy and a trapped electron.The production efficiency of the V_<si>-type center in SiC crystals irradiated at several temperatures by the KUR-LTL has a maximum about 200K.The characterization of the irradiation temperature dependence of the V_<si>-type center is evidently different from that of oxides or metals which has negative steep gradient from 20K to 400K.On the other hand, the change of electric resistivity in the neutron-irradiated SiC samples was observed. The resistivity for as-irradiated samples increases with fast neutron fluence. The increase of resistivity can be explained by the decreases of the carrier density. From elevated temperature annealing behaviors, it has been clarified that the electric resistivity and the carrier mobility have two annealing stages (350K 500K). By the 800K annealing, both of them recovered about 40%, whereas the change of the carrier density did not found.Also, the effect of the neutron transmutation doping of SiC has been studied. The result shows that a part of ^<30>P which is produced by ^<29>Si(n, gamma)^<30>Si reaction, can effectually act as the electron donor.