基于4H-SiC肖特基势垒二极管的γ射线探测器

γ-ray detector based on n-type 4H-SiC Schottky barrier diode

针对极端环境下耐高温和耐辐照半导体核探测器的研制需求,采用外延层厚度为100μm的4H碳化硅(4H-SiC)制备成肖特基二极管探测器,研究了该探测器对^(241)Am源γ射线的能谱响应.采用磁控溅射金属Ni制备了肖特基二极管的欧姆接触和肖特基接触,利用室温电流-电压和电容-电压测试研究了二极管的电学特性.欧姆特性测试表明,1050°C退火时,欧姆接触特性最好.从正向电流-电压曲线得出二极管肖特基势垒高度为1.617 eV,理想因子为1.127,表明探测器具备良好的热电子发射特性.从电容-电压曲线获得二极管外延层净掺杂浓度为2.903×10^(14)cm^(-3),并研究了自由载流子浓度在外延层中的纵向分布.在反向偏压为500 V时,二极管的漏电流只有2.11 nA,具有较高的击穿电压.测得在-300 V条件下,SiC二极管探测器对能量为59.5 keV的γ射线的能量分辨率为9.49%(5.65 keV).

Silicon carbide（SiC） is a wide band-gap, high-temperature-resistant, and radiation-resistant semiconducting material, which can be used as a radiation detector material in harsh environments such as high radiation background and high temperatures. Schottky barrier diode radiation detectors are fabricated using 100 μm-thick n-type 4H-SiC epitaxial layers for low energy γ-ray detection. The spectrum responses of 4H-SiC Schottky barrier detectors are investigated by irradiation of γ-ray from ^241Am source. Schottky diodes are prepared by magnetron-sputtering 100 nm-thick nickel on epitaxial surface（Si face） to obtain Schottky contact and Ni/Au on substrate surface（C face） to obtain Ohmic back contact, respectively. Room temperature current-voltage（I-V） and capacitance-voltage（C-V） curves are measured to study the properties of Schottky diodes. Ohmic characteristic measurement shows that the Ohmic contact is formed after annealing in a temperature range of 900–1050℃, and the lowest specific contact resistivity of 2.55 × 10^-5Ω·cm^2 is obtained after annealing at 1050℃. The forward I-V curve reveals that the Schottky barrier height and the ideality factor are 1.617 e V and 1.127, respectively, indicating that the main current transportation process is the thermal electron emission. From the C-V curve, besides the net dopant concentration being inferred to be 2.903 × 10^14 cm^-3,the profile of the free carrier concentration in epitaxial layer is also studied. A comparision of the reverse I-V curves of Si C Schottky diodes with different epitaxial layer thickness shows that the diode with 100 μm-thick epitaxial layer has a constant reverse leakage current when the bias voltage is less than 400 V, showing good rectification characteristics.By applying a reverse bias of 500 V, the diode has a leakage current of 2.11 nA, exhibiting a relatively high breakdown voltage. The depletion layer width of Si C detector is calculated to be 94.4 μm at 500 V, indicating that the epitaxial layer is almost fully depleted. The signal of Si C detector through preamplifier displays a relatively low amplitude pulse（15 mV）. A typical γ-ray spectrum response from Si C detector shows 9.49%（5.65 keV） energy resolution for 59.5 keV with a reverse bias of 300 V. The potential causes of poor count rate and energy resolution of fabricated detectors are analyzed in this article. The lower count rate is mainly caused by the narrow depletion layer, resulting in fewer photons deposited in sensitive region which can generate carriers. The poor energy resolution of SiC detector can be attributed to the electronic noise of read-out circuit, the pre-match amplifier circuit for detector needs to be improved, in addition,the extra defects existing in detector caused by increasing thickness of epitaxial layer can also deteriorate the detector performance.