IBIC analysis of SiC detectors developed for fusion applications

Highlights

• Radiation hardness of SiC diodes is studied for the detection of 3.5 MeV He ions.

• The CCE and energy resolution are not degraded for fluences up to 1 × 10¹¹ cm^-2.

• Degradation of the spectrometric properties is observed at fluences ≥ 3 × 10¹¹ cm².

• Minority carrier lifetime strongly deteriorate with increasing particle fluence.

Abstract

In this work, we consider a 4H-SiC detector as a plasma diagnostic system for the detection of fusion-born alpha particles in future nuclear fusion reactors. A nuclear microprobe was used to locally irradiate micrometer-sized regions of the detector with 3.5 MeV He ions to fluences from 5 × 10⁹ to 5 × 10¹¹ cm^-2. Ion Beam Induced Charge (IBIC) microscopy was employed to study its degradation in Charge Collection Efficiency (CCE) and energy resolution after irradiation. At high reverse-bias voltages, both parameters remain practically unaffected for fluences up to 1 × 10¹¹ cm^-2, while a significant deterioration of the spectroscopic performance was observed above 3 × 10¹¹ cm^-2. A theoretical drift-diffusion model, in combination with Shockley-Read-Hall recombination statistics, was used to obtain the holes lifetime from the fitting of the experimental CCE values measured at different reverse voltages. Holes lifetime was found to strongly decrease with increasing particle fluence, changing from 57 ns in pristine detectors to 0.2 ns after irradiation with a fluence of 1 × 10¹¹ cm^-2.

Introduction

In nuclear fusion plasma devices, fast (i.e. suprathermal) ions generated by heating systems and fusion-born alpha-particles must be kept well confined until they transfer their energy to the background plasma. This is especially important in large future fusion devices such as ITER (International Thermonuclear Experimental Reactor) and DEMO (DEMOnstration Power Station), where even a loss of a small fraction of energetic ions must be prevented (Fasoliet al., 2007). In ITER, the monitoring of escaping 3.5 MeV alphas produced in the deuterium-tritium (D-T) reaction is still a requirement of the outmost importance (Fasoliet al., 2007; Garcia-Munozet al., 2016). In present fusion devices, based on magnetic confinement of deuterium-deuterium (D-D) plasmas, the main diagnostic used to study the velocity-space of the escaping ions is the scintillator-based Fast Ion Loss Detector (FILD) (Garcia-Munozet al., 2009). Although FILD systems are currently used on virtually all large fusion devices worldwide, the foreseen harsh operation conditions of ITER (high neutron, gamma and heat fluxes) (Fasoliet al., 2007) will compromise, or even invalidate, the use of some of the diagnostic and measurement systems that are employed today. For example, if employing FILD, the temperature of the inner side of the probe head, located near the plasma edge, should not exceed the quenching temperature of the scintillator material, which is ~350 °C for most of the relevant scintillator materials (Rodríguez-Ramos et al., 2017).

An attractive alternative for lost-alpha particle monitoring in ITER would be the use of a semiconductor material as the active component of the probe head. Semiconductor-based detectors provide an intrinsic energy resolution by pulse height analysis, which implies that they have to be used in counting mode. They could have an additional advantage from the integration point of view. Indeed, a transmission line for the light delivery from the detection point to the port plug, in the case of a scintillator-based detector, may occupy more space than the electrical signal delivery by cables, in the case of a semiconductor-based system. This would allow installing a larger number of detectors in each poloidal cross section.

Due to its wide bandgap (3.27 eV for the 4H polytype) and large thermal conductivity (3.7 W/cm·K), silicon carbide (SiC) is one of the most attractive semiconductor materials for the development of, e.g., high operating-temperature electronic devices (Palmour et al., 1987). Moreover, because of its high saturated drift velocity (2 × 10⁷ cm/s) and high atomic displacement energy (E_d(Si) = 35 eV, E_d(C) = 22 eV), SiC devices have demonstrated much higher radiation tolerance compared to silicon (Ohshimaet al., 2012). For instance, during gamma-ray irradiation, no significant change in the electrical characteristics of SiC transistors was observed up to 10⁵ Gy, whereas the Si MOSFETs have shown clear degradation (Ohshimaet al., 2012). On the other hand, the decrease in Charge Collection Efficiency (CCE) during irradiation with a 17 MeV proton beam was considerably larger for Si detectors in comparison to SiC Schottky barrier diodes (SBD) (Garcia Lopez et al., 2016). A comprehensive review on the use of SiC as radiation detector can be found in (Nava et al., 2008a).

The behavior of 4H-SiC SBDs against irradiation with alpha particles has been recently studied by Pastuovic et al. (2015). In that work, the detectors were bombarded with 2 and 4 MeV He ions (with the detectors unbiased) and the deep traps created were characterized using Deep Level Transient Spectroscopy (DLTS). A significant degradation of the CCE, studied by the Ion Beam Induced Charge (IBIC) technique (Vittone et al., 2016), was observed for He fluences above 10¹¹ cm^-2.

In this paper, we have extended the previous studies, focusing on the possible use of a 4H-SiC pn junction diode (PND) as a plasma diagnostic system for the detection of alpha-born particles, which will be the main product of the D-T fusion reaction in the future ITER reactor. In order to assess the actual response of the detector in realistic operation conditions, the PND was submitted to 3.5 MeV He bombardment, with the detector biased at its usual polarization voltage. In the same way, IBIC analysis was carried out using also 3.5 MeV He to evaluate the degradation of the spectrometric and transport properties of the PND, such as CCE, energy resolution, and the minority-carriers diffusion length. In this preliminary study, both irradiation and the IBIC measurements were performed at room temperature, which has permitted the validation of the device and the employed methodology. Further analysis at high temperature is being performed presently and will be reported in a forthcoming paper.