Silicon Carbide (SiC) is regarded as a promising candidate for electronic devices used in harsh radiation environments (Rad-hard devices) such as in space, accelerator facilities and nuclear power plants [1-5]. In order to apply SiC to such rad-hard devices, we have to know the radiation response of the characteristics of SiC devices, because semiconductor devices show destructive and non-destructive malfunctions and/or degradation their characteristics due to irradiation. For radiation effects on semiconductor devices, three major effects, Single Event Effects (SEEs), Total Ionizing Dose (TID) effect, and Displacement Damage Dose (DDD) effects are known.
When charged particles such as heavy ions are irradiated into semiconductors, dense charge (electron-hole pairs) is generated in semiconductors along to the ion track. The malfunctions of electronic devices such as LSIs and power devices caused by charge generated by charged particles are called SEEs. The SEEs occur even by only one particle incidence, and there are both nondestructive (soft errors) and destructive (hard errors) SEE failures [6-8]. The soft errors arise if the amount of charge collected by devices is large enough to reverse or flip the data state of a memory cell, register, latch, or flip-flop. Since the soft errors are not destructive, the function of semiconductor devices can be recovered by writing new data to the bit and/or resetting of devices. For example, the Single Event Upset (SEU) and the Multiple Bit Upset (MBU) in a Static Random Access Memory (SRAM) and a Dynamic Random Access Memory (DRAM), the Single Event Functional Interrupt (SEFI) in Field Programmable Gate Array (FPGA) or DRAM control circuitry are known as the soft errors. Recently, the Single Event Transient (SET) arises as a serious issue for analog electronics and digital logic cells. In general, the SETs in analog electronics are referred to as ASETs, and those in digital combinatorial logic are referred to as DSETs. In contrast, the Single Event Latch-up (SEL), the Single Event Burnout (SEB), and the Single Event Gate Rupture (SEGR) in power electronic devices are known as the hard errors.
Electron-hole pairs are induced in insulator layers of Metal-Insulator-Semiconductor (MIS) structure devices, such as Metal-Oxide-Semiconductor (MOS) devices by irradiation, and as a result, charge trapped in insulator (oxide) and/or traps near the interface between oxide and semiconductor (interface traps) are generated. Since such charge trapped in insulator and interface traps act give harmful influence to transport properties of semiconductors, the electrical characteristics of MIS devices are degraded by their generation [9, 10]. For example, the shift of threshold voltage (
When energetic particles are irradiated into semiconductor crystals, atoms at lattice sites are scattered into non-lattice sites (knock-on effects). As a result, vacancies and interstitials are created in semiconductor crystals. This is the origin of the DDD effect. However, in reality, the structure of residual defects is not so simple and a wide variety of defects such as divacancies, vacancy clusters, and vacancy-impurity complexes exists in crystals because generated vacancies and interstitials thermally diffuse and finally they become stable defects. In general, such defects act as scattering/recombination centers to free carriers, and as a result, the electrical characteristics of semiconductors devices are degraded. In the case of the DDD effect, similar to the TID effect, the degradation of the characteristics of semiconductor devices becomes larger with increasing fluence of radiation. The degradation of the electrical performance of solar cells installed in space satellites is known as one of the examples of the DDD effect [11-14].
In this chapter, the effects of radiation on the electrical characteristics of SiC devices are described from the point of view of the TID effect and the SEEs.
2. Gamma-ray irradiation effects on SiC MOSFETs
Figure 1 shows the change in the subthreshold region of drain current (
According to Mcwhorter and Winokur , the density of charge trapped in gate oxide (Δ
Since the subthreshold curve between
where „post“ and „pre“ mean after and before irradiation, respectively.
In order to obtain the value of
The value of Δ
Figure 2 (a) shows the Δ
These behaviors indicate that both positively and negatively charges are generated in gate oxide for the H2 SiC MOSFETs by gamma-ray irradiation. It was reported from the change in capacitance – voltage characteristics of 6H-SiC MOS capacitors due to gamma-ray irradiation that negative and positive trapped charges were generated near SiO2/SiC interface and in oxide at 40 nm from the interface, respectively . Although the mechanism of H2-anneling effect on the gate oxide and the interface between oxide and SiC has not yet been clarified, since the initial value of
The values of Δ
The μch for Si MOSFETs is known to decrease with increasing absorbed dose . In order to confirm this for SiC MOSFETs, μch for the H2 SiC MOSFETs were plotted as a function of absorbed dose (Fig. 5). For comparison, the result reported for Si MOSFETs are also plotted in the figure . The μch for the H2 SiC MOSFETs does not change up to 20 kGy and the value decreases with increasing absorbed dose above 60 kGy. Then, the value of μch reduces to be 50 % of the initial value at 530 kGy. On the other hand, μch for the Si MOSFETs decreases with increasing absorbed dose and becomes 50 % of the initial value by irradiation at 10 kGy. Although the initial value of μch for Si MOSFETs (600 cm2/Vs) is much higher than the initial value of μch for the H2 SiC MOSFET (~ 50 cm2/Vs), the value for Si MOSFETs is assumed to be almost zero after irradiation at 100 kGy whereas the H2 SiC MOSFETs still keep 25 cm2/Vs of μch even after irradiation at 530 kGy. In addition, it is mentioned that the stability of their electrical performance against irradiation is also important for Rad-hard devices. Therefore, it can be concluded that SiC MOSFETs are quite tolerant against radiation in comparison with Si MOSFETs. For the degradation mechanism of μch, Ohshima et al. reported  that the relationship between the decrease of μch and Δ
Next, the effects of the surface morphology on μch of SiC MOSFETs irradiated with gamma-rays will be discussed. In this study, MOSFETs were fabricated on n-type 6H-SiC epitaxial layers using the same fabrication process except the procedures of high temperature annealing after implantation . Thus, although all samples were annealed at 1650°C for 3 min in an Ar atmosphere, the surface of one series of samples was covered with carbon films (C-coating) during the annealing to avoid the degradation of the surface morphology , and the other series of samples were annealed without the carbon coating (non-coating). After the annealing, the carbon films were removed by the oxidation at 800°C for 30 min in O2 gas. Gate oxide of both series of the MOSFETs were formed by pyrogenic oxidation (H2:O2 = 1:1) at 1100°C for 30 min. For the details of the fabrication process, please see Ref. . The initial values of μch for C-coating and non-coating SiC MOSFETs are 41 and 44 cm2/Vs, respectively. For the surface morphology, the values of root mean square (RMS) for the C-coating and non-coating SiC are obtained to be 0.67 and 1.36 nm, respectively, from AFM measurements, whereas the RMS was 0.25 nm before annealing.
Figures 6 (a) and (b) show μch and Δ
3. Radiation hardness of SiC devices
In this section, the change in the electrical characteristics of SiC transistors such as Static Induction Transistors (SITs), Metal-Semiconductor (MES) FETs and MOSFETs due to gamma-ray irradiation will be compared to Si MOS FETs from the point of view of the radiation hardness. Figure 7 shows Δ
Next, the change in the electrical characteristics of the SiC SITs by gamma-ray irradiation is expressed. The SiC SITs have an on-resistance of 0.15 Ω and a blocking voltage of 900 V at
The on-state characteristics were measured under
4. Charge induced in SiC diodes by Ion irradiation
Since destructive or/and non-destructive malfunctions called SEEs occurs in electronic devices by charge (electron-hole pairs) generated by charged particle incidence, especially heavy ions. The SEEs on semiconductor devices are one of the most major issues for space applications. On the other hand, for high energy physics using accelerators with high luminosity, such as J-PARC and Super-LHC, Rad-hard particle detectors are expected to be developed. For the development of Rad-hard particle detectors as well as Rad-hard devices for space applications, it is important to clarify the behavior of charge generated in devices by charged particle incidence. In a previous study , Nava et al. reported that the Charge Collection Efficiency (CCE) obtained from 4H-SiC Schottky diodes by alpha particle incidence was estimated to be 100 %. It was also reported that 4H-SiC Schottky diodes could detect X-rays from radio isotopes [31,32]. Besides, the neutron detection by SiC diodes was investigated previously [33, 34]. As for light ions and X-rays irradiation into SiC, relatively large number of studies has been already reported. On the other hand, from the point of view of SEEs, study of ion irradiation on electronic devices using heavy ions is important. In this section, charge induced in SiC diodes by heavy ion incidence is reviewed on the basis of our previous studies [35-40].
In order to obtain the information on charge induced in electronic devices, Ion Beam Induced Charge (IBIC) measurements is thought to be one of the useful methods. However, the decrease in collected charge during IBIC measurements should be considered for the accurate evaluation of charge induced by ion beams, since the device characteristics are degraded by radiation damage created in samples by ion incidence . Therefore, single-ion hit Transient Ion Beam Induced Current (TIBIC) was developed at JAERI Takasaki in order to realize the evaluation of ion-induced current with minimizing the influence of damage . Figure 10 shows the schematic set-up of the TIBIC system installed at JAEA Takasaki and the photo of the TIBIC system. The TIBIC collection system connects with a heavy ion microbeam line from the 3MV Tandem accelerator, and consists of a single event triggering system and a fast switch beam shutter system. The transient current signals induced by ions can be detected using a digital sampling oscilloscope (Tektronix 3 GHz TDS694C or 15 GHz TDS6154C). The details of the single ion hit TIBIC collection system are described in Ref. . Since the TIBIC system connects with a beam scanning system, spatial images of transient current signals can be obtained.
Figure 11 shows TIBIC signals obtained from 6H-SiC n+p diodes with applied bias of 30, 90 or 150 V. Si ions with 12 MeV were used as probe beams. In this study, the 6H-SiC n+p diodes with 100 - 300 μm diameters were fabricated on p-type substrates with p-type epitaxial layers (Al doping concentration between 8x1014 and 3.5x1015 /cm3). The n+ region was formed by three-fold implantation (60, 90, 140 keV) of phosphorus (P) ions at 800°C and subsequent annealing at 1650°C for 3 min in argon (Ar) atmosphere. The thickness and a mean P concentration of the implanted layer are ~100 nm and 5x1019 /cm3, respectively. During the annealing, the sample surface was covered with a carbon film to avoid the degradation of the surface morphology . The details of the diode fabrication process are described elsewhere . The peak height of the TIBIC signals increases with increasing applied bias, and the value becomes to 0.50 from 0.19 mA when applied bias increases to 150 from 30 V. The fall-time, which is defined as the time from 90 % to 10 % of the current transient, shorten with increasing applied reverse bias, and the value decreases to 0.48 from 0.98 ns when applied bias increases to 150 from 30 V. These results can be interpreted in terms of an increase of the electric field in the depletion layer due to increasing applied bias. It is also mentioned the leakage currents of the diodes were in order of 10-11 A at an applied reverse bias of 150 V, and no significant differences in
By the integration of a TIBIC signal, charge collected by a diode can be estimated. Charge collected by the 6H-SiC n+p diodes as a function of applied bias is shown in Fig. 12. In this study, Si ions with different energies were applied as probe beams, and the value of energy of Si ions are described in the figure. Charge collected by the diodes increases with increasing applied bias, and the value of collected charge saturates in a higher bias region. For example, the saturation is observed above 40 and 60 V for 15 and 18 MeV, respectively. Charge generated in the depletion region of a diode can be collected by its electric field (Drift component). On the other hand, charge generated in deeper than the depletion region diffuses, and only charge reaching the repletion region can be collected by a diode (Diffusion component) whereas some generated carriers recombine during diffusion. Thus, if the depletion region is shorter than the projection range of ions, the decrease in collected charge is observed due to the recombination of generated carriers during diffusion. Since ions with higher energy have a longer projection range, the results obtained in Fig. 12 can be qualitatively interpreted in terms of the drift and the diffusion components. However, in reality, since an extended drift region is temporarily created in a deeper region than the depletion region, the saturation of collected charge occurs even in the case that the depletion region is shorter than the ion projection range .
At a bias of 150V, the depletion region is estimated to be 7 μm, and this is longer than the ion projection range of Si ions at 18 MeV which is estimated to be 4.8 μm by a Monte Carlo simulation code, SRIM . Thus, at a bias of 150 V, all charge generated in the 6H-SiC diodes by Si ion incidence can be collected by the electric field in the depletion layer. The CCE for the 6H-SiC diodes is estimated from the value of charge collected at a bias of 150 V. Here, the value of CCE is defined as
where Qexp and Qideal are the value of charge experimentally obtained at 150 V and the ideal value of charge generated in SiC, respectively. The value of Qideal is obtained by the equation
In order to understand the degradation of the CCE due to not energy loss near the surface regions, the effect of ion species on the CCE was investigated. Figure 13 shows the relationship between ions species with the same energy (12 MeV) and the value of the CCE. The value of the CCE is obtained from the integration of TIBIC signals for the 6H-SiC n+p diodes at a bias of 150 V. The CCE for the diodes probed by O ions is estimated to be 90 %, and this value is the highest of all ion species in Fig. 13. With increasing atomic number, the value of the CCE decreases. The CCE of 42 % is observed by Au ion incidence. The degradation of the CCE for SiC diodes by Au ion incidence was also reported . Zajic et al. suggested that high density of e-h pairs is generated by heavy ions, and generated e-h pairs are easy to recombine in such dense plasma .
The carrier density generated in SiC, and the distributions of e-h pairs are calculated on the basis of Kobetich and Katz (KK) theorem . In this calculation, the KK model improved using empirical equations reported by Fageeha et al.  was applied since the KK model overestimates the density of e-h pairs at the core of the ion track. The calculated results of the density of e-h pairs generated in SiC by (Left) 12 MeV-O and (Right) -Au ion irradiation are shown in Fig. 14. In the case of 12 MeV-O ion incidence, the radius of the ion track at the sample surface and projection range of ions are estimated to be ≈ 40 nm and 5.2 μm, respectively. On the other hand, the ion track radius at the surface and the ion range for 12 MeV-Au ions are estimated to be ≈ 2 nm and 1.9 μm, respectively. Since the energy (12 MeV) is the same for both O and Au ions, the total number of e-h pairs generated in the ion track region is the same between O and Au ions. Thus, the density of e-h pairs in SiC irradiated with Au ions is much higher than that irradiated with O ions, and the estimated density of e-h pairs in SiC irradiated with 12 MeV-Au ions is a several orders of magnitude higher than that in SiC irradiated with 12 MeV-O ions. In such a high density of e-h pairs, the ambipolar effect occurs easily and the electric field temporarily weakens. As a result, the amount of the recombination between electrons and holes increases. For the dynamics of carriers generated in SiC by heavy ion incidence, please see Ref. . The result obtained in this study indicates that it is important to consider the decrease in the CCE for SiC particle detectors when heavy ions are detected. From the point of view of SEEs in SiC, the decrease in collected charge is thought to be one of the advantages for the development of Rad-Hard devices. The similar charge collection behaviours have been also obtained for SiC p+n diodes, although only results obtained from SiC n+p diodes were introduced in this article .
For the effects of ion incidence on MOS capacitors fabricated on SiC, it was reported that the peak amplitude of TIBIC signals decreased and the fall time increased with increasing number of incident ions [49-51]. Furthermore, the peak of TIBIC signals can be refreshed to its original value by applying a forward bias of + 1V to the gate electrode. From the measurement of the capacitance of SiC MOS capacitors during O ion irradiation, the value of capacitance was found to increase with increasing number of incident ions. This indicates that the depletion length of the MOS capacitors becomes shorten with increasing number of incident ions. Since large amounts of charge are induced by heavy ion incidence and some of them might flow to the interface between SiO2 and SiC, the degradation of TIBIC signals can be explained by a change in the net bias applied to the gate oxide due to the creation of the inversion region or/and charging up deep traps. The refreshment of TIBIC signals by applying a forward bias can be also interpreted in terms of releasing charge from the interface or/and deep traps. For the effects of heavy ion irradiation on 6H-SiC MOSFETs, Onoda et al. Reported from experimental results and their simulation using the Technology Computer Aided Design (TCAD)  that the charge collection behaviours were affected by drift, funnelling, diffusion, and recombination, and especially, the enhancement of transient currents was observed due to the parasitic bipolar action. It was also reported that the enhanced charge collection was observed for 4H-SiC MESFETs by heavy ion incidence . According to device simulations using the TCAD, it was concluded that the enhanced charge collection effect can be interpreted in terms of not only the bipolar action but also the channel modulation effects. For the DDD effect in SiC devices, it was reported that the value of the CCE for SiC n+p diodes and the majority carrier concentration in them decreased with increasing gamma-rays, electrons or protons and the damage factor of the CCE and the carrier removal rate can be scaled by Non Ionizing Energy Loss (NIEL) [53-55].
In order to develop Rad-hard devices based on SiC, the radiation response of SiC devices have to be understood. In this chapter, effects of gamma-rays and swift heavy ions on SiC devices were reviewed. Firstly, the gamma-ray irradiation effects on SiC MOSFETs were introduced, and the degradation of their characteristics was discussed on the basis of charge generated in gate oxide and interface traps by irradiation. Then, the radiation resistance of SiC transistors, MOSFETs, MESFETs and SITs was compared to Si transistors. SiC transistors showed higher radiation resistance than Si transistors, and SiC SITs could be operated up to 10 MGy. This indicates that SiC SITs have extremely high radiation tolerance from the point of view of TID effects. Charge generated in 6H-SiC n+p diodes by heavy ion incidence was evaluated using TIBIC. The signal peak of the transient current increased, and the fall-time decreased with increasing applied reverse bias. The high CCE values were observed when ions with relatively light mass such as O and Si ions were applied as probe ions. However, the CCE decreased with increasing atomic number, and the value reduced to approximately 40 % when 12 MeV-Au ions were applied as probe ions. From the calculation based on the modified KK model, it was found that the density of e-h pairs in SiC irradiated with heavy ions, such as Ni and Au, is much higher than that in SiC irradiated with O and Si ions. Therefore, the decrease in the CCE by the irradiation of ions with heavy mass was interpreted in terms of the recombination of e-h pairs in plasma.
This study of gamma-ray irradiation effects on SiC SIT was supported by the Strategic Promotion Program for Basic Nuclear Research by the Ministry of Education, Culture, Sports, Science and Technology of Japan. Also, the study of charge induced in SiC pn diodes and MOS capacitors by heavy ion incidence was partially supported by the Ministry and Education, Science, Sports and Culture, Grant-in-Aid for Scientific Research (B), 2006, 18360458 and (B), 2009, 21360471, respectively.