**4. Degradation of SiC Performance under the Action of Radiation**

When a charged particle is decelerated in the semiconductor matrix, the released energy can shift the lattice atoms away from the equilibrium position. This yields the so-called primary radiation defects (Frenkel pairs), vacancies in the lattice and interstitial atoms. Most of formed defects recombine, and the rest of them create levels (deep centers) in the band energy gap of a semiconductor. Also possible is the interaction of primary defects with each other and with impurity atoms to give secondary radiation defects. As a rule, the secondary radiation defects are formed upon an increase in temperature, which is accompanied by the annealing out of the remaining primary defects.

– As the irradiation dose increases, radiation defects gradually accumulate, which causes degradation of a semiconductor device, for example, see [36–38]. In SiC pn structures, as well as in pn structures based on other semiconductor materials, irradiation leads to the following effects.


Figure 4 shows dependences of the quantum efficiency of a SiC UV photodetector before and after the irradiation with heavy ions. The decrease in the quantum efficiency is due to that in the carrier diffusion length.

Figure 5 shows how the conductivity of Schottky diode bases decreases upon irradiation with protons and electrons. The formation of compensating acceptor levels leads to a 6–7 orders of magnitude decrease in the carrier concentration in the base region. In this case, the carrier mobility decreases only slightly.

−2 **Figure 4.** Spectral dependences of the quantum efficiency of a 4H-SiC photodetector with Schottky barriers: (black) initial sample and (red) sample irradiated with 167 MeV Xe ions at a fluence of 6 × 10 9 cm−<sup>2</sup> . Room temperature. –

**Figure 5.** Base conductivity of a Schottky diode (600 V) after (1) protons and (2) electron irradiation. Different symbols correspond to different diodes from the same batch.

−1 – −1 Figure 6 shows current-voltage characteristics of a Schottky diode with breakdown voltage of 1200 V after the proton irradiation. The irradiation affects only slightly the voltage dependence of the forward current in the exponential area of the current-voltage characteristic. The irradiation in the pre-exponential (currents of 10 <sup>−</sup>12–10 <sup>−</sup><sup>14</sup> A) and post-exponential (high currents) areas drastically affects the current-voltage characteristic. At high currents, the base resistance increases due to the decrease in the free-carrier concentration. Leakage currents grow at low currents.

−1 –

−1

**Figure 6.** Forward current-voltage characteristic of a Schottky diode (1200 V class) at various doses of irradiation with 15 MeV protons [36].

The reverse current-voltage characteristics of Schottky diodes before and after the proton irradiation were investigated in [33]. It was shown that the leakage currents decrease at low reverse voltages, which is apparently due to the total increase in the resistance of the structure. At high reverse voltages, the reverse currents do increase, which can be attributed to the appearance in the space-charge layer of deep centers (radiation defects) via which carriers are generated.

Somewhat more complicated is the result of irradiation of SiC MOSFETs (a typical structure of such a device is shown in Figure 7). First, as in the case of a pn structure and Schottky diode, the free-carrier concentration decreases and the resistance of the drift region grows. Second, the devices have a subgate insulator layer (SiO2) in which the charge state of traps changes under irradiation. This may lead to an increase in the output current of a transistor at small irradiation doses. Both of these effects are well represented in the current-voltage characteristics presented in Figure 8.

μ μ **Figure 7.** Cross-section of an elementary cell of a 4H-SiC MOSFET. The gate length Lg is 0.5 µm, the oxide thickness d is 60 nm, and the drift (blocking) layer thickness Hd is 9 µm [37].

SiC MOSFET. The gate length Lg is 0.5 μm, the

oxide thickness d is 60 nm, and the drift (blocking) layer thickness Hd is 9 μm [3

**Figure 8.** Output characteristics Id(Vd) of a SiC MOSFETs (1.2 kV class) under study at various irradiation doses. The gate voltage Vg = 5 V [37].

#### **5. Comparison of the Radiation Hardnesses of Si and SiC**

Since silicon carbide is often viewed as a possible replacement for silicon in power devices, it is interesting and useful to compare the radiation hardness of the two materials. In our opinion, two approaches to such a comparison are possible.

First, two SiC- and Si-based diodes with the same breakdown voltage can be compared

$$\mathbf{U}\_{\text{brSi}} = \mathbf{U}\_{\text{brSiC}} = > (\mathbf{E}\_{\text{crSi}} \cdot \mathbf{W}\_{\text{Si}}) / 2 = (\mathbf{E}\_{\text{crSiC}} \cdot \mathbf{W}\_{\text{SiC}}) / 2 = > \mathbf{W}\_{\text{cr}} = \mathbf{W}\_{\text{Si}} \cdot \mathbf{E}\_{\text{crSiC}} / \mathbf{E}\_{\text{crSi}} \tag{10}$$

crSiС Here, Ubr is the breakdown voltage, Ecr the critical electric field, and W the spacecharge layer thickness at Ubr .

≈ 10, we obtain, with consideration for the fact that W ≈ √N<sup>d</sup> − N<sup>a</sup> (d−a)Si (d−a)SiC − Because the critical field in silicon carbide exceeds by an order of magnitude that in silicon, EcrSiC/EcrSi ≈ 10, we obtain, with consideration for the fact that W ≈ <sup>√</sup>N<sup>d</sup> <sup>−</sup> <sup>N</sup><sup>a</sup> , N(d−a)Si = 100 N(d−a)SiC, where Nd−<sup>a</sup> is the concentration of the uncompensated impurity in the base.

Thus, at the same breakdown voltage, the SiC diode is doped to a level exceeding by two orders of magnitude that for the Si diode. Consequently, even at equal values of Vd, the compensation of SiC diodes requires a 100 times higher irradiation doses, compared with Si diodes.

Second, the radiation hardness of SiC and Si diodes with the same base thickness can be compared [39]. This is important for fabrication of charged-particle detectors, in which the applied reverse voltage is limited and the maximum thickness of the space-charge layer should be obtained. In this case, the carrier-removal rates are directly compared.

It can be seen in Table 1 that the value of V<sup>d</sup> for SiC is only two times smaller than for Si. Because the energy gap E<sup>g</sup> of SiC is nearly three times that for silicon, a question arises why the difference between the values of V<sup>d</sup> for these two materials is so insignificant.

– ≥1200 Table 2 presents the results of an analysis of the annealing-out of radiation defects in 4H and 6H silicon carbide irradiated with various kinds of ions. It can be seen that there are two characteristic temperature ranges in which this annealing occurs: 200–800 ◦C and ≥1200 ◦C. Such a situation is also characteristic of other semiconducting materials. In the first stage of annealing, most of the primary radiation defects recombine, with the remaining forming substantially more temperature-resistant complexes, which are annealed out at significantly higher temperatures. However, the position of these annealing stages along the temperature scale depends on the properties of a semiconductor, including its energy gap.


**Table 2.** Annealing temperatures of radiation defects in SiC after various kinds of irradiation [38].

e stands for irradiation with electrons; p, for irradiation with protons; and He, for irradiation with helium nuclei.

Figure 9 schematically demonstrates how the concentration of radiation defects (Rd) in silicon and silicon carbide varies with temperature.

**Figure 9.** Schematic comparison of how the radiation defects are annealed-out in SiC and Si [38].

The figure shows that, at room temperature, these two materials are in different physical states with respect to the annealing-out of radiation defects. For silicon, the primary annealing stage has already been completed, whereas for SiC it has not yet begun. Thus, even if the concentration of introduced radiation defects was lower immediately after the irradiation (the irradiation temperature is conditionally 0 K), the concentration of defects in silicon when heated to room temperature became lower than that in SiC.

This can explain such a small difference between V<sup>d</sup> in SiC and Si and gives impetus to a desire to verify this assumption and irradiate silicon carbide at elevated temperatures.

#### **6. Irradiation of SiC at Elevated Temperatures**

– Previous experiments with III–V materials have demonstrated that the irradiation temperature can cardinally change the radiation hardness of materials and devices [46]. We examined, for the first time, the influence exerted by the electron irradiation temperature on high-power (blocking voltage 1700 V, working current 10 A) 4H-SiC Schottky diodes within the range 23–500 ◦C [47–49].

To perform high-temperature irradiations, we designed and constructed a special target chamber. This chamber enabled us to work with irradiations in air at temperatures ranging from room temperature to 600 ◦C. The accuracy of maintaining the sample temperature during irradiation was ±5 ◦C. The heating rate was maintained at 0.5 deg/s and the cooling rate was about 0.25 deg/s.

– –

It was found that, at comparatively small values of *Φ* ≈ 10 16 cm−<sup>2</sup> , raising the irradiation temperature from room temperature to 300 ◦C affects, comparatively slightly, the electron removal rate. With increasing dose, the difference between the base resistivities upon irradiation at room and elevated temperatures monotonically grows and exceeds three orders of magnitude at *Φ* ≈ 6 × 10 16 cm−<sup>2</sup> (Figure 10). *Φ* ≈ 10 −2 *Φ* ≈ 6 × −2

– *Φ* ≈ 6 × −2 the base resistivity ρ depends on the inverse irradiation tempera **Figure 10.** Forward current–voltage characteristics of diodes upon their irradiation with 0.9 eV electrons at three different irradiation temperatures *T*<sup>i</sup> . the dose *Φ* ≈ 6 × 10 16 cm−<sup>2</sup> . The inset shows how the base resistivity ρ depends on the inverse irradiation temperature [47].

We also examined the effect of a high-temperature irradiation with 15 MeV protons on parameters of high-voltage 4H-SiC Schottky diodes at doses in the range from 7 × 10 13 to 2 × 10 14 cm−<sup>2</sup> .

−2 −2 ≈ 0.35 ≈ 0.8 V. At the same reference forward voltage U = 2 V, the decrease in up to U ≥ 2 V. diodes, the forward current I at the reference forward voltage U = 2 V is I ≈ 12 A (see After the irradiation with a dose of 10 14 cm−<sup>2</sup> at room temperature, the forward current at a forward voltage U = 2 V decreases by ~10 orders of magnitude (Figure 11). In this case, the cutoff voltage Uc, equal to ~0.6 V in unirradiated devices, decreases to U<sup>c</sup> ≈ 0.35 V. By contrast, irradiation with the same dose at a temperature of 500 ◦C results in that U<sup>c</sup> increases to U<sup>c</sup> ≈ 0.8 V. At the same reference forward voltage U = 2 V, the decrease in current, as compared with the value for unirradiated devices, was ~4 orders of magnitude. In the whole range of doses and irradiation temperatures under study, the forward current-voltage characteristic of the diodes is linear at U > U<sup>c</sup> up to U ≥ 2 V. In unirradiated diodes, the forward current I at the reference forward voltage U = 2 V is I ≈ 12 A (see details, datasheet, quote on part number: CPW3-1700-S010B-WP).

The resulting set of experimental data indicates an increase in the radiation resistance of diodes with an increase in the temperature of irradiation. The physical reason for this temperature dependence is a decrease in the stationary concentration of radiation defects (RD), which are responsible for compensation of the base conductivity of the Schottky diodes under study, with an increase in the irradiation temperature.

As is known, the main RDs that create deep acceptor levels in n-SiC are mainly carbon vacancies [37,50]. The rate of generation of primary RDs (which are vacancies and interstitial atoms in both silicon carbide sublattices) in the temperature range under study is practically independent of the irradiation temperature [51,52].

– *Φ* −2 **Figure 11.** Forward current–voltage characteristics of the diodes upon their irradiation with 15 MeV protons at three different irradiation temperatures *T*<sup>i</sup> , the dose *Φ* = 1 × 10 14 cm−<sup>2</sup> [48].

However, the further fate of the generated vacancies (secondary defect formation) can significantly depend on temperature. As the temperature rises, the vacancy mobility increases and the recombination radius with a genetically related interstitial atom increases. Therefore, the fraction of vacancies that have escaped recombination and created deep acceptor levels is greatly reduced. According to our picture of irradiation, this proportion is about 25%, then at 300–400 ◦C it decreases by a factor of 2–3.

In principle, a second possible reason cannot be ruled out, which is a change in the spectrum of secondary radiation defects created during hot irradiation. A change in the X-ray diffraction spectrum was previously observed under hot electron irradiation of silicon and A3B<sup>5</sup> materials [47,52].

Thus, we can conclude that the previously made assumption that the radiation hardness of silicon carbide will grow with an increase in the irradiation temperature is valid. In our opinion, this is an important conclusion because SiC is considered to be a promising material, especially for development of high-power and high-voltage devices.
