Displacement Damage Effects in Silicon Carbide Power Devices +

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Silicon carbide (particularly 4H-SiC), with its wide bandgap (3.3eV), high critical breakdown electric field (~10 times that of Si), high thermal conductivity, and high displacement threshold energy, is an ideal material to replace silicon for applications in high-temperature, high-frequency, and harsh radiation environments. However, displacement damage in radiation environments (caused by high-energy particles such as neutrons and protons) can displace lattice atoms via collisions, generating point defects like vacancies, interstitials, and their complexes. These defects introduce deep-level centers within the bandgap, acting as carrier traps or recombination centers, thereby significantly altering the electrical and optical properties of the material. Therefore, a deep understanding of the displacement damage mechanisms is crucial for evaluating and enhancing the reliability of SiC devices in radiation environments.

1. 4H-SiC Displacement Damage Mechanisms and Defect Characterization

Displacement damage originates from elastic collisions between high-energy particles and lattice atoms, causing atoms to be displaced from their lattice sites, forming Frenkel pairs (vacancy-interstitial pairs) and their complex derivatives. These defects introduce deep-level centers within the semiconductor bandgap, profoundly affecting carrier generation, recombination, and transport processes.

1.1 Defect Characteristics Under Different Particle Irradiation

Research by Hazdra & Popelka shows that deep-level transient spectroscopy (DLTS) analysis can clearly identify deep-level defects introduced by different particles in n-type 4H-SiC epitaxial layers. As shown in Fig. 1, proton, electron, and neutron irradiation all introduce new characteristic peaks in the DLTS spectra. Electron and light ion (proton, alpha particle) irradiation primarily produce two dominant acceptor levels: E2 (Ec – 0.60eV) and E3 (Ec – 0.72eV). In contrast, fast neutron irradiation introduces deeper levels, such as RD1/2 (Ec – 0.88eV) and RD4 (Ec – 1.45eV), which are typically associated with more stable damage clusters. Acting as electron traps or recombination centers, the specific role of these deep levels depends on their position in the energy band and their electronic configuration.

Fig. 1 DLTS spectra of 4H-SiC after proton, electron, and neutron irradiation

1.2 Thermal Stability and Evolution of Defects

Most defects introduced by irradiation in 4H-SiC at room temperature are unstable. Hazdra & Popelka demonstrate that defects begin to anneal or transform into more stable configurations even at temperatures below 200°C, which is the typical operating temperature range for power devices. For instance, the E1, E2, and E3 defects generated by electron irradiation gradually transform into the S1, Z1/Z2, and S2 centers. After annealing above 370°C, ultimately only two defect centers dominate: the Z1/Z2 center, which limits carrier lifetime, and the EH6/7 center, which acts as an efficient carrier generation center. Both are closely related to carbon vacancies and only anneal out at temperatures above 1300°C. This thermal evolution behavior of defects is crucial for understanding device performance changes during high-temperature operation or after annealing.

Fig. 2 Impact of low-temperature isochronal annealing on the DLTS spectrum of electron-irradiated 4H-SiC

1.3 Damage Revealed by Optical Characterization

In addition to electrical characterization, optical methods are extremely sensitive for detecting low-concentration defects. Using time-resolved photoluminescence (TRPL) technique, Migliore et al. (2024) found that 2.5 MeV electron (β-ray) irradiation significantly shortens the lifetime of the 3.2eV exciton luminescence band in 4H-SiC epitaxial layers. This lifetime shortening is attributed to the enhancement of non-radiative recombination channels by irradiation-induced point defects (such as vacancies or interstitials). Notably, in samples with more intrinsic defects (e.g., stacking faults), this effect appears at lower irradiation fluences. In contrast, X-ray irradiation at the same dose showed no observable change in exciton lifetime, strongly proving that the damage induced by electron irradiation belongs to the category of displacement damage, not merely ionization effects.

2. Impact of 4H-SiC Displacement Damage on Carrier Transport Properties

The introduced radiation-induced defects fundamentally degrade the carrier transport capability of semiconductors through three interrelated mechanisms: carrier removal, mobility degradation, and minority carrier lifetime reduction.

2.1 Carrier Removal and Concentration Reduction

Acceptor-type defects generated by irradiation compensate shallow donors in n-type materials, leading to a decrease in effective carrier concentration. For particles that produce uniform damage (e.g., electrons, neutrons), the carrier concentration decreases linearly with fluence: n = n₀ – K_N Φ, where K_N is the carrier removal rate. C-V profiling measurements directly confirm this. As shown in Fig. 3, with increasing neutron or electron fluence, the free electron concentration in n-type epitaxial layers decreases linearly. Further C-V testing by Chao et al. (2019) indicates that when the neutron fluence reaches 1.3×10¹⁵ cm⁻², the effective carrier density (N_B) in SiC SBD and MOS capacitors decreases by more than an order of magnitude (from ~10¹⁶ cm⁻³ to the 10¹⁵ cm⁻³ range), directly demonstrating the severe compensation effect of displacement damage on doping.

Fig. 3 Trend of free carrier concentration decrease versus neutron and electron irradiation fluence in n-type epitaxial layers of different SiC devices

2.2 Mobility Degradation

Defects not only reduce the number of carriers but also act as scattering centers, lowering carrier mobility. Through simulation analysis of JBS diode on-state characteristics, Hazdra and Popelka (2019) quantitatively provided the relationship of electron mobility (μ_n) decreasing with increasing radiation defect concentration (N_T). This effect is more pronounced in SiC than in Si. The decrease in mobility directly leads to an increase in the semiconductor material’s resistivity (or the device’s on-resistance), which is one of the primary reasons for performance degradation in unipolar devices.

2.3 Carrier Lifetime Shortening

For bipolar devices, minority carrier lifetime (τ) is a key parameter. Recombination centers introduced by irradiation (such as Z1/Z2 and their precursors E2/E3) drastically shorten the carrier lifetime, following the relationship: 1/τ = 1/τ₀ + K_TΦ, where K_T is the lifetime degradation coefficient. Due to the high defect introduction rate in SiC, the K_T value is significantly larger than in silicon. Neutron irradiation rapidly reduces the high-injection carrier lifetime of a 4.5kV SiC PIN diode from the microsecond range to below 200ns, severely weakening the conductivity modulation effect.

Fig. 4 Effect of neutron irradiation on the high-level carrier lifetime of a 4.5kV SiC PIN diode (measured by OCVD)

3. Analysis of Electrical Characteristic Degradation in 4H-SiC Devices

The impact of displacement damage on device performance varies significantly depending on the device structure and operating principle.

3.1 JBS Diode (Unipolar Operation Mode)

The degradation of JBS diodes is primarily manifested as the deterioration of forward conduction characteristics. As shown in Fig. 5, electron irradiation causes a decrease in the slope of its forward I-V curve and an increase in the on-state voltage drop. This stems from the increase in resistivity due to both carrier removal and mobility degradation in the drift region. At high doses (>500kGy), the drift region becomes fully compensated, leading to device functional failure. However, its blocking characteristics (breakdown voltage and leakage current) are minimally affected by irradiation, attributed to the fact that defects generated by electron irradiation are primarily shallow-level traps rather than deep-level generation-recombination centers. Furthermore, Fig. 6 shows that low-temperature annealing (150-200°C) can partially restore conductivity, directly related to the annealing behavior of unstable irradiation-induced defects.

Fig. 5 Effect of 4.5MeV electron irradiation on the forward I-V characteristics of a 1.7kV 4H-SiC JBS diode

Fig. 6 Comparison of blocking I-V characteristics of a JBS diode before and after 500kGy electron irradiation

3.2 PIN Diode (Bipolar Operation Mode)

As a bipolar device, the PIN diode is extremely sensitive to carrier lifetime degradation. Fig. 7 clearly shows that its forward voltage drop deteriorates much more severely after neutron irradiation compared to the JBS diode. The drastic shortening of carrier lifetime weakens the conductivity modulation effect, preventing most of the drift region from conducting, leading to a sharp increase in voltage drop. Although lifetime shortening reduces the stored charge during turn-off, potentially lowering switching losses, its negative impact on forward characteristics is dominant.

Fig. 7 Effect of neutron irradiation on the forward I-V characteristics of a 4.5 kV SiC PIN power diode

3.3 MOSFET

The degradation mechanism of MOSFETs is the most complex, involving both displacement damage in the bulk material and TID effects in the gate dielectric. Research reveals the complex changes in its parameters under electron irradiation: at low doses (1-20 kGy), positive charge trapping in the gate oxide causes a negative shift in threshold voltage (V_TH), and on-resistance (R_DS(on)) slightly decreases; at high doses (200 kGy), the combined effects of accumulated negative charge trapping at the interface and damage in the drift region cause V_TH to recover and R_DS(on) to increase significantly. This V_TH “turnaround” phenomenon directly affects blocking capability.

Fig. 8 Transfer characteristic curves of a SiC MOSFET under different dose electron irradiations (V_DS=50mV)

Fig. 9 Effect of total electron irradiation dose on the threshold voltage, on-resistance, and leakage current of a SiC MOSFET

Fig. 10 Effect of different dose electron irradiations on the blocking characteristics of a SiC MOSFET (V_GS = 0V and -1V)

3.4 JFET

Lacking a gate oxide layer, JFET degradation mainly stems from carrier removal caused by displacement damage. Neutron irradiation compensates doping in both the channel and drift region, leading to an increase in V_TH and a decrease in transconductance. When the drift region is fully compensated, the R_DS(on) increases sharply, causing device failure. Similar to JBS diodes, its blocking characteristics show a slight increase in breakdown voltage after irradiation.

4. 4H-SiC Material Structural Damage and Recovery

Displacement damage ultimately manifests as changes in the crystal structure of the material. Using high-brightness synchrotron X-ray Laue diffraction and rocking curve analysis, Chao et al. investigated 4H-SiC crystals after neutron irradiation. As shown below, after high neutron fluence (1.6×10¹⁶ cm⁻²) irradiation, diffraction spots elongated into arcs along specific crystal plane directions, and the X-ray diffraction rocking curve peak for the (0008) plane significantly broadened with reduced intensity. This directly proves that displacement damage causes lattice distortion and a reduction in long-range order, providing structural evidence for the degradation of electrical performance.

Fig. 11 Diffraction patterns obtained via high-brightness synchrotron X-ray Laue diffraction: (a) Unirradiated 4H-SiC crystal, showing sharp diffraction spots; (b) 4H-SiC crystal irradiated with a fast neutron fluence of 1.6×10¹⁶ cm⁻², showing diffraction spots elongated into arcs along the (1, -1, L) crystal plane

Fig. 12 Corresponding X-ray diffraction rocking curve for the (0008) reflection (2θ = 75.536°)

Regarding defect recovery, Migliore et al. found that defects generated by 2.5MeV electron irradiation, which cause exciton lifetime shortening, remained stable even after heat treatment in air at 900°C, showing no signs of recovery. This aligns with the conclusion by Hazdra et al. that carbon-vacancy-related defects like Z1/Z2 possess high thermal stability, indicating that certain displacement damages, once formed, are difficult to eliminate under conventional operating temperatures.

Although different high-energy particles cause damage in 4H-SiC through the common mechanism of generating acceptor-type deep-level defects (leading to carrier removal, mobility degradation, and lifetime shortening), the electrical responses of devices with different structures such as JBS, PIN, MOSFET, and JFET exhibit significant differences. However, the difficulty in eliminating stable defects related to carbon vacancies constitutes a fundamental challenge for radiation reliability. Future efforts require multi-dimensional collaborative optimization of materials and devices to address this.

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References:

1. Hazdra, P., & Popelka, S. (2019). Displacement damage and total ionisation dose effects on 4H‐SiC power devices. IET Power Electronics, 12(15), 3910-3918.
2. Migliore, F., Alessi, A., Principato, F., Girard, S., Cannas, M., Gelardi, F. M., … & Agnello, S. (2024). β-rays induced displacement damage on epitaxial 4H-SiC revealed by exciton recombination. Applied Physics Letters, 124(4).
3. Chao, D. S., Shih, H. Y., Jiang, J. Y., Huang, C. F., Chiang, C. Y., Ku, C. S., … & Lee, C. Y. (2019). Influence of displacement damage induced by neutron irradiation on effective carrier density in 4H-SiC SBDs and MOSFETs. Japanese Journal of Applied Physics, 58(SB), SBBD08.