Effects of irradiation by protons, neutrons, and gamma particles on electrical properties of 4H-SiC diodes and LGAD sensors

Technical Summary: Effects of Irradiation by Protons, Neutrons, and Gamma Particles on Electrical Properties of 4H-SiC Diodes and LGAD Sensors

Problem Statement
Silicon carbide (4H-SiC) is a wide-bandgap semiconductor with intrinsic properties—such as high displacement threshold energy, a large critical electric field, and low intrinsic carrier concentration—that make it attractive for radiation-hard detector applications. While 4H-SiC Low-Gain Avalanche Detectors (LGADs) have demonstrated internal gain and promising performance, systematic studies regarding their behavior under extreme radiation environments remain limited. Specifically, there is a need to understand how high hadron fluences and extreme neutron irradiation affect the effective space charge, compensation mechanisms, and the evolution of the depletion region in 4H-SiC devices.

Methodology
The study investigated 4H-SiC P+-in-N (PN) diodes and LGAD sensors (LGAD1 and LGAD2) fabricated by onsemi on 6-inch wafers. The devices featured a highly doped N-type substrate, a high-resistivity N-type epitaxial layer (30–100 µm thick), and specific ion-implanted layers for multiplication and junction termination.

Samples were subjected to three distinct irradiation types:

1. Proton Irradiation: Performed at CERN IRRAD using 24 GeV/c protons, targeting fluences up to $1 \times 10^{16}$ protons/cm².
2. Neutron Irradiation: Conducted at the TRIGA Mark II reactor (Jožef Stefan Institute) using reactor neutrons, targeting fluences up to $1.0 \times 10^{18}$ 1 MeV neq/cm².
3. Gamma Irradiation: Performed using a $^{60}$Co source at UJP PRAHA a.s., delivering total ionizing doses (TID) up to 300 kGy.

Post-irradiation, electrical characterization was performed at room temperature ($+20^\circ$C) using Current–Voltage (IV) and Capacitance–Voltage (CV) measurements. Bulk capacitance was measured using a series configuration (CSRS) with a 20 kHz, 1.0 V AC signal. Samples were stored at temperatures below $-20^\circ$C (or allowed to decay for neutrons) to minimize unintended annealing prior to testing.

Key Results

• Proton Irradiation:

  • Leakage Current: At low fluences, leakage current behavior was similar to unirradiated devices. However, for fluences exceeding $1 \times 10^{14}$ protons/cm², the reverse leakage current decreased significantly. The IV characteristics became nearly symmetric with respect to voltage polarity.
  • Capacitance: The bulk capacitance became nearly independent of bias voltage at high fluences, stabilizing near the geometric capacitance of the fully depleted epitaxial layer (~15.4 pF).
  • Interpretation: These changes indicate strong radiation-induced compensation of the originally N-type material by deep acceptor-like defects (e.g., Z1/2 and EH6/7 centers), leading to a reduction in effective free carrier concentration and a uniform electric field distribution across the epitaxial layer.

• Neutron Irradiation:

  • Leakage Current: Leakage current increased systematically with fluence, reaching ~10 nA at $1.0 \times 10^{18}$ neq/cm².
  • Capacitance: While lower fluences ($2.3 \times 10^{17}$ neq/cm²) showed behavior similar to proton irradiation, extreme fluences ($5.2 \times 10^{17}$ and $1.0 \times 10^{18}$ neq/cm²) caused a dramatic drop in bulk capacitance to ~2.2 pF.
  • Interpretation: This capacitance reduction corresponds to an effective depletion depth of approximately 350 µm, far exceeding the nominal 50 µm epitaxial thickness. This implies the depletion region penetrates deeply into the highly doped substrate ($10^{19}$ cm$^{-3}$), suggesting substantial radiation-induced modification of the space charge density in the substrate to an effective level of $10^{12}$–$10^{13}$ cm$^{-3}$. The observed leakage current is attributed to the enlarged depletion volume combined with field-enhanced and surface-related generation mechanisms, rather than simple bulk thermal generation.

• Gamma Irradiation:

  • Leakage Current: The reverse leakage current remained statistically comparable to unirradiated devices even at 300 kGy.
  • Capacitance: Significant modifications were observed only at the highest dose (300 kGy). For PN diodes, the capacitance decrease with bias became steeper. For LGADs, the capacitance became nearly constant across the reverse bias range (~17 pF).
  • Interpretation: Gamma irradiation primarily affects the effective doping concentration in the epitaxial and multiplication layers through ionization-related mechanisms (e.g., charge traps in oxides/interfaces) without significantly altering the depletion depth or leakage current.

Significance and Conclusions
The paper demonstrates that 4H-SiC devices exhibit exceptional radiation hardness, but the nature of the damage is highly dependent on the radiation type.

• Protons primarily induce compensation within the epitaxial layer, modifying the effective space charge and reducing leakage current at high fluences.
• Neutrons cause a qualitative change by drastically expanding the depletion region into the substrate due to strong space charge modification, significantly increasing the active volume contributing to leakage.
• Gamma rays predominantly influence effective doping and electric field distribution via ionization damage, with minimal impact on leakage current.

The authors conclude that while radiation-induced compensation strongly modifies the effective space charge in 4H-SiC at high hadron fluences, the leakage current is governed by a combination of enlarged depletion volume and field-enhanced generation mechanisms. These findings confirm the potential of 4H-SiC PN and LGAD sensors for operation in extreme radiation environments, provided the specific effects of neutron-induced depletion expansion and ionization-induced doping reduction are accounted for.