Carrier Capture Mechanism, Characterization, and Passivation of Key Defects in 4H-SiC +

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4H-SiC, as a wide bandgap semiconductor, has broad application prospects in power electronic devices, radiation detection, and quantum sensing due to its high breakdown field strength, high thermal conductivity, and high electron mobility. However, its performance and reliability are severely constrained by intrinsic point defects and impurity defects in the crystal, especially carbon vacancies (V_c), silicon vacancies (V_Si), and boron related defects, which introduce deep energy levels that become the main limiting factor for carrier lifetime. Understanding the carrier capture mechanism of these defects, developing high-precision characterization methods, and exploring effective passivation strategies have become the core topics in current research on 4H-SiC materials.

1. Carrier Trapping Mechanism of Key Defects in 4H–SiC

1.1 Carbon Vacancies and Deep Energy Levels

Carbon vacancies are the most common and thermally stable intrinsic point defects in 4H-SiC, and their introduction of Z₁/₂ and EH₆/₇ deep energy levels in the bandgap has been confirmed as the main carrier lifetime killers. The Z₁/₂ energy level is located about 0.64eV below the bottom of the conduction band, corresponding to the (0/−2) charge state transition of Vc; The EH₆/₇ energy level is located about 1.54 eV below the bottom of the conduction band, corresponding to the (+1/0) transition of Vc. Through first principles calculations, it was found that the metastable spin triplet state plays a key transition state role in the non radiative carrier capture pathway of carbon vacancies, especially in the hole capture process of donor energy levels. This metastable state can be formed through spin selective carrier capture during material growth, significantly affecting the signal ratio relationship between EH₆/₇ and Z₁/₂.

1.2 Silicon Vacancies and Boron Related Defects

Silicon vacancies in 4H-SiC mainly manifest as electron acceptors, with their deep energy levels S₁ and S₂ corresponding to the (-3/-) and (=/-) charge states of Vsi, respectively. In addition, boron impurities form shallow level B₁/B₂ and deep level D-center in n-type 4H-SiC, corresponding to boron atoms occupying silicon and carbon sites, respectively. The D-center exhibits a single peak in the DLTS spectrum, but two components, D₁ and D₂, can be distinguished through high-resolution GS4 weight functions and capture rate measurements, located at Ev+0.57eV and Ev+0.47eV, respectively.

2. Progress in Characterization Technology for 4H-SiC Energy Level Defects

2.1 DLTS and Laplace DLTS

Deep level transient spectroscopy is the main method for studying deep level defects in semiconductors, which can provide information such as activation energy, capture cross-section, and concentration of defects. However, the energy resolution of traditional DLTS is limited, making it difficult to distinguish defect energy levels with similar energies. Laplace DLTS (L-DLS) has improved its energy resolution to the meV level through isothermal measurement and signal averaging processing, successfully distinguishing the two components of Z₁ (Ec-0.59eV) and Z₂ (Ec-0.67eV) in Z₁/₂.

Fig. 1 L-DLS spectra of n-type 4H-SiC SBD measured at room temperature, clearly displaying Z₁ and Z₂

2.2 Capture Rate Measurement and Thermodynamic Analysis

By measuring the capture rate, the true capture cross-section and capture energy barrier can be extracted. Combined with the performance parameters obtained from thermal emission experiments, the entropy change and free energy of defects can be further calculated. For example, analysis of the capture rate of Z₁/₂ energy levels shows an electron capture cross section of approximately 10⁻¹³ ~10⁻¹² cm², which is consistent with experimental values. The capture experiment of D-center further revealed that it contains two different capture mechanisms, corresponding to the D₁ and D₂ energy levels, respectively.

Fig. 2 D-center capture curves: (a) Relationship between the filling pulse duration of DLTS signal S and the peak value of D center; (b) Arrhenius plot of pore capture cross-section σp and 1/T; (c) Differential junction DLTS spectra recorded with pulses at different times; (d) Trap features come from the Arrhenius plots of peaks D1 and D2

3. Passivation Effect of Hydrogen and Oxygen in 4H–SiC

3.1 Hydrogen Passivation Mechanism

Hydrogen is not only a common unintentional doping element in 4H-SiC, but also effectively passivates vacancy defects. Yu et al. found through HSE06 hybrid functional calculations that hydrogen atoms bind to dangling bonds in carbon or silicon vacancies, significantly reducing the formation energy of defects and altering their charge states and carrier capture characteristics. As the number of hydrogen atoms increases, Vc gradually transitions from a bipolar defect to a donor type defect that only captures holes, ultimately losing its ability to capture charge carriers completely at Vc-4H.

Fig. 3 Formation energy and transition energy levels of Vc – nH (n=0-4)

3.2 Oxygen Passivation Mechanism

Oxygen atoms inevitably enter the 4H-SiC lattice during thermal oxidation and form stable complexes with vacancies. Oxygen passivation can cause Vc to transition from bipolar defects to donor defects, while Vsi transitions from acceptor defects to bipolar defects. In addition, oxygen tends to preferentially bind with silicon dangling bonds, further affecting the electronic structure of vacancy complexes.

The carrier capture mechanism of point defects in 4H-SiC is complex and diverse, especially the metastable spin state of Vc plays a key role in non radiative recombination. The spectroscopic techniques represented by DLTS, L-DLTS, and capture rate measurement provide powerful tools for defect identification and thermodynamic analysis. The introduction of hydrogen and oxygen can effectively passivate vacancy defects, regulate their charge states and capture behavior, and provide theoretical basis and experimental guidance for optimizing the performance of 4H-SiC devices.

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References:

1. Capan, I., & Brodar, T. (2022). Majority and minority charge carrier traps in n-type 4H-SiC studied by junction spectroscopy techniques. Electronic Materials, 3(1), 115-123.
2. Jiang, X., Huang, Y., Wang, R., Pi, X., Yang, D., & Deng, T. (2025). Carrier lifetime killer in 4H-SiC: carrier capture path via carbon vacancies. Journal of Materials Chemistry C, 13(11), 5575-5581.
3. Samperi, O., Vines, L., Hallén, A., & Fragalà, M. E. (2024, August). Charge carrier capture by prominent defect centers in 4H-SiC. In Defect and Diffusion Forum (Vol. 434, pp. 173-182). Trans Tech Publications Ltd.
4. Yu, X., Xu, X., Jiang, H., Wei, Y., Ying, T., Li, W., … & Li, X. (2025). Passivation effect of hydrogen and oxygen on the carrier capture of vacancies in 4H-SiC. Computational Materials Science, 246, 113365.