Impact of Carbon Injection on Electrical Properties and Defect Structures in 4H-SiC +

4H-SiC epitaxial wafers are available for research and industrial applications. More technical parameters, please contact the sales team at [email protected]

As a wide-bandgap semiconductor material, 4H-silicon carbide (4H-SiC) exhibits outstanding performance under extreme conditions such as high temperature and high voltage, making it highly promising for power electronic device applications. However, point defects commonly present in 4H-SiC epitaxial layers, especially carbon vacancies (V_C), significantly reduce the minority carrier lifetime and thus limit device performance. To eliminate V_C, various methods have been proposed by researchers, all of which share the common approach of injecting carbon interstitials (C_i) to annihilate V_C through the reaction: C_i+V_C → perfect lattice. Nevertheless, the carbon injection process may introduce new carbon-related defects that introduce energy levels within the bandgap, thereby affecting the electrical properties of the material. This review outlines the effects of carbon injection on defect formation, carrier traps, and minority carrier lifetime in 4H-SiC.

1. Experimental Methods: Carbon Injection and Characterization

In the study, carbon injection was performed using a carbon cap (C-cap) method: a photoresist was coated on the surface of 4H-SiC samples and graphitized by vacuum heat treatment at 900°C to form a carbon source layer; subsequently, annealing was carried out at 1250°C under an Ar atmosphere for different durations (1.5h, 2.5h, 6.6h). Deep-level transient spectroscopy (DLTS) and minority carrier transient spectroscopy (MCTS) were employed to monitor changes in majority and minority carrier traps during the injection process, combined with time-resolved photoluminescence (TRPL) to measure the depth distribution of minority carrier lifetime.

2. Impact of Carbon Injection on Majority Carrier Traps of 4H-SiC

DLTS results show that the as-grown samples primarily contain the Z_1/2 level (corresponding to the (0/2-) charge transition level of V_C) and the R1 level (approximately 0.41eV). After carbon injection, the concentration of Z_1/2 decreases significantly below the DLTS detection limit (<1×10¹¹ cm⁻³), indicating effective annihilation of V_C. Simultaneously, a series of new trap levels emerge, including E_0.38(E_C-0.39eV), ON0a (E_C-0.58eV), ON0b (E_C-0.69eV), ON1 (E_C-0.89eV), and ON2 (E_C-1.15eV). Among these, E_0.38 disappears after bias annealing (600K), suggesting it originates from unstable single carbon interstitial defects (C_i), whereas the ON family of levels exhibits higher thermal stability, likely originating from carbon clusters.

Fig. 1 DLTS spectra of 4H-SiC after different annealing durations, showing the emergence of new trap levels (E₀.₃₈, E₀.₅₉, etc.) and the disappearance of the Z_1/2 level following carbon injection

3. Impact of Carbon Injection on 4H-SiC Minority Carrier Traps

MCTS measurements reveal that the as-grown samples contain a shallow boron level (B, ~E_V+0.27eV) and a deep-level D-center (~E_V+0.61eV, possibly related to B_C). After carbon injection, the concentration of the D-center decreases significantly or disappears entirely, and the intensity of the shallow boron level also drops by approximately (3–5)×10¹² cm⁻³. With prolonged injection time (6.6 h), new minority carrier traps T1 (E_V+0.46eV), T2 (E_V+0.56eV), and T3 (broad peak, ~ E_V+1.17eV) appear in the lower half of the bandgap, which may be related to carbon clusters.

Fig. 2 After carbon injection, (a) MCTS spectra show the reduction of boron-related levels (B, D), and (b) the formation of new carbon-related traps (T1, T2, T3)

TRPL depth profiling indicates that the minority carrier lifetime of the as-grown sample reaches a maximum (~2.4μs) at about 80 μm from the surface, with lower lifetimes in the near-surface region due to surface recombination. Short-term carbon injection (1.5h, 2.5h) leads to a decrease in lifetime across the entire epilayer, which may result from changes in surface recombination velocity induced by high-temperature treatment. However, after long-term injection (6.6h), the lifetime in the near-surface region (0–50μm) increases by 10–20% compared to the as-grown sample. This improvement may be related to the annihilation of V_C and the D-center following C_i injection. Calculations show that after annealing at 1250°C for 6.6 h, the diffusion length of Cⁱ can reach ~52μm, sufficient to influence the defect structure in the near-surface region.

Fig. 3 Depth distribution of minority carrier lifetime measured by TRPL after different annealing durations

4. 4H-SiC Epi Microstructure with Carbon Injection Defects

The defect levels introduced by carbon injection exhibit diversity: E₀.₃₈ likely corresponds to single C_i with a migration barrier of approximately 2.35eV, close to the theoretically predicted diffusion barrier of C_i along the c-axis (2.20eV). The ON family of levels (ON0a, ON0b, etc.) aligns with defect levels observed during thermal oxidation, suggesting they originate from carbon clusters such as carbon di-interstitials ((C_i)₂) or carbon antisite ring structures ((C₃)_Si). Laplace-DLTS further reveals that E₀.₃₈ consists of two sub-levels (E_C-0.33eV and E_C-0.39eV), possibly corresponding to C_i at hexagonal (h) and pseudo-cubic (k) lattice sites.

The study also found that carbon injection affects boron impurities: the reduction of shallow boron and D-center intensities may be related to complex formation between C_i and boron. This indicates that carbon injection not only regulates intrinsic defects but also interacts with dopants, further influencing the electrical properties of the material.

In summary, carbon injection serves as a means to modulate the defect structure in 4H-SiC, effectively eliminating harmful V_C and boron-related deep levels, thereby enhancing minority carrier lifetime, particularly in the near-surface region. However, the injection process introduces various carbon-related defects (such as the E_0.38, ON, and T families of levels) with distinct thermal stabilities and electrical behaviors, indicating that the migration and aggregation of carbon in 4H-SiC are complex processes that may involve single interstitials, di-interstitials, and larger carbon clusters. Future work should combine theoretical calculations and more refined experimental characterization to clarify the atomic configurations of these defects and their comprehensive impact on device performance.

Whether you need SiC wafer for research or for industrial applications, please contact us email at [email protected] and [email protected].

 

References:

1. Karsthof, R., Etzelmüller Bathen, M., Kuznetsov, A., & Vines, L. (2022). Formation of carbon interstitial-related defect levels by thermal injection of carbon into n-type 4H-SiC. Journal of Applied Physics, 131(3).
2. Bathen, M. E., Karsthof, R., Galeckas, A., Kumar, P., Kuznetsov, A. Y., Grossner, U., & Vines, L. (2024). Impact of carbon injection in 4H-SiC on defect formation and minority carrier lifetime. Materials Science in Semiconductor Processing, 176, 108316.