Dual Role of Hydrogen in 4H-SiC: from Dopant Control to Quantum Defect Passivation

Dual Role of Hydrogen in 4H-SiC: from Dopant Control to Quantum Defect Passivation +

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4H-SiC, as a third-generation wide bandgap semiconductor, shows great potential in high-temperature, high-frequency, high-power electronic devices, and emerging quantum technologies. During its crystal growth (e.g., by Physical Vapor Transport, PVT) and epitaxy (e.g., by Chemical Vapor Deposition, CVD), hydrogen (H₂) is often used as a carrier or process gas. Furthermore, hydrogen can be readily introduced into the material during device fabrication processes such as ion implantation and plasma treatment. Consequently, hydrogen, as a common impurity, profoundly influences the electrical properties, defect evolution, and optical characteristics of 4H-SiC. Understanding the role of hydrogen is crucial for developing impurity engineering strategies for 4H-SiC.

1. Stable Configurations and Basic Behavior of Hydrogen

Theoretical calculations have revealed various possible sites for hydrogen within the 4H-SiC lattice, including the carbon antisite (H), silicon antisite (H), and several interstitial sites, such as the bond-centered site (H), the silicon tetrahedral interstitial site (H), and the carbon tetrahedral interstitial site (H).

Fig. 1 Schematic diagram of typical atomic configurations of hydrogen in 4H-SiC

Formation energy calculations indicate that the formation energies of interstitial hydrogen are generally lower than those of substitutional hydrogen. In p-type 4H-SiC, His the dominant configuration, whereas in n-type material, H is the most stable. Both of these interstitial hydrogen configurations exhibit negative-U behavior, with their charge states transitioning directly from +1 to -1 at Fermi levels (E_F) of 2.80 eV and 0.34 eV, respectively. They also introduce deep levels within the bandgap, capable of trapping charge carriers, which is the fundamental reason for their ability to affect the electrical and optical properties of 4H-SiC.

2. Compensation and Control of Doping Behavior by Hydrogen

2.1 Compensation of N-Type Doping

In nitrogen (N)-doped n-type 4H-SiC, negatively charged H acts as a compensation center, counteracting the electrons provided by nitrogen donors. When the unintentionally introduced hydrogen concentration is high, it can pin the Fermi level, severely hindering any further increase in electron concentration. Calculations show that the compensation effect of hydrogen on high-concentration nitrogen doping (e.g., 10¹⁹ cm⁻³) becomes significant only when the hydrogen concentration exceeds 10¹⁸ cm⁻³, at which point the Fermi level is pinned at approximately 1.4 eV. Furthermore, the formation energy of the N_C-H complex is high and its binding energy is negative, indicating that hydrogen is unlikely to passivate nitrogen donors under thermal equilibrium conditions. Its influence primarily stems from the compensation effect of its interstitial states.

2.2 Impact on P-Type Doping

In aluminum (Al)-doped p-type 4H-SiC, the intrinsic carbon vacancy (V_C) is the primary compensation center, with a formation energy much lower than that of H. Therefore, compared to V_C, the compensation effect of hydrogen on p-type doping is negligible. Simultaneously, the formation energy of the Al_Si-H complex is also higher than that of isolated Al_Si and interstitial hydrogen, suggesting that hydrogen does not readily passivate aluminum acceptors under thermal equilibrium growth conditions. However, passivation may occur during non-equilibrium processes like hydrogen ion implantation, but due to its low binding energy, this passivation can be easily removed by subsequent thermal annealing.

2.3 Suppression of Unintentional Nitrogen Doping

In the growth of high-purity semi-insulating (HPSI) 4H-SiC crystals, controlling unintentional doping of nitrogen and boron (B) is key to achieving high resistivity. Research has found that introducing hydrogen during PVT growth can significantly suppress nitrogen incorporation, while having little effect on boron. The mechanism involves hydrogen adsorbing on the growth surface and reacting with nitrogen to form stable NH₃ gas, which escapes, thereby reducing the chemical potential of nitrogen and increasing its formation energy in 4H-SiC. This raises the critical partial pressure of nitrogen allowed for growing HPSI 4H-SiC by about two orders of magnitude, relaxing the requirements on the vacuum level of the growth system and the purity of source materials.

3. Passivation Effects of Hydrogen on Key Defects in 4H-SiC

3.1 Passivation of Carbon Vacancies: Enhancing Carrier Lifetime

V_C is widely recognized as a “carrier lifetime killer” in 4H-SiC bipolar devices, as its deep levels effectively trap non-equilibrium carriers, leading to degraded device performance. Theoretical studies indicate that introducing hydrogen via non-equilibrium methods (e.g., hydrogen ion implantation) can form V_C-nH (n=1-4) complexes. Among these, the V_C-4H complex can completely eliminate the defect levels of V_Cwithin the bandgap.

Fig. 2 Formation energies and binding energies of VC-nH complexes

Fig. 2 Formation energies and binding energies of V_C-nH complexes

Although the formation energy of V_C-4H is high under thermal equilibrium conditions, it possesses a positive binding energy within a specific Fermi level range (0.44 – 3.03 eV), indicating that once formed, this complex is difficult to dissociate. This implies that hydrogen passivation achieved through non-equilibrium processes is an effective strategy for eliminating V_Cdefect states and enhancing the carrier lifetime of 4H-SiC.

3.2 Passivation of Silicon Vacancies: An Obstacle for Quantum Applications

V_Siis a color center in 4H-SiC exhibiting room-temperature single-photon emission and long spin coherence times, making it a candidate for quantum information technologies. However, hydrogen readily binds with V_Sito form V_Si-nH (n=1-4) complexes, altering its electronic structure and optical properties. For instance, the defect levels of (V_Si-3H)^– differ from those of the isolated V^–_Si, leading to changes in photoluminescence (PL) wavelength and intensity, or even complete quenching of its luminescence.

Proton irradiation experiments confirm that when hydrogen implantation is performed at elevated temperatures (e.g., 400°C), the enhanced mobility of hydrogen increases the likelihood of encountering irradiation-induced V_Siand forming complexes, resulting in a significant weakening of the V_Si-related PL signals (V1 and V2 lines). These V_Si-nH complexes all have positive binding energies; once formed, they are very stable and require thermal annealing to dissociate them and restore the uniform luminescence of the V_Si color centers.

3.3 Influence on the Formation of Nitrogen-Vacancy Color Centers

The N_CV_Si^–color center has attracted significant attention due to its emission in the near-infrared band (around 1300 nm). Research on forming this center via hydrogen ion implantation revealed a unique threshold fluence phenomenon (~5.0×10¹⁵ cm⁻²) after annealing at 1000°C.

At fluences below this threshold, the PL signal is extremely weak, attributed to the formation of complexes like V_Si-H, which quench the luminescence of N_CV^– _Si. When the fluence exceeds the threshold, the carrier removal effect induced by irradiation transforms the material from n-type to weak n-type or intrinsic. This changes the stable charge state of hydrogen from H^–_i to H⁰_i or H⁺_i, reducing its tendency to bind with defects and making it more likely to desorb from the material. This, in turn, allows the N_CV^– _Si color center to form effectively and emit light. This phenomenon highlights the critical influence of hydrogen charge state on its behavior and its complexity in the fabrication of quantum color centers.

In summary, hydrogen plays a dual role in 4H-SiC: it is a beneficial passivant that can neutralize carbon vacancies and improve device performance, but it is also a detrimental impurity that can compensate doping and quench the luminescence of quantum color centers.

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References:

Huang, Y., Wang, R., Zhang, Y., Yang, D., & Pi, X. (2022). Assessing the effect of hydrogen on the electronic properties of 4H-SiC. Chinese Physics B, 31(5), 056108.
Huang, Y., Wang, R., Zhang, N., Zhang, Y., Yang, D., & Pi, X. (2022). Effect of hydrogen on the unintentional doping of 4H silicon carbide. Journal of Applied Physics, 132(15).
Bathen, M. E., Galeckas, A., Coutinho, J., & Vines, L. (2020). Influence of hydrogen implantation on emission from the silicon vacancy in 4H-SiC. Journal of Applied Physics, 127(8).
Narahara, T., Sato, S. I., Kojima, K., Hijikata, Y., & Ohshima, T. (2021). Influences of hydrogen ion irradiation on NcVsi− formation in 4H-silicon carbide. Applied Physics Express, 14(2), 021004.