Key Takeaways & Executive Findings
- •• • Nitrogen annealing (reducing ambient) increases electron concentration from 6.37×10^17 cm^-3 to 7.48×10^18 cm^-3, a 11.7-fold rise, while raising interface state density (Dit) to 3.36×10^15 eV^-1 cm^-2; this enables degenerate n-type Ga2O3 for low-resistance Ohmic contacts, critical for reducing conduction losses in power switching devices. • • Air annealing (oxidizing ambient) reduces electron concentration to 3.01×10^16 cm^-3 and suppresses Dit to 1.74×10^14 eV^-1 cm^-2, a 19.3-fold reduction versus nitrogen-annealed samples; this yields a highly resistive layer suitable for current blocking layers (CBL) in vertical MOSFETs, directly addressing the need for enhancement-mode operation. • • The ET2 trap (VGa-VO complex) density increases from 6.13×10^15 to 1.1×10^16 cm^-3 after nitrogen annealing, while ET1 (VGa) is reduced; this defect transformation converts deep acceptors into neutral complexes, mitigating compensation and enabling controllable carrier concentration—essential for reproducible doping in Ga2O3 epitaxy. • • FeGa-related ET3 (EC−0.84 eV) is concentrated near the surface and its detectability is enhanced by nitrogen annealing due to Fermi-level-induced reduction of formation energy; air annealing suppresses ET3, demonstrating that thermal budget directly modulates unintentional Fe contamination, a key reliability concern for high-field devices.
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Abstract
Defect engineering is pivotal in comprehending physical mechanisms that govern carrier transport and device performance. The defect evolution and carrier manipulation in Sn-doped Ga2O3 bulk crystals subjected to different thermal treatments were investigated, utilizing depth-profiled deep-level transient spectroscopy (DLTS) and frequency-dependent capacitance-voltage (C-V-f) techniques. In untreated Sn-doped Ga2O3 with an electron concentration of 6.37×10^17 cm^-3, two dominant electron traps, ET1 (EC−0.68 eV) and ET2 (EC−0.76 eV), were identified, corresponding to gallium vacancy (VGa) and the neutral complex of VGa-VO, respectively, and characterized as bulk traps. FeGa-related defects, ET3 (EC−0.84 eV), were concentrated near surface. Nitrogen annealing significantly reduced ET1, increased ET2 density from 6.13×10^15 to 1.1×10^16 cm^-3, and raised the interfacial state density (Dit) to 3.36×10^15 eV^-1 cm^-2, accompanied by an elevated electron concentration of 7.48×10^18 cm^-3. In contrast, air annealing enhanced ET1, with a density of 1.42×10^16 cm^-3, suppressed of ET2/ET3 traps, resulting in a lower Dit of 1.74×10^14 eV^-1 cm^-2, and a reduced electron concentration to 3.01×10^16 cm^-3. The findings reveal that a reducing environment induces VO formation and converts discrete VGa acceptors into neutral VGa-VO complexes, leading to downward surface band bending and electron accumulation. Conversely, VGa-VO complexes are dissociated into VGa acceptors in oxidizing conditions, leading to an upward surface band bending and electron compensation. This work underscores the carrier concentration manipulation by defect engineering in Ga2O3, offering insights essential for developing high-performance gallium oxide electronics.
1. Introduction
Commercial deployment of ultra-wide bandgap Ga2O3 power electronics has been stalled by the inability to reliably control carrier concentration and surface band bending. Unintentional n-type conductivity, typically in the 10^17–10^18 cm^-3 range, prevents the formation of semi-insulating current blocking layers (CBL) required for enhancement-mode MOSFETs. Existing approaches—solid-state diffusion of acceptors via oxygen annealing or ion implantation of N and Mg—suffer from poor reproducibility, low activation efficiency, and surface damage. The absence of a spectroscopic understanding of how deep-level defects evolve under thermal treatment has left device engineers without a predictive framework for tuning conductivity.
This study addresses the bottleneck by systematically correlating thermal ambient (nitrogen vs. air) with depth-profiled deep-level transient spectroscopy (DLTS) and frequency-dependent capacitance-voltage (C-V-f) data on Sn-doped β-Ga2O3 (100) single crystals. The authors quantify the interconversion between gallium vacancy (VGa) acceptors and neutral VGa-VO complexes, and link these defect reactions to surface band bending and carrier profiles. The resulting process-structure-property relationships provide a rational basis for engineering Ohmic contacts or resistive layers, directly impacting the design of high-performance gallium oxide electronics.
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Jiahao Yao, Yiyuan Liu, Zhaoxuan Fang, Zhengpeng Wang, Na Sun, Hehe Gong, Wenxiang Mu, Zhitai Jia, Xutang Tao, Fangfang Ren, Shulin Gu, Rong Zhang, Jiandong Ye (2025). Deep-level defects and carrier manipulation in Sn-doped β-Ga2O3 (100) single crystals. SCIENCE CHINA Materials. https://doi.org/10.1007/s40843-025-3387-6
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Frequently Asked Questions
What is the dominant failure mechanism when Ga2O3 power devices are subjected to high-temperature reducing environments?
Nitrogen annealing at elevated temperatures generates oxygen vacancies (VO) that compensate VGa acceptors by forming neutral VGa-VO complexes. This increases electron concentration to 7.48×10^18 cm^-3 and raises interface state density to 3.36×10^15 eV^-1 cm^-2, causing downward band bending and electron accumulation. The resulting degenerate surface can lead to uncontrolled leakage and loss of gate control in MOSFETs.
How does air annealing compare to nitrogen annealing in terms of defect suppression and resistivity control?
Air annealing (oxidizing) annihilates VO donors and enhances gallium out-diffusion, increasing VGa concentration. It suppresses ET2/ET3 traps, reduces Dit to 1.74×10^14 eV^-1 cm^-2, and lowers electron concentration to 3.01×10^16 cm^-3. This produces a highly resistive layer suitable for current blocking, whereas nitrogen annealing yields a conductive, degenerate surface.
What is the quantitative impact of Fermi level position on FeGa-related ET3 defect formation?
Nitrogen annealing raises carrier concentration, shifting the Fermi level closer to the conduction band and reducing FeGa formation energy, which enhances ET3 detectability. Air annealing reduces carrier concentration, pushing the Fermi level away from the conduction band, increasing FeGa formation energy and suppressing ET3 to near disappearance. This demonstrates that thermal budget directly controls unintentional Fe contamination.
Can these defect engineering strategies be scaled to production-grade Ga2O3 substrates without compromising crystal quality?
The study uses bulk single crystals and depth-profiled DLTS, showing that defect evolution occurs within the first 200 nm near surface and up to 6 μm in bulk. Nitrogen annealing for 24 h is practical for forming Ohmic contacts, while air annealing above 1400°C generates resistive layers. Scalability depends on controlling annealing atmosphere and temperature uniformity, but the observed defect interconversion is reversible and reproducible, indicating potential for industrial adoption.
What are the implications of the ET1 and ET2 trap densities for device reliability under high-field stress?
ET1 (VGa) at EC−0.68 eV and ET2 (VGa-VO) at EC−0.76 eV are bulk traps. Nitrogen annealing reduces ET1 but increases ET2 to 1.1×10^16 cm^-3, which can act as trapping centers under high field, causing dynamic on-resistance degradation. Air annealing increases ET1 to 1.42×10^16 cm^-3 but suppresses ET2, potentially trading off bulk trapping for surface depletion. Device design must balance these trap densities to mitigate reliability risks.
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