Electrical Stability of Cr2O3/{eta}-Ga2O3 and NiOx/{eta}-Ga2O3 Heterojunction Diodes

This work reports the electrical characteristics comparison study between Cr2O3 and NiOx based heterojunction diodes (HJD) on halide vapor phase epitaxy (HVPE) grown \b{eta}-Ga2O3 epitaxial layers. Both as-fabricated Cr2O3 and NiOx HJDs exhibited forward current density in a range of 130-150 A/cm^2 at 5 V with rectifying ratios >10^10 and a reverse leakage current density at 10^-8 A/cm^2 at -5 V. The differential specific on-resistance of Cr2O3 and NiOx HJDs was 12.01 mΩcm^2 and 12.05 mΩcm^2, respectively. Breakdown voltages of Cr2O3 HJDs ranged from 1.4-1.9 kV and 1.5-2.3 kV for NiOx HJDs. Theoretical band alignment between Cr2O3 and \b{eta}-Ga2O3 was calculated from first principles. The ambient exposed NiOx/HVPE \b{eta}-Ga2O3 HJDs forward current density degraded after 10 days while that of Cr2O3/HVPE \b{eta}-Ga2O3 HJDs remained nearly unchanged after the same amount of time. It was later confirmed that the ambient exposed sputtered NiOx sheet resistance (Rsh) degradation gave rise to the reduction of the forward current density of the NiOx based HJDs, and water (H2O) was qualitatively determined to be the agent attributed to the forward conduction degradation by measuring the Rsh of NiOx-on-sapphire reference wafer after exposing it to different environments. The Cr2O3/HVPE \b{eta}-Ga2O3 HJD also exhibited enhanced thermal stability compared to the NiOx/\b{eta}-Ga2O3 heterostructures at elevated temperatures. Interfacial nickel gallate (Ga2NiO4) phase formation expected from phase diagrams can explain the reduced thermal stability of NiOx/\b{eta}-Ga2O3 HJDs. This study indicates that Cr2O3 is a stable p-type oxide for the realization of robust multi-kV \b{eta}-Ga2O3 HJDs.

💡 Research Summary

This paper presents a comprehensive comparative study on the electrical performance and, more critically, the long-term stability of heterojunction diodes (HJDs) based on two p-type oxides, chromium oxide (Cr2O3) and nickel oxide (NiOx), on halide vapor phase epitaxy (HVPE)-grown β-Ga2O3.

The study first establishes that as-fabricated devices from both materials exhibit remarkably similar and excellent initial electrical characteristics. Both Cr2O3 and NiOx HJDs demonstrated high forward current density (130-150 A/cm² at 5V), high rectification ratios (>10¹⁰), low reverse leakage current (~10⁻⁸ A/cm² at -5V), and low differential specific on-resistance (~12 mΩ·cm²). Breakdown voltages were also competitive, ranging from 1.4-1.9 kV for Cr2O3 HJDs and 1.5-2.3 kV for NiOx HJDs. First-principles calculations using the HSE hybrid functional revealed a type-II band alignment at the Cr2O3/β-Ga2O3 interface with valence and conduction band offsets of 1.85 eV and 0.48 eV, respectively, supporting its functionality as a heterojunction.

The core findings of the work, however, lie in the stark contrast in environmental and thermal stability between the two material systems. When exposed to ambient air, the forward current density of NiOx-based HJDs degraded severely over 10 days, dropping to less than 1 A/cm², while Cr2O3-based HJDs maintained nearly their original performance. Through controlled experiments on sputtered oxide films on sapphire reference wafers, the researchers identified that the degradation was rooted in a fundamental instability of the NiOx layer itself. The sheet resistance of NiOx increased significantly over time in ambient air. By exposing NiOx films to different controlled environments (vacuum, N2, O2, and H2O), they qualitatively determined that water vapor (H2O) in the air is the primary agent responsible for this electrical degradation.

Furthermore, the thermal stability of Cr2O3 HJDs was found to be superior. Temperature-dependent J-V measurements from 25°C to 175°C showed that Cr2O3 HJDs had a lower temperature-resistance coefficient compared to previously reported values for NiOx HJDs. The study links the thermal instability of the NiOx/β-Ga2O3 interface to interfacial reactions. By consulting ternary phase diagrams from the Materials Project database, it is shown that the Ga-Ni-O system has a stable ternary phase, nickel gallate (Ga2NiO4), which can form at the heterojunction interface at elevated temperatures, degrading the device. In contrast, the Ga-Cr-O system shows no stable ternary phase between β-Ga2O3 and Cr2O3, explaining the enhanced thermal stability of Cr2O3-based heterostructures.

In conclusion, while both oxides enable high-performance multi-kV β-Ga2O3 heterojunction diodes initially, Cr2O3 demonstrates significantly superior stability against ambient exposure (specifically moisture) and high-temperature operation. This makes Cr2O3 a more robust and reliable p-type oxide candidate for the realization of practical, high-power β-Ga2O3-based electronic devices. The work underscores the importance of evaluating material stability and interfacial chemistry alongside initial electrical metrics for power device development.