【Member Papers】Trap-mediated current transport in tunneling layer of Ni/Al₂O₃/β-Ga₂O₃ diodes: effect of proton irradiation

Associate Professor Leidang Zhou's Team from Xi'an Jiaotong University has published a paper titled “Trap-mediated current transport in tunneling layer of Ni/ Al₂O₃/β-Ga₂O₃ diodes: effect of proton irradiation” in the academic journal Applied Surface Science.

Background

High-energy protons prevalent in space environments can easily cause severe displacement damage and single-particle effects, posing a significant challenge to the reliability of β-Ga₂O₃ devices. Therefore, the pursuit of high-performance β-Ga₂O₃ devices with potential radiation tolerance and elucidating the underlying mechanisms of proton irradiation effects on the device performance are critical issues requiring in-depth investigation for the current application and radiation-hardening design of β-Ga₂O₃ devices. The electrical characteristics and radiation resistance of β-Ga₂O₃ devices are influenced by heterojunction terminal design. In contrast to Schottky barrier diodes (SBDs), β-Ga₂O₃ metal-insulator-semiconductor diodes (MISDs) with a nanometer-thick insulator layer for interface modification demonstrate significant improvements in leakage current suppression and breakdown voltage, as exemplified by MISDs based on AlN, Al reaction interface layers, Nb₂O₅, and TiO₂. Notably, GaN-based MIS-high electron mobility transistors (MIS-HEMTs) demonstrate superior proton radiation resistance over conventional HEMTs, which implies the intrinsic radiation-hardening potential of the MIS structure, i.e., a thin Al₂O₃ layer. Nevertheless, systematic investigation into the effects of proton irradiation on β-Ga₂O₃ MISDs remains scarce.

Abstract

The radiation tolerance of ultrawide-bandgap gallium oxide (β-Ga₂O₃) devices has been a critical research frontier for their application in extreme irradiation environments. While metal-insulator-semiconductor diodes (MISDs) exhibit superior reliability compared to Schottky barrier diodes (SBDs), their radiation tolerance has not been comprehensively unveiled. Herein, high-performance Ni/Al₂O₃/β-Ga₂O₃ MISDs are fabricated by incorporating 3-nm Al₂O₃ layer and employing supercritical fluid treatment. The subsequent 5 MeV proton irradiation of MISDs reveals concurrent degradation in both forward and reverse characteristics, distinct from that observed in SBDs. The divergence primarily stems from proton-induced generation of oxygen vacancy electron traps within Al₂O₃. On one hand, the comparative analysis between proton and γ-ray irradiation damage unravels the competitive interplay between electron traps within Al₂O₃ and series resistance in governing the on-state current. On the other hand, the transition of the leakage current after proton irradiation from a positive to a negative temperature dependence at high bias confirms a shift in the dominant leakage mechanism, which is no longer consistent with Poole-Frenkel emission and Fowler-Nordheim tunneling. This research provides detailed insights into the irradiation damage mechanism of β-Ga₂O₃ devices with MIS structure, guiding the establishment of design strategies for highly reliable β-Ga₂O₃ devices suitable for future space applications.

Highlights

High-performance Ni/Al₂O₃/β-Ga₂O₃ MISDs were fabricated by incorporating 3-nm Al₂O₃ layer and employing supercritical fluid treatment.

The distinct performance degradation under proton irradiation between MISDs and Ni/β-Ga₂O₃ SBDs originates from radiation-induced traps within Al₂O₃.

The current transport mechanism in as-fabricated and irradiated MISDs is elucidated.

A physical model is established to differentiate between the on-state current responses to proton and γ-ray irradiation.

Conclusion

The study systematically investigates the proton irradiation damage mechanism in high-performance Ni/Al₂O₃/β-Ga₂O₃ MISDs fabricated by incorporating 3-nm Al₂O₃ layer and employing SCF treatment. The main conclusions are as follows:

(i) After proton irradiation, MISDs exhibited a 44% smaller increase in R_on,sp than SBDs. Nevertheless, despite their initial leakage superiority, MISDs suffered from a substantial leakage current increase with intensified inhomogeneity, in sharp contrast to the reduced leakage current observed in SBDs.

(ii) Both ionization and displacement damage induced by proton irradiation in Al₂O₃ contribute to the increase of N_t and N_eff, whereas the carrier removal effect induced by displacement damage dominates in β-Ga₂O₃. Furthermore, irradiation-induced defects modify the band alignment of Al₂O₃/β-Ga₂O₃ heterojunction, as manifested by an increase of ΔE_C and a transition of ΔE_V from positive to negative.

(iii) The forward current primarily follows TFE model in the subthreshold region and TAT model under on-state conditions. The on-state current is determined by the competitive interplay between the electron traps in Al₂O₃ and R_s in β-Ga₂O₃, contributing to the opposite trends in on-state current after proton and γ-ray irradiation.

(iv) The leakage current of as-fabricated MISDs adheres to the typical transition in mechanisms from PFE at low bias to FNT at higher bias. In contrast, the leakage current of proton-irradiated MISDs exhibits a positive temperature dependence at low bias and a negative dependence at higher bias, confirming the dominance of the TAT process.