【Member Papers】Enhanced secondary electron yield of magnetron sputtered Ga₂O₃ ultra-wide bandgap films via high-temperature annealing and AlN surface modification

Background

      Vacuum photodetectors such as image intensifiers and photomultiplier tubes rely on secondary electron emission (SEE) for signal multiplication, and high secondary electron yield (SEY) materials serve as core dynode layers which determine signal-to-noise ratio of weak light imaging and single photon detection. Traditional MgO and Al₂O₃ dynode materials suffer slow electron replenishment and severe surface charging, leading to structural breakdown during long-term operation. Monoclinic β-Ga₂O₃  ultra-wide bandgap semiconductor owns a 4.6–4.9 eV bandgap and widely tunable resistivity from 10⁻² to 10⁶ Ω・cm, capable of rapidly dissipating surface charge, with excellent thermal and moisture resistance, which makes it ideal for next-generation SEE dynode materials. Existing researches on Ga₂O₃  SEE performance have obvious limitations: as-deposited sputtered films contain massive carbon contaminants and intrinsic oxygen vacancies, leading to high electron escape barrier and low pristine SEY of only 2.54. Single high-temperature annealing can merely slightly improve emission capacity, lacking synergistic AlN surface modification strategy with low electron affinity. Previous literatures only investigate annealing or coating separately, without systematic quantitative analysis on combined process including annealing, AlN deposition and Ar⁺ sputtering. No precise fabrication route for 4 nm quasi-continuous AlN capping layer is reported, and the modulation mechanism of interfacial Al-O-N hybrid phases on secondary electron escape remains unclear, forming a major research gap for mass production of high-performance vacuum electron multipliers.

Abstract

      Gallium oxide (Ga₂O₃ ) semiconductor, possessing an ultra-wide bandgap, combines strong secondary electron emission (SEE) capabilities with controllable electrical conductivity. Hence, Ga₂O₃  is supposed to find application in dynodes achieving electron multiplication and charge replenishment. In this work, Ga₂O₃  films were deposited by magnetron sputtering, and high-temperature annealing was used to modulate SEE yield (SEY). Experimental results indicate the 1000 °C annealing in oxygen atmosphere enhances the crystallinity of Ga₂O₃  films and partly eliminates the surface organic contamination, then improves film’s maximum SEY (δₘ ) from 2.54 to 3.21. After coating AlN thin film, which possesses low electron affinity, on Ga₂O₃  sample, surface condition and SEY were further improved. Elemental analysis indicates that following the deposition of AlN, Al element exists on the surface in three forms: Al-N, Al-O, and Al-O-N. And δₘ  increased from 2.54 to 3.29. Following annealing and Ar⁺ sputtering, organic contaminants were further removed, restoring the pristine AlN/Ga₂O₃  layer structure. SEY was further enhanced, the AlN-modified Ga₂O₃  sample raised δₘ  to 3.35 after annealing, and to 3.44 after annealing and Ar⁺ sputtering. The ultra-wide bandgap Ga₂O₃  material demonstrate promising potential as practical SEE materials for vacuum electron multiplication applications. Moreover, this work provides a valuable reference for expanding SEE applications of other ultra-wide bandgap materials.

Highlights

      Propose dual optimization strategy combining high-temperature oxygen annealing and ultrathin AlN surface modification to boost SEY of magnetron sputtered β-Ga₂O₃ films.

      Verify that 1000 ℃ oxygen annealing improves crystallinity and removes surface carbon impurities, δₘ  rises from 2.54 to 3.21 for bare Ga₂O₃  film.

      Reveal three Al-containing interfacial phases (Al-N, Al-O, Al-O-N) formed after AlN coating, and realize δₘ =3.29 for AlN-modified sample.

      Achieve cumulative 35.4% SEY enhancement via sequential annealing+AlN deposition+Ar⁺ sputtering, final maximum δₘ  up to 3.44.

Conclusion

      In summary, Ga₂O₃  films were successfully deposited onto low-resistivity n-Si(100) substrates via RF magnetron sputtering, and their SEE performance was comprehensively engineered through post-deposition thermal treatment and AlN surface modification. The as-deposited Ga₂O₃  films of 350 nm were used for surface-sensitive SEY measurements, whereas thicker variants (approximately 1 μm) were synthesized exclusively to ensure sufficient signal intensity for XRD and EDS analyses. High-temperature thermal treatment at 1000 ◦C induced profound structural densification within the matrix, as macroscopically underscored by a 16%-18% contraction in film thickness relative to the as-deposited state. Meanwhile, annealing increased the average grain diameter by 8.7% (from 41.1 nm to 44.7 nm) and the median by 8.9% (from 32.6 nm to 35.5 nm). While the β-Ga₂O₃  phase emerged following high-temperature O₂ annealing (600-1000 °C), substrate-related spectral features were simultaneously resolved in the XRD patterns. Quantitative EDS profiling revealed a sub-stoichiometric Ga:O atomic ratio of 2:2.84 post-annealing. High-temperature annealing systematically improved the δₘ  of the Ga₂O₃  films from 2.54 to 3.21, representing a 26.3% enhancement. Furthermore, surface modification via a 4.0 nm AlN nominal layer boosted the low-energy emission regime, shifting δₘ  from 2.54 to 3.29 and reducing the Eₘ from 473 eV to 306 eV. Through the synergistic optimization of AlN modification and post-annealing, the δₘ  was successfully elevated to 3.35.

      These performance improvements are directly tied to structural and compositional evolutions within the Ga₂O₃  surface. The sub-stoichiometric Ga:O ratio confirms the generation of surface V_O which, along with the selective thermal elimination of carbonaceous contaminants, lowers the surface electron escape barrier. Given the nominal thickness verified by ToF-SIMS profiling, the AlN modifier resides as a functionalized, quasi-continuous layer across the Ga₂O₃  surface. Complementary core-level XPS and ToF-SIMS profiling confirmed that the 1000 ◦C reconstruction induced interfacial atomic interdiffusion—marked by a prominent intensity plateau within the 40–87 s sputtering window. This interdiffusion yielded a composite surface where the high SEE from the low-electron-affinity AlN surface modification layer is sustained by rapid electron replenishment from the underlying Ga₂O₃  bulk, effectively suppressing surface charging.

      These findings indicate that high-temperature annealing yields a highly stable composite SEE surface defined primarily by Ga-O and Al-N hybridization frameworks, while secondary Al-O and Al-O-N groups act as minor surface-modulating components. Ultimately, this dual-engineering strategy provides a powerful and reproducible methodology for tailoring the SEY properties of Ga₂O₃ -based electron multipliers. This practical scheme can be universally extended to enhance other UWBG material systems, expanding the library of robust material options for next-generation high-power electronic, RF, and electron multiplication devices.

Project Support

      Project supported by the Integrated Circuit Manufacturing Materials Innovation Consortium Project of Gansu Province (Grant No. 25ZDGE004), the Fundamental Research Funds for the Central Universities (Grant. No. xzy012025052), the National Natural Science Foundation of China (Grant. No. 62101425), the Major scientific research instrument development project of National Natural Science Foundation of China (Grant No. 52127817) and the Shenzhen Science and Technology Program (Grant No. KJZD20231023100501003).