Exploiting Mg-Interdiffusion-Driven Work-Function Reduction in Ti/Mg/Ti Multilayers to Achieve Low-Resistivity Ohmic...

Low-resistivity Ohmic contact formation in β-gallium oxide (β-Ga₂O₃) is crucial for high-efficiency power electronics and deep-ultraviolet (DUV) optoelectronic devices. In this study, we successfully developed a Ti/Mg/Ti multilayered metal stack (20 nm/50 nm/20 nm) as an Ohmic contact to (001) β-Ga₂O₃. The Mg interlayer, with its low work function relative to Ti and the electron affinity of β-Ga₂O₃, effectively reduces the Schottky barrier, enabling low contact resistivity. The Ti layers on both sides of the Mg layer act as protective caps, preventing Mg oxidation and improving chemical stability. Ultraviolet photoelectron spectroscopy (UPS) analysis showed that the Ti/Mg/Ti metal stack achieved a stabilized work function of ∼3.94 eV after 400 °C annealing, close to β-Ga₂O₃'s electron affinity, facilitating efficient electron injection. X-ray photoelectron spectroscopy (XPS) confirmed interfacial stability, indicating that the stack protects the reactive Mg layer while compensating for the higher work function of Ti. Moreover, cross-sectional transmission electron microscopy and elemental mapping analysis reveal that the in situ formed interfacial TiO_x layer on (001) β-Ga₂O₃ is relatively thin (2.5 nm) and homogeneous. Importantly, Mg diffusion into Ti layers highly affects the reduction of the effective work function. Transmission electron microscopy and elemental analysis further confirm that Mg diffuses into the Ti layers, leading to the formation of a Ti-Mg alloy. This alloying effect significantly reduces the effective work function of the contact, thereby facilitating electron injection and lowering the contact resistance. Transmission line model (TLM) measurements revealed a contact resistivity of 6.25 × 10^-4 Ω·cm² for the 400 °C annealed Ti/Mg/Ti metal stack, which is significantly lower than that of Ti or Mg alone. Moreover, TLM measurements performed under cryogenic temperatures revealed that the Ti/Mg/Ti contact stack annealed at 400 °C in an argon (Ar) atmosphere exhibits minimal temperature dependence, indicating that carrier transport is predominantly governed by tunneling. A similar tunneling-dominated behavior was also observed for samples annealed in vacuum and nitrogen (N₂) atmospheres. In contrast, contacts annealed in air showed a strong temperature dependence, confirming that thermionic emission is the dominant transport mechanism in this case. These electrical observations are fully supported by XPS analysis, which shows significant oxidation of both Mg and Ti for the air-annealed samples, reduced oxidation for N₂ and vacuum annealing, and the lowest oxidation level for Ar annealing. The reduced oxidation in inert and oxygen-free environments enhances tunneling, whereas the high oxide formation in air suppresses tunneling and strengthens thermionic emission. Overall, the combined TLM and XPS results clearly demonstrate that the annealing atmosphere plays a critical role in defining the contact transport mechanism, with Ar providing the most stable and tunneling-favorable interface. Finally, the proposed metal stack-based Ohmic contact (Ti/Mg/Ti contact stack annealed at 400 °C in an Ar atmosphere) was incorporated into β-Ga₂O₃-based metal-semiconductor-metal (MSM) photodetectors to assess device performance. Under 2 μW of 255 nm DUV illumination, the devices exhibited a high responsivity of 14,240.7 A/W. These findings demonstrate that the Ti/Mg/Ti multilayer approach effectively combines low work function engineering with oxidation protection to achieve low-resistance, chemically stable Ohmic contacts. This strategy offers a promising route for enhancing β-Ga₂O₃-based high-power and DUV optoelectronic devices.