【International Papers】Structural and electrical properties of (100) β-(ScₓGa₁₋ₓ)₂O₃/Ga₂O₃ heterojunctions

      Researchers from the Institute of Science Tokyo have published a dissertation titled "Structural and electrical properties of (100) β-(Sc_xGa_1−x)₂O₃/Ga₂O₃ heterojunctions" in Journal of Applied Physics.

Background

      Ultrawide bandgap semiconductors have been a recent subject of intense investigation for the development of electric devices operable under harsh conditions, such as high voltages, high powers, high temperatures, and high radiations. Among candidates, monoclinic β-Ga₂O₃, the most stable polytype of Ga₂O₃, is promising owing to the availability of melt-grown large bulk single crystals with tunable electrical properties from semi-insulating to highly conductive and its sufficiently large bandgap energy (E_g) of 4.4–5.0 eV. Despite its distinctive crystal structure, bandgap engineering is viable in β-Ga₂O₃ through cation substitution, akin to other group III compound semiconductors such as wurtzite-type InN–GaN–AlN or corundum-type Al₂O₃–Ga₂O₃ systems. This approach enables the growth of a pseudomorphic heterostructure applicable to electronic devices. In particular, modulation-doped heterostructures, where a donor impurity is selectively doped into the barrier layer, are attractive since they form a two-dimensional electron gas (2DEG) at the interface. High-density 2DEG in β-Ga₂O₃ is predicted to exceed the bulk electron mobility owing to the screening of phonon scattering. Therefore, they can be applied in heterojunction field-effect transistors (HFETs) capable of high-frequency or high-voltage operation.

Abstract

      Continuous composition-spread and heavily Si-doped β-(Sc_xGa₁₋x)₂O₃ (0.04 ≤ x ≤ 0.20) films were epitaxially grown on (100) β-Ga₂O₃ substrates by pulsed-laser deposition. The x dependence of structural and electrical properties was examined at room temperature. Vertical Schottky barrier diodes (SBDs) of Pt/β-(Sc_xGa₁₋x)₂O₃ with x ≤ 0.10 showed ideality factors of 1.6–1.7 and series resistance less than 10 mΩ cm². In addition, the effective Schottky barrier height increased by ∼0.2 eV with increasing x from 0 to 0.10. The parasitic series resistance in SBDs with x > 0.10 increased exponentially as x increased, which was attributed to carrier localization in β-(Sc_xGa₁₋x)₂O₃ epilayers. This behavior was correlated with a sharp increase in the density of incoherent twin boundaries inherent in (100)-oriented β-Ga₂O₃-based epilayers. Moreover, the β-(Sc_xGa₁₋x)₂O₃/Ga₂O₃ interface with 0.04 ≤ x ≤ 0.11 accommodated two-dimensional electrons with a sheet density of 5–6 × 10¹² cm⁻², which is comparable to that observed at the (010) β-(Al_xGa₁₋x)₂O₃/Ga₂O₃ interface. Our study demonstrates extended material designs for realizing modulation doping in β-Ga₂O₃ and its application to heterojunction field-effect transistors.

Conclusion

      We investigated various properties of (100)-oriented β-(Sc_xGa₁₋x)₂O₃ epilayers as a function of x, including the electrical characteristics of SBDs with Pt electrodes and 2DEG at the interface with Ga₂O₃ substrates. In J–V curves for the SBDs, clear rectification behaviors were confirmed for samples with x ≤ 0.12, while an exponential increase in resistivity above x = 0.10 was ascribed to the insulation behavior of β-(Sc_xGa₁₋x)₂O₃ epilayers. In addition, we found that the linear increase in ϕ_SBH,eff corresponds to ΔE_g(x). From AFM and EBSD inspections, the surface morphology and microscopic crystal orientation were found to influence the temperature-dependent transport properties as well as the insulation behavior. In short, all these results could be explained in terms of the characteristic size of grains in between ITBs inherent in (100)-oriented β-Ga₂O₃-based epilayers and ionized donor concentration, both of which decreased as x increased. The carrier density profiles at the (100) β-(Sc_xGa₁₋x)₂O₃/Ga₂O₃ interface revealed the formation of a 2DEG with a sheet density of 5–6 × 10¹² cm⁻² for x ≤ 0.11. This value is comparable to that obtained for (010) β-(Al_xGa₁₋x)₂O₃/Ga₂O₃. Although the doping efficiency of Si donors in the present samples is limited, particularly above x = 0.10, our results contribute to the continuous development of high-performance heterojunction devices based on β-Ga₂O₃.