【International Papers】Absorption anisotropy of orthorhombic single-domain κ-Ga₂O₃ from infrared to ultraviolet: Phonons and bandgaps

      Researchers from the Otto-von-Guericke-Universität Magdeburg have published a dissertation titled "Absorption anisotropy of orthorhombic single-domain κ-Ga₂O₃ from infrared to ultraviolet: Phonons and bandgaps" in Applied Physics Letters.

Background

      Gallium oxide (Ga₂O₃) is an ultra-wide-bandgap semiconductor with several polymorphs (α, β, γ, δ, and κ phases). Due to its high breakdown field and large bandgap, it has attracted strong interest for high-power electronic devices and solar-blind photodetectors. Among these polymorphs, the orthorhombic κ-phase (Pna2₁) is particularly noteworthy because of its spontaneous polarization and ferroelectricity, which enable the formation of a two-dimensional electron gas (2DEG). This makes κ-Ga₂O₃ a promising material for high-electron-mobility transistors (HEMTs) and related applications.

      However, the growth of κ-Ga₂O₃ thin films presents a major challenge—the formation of rotational domains. Conventional κ-Ga₂O₃ layers often consist of multiple domains rotated by 120°, known as multi-domain κ-Ga₂O₃, which results in an apparent in-plane isotropy. Consequently, previous optical studies could only resolve the anisotropy parallel and perpendicular to the (001) direction, without distinguishing the intrinsic in-plane optical anisotropy of the orthorhombic crystal structure.

      In this context, the present study employs spectroscopic ellipsometry from the infrared (IR) to ultraviolet (UV) spectral range to determine the anisotropic dielectric functions and absorption coefficients of single-domain κ-Ga₂O₃. The analysis reveals the infrared-active phonon modes with B₁ and B₂ symmetry, as well as the anisotropic interband transitions and high-frequency dielectric constants in the visible–ultraviolet region, providing comprehensive insight into the optical anisotropy of this orthorhombic semiconductor.

Abstract

      We present the anisotropic dielectric functions and absorption coefficients of orthorhombic single-domain (001) κ-Ga₂O₃. Single-domain κ-Ga₂O₃ grown by mist chemical vapor deposition on ε-GaFeO₃ was investigated by spectroscopic ellipsometry, from the infrared to the ultraviolet spectral range. We find infrared-active phonon modes of B₁ and B₂ symmetry. The dielectric functions in the visible–ultraviolet range yield the high frequency dielectric constants ε_∞ and the anisotropic fundamental interband transitions at 4.82 and 5.75 eV (⁠ ε_xx⁠), 5.09 eV (⁠ ε_yy⁠), and 5.43 eV (⁠ ε_zz⁠). The results are compared to theoretical calculations, and earlier studies on multi-domain κ-Ga₂O₃.

Conclusion

      In conclusion, single-domain κ-Ga₂O₃ grown by mist-CVD on ε-GaFeO₃ was investigated by spectroscopic ellipsometry across the IR to the UV spectral range, to determine its anisotropic DFs. The IR-DFs yield B₁ and B₂ phonon modes. The UV-DFs reveal the anisotropy of the high frequency dielectric constants as well as the absorption onsets. By evaluating the UV-DFs using a line shape fit, we find the first and partly second interband transition energies, which are in agreement with certain theoretical predictions and previously reported experimental results on multi-domain κ-Ga₂O₃.

FIG. 1.Ellipsometric angles Ψ_pp (left) and Δ_pp (right) of single-domain κ-Ga₂O₃ in the infrared spectral range with the (010)-axis perpendicular (bottom) and parallel to the plane of incidence (top) for different angles of incidence 30°, 50°, and 70° in red, blue, and green, respectively, together with their corresponding point-by-point (pbp) fits in black.

FIG. 2.Point-by-point complex dielectric functions (solid lines) in the spectral range of the infrared-active optical phonons of single-domain κ-Ga₂O₃. The real (top curves, axis to the left) and imaginary parts (bottom curves, axis to the right) of ε_xx and ε_yy are shown in blue and red, respectively. Additionally, line shape models (black dashed) are also shown.

FIG. 3.Ellipsometric angles Ψ_pp (left) and Δ_pp (right) of single-domain κ-Ga₂O₃ in the visible–ultraviolet spectral range with the (010)-axis perpendicular (bottom) and parallel to the plane of incidence (top) for different angles of incidence 30°, 60°, 45°, and 75° shown in red, green, blue, and pink, respectively, together with their corresponding point-by-point (pbp) fits in black.

 

DOI：

doi.org/10.1063/5.0302622