【International Papers】Tuning of crystal structure and electronic band gap of the monoclinic Ga₂O₃ by simultaneous alloying with Al₂O₃ and In₂O₃

      Researchers from the Lviv Polytechnic National University have published a dissertation titled " Tuning of crystal structure and electronic band gap of the monoclinic Ga₂O₃ by simultaneous alloying with Al₂O₃ and In₂O₃" in Scientific Reports.

Background

      Gallium oxide (Ga₂O₃) is a well-established ultra-wide bandgap semiconductor that has attracted considerable interest due to its exceptional properties, including a large bandgap of about 4.9 eV, a high breakdown field of 8 MV/cm, and excellent thermal stability (see Zhang et al.1 and references therein). These properties position Ga₂O₃ as a highly promising material for diverse applications, including high-power electronic devices, solar-blind ultraviolet (UV) photodetectors and scintillators. Ga₂O₃ exists in several structural polymorphs, including the corundum-like (α), monoclinic (β), defective spinel (γ), and two variations (ε and δ) of orthorhombic structure. Among these, the monoclinic phase (β-Ga₂O₃) stands out as the most prevalent as it is the most thermodynamically stable and, hence, readily obtainable under ambient conditions..

      Monoclinic β-Ga₂O₃ has a direct bandgap of 4.87 eV with a small contribution of indirect transitions of 4.83 eV. The valence band is mainly formed by the occupied O 2p derived states slightly hybridized with Ga 3d, 4p, and 4 s orbitals, while the conduction band is mainly composed of Ga 4 s derived states (see Zhang et al. and references therein for details).

      When activated with Cr³⁺ ions, β-Ga₂O₃ produces a broad and efficient near-infrared (NIR) emission at about 750 nm suitable for NIR tunable lasers, NIR phosphor-converted light-emitting diodes (pc-LEDs), high pressure and temperature sensors..

Abstract

      The paper presents the study of phase and structural behavior of pseudoternary compound (Ga_1-x-yIn_xAl_y)₂O₃ with focus on the compositional cross-section where the x/y ratio is a fixed at 0.31/0.69. This specific ratio ensures that the average cation radius, equal to that of Ga³⁺ ions, remains unchanged. Through the combination of experimental XRD studies and DFT calculations, the stability region of the monoclinic phase within the Ga₂O₃–Al₂O₃–In₂O₃ ternary system was established. Detailed analysis of the crystal lattice parameters and unit cell volume of the monoclinic structure was carried out across a wide range of compositions. An empirical relationship was derived linking the monoclinic lattice parameters to the average ionic radius of the cations (Ga³⁺, Al³⁺, In³⁺) enabling prediction of lattice parameters in monoclinic (Ga_1-x-yIn_xAl_y)₂O₃ solely from chemical composition. The experimental crystal structure studies and the electronic structure calculations suggest that in the monoclinic (Ga_1-x-yIn_xAl_y)₂O₃ structure the tetrahedral positions of Ga1 atoms are preferentially occupied by Ga³⁺ and Al³⁺ cations, while the octahedral Ga2 sites accommodate a mixture of Ga³⁺, Al³⁺ and In³⁺ cations. Additionally, the presence of unidentified phase(s) was confirmed in the central region of the Ga₂O₃–Al₂O₃–In₂O₃ triangle. Comparison of the calculated optical absorption spectra and the Tauc-plots derived from the diffuse reflectance spectra indicate that the monoclinic (Ga_1−x−yIn_xAl_y)₂O₃ compounds have a direct band gap.

Conclusions

      The phase and structural behavior in the Ga₂O₃–Al₂O₃–In₂O₃ pseudoternary system has been studied in detail trough the experimental XRD studies of over 50 microcrystalline samples prepared by the solution combustion method. For the first time, the range of existence of a doubly substituted monoclinic solid solution (Ga_1-x-yIn_xAl_y)₂O₃ with varying x and y values was established, and a complete set of crystal structure parameters was determined.

      Special attention was given to the compositional cross-section with a fixed x/y ratio of 0.31/0.69, i.e. corresponding to the (Ga_1-zAl_0.69*zIn_0.31*z)₂O₃ compositions, which ensures the maintenance of the average cation radius equal to that of Ga³⁺ ions. It has been confirmed that a single-phase monoclinic structure in this series exists in the concentration range 0 ≤ z ≤ 0.4. At higher In(Al) contents, diffraction peaks of unknown phases appear in the X-ray patterns, which gradually increase with decreasing gallium content in the Ga_1-zAl_0.69*zIn_0.31*z)₂O₃ and become dominant at compositions z = 0.8 and 0.9. The presence of a new unknown phase (Ga_1-x-yIn_xAl_y)₂O₃ in the Ga₂O₃-Al₂O₃-In₂O₃ system has been revealed for the first time. Further research and interpretation of new crystal phases are ongoing and will be published separately.

      The crystal lattice parameters and unit cell volume of the monoclinic structure were analyzed in detail across a wide range of compositions, including the non-single-phase compositions, including multiphase samples, demonstrating the possibility of targeted tuning of the monoclinic structure by simultaneously replacing gallium cations in both octahedral and tetrahedral positions. An empirical expression relating the monoclinic lattice parameters to the average radius of M³⁺ (M = Ga, Al, In) cations was established, allowing the prediction of the crystal lattice parameters of β-(Ga_1-x-yIn_xAl_y)₂O₃ based solely on its chemical composition (i.e. the average ionic radius of the cations).

      The crystal structure refinement suggests that the tetrahedral positions of Ga1 atoms in the monoclinic (Ga_1-x-yIn_xAl_y)₂O₃ structures are predominantly occupied by Ga³⁺ and Al³⁺ cations, while the octahedral Ga2 positions contain a mixture of Ga³⁺, Al³⁺ and In³⁺ cations. The incorporation of Al³⁺ and In³⁺ cations into the monoclinic (Ga_1-x-yIn_xAl_y)₂O₃ structure has also been confirmed by analyzing the interatomic distances within MO₄ tetrahedra and MO₆ octahedra (M = Al, In, Ga) in the (Ga_1-zAl_0.69*zIn_0.31*z)₂O₃ structures as a function of the Al and In content (z value).

      The electronic structure calculations confirmed that the tetrahedral Ga³⁺ lattice positions are the preferentially substituted by Al³⁺ ions, rather than In³⁺ ones. A comparison of the calculated optical absorption spectra with Tauc-plots derived from experimental diffuse reflectance spectra led to a conclusion that (Ga_1−x−yIn_xAl_y)₂O₃ compounds possess a direct band gap.