Indium oxide (In₂O₃) is a promising ultra-wide bandgap semiconductor with a bandgap of approximately 3.7 eV, positioning it between conventional wide bandgap materials like GaN and emerging ultra-wide bandgap candidates such as Ga₂O₃. Its high electron mobility, exceeding 200 cm²/V·s in thin-film form, makes it particularly attractive for high-frequency and high-power electronic applications. The metastable cubic phase of In₂O₃, known as the bixbyite structure, is the most commonly studied due to its compatibility with epitaxial growth techniques and favorable electronic properties. However, challenges such as oxygen vacancy management, phase stability, and Ohmic contact formation must be addressed to fully exploit its potential.

Epitaxial growth of high-quality In₂O₃ thin films is critical for device applications. The cubic phase can be stabilized on lattice-matched substrates such as yttria-stabilized zirconia (YSZ) or gadolinium gallium garnet (GGG), which provide a close lattice match to the bixbyite structure. Molecular beam epitaxy (MBE) and pulsed laser deposition (PLD) are the most common techniques for growing single-crystalline In₂O₃ films. MBE offers precise control over stoichiometry and doping, while PLD is advantageous for achieving high-quality films at relatively lower temperatures. The growth temperature typically ranges between 300°C and 600°C, with lower temperatures favoring the cubic phase and higher temperatures risking phase transformation to the less desirable rhombohedral phase. Oxygen partial pressure during growth is another critical parameter, as it influences the concentration of oxygen vacancies, which act as unintentional n-type dopants.

Ohmic contact formation to In₂O₃ is challenging due to its wide bandgap and the need for low-resistance interfaces. Titanium-based metallization schemes, such as Ti/Au or Ti/Al/Ni/Au, have shown promise, with specific contact resistances reaching as low as 10⁻⁵ Ω·cm² after annealing at temperatures around 400°C to 500°C. The annealing process promotes interfacial reactions that reduce the Schottky barrier height, enabling efficient carrier injection. However, excessive annealing can lead to indium out-diffusion and contact degradation, necessitating careful optimization. Alternative approaches include the use of transparent conducting oxides like indium tin oxide (ITO) as contact layers, which can simultaneously serve as both Ohmic contacts and optical windows in optoelectronic devices.

The high electron mobility of In₂O₃ is a key advantage for high-frequency transistors. Field-effect transistors (FETs) based on In₂O₃ have demonstrated cutoff frequencies (fT) exceeding 40 GHz in sub-micron gate-length devices, outperforming many oxide semiconductors and approaching the performance of GaN-based devices. The high electron velocity, estimated at around 1.5 × 10⁷ cm/s under high fields, contributes to these impressive figures of merit. However, the relatively low bandgap compared to Ga₂O₃ (∼4.8 eV) limits the breakdown voltage, making In₂O₃ more suitable for high-frequency rather than high-power applications. The Baliga figure of merit (BFOM), which quantifies the trade-off between on-resistance and breakdown voltage, is lower for In₂O₃ than for Ga₂O₃, but its superior mobility makes it competitive in high-speed switching applications.

Oxygen vacancy management is a critical challenge in In₂O₃ devices. Oxygen vacancies are native defects that act as shallow donors, providing high background carrier concentrations in the range of 10¹⁸ to 10¹⁹ cm⁻³. While this facilitates low-resistance Ohmic contacts, it also complicates device design by making it difficult to achieve fully depleted channels in FETs. Post-growth annealing in oxygen-rich environments can reduce vacancy concentrations, but excessive oxidation can degrade electrical properties. Doping with elements such as tin or zinc can help control carrier concentrations, but these dopants may also introduce scattering centers that reduce mobility. Advanced defect passivation techniques, such as hydrogen plasma treatment, have shown promise in mitigating the effects of oxygen vacancies without compromising mobility.

Stability under high electric fields is another concern for In₂O₃ devices. While the material exhibits reasonable thermal stability up to temperatures of around 500°C, prolonged operation at high fields can lead to defect generation and mobility degradation. The cubic phase is metastable, and under high stress or temperature, it may transform into the rhombohedral phase, which has inferior electronic properties. Encapsulation layers such as Al₂O₃ or SiO₂ can improve environmental stability, but long-term reliability studies are still needed to assess device lifetimes under operational conditions.

Comparing In₂O₃ with Ga₂O₃ reveals trade-offs between bandgap and electron velocity. Ga₂O₃ has a wider bandgap, enabling higher breakdown voltages and better suitability for power electronics. However, its electron mobility is significantly lower, typically below 200 cm²/V·s, limiting its high-frequency performance. In₂O₃, with its higher mobility, is better suited for RF and mixed-signal applications where speed is prioritized over voltage handling. The two materials may find complementary roles in future semiconductor technologies, with Ga₂O₃ dominating high-power applications and In₂O₃ excelling in high-frequency circuits.

In summary, In₂O₃ is a compelling ultra-wide bandgap semiconductor with high electron mobility and compatibility with epitaxial growth techniques. Its metastable cubic phase can be stabilized on lattice-matched substrates, enabling high-quality thin-film growth. Ohmic contact formation has been demonstrated with low-resistance metallization schemes, though careful annealing is required. The material’s high electron velocity makes it attractive for high-frequency transistors, though its lower bandgap compared to Ga₂O₃ limits its use in high-power applications. Oxygen vacancy management remains a key challenge, requiring advanced defect engineering approaches. With further development, In₂O₃ could play a significant role in next-generation electronic and optoelectronic devices.