Low-Temperature As-Grown Crystalline β-Ga2O3 Films via Plasma-Enhanced Atomic Layer Deposition

Analysis of Thermal Stress in Vanadium Dioxide Thin Films by Finite Element Method

The buckling, de-lamination, and cracking of the thin film/substrate system caused by thermal stress is the main obstacle for functional failure. Moreover, the thermal stress of vanadium dioxide (VO₂) thin film may be more complicated due to the stress re-distribution caused by phase transition. Therefore, the thermal stress of VO₂ thin films deposited on four substrates with different materials (fused silica, silicon slice, sapphire, and glass) has been studied by finite element method in the present work. The influences of external temperature, substrate, and interlayer on thermal stress were analyzed. It was found that the substrates can greatly affect the thermal stresses, which were mainly caused by the mismatch of coefficient of thermal expansion (CTE). The thermal stress had a linear relationship with the external temperature, but this tendency would be redistributed or even change direction when phase transition occurred. The simulated results were in tandem with the analytical method. Meanwhile, the radial stress and shear stress distribution under the influence of phase transition were calculated. In addition, the reduction of thermal stress and shear stress showed that the appropriate interlayer can enhance the adhesive strength effectively.

Compact Ga2O3 Thin Films Deposited by Plasma Enhanced Atomic Layer Deposition at Low Temperature

Amorphous Gallium oxide (Ga₂O₃) thin films were grown by plasma-enhanced atomic layer deposition using O₂ plasma as reactant and trimethylgallium as a gallium source. The growth rate of the Ga₂O₃ films was about 0.6 Å/cycle and was acquired at a temperature ranging from 80 to 250 °C. The investigation of transmittance and the adsorption edge of Ga₂O₃ films prepared on sapphire substrates showed that the band gap energy gradually decreases from 5.04 to 4.76 eV with the increasing temperature. X-ray photoelectron spectroscopy (XPS) analysis indicated that all the Ga₂O₃ thin films showed a good stoichiometric ratio, and the atomic ratio of Ga/O was close to 0.7. According to XPS analysis, the proportion of Ga³⁺ and lattice oxygen increases with the increase in temperature resulting in denser films. By analyzing the film density from X-ray reflectivity and by a refractive index curve, it was found that the higher temperature, the denser the film. Atomic force microscopic analysis showed that the surface roughness values increased from 0.091 to 0.187 nm with the increasing substrate temperature. X-ray diffraction and transmission electron microscopy investigation showed that Ga₂O₃ films grown at temperatures from 80 to 200 °C were amorphous, and the Ga₂O₃ film grown at 250 °C was slightly crystalline with some nanocrystalline structures.

Large-Sized Nanocrystalline Ultrathin β-Ga2O3 Membranes Fabricated by Surface Charge Lithography

Large-sized 2D semiconductor materials have gained significant attention for their fascinating properties in various applications. In this work, we demonstrate the fabrication of nanoperforated ultrathin β-Ga₂O₃ membranes of a nanoscale thickness. The technological route includes the fabrication of GaN membranes using the Surface Charge Lithography (SCL) approach and subsequent thermal treatment in air at 900 °C in order to obtain β-Ga₂O₃ membranes. The as-grown GaN membranes were discovered to be completely transformed into β-Ga₂O₃, with the morphology evolving from a smooth topography to a nanoperforated surface consisting of nanograin structures. The oxidation mechanism of the membrane was investigated under different annealing conditions followed by XPS, AFM, Raman and TEM analyses.

Ga2O3 and Related Ultra-Wide Bandgap Power Semiconductor Oxides: New Energy Electronics Solutions for CO2 Emission Mitigation

Currently, a significant portion (~50%) of global warming emissions, such as CO₂, are related to energy production and transportation. As most energy usage will be electrical (as well as transportation), the efficient management of electrical power is thus central to achieve the XXI century climatic goals. Ultra-wide bandgap (UWBG) semiconductors are at the very frontier of electronics for energy management or energy electronics. A new generation of UWBG semiconductors will open new territories for higher power rated power electronics and solar-blind deeper ultraviolet optoelectronics. Gallium oxide—Ga₂O₃ (4.5–4.9 eV), has recently emerged pushing the limits set by more conventional WBG (~3 eV) materials, such as SiC and GaN, as well as for transparent conducting oxides (TCO), such asIn₂O₃, ZnO and SnO₂, to name a few. Indeed, Ga₂O₃ as the first oxide used as a semiconductor for power electronics, has sparked an interest in oxide semiconductors to be investigated (oxides represent the largest family of UWBG). Among these new power electronic materials, Al_xGa_1-xO₃ may provide high-power heterostructure electronic and photonic devices at bandgaps far beyond all materials available today (~8 eV) or ZnGa₂O₄ (~5 eV), enabling spinel bipolar energy electronics for the first time ever. Here, we review the state-of-the-art and prospects of some ultra-wide bandgap oxide semiconductor arising technologies as promising innovative material solutions towards a sustainable zero emission society.