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This study investigates the electrical and structural properties of metal–oxide–semiconductor capacitors (MOSCAPs) with in situ metal-organic chemical vapor deposition-grown Al2O3 dielectrics deposited at varying temperatures on (010) β-Ga2O3 and β-(AlxGa1−x)2O3 films with different Al compositions. The Al2O3/β-Ga2O3 MOSCAPs exhibited a strong dependence of electrical properties on Al2O3 deposition temperature. At 900 °C, reduced voltage hysteresis (∼0.3 V) with improved reverse breakdown voltage (74.5 V) was observed, corresponding to breakdown fields of 5.01 MV/cm in Al2O3 and 4.11 MV/cm in β-Ga2O3 under reverse bias. In contrast, 650 °C deposition temperature resulted in higher voltage hysteresis (∼3.44 V) and lower reverse breakdown voltage (38.8 V) with breakdown fields of 3.69 and 2.87 MV/cm in Al2O3 and β-Ga2O3, respectively, but exhibited impressive forward breakdown field, increasing from 5.62 MV/cm at 900 °C to 7.25 MV/cm at 650 °C. High-resolution scanning transmission electron microscopy (STEM) revealed improved crystallinity and sharper interfaces at 900 °C, contributing to enhanced reverse breakdown performance. For Al2O3/β-(AlxGa1−x)2O3 MOSCAPs, increasing Al composition (x) from 5.5% to 9.2% reduced net carrier concentration and improved reverse breakdown field contributions from 2.55 to 2.90 MV/cm in β-(AlxGa1−x)2O3 and 2.41 to 3.13 MV/cm in Al2O3. The electric field in Al2O3 dielectric under forward bias breakdown also improved from 5.0 to 5.4 MV/cm as Al composition increased from 5.5% to 9.2%. The STEM imaging confirmed the compositional homogeneity and excellent stoichiometry of both Al2O3 and β-(AlxGa1−x)2O3 layers. These findings demonstrate the robust electrical performance, high breakdown fields, and excellent structural quality of Al2O3/β-Ga2O3 and Al2O3/β-(AlxGa1−x)2O3 MOSCAPs, highlighting their potential for high-power electronic applications. 

Growths of monoclinic (Al_xGa_1−x)₂O₃thin films up to 99% Al contents are demonstrated via metalorganic chemical vapor deposition (MOCVD) using trimethylgallium (TMGa) as the Ga precursor. The utilization of TMGa, rather than triethylgallium, enables a significant improvement of the growth rates (>2.5 μm h⁻¹) of β‐(Al_xGa_1−x)₂O₃thin films on (010), (100), and (01) β‐Ga₂O₃substrates. By systematically tuning the precursor molar flow rates, growth of coherently strained phase pure β‐(Al_xGa_1−x)₂O₃films is demonstrated by comprehensive material characterizations via high‐resolution X‐ray diffraction (XRD) and atomic‐resolution scanning transmission electron microscopy (STEM) imaging. Monoclinic (Al_xGa_1−x)₂O₃films with Al contents up to 99, 29, and 16% are achieved on (100), (010), and (01) β‐Ga₂O₃substrates, respectively. Beyond 29% of Al incorporation, the (010) (Al_xGa_1−x)₂O₃films exhibit β‐ to γ‐phase segregation. β‐(Al_xGa_1−x)₂O₃films grown on (01) β‐Ga₂O₃show local segregation of Al along (100) plane. Record‐high Al incorporations up to 99% in monoclinic (Al_xGa_1−x)₂O₃grown on (100) Ga₂O₃are confirmed from XRD, STEM, electron nanodiffraction, and X‐ray photoelectron spectroscopy measurements. These results indicate great promises of MOCVD development of β‐(Al_xGa_1−x)₂O₃films and heterostructures with high Al content and growth rates using TMGa for next‐generation high‐power and high‐frequency electronic devices. 

We report the detailed mechanism behind the β to γ phase transformation in Sn-doped and Si-implanted Ga2O3 that we determined based on the direct observation of the atomic scale structure using scanning transmission electron microscopy (STEM). Quantitative analysis of the STEM images revealed that the high concentration of impurity atoms favored the formation of interstitial–divacancy complexes, which then leads to the secondary relaxation that creates additional interstitial atoms and cation vacancies, resulting in a local structure that closely resembles γ-Ga2O3. We explain the mechanism of how the impurity atoms facilitate the transformation, as well as the detailed sequence of the local γ phase transformation. The findings here offer an insight on how the lattice respond to the external stimuli, such as doping and strain, and transform into different structures, which is important for advancing Ga2O3 but also a variety of low symmetry crystals and oxides with multiple polymorphs. 

Abstract (Al_xGa_1–x)₂O₃ is an ultrawide‐bandgap semiconductor with a high critical electric field for next‐generation high‐power transistors and deep‐ultraviolet photodetectors. While (010)‐(Al_xGa_1–x)₂O₃ films have been studied, the recent availability of (100), (01)‐Ga₂O₃ substrates have developed interest in (100), (01)‐(Al_xGa_1–x)₂O₃ films. In this work, an investigation of microscopic and spectroscopic characteristics of (100), (01), (010)–(Al_xGa_1–x)₂O₃ films is conducted. A combination of scanning transmission electron microscopy, atom probe tomography (APT), and first‐principle calculations (DFT) is performed. The findings reveal consistent in‐plane chemical homogeneity in lower aluminum content (x = 0.2) films. However, higher aluminum content (x = 0.5), showed inhomogeneity in (100), (010)–(Al_xGa_1–x)₂O₃ films attributed to their spectroscopic properties. The study expanded APT's capabilities to determine Ga─O and Al─O bond lengths by mapping their ion‐pair separations in detector space. The change in ion‐pair separations is consistent with varying orientations, irrespective of aluminum content. DFT also demonstrated a similar trend, concluding that Ga─O and Al─O bonding energy has an inverse relationship with their bond length as crystallographic orientations vary. This systematic study of growth orientation dependence of (Al_xGa_1–x)₂O₃ films’ microscopic and spectroscopic properties will guide the development of new (100) and (01)‐(Al_xGa_1–x)₂O₃ along with existing (010)–(Al_xGa_1–x)₂O₃ films. 

The in situ metalorganic chemical vapor deposition (MOCVD) growth of Al 2 O 3 dielectrics on β-Ga 2 O 3 and β-(Al x Ga 1−x ) 2 O 3 films is investigated as a function of crystal orientations and Al compositions of β-(Al x Ga 1−x ) 2 O 3 films. The interface and film qualities of Al 2 O 3 dielectrics are evaluated by high-resolution x-ray diffraction and scanning transmission electron microscopy imaging, which indicate the growth of high-quality amorphous Al 2 O 3 dielectrics with abrupt interfaces on (010), (100), and [Formula: see text] oriented β-(Al x Ga 1−x ) 2 O 3 films. The surface stoichiometries of Al 2 O 3 deposited on all orientations of β-(Al x Ga 1−x ) 2 O 3 are found to be well maintained with a bandgap energy of 6.91 eV as evaluated by high-resolution x-ray photoelectron spectroscopy, which is consistent with the atomic layer deposited (ALD) Al 2 O 3 dielectrics. The evolution of band offsets at both in situ MOCVD and ex situ ALD deposited Al 2 O 3 /β-(Al x Ga 1−x ) 2 O 3 is determined as a function of Al composition, indicating the influence of the deposition method, orientation, and Al composition of β-(Al x Ga 1−x ) 2 O 3 films on resulting band alignments. Type II band alignments are determined at the MOCVD grown Al 2 O 3 /β-(Al x Ga 1−x ) 2 O 3 interfaces for the (010) and (100) orientations, whereas type I band alignments with relatively low conduction band offsets are observed along the [Formula: see text] orientation. The results from this study on MOCVD growth and band offsets of amorphous Al 2 O 3 deposited on differently oriented β-Ga 2 O 3 and β-(Al x Ga 1−x ) 2 O 3 films will potentially contribute to the design and fabrication of future high-performance β-Ga 2 O 3 and β-(Al x Ga 1−x ) 2 O 3 based transistors using MOCVD in situ deposited Al 2 O 3 as a gate dielectric. 

Metalorganic chemical vapor deposition (MOCVD) growths of β-Ga 2 O 3 on on-axis (100) Ga 2 O 3 substrates are comprehensively investigated. Key MOCVD growth parameters including growth temperature, pressure, group VI/III molar flow rate ratio, and carrier gas flow rate are mapped. The dependence of the growth conditions is correlated with surface morphology, growth rate, and electron transport properties of the MOCVD grown (100) β-Ga 2 O 3 thin films. Lower shroud gas (argon) flow is found to enhance the surface smoothness with higher room temperature (RT) electron Hall mobility. The growth rate of the films decreases but with an increase of electron mobility as the VI/III molar flow rate ratio increases. Although no significant variation on the surface morphologies is observed at different growth temperatures, the general trend of electron Hall mobilities are found to increase with increasing growth temperature. The growth rates reduce significantly with uniform surface morphologies as the chamber pressure increases. By tuning the silane flow rate, the controllable carrier concentration of (100) β-Ga 2 O 3 thin films between low-10 17  cm −3 and low-10 18  cm −3 was achieved. Under optimized growth condition, an (100) β-Ga 2 O 3 thin film with RMS roughness value of 1.64 nm and a RT mobility of 24 cm 2 /Vs at a carrier concentration of 7.0 × 10 17  cm −3 are demonstrated. The mobilities are primarily limited by the twin lamellae and stacking faults defects generated from the growth interface. Atomic resolution scanning transmission electron microscopy reveals the formation of twin boundary defects in the films, resulting in the degradation of crystalline quality. Results from this work provide fundamental understanding of the MOCVD epitaxy of (100) β-Ga 2 O 3 on on-axis Ga 2 O 3 substrates and the dependence of the material properties on growth conditions. The limitation of electron transport properties of the (100) β-Ga 2 O 3 thin films below 25 cm 2 /Vs is attributed to the formation of incoherent boundaries (twin lamellae) and stacking faults grown along the on-axis (100) crystal orientation.