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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. 

A new record‐high room‐temperature electron Hall mobility (μ_RT = 194 cm² V⁻¹ s⁻¹atn ≈ 8 × 10¹⁵ cm⁻³) for β‐Ga₂O₃is demonstrated in the unintentionally doped thin film grown on (010) semi‐insulating substrate via metal‐organic chemical vapor deposition (MOCVD). A peak electron mobility of ≈9500 cm² V⁻¹ s⁻¹is achieved at 45 K. Further investigation on the transport properties indicates the existence of sheet charges near the epilayer/substrate interface. Si is identified as the primary contributor to the background carrier in both the epilayer and the interface, originating from both surface contamination and growth environment. The pregrowth hydrofluoric acid cleaning of the substrate leads to an obvious decrease in Si impurity both at the interface and in the epilayer. In addition, the effect of the MOCVD growth condition, particularly the chamber pressure, on the Si impurity incorporation is studied. A positive correlation between the background charge concentration and the MOCVD growth pressure is confirmed. It is noteworthy that in a β‐Ga₂O₃film with very low bulk charge concentration, even a reduced sheet charge density plays an important role in the charge transport properties.