BGaN is an emerging ultrawide bandgap semiconductor with important applications ranging from power electronics to ultraviolet light emitters. To date, BGaN boron composition has been limited to <10% in the wurtzite phase. Herein, a 200 nm thick high quality mixed-phase BGaN film was grown via horizontal–reactor metalorganic chemical vapor deposition with boron composition exceeding 10%. The growth was performed under low temperature and pressure conditions of 600 °C and 75 Torr, respectively, with a growth rate of 0.29 µm/h. Triethylborane and triethylgallium were used as the source gases for boron and gallium, respectively. Pure nitrogen gas was used as the carrier for all reactants. A root mean square roughness value of 2.56 nm was determined using an atomic force microscopy scan on an area of 5 × 5 µm2. X-ray diffraction (XRD) 2θ–ω scans show a nearly lattice-matched BGaN/AlN film corresponding to a boron composition of ∼10%. A mixed wurtzite and zincblende phase was confirmed via an XRD pole figure and transmission electron microscopy. Additionally, the high crystalline quality of the mixed (002)wz/(111)ZB planes was shown using an XRD rocking curve with 810 arcsec full width at half maximum. The boron composition was precisely measured as 15% using Rutherford backscattering spectrometry combined with nuclear reaction analysis.