Localized epitaxy of gallium nitride (GaN) on silicon (Si) is studied, with the aim of achieving material compatible with 1200 V vertical devices, in particular an unintentional doping level <1 × 10¹⁶ cm⁻³, which is essential to have high quality devices. In this study, three mask materials (SiN, SiO₂, Al₂O₃) are examined and the unintentional doping concentration in GaN layers grown by metal organic vapor phase epitaxy (MOVPE) is investigated by scanning spreading resistance microscopy (SSRM). The results from SSRM are verified by secondary ions mass spectroscopy (SIMS) and show that an unintentional doping concentration lower than 10¹⁶ cm⁻³ is achieved when Al₂O₃ is used as the mask material. This value is lower than any previous studies of localized GaN growth, and in addition, this is the first time that SSRM has been used to observe such low doping concentrations in GaN.