Selective area doping (SAD) of gallium nitride (GaN), especially p-type doping, is desirable for high-power applications but yet challenging. The lack of this process greatly limits the power device design flexibility, so the reported device figure-of-merits are well below the theoretical calculation/simulation. Selective-area etching followed by regrowth is the approach we choose to overcome this obstacle. For the etching step, instead of adopting conventional Cl-based plasma etching process, we explored in-situ TBCl etching, studied the etching mechanism, and optimized the process to prepare smooth surface and trenches. Room-temperature PL, XPS, and electrical characterizations indicate that TBCl etching creates a low-defect surface. Moreover, selective area growth of p-GaN in a patterned trench was analyzed by atom probe tomography (APT). We found a reverse proportionality between the local growth rate and Mg doping concentration, and the development of fast-growing semi-polar facet from the sidewall. As a consequence, non-uniform Mg doping in non-planar growth is observed.