GaN nanocolumns of exceptional crystalline quality have been grown by molecular beam epitaxy on both silicon (111) and sapphire (0001) substrates. Reflection high energy electron diffraction produces a unique diffraction pattern for in situ verification of columnar growth. Subsequent molecular beam epitaxial overgrowth of the nanocolumns has been used to improve the quality of thin film GaN layers when compared to GaN films grown directly on sapphire substrates. Transmission electron microscopy was used to confirm the absence of threading dislocations in the selected columns. Scanning electron microscopy of overgrown material demonstrated surface morphology similar to thin films grown in the intermediate (Ga-rich) growth regime, or a pattern of densely packed. hexagonal structures, depending on growth conditions. Low temperature photoluminescence (PL) spectra demonstrated a greater than two orders of magnitude improvement in PL intensity of overgrown film versus direct film growth. (c) 2007 American Vacuum Society.