Electrical properties of undoped GaN films grown by maskless epitaxial lateral overgrowth

Electrical properties, deep traps spectra, microcathodoluminescence (MCL) spectra measurements, MCL imaging, and electron beam induced current (EBIC) imaging were performed for undoped GaN films grown by metalorganic chemical vapor deposition using maskless epitaxial lateral overgrowth on basal plane sapphire. The films showed a low dislocation density of ∼10⁸ cm^-2 in the laterally overgrown wings and an order of magnitude higher dislocation density in vertical growth seed regions, as determined by MCL and EBIC imaging. The polarity of EBIC signal measurements and the room temperature capacitance-voltage characteristics suggested that the high-dislocation-density seed regions were high-resistivity p-type, with the Fermi level pinned near E_v + 0.4 eV, as determined by admittance spectroscopy. The wing regions were n-type, with low residual donor concentration of some 10¹⁴ cm^-3 near the surface. The donor concentration further decreased upon movement towards the sapphire substrate. Some possible explanations of the observed effects are discussed.