  N-polar aluminum nitride (AlN) is an important building block for
next-generation high-power RF electronics. We report successful homoepitaxial
growth of N-polar AlN by molecular beam epitaxy (MBE) on large-area
cost-effective N-polar AlN templates. Direct growth without any in-situ surface
cleaning leads to films with inverted Al-polarity. It is found that Al-assisted
cleaning before growth enables the epitaxial film to maintain N-polarity. The
grown N-polar AlN epilayer with its smooth, pit-free surface duplicates the
structural quality of the substrate as evidenced by a clean and smooth growth
interface with no noticeable extended defects generation. Near band-edge
photoluminescence peaks are observed at room temperature on samples with
MBE-grown layers but not on the bare AlN substrates, implying the suppression
of non-radiative recombination centers in the epitaxial N-polar AlN. These
results are pivotal steps towards future high-power RF electronics and deep
ultraviolet photonics based on the N-polar AlN platform.
