  N-polar AlN epilayers were grown on the N-face of single crystal bulk AlN
substrates by plasma assisted molecular beam epitaxy (PA-MBE). A combination of
in situ thermal deoxidation and Al-assisted thermal desorption at high
temperature helped in removing native surface oxides and impurities from the
N-polar surface of the substrate enabling successful homoepitaxy. Subsequent
epitaxial growth of AlN layer on the in situ cleaned substrates, grown in
sufficiently high Al droplet regime, exhibited smooth surface morphologies with
clean and wide atomic steps. KOH etch studies confirmed the N-polarity of the
homoepitaxial films. Secondary ion mass spectrometry profiles show Si and H
impurity concentrations below the noise levels, whereas O and C impurities
concentrations of ~ 8x10^{17} atoms/cm^3 and ~ 2x10^{17} atoms/cm^3 are
observed respectively. Though the structural defect densities are low, they
interestingly appear as inversion domains of different dimensionalities.
