  Nitrogen-polar III-nitride heterostructures offer advantages over metal-polar
structures in high frequency and high power applications. However, polarity
control in III-nitrides is difficult to achieve as a result of unintentional
polarity inversion domains (IDs). Herein, we present a comprehensive structural
investigation with both atomic detail and thermodynamic analysis of the
polarity evolution in low- and high-temperature AlN layers on on-axis and
4$^{\circ}$ off-axis Carbon-face 4H-SiC (000$\bar{1}$) grown by hot-wall metal
organic chemical vapor deposition. A polarity control strategy has been
developed by variation of thermodynamic Al supersaturation and substrate
misorientation angle in order to achieve desired growth mode and polarity. We
demonstrate that IDs are totally suppressed for high-temperature AlN nucleation
layers when step-flow growth mode is achieved at the off-axis. We employ this
approach to demonstrate high quality N-polar epitaxial AlGaN/GaN/AlN
heterostructures.
