Because of the lack of inversion symmetry along the c-axis of wurtzite crystals, basal-plane growth of III-nitride materials can occur in either one of two distinct polarities: (0001), known as metal polarity, and (000-1) known as nitrogen (N) polarity. With Metal-Organic Vapour Phase Epitaxy (MOVPE), the metal polarity is by far the most commonly used, as it generally results in smoother surfaces. In fact, for long time, mixed or purely N-polar materials were regarded as an undesirable consequence of non-optimized growth conditions, recognizable by their characteristically rough surface morphology dominated by hexagonal hillocks, and totally unsuitable for device fabrication.

However, despite the difficulties in growing smooth N-polar III-nitrides, this orientation has many potential advantages. In High Electron Mobility Transistors (HEMTs), the inversion of the polarization fields favours better ohmic contacts and channel scaling. In bipolar devices, where the long diffusion tail of Mg forces the p-type layer to be grown after the n-type one, N-polar optoelectronic devices have built-in and polarization-induced fields oriented along the same direction, which reduces the quantum confined stark effect in forward-biased LEDs, and is more suitable for charge separation or carrier multiplication in reverse-biased photodiodes or avalanche detectors.

Due to these advantages, the growth of N-polar GaN on sapphire and SiC has been intensively studied over the last two decades, and smooth surfaces are now possible on vicinal substrates. However, despite the considerable progress made in this field, more work is still needed to improve current N-polar power and RF devices, and to be able to bring N-polar optoelectronic devices to market. In particular, N-polar GaN epilayers grown on foreign substrates such as sapphire or SiC, are still of significantly worse crystal quality than their Ga-polar counterparts. In addition to that, the MOVPE growth of smooth N‑polar AlN or Al-rich AlGaN materials, which would allow for higher-power density HEMTs, is still a challenge.

In this talk, the most recent results on N-polar III-nitride materials achieved at Tyndall National Institute of Cork will be reviewed. A two-step growth technique for achieving a significant reduction of threading dislocations in N-polar GaN will be presented, as well as the growth of smooth N-polar AlN templates on sapphire, demonstrated in collaboration with Nagoya University.