GaN, AlN and InGaN have a polar wurtzite structure and epitaxial films of these materials typically grow along the polar axis. Although the polarity of these nitrides has been studied by quite a number of techniques, many results in the literature are in conflict. In this paper an attempt is made to lay out a set of polarity assignments to provide a context for discussion of these results. A “standard framework” is proposed to correlate the disparate results, and the framework is used to draw general conclusions about the polarity of bulk crystals, VPE and MBE epitaxial films, and devices.

We investigate the electron affinity of aluminum nitride surfaces prepared by nitrogen sputtering and annealing via x-ray, ultraviolet, and inverse photoemission spectroscopy. The combination of these techniques leads to a precise determination of the relative positions of the Fermi level, valence-band maximum, conduction-band minimum, and vacuum level at the semiconductor surface. We demonstrate that, in spite of the presence of a sharp photoemission onset feature previously associated with negative electron affinity, the electron affinity is clearly positive on these surfaces.

We have used plasma molecular beam epitaxy on (0 0 0 1) and (0 0 0 ) ZnO substrates to induce epitaxial growth of GaN of a known polarity. The polarity of the ZnO substrates can be easily and unambiguously determined by measuring the sign of the piezoelectric coefficient. If we assume that N-face GaN grows on O face ZnO and that Ga-face GaN grows on Zn face ZnO, then we can study the growth of both Ga and N faces. The most striking difference is the doping behavior of the two faces. Growth on the Ga-face is characterized by a higher carrier concentration and a lower threshold for Ga droplet formation.

The (1 0 0) face of γ -LiAlO 2 has attracted attention as a possible substrate for GaN epitaxial growth. This is partly because this face has an excellent lattice and structural match to (1 0 0) GaN. This orientation would have a misfit of only -1.4% along the c-direction and -0.1% along the b-direction of LiAlO 2 . We find that in practice this orientation relationship does not occur; instead, (0 0 0 1) oriented GaN grows with a small tilt (0.6° towards the c-direction) between the film and substrate. Although the misfit along the substrate b direction is large (-6.3%) for this orientation, the tilt perfectly accommodates the -1.4% misfit in the c direction. We present characterization of these films by RHEED, X-ray diffraction, and TEM. We propose that the tilt is driven by a reduction of interface energy which occurs in polar, incoherent interfaces.

We report the use of ScAlMgO4 as a substrate for the epitaxial growth of wurzitic GaN. The low misfit (+1.8%) allows coherent epitaxy of GaN, as observed by RHEED. The congruent melting of ScAlMgO4 makes Czochralski growth possible, suggesting that large, high quality substrates can be realized. Boules about 17mm in diameter are reported. We have used nitrogen-plasma molecular beam epitaxy to grow GaN epitaxial films onto ScAlMgO4 substrates. Band-gap photoluminescence has been observed from some of these films, depending primarily on the deposition conditions. A 3×3 superstructure has been observed by RHEED on the GaN surfaces. Structural analysis by x-ray diffraction indicates very good in-plane alignment of the GaN films. We also report thermal expansion measurements for ScAlMgO4.