Anisotropic chemical etching is an important means for characterizing the polarity and defect density of single crystals. In this letter, we present the results of our studies on the etching of bulk AlN crystals in aqueous potassium hydroxide solution. The nitrogen polarity (0001) basal plane initially etched rapidly, while the aluminum polarity basal plane, and prismatic (100) planes were not etched. The etch rate of the nitrogen polarity basal plane eventually decreased to zero, as the surface became completely covered with hexagonal hillocks which were bounded by {101} planes. The hillock density for the self-seeded AlN crystals studied was typically in the range of 5×107cm −2 to 109cm −2. From our analysis of etched AlN crystals, we infer that freely nucleated crystals predominately have the nitrogen to aluminum direction pointing out from the nucleation surface, that is the ends of the AlN crystals facing the source are aluminum polarity.

In the growth of GaN from nitrogen dissolved in Ga under high N2 pressure, two main habits are observed: plate-like and needle-like. The plate-like crystals can be divided into those having (0001), (000) and {100} faces and those with the additional {101} and {102} faces. The needle-like crystals belong to three classes: with or without (0001) faces and a third with unusual, star-like needles. The plate-like and needle-like habits and transformation between these habits are discussed in greater detail. It is shown that it is possible to evaluate the relative growth rates corresponding to such transitions.

Free standing GaN platelets were fabricated by hydride vapor phase epitaxy (HVPE). The platelets having a current maximum size of 7×6×0.1 mm3 were obtained by HVPE growth of ~100 μm thick GaN layers on SiC substrates and subsequent removal of the substrates by reactive ion etching (RIE). Surface of the GaN platelets was characterized by reflectance high energy election diffraction (RHEED), and Auger electron spectroscopy (AES). Crystal structure and optical properties of the platelets were studied by x-ray diffraction and photoluminescence (PL), respectively. Raman spectroscopy was also applied for material characterization. Residual strain was detected in the crystals. The stress was eliminated by high temperature anneal.