GaN epilayers are grown by molecular beam epitaxy using NH3 as nitrogen precursor on sapphire substrates. When a low temperature buffer layer is used, the GaN films grow with the Ga polarity. However, the polarity can be changed from Ga to N by the deposition of a Mg monolayer during growth interruption. Such a polarity inversion may also occur when a large Mg flux is supplied during growth. Secondary ion mass spectroscopy shows that the critical Mg concentration for the polarity inversion must be larger than 3x1020 cm-3. The interface between the two polarities has been characterized by transmission electron microscopy (TEM). It demonstrates a faceted morphology. The polarity inversion is perfectly controlled over a 2 inch wafer and no dislocation is introduced. Actually, the quality of the N-polarity GaN layer replicates that of the Ga-polarity underlayer. Despite some indications about the role of the Mg, the actual mechanism responsible for the polarity inversion is still unclear. Furthermore, if the growth proceeds on the N-polarity with a large Mg flux, the GaN layer undergoes a crystal phase transition from hexagonal to cubic. Surprisingly, this is achieved for standard growth temperatures (800oC) for which the cubic phase should be unstable. Moreover, high-resolution TEM images show a perfectly smooth interface between h- and c-GaN.
On a wafer with a single Ga/N polarity inversion, stripes have been etched up to the Ga polarity. At this stage the sample exhibits an in-plane Ga/N periodic polarity modulation. Then, GaN is overgrown in order to get thick periodic polarity layers, which are dedicated to non-linear optics. Indeed, the lateral modulation of the GaN polarity opens the way for quasi-phase matching structures. The verticality of the polarity domain boundaries has been checked by TEM as a function of the stripe orientation. Smooth (10-10) planes are obtained for properly chosen stripe orientation. This is confirmed by selective wet etching, which reveals only the Ga polarity material. We will show that this approach allows the fabrication of high-quality three-dimensional structures without plasma etching step.