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Wurtzite Ill-nitrides crystallize uniaxially and non-center-symmetrically. They exhibit large spontaneous and strain-induced polarization effects [1]. The large amount of polarization charge appearing at their heterointerfaces and the strong electric fields associated with it is unique to Ill-nitride heterostructures and has a dramatic effect on their optical and electrical properties. When a GaN film is grown on the c-plane of sapphire substrates by molecular beam epitaxy (MBE), it does not share the same atomic stacking order with sapphire. Consequently, the crystal direction [0001] of a GaN film can be either parallel or anti-parallel to the growth direction, leading to epilayers with two different polarities, Ga-and N-polar films. Investigations have shown that these two polar films have vastly differing growth, surface, and other properties [2]. Therefore, proper control of the film polarity during growth, as well as characterization of the materials with different polarities, are very important considerations in the application of these materials.

The availability of reliable and quick methods to investigate defects in GaN films is of great interest. Photo-electrochemical (PEC), and hot wet etching using both H3PO4 acid and molten KOH have been used to study structural defects in GaN layers grown by hydride vapor phase epitaxy and molecular beam epitaxy. The purpose of this work is to determine whether, and under what conditions, these different methods of investigation are consistent and to get to a more accurate estimation of the defect density. As-grown and etched surfaces were investigated by atomic force microscopy (AFM), and plan-view and cross-sectional transmission electron microscopy (TEM).

We compared photoluminescence (PL) and cross-sectional transmission electron microscopy (TEM) characteristics of GaN samples with Ga and N polarities grown by molecular beam epitaxy (MBE) on sapphire substrates. Ga-polar films grown at low temperature typically have very smooth surfaces, which are extremely difficult to etch with acids or bases. In contrast, the N-polar films have rougher surfaces and can be easily etched in hot H3PO4 or KOH. The quality of the X-ray diffraction spectra is also much better in case of Ga-polar films. Surprisingly, PL efficiency is always much higher in the N-polar GaN, yet the features and shape of the PL spectra are comparable for both polarities. We concluded that, despite the excellent quality of the surface, MBE-grown Ga-polar GaN layers contain higher concentration of nonradiative defects.