Polarity of GaN nanowires grown by plasma-assisted molecular beam epitaxy on Si(111)

Based on the breakdown of Friedel's law, resonant x-ray diffraction is shown to be a suitable method to determine polarity of non-centrosymmetrical wurtzite gallium nitride (GaN) nanowires (NWs) at a macroscopic scale. It is demonstrated that such GaN NWs grown by plasma-assisted molecular beam epitaxy on bare Si(111) are N-polar, consistent with results obtained by convergent beam electron diffraction. The N-polarity feature is attributed to the formation of a thin SiₓN₁₋₋ₓ layer on the Si surface before growth. The use of a thin AlN buffer layer does not modify the GaN NW polarities, suggesting that NWs actually grow between the AlN grains rather than on top of them.