Institute of Physics, Polish Academy of Sciences

Results posted here concern studies on GaN nanowires crystallised by molecular beam epitaxy coated with the wide bandgap oxides: AlO_x and HfO_x shells of nominal thickness varying from 1 to 20 nm, obtained via atomic layer deposition (ALD) . The scanning electron microscopy images show that the shells of different nominal thicknesses exhibit varying morphology. Specifically, as the number of ALD deposition cycles decreases, the shells take the form of islands. This leads to partial coverage of the nanowires. To explore the changes in the GaN core’s crystal lattice, strain was calculated based on X-ray diffraction, photoluminescence, and Raman spectroscopy. A great agreement between all techniques was found, indicating the presence of in-plane compressive strain.

Moreover, in both photo- (PL) and cathodoluminescence (CL) spectra, luminescence enhancement was observed for the nanowires with partial coatings. The observation was confirmed through temperature-dependent PL and CL measurements . It was expected that the shells would enhance luminescence by passivating surface states and inducing a flat-band effect. However, we found that thick shells may reduce signal intensity by compromising structural quality, increasing strain gradient, and scattering light. Hence, the optimum shell thickness occurs when a balance between all these factors is achieved. What is more, the partial coating was confirmed to successfully protect nanowires against photoadsorption, despite their non-homogenity. We propose that the shells grow in the Stranski-Krastanov mode, which assumes that the additional monolayer is deposited prior to the formation of islands, providing sufficient protection for the cores.