Catalyst-free GaN nanowires (NWs) are known for the high quality of their crystal structure. They are intensively investigated worldwide to fabricate improved optoelectronic devices. Plasma-assisted molecular beam epitaxy (MBE) is a well-established technique to elaborate these NWs. The control of their crystal polarity is an important issue. Indeed, in such wurtzite crystals, a large number of properties such as growth kinetics, impurity or dopant incorporation and direction of piezoelectric field are driven by the crystal polarity.
While it is well established that N-rich conditions are necessary to form self-catalyzed GaN NWs, several possible routes are used to initiate their growth. In our process, we use a small amount of Al to form AlN platelets at the Si surface as evidenced by grazing incidence X-Ray diffraction (GIXRD) and scanning electron microscopy (SEM). We observe that these AlN platelets act as pedestals for the subsequent growth of GaN NWs. Consequently, the GaN NWs adopt the polarity of the AlN pedestals. Convergent beam electron diffraction (CBED) and chemical etching [1] reveal a single N-type polarity for all the NWs. We propose that the particular nucleation mechanism of the AlN pedestals is responsible for this N-polarity.
Finally, we use thin AlN layers as time markers inside the NWs to investigate the chronology of their formation. We observe at the atomic scale the shape and position of these AlN markers, longitudinally and along a radial cross-section by high-angle annular dark field scanning transmission electron microscopy (HAADF STEM) using a Cs-probe aberration corrected STEM Jeol 2200 FS. We deduce the kinetics of NW elongation which indicates that diffusion of Ga adatoms along the NW sidewalls contributes to the NW axial growth. At longer growth duration, shell formation around the initial cores is evidenced. Their growth mechanism is revealed by the morphology of the markers: bunches of monolayers are formed at the bottom of the NW and propagate along its sidewall facets, toward the top facet. The core and shell compete with each other in collecting Ga adatoms from the sidewalls. In general, the shell grows faster than the core and finally they merge with each other [3]
1. L. Largeau, E. Galopin, N. Gogneau, L. Travers, F. Glas, J.C. Harmand, Cryst. Growth & Design, 12, 2724 (2012).
2. E. Galopin, L. Largeau, G. Patriarche, L. Travers, F. Glas, J.C. Harmand, Nanotechnology 22, 245606 (2011)