Résumé
The growth of spontaneous (catalist-free) self-assembled and ordered (localized) III-Nitride nanocolumns by Molecular Beam Epitaxy is addressed. A description of the nucleation process and the nanocolumn growth is given in relation to the growth conditions and the substrate/buffer used. Nucleation proceeds via Volmer-Weber or Stranski-Krastanov mechanisms depending on the initial lattice mismatch between the nanocolumns material and substrate. It is experimentally observed that a high initial mismatch (i.e. GaN on bare Si) leads to isolated, well defined nanocolumns, whereas a smaller one (i.e. GaN on AlN buffered Si) produces a mixture of nanocolumns and a "faceted matrix" of continuous, rough material. Polar wurtzite InN nanocolumns have been grown along the c-plane, on Si(111) and Si(100) substrates. In addition, non-polar InN nanocolumns have also been grown along the a-plane on m-plane GaN/r-plane Al2O3 templates. In this case case, a well defined a-plane orientation is determined by HR-TEM and from the dependence of the E2 Raman mode with the polarization angle. HR-SEM pictures reveal top nanocolumn surfaces showing a- and m- plane facets. PL measurements at low temperature give a emission energy of 0.69 eV, a typical signature of a very high quality material. Though self-assembled nanocolumns are quite useful to understand the basic growth mechanisms and material properties, the growth of ordered arrays of nanocolumns is essential to fabricate real devices, like LEDs and detectors, because of the need of homogeneous geometry and characteristics of individual nanodevices. Details will be given on the use of thin Ti masks with nanoholes of different diameter and pitch and on the specific conditions for localized growth, far from those to grow self-assembled nanocolumns. Different approaches of ordered arrays will be revised with emphasis on the applications to very efficient optoelectronic devices, like phosphor-free white LEDs, and multijunction solar cells.