Résumé
The extraordinary high crystal quality of III-nitrides nanocolumns makes them very attractive in both fundamental and applied fields. Self-assembled GaN nanocolumns have been grown on Al2O3, Si(111) or Si(100) in the past. A systematic study of the diameter, density and growth rate of the nanocolumns depending on impinging Ga and N fluxes and growth temperature has been recently published. While the self-assembled growth approach is relatively-easily controlled (substrate temperature and III/V ratio), its random nature leads to significant dispersion (height and diameter) among nanocolumns, making strongly difficult the planar processing and contact of each nanocolumn at the time of fabricating nanodevices. On the other hand, the selective (ordered) growth, making use of masks or patterning on the substrate, can localize the growth, yielding a periodic, homogeneous distribution of nanostructures, what makes the processing much easier. Selective area growth using Ti mask with nanohole arrays has been recently demonstrated. This work focuses on the influence that different factors have on the successful selective area growth of GaN nanocolumns. These parameters include the fabrication method, design and nitridation of the Ti mask, as well as the standard plasma-assisted MBE growth parameters such as the growth temperature and the impinging Ga/N flux ratio. Nanohole arrays (diameter typically in the range 70 nm to 150 nm) were fabricated by e-beam lithography and etching of a continuous thin (5 nm) Ti layer or by lift-off techniques using colloidal masks, on commercial GaN templates and Si(111) substrates. Due to the high growth temperature needed to promote the selective growth (above 900ºC), a two-step nitridation process of the Ti mask was first optimized in order to avoid its degradation. Systematic studies of the growth temperature (from 900ºC to 1040ºC), the Ga flux (between 5 and 20 nm/min) and the nitrogen (from 8 to 20 nm/min) were performed.