Carrier localization and surface effects in InGaN nanocolumns grown by plasma-assisted molecular beam epitaxy.
Résumé
InGaN-based quasi-one-dimensional systems (nanowires, nanorods and nanocolumns) attract increasing interest because of their ability to produce defect-free light emitters covering the entire visible range. Integration to solid-state optoelectronic devices makes epitaxial growth of vertical structures more suitable for light emission purposes, all the more that they open the way to the fabrication of complex heterostructures, such as quantum disks. In this study, we demonstrate the controlled growth of InGaN nanocolumns (NCs) by plasma-assisted molecular beam epitaxy on Si(111) substrates. The seeding of InGaN NCs by underlying GaN NCs preserves the NC morphology, while enabling the control of In content in the range of 2 to 30 %, as directly deduced from photoluminescence (PL) experiments. Similar to InGaN compact epilayers, the PL linewidth increases with In content, as a result of increased alloy disorder and carrier localization at potential fluctuations. Temperature-dependent PL emphasize the impact of this localization on the competition between radiative and nonradiative recombination processes. We find that the larger the In content the smaller the PL intensity drop measured between 8 and 300K. The well-known s-shaped T-dependence of the PL peak energy is also observed, proving, in particular, that recombination of localized carriers is still the dominant process at room temperature for In-rich InGaN NCs. Room-temperature PL experiments provided another -potentially important- result regarding both the chemical composition of InGaN NCs and their radiative efficiency. Indeed, we show how specific surface modifications of the NCs were used to reveal differences of In content between the surface and the core of the NCs, with strong impact on the room-temperature PL intensity.