Quantum dots (QDs) are the result of quantum confinement in the three spatial directions, as such they exhibit remarkable properties, of which the most important is perhaps the absence of a continuum of states. The allowed energetic levels are discrete, similarly to the energy dispersion in single atoms. As a result, for an individual QD, single photon emission can be achieved. In particular, GaN QDs display an enhanced thermal robustness in comparison to conventional III-V QDs and remain bright even at room temperature. Furthermore, as opposed to other thermally stable QD systems such as colloidal QDs, GaN QDs are epitaxially grown and hence can be easily integrated into photonic devices. However, III-nitride technology is comparatively less mature than traditional III-V QDs and several challenges remain regarding material quality, particularly in terms of structural defects, point defects, and doping of high Al content AlGaN.
With the aim of integrating GaN QDs into photonic nanostructures such as waveguides and cavities based on membrane photonic crystals (PhC), the thesis is oriented towards the development of GaN QDs on thin AlN layers on Si(111). III-nitrides enjoy a high etching selectivity with Si, which allows the fabrication of membrane PhC. The thesis begins by developing high quality AlN on Si(111) and describing the growth of AlN by NH3-molecular beam epitaxy (MBE) at temperatures above 1000 °C. At the limit between layer by layer and step flow growth, very smooth AlN can be achieved even for thin layers on Si(111). These layers, together with the knowledge acquired in the growth of AlN serve as a stepping stone for the growth of GaN QDs.
GaN QDs have been produced by NH3-MBE since the first demonstration in 1999, by a modified Stranski-Krastanov (m-SK) growth that relies on a growth interruption. Here we develop the growth of GaN on AlN so that a spontaneous transition into a SK growth occurs. This is analogous to the seminal InAs/GaAs system and as such, the same principles for QD size and density control hold. III-nitrides are often grown heteroepitaxially and the resulting layers have a large threading dislocations density (TDD) (10^9 - 10^10 1/cm²). Dislocations offer nucleation centers due to a strain dipole that limits the control over the size and density of the QDs. Then by employing, AlN single crystal substrates with TDD in the order of 10^3 1/cm² this limitation is overcome. In this manner, we demonstrate size and density control by varying the amount of deposited GaN and the diffusion length. QD densities ranging from 10^8 to 10^11 1/cm² are achieved. Finally, an approach capable of yielding low QD densities even on highly dislocated substrates based on ripening and evaporation is presented.
The optical properties of GaN/AlN QDs were investigated. They display high internal quantum efficiencies at 300 K in the order of 40-60 %. The control over the QD size and density is demonstrated by PL measurements. Single QDs are inquired thanks to the possibility of low QD densities. A sample consisting of GaN QDs in AlN on Si(111) was studied in depth. These QDs exhibit multiexcitonic complexes and remain bright even when increasing the temperature to 300 K. At this temperature, single photon emission is revealed by second-order autocorrelation measurements g(2) (¿) with g(2) (0) = 0.17 ± 0.03. Finally, GaN QDs are demonstrated to be bright at room temperature with absolute counts in the order of 10^6 1/s.
EPFL2019
