Thesis presented July 09, 2014
Abstract:
This report focuses on the fabrication of GaAs nanowires and GaAs/AlGaAs core-shell structures by molecular beam epitaxy, deposited on Si (111) substrates in order to provide arrays of wires for innovative solar cells and bottom-up photonic wires for efficient single photon emitters.
The first part of this work is a systematic study of the key parameters which control the one- dimensional growth of bare GaAs NWs with a self-assisted vapor-liquid-solid growth process, namely the As-to-Ga flux ratio, the substrate temperature, and the deposition rate.
The second part concentrates on the growth and characterization of GaAs wires covered with a shell of AlGaAs alloy (35 % Al) in order to get rid of the surface recombinations. These shells were fabricated under As-rich condition with ratio As/Ga >10 in order to consume the Ga- droplets completely and to promote a radial growth. The obtained axial-to-radial growth ratio is 6. The optical characterizations on ensemble were carried out at low temperature via the cathodoluminescence (CL), photoluminescence (PL), and time-resolved PL measurements. The results show that the lifetime of carriers and luminescence intensity increase significantly with shell coverage. About 7 nm thick shell is enough to optimize the passivation and suppress the surface state recombination. A thin outer cap of GaAs is required in order to prevent some oxidation of the AlGaAs alloy shell.
In addition, the exciton diffusion lengths of these NWs, studied via the spatially resolved CL, are in the range of 0.7 - 1.5 µm for NWs with shell thicknesses between 20 - 50 nm. These values are smaller for thicker shells due to the defect formation, leading to limit the quality of core-shell interface. The shift in optical emission experiments provides the information of the strain generation of core-shell when we vary the shell thickness. The piezoelectric field was noticed in these samples.
Keywords:
Semiconductor, molecular beam epitaxy, nanowires, core-shell structure, arsenide
On-line thesis.