Electro-Optical Characterization of Passivated InAsP/InP Quantum Discs-in-Nanowire Heterostructures

Abstract: Semiconductor nanowires, with their quasi one-dimension geometry, inherently provide strain relaxation for heterogeneous integration and also strong light mode confinement. Nanowire-based photonic devices have also been demonstrated to offer significantly enhanced quantum efficiency, gain and reduced noise. This project deals with electro-optical investigations of InP nanowires with axially embedded InAsP quantum discs. The advantages of InP/InAsP nanowire-based photodetectors include direct growth on lattice-mismatched silicon platforms due to the small nanowire footprint, significant bandgap tuning, strong confinement in quantum discs and optical resonances providing a large optical absorption despite the small material volume compared to a thin film. However, due to the large surface-to-volume ratio of nanowires, surface states can strongly affect the electro-optical performance of nanowire-based heterostructure devices. Surface states can induce e.g. Fermi-level-pinning, non-radiative recombination and significant surface leakage currents which can deteriorate the performance of nanowire-based devices. Despite of all passivation attempts, surface states are still a challenge for InP/InAsP nanowire heterostructures. In this thesis work, the surfaces of InP nanowires, with embedded single- or 20 InAsP quantum discs, were passivated using atomic layer deposition of SiO2 or POx/Al2O3. Single passivated nanowires were optically characterized by photoluminescence and the results were compared with as-grown nanowires. To evaluate the effect of passivation on the corresponding electrical properties of the nanowires, nano-probe current-voltage measurements were carried out on single nanowires still standing on the growth substrate. Our results show that passivation with POx/Al2O3 improves the optical properties of the nanowires. POx/Al2O3 passivation leads to improved electrical characteristics in the nanowires with 20 quantum discs, while SiO2 enhances the electrical properties of nanowires with a single embedded quantum disc. The results will be used for future optimization of photodetectors.