| dc.description.abstract | Recent development on high-crystalline and almost defect-free one-dimensional nanowire (NW) structures of III-V semiconductors through various epitaxial growth techniques leads to the possibility of producing a new generation of optoelectronic devices such as low-cost and high-efficiency solar cells and ultraviolet light emitters. The parallel emergence of the wonder material graphene over the last two decades with superior electrical transport characteristics, high optical transparency, high heat dissipation, strong mechanical and chemical stability may extend such possibilities beyond the capabilities of today’s semiconductor technology if these two material systems are combined.
This doctoral thesis combines these two material systems to build an unique hybrid platform which could open up for numerous new possibilities for a new generation of optoelectronic applications in the future. The focus of this thesis is categorized in two directions. Firstly, it focuses on the fundamental studies on graphene/NW junctions, prioritized to gain in-depth understanding of the electrical transport properties, energy band alignment, optical absorptions, and carrier extractions dynamics, etc. Secondly, it concentrates on device applications using the vertical arrangement of the NWs into an array pattern, in which GaAs/AlGaAs NWs and GaN/AlGaN self-assembled nanocolumns (NCs) are used to demonstrate functional solar cells and ultraviolet light-emitting diodes, respectively.
In order to perform in-depth fundamental studies on graphene/NW junctions, a major attention was given to fabricate reliable quality single GaAs NW/graphene contact devices through the development of a position-controlled micro-transfer technique of graphene with embedded NWs. Significantly improved electrical transport properties are consistently observed from the embedded configuration of the NWs, which is then utilized to fabricate single NW devices throughout this thesis. The electrical properties of the embedded NW devices are explained by using a ‘parallel resistors’ and a ‘depletion limited’ model based on the current crowding effect in the graphene and/or the metal contacted NW cross-section. A bias- and polarization-dependent scanning photocurrent microscopy technique was also employed in this thesis to investigate the optoelectronic properties of the graphene/GaAs NW junctions by collecting spatially resolved free carriers as an effect of local photo-excitations. The results show that graphene forms an inverted band bending when contacted with p- and n-GaAs NWs, where the band bending is downwards with p-GaAs and upwards with n-GaAs NWs.
In order to demonstrate functional solar cells using vertically arranged GaAs NW arrays, both an axial and a core-shell geometry of the NWs have been utilized. A major attention was given to demonstrate how the III-V photoconversion material can be used more effectively by utilizing an axial p-i-n junction GaAs/AlGaAs NW array grown by molecular beam epitaxy on a Si substrate, leading to an ultrahigh power-per-weight ratio. The GaAs NW array solar cell shows a photoconversion efficiency of ~7.7% with only ~1.3% areal coverage of the NWs under 1 Sun intensity. This corresponds to a power-per-weight ratio of the active III-V photoconversion material as high as 560 W/g, showing great promise for high-efficiency and low-cost III-V NW solar cells and III-V NW/Si tandem solar cells.
Considering the huge potential of enhancing the photoconversion efficiency by using the core-shell geometry of NWs, a radial p-i-n junction GaAs/AlGaAs NW array solar cell grown on a Si substrate has also been studied in this thesis. However, a relatively high leakage current density was measured from this solar cell, resulting in a photoconversion efficiency of only ~2.1%. The origin of this high leakage current is analyzed through multiple electrical measurements such as nanoprobing of as-grown individual NWs and multi-contact single NWs, as well as structural characterization of the fabricated NW array solar cell through transmission electron microscopy. These measurements reveal a high variation in electrical properties from NW-to-NW in an array and a regular occurrence of NWs with off-centered nucleation for the p-GaAs NW core in the substrate hole-mask, which are predominantly responsible for the high leakage current density and poor PCE from the NW array solar cell.
In addition, flip chip ultraviolet light-emitting diodes are studied in this thesis using self-assembled GaN/AlGaN NCs and graphene, the latter both as a growth substrate as well as a transparent conducting electrode. High crystalline quality of the NCs and the presence of an intrinsic GaN quantum disk in the active region of the NCs are revealed from the structural characterizations of the device. The electroluminescence spectrum shows the presence of a blue-shifted emission at 350 nm from the GaN quantum disk in the active region with no defect-related yellow emission. This proof-of-concept device shows the potential of this hybrid NW/graphene platform for the fabrication of next-generation nano-optoelectronic and electronic devices. | en_US |
