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Title: Flexible and substrate-free optoelectronic devices based on III-V semiconductor nanowires
Author: Baig, Sarwat
ISNI:       0000 0004 7961 9577
Awarding Body: University of Cambridge
Current Institution: University of Cambridge
Date of Award: 2019
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III-V nanowires have been the subject of intense research interest for the past 20 years, as their unique optical and electronic properties, which arise from their nanoscale dimensions and composition, make them particularly suited for high-performance opto-electronic devices. Since epitaxial growth is on expensive, brittle, crystalline substrates, the field of flexible devices has been little explored in the context of III-V nanowires. In order to fully exploit these properties and move away from conventional wafer based electronics to flexible electronics, hybrid devices consisting of organic and inorganic components must be developed to harness the benefits from both materials systems. Embedding high performance vertically aligned III-V nanowires in a flexible matrix enables applications where there is a need for substrate-free, flexible devices. The work in this thesis looks to address this by (1) developing a repeatable method of producing nanowire-polymer thin films and (2) demonstrating how these thin films could be fabricated into different opto-electronic devices. The thin films are made by encapsulating the nanowires in Parylene C, which are then be peeled off from the growth substrate, thus retaining the vertical alignment of the nanowires. These thin films are used to fabricate a THz modulator and a solar cell. Single and multi-layer THz modulators are fabricated from nanowire-Parylene C thin films laminated together. 1,2,4,8, and 14-layer modulators are compared, with the 14-layer modulator displaying the best performance. A high switching speed (< 5 ps), modulation depth (-8 dB), extinction (13%) and dynamic range (-9 dB) and broad bandwidth operation (0.1 THz-4 THz) are obtained. This surpasses the performance of several devices in the literature and presents the first THz modulator which combines a large modulation depth, broad bandwidth, picosecond time resolution for THz intensity and phase modulation, which makes it an ideal candidate for ultrafast THz communication. In addition to the THz work, the fabrication process towards a flexible solar cell is also developed. This consists of optimising the dry etching, and annealing-free contacting processes to give nanowire devices that show good ohmic IV characteristics. Following this work, a proof-of-concept Schottky barrier solar cell is fabricated using the knowledge gleaned from this development work. This preliminary device gives a conversion efficiency of 0.02% and a fill factor of 0.3, with scope for device performance improvement by using nanowires that are grown and optimised specifically for solar cell operation.
Supervisor: Joyce, Hannah Sponsor: EPSRC
Qualification Name: Thesis (Ph.D.) Qualification Level: Doctoral
Keywords: III-V nanowires ; Photovoltaic devices ; THz modulator