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Title: Tailoring the electronic structures of zinc oxide based nanowires for optoelectronic applications
Authors: Zhao, Xin
Keywords: DRNTU::Science::Physics::Optics and light
Issue Date: 2017
Source: Zhao, X. (2017). Tailoring the electronic structures of zinc oxide based nanowires for optoelectronic applications. Doctoral thesis, Nanyang Technological University, Singapore.
Abstract: ZnO nanowires (NWs) have attracted tremendous attention in the past 15 years after the first demonstration of optically pumped ultraviolet laser by Yang et al.1, 2 As a wide band gap (3.3 eV, RT) material possessing room-temperature stable excitons (60 meV binding energy), ZnO NWs have been treated as one of the most promising platform for next-generation optoelectronic devices.3, 4 The global lightening market calls for materials with controllable doping, energy band gap, as well as novel device design schemes to achieve high-efficiency devices emitting light with arbitrary mono or mixed colors.5, 6 In order to fulfill these requirements, we have synthesized ZnO NWs with designed electronic structures including 1) the n-/p-type doped MgZnO alloy wires where we tuned the carrier concentration and band gap simultaneously, 2) core/doped-shell NWs to spatially tuned band structure via integrated utilization of two growth methods: hydrothermal synthesis and pulsed laser deposition (PLD). The synthesized ZnO and ZnMgO NWs were characterized mainly by methodologies such as photoluminescence (PL) and X-ray photoelectron spectroscopy (XPS), etc. Self-consistent models were proposed to shine some light on the physical correlations between the carrier density, surface depletion, and luminescence, which could provide instructions on designing novel light emitting devices. Furthermore, taking into account conjugated factors aforementioned, we proposed a novel design scheme of ZnO NW-based white-light-emitting-diodes via Förster resonant energy transfer (FRET) from ZnO defect levels to quantum dots (QDs) and a prototype device was demonstrated for the first time. The core part of the thesis could be divided into three main parts accordingly as follows: Part-I Synthesis of MgZnO NWs with controlled conduction type via Ga and P doping In Chapter 3, Ga and P-doped MgZnO nanostructures were synthesized by PLD to explore the possibility of modulating the band gap and conduction type simultaneously. The motivation of this work lay in the great importance of tuning the band offset ratio as well as extending the emission and transparent region of doped ZnO NWs. The Ga:MgZnO NWs array grew perpendicular to the sapphire substrates and the following XPS and PL confirmed the existence of gallium and magnesium. Under pulsed laser excitation, the doped wires exhibited Fabry-Perot lasing at 386 nm with a quality factor Q exceeding 1000. For the P-doped MgZnO, the product displayed as very dense bundle of NWs with an hierarchical arrangement. The following XPS confirmed the successful incorporation of Mg and high concentration of P, which act as the key to compensate the background carriers and achieve p-type. Photoluminescence further confirmed the band gap tuning and existence of acceptor-related optical transitions. The stable p-type conductivity was proved by the fabricated homojunction n-ZnO thin film/P:ZnMgO NW diode, which shown rectification behavior even after three-month storage in air. Part-II Forming ZnO/Ga:ZnO core-shell structure via doping gallium the surface to modulate the optical properties. ZnO/Ga:ZnO core-shell NWs were fabricated to spatially tailor the electronic band gap via introducing the n+ region near the NWs surface. This Ga-doped shells would act as the hole blocking layer preventing the non-radiative recombination of holes with trapped electrons induced by surface depletion effect. We have proposed an integrated electronic band structure model considering the conjugating factors such as mid-gap defects, surface depletion, and carrier concentration-induced band gap bending to explain the successful suppression of deep-level emissions (DLE). The following XPS measurement and temperature-dependent PL could fit the model very well. In short, this novel scheme and integrated model provided non-trivial physical insights on NW-based light emitting devices design and fabrication. Part-III Excitonic energy recycling from ZnO defect states: towards electrically driven NW-QDs hybrid white light-emitting-diodes. It is widely known that QDs, though be treated as eminent lumophores due to their high quantum yield and color tunability, are hinder in the path towards practical light emitting devices owing to the surface ligand-induced carrier injection difficulty. On another hand, as promising UV emitting material, ZnO NWs suffer from the undesirable visible emission due to the DLE, which result in uncontrollable emission color and waste of exciton energy. To address this problem, we proposed a novel device designing scheme for w-LEDs, which incorporated CdSe QDs with p-GaN/n-ZnO NWs heterostructures. For the first time, QDs were excited by the recombination of carriers captured by deep-level states in electrically injected ZnO NWs. The device exhibited an achromatic emission with chromaticity coordinate (0.327, 0.330) and color temperature 5783 K, equal to the sun.
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