Study On The Growth Technique And Optoelectronic Properties Of Intrinsic Acceptor-rich ZnO Single-crystal Microtubes

ZnO is a representative of the 3^rd generation semiconductors,its high-quality n-type doping ZnO is easy to achieve due to its intrinsic n-type conductivity.However,the fabrication of reliable p-type ZnO is a major challenge to realize ZnO-based electronic device applications.Undoubtedly,the stable acceptor is the base of reliable doped p-ZnO.In this thesis,we proposed a novel technique to grow free-standing undoped acceptor-rich ZnO（A-ZnO）microtubes with high quality,stability,and reproducibility.On this basis,the corresponding electronic and optical properties of ZnO microtubes were studied,and a series of novel applications were prepared.Firstly,the optical vapour supersaturated precipitation（OVSP）method was presented to fabricate A-ZnO microtubes based on an optical floating zone furnace.The Zn vapor supersaturated precipitation and axial photo-thermal-decomposition were proposed to interpret the microrods growth and microtubes formation,respectively.Uniform temperature field and oxygen-enriched atmosphere were found to be the necessary conditions for the growth of A-ZnO microtubes in acceptor-rich by OVSP method.The effects of major growth parameters（e.g.lamp power,filament geometry and growth platform shape）on temperature field at the growth platform of precursor rod were investigated by a finite element model as well.The lamp power of 65%（1500W）,thick single-filament and appropriate conical growth platform shape were beneficial to achieve a uniform temperature field for consistent microtubes finish-quality and prevent twinmicrotubes formation.Secondly,the optical and electric characteristics of A-ZnO microtubes were studied.The results show that A-ZnO microtubes contain abundant of stable acceptors,which originate from intrinsic zinc vacancy defects at the top of valence band of 127meV.In addition,In/Ga alloy was found to be an appropriate electrode material for A-ZnO microtubes to realize ohmic contact.On this basis,the undoped A-ZnO microtube was partially deposited by ZnO:Sn film to form a mimetic p-n homojunction,which demonstrated a rectification behaviour with the threshold voltage of 0.7 V,turn-on voltage of 2.1 V,reverse breakdown voltage>15 V,and reverse saturation current of<10μA,respectively.Then,the Ultra-thin-walled（UTW-）ZnO microtubes with a diameter of⁵0μm and a facet wall thickness of⁷50 nm were fabricated using optimized OVSP method.Spectral results show that the UTW-ZnO microtube could be used as a novel microcavity that supports various optical modes.Therefore,the optical applications on UTW-ZnO microtube were explored:（1）The temperature-controlled multicolor,free-quenching,and high-efficiency luminescence was achieved,which ranged from the visible band to near-white（0.30,0.39）then to bluish-violet（0.17,0.12）.（2）UV lasing was also realized in the single UTW-ZnO microtube under an unprecedented low threshold of 5.50μW.（3）The UTW-ZnO microtubes-based microfluidic chips was demonstrated for recyclable on-chip degradation,where high concentration methylene blue solution could be completely degraded under solar irradiation of 20 mins.Finally,the self-absorption ratio in the UTW-ZnO microtube was increased from34.1%to 77.2%by optimizing the excitation-microcavity-detection geometry.Meanwhile,the possibility of exciton-exciton collision was raised although the recombination channel for free exciton was suppressed,which could boost the low-threshold amplified spontaneous emission.Based on the quenching effect of Ag nanoparticles on photoluminescence of UTW-ZnO microtubes,the self-absorption coefficients of fluorescent photons in microcavities were calculated and analyzed.It is also found that the enhancement ratio for UV band emissions by Purcell effect in UTW-ZnO microtubes could be up to 40 folds higher than that using localized surface plasmon resonance via Ag nanoparticles.The results of this thesis not only provide technical support for the further research on the preparation of p-type ZnO and its devices,but also pave new ways for the design and application of novel semiconductor optical microcavities.