Study On The Performance Modulation With Multi-field Coupling Effect In ZnO Nanodevices

In recent years, oxide interface becomes research focus in semiconductor interface engineering field because metal oxides possess a variety of properties that are unattainable in silicon and Ⅲ-Ⅴ materials. Among numerous oxides being explored, wurtzite ZnO is one of the most studied for its wide band gap, high exciton binding energy and electron mobility. More importantly, coexisting of the multiple characteristics and their interactions in this material offers a potent playground for the coupling effect research, giving birth to a new research area-piezotronics and piezophototronics. In this thesis, we systematically studied the synthesis of ZnO nanomaterials, fabrication of piezo-phototronic devices and their service behaviour. Performance modulation with external stimuli was demonstrated and corresponding mechanism was investigated.ZnO nanowire synthesis following vapor-liquid-solid and vapor-solid mechanism was accomplished with CVD technique. For hydrothermal method, the influence of vanadium doping, PEI additive and multiple growth times on ZnO morphology was investigated. The finite element modelling result indicates that longer nanowire with smaller diameter gives better piezoelectric property. Improved piezoelectric coefficient d33 16.7 pm V-1 was achieved in vanadium-doped ZnO nanorod, and ferroelectric like behaviour was obtained using PFM characterization. By combining two-beam laser interference lithography and hydrothermal method, large area vertically aligned ZnO nanorod array with uniform morphology was synthesized.Surface-polarity dependent piezotronic effect and electrical transport tuning with coupled piezo-charges and photoexcitation were demonstrated with self-modified C-AFM. For Zn-polar ZnO nanorod, the piezotronic effect raises Schottky barrier height (SBH) at Pt/Ir-ZnO interface under compressive force, while opposite result is obtained for O-polar ZnO and negligible SBH change is observed for m-plane ZnO. The 355 nm UV light illumination lowers SBH due to increased electron concentration. For Zn-polar ZnO nanorod, by choosing a 0.46 μN compressive force and illumination density of 0.52 mW/cm2, the Ⅰ-Ⅴ curve matches well with the original one obtained without force in dark.Fabrication of self-powered photodetector was demonstrated on the basis of ZnO/electrolyte heterojunction. The device shows an on-off ratio of about 102 with the response time 0.11 s. When applying a 0.15% compressive strain, the photocurrent increased by about 48% with high reproducibility. Electrochemical impedance spectroscopy results indicate that the depletion width increased to 6.3 nm from 4.6 nm under this condition. Increased number of effective excitons and shorter transport length give to this enhanced photocurrent.Self-powered photodetector on the basis of all-oxide Cu2O/ZnO heterojunction was fabricated with responsivity 2.32 mA/W, response and recovery time in the range of millisecond. The capacitance-voltage measurement demonstrates that piezopolarization charges generated in ZnO could affect the depletion region width in CU2O. The results also indicate the illumination density-dependent photoresponse enhancement due to the screening effect, a 2.2% response enhancement per 0.1% strain could be obtained under the illumination density of 17.2 mw/cm2, while only 1.2% for 87.8 mw/cm2.Piezotronic strain sensor with metal-semiconductor-metal structure was fabricated. In dark, the device demonstrates a response ratio of about 200 and a response time of 0.3 s when applying a 0.53% compressive strain. When illuminating the 365 nm UV light, performance of the strain sensor deteriorated dramatically to 0.17% with the increase of illumination density due to enhanced screening effect and vanishment of rectifying behavior at the interface. It is demonstrated that when the illumination density is larger than 0.7 mW/cm2 and electron concentration in ZnO larger than 1018-1019 cm-3, the piezoelectrically generated immobile charges could be almost totally neutralized.