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Studies On Preparation And Performance Characteristics Of ZnO Based Heterojunction Optoelectronic Devices

Posted on:2017-02-27Degree:DoctorType:Dissertation
Country:ChinaCandidate:Y W ShenFull Text:PDF
GTID:1108330485950037Subject:Materials Science and Engineering
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Ultraviolet (UV) optoelectronic devices have widely and promising applications in green lighting, optical communication and UV detection, etc. As one of the new third generation semiconductor, Zinc Oxide (ZnO) is an important II-VI group direct-bandgap semiconductor material. ZnO has a wide bandgap of 3.37 eV at room temperature, and also a large exciton binding energy of 60 meV, which is much larger than the thermal activation energy of 26 meV. In addition, ZnO exhibits good photoconductive characteristics, high optical gain coefficient of 320 cm-1 and strong absorption in UV band. At the same time, it also has excellent piezoelectric and pyroelectric properties, as well as rich in raw materials, low cost, non-toxic, environmentally friendly, etc. Therefore, as an important material in semiconductor UV optoelectronic devices, ZnO has huge potential in applications such as light emitting and UV detection.In this thesis, we have studied the photoelectrical properties of ZnO thin film and ZnO nanorod array in several aspects. For ZnO/p-GaN film heterojunction, we studied its ultraviolet light emitting phenomenon, the photovoltaic effect, and thus designed and demonstrated a blue light-emitting diode and a self-powered high-speed UV detection device. For ZnO nanorods/NiO heterojunction, we performed an in-depth study of its self-powered photoelectrical characteristics and corresponding response mechanism. For ZnO/Spiro-MeOTAD flexible organic-inorganic hybrid heterojunction, we systematically studied its photoelectrical performance and the influence of strain modulation on the photoresponse properties.By optimizing the experimental conditions and regulating the synthesis parameters, ZnO thin film and ZnO nanorod arrays were prepared using radio-frequence magnetron sputtering and hydrothermal synthesis method, respectively. The influence of precursor concentration for hydrothermal method on ZnO morphologies was investigated. The as-grown ZnO thin film and ZnO nanorod array both have a hexagonal wurtzite structure, a preferential growth direction of [0001], and excellent optical properties with a dominant near-band-edge UV emission peak and a wide weak visible emission peak. The electron concentration and carrier mobility of ZnO films deposited by magnetron sputtering technique were calculated using hall effect measurement to be 3.13× 1018 cm-3 and 14.6 cm2 V-1 s-1, respectively.The electroluminescent properties of a heterojunction light-emitting diodes constructed from n-ZnO and p-GaN film were studied. The series resistance of diode device is about 102 Ω, and the threshold emission forward-voltage approximately 2.7 V. Electroluminescence spectra show that a stable high-brightness blue light emission centered at 460 nm can be achieved when the voltage is higher than 2.7 V. The emission efficiency reached 0.89%. Additionally, the curve of light-output versus injection current followed the power law of L~Im, which revealed a superliner dependence at lower currents and became almost linear at higher currents, indicating that a limited influence of nonradiative defects on the light emission.The self-powered photoresponse performance of a heterojunction based on ZnO film and p-type GaN film was also investigated. Under UV illumination, the device exhibited good photovoltaic properties and a fast, consistent and repeatable UV photoresponse with a response time less than 0.3 s at zero bias. However, no noticeable photocurrent response can be observed for the 514 nm visible light illumination. Upon 365 nm UV illumination with a power density of 1.0 mW/cm2, the device’s open-circuit voltage (Voc) and short-circuit current (Isc) are~1.32 V, and 5.55 μA, respectively. In addition, the photocurrent increases linearly with the light intensities for both 365 nm and 254 nm UV light and no saturation phenomenon appears. The spectral responsivity curves of the device at zero bias show a maximum responsivity of 25 mA/W as well as high UV to visible rejection ratio of 102 under 365 nm UV light irradiation.The self-powered photosensing performance of a heterojunction UV sensor based on ZnO nanorod array/p-NiO film was further studied. Optoelectronic test results show that the fabricated heterojunction device can operate in photovoltaic mode and exhibits a remarkable photocurrent of~0.3 μA for a low 355 nm UV irradiance of 3.2 mW/cm2. under 0.1 mV forward bias, the photocurrent alternated between positive and negative at the moment of switching the UV light on and off, which demonstrated a high-speed on/off binary-response arising from the photovoltaic behavior and low turn-on voltage of the ZnO/NiO device. The zero-biased device’s responsivity increased rapidly when the UV irradiance intensity was lower, then decreased with the increase of irradiance intensity, and a maximum of 0.44 mA/W for 0.4 mW/cm2 irradiance was achieved.The self-powered photodetection performance and relative strain modulation effect of a flexible organic-inorganic hybrid heterojunction based on ZnO film and Spiro-MeOTAD was studied. Electrical performance test indicate that the heterojunction exhibits a typical diode-like rectifying behavior with a fairly low turn-on voltage of 0.8 V and a high rectification ratio of 7.69 × 102 at ±1 V. The obtained device has a distinct light response and significant photovoltaic effect under UV light illumination. I-t curves show that the current response is fast, consistent and repeatable. Besides, the modulation effect of strain-induced piezopotential on the performance was further investigated. The photoresponse performance can be improved step by step when the applied tensile strain increases gradually. Together with responsivity and detectivity, the photocurrent can be increased about 1-fold upon applying a 0.753% tensile strain. While in contrast, the photoresponse performance deteriorates when compressive strain increases. This modulation effect could be due to that the strain-induced piezo-polarization charges in wurtzite-structured ZnO could tune/control the strength of build-in electric field at the heterojunction depiction region, leading to the enhanced or weakened separation efficiency of photogenerated electron-hole pairs.
Keywords/Search Tags:zinc oxide, heterojunction, light emitting diodes, ultraviolet photodetector, self-powered, piezo-phototronic effect
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