Semiconductor Nanowire Based Piezoelectric Energy Harvesters: Modeling, Fabrication, and Characterization
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Semiconductor nanowire (NW) arrays’ unique advantages over bulk forms, including enhanced surface area, high mechanical flexibility, high sensitivity to small forces, better charge collection, and enhanced light absorption through trapping, make them ideal templates on which to build other structures. This research on the piezoelectric behavior of NWs used in high-performance energy harvesters is based on device modeling, fabrication, and characterization. These activities optimize the electrical properties of a NW device in response to a compression/release force applied to the NWs. The dissertation first discusses the piezoelectric and semiconductor properties of wurtzite compound nanomaterials, emphasizing III-nitride semiconducting InN and GaN NWs. Static analysis identifies the role of carrier density, temperature, force, length/diameter ratio, and Schottky barrier height. Piezoelectric nanogenerators (NGs) based on vertically aligned InN nanowires (NWs) are fabricated, characterized, and evaluated. In these NGs, arrays of exclusively either p-type or intrinsic InN NWs prepared by plasma-assisted molecular beam epitaxy (MBE) demonstrate similar average piezoelectric properties. The p-type NGs show 160% more output current and 70% more output power product than the intrinsic NGs. The features driving performance enhancement are reduced electrostatic losses due to a higher NW areal density and longer NWs, and improved electromechanical energy conversion efficiency due to smaller NW diameters. These findings highlight the potential of InN based NGs as a power source for self-powered systems and the importance of NW morphology in overall NG performance. The second part is devoted to demonstrate a series of flexible transparent ZnO p-n homojunction nanowire (NW)-based piezoelectric nanogenerators (NGs) with different p-doping concentrations. The lithium-doped segments are grown directly and consecutively on top of intrinsic nanowires (n-type). When characterized under cyclic compressive strains, the overall NG performance is enhanced by up to eleven-fold if the doping concentration is properly controlled. This improvement is attributable to reduction in the mobile charge screening effect and optimization of the NGs’ internal electrical characteristics. Experimental results also show that an interfacial MoO3 barrier layer, at an optimized thickness of 5-10 nm, reduces leakage current and substantially improves piezoelectric NG performance. The third part presents the first cascade-type compact hybrid energy cell (CHEC) that is capable of simultaneously or individually harvesting solar and strain energies. It is made of an n-p junction NW-based piezoelectric nanogenerator to harvest strain energy and an nc/a-Si:H single junction cell to harvest solar energy. The CHECs ability to harvest energy effectively simultaneously, and complementary is demonstrated by deploying six CHECs to power LEDs and a wireless strain gauge sensor node. Under ~10 mW/cm2 illumination and vibrations of 3 m/s2 at 3 Hz frequency, the output current and voltage from a single 1.0 cm2 CHEC are 280 μA and 3.0 V, respectively; enough to drive many low power commercial electronics. This dissertation aims to deepen understanding of the piezoelectric behavior of semiconductor NWs on hard and flexible substrates. Thus, this research in the field of nanopiezoelectrics could have a substantial impact on many areas, ranging from the fundamental study of new nanomaterial properties and mechanical effects in nanostructures to diverse applications like aerospace.