Nanomaterials, Devices and Interface Circuits: Applications for Optoelectronic and Energy Harvesting
thesisposted on 2013-10-24, 00:00 authored by Mohsen Purahmad
Metal oxide semiconductor nanowires (NWs) have received a lot of research attention because of their unique and wide-ranged optical, electronic, and piezoelectric properties. Among the metal oxide semiconductor NWs zinc oxide (ZnO) NWs have been the focus of much research due to their unique optical, electrical and piezoelectric properties. ZnO is a compound semiconductor with a wide direct band gap (3.37 eV) and a large exciton binding energy (60 meV) which makes this material a promising candidate for optoelectronic applications such as short-wavelength semiconductor lasers and light-emitting diodes. ZnO NWs have additional functionality arising from their size which allows for further flexibility in band gap engineering. In addition to the novel optical properties of ZnO NWs, utilization of ZnO NWs as nano-scale energy harvester devices has made them a very exciting topic of research. ZnO can possess two crystal structures, cubic zinc blende (ZB) and hexagonal wurtzite (WZ) in which each atom is tetrahedrally coordinated by atoms of the opposite species. In the crystal structure of ZnO, Zn2+ cations and O2– anions are tetrahedrally coordinated in crystal structure of ZnO, and the centers of the positive and negative ions overlap. Therefore, if a stress is applied at an apex of the tetrahedron, the centers of the cations and anions are relatively displaced, resulting in a dipole moment and consequently, polarization from all of the units result in a macroscopic potential drop along the straining direction in the crystal. Due to the large surface-to-volume ratio of semiconductor NWs the surface effects such as geometric properties, surface states, surface roughness, and surface passivation can strongly modify the optical and electrical properties of nano-structure materials such as ZnO NWs. Hence, understanding the surface effects, on the transport behavior of ZnO nanostructures is crucial for reliable device fabrication. In the first part of this dissertation, the effect of surface passivation on the near-band-edge emission (NBE) of as grown ZnO nanowires has been studied. It was shown that decorating the ZnO nanowires with sputtered metallic nano-particles can strongly enhance the NBE of as grown ZnO nanowires. Since the ZnO NWs have a weak NBE, numerous studies have been done to enhance the NBE and photoluminescence (PL) efficiency of ZnO NWs. Different methods such as polymer coating of ZnO NWs and hydrogen plasma treatment are seen to boost the NBE and photoluminescence efficiency of ZnO NWs. Recently, among the different passivation methods the effect of metallic nanoparticles (NPs) on PL properties of ZnO NWs have been the focus of much research. In most studies an enhancement of NBE was observed and the results were interpreted in terms of surface plasmons, unintentional hydrogen incorporation and the nature of the contact formed between the metal and ZnO NWs. Our study demonstrates that decorating the ZnO NWs with metal NPs in the presence of high energy Ar atoms cleans the surface of ZnO NWs from near surface traps and surface adsorbed species, thus it leads to a strong enhancement of NBE emission and a relative reduction of visible peak. Furthermore, we have investigated the effect on the intensity of PL spectra of deposited nanostructure materials such as ZnO NWs when deposited on different substrates (gallium arsenide, silicon, glass and indium tin oxide-coated glass). The experimental results show that the PL intensity is the highest for the gallium arsenide (GaAs) and the least from the indium tin oxide-coated glass (ITO-coated glass) substrate for both the quantum dots and the ZnO NWs. These experimental results were simulated by taking into account the scattered field from different substrates and were seen to be in agreement to the experimental results. We found that the PL intensity is a function of scattered light from the surface of substrate rather than the type of heterojunction formed between the deposited material and substrate. As discussed in addition to novel optoelectronic applications of ZnO nanowires, utilization of ZnO nanowires as nano-scale piezoelectric energy harvesters have made them a hot topic of research. In the second part of this thesis, the piezoelectric properties of ZnO nanowires have been the focus of the study. Since demonstration of the first nano-scale energy harvester device based on one single ZnO NWs a comprehensive model which can explain the generation of strain induced piezoelectric field in the presence of free carriers has not been proposed. The piezoelectric constitutive equations, which are used to calculate the strain induced piezoelectric potential, can be applied in a medium with zero or negligible free carriers. Therefore, in case of piezoelectric metal oxide semiconductor materials such as ZnO NWs with a free carrier density about 1018 cm-3 the constitutive equations cannot be applied directly. Here, we have developed a model which strongly conciliates some strongly divergent opinions behind operation of the semiconductor piezoelectric nano-generators. In order to develop such a physics-based model, first the electrostatic potential and depletion width in piezoelectric semiconductor NWs are derived by considering a non-depleted region and a surface depleted region and solving the Poisson equation. By determining the piezoelectric induced charge density, in terms of equivalent density of charges, the effect of piezoelectric charges on the surface depletion region and the distributed electric potential in NW have been investigated. The numerical results demonstrate that the ZnO NWs with smaller radii have a larger surface depletion region which results in a stronger surface potential and depletion region perturbation by induced piezoelectric charges. Furthermore, we have also studied another type of piezoelectric materials which have a negligible free carrier concentration and constitutive equations can be applied directly. In this domain, we have investigated the effect of applied stress on the generated electrical pulse of Polyvinylidene fluoride (PVDF). PVDF is a piezoelectric polymer that has been the focus of much research in many piezoelectric applications due to its high piezoelectric coefficient. The electromechanical properties of PVDF can be defined by the constitutive equations. In this study, the effect of applied stress frequency on the electrical response of PVDF-TrFE has been investigated. The experimental results demonstrate that increasing the frequency of applied stress results in increasing of pulse width of the electrical response. A model based on the mechanical creep and relaxation of PVDF has been proposed, which agrees well with the observed experimental results. In the last part of our study on piezoelectric energy harvesters the low power interface circuits which are one of the fundamental building blocks of any self-powered devices has been studied. Utilization of piezoelectric energy harvesters to power electronic devices has attracted significant attention recently. However, the power generated by a piezoelectric energy harvester is too small to power an electronic device directly. Hence, a low power, efficient interface circuit between the energy harvester and a storage unit is essential in any piezoelectric energy harvesting system. Here, a new interface circuit topology for piezoelectric energy harvesting applications is proposed and various design factors for circuit-level optimization are discussed. In the proposed interface circuit a peak detector circuit operating in the sub-threshold region with power dissipation around 160 nW together with a delay circuit form the control block, which is one of the more important units of the piezoelectric energy harvesting systems.