Experimental and Analytical Analysis of Nanostructured Materials and Devices
thesisposted on 2017-11-01, 00:00 authored by Mojgan Mazouchi
The nanoscale regime devices have attracted considerable research attention, due to the wide range of applications resulted from the size reduction of material structures and devices. To make essential progress towards developing novel devices further study of properties of nanomaterials and nanodevices is required. Among the nanostructured materials, metal oxide semiconductor nanowires have been the focus of research studies due to their unique electrical, optical and chemical properties. Due to the large surface-to-volume ratio of metal oxide semiconductor NWs, the surface effects such as surface defect states and geometric properties can strongly modify the optical, electrical and chemical properties of materials such as In2O3 NWs. In2O3 is a transparent n-type metal oxide semiconductor with a wide direct band gap around 3.7 eV at room temperature, that tends to cause oxygen deficiency and become conductive especially if synthesized in an oxygen-poor environment. These properties make this material a promising candidate for optoelectronic applications such as solar cells, light-emitting diodes and toxic-gas detectors. Thus, understanding the electrical transport behavior of In2O3 nanowires is critical to fabricate the reliable nanostructure devices. In the first part of this dissertation, we present a detailed study of the growth conditions of In2O3 NWs using a carbothermal reduction method. Further, we present the study of electrical conduction mechanisms of In2O3 nanowires at different temperatures, in dark and under UV illumination. Moreover, the discussion of LC is critical in design and fabrication of high efficiency multijunction solar cells. Thus, in this thesis we propose a theoretical approach to compute the voltage increase of multijunction cells due to LC and we explore the change in recombination current density due to LC in detail. In order to study the LC effects on multijunction solar cell, first we study and investigate the LC effects on both photocurrent-matched and photocurrent-mismatched double-junction solar cell and triple-junction solar cell through the fundamental physical theories and h-spice circuit simulations. Afterward, we extend the analytical model so that we can predict the effect of LC on n series-connected multijunction solar cells. Further, we investigate the effects of LC on voltage in the voltage range between V_MPP to V_OC and we study the dependency of voltage on both the LC efficiency and the number of junctions in the aforementioned voltage range. In recent years, enormous advances in nanotechnology have enabled the fabrication of nanostructures and increased the feasibility of acoustic wave confinement. As a result of their smaller dimensions, these structures portend applications at higher frequencies than those for microscale structures. Thus, the effect of downscaling from the micro-scale down to the nano-scale on the acoustic response of the nanostructure waveguides and resonators has attracted a great deal of attention. To address the technological issues in order to design high frequency nanoresonators and nanoelectromechanical systems (NEMS), it is crucial to gain a solid understanding of phonon behavior on the nanoscale objects. In the last part of this thesis, we study the confinement of resonant acoustic field for the case where energy trapping is caused by thickening the center region of the plate. We use the elastic continuum mechanics model to determine the quantized acoustic-phonon modes in an isotropic nanowaveguide. We obtain and present the acoustic-phonon amplitudes and relative frequency dispersion relations analytically for both the odd symmetry shear modes and even symmetry shear modes. Further, we quantize the acoustic-phonon modes in the non-piezoelectric nanowaveguide. Furthermore, we discuss the limit of the quality factor and frequency (fQ) product achievable by a resonator and we present the fQ product calculation for silicon resonators working in sub-terahertz range for three different orientations. Furthermore, we present the detailed study of acoustic acoustic phonon SH modes in an unbounded hexagonal Nitride-based piezoelectric nanoresonator. We analyze the SH mode propagation in an X-cut hexagonal elastic plate to provide an analytical solution for general quantized acoustic nanoresonators. For the first time, we derive and present the quantized acoustic shear horizontal (SH) modes in a piezoelectric nanoresonator, which is of importance for modelling and understanding charge carrier-acoustic phonon interactions. Further, we derive the phonon-mode amplitude for each symmetry mode by using the energy stored in the quantized vibrational mode. We present the obtained acoustic-phonon frequency dispersion relations for odd symmetry and even symmetry SH modes. Further, we calculate and present the fQ product of nitride-based piezoelectric nanoresonator such as AlN and GaN. Eventually, we study the electrical surface perturbation in the nanoresonator which has the significant interest for sensing application and we derive and calculate the resulting resonance frequency shift caused by the electrical surface perturbation.