A study on the synthesis and electrical properties of one-dimensional Zinc-based semiconductor structures

Abstract

Nanoscale one-dimensional (1D) structures have stimulated great interest recently owing to their unique electronic, optical, and mechanical properties as a result of their low-dimensionality and the quantum confinement effect. Their potential applications as building blocks, interconnects and functional units in electronic and optoelectronic devices and sensors have also been demonstrated. Even though there are ongoing efforts to realize nano-devices using 1D nanowires building-block, the research on the control methods of chemical composition, structure, and size at nanoscale are still required with rational synthesis including reproducibility. The work described in this thesis focuses on understanding the basic synthesis of Zinc-based semiconductor structures of interest for growth behavior and hetero-structure. The goal is to investigate nanostructures relevant to specific growth conditions, and then through careful analysis of these structures, gain new insights into the growth behavior governing their nucleation and growth. In particular, this work is focused on the growth mechanism of ZnS nanowires, the analysis of nanostructures using 3D TEM tomography and 3D printing, the optical properties of ZnS / diamond-like carbon core-shell heterostructure nanowires and the electrical characteristics of a ZnO microwire. Firstly, we report sublimation of crystalline ZnS nanowires at elevated temperatures in vacuum imaged by in situ transmission electron microscopy. The ZnS nanowires, 20-80 nm in diameter, were heated using a controllable heating system, and their melting temperature was studied. The results showed a significant reduction of the melting temperature of about 400°C, depending on the diameter of the nanowire, compared to the bulk melting point of 1185 °C. In addition, the in-situ heating experiment showed that the SLV process proceeds exactly in the reverse direction of VLS, and the synthesis mechanism of Ag2S catalyzed ZnS NW was investigated. Secondly, the work focus on recent developments in the field of 3D imaging at the nanoscale, when applied to nanomaterials and nanostructures. I demonstrate that recent progress in the use of electron microscopy techniques based on tomography allows one to fill the gap between the development of new materials and their structures and characterization. A special emphasis is put on two new 3D approaches: quantitative and analytical 3D tomography. Electronic tomography studies the 3D form of nanomaterials and provides a comprehensive insight into the structure and interface of nanomaterials. Here, we report 3D characteristics of ZnS nanostructures using Ag catalyst using electron tomography using bright field image. Thirdly, we fabricated ZnS/diamond-like carbon (DLC) core-shell heterostructure nanowire using a simple two-step process: the vapor-liquid-solid method combined with radio frequency plasma enhanced chemical vapor deposition (rf PECVD). As a core nanowire, ZnS nanowires with face-centered cubic structure were synthesized with a sputtered Au thin film, which exhibit a length and a diameter of ~10µm and~30-120nm. After rf PECVD for DLC coating, The length and width of the dense ZnS/DLC core-shell nanowires were a range of ~10μm and 50-150nm, respectively. In addition, ZnS/DLC core-shell nanowires were characterized with scanning transmission electron microscopy. From the results, the products have flat and uniform DLC coating layer on ZnS nanowire in spite of high residual stress induced by the high sp3 fraction. To further understanding of the DLC coating layer, Raman spectroscopy was employed with ZnS/DLC core-shell nanowires, which reveals two Raman bands at 1550 cm-1 (G peak) and 1330 cm-1 (D peak). Finally, we investigated the infrared transmittance property using Fourier transform infrared spectrometry. The results confirm that products increased the infrared transmittance property of the ZnS nanowires by 1.1-2.8%. Lastly, we investigate the influence of the contact interface on the electrical properties of a ZnO microwire (MW) with silver (Ag) paste electrodes. The ZnO MW devices that are produced by dropping Ag paste on the ZnO MW surface followed by a curing step at an elevated temperature exhibit linear current-voltage characteristics, whereas the devices with Ag paste electrodes dropped upon a heated ZnO MW exhibit a non-linear electrical behavior. The results of electron microscopy and cathodoluminescence show the effect of the contact interface properties, such as interfacial defects and/or charge trap sites, between the ZnO MW and Ag paste electrodes. An energy band model is suggested to explain the charge transport mechanism for different types of Ag contacts on the ZnO MW.