Characteristics of nanowire and nanoparticle framework for functional nano-electronic applications

Abstract

This dissertation describes an experimental study on fabrication and characteristics of semiconductor and metal nanoparticle (NP), metal oxide nanowire (NW), and its application for the unique functional nano-electronic devices such as non-volatile nano floating gate memory (NFGM), nanowire field effect transistor (NWFET), and nano generator.

Part I : Semiconductor and Metal Nanoparticels

At first, in order to develop the optimum fabrication for semiconductor and metal NPs, we invented efficient method for the NPs formation by using a Nd: YAG LASER (wavelength=355 nm) irradiation method. A LASER irradiation method is the simple and direct fabrication of the NPs without performing any micro- or nano-pattering process. Scanned LASER irradiation of low power causes localized segregation of ultrathin as-deposited films by the LASER-induced heat, resulting in the fabrication of NPs. The size of NPs can be easily controlled by initial deposition thickness of materials. In addition, this technique gives narrow spatial distribution of the NPs, resulting in an excellent degree of scalability for the size. Experimentally, observations by scanning electron microscopy (SEM), atomic force microscopy (AFM), and transmission electron microscopy (TEM) clearly confirm highly uniform controllable nanoscale NPs. Second, semiconductor and metal NPs were investigated for use in charge storage for metal oxide semiconductor (MOS) devices with thin HfO2 tunneling and control oxide layer. The CoSi2 NPs were fabricated without a post-annealing process by exposure of Co/Si/HfO2 tunneling oxide/Si stacks to an external UV LASER. The thickness of the Co and Si layers were intentionally controlled to obtain ideal CoSi2 NPs. Observations from X-ray photoelectron spectroscopy (XPS) and high resolution transmission electron microscopy clearly confirm the formation of CoSi2 in our process. These CoSi2 NPs in MOS devices exhibited a memory window of 3.4 V under +11V / -9 V in capacitance-voltage curves as well as good retention and endurance times, thereby demonstrating that they show promise for nonvolatile applications. In addition, the memory behavior of natively oxidized AlOx shell – Al NPs in MOS structure were investigated for high retention characteristics. Among many of the discussions regarding improvements in retention characteristics, an attempt of tunneling barrier engineering (TBE) using a native metal oxide shell was chosen. TEM measurements clearly demonstrate the formation of an AlOx shell (thickness 1 – 1.5 nm), surrounding Al (size 5 – 7 nm) core NPs in the MOS structure. Electrical measurements exhibited a memory window of 3.6 V, together with promising charge retention characteristics of about 10 years. A possible band model needed for enhanced retention characteristics was given by considering the electron / hole barrier width and the additional interface states through the AlOx shell as a method of tunneling barrier engineering. Third, spin filtered charge trap devices called as spin capacitor which modifying a conventional MOS structure to incorporate spin trap sites in the oxide layer and ferromagnetic electrode was experimentally investigated. The basic concept of spin capacitor is starting from a modified by MOS capacitor with floating gate and subsequently detected thorough change of the capacitance curve in accordance with external magnetic field. We consider an identical structure, but having ferromagnetic NPs as floating gate and ferromagnetic thin film as gate electrode. A ferromagnetic cobalt (Co) NPs were used to trap the spin filtered charges. As magnetic properties, the switching magnetic fields are 60 Oe for the Co NPs. The fundamental characteristics of the spin capacitor were analyzed using C-V curve as a function of external magnetic field. Consequently, the C-V and I-V curve reliably exhibited spin dependent MOS behaviors with floating gate.

Part II : Metal Oxide Nanowires and Doping Process

One-dimensional (1D) nanostructures such as nanowire (NW) and nanotube have attracted great interest over the past decade because of their specific physical properties and their potential as building blocks for next-generation nano-electronic devices. Among various 1D materials, zinc oxide (ZnO), which has a direct and wide band gap, is a promising candidate for light-emitting diodes, UV/gas sensors, transistor channels, and other devices that can utilize the unique vertical alignment characteristics and highly ordered single crystalline properties of NW structures. First, structural properties of ZnO NWs synthesized by hydro thermal process were investigated. The ZnO NWs were synthesized on ZnO thin film seed layers via an aqeous solution method with zinc nitrate hexahydrate (Zn(NO3)2•6H2O) and hexamethylenetetramine (HMT). The growth speed and the shape of the ZnO NWs were determined for various mole concentrations. The structural analysis of the ZnO NWs was performed using X-ray diffraction, SEM, and TEM measurements. The correlation of structural results wit growth conditions, such as the mole concentration and the growth temperature of chemical precursors based on Gibbs free energy. Second, doping process of ZnO nanowires for fuctional nano-electronic applications was investigated. The undoped ZnO NWs are intrinsically n-type, their use in practical devices has been hindered, and much effort has been dedicated toward the development of p-type ZnO NWs. In particular control and manipulation of the doping process is increasingly becoming a key approach that has been taken for the realization of p-type ZnO NWs. To realze p-type ZnO NWs, the initial dopant candidates tested included group-V elements substitute for O and group-III elements to substitute for An, despite the large size mismatches in both cases. Therfore, group I species as lithium (Li) and silver (Ag) have been used to synthesize p-type ZnO NWs based on the expectation thate these elements would function as shallow acceptors in ZnO host materials. In case of Li dopants, Li has the smallest ionic radius (0.76 Å) that is very close to that of Zn (0.74 Å). TEM meausrements of the c-axis oriented and highly vertically aligned NWs demonstrated that Li defect in as-grown ZnO:Li NWs can occupy the empty cages of the wurtzite structure at octahedral sites, and that Li substitution of Zn occurred because of thermally-induced migration due to post-annealing in the presence of oxygen. The stable formation of p-type ZnO:Li NWs using a NWFET and a simple n-type thin film / p-type annealed ZnO:Li NWs homojunction diode. In case of Ag dopants, zinc nitrate hexahydrate (Zn(NO3)2•6H2O) and hexamethylenetetramine (HMT) solution was prepared in deionized water at 25 mmol and silver nitrate AgNO3 solution was also prepared in deionized water at 1 mmol. Low temperature photoluminescence (PL) spectra at 10 K gained significant insight into the effects of Ag-doping. The ZnO:Ag NWs showed four dominant peaks at 3.357, 3.318, 3.252 and 3.180 eV, representing neutral acceptor-bound exciton peak (AoX), free electron to the acceptor transition peak (FA), and donor-to-acceptor pair transition peak (DAP) and donor-to-acceptor pair transition phonon replica peak (DAP-1LO), respectively. Finally, we demonstrate simple and effective approaches to improve the efficiency of piezoelectric energy harvesting through lithium and silver dopants. The output voltage and current of an optimized nano generator (NG) are drastically improved by over 40 times compared to those of a NG based on undoped ZnO NWs, respectively. Furthermore, we demonstrate that output power of our NG is sufficient to drive electrophoretic displays. This approach is expected to be an attractive potential strategy, providing the high possibility for realizing multi-functional, self-powered devices with high performance.