Abstract
The Ph.D. thesis is entitled as “Synthesis of Ge-Related Semiconductor Nanostructures for their Applications”. In this dissertation, we put efforts to explore and expertise in contemporary one-dimensional nanostructures research. The research is focused on Ge based nanosystems and its novel applications towards present/future technology. We carried out detailed investigations on the following; a) Tuning the experimental parameters for synthesis of Ge nanowires. b) Synthesis of doped and undoped GeOX nanowires and nanotubes and investigation of dependence of change in optical properties on dopant into GeOX nanowires and nanotubes compared to that of the undoped GeOX nanowires and nanotubes. c) Germanium nanochains were fabricated and utilized to fabricate the single electron transistor at room temperature. d) Crdoped germanium nanotowers have been synthesis and its growth mechanism was discussed. e) Cr-doped core-shell Ge/GeOX nanowires were synthesized and its room temperature ferromagnetic properties were investigated. Systematic investigation of growth conditions to control morphologies of germanium was carried out at the outset of this work. We achieved growth of single crystalline germanium nanowires which can be utilized for optical and electrical properties. X We have investigated the doping impurities like Cr, B and Ga into GeOX nanowires and nanotubes and have tune the optical properties as compared to undoped GeOX nanowires and nanotubes. In the beginning, germanium oxide nanowires were grown by using simple vapor transport method. By using Cr source and mixing with the germanium sources and keeping same parameters that for the nanowires, Cr-doped GeOX nanowires were synthesized. In the second experiment, germanium oxide nanotubes were obtained at higher temperature and B and Ga were doped using their respective sources with germanium sources. The red shift in the photoluminescence spectra shows that the optical properties of the GeOX nanowires can tune by doping Cr in GeOX nanowires. Similarly Ga and B were doped in GeOX nanotubes. Similar to nanowires the comparison between undoped and doped (Ga and B) GeOX nanotubes has been investigated. The doping of Ga and B in GeOX nanotubes resulted in the red shift of luminescence spectra as compared to undoped GeOX nanotubes. The red shift of spectra suggests the possible control of defects in nanotubes and oxygen vacancies in GeOX nanotubes. We demonstrate a conceptually novel approach to utilize the germanium nanochains for fabrication of single electron transistor. Ge nanochains were grown by using a simple vapor transport method. The appropriate dimensions of Ge bulbs (cores) insulated by GeOX necks have motivated us to investigate its I-V characteristics at RT. A MTJ single electron transistor (SET) device consisting of N quantum dots (Ge bulbs) isolated with N+1 potential barriers (GeOX necks) were fabricated using focus ion beam technique. Moreover, Al-base SET device was fabricated, which operated at above 4.2 K to show periodic fluctuation in Id-Vds curve with XI variable gate. Interestingly, Ge nanochain with two Ge bulbs connected with GeOX neck not only show uniform Coulomb staircase behavior in I-V characteristics, but it also shows periodic gate modulation at RT, which is excellent in order to utilize SET in electronic devices for various applications. Also, after comparing I-V behavior of STJ and MTJ at RT, it was clear that Ge nanochain MTJ SET has improved the threshold voltage (Vth). The easily reproducing Ge nanochain to fabricate the SET device, its periodic dependence of single electron tunneling on applied gate voltage (Vg) at RT and the enhancement of threshold voltage can open up a new era in SET devices in order to utilize SET in electronic devices for various applications like single electron memory, high sensitive electrometer and radio-frequency mixer. The influence of impurities like Cr on the morphology of the germanium nanostructure was investigated. The growth mechanism of Cr-doped germanium nanotowers has been demonstrated. Two kinds of Ge nanostructures (Ge NWs and Cr-doped Ge NTrs) were grown on silicon wafer via a vapor transport method. The morphological and structural features of both the nanostructures have been investigated to clarify the role of Cr source. To demonstrate the growth mechanism of Cr-doped Ge Ntrs, three experiments have been performed at different reaction times using similar experimental conditions used for nanotowers. Vapor-liquid-solid (VLS) growth mechanism accounts for the growth of NTrs which are initiated by thin nanowires with gold catalyst. The lateral growth of polygonal facets (bottom of the NTr) followed by simultaneous longitudinal (1D) growth below the critical length (LC) has been explained by vapor-solid (VS) mechanism along with the VLS mechanism. The Cr atoms may be remarkably XII effective on determining diffusion length of the Ge adatoms and could be responsible for the formation of NTr structure. Moreover Cr dopant can also play vital and complementary role to generate the magnetic ordering at high temperature in Ge matrix. In this work, the room temperature ferromagnetism was observed in Cr-doped Ge NTrs. Thus Ge-Cr complex nanostructures, that is, NTrs will be very promising materials for future devices and can open a new door to fabricate Ge based dilute magnetic semiconductor (DMS) materials as a hard magnet for high temperature spintronics devices. The Cr-doped core-shell Ge/GeOX nanowires were fabricated using a simple vapor transport method. Room temperature ferromagnetism of core-shell Ge/GeOX nanowires was discovered. Interestingly core-shell nanowires shows high curie temperature (TC > 300 K). The values of remanence (MR) and coercivity (HC) are extraordinarily dominating as compared to previous germanium DMSs nanomaterials. Thus Ge/GeOX core-shell nanowires due to their higher characteristic values of FM ordering can be utilized as a hard magnet to fabricate spintronic device.