Nanowire under tensile strain

Today, most modern technology relies on materials with reduced dimensionalities, such as thin films (2D), nanowires (1D), and quantum dots (0D). An extreme success in synthesis, characterization, and application has been achieved within the past 30 years. Although the electrical and optical properties of nanostructures have been extensively studied, the effects of high mechanical strain levels on the electronic and optical properties have been largely overlooked up to now. In situ tuning of high strain levels has proved to be challenging. The nanostructures have to be interfaced with electrodes which guarantee reliable contacts even when subjected to such mechanical forces, where surface imperfections may initiate fractures. Most of the approaches, such as the commonly used four point bending method, are applicable to study e.g. the piezoresistive behavior of NWs only at low strain levels. Therefore we merge: (a) on the personal level two important research groups out of Japan and Austria, as well as (b) on the scientific level bottom up and top down techniques: self assembled synthesis of NWs via the vapor- liquid-solid growth mechanism and a highly sophisticated electrostatic actuated nano tensile testing device. Thus, we explore a special setup that enables the application of pure tensile strain without a shear component while measuring simultaneously the electrical and optical properties of thereby ultra-strained NWs. The synthesis techniques will be based on the vapor-liquid-solid process to integrate well defined nanowires within the electrostatic actuated nano tensile testing device. Such nanowires appeared to be relatively free of extended volume defects and may withstand high stress levels before the strain will be relieved by the formation of dislocations and other defects. The ability to fabricate single-crystal NWs that are widely free of structural defects will it make possible to apply ultra-high strain without fracturing, therefore in any application where crystallinity and strain are important, the idea of making NWs should be of a high value. Such strained nanowires in the electrostatic actuated nano tensile testing device will be subjected to various analysis techniques like field emission scanning electron microscopy, -Raman spectroscopy, spatially resolved photoluminescence as well as temperature dependent electrical transport investigations. Three promising systems will be investigated within our research: (i) Si-, Ge- and SiC-nanowires, (ii) the respective silicides and germanides of Pt, Ni and Co, and (iii) axial NW heterostructures such as NiSi/Si/NiSi- NWs. The main focus of this project is the investigation of the correlation between strain and functionality of the NWs, in view to enable novel electronic, photonic, or ferroic devices. A long term goal of the project includes the realization of a prototype of silicon-compatible strain gauges or a high performance piezoelectric MOSFET device.

Today, most modern technology relies on materials with reduced dimensionalities, such as thin films (2D), nanowires (1D), and quantum dots (0D). Although the electrical and optical properties of such nanostructures have been extensively studied, the effects of high mechanical strain levels on the electronic and optical properties have been largely overlooked up to now. In situ tuning of high strain levels has proved to be challenging as the nanostructures have to be interfaced with electrodes which guarantee reliable contacts even when subjected to high mechanical forces. To achieve this goal we merged: (a) on the personal level two research groups out of Japan and Austria, as well as (b) on the scientific level bottom up and top down techniques: self-assembled synthesis of nanowires (NWs) via the vapor-liquid-solid (VLS) growth mechanism and a highly sophisticated electrostatic actuated nano tensile testing devices. Thus within the NANOTEST project, we explored a particular setup that enables the application of pure tensile strain without a shear component while measuring simultaneously the electrical and optical properties of thereby ultra-strained NWs.Therefore strained Si and Ge NWs in the nano tensile testing device were subjected to various analysis techniques like field emission scanning electron microscopy, -Raman spectroscopy, spatially resolved photoluminescence (PL) as well as temperature dependent electrical transport investigations.The main focus of this project was the investigation of the correlation between strain and functionality of the NWs, in view to enable novel electronic and photonic devices. Thus we demonstrated the fabrication and application of an electrostatic actuated test device enabling strain engineering in individual suspended NWs. To demonstrate the potential of this approach we investigated the piezoresistivity of VLS grown Si and Ge NWs. A giant piezoresistive effect was observed, resulting in a fivefold and more than tenfold increase in conductivity for 3% uniaxially strained Si and Ge NWs, respectively. A model based on stress induced carrier mobility change and surface charge modulation is proposed to interpret the actual piezoresistive behavior of the NWs. By controlling the nature and density of the surface states via passivation the intrinsic piezoresistance of the Si NWs is found to be comparable with that of bulk material. This demonstrates the indispensability of application-specific NW surface conditioning and the modulation capability of Si NWs properties for sensor applications.Direct semiconductor NWs are also of particular interest as these have already demonstrated e.g. continuous wave laser emission1 and ultrafast modulation capabilities.2 However individual NW laser devices currently suffer from fixed emission spectra determined by the material band gap. In cooperation with the group of Carsten Ronning from the Univ. Jena, exemplarily laser devices were fabricated by integrating individual CdS NWs into the nano straining device. The middle part of the NW is thus bridging a length tunable gap, allowing to apply uniaxial strain to the gap region of the NW. The strain distribution along the NW was verified using spatially resolved -PL and Raman spectroscopy. Subsequently, the NW lasing performance was measured as a function of the applied strain indicating a significant laser mode red-shift with increasing strain, as the gain envelope was shifted to smaller emission energies for all pump powers above the laser threshold. Thus, this NW laser device enables the realization of dynamically tunable nanoscale coherent light sources.Finally, a device concept utilizing the strain-induced gradient in bandgap along a tapered NW was used to demonstrate strain-induced charge separation in the test bed of a Ge NW model system. The possibility of strain-tunable solar cells from cheap and abundant materials which have not been considered for photovoltaic applications so far is believed to foster further research in the field.