Monolitic Integration of Nanowires

Over the past decade, one-dimensional nanostructures have been proven as powerful building blocks in active nanometer-scale devices. Aside from carbon nanotubes, semiconducting nanowires (NWs) are one of the most promising approaches considered for scaling down optic-, electronic-, magnetic- and sensor devices. Moreover, in comparison to other nanotechnology device option, semiconductor NWs may have the potential to be implemented on an existing semiconductor infrastructure and can thus benefit from continous improvements in CMOS technology. Apart from device shrinking, the monolithic integration of the (superior) III-V semiconductors which are particularly attractive for optoelectronic device applications into the mature silicon technology combine the best parts of different technologies and will lead to enhanced functionality. Carrier confinement in heterostructures provides further an opportunity to increase the efficiency of radiative recombination of electrons and holes. Fundamental issues such as lattice and thermal expansion mismatch and the formation of antiphase domains which have prevented the epitaxial integration of III-V with group IV semiconductors could be avoided by a nanowire approach. By reducing the contact area the crystal lattice of the III-V material will be elastically deformed near the interface, and due to the small dimension the strain could be accommodated at the nanowire surface. The monolithic integration of hierarchical NW heterostructures with Si devices and the electrical and optical characterization of these functional building blocks is the final goal of this project. Initially, we will explore the well controlled formation of NWs and heterostructures combining LPCVD and MBE techniques. The control over the so called vapor-liquid-solid (VLS) NW growth is mediated through the usage of nanoscopic metal templates. In a second step we will integrate individual NWs and defined assemblies of NWs in test modules which enable to extract their fundamental electrical and optical properties being a significant step for the exploitation of such structures as a platform for novel electronic, spintronic or magneto-optic devices and sensor applications. For this purpose several aspects of contact formation to the 1D nanostructures as well as abrupt heterojunction formation have to be solved. Two different, partly self-aligned processes of test module formation mainly based on SOI technology are under consideration. By means of a detailed research plan, a series of material combinations will be successively processed and evaluated to achieve three main goals: (i) to develop suitable processes for the formation of NW heterostructures (hybrid systems, vertical and radial heterostructures and hierarchical branched structures) (ii) to gain basic understanding of hetero-junction interface as well as metal/nanowire contact properties (iii) to develop suitable measurement methodologies to qualitatively as well as quantitatively explore electrical and optical characteristics of NW/Si-technology hybride systems and thereby allow a benchmarking. The growth of NW heterostructures and integration in the various test modules will be accompanied by analytical support activities (HRTEM, EDX, EELS, AES, SIMS, XRD,) providing feed-back for process optimization. The near-term relevance of this project is of a scientific nature. The project specific NW heterostructures can be considered as primary tools for the exploration of the physics and the chemistry on the few-nanometer scale. Due to our scientific mission, nanoscale electronic device research is our primary focus. Since the fundamental investigation of NW heterostructures, especially of their electrical properties, is not possible without implementing the materials in any kind of building blocks or devices, the results may also lead to novel, functional nanoelectronic device concepts that might find their way into discussion as potential device candidates for long-term future technology nodes. Aside of the scientific output, a major aspect of the proposal is to provide young researchers and especially PhD candidates the opportunity to take part in a project on nanotechnology will allow them to develop a scientific independence and to gain hands-on experience in a prospective area in the core of future science.

Over the past decade, 1D-nanostructures have been proven as powerful building blocks in active nanometer-scale devices. Aside from carbon nanotubes, semiconducting nanowires (NWs) are one of the most promising approaches considered for scaling down optic-, electronic-, magnetic- and sensor devices. Moreover, in comparison to other nanotechnology device option, semiconductor NWs may have the potential to be implemented on an existing semiconductor infrastructure and can thus benefit from continuous improvements in ICs technology.Within this project we explored several approaches for the monolithic integration of NWs within functional Si based devices. Further we assembled a new setup that enabled the electrical and optical characterization of individual NWs within these functional building blocks. The reliable integration required precise control over the so called vapor-liquid-solid (VLS) NW growth, mediated through the usage of nanoscopic metal templates. We tested the applicability of a novel precursor with respect to reasonable growth rates, feasibility of epitaxial NW growth and versatility with respect to diverse catalysts. Epitaxial growth of Si-NWs was achieved using octochlorotrisilane as Si precursor and Au as catalyst. Further, by optimizing the growth conditions, effective NW synthesis was shown for alternative catalysts, in particular, Cu, Ag, Ni, and Pt with the latter two being compatible to complementary metal-oxide-semiconductor technology. The feasibility of Ga as a catalyst for NW growth deriving from an implantation process in silicon was impressively shown leading to ultra-fast growth of Si-NWs with a length of several tens of micrometers.Several aspects of contact formation to the 1D-nanostructures as well as abrupt heterojunction formation were also in the focus of this project. Exemplarily we formed Cu3Ge/Ge/Cu3Ge-NW heterostructures with atomically sharp interfaces between the quasi-metallic Cu3Ge segments and the intrinsic Ge-NW. The formation of such perfect heterostructures enabled us to fabricate short channel metal oxide semiconductor transistors down to the sub-10 nm regime were the channel length is controlled by chemical reaction inside the NW and not restricted by a lithographical process.According to the main goal of the MINEHYDE project we integrated Si, Ge, GaAs and GaP into pre-patterned Si based devices. Finally we performed extensive electrical and electro-optical characterization of individual NWs under tensile strain conditions. The measurements were performed primarily on crystalline NWs, monolithically integrated into a micromechanical 3-point strain module. Compared to bulk materials, an anomalously high piezoresistive coefficient has been found for Si and Ge. Spectrally resolved photocurrent and photoluminescence measurements on strained Ge and GaAs NWs gave experimental evidence on the strain-induced modifications of the band structure. Such monolithically integrated NWs are recognized as an ideal platform for the exploration of strain-related electronic and optical effects and may contribute significantly to the realization of novel optoelectronic devices, strain-enhanced field-effect transistors, or highly sensitive strain gauges.