Beam Assisted Nanowire Growth

Over the past decade, one-dimensional nanostructures have been proven as powerful building blocks in active nanometer-scale devices. Aside from carbon nanotubes, nanowires (NWs) are one of the most promising approaches considered for scaling down optic-, electronic-, magnetic- and sensor devices. Low-dimensional nanostructures are usually fabricated using either a top down or a bottom up strategy. The former technique is extremely flexible, but suffers from limitations in minimum feature size and uniformity. The latter one, utilizing spontaneous self-ordering effects, is limited by the broad size distribution and the lack of control of the positioning of the self-organized nanostructures. Both ease and reproducibility of the processes involved are key factors for its practical use. In this context the discovery of the appearance of nanostructures induced by ion bombardment has attracted growing interest. In particular resist-less focused ion beam (FIB) techniques are most suited for the combination of top-down structuring with selective bottom-up self-assembling techniques. One of the most recent topics in FIB-solid interaction is, as erosion proceeds, the evolution of self-organized nanoscale pattern on solid surfaces. The diameters of these nanostructures are comparable to the sizes of biological and chemical species, and thus intuitively represent excellent primary transducers for producing signals that ultimately interface with macroscopic instruments. Additionally, nanostructure based sensors exhibit a fast response with a substantially higher sensitivity and selectivity than polycrystalline and crystalline bulk film based sensors. As NWs show up a high surface-to-volume ratio the impact of the ambient atmosphere on the conducting behaviour is extreme. Such sensors could be much smaller than standard optical-based detectors and simpler to use. Thousands of sensors could be packed into a hand-held device, which could produce results almost instantaneously. Based on previous own research this work advances towards new directions of FIB induced NW synthesis and their integration in CMOS compatible sensor prototypes. Initially, we will explore the nanopattern formation due to exposure with a Ga-FIB, several other ion beam species, beams of multi-charged ions as well as a novel fullerene source. In the same way we will investigate the capability of these methods for NWs formation. By means of a detailed research plan, a series of ion beam-material combinations will be successively investigated and evaluated to achieve three main goals: (i) to gain basic understanding of ion beam induced self assembling processes (ii) to develop suitable processes for the reproducible formation of NWs (iii) to develop suitable processes for CMOS compatible NWs formation. In a second step we will integrate individual as well as assemblies of NWs in resitivity type gas sensors and nanoscale pH-probes. For this purpose several aspects of contact formation, post-synthesis treatment such as annealing or enwrapping of the NWs by atomic layer deposition will be investigated. A CMOS compatible self- aligned processesing of the resitivtiy type gas sensor and nanoscale pH-probe are under consideration. The near-term relevance of this project is of a scientific nature. The project specific FIB generated nanopattern can be considered as primary tools for the exploration of the physics and the chemistry on the few-nanometer scale. Moreover, to the best of our knowledge no attempts have been published regarding the integration of such NWs into prototype devices. A long-term impact is of technological nature. The trends in sensor technology meet the goals of the proposed NW sensor, i.e. miniaturization and integration. Microsensors and especially nanosensors have lower manufacturing cost (self-assembly, mass-production, less materials), a wider exploitation of IC technology (integration), a wider applicability to sensor arrays, and a lower weight (better portability). Last but not least offers the project interesting topics to potential PhD candidates so that highly qualified students can be attracted. This project will enable them to achieve a PhD and gain a comprehensive understanding of nanostructuring and sensing methods.

One of the most recent topics in focused ion beam (FIB) matter interaction is, as erosion proceeds, the evolution of self-organized nanostructures. By means of a detailed research plan, a series of ion beam-material combinations were investigated to evaluate three main goals: (i) to gain basic understanding of ion beam induced self-assembling processes (ii) to develop suitable processes for the reproducible formation of nanostructures (iii) to develop suitable processes for FIB generated NW integration in CMOS compatible sensors.With respect to nanostructure formation via FIB-material interaction Si, Ge, Sb, GaSb, HOPG, and Bi appeared to be the main materials of interest. We investigated in great detail the influence of processing parameters such as beam energy, ion fluence, angle of ion beam incidence and scanning strategy at room temperature as well as for the heated sample. Complementary experiments with alternative ion sources were conducted at the Forschungszentrum Dresden-Rossendorf via Support of Public and Industrial Research using Ion beam Technology (SPIRIT) project.One of the most remarkable results were achieved via FIB exposure of highly ordered pyrolithic graphite (HOPG). Using a particular FIB scanning strategy and a heated substrate enables the synthesis of free-standing graphite nanosheets. Aside of the remarkable formation process such structures exhibit photoluminescence (PL) which is currently investigated with great emphasis in cooperation with the EPFL in Lausanne.FIB treatment of bulk Ge samples and subsequent annealing resulted in free-standing germanium nano-webs with a thickness below 20 nm. Kryo-measurements performed in cooperation with the University in Delft revealed also luminescence of such nanostructures in the near-infrared region. These systematic investigations of FIB matter interaction revealed also an effect by FIB implantation into Ge nanowires, which have been unknown so far. The experimental outcomes indicated spontaneous Ga dopant activation during the ion implantation to be responsible for the observed conductivity increase and was published in nano letters.The ultimate goal of the BANG project was to explore new directions of FIB induced nanowire synthesis and their integration in CMOS compatible sensor prototypes. This was realized via FIB induced nanowires formation for gas sensor applications, which was published in the journal nanotechnology. Therefore antimony NWs with uniform diameters of only 25 nm and lengths up to several microns were synthesized at predefined positions in a FIB induced self-assembling process without any additional material source. Such antimony nanowires exhibit selective sensing properties for ethanol and H2O with exceptional sensitivities of more than 17000 and 60000, respectively.