Optimization of CNT Contacts in Suspended CNTFETs and Post Dry-Transfer Processing

Carbon nanotube based devices have been demonstrated to display exceptional gas, mass, and force sensing capabilities as a result of their unique mechanical and electrical properties. Continuous advances in carbon nanotube field-effect transistor based electronic components have also contributed to the continued research effort into nanotube integration.



The large scale carbon nanotube integration and application, as of currently, is primarily limited by low device yield and large device-to-device performance variations. Various chirality specific carbon nanotube growth and in-situ imaging and visualization methods have therefore been investigated, to enable selective integration. Here, the mechanical dry-transfer of individual carbon nanotubes was employed. This method allows for the prior carbon nanotube characterization and a pick-and-place approach.



Fabrication of devices by dry-transfer with various architectures confirmed that not only carbon nanotube to carbon nanotube variations are present, but device-to-device differences can also be observed when multiple segments of a single carbon nanotube are measured. Differences in the on-current, threshold voltage and subthreshold-slope were observed.



In order to determine the origin of these fluctuations, the effect of various post-transfer processing methods on the current level, statistical current distribution and noise were investigated. The post-transfer processing methods employed within this thesis were comprised of annealing, selective metallization and selective passivation.



Annealing and selective metallization could be shown to significantly enhance the on-current and reduce noise. An average current increase of more than two orders of magnitude could be demonstrated. Particularly for small gap semiconducting and metallic carbon nanotubes a significant improvement could be observed.



By means of surface analysis, Raman spectroscopy monitoring and pressure dependent measurements, the most important factors causing transfer characteristics variation could be isolated. Specifically, spontaneous hydrocarbon adsorption onto the palladium electrode surface prior to the transfer, and local surface energy and work function variations due to the polycrystalline nature, could be identified as contributing factors. Annealing and selective metallization were shown to be effective in eliminating unwanted tunnel barriers at the carbon nanotube-electrode interface. Due to the high residual carbon content of the atomic layer deposition based metallization process, good wetting of the thin film on carbon nanotubes was observed. This is crucial in order to achieve uniform coverage of the contact area.



High pressure measurements were shown to result in irreversible carbon nanotube reconfiguration on the electrodes, improving their mechanical and electrical stability.



In conclusion, the relevant factors limiting the performance of mechanical dry-transfer based carbon nanotube transistors were identified and post processing techniques to overcome these issues were suggested and, as far as possible within the scope of this thesis, demonstrated.