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Title: Carbon based nanomaterials for future microelectronics
Author: Balakrishnapillai , Premlal
Awarding Body: University of Sheffield
Current Institution: University of Sheffield
Date of Award: 2013
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The dimensional scaling of CMOS technology is approaching its fundamental limits and alternate device architectures and more functional channel materials ensuring superior operation al sub l0mm length scales are required to realise post CMOS applications. Among the quest for alternate materials. carbon based nanostructures such as 1•0 carbon nanotubes and graphene nanoribbons are considered as one of the many possible candidates. Demonstration of these one dimensional graphitic prototypes in microelectronic industry are strongly hindered by several factors such as the difficulty in controlling band gap. precise positioning and manufacturing on wafer scale, controlling carrier type and carrier concentration. deposition of CMOS compatible gate di electric and formation of low resistive contacts. Amongst these. an absence of a finite band gap and issues in controlling carrier concentration in graphene presents a major challenge for this material to be considered in logic devices. Moreover. both carbon nanotubes and graphene nanoribbons exhibit a diameter/width dependent band gap. At the nanoscale however. estimating the band gap both from experiment and theory is fraught with difficulties related to the underlying assumptions. Hence an accurate theoretical prediction of the performance of these material in future microelectronic devices is essential at this point since the International Technology Road map for Semiconductors (ITRS) -2011 edition strongly recommends carbon based nanostructures as an alternate channel material al for post CMOS logic applications. Also the technology requires novel band gap engineering techniques and doping g strategies without destruction of the intrinsic properties to enable application of graphene in various sectors such as microelectronics. photonics, photovoltaics and biosensors. In this thesis. suitable device geometries of such carbon based materials and their performance evaluation using realistic band gap values is investigated. Zig-zag nanotubes in the diameter range 0.55 - 1.26nm are considered in the study. Both MOSFET and tunnel FET device geometrics arc considered, but more focus is given To the tunnel-FET considering its energy efficient operation. The evaluation is made using non-equilibrium Greens function based numerical simulation with band gap values which have been calculated from the slate of an many-body perturbation theory GW method by P.Umari el al. Comparison of on-off ratio. device delay and saturation current in nanotube tunnel FETs reveals a considerable difference from previous evaluations made in the literature. Analysis reveals that. among the semiconducting zig-zag chiralities considered here, (11,0) nanotube devices in the tunnel FET configuration exhibits the best on-off ratio. device delay and saturation current which meets the ITRS-20Il requirement of low-operating power technology for 2020. Comparison of nanotube tunnel FETs with graphene nanoribbon tunnel FETs reveals that nanotube FETs deliver high on-off ratio and saturation current and exhibit comparable device delay. Among the considered families of nanoribbons, a l6AGNR exhibits best on -off ratio and device delay which operates within the future ITRS requirement. An atomic level investigation of the fundamental properties of a new type of metalgraphene system fanned by intercalation of gold atoms in epitaxial graphene is also presented. The effects of gold deposition on monolayer graphene (MG) epitaxied on SiC (0001) substrate are examined via Scanning Tunneling Microscopy (STM) and Spectroscopy (STS). Gold atoms exhibit mainly two types of self- assembly process below the graphene layer and the resulting gold-graphene systems exhibits contrasting electronic properties from each other. Insertion of a monolayer of gold below the monolayer graphene opens up a 100meV band gap in the electronic spectrum of graphene and creates a finite p-doping in graphene. While gold atoms intercalated in the form of atomic clusters shows negligible doping effect. Finally, signatures of a superlattice structure composed of a quasiperiodic arrangement of atomic gold clusters below an epitaxied graphene layer are examined Using dispersive Raman spectroscopy. The gold- graphene system exhibits a laser excitation energy dependant red shift of the 2D mode as compared to pristine epitaxial graphene. The phonon dispersion in pristine and gold intercalated graphene are mapped using experimentally observed Raman signatures and third-nearest neighbour (3NN) tight binding band structure model. This reveals that the observed behaviour is caused by modifications of the phonon. dispersion rather than changes in electronic structure. The intercalated gold atoms are found to restore the phonon band structure of epitaxial graphene towards that of free standing graphene.
Supervisor: Not available Sponsor: Not available
Qualification Name: Thesis (Ph.D.) Qualification Level: Doctoral
EThOS ID:  DOI: Not available