Kurzbeschreibung
(Englisch)
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The properties of nanosized object interfaces often dominates their own electrical performances. The key step of integra-tion of Single Walled Carbon Nanotubes (SWCNTs) into functional devices is the engineering of their electrical contacts providing reproducible and well defined properties of interfaces, their long term stability and specific device requested characteristics. Electronic devices based on carbon nanotubes have shown that their performance is influenced by a po-tential barrier existing at the SWCNT-metal contact interfaces, which governs the charge injection into the carbon nano-tubes. This project focuses on improvement of electrical properties of SWCNT-metal interfaces in SWCNT - Field Effect Transistor (FET) configuration for nanotube use as a sensing element in chemical sensor devices. The improvement will be reached with progresses in three objectives; growth of SWCNTs with controlled material characteristics fitting to reqirements for their specific functionality as sensing element, selection of optimal metal type and morphology of metal-nanotube contacts and finaly in developing technological process flow which reduce possible contamination of nanotube surface prior to metal deposition as well as protecting metal-nanotube interface from environmental influence. The functionality of chemical sensor with improved and environmentaly protected metal-nanotube interface will be demonstrated.
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Partner und Internationale Organisationen
(Englisch)
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BE, BG, CH, DE, DK, EL, ES, FI, FR, HU, IE, IT, PL, PT, RO, SE, SI, TR, UK
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Abstract
(Englisch)
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For electronic devices using high quality carbon nanotubes (CNTs), the electrical carrier transport is often determined by the properties of the electrical contacts. The contact properties can be significantly affected by the device fabrication. One of the critical issues for the resist-based fabrication (photo or e-beam lithography) is resist-induced contamination. In order to maintain the cleanliness of the metal-SWNT interface, we used a thin layer of alumina to protect SWNTs during the device fabrication. Owing to the preserved clean contacts, the device on-resistance was reduced by 46 %. For further improvement, resist re-deposition on the future contact areas was avoided by additional oxygen plasma treatment. This cleaning step was optimized to remove the resist residues from the surface of the alumina protective layer prior to alumina etching for the contact area definition at simultaneously preserving the original nanotube quality. The effect of oxygen plasma on nanotubes depends on the thickness of the alumina protective layer. Alumina layers with a thickness larger than 20 nm are sufficient to protect SWNTs against the oxygen plasma treatment as proved by the SWNTs' Raman spectra. Devices prepared by the process implementing alumina protective layer combining with oxygen plasma treatment show improved electrical characteristics. The median on-resistance was determined to be 66% smaller than the devices fabricated by the process where nanotubes have been directly exposed to the resists. The inter quartile dispersion (between 25 and 75 percentile) was reduced ten times. The role of the Cr adhesion layer on the electrical characteristics of Au-contacted CNFETs was also investigated. Samples were fabricated using different thicknesses of Cr varying from 0 to 8 nm capped by 120-nm Au (electrode). The device on-current was reduced gradually with the increase of Cr thickness. This effect is attributed to the formation of a continuous Cr layer covering SWNT and thus a change of the metal work function at the metal - SWNT interface. Additionally, several devices with different channel lengths were fabricated on the same SWNT. The device contact and channel resistances were estimated from the transmission line model and determined to be 6.8 k? and 25.7 k?/µm, respectively. This implies a very high quality of the electrical contacts (approaching the theoretical limit) prepared by the presented fabrication approach. Finally, a CNFET large-scale fabrication process was developed. More than 2000 semiconducting devices were fabricated on a 4-inch wafer. Two types of metals, Pd and Au, were used separately on the same wafer for contacts using a 0.4-nm Cr adhesion layer. The median on-resistance of the CNFETs using Pd contacts was 64 k?, 28% lower than the corresponding value of CNFETs using Au contacts. This suggests that in combination with improved fabrication process, Pd is a superior metal for CNT electrical contacts.
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