Single-Walled Carbon Nanotube Devices
Development of Technological Grounds for Creation of High Performance Single-Walled Nanotube Electronic Devices
Tech Area / Field
- PHY-SSP/Solid State Physics/Physics
- INF-ELE/Microelectronics and Optoelectronics/Information and Communications
- MAT-ALL/High Performance Metals and Alloys/Materials
- MAT-SYN/Materials Synthesis and Processing/Materials
8 Project completed
Senior Project Manager
Mitina L M
Institute of Microelectronics Technology and High Purity Materials, Russia, Moscow reg., Chernogolovka
- State Enterprise Krasnaya Zvezda, Russia, Moscow
- URA 0073/Universite Paris-Sud / Laboratoire de Physique des Solides, France, Orsay\nKERI - Korea Electrotechnology Research Institute, Korea, Gyeongsan\nUniversität Ulm / Central Facility of Electron Microscopy, Germany, Ulm
Project summaryThe goal of the project is to develop vital technological steps necessary for the creation of a new generation of electronic devices based on the promising properties of carbon nanotubes.
Since discovered by Ijima in the early 1990s [S.Iijima, Nature, 1991, 354, 56], single-walled carbon nanotubes (SWNTs) have excited a great scientific and practical interest with many important potential applications because of their unique structural, mechanical and electron properties [Dresselhaus M.S., Dresselhaus G., Eklund P.C. Science of fullerenes and carbon nanotubes, Academic Press, San Diego, 1996]. Nanotubes have a great potential for advanced electronic devices. Perspectives of application of nanotubes as a base for molecular electronics are now being actively discussed, and attempts to create nanotube-based electronic devices are now undertaken by researches during several years. It has been generally accepted that Si-based CMOS technology will reach its scaling limit in the next decade. It is why exploration of the potential of carbon nanotubes as building blocks in future nanoelectronics is needed. With exceptional electrical properties, such as quasi-ballistic transport [Wind S., Appenzeller J., Avouris Ph., Phys. Rev. Lett. 2003, 91, 58301; Javey A., Guo J., Paulsson M., Wang Q., Mann D., Lundstrom M., Dai H., Phys.Rev.Lett. 2004, 92, 106804], high carrier mobility in the diffusive regime [Durkop T., Getty S.A., Cobas E., Fuhrer M.S. Properties and applications of high-mobility semiconducting nanotubes. Nano Lett. 2004, 4, 35], nanotube field effect transistors (FET) nowadays exhibit characteristics rivaling those of state-of-the-art Si-based MOSFETs.
But despite a remarkable progress in the creation of model examples of such devices, essential technological and material science tasks have to be solved to improve their performance significantly and to bring the nanotube-based technology near the industrial standards. Single-walled nanotubes (SWNTs) attracting now a particular interest can be of metallic or semiconductor type conductivity depending on particular combinations of chirality angle and nanotube diameter. Besides, the band gap value of semiconducting nanotubes depends on their diameter. One of the main obstacles for the application of the nanotubes as a promising one-dimensional conductor for wave electron transport is the impossibility up to now to synthesize nanotubes of a given diameter and chirality in a single growth experiment. Thus, the controlled creation of nanotubes of necessary diameter and chirality and, consequently, of desirable electronic properties is an unsolved task up to now. It was noticed earlier that the resulting diameter of nanotubes depends on the size of catalytic nanoparticles [Zhang Y., Li Y., Kim W., Wang D., Dai H. Imaging as-grown single-walled carbon nanotubes originated from isolated catalytic nanoparticles. Appl. Phys. A, 2002, 74, 325-328], thus the problem of controlled nanotube growth appears to be closely connected with the formation of catalytic particles of a definite size and structure, which have to be uniform in a given growth experiment. An important problem in the creation of such catalytic particles is prevention of coalescence of catalytic islands at heating of the sample to quite a high temperature (~900 C) during synthesis of nanotubes.
The project is aimed to find a solution of the problem of controlled nanotube growth. The following studies will be performed:
- Study of the different ways for formation of the nano-sized catalytic particles will be performed to increase the particle uniformity during synthesis. Among such ways, planned to be studied in this Project, is isolation of inpidual catalytic particles with the use of electron lithography, deposition of additional thin films to decrease catalyst atom diffusion mobility, formation of catalytic clusters by the thermal deconvolution of large molecules containing necessary amount of catalyst atoms will be studied.
- Direct CVD synthesis of SWNTs by catalytic pyrolysis of hydrocarbons on the required substrate will be applied to control the correlation of the synthesis process parameters with the structure of nanotubes and catalytic particles. It is important that the CVD synthesis process developed in our previous works [Khodos I.I. et. al, Growth of carbon nanotubes on catalytic nanoparticles, Proc. 7th Internat. Conf. on nanostructured materials (NANO 2004), Wiesbaden, Germany (2004), p.56], allowed us to grow the nanotubes consisting of only SWNTs that are not contaminated practically with amorphous carbon.
- Atomic structure of the nanotubes and catalytic particles, chirality angle of the nanotube as well as their correlation with the conditions of nanotube synthesis and of under/upper layer materials will be revealed by applying high-resolution transmission electron microscopy (HRTEM) imaging, particularly, with the instrument having the spherical aberration corrector (resolution limit is below 0.1 nm). Among various structure study techniques only HRTEM allows to define in details the structure of the nanotubes up to the atom positions. Although because of cylindrical structure of nanotubes, interpretation of images to find out the positions of inpidual atoms requires modeling of atomic structure of nanotube and corresponding HRTEM images. Nanotube structure modeling together with electron microscopical image simulation will be performed in the project to reveal local features of the atomic arrangement.
- A combined structural investigation of the grown nanotubes and the catalyst will be performed together with the conductivity measurements of the same nanotubes. The totality of these properties can be studied for a single nanotube as well. Such combined study is possible due to an original sample construction developed by the project participants. It consists in using self-supported nanotubes thrown over a through slit in a thin insulating SiO2/Si3N4 membrane serving as a support [Kasumov A.Yu., Khodos I.I., Ajayan P.M., Colliex C. Electrical resistance of a single carbon nanotube. // Europhys. Lett. 1996, 34. 429-434]. The approach has already been used by the project participants in the study of conductivity of inpidual multi- and single-walled carbon nanotubes and nanotube ropes with the characterization of the same nanotubes by high resolution transmission electron microscopy (TEM) [Kasumov A.Yu., Bouchiat H., Reulet, Stephan, Khodos I.I., Gorbatov Yu.B., Colliex C., Conductivity and atomic structure of isolated multiwalled carbon nanotubes, Europhys.Lett. 1998, 43, 89-94; Kasumov A.Yu. et al. Supercurrents through single-walled carbon nanotubes, Science 1999, 284, 1508-1510]. We used the laser ablation technique in our previous work to transfer the forehand-obtained nanotubes from the substrate and arrange them across the through slit. Our approach was then further developed to perform catalytic synthesis of nanotubes directly on the substrate and, moreover, on the desirable local places of the sample.
- Aligned nanotube arrays are required to arrange nanotubes between metal contacts and to fabricate a large quantity of nanotube devices. Though the task is not fully solved for the nanotubes grown parallel to the substrate surface, an achievable success was demonstrated. Orientation effect can be obtained if gas flow atmosphere is used for nanotube growth [Sh.Huang, M.Woodson, R.Smalley, J.Liu. Growth mechanism of oriented long single walled carbon nanotubes using “fast-heating” chemical vapor deposition process.Nano Lett. 2004, 4, 1025-1028]. Aligned single-walled carbon nanotubes were grown atop α-plane and r-plane sapphire substrates [H.Song, X.Liu, C.Zhou. J.American Chem.Soc. 2005, 127, 5294-5295]. Aligning effect of electric field of the order 1 V/µm was also observed [Ural A., Li Y. and Dai H. Electric-field-aligned growth of single-walled carbon nanotubes on surfaces. Appl.Phys.Lett. 2002, 81, 3464-3466]. High density of vertically oriented nanotubes can be obtained if electric field of an appropriate value is applied to substrate and high density of small-size catalytic particles is ensured [Murakami Y., Chiashi S., Miyauchi Y et. al. Chem.Phys.Lett. 2004, 385, 298-303]. Developing of these techniques specifically for our sample construction and synthesis process is planned in the project.
- It was shown that while the devices with metallic nanotubes were close to the fundamental limit of resistance for 1D wave conductor (6.5 kΩ), the typical resistances reported for semiconducting nanotubes were of the order 1 MΩ. This suggests the presence of Schottky barrier (SB) at the contacts dominating the source-drain current. The SB height is determined by the nanotube diameter and the nature of the source/drain metal contacts. Many efforts were applied to achieve high performance p-type semiconductor nanotube FETs through contact optimization, dielectric integration and lateral scaling. Progress with n-FETs is slow partly due to the difficulty in affording low Schottky barrier contacts for high on-states and in achieving high on/off ratios with small diameter (or large band gap) tubes [Javey A., Tu R., Farmer D.B., Guo J., Gordon R.G.,Dai H. Nano Lett. 2005, 5, 345] at the same time. The possible way to decrease SB and increase parameters of n-type SWNT FETs with performance surpassing the state-of-the-art Si n-type MOSFET includes chemical doping of source and drain regions, improving of dielectric layers, and working out new device design [A.Javey, R.Tu, D.B.Farmer, J.Guo, R.G.Gordon, and H.Dai. Nano lett, 2005, 5, 345]. Besides, optimization of combination of tube diameter and type of material of conducting electrodes could provide a better performance of nanotube-based devices.
- Requirements to reduce parasitic capacitance between electrodes and silicon substrate and to increase the density of the devices stimulate to use electron lithography for electrode preparation. Areas of micron- and sub-micron size will be prepared in the project. In combination with controlled density nanotube synthesis this will allow to prepare from single to several nanotubes per an electrode. Density correction of the grown nanotubes can be done applying strongly focused beam of the transmission electron microscope. Development of the technique of local nanotube synthesis on the prepared locations opens a perspective to design ensembles of nanotube-based nanodevices reproducibly.
The following studies are planned to fulfill the goal of the project:
- Studying the interaction of the catalyst nanoparticles with the under- and cover layers to optimize the state of the catalyst and nanotube growth,
- Increasing the uniformity of the nanotube diameter distribution
- Growth of nanotubes of controlled density,
- Creation of good metallic contacts to nanotubes
- Growth of oriented nanotubes,
- Deposition of insulating layers with good characteristics,
- Formation of micro- and sub-micro sized metallic areas used as electrodes of the nanotube-based devices.
It can be concluded that the carbon nanotubes as one the most interesting objects studied up to now by the solid state physics, remain to be the today’s problem of physics and technology. Much deeper insight into the structural origins of their fundamental properties is required. It can be gained by application of the advanced techniques for the structure research along with the studying the physical properties of nanotubes. Further development of technological approaches is necessary as well. The design of the nanotube based electronic devices of a new generation with advanced parameters may become possible as a result of the research.
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