IBM has earned U. S. Patent 7,628,974 for its method to control single wall carbon nanotube (SWNT) diameter growth during manufacturing by either chemical vapor deposition (CVD) or plasma enhanced chemical vapor deposition (PECVD).
Carbon nanotube based field effect transistors (CNTFETs) show great promise for device applications. Recently CNTFETs with excellent electrical characteristics comparable to state-of-the-art silicon MOSFETs have been demonstrated. The electrical characteristics of CNTFETs however depends largely on the band-gap of the single wall carbon nanotube (SWNT) forming the channel of the transistor. Since the band-gap of SWNTs has a strong dependence on the diameter, accurate control of the diameter is essential to the success of any device technology based on carbon nanotubes.
A crucial difficulty in obtaining individual SWNTs by CVD is control of nanometric catalyst particle size at growth temperatures of 700-1000.degree. C. It has been theorized that the particle size of the growth catalyst used can define the diameter of as grown carbon nanotubes. This hypothesis has been supported by the observation that catalytic particles at the ends of CVD grown SWNT have sizes commensurate with the nanotube diameters Catalysts typically employed are transition metals, notably Fe, Mo, Co, NI, Ti, Cr, Ru, W, Mn, Re, Rh, Pd, V or alloys thereof. However, the synthesis of small catalyst particles with a narrow diameter distribution is complicated and difficult to control.
IBM controls the diameter of CVD or PECVD grown CNTs based on the control of the residence time of the gases in the reactor such as by controlling the pressure, or the gas flow rates, or a combination of both, independent of catalyst particle size, according to inventors Alfred Grill, Deborah Neumayer and Dinkar Singh.
The gas residence time is a measure of the average time of the gas in the reactor. Thus, if the flow is constant and the pressure increases, the residence time increases, and if the pressure is constant and the flow increases the residence time decreases. The inventors unexpectedly discovered that by varying the residence they can influence the nanotube diameter. If the residence time is too high, only pyrolytic carbon is deposited and if the residence time is too low, nothing is deposited. The residence time is typically about 1 minute to about 20 minutes and more typically about 1 to about 10 minutes. The residence time is typically determined by controlling the pressure, flow or both the pressure and flow in the reactor. By varying the residence time (e.g growth pressure and/or flow rates) of the CNT precursor gases in the CVD or PECVD reactor, nanotube diameter can be varied from about 0.2 nanometers to several nanometers to 100 nanometers.
FIGS. 1A-1B show scanning electron microscope images of CNTs grown at atmospheric pressure using identical catalysts, but different gas flows. (FIG. 1A shows that higher gas flow results in relatively thin tubes, while FIG. 1B shows that lower gas flows in result in relatively thick tubes).
A further aspect of the patent relates to fabricating a SWNT or array of SWNTs having well defined diameters and origins by the above disclosed processes wherein the SWNTs form the channel of a field effect transistor. A field-effect transistor having source and drain regions and a channel located between the source and drain regions is obtained by a process comprising: a) depositing a thin film of catalyst; b) lithographically patterning the thin film of catalyst to provide catalyst only in the source or drain region or both; c) removing unwanted catalyst from the channel region defined by the lithographic pattern; and d) growing nanotube with a well controlled diameter ranging from about 0.2 nanometers to about 100 nanometers by controlling the residence time of gases in the reactor used for the growing of the nanotube and wherein the channel region extends from the source region to the drain region.
