This proposal presents a novel idea for a device which takes advantage of the exceptional electronic transport properties of Single-Walled Carbon Nanotubes (SWNTs). SWNTs are basically graphene sheets (graphite consists of a stack of such sheets) that have been folded into tubes of very regular structure, with diameter of typically 1.5 nm. SWNTs of sufficient quality have only been fabricated and characterized in the last three to four years. It has been found that mean free paths (mfp, the distance an electron travels before scattering) for electrons in SWNTs can be as long as 1 um at room temperature, both in metallic and semiconducting versions. The proposed device is a Hot Electron Bolometer (HEB) Heterodyne Detector, for operation up to at least 1 THz. The device would consist of a short (200 nm to 400 nm) SWNT contacted with metallic ohmic contacts in both ends. Under DC bias, such a device is expected to have a temperature-dependent resistance, which will enable it to detect terahertz radiation, while converting the terahertz radiation to a lower frequency (GHz) which can be amplified with a broadband microwave amplifier. HEB heterodyne detectors ("mixers") using superconducting NbN devices are an established technology, which has resulted in increasing the sensitivity of astronomy receivers in the terahertz range by an order-of-magnitude. PI Yngvesson and Co-PI Gerecht are leaders in this international development work. The unique property of the proposed SWNT HEB detector is that it is predicted to have a time-constant two orders-of-magnitude faster than the superconducting version (the bandwidth over which it works would be several hundred GHz). This is due to the much faster electron transport in the SWNTs, approaching and reaching "ballistic transport" (when there is no scattering in the nanotube). It is thus critical to understand electronic transport in such devices, and the conditions under which the device responds to terahertz radiation. The power required from the Local Oscillator (LO) is expected to be very low (1 uW or less), and liquid nitrogen temperature operation is expected. The case for the SWNT HEB is supported by a recent result, describing a device that acts as an HEB, while relying on ballistic transport to reach 40 GHz bandwidth. The active medium in that case was twodimensional electron gas (2DEG, a gallium arsenide semiconductor type medium), but no theoretical explanation is available for its operation. A very important component of this proposal is thus to perform simulations of both the 2DEG device and the SWNT devices. Co-PI Fischetti is an internationally recognized expert on ballistic transport in several semiconductor devices. He is joining the UMass/Amherst faculty from the IBM T.J. Watson Research Center, and will perform these studies for the proposed effort.
The proposed plan is to perform measurements at about 100 GHz in order to confirm the process we hypothesize, and measure the time-constant, noise output and LO power of a series of SWNT devices, selected based on DC tests. The devices will be fabricated and integrated with antenna structures for coupling to the radiation, through collaboration with the National Institute of Standards and Technology, Boulder, CO, (Co-PI Gerecht), and the IBM T.J. Watson Research Center (Collaborator J. Appenzeller). The theoretical analysis will use Monte-Carlo simulations, and a preliminary effort toward a more rigorous approach based on Non-Equilibrium Green's Functions. The potential applications for the proposed device are to Focal Plane Imaging Systems for up to 1 THz, which can be used for security-related detection, as well as medical imaging. The devices could also be applied to terahertz spectroscopy for detection of chemical species.
BROADER IMPACT OF THE PROPOSED RESEARCH. The research proposed will be performed at an educational institution, the University of Massachusetts. The Terahertz group has a long record of involving students at all level down to high school in the research activities, including many women students. For instance, many students helped to install a terahertz receiver at the South Pole. The College of Engineering has very active minority and women engineering programs. The PI and one Co-PI teach regular courses in this program. Unique terahertz infrastructure is being built up and the research includes collaboration with NIST as well as IBM. The applications mentioned above can potentially provide substantial benefits to society.
