NER: Ultrafast Terahertz Hot Electron Bolometer Heterodyne Detectors Based on Single Wall Carbon Nanotubes

本文提出了一种利用单壁碳纳米管（SWNTs）独特的电子输运特性的器件的新思路。单壁碳纳米管基本上是石墨烯片（石墨由一堆这样的片组成），它们被折叠成非常规则的结构，直径通常为1.5纳米。高质量的单壁碳纳米管在过去三到四年里才被制造出来并进行了表征。已经发现，在室温下，在金属和半导体版本中，单壁碳纳米管中的电子的平均自由程（mfp，电子在散射前移动的距离）可以长达1um。所提出的装置是一个热电子辐射热计（HEB）外差探测器，其工作频率至少为1太赫兹。该装置将由一个短的（200纳米至400纳米）SWNT组成，两端与金属欧姆触点接触。在直流偏置下，这种器件预计具有温度相关电阻，这将使其能够检测太赫兹辐射，同时将太赫兹辐射转换为较低频率（GHz），可以用宽带微波放大器放大。使用超导NbN器件的HEB外差探测器（“混合器”）是一项成熟的技术，它使天文学接收器在太赫兹范围内的灵敏度提高了一个数量级。PI Yngvesson和Co-PI Gerecht是这项国际发展工作的领导者。所提出的SWNT HEB探测器的独特特性是，预计它的时间常数比超导版本快两个数量级（其工作带宽将达到几百GHz）。这是因为单壁碳纳米管中的电子传输速度要快得多，接近并达到了“弹道传输”（当纳米管中没有散射时）。因此，了解这种设备中的电子输运以及设备响应太赫兹辐射的条件至关重要。本振（LO）所需的功率预计非常低（1 uW或更低），并且预计液氮温度操作。SWNT HEB的案例得到了最近一项研究结果的支持，该研究描述了一种作为HEB的设备，同时依靠弹道传输达到40 GHz带宽。在这种情况下，活性介质是二维电子气体（2DEG，一种砷化镓半导体类型介质），但没有理论解释其操作。因此，该提案的一个非常重要的组成部分是对2DEG器件和SWNT器件进行模拟。Co-PI Fischetti是国际公认的半导体器件弹道传输专家。他将从IBM T.J.沃森研究中心加入马萨诸塞大学/阿默斯特学院，并将为拟议的努力进行这些研究。提出的计划是在大约100 GHz的频率下进行测量，以证实我们的假设，并测量一系列基于直流测试选择的SWNTdevices的时间常数、噪声输出和本LO功率。通过与科罗拉多州博尔德市国家标准与技术研究所（CO - pi Gerecht）和IBM T.J.沃森研究中心（合作者J. Appenzeller）的合作，这些设备将与天线结构集成并制造，以耦合到辐射。理论分析将使用蒙特卡罗模拟，并初步努力采用基于非平衡格林函数的更严格的方法。该装置的潜在应用是高达1太赫兹的焦平面成像系统，可用于安全相关检测以及医学成像。该装置还可以应用于太赫兹光谱，用于化学物质的检测。拟议研究的更广泛影响。这项提议的研究将在马萨诸塞州大学（University of Massachusetts）这一教育机构进行。长期以来，太赫兹小组一直在让各个层次的学生（低至高中）参与研究活动，其中包括许多女学生。例如，许多学生帮助在南极安装了一个太赫兹接收器。工程学院有非常活跃的少数民族和女性工程项目。PI和一名副PI在这个项目中教授常规课程。独特的太赫兹基础设施正在建立，研究包括与NIST和IBM的合作。上面提到的应用可能会给社会带来巨大的好处。

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 fasterthan 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 isan 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 performthese studies for the proposed effort.The proposed plan is to perform measurements at about 100 GHz in order to confirm the processwe hypothesize, and measure the time-constant, noise output and LO power of a series of SWNTdevices, 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 receiverat 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.