Detection of few-photon signals requires photodetectors with high gain, typically achieved with bulk devices such as avalanche photodiodes, superconducting systems, and photomultiplier tubes. Recently, there has been interest in exploiting the properties of nanoscale materials and devices to realize new types of high gain photodetectors. In this presentation, a new photodetector is reported based on C60-sensitized aligned carbon nanotube (CNT) transistors with extremely high responsivity of 108 A W-1 (gain > 108) in the UV and visible, and 720 A W-1 (gain = 940) in the IR. In contrast to most sensitized phototransistors that operate on the photogating effect, the new photodetector operates on modulation of the electron scattering in the CNTs, leading to negative photoconductivity. Comparison with similar photodetectors using random CNT networks show the benefit of using aligned CNTs. At room temperature the aligned CNT photodetectors are demonstrated to detect a few tens of photons per CNT, and recent results with single CNT devices show detection of 200 photon pulses. Quantum transport modeling reveals the photodetection mechanism and establishes a path towards single photon detection.
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