We demonstrate that the conductance of a field-effect transistor ͑FET͒ gated by a layer of nanometer-sized quantum dots is sensitive to the absorption of single photons. Rather than relying upon an avalanche process, as in conventional semiconductor single-photon detectors, the gain in this device derives from the fact that the conductivity of the FET channel is very sensitive to the photoexcited charge trapped in the dots. This phenomenon may allow a type of three-terminal single-photon detector to be developed based upon FET technology. © 2000 American Institute of Physics. ͓S0003-6951͑00͒01525-4͔Time-resolved single-photon detection is required for such diverse applications as medical diagnosis and imaging, chemical analysis, laser ranging, and materials characterization. 1 Photonic technology is also important for future applications in optical quantum cryptography and computing. Conventionally, single photons are detected by multiplication of a photogenerated carrier by an avalanche process, either in a vacuum photomultiplier tube, or, in the case of the semiconductor avalanche photodiode, a reversebiased junction ͑see, for example, Refs. 1-3͒. Here, we propose and demonstrate detection of single photons based upon an entirely different principle using a field-effect transistor ͑FET͒ containing quantum dots.Self-organizing growth techniques, based upon carefully controlled strained layer epitaxy, allow the formation of a high density of quantum dots with nanometer dimensions and relatively homogeneous size distribution. A particularly attractive aspect of this technique is that it can be used to produce quantum-dot layers within an epitaxially grown device structure. It has been shown that such a layer of selfassembled quantum dots can radically alter the transport properties of a nearby two-dimensional electron gas ͑2DEG͒. 4-6 Under illumination the quantum dots can trap photoexcited carriers, providing the possibility of using such dots as an optically addressed data storage medium. 7-11 Such structures have also been demonstrated as nonquantum detectors of midinfrared radiation through excitation of the bound-to-continuum intersubband photoconductivity. 12 It has been proposed 7 and demonstrated 8,10 that the inhomogeneous broadening of the quantum-dot optical transitions can be used to store more than one bit of information at each laser spot position, by tuning the laser wavelength to access dots with different transition energies. This requires a scheme to detect the presence of photoexcited charge in a single ͑or just a few͒ quantum dot. The results presented here demonstrate that by using a FET structure it is indeed possible to detect the presence of a single photoexcited carrier in a single quantum dot. Figure 1 shows a schematic of the device structure. It consists of a GaAs/Al 0.33 Ga 0.67 As modulation-doped FET containing a layer of InAs quantum dots separated from the 2DEG in the GaAs channel by a thin Al 0.33 Ga 0.67 As barrier. Since there are bound states of the quantum dots lying to lower ...

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We propose and demonstrate a type of GaAs/AlGaAs modulation-doped field effect transistor (FET) which is sensitive to single photons. The FET contains a layer of InAs quantum dots formed using an in-situ, self-organising method, adjacent to the channel and separated from it by a thin AlGaAs barrier. Capture of a single photo-excited carrier by a quantum dot leads to a sizeable change in the source-drain current through the transistor, allowing the detection of a single photon. We show this is because the mobility of the electron channel is extremely sensitive to the charge trapped in the dots. This discovery may allow a new type of single photon detector to be developed which does not rely upon avalanche processes.

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