Takeji Ueda

Charge-sensitive infrared phototransistors : Characterization by an all-cryogenic spectrometer

Charge-sensitive infrared phototransistors (CSIPs) with a 16×4 μm2 active area, which are fabricated in a GaAs/AlGaAs double-quantum-well structure, are studied with an all-cryogenic spectrometer operated at 4.2 K. Extremely low level of background radiation makes reliable determination of detector characteristics at 4.2 K possible: The detection band is found to be centered at the wavelength λ=14.7 μm with a bandwidth (full width at maximum) Δλ=1 μm. The quantum efficiency (η), the current responsivity (R), the noise equivalent power (NEP), and the specific detectivity (D∗) are derived to be η=(2±0.5)%, R=4×104–4×106 A/W, NEP≅6.8×10−19 W/Hz1/2, and D∗≅1.2×1015 cm Hz1/2/W. The dynamic range of detection is demonstrated to exceed 106 (approximately attowatts to picowatts), but the upper limit of the radiation power is limited by the radiation source intensity. The intrinsic dynamic range of the detector is suggested to reach 1013 (approximately attowatts to microwatts). The detection speed is suggested to ...

Photon-counting microscopy of terahertz radiation

Photon-counting imaging of terahertz radiation is demonstrated with a spatial resolution of 50μm for a free-space wavelength of 132μm by incorporating a quantum-dot single photon detector into a scanning confocal optical system. Terahertz radiation of 10−19–10−16W or 102–105photons∕s is imaged. Applying the microscope to the study of semiconductor quantum Hall devices, a threshold behavior of the cyclotron emission is found and the mechanism is discussed.

Infrared phototransistor using capacitively coupled two-dimensional electron gas layers

A narrow-band infrared phototransistor (14.8μm) is designed and realized based on a GaAs∕AlGaAs double-layer structure. An isolated island formed from the first quantum well (QW) works as a gate, which is capacitively coupled to the remote two-dimensional electron gas (2DEG) layer working as the source/drain channel. Incident radiation excites the intersubband transition within the isolated QW island. Excited electrons tunnel out of the QW causing it to positively charge up. This affects the conductance of the remote 2DEG channel, yielding detectable photosignals. The present detection mechanism makes it possible to design semiconductor infrared detectors with higher sensitivities along with custom designed tunability. The mechanism also holds potentiality of single-photon detection in the infrared region.

Metal Hole Arrays as Resonant Photo-Coupler for Charge Sensitive Infrared Phototransistors

Metallic photo-couplers utilizing surface plasmon polariton (SPP) excitation have been studied experimentally and theoretically for improving the quantum efficiency of charge sensitive infrared phototransistors (CSIP). Metallic hole arrays deposited on top of the photo-active area of CSIPs (wavelength of 14.7 ¿m) induce intensified near fields for the intersubband transition in a GaAs quantum well at 100 nm below the metal/substrate interface. Cross-hole arrays yield the highest efficiency of 7%, which is by a factor of about four higher than the previously achieved value with square patch arrays.

A sensitive double quantum well infrared phototransistor

An infrared phototransistor (∼14.5μm) on a GaAs∕AlGaAs double quantum well (QW) heterostructure is studied. A confined upper QW behaves as a photoactive gate to a conducting channel formed by the lower QW. By properly biasing the narrow gates for isolating the upper QW island, the lateral tunneling rate of cold electrons on upper QW can be tuned and hence the lifetime of photocarriers on the QW island can be controlled. Associated with this controllable lifetime, photoresponse takes a sharp maximum, which reaches as high as ∼103A∕W. Analysis in terms of a simple model suggests that the peak response originates from the interplay∕trade-off between the lifetime of photocarriers and the efficiency of photodetection process. The photodetection efficiency substantially varies as a consequence of large band bending induced by the 300K thermal background radiation. The long (approximately millisecond order) and controllable lifetime in our device paves the way for future development of photon counters in the lon...

Reset Operation of Quantum-Well Infrared Phototransistors

An improved operation of charge-sensitive infrared (IR) phototransistor (CSIP) is demonstrated by adding a reset function. The phototransistor is fabricated in a GaAs/AlGaAs double quantum-well (QW) structure. The upper QW is lithographically defined to form an isolated island. Under IR illumination (lambda ~ 14 mum), excited electrons escape from the isolated QW island. The QW island is thereby positively charged up, which, in turn, causes the conductance through the lower QW to increase. The performance of the detector, however, was restricted by the reduction of sensitivity, which arises from a distortion in the electrostatic potential profile caused by the accumulation of positive charges on the upper QW island. This drawback is removed by introducing a front gate on a channel leading to the isolated QW island. Applying positive pulses (duration 1 mus) to the gate, neutralizes the isolated QW island and reset the CSIP to the highly sensitive state. Typically, a current responsivity on the order of ~104 A/W is realized along with a dynamical range as large as > 109.

A passive long-wavelength infrared microscope with a highly sensitive phototransistor

A passive scanning confocal microscope in the long-wavelength infrared (LWIR) region has been developed for sensitive imaging of spontaneous LWIR radiation by utilizing an ultrahighly sensitive detector, called the charge-sensitive infrared phototransistor (CSIP). The microscope consisted of room-temperature components including a Ge objective lens and liquid helium temperature components including a confocal pinhole, Ge relay lenses, and CSIP detector. With the microscope, thermal radiation (wavelength of 14.7 microm) spontaneously emitted by the object was studied with a spatial resolution of 25 microm. Clear passive LWIR imaging pictures were obtained by scanning a sample consisting of glass, Al foil, Ag paste, and Au. Clear passive LWIR image was also obtained even when the sample surface was covered by a GaAs or Si plate. This work suggests usefulness of CSIP detectors for application of passive LWIR microscopy.

Effects of impurity scattering on the quantized conductance of a quasi-one-dimensional quantum wire

We report an experimental observation of how the presence of an impurity in a quasi-one-dimensional wire influences the quantized conductance. The impurity is a chemically etched nanohole, relative to which the walls of the wire can be tuned via external gate voltages. Depending on the positions of the sidewalls, resonance features are observed in the quantized conductance due to either the multiple scatterings between the impurity and the wall of wire or the channel interference. Meanwhile, the differential conductance exhibits the well-known half-plateau features in a single channel wire or saturates in a wire with coupled two channels.

A modified scheme of charge sensitive infrared phototransistor

Charge sensitive infrared phototransistors (CSIP) realized in a GaAs/AlGaAs double quantum well (QW) structure have so far exploited the tunneling of excited electrons from an isolated island of upper QW to the lower two-dimensional electron gas layer. Another type of CSIP is developed by using a GaAs/AlGaAs double QW crystal, in which inter-QW tunneling is suppressed. Instead of “vertical” tunneling, excited electrons in the upper QW flow in and out the isolated island “laterally” via translational motion through gate-induced potential barriers. The scheme is demonstrated for wavelengths ≈14.6 μm but is suitable for expanding toward longer wavelengths.

Charge sensitive infrared phototransistor for 45 μm wavelength

The detection wavelength of charge-sensitive infrared phototransistors (CSIPs), originally developed for 15 μm wavelength radiation, is expanded to longer wavelengths of ∼45 μm. The CSIPs are fabricated on GaAs/AlGaAs crystals with bilayer two-dimensional electron gas. Photoresponse at targeted wavelengths is confirmed. Low quantum efficiency of photoresponse, on the order of 10−4, has been ascribed to electron traps (Al–O complex) contained in an AlGaAs barrier layer. Several possible approaches for improving the detector performance are suggested.