Charge-sensitive Infrared Phototransistors Research Articles

Above-77 K operation of charge sensitive infrared phototransistor with dynamically controlled optical gate

Charge sensitive infrared phototransistors (CSIPs) show great promise for sensitive mid-infrared photodetection, extending up to single-photon counting, owing to the built-in amplification mechanism. However, the operating temperature of previously reported CSIPs has been limited to below 30 K. In this work, we propose a technique that enhances the operating temperature to above liquid nitrogen temperature by dynamically controlling the electrostatic potential of the optical floating gate (FG). This control effectively suppresses the annihilation of photogenerated holes in the FG, mitigating the vertical recombination process of thermally excited electrons. We detected the photosignal up to ∼85 K under a photon flux of Φ∼3.6×108 s−1. An outstanding photoresponsivity (R=39.11 A/W) to external blinking light at the peak wavelength of λ=11μm is achieved at 77 K. Our work not only extends the practical application of CSIPs, meeting the high demand for high temperature operation, but also offers more flexibility in fabricating more general highly sensitive phototransistors.

Auto-Calibrated Charge-Sensitive Infrared Phototransistor at 9.3 µm.

Charge-sensitive infrared photo-transistors (CSIP) are quantum detectors of mid-infrared radiation (λ=4 µm-14 µm) which have been reported to have outstanding figures of merit and sensitivities that allow single photon detection. The typical absorbing region of a CSIP consists of an AlxGa1-xAs quantum heterostructure, where a GaAs quantum well, where the absorption takes place, is followed by a triangular barrier with a graded x(Al) composition that connects the quantum well to a source-drain channel. Here, we report a CSIP designed to work for a 9.3 µm wavelength where the Al composition is kept constant and the triangular barrier is replaced by tunnel-coupled quantum wells. This design is thus conceptually closer to quantum cascade detectors (QCDs) which are an established technology for detection in the mid-infrared range. While previously reported structures use metal gratings in order to couple infrared radiation in the absorbing quantum well, here, we employ a 45° wedge facet coupling geometry that allows a simplified and reliable estimation of the incident photon flux Φ in the device. Remarkably, these detectors have an "auto-calibrated" nature, which enables the precise assessment of the photon flux Φ solely by measuring the electrical characteristics and from knowledge of the device geometry. We identify an operation regime where CSIP detectors can be directly compared to other unipolar quantum detectors such as quantum well infrared photodetectors (QWIPs) and QCDs and we estimate the corresponding detector figure of merit under cryogenic conditions. The maximum responsivity R = 720 A/W and a photoconductive gain G~2.7 × 104 were measured, and were an order of magnitude larger than those for QCDs and quantum well infrared photodetectors (QWIPs). We also comment on the benefit of nano-antenna concepts to increase the efficiency of CSIP in the photon-counting regime.

Dual-color charge-sensitive infrared phototransistors with dynamic optical gate

Infrared multispectral photodetectors with high performance show great potential in a broad range of applications. Here, sensitive and controllable dual-color photodetection at 10.6 and 15.7 μm is demonstrated by using a charge sensitive infrared phototransistor (CSIP) with dynamical optical gate. The CSIP device is fabricated in a GaAs/AlGaAs double quantum well (QW) crystal with both upper and lower QWs (7 and 11 nm thick, respectively) being photosensitive via intersubband absorption and, hence, each QW corresponding to one target wavelength (10.6 and 15.7 μm, respectively). Moreover, the upper QW serves as a photosensitive floating gate (FG), while the lower QW functions as the conducting channel of the phototransistor. By periodically lifting-up (lowering-down) the electrostatic potential of FG, the photoresponse at 10.6 (15.7 μm) associated with photoexcitation in upper (lower) QW can be achieved. This electrically controllable photoresponse together with intrinsically high photoconductive gain (∼102) provides a scheme to realize sensitive dual-color photodetection for infrared optoelectronic applications.

Linear array of charge sensitive infrared phototransistors for long wavelength infrared detection

Infrared has been deeply involved in frontier research, modern technologies, and human society, which requires sensitive infrared detection and sensing, in particular, array imaging. A charge-sensitive infrared phototransistor (CSIP) device is made of a GaAs/AlGaAs double quantum well and has been shown to exhibit much larger infrared photoresponsivity than conventional infrared photodetectors, attributable to its built-in amplification mechanism of large transconductance. In this work, we propose and demonstrate a linear array architecture of long wavelength CSIPs with each pixel directly addressable via its reset gate. The performance of a prototype 1×8 linear array of CSIPs is studied at 5 K, and each pixel shows a large photoresponsivity of >50 A/W at the peak wavelength of λ=11 μm, nearly two orders higher than conventional detectors. Using a home-made multi-channel pulse generator, the array is operated at a typical frame rate of ∼5 ms, without the necessity of using cryogenic readout circuits. Besides, the fabrication of the proposed CSIP array requires only state-of-the-art planar technology; our work, therefore, provides a promising solution to realize very sensitive and small-scale array infrared imaging for sensitive long-wavelength infrared applications.

Application of surface plasmon polariton grating in high sensitive infrared detectors

Free electrons in metals can couple with electromagnetic waves and form surface plasmon polariton (SPP) on metal surface, which can effectively couple and bound the radiation energy to the metal surface, leading to significant field enhancement effect in the near field range. Based on the metal periodic structure, the SPP grating can be used to design resonance wavelength flexibly while utilizing the near field enhancement effect, so it has been widely used in the research and development of high sensitive infrared detectors. The charge sensitive infrared phototransistor (CSIP) detector with semiconductor double quantum well structure is a new type of highly sensitivity infrared detector. It utilizes photo-grating effect to realize the multiplier effect of infrared photoelectronic conversion process, therefore, CSIP has the advantages of large tunability, high sensitivity and high photoresponse. This paper reviews the research progress of design and application of SPP grating in CSIP detectors. The coupling physical mechanism, field enhancement effect and polarization conversion efficiency of metal hole array gratings are described. The SPP grating can be optimized to improve the coupling quantum efficiency of CSIP devices. Then, the structure, technological process, fluorescence spectrum, current-voltage ( I - V ) measurement and step-scan spectroscopy are described to demonstrate photoelectric detection performance of CSIP infrared detectors. Finally, the future development and application trend of SPP coupled CSIP devices are prospected.

Terahertz response in the quantum-Hall-effect regime of a quantum-well-based charge-sensitive infrared phototransistor

The characteristics of a charge-sensitive infrared phototransistor (CSIP) based on a GaAs/AlGaAs multiple quantum-well (QW) structure are studied under a magnetic field. In the CSIP, the upper QWs serve as a floating gate that is charged by photoexcitation. The photoinduced charges are detected using the resistance of the lowest QW conducting channel. The conducting channel exhibits the integer quantum Hall effect (QHE) in a perpendicular high magnetic field, yielding the magnetic field dependence of the terahertz (THz) response ΔR. We found two different features of ΔR. One is that ΔR switches sign across the QHE plateau, which is explained simply by an increased electron density in the conducting channel. The other feature is observed as an enhanced positive ΔR when a potential barrier is formed in the conducting channel. The latter mechanism can be interpreted as the promotion of edge/bulk scattering due to photoinduced charges. These findings suggest ways to enhance the THz response by using magnetic fields and potential barriers.

Charge-Sensitive Infrared Phototransistor (CSIP)

Charge-Sensitive Infrared Phototransistor (CSIP)

Improved Performance of Ultrahigh-Sensitive Charge-Sensitive Infrared Phototransistors (CSIP)

Charge-sensitive infrared phototransistor (CSIP) is a highly sensitive semiconductor terahertz (THz) detector with single-photon sensitivity. Due to the excitation mechanism via intersubband transition in a quantum well, the CSIP requires careful design of the photo-coupler and proper light illumination method to achieve high quantum efficiency. We have improved the quantum efficiency by introducing the radiation from the backside of the CSIP substrate, which leads to efficient surface plasmon excitation in the photo-coupler.

Highly photoresponsive charge-sensitive infrared phototransistors with a dynamically controlled optical gate

Charge-sensitive infrared phototransistors (CSIPs) with a built-in field-effect-induced amplification mechanism have much higher infrared photoresponsivity (≥103 A/W) than conventional detectors, which is often restricted by background black-body radiation induced saturation. Here, we report that dynamically controlling the electrostatic potential of the photosensitive floating gate of a CSIP can counterbalance this background-induced saturation effect. As a result, the CSIP photoresponsivity can be improved by about one order of magnitude, reaching as high as ∼1.2×104 A/W to external blinking light. Our work suggests that time-domain manipulation could be an agile degree of freedom in optimizing the CSIP performance and provide insight into operating more general phototransistors for a wide variety of optoelectronic applications.

Charge-sensitive infrared phototransistors with integrated plasmonic photocouplers

A plasmonic coupler, which consists of periodic metal gratings and a tapered waveguide, is integrated to a charge-sensitive infrared phototransistor (CSIP) for 45 µm wavelength radiation. The period and height of the gratings are about 42 µm and 4 µm, respectively. The gratings convert normally incident radiation into a surface wave, which is converged to the active region of the CSIP through the tapered waveguide. Both rise rate and signal amplitude become larger, due to the presence of the plasmonic coupler.

Terahertz single-photon detectors based on quantum wells

Semiconductor charge-sensitive infrared phototransistors (CSIPs) based on quantum wells are described. They are the only detectors that are able to count single photons in the terahertz region at present. In terms of the noise equivalent power (NEP), the detectors show experimental values of 7 × 10−20 W/Hz1/2, while theoretically expected values are even much lower. These NEP values are by several orders of magnitude lower than any other state-of-the-art highly sensitive detectors. In addition to the outstanding sensitivity, the detectors are featured by strong advantage of huge current responsivity (>1 × 105 A/W) and low output impedance (<10 kΩ). This excellent performance in the above has been obtained for λ = 12–28 μm. By introducing a modified scheme of detection (called “lateral-escape”) along with an improved coupler structure (bowtie antenna), we have achieved similar excellent performance for 45 μm. The CSIP provides extremely promising detectors for a variety of applications covering a wide spectral range of 12–100 μm.

An Anomalous Conductance Decrease in Charge Sensitive Infrared Phototransistor

For charge-sensitive infrared phototransistors (CSIP), it is observed that “conductance decrease,” which is contrary to the standard “conductance increase” photon response, can also happen after absorbing infrared light. By experimental modeling the charge-up detection mechanism of CSIP via a capacitive way, we clarify that “conductance decrease” should be attributed to the significantly reduced low quantum well electron mobility after the photon-charging process, rather than a reversed electron transfer. This experimental result clearly indicates that photon-induced charges are able to modify the electron mobility in those “charge-sensitive sensor” types of semiconductor quantum single-photon detectors.

A new scheme for sensitive detection of terahertz photons

Charge sensitive infrared photo transistors (CSIPs) made in GaAs/AlGaAs bilayer two-dimensional electron systems (2DESs) serve as sensitive photodetectors in the mid- and long-wavelength infrared regions. A new design of CSIP is proposed to expand the wavelength range to longer wavelengths (λ > 36 μm). Remarkably improved detector performance is demonstrated for λ ≈ 39 μm. In CSIPs electrons are photo-excited in a floating gate (FG) served by an isolated region of upper layer 2DESs. In the new design (i) a bow-tie antenna couples incident radiation to an FG far smaller in size (2–3 μm) than the wavelength and (ii) excited electrons ‘laterally’ escape from the FG via tunneling through a barrier formed by biased metal cross gates. The charge state of the FG is sensed by a source–drain channel in the lower layer of the 2DES. The sensitivity and the quantum efficiency have been greatly improved, indicating that CSIPs are promising detectors in an expanded wavelength range exceeding 36 μm.

Passive THz Near-Field Imaging and its Applications for Engineering

We have recently developed a THz near-field microscope with an ultrahighly sensitive detector, CSIP (charge-sensitive infrared phototransistor). The microscope probes spontaneous evanescent field on samples derived from local phenomena and the signal origin from metals was previously revealed to be thermal charge/current fluctuations. The intensity of passive near-field signal is very well consistent with Bose-Einstein distribution, which corresponds to the sample temperature. In this study, we demonstrate nano-thermometry with the microscope by monitoring passive near-field signals on a biased NiCr pattern. The obtained signals correspond to the local temperature and the result shows that the inner side of the line curve is much brighter than outer side. It can be easily interpreted by Kirchhoff’s law. The spatial resolution is 60 nm, which cannot be experimentally achieved by any other optical thermometry. This demonstration strongly suggests that our microscope is very well suited for real-time temperature mapping of complicated circuit patterns, and others like bio-samples.

Some Failure Mechanisms in Charge-Sensitive Infrared Phototransistors

For an infrared photon detector, such as charge-sensitive infrared phototransistors (CSIPs), we propose and use a capacitive charging method to study some failure mechanisms that disable the photon response of CSIPs. Two failure mechanisms are highlighted, namely interquantum well (QW) leakage and low tunneling probability for intersubband-transition-excited electrons. A correlation between the Al content in the inter-QW AlGaAs barrier and the failure mechanism type are discussed. On the other hand, the previously unexplained puzzle that the success of photon response is only weakly dependent on the QW electron mobility that is attributed to an imperfect inter-QW barrier.

Ultrasensitive Far-Infrared Phototransistors Fabricated in Superlattice Structures

Charge sensitive infrared phototransistors (CSIPs), ultrasensitive detectors in the wavelength range of 10–50 µm, are fabricated in GaAs/AlGaAs superlattice structures. The superlattice structures consist of alternately grown 2-nm-thick Al0.3Ga0.7As tunnel barriers and thickness-modulated GaAs layers from 2 to 10 nm. The tilted miniband formed in a superlattice structure serves as a potential slope similar to formally used compositionally graded barriers. The utilization of the superlattice in a CSIP is expected to give advantages of easier design, less impurities, and better repeatability in crystal growth, and therefore provides more reliable device performance. The novel structure paves the way for the future mass production of CSIPs.

Ultrasensitive Far-Infrared Phototransistors Fabricated in Superlattice Structures

Charge sensitive infrared phototransistors (CSIPs), ultrasensitive detectors in the wavelength range of 10–50 µm, are fabricated in GaAs/AlGaAs superlattice structures. The superlattice structures consist of alternately grown 2-nm-thick Al0.3Ga0.7As tunnel barriers and thickness-modulated GaAs layers from 2 to 10 nm. The tilted miniband formed in a superlattice structure serves as a potential slope similar to formally used compositionally graded barriers. The utilization of the superlattice in a CSIP is expected to give advantages of easier design, less impurities, and better repeatability in crystal growth, and therefore provides more reliable device performance. The novel structure paves the way for the future mass production of CSIPs.

Requirements on Photon Counting Detectors for Terahertz Interferometry

Future Terahertz astronomy requires higher angular resolution observations. One possibility is to use photon counting interferometry, which is similar to the intensity interferometry demonstrated by Hanbury Brown and Twiss in 1956. In terahertz frequencies, it will be possible to measure all the photon arrivals and can identify bunched photons from thermal sources to measure correlation amplitude and time delay of the photon arrivals. By obtaining the complex visibility, aperture synthesis images can be obtained. I name this technology as photon counting terahertz interferometry (PCTI). From scientific case study, requirements to the detectors are high photon counting rate and accurate time resolution. I discuss on requirements on possible detector technologies such as superconducting single photon detectors (SSPD), superconducting tunnel junction (STJ) detectors and charge sensitive infrared photo-transistors (CSIP).

Probing thermal evanescent waves with a scattering-type near-field microscope**This paper was presented at ISMQC-2010, the 10th International Symposium on Measurement and Quality Control, held in Osaka, Japan, on 5–9 September 2010. Two other papers from that meeting also appear in this issue.

Long wavelength infrared (LWIR) waves contain many important spectra of matters like molecular motions. Thus, probing spontaneous LWIR radiation without external illumination would reveal detailed mesoscopic phenomena that cannot be probed by any other measurement methods. Here we developed a scattering-type scanning near-field optical microscope (s-SNOM) and demonstrated passive near-field microscopy at 14.5 µm wavelength. Our s-SNOM consists of an atomic force microscope and a confocal microscope equipped with a highly sensitive LWIR detector, called a charge-sensitive infrared phototransistor (CSIP). In our s-SNOM, photons scattered by a tungsten probe are collected by an objective of the confocal LWIR microscope and are finally detected by the CSIP. To suppress the far-field background, we vertically modulated the probe and demodulated the signal with a lock-in amplifier. With the s-SNOM, a clear passive image of 3 µm pitch Au/SiC gratings was successfully obtained and the spatial resolution was estimated to be 60 nm (λ/240). The radiation from Au and GaAs was suggested to be due to thermally excited charge/current fluctuations and surface phonons, respectively. This s-SNOM has the potential to observe mesoscopic phenomena such as molecular motions, biomolecular protein interactions and semiconductor conditions in the future.

Charge-Sensitive Infrared Phototransistors Developed in the Wavelength Range of 10–50 µm

Charge-sensitive infrared phototransistors (CSIPs) have been developed in a wavelength range of 12 to 45 µm by using double-quantum-well GaAs/AlGaAs heterostructures. Important parameters for designing the devices are discussed. The detectors are operated at 4.2 K. Quantum efficiency is affected by the crystal quality, which is supposed to be primarily determined by the imperfection in barrier regions. The specific detectivity, substantially determined by the quantum efficiency, is in the range of D* = 1012–1016 cm Hz1/2/W.