Zhenghua An

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 ...

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.

Numerical study of self-heating effects of MOSFETs fabricated on SOAN substrate

A two-dimensional numerical analysis is performed to investigate the self-heating effects of metal-oxide-silicon field-effect transistors (MOSFETs) fabricated in silicon-on-aluminum nitride (SOAN) substrate. The electrical characteristics and temperature distribution are simulated and compared to those of bulk and standard silicon-on-insulator (SOI) MOSFETs. The SOAN devices are shown to have good leakage and subthreshold characteristics. Furthermore, the channel temperature and negative differential resistance are reduced during high-temperature operation, suggesting that SOAN can mitigate the self-heating penalty effectively. Our study suggests that AlN is a suitable alternative to silicon dioxide as the buried dielectric in SOI, and expands the applications of SOI to high temperature.

Widely Tunable Terahertz Phase Modulation with Gate-Controlled Graphene Metasurfaces

Combining metasurfaces with gate controlled graphene, we experimentally demonstrate ±180° phase modulation can be realized at certain frequencies in THz domain, and describe a practical scheme to achieve full-range active phase modulation with such graphene metasurfaces.

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.

Dependence of structures and properties of carbon nanotube fibers on heating treatment

The dependence of structures and mechanical and electrical properties of carbon nanotube fibers on heating treatments have been first investigated in both argon and air. The relationships between structures and properties have been also explored. These discoveries may be used to design and improve carbon nanotube fibers.

Plasmonic light harvesting for multicolor infrared thermal detection.

Here we combined experiments and theory to study the optical properties of a plasmonic cavity consisting of a perforated metal film and a flat metal sheet separated by a semiconductor spacer. Three different types of optical modes are clearly identified-the propagating and localized surface plasmons on the perforated metal film and the Fabry-Perot modes inside the cavity. Interactions among them lead to a series of hybridized eigenmodes exhibiting excellent spectral tunability and spatially distinct field distributions, making the system particularly suitable for multicolor infrared light detections. As an example, we design a two-color detector protocol with calculated photon absorption efficiencies enhanced by more than 20 times at both colors, reaching ~42.8% at f1 = 20.0THz (15μm in wavelength) and ~46.2% at f2 = 29.5THz (~10.2μm) for a 1μm total thickness of sandwiched quantum wells.

Quasi-whispering gallery modes of exciton-polaritons in a ZnO microrod

We report the photoluminescence (PL) investigations of quasi-whispering gallery mode (quasi-WGM) polaritons in a ZnO microrod at room temperature. By using the confocal micro-PL spectroscopic technique, we observe the clear optical quasi-WGMs. These quasi-WGMs appeared in the ultraviolet (UV) emission region where the cavity modes strongly couple with excitons and form polaritons. The quasi-WGMs polaritons can be well described by the plane wave interference and the coupling oscillator model.