Edge-Terminated AlGaN/GaN/AlGaN Multi-Quantum Well Impact Avalanche Transit Time Sources for Terahertz Wave Generation.

In our pursuit of high-power terahertz (THz) wave generation, we propose innovative edge-terminated single-drift region (SDR) multi-quantum well (MQW) impact avalanche transit time (IMPATT) structures based on the AlxGa1-xN/GaN/AlxGa1-xN material system, with a fixed aluminum mole fraction of x = 0.3. Two distinct MQW diode configurations, namely p+-n junction-based and Schottky barrier diode structures, were investigated for their THz potential. To enhance reverse breakdown characteristics, we propose employing mesa etching and nitrogen ion implantation for edge termination, mitigating issues related to premature and soft breakdown. The THz performance is comprehensively evaluated through steady-state and high-frequency characterizations using a self-consistent quantum drift-diffusion (SCQDD) model. Our proposed Al0.3Ga0.7N/GaN/Al0.3Ga0.7N MQW diodes, as well as GaN-based single-drift region (SDR) and 3C-SiC/Si/3C-SiC MQW-based double-drift region (DDR) IMPATT diodes, are simulated. The Schottky barrier in the proposed diodes significantly reduces device series resistance, enhancing peak continuous wave power output to approximately 300 mW and DC to THz conversion efficiency to nearly 13% at 1.0 THz. Noise performance analysis reveals that MQW structures within the avalanche zone mitigate noise and improve overall performance. Benchmarking against state-of-the-art THz sources establishes the superiority of our proposed THz sources, highlighting their potential for advancing THz technology and its applications.

On-Chip Modification of Titanium Electrothermal Characteristics by Joule Heating: Application to Terahertz Microbolometer.

This study demonstrates the conversion of metallic titanium (Ti) to titanium oxide just by conducting electrical current through Ti thin film in vacuum and increasing the temperature by Joule heating. This led to the improvement of electrical and thermal properties of a microbolometer. A microbolometer with an integrated Ti thermistor and heater width of 2.7 µm and a length of 50 µm was fabricated for the current study. Constant-voltage stresses were applied to the thermistor wire to observe the effect of the Joule heating on its properties. Thermistor resistance ~14 times the initial resistance was observed owing to the heating. A negative large temperature coefficient of resistance (TCR) of -0.32%/K was also observed owing to the treatment, leading to an improved responsivity of ~4.5 times from devices with untreated Ti thermistors. However, this does not improve the noise equivalent power (NEP), due to the increased flicker noise. Microstructural analyses with transmission electron microscopy (TEM), transmission electron diffraction (TED) and energy dispersive X-ray (EDX) confirm the formation of a titanium oxide (TiOx) semiconducting phase on the Ti phase (~85% purity) deposited initially, further to the heating. Formation of TiOx during annealing could minimize the narrow width effect, which we reported previously in thin metal wires, leading to enhancement of responsivity.

Refractive Index Measurement Using SOI Photodiode with SP Antenna toward SOI CMOS-Compatible Integrated Optical Biosensor.

This paper proposes a new optical biosensor composed of a silicon-on-insulator (SOI) p-n junction photodiode (PD) with a surface plasmon (SP) antenna. When the phase-matching condition between two lateral wavelengths of the diffracted light from the SP antenna and the waveguiding mode in the SOI PD is satisfied, we observe sharp peaks in the spectroscopic light sensitivity. Since the peak wavelength depends on the RI change around the SP antenna corresponding to the phase-matching condition, the SOI PDs with an SP antenna can be applied to the optical biosensor. The RI detection limit is evaluated in the measurements with bulk solutions, and 1.11 × 10-5 RIU (refractive index unit) can be obtained, which is comparable to that of a surface plasmon resonance (SPR) sensor, which is well known as a representative optical biosensor. In addition, the response for intermolecular bonds is estimated by the electromagnetic simulations using the finite-difference time-domain (FDTD) method to clarify its ability to detect biomolecular interactions. The results of this paper will provide new ground for high-throughput label-free biosensing, since a large number of SOI PDs with an SP antenna can be easily integrated on a single chip via an SOI complementary metal-oxide-semiconductor (CMOS) fabrication process.

Responsivity and NEP Improvement of Terahertz Microbolometer by High-Impedance Antenna.

The antenna-coupled microbolometer with suspended titanium heater and thermistor was attractive as a terahertz (THz) detector due to its structural simplicity and low noise levels. In this study, we attempted to improve the responsivity and noise-equivalent power (NEP) of the THz detector by using high-resistance heater stacked on the meander thermistor. A wide range of heater resistances were prepared by changing the heater width and thickness. It was revealed that the electrical responsivity and NEP could be improved by increasing the heater's resistance. To make the best use of this improvement, a high-impedance folded dipole antenna was introduced, and the optical performance at 1 THz was found to be better than that of the conventional halfwave dipole antenna combined with a low-resistance heater. Both the electrical and optical measurement results indicated that the increase in heater resistance could reduce the thermal conductance in the detector, thus improved the responsivity and NEP even if the thermistor resistance was kept the same.

FDTD Study on Evolution of Trimer Silver@Silica Nanospheres to Dimer for SERS Characteristics.

Light enhancement occurs strongly within the plasmonic clusters by interaction with surface plasmons. Surface-enhanced Raman spectroscopic (SERS) characteristics of a series of silver@silica trimer core-shell (CS) nanosphere (NS) clusters are investigated in this paper. It is significant to understand the electric field (EF) enhancement mechanism behind the SERS technique. The effect of symmetry breaking is studied for the series starting from the highly symmetric trimer cluster and transformed to linear dimer geometry which progresses through the gradual reduction in the vertex NS. The optical activity such as the evolution of LSPR peak is discussed, the formation of hot spots is demonstrated and the strength of the local EF enhancement is calculated and correlated with the plasmon dipolar modes by using plasmon hybridization theory to understand the underlying physical concepts.

Tunable graphene nanopatch antenna design for on-chip integrated terahertz detector arrays with potential application in cancer imaging.

Aim: Further to our reports on chip-integrable uncooled terahertz microbolometer arrays, compatible with medium-scale semiconductor device fabrication processes, the possibility of the development of chip-integrable medical device is proposed here. Methods: The concept of graphene-based nanopatch antennas with design optimization by the finite element method (FEM) is explored. The high-frequency structure simulator (HFSS) utilized fine FEM solver for analyzing empirical mode decomposition preprocessing and for modeling and simulating graphene antennas. Results: Graphene nanopatch antennas exhibited tunable features with varying patch dimensions and dependence on substrate material permittivity. Conclusion: This work implements reconfigurable graphene nanopatch antenna compatible with terahertz microbolometer arrays. This design concept further develops on-chip medical devices for possible screening of cancer cell with terahertz image processing.

Angle-Sensitive Detector Based on Silicon-On-Insulator Photodiode Stacked with Surface Plasmon Antenna.

We present a pixel-level angle sensitive detector composed of silicon-on-insulator (SOI) photodiode (PD) stacked with a gold surface plasmon (SP) antenna to affect the direction of the incoming light. The surface plasmons are excited in the grating-type SP antenna and enhance the diffraction efficiency of the grating. The diffracted light is coupled strongly with the propagation light in the SOI waveguide when the phase matching condition is satisfied. The phase matching takes place at a specific angle of light incidence, and the discrimination of the light based on the incident angle is achieved. As spatial patterns in the polar coordinate of the elevation-azimuth angles (θ, ϕ) of the incident light, we present the phase matching condition theoretically, the absorption efficiency in the SOI by simulation, and also the quantum efficiency of the SOI PD experimentally for different SP antennas of one-dimensional (1D) line-and-space (L/S) and two-dimensional (2D) hole array gratings under various polarization angles. 1D grating offers a polarization sensitive angle detection and 2D grating exhibits angle detection in two orthogonal directions, enabling a polarization independent angle sensitivity. A good agreement among the theory, simulation, and experiment are attained. The proposed device features relatively high quantum efficiency as an angle-sensitive pixel (ASP) and gives wider opportunities in applications such as three-dimensional (3D) imaging, depth-of-field extension, and lensless imaging.

Performance Comparison of SOI-Based Temperature Sensors for Room-Temperature Terahertz Antenna-Coupled Bolometers: MOSFET, PN Junction Diode and Resistor.

Assuming that the 0.6-µm silicon-on-insulator (SOI) complementary metal-oxide-semiconductor (CMOS) technology, different Si-based temperature sensors such as metal-oxide-semiconductor field-effect transistor (MOSFET) (n-channel and p-channel), pn-junction diode (with p-body doping and without doping), and resistors (n+ or p+ single crystalline Si and n+ polycrystalline Si) were designed and characterized for its possible use in 1-THz antenna-coupled bolometers. The use of a half-wave dipole antenna connected to the heater end was assumed, which limited the integrated temperature sensor/heater area to be 15 × 15 µm. Our main focus was to evaluate the performances of the temperature sensor/heater part, and the optical coupling between the incident light and heater via an antenna was not included in the evaluation. The electrothermal feedback (ETF) effect due to the bias current was considered in the performance estimation. A comparative analysis of various SOI bolometers revealed the largest responsivity (Rv) of 5.16 kV/W for the n-channel MOSFET bolometer although the negative ETF in MOSFET reduced the Rv. The noise measurement of the n-channel MOSFET showed the NEP of 245 pW/Hz1/2, which was more than one order of magnitude smaller than that of the n+ polycrystalline Si resistive bolometer (6.59 nW/Hz1/2). The present result suggests that the n-channel MOSFET can be a promising detector for THz applications.

Evaluation of multidrug cancer chronotherapy based on cell cycle model under influences of circadian clock.

The intracellular circadian clock mechanisms are known to affect various substantial cellular machinery such as cell cycle progression, inflammatory response, apoptosis, and DNA repair. Cancer growth in various tissues is still under circadian control, which may be at least partly underlain by the survived connections between the intracellular machinery and the clock. Considering such findings, chronotherapy has been applied to cancer treatments, in which anti-cancer drugs are administered in scheduled circadian times so as to resolve the trade-off between damages against the normal and cancer cells. However, any effective administration strategy has not yet been established especially in a quantitative sense. In this study, we develop an automaton model of cell division cycle interacting with circadian clock and suffering from a probability of cell death. A cancer cell is modeled by shortening/ lengthening the cell division interval and a transition to motility state under starving condition. Population proliferating dynamics in 3D space are simulated under the diffusion of nutrient factor and the anti-cancer drugs from a vessel. The simulation results show that the drug administration schedule could differentiate the damages against proliferation of normal and cancer cells. This implies the existence of optimal timing for the drug administration, which could provide an efficient strategy of chronotherapeutic treatment of cancer.

Effects of substrate voltage on noise characteristics and hole lifetime in SOI metal-oxide-semiconductor field-effect transistor photon detector.

Low-frequency noise and hole lifetime in silicon-on-insulator (SOI) metal-oxide-semiconductor field-effect transistors (MOSFETs) are analyzed, considering their use in photon detection based on single-hole counting. The noise becomes minimum at around the transition point between front- and back-channel operations when the substrate voltage is varied, and increases largely on both negative and positive sides of the substrate voltage showing peculiar Lorentzian (generation-recombination) noise spectra. Hole lifetime is evaluated by the analysis of drain current histogram at different substrate voltages. It is found that the peaks in the histogram corresponding to the larger number of stored holes become higher as the substrate bias becomes larger. This can be attributed to the prolonged lifetime caused by the higher electric field inside the body of SOI MOSFET. It can be concluded that, once the inversion channel is induced for detection of the photo-generated holes, the small absolute substrate bias is favorable for short lifetime and low noise, leading to high-speed operation.

SOI metal-oxide-semiconductor field-effect transistor photon detector based on single-hole counting.

In this Letter, a scaled-down silicon-on-insulator (SOI) metal-oxide-semiconductor field-effect transistor (MOSFET) is characterized as a photon detector, where photogenerated individual holes are trapped below the negatively biased gate and modulate stepwise the electron current flowing in the bottom channel induced by the positive substrate bias. The output waveforms exhibit clear separation of current levels corresponding to different numbers of trapped holes. Considering this capability of single-hole counting, a small dark count of less than 0.02 s(-1) at room temperature, and low operation voltage of 1 V, SOI MOSFET could be a unique photon-number-resolving detector if the small quantum efficiency were improved.

Metal--semiconductor transition in single-walled carbon nanotubes induced by low-energy electron irradiation.

We report the effect of low-energy (1 keV) electron beam irradiation on gated, three-terminal devices constructed from metallic single-walled carbon nanotubes. Pristine devices, which exhibited negligible gate voltage response at room temperature and metallic single-electron transistor characteristics at low temperatures, when exposed to an electron beam, exhibited ambipolar field effect transistor (room temperature) and single-electron transistor (low temperature) characteristics. This metal-semiconductor transition is attributed to inhomogeneous electric fields arising from charging during electron irradiation.