International Journal of High Speed Electronics and Systems

Terahertz (THz) gas photonics uses gas as THz emitter and sensor for time-domain spectroscopy. Unique properties of the gas promise scalable, strong THz wave generation with broad spectral range covering the entire THz gas (0.3 THz to 35 THz). The systematic study of THz wave generation and detection in different gases shows that the generation efficiency is monotonically decreasing with the ionization potential of the gas molecules while the detection efficiency is linearly proportional to the third order nonlinear coefficient of the gas molecules. We also discuss the development of THz wave detection using laser-induced fluorescence and coherent control with THz gas photonics.

Efficiency droop and green gap are terms that summarize performance limitations in GaInN/GaN high brightness light emitting diodes (LEDs). Here we summarize progress in the development of green LEDs and report on time resolved luminescence data of polar c-plane and non-polar m-plane material. We find that by rigorous reduction of structural defects in homoepitaxy on bulk GaN and V-defect suppression, higher efficiency at longer wavelengths becomes possible. We observe that the presence of donor acceptor pair recombination within the active region correlates with lower device performance. To evaluate the aspects of piezoelectric polarization we compare LED structures grown along polar and non-polar crystallographic axes. In contrast to the polar material we find single exponential luminescence decay and emission wavelengths that remain stable irrespective of the excitation density. Those findings render high prospects for overcoming green gap and droop in non-polar homoepitaxial growth.

We report on terahertz detection (from 0.2 THz to 2.4 THz) by Si FinFETs of different widths (with 2, 20, and 200 fins connected in parallel). FinFETs (with a small number of fins and with feature sizes as short as 20 nm to 40 nm) showed a very high responsivity (far above that previously measured for standard CMOS). We explain this improvement by negligible narrow channel effects.

Recent progress and state of GaSb based type-I lasers emitting in spectral range from 2 to 3.5 μm is reviewed. For lasers emitting near 2 μm an optimization of waveguide core width and asymmetry allowed reduction of far field divergence angle down to 40-50 degrees which is important for improving coupling efficiency to optical fiber. As emission wavelength increases laser characteristics degrade due to insufficient hole confinement, increased Auger recombination and deteriorated transport through the waveguide layer. While Auger recombination is thought to be an ultimate limiting factor to the performance of these narrow bandgap interband lasers we demonstrate that continuous improvements in laser characteristics are still possible by increasing hole confinement and optimizing transport properties of the waveguide layer. We achieved 190, 170 and 50 mW of maximum CW power at 3.1, 3.2 and 3.32 μm wavelengths respectively. These are the highest CW powers reported to date in this spectral range and constitute 2.5-fold improvement compared to previously reported devices.

The volume of telecommunications traffic keeps growing at an exponential rate. The optical-communications industry, the linchpin of modern telecommunications, in its quest of keeping up with this growth simply must increase the number of wavelengths in the wavelength-division multiplexing (WDM) configuration. The result of this increase would mean, too, that the number of transmitters and receivers that could be placed on one board would increase as well; hence, the density of their packaging would come to micro- and even nano-scale. At the receiver end, which we will consider in this paper, manufacturers are now able to fabricate an array of photodiodes (PDs) on a single wafer, reducing the size of an individual PD to hundreds of nm. The main obstacle in approaching this scale from a communications link point of view, however, is the diffraction limit. One possible solution to this problem is the use of plasmonics. This paper discusses a possible approach to using surface plasmon polaritons (SPPs) for WDM demultiplexing, presents a possible scheme to implement this approach, and offers our analysis of this scheme along with suggestions for future developments.

An attempt is made to correlate electrical measurement results to specific defects in the dielectric stacks of high-k/metal gate devices. Defect characteristics extracted from electrical data were compared to those obtained by ab initio calculations of the dielectric structures. It is demonstrated that oxygen vacancies in a variety of charge states and configurations in the interfacial SiO₂ layer of the high-k gate stacks contribute to random telegraph noise signal, time-dependent dielectric breakdown, and the flatband voltage roll-off phenomenon.

SOI technology offers ample room for scaling, performance improvement, and innovations. The current status is reviewed by focusing on several technological options for boosting the transport properties in SOI MOSFETs. The impact of series resistance, high-K dielectrics, and metal gate in advanced transistors is discussed. Carrier mobility measurements as a function of channel length and temperature reveal the beneficial effect of strain, mitigated however by various types of defects. The experimental data is exclusively collected from state-of-the-art, ultrathin body, fully depleted MOSFETs. Simple models are presented to clarify the mobility behavior.

Hall electron mobility in buried QW InGaAs channels, grown on InP substrates with HfO₂ gate oxide, is analyzed experimentally and theoretically as a function of top barrier thickness and composition, carrier density, and temperature. Temperature slope α in μ ~T^α dependence is changing from α=-1.1 to +1 with the reduction of the top barrier thickness indicating the dominant role of remote Coulomb scattering (RCS) in interface-related contribution to mobility degradation. Insertion of low-k SiO_x interface layer formed by oxidation of thin in-situ MBE grown amorphous Si passivation layer has been found to improve the channel mobility, but at the expense of increased EOT. This mobility improvement is also consistent with dominant role of RCS. We were able to a obtain a reasonable match between experiment and simple theory of the RCS assuming the density of charges at the high-k/barrier interface to be in the range of (2-4)×10¹³ cm^-2.

Insulated gate n-channel enhancement mode InGaAs field effect transistors with the GdScO₃ high-k dielectric have been fabricated and studied. The low frequency noise was high indicating a high interface density of traps. Trap density and its dependence on the gate voltage have been extracted from the noise and conductance measurements.

Implantable biomedical sensors and continuous real time in vivo monitoring of various physiological parameters requires low-power sensor electronics and wireless telemetry for transmission of sensor data. In this article, generic blocks required for such systems have been demonstrated with design examples. Ideally neural or electro-chemical sensor signal monitoring units comprise of low noise amplifiers, current or voltage mode analog to digital domain data conversion circuits and wireless telemetry circuits. The low-noise amplifier described here has a novel open loop amplifier scheme used for neural signal recording systems. The design has been implemented using 0.5-μm SOI-BiCMOS process. The fabricated chip can work with 1 V supply and consumes 805 nA. The current mode analog to digital conversion signal processing circuitry takes the current signal as an input and generates a pulse-width modulated data signal. The data signal is then modulated with a high frequency carrier signal to generate FSK data for wireless transmission. The design is fabricated in 0.5-μm standard CMOS process and consumes 1.1 mW of power with 3.5 V supply.

GaAs and InP based III-V compound semiconductor nanowires were grown epitaxially on GaAs (or Si) (111)B and InP (111)B substrates, respectively, by metalorganic chemical vapor deposition using Au nanoparticles as catalyst. In this paper, we will give an overview of nanowire research activities in our group. In particular, the effects of growth parameters on the crystal structure and optical properties of various nanowires were studied in detail. We have successfully obtained defect-free GaAs nanowires with nearly intrinsic exciton lifetime and vertical straight nanowires on Si (111)B substrates. The crystal structure of InP nanowires, i.e., WZ or ZB, can also be engineered by carefully controlling the V/III ratio and catalyst size.

Here we report our research on quantum-dot structures with collective barriers surrounding groups of quantum dots (planes, clusters etc) and preventing photoelectron capture. Employing Monte-Carlo simulations, we investigate photoelectron kinetics and calculate the photoelectron lifetime as a function of geometrical parameters of the structures, dot occupation, and electric field. Results of our simulations demonstrate that the capture processes are substantially suppressed by the potential barriers and enhanced in strong electric fields. Detailed analysis shows that the effects of the electric field can be explained by electron heating, i.e. field effects become significant, when the shift of the electron temperature due to electron heating reaches the barrier height. Optimized photoelectron kinetics in quantum-dot structures with collective barriers allows for significant improvements in the photoconductive gain, detectivity, and responsivity of photodetectors based on these structures.

Graphene nanoribbons exhibit band gap modulation when subjected to strain. While band gap creation has been theoretically investigated for uniaxial strains, other deformations such as nanoribbon twist have not been considered. Our main objective in this paper is to explore band gap opening in twisted graphene nanoribbons that have metallic properties under tight-binding approximation. While simple considerations based on the Hückel model allow to conclude that zigzag graphene nanoribbons exhibit no band gap when subjected to twist, the Hückel model overall may be inaccurate for band gap prediction in metallic nanoribbons. We utilize Density Functional Theory Tight-Binding Approximation together with a requirement that energy of twisted nanoribbons is minimized to evaluate band gap of metalic armchair nanoribbons. Besides considering twisting deformations, we also explore the possibility of creating band gap when graphene nanoribbons are subject to inhomogeneous deformation such as sinusoidal deformations.

We report results of the experimental investigation of the low-frequency noise in graphene transistors. The graphene devices were measured in three-terminal configuration. The measurements revealed low flicker noise levels with the normalized noise spectral density close to 1/f (f is the frequency) and the Hooge parameter α_H ~10^-3. Both top-gate and back-gate devices were studied. The analysis of the noise spectral-density dependence on the gate biases helped us to elucidate the noise sources in these devices. We compared the noise performance of graphene devices with that of carbon nanotube devices. It was determined that graphene devices works better than carbon nanotube devices in terms of the low-frequency noise. The obtained results are important for graphene electronic, communication and sensor applications.

In this study, nc-ZnO films deposited in a Pulsed Laser Deposition (PLD) system at various temperatures were used to fabricate high performance transistors. As determined by Transmission Electron Microscope (TEM) images, nc-ZnO films deposited at a temperature range of 25°C to 400°C were made of closely packed nanocolums showing strong orientation. The influences of film growth temperature and post growth annealing on device performance were investigated. Various gate dielectric materials, including SiO₂, Al₂O₃, and HfO₂ were shown to be suitable for high performance device applications. Bottom-gate FETs fabricated on high resistivity (>2000 ohm-cm) Si substrates demonstrated record DC and high speed performance of any thin film transistors. Drain current on/off ratios better than 10¹² and sub-threshold voltage swing values of less than 100mV/decade could be obtained. Devices with 2μm gate lengths produced exceptionally high current densities of >750mA/mm. Shorter gate length devices (LG=1.2μm) had current and power gain cut-off frequencies, f_T and f_max, of 2.9GHz and 10GHz, respectively.

Zinc Oxide (ZnO) nanoparticles were created by a top-down wet-chemistry synthesis process (ZnO-A) and then coated with polyvinyl-alcohol (PVA) (ZnO-U). In ZnO-U, strong UV emission was apparent while the parasitic green emission, which normally appears in ZnO suspensions, was suppressed. A standard lift-off process via e-beam lithography was used to fabricate a detector by evaporating Aluminum (Al) as ohmic electrodes on the ZnO nanoparticle film. Photoconductivity experiments showed that linear current-voltage response were achieved and the ZnO-U nanoparticles based detector had a ratio of UV photo-generated current more than 5 times better than that of the ZnO-A based detector. In addition, non-linear current-voltage responses were observed when interdigitated finger Gold (Au) contacts were deposited on ZnO-U. The UV generated current to dark current ratios were between 4 and 7 orders of magnitude, showing better performance than the photodetector with Al contacts. ZnO-U were also deposited on Gallium Nitride (GaN) and Aluminum Gallium Nitride (AlGaN) substrates to create spectrally selective photodetectors. The responsivity of detector based on AlGaN is twice that of commercial UV enhanced Silicon photodiodes. These results confirmed that ZnO nanoparticles coating with PVA is a good material for small-signal, visible blind, and wavelength selective UV detection.

Carbon-based nanoelectromechanical devices are approaching applications in electronics. Switches based on individual carbon nanotubes deliver record low off-state leakage currents. Arrays of vertically aligned carbon nanotubes or nanofibers can be fabricated to constitute varactors. Very porous, low density arrays of quasi-vertically aligned arrays of carbon nanotubes behave mechanically as a single unit with very unusual material properties.

Despite the recent progress in understanding the geometric structures of defects and edges in a graphene device (GD), how such defects and edges affect the transport properties of the device have not been clearly defined. In this study, the surface geometric structure of a GD was observed with an atomic force microscope (AFM) and the spatial variation of the transport current by the gating tip was measured with scanning gate microscopy (SGM). It was found that geometric corrugations, defects and edges directly influence the transport current. This observation is linked directly with a proposed scattering model based on macroscopic transport measurements.

III-Nitride heterostructure field-effect transistors (HFETs) demonstrated a new paradigm in microwave switching and control applications due to unique combination of extremely low channel resistance (leading to low loss), very high RF power, low off-state capacitance, broad range of operating temperatures, chemical inertness and robustness. The paper reviews novel approaches and recent advances in III-Nitride technology for RF switching devices leading to higher operating frequencies and even lower insertion loss.