Small Junction Area Research Articles

Area-dependent gain and noise characteristics of mid-wavelength infrared HgCdTe planar electron avalanche photodiodes

This paper mainly investigates the area-dependent gain and noise characteristics of mid-wavelength infrared (MWIR) Hg0.7Cd0.3Te planarelectron avalanche photodiodes (e-APDs) operated at 80 K. The 10-μm-radius diode exhibits low dark current in the magnitude of 10–13 A below −5.5 V, high gain up to 1270 at −10 V, and low excess noise factor between 1 and 1.2. The optimal performances are compromised by tunneling current, which should be further suppressed. Studies on variable-area diodes show that larger diodes have a reduced gain due to a smaller contribution from edge gain, as well as an increased 1/f noise and corner frequency due to higher defect density. From the gain and noise perspectives, HgCdTe e-APDs with smaller junction areas are more suitable for focal plane array (FPA) applications.

S/N ratio improvement of a nanocuboid array photodetector based on a Au/n-Si Schottky junction for broadband near-infrared light

In this paper, an improvement in the signal-to-noise (S/N) ratio of a nanocuboid array near-infrared (NIR) photodetector is demonstrated by using a combination of a small Schottky junction area and a converging lens. The combination of the small Schottky junction area and the lens showed a higher S/N ratio than a Schottky area of 1 cm2 for wavelengths above 1.2 μm, indicating 3.5 and 9.5 dB improvements in the S/N ratio at wavelengths of 1.2 and 1.5 μm, respectively. The structural parameters of the nanocuboids were determined to be 600 nm in length, 535 nm in height, and 900 nm in periodicity with high absorption efficiency in the broadband NIR region. The photocurrent signal was obtained over a broadband in the NIR region from 1.1 to 1.5 μm in wavelength by converging light with a convex lens above the nanocuboid array with a Schottky junction area of 1 mm2.

High-speed integrated micro-LED array for visible light communication.

In this Letter, we report high-speed integrated 14 µm in diameter micro-light-emitting diode (μLED) arrays with the parallel configuration, including ${2} \times {2}$2×2, ${2} \times {3}$2×3, ${2} \times {4}$2×4, and ${2} \times {5}$2×5 arrays. The small junction area of μLED (${\sim}{191}\;\unicode{x00B5}{\rm m}^2$∼191µm2) in each element facilitates the operation of higher injection current density up to ${13}\;{{\rm kA/cm}^2}$13kA/cm2, leading to the highest modulation bandwidth of 615 MHz. The optical power of ${2} \times {5}$2×5 array monotonically increases (${\sim}{10}$∼10 times higher) as the number of arrays increases (1 to 10), while retaining the fast modulation bandwidth. A clear eye diagram up to 1 Gbps without any equalizer further shows the capability of this high-speed transmitter for VLC. These results mean that tailoring the optical power of μLEDs in a parallel-biased integrated array can further enhance the data transmission rate without degradation of the modulation bandwidth.

Unshielded SQUID Sensors for Ultra-Low-Field Magnetic Resonance Imaging

We have implemented a new field-tolerant low-critical-temperature superconducting quantum interference device current sensor with submicrometer Josephson junctions. The small junction area enables the fabrication of devices with lower sensor noise and higher tolerance against magnetic fields. An application benefitting from these properties is ultra-low-field magnetic resonance imaging (ULF MRI). In the currently ongoing BREAKBEN project, new technology is being pushed forward by combining ULF MRI with magnetoencephalography in a single instrument. We describe the sensor architecture aimed for this purpose and justify the guidelines behind the design. Experimental characterization reveals that, with the aid of reasonable amount of heating, unshielded sensor operation after high magnetic pulses at least up to 150 mT appears to be possible.

Highly-Sensitive Thin Film THz Detector Based on Edge Metal-Semiconductor-Metal Junction

Terahertz (THz) detectors have been extensively studied for various applications such as security, wireless communication, and medical imaging. In case of metal-insulator-metal (MIM) tunnel junction THz detector, a small junction area is desirable because the detector response time can be shortened by reducing it. An edge metal-semiconductor-metal (EMSM) junction has been developed with a small junction area controlled precisely by the thicknesses of metal and semiconductor films. The voltage response of the EMSM THz detector shows the clear dependence on the polarization angle of incident THz wave and the responsivity is found to be very high (~2,169 V/W) at 0.4 THz without any antenna and signal amplifier. The EMSM junction structure can be a new and efficient way of fabricating the nonlinear device THz detector with high cut-off frequency relying on extremely small junction area.

Theoretical study on high-frequency graphene-nanoribbon heterojunction backward diode

We propose and analyze a heterojunction backward diode for millimeter- or terahertz-wave detection using edge-modified graphene nanoribbons (GNRs). According to the electron-affinity difference between a hydrogen-terminated GNR and a fluorine-terminated GNR, it is possible to construct a staggered-type lateral heterojunction diode. First-principles calculations reveal that because of band-to-band tunneling, the diode has a nonlinear current of the order of kA/m. The small junction area contributes to the reduction of the intrinsic junction capacitance. Equivalent-circuit analyses show that when the total capacitance is reduced below 100 aF, the diode exhibits a voltage sensitivity of 3.79 × 103 V/W at 300 GHz.

Platinum multicubes prepared by ni(2+) -mediated shape evolution exhibit high electrocatalytic activity for oxygen reduction.

Pt(100) facets are generally considered less active for the oxygen reduction reaction (ORR). Reported herein is a unique Pt-branched structure, a multicube, whose surface is mostly enclosed by {100} facets but contains high-index facets at the small junction area between the adjacent cubic components. The synthesis is accomplished by a Ni(2+) -mediated facet evolution from high-index {311} to {100} facets on the frameworks of multipods. Despite the high {100} facet coverage, the Pt multicubes exhibit impressive ORR activity in terms of half-wave potential and current density nearly to the level of the most active Pt-based catalysts, while the durability of catalysts is well retained. The facet evolution creates a set of samples with tunable ratios of high-index to low-index facets. The results reveal that the excellent ORR performance of Pt multicubes is a combined result of active sites by high-index facets and low resistance by flat surface. It is anticipated that this work will offer a new approach to facet-controlled synthesis and ORR catalysts design.

Platinum Multicubes Prepared by Ni2+‐Mediated Shape Evolution Exhibit High Electrocatalytic Activity for Oxygen Reduction

AbstractPt(100) facets are generally considered less active for the oxygen reduction reaction (ORR). Reported herein is a unique Pt‐branched structure, a multicube, whose surface is mostly enclosed by {100} facets but contains high‐index facets at the small junction area between the adjacent cubic components. The synthesis is accomplished by a Ni2+‐mediated facet evolution from high‐index {311} to {100} facets on the frameworks of multipods. Despite the high {100} facet coverage, the Pt multicubes exhibit impressive ORR activity in terms of half‐wave potential and current density nearly to the level of the most active Pt‐based catalysts, while the durability of catalysts is well retained. The facet evolution creates a set of samples with tunable ratios of high‐index to low‐index facets. The results reveal that the excellent ORR performance of Pt multicubes is a combined result of active sites by high‐index facets and low resistance by flat surface. It is anticipated that this work will offer a new approach to facet‐controlled synthesis and ORR catalysts design.

Device Area Scale‐Up and Improvement of SWNT/Si Solar Cells Using Silver Nanowires

DOI: 10.1002/aenm.201400186 Si solar cells. This difference becomes more problematic during the scale up to larger size cells, because the total resistive power loss per unit area ( P R ) scales with the cell area. [ 17 ] P R is given by / / R S max 2 s max 2 S max 2 P R I A R J A A R AJ ( ) = = = , where R s is the series resistance in the device, A is the device area, I max and J max are the cell current and current density in the maximum power output respectively. This equation illustrates that the P R is proportional to the product of the series resistance and the area ( R s A ), which directly scales with the cell area. As a result, all the CNT/Si and graphene/Si solar cells with high effi ciency (>10%) reported in the literature are limited with very small junction area (<15 mm 2 ), while the cells with large junction area (≈50 mm 2 ) suffers from very poor effi ciencies (≈1.3%) ( Table 1 ). Besides the effi ciency, a drastic degradation of other device parameters is also observed with a scale-up of the junction-area. For example, Shi et al. reported that the fi ll factor decreased from 72% to 50% when the junction area of graphene/Si solar cells increased from 4.7 mm 2 to 14.5 mm 2 . [ 10 ] A similar reduction of fi ll factor from 45% to 38% was also observed in the scale up of organic solar cells from 2.4 mm 2 to 1.13 cm 2 . [ 18 ]

Effect of Proton Irradiation on DC Performance and Reliability of Circular-Shaped AlGaN/GaN High Electron Mobility Transistors

AlGaN/GaN High Electron Mobility Transistors (HEMTs) show great promise for applications such as military radar and satellite-based communications systems. Circular devices, with smaller junction areas and higher effective gate width/gate length, are better suited for high power, high frequency application. Since the circular devices have been shown to improve the device performance, the effect of radiation on their reliability is an important topic. There are many studies characterizing the performance under proton, neutron, and electron irradiations for rectangular AlGaN/GaN HEMTs. However, the studies for circular-shaped devices are limited. In this work, we report the effect of proton irradiation on circular-based AlGaN/GaN HEMT. DC performance, breakdown voltage, and gate lag measurement were done to characterize the devices.AlGaN/GaN HEMT heterostructures were grown on c-plane sapphire by molecular beam epitaxy (MBE). The layer structure included a thin AlGaN nucleation layer followed with a 2 µm thick undoped GaN buffer topped by a 25 nm thick unintentionally doped Al0.28GaN layer. Mesa isolation was achieved by using an inductively coupled plasma system with Ar/Cl2-based discharges. The Ohmic contacts were formed by lifting-off e-beam evaporated Ti (200 Å)/Al (1000 Å)/Ni (400 Å)/Au (800 Å). Gates with the dimension of 1.5 µm × 314 µm were defined by lift-off of e-beam deposited Ni/Au metallization. The contacts were annealed at 850°C for 45 s under a flowing N2 ambient in Heatpulse 610T system. The final step was deposition of e-beam evaporated Ti/Au (300 Å/1200 Å) interconnection contacts. Devices were exposed to protons with 5 MeV and different doses of 2×1014 proton/cm2. The device DC performance was characterized using an HP 4156 parameter analyzer. The breakdown voltage was conducted with a Tektronix 370A curve tracer. FLOODS TCAD finite-element solver was used to simulate the electrical field around the gate edge to confirm the experimental result.As shown in Figure 1, drain current IVs, IDSS, decreased from 454 to 398 mA/mm after irradiation with a proton dose of 2×1014 cm-2. Moreover, resistance between source and drain increased and extrinsic transconductance decreased after irradiation. However, the drain breakdown voltage increased from 150 to 230 V. The improvement of the breakdown voltage was due to the reduction of electric field at the edge of the gate, caused by the defects generated by proton irradiation at AlGaN/GaN interface, which was confirmed with simulation, as shown in Figure 2.Both gate and drain pulse measurements were also performed to verify the introductions of implanted defects into the HMET structure, which will be discussed in the talk.

${\rm Si}_{\rm x}{\rm Ge}_{1\hbox{-}{\rm x}}$ Epitaxial Tunnel Layer Structure for P-Channel Tunnel FET Improvement

The tunnel field-effect transistor (FET) is a promising candidate for use in ultralow-power applications because of its distinct operation principle, namely, band to band tunneling (BTBT). However, the ON-state current of the tunnel device is extremely low because of the poor tunneling efficiency of the BTBT. In this paper, a novel epitaxial tunnel layer (ETL) structure combining vertical tunneling orientation was proposed. The ETL structure performs more favorably than does the traditional lateral tunnel FET structure in an all-silicon device. By using low bandgap materials in the ETL, the ON-state BTBT current increases and an extremely low intrinsic OFF-state current is maintained because of the small low bandgap junction area. The onset voltage of the bipolar BTBT can also be postponed using ETL band engineering. The optimized parameters of the SixGe <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">1-x</sub> ETL tunnel FET structure increase the ON-state current 10 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">7</sup> -10 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">8</sup> times compared with that of the traditional lateral silicon tunnel FET. The minimal subthreshold swing (SS) and ON/OFF current ratio also improve, the SS decreases from 47 mV/decade to 29 mV/decade, and the ON/OFF current ratio increase from 10 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">5</sup> to 10 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">10</sup> . In this paper, the effects of the ETL parameters on device performance are discussed in detail.

Noise characteristics of InAs/GaSb superlattice infrared photodiodes

AbstractA sophisticated noise measurement setup employing a switching unit to measure statistically relevant numbers of InAs/GaSb superlattice photodiodes has been developed. The noise current resolution limit of 20 fA Hz‐1/2 enables the characterization of focal plane array‐sized InAs/GaSb superlattice homojunction photodiodes for the long‐wavelength infrared at 77 K. To resolve midwavelength infrared photodiodes a junction area of about (400 µm)2 is required. Without switching unit and by using a dedicated low noise amplifier, noise currents down to 2 fA Hz‐1/2 can be achieved, allowing the noise characterization of mid‐wavelength photodiodes with smaller junction areas. With this setup long‐wavelength and mid‐wavelength InAs/GaSb superlattice photodiodes, with generation‐recombination limited dark current behavior, are investigated at 77 K. The measured diode noise follows the theoretically predicted shot noise level within the white noise part of the spectra. (© 2013 WILEY‐VCH Verlag GmbH &amp; Co. KGaA, Weinheim)

A Centrifugal Microfluidics Platform for Potential Application on Immobilization-Free Bead-Based Immunoassays

In this work, we demonstrate a simple yet convenient centrifugal microfluidics, or lab-on-a-disc (LOAD) platform for bead-based immunoassays. The disc contains a network of passive valves for sample manipulation and a bead-aggregating barrier structure to instead of commonly used immobilization method. The narrow-channel structure not only provides a geometric barrier for retaining the beads, but also significantly increased fluidic resistance which results in lower flow rate and longer reaction time for each reagent in the reaction chamber. Centrifuge-forced beads densification in the small junction area and signal concentrating allows high signal sensing capability. The acceleration speed is optimized for largest signal-to-noise ratio. The device uses different rotation speed to trigger a cascade of flow events. The integrated platform contains all the necessary functional elements for conducting one-step parallel assays through a sequential protocol. For our test device, the total time required to complete an immunoassay experiment is typically under 6 minutes. Through this work we have also shown that the use of a geometric barrier for the aggregation of signaling beads is an effective approach for the enhancement of optical signals in common immunoassays. This flexible design is suitable for a broad range of bead-based immunoassays with low-cost and high-speed operation.

Decoherence of superconducting qubits caused by quasiparticle tunneling

In superconducting qubits, the interaction of the qubit degree of freedom with quasiparticles defines a fundamental limitation for the qubit coherence. We develop a theory of the pure dephasing rate \Gamma_{\phi} caused by quasiparticles tunneling through a Josephson junction and of the inhomogeneous broadening due to changes in the occupations of Andreev states in the junction. To estimate \Gamma_{\phi}, we derive a master equation for the qubit dynamics. The tunneling rate of free quasiparticles is enhanced by their large density of states at energies close to the superconducting gap. Nevertheless, we find that \Gamma_{\phi} is small compared to the rates determined by extrinsic factors in most of the current qubit designs (phase and flux qubits, transmon, fluxonium). The split transmon, in which a single junction is replaced by a SQUID loop, represents an exception that could make possible the measurement of \Gamma_{\phi}. Fluctuations of the qubit frequency leading to inhomogeneous broadening may be caused by the fluctuations in the occupation numbers of the Andreev states associated with a phase-biased Josephson junction. This mechanism may be revealed in qubits with small-area junctions, since the smallest relative change in frequency it causes is of the order of the inverse number of transmission channels in the junction.

Identifying Sources of Decoherence in a dc SQUID Phase Qubit With a Sub-$\mu{\rm m}$ Junction and Interdigitated Capacitor

We fabricated a dc SQUID phase qubit with a sub- μm Al/AlOx/Al qubit junction and an interdigitated shunting capacitor on a sapphire substrate. The qubit junction had a critical current of 135 nA, and the isolation junction had a critical current of 8.3 μA. The shunting capacitance was about 1.5 pF. To reduce the unwanted effects of two-level systems and increase the relaxation time T1, we have removed unnecessary dielectrics, used a small qubit junction area (450 nm × 500 nm), isolated the qubit from the leads with an on-chip LC filter, and fabricated the device on a bare sapphire substrate. However, at a temperature of 20 mK, we found T1 ≈ 300 ns and the coherence time T2 ≈ 110 ns, which was much lower than one would expect from loss attributed to the leads and to dielectrics in the tunnel junction and substrate. Measurements of T1 versus applied flux (which tuned the qubit frequency) revealed a correlation between the strength of the coupling of the microwave excitation line to the qubit and the rate of energy dissipation in the qubit. This result suggests that the relaxation time was being limited by coupling to the microwave line.

Low-Temperature Scanning Tunneling Microscopy of Ring-Like Surface Electronic Structures Around Co Islands on InAs(110) Surfaces

We report on the experimental observation by scanning tunneling microscopy at low temperature of ring-like features that appear around Co metal islands deposited on a clean (110) oriented surface of cleaved p-type InAs crystals. These features are visible in spectroscopic images within a certain range of negative tunneling bias voltages due to the presence of a negative differential conductance in the current-voltage dependence. A theoretical model is introduced, which takes into account non-equilibrium effects in the small tunneling junction area. In the framework of this model the appearance of the ring-like features is explained in terms of interference effects between electrons tunneling directly and indirectly (via a Co island) between the tip and the InAs surface.

Dislocation‐induced deep levels in ELO InP revealed by point contact photocapacitance measurements

30K-photocapacitance and excitation photocapacitance methods were applied to reveal the dislocation-induced deep levels in coalescent epitaxial lateral overgrowth layers of InP. Point-contact Schottky barrier junctions with small junction areas were formed on dislocated and dislocation-free regions. In the dislocation-free layers, the dominant deep level was located at Ec-1.30 eV, whereas in the dislocated area, dominant deep levels were detected at Ec-0.86 eV and 1.05 eV. A neutralized state was also detected at 0.66 eV+Ev. Excitation photocapacitance results have shown that the defect configuration coordinate diagram of the dislocation-induced deep levels was considered with large Frank-Condon shifts of 0.28 eV. (© 2007 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

Dislocation-induced deep electronic states in InP: Photocapacitance measurements

Photocapacitance and excitation photocapacitance methods were applied to reveal the dislocation-induced deep levels in coalescent epitaxial lateral overgrowth layers of InP. Point-contact Schottky barrier junctions with small junction areas were formed on dislocated and dislocation-free regions by using wedge wire-bonding of Au, and photocapacitance measurements were then carried out at $30\phantom{\rule{0.3em}{0ex}}\mathrm{K}$. In the dislocation-free layers, the dominant deep level was located at $1.30\phantom{\rule{0.3em}{0ex}}\mathrm{eV}$ below the conduction band, whereas in the dislocated area, dominant deep levels were detected at $0.86\phantom{\rule{0.3em}{0ex}}\mathrm{eV}$ $(\ensuremath{\lambda}=1.44\phantom{\rule{0.3em}{0ex}}\ensuremath{\mu}\mathrm{m})$ and $1.05\phantom{\rule{0.3em}{0ex}}\mathrm{eV}$ $(\ensuremath{\lambda}=1.18\phantom{\rule{0.3em}{0ex}}\ensuremath{\mu}\mathrm{m})$ below the conduction band. A neutralized state was also detected at $0.66\phantom{\rule{0.3em}{0ex}}\mathrm{eV}$ above the valence band. From the detailed excitation photocapacitance results, it is shown that the defect configuration coordinate diagram of the dislocation-induced deep levels was considered with large Frank-Condon shifts $({d}_{\mathrm{FC}})$ of $0.28\phantom{\rule{0.3em}{0ex}}\mathrm{eV}$. This means that the atomic configurations around the deep levels are highly relaxed, as expected from the structures of the dislocation cores.

Thouless energy of a superconductor from non local conductance fluctuations

We show that a spin-up electron from a normal metal entering a superconductor propagates as a composite object consisting of a spin-down hole and a pair in the condensate. This leads to a factorization of the non local conductance as two local Andreev reflections at both interfaces and one propagation in the superconductor, which is tested numerically within a one dimensional toy model of reflectionless tunneling. Small area junctions are characterized by non local conductance fluctuations. A treatment ignoring weak localization leads to a Thouless energy inverse proportional to the sample size, as observed in the numerical simulations. We show that weak localization can have a strong effect, and leads to a coupling between evanescent quasiparticles and the condensate by Andreev reflections ``internal'' to the superconductor.

Dielectric Breakdown in Underoxidized Magnetic Tunnel Junctions: Dependence on Oxidation Time and Area

Magnetic tunnel junctions (MTJs) with partially oxidized 9 \AA AlO$_x$-barriers were recently shown to have the necessary characteristics to be used as magnetoresistive sensors in high-density storage devices. Here we study dielectric breakdown in such underoxidized magnetic tunnel junctions, focusing on its dependence on tunnel junction area and oxidation time. A clear relation between breakdown mechanism and junction area is observed for the MTJs with the highest studied oxidation time: samples with large areas fail usually due to extrinsic causes (characterized by a smooth resistance decrease at dielectric breakdown). Small area junctions fail mainly through an intrinsic mechanism (sharp resistance decrease at breakdown). However, this dependence changes for lower oxidation times, with extrinsic breakdown becoming dominant. In fact, in the extremely underoxidized magnetic tunnel junctions, failure is exclusively related with extrinsic causes, independently of MTJ-area. These results are related with the presence of defects in the barrier (weak spots that lead to intrinsic breakdown) and of metallic unoxidized Al nanoconstrictions (leading to extrinsic breakdown).