Backward Diodes Research Articles

An Indirect Impedance Characterization Method for Monolithic THz Antennas Using Coplanar Probe Measurements

We develop a simple and robust impedance characterization method for planar THz antennas with micron- and submicron-size port geometries. Such antennas are often encountered in THz sensing applications where an ultrafast electronic device, such as a Schottky junction or a heterostructure backward diode, is integrated with a planar antenna structure. Standard probe characterization of such antennas at the device port is not currently possible due to the large contact areas required. The proposed method allows for indirect characterization of antenna impedance seen at the device port using measurements collected at a more suitable, remote location on the antenna plane. Three S11 measurements are performed using contact probes at a larger pad on the antenna periphery, using three terminations (short, open, and a resistive load) of the port under consideration. Through a simple relation, the measured data set is used to compute the port-impedance indirectly. Experimental results are presented to illustrate the accuracy of the proposed approach.

Analysis of a monolithic integrated rectenna by using an InGaAs/InAlAs triple‐barrier resonant tunneling diode for zero bias detection of submilimeter‐waves

AbstractZero bias millimeter‐wave detection has been investigated by using backward diodes with InGaAs‐based or Sb‐based interband tunneling and Schottky contact InGaAs‐ based double‐barrier resonant tunneling diodes (DBRTDs). These were intended to be typical cases where the antenna and the detector are individually designed and mutually connected. On the other hand, in this paper we focus on constructing a monolithic type of rectenna (rectifying antenna) for sub‐millimeter wave detection. Our objective is to propose and analyze zero bias detection properties with an InGaAs/InAlAs triple‐barrier resonant tunneling diode (TBRTD) integrated with a bow‐tie type of broadband self‐complementary antenna. (© 2012 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

The absorption spectrum of methane at 80 and 294 K in the icosad (6717–7589 cm −1): Improved empirical line lists, isotopologue identification and temperature dependence

Using a cryogenic cell and a series of Distributed Feed Back (DFB) diode lasers, new high resolution spectra of methane have been recorded at 80 K and room temperature by differential absorption spectroscopy (DAS) between 6717 and 7589 cm −1 (1.49–1.32 μm). The investigated spectral region corresponds to the very congested icosad, which is not tractable by theory. Empirical lists of 19,940 and 24,001 lines were constructed from the 80 K and room temperature spectrum, respectively. The room temperature list adds about 8500 features to the empirical list of 15,375 lines at 296 K adopted in the HITRAN database from the original work of L. Brown (Brown, L. Empirical line parameters of methane from 1.1 to 2.1 μm. JQSRT 2005;96:251–70). A number of relatively strong CH 4 lines located near strong water lines were found missing in the HITRAN line list. The improved sensitivity allowed adding more than 7000 lines to our previous list of about 12,000 transitions at 80 K (Campargue A, Wang L, Kassi S, Mašát M, Votava O. Temperature dependence of the absorption spectrum of CH 4 by high resolution spectroscopy at 81 K: (II) The Icosad region (1.49–1.30 μm). JQSRT 2010;111:1141–51). In order to facilitate identification of the transitions of the different methane isotopologues present in “natural” isotopic abundance, spectra of highly enriched CH 3D and 13CH 4 samples were recorded with the same experimental setup, both at room temperature and at 80 K. From the variation of the line strengths between 80 K and 294 K, the low energy values of about 12,000 transitions were determined. They allow accounting for the temperature dependence of 84 and 93% of the methane absorbance in the region, at room temperature and 80 K, respectively. As a result, we provide as supplementary material two complete line lists at 80 K and 294 K, including CH 3D and 13CH 4 identification and lower state energy values.

A PHYSICS-BASED TUNNELING MODEL FOR SB-HETEROSTRUCTURE BACKWARD TUNNEL DIODE MILLIMETER-WAVE DETECTORS

InAs / AlSb / GaSb heterostructure backward tunnel diodes are sensitive millimeter-wave detectors that achieve their high sensitivity by leveraging interband tunneling current in a broken-gap heterostructure. A new physics-based tunneling model that accurately reproduces experimental results is proposed in this work. This model, which uses Kane's eight-band k · p band structures and tunneling coefficients calculated using the transfer matrix method, predicts IV curves that closely match measured data. It also predicts critical device parameters for detector performance (including curvature coefficient and junction resistance) as a function of device structure, and good agreement with experiment results has been obtained. It accurately predicts the junction resistance's exponential dependence on tunnel barrier thickness, as well as more subtle effects such as the impact of cathode doping profile adjustment and anode ternary composition. The predictive capabilities of this model are promising to provide guidance for future detector heterostructure optimization and may be useful in modeling the characteristics of other heterostructure designs and devices using 6.1 Å lattice constant materials.

A Double-Heterojunction Bipolar Transistor Having a Degenerately Doped Emitter and Backward-Diode Base Contact

A double-heterojunction bipolar transistor having a degenerately doped emitter layer is investigated. The base-emitter Esaki diode introduces very low input impedance in the off- state but does not degrade the current gain at high forward bias. The heavily doped emitter layer makes it possible to scale the emitter-layer thickness considerably. The heavily doped emitter layer also allows contacting the base via the base-emitter backward diode. The high-frequency performance of the device is presented.

Sub-Micron Area Heterojunction Backward Diode Millimeter-Wave Detectors With 0.18 ${\rm pW/Hz}^{1/2}$ Noise Equivalent Power

InAs/AlSb/AlGaSb heterojunction backward diodes are promising detectors for millimeter-wave imaging applications due to their high sensitivity, low noise, and high cutoff frequency. By using a device heterostructure with a thin (11 Å) barrier layer, δ-doped cathode, and optimized AlxGa1-xSb anode composition (x=12%), in conjunction with submicron (0.4×0.4 μm2) active area, fabricated detectors have demonstrated DC curvatures of -47 V-1 and record unmatched sensitivities of 4600 V/W at 94 GHz. Impedance-matched sensitivities of 49,700 V/W at 94 GHz are projected from on-wafer S-parameter and sensitivity measurements. These detectors have measured junction resistances of 814 Ω·μm2 and capacitances of 15 fF/μm2. A record low NEPmin of 0.18 pW/Hz1/2 has been projected under conjugate matching conditions. This study demonstrates the potential of Sb-heterostructure backward diodes as ultra-low-noise millimeter-wave direct detectors.

Subwavelength electromagnetic diode: One-way response of cascading nonlinear meta-atoms

We propose a scheme for subwavelength electromagnetic diode by employing cascading nonlinear meta-atoms. One-way response is conceptually demonstrated on a microwave transmission line comprising of three metallic ring resonators acting as meta-atoms and a varactor as the nonlinear medium inclusion. Experiments show that our implementation can operate simultaneously as forward diode and backward diode at different frequencies. A transmission contrast of up to 14.7 dB was achieved between forward and backward transmission. Subwavelength size of our diode should be useful for miniaturization of integrated optical nanocircuits.

A Fully Tunable Single-Walled Carbon Nanotube Diode

We demonstrate a fully tunable diode structure utilizing a fully suspended single-walled carbon nanotube. The diode's turn-on voltage under forward bias can be continuously tuned up to 4.3 V by controlling gate voltages, which is ∼6 times the nanotube band gap energy. Furthermore, the same device design can be configured into a backward diode by tuning the band-to-band tunneling current with gate voltages. A nanotube backward diode is demonstrated for the first time with nonlinearity exceeding the ideal diode. These results suggest that a tunable nanotube diode can be a unique building block for developing next generation programmable nanoelectronic logic and integrated circuits.

Quantum Dot Size Dependent J−V Characteristics in Heterojunction ZnO/PbS Quantum Dot Solar Cells

The current-voltage (J-V) characteristics of ZnO/PbS quantum dot (QD) solar cells show a QD size-dependent behavior resulting from a Schottky junction that forms at the back metal electrode opposing the desirable diode formed between the ZnO and PbS QD layers. We study a QD size-dependent roll-over effect that refers to the saturation of photocurrent in forward bias and crossover effect which occurs when the light and dark J-V curves intersect. We model the J-V characteristics with a main diode formed between the n-type ZnO nanocrystal (NC) layer and p-type PbS QD layer in series with a leaky Schottky-diode formed between PbS QD layer and metal contact. We show how the characteristics of the two diodes depend on QD size, metal work function, and PbS QD layer thickness, and we discuss how the presence of the back diode complicates finding an optimal layer thickness. Finally, we present Kelvin probe measurements to determine the Fermi level of the QD layers and discuss band alignment, Fermi-level pinning, and the V(oc) within these devices.

A Degenerately Doped $\hbox{In}_{0.53}\hbox{Ga}_{0.47} \hbox{As}$ Bipolar Junction Transistor

An InGaAs bipolar junction transistor having degenerately doped base and emitter layers is reported. The high emitter efficiency is attributed to the asymmetry between the density of states of the conduction and valence bands. A high-frequency transistor having base and emitter metals simultaneously deposited on the emitter layer is demonstrated. The base contact backward diode resistance was 125 Ω · μm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> .

Energy Band Control of GaAsSb-Based Backward Diodes to Improve Sensitivity of Millimeter-Wave Detection

The effect of energy band control with varying carrier concentration in GaAsSb-based backward diodes was investigated and diode parameters were analyzed to enhance voltage sensitivity. The backward diodes consisted of a heterojunction of p-GaAs0.51Sb0.49/i-In0.52Al0.48As/n-In0.63Ga0.37As/n-In0.53Ga0.47As, and they were mostly lattice-matched to an InP substrate. The degree of energy band bending at the depletion layer in n-In0.63Ga0.37As was varied on the basis of the carrier concentration in the n-In0.53Ga0.47As band control layer. Voltage sensitivity depends on carrier concentration since the concentration affects the band bending structure at the junction. The parameter analysis indicated that junction capacitance decreased when the carrier concentration in the band control layer decreased. When the carrier concentration in the band control layer was as low as 5×1018 cm-3, with the diode mesa at a diameter of 2.0 µm, an unmatched voltage sensitivity of 1495 V/W was obtained.

A Novel Approach for Improving Off-Axis Pixel Performance of Terahertz Focal Plane Arrays

We present an antenna engineering approach to significantly improve off-axis detection properties of terahertz focal plane arrays (FPAs) behind an extended-hemispherical lens. In particular, antennas in the off-axis pixels in a large FPA were redesigned to exhibit tilted receiving patterns. The new antenna structure corrects for beam distortions in off-axis sensors due to the small lens size. Specifically, this approach is well suited for monolithic antenna structures integrated with high-sensitivity heterostructure backward diodes. Although the approach is demonstrated for 100- and 500-GHz FPAs, the presented technique is equally applicable to other planar antenna topologies and sensor materials that operate well into the terahertz regime. In addition, standard RF impedance-matching techniques were used to improve responsivity performance of the diode-coupled FPAs. The beam-correction techniques allow for a factor-of-5 larger size FPA for a fixed lens size. We also note that a tradeoff between Gaussicity and directivity becomes necessary for larger lenses. Thus, increasing the size of the FPA, without resorting to increasing the lens size, enables improved coupling efficiencies (Gaussicity) as well as better pixel resolution (directivity).

Determination of the low energy values of 13CH 4 transitions in the 2 ν3 region near 1.66 μm from absorption spectra at 296 and 81 K

The high resolution absorption spectra of 13CH 4 were recorded at 81 K by differential absorption spectroscopy using a cryogenic cell and a series of distributed feed back (DFB) diode lasers and at room temperature by Fourier transform spectroscopy. The investigated spectral region corresponds to the high energy part of the 13CH 4 tetradecad dominated by the 2 ν 3 overtone near 5988 cm −1. Empirical line lists were constructed containing, respectively, 1629 13CH 4 transitions detected at 81 K (5852–6124 cm −1) and 3481 features (including 85 lines of 12CH 4) measured at room temperature (5850–6150 cm −1); the smallest measured intensities are about 3 × 10 −26 and 4 × 10 −25 cm/molecule at 81 and 296 K, respectively. The lower state energy values were derived for 1196 13CH 4 transitions from the variation of the line intensities between 81 and 296 K. These transitions represent 99.2% and 84.6% of the total absorbance in the region, at 81 and 296 K, respectively. Over 400 additional weak features were measured at 81 K and could not be matched to lines observed at room temperature. The quality of the resulting empirical low energy values is demonstrated by the excellent agreement with the already-assigned transitions and the clear propensity of the empirical low J values to be close to integers. The two line lists at 81 and at 296 K provided as Supplementary material will enable future theoretical analyses of the upper 13CH 4 tetradecad.

High sensitivity absorption spectroscopy of methane at 80K in the 1.58μm transparency window: Temperature dependence and importance of the CH3D contribution

Abstract The high resolution absorption spectrum of methane in the 1.58 μm transparency window has been recorded at room temperature and at 79 K by CW-Cavity Ring Down Spectroscopy using a cryogenic cell and a series of Distributed Feed Back (DFB) diode lasers. The achieved sensitivity ( α min ∼ 3 × 10 −10 cm −1 ) has allowed for a detailed characterization of the 6289–6526 cm −1 region which corresponds to the lowest opacity of the transparency window. A list of 6868 and 4555 transitions with intensities as weak as 1 × 10 −29 cm/molecule was constructed from the recordings at 297 and 79 K, respectively. By comparison with a spectrum of CH 3 D recorded separately by Fourier Transform Spectroscopy, 1282 and 640 transitions of monodeuterated methane, CH 3 D, in natural abundance in our sample were identified at 297 and 79 K, respectively. The rotational temperature determined from the intensity distribution of the 3 ν 2 band of CH 3 D (79.3 K) was found in good agreement with the temperature value previously obtained from the Doppler line broadening. The reduction of the rotational congestion by cooling down to 79 K reveals a spectral region near 6300 cm −1 where CH 3 D transitions are dominant. The low energy values of the transitions observed both at 79 K and at room temperature were derived from the variation of their line intensities. These transitions with lower energy determination represent 93.9% and 68.4% of the total absorbance in the region, at 79 K and room temperature, respectively. The quality of the obtained empirical low energy values is demonstrated for CH 4 by the marked propensity of the empirical low J values to be close to integers. The line lists at 79 K and room temperature provided as Supplementary Material allow accounting for the temperature dependence of methane absorption between these two temperatures. The investigated region covering the 5 ν 4 band of the 12 CH 4 isotopologue will be valuable for the theoretical treatment of this band which is the lowest energy band of the icosad.

Temperature dependence of the absorption spectrum of CH 4 by high resolution spectroscopy at 81 K: (II) The icosad region (1.49–1.30 μm)

The high resolution absorption spectrum of methane has been recorded at liquid nitrogen temperature by differential absorption spectroscopy between 6717 and 7351 cm −1 (1.49–1.36 μm) using a cryogenic cell and a series of distributed feed back (DFB) diode lasers. The investigated spectral region corresponds to the very congested low energy part of the icosad for which the HITRAN database provides neither rovibrational assignments nor the lower state energies. The positions and strengths at 81 K of 9389 transitions were obtained from the spectrum analysis. The minimum value of the measured line intensities (at 81 K) is on the order of 10 −26 cm/molecule. From the variation of the line strength between 81 K and 296 K, the low energy values of a total of 4646 transitions were determined. They represent 79.4% and 68.4% of the total absorbance in the region at 81 and 296 K, respectively, and include 28 transitions assigned to the ν 2+4 ν 4 band near 6765 cm −1. The reliability of the method based on the association of lines with coinciding centers in the 81 K and 296 K spectra is discussed. The results of the present analysis have been combined with previously analyzed high energy part of the icosad dominated by the ν 2+2 ν 3 band near 7510 cm −1. The line list for the whole icosad (6717–7655 cm −1) consists of 12 865 transitions at 81 K.

Temperature dependence of the absorption spectrum of CH 4 by high resolution spectroscopy at 81 K: (I) The region of the 2ν 3 band at 1.66 μm

In a recent contribution, (Gao B, Kassi S, Campargue A. Empirical low energy values for methane transitions in the 5852–6181 cm −1 region by absorption spectroscopy at 81 K. J Mol Spectrosc 2009;253:55–63.), the low energy values of methane transitions between 1.71 and 1.62 μm were derived from the variation of the line intensities between 296 and 81 K. The line intensities at 81 K were retrieved from the high resolution absorption spectrum of methane recorded at liquid nitrogen temperature by direct absorption spectroscopy using a cryogenic cell and a series of distributed feed back (DFB) diode lasers. For the line intensities at 296 K, the values provided by the HITRAN database were used. As a consequence of the relatively high intensity cut off (4×10 −24 cm/molecule) of the HITRAN line list in the considered region, the lower energy values were derived for only 845 of the 2187 transitions measured at 81 K. In the present work, our line list was extended by the retrieval of many weak line intensities leading to a set of 3251 transitions. The minimum value of the measured line intensities (at 81 K) is on the order of 10 −26 cm/molecule. In relation with the project “Greenhouse Gases Observing Satellite” (GOSAT), a much more complete line list for CH 4 at 296 K has become available (intensity cut off of 4×10 −26 cm/molecule). By applying the two temperature method to our line intensities at 81 K and GOSAT intensities at 296 K, the lower energy values of 2297 transitions could be derived. These transitions represent 99.1% and 90.8% of the total absorbance in the region, at 81 and 296 K respectively. This line list provided as Supplementary Material allows then accounting for the temperature dependence of CH 4 absorption below 300 K. The investigated spectral range is dominated by the 2ν 3 band near 6005 cm −1 which is of particular interest for atmospheric retrievals. The factor 2 narrowing of the Doppler linewidth from room temperature down to 81 K has allowed the resolution of a number of 2ν 3 multiplets and improving the line intensity retrievals. A detailed comparison with GOSAT and HITRAN line lists has revealed a number of possible improvements.

GaN/ZnO nanorod light emitting diodes with different emission spectra

Light emitting diodes (LEDs) consisting of p-GaN epitaxial films and n-ZnOnanorods have been fabricated and characterized. The rectifying behavior andemission spectra were strongly dependent on the electronic properties of both GaNfilm and ZnO nanorods. Light emission under both forward and reverse bias wasobtained in all cases, and emission spectra could be changed by annealing theZnO nanorods. The emission spectra could be further tuned by using a GaNLED epiwafer as a substrate. Both forward and backward diode behavior hasbeen observed and the emission spectra were significantly affected by both theproperties of the GaN substrate and the annealing conditions for the ZnO nanorods.

Experimental low energy values of CH 4 transitions near 1.33 μm by absorption spectroscopy at 81 K

The high resolution absorption spectrum of methane has been recorded at liquid nitrogen temperature by direct absorption spectroscopy between 1.36 and 1.30 μm (7351–7655 cm −1) using a cryogenic cell and a series of distributed feed back (DFB) diode lasers. The investigated spectral range corresponds to the high energy part of the icosad dominated by the ν 2+2 ν 3 band near 7510 cm −1. The positions and strengths at 81 K of 3473 transitions were obtained from the spectrum analysis. The minimum value of the measured line intensities (at 81 K) is on the order of 10 −26 cm/molecule, i.e. significantly lower than the intensity cut off of the HITRAN database in the region (4×10 −25 cm/molecule at 296 K). From the variation of the line strength between 81 and 296 K, the low energy values of 1273 transitions could be determined. They represent 69% and 81% of the absorbance in the region at 296 and 81 K, respectively. The obtained results are discussed in relation with the few rovibrational assignments previously reported in the region.

GaAsSb-based backward diodes for highly sensitive millimetre-wave detectors

Highly sensitive millimetre-wave detectors based on p-GaAs0.51Sb0.49/i-In0.52Al0.48As/n-InxGa1-xAs backward diodes under zero-bias operation were developed on an InP substrate. Voltage sensitivity of 12300 V/W at 94 GHz was achieved with the GaAsSb-based diode which has a circular mesa with a diameter of 0.9 µm. To improve the sensitivity, the indium composition of the n-InxGa1-xAs layer and thickness of the i-In0.52Al0.48As layer were optimised. Furthermore, a self-aligned process was performed to minimise parasitic resistance at an ohmic contact layer. These GaAsSb-based diodes that are lattice-matched to InP are promising for easy integration with high-performance InP low-noise amplifiers.

Key role of Cu–Se binary phases in electrodeposited CuInSe 2 precursors on final distribution of Cu–S phases in CuIn(S,Se) 2 absorbers

Using micro-Raman spectroscopy, we demonstrate that the formation of the CuS binary phase in CISEL™ cells absorbers is highly determined by the presence of a Cu–Se binary phase in the precursors and depends on the Cu/In ratio. A selective sulphurization mechanism of the Cu–Se phase is proposed as the origin of CuS platelets. For low Cu/In ratio both binaries are associated to the surface and do not affect the device performance. Conversely, when the binary phase in the precursor is present close to the back contact (for high Cu/In ratio), the CuS binary phase is also formed at this region after sulphurization. Strongly associated to this Cu-rich binary phase, a spinel-type phase is also observed in the sulphurized samples. This phase has n-type conductivity, which has a strong impact on the characteristics of the solar cells. The relationship between the presence of these phases and the electrical properties of the final devices is described, showing that the reduction of the V oc and the increase of the R s are related to the formation of a back diode by the spinel-type layer, as consequence of an undesirable n-p-n structure.