Identical Layer Structure Research Articles

Electro-absorption modulated 53 Gbps widely tunable laser based on half-wave V-coupled cavities.

In this paper, a 53 Gbps widely tunable transmitter is experimentally demonstrated for the first time, to our knowledge. An InGaAlAs/InP multiple-quantum-well (MQW) wafer is used with an identical layer structure for both the V-coupled cavity laser (VCL) and the electro-absorption modulator (EAM). The VCL uses a shallow-etched waveguide to reduce loss, while the EAM uses a deep-etched waveguide to increase the 3-dB modulation bandwidth. With the temperature varying from 19.5 to 30°C, the transmitter achieves wavelength tuning of 42 channels with a spacing of 100 GHz, corresponding to a tuning range of 32.6 nm from 1538.94 to 1571.54 nm. The static extinction ratio (ER) for all channels is higher than 14 dB. The measured 3-dB electro-optic (E0) bandwidth of the transmitter is over 40 GHz, which fits well with the calculated 3-dB bandwidth. At a fixed peak-to-peak driving voltage of 2.4 V, all channels exhibit clearly an open eye diagram with a 53 Gbps non-return-to-zero (NRZ) signal, while the dynamic ER is higher than 4.5 dB.

Evaluation of Pulsed I–V Analysis as Validation Tool of Nonlinear RF Models of GaN-Based HFETs

This paper evaluates the applicability of pulsed I – V measurements as a tool for accurately extracting nonlinear gallium nitride (GaN)-based heterojunction field-effect transistor (HFET) models. Two wafers with the identical layer structure but different growth conditions have been investigated. A series of I – V measurements was performed under dc and pulsed conditions demonstrating a dramatic difference in the kink effect and current collapse (knee walkout) suggesting different trapping behaviors. However, when radio frequency (RF) I – V waveform measurements, utilizing active harmonic load–pull, were used to study the impact of these traps on the RF performance, both wafers gave good overall RF performance with no significant difference observed. This absence of correlation between pulsed I – V measurement results and RF performance raises a question about the applicability of pulsed I – V measurements alone as a tool for extracting nonlinear device models in the case of GaN HFETs.

3,4-Lutidinium salts with the diiodidoaurate(I) anion: structure of [(3,4-lut)2H]+·[AuI2]−and of two polymorphs of [(3,4-lut)2H+]2·[AuI2]−·I−

Reactions between potassium tetraiodidoaurate(III) and pyridine (py, C5H5N) or 3,4-lutidine (3,4-dimethylpyridine, 3,4-lut, C7H9N) were tested as possible sources of azaaromatic complexes of gold(III) iodide, but all identifiable products contained gold(I). The previously known structure dipyridinegold(I) diiodidoaurate(I), [Au(py)2]+·[AuI2]-, (3) [Adams et al. (1982). Z. Anorg. Allg. Chem. 485, 81-91], was redetermined at 100 K. The reactions with 3,4-lutidine gave three different types of crystal in small quantities. 3,4-Dimethylpyridine-3,4-dimethylpyridinium diiodidoaurate(I), [(3,4-lut)2H]+·[AuI2]-, (1), consists of an [AuI2]- anion on a general position and two [(3,4-lut)2H]+ cations across twofold axes. Bis(3,4-dimethylpyridine-3,4-dimethylpyridinium) diiodidoaurate(I) iodide, [(3,4-lut)2H+]2·[AuI2]-·I-, (2), crystallizes as two polymorphs, each forming pseudosymmetric inversion twins, in the space groups P21 and Pc (but resembling P21/m and P2/c), respectively. These are essentially identical layer structures differing only in their stacking patterns and thus might be regarded as polytypes.

Reverse Gate-Current of AlGaN/GaN HFETs: Evidence of Leakage at Mesa Sidewalls

Reverse gate-current of AlGaN/GaN heterostructure field-effect transistor is studied over a wide range of lattice-temperatures from 150 to 490 K. For gate-source voltages approaching zero, volt signatures of gate-to-2-D electron gas leakage through the sidewalls of the mesa are observed. For exploring this leakage path, a set of devices built on a number of alternative isolation features of different geometries, and with different number of gate-covered sidewalls, are investigated. Among these devices, which were realized on an identical layer structure (produced following the same fabrication technology with identical gate-length and -width), as the number of isolation-feature sidewalls overlapping with the gate metal increases, an increase in the gate-current is observed. The identified sidewall leakage is not only consequential in devices built on isolation-feature geometries presenting more than two sidewalls, but it can also compete with mechanisms such as Poole–Frenkel (PF), Fowler–Nordheim, and trap-assisted tunneling (TAT), which are traditionally considered in devices built on cubic mesas. In this paper, the relevance of these other transport mechanisms is also re-evaluated. It is observed that at temperatures below 320 K, among all of the explored devices, sidewall leakage becomes more dominant than the PF and trap-assisted tunneling (TAT) processes.

Growth, steady-state, and time-resolved photoluminescence study of CdTe/MgCdTe double heterostructures on InSb substrates using molecular beam epitaxy

CdTe/MgCdTe double heterostructures (DHs) are grown on InSb substrates using molecular beam epitaxy and reveal strong photoluminescence with over double the intensity of a GaAs/AlGaAs DH with an identical layer structure design grown on GaAs. Time-resolved photoluminescence of the CdTe/MgCdTe DH gives a Shockley-Read-Hall recombination lifetime of 86 ns, which is more than one order of magnitude longer than that of typical polycrystalline CdTe films. These findings indicate that monocrystalline CdTe/MgCdTe DHs effectively reduce surface recombination, have limited nonradiative interface recombination, and are promising for solar cells that could reach power conversion efficiencies similar to that of GaAs.

Analysis of the slope efficiency for terahertz quantum-cascade lasers

The slope efficiency is, in addition to the threshold current density and operating voltage, a decisive target value for the operation of quantum-cascade lasers (QCLs) in order to achieve an optimal total efficiency for the conversion of electrical input power into optical output power. We analyze the light-current characteristics for a set of similar, high-power, bound-to-continuum terahertz QCLs. The calculated internal slope efficiency shows a clear dependence on the height and thickness of the barriers. In contrast to the simulations, we found experimentally a significant difference in the threshold current densities and slope efficiencies for QCLs with nominally identical layer structures, which we mainly attribute to different line broadening.

Interface reactions and control of diffusion at the interface between SiC fibres and layer of deposited Fe–9Cr base alloy

The ferromagnetic 9Cr steel EUROFER 97 is considered as a structural material for the first wall of future fusion reactors. To solve a problem of limitation of the maximum operation temperature of 550 °C for EUROFER 97, a new metal matrix composite SiC/EUROFER 97 is proposed. To create a thermally stable interface between the SiC fibres and the steel matrix, the following interlayers were tested as a diffusion barrier: Cr, Re, W and Re/W. Samples with these interlayers were investigated before and after annealing at 750 °C which is supposed to be the operation temperature of the composite material. To simplify examination of the coatings using available analysis methods (SEM, IBA and XRD), the fibre/steel system was approximated by planar samples with identical layer structure. The experiments with both SiC fibres and planar substrates revealed W to be an efficient diffusion barrier.

Comparative Studies on Double δ -Doped Al0.3Ga0.7As ∕ InxGa1 − xAs ∕ GaAs Symmetrically Graded Doped-Channel Field-Effect Transistors

This work provides comparative studies of a double δ-doped symmetrically graded doped-channel field-effect transistor (DD-DCFET) with respect to a conventional double δ-doped pseudomorphic high electron mobility transistor (pHEMT) and a conventional DCFET structure. All threes samples, grown by the low-pressure metallorganic chemical vapor deposition (LP-MOCVD) system, have identical layer structures except for their different doping schemes. Comprehensive investigations on the static, microwave, and temperature-dependent characteristics have been made. Possessing the advantages of DCFETs and pHEMTs, the proposed DD-DCFET has demonstrated comprehensively superior linearity, current drive, voltage gain, high-frequency characteristics, and thermal stability characteristics. It is promisingly suitable for millimeter-wave integrated circuit applications.

Performance study of SQW GaInNAs Fabry‐Perot lasers

AbstractThe work presented here is concerned with studies of the operational characteristics of single‐quantum‐well GaInNAs/GaAs Fabry‐Perot lasers, where identical layer structures with differing cavity lengths are evaluated both experimentally and theoretically. Standard techniques are used to determine the temperature dependence, in the pulsed mode, of I‐V, L‐I and spectral electroluminescence. Important parameters, such as the threshold current and T0, are determined and tabulated for all the structures. The differential and internal quantum efficiency and hence the internal losses for each of the lasers are obtained from the experimental data. The latter is compared with the gain spectra measured using the Hakki‐Paoli method and calculated from a simple theory. From the spectral measurements above the lasing threshold, we obtain the temperature dependence of the GaInNAs quantum well effective gap as 0.3 meV/K, which is in good agreement with results published by other groups. However, a better fit is obtained using the Varshni temperature dependence using values of E0 = 1.058 eV, α = 4.8 × 10–4 eVK‐2, β = 284 ± 167 K. To put our results in perspective, we also present an up‐to‐date review of similar SQW lasers and suggest improvements to the current structure for enhanced threshold and power characteristics. (© 2007 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Atomic hydrogen-assisted molecular beam epitaxy for the fabrication of multi-quantum-well solar cells

The atomic hydrogen-assisted molecular beam epitaxy (H-MBE) technique has been applied to fabricate multi-quantum-well (MQW) based compound semiconductor solar cells. A number of different InGaAs/GaAs MQW structures were inserted within the photon absorption region of a p–i–n GaAs junction diode. The spectral response characteristics of each device with varying number, thickness, and In composition of MQWs were measured and analyzed to investigate the effects of atomic H on the device performance. In the photon wavelength region of 800–1000 nm, the spectral response of H-MBE-grown cells was increased by >25% for the GaAs homojunction control cell, and by as much as ∼35% for a 10-period MQW cell (In0.07Ga0.93As: GaAs=50 nm:50 nm) as compared to the conventionally MBE-grown cells with identical layer structures.

Influence of the undoped spacer layer thickness on the DC characteristics of n-type GaAs/AlAs MESFETs

We have investigated the influence of the thickness of an undoped spacer layer under the channel on the current-voltage characteristics of GaAs MESFETs. To that purpose we have grown the active layers of the MESFETs on top of a 50 nm AlAs layer to separate the device effectively from the undoped GaAs substrate. Substrate leakage currents are of the order of A at 30 V source-substrate bias. DC characteristics are measured for four MESFETs with identical layer structure and doping but different spacer layer thicknesses. Comparison shows that MESFETs with thin spacer layers can be useful for low-noise applications but lose in transconductance and mobility values.

Pixels consisting of a single vertical-cavity laser thyristor and a double vertical-cavity phototransistor

Bidirectionally cascadable optical pixels that consist of a single-vertical-cavity surface-emitting laser (VCSEL) thyristor and a double-vertical-cavity phototransistor are proposed. Despite almost identical layer structures, each device characteristic can be independently optimized by introducing a lambda /2-spacer layer into the phototransistor section. A lasing threshold of 0.8 mA and a slope efficiency of 0.25 W/A are obtained for the laser thyristor, and a flat-topped photocurrent spectrum over 30 A and a photocurrent gain of 70 A/W are obtained for the phototransistor at the resonant wavelength. This work demonstrates the possibility of monolithic integration using thermal desorption and a regrowth technique and the suitability of these devices for massively parallel optical interconnections.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>

Compounds containing layers of composition [(Sb IIIF) XO 4] ( X = P, As). Crystal structures of Na(SbF)PO 4 · 1.5H 2O, Na(SbF)AsO 4, NH 4(SbF)PO 4 · H 2O, and NH 4(SbF)AsO 4 · 3H 2O

The title compounds have been prepared in water by reaction of SbF 3 with dihydrogen phosphates or arsenates and characterized by single crystal X-ray work, IR, Raman, and Mössbauer spectroscopy. They have identical layer structures. Layers of composition [(SbF) XO 4] − ( X = P, As) were formed by sharing four corners between XO 4 tetrahedra and SbFO 4 pseudooctahedra. The lengths of the terminal SbF bond (with the lone pair in a trans-position) and the SbO bonds are 192 and 219 pm, respectively. The stacking of the layers and the interlayer distance depend on the cations and the number of intercalated water molecules. In Na(SbF)AsO 4 the Na + ion is coordinated by only two oxygen atoms within 300 pm. Crystal data: Na(SbF)PO 4 · 5H 2O, monoclinic, P2 1 m , a = 656.2(5), b = 654.1(5), c = 867.9(3) pm , β = 92.43(1)°, 889 reflections, 81 parameters, R = 0.044, R w = 0.046. NH 4(SbF)PO 4 · H 2O, tetragonal, I4 m , a = 656.6(3), c = 1439.8(5) pm , 680 reflections, 31 parameters, R = 0.023, R w = 0.021. Na(SbF)AsO 4, tetragonal, P4 ncc , a = 671.8(1), c = 1756.4(4) pm , 1056 reflections, 28 parameters, R = 0.052, R w = 0.065. NH 4(SbF)AsO 4 · 3H 2O, tetragonal, P4 ncc , a = 683.8(2), c = 1873.0(7) pm , 1194 reflections, 30 parameters, R = 0.042, R w = 0.050.

Interpreting Substituent Effects on the Crystal Packing of Long-Chain Diacyl Peroxides. The Crystal Structures of D1 (11 Bromoundecanoyl) Peroxide and Di (Undecanoyl) Peroxide

Abstract Although crystals of di(11-bromoundecanoyl) peroxide and di(undecanoyl) peroxide have different space groups (P43212 and C2221), the molecules pack in almost identical layers. They differ only in the nature of stacking across interfaces involving the terminal groups. Because the 90° twist about the O-O bond locks neighboring molecules together within the layer, each peroxide shows a single solid phase from 5K to the melting point. Analysis of the stacking pattern in terms of the six possible orientational relationships suggests special stability for an L-shaped motif of C-Br-Br-C. Other substituents create different stackings of the same layer structure to give three crystal classes and five space groups among 14 compounds. Unsymmetrical peroxides are useful both for forcing a variety of substituted chains (particularly odd-even homologues) to pack with identical layer structures, and for controlling the stacking pattern. Because structural differences are localized in the vicinity of the substituents, this series of “substitutional polytypes” will allow systematic investigation of substituent effects on the physical and chemical properties of solids.

On the structure of the biaxial and uniaxial smectic-B-type phases of 1̄4̄S5, and the apparently second-order phase transition between them

The biaxial smectic-B-type phase of 4-n-pentylbenzenethio-4′-n-tetradecyloxybenzoate (1̄4̄S5) has been studied along with its uniaxial smectic-l B phase. The results are interpreted in terms of identical layer structures in both phases (with the molecules tilted with respect to the layer normal) but with different kinds of disorder (within the layers and between the layers). The transition between the two phases appears to be second order.