Pulsed Operating Conditions Research Articles

Low-threshold visible InP quantum dot and InGaP quantum well lasers grown by molecular beam epitaxy

III-V lasers based on self-assembled quantum dots (QDs) have attracted widespread interest due to their unique characteristics, including low threshold current density (Jth), low sensitivity to backreflections, and resistance to threading dislocations. While most work to date has focused on 1.3 μm InAs/GaAs QDs, InP QDs have also aroused interest in lasers emitting at visible wavelengths. Molecular beam epitaxy (MBE) enables the growth of high-density InP/AlGaInP QDs on exact (001)-oriented GaAs substrates but requires a relatively low substrate temperature of <500 °C. The low substrate temperature used for phosphide growth in MBE leads to degraded optical properties and makes post-growth annealing a crucial step to improve the optical quality. Here, we report the exceptional thermal stability of InP/AlGaInP QDs grown using MBE, with up to 50× improvement in room temperature photoluminescence intensity with the optimization of annealing temperature and time. We also demonstrate the room temperature pulsed operation of InP multiple quantum dot (MQD) lasers on GaAs (001) emitting close to 735 nm with Jth values of 499 A/cm2 after annealing, a factor of 6 lower than their as-grown counterparts and comparable to such devices grown by MOCVD. In0.6Ga0.4P single quantum well (SQW) lasers on GaAs (001) also exhibit a substantial reduction in Jth from 340 A/cm2 as-grown to 200 A/cm2 after annealing, emitting at 680 nm under pulsed operation conditions. This work shows that post-growth annealing is essential for realizing record-performance phosphide lasers on GaAs grown by MBE for applications in visible photonics.

Heavily Ge-doped GaN as transparent current spreading layer for blue tunnel junction light emitting diodes

We report on metalorganic vapor phase epitaxy of highly conductive germanium-doped GaN layers and their application for blue tunnel-junction light emitting diodes (TJ-LEDs). Using Ge as donor, carrier densities of up to 2 × 1020 cm−3 and low bulk resistivities down to 3 × 10−4 Ωcm are achieved. Under optimum growth conditions, no degradation of the crystalline quality is observed and layers exhibit high transparency making GaN:Ge very attractive as current spreading layer in light emitting devices. We have realized GaN-based TJ-LEDs by capping conventional InGaN LED structures with highly doped GaN:Ge. Such TJ-LEDs withstand operation currents up to 20 kA/cm2 in continuous and up to 60 kA/cm2 in pulsed operation conditions. Moreover, TJ-LEDs exhibit homogeneous electroluminescence and light output through the top surface that is increased by more than 60% as compared to conventional LEDs with transparent indium tin oxide contacts. The impact of the doping profile, carrier gas conditions, and acceptor activation by annealing for low-resistive TJ characteristics is discussed. Light output and current voltage characteristics of blue-light emitting devices with GaN-TJ are presented at low and high-current densities.

Towards Optimised Cell Design of Thin Film Silicon-Based Solid-State Batteries via Modelling and Experimental Characterisation

To realise the promise of solid-state batteries, negative electrode materials exhibiting large volumetric expansions, such as Li and Si, must be used. These volume changes can cause significant mechanical stresses and strains that affect cell performance and durability, however their role and nature in SSBs are poorly understood. Here, a 2D electro-chemo-mechanical model is constructed and experimentally validated using steady-state, transient and pulsed electrochemical methods. The model geometry is taken as a representative cross-section of a non-porous, thin-film solid-state battery with an amorphous Si (a-Si) negative electrode, lithium phosphorous oxynitride (LiPON) solid electrolyte and LiCoO2 (LCO) positive electrode. A viscoplastic model is used to predict the build-up of strains and plastic deformation of a-Si as a result of (de)lithiation during cycling. A suite of electrochemical tests, including electrochemical impedance spectroscopy, the galvanostatic intermittent titration technique and hybrid pulse power characterisation are carried out to establish key parameters for model validation. The validated model is used to explore the peak interfacial (a-Si∣LiPON) stress and strain as a function of the relative electrode thickness (up to a factor of 4), revealing a peak volumetric expansion from 69% to 104% during cycling at 1C. The validation of this electro-chemo-mechanical model under load and pulsed operating conditions will aid in the cell design and optimisation of solid-state battery technologies.

Status and challenges for the concept design development of the EU DEMO Plant Electrical System

The EU DEMO Plant Electrical System (PES) main scopes are to supply all the plant electrical loads and to deliver to the Power Transmission Grid (PTG) the net electrical power generated. The studies on the PES during the Pre-Concept Design (PCD) Phase were mainly addressed to understand the possible issues, related to the special features both of the power generated, with respect to a power plant of the same size, and of the power to be supplied to the electrical loads. For this purpose, the approach was to start the design of the different PES components adopting technologies already utilized in fusion experiments and in Nuclear Power Plants (NPP) to verify their applicability and identify possible limits when scaled to the DEMO size and applied to the specific pulsed operating conditions. This work is not completed, however several issues have been already identified related to the pulsed operation of the turbine generator, the large amount of recirculation power, the very high peaks of active power required for the plasma formation and control, the huge reactive power demand, if thyristor converter technology was adopted to supply the superconducting coils, etc.. The paper gives an overview on the features and scope of the PES and its subsystems, on the main achievements during the Pre-Concept Design (PCD) Phase, on the challenges for the development of the conceptual design in the next framework program and on the plan to face them.

A time to failure evaluation of AlGaN/GaN HEMT transistors for RF applications

This paper reports an evaluation of time to failure (TTF) of GaN transistors for 5G and RADAR applications. The TTF on Arrhenius curves are extrapolated from performed RF pulsed life tests with different input powers and duty cycles. The paper explains the temperature estimation method used in this study for operating conditions of the aging tests. A degradation mechanism of surface pitting is extrapolated from previous works. With our experiments, tests can show that the supposed scaling DC to RF life tests could be now extended for duty cycles. The high reliability of HEMT AlGaN/GaN on RF pulsed operating conditions is shown for a power bar of RF transistors.

Monolithic growth of GaAs laser diodes on Si(001) by optimal AlAs nucleation with thermal cycle annealing

A GaAs quantum-well laser diode was directly grown on silicon (001) substrate by a hybrid technique comprising AlAs nucleation and thermal cycle annealing. The hybrid technique provided the advantages of superior surface roughness, high quantum efficiency, and low threading dislocation density (TDD) of a thin buffer. The TDD was quantitatively characterized through the electron channeling contrast imaging method. Directly grown GaAs on Si exhibited a TDD of 5.45 × 107 /cm2 with small thickness of approximately 1.5 µm. The roughness and quantum efficiency of GaAs on Si was enhanced by adopting the nucleation layer of AlAs. We found that there exists an optimal thickness of AlAs nucleation to be 1.68 nm through structural and optical analysis. Based on optimized GaAs on Si, the GaAs quantum-well laser diode was directly grown with a TDD of 2.5 × 107 /cm2. Whole epitaxial layers were grown by metalorganic chemical vapor deposition. An edge-emitting broad stripe laser diode was successfully fabricated with a cavity length and width of 1120 µm and 60 µm, respectively. The continuous-wave lasing at room temperature was realized with a threshold current density of 643 A/cm2 and maximum output power of 19.7 mW at a single facet, where a threshold current density of 317 A/cm2 was obtained under pulsed operation condition. This result would constitute a building block to realize silicon-based on-chip light sources.

Industrial-scale manufacturing experience of titanium beryllide block for DEMO blanket application

Abstract For the first time, the TiBe12 blocks have been manufactured by powder metallurgy using industrial equipment at the Ulba Metallurgical Plant. Such blocks will be used for neutron multiplication in the helium-cooled breeding blanket for the EU DEMO fusion reactor. The vacuum annealing of a cold-pressed composite of Be and Ti powders at 600–1275 °C approved the conditions for TiBe12 synthesis. After hot isostatic pressing at 1150 °C, a single-phase TiBe12 structure was formed, but the sample fragmented into several pieces. These beryllide pieces were used to manufacture TiBe12 powder. Vacuum hot pressing of the obtained powder resulted in a solid titanium beryllide workpieces of O150 mm × 170 mm. Electrical discharge machining and waterjet cutting were used to machine the external and internal surfaces in accordance with the design requirements. The obtained block has a density of 98.3 % of the theoretical density of TiBe12. Microstructural analysis revealed a bimodal grain size distribution with grains of 5–10 μm and 20–40 μm. Electrical conductivity and thermal expansion coefficient of TiBe12 are similar to other beryllides. To mimic the pulsed operating conditions in DEMO, a sample of titanium beryllide was rapidly heated to 900 °C and cooled with a rate of 12 K/s. After 50 cycles, the sample had no cracks or severe surface oxidation.

InAlGaN/GaN with AlGaN back-barrier HEMT technology on SiC for Ka-band applications

This paper presents performances achieved with InAlGaN/GaN HEMTs with 0.15 µm gate length on SiC substrate. Technology Computer Aided Design simulations were used to optimize the heterostructure. Special attention was paid to the design of the buffer structure. I-V measurements with DC and pulsed bias voltages were performed. CW measurements at millimeter waves were also carried out and are detailed in the following sections. The technology, optimized for power applications up to 45 GHz, demonstrates a current gain cut-off frequency FTof 70 GHz and a maximum available gain cut-off frequency FMAGof 140 GHz. CW Load-pull power measurements at 30 GHz enable to achieve a maximum PAE of 41% associated with an output power density of 3.5 W/mm when biased at VDS= 20 V. These devices, with an improved buffer structure show, reduced recovery time in pulsed operating conditions. These improved characteristics should have a positive impact for pulsed or modulated signal applications.

Pulsed characterization of a UV LED for pulsed power applications on a silicon carbide photoconductive semiconductor switch.

The electrical and optical characteristics of a high-power UV light emitting diode (LED) (365 nm wavelength) were evaluated under pulsed operating conditions at current amplitudes several orders of magnitude beyond the LED's manufacturer specifications. Geared towards triggering of photoconductive semiconductor switches (PCSSs) for pulsed power applications, measurements were made over varying pulse widths (25 ns-100 μs), current (0 A-250 A), and repetition rates (single shot-5 MHz). The LED forward voltage was observed to increase linearly with increasing current (∼3.5 V-53 V) and decrease with increasing pulse widths. The peak optical power observed was >30 W, and a maximum system efficiency of 23% was achieved. The evaluated LED and auxiliary hardware were successfully used as the optical trigger source for a 4H-SiC PCSS. The lowest measured on-resistance of SiC was approximately 67 kΩ.

Radiation Resistant Insulation Systems for the ITER Toroidal Field Coils

Insulation systems for fusion magnets have to operate under intense radiation, mainly fast neutrons and γ-radiation. At the ITER design fluence of 1x10 22 m -2 (E>0.1 MeV), conventional epoxy resins, which were used, e.g., for the ITER TF model coil, start to disintegrate. As a consequence, research programs were launched under EFDA, F4E and ITER contracts in order to find radiation harder resins as well as to investigate the properties of cyanate esters (CE) and CE/epoxy blends. The CE content was varied in the range between 100 % and 20 %. Test materials, consisting of wrapped R-glass fiber / polyimide tapes and VPI impregnated with various resins, were produced by European industry and tested at 77 K. Tensile and short beam shear tests were carried out to characterize the mechanical properties prior to and after irradiation to fast neutron fluences of up to 5x10 22 m -2 (E>0.1 MeV) in the TRIGA reactor (Vienna). In addition, tension-tension fatigue measurements were performed in the load-controlled mode to simulate the pulsed operation conditions of ITER. It turned out that these CE based insulation systems offer excellent properties at the ITER design fluence including an adequate safety margin, especially when a mixing ratio of 40% CE and 60 % epoxy resin is employed. Slight modifications of the resin, mainly regarding an extension of the pot-life to more than 100 hours, were made in view of the dimensions and the complexity of the ITER TF coils. The corresponding results confirm their excellent performance and will be reported. Radiation resistant materials have also been developed by US and Japanese Industry. The qualification of these insulation systems is currently under way and comprises 40:60 CE / epoxy blends from different suppliers (Huntsman, CTD). Test materials were fabricated under the same conditions in order to provide a comparable data base. The results will be presented and discussed.

High Efficiency InGaN Blue Light-Emitting Diode With ${>}{\rm 4}\hbox{-}{\rm W}$ Output Power at 3 A

This letter reports high-power and high-efficiency characteristics of the InGaN-based blue light-emitting diode (LED) operating at > 10-W electrical input power in a single-chip package. The LED chip is fabricated as a vertical-injection structure with chip dimensions of 1.8 mm × 1.8 mm. InGaN/GaN short-period superlattice (SL) structures are employed below multiple-quantum-well active region as current spreading layers. It is found, by simulation, that SL layers are quite effective in improving current spreading and uniformity in carrier distribution. When the characteristics of the fabricated LED package are measured under pulsed operation conditions, efficiency droop is found to be greatly reduced in the LED structure with SL layers. A record high light output power of 4.18 W and external quantum efficiency of 51% are demonstrated at 3-A injection current.

Structural Analysis for EHF Hypervapotron Twin Fingers and Beryllium Tile Size Study

The impact of Be tile size on the stress exerted on the CuCrZr heat sink for the ITER EHF finger was examined. The study especially focused on the areas beneath the tiles that are exposed to the high convective heat flux. For reference, in a Be tile size of 50x50x8 mm3, the calculated equivalent strain range using elastic analysis for the path of interest through the side wall of the CuCrZr heat sink resulted in a peak value at the inner wall of ~0.492%. The corresponding fatigue lifetime of the heat sink locally is unacceptably low, 1400 cyclic operations. By using smaller tiles, lower stress amplitudes are observed due to a smaller deformation. In this paper, the total strain range under ITER projected pulsed operating conditions is analyzed for a range of Be tile sizes. The analysis model uses a complete pair of twin fingers as opposed to a sub-model of two tiles. The paper documents the calculated cyclic lifetime of the ITER EHF CuCrZr heat sink with respect to Be tile size and peak heat loads by evaluating the total strain range both from elastic and time independent elasto-plastic analyses for repeated cycle.

Simulation of thermal management in AlGaN/GaN HEMTs with integrated diamond heat spreaders

We investigated the impact of diamond heat spreading layers on the performance of AlGaN/GaN high-electron-mobility-transistors (HEMTs). A finite element method was used to simulate the thermal and electrical characteristics of the devices under dc and pulsed operation conditions. The results show that the device performance can be improved significantly by optimized heat spreading, an effect strongly dependent on the lateral thermal conductivity of the initial several micrometers of diamond deposition. Of crucial importance is the proximity of the diamond layer to the heat source, which makes this method advantageous over other thermal management procedures, especially for the device in pulsed operation. In this case, the self-heating effect can be suppressed, and it is not affected by either the substrate or its thermal boundary resistance at the GaN/substrate at wider pulses. The device with a 5 µm diamond layer can present 10.5% improvement of drain current, and the self-heating effect can be neglected for a 100 ns pulse width at 1 V gate and 20 V drain voltage.

Ultrawide-bandwidth, superluminescent light-emitting diodes using InAs quantum dots of tuned height

An ultrawide-bandwidth, superluminescent light-emitting diode (SLED) utilizing multiple layers of dots of tuned height is reported. Due to thermal effect, the superluminescent phenomenon is observed only under pulse-mode operation. The device exhibits a 3 dB bandwidth of 190 nm with central wavelength of 1020 nm under continuous-wave (cw) conditions. The maximum corresponding output power achieved in this device under cw and pulsed operation conditions are 0.54 mW and 17 mW, respectively.

High-power single-chip InGaN blue light-emitting diode with 3.3 W output power

The characteristics of a high-power single-chip blue light-emitting diode (LED) operating with >10 W input power are reported. The LED chip was fabricated in the form of a vertical-injection structure with chip dimensions of 1.8×1.8 mm. Electrode patterns at the n-GaN surface were designed to optimise the output power and operation voltage. Electrical and optical characteristics of the LED were measured up to 3 A injection current under pulsed operation condition. Output power and forward voltage at 3 A were obtained to be 3.3 W and 3.67 V, respectively, which demonstrates the wall-plug efficiency of >30% with 10 W input power. Even higher output power and efficiency are expected from the single-chip LED by reducing the efficiency droop of InGaN-based LEDs.

Characterization of advanced cyanate ester/epoxy insulation systems before and after reactor irradiation

Insulation systems for fusion magnets have to operate in a harsh environment, especially also under intense radiation. Over the past years, cyanate ester (CE) resins have been playing an increasingly important role because of their enhanced temperature and radiation resistance compared to conventional epoxy resins. Blending CE with epoxy resins offers the possibility to manufacture radiation resistant insulations at a low price compared to pure CE materials. Therefore, it is of special interest to study the influence of the CE content and of the epoxy resin on the mechanical properties to find materials, which are suitable and economically reasonable for the specific demands of such magnets. In this study R-glass fiber/Kapton reinforced cyanate ester/epoxy blends with different CE content were investigated. Each material was exposed to conditions matching those expected for the ITER TF coil insulation as closely as possible. In order to characterize the mechanical properties, short-beam shear and static tensile tests were carried out at 77 K prior to and after irradiation to fast neutron fluences of up to 5 × 10 22 m −2 ( E > 0.1 MeV), in the TRIGA reactor (Vienna) at ambient temperature (340 K). In addition, tension–tension fatigue measurements were performed in the load-controlled mode to simulate the pulsed operation conditions of ITER.

Electra KrF Laser Hibachi Foil Cooling with Near-Wall Jets

Active cooling of the transmission foil separating the vacuum diodes from the laser cell in the Electra KrF Laser is necessary to prevent its failure under repetitively pulsed (5Hz) operating conditions. This paper investigates the effectiveness of forced convection cooling using near-wall jets as a means of protecting the foil. Two different near-wall jet configurations are examined. The first one uses a planar, 1mm-thick, high-speed jet flowing parallel to the laser gas stream along the entire width of the hibachi foil structure. The second one uses small, 0.8mm-diameter circular jets positioned in staggered locations, 12.7mm apart, along the ˜30cm height of each of the 24 hibachi rib spans with flow perpendicular to the laser gas stream. Bench-top experiments simulating a single 3.4cm×30cm foil span between two neighboring ribs have been conducted. For both jet configurations, experiments have been performed at different jet velocities and heat inputs. The goal of these experiments is to demonstrate quantitatively that near-wall jets can effectively cool the Electra hibachi foil under prototypical pulsed operating conditions without adverse impact on beam quality or laser efficiency. Preliminary tests with a full-size hibachi on Electra have shown this to be true.

Coupled transient thermo-fluid/thermal-stress analysis approach in a VTBM setting

A virtual test blanket module (VTBM) has been envisioned as a utility to aid in streamlining and optimizing the US ITER TBM design effort by providing an integrated multi-code, multi-physics modeling environment. Within this effort, an integrated simulation approach is being developed for TBM design calculations and performance evaluation. Particularly, integrated thermo-fluid/thermal-stress analysis is important for enabling TBM design and performance calculations. In this paper, procedures involved in transient coupled thermo-fluid/thermal-stress analysis are investigated. The established procedure is applied to study the impact of pulsed operational phenomenon on the thermal-stress response of the TBM first wall. A two-way coupling between the thermal strain and temperature field is also studied, in the context of a change in thermal conductivity of the beryllium pebble bed in a solid breeder blanket TBM due to thermal strain. The temperature field determines the thermal strain in beryllium, which in turn changes the temperature field. Iterative thermo-fluid/thermal strain calculations have been applied to both steady-state and pulsed operation conditions. All calculations have been carried out in three dimensions with representative MCAD models, including all the TBM components in their entirety.

Mechanical strength of various cyanate ester/epoxy insulation systems after fast neutron irradiation to the ITER design fluence and beyond

To ensure secure operation of the magnet coils, the insulation system must keep its mechanical strength over the whole plant lifetime in a gamma and fast neutron radiation environment. Recent results on cyanate ester (CE)/epoxy blends demonstrated their mechanical integrity after irradiation to the ITER design fluence of 1 × 10 22 m −2 ( E > 0.1 MeV). For economic reasons, the cyanate ester content should be kept as low as possible due to the higher price compared to traditional epoxy resins. Therefore, the optimal composition of cyanate ester and epoxy is of great importance. In this study R-glass fiber/Kapton reinforced cyanate ester/epoxy blends with different epoxy resins and epoxy content were investigated. Short-beam shear and static tensile tests were carried out at 77 K prior to and after irradiation to a fast neutron fluence of 1 × 10 22 and 2 × 10 22 m −2 ( E > 0.1 MeV), which was done in the TRIGA reactor (Vienna) at ambient temperature (340 K). In addition, tension–tension fatigue measurements were performed in the load- and strain- controlled mode to simulate the pulsed operation conditions of ITER.

230 W C-band GaN-FET power amplifier

A C-band high-power amplifier with two GaN-based FET chips exhibits record output powers under continuous-wave (CW) and pulsed operation conditions. At 5.0 GHz, the developed GaN-FET amplifier delivers a CW 208 W output power with 11.9 dB linear gain and 34% power-added efficiency. It also shows a pulsed 232 W output power with 8.3 dB linear gain.