196,419 research outputs found
Development of advanced Gunn diodes and Schottky multipliers for high power THz sources
No full textAn advanced step-graded Gunn diode (∼ 100 GHz fundamental frequency) has been developed using a joint modelling-experimental approach to test GaAs based Gunn oscillators at sub-millimetre wavelengths. These devices are to be used as high power (multi-mW) Terahertz sources in conjunction with multipliers using Schottky diodes as the non-linear elements. The modelled-measured results of low series resistance Schottky diodes with non-alloyed contacts are also discussed. ©2010 IEEE
Time-domain analysis of sub-micron transit region GaAs Gunn diodes for use in Terahertz frequency multiplication chains
No full textSimulated RF time-domain characteristics for advanced Gunn diodes with hot electron injection and sub-micron transit region lengths for use at frequencies over 100GHz are reported. The physical models used have been developed in SILVACO and are compared to measured results. The devices measured were originally fabricated to investigate the feasibility of GaAs Gunn diode oscillators capable of operating at D-band frequencies and ultimately intended for use in high power (multi-mW) Terahertz sources (∼0.6THz) when used in conjunction with novel Schottky diode frequency multiplier technology. The device models created using SILVACO are described and the DC and time-domain results presented. The simulations were used to determine the shortest transit region length capable of producing sustained oscillation. The operation of resonant disk second harmonic Gunn diode oscillators is also discussed and accurate electromagnetic models created using Ansoft High Frequency Structure Simulator presented. Novel methods for combining small-signal frequency-domain electromagnetic simulations with time-domain device simulations in order to account for the significant interactions between the diode and oscillator circuit are described. © 2010 SPIE
Surfactant-mediated growth of InAs-GaAs superlattices and quantum dot structures grown at different temperatures
No full textThe structural and optical qualities of superlattice InAs-GaAs structures and quantum dots (QDs), grown by molecular beam epitaxy (MBE) at low (250 °C) and normal (∼450 °C) growth temperatures, have been investigated. The InAs layers (3 monolayers) were grown under conditions where only the indium beam impinged upon the growth surface (surfactant growth mode). This growth mode still resulted in the formation of QDs at normal growth temperatures, but with dot sizes that were much smaller than those for "normal" growth of 3 ML InAs-GaAs QD structures. In addition, at low temperature under such "arsenic-free" conditions a very high quality InAs-GaAs superlattice structure with 3 ML of InAs was formed, as demonstrated by transmission electron microscopy (TEM). This is a direct confirmation that the critical thickness of InAs can be extended well beyond the 1.7 ML limit seen at higher growth temperatures. © 2008 Elsevier Ltd. All rights reserved
Interaction of low-temperature surfactant-grown InAs superlattice layers with arsenic precipitates
No full textDouble crystal X-ray diffraction (DCXRD), transmission electron microscopy (TEM) and photoluminescence (PL) measurements were used to study the effect of post-growth annealing temperature on the structural properties of novel low growth (250 °C) and normal growth (450 °C) temperature InAs surfactant-mediated grown materials. Under conditions of "arsenic-free" growth, high-quality InAs-GaAs superlattices are obtained at low temperatures (LT) even at thicknesses as high as 3 monolayers (MLs) for InAs. The interaction of the built-in strain fields and the point defects in LT-GaAs both before and after annealing has been studied in detail. DCXRD studies show that the thickness of the LT-grown InAs layers decreased by up to 0.7 ML as the annealing temperature increased to 550 °C. There is evidence from the DCXRD and TEM that the LT InAs-GaAs superlattice structure starts to distort at annealing temperatures above 450 °C. In comparison, the sample grown at normal temperature, 450 °C, still retained the periodicity of the superlattice layers up to an annealing temperature of 650 °C without any change in the thickness of either the InAs (wetting layer) or GaAs. © 2008 Elsevier Ltd. All rights reserved
Advanced Gunn diode as high power terahertz source for a millimetre wave high power multiplier
No full textAn advanced step-graded Gunn diode is reported, which has been developed through joint modelling-experimental work. The ~ 200 GHz fundamental frequency devices have been realized to test GaAs based Gunn oscillators at sub-millimetre wave for use as a high power (multi mW) Terahertz source in conjunction with a mm-wave multiplier, with novel Schottky diodes. The epitaxial growth of both the Gunn diode and Schottky diode wafers were performed using an industrial scale Molecular Beam Epitaxy (V100+) reactor. The Gunn diodes were then manufactured and packaged by e2v Technologies (UK) Plc. Physical models of the high power Gunn diode sources, presented here, are developed in SILVACO. © 2009 SPIE
Effects of Thermal Annealing on Current Degradation in Enhancement Mode Pd gate InAlAs/InGaAs/InP pHEMTs
No full textConventional InGaAs/InAlAs pHEMTs, which work mostly in depletion mode, are limited by their high power consumption and requirements for a negative voltage supply. The realization of reliable enhancement mode pHEMTs (E-pHEMTs) using all postive supplies has proven difficult so far. Previously, E-pHEMTs have been reported using platinum (Pt) and palladium (Pd) gate metallization diffusion on GaAs based pHEMTs, and also by using Pt gate on InP based pHEMTs, but no Pd gate on InP based pHEMTs has been reported to date. In this work, we have investigated a novel quasi-enhancement mode Pd gate delta-doped InGaAs/InAlAs pHEMT, which has the advantages of
high breakdown voltage
Electrical characteristics of highly strained InGaAs/ InAIAs 2μm quantum cascade light -emitting devices
No full textGrowth of electroluminescent devices operating at 2μm based on quantum cascade (QC) design of strain-compensated InxGal xAs/InyAl1-yAs has been undertaken. Experimentally measured current- voltage characteristics are in good agreement with theoretical predictions
Material characterization of highly strained and partially strain compensated In<sub>x</sub>Ga<sub>1-x</sub>As/In<sub>y</sub>Al<sub>1-y</sub>A<sub>s</sub> quantum cascade light emitting diodes grown by MBE for emission in the near infrared (2-4 μm)
No full textMaterial characterization of quantum cascade (QC) structures aimed at producing emission in the near to mid infrared wavelengths (2-4 μm) is presented. It is proposed that this material system (grown by MBE under stoichiometric growth conditions) can be developed to produce a quantum cascade laser (QCL) operating at communication wavelengths below 2 μm. The InxGa1-xAs/InyAl1-yAs on a semi-insulating InP substrate material system demonstrates compressive strain in the quantum well (QW) material up to 1.85% (x=0.8) and partially compensating tensile strain in the barrier material of up to 1.86% (y=0.25) both with respect to InP. Presented is a comparison of lattice matched and highly strained devices. Experimental data is provided to demonstrate the excellent optical and electrical characteristics of the material (photoluminescence signals at room temperature and I-V measurements down to 20 K). The results are encouraging for the development of this material system to produce the first QC emission approaching 2 μm using current InGaAs/InAlAs on InP MBE tooling technology.</p
Current-voltage and light-current characteristics in highly strained InGaAs/lnAlAs quantum cascade laser structures
No full textGrowth of electroluminescent devices based on strain-compensated In xGa1-xAs/ InyAl1-yAs has been undertaken. The very high conduction band offset of the strained material allows the design of such devices with very short emission wavelength. Device design and material characterisation for 2 μm emission has been undertaken. An analysis of the direct current amplitude modulation response of the strained structure is also performed. Strong room temperature emission with peaks around 1.55 μm dependent upon driving current has been observed under continuous wave reverse operational bias. This reverse bias emission was observed owing to the presence of interface states. Moreover experimentally measured current-voltage characteristics in forward bias are found to be in good agreement with theoretical predictions.</p
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