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  • Type-I mid-infrared InAs/InGaAs quantum well lasers on InP-based metamorphic InAlAs buffers


    InAs/InGaAs quantum well laser structures have been grown on InP-based metamorphic In0.8Al0.2As buffers by gas source molecular beam epitaxy. The effects of barrier and waveguide layers on the material qualities and device performances were characterized. X-ray diffraction and photoluminescence measurements prove the benefits of the strain compensation in the active quantum well region on the material quality. The device characteristics of the lasers with different waveguide layers reveal that the separate confinement heterostructure plays a crucial role on the device performances of these metamorphic lasers. Type-I emissions in the 2–3 µm range have been achieved in these InP-based metamorphic antimony-free structures. By combining the strain-compensated quantum wells and separate confinement heterostructures, the laser performances have been improved and laser emission up to 2.7 µm has been achieved. Source:IOPscience For more information, please visit our website: www.semiconductorwafers.net, send us email at angel.ye@powerwaywafer.com or powerwaymaterial@gmail.com

  • Characterization of GaSb photodiode for gamma-ray detection


    We extract the carrier mobility-lifetime products for epitaxially grown GaSb and demonstrate the spectral response to gamma rays of a GaSb p–i–n photodiode with a 2-µm-thick absorption region. Under exposure from 55Fe and 241Am radioactive sources at 140 K, the photodiode exhibits full width at half maximum energy resolutions of 1.238 ± 0.028 and 1.789 ± 0.057 keV at 5.89 and 59.5 keV, respectively. We observe good linearity of the GaSb photodiode across a range of photon energies. The electronic noise and charge trapping noise are measured and shown to be the main components limiting the measured energy resolutions. Source:IOPscience For more information, please visit our website: www.semiconductorwafers.net, send us email at angel.ye@powerwaywafer.com or powerwaymaterial@gmail.com

  • Growth of GaN epitaxial films on polycrystalline diamond by metal-organic vapor phase epitaxy


    Heat extraction is often essential in ensuring efficient performance of semiconductor devices and requires minimising the thermal resistance between the functional semiconductor layers and any heat sink. This paper reports the epitaxial growth of N-polar GaN films on polycrystalline diamond substrates of high thermal conductivity with metal-organic vapor phase epitaxy, by using a Si x C layer formed during deposition of polycrystalline diamond on a silicon substrate. The Si x C layer acts to provide the necessary structure ordering information for the formation of a single crystal GaN film at the wafer scale. It is shown that a three-dimensional island (3D) growth process removes hexagonal defects that are induced by the non-single crystal nature of the Si x C layer. It is also shown that intensive 3D growth and the introduction of a convex curvature of the substrate can be deployed to reduce tensile stress in the GaN epitaxy to enable the growth of a crack-free layer up to a thickness of 1.1µm. The twist and tilt can be as low as 0.65° and 0.39° respectively, values broadly comparable with GaN grown on Si substrates with a similar structure. Source:IOPscience For more information, please visit our website: www.semiconductorwafers.net, send us email at angel.ye@powerwaywafer.com or powerwaymaterial@gmail.com

  • InAs/InSb nanowire heterostructures grown by chemical beam epitaxy


    We report the Au-assisted chemical beam epitaxy growth of defect-free zincblende InSb nanowires. The grown InSb segments are the upper sections of InAs/InSb heterostructures on InAs(111)B substrates. We show, through HRTEM analysis, that zincblende InSb can be grown without any crystal defects such as stacking faults or twinning planes. Strain-map analysis demonstrates that the InSb segment is nearly relaxed within a few nanometers from the interface. By post-growth studies we have found that the catalyst particle composition is AuIn2, and it can be varied to a AuIn alloy by cooling down the samples under TDMASb flux. Source:IOPscience For more information, please visit our website: www.semiconductorwafers.net, send us email at angel.ye@powerwaywafer.com or powerwaymaterial@gmail.com

  • Tilted angle CZT detector for photon counting/energy weighting x-ray and CT imaging


    X-ray imaging with a photon counting/energy weighting detector can provide the highest signal to noise ratio (SNR). Scanning slit/multi-slit x-ray image acquisition can provide a dose-efficient scatter rejection, which increases SNR. Use of a photon counting/energy weighting detector in a scanning slit/multi-slit acquisition geometry could provide highest possible dose efficiency in x-ray and CT imaging. Currently, the most advanced photon counting detector is the cadmium zinc telluride (CZT) detector, which, however, is suboptimal for energy resolved x-ray imaging. A tilted angle CZT detector is proposed in this work for applications in photon counting/energy weighting x-ray and CT imaging. In tilted angle configuration, the x-ray beam hits the surface of the linear array of CZT crystals at a small angle. This allows the use of CZT crystals of a small thickness while maintaining the high photon absorption. Small thickness CZT detectors allow for a significant decrease in the polarization effect in the CZT volume and an increase in count rate. The tilted angle CZT with a small thickness also provides higher spatial and energy resolution, and shorter charge collection time, which potentially enables fast energy resolving x-ray image acquisition. In this work, the major performance parameters of the tilted angle CZT detector, including its count rate, spatial resolution and energy resolution, were evaluated. It was shown that for a CZT detector with a 0.7 mm thickness and 13° tilting angle, the maximum count rate can be increased by 10.7 times, while photon absorption remains >90% at photon energies up to 120 keV. Photon counting/energy weighting x-ray imaging using a tilted angle CZT detector was simulated. SNR improvement due to optimal photon energy weighting was 23% and 14% when adipose contrast element, inserted in soft tissue with 10 cm and 20 cm thickness, respectively, was imaged using 5 energy bins and weighting factors optimized for the adipose. SNR improvement was 42% and 31% when CaCO3 contrast element, inserted in soft tissue with 10 cm and 20 cm thickness, respectively, was imaged using 5 energy bins and weighting factors optimized for CaCO3. The SNRs of the photon counting single-kVp dual-energy subtracted images of CaCO3 and adipose were higher by 2.04 and 2.74 times, respectively, as compared to currently used dual-kVp dual-energy subtracted images. Experiments with a CZT crystal with 2 mm thickness have shown significant decrease in the tailing effect of the CZT pulse spectrum at 59 keV and 122 keV photon energies, when the tilting angle configuration was used. Finally, feasibility of the tilted angle CZT detector for photon counting cone beam breast CT imaging was demonstrated. Source:IOPscience For more information, please visit our website: www.semiconductorwafers.net, send us email at angel.ye@powerwaywafer.com or powerwaymaterial@gmail.com

  • Single-crystal growth and thermoelectric properties of Ge(Bi,Sb)4Te7


    The thermoelectric properties between 10 and 300 K and the growth of single crystals of n-type and p-type GeBi4Te7, GeSb4Te7 and Ge(Bi1−xSbx)4Te7 solid solution are reported. Single crystals were grown by the modified Bridgman method, and p-type behavior was achieved by the substitution of Bi by Sb in GeBi4Te7. The thermopower in the Ge(Bi1−xSbx)4Te7 solid solution ranges from −117 to +160 μV K−1. The crossover from n-type to p-type is continuous with increasing Sb content and is observed at x ≈0.15. The highest thermoelectric efficiencies among the tested n-type and p-type samples are ZnT = 0.11 and ZpT = 0.20, respectively. For an optimal n–p couple in this alloy system the composite figure of merit is ZnpT = 0.17 at room temperature. Source:IOPscience For more information, please visit our website: www.semiconductorwafers.net. send us email at angel.ye@powerwaywafer.com or powerwaymaterial@gmail.com

  • Graphene on silicon carbide can store energy


    The thinnest material ever produced, graphene, consists of a single layer of carbon atoms. They form a chicken-wire structure one atom thick, with unique properties. It is around 200 times stronger than steel, and highly flexible. It is transparent, but gases and liquids cannot pass through it. In addition, it is an excellent conductor of electricity. There are many ideas about how this nanomaterial can be used, and research into future applications is intense. "Graphene is fascinating, but extremely difficult to study," says Mikhail Vagin, principal research engineer at the Department of Science and Technology and the Department of Physics, Chemistry and Biology at Linköping University. One of the factors contributing to the difficulty in understanding the properties of graphene is that it is what is known as an "anisotropic" material. This means that its properties when measured on the plane surface of the carbon atom layer differ from those measured at the edges. Furthermore, attempts to understand the behaviour of graphene at the atomic level are complicated by the fact that it can be produced in several ways. The properties of graphene in small flakes, which have many edges, differ in several ways from those of graphene produced as sheets with an area around 1 cm2. The researchers who carried out the study used graphene created on a crystal of silicon carbide by a method developed at Linköping University. When silicon carbide is heated to 2000 °C, silicon atoms on the surface moves to the vapor phase and only the carbon atoms remain. The graphene does not react easily with its surroundings due to the high quality of the graphene layer and its innate inertness, while applications often rely on controlled interaction between the material and the surroundings, like gas molecules. An on-going discussion among researchers in the field is whether it is possible to activate the graphene on the flat surface or whether it is necessary to have edges. The LiU researchers investigated what happens when defects in the surface are introduced in a controlled manner, and in this way attempted to understand in more detail how the properties of graphene are related to its structure. "An electrochemical process known as 'anodising' breaks down the graphene layer such that more edges are created. We measured the properties of anodised graphene and discovered that the capacity of the material to store electricity was quite high," says Mikhail Vagin. More work is necessary before the new knowledge can be used, and to produce the same effect at a larger scale. The scientists plan to follow up the research in several ways. "Graphene on silicon carbide can be made in larger areas than other types of graphene. If we can change the properties of the material in a controlled manner, it may be possible to tailor the surface for other functions. It may be possible, for example, to create a sensor that has its own built-in battery," says Mikael Syväjärvi, principal resear...

  • Optical emission spectroscopy of gallium phosphide plasma-enhanced atomic layer deposition


    The capability of optical emission spectroscopy for in situ study and control of plasma-enhanced atomic-layer deposition (PE-ALD) of gallium phosphide from phosphine and trimethylgallium carried by hydrogen was explored. The gas composition changing during the PE-ALD process was monitored by in situ measurements of optical emission intensity for phosphine and hydrogen lines. For PE-ALD process where phosphorus and gallium deposition steps are separated in time a negative influence of excess phosphorus accumulation on the chamber walls was observed. Indeed, the phosphorus deposited on the walls during PH3 decomposition step is etched by hydrogen plasma during the next trimethylgallium decomposition step leading to uncontrollable and unwanted conventional plasma-enhanced chemical vapor deposition. To reduce this effect, it has been proposed to introduce a step of hydrogen plasma etching, which allows one to etch excess phosphorus before the beginning of gallium deposition step and achieve atomic-layer deposition growth mode. Source:IOPscience For more information, please visit our website: www.semiconductorwafers.net, send us email at angel.ye@powerwaywafer.com or powerwaymaterial@gmail.com

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