Recent advances in nonsilica fiber technology have prompted the development of suitable materials for devices operating beyond 1.55 m. The III-V ternaries and quaternaries ͑AlGaIn͒͑AsSb͒ lattice matched to GaSb seem to be the obvious choice and have turned out to be promising candidates for high speed electronic and long wavelength photonic devices. Consequently, there has been tremendous upthrust in research activities of GaSb-based systems. As a matter of fact, this compound has proved to be an interesting material for both basic and applied research. At present, GaSb technology is in its infancy and considerable research has to be carried out before it can be employed for large scale device fabrication. This article presents an up to date comprehensive account of research carried out hitherto. It explores in detail the material aspects of GaSb starting from crystal growth in bulk and epitaxial form, post growth material processing to device feasibility. An overview of the lattice, electronic, transport, optical and device related properties is presented. Some of the current areas of research and development have been critically reviewed and their significance for both understanding the basic physics as well as for device applications are addressed. These include the role of defects and impurities on the structural, optical and electrical properties of the material, various techniques employed for surface and bulk defect passivation and their effect on the device characteristics, development of novel device structures, etc. Several avenues where further work is required in order to upgrade this III-V compound for optoelectronic devices are listed. It is concluded that the present day knowledge in this material system is sufficient to understand the basic properties and what should be more vigorously pursued is their implementation for device fabrication.
Enhancement in below bandgap room temperature infrared transmission has been observed in tellurium (Te)-doped GaSb bulk crystals. The effect of Te concentration on the transmission characteristics of GaSb has been experimentally and theoretically analysed. Undoped GaSb is known to exhibit p-type conductivity with residual hole concentration of the order of (1-2) × 10 17 cm −3 at room temperature due to the formation of native defects. For such samples, inter-valence band absorption has been found to be the dominant absorption mechanism. The residual holes could be compensated by n-type dopants such as Te. With increasing Te concentration, free carrier absorption due to electrons and inter-valley transitions in the conduction subband become significant. The dependences of various absorption mechanisms as a function of wavelength have been discussed in this paper.
The supercritical carbon dioxide (S-CO 2 ) based Brayton cycle is a good alternative to conventional power cycles because of high cycle efficiency, compact turbo machinery and compact heat exchangers. In this cycle, the majority of heat transfer (approximately 60-70% of total cycle heat transfer) occurs in the regenerator. For the regenerator, micro-channel heat exchanger is an attractive option because of its high surface-area-to-volume ratio. In this study, the performance of a printed circuit heat exchanger (PCHE) with straight and zigzag channels is evaluated. The study is performed for fully turbulent conditions. The channel diameter and the operating Reynolds number play significant roles in the overall heat transfer and pressure drop of hot and cold channels of S-CO 2 . For zigzag channels, it is found that a larger bend angle and smaller linear pitch perform better than a smaller bend angle and large linear pitch combination.Correlations for Nusselt number and friction factor are developed using ANSYS Fluent and are subsequently utilized in one dimensional (1D) thermal modelling of the heat exchanger. For the same thermal capacity, the model indicates that the zigzag channel PCHE volume is significantly smaller than that of a straight channel PCHE because of higher heat transfer coefficient.However, the pressure drop incurred in the former design is larger.
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