Single-pulse chemical shock tube for ignition delay measurements

We describe a single-pulse chemical shock tube CST2 established for measuring the reaction rate of chemical reactions and ignition delay for fuels at high temperature along with the procedure for its calibration. The suitability of the facility for measuring the ignition delay is demonstrated by measuring the ignition delay for the ethane-oxygen gas mixture in the temperature range 1250-1611 K by recording the ignition-induced pressure jump and emission from CH radical simultaneously. The results obtained in the present study compare well with the earlier reported values

Synergistic effect of reactor chemistry and compressive stress on dislocation bending during GaN growth

The synergistic effect of compressive growth stresses and reactor chemistry, silane presence, on dislocation bending at the very early stages of GaN growth has been studied using in-situ stress measurements and cross-sectional transmission electron microscopy. A single 100 nm Si-doped GaN layer is found to be more effective than a 1 mu m linearly graded AlGaN buffer layer in reducing dislocation density and preventing the subsequent layer from transitioning to a tensile stress. 1 mu m crack-free GaN layers with a dislocation density of 7 x 10(8)/cm(2), with 0.13 nm surface roughness and no enhancement in n-type background are demonstrated over 2 inch substrates using this simple transition scheme. (C) 2013 AIP Publishing LLC

Wafer-scale epitaxial germanium (100), (111), (110) films on silicon using liquid phase crystallization

A wafer-scale method to obtain epitaxial germanium (Ge) on crystalline silicon (Si) using liquid-phase-crystallization (LPC) is presented. The technique provides a simple yet versatile method to grow epitaxial germanium on silicon with any crystallographic orientation: (100), (110) or (111). The process starts with amorphous Ge, which is melted and cooled in a controlled manner to form epitaxial germanium. LPC Ge films are continuous with an average grain-size of 2-5 mu m. Rocking scan confirms that the LPC Ge is oriented with a threading dislocation density of similar to 109 cm(-2). The phi-scan confirms that LPC germanium is epitaxial with Ge (100), Ge (110) and Ge (111) showing four-fold, two-fold, and three-fold symmetry, respectively. The epitaxial quality of the Ge is influenced by the cleanliness of the Ge/Si interface; rate of cooling and ambient gas during LPC; and Ge layer thickness. Best films are obtained for 1 mu m thick LPC Ge(100), cooled at similar to 3-4 C/min in hydrogen ambient. Electron Hall mobility in these LPC Ge films is 736cm(2)/Vs, a high value that confirms the electronic quality of LPC Ge film. (c) 2018 Author(s). All article content, except where otherwise noted, is licensed under a Creative Commons Attribution (CC BY) license

Integrating AlGaN/GaN high electron mobility transistor with Si: A comparative study of integration schemes

AlGaN/GaN high electron mobility transistor stacks deposited on a single growth platform are used to compare the most common transition, AlN to GaN, schemes used for integrating GaN with Si. The efficiency of these transitions based on linearly graded, step graded, interlayer, and superlattice schemes on dislocation density reduction, stress management, surface roughness, and eventually mobility of the 2D-gas are evaluated. In a 500 nm GaN probe layer deposited, all of these transitions result in total transmission electron microscopy measured dislocations densities of 1 to 3 x 10(9)/cm(2) and <1 nm surface roughness. The 2-D electron gas channels formed at an AlGaN-1 nm AlN/GaN interface deposited on this GaN probe layer all have mobilities of 1600-1900 cm(2)/V s at a carrier concentration of 0.7-0.9 x 10(13)/cm(2). Compressive stress and changes in composition in GaN rich regions of the AlN-GaN transition are the most effective at reducing dislocation density. Amongst all the transitions studied the step graded transition is the one that helps to implement this feature of GaN integration in the simplest and most consistent manner. (C) 2015 AIP Publishing LLC

Structure and morphology studies of chromium film at elevated temperature in hypersonic environment

This paper presents the after shock heated structural and morphological studies of chromium film coated on hypersonic test model as a passive drag reduction element. The structural changes and the composition of phases of chromium due to shock heating (2850 K) are characterized using X-ray diffraction studies. Surface morphology changes of chromium coating have been studied using scanning electron microscopy (SEM) before and after shock heating. Significant amount of chromium ablation and sublimation from the model surface is noticed from SEM micrographs. Traces of randomly oriented chromium oxides formed along the coated surface confirm surface reaction of chromium with oxygen present behind the shock. Large traces of amorphous chromium oxide phases are also observed

Curvature Management in Buffer Layer for Device Quality GaN Growth on Si (111)

An efficient buffer layer scheme has been designed to address the issue of curvature management during metalorganic chemical vapour deposition growth of GaN on Si (111) substrate. This is necessary to prevent cracking of the grown layer during post-growth cooling down from growth temperature to room temperature and to achieve an allowable bow (<40 m) in the wafer for carrying out lithographic processes. To meet both these ends simultaneously, the stress evolution in the buffer layers was observed carefully. The reduction in precursor flow during the buffer layer growth provided better control over curvature evolution in the growing buffer layers. This has enabled the growth of a suitable high electron mobility transistor (HEMT) stack on 2'' Si (111) substrate of 300 m thickness with a bow as low as 11.4 m, having a two-dimensional electron gas (2DEG) of mobility, carrier concentration, and sheet resistance values 1510 cm(2)/V-s, 0.96 x 10(13)/cm(2), and 444 /, respectively. Another variation of similar technique resulted in a bow of 23.4 m with 2DEG mobility, carrier concentration, and sheet resistance values 1960 cm(2)/V-s, 0.98 x 10(13)/cm(2), and 325 /, respectively

Dislocation bending and stress evolution in Mg-doped GaN films on Si substrates

P-type doping using Mg is essential for realizing a variety of electronic and optoelectronic III-nitride devices involving hetero-epitaxial thin films that also contain a significant number of dislocations. We report on the effect of Mg incorporation on dislocation and stress evolution during the growth of GaN thin films by using in situ curvature measurements and ex situ transmission electron microscopy. A complete picture involving the interplay between three effects-dopant size effect, dislocation bending, and polarity inversion is presented. Mg aids dislocation bending, which in turn generates tensile stresses in Mg-doped GaN films As a result, the compressive stress expected due to the dopant size difference effect can only be discerned clearly in films with dislocation densities below 5 x 109 cm(-2). Polarity inversion at doping exceeding 1019 cm(-3) is associated with a sharp drop in screw dislocation density. A kinetic stress evolution model has been developed to capture dislocation bending and size difference effects, and a match between calculated bending angle from the model and that measured from TEM images is obtained. Published by AIP Publishing

Single-pulse chemical shock tube for ignition delay measurements

We describe a single-pulse chemical shock tube CST2 established for measuring the reaction rate of chemical reactions and ignition delay for fuels at high temperature along with the procedure for its calibration. The suitability of the facility for measuring the ignition delay is demonstrated by measuring the ignition delay for the ethane–oxygen gas mixture in the temperature range 1250–1611 K by recording the ignition-induced pressure jump and emission from CH radical simultaneously.The results obtained in the present study compare well with the earlier reported values