Step instability and island formation during annealing of pseudomorphic InGaAs/GaAs layers

The morphological stability of compressively strained In0.27Ga0.73As/GaAsIn0.27Ga0.73As/GaAs pseudomorphic layers has been investigated during annealing. Large three-dimensional islands form at the beginning of annealing on initially flat surfaces, likely to relieve strain energy. The islands disappear with increasing annealing, being reabsorbed into the terraces. At the same time, the step line destabilizes forming cusps that inject two-dimensional vacancy islands into the terrace. At high temperatures, this process leads to a severe deterioration of the morphology that is not due to decomposition. The island dissolution and the development of the step instability are likely alternative paths towards the reduction of surface energy. © 2003 American Institute of Physics.Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/71229/2/APPLAB-83-22-4518-1.pd

Relative Performance of Ca, Ba-FeSi Inoculants to Chill Control in Low-S Grey Cast Irons

Low sulphur irons (< 0.05%S) production is more and more promoted in many parts of the world, as small and less efficient cupolas were replaced by the new generation of induction furnaces, while a single low sulphur base iron is very attractive for grey/ductile/compacted irons production. The problem is that at low Slevels, grey irons usually solidify with high eutectic undercooling, favorable for carbides formation, especially in thin wall castings (automotive industry). Relative performance of different Ca, Ba and Al bearing FeSi alloys was calculated to evaluate their efficiency to control chill tendency, in critical base irons [< 0.035%S, (%Mn) x (%S) < 0.02, 0.002%Al]. Relative clear/mottled/total chill measurement criteria were applied, for chill wedges with different cooling modulus (CM = 0.11 – 0.35 cm). The results showed that some inoculants performed better than the other alloys bearing the same base inoculating elements and have different positions for different chill evaluation criteria and wedge size (cooling modulus) parameters reference. An optimum association of Ca, Ba and Al contents at a proper Ba/Ca ratio is more efficient comparing to the increasing of the inoculating elements leveling FeSi-based alloys for inoculation of lower sulphur, electrically melted irons

Structure Characterization of Ca/Ba, Ca-FeSi Inoculated, Low Sulphur, Electric Melted, Thin Wall Grey Iron Castings

Much of the base iron in grey iron foundries is electrically melted in an acid lined induction furnace. The performance of the induction furnace allows superheating above 1500 oC, which is appropriate for thin wall casting production. With higher levels of superheat, the base iron characteristics are totally different from cupola melted iron, resulting in changes to the final casting microstructure. Previous experiments illustrated that eutectic undercooling of this type of base iron is excessively high, demonstrating an increased need for inoculation. The high dissolution rate of residual graphite in superheated iron and difficulties in forming complex (Mn,X)S compounds as active nucleation sites of graphite can be due to very low residuals of Al (< 0.003%) and Zr (< 0.0003%), especially at less than 0.03%S content. This results in increased tendencies for chill and undercooled graphite morphologies, even in inoculated irons. The structural characteristics of low-S (0.025%), low-Al (<0.003%) and 4.0 wt.% carbon equivalent for electrically melted grey irons were studied at different solidification cooling rates in wedge castings up to 20mm wall thickness, using Ca and (Ba + Ca) inoculating elements in FeSi based alloys with the same Si and Al contents. Under these conditions, Ca inoculation had minimal effect at less than 8mm wall thickness, while a Ca-Ba combination improved most of the structural parameters, including those in thin wall castings: less than 10% carbides for 2.5 mm and no carbides at more than 5 mm section size, which also showed the highest graphite amount with a uniform distribution over the casting section

Cooling Rate Dependence of STructures Characteristics in Ce-Inoculated Low-S Grey Irons

The efficiency of Ce,Ca,Al-FeSi alloy was tested for lower addition rates (0.15-0.25wt.%), as, traditionally, high inoculant addition rates have been used in low sulphur grey cast irons, comparing to the base iron and conventional inoculated irons (Ba,Ca,Al-FeSi commercial alloy). The present work explores chill and associated structures in hypoeutectic grey iron chill wedges, with cooling modulus 0.21 cm and a large variation of the cooling rate, from the apex to the base of W2 samples [ASTM A 367, furan resin mould]. The chill tendencies of the experimental irons correlate well with the structure characteristics, displayed as the carbides/graphite ratio of undercooled graphite morphologies. Carbide sensitivity is lower with increasing wedge width, but depends on whether the state of the iron is as base iron or inoculated with different alloys. Undercooled graphite was present for both un-inoculated irons and higher cooling rate inoculated irons. As expected, inoculation as well as an increase in wall thickness of the same wedge sample led to improved undercooled graphite control. The difference in effects of the two inoculants addition is seen as the ability to decrease the amount of carbides and undercooled graphite, with Ce-bearing FeSi alloy outperforming the conventional inoculant, especially at the low alloy addition and high cooling rate solidification

Surface reconstructions of In-enriched InGaAs alloys

The atomic structure of In0.81Ga0.19As/InPIn0.81Ga0.19As/InP alloy layers was examined using in situ scanning tunneling microscopy. The (2×3) reconstruction observed during growth by reflection high-energy electron diffraction represents a combination of surface structures, including a β2(2×4) commonly observed on GaAs(001) and InAs(001) surfaces, and a disordered (4×3) that is unique to alloy systems. The proposed (4×3) structure is comprised of both anion and cation dimers. Empty and filled states images show that the features reverse contrast with sample bias, in agreement with the model. © 2003 American Institute of Physics.Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/70065/2/APPLAB-83-7-1361-1.pd

Interaction and Cooperative Nucleation of InAsSbP Quantum Dots and Pits on InAs(100) Substrate

An example of InAsSbP quaternary quantum dots (QDs), pits and dots–pits cooperative structures’ growth on InAs(100) substrates by liquid phase epitaxy (LPE) is reported. The interaction and surface morphology of the dots–pits combinations are investigated by the high-resolution scanning electron microscope. Bimodal growth mechanism for the both QDs and pits nucleation is observed. Cooperative structures consist of the QDs banded by pits, as well as the “large” pits banded by the quantum wires are detected. The composition of the islands and the pits edges is found to be quaternary, enriched by antimony and phosphorus, respectively. This repartition is caused by dissociation of the wetting layer, followed by migration (surface diffusion) of the Sb and P atoms in opposite directions. The “small” QDs average density ranges from 0.8 to 2 × 109 cm−2, with heights and widths dimensions from 2 to 20 nm and 5 to 45 nm, respectively. The average density of the “small” pits is equal to (6–10) × 109 cm−2 with dimensions of 5–40 nm in width and depth. Lifshits–Slezov-like distribution for the amount and surface density of both “small” QDs and pits versus their average diameter is experimentally detected. A displacement of the absorption edge toward the long wavelength region and enlargement toward the short wavelength region is detected by the Fourier transform infrared spectrometry

Thermal Analysis of Inoculated Grey Cast Irons

A research was done to investigate the effect of 0.05…0.25 wt.% addition rate of Ca, Zr, Al – FeSi alloy, in ladle and in-mould inoculation of grey cast irons. In the present paper, the conclusions drawn are based on thermal analysis. For solidification pattern, some specific cooling curves characteristics, such as undercooling degree at the beginning of eutectic solidification and at the end of solidification, as well as recalescence level, were identified to be more influenced by the inoculation technique. In order to secure stable and controlled processes, representative thermal analysis parameters could be used, especially in thin wall grey iron casting production

Use of Boron Nitride for Field Effect Electron Emission

Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/76687/1/AIAA-2007-5253-734.pd