Conical Inserts Research Articles

Reduction of visible defect densities in molecular beam epitaxy grown GaAs using a high-capacity, low flux transient Ga source with novel crucible inserts

A new design of large capacity crucible with novel inserts has been evaluated with respect to visible defect density, layer uniformity, and flux transient. The crucible was tested in a commercially available furnace with two independently controlled heater windings. The inserts were two disks, each having a different set of holes, stacked inside the crucible and permitting no line of sight between the Ga melt and the substrate. The source was tested side-by-side with a similar furnace equipped with a commercially available crucible and conical insert set designed for low shutter transient and low flux drift. Defect densities were measured on epitaxial layer structures having a total of 1 μm of GaAs. The new design showed significantly lower total defect densities than a control growth with the standard cell, producing as few as ∼1/3 the defects (43±6 vs 155/cm2). Flux transients were the same with the two sources (<2%). Uniformity was tested by using photoluminescence to measure the thickness of GaAs quantum wells across a 75 mm diameter wafer. While the old design produces a center-to-edge decrease in thickness of ∼2.5%, the new source produces a decrease in thickness of ∼6.5%.

Corrosion-resistant exhaust hood with a conical insert

Corrosion-resistant exhaust hood with a conical insert

Improved growth uniformity in molecular-beam epitaxy: Alternative strategies

We use computer modeling to evaluate alternative strategies for the improvement of molecular-beam epitaxy (MBE) growth uniformity. Using GaAs growth in a V80H MBE system as an example, we evaluate the effect of shifting the effusion cell away from the substrate on layer uniformity, Ga melt temperature, and yield. Four different crucible systems are compared. We find that, for the V80H system, pulling back a straight wall crucible brings substantial uniformity improvement only when the crucible orifice is capped with an annular disk. This modification is not necessary for a conical crucible or straight wall crucible with a conical insert. The increase in the cell operating temperature due to the pull back is found to be about 50 °C for a 10 cm cell shift for all the crucible designs. Depending on the cell, such a shift can improve layer uniformity over a 3 in. wafer by up to a factor of 5.

Improved thickness uniformity in molecular beam epitaxial growth of GaAs using a tilted conical insert crucible

Significant improvements in the uniformity of epitaxial layers of GaAs, deposited using a V80-H molecular beam epitaxy system, can be obtained by modifying the crucible so that the flux is directed towards the center of rotation of the substrate. This has been obtained with a dual crucible system, consisting of a tilted conical insert inside a large capacity straight sided crucible. With this combination the layer uniformity can be improved to better than ±1.5% over the useable area of a 76 mm diam substrate and to ±0.7% for 50 mm diam substrates. These results are in excellent agreement with the predictions of recent flux modeling calculations.

Effect of container type on pH and alkalinity stability

Errors in pH measurement of 0.1-0.2 units can be propagate large errors in the predicted aqueous speciation. When accurate on-site analyses are impossible to obtain, changes in pH and alkalinity between the time of sampling and after transport to a laboratory must be minimized in many sampling programs. Several container and cap combinations were evaluated for their ability to preserve the pH and alkalinity of test waters that were out of equilibrium with atmospheric carbon dioxide over time periods representative of mail or other surface transport. Provided that no air bubbles were present when the sample bottle was filled and sealed, polyvinyl chloride bottles with caps having a conical polyethylene insert yielded the most satisfactory results. High density linear polyethylene bottles were also generally satisfactory. Cap design was observed to be very important. Guidelines for analysis and preservation are given, and if followed, laboratory measurements can be reliably substituted for field measurements in many instances.

A strain-gauge, brushless torque meter

A universal, brushless strain-gauge torque meter employing a force-balance system designed and developed. Full scale reading of recording systems at any given torque range secured by use of changeable conical seat inserts and semi-conductor strain gauges.

Plugs and nozzles for intermediate ladles in continuous steel casting with automatic jet control

We prepared and tested in service refractories satisfying the requirements of automatic casting from the intermediate ladle in continuous steel-casting plants. The high-alumina conical plugs and unfired magnesite inserts fixed to fireclay nozzles can be recommended for further use.

The placement of inserts to correct flow in bins

Many existing installations for storing particulate solids include hoppers from which funnel flow takes place, and although it may be desirable to replace them by mass flow hoppers this may not be economic. It may be possible to concert the hopper into a mass flow hopper, or to reduce the tendency to form stable arches or pipes, by the use of a conical insert. A method is given for the design of the insert, and methods of estimating the pressure on the insert are also described.

Sodium iodide crystal of welltype for in-vivo gamma ray measurements.

An improved scintillation counter collimator system was devised based on the non-uniform response of a well-type sodium iodide crystal fitted with a tapered collimator and a removable conical lead insert in the well. The tracer dose administered to the patient can be reduced by at least a factor four as the sensitivity was 6 times higher than a conventional 1.8 cm diameter cylindrical collimator at the same resolving power (I131). The instrument can easily be converted into a conventional well-type scintillation counter.