Patrick G. Barber

A procedure to visualize the melt-solid interface in Bridgman grown germanium and lead tin telluride

Abstract An X-ray and gamma-ray technique has been developed that enables the melt-solid interface to be observed during crystal growth in a Bridgman furnace. This technique has been used to observe the interface movement and shape in germanium and in lead tin telluride.

Growth rates and interface shapes in germanium and lead tin telluride observed in-situ, real-time in vertical Bridgman furnaces

Using the advanced technology developed to visualize the melt-solid interface in low Prandtl number materials, crystal growth rates and interface shapes have been measured in germanium and lead tin telluride semiconductors grown in vertical Bridgman furnaces. The experimental importance of using in-situ, real time observations to determine interface shapes, to measure crystal growth rates, and to improve furnace and ampoule designs is demonstrated. The interface shapes observed in-situ, in real-time were verified by quenching and mechanically induced interface demarcation, and they were also confirmed using machined models to ascertain the absence of geometric distortions. Interface shapes depended upon the interface position in the furnace insulation zone, varied with the nature of the crystal being grown, and were dependent on the extent of transition zones at the ends of the ampoule. Actual growth rates varied significantly from the constant translation rate in response to the thermophysical properties of the crystal and its melt and the thermal conditions existing in the furnace at the interface. In the elemental semiconductor germanium the observed rates of crystal growth exceeded the imposed translation rate, but in the compound semiconductor lead tin telluride the observed rates of growth were less than the translation rate. Finally, the extent of ampoule thermal loading influenced the interface positions, the shapes, and the growth rates.

Use of moments of momentum to predict the crystal habit in potassium hydrogen phthalate

A relatively simple calculation of the moments of momentum predicts the morphological order of crystal faces for potassium hydrogen phthalate. The effects on the habit caused by the addition of monomeric, dimeric, and larger aggregates during crystal growth are considered. The first six of the seven observed crystal faces are predicted with this method.

Crystal growth precursors of YBa2Cu3O7−x predicted from crystal habits by use of moments of momentum

Abstract Moments of momentum (MoM) calculations using chemical species expected to exist in the melts from which crystals of YBa 2 Cu 3 O 7− x grow predict some of the same crystal habits as found by the periodic bond chain method. Since momentum methods utilize solution species and their distributions, the results provide insight as to species in the melt that are potential precursors to actual crystal growth. Whether copper (II) oxide is present as a square planar or square pyramidal complex in the melt affects the crystal habit with the latter complex suppressing the appearance of {010} faces.

Modelling melt-solid interfaces in Bridgman growth

Abstract Doped epoxy models with abrupt interfaces were prepared to test radiographic and computer enhancement procedures used to study the images of melt–solid interfaces during crystal growth in Bridgman furnaces. A column averaging procedure resulted in improved images that faithfully reproduced the positions and shapes of interfaces even at very low density differences. These techniques were applied to lead tin telluride growing in Bridgman furnaces.

Characterization of the Bridgman crystal growth process by radiographic imaging

The normal characterization of a Bridgman crystal growth procedure is to note the ingot preparation, ampoule size and material, the ampoule translation rate, and the furnace zone temperatures. Even though these are important parameters which must be recorded and controlled they may not represent a sufficient set to produce repeatable growth results. One of the primary growth variables, the actual growth rate, is often ignored. Even though it is well recognized that the actual growth rate is not equal to the ampoule translation rate the difference is usually assumed small and the stochastic nature of the difference has not been previously reported. These variations between pull rate and growth rate are especially acute in unseeded growth subjected to deep supercooling. This paper discusses both elemental (Ge) and alloy (PbSnTe) crystal growth that is monitored, via radiography, to reveal both the interface position and shape in real time. Both seeded and unseeded growth are examined. The actual growth rate is shown to be a strong function of the degree of supercooling. Actual growth rates that exceed the pull rate by a factor of greater than two have been observed and the interface shape has been observed to change from concave to flat to convex during the growth.

Moments of momenta as predictors of “molecular” species in crystal growth solutions

Abstract Crystal growth occurs from supersaturated or supercooled, thermodynamically unstable fluids in which a variety of aggregated molecular species are possible. The moments of momenta of these various species in the fluid phase influence the resulting habits of the crystals. The distribution of these molecular species can be deduced from the observed crystal habits, since growth can only occur in response to the rates at which suitably oriented growth units arrive at growing faces. Biphenyl, potassium hydrogen tartrate, and sucrose are presented as examples.

A clarified gel for crystal growth

Abstract The use of ion exchange resins to prepare clarified silicate gels suitable for crystal growth is described.

Experimental verification of agreement between thermal and real time visual melt-solid interface positions in vertical Bridgman grown germanium

Abstract Measurements of the liquid-solid interface position during crystal growth were made by observing the discontinuity of the temperature gradient with movable thermocouples in a centerline, quartz capillary placed inside a sealed quartz ampoule of germanium in a vertical Bridgman furnace. Simultaneously, in situ, real time visual observations, using X-ray imaging technology, determined the position of the melt-solid interface. The radiographically detected interface position was several millimeters from the thermal interface position and the direction of displacement depended upon the direction of thermocouple insertion. Minimization of this spurious heat flow was achieved by using an unclad thermocouple that had each of its two wire leads entering the capillary from different ends of the furnace. Using this configuration the visual interface coincided with the thermal interface. Such observations show the utility of using in situ, real time visualization to record the melt-solid interface shape and position during crystal growth; and they suggest improvements in furnace and ampoule designs for use in high thermal gradients.

Estimating crystal quality in semiconductors using voltammetry

Abstract A voltammeter can be used to differentiate the crystalline quality of the compound semiconductor, lead–tin–telluride. A modified differential pulse stripping voltammetric analysis procedure is used with a millimolar bromine/bromide etchant. Higher quality crystals, which had been grown using a magnetically damped Bridgman technique, required longer times for the onset of etching, had smaller rates of etching, and attained smaller depths of etching than did poorer quality crystals grown without magnetic dampening. This analysis procedure is faster, more amenable to quantitative results, and much safer than traditional etching techniques, which use highly corrosive chemicals. The results of the voltammetric analysis were verified using a standard Norr etch in a traditional electrochemical etching procedure and counting dislocation densities.