R.L. Barns

Crystal stoichiometry of Czochralski grown rare-earth gallium garnets

The observed lattice parameters of Czochralski grown rare-earth gallium garnets are reported and were found to be larger than those of stoichiometric ceramic samples. The difference is attributed to some of the rare-earth ions occupying the octahedral site which results from a shift of the congruent melting composition toward the rare-earth rich region of the phase diagram. The degree of octahedral rare-earth substitution was found to be a function of the ionic radius and ranges from x=0.01 for Nd3Ga5O12 to x=0.09 for Y3Ga5O12 in (RE3[RExGa2−x]Ga3O12).

Crystal growth of high temperature superconductors — Problems, successes, opportunities

Abstract The chemistry of oxide superconductors, especially those of the YBa 2 Cu 3 O x family, is discussed with respect to their crystal growth. YBa 2 Cu 3 O x melts incongruently, loses oxygen at elevated temperatures and reacts with water. Present knowledge of the appropriate phase diagrams is reviewed. Major important methods of crystal growth are summarized and their present applicability to YBa 2 Cu 3 O x is discussed. In addition, recent results on the melt growth of bulk crystals from melt compositions near the ternary eutectic and on the growth of thin oriented, partly epitaxial films by vapor growth using thermal and e-beam vapors and MBE are described.

Czochralski growth and characterization of GaSb

Abstract Single crystal GaSb is important as a substrate for long wavelength ( ⩾ 1.5 μm) detectors and lasers. Procedures for the reproducible synthesis of GaSb, and Czochralski growth of single crystals with varied stoichiometry are discussed. The distribution of dopants including Te, Se, Si and Ge was studied and the dependence of carrier concentration on dopant concentration (determined by atomic absorption analysis) is reported. In the Te concentration range studied, 1.5 × 10 17 -3 × 10 19 cm -3 , the carrier concentration is always substantially less than the Te concentration ranging from ≈ 0.25 at low Te to ≈ 0.14 at high Te. Room temperature electron mobilities > 3000 cm 2 V -1 s -1 are obtained at low carrier concentrations. The interface shape and its change with fraction frozen is shown to affect the impurity distribution. Normal freeze behavior is qualitatively observed for Te partition as a function of fraction of melt frozen. Equations for the effect on impurity distribution of the changing Ga/Sb ratio in the solid as freezing proceeds are developed and compared with experimental results. It is shown that the most important correction to the distribution constant is based on interface shape changes and on and off facet growth as freezing proceeds.

Nonstoichiometry and crystal growth of gadolinium gallium garnet

Abstract Lattice parameter and weight loss measurements have confirmed the existence of compositional shifts due only to Ga 2 O 3 vaporization losses in gadolinium gallium garnet single crystals grown by pulling from the melt. The activation energy for vaporization into an atmosphere indicates that gas phase condensation above the melt surface is probably the rate-determining mechanism. Gadolinium gallium garnet was found to possess a room-temperature existence region over excess Gd 2 O 3 compositions only. All crystalls grown to data have therefore been found to be nonstoichiometric.

Crystal stoichiometry and growth of rare-earth garnets containing scandium

Abstract Rare-earth garnets containing scandium of the general formula type RE3Sc2Al3O12 and RE3Sc2Ga3O12 have been grown by the Czochralski technique. The distribution coefficient for Nd in these garnet systems indicate that it is a linear function of the host lattice parameter and independent of the host rare-earth ion. Lattice parameter measurements of crystals grown from stoichiometric melts indicate that the crystal composition is not stoichiometric, but that they contain an excess of Sc for the aluminum garnets and rare-earth for the gallium garnets.

Dislocation-free and low-dislocation quartz prepared by hydrothermal crystallization

Abstract A reproducible technique for the hydrothermal crystallization of low dislocation-density and dislocation-free α-quartz on (0001) and (01 1 1) seeds is described. Dislocations are shown to propagate from pre-existing dislocations in seeds and from particulate inclusions (lithium sodium iron silicate) grown into the crystal. To prepare dislocation-free material in high yields dislocation free seeds which are carefully etched to remove damage and growth in noble-metal-lined autoclaves to reduce iron containing inclusions are necessary. Etching studies showed that shallow etch pits are probably associated with ordinary dislocations while etch pipes are associated with dislocations found under growth conditions were iron decoration is probable. Growth rate had no obvious effect on perfection over a growth rate range of 2–4X. Problems associated with a unique Burgers vector determination are outlined. The relationship of dislocations and strain to cracking previously observed in growth on (01 1 1) seeds is discussed.

Highly textured and single crystal Bi2CaSr2Cu2Ox prepared by laser heated float zone crystallization

Abstract A CO 2 laser heat source was used to float zone 0.36 cm diameter ceramic rods of Bi 2 CaSr 2 Cu 2 O x . Well behaved, stable molten zones were obtained in air and in O 2 pressures up to 2.6 atm. Highly textured elongated grains ∼ 1 cm long with the c -axis normal to the rod axis were obtained at slow (0.19 cm/h) growth rates. Crystals 3×1.2×0.3 mm were easily cleaved from such specimens. In the best cases (slow growth rates), X-ray powder diffraction indicated ∼ 100% Bi 2 CaSr 2 Cu 2 O x , the superconducting transition temperature was as high as 87 K, and diamagnetism measurements indicated 60–70% superconducting phase. Meissner measurements however, in common with most Meissner measurements on Bi-containing superconductors, indicated substantially less superconducting material. Critical currents were comparable to untextured ceramic and flux grown single crystals. Float zone crystallization shows great promise for preparing single crystals and textured ceramics of incongruently melting superconductors, even when detailed phase diagrams are not known.

Production and perfection of “z-face” quartz

Abstract Synthetic quartz grown on minor rhombohedral seed plates is most economical for the fabrication of resonators for certain monolithic crystal filters. Laboratory studies leading to the choice of commercial growth conditions are reviewed. Conditions for growth in large autoclaves are described. The role of fill, temperature, temperature difference and mineralizer is discussed. The growth rate under present conditions in (OH) - solutions is 32–38 mil/day and acoustic Q s adequate for applications (>10 6 ) are routinely obtained at rates of 35 mil/day. OH concentration in minor rhombohedral growth depends on growth rate in a manner similar to that reported for basal plane growth. New evidence for the inclusion of non-charge compensated OH, probably in the form of H 2 O inclusions, is advanced. Statistical data on a number of runs are presented showing the uniformity of growth rate throughout the growth vessel. As a means for improving seed quality and the yield and perfection of grown material, a new polariscope designed for use in this work is described. It is shown that as much as a 20% increase in yield can be obtained by choosing low strain seeds. Defects in seed plates are usually propagated into grown quartz and strain was found to increase with growth rate.

Are icicles single crystals

Abstract During the winter of 1977–1978, about 60 icicles were examined between crossed polarizers (12 inch sq.). Thus, specimens more than a foot long could be examined. Thin sections parallel and normal to the long axis were used. The thin sections (1/8 inch–1/4 inch) were prepared by using a warmed aluminium plate to form plane-parallel surfaces by melting. The growth rate of selected specimens was from less than 1 inch to more than 14 inch/day. A wide variety of textures was observed. Crystallite size ranged from less than 0.03 to more than 8 inch. It was found that single crystal regions as large as 8 inch by 1 inch can be produced in icicle growth. Crystal orientation was checked in some cases (using the optical interference figure) and in no case was the c-axis found to be near the icicle axis. Representative photographs are presented, comparisons are made with ordinary ice-cube texture and the growth mechanism is discussed. It is interesting to observe that nature, apparently without too much effort, regularly grows millions of ice crystals of a size which most crystal growers would be proud. Suggestions are made for the use of the simple techniques described for further studies of icicles by students interested in the phenomena of crystallization.

Czochralski crystal growth in the system PtMnxSb2-x

Abstract Czochralski growth of the system PtMn x Sb 2− x was investigated to explore its magnetic semiconducting potential. Using the LEC technique, PtSb 2 single crystals, p-type with ~ 10 18 carriers cm -3 , were obtained from BN crucibles; doping with Te exhibited a k (eff) of about one and produced n-type material. Melts of a composition PtMnSb crystallized as PtMnSb in single crystal form. Melt compositions of PtMn 0.25 Sb 1.75 resulted in the formation of PtSb 2 and with continuing growth in the appearance of several different phases, suggesting the likely absence of congruently melting compositions with x