44 research outputs found
Growth Techniques for Bulk ZnO and Related Compounds
ZnO bulk crystals can be grown by several methods. 1) From the gas phase,
usually by chemical vapor transport. Such CVT crystals may have high chemical
purity, as the growth is performed without contact to foreign material. The
crystallographic quality is often very high (free growth). 2) From melt fluxes
such as alkaline hydroxides or other oxides (MoO3, V2O5, P2O5, PbO) and salts
(PbCl2, PbF2). Melt fluxes offer the possibility to grow bulk ZnO under mild
conditions (<1000 deg. C, atmospheric pressure), but the crystals always
contain traces of solvent. The limited purity is a severe drawback, especially
for electronic applications. 3) From hydrothermal fluxes, usually alkaline
(KOH, LiOH) aqueous solutions beyond the critical point. Due to the amphoteric
character of ZnO, the supercritical bases can dissolve it up to several per
cent of mass. The technical requirements for this growth technology are
generally hard, but this did not hinder its development as the basic technique
for the growth of {\alpha}-quartz, and meanwhile also of zinc oxide, during the
last decades. 4) From pure melts, which is the preferred technology for
numerous substances applied whenever possible, e.g. for the growth of silicon,
gallium arsenide, sapphire, YAG. The benefits of melt growth are (i) the high
growth rate and (ii) the absence of solvent related impurities. In the case of
ZnO, however, it is difficult to find container materials that are compatible
from the thermal (fusion point Tf = 1975 deg. C) and chemical (required oxygen
partial pressure) point of view. Either cold crucible (skull melting) or
Bridgman (with reactive atmosphere) techniques were shown to overcome the
problems that are inherent to melt growth. Reactive atmospheres allow to grow
not only bulk ZnO single crystals, but also other TCOs such as {\beta}-Ga2O3
and In2O3.Comment: 10 pages, 7 figures, talk on MRS Fall 2011 Bosto
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Crystal growth and thermodynamic investigation of Bi2M2+O4 (M = Pd, Cu)
Phase equilibria that are relevant for the growth of Bi2MO4 have been studied experimentally, and the ternary phase diagrams of Bi2O3–PdO2–Pd and Bi2O3–Cu2O–CuO and its isopleth section Bi2O3–CuO were redetermined. It is shown that every melting and crystallization process is always accompanied by a redox process at the phase boundary and that for both title compounds, the valence of the transition metal is lowered during melting. Vice versa, during crystal growth, O2 must be transported through the melt to the phase boundary. Based on these new insights provided by our thermodynamic studies, Bi2CuO4 single crystals with a length of up to 7 cm and a diameter of 6 mm were grown by the OFZ technique to be used for investigations of magnetic, electronic and thermal transport properties. The grown crystals were characterized by powder X-ray diffraction, Laue, magnetization and specific heat measurements