The growth of ZnO crystals from the melt

Abstract

The peculiar properties of zinc oxide (ZnO) make this material interesting for very different applications like light emitting diodes, lasers, and piezoelectric transducers. Most of these applications are based on epitaxial ZnO layers grown on suitable substrates, preferably bulk ZnO. Unfortunately the thermochemical properties of ZnO make the growth of single crystals difficult: the triple point 1975 deg C., 1.06 bar and the high oxygen fugacity at the melting point p_O2 = 0.35 bar lead to the prevailing opinion that ZnO crystals for technical applications can only be grown either by a hydrothermal method or from "cold crucibles" of solid ZnO. Both methods are known to have significant drawbacks. Our thermodynamic calculations and crystal growth experiments show, that in contrast to widely accepted assumptions, ZnO can be molten in metallic crucibles, if an atmosphere with "self adjusting" p_O2 is used. This new result is believed to offer new perspectives for ZnO crystal growth by established standard techniques like the Bridgman method.Comment: 6 pages, 6 figures, accepted for J. Crystal Growt

The Influence of Atmosphere on Oxides Crystal Growth

“…3). To sum up, although iridium is more sensitive to oxidation than platinum (E= -0.1474 V for Ir and E= -0.4422 V for Pt (Klimm et al, 2008c), it is the most chemically resistant of all metals. The high melting point of iridium (2716 K) and its resistance to attack by stable oxide melts is maintained up to temperatures above 2250 K make it a particularly suitable material for oxides crystal growth from hot melt which preclude the use of platinum alloys or rhodium.…”

“…Bulk ZnO crystals can be grown using melt (Klimm et al,2008a), hydrothermal (Suscavage et al,1999) and vapor phase growth techniques (Look et al, 2002). Unfortunately, traces of the solvent are always incorporated in such hydrothermal crystals and hydrogen and lithium are found typically in concentrations of several 10 18 cm -3 .…”

“…Single crystals of ZnO can be grown from the melt at high oxygen pressure and high temperature. The Czochralski (Klimm et al, 2008a) and Bridgman (Jacobs et al, 2009) methods are the most common melt-growth techniques used for production of the bulk single-crystal. The advantages of the Czochralski and Bridgman growth methods are relatively high growth rates (in the range of several millimeters per hour) and the nearly thermodynamically equilibrium conditions are capable to generate bulk crystals of high structural perfection.…”

“…They found that Crystal growth atmosphere composition and melt stoichiometry are strongly influenced on the quality of crystals as well as solid-liquid interface shape. However, the growth of oxides such as Gd 3 Ga 5 O 12 [Brandle et al, 1972], ZnO [Klimm et al, 2008a], SrLaGaO4 [Pajaczkowska et al, 2001], single crystals by standard melt growth techniques are difficult due to their high melting point and thermal decomposition upon heating. Otherwise, if the composition of melt would be serious, the chemical composition of these oxides melt will depart from the congruent point and superfluous components would become the harmful impurities for crystal growth.…”

Growth of oxide compounds under dynamic atmosphere composition

“…Fortunately, the p O2 (T ) supplied by CO 2 is sufficient to stabilize ZnO. In companion papers was reported recently on the melt growth of ZnO boules from Ir crucibles in a Bridgman-like setup [22,23,24,25]. During crystal growth T gradients must be established, hence compatibility of the materials must be guaranteed over a certain T range.…”