Growth Techniques for Bulk ZnO and Related Compounds
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Growth Techniques for Bulk ZnO and Related Compounds Detlef Klimm, Detlev Schulz, Steffen Ganschow, Zbigniew Galazka, Reinhard Uecker Leibniz Institute for Crystal Growth, Max-Born-Str. 2, 12489 Berlin, Germany ABSTRACT 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 (1200°C in vacuum (10−6 bar), >1300°C in argon, and >1450°C in air [2, Fig. 7 there]. For T > 1600°C the vapor pressure becomes so large that reasonable transport rates for sublimation growth (physical vapor transport, PVT) can be obtained [3]. It should be noted, however, that such crystals are typically only on the millimeter scale. 1975°C 1800°C
1600°C
1400°C
1200°C
1100°C
log[p]
0
Zn
-2
-4
-6
O2
O
4.25
5.25
6.25
10000 K/T
7.25
Fig. 2: Vapor pressure of different species over ZnO. 1975°C is the melting point of ZnO. The horizontal line corresponds to 1 bar.
Significantly better crystals can be grown by chemically assisted vapor phase transport (CVT). In Erlangen (Germany) the growth of bulk ZnO by different techniques was performed since the 1930s (see next subsection), and there E. Scharowsky [4] performed a PhD work based on an apparatus that is schematically drawn in Fig. 3. There liquid Zn metal (melting point 419°C) was held inside a ceramic tube at ca. 600°C. It should be noted that the volatility of Zn is way higher compared to ZnO, and reaches at 600°C already 15 mbar (boiling point at ambient pressure is ca. 1200°C). A flow of ca. 1100 ml/min oxygen-free nitrogen (with ca. 60 ml/min hydrogen added to avoid accidental Zn oxidation) transports the evaporated zinc via a conical tube outlet to a second, larger furnace that is held at higher temperature (maximum 1200°C). This furnace is partially open and allows the inflow of air. The N2 flux must be adjusted in such a manner that backflow of air to the Zn metal is prohibited, but on the other side a too strong “blow” prohibits crystal growth at the rim of the cone. Under optimum conditions colorless ZnO needles with up
to 4 cm length and a few tenth of a millimeter diameter with almost perfect hexagonal cross section could be obtained. Sometimes lancet shaped single crystals were harvested. A too large gas flow results in microcrystalline ZnO fume, under a too large H2 flux the crystals become yellowish. air Zn
ZnO
N2 + H2 600°C 1150°C
Fig. 3: The setup that was used by Scharowsky (1953) [4] for the CVT growth of ZnO single crystals.
Later this technique was modified by Helbig [5] who used annealed ZnO feed as source material. With similar N2/H2 flow rates through the feedstock, and a separately controlled flow of pure O2 to the growth zone, ZnO crystals
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