ZnO crystals can be grown from alkaline aqueous solution not only by the standard hydrothermal technique at temperatures between 350 °C and 400 °C, but also by chemical bath deposition (CBD) at temperatures below 100 °C. In the presence of ZnO and ScAlMgO4 (SCAM) substrates almost all ZnO deposits on the substrate, with different habits, however. Under optimized conditions even homoepitaxial layers can be obtained, while rod-like structures are obtained on SCAM substrates. The chemistry and the driving forces behind the two processes are considered in detail and the temperature dependence of the solution composition has been calculated. The driving force for the ZnO crystal growth in the standard hydrothermal technique is the difference in the ZnO solubility in alkaline solutions at different temperatures. In contrast, the driving force for the chemical bath deposition of ZnO at low temperatures is the decay of zinc ion complex molecules with increasing temperature.
Keywords: Zinc oxide; Chemical bath deposition; Hydrothermal growth; Nanocrystals
Fig. 1. Scanning electron micrograph of ZnO nanorods and small islands grown on ScAlMgO4 substrates.
Fig. 2. Aligned growth of ZnO nanowhiskers on a ZnO substrate with non-specific crystallographic orientation.
Fig. 3. Homoepitaxial layer growth on ZnO substrates. Intentionally a disturbed area was selected, which gives an impression of the layer thickness. It is expected that under optimized conditions highly perfect closed layers can be grown over the whole substrate area.
Fig. 4. Calculated change of the masses of the most abundant Zn containing chemical species with increasing temperature. The solution was prepared from H2O, Zn(CH3COO)2·2H2O and concentrated NH3 solutions. Masses and volumes are given in Section 2 of the text.
Fig. 5. pH value and total masses of Zn containing species in solution and in solid state as a function of temperature. The pH value of the solution remains almost constant at about 10.2, but with increasing temperature more and more Zn containing solute is transformed into solid material.
Comparison between the growth of ZnO crystals by the standard hydrothermal technique and by chemical bath deposition
Solid State Sciences
Volume 12, Issue 3, March 2010, Pages 333-338
5th European Workshop on Piezoelectric Materials