Chemical Vapor Deposition Growth of Zinc Oxide on Sapphire with Methane: Initial Crystal Formation Process

Using a new high-temperature chemical vapor deposition (CVD)-based growth process for high-quality zinc oxide (ZnO) layers, the initial stages for heteroepitaxial growth on sapphire substrates with an aluminum nitride nucleation layer was investigated. A series of samples were grown with various supplies of zinc vapor, which can easily be controlled by the amount of the precursor gas methane (CH4) used to reduce the ZnO powder. In the substrate region, the zinc vapor was reoxidized by pure oxygen, which initially led to the formation of ZnO islands on the substrate, and for longer growth duration to the desired highly crystalline ZnO layers. To determine the details about this initial layer formation process, atomic force microscopy and scanning electron microscopy were used. We find that the ZnO microcrystals coalesce very fast and form a smooth and closed layer after a growth time of 10 min only. Electron backscatter diffraction measurements on this early stage of the layer formation show a perfect c-orientation of the ZnO microcrystals. Also high- resolution X-ray diffraction measurements support the perfect alignment of the ZnO layer and show a drastic increase in crystal quality over growth time. This increase in crystal quality is also demonstrated by low-temperature photoluminescence (PL) measurements, in which the spectra show sharp lines for the donor-bound excitons already for the ZnO microcrystals. The PL spectra also show clearly that the tendency of forming basal plane stacking faults is quite high when the ZnO microcrystals are starting to coalesce but anneal out very fast after coalescence.