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 (CH 4 ) 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 highresolution 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.

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The growth of single-crystalline high-quality zinc oxide (ZnO) layers by a methane (CH 4 )-based chemical vapor deposition (CVD) growth process on sapphire substrates with an aluminum nitride (AlN) nucleation layer was investigated. We achieved monocrystalline ZnO layers free of rotational domains, which show in high-resolution X-ray diffraction (HRXRD) measurements a very narrow (∼110 arcsec) full width at halfmaximum (fwhm) in ω scans for the ZnO (0002) reflection. The influence of growth time and layer thickness on crystal properties such as surface roughness, dislocation density, and optical properties was investigated. We find low edge and screw dislocation densities of around 6.4 × 10 8 and 2.1 × 10 7 cm −2 , respectively. In low-temperature photoluminescence (PL) spectra the fwhm of the donor-bound exciton emission drops to about 170 μeV for increasing layer thickness. Moreover, these layers have a smooth surface with a surface roughness RMS value of 4 nm and a very low donor concentration of about 1.7 × 10 15 cm −3 . We also studied the influence of substrate miscut on crystal growth properties and found no significant influence. The results prove the high potential of methane-based chemical vapor deposition for the production of high-quality ZnO layers.

This work presents a H2S selective resistive gas sensor design based on a chemical field effect transistor (ChemFET) with open gate formed by hundreds of high temperature chemical vapour deposition (CVD) grown zinc oxide nanowires (ZnO NW). The sensing ability of pristine ZnO NWs and surface functionalized ZnO NWs for H2S is analysed systematically. ZnO NWs are functionalized by deposition of discontinuous gold (Au) nanoparticle films of different thicknesses of catalyst layer ranging from 1 to 10 nm and are compared in their gas sensing properties. All experiments were performed in a temperature stabilized small volume compartment with adjustable gas mixture at room temperature. The results allow for a well-founded understanding of signal-to-noise ratio, enhanced response, and improved limit of detection due to the Au functionalisation. Comprehension and controlled application of the beneficial effects of Au catalyst on ZnO NWs allow for the detection of very low H2S concentrations down to 10 ppb, and a theoretically estimated 500 ppt in synthetic air at room temperature.