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ZnO films of high second order susceptibility. SEM images of two samples with -axis oriented nanocrystallites on -plane sapphire, grown at 400 °C (a) and 500 °C (b) with a thickness of 150 and 300 nm, respectively. The arrows mark the (0001) orientation (-axis) of the crystallites, whereas their -axes would be directed normal to the paper plane. The orientation was not concluded from these images itself, but from the measurements cf. Figs. 2 and 3.
X-ray diffraction curves (-scans) from (a) ZnO and (b) and ZnO . From the relative peak positions, the orientation of the ZnO grains with respect to the substrate is obtained. At the top, two rocking curves from ZnO are shown. Bullets: sample as in Fig. 1(a), open circles: sample as in Fig. 1(b).
Polarization characteristics of SHG at normal incidence for the ZnO film in Fig. 1(a). Experimental data (open squares) is compared to a simple theoretical model (, thick gray). Depending on polarization, two different components of the susceptibility tensor contribute (-polarization, dashed and -polarization, dotted). The peak positions at 90° and 270° correspond to a ZnO -axis orientation parallel to the electric field vector.
Measured (symbols) and simulated (lines) dependence of the SHG intensity on tilt angle. The asymmetry stems from saw-tooth like facetting of the crystallites. Both, -axis and -axis oriented ZnO films have been considered for tilt in the - and the -polarization plane. Theoretical curves are based on identical -tensor values, assuming a 6° tilt angle of the crystallite facettes relative to the surface. Film1a and film1b refer to the two different samples in Fig. 1. The -axis values are shown for comparison.
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