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(a) Right: design of the QW structure. The thicknesses of the lower ZnMgO barrier, the QW, and the upper barrier are 2.6, 3.3, and 22 nm, respectively. Left: RHEED specular beam intensity oscillations recorded during the growth at of the structure parts, as indicated by the dashed lines. The growth rates are 0.48 ML/s (ZnMgO) and 0.43 ML/s (ZnO QW). The Mg content of the low-temperature ZnMgO barrier as deduced from the reduction of the period of the RHEED oscillations amounts to 11%. (b) AFM image of the surface morphology of the upper ZnMgO barrier layer. The height profile is taken along the line. The step heights correspond to 1 and 2 ML of ZnMgO, respectively. The root mean square roughness is 0.5 nm on a scan area of .
PL (solid) and PL excitation (dashed) spectra of the ZnO/ZnMgO QW structure presented in Fig. 1. To record the PL spectrum, the sample was excited at 3.8 eV. The PL excitation spectrum was measured, stetting the detection energy to the low-energy tail of the QW emission. The measurements are performed at 5 K.
HRTEM imaging and transmission electron diffraction of ZnO layers grown on at a temperature of . HRTEM imaging was performed along two orthogonal directions: (a) [01.0] and (b) [00.1]. The position of the interface between ZnO buffer and LT ZnO is marked in both images. In (a), BSFs are identified (see inset up right). For a better visualization of the stacking sequence, the image contrast is inverted.
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