Controlled texture of reactively rf-sputtered ZnO thin films

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Abstract
Detailed analysis by reflection electron diffraction (RED), x-ray diffraction (XRD), and scanning electron microscopy (SEM) was performed on thin ZnO layers which were formed under reactive and nonreactive rf-sputtering conditions. A variety of textures and morphologies were observed. 100% reproducible piezoelectric layers, preferred oriented with [002] perpendicular to the layer within 7°, could be obtained by reactive sputtering from a zinc target at rf power of 150 W, oxygen-argon atmosphere of 8×10−3 Torr with 35% O2, and with the glass substrate being kept at room temperature by a cooling device. No differences in the surface-acoustic-wave properties were found between reactively and nonreactively sputtered ZnO layers which had similar texture and morphology. SEM techniques proved to be extremely misleading in the study of this type of layer; there is no relationship between an observed columnar structure and the texture of the layer which is determined by RED, and also between the column thickness (∼1 μm) and the actual crystallite size (∼200 Å) which is determined by XRD. It is shown that the observed columns must be polycrystalline.