Seed Layer Assisted Hydrothermal Deposition of Low-resistivity ZnO Thin Films
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Seed Layer Assisted Hydrothermal Deposition of Low-resistivity ZnO Thin Films Eugene Chubenko1, Vitaly Bondarenko1, Amir Ghobadi2, Gamze Ulusoy3, Kağan Topalli3, Ali Kemal Okyay2,3 1 Belarusian State University of Informatics and Radioelectronics, Minsk, 220013, Belarus 2 Department of Electrical and Electronics Engineering, Bilkent University, Bilkent, Ankara, TR 06800, Turkey 3 UNAM-National Nanotechnology Research Center and Institute of Materials Science and Nanotechnology, Bilkent University, Bilkent, Ankara, TR 06800, Turkey ABSTRACT In this work, we describe the combination of hydrothermal and atomic layer deposition (ALD) for growing low-resistivity ZnO polycrystalline continuous films. The effect of the thickness of ALD seed layers on the morphology of the hydrothermal ZnO films was studied. It was shown that ZnO films hydrothermally deposited on very thin seed layer consist of separate nanorods but in the case of 20 nm seed layer ZnO films transform to uniform continuous layers comprising of closely packed vertically aligned crystallites. Photoluminescence spectra were shown to exhibit broad band behavior in the visible range, corresponding to radiative recombination processes via oxygen defects of ZnO crystalline lattice, and narrow band in the UV region, associated with band-to-band recombination processes. It was shown that the resistivity of the obtained ZnO films is decreased gradually with the increase of ZnO films thickness and determined by the presence of crystal lattice defects in the seed layer. INTRODUCTION ZnO is a promising material for optoelectronics, photovoltaics and sensor applications [1, 2]. ZnO has unique combination of physical and chemical properties: wide direct band-gap, high exciton binding energy [3, 4] and transparency in optical and near IR-region [1, 5], prominent photocatalytic activity [6]. The key advantage of ZnO is that it can be fabricated by numerous techniques in the form of continuous films and nanostructures arrays even without lithography assistance [2, 3]. Molecular beam epitaxy, chemical vapor deposition, atomic layer deposition (ALD) and other vacuum based methods allow to deposit ZnO films with good crystalline quality. However, they become very expensive in the case of large substrates and have high operational temperature which is crucial for producing the integrated heterostructures [3]. Liquid phase electrochemical and chemical depositions are not so technologically demanding, but produce ZnO films with more structural defects and uncontrolled impurities [2, 7, 8]. Combination of vacuum based and liquid phase methods gives an opportunity to fabricate ZnO films with special structural and physical properties [2]. In this work, hydrothermal deposition as representative of liquid phase methods was applied to deposit thick ZnO continuous films on thin ZnO seed layers formed by ALD method on the Si and SiO2 substrates. It was supposed that seed layer could reduce the substrate influence on morphology of hydrothermal ZnO films, provides uniformity, good adhesion
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