Materials Chemistry and Physics 127 (2011) 364–370 Contents lists available at ScienceDirect Materials Chemistry and Physics journal homepage: www.elsevier.com/locate/matchemphys Role of hydrogen addition in the plasma phase in determining the structural and chemical properties of RF sputtered ZnO films Gloria Gottardi a,∗ , Ruben Bartali a , Victor Micheli a , Nadhira Laidani a , Damiano Avi b a Fondazione Bruno Kessler, Center for Materials and Microsystems, PAM Unit, via Sommarive 18, I - 38123 Povo, Trento, Italy b University of Trento, Physics Department, Atomic and Molecular Physics Lab., Via Sommarive 14, I - 38123 Povo, Trento, Italy article info Article history: Received 1 September 2010 Received in revised form 8 February 2011 Accepted 9 February 2011 Keywords: Zinc oxide Sputtering Crystal structure Surface characterization abstract In the present work, ZnO thin films were RF sputtered from a pure ZnO target, without external heating, in H 2 :Ar plasma at different H 2 concentrations (0–50%). Aim of the study was the identification of the effects of H incorporation on the film growth and properties. During the deposition experiments, optical emission (OES) spectra were recorded to monitor any variation in the plasma chemical species relative to different process or gas mixture settings. X-ray photoelectron spectroscopy (XPS) and attenuated total reflection Fourier-transformed infrared spectroscopy (ATR-FTIR) were used to study the bulk and surface chemical composition of the films, while X-ray diffraction (XRD) analysis allowed lattice structure and grain size determination. The introduction of hydrogen in the plasma phase appears to strongly affect the structural and chemical properties of ZnO films. Both FTIR spectra and X-ray diffraction patterns showed that all the films crystallized in the hexagonal würtzite form. Nevertheless, while samples deposited in pure Ar plasma are highly textured, present- ing just one dominant preferred orientation along the [0 0 2] axis, films sputtered in H 2 :Ar atmosphere exhibit multiple growth directions with crystallites of noticeably reduced dimensions. Such a structural modification turns up together with clear variations in the films surface chemical state which appears to deviate from the pure oxide (Zn–O). By combining XPS, ATR-FTIR and OES data we could correlate such variations with the process induced H incorporation in the crystal structure in the form of hydroxide species. © 2011 Elsevier B.V. All rights reserved. 1. Introduction Fundamental and applied investigation of ZnO has been recently experiencing a renaissance due to the prospective use of such material in various technological domains and, in particular, as transparent conductive oxide (TCO) to be used in smart windows, flat-panel displays and electrodes for high efficiency solar cells [1]. The increasing attention towards this material stems from the unique combination of distinctive physical properties, low price, fair abundance and non-toxicity which place zinc oxide among the most interesting candidates for industrial applications, especially if compared to other oxides like CdO, ITO and TiO 2 [2]. Nevertheless, an effective exploitation of ZnO in such applications is currently still hampered by a lack of control over its physical properties, specifically its conductivity and majority-carrier type [3,4]. Unintentional and undesirable levels of conductivity in ZnO (and in general in wide-band-gap semiconductors) have been discussed ∗ Corresponding author. Tel.: +39 0461 314475/460; fax: +39 0461 810851. E-mail address: ggottard@fbk.eu (G. Gottardi). for years and traditionally always attributed to native point defects like interstitial zinc (Zn i ), Zn-on-O antisite (Zn O ) and oxygen vacan- cies (V O ) [5,6]. However, both theoretical and experimental works by Van De Walle, Cox and co-workers [3,7] have suggested an unex- pected extrinsic source for such unintentional n-type conductivity. They showed in fact that hydrogen, differently than in most semi- conductors where it occurs as an amphoteric impurity (an impurity that can be either a donor or an acceptor, thus counteracting the prevailing conductivity), behaves exclusively as a donor in ZnO, acting as an electrically active dopant impurity [3,5]. If we consider that it’s almost impossible to completely avoid H in any crystal growth environment, the importance of a clear understanding of the mechanisms governing its incorporation and interaction with the material becomes therefore straightforward. In this work, ZnO thin films were RF sputtered from a pure ZnO target, without external heating, in H 2 :Ar plasma at differ- ent H 2 concentrations (0–50%). The influence of the gas mixture hydrogen content on the oxide growth process and on its bulk and surface properties was in this way investigated. To this aim, dur- ing the sputtering process the plasma chemical species were in situ monitored in function of the different gas mixture settings by opti- 0254-0584/$ – see front matter © 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.matchemphys.2011.02.020