Electrical characterization of Cr Schottky contacts on undoped and Ni-doped p-ZnO films grown by pulsed laser deposition on Si (1 0 0) substrates M. Stamataki a , D. Tsamakis a,⇑ , J.P. Xanthakis a , H.A. Ali b , S. Esmaili-Sardari b , A.A. Iliadis b a School of Electrical and Computer Engineering, National Technical University of Athens, 9 Heroon Polytechniou Str., Zographou Campus, 15780 Athens, Greece b Department of Electrical and Computer Engineering, University of Maryland, College Park, MD 20742, USA article info Article history: Received 22 December 2011 Received in revised form 25 October 2012 Accepted 28 November 2012 Available online 7 December 2012 Keywords: ZnO Pulsed laser deposition (PLD) Schottky AFM Electrical measurements abstract Wide band gap semiconducting layers of undoped and Ni-doped p-type ZnO thin films were developed on (1 0 0) silicon substrates by pulsed laser deposition (PLD) at a constant oxygen partial pressure of 1.0  10 4 Torr and at constant temperature of 250 °C. The electrical properties of the films were studied by resistivity and Hall coefficient measurements at room temperature. P-type conductivity was found to be dominant in these films at room temperature although the incorporation of Ni dopants caused a reduction in the p-character of ZnO films. The surface morphology of the films was evaluated by contact mode atomic force microscopy (AFM) in air. Cr-Schottky contacts were deposited on these p-type ZnO films, and the rectifying properties of the contacts were evaluated. The current–voltage (I–V) character- istics of the Cr/ZnO Schottky diodes were measured at room temperature. Due to series resistance, the Cheung and Cheung functions were used to determine the barrier height and ideality factor of the diodes while Norde function was used to determine barrier height. Ideality factors of 1.07 and 1.22 were obtained for the Cr/undoped ZnO and Cr/Ni-doped ZnO diodes, respectively, while the barrier height was 0.59 eV by the Norde model, and in good agreement with 0.62 eV of the Cheung–Cheung model, for both undoped and Ni-doped samples. Ó 2012 Elsevier B.V. All rights reserved. 1. Introduction ZnO is an attractive wide band gap semiconductor and a prom- ising material for transparent electronic applications such as short- wavelength LEDs [1,2], lasers [1] and UV detectors [3] because it can be obtained chemically stable and easily deposited on different semiconductor substrates such as sapphire and GaAs [4–6]. The use of ZnO in a variety of applications is mainly due to its impor- tant properties of a direct band gap with an energy gap of 3.37 eV at room temperature and a larger exciton binding energy (60 meV) than GaN (25 meV) [7]. ZnO grows nominally n-type which is believed to be due to large concentrations of intrinsic de- fects in the form of oxygen vacancies and several articles deal with extrinsic doping of ZnO to achieve controllable and stable p-type doping using various dopants [8–10]. Furthermore, several recent articles reported undoped intrinsic p-type ZnO grown by various techniques [11–21]. The observed intrinsic p-type conductivity de- pended on growth conditions and the activation temperature of the intrinsic defects, suggesting a self-compensation process be- tween the intrinsic donor and acceptor-like defects [15]. Schottky diodes, are necessary for applications in photodetectors and sen- sors [22]. N-type ZnO-based Schottky diodes have been demon- strated using low reactive metals such as Pt, Au, Pd with a barrier height of 0.6–0.9 eV [23,24]. Au as a contact metal on n- type ZnO can produce Schottky contacts due to its higher work function than that of the semiconductor’s (U m > U s ). However, in p-type ZnO it was reported to act as a low resistance Ohmic contact after annealing [25,26]. As deposited Au contacts on our p-type ZnO films grown on Si (not shown here) did not show diode recti- fying characteristics but nearly linear characteristics dominated by series resistance. Chromium (Cr) has been typically used as a Scho- ttky contact on p-type GaN [27] and p-type GaAs [28] thin films with successful results showing also remarkable thermal stability. Recently, Cr has been used as Schottky contact on laser annealed ZnO films that showed p-type conductivity after laser annealing [20]. In order to explore the capabilities of the intrinsic p-type ZnO films further, Ni doping was developed. The incorporation of transition metals (TMs) like Ni, Ti, V, Co and others can have advan- tages in developing the electrical, optical and magnetic properties of ZnO for device applications and has been investigated by several authors [29–36]. Although Ni is considered to be an n-type dopant, its role is still not well understood for the n-type ZnO matrix [29– 33,35]. The incorporation of Ni dopants in our undoped intrinsic p-type ZnO films reported for the first time here provides an important comparison between the undoped intrinsic and Ni- doped p-ZnO, in terms of electrical and transport properties 0167-9317/$ - see front matter Ó 2012 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.mee.2012.11.021 ⇑ Corresponding author. Tel.: +30 2107723846/2576; fax: +30 2107722281. E-mail address: dtsamak@central.ntua.gr (D. Tsamakis). Microelectronic Engineering 104 (2013) 95–99 Contents lists available at SciVerse ScienceDirect Microelectronic Engineering journal homepage: www.elsevier.com/locate/mee