Electrical conduction effects at low temperatures in undoped ZnO thin films grown by Pulsed Laser Deposition on Si substrates Ν. Brilis a, ⁎ , D. Tsamakis a , H. Ali b , S. Krishnamoorthy b , A.A. Iliadis b,c a School of Electrical Engineering and Computer Science, National Technical University of Athens, Iroon Polytechniou 9 Zografou, 15773 Athens, Greece b Department of Electrical & Computer Engineering, University of Maryland, College Park, Maryland 20742, USA c Department of Information and Communication Systems Engineering, University of the Aegean, Karlovasi, 83200 Samos, Greece Received 13 March 2007; received in revised form 16 December 2007; accepted 17 December 2007 Available online 23 December 2007 Abstract The electrical properties of undoped ZnO films grown by Pulsed Laser Deposition on Si substrates at growth temperatures between 150 and 250 °C and low O 2 partial pressures, were studied by resistivity and Hall coefficient measurements in the temperature range of 80 to 350 K. We report acceptor band and hopping conduction effects in these ZnO films, for the first time. P-type conduction is found to be dominant in these films for temperatures higher than approximately 235 K, while at temperatures lower than 235 K the films exhibit a conversion from p- to n-type conductivity. The electrical conductivity studies revealed a conduction mechanism by hopping in the acceptor band in the temperature range between 80 and 270 K. For temperatures higher than 270 K a thermally activated behavior, between the native acceptor band (Zn vacancies) and the valence band is dominant. The acceptor activation energy values extracted from the lnp versus T - 1 curves were found to be of 0.1 eV. The electrical mobility values fall-off rapidly from 90 to 8 cm 2 /V s for temperatures below 270 K, providing evidence that a mobility edge exists between transport in the valence band and transport in the acceptor band. © 2008 Elsevier B.V. All rights reserved. Keywords: Zinc-oxide; Acceptor band; Laser ablation; Hopping conductivity; Si 1. Introduction ZnO is a very promising material for electronic and optoelectronic devices [1–3], having also a good potential for applications in spintronic devices [4]. It is a wide band-gap semiconductor material having a large exciton binding energy (60 meV), which is important for applications in highly efficient room temperature UV Lasers and Light Emitting Diodes [4–6], as well as transparent electronic devices [7]. Undoped ZnO usually exhibits n-type conduction with high electron concentrations, which is attributed to a stoichiometric deviation from the native defects, such as oxygen vacancies (deep donors), Zn interstitials [8–10], and/or shallow hydro- genic donor defects that have been reported in the past few years [11–13]. The growth of stable p-type ZnO films doped with deep acceptors [7,13] demonstrating reproducible properties is a very difficult task mainly due to the self-compensation effects caused by the native shallow donor states [8,9]. The p-type conductivity of ZnO is necessary for applications in p–n junc- tion devices, such as UV diode lasers and other optoelectronic devices [5]. Such applications also require the use of p-type ZnO films having hole concentrations higher than 10 18 cm - 3 . Undoped p-type ZnO films grown by Pulsed Laser Deposition (PLD), Chemical Vapor Deposition and Metalorganic Vapor Phase Epitaxy techniques in O 2 rich conditions have been reported [14,15]. Our recent report indicates that PLD growth at low O 2 overpressures and low temperatures produces stable undoped p-type ZnO [16]. The p-character of these ZnO films is believed to be associated with the high competition effects occurring between the native shallow or deep donor and accep- tor defects [8,10]. The compensation effects are revealed by the temperature dependence of the apparent carrier concentration deduced by Hall effect measurements. High compensation Available online at www.sciencedirect.com Thin Solid Films 516 (2008) 4226 – 4231 www.elsevier.com/locate/tsf ⁎ Corresponding author. National Technical University of Athens, Electronic Materials Lab, 9 Heroon Polytechniou Str. Zographou Campus, GR-15773, Athens, Greece. Tel.: +30 210 772 2576. E-mail address: brilis_nikos@yahoo.gr (Ν. Brilis). 0040-6090/$ - see front matter © 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.tsf.2007.12.137