Investigation of space charge at pentacene/Au interface with UV/ozone treatment by a near-field microwave microprobe Eunju Lim a , Takaaki Manaka a , Mitsumasa Iwamoto a , Barry Friedman b , Arsen Babajanyan c , Songhui Kim c , Youngwoon Yoon c , Seunghwan Kim c , Kiejin Lee c, ⁎ a Department of Physical Electronics, Tokyo Institute of Technology, 2-12-1 O-okayama, Meguro-ku, Tokyo 152-8552, Japan b Department of Physics, Sam Houston State University, Huntsville, Texas 77341, USA c Department of Physics and Interdisciplinary Program of Integrated Biotechnology, Sogang University, Seoul 121-742, Republic of Korea Available online 29 April 2007 Abstract The near-field microwave microprobe (NFMM) and Kelvin-probe measurements were performed to evaluate the conductivity in terms of surface potential of pentacene films on Au electrode. The UV/ozone treated and untreated Au electrodes were prepared to reveal the relationship between the electrical conductivity and interfacial electrostatic charging phenomena. For the pentacene film deposited on the Au electrode with UV/ozone treatment, holes were displaced from Au electrode to pentacene film, resulting the accumulation of apparent positive charges in the interfacial region of pentacene film around Au electrode. The NFMM measurement revealed that the reflection coefficient of microwave, S 11 increased with UV/ozone treatment, indicating the resistance decrease of the surface. Accumulation of positive charge in the interfacial region of Au electrode/pentacene film is one of the essential reasons for the increase of the conductivity of pentacene film. © 2007 Elsevier B.V. All rights reserved. PACS: 68.37.-d; 68.37.Uv; 68.55.-a; 68.60.-p Keywords: Near-field; Microwave microprobe; Kelvin-probe; Pentacene; Space charge; UV/ozone treatment 1. Introduction Recently, organic semiconductor devices, such as organic thin-film transistors (OTFTs) and organic light emitting diodes (OLEDs) have attracted considerable attention [1–3]. Carrier injection, carrier separation at metal/organic interfaces and carrier transport in the organic films are key processes in organic semiconductor devices [3]. Pentacene is one of the promising organic materials for the practical applications for OTFTs, OLEDs and photovoltaic cells because of its high mobility and high electrical conductivity [4]. The pentacene FET using the Au source and drain electrodes exhibits the typical p-type semiconductor behavior with a mobility over 1 cm 2 /Vs and an on–off ratio of N 10 6 [5]. Many studies have investigated the improvement of film quality to enhance device performance [6]. However, it is equally important to understand the metal/organic interface to improve device performance. In a previous report, a positive surface potential was observed for the pentacene film deposited on gold (Au) surface [7]. This is possibly a result of displacement of excess charges at the interface because of a change in the apparent work function of the metal surface. The space charge at Au/pentacene thin film interfaces could be qualitatively evaluated by a surface potential measurement. However, in order to further understand electro- static phenomena at the metal/organic interface instead of the previously reported Kelvin-probe surface potential method, we take advantage of the non-contact and nondestructive evalua- tion capabilities of a near-field microwave microprobe (NFMM) [8–11]. It is noteworthy that NFMM can selectively probe mobile carriers contributing to the conductivity at the metal/ organic interface in response to microwave. In this paper, we study the conductivity changes of Au thin films and the surface potential properties at the interface of a pentacene film on an Au substrate with and without UV/ozone treatments using an NFMM. We used an NFMM coupled to a high-quality dielectric resonator with a distance regulation system at an operating frequency f = 4.4–4.5 GHz. The electrical Available online at www.sciencedirect.com Thin Solid Films 516 (2008) 2573 – 2576 www.elsevier.com/locate/tsf ⁎ Corresponding author. Tel.: +82 2 705 8429. E-mail address: klee@sogang.ac.kr (K. Lee). 0040-6090/$ - see front matter © 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.tsf.2007.04.091