14854
J. Phys. Chem. B 2010, 114, 14854–14859
Effect of Oxygen Plasma Treatment on Crystal Growth Mode at Pentacene/Ni Interface in Organic Thin-Film Transistors Bang Joo Song, Kihyon Hong, Woong-Kwon Kim, Kisoo Kim, Sungjun Kim, and Jong-Lam Lee* DiVision of AdVanced Materials Science and Department of Materials Science and Engineering, Pohang UniVersity of Science and Technology (POSTECH), Pohang, Kyungbuk, 789-784, Korea ReceiVed: July 9, 2010; ReVised Manuscript ReceiVed: September 26, 2010
We report how treatment of nickel (Ni) with O2 plasma affects the polarity of Ni surface, crystallinity of pentacene film on the Ni, and electrical properties of pentacene organic thin-film transistors (OTFTs) that use Ni as source-drain electrodes. The polar component of surface energy in Ni surface increased from 8.1 to 43.3 mJ/m2 after O2-plasma treatment for 10 s. From X-ray photoelectron spectra and secondary electron emission spectra, we found that NiOx was formed on the O2-plasma-treated Ni surface and the work function of O2-plasma-treated Ni was 0.85 eV higher than that of untreated Ni. X-ray diffraction and atomic force microscopy measurements showed that pentacene molecules are well aligned as a thin-film and grains grow much larger on O2-plasma-treated Ni than on untreated Ni. This change in the growth mode is attributed to the reduction of interaction energy between pentacene and Ni due to formation of oxide at the Ni/pentacene interface. Thus, O2-plasma treatment promoted the growth of well-ordered pentacene film and lowered both the hole injection barrier and the contact resistance between Ni and pentacene by forming NiOx, enhancing the electrical property of bottom-contact OTFTs. Introduction Bottom contact (BC) organic thin-film transistors (OTFTs) are promising candidates for devices such as pressure sensors, light sensor, and flexible displays because of the possibility of etching fine patterns on them using lithography.1-5 However, BC-OTFTs have low field-effect mobility due to high contact resistance (Rc) between source/drain (S/D) electrodes and the organic semiconductor (OSC).6-11 A hole injection barrier at the interface between S/D electrodes and OSC, and the inconsistent orientations of OSC molecules on metal and dielectric layers, cause poor hole injection efficiency from source electrode to OSC; this is the main cause of high Rc. For these reasons, selection of S/D electrodes materials and treatment of electrodes surfaces can contribute to reducing Rc of OTFTs. Because the quality of pentacene film on metal electrode is directly related to charge injection and charge transport at the metal/pentacene interface, understanding the interaction between pentacene and the metal electrode is very important. Kim et al. and Chang et al. have reported that surface treatment on Au electrode can control the growth mode of pentacene film.12,13 Gold (Au) is a typical material used as an electrode for pentacene-based organic thin film transistor (OTFTs) due to its chemical inertness and high work function (∼5.1 eV). However, when pentacene is deposited on Au, the vacuum level shifts downward at the interface, making a hole injection barrier at the Au/pentacene interface.14 This hole injection barrier could be reduced by O2-plasma treatment on the surface of Au electrode prior to the pentacene deposition. The O2 plasma resulted in the formation of a thin Au2O3 at the surface and made the work function increase.13 However, the Au2O3 was unstable and it spontaneously dissociated to gold elements with aging time.15,16 Thus, Au might not be appropriate in real * Corresponding author. E-mail:
[email protected]. Tel: +82(0)54279-2152. Fax: +82(0)54-279-5242.
application due to this instability as well as due to the cost and patterning problems. To solve these problems, the transition metals with high work function, such as cobalt (Co), palladium (Pa), copper (Cu), and silver (Ag), have been intensively studied to use as a conducting electrode in OTFTs.17-20 However, the pentacene film deposited on the transition metal showed a rough surface and very little ordering. Due to the π-orbitals of pentacene molecule and the high-density d-orbitals of the transition metal atoms, there exist of strong molecule-substrate interaction at the interface of pentacene with transition metal. The strong molecule-substrate interaction provides plenty of nucleation centers for thin-film growth of pentacene molecule and reduces its domain size effectively. Thus, pentacene molecules in the first monolayer lie flat on the transition metal surface and additional molecules will nucleate on this monolayer to form disordered with “lyingdown” geometry, leading to the reduction of electrical properties, via poor crystallinity with many kinds of interfacial defects.21 There were a number of attempts to improve the crystallinity of pentacene film to get a high performance of OTFTs. When the pentacene films were deposited on the preannealed (>600 °C) molybdenum (Mo) electrode on n-type Si substrate, the film became dense and its crystallinity was improved. The OTFTs fabricated on the annealed electrode showed 10 times higher on/off ratio and field effect mobility than that on as-deposited Mo electrode.22 However, the annealing temperature was too high to be applicable to OTFTs on the glass and/or plastic substrates. Self-assembled monolayer treatment on electrodes could improve the crystallinity of pentacene films.23 Octadecyltrichlorosilane (OTS) treatment on palladium electrode can improve the field-effect mobility of OTFTs.18 However, the OTS treatment required an additional annealing process at high temperatures (>100 °C, ∼3 h). Thus, such high-temperature annealing is not suitable for application to flexible OTFTs with plastic substrate. The Ni oxide film (NiOx), produced by reactive
10.1021/jp106364v 2010 American Chemical Society Published on Web 10/28/2010
Pentacene Deposited on O2-Ni Electrode
Figure 1. Bottom contact OTFT structure with Ni source-drain electrodes. The transistor has channel length of 5 µm, channel width of 200 µm, and pentacene thickness of 50 nm.
radio frequency sputtering under an O2 ambient, plays a role in reducing the carrier injection barrier between electrode and pentacene.24 However, the resistivity (106-108 µΩ · cm) of NiO was as high as 106-108 µΩ · cm, resulting in the high threshold voltage of OTFTs. Copper oxide (CuO) is a p-type semiconductor with high work function (>5.0 eV).25 Using a thermally evaporated CuO thin film on Au electrode could improve the field-effect mobility of OTFTs.26 The improvement can be explained in term of polarity of metal electrode and crystal growth mode of pentacene. Pentacene is a nonpolar organic molecule.27 The polarity at the surface of metal oxide (CuO, polar) is higher than that at the metal surface (nonpolar) because oxygen atoms have high electronegativity. The polar surface of metal oxide (CuO) repels nonpolar pentacene, and π-π interactions between pentacene molecules are stronger than the interaction of pentacene with the electrode, leading to standingup geometry of pentacene molecules. Thus, pentacene film deposited on metal oxide (CuO) showed the better crystallinity, resulting in the improved electrical property of OTFTs. However, thermally evaporation could cause a nonstoichiometry composition of Cu and O in the CuO film, which prohibited the thermally evaporated CuO from the application to industry. O2-plasma treatment is effective in producing a thin metal oxide (
