A poly(styrene-co-methyl methacrylate)/room-temperature sputtered hafnium oxide bi-layer dielectrics as gate insulator for a low voltage organic thin-ﬁlm transistors Ting-Hsiang Huang a , Kou-Chen Liu b , Zingway Pei a,⇑ , Wen-Kai Lin a , Shu-Tong Chang a a Graduate Institute of Optoelectronic Engineering, Department of Electrical Engineering, National Chung Hsing University, Taichung 402, Taiwan, ROC b Institute of Electro-Optical Engineering, Chang Gung University, Tao-Yuan 333, Taiwan, ROC article info Article history: Received 31 August 2010 Received in revised form 7 April 2011 Accepted 11 May 2011 Available online 26 May 2011 Keywords: Organic thin ﬁlm transistor Hafnium oxide Random copolymer Gate dielectric abstract In this study, we present a low voltage pentacene organic thin ﬁlm transistor (OTFT) with poly(styrene-co-methyl methacrylate) grafted hafnium oxide (PS-r-PMMA/HfO x ) as gate dielectrics. The HfO x was sputtered at room temperature to approach low temperature and meet low cost requirements of organic electronics. The thickness of the PS-r-PMMA can be controlled to be extremely thin (<10 nm). Consequently, the gate overdriving volt- age as low as 3 V was achieved for an OTFT. Furthermore, by applying PS-r-PMMA on top of the HfO x surface, the traps on the HfO x surface and possible pinholes of the HfO x can be pas- sivated. Therefore, the gate dielectric properties in terms of interface states and leakage current were improved. The leakage current reduced to approximately two orders of mag- nitude and the interface states reduced to nearly one order of magnitude. The reduction of the interface state density was observed by capacitance–voltage measurement. As a result, the subthreshold swing and on/off current ratio were improved signiﬁcantly from 1.01 V/ dec. and 10 3 to 0.41 V/dec. and 10 5 , respectively. Ó 2011 Elsevier B.V. All rights reserved. 1. Introduction In recent years, organic thin ﬁlm transistors (OTFTs) have become promising candidates for portable and ﬂexi- ble electronics due to their distinct advantages such as large area, lightweight, low cost, and low fabrication temperature. For portable applications, an OTFT with low driving voltage is the key issue for ensuring low power consumption. The driving voltage of an OTFT is determined mainly by the thickness and the dielectric constant of the gate dielectric. To achieve a low voltage operation, the dielectrics applied to an OTFT, including a self-assembled monolayer (SAM) [1], high dielectric constant (k) metal oxide [3,16], multi-component insulator [4], and hybrid or- ganic/inorganic materials [5,6] lead to gate capacitance enhancement for accumulating enough carriers in the channel at low voltage. Metal oxides such as HfO 2 ,Y 2 O 3 , and Ta 2 O 5 [3,5,7,16] were attractive for the gate dielectric due to their high dielectric constant providing high gate capacitance. However, the surface of a high-k metal oxide dielectric con- tains numerous hydroxyl groups (OH  ) at semiconductor/ dielectric interfaces that act as trap states in ambient envi- ronments [8,9]. The collision of channel carriers with these hydroxyl groups reduces carrier mobility. Furthermore, the interaction between these hydroxyl groups and ambient water molecules creates either donor-like or acceptor-like states causing the trapping and de-trapping process to reduce the device current and alter the subthreshold behav- ior of an OTFT. In addition to the traps, a lower energy gap of 1566-1199/$ - see front matter Ó 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.orgel.2011.05.014 ⇑ Corresponding author. Present address: Graduate Institute of Opto- electronic Engineering, Department of Electrical Engineering, National Chung Hsing University, 250, Kuo Kuang Rd., Taichung 40227, Taiwan, ROC. Tel.: +886 422851549x801; fax: +886 422851410. E-mail address: zingway@dragon.nchu.edu.tw (Z. Pei). Organic Electronics 12 (2011) 1527–1532 Contents lists available at ScienceDirect Organic Electronics journal homepage: www.elsevier.com/locate/orgel