High performance n-type organic thin-film transistors with inert contact metals
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1154-B07-07
High performance n-type organic thin-film transistors with inert contact metals Sarah Schols1,2, Lucas Van Willigenburg1, Robert Müller1, Dieter Bode1,2, Maarten Debucquoy1,2, Jan Genoe1, Paul Heremans1,2, Shaofeng Lu3 and Antonio Facchetti3 1
IMEC v.z.w., PME-LAE, Kapeldreef 75, 3000 Leuven, Belgium Katholieke Universiteit Leuven, ESAT-INSYS, Kasteelpark Arenberg 10, 3000 Leuven, Belgium 3 Polyera Corporation, 8025 Lamon Avenue, Skokie, IL 60077, USA 2
ABSTRACT Thin film growth by high vacuum evaporation of the n-type organic semiconductor 5, 5'''diperfluorohexylcarbonyl-2,2':5',2'':5'',2'''-quaterthiophene (DFHCO-4T) on poly-(αmethylstyrene)-coated n++-Si/SiO2 substrates is investigated at various deposition fluxes and substrate temperatures. Film characterization by atomic force microscopy reveals typical Stransky-Krastanov growth. Transistors with Au source-drain top contacts and optimized DFHCO-4T deposition conditions attain an apparent saturation mobility of 4.6 cm²/Vs, whereas this parameter is 100× lower for similar transistors with LiF/Al top contacts. We explain this lower performance by the formation of a thin interfacial layer with poor injection properties resulting from a redox reaction between Al and DFHCO-4T.
INTRODUCTION Driven by potential applications of complementary logic, the field of electron-channel (ntype) organic thin-film transistors (n-OTFT) has recently gained a lot of attention. Using n-type materials such as fullerene[1], naphthalene[2], perylene[3] and oligothiophene[4] derivatives, high performance n-OTFTs have been reported. 5,5'''-diperfluorohexylcarbonyl-2,2':5',2'':5'',2'''quaterthiophene (DFHCO-4T, Figure 1) is an example of such a promising electron-conducting organic semiconductor. Recently, DFHCO-4T was shown to obtain an electron field-effect mobility of 1.7 cm²/Vs.[5] A top-contact geometry using Au source and drain contacts was used in these devices. In this paper, we discuss the optimization of DFHCO-4T growth and compare the performance of DFHCO-4T transistors with different top-contact metals. While most n-OTFTs need low-workfunction metal contacts such as Mg, Ca or LiF/Al for efficient electron injection into the semiconductor lowest unoccupied molecular orbital (LUMO),[5,6] we find that DFHCO-4T n-OTFTs only function properly with contacts that have a low chemical reactivity such as Au and Ag. Electron mobilities as high as 4.6 cm2/Vs are reported for Au top-contact transistors using optimized DFHCO-4T growth conditions. This fact is of high technological relevance because for use in complementary logic it is preferable to use a single type of source and drain metal for both the p-type and the n-type OTFTs. The reduced performance with easily oxidizable metals such as Al and Yb is explained as a consequence of an electron-transfer reaction occurring at the metal/DFHCO-4T interface.
Figure 1. Molecular structure of DFHCO-4T (ActivInkTM 0800).
EXPERIMENTAL Devices were fabricated on highly doped n++-Si wafers with a 140-nm thick thermally grown SiO2
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