Nanoscale Measurements of Electronic Properties in Organic Thin Film Transistors
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I4.5.1
Nanoscale Measurements of Electronic Properties in Organic Thin Film Transistors
Oren Tal and Yossi Rosenwaks Department of Physical Electronics, Faculty of Engineering, Tel Aviv University, Tel Aviv 69978, Israel.
Yohai Roichman and Nir Tessler Department of Electrical Engineering Dept., Technion Israel institute of technology, Haifa 32000, Israel.
Calvin K. Chan and Antoine Kahn Department of Electrical Engineering, Princeton University, Princeton NJ 08544, USA
Abstract Kelvin probe force microscopy was used for extraction of the threshold and the pinch off voltages in organic thin film transistors. The first was determined by direct detection of the charge accumulation onset and the latter by a direct observation of the pinch off region formation. In addition, an effective threshold voltage shift can be extracted from the pinch-off voltage as a function of charge concentration. The dependence of the effective threshold voltage on the gate voltage must be considered when calculating charge carrier concentrations in organic thin film transistors.
I4.5.2
It is well established that charge carrier mobility in disordered organic semiconductors strongly depends on the charge concentration1. The primary experiment used to study this phenomenon is the transfer characteristics of organic field effect transistors (OFET)2-4. Quantifying the dependence of mobility on charge density is important for studying the underlying physics of the devices and for optimizing materials by extracting an effective disorder parameter5. The first important step in the analysis of an OFET is the determination of the threshold voltage (Vt) defined as the onset of charge accumulation. Since the mobility is charge density-dependent, the classical shape of the IS-VG curve is altered2 and the determination of Vt by back extrapolation may not be correct4,5. We have overcome the current detection problem by measuring the onset of the gate screening by charge accumulation in the channel6. In this case, a potential mapping of the channel region is done using Kelvin Probe Force Microscopy (KPFM)7. Moreover, we present a method for direct determination of the pinch-off voltage ( VPO ) using KPFM. VPO is defined as the drain-source voltage (VD) for which there is zero charge at the drainchannel interface and it is used to extract Vteff, the effective threshold voltage, at different gate voltages (VG). Vt eff is used instead of a standard threshold voltage, since the OFET acts as if it has a different threshold voltage at each gate voltage. The conventional threshold voltage is determined by the onset of the channel charge.
Experimental Apparatus and Methods OFET substrates were fabricated at the Technion Institute of Technology. These substrates consisted of a heavily doped p-type silicon gate electrode, a 90 nm thermally grown silicon-oxide gate insulator, and 50nm gold strips evaporated on the oxide to form the source and drain electrodes. A 50 nm thick film of N,NI -diphenylN,NI -bis(1-naphthyl)-1,10 -biphenyl-4,4II -diamine (a-NPD) wa
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