PhD thesis - Evans Research Group - University of Wisconsin-Madison

30believed to result from impurities and structural disorders introduced during growth oforganic semiconductors [24]. Above the threshold voltage, the charges fill the trap sites. Ahigh threshold voltage is thus related to a high trap density [23, 24]. The trap density canbe expressed by [24]:NtrapCgdVT= (2. 2)eHere, N trap represents the concentration of traps.We suspect that the traps are not uniformly distributed. When islands finally touch eachother, the contacts could produce a higher trap density at the junction due to themisorientation between islands, and a higher threshold voltage at the contact can beexpected.The current onset after 1 ML at our percolation threshold measurements is thought to bedue to the delay caused by a very high threshold voltage at the junction. Although thecoverage of pentacene on SiO 2 exceeded the theoretically predicted percolation threshold,0.67, a high structural disorder at the contact between islands induces a high trap densityand this prevents mobile charge carriers from accumulating at the junction, resulting in ahigh contact resistance at the junction. The threshold voltage for the device is thus notattainable in the scan range of the gate voltage in our percolation experiments.The threshold voltage below 1 ML in Fig. 2.1(b) was close to 80 V. This is much highervalue compared with those of thick pentacene FETs. The threshold voltages of thickpentacene films are in the range between -30 and 30 V. The trap density of 1 ML pentaceneis 9×10 12 cm -2based on equation (2.2). The molecular density of the single crystalpentacene is 2.9×10 21 cm -3 [25] and the thickness of 1 ML pentacene is 1.5 nm. From the