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

38Charge transport in the accumulation layer close to the SiO 2 surface can be systemicallystudied by measuring the mobility as a function of the pentacene coverage. The sourcedraincurrent in the contact limited device of two contact FET didn’t reflect the change inpentacene islands after 1 ML during growth. Using four-terminal devices, we studied howstructural changes in pentacene islands can contribute to the transport of holes in pentacenefilms around 1 ML. We deposited pentacene onto three van der Pauw geometry deviceswith different thickness. After deposition we removed these samples from the vacuumchamber and, after wiring them in air, measured the sheet resistance as a function of gatevoltage under vacuum in the deposition chamber. The probe current for the sheetconductance measurements in Fig. 2.5 was 4.2 nA.Fig. 2.5 (b) shows the AFM images of pentacene at the coverage ranging from 0.99 MLto 1.6 ML. At the coverage of 0.99 ML pentacene islands on SiO 2 are bright and the SiO 2substrate dark. At this coverage, vacant sites between pentacene islands are still beingfilled by additional pentacene and second-layer islands are visible on top of the first layer.The mobility of holes in the 0.99 ML pentacene layer was 0.016 cm 2 /Vs.At higher coverages, as more pentacene molecules were added, the vacant sites betweenislands were almost completely filled and second layer islands grew laterally. At thecoverage of 1.32 ML the dark-colored substrate is rarely seen and islands with dendriticshapes on the completed layer are observed. At the coverage of 1.6 ML the first layer wascompleted. For a 1.32 ML film, the mobility was a factor of two higher than at the lowercoverage even though the pentacene coverage had only increased by 30%. With 1.6 ML thesecond layer islands were larger, with the beginnings of the third layer on top of them (Fig.