Structure-performance relationship in pentacene/Al2O3 thin-film transistors

Pentacene based field-effect transistors were fabricated with alumina as the gate insulator. In some of the devices, the alumina surface was modified with a fatty acid self-assembled monolayer (SAM). Atomic force microscopy (AFM) shows that the growth of the vacuum deposited pentacene layer is strongly influenced by the surface treatment. On bare alumina, the first monolayer follows a two-dimensional growth, while the subsequent layers grow three dimensionally. By contrast, the growth is three-dimensional all along on a SAM modified surface. X-ray diffraction measurements indicate that the density of dislocations is higher when the film is deposited on bare alumina. The hole mobility of the pentacene film was measured as a function of gate bias. Again, very different behaviors were observed depending on the preparation of the alumina surface. On bare alumina, the mobility is found to increase continuously with gate voltage, while on a SAM modified surface it decreases after an initial increase. This is interpreted by noting that as the gate bias increases, charges tend to concentrate near the insulator-semiconductor interface. Accordingly, the observed features indicate that on bare alumina, mobility is higher close to the interface than in the bulk, while the reverse is true for a SAM treated oxide. (C) 2004 Elsevier B.V. All rights reserved.