44251-AC10
Field Effect Transistors Based on Discrete Organic Semiconductor Grains
This project funded one graduate student who is pursuing the use of scanning probe microscopy techniques to understand the microstructure of ultrathin crystalline organic semiconductor films and to relate the microstructure to electrical properties. This student, David Ellison, is currently in his fourth year of graduate study in chemical engineering. His thesis work involves the use of Kelvin probe force microscopy (KFM) to image the surface potential distribution in ultrathin films of the organic semiconductor pentacene grown on gate dielectric substrates. He has found that the surface potential varies with the microstructure. In particular, using a new technique termed transverse shear microscopy (TSM), he and fellow graduate student Vivek Kalihari have shown that pentacene layers grow with both epitaxial and non-epitaxial relationships, i.e., the second monolayer of pentacene can grow epitaxially on the first layer, or non-epitaxially. The epitaxial domains have a different surface potential than the non-epitaxial domains. This is important because the surface potential influences the transport of charge at the pentacene/dielectric interface in organic field effect transistors (OFETs). What Ellison has shown is that the microstructure of ultrathin pentacene films at this interface is complex and that it is correlated with variations in a key electrical property, the surface potential. This finding provides a concrete example of a structure-property relationship that will influence the electrical performance of this material in devices.
Ellison has also pursued the use of chemical etching techniques to reveal line dislocations and point defects in pentacene films. This work is currently being drafted for publication. The key point is that it is possible to visualize defect densities by combining etching with scanning probe microscopy techniques.
In yet a third vein, Ellison is using KFM to map potential variations across discrete grain boundaries in ultrathin organic semiconductor films. In these experiments, he mounts a working OFET into an atomic force microscope and controls the carrier density in the film by means of a gate voltage. In this way he can measure the grain boundary potential as a function of gate induced charge density.
