 UW-Madison MRSEC |
Interface Formation and Epitaxial Order in Nanoscale Organic Electronics |
Principal Investigators:
Paul Evans - evans@engr.wisc.edu
Thomas Kuech - kuech@engr.wisc.edu
| The integration of inorganic and organic electronics offers unique and unexploited opportunities in nanoscale design, leading to the development of science at the intersection of chemistry and electronic materials which will have a large technological impact. Organic electronics holds out the tantalizing possibility of combining control of the properties of materials through organic chemistry with the scalability of planar electronic processing. The optimum integration of organic molecules is the key issue and area of opportunity. Although inorganic materials, primarily Si for logic and III-V compounds for optoelectronics, will continue their dominant role for the foreseeable future, organic materials have unique and tunable properties that are unavailable in inorganic materials and bring the added technical advantages of low-temperature and potentially low-cost processing. The physical interface between organic and inorganic materials is supremely important in these devices and involves issues of molecular structure, bonding, chemistry, and electronic transport. At present, there is a poor quantitative connection between molecular order, surface and interface formation, and the properties of subsequent devices. The natural length scale of the problem is set by the molecular dimension and by interactions at the interface - both require nanometer-scale understanding and control. We are looking into two aspects of these integration problems. One avenue of investigation is the establishment of structural order in the organic FET channel material using the gate insulator as a structural template through a variety of crystal growth approaches. Our strategy combines conventional lithographic techniques and the growth of a crystalline gate insulator with the in situ deposition and electrical characterization of pentacene, as a model material. More extensive electrical, structural and optical characterization will be accomplished ex situ with capped films. The structure of the first few layers can be drastically different from bulk materials but it is as yet unresolved what systematic consequence this has for the electrical conduction in a transistor geometry. We will determine the structural order and the change in electrical transport, during deposition to look for critical transitions in the formation of the channel. Transport through inorganic-organic interfaces will also be studied. Many of the electronic properties of inorganic semiconductors can be controlled through band structure engineering, using heteroepitaxy and electronic doping to control, for example, the conductivity and electronic density of states. We will apply this control to fabricate epitaxial organic devices on specially prepared wide-bandgap semiconductors such as GaN. For example, there is a unique electronic structure that has not been exploited between GaN and many of the electronic organic molecules and could lead to orders of magnitude improvement in the electrical transport at this critical interface enabling new applications in solid-state lighting and organic electronics. Our studies will focus initially on the valence band-to-HOMO band line-up in pentacene and PPV with p-GaN as model systems. The predicted band line-up, leading potentially to a true hole-based ohmic contact, will be determined through UPS. The chemical composition and bonding at the interface will be determined via XPS and FTIR studies. Results of these studies will be used to determine new strategies concerning the interplay and connection between the electronic systems of the organic and inorganic materials. |
