Growth and Structure of Organic Semiconductors

Organic semiconducting materials such as acenes, perylene-derivatives, and phthalocyanines are presently receiving much attention due to their attractive optical and structural properties. Several spectacular phenomena and device applications have been demonstrated, such as diodes, field-effect transistors up to all-organic integrated circuits, photodiodes, photovoltaic cells, and light emitting diodes [1]. In fact, many of the devices and phenomena known from inorganic semiconductors, which are determining much present-day technology, have been realized with organics, and we are presently facing what may be called a "molecular electronics revolution".

While it is clear that for certain applications like high-speed electronics inorganic materials are superior in terms of performance (due to their higher charge carrier mobility), organic materials can have advantages in certain other respects [2]. One of the attractive features of organics is the potential tunability of their functional properties by means of chemical synthesis. For instance, the emission colour in optoelectronic devices can be tuned by attaching or replacing a certain functional group while leaving the "core" of the molecule unchanged. New and different parameter ranges, e.g., for the effective mass of the charge carriers and the dielectric function and, thus, new areas of physics, are accessible.

For device applications, it is generally agreed that the use of single crystals is less practical, and consequently much effort is devoted to the growth of highly ordered thin films [4]. To this end, organic molecular beam deposition (OMBD) holds the promise for optimum control of the structure and purity. Yet, there are several challenges to face, which are both of practical relevance and of fundamental interest:

• growth of objects with significant internal degrees of freedom (molecules as opposed to atoms)
• epitaxy with large unit cell mismatch and its relationship with the phase diagramme
• interdiffusion at the organic-inorganic interface
• charge transfer at the interface
• nature of the interaction / bond at the interface

Our recent work in this field focuses on several aspects of these systems:

• the growth kinetics of organic molecules
• the phases and phase transitions of organic films
• adsorption and wetting behaviour
• the epitaxy and stability of the organic-inorganic interface
• interactions and electronic effects at the organic-inorganic interface
• the optical properties of organic thin films