Films of organic molecular semiconductors are grown by organic molecular beam epitaxy (OMBE) under different conditions, such as pressure, substrate type and temperature, absence or presence of external fields, for the study of the growth process itself and of the intrinsic properties of the molecular materials in the solid state and, in particular, in the form of thin layers, suitable for device applications. Several molecules, such as oligothiophenes, oligocenes, acridines, and porphyrines are studied. The OMBE growth process is monitored in-situ by reflectance anisotropy spectroscopy (RAS), which also gives insight on the evolution of the electronic properties of the films during growth. The morphology and structural properties of the samples, closely related to the growth mode, are then studied ex-situ, mainly by atomic force microscopy; finally, the optical and transport properties of the molecular films are studied in comparison with the single crystal properties.
Films of different molecules are grown on high quality single crystals of the same or similar molecular organic compounds, so as to reach homo- and hetero-epitaxy, therefore preparing artificial structures with high quality interfaces and controlled properties. Thin films are also stacked in multilayers on different inorganic and organic substrates. The morphology and structure of each layer, the interface quality, and the electronic states of the whole structure are studied by scanning probe microscopies and by optical techniques.
Single crystals of the same molecular compounds are grown from solution, from the vapour phase, and from floating drop, a technique developed in order to obtain crystals of higher quality in terms of structure and surface control, with shape and size suitable for their use as substrates for OMBE and for the structural and optical studies.
The OMBE apparatus consists of the introduction chamber, a second chamber for the sample metallization, and the deposition chamber, where six sources are installed for different compounds, a quartz microbalance controls the film thickness, and the RAS apparatus is installed; a fourth chamber for in-situ optical characterization is also available. Optical spectroscopies, such as absorption, reflection, photoluminescence and ellipsometry, are used for the study of thin films and multilayers down to few K, also in comparison with the properties detected in-situ by RAS. Atomic force microscopy is used for the morphology characterization of all the samples and for the study of the film growth process.