Evaluation of organic sub-monolayers by X-ray based measurements under gracing incident conditions

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Published on 28 March 2009

The structural investigations of model organic systems like pentacene in the monolayer regime is very important for fundamental understanding of the initial nucleation process together with the electronic performance of transistor devices. The fact that the transistor performance saturates after deposition of some monolayers of the active organic material motivates a basic investigation of the submonolayer and monolayer regime in more detail. In this paper a method for the evaluation of the island formation and the island growth within the first monolayer is introduced. The method is based on X-ray scattering under grazing incident condition by means of specular X-ray reflectivity and off-specular X-ray scattering. From the specular reflectivity the electron density can be obtained which is directly correlated with the coverage of a submonolayer. Within the presented experiment coverages ranging from 7% up to 97% could be identified and are in excellent agreement with atomic force microscope results. Lateral information on the islands is obtained by rocking curve and detector scan measurements under grazing incident condition. The observed correlation peaks are evaluated by using Distorted Wave Born approximation, whereby mean island sizes ranging from 300nm to 1.5µm and mean island separation of about 2µm could be determined for the various samples. The obtained results encourages the use of this type of investigation for in-situ growth experiments to obtain a better understanding of the first monolayer formation.