Growth Front Roughening of Room Temperature Deposited Oligomer Thin Films
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GROWTH FRONT ROUGHENING OF ROOM TEMPERATURE DEPOSITED OLIGOMER THIN FILMS D. Tsamouras1, G. Palasantzas2, J. Th. M. De Hosson2*, and G. Hadziioannou1 1 Department of Polymer Chemistry and 2Department of Applied Physics, Materials Science Center, University of Groningen, Nijenborgh 4, 9747 AG Groningen, The Netherlands. ABSTRACT Growth front scaling aspects are investigated for PPV-type oligomer thin films vapordeposited onto silicon substrates at room temperature. For film thickness d∼15-300 nm, commonly used in optoelectronic devices, correlation function measurement by atomic force microscopy yields roughness exponents in the range H=0.45±0.04, and an rms roughness amplitude which evolves with film thickness as a power law ∝ d with b=0.28±0.05. The non-Gaussian height distribution and the measured scaling exponents (H and β) suggest a roughening mechanism close to that described by the Kardar-Parisi-Zhang scenario. INTRODUCTION Thin films grown under non-equilibrium conditions show in many cases scaling behavior, attracting considerable interest from the fundamental and the applications point of view, in relation to their physical properties [1-5]. During recent years there has been an increasing technological interest in organic thin films, either polymers or oligomers, as the active layer in molecular devices such as light-emitting-diodes (LEDs), photovoltaic devices (PVs) and field-effect-transistors (FETs) [2,6]. Injection, transport and recombination of charge carriers depend among other parameters on molecular packing, range of grain boundaries and roughness of the formed interfaces [7,8]. Therefore, control of the film morphology is of primary concern for the optimization of electro-optical properties of organic-based photonic devices [6]. So far, only scant research has been concentrated on growth properties of organic thin films showing the possibility of various scaling relations controlling their growth morphology [3-5]. These studies have shown that the rms roughness amplitude s evolves with film thickness as a power law ∝ d with b the growth exponent in the range 0.25 (with A: constant; A∼ / H ) [13]. The roughness exponent H describes the degree of surface irregularity at short roughness wavelengths (r2.9 nm (molecule length), significant lateral that yields correlation develops during growth. The calculation of g(r) requires the use of a scan size at least ten times larger than the significantly large cluster sizes seen in the AFM images in order to capture all of the necessary lateral roughness wavelengths [14]. The height distribution P(h) shows deviations from the best Gaussian fit especially in the negative tail. To quantify further this point we calculated the Skewness S = ∫ h 3 [ P(h ) / ∫ P( h)dh ]dh / 3 , which is a measure of the symmetry of a distribution around a reference surface level. For a Gaussian distribution S=0, while for the present case (i.e., figure 1) we obtain 0.2 0, indicating violation of the h→-h symmetry during growth. The latter implies the presence of a non-linearity th
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