Smoothing and coherent structure formation in organic-organic heterostructure growth

We use in situ real-time X-ray reflectivity and complementary atomic force microscopy to monitor crystallinity and roughness evolution during growth of organic heterostructures, i.e. perfluoropentacene (PFP) on diindenoperylene (DIP) and pentacene (PEN) on PFP. For both systems, surface smoothing during thermal evaporation of the second material on top of the first is observed. The smoothing can be rationalized by a, compared to homoepitaxy, lowered step edge barrier for one species diffusing on the other. In addition, we find an exceptionally well-ordered interface for PEN-on-PFP growth and PEN growth with anomalously low roughening, along with coherent scattering over the entire thickness. Copyright c © EPLA, 2010 Introduction. – The surface morphology and roughness of thin films and crystals depend on competing mechanisms, which either roughen or smooth the film surface during growth. Important roughening mechanisms, which tend to be dominating in most cases, are kinetic roughening based on shot noise and roughening due to mound growth, which is facilitated by reduced interlayer transport, often associated with a step edge, or EhrlichSchwoebel barrier. While these issues have been relatively well studied for growth of simple atomic species [1–3], comparatively little is known about organic systems. These are expected to exhibit a fundamentally different growth behavior, due to their different dominating interactions (van der Waals) and thus different response to strain and also due to their internal degrees of freedom (tilt angle as well as vibrational excitations and thus additional thermalisation channels). In crystalline organic thin-film growth, usually high diffusion lengths in the range of several micrometers [4] are possible, but also high step edge barriers of ≈0.7 eV have been reported [5,6]. Recently several interesting and new features for organic growth have been reported including anomalous rapid roughening [7–9], deep grain boundaries [10] and thickness-dependent step edge (a)E-mail: [email protected] barriers [5,11,12]. Theoretical models show that by neglecting the step edge barrier roughening would be strongly reduced [13]. These findings suggest that mound growth, induced by high step edge barriers, often dominates the roughness evolution for crystalline organic thin films. The situation is more complex for growth of organicorganic heterostructures [14], which are important for many device applications like organic photovoltaic cells or organic light-emitting diodes. In this letter we present the roughness evolution for organic-organic heterostructures and evidence for smoothing. We also show that if the out-of-plane crystal lattice spacing of both materials is sufficiently similar, the smoothing effect can be accompanied by a structure that scatters X-rays coherently even from the organic-organic interface. Experimental. – Thin films of diindenoperylene (DIP; C32H16; fig. 1(a)), perfluoropentacene (PFP; C22F14; fig. 1(a)) and pentacene (PEN; C22H14) were evaporated on silicon wafers with a 1.5 nm thick native oxide layer in ultra-high vacuum (base pressure < 5 · 10−9 mbar) by thermal evaporation. The substrate temperature T was held at 30 ◦C. The growth rate for all molecules was between 0.1 and 0.2 nm/min and monitored via X-ray reflectivity. In situ X-ray reflectivity