Solid State Communications, Vol. 97, No. 1 I, pp. ?65-969, 1996 Printed m COP.YnpIht w;,=;$y;;+;~ re.at Bntam. All nghts reserved zyxwvuts 0038-1098(95)00749-0 SCALING OF SURFACE ROUGHNESS IN EVAPORATED CALCIUM FLUORIDE FILMS G.W. Mbise,* G.A. Niklassont and C.G. Granqvist Department of Technology, Uppsala University, P.O. Box 534, S-751 21 Uppsala, Sweden zyxwvutsrqponmlk (Received 30 October 1995 by B. Lundqvist) The surface roughness of evaporated CaF2 films was studied by Atomic Force Microscopy. Scaling exponents for kinetic roughening, cr and 0, were found to be close to 1 and l/2, respectively. These values were interpreted in terms of unstable growth of columnar structures due to Schwoebel barriers for diffusing atoms at steps in the surface. Keywords: A. thin films, A. surfaces and interfaces. 1. INTRODUCTION THIS PAPER reports on thickness-dependent surface reliefs of evaporated CaF2 films. Scaling exponents for the kinetic roughening were extracted and interpreted on the premise that surface diffusion is important for understanding the growth dynamics. Recently, the interest in thin film growth has soared as a consequence of the realization that the surface roughness often exhibits fractal [l] scaling properties. In particular, much attention has been directed towards the self-affine scaling of growing surfaces generated by various deposition methods (such as ballistic and random deposition) [2-41. Experimental studies of scaling encompass films made by evaporation [5], sputtering [6], molecular beam epitaxy [7, 81 and electrodeposition [9], as well as films modified by ion bombardment [lo]. Excepting epitaxial growth, the results are generally poorly understood, and it is patently clear that more comprehensive and careful studies are much needed. From the perspective of applications, surface rough- ness is of much interest since it is directly related to optical scattering. Here we report on a detailed investigation of CaFz hhns grown by thermal evaporation, i.e., by a technique believed [l l] to be described by ballistic deposition. We use Atomic Force Microscopy (AFM), capable of atomic resolution, to analyze surface reliefs in directions * Present and permanent address: Physics Depart- ment, University of Dar es Salaam, P.O. Box 350 63, Dar es Salaam, Tanzania. t Author for correspondence. perpendicular to and along the substrate. Earlier work on the roughness of CaF2 tihns, by Rasigni et al. [12, 131 and others [14], employed less accurate techniques. 2. EXPERIMENTAL Our calcium fluoride films were made in a con- ventional diffusion pumped evaporation unit with a LN2 trap. The base pressure was 8 x 10e4 Pa. CaF2 pellets (purity 99.95%) were evaporated from a resistively heated W boat with normal incidence onto Corning 7059 glass substrates 3Ocm above the vapor source. The deposition rate was measured on a calibrated quartz crystal microbalance (SLOAN MDC 9000). The rate was0 zyxwvutsrqponmlkjihgfedcbaZ 10 zyxwvutsrqponmlkjihgfe As-‘, and the film thickness h lay between 250 A and 12 000 A. After removal of the films from the evaporation system, their surface reliefs were analyzed by AFM (Park SFM-BD2-210) operating in air. The force between a pyramidal S&N4 tip and the sample was - 10 nN in repulsive mode. TRACK mode was used, and a typical scan frequency was 0.7 Hz. AFM measure- ments can be plagued by uncertainties and artefacts [ 151. The tip shape is a cause of concern [ 161 but is unlikely to seriously distort the images except at small length scales. Lateral forces [17], which in principle can produce a torque that significantly influences the imaging, are not likely to be a major effect. Hysteresis, and the fact that the tip truly scans a spherical surface, are believed to be of subordinate inhuence. Reliefs were imaged at 256 x 256 pixels. Initial scans over 3 x 3 pm were taken to ensure that dirt particles, hillocks and other artefacts were avoided. 965