Growth Morphology of Vicinal Hillocks on the {101} Face of KH 2 PO 4 : Evidence of Surface Diffusion
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ABSTRACT The growth morphologies of vicinal hillocks on KH2PO4 {101) surfaces have been investigated using atomic force microscopy. Both 2D and spiral dislocation growth hillocks are observed on the same crystal surface at supersaturations of -5 %. Growth occurs on monomolecular 5 A steps both by step-flow and through layer-by-layer growth. The distribution of islands on the terraces demonstrate that surface diffusion is an important factor during growth. Terraces that are less than the diffusion length do not contain any islands. This, together with the length scale of the inter island spacing and the denuded zones provide an estimate of the diffusion length. In situ experiments at very low supersaturation (-0.1 %) show that growth is a discontinuous process due to step pinning. In addition, in situ images allow for the direct determination of the fundamental growth parameters ac,the step edge energy, and 03,the kinetic coefficient. INTRODUCTION In recent years the nanometer-scale morphology of crystalline surfaces has received considerable attention. This interest is due to the large role that the nanoscale features play in the control of materials properties and performance. Most studies have considered the growth of surfaces by molecular beam epitaxy or chemical vapor deposition where the system is generally far from equilibrium both in terms of the flux of impinging molecules and the chemical potential. It has been shown both theoretically [1,2] and experimentally [2] that, in this regime, growth
progresses either by step-flow at pre-existing steps on vicinal surfaces or through layer-by-layer and multi-layer growth on nucleating islands. For example, during the growth of Si at typical deposition conditions, the critical nucleus consists of only a single atom, as shown in scanning tunneling microscopy studies [3]. Few studies [4-8] have given attention to the nano-meter scale morphology of single crystal surfaces grown at low-to-moderate supersaturation despite the fact that most bulk single crystals are grown in this manner. In this regime stable islands consist of tens of molecules and the classic dislocation controlled mode of growth first described by Burton, Cabrerra and Frank (BCF) [9] should be applicable. Recently, we reported AFM results on the growth morphology of KH2PO4 (KDP) grown at low to moderate supersaturations [4]. It was shown that the (101) surface advances on monomolecular steps even on dislocation-induced vicinal hillocks where the Burgers vector exceeds one unit step. Local supersaturations were calculated from the measured Burgers vectors and the hillock slopes. These results were then compared to the predictions of BCF-type theories. For Burgers vectors of more than one unit step, the hillocks were observed to contain hollow cores. The size of these cores compared well with theoretical predictions, and the shape demonstrated that the step edge energy is isotropic. We showed that, at moderate supersaturations (-5-10 %), the growth of KDP ( 101 ) surfaces occurs both on dislocation indu
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