Substrate-step-induced effects on the growth of CaF2 on Si (111)

Furthermore, Fig. 5b demonstrates that the growth oftype-D islands is stopped at the Si substrate <strong>step</strong>. The type-DCaF stripe is only on top of the upper Si terrace; no materialis on top of the type-S CaF stripe on the lower terracerunning perpendicular to the type-D stripe. This effect is observedalso for type-T islands (Fig. 5a). Therefore, it can beconcluded that Si substrate <strong>step</strong>s act as growth barriers. Thiswill be discussed in more detail below.(a)1593 Initial stages of growth on the CaF interface layerFigure 6 presents an STM micrograph of a 1.7-TL CaF 2 filmdeposited at 600 ◦ C on a Si substrate with very short terraces.The surface exhibits <strong>step</strong>s of both mono-atomic and bi-atomicheights.Clearly, one can distinguish two different kinds of terraces.On the one hand, type-R terraces are covered by“rough” films with many point defects. On the other hand,type-S terraces are extremely smooth and defect free. By investigatingvarious deposition amounts, it is clear that type-Rand type-S terraces are covered by the CaF interface layer andCaF 2 films, respectively.Point defects in CaF islands have been reported previously[33]. They are attributed to the ejection of Si atomsfrom the 7 × 7 reconstructed surface due to the transformationof the 1×1 Si interface layer underneath the CaF island.These Si atoms are incorporated into the CaF layer. For theadditionally deposited CaF 2 molecules diffusing on the interfacelayer, however, these defects do not act as strong bindingcenters since no nucleation occurs at these sites. At least thenucleation process is strongly suppressed, since there are noislands observed in the STM micrograph.Figure 7a presents a UHV–AFM micrograph recorded insitu after growth of 1.4-TL CaF 2 at 750 ◦ C. Obviously, thefilm covers the entire Si substrate and generally mimics theregularly <strong>step</strong>ped substrate structure.The normal force on the AFM cantilever was used torecord the film topography. Lateral forces are recorded sim-500nm(b)(c)RSFig. 7. a AFM micrograph obtained after deposition of 1.4-TL CaF 2 at750 ◦ C. The film covers entirely Si substrate terraces. In addition, very fewtriangular protrusions are observed. b Friction force micrograph of the areamarked in a. The friction contrast is used to distinguish CaF (strong frictionsignal, light colored terraces) andCaF 2 layers grown on top of the CaF interfacelayer (small friction signal, dark colored terraces). c Lateral frictionforce obtained from the line scan marked in b50nmRRFig. 6. STM micrograph obtained after deposition of 1.7-TL CaF 2 at600 ◦ C. The arrows indicate double <strong>step</strong>s. The residual <strong>step</strong>s are monoatomic.Two types of terraces can be distinguished: R – terraces coveredwith a rough CaF layer; S – smooth terraces covered with a CaF 2 film. TheCaF 2 film is grown with double layer thickness on top of the CaF interfacelayerultaneously. Figure 7b shows the lateral force for the areamarked by the white rectangle in Fig. 7a. Since, duringrecording, the normal force was kept constant by a feedbackloop, the contrast due to the lateral forces is caused by differentfriction forces between probe and film. The stronglypronounced signal (white lines) originates from the <strong>step</strong>s. Inaddition, there is a clear contrast between different terraces.Terraces with an exposed CaF interface layer have a largerfriction signal than terraces covered with CaF 2 multilayers[34]. At the moment it is not clear whether the increasedfriction is due to the <strong>induced</strong> dipole forces between the cantilevertip and the CaF interface layer or due to topological<strong>effects</strong>, e.g. the defects observed by STM (Fig. 6) which cannotbe observed by AFM due to its limited lateral resolution.