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

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160As emphasized previously, no free-standing islands areobserved in either experiment. Therefore, it seems that theentire film grows in the <strong>step</strong>-flow growth mode. As shownabove, this growth mode is expected for the CaF interfacelayer for the terrace size of the Si substrate used here. For theCaF 2 layer, however, this cannot be true, since only very fewof the terraces are type-S terraces. Thus the CaF 2 does notform nuclei at every <strong>step</strong>. Consequently, the additional CaF 2double layer does not grow in the <strong>step</strong>-flow growth mode.This effect of very inhomogeneous CaF 2 multilayergrowth can be attributed to the B orientation of the CaF 2 film.After deposition of the CaF interface layer, the <strong>step</strong>s are nolonger preferential nucleation sites. For instance, at monoatomicSi substrate <strong>step</strong>s, additional CaF 2 molecules cannotbind strongly to the CaF molecules on the upper terrace, since(a)(b)(c)a filma <strong>step</strong>MM CaF[111]top Fbottom F[111][112][ 110]Si[112]M si[112]Fig. 8a–c. Models of second-layer CaF 2 molecules with B orientation adjacentto Si substrate <strong>step</strong>s. a Comparison of the bond length in [11¯2]direction for the binding at <strong>step</strong>s a <strong>step</strong> and within films projected a film formono-atomic <strong>step</strong>s. b Triangular B-oriented CaF 2 island in front of a CaFcovered mono-atomic <strong>step</strong>. c Comparison of the binding at double height<strong>step</strong>s for binding to the CaF film (M CaF ) and binding to the Si <strong>step</strong> (M Si )the lateral distance between the CaF 2 molecule on the regularsite on the lower terrace and the CaF molecule on the upperterrace is √ 7/3 of the regular bond length. The binding distanceis too large to have significant interaction between thetwo molecules.The same effect occurs also for multiple substrate <strong>step</strong>s,as demonstrated by Fig. 8b for a Si double <strong>step</strong>. The additionalCaF 2 molecule cannot be simultaneously on a regularsite with respect to the CaF monolayer (M CaF ) or with respectto the Si <strong>step</strong> atoms (M Si ).4 Film growthIn the preceding sections, it was established that the CaFinterface layer can grow in the <strong>step</strong>-flow growth mode at sufficientlyhigh temperatures (dependent on the terrace width).This mechanism does not work for the subsequent CaF 2 (double)layers, as will be demonstrated in this section.Figure 9 shows UHV–AFM micrographs recorded forCaF 2 films of various thicknesses deposited at 750 ◦ C. Duringthe first stages of film growth, the topography is governedby terrace-<strong>step</strong> structures <strong>induced</strong> by the Si substrate. In addition,however, one observes some triangular protrusions.They grow with increasing coverage and overgrow substrate<strong>step</strong>s.As reported in the previous section, the AFM topographysignal is not sensitive to different species of the CaF 2film, namely the CaF interface layer and the CaF 2 multilayergrown on top of the interface layer. Therefore, in addition, thefriction force signal is used to distinguish between the twospecies. Generally, all terraces are covered exclusively eitherby the CaF interface wetting layer or by thick CaF 2 ‘islands’.Thus, here, ‘islands’ denote CaF 2 stripes whose boundariesfollow substrate <strong>step</strong>s. The confining mechanism will be discussedbelow.Figure 10a shows the analysis of the friction force micrographsrecorded from CaF 2 films deposited at 500 and750 ◦ C. For the high-temperature deposition large parts of theinterface layer are exposed even after deposition of very thickfilms. For deposition at 500 ◦ C, the exposed interface layervanishes much faster with increasing film thickness. Practically,the interface layer is overgrown after deposition of10-TL CaF 2 .Combining both the deposited CaF 2 coverage and the exposedCaF interface coverage, one can estimate the averageheight, t, oftheCaF 2 ‘islands’. The result is presented inFig. 10b. The dashed line shows the height expected for perfectlayer-by-layer growth. On the one hand, the film grownat 500 ◦ C approaches this growth mode after deposition of10-TL CaF 2 , confirming the growth of a closed film for thiscoverage. On the other hand, the CaF 2 ‘islands’ grown at750 ◦ C are always thicker. Thus, CaF 2 grows in the Stranski–Krastanov growth mode at these high temperatures, and thelateral growth is strongly suppressed perpendicular to the<strong>step</strong>s.In the previous section, it was emphasized that substrate<strong>step</strong>s are not preferential nucleation sites for growth on topof the CaF interface layer. In addition, it is obvious from themorphology studies presented in this section that CaF 2 hasdifficulty in overgrowing the substrate <strong>step</strong>s (except for thetriangular protrusions). Again, the B orientation of the CaF 2
500nm(a)161(b)CaF 2CaFSi(111)tFig. 10. a Exposed CaF interface coverage as obtained from friction forcemicrographs. The interface layer is overgrown more difficultly at higher depositiontemperatures. b Average thickness of the CaF 2 islands obtainedfrom a and assuming that the CaF interface layer completely wets the Sisubstrate(c)Fig. 9a–c. AFM micrographs obtained after CaF 2 growth at 750 ◦ C: a 5TL,b 10 TL, and c 43 TL. With increasing coverage, the number of triangularprotrusions increases significantly. For the large coverage presented in c,the coalescence of the islands has startedfilm explains that substrate <strong>step</strong>s act as growth barriers. Thiseffect is demonstrated in Fig. 11. If the substrate has singleor double <strong>step</strong>s (Fig. 11a and b, respectively) the lattices ofboth ‘islands’ do not match. This is different if the substrate<strong>step</strong> has a TL height (or any integer multiple of it) as seen inFig. 11c. Here, the lateral shift due to the B orientation can becompensated. Since most of the substrate <strong>step</strong>s have heightsdifferent to (integer multiples of) the TL height, most of themact as growth barriers promoting the inhomogeneous growthof the CaF 2 film.Since CaF 2 islands cannot nucleate homogeneously at<strong>step</strong>s, nucleation on top of the underlying CaF interfacelayer and the pre-grown CaF 2 multilayer is the dominatingatomic process for the film growth. As soon as one islandhas been nucleated on a terrace it grows immediately intwo dimensions until its boundary touches the <strong>step</strong>s confiningthe terrace. Thereafter, the growth perpendicular to the<strong>step</strong>s is stopped and the island grows exclusively parallel tothem.This mechanism explains the strongly improved homogeneousgrowth at lower temperatures. The nucleus densityincreases with decreasing deposition temperature. Therefore,the film starts to grow simultaneously at more sites thanat higher temperatures, and more terraces (within the AFMscanning range) are covered by CaF 2 ‘islands’. This modelpredicts that the film grows homogeneously if the temperaturefalls below the critical temperature when the average islanddistance is less than the terrace width.For the terrace width of the substrates studied here(100–200 nm), this condition is fulfilled if the film is grownat 400 ◦ C, as demonstrated by the AFM micrograph presentedin Fig. 12a. The entire surface is covered by a homogeneousCaF 2 film. The observed multi-<strong>step</strong>s are due to theSi substrate. On top of the terraces, one obtains exclusivelymono-atomic islands. In addition, holes of mono-atomic (H)depth are observed. Therefore, the film grows almost perfectlyin the layer-by-layer growth mode.
Page 4 and 5: 158[111](a)(a)Si[112]D S T(b)[ 110]
Page 8 and 9: HH162250nmHP P[111](a)[112](a)100nm
Page 10 and 11: 164a(nm)b(nm)2.01.51.00.500602 4 64
Page 12: 166increasing coverage. Therefore,

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

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