Much progress has been made in recent years in the understanding of island shapes in epitaxial growth systems. Depending on fundamental growth parameters such as surface temperature and deposition rate, the deposited atoms form aggregates that may have a variety of shapes such as strongly ramified isotropically (fractal), anisotropically ramified with branching along some preferred symmetry directions (dendritic) or polygonal, compact shapes (faceted). 

Fractal and faceted shapes have been repeatedly observed in a number of metallic systems. In particular, previously published STM (Scanning Tunneling Microscopy) results on homoepitaxial growth of Pt(111) surfaces [1] provide evidence of fractal growth at low temperatures (~ 200 K) due to diffusion-limited aggregation of the adsorbed atoms. At higher temperatures (300 - 600 K), the growth shape is polygonal in most cases (triangular islands) but dendritic-skeletal island shapes were also observed. The occurrence of faceted versus dendritic morphologies is a consequence of specific anisotropies of the growth process.

At variance from the growth of (111) surfaces, the observed morphologies in homoepitaxial growth of (001) metallic surfaces have been compact square islands, square pyramidal structures or completely irregular deposits but, to our knowledge, dendritic morphologies, which we are reporting here for Ag(001), have never been reported. Another observation presented here is on the effect of a surfactant on the shapes of the islands. Surfactants are used in epitaxial growth to enhance wetting and reduce the surface roughness of growing films. Mostly, this is caused by enhancing the interlayer mass transport across different surface levels, in agreement with our observations described below.

The measurements were done at the surface diffraction beamline ID3. The incoming angle and the exit angle of the specularly reflected X-rays with the sample surface were equal and adjusted such that the phase difference between the scattered amplitudes of two consecutive (001) planes was precisely rad. Under these conditions, the diffracted intensity is largely sensitive to the short-range correlations at the surface such as those arising from the distribution of surface terraces.

2-D images of the scattered intensity were monitored with a commercial CCD camera mounted on the arm of the diffractometer. Images of the scattered X-ray beam were taken during film growth at a rate of 3.7 s per frame (2.5 s exposure to the X-rays plus 1.2 s readout). A typical experiment consisted of setting a sample temperature and recording successive 2-D images during growth, for film thickness between 0 and ~ 25 atomic layers (growth rate: 1 atomic layer every 50 seconds).

We recorded many "movies" of the growth process under different conditions. The interested reader may see one at

At temperatures around 323 K we observed, after growing several atomic layers, the diffuse scattering depicted in Figure 52. The Maltese cross-shaped distribution has been interpreted as arising from a distribution of dendritic islands as the ones depicted in the right panel in the figure. This morphology appears to be kinetically determined since it evolves towards the commonly observed equilibrium shape (polygonal) when growth is interrupted and the surface temperature is increased.

Figure 52
Fig. 52: (Left) X-ray diffuse scattering distribution after the growth of ~ 20 layers of Ag on Ag (001) at 320 K taken with 5 minutes exposure time to enhance the quality; (Right) Real space distribution of a multilevel random distribution of cross-shaped islands. The levels are differentiated by different grey tones. The inset shows the corresponding calculated diffraction pattern.

If prior to the growth, the surface was covered with 0.2 atomic layers of In atoms, then under the same growth conditions as before, the diffuse scattering pattern obtained after growing several atomic layers was the one depicted in Figure 53. The real space distribution of islands which generate that diffuse scattering are shown in the right panel. The islands are now square, the equilibrium morphology expected from the Wulff construction.

Figure 53
Fig. 53: (Left) X-ray diffuse scattering distribution after the growth of ~ 20 layers of Ag on Ag(001) covered with a small concentration of surfactant atoms prior to growth (Exposure time: 5 minutes); (Right) Real space distribution of multileveled square islands with their equilibrium square shape in contrast with dendritic shapes in Figure 52. The inset shows the calculated diffraction pattern.

Interestingly enough, annealing of a film grown without surfactant (i.e. exhibiting the dendritic island morphology) causes the diffuse scattering to evolve from the shape in Figure 52 to that in Figure 53. This means that the dendritic islands evolve to their equilibrium shape when the temperature is raised. This result illustrates that the dendritic island shapes result from kinetic limitations and that the surfactant enhances surface mass transport to allow the islands to acquire their equilibrium morphology.

[1] T. Michely and G. Comsa in Morphological Organization in Epitaxial Growth and Removal, Z. Zhang and M. Lagally eds, Direction in Condens. Matter Physics 14, World Scientific, Singapore (1998).

Principal Publication and Authors
J. Alvarez (a), E. Lundgren (b), X. Torrelles (c) and S. Ferrer (d), Surface Science 464, 165-175 (2000).
(a) UAM, Madrid (Spain)
(b) Inst. Fur Allgemeine Phys., TU Wien (Austria)
(c) Inst. Ciencia Materials, Barcelona (Spain)
(d) ESRF