Discrete Row Growth at Vicinal Surfaces

Vittorio Marsico, Martial Blanc, Klaus Kuhnke, Klaus Kern
1997 Physical Review Letters  
Discrete row growth during the initial stage of molecular beam epitaxy of rare gases and metals on the vicinal Pt(997) surface has been observed. The row-by-row growth is revealed by intensity oscillations in thermal-energy atom scattering at grazing incidence. Thermodynamic modeling provides an estimate of the excess binding energy close to the step edges. [S0031-9007(96)01999-0] PACS numbers: 79.20.Rf, 82.65.Dp Manipulating the morphology of epitaxial films through detailed control of the
more » ... control of the growth kinetics has attracted much interest recently. A primary goal is the layer-by-layer growth of smooth films with abrupt interfaces. The most widely used techniques for monitoring the growth mode are diffraction techniques [1] [2] [3] [4] , where the occurrence of intensity oscillations provides unique evidence for the desired two-dimensional (2D) growth. These oscillations in the diffracted or specularly reflected intensity reflect the periodically varying step density of homogeneously nucleating and successively coalescing 2D islands. The presence of substrate steps can suppress the homogeneous nucleation on terraces in favor of heterogeneous step nucleation still permitting smooth two-dimensional growth. Binding energies for adatoms at step sites are in general larger than on terrace sites due to the increased coordination. As a consequence 2D islands preferentially nucleate at steps if the average adatom diffusion length is larger than the terrace width [5] [6] [7] . In the submonolayer range this can be exploited to grow quasi-one-dimensional systems like quantum wires using substrate step arrays as a template [8, 9] . Similar to the growth mode classification in thin film epitaxy different step decoration modes can be distinguished, the occurrence of which depend on the detailed interaction of the adsorbate with the substrate step [10] . In the present Letter we demonstrate that the step decoration modes can be studied by specular thermal energy helium scattering at grazing incidence. For a regularly stepped Pt(997) surface we find during the adsorption of rare gases and during the deposition of metals in the submonolayer range oscillations in the reflected helium intensity. These intensity oscillations reflect the sequential growth of rows at the steps (named here discrete row growth or row-by-row growth) during deposition. The vicinal Pt(997) surface, consisting of about 20 Å wide (111) terraces separated by ͑111͒ monatomic steps, has been chosen as substrate because it is known to exhibit a regular step-terrace ordering with a narrow terrace width distribution [11, 12] . The initial growth of the rare gas Xe and the metal Ag on this surface has been studied with a novel triple axis He-surface spectrometer [13] in grazing incidence scattering geometry. While the rare gases are adsorbed on the Pt(997) surface from the ambient gas phase, silver is deposited with a molecular beam effusion cell. Xe films on Pt(997) grow in a 2D growth mode. This can be seen from the oscillations in specularly reflected He intensity shown in Fig. 1 (note that the scattering geometry is far from grazing incidence and specular to the (111) terraces). As is well known from the literature [2, 3, 14] each intensity maximum corresponds to the completion of a xenon layer. The damping of the oscillations is mainly due to the large Debye-Waller FIG. 1. He scattering intensity normalized to the intensity at zero coverage as a function of Xe coverage at nongrazing angles. Incidence angle of the He beam with respect to the surface normal u i 53.9 ± , exit angle u f 41.0 ± . The geometry corresponds to specular reflection with respect to substrate terraces, f i f f , and a diffraction order n 23 with respect to terrace periodicity. Xe ambient pressure 1.7 3 10 28 mbar, surface temperature T 34 K, He beam wavelength l He 1.03 Å. 94 0031-9007͞96͞78(1)͞94(4)$10.00
doi:10.1103/physrevlett.78.94 fatcat:ekbhja4sfjbe3mrtjipoo3jnvm