Evolution of Mound Morphology in Reversible Homoepitaxy on Cu(100)

Abstract

Evolution of mound morphology in reversible homoepitaxy on Cu(100) was studied via spot-profileanalysis (SPA) LEED and scanning tunneling microscopy. The mound separation shows coarsening vs growth time with L͑t͒ ϳ t 1͞4 , in support of theory based on capillarity between mounds. The growth ultimately reaches a steady state characterized by a selected mound angle of ϳ5.6 ± . We suggest that this results from a downhill current driven by step edge line tension in balance with an uphill current due to the Schwoebel barrier effect. Also, we have clarified the interpretation for the evolution of the SPA-LEED profile from a ring structure to a single time-invariant peak. [S0031-9007 (97)02851-2] PACS numbers: 68.55.Jk, 61.14.Hg, 61.16.ChRecently, a number of epitaxial growth experiments have shown a pyramidlike mound morphology with a welldefined mound separation and a selected mound slope [1][2][3][4][5][6]. The origin of this phenomenon is ascribed to a growth instability caused by the step barrier (or Schwoebel barrier) which resists step-down diffusion of deposited atoms [7]. As a result, the probability for the nucleation of upper-level islands onto lower-level islands is enhanced. Repeated application of this process in successive layers leads to the observed pyramidlike mound morphology. As deposition proceeds, the mounds grow bigger and steeper (i.e., unstable), and coalescence will occur with the filling of gaps between mounds and development of new top layers. This results in the coarsening of the island size distribution, i.e., the distribution becomes dominated by larger islands at the expense of smaller ones, as characterized by the increase in the average mound separation L͑t͒ with growth time t. In general, L͑t͒ is found to follow a power law [3-6,8-10],with the exponent n depending on the coarsening mechanism.Mullins first obtained Eq. (1) with n 1͞4 by solving a continuity equation, ≠h͞≠t 2= 4 h, where h is the surface height and the = 4 h term describes capillary-induced mound coalescence which eliminates smaller mounds in favor of larger ones [10]. Recently, Siegert and Plischke [8] and Hunt et al.[9] also obtained n ϳ 0.25 from numerical integration of the continuity equation with additional terms incorporating the Schwoebel barrier effect. This kind of coalescence only occurs when a local equilibrium can be established between diffusing atoms and growing mounds. The process requires detachment of an atom from the step edges of small mounds in a time scale roughly equal to ϳ1͞F (F is the flux), i.e., a reversible growth. However, if such a detachment is not allowed, i.e., for an irreversible growth, Stroscio et al. obtained n ϳ 0.18, for which they introduced a term for a so-called local "corner" free energy in the continuity equation instead of the = 4 h term [4]. This exponent agrees with their measured value of n ϳ 0.16 from the Fe͞Fe(100) growth at a temperature T 293 K [4] at which the growth has previously been shown to be irreversible [11], and also is supported by a recent kinetic Monte Car...

Interface Dynamics and Far-From-Equilibrium Phase Transitions in Multilayer Epitaxial Growth and Erosion on Crystal Surfaces: Continuum Theory Insights

“…Cu(001) surfaces [44]. A similar effect for positive VA is seen in the facet edge widths, with convex edges narrower and concave edges broader (relative to their widths at zero VA) -cf.…”

“…These findings, in agreement with the experimental data [30,31], provide the first clear evidence that VA plays a significant role in epitaxial growth and erosion on crystal surfaces. It has long been recognised, for example on (001) surfaces, that VA produces differences between the shapes of four-sided pyramids and pits (inverted pyramids) [44]. However, VA effects on interface structure and dynamics are still debated, and a better (but likely incomplete) understanding has only begun to emerge quite recently for the case of high symmetry (001) surfaces [29].…”

“…Epitaxial growth and erosion phenomena, such as discussed in this review, are no exceptions. For example, VA effects are clearly visible on the interfaces of homoepitaxially grown films, through the difference between the shapes of pyramids and pits (upside-down pyramids) that develop during the homoepitaxial growth on (001) and (111) crystal surfaces [44] -recall Fig. 1(b).…”

“…On square symmetry surfaces (001) surfaces, the ESV instability [4] frequently induces the formation of nearly periodic (defected) checkerboard arrangements of alternating four-sided pyramids and pits (inverted pyramids), with sizes growing in time as t 1/4 as exemplified by Cu(001) epitaxial growth [44]. As discussed in § 3.2, this coarsening law may be understood from the dynamics of topological defects (dislocations) of the regular checkerboard structure [18,19].…”

“…Pyramid-like structures have often been observed -e.g. on the homoepitaxy of GaAs [6,11], Cu [5,44], Ge [12] and Fe [13,14], all grown on singular (001) substrates, and on the Rh(111) surface [15]. The lateral size λ and height w of these pyramids grow in time as power laws with the same exponent.…”