Variation of growth morphology with chemical composition of terraces: Ag on a twofold surface of a decagonal Al-Cu-Co quasicrystal

Abstract

Growth of Ag thin films on the twofold surface of a decagonal Al-Cu-Co quasicrystal is characterized by scanning tunneling microscopy, at different temperatures, and for coverages ranging from submonolayer to 11 monolayers. From prior work, three types of clean surface terraces are known to exist. By correlation with a bulk structural model, the major difference between them lies in their transition-metal (TM) content, two being aluminum-rich (0 and 15 at. % TM) and one being TM-rich (40-50 at. % TM). The present article focuses on understanding the difference between Ag film morphologies on these terminations, in terms of their chemical content. Growth is found to be smoother on the TM-rich terraces and rougher on the Al-rich ones. The first Ag atomic layer is even pseudomorphic on the TM-rich terraces. Roughness variation with temperature shows that the equilibrium morphology is two dimensional for TM-rich terraces and three dimensional for Al-rich terraces. The explanation of different growth modes in terms of different terrace compositions is supported by calculations of the adhesion energy of a Ag slab with Ag, Al, Cu, and Co slabs, using density-functional theory. For the Al-rich terraces, the roughness variation with temperature also indicates reentrant growth, i.e., anomalously smooth growth at low temperature due to kinetic effects. Growth of Ag thin films on the twofold surface of a decagonal Al-Cu-Co quasicrystal is characterized by scanning tunneling microscopy, at different temperatures, and for coverages ranging from submonolayer to 11 monolayers. From prior work, three types of clean surface terraces are known to exist. By correlation with a bulk structural model, the major difference between them lies in their transition-metal ͑TM͒ content, two being aluminum-rich ͑0 and 15 at. % TM͒ and one being TM-rich ͑40-50 at. % TM͒. The present article focuses on understanding the difference between Ag film morphologies on these terminations, in terms of their chemical content. Growth is found to be smoother on the TM-rich terraces and rougher on the Al-rich ones. The first Ag atomic layer is even pseudomorphic on the TM-rich terraces. Roughness variation with temperature shows that the equilibrium morphology is two dimensional for TM-rich terraces and three dimensional for Al-rich terraces. The explanation of different growth modes in terms of different terrace compositions is supported by calculations of the adhesion energy of a Ag slab with Ag, Al, Cu, and Co slabs, using density-functional theory. For the Al-rich terraces, the roughness variation with temperature also indicates reentrant growth, i.e., anomalously smooth growth at low temperature due to kinetic effects.

Temperature Dependence of Ag Film Roughening during Deposition on Quasicrystal and Approximant Surfaces

“…As a result, one nds rougher growth on the former and smoother growth on the latter. Furthermore at 5 ML, one nds that lm roughness decreases monotonically with decreasing T for the Al-rich termination (as for the Ag on AlPdMn quasicrystal), and reentrant smooth growth for the TM-rich termination (as for Ag on the ξ -approximant) [21].…”

“…This difference can reect dierent surface energies for the two substrates, dierent degrees of epitaxial match between lm and substrate, and strong QSE for the AlPdMn quasicrystal surface. For completeness, we briey comment on another study of Ag lm growth on a decagonal AlCuCo quasicrystal [21]. Interestingly, this surface exposes two distinct types of terraces: one is an Al-rich termination, and the other a transition metal (TM) rich termination.…”

From Initial to Late Stages of Epitaxial Thin Film Growth: STM Analysis and Atomistic or Coarse-Grained Modeling

“…One could imagine quasi-periodicity may facilitate an epitaxial match [80] with some multiple of the smaller unit cell dimension in the orthogonal direction (although we do not currently have experimental evidence to support this claim). Our key experimental observation for Ag deposition at 365 K [81] is that growth is smoother on the TM-rich terrace compared to the Al rich terrace. Further, upon annealing to 420 K, the film of the TM-rich terrace becomes smoother, and that on the Al-rich terrace becomes rougher [81].…”

“…Our key experimental observation for Ag deposition at 365 K [81] is that growth is smoother on the TM-rich terrace compared to the Al rich terrace. Further, upon annealing to 420 K, the film of the TM-rich terrace becomes smoother, and that on the Al-rich terrace becomes rougher [81]. See Fig.…”

“…Right: dewetting and roughening via annealing on pure-Al terrace. [81] Insight into this terrace-dependent thermodynamics can be extracted from a benchmark DFT analysis of related simple heteroepitaxial systems [81]. Specifically, we consider the adhesion of fcc Ag slabs to strained fcc Al, Cu, and Co slabs with various orientations.…”