Epitaxial growth of Ag on W(110)

Abstract

Epitaxial growth of Ag on W(110) at room temperature was studied by scanning tunneling microscopy (STM) and polarization-dependent photoemission. At coverages far below one monolayer Ag atoms populate bcc sites of the substrate and form close-packed islands of monolayer thickness. With increasing coverage geometrical misfit between Ag(111)-like layers and W(110) generates surface stress along $\mathrm{W}[1\overline{1}0]$. This is released by formation of domain walls parallel W[001] which are observed with a distance between about $25\phantom{\rule{0.3em}{0ex}}\mathrm{\AA{}}$ and $30\phantom{\rule{0.3em}{0ex}}\mathrm{\AA{}}$, depending on the details of the growth process. At one monolayer coverage most of the Ag atoms still reside in or very near to bcc substrate positions, but now the strain release pattern is changed: solitons aligned along $\mathrm{W}[\overline{1}12]$ are formed at an average distance between $35\phantom{\rule{0.3em}{0ex}}\mathrm{\AA{}}$ and $50\phantom{\rule{0.3em}{0ex}}\mathrm{\AA{}}$. The details of the soliton arrangement depend critically on the degree of equilibration and the presence of holes in the monolayer film which allow an additional stress release. This is evident from a comparison with results of STM studies performed at the closed and carefully annealed Ag monolayer [Kim et al., Phys. Rev. B 67, 223401 (2003)]. Further deposition of Ag starts growth of a second monolayer by formation of islands which increase in size with coverage. At a nominal coverage of 1.5 monolayers the strain relieve pattern changes again: some corrugation lines are oriented along W[001] as in the submonolayers, but other orientations related to Ag(111) directions appear as well. This indicates that several possibilities are available at similar energy costs and that the transition from the W substrate potential to a Ag potential seen by the second layer is very soft. Finally at a nominal coverage of several monolayers, Stranski-Krastanov growth is observed producing Ag(111)-like terraces with one of the dense-packed Ag rows oriented parallel to $\mathrm{W}[1\overline{1}1]$.