Effect of Layer-Dependent Adatom Mobilities in Heteroepitaxial Metal Film Growth: Ni/Ru(0001)

Abstract

Scanning tunneling microscopy observations show that (i) the lattice mismatch between Ni and Ru leads to a sequence of increasingly relaxed structures in Ni films grown on Ru(0001), and (ii) the density of Ni islands changes drastically with the thickness of the underlying Ni film. The latter is associated with an increase in Ni mobility; from monolayer to three-layer Ni films a reduction in Ni adatom diffusion barrier of 300 meV is estimated. Such effects are shown to significantly affect film growth and, in this case, to promote smoother growth. New possibilities for obtaining smoother film growth for heteroepitaxial systems, in general, are discussed. PACS numbers: 68.55. -a, 61.16.Ch Epitaxial growth affords the opportunity to make materials with morphologies and chemical compositions which would not ordinarily occur in nature [1 -4]. By varying the experimental parameters of temperature, deposition rate, and, more recently, additive (surfactant) concentrations the characteristics of the growing film can be controlled to a certain extent [5 -12]. For heteroepitaxy further effects come into play, namely, the change in film structure due to the removal of strain, and the change in electronic properties with increasing film thickness. The strain results from the lattice mismatch between the substrate and the growing film. In this Letter we demonstrate that these surface modifications can have a drastic effect on the adatom mobilities, leading to layerdependent island densities. The structure of and adatom mobilities on films of different layer thicknesses were determined by scanning tunneling microscopy (STM). For these experiments we first prepared well-defined Ni films on a Ru (0001) substrate by deposition at 550 K. This temperature is sufficiently high to produce well-ordered surfaces and rather large terraces, but not high enough to create the equilibrium morphology of that system, which consists of 3D crystallites growing on a single Ni layer (Stranski-Krastanov growth) and which is obtained after deposition at or annealing to higher temperatures [13,14].On these thin film substrates we subsequently deposited small amounts of Ni in a second dose at room temperature. From the distribution and density of the Ni islands obtained after the second dose we gain insight into adatom diffusion on and among the various well-defined Ni layers. The results lead us to introduce layer dependent mobilities and island densities as a general concept for heteroepitaxy with substantial implications for growth descriptions and film morphologies.Deposition and STM imaging were carried out at p ( 3 x 10 " Pa, on carefully cleaned Ru(0001) substrates.Further details on the experimental setup and procedures and on the growth behavior of that system will be described elsewhere [14]. STM images were recorded in a pocket-sized STM at tunnel currents around 1 nA and bias voltages of 100 mV. The images are presented in either a top view representation, v .ih darker areas corresponding to lower levels, or in a bird's eye view with illu...

Surface structure of nickel oxide layers on a Rh(111) surface

“…The first Ni layer (Fig. 1a,b) exhibits a characteristic network of line protrusions, running along the three equivalent b110> substrate directions, which have been assigned to domain boundaries separating pseudomorphic areas with fcc and hcp stacking [21,22]. This causes a partial strain relief in the first Ni layer (note that the bulk lattice constants of the Rh(111) and Ni(111) surfaces are 2.69 Å and 2.49 Å, respectively, which corresponds to a 8% misfit).…”

Microscopic view of epitaxial metal growth: nucleation and aggregation

“…Dislocations can therefore strongly in¯uence nucleation island densities [150,151,189]. Their in¯uence on nucleation can reach as far as driving the most perfect layerby-layer growth observed so far.…”

“…15. The three STM images were obtained for low temperature nucleation of Ag on Pt (1 1 1), of Ag on the ®rst Ag monolayer grown on Pt (1 1 1), and of Ag on ÐÐÐÐ 19 Such a layer dependence was also reported for NiaRu(0 0 0 1) [151]. In that case it is due to diffusion on inhomogeneous surfaces, where strain is relieved in moire  or dislocation patterns.…”

Nucleation Kinetics on Inhomogeneous Substrates: Al/Au(111)

“…While nucleation on homogeneous substrates has been extensively addressed in experiment and theory, a quantitative investigation of diffusion and nucleation on inhomogeneous substrates is presently lacking. Several observations reported in the literature [4] suggest that dislocations can represent considerable repulsive barriers towards diffusing adatoms by which they may control nucleation and growth.…”