Quantitative Determination of the Metastability of Flat Ag Overlayers on GaAs(110)

Abstract

Atomically flat ultrathin Ag films on GaAs(110) can be formed through a kinetic pathway. However, such films are metastable and will transform to 3D islands upon high temperature annealing. Using scanning tunneling microscopy, we have measured quantitatively the layer-resolved metastability of flat Ag overlayers as they evolve toward their stable state, and deduced the corresponding kinetic barrier the system has to overcome in reaching the stable state. These results indicate that the metastability of the Ag overlayer is defined by the quantum nature of the conduction electrons confined within the overlayer. DOI: 10.1103/PhysRevLett.88.016102 PACS numbers: 68.55.Jk, 68.35.Ct, 68.37.Ef, 73.21.Fg In today's search for novel materials, metastable configurations of complex systems [1] are attracting much interest, because they often possess intriguing properties that may lead to innovative technological advancements. However, as a prerequisite for any potential device application, we must explore the degree of metastability of such materials, namely, how far they are from their thermodynamically stable configuration. In particular, it is desirable to know the energy barrier preventing a specific metastable configuration of a given macroscopic system from reaching its thermodynamically stable state, and to identify the dominant physical factors defining the metastable state.One example of such a metastable system is Ag overlayers on a GaAs(110) surface, which can be grown atomically flat using a two-step procedure [2][3][4]. This particular growth procedure (low-temperature deposition followed by thermal annealing) allows the film to follow a special kinetic pathway and reach a fully wetting and atomically flat two-dimensional morphology, although in thermal equilibrium the Ag-GaAs system is nonwetting and minimizes the interface area by forming three-dimensional mounds [5].In this Letter we determine quantitatively the energetics that govern the wetting-nonwetting transition of the Ag overlayers grown on GaAs(110) as well as the underlying physical factors defining the metastability of the flat films. More specifically, we measure the layer-resolved relative energies of the flat overlayer in different metastable configurations, and deduce the kinetic barrier the initial system must overcome in order to transform from the wetting to the nonwetting morphology. Finally, a comparison with existing theoretical models indicates that the metastable configuration of the Ag overlayer is defined by the confined motion of the conduction electrons within the overlayer.The experiments were performed in an ultrahigh vacuum system with a base pressure of 3 3 10 29 Pa. The Ag films were deposited on in situ cleaved GaAs(110) surfaces at temperatures below 140 K, followed by annealing to room temperature. The thickness of the film is monitored by a calibrated quartz crystal. At this stage we have to recall some important features: First, for all the film thicknesses investigated in this work, the as-deposited film at low temper...

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