Direct Measurement of Surface Diffusion Using Atom-Tracking Scanning Tunneling Microscopy

Abstract

The diffusion of Si dimers on the Si(001) surface at temperatures between room temperature and 128 ± C is measured using a novel atom-tracking technique that can resolve every diffusion event. The atom tracker employs lateral-positioning feedback to lock the scanning tunneling microscope (STM) probe tip into position above selected atoms with subangstrom precision. Once locked the STM tracks the position of the atoms as they migrate over the crystal surface. By tracking individual atoms directly, the ability of the instrument to measure dynamic events is increased by a factor of ϳ1000 over conventional STM imaging techniques. PACS numbers: 68.35.Fx, 61.16.Ch, 68.10.Jy The evolution of surface morphology during growth or etching depends on the detailed interplay between a myriad of atomic-scale kinetic processes. In order to control particular surface morphologies (e.g., to suppress or enhance overlayer island formation) it is important to achieve an understanding of the balance between thermodynamics and various kinetic processes. The equilibrium surface morphologies are controlled by thermodynamics, that is, the binding or configurational free energies, while the rates at which dynamic events occur on the surface are controlled by the details of the activation barriers-kinetics.To study the evolution of surface morphology investigators often utilize rate-equation analysis [1], Monte Carlo [2], or molecular dynamics simulations [3] which explicitly require the input of specific atomic-scale energy parameters. The values for the input parameters are derived in a variety of ways ranging from first-principles total energy calculations, through semiempirical calculations, e.g., from the embedded-atom method, to purely empirical values chosen to reproduce particular measured morphological features. Of course, the ideal parameter values are derived from quantitative experimental measurements of particular system-specific atomic-scale processes. The quantitative measurements of individual atomic-scale energy parameters not only serve as inputs to realistic simulations and model calculations, but also enable the validation and refinement of such calculations. Such processes on certain metal systems have been studied for some time using field ion microscopy [4].Because of its inherent atomic-scale resolution and access to a broad range of materials, the scanning tunneling microscope (STM) is ideally suited to studies of individual atomic-scale processes on surfaces, particularly since the advent of variable-temperature versions which enable the imaging of the surface as the rates of specific processes are varied by altering the sample temperature. However, conventional STM image acquisition is limited by the rate at which dynamic events can be resolved. In this work I report on the use of a novel atom-tracking technique that allows direct quantitative measurements of the diffusion of adsorbed silicon dimers on the Si(001) surface from room temperature to 128 ± C over which the diffusion rate changes by 4 orders of mag...

Nucleation and growth of thin films of rod-like conjugated molecules

“…As described above, classic surface science treats zero-dimensional particles with a few exceptions such as Si dimers [46]. The extended shape of the used molecules does not only allow them to obtain different orientations in space but also influences the way they interact with the surrounding.…”

Non-Arrhenius Behavior of Surface Diffusion near a Phase Transition Boundary

“…We find that the single-particle waiting-time distribution W (t) gives the most detailed picture of the microscopic processes. It has been recently demonstrated by Swartzentruber [9] that this distribution function in the presence of several different microscopic activation barriers can indeed be measured using the STM. From the long-time tail of W (t) one can obtain information on the energetics of the rate-limiting processes of diffusion in the form of an effective activation barrier E W A .…”

“…Further, it is experimentally available through e.g. STM measurements [9].In this Letter, we have carried out Monte Carlo (MC) simulations for a model of oxygen on the W(110) surface [10,11]. In this system, the substrate remains unreconstructed [12], the oxygen atoms have well-defined adsorption sites [13], and desorption of oxygen occurs only at temperatures 1600 K or above [12].…”

Simulations of denuded-zone formation during growth on surfaces with anisotropic diffusion

“…[7][8][9]13,17 We then solve the time-dependent diffusion equation to model how the adparticle concentration c(x,y,t) increases with time and then estimate the time-integrated island density (x,y,t) across the terrace using an expression for the island nucleation rate proposed by Theis and Tromp. 9 The solution is obtained up to the time n when the area-integrated total number of islands ⍀(t) is equal to the measured number of islands for that growth condition.…”

“…Recently, scanning tunneling microscopy ͑STM͒ has been used to directly monitor diffusion anisotropy by following the motions of individual diffusing adparticles at low growth temperatures. 7,8 However, this technique is so far not applicable at higher temperatures where most semiconductor growth and processing is done and where diffusion is too rapid for existing STM's to follow. Hence it remains useful to study diffusion anisotropy indirectly through the investigation of denuded zones.…”