Island-size distributions in submonolayer epitaxial growth: Influence of the mobility of small clusters

Bartelt, M. C.; Günther, Sebastian; Kopatzki, E.; Behm, R. J.; Evans, James W.

doi:10.1103/physrevb.53.4099

Cited by 70 publications

(66 citation statements)

References 26 publications

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“…For example, one may wonder whether the usual growth theories [3][4][5] can be used by replacing the continuous flux by the average value of the chopped flux over a cycle. In the following, we will show that this is not the case, and that the growth of the film is profoundly changed by the modulation of the incident flux for the case of growth with irreversible aggregation (critical island size 1, see [3][4][5][7][8][9]. Conversely, we show what kind of information can be derived from experiments carried under these conditions.…”

Section: Introductionmentioning

confidence: 94%

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The effect of a modulated flux on the growth of thin films

Jensen

Niemeyer

1997

Surface Science

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Thin films are usually obtained by depositing atoms with a continuous flux.We show that using a chopped flux changes the growth and the morphology of the film. A simple scaling analysis predicts how the island densities change as a function of the frequency of the chopped flux in simple cases where aggregation is irreversible. These predictions are confirmed by computer simulations.We show that the model can be used to obtain information on the diffusion or the evaporation of the adatoms. The model is also useful to understand the growth of thin films prepared by pulsed sources.Typeset using REVT E X 1 One of the main interests of usual deposition techniques such as Molecular Beam Epitaxy [1] is that the structure of the deposited films is to a large extent determined by kinetic factors, as opposed to thermodynamic equilibrium. This allows to "play games" [2] with the different growth parameters (incident flux of particles, diffusion coefficient of an adatom . . . ) in order to obtain different film morphologies. A simple example is given by the quantity of islands grown on a substrate at low enough temperatures : it is known that the number of islands at saturation is given by (F/D) 1/3 [3][4][5] where F is the incident flux and D the diffusion coefficient. Then, by increasing the flux or decreasing the diffusion constant (by lowering the substrate temperature), one can adjust the saturation number of islands grown on the substrate. In this sense, each kinetic factor is a "handle" on the system, allowing to control the morphology of the films. We introduce in this Letter a new kinetic handle, which should enable a larger control over film growth : the chopping of the incident flux. We note that this flux modulation is intrinsic to other deposition techniques such as cluster laser vaporization (the laser is pulsed [6]). It is therefore important to understand how growth proceeds in the presence of a modulated flux if one is to be able to interpret experiments performed in these conditions. For example, one may wonder whether the usual growth theories [3][4][5] can be used by replacing the continuous flux by the average value of the chopped flux over a cycle. In the following, we will show that this is not the case, and that the growth of the film is profoundly changed by the modulation of the incident flux for the case of growth with irreversible aggregation (critical island size 1, see [3][4][5][7][8][9]. Conversely, we show what kind of information can be derived from experiments carried under these conditions.The basic idea of our method is that if instead of using a continuous flux we use a chopped flux to grow a film, the number of islands formed on a substrate will depend on the chopping frequency f and on d, the fraction of the period the flux is "on" (see Fig. 1).This dependence is due to the fact that the free particle concentration on the surface does not reach its steady state concentration instantaneously, but only after a time which we will call τ m . Then, if the timescale of the chopping (...

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Section: Introductionmentioning

confidence: 94%

“…We assume here, for simplicity, that adatom-adatom aggregation is irreversible and that only monomers can move on the surface [3][4][5][7][8][9]. Then, the important timescale for growth is given by the time needed for monomers to reach their steady state concentration or to disappear.…”

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confidence: 99%

The effect of a modulated flux on the growth of thin films

Jensen

Niemeyer

1997

Surface Science

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“…PRB 59initial stages of island nucleation. Also, it is known that the form of the island size distribution is sensitive to the details of island shape ͑including ramification 19 or anisotropy 20 ͒, to significant diffusion of dimers and other small clusters, 21 to the onset of reversibility in island formation, 4,22 and to anisotropy in terrace diffusion. 6,23 Thus we are examining the extent to which this sensitivity reflects underlying changes in the form of the size dependence of adatom capture.…”

Section: Discussionmentioning

confidence: 99%

Adatom capture by arrays of two-dimensional Ag islands on Ag(100)

et al. 1999

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We examine the capture of diffusing Ag adatoms by arrays of two-dimensional Ag islands subsequent to deposition on Ag(100) at room temperature. This is achieved by a combination of scanning tunneling microscopy experiments, kinetic Monte Carlo simulations, and diffusion equation analyses. The dependence of the capture rates on Ag-island size is shown to reflect larger island-free regions surrounding the larger islands, i.e., a strong correlation between island sizes and separations. This feature, and the influence of the local environment of the islands on capture, are elucidated by introducing suitable tessellations of the surface into "capture zones" for each island. We show that a Voronoi-type tessellation based on the distance from the island edges accurately reflects adatom capture. However, a tessellation exactly describing adatom capture is only obtained from a solution of the steady-state equation describing adatom deposition, diffusion, and capture by an array of islands distributed as in experiment. The stochastic nature of adatom capture is also quantified by analysis of the dependence on the deposition location of the probability for diffusing adatoms to be captured by a specific island. The experimental island size dependence of adatom capture is found to be entirely consistent with that obtained from a "canonical" model for the irreversible nucleation and growth of square islands. Disciplines Mathematics | Physical Chemistry CommentsThis article is from Physical Review B 59, no. 4 (1999) We examine the capture of diffusing Ag adatoms by arrays of two-dimensional Ag islands subsequent to deposition on Ag͑100͒ at room temperature. This is achieved by a combination of scanning tunneling microscopy experiments, kinetic Monte Carlo simulations, and diffusion equation analyses. The dependence of the capture rates on Ag-island size is shown to reflect larger island-free regions surrounding the larger islands, i.e., a strong correlation between island sizes and separations. This feature, and the influence of the local environment of the islands on capture, are elucidated by introducing suitable tessellations of the surface into ''capture zones'' for each island. We show that a Voronoi-type tessellation based on the distance from the island edges accurately reflects adatom capture. However, a tessellation exactly describing adatom capture is only obtained from a solution of the steady-state equation describing adatom deposition, diffusion, and capture by an array of islands distributed as in experiment. The stochastic nature of adatom capture is also quantified by analysis of the dependence on the deposition location of the probability for diffusing adatoms to be captured by a specific island. The experimental island size dependence of adatom capture is found to be entirely consistent with that obtained from a ''canonical'' model for the irreversible nucleation and growth of square islands.

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“…With the present system, the stability of dimers is guaranteed up to elevated temperatures due to the high dimer dissociation energy, determined by ab initio calculations to 0.54 6 0.06 eV [21]. Dimer mobility can be similarly excluded for the present temperature range, based on the theoretical value of the dimer migration barrier [21] and on the scaling laws in the presence of cluster diffusion [22,23]. The saturation island density is thus solely controlled by the tracer diffusion coefficient and the deposition flux F. Then the following scaling law relates D ‫ء‬ to n x [19,24]:…”

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Dynamics of Surface Migration in the Weak Corrugation Regime

et al. 2000

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We report a systematic study for metal-on-metal surface migration in the weak corrugation regime, i.e., with migration barriers falling below ഠ100 meV. The migration characteristics are elucidated by variable-temperature scanning tunneling microscopy observations in the 50 -200 K temperature range, which are analyzed by means of nucleation theory. The results demonstrate that, upon entering the weak corrugation regime, the dynamics of the systems are characterized by increasingly reduced effective preexponential factors, while Arrhenius behavior prevails. PACS numbers: 68.35.Fx, 61.16.Ch, 68.35.Ja, The thermal migration of adsorbed atoms on the periodic lattice of a surface offers an ideal test case to systematically investigate the energetics and dynamics of a rate phenomenon in chemical physics. Surface migration requires the overcoming of an energy barrier E m separating two identical states, a process usually obeying an Arrhenius law [1][2][3]. In particular, the mobility of atoms on metal surfaces has been extensively studied due to its importance for a variety of processes in science and technology, such as heterogeneous catalysis or epitaxial growth [4][5][6]. The phenomenology is simple: Atoms or small molecules are preferentially adsorbed at specific highsymmetry sites of the surface periodic atomic lattice, which are separated by energy barriers being generally a mere fraction of the adsorption energy. The lateral mobility of the adspecies is driven by the substrate heat bath providing stochastic excitations whose characteristic energy is given by the Debye temperature (ϳ25 meV for metals). When the density is low, a two-dimensional random motion of isolated adspecies or tracer diffusion [5] is operational; i.e., adsorbates hop from energy minima to neighboring energy minima. This process can be adequately described in terms of transition state theory (TST) [7], when the thermal energies are much smaller than the migration barrier, i.e., k B T ø E m , and when the coupling of the adspecies to the substrate is sufficiently effective to ensure equilibration between the individual jumps. These conditions normally hold for metallic adsorbates on metal surfaces, and the corresponding tracer diffusion coefficient D ‫ء‬ is given by [5]where b 1͞k B T, a is the periodicity of the surface lattice, and n 0 is the attempt frequency, frequently associated with the lateral vibrational mode of the adatom; D 0 is the preexponential factor.The situation becomes more intricate when the thermal energy increases such that k B T ϳ E m and it is anticipated that TST breaks down, until for k B T ¿ E m a free twodimensional Brownian motion of the adatoms is expected, where the diffusivity is directly proportional to the temperature [8]. Another complication may arise when the coupling of the adatom to the substrate is weak or strong. The resulting effect is twofold: On the one hand, a decrease of the attempt frequency is expected, which lowers the mobility [9][10][11]. On the other hand, for the case of weak coupling, adspec...

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Island-size distributions in submonolayer epitaxial growth: Influence of the mobility of small clusters

Cited by 70 publications

References 26 publications

The effect of a modulated flux on the growth of thin films

The effect of a modulated flux on the growth of thin films

Adatom capture by arrays of two-dimensional Ag islands on Ag(100)

Dynamics of Surface Migration in the Weak Corrugation Regime

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