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

Abstract

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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