Cluster Size Distributions in Different Temperature Regimes: The System Ga on GaAs(001)
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CLUSTER SIZE DISTRIBUTIONS IN DIFFERENT TEMPERATURE REGIMES: THE SYSTEM Ga ON GaAs(001) Y. REN*, M. ZINKE-ALLMANG*, L.C. FELDMAN+ AND W. VAN SAARLOOS# * Department of Physics, University of Western Ontario, London, Ontario, Canada N6A 3K7 + AT&T Bell Laboratories, 600 Mountain Ave., Murray Hill, N.J. 07974, U.S.A. # Instituut - Lorentz, University of Leiden, P.O. Box 9506, 2300 RA Leiden, The Netherlands
ABSTRACT In this paper we discuss results for the clustering of Ga on GaAs(001). The dominant dynamic process which drives this system toward this three dimensional equilibrium changes from Ostwald ripening at low deposition rates to coalescence at higher deposition rates. The experimental data allow us to test several theoretical predictions for cluster size distributions based on a detailed study of the microscopic processes. These include, at higher deposition rates, the observation of local ripening effects and diffusion limited growth.
INTRODUCTION Phase formation processes on surfaces have received significant attention in the materials science community since they provide crucial information on the structural details of thin film morphologies. Final film structures for typical growth conditions (MBE growth) are often determined by cluster growth and coalescence, i.e. growth of clusters into each other upon contact, e.g. for GaAs films on Si [1]. A number of experimental studies have addressed the cluster nucleation regime [2] and a late-stage growth process, Ostwald ripening [3] (growth of larger clusters at the expense of smaller clusters driven by the Gibbs-Thomson effect). In this paper, we use new data on the system Ga/GaAs(001) to emphasize that there are other processes in the late stage, such as coalescence, which may result in qualitatively different morphologies for the final structure. This is particularly important at higher deposition rates since deviations from the ripening concept become more probable [4]. Fig. 1 shows two micrographs of Ga on a GaAs(001) surface after treatment at two different temperatures. The upper morphology is obtained by annealing to 660 'C for 5 minutes and the lower micrograph is obtained by annealing to 585 °C for 10 minutes. The low temperature growth establishes a condition with no significant As loss during clustering, i.e. conforms to mass conservation. The high temperature sample is grown under conditions of steady As loss, i.e. steady increase of Ga. The observed morphologies are strongly different with uniformly sized clusters in case (b) and a wide range of sizes with a bimodal distribution in case (a). In this paper we discuss the different growth mechanisms which give rise to these fundamentally different morphologies.
EXPERIMENTAL TECHNIQUES AND RESULTS Sample preparation is done in two different ultra-high vacuum systems (base pressure < 5 x 10-9 Pa) equipped with standard surface analytical tools [5]. Samples are mounted on Mo backings with In and are either heated radiatively (Bell Labs) or by direct current (U Western Ontario). Temperature control is
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