<title>Thin film growth effects related to the propensity for clustering</title>

Abstract

ABSTRACT Control of the morphology of epitaxial thin films is a prerequisite for device applications. Particularly crucial is the formation of smooth and defect—free hetero—interfaces at elevated temperatures where cluster formation is thermodynami- cally favoured. In particular knowledge of the early stage of clustering is crucial as nucleation often results in aggregatesof a few atoms which could still be integrated during further growth. Only few data exist for this stage to date due to smallcluster sizes and the transient time behaviour which complicate measurements with most standard surface techniques. An alternative approach is described to study the early stage of cluster growth as a by—product in late stage growth experiments. When applicable, like in the case of Ga clustering on GaAs(OO1) as discussed in this paper, simpler technicalaccess and higher precision in the determination of a time scale favour this method.1. INTRODUCTIONClusters on surfaces are in most cases thermodynamkally favoured over uniform films or random adatom distributions.To understand their dynamics is important for heteroepitaxial growth since clustering is a major break-down mechanismof uniform film formation at the interface. The fundamental knowledge we have of clustering kinetics and its dependenceon fundamental surface processes such as surface diffusion, adsorption and desorption, has been developed from studiesof non—equilibrium thermodynamics where clustering is viewed as a surface phase transition and phase separation. Theconnection between both concepts, thin film growth and non-equilibrium phase separations, is illustrated in Fig. 1. Thegraph shows the miscibility gap (area below solid line) for an idealized binary system. An example for such a system issolute precipitation in a solvent Equivalent systems exist on surfaces with the number of adatoms per unit area definingthe concentration axis. Typical thin film growth is an isothermal process (arrow b) where continuous deposition bringsthe system from a stable, undersaturatedadatom concentrationinto the supersaturated regime. The equivalent process usedin theoretical studies is a temperature quench' (arrow a) transforming a one—phase system suddenly into a non—stableconfiguration. As a result the system becomes metastable against nucleation which marks the formation of a second phase,i.e., the dense cluster phase. Applying the concepts developed for the formation of non-equilibrium configurations allowsus to describe break-down of thin film growth due to clustering.A general and comprehensive treatment of clustering phenomena is not available. Instead, we have to divide the probleminto stages, including (i) nucleation, (ii) early stage cluster growth as a result of local processes near the cluster and (fli)late stage growth with global dependence on adatom concentration and cluster distribution. The late regime has receivedmost attention from expenmentalists as the larger cluster sizes can be studied easier. Examples are studies of thin filnisemiconductor growth of GaAs on Si, Ge on Si4'5 or heterolayers of Ga/GaAs6 and InfInP during group III deposition.A topic which requires further investigation is the early stage growth regime which links nucleation and the late stage.78