Scanning Tunnelling Microscopy Investigation of Surface Morphology in the Growth of GaAs(001)
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SCANNING TUNNELLING MICROSCOPY INVESTIGATION OF SURFACE MORPHOLOGY IN THE GROWTH OF GaAs(001) E.J. HELLER AND M.G. LAGALLY University of Wisconsin - Madison, Madison, WI 53706 ABSTRACT The surface morphology of MBE - grown GaAs(001) has been investigated using scanning tunnelling microscopy (STM) and reflection high - energy electron diffraction (RHEED). STM shows that the missing - dimer - row structure of the (2 x 4)/c(2 x 8) reconstruction consists of rows of clusters of two As dimers separated by rows of two missing dimers, in agreement with previous reports. Layers grown on nominally flat substrates display a multi - level system of terraces elongated along [ 110] suggesting that growth occurs primarily by sticking at B - type steps. For films grown under certain growth conditions, B - type steps on vicinal substrates exhibit a dendritic step morphology, which may be an example of a step flow growth instability consistent with limited Ga diffusion over steps. INTRODUCTION The properties of many novel semiconductoi structures are influenced by surface and interface morphology introduced during film growth. Kinetic mechanisms of transport, sticking, and desorption control this morphology in varyingly complex ways, in all methods of deposition [1,2]. The simplest of these, in terms of identifying and separately quantifying specific kinetic mechanisms, is molecular - beam epitaxy (MBE). Even for MBE, tools to identify growth mechanisms at the atomic scale have not been available until quite recently. The most universally investigated compound semiconductor, GaAs(001), which also is the first semiconductor material grown by MBE [3], represents a good model system for such studies. Despite the large literature on GaAs(001), there have been relatively few efforts to quantify such kinetic mechanisms as surface diffusion, transport over steps, and sticking and desorption from steps [4-8]. These have been principally through the use of reflection high - energy electron diffraction (RHEED). Initial RHEED observations concentrated on identifying the surface reconstruction and on characterizing qualitatively the surface morphology [4]. More recently, RHEED has been used to investigate GaAs growth. In particular, efforts at quantitative measurement of step roughness [5,6] and Ga surface diffusion during growth have been made [7,8]. It has been recognized, however, that the interpretation of these results is complicated by a lack of understanding of the interaction of adatoms with and their transport over steps [5]. The scanning tunnelling microscope (STM) has been used extensively to study growth and transport mechanisms of Si on Si(001) [9] and the energetics of steps on Si(001) [10]. Studies of 2 - D island formation have highlighted differences in sticking at and transport over steps as well as differences between growth and equilibrium shapes of islands [11]. STM observations of MBE - grown GaAs(001) films have, however, concentrated only on the atomic structure of several reconstructions [12,13] and on the local struct
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