Dynamics of Ga Clusters on GaAs(001) Surface
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Dynamics of Ga Clusters on GaAs(001) Surface Shiro Tsukamoto and Nobuyuki Koguchi National Research Institute for Metals 1-2-1 Sengen, Tsukuba, Ibaraki 305-0047, JAPAN ABSTRACT Dynamics of Ga clusters and GaAs epitaxial growth on a GaAs (001) surface were successfully observed by a system in which scanning tunneling microscopy (STM) and molecular beam epitaxy (MBE) were incorporated into one unit rather than in separate chambers. With the substrate temperature of 528 oC, reflection high-energy diffraction (RHEED) patterns showed a (4x6) Gastabilized surface reconstruction and dynamics of steps and islands were clearly observed. The detaching and attaching of small Ga clusters might cause the change of steps and islands. It seems that the small Ga clusters migrated with the diameter of about 0.8 to 1.2 nm and around the steps and islands. These clusters could be observed only when they were detached from or attached to the steps and islands. Moreover, even under the substrate temperature of 440 oC and the As4 partial pressure of 2x10-6 torr, STM images were clearly observed. After 0.1 ML Ga was additionally supplied to the sample by migration enhanced epitaxy mode, step flow growth occurred, resulting in an additional GaAs layer grown on the B-step side. Moreover, the c(4x4) As-stabilized surface reconstruction was moderate. It seems that there is an equilibrium additional layer of As amorphous adatoms on the c(4x4) surface reconstructions. INTRODUCTION The complexity and variety of surface reconstructions observed on semiconductors has been an intriguing problem in surface science. Recently, combined MBE and STM systems have proven to be a very powerful technique for the observation of real-space semiconductor surfaces, especially GaAs(001), with extremely high lateral and vertical resolution[1-5]. Avery et al. reported STM studies of submonolayer islands in the precoalescence regime grown by MBE on the three low-index surfaces of GaAs[6,7]. However, the GaAs growth dynamics are still poorly understood at the atomistic level. Since, using this combined system, samples need to be cooled and transferred to a cleaner environment for STM analysis, the surface, especially B-step edges that provide the most active sites on GaAs(001)[8-10], are evolved and no longer representative of the true growth dynamics. Moreover, reflection high-energy electron diffraction (RHEED) is well known as a real-time in-situ surface observation technique for the MBE growth. However, the RHEED patterns show not real-space but reciprocal lattice images of surface reconstructions. These patterns are good for the 2D growth but, in the 3D cases, there is much missing information, especially that of atomic-level 3D structures. In order to observe the atomic-level 3D structures, we need a system in which STM and MBE are not in separate chambers, but incorporated in one unit; so-called, “scanning tunneling molecular beam epitaxy (STMBE) system”[11,12]. In this paper, the dynamics of Ga clusters and GaAs epitaxial growth on the GaAs (001) surface was
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