GaAs-on-Ge Heteroepitaxy by Atomic Hydrogen-Assisted Molecular Beam Epitaxy (H-MBE)
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on-Si material system (Aa / a -4.0% and Aa - 60%), which has been intensively studied for its potential applications in integrated optoelectronics. However, the problem of heavy segregation and interdiffusion of Ge and As at the GaAs/Ge interface arises, if GaAs is grown at high growth temperatures (> 450"C) [2] and consequently, the initial stages of the growth generally proceed by a three-dimensional (3D) islanded growth mode resulting in rough surfaces and interfaces [3,4]. The purpose of this work is to achieve a high-quality epitaxy of GaAs-on-Ge system at low growth temperatures of - 350 °C, by atomic hydrogen-assisted molecular beam epitaxy (H-MBE), in attempt to reduce Ge segregation to a minimum and to realize a layer-by-layer two-dimensional (2D) growth mode from the initial stages of the growth. A high-quality heteroepitaxy is expected in H-MBE technique as atomic H is known to act as an effective surfactant modifying the kinetics and energetics of the growth [5,6], which are practically difficult to control on atomic-scale unless a third element like a surfactant is introduced into the growth system [7]. Furthermore, irradiation of atomic H during the MBE growth brings a number of additional favorable effects such as removal of surface contaminants especially carbon, and hence surface cleaning, and inactivation of defect centers [8,9]. And as a result, a layer-by-layer 2D growth and high material quality can be achieved even in heteroepitaxy of highly lattice-mismatched systems such as Ge/Si [7], and GaAs/ Si [8,9]. It is shown with the support of reflection high-energy electron diffraction (RHEED) and * On leave from Institute of Materials Science, University of Tsukuba, 1-1-1 Tennodai, Tsukuba, Ibaraki, Japan.
E-mail address: [email protected], [email protected] 203 Mat. Res. Soc. Symp. Proc. Vol. 399 01996 Materials Research Society
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Fig. 1.H-MBE growth system used in this work. atomic force microscopy (AFM) data that an enhanced layer-by-layer 2D growth can actually be realized and (2x4) GaAs(001) surface can be achieved in low-temperature heteroepitaxy on vicinal Ge(001) substrates at 400 °C by H-MBE. EXPERIMENT The growth was carried out in our conventional MBE system, a schematic of which is shown in Fig. 1. A continual generation and irradiation of atomic H inside the growth chamber is done by a cracker that fits into one of the effusion cell ports of the chamber. The hydrogen cracker consists of a spirally-wound tungsten filament geometry, which when heated > - 1500°C, will dissociate the molecular hydrogen into atomic H [10]. The surface structures during the substrate cleaning and growth were monitored by RHEED. The surface morphologies were characterized by AFM in ambient atmosphere immediately after each growth, and the depth profiles of each compositional element were measured by SIMS with Cs+ ions. Vicinal Ge(001) substrates tilted 2°-off toward [110] azimuth were arbitrarily chosen for this work. Befo
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