Molecular beam epitaxy (MBE) denotes epitaxial growth of thin semiconductor, metal, or oxide films from atomic or molecular beams and was introduced in the late 1960s by J. R. Arthur and A. Y. Cho for growth of III/V-semiconductors. Overviews are given, for instance, in [1-4]. The term epitaxy (greek: "epi" "above" and "taxis" "in ordered manner") describes crystalline growth with order given by the substrate. The samples studied here were fabricated in a MBE cluster system with two semiconductor growth chambers (Riber 32P and Riber C21). The base pressure inside the MBE chambers is in the low 10-11 mbar range in order to avoid unintentional doping with background impurities. The molecular or atomic beams are thermally evaporated from ultra-pure elements in so-called effusion cells. The MBE chambers are equipped with several effusion cells for evaporation of the group III elements Ga, Al, and In, the group V element As, the dopants Si and C, as well as with a Mn cell for fabrication of diluted magnetic semiconductors. This cell configuration allows the growth of heterostructures composed of the compound semiconductors GaAs, AlAs, InAs, and alloys of these materials. With cell shutters in front of the effusion cells, switching of the respective flux takes less than 0.5 s. In combination with a typical MBE growth speed of about one monolayer (ML) per second this enables the vertical structuring of semiconductor crystals on the atomic scale. We use 2 inch GaAs, InAs or InP wafers as substrates for the MBE growth. Most samples are grown on (001) GaAs substrates.
The time evolution of the surface morphology on the growing crystal was examined in situ using reflection high-energy electron diffraction (RHEED). RHEED is a very powerful method and has been established as a standard technique for instance to study the GaAs surface morphology [5,6] during MBE, intensity oscillations during GaAs layer-by-layer growth [7-11], and the spontaneous formation of InAs quantum dots in Stranski-Krastanov mode [12-15]. In our RHEED experiments we use a 12 keV electron source in combination with a CCD camera and an image processing program on a personal computer for data acquisition. An ex situ analysis of the created nanostructures was performed using atomic force microscopy (AFM) and transmission electron microscopy (TEM).
References
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