Growth Processes and Surface Phase Equilibria in Molecular Beam Epitaxy
The book considers the main growth-related phenomena occurring during epitaxial growth, such as thermal etching, doping, segregation of the main elements and impurities, coexistence of several phases at the crystal surface and segregation-enhanced diffusi
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2.1 MBE Apparatus First of all we shall address ourselves to the basics of molecular beam epitaxy. M B E is a technique for the epitaxial growth of materials by means of the chemical interaction of one or several molecular or atomic beams of different intensities and compositions that occurs on the surface of a heated singlecrystalline substrate. A schematic representation of suitable MBE apparatus is shown in Fig. 2.1. The source materials are placed in evaporation cells that are composed of a crucible of shape and dimensions ensuring the required angular distribution of atoms or molecules in a beam, a resistive heater, and thermal screens. The angular distribution of the beam and the distances between the sources and the substrate determine the homogeneity of the parameters of epilayers and heterostructures grown by this technique. A manipulator with
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RHEED Substrate ElectronGun on a SubstrateHolder UlrahighVacuum ~ %.....~iUItraChhghb vacuum
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2. Basics of MBE Growth
a substrate holder is used to enable the required position of the substrate relative to the cells to be obtained, and to heat it to the necessary temperature. The homogeneity of the grown films is often improved by rotation of the substrate. The molecular beam condition (where the free path of the particle is larger than the geometrical size of the chamber) is ensured, for typical distances between the sources and the substrate, if the total pressure does not exceed 10 -4 Tort. However, all MBE systems are, as a matter of course, provided with a means of reaching and maintaining an ultrahigh vacuum ( ~ 10 -11 Torr) and the operation is usually oil-free. One of the reasons why MBE systems have to be oil-free is the need to ensure that the substrate is atomically clean before growth. Even so, a low level of background doping and control over the properties of the grown materials and structures can be assured only if uncontrolled fluxes of atoms reaching the substrate surface are as weak as possible. An ultrahigh vacuum is essential for this purpose, but it is not a sufficient condition; two other conditions need to be applied. First, any vacuum represents an equilibrium between the rate of gas evolution and the rate of pumping, so that it is necessary to use construction and crucible materials with the lowest rate of gas evolution. The usual crucible material for MBE of III-V compounds is boron nitride, which combines a low rate of gas evolution with weak chemical activity right up to temperatures of the order of 1500 ~ Second, it is important to ensure cryogenic screening around the substrate so as to minimize stray fluxes of atoms and molecules from the walls of the chamber, which are at room temperature, and from the heated components of the apparatus. It is necessary also to monitor not only the total pressure but also the partial composition of the atmosphere and, if necessary, to alter it (by, for example, exposure of the
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