Epitaxial Growth of EuTe on PbTe (111) Influenced by Strain-Induced Coherent 3D Islanding
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short period magnetic superlattices [6]. In addition, EuTe is a wide band gap semiconductor, as opposed to the narrow band gap PbTe lead salt compound and, therefore, quantum well structures with very large confinement energies can be obtained [7]. In terms of heteroepitaxial growth, both materials crystallize in the rock salt crystal structure and their lattice mismatch is about 2 %, which makes them a representative of a moderately lattice-mismatched heteroepitaxial system. 2. EXPERIMENTAL Our investigations were carried out in a Riber MBE growth chamber and custom-built preparation and load-lock chambers. PbTe is evaporated from a compound effusion cell since PbTe evaporates congruently mainly in the form of PbTe molecules. For EuTe growth, separate Eu and Te2 effusion cells are used. Te2 has to be supplied in excess because of its high vapor pressure at the used substrate temperatures in the range of 230'C - 350'C. Absolute beam flux rates from the effusion cells were measured with a quartz crystal thickness monitor, and the substrate temperature was calibrated using the Te2 condensation points as a reference. In situ reflection high energy electron diffraction (RHEED) surface studies were carried out with a 35 keV electron gun. The RHEED patterns were recorded with a video camera and analyzed with an image processing system. It is used for the determination of intensity profiles and the time dependence of the intensities of different diffraction features in the RHEED patterns during growth. In all experiments, first a thick PbTe buffer of about 4 gtm was deposited at a substrate temperature of 350*C on freshly cleaved (111) BaF 2 substrates. This ensures complete strain relaxation of the buffer and a very high crystalline perfection of the epitaxial layer [8] with extremely smooth surface morphologies [9]. 3. RHEED SURFACE STUDIES OF HETEROEPITAXIAL GROWTH The surface phase diagram for EuTe growth on PbTe (111) is shown in
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Fig. 1 for a constant Eu flux rate of 0.4
monolayers(ML)/sec, which corresponds to the EuTe growth rate. Depending on the substrate temperature and impinging Te2 beam flux rate, the EuTe (111) surface is either Te- or Eu-stabilized, each surface state exhibiting a different type of surface reconstruction [10]. The Te-stabilized surface has a multi-domain (3x4) surface reconstruction, which is indicated by the appearance of 1/3 and 1/4 order streaks in the azimuth directions and of 1/2 order streaks in the azimuths. It is stable only for substrate temperatures less than 20 30°C above the Te - condensation point at which the reevaporation rate of Te2 is equal to the net impinging Te2 rate ( solid line in Fig. 1). At substrate temperatures more than 30°C above the Te2 condensation points, the redesorption rate of the chemisorbed Te on the surface is already so high, that the equilibrium Te surface coverage becomes very small and the surface changes to the Eu-stabilized state. In this case, a clear (2413 x 243)R30°
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