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The Gan heteroepitaxy on 6H-SiC is affected by the bad morphology of the substrate surface. We performed a hydrogen etching at 1550oC on the 6H-SiC(0001) substrates to obtain atomically flat terraces. An improvement of the structural properties of GaN grown by MBE on such substrates after deposition of a LT-AlN buffer layer is observed. A value of less than 220 arcsec of the FWHM of the XRD rocking curve, showing a reduced screw dislocations density, is comparable with the best results reported until now for thick GaN samples. Photoluminescence showed a structured near band edge emission spectrum with evidence of the A, B and C free exciton recombinations.
Heteroepitaxy of β-FeSi2 on (100) and (111) silicon surfaces has been achieved by gas source molecular beam epitaxy (GSMBE). Fe(CO)5 and SiH4 are used as sources for the silicide growth in the substrate temperature range 400 – 750°C. Depending on growth temperature different growth modes are observed. Concerning morphology two best temperatures were identified for the growth of β-FeSi2 on Si(111). Tg = 550°C - the epitaxial relationship with the substrate is β-FeSi2(100)||Si(111) as shown by High Resolution Transmission Electron Microscopy (HRTEM) and Low Energy Electron Diffraction (LEED); a thin γ-FeSi2 (metallic cubic phase) layer is seen to be stabilized at the interface acting as a buffer layer for the further growth of the orthorhombic semiconducting phase. Tg = 700°C - the same epitaxial relationship as in SPE grown layers is observed: β-FeSi2(101) or β-FeSi2(110)||Si(111) and no γ-FeSi2 is present at the substrate interface. In accord with the lower growth temperature, the silicide layers show slightly higher surface roughness. On Si(100) the epitaxial relationship with the substrate is β-FeSi2(100)||Si(100) in the whole temperature range. On both Si substrates domains with different azimuthal orientations are observed by LEED. In-situ electronic characterization is performed by photoelectron and electron energy loss spectroscopies. Electrical characterization at room temperature shows relatively high mobility values (up to 70cm2/Vs) but the complex temperature behaviour of the Hall constant suggests the presence of both carrier types in the β-FeSi2 grown layers. Measurements of the absorption coefficient at RT show an indirect minimum gap for β-FeSi2.
GaAs has been grown on (100) and (111) Si and GaP substrates by MOCVD during the same run at a substrate temperature of 550°C. The defect structure of the interface and the epitaxial GaAs layers has been analysed by transmission electron microscopy (TEM). Depending on the substrate type and orientation different defects and defect orientations were observed which can be attributed to different types of misfit dislocations and the thermal mismatch between the two materials.
The growth mode and the relaxation of MBE grown InxGa1-xiAs layers (0.13 ≦ x ≦ 1.0, nominal film thickness 5 nm) on GaAs(100) substrates with a lattice-parameter mismatch were investigated by transmission electron microscopy (TEM). The transition between two- and three-dimensional growth occurs at x ≈ 0.4. The three-dimensional growth mode for x ≥ 0.6 results in a wide spectrum of island sizes. In contrast to the two-dimensional growth mode, the strain state of a three-dimensionally growing layer is completely inhomogeneous because the relaxation of the strain is correlated with the island size. The reduction of elastic strain for islands is reasonably described by an energy balance model.
The Gan heteroepitaxy on 6H-SiC is affected by the bad morphology of the substrate surface. We performed a hydrogen etching at 1550°C on the 6H-SiC(0001) substrates to obtain atomically flat terraces. An improvement of the structural properties of GaN grown by MBE on such substrates after deposition of a LT-AIN buffer layer is observed. A value of less than 220 arcsec of the FWHM of the XRD rocking curve, showing a reduced screw dislocations density, is comparable with the best results reported until now for thick GaN samples. Photoluminescence showed a structured near band edge emission spectrum with evidence of the A, B and C free exciton recombinations.
The mechanical behavior of Fe-Cr-Al-alloys was investigated at different temperatures and after several heat-treatments in order to determine critical conditions for the ductility of the material. It was found that a significant embrittlement takes place during deformation at higher temperatures around 400 °C as well as after heat-treatments at temperatures above 1000 °C in combination with very high or very low cooling rates. Possible reasons for this behavior are discussed with respect to the results of transmission electron microscopy (TEM), Auger spectroscopy (AES), positron annihilation spectroscopy (PAS) and measurements of the electrical resistivity.
High-angle annular dark-field (HAADF) scanning transmission electron microscopy (STEM) images of electron-transparent samples show dominant atomic number (Z-) contrast with a high lateral resolution. HAADF STEM at low electron energies <30 keV is applied in this work for quantitative composition analyses of InGaAs quantum wells. To determine the local composition, normalized experimental image intensities are compared with results of Monte Carlo simulations. For verification of the technique, InGaAs/GaAs quantum-well structures with known In concentration are used. Transmission electron microscopy samples with known thickness are prepared by the focused-ion-beam technique. The method can be extended to other material systems and is particularly promising for the analysis of materials that are sensitive toward knock-on damage.