S. Müller

SiC-bulk growth by physical-vapor transport and its global modelling

4H- and 6H-SiC bulk crystals have been prepared by physical vapor transport (PVT) both in resistively and inductively heated growth reactors. Epitaxial SiC layers were grown on the wafers by chemical vapor deposition. Structural and electrical material properties of the 1–1.4 inch boules and epitaxial layers were investigated by defect etching and optical microscopy, stress birefringence and Hall effect. Single crystalline material exhibits a low micropipe density MPD ≈ 70 cm−2 and stress level. Blocking characteristics of the epitaxial layers have been determined electrically revealing high breakdown fields of 1.8–1.9 MV/cm. Finally simulation results applying a process model of SiC PVT crystallization including heat and mass transfer and chemical reactions are presented.

UNCLE: a code for constructing cluster expansions for arbitrary lattices with minimal user-input

We present a new implementation of the cluster expansion formalism. The new code, UNiversal CLuster Expansion (UNCLE), consolidates recent advances in the methodology and leverages one new development in the formalism itself. As a core goal, the package reduces the need for user intervention, automating the method to reduce human error and judgment. The package extends standard cluster expansion formalism to the more complicated cases of ternary compounds, as well as surfaces, including adsorption and inequivalent sites.

High precision LEED structure analysis of ultra-thin epitaxial fcc Fe films on Cu(100)

Abstract We present structure determinations of ultra-thin epitaxial fcc Fe films on Cu(100) by low energy electron diffraction in the coverage regime of 5–10 monolayers. Special emphasis is on the precision of both the measurement of diffraction intensities and their full dynamical analysis. We demonstrate that it is possible to achieve an almost perfect fit between theory and experiment mirrored by R-factors as low as RP = 0.090, RZJ= 0.026 and R2 = 0.028 which to our knowledge are the best reported for non-trivial structures. The structure of the films, which stays practically constant in the coverage regime considered, is characterized by a substantial surface reconstruction, an expanded spacing between the first two iron layers and an unstrained in-plane lattice constant aP(Fe) = 2.52 A rather than that for the substrate (aP(Cu) = 2.55 A). The surface reconstruction, which always moves to the top layer when the film thickness is made to grow, can be interpreted as a precursor for the transition to bcc Fe which takes place at higher coverages. The expansion of the top layer spacing, which is consistent with the reconstruction, is fully in line with published surface magnetic measurements which exhibit ferromagnetic coupling in the first two layers.

Crystallography of ultrathin iron, cobalt and nickel films grown epitaxially on copper

Though correlations between surface structure and magnetism are evident, surface structure determinations of ultrathin magnetic films in the crystallographic sense are available only for a few cases. We investigate the three examples Ni/Cu(100), Co/Cu(111) and Fe/Cu(100) which each exhibit only little film-substrate lattice misfit but which are rather different as regards the native lattice symmetry of the materials involved. Correspondingly, the structures of the films are rather different. They are described for various film thicknesses in each case and a comparison with the magnetic properties is made.

Interface structure and stabilization of metastable B2-FeSi/Si(111) studied with low-energy electron diffraction and density functional theory

We present a combined experimental and theoretical investigation of the interface between a B2-type FeSi film and Si(111). Using an ultra-thin B2-FeSi film grown on Si(111), the interface is still reached by electrons, so quantitative low-energy electron diffraction (LEED) could be applied to determine the bonding geometry experimentally. As a result, the local configuration at the shallow buried interface is characterized by near-substrate Fe atoms being 8-fold coordinated to Si atoms and by the silicide unit cell being rotated by 180° with respect to the Si unit cell (B8 configuration). The interface energetics were explored by total-energy calculations using density functional theory (DFT). The B8-type interface proves to be the most stable one, consistent with the experimental findings. The atomic geometries obtained experimentally (LEED) and theoretically (DFT) agree within the limits of errors. Additionally, the calculations explain the stabilization of the B2 phase, which is unstable as bulk material: the analysis of the elastic behaviour reveals a reversed energy hierarchy of B2 and the bulk stable B20 phase when epitaxial growth on Si(111) is enforced.

Reinterpreting the Cu–Pd phase diagram based on new ground-state predictions

Our notions of the phase stability of compounds rest to a large extent on the experimentally assessed phase diagrams. Long ago, it was assumed that in the Cu–Pd system for xPd≤25% there are at least two phases at high temperature (L12 and a L12-based superstructure), which evolve into a single L12-ordered phase at low temperature. By constructing a first-principles Hamiltonian, we predict a yet undiscovered Cu7Pd ground state at xPd = 12.5% (referred to as S1 below) and an L12-like Cu9Pd3 superstructure at 25% (referred to as S2). We find that in the low-temperature regime, a single L12 phase cannot be stable, even with the addition of anti-sites. Instead we find that an S2-phase with S1-like ordering tendency will form. Previous short-range order diffraction data are quantitatively consistent with these new predictions.

Formation of micropipes in SiC under kinetic aspects

Abstract We measure the radii of micropipes at the {0001} surface of modified Lely grown 6H-SiC and the total step height of the accompanying growth spirals by using atomic force microscopy. The micropipes lie in the center of spirals; the total step height ranges between one and 19 unit-cells (1.5–28.5 nm). We fit Franks theory of hollow core dislocations as modified with regard to kinetic effects by Cabrera and Levine to these experimental results and obtain values for surface energy and supersaturation near the emergence point of the micropipe.

First-principles modeling of temperature- and concentration-dependent solubility in the phase-separating alloy FexCu1−x

We present a novel cluster-expansion (CE) approach for the first-principles modeling of temperature and concentration dependent alloy properties. While the standard CE method includes temperature effects only via the configurational entropy in Monte Carlo simulations, our strategy also covers the first-principles free energies of lattice vibrations. To this end, the effective cluster interactions of the CE have been rendered genuinely temperature dependent, so that they can include the vibrational free energies of the input structures. As a model system we use the phase-separating alloy Fe-Cu with our focus on the Fe-rich side. There, the solubility is derived from Monte Carlo simulations, whose precision had to be increased by averaging multiple CEs. We show that including the vibrational free energy is absolutely vital for the correct first-principles prediction of Cu solubility in the bcc Fe matrix: The solubility tremendously increases and is now in quantitative agreement with experimental findings.

Status of 4H-SiC Substrate and Epitaxial Materials for Commercial Power Applications

The performance enhancements offered by the next generation of SiC high power devices offer potential for enormous growth in SiC power device markets in the next few years. For this growth to occur, it is imperative that substrate and epitaxial material quality increases to meet the needs of the targeted applications. We will discuss the status and requirements for SiC substrates and epitaxial material for power devices such as Schottky and PiN diodes. For the SiC Schottky device where current production is approaching 50 amp devices, there are several material aspects that are key. These include; wafer diameter (3-inch and 100-mm), micropipe density ( −2 for 3-inch substrates and 16 cm −2 for 100-mm substrates), epitaxial defect densities (total electrically active defects −2 ), epitaxial doping and epitaxial thickness uniformity. For the PiN diodes the major challenge is the degradation of the Vf characteristics due to the introduction of stacking faults during the device operation. We have demonstrated that the stacking faults are often generated from basal plane dislocations in the active region of the device. Additionally we have demonstrated that by reducing the basal plane dislocation density, stable PiN diodes can be produced. At present typical basal plane dislocation densities in our epitaxial layers are 100 to 500 cm −2 ; however, we have achieved basal plane dislocation densities as low as 4 cm −2 in epitaxial layers grown on 8° off-axis 4H-SiC substrates.

Predicting the segregation profile of the Pt25Rh75(100) surface from first-principles

The segregation profile of the Pt(25)Rh(75)(100) surface is studied by the combination of density functional theory calculations with the cluster-expansion method and Monte Carlo simulations. We construct the stability diagram for the surface layers, which allows the prediction of the most stable atomic configuration for a given average concentration in those layers. On this basis, we apply the cluster-expansion Hamiltonian in grand-canonical Monte Carlo simulations for the prediction of the temperature-dependent concentration profile. The experimentally found enrichment of Pt in the top layer and depletion in the second layer is nicely confirmed by the calculations.