Ch. Dieker

Photoluminescence and electroluminescence of SiGe dots fabricated by island growth

We present a study of photo‐ and electroluminescence of SiGe dots buried in Si and compare them with structures containing smooth SiGe layers. The SiGe dot structures were fabricated by low‐pressure chemical vapor deposition using the Stranski–Krastanov growth mode (island growth). We show that the localization of excitons in the dots leads to an increase of the luminescence efficiency at low excitation compared to smooth SiGe layers (e.g., quantum wells). At higher excitation the efficiency decreases which is attributed to nonradiative Auger recombination processes in the dots.

Ion beam synthesis of buried α-FeSi2 and β-FeSi2 layers

Using high dose implantation of Fe+ into (111)Si, followed by rapid thermal annealing (RTA) at 1150 °C for 10 s, we fabricated continuous buried layers of the metallic α‐FeSi2 phase. Rutherford backscattering experiments indicate that these layers contain a large number of Fe vacancies, up to 18%. By implanting through a SiO2 mask, we produced Schottky diodes with idealty factors of 1.4±0.1 and a Schottky barrier height of ΦB=0.84±0.03 eV on (111) n‐Si. In this letter we report for the first time the formation of the semiconducting stoichiometric FeSi2 (β‐FeSi2) phase by annealing the buried α‐FeSi2 layers below the phase transition temperature of 937 °C; specifically at 750 °C for 20 h.

Optical and structural investigation of SiGe/Si quantum wells

In this letter we report photoluminescence and structural results obtained on asymmetrically strained Si0.7Ge0.3/Si single and multiple quantum wells epitaxially grown by low pressure chemical vapor deposition. Well‐resolved peaks were obtained which can be attributed to quantum well excitons and their transversal optical phonon replica. A good correlation between peak properties and structure results was found. From the photoluminescence peak energies a valence band offset of 0.27 eV and an effective hole mass of 0.25 were estimated.

Multilayer mirror for x rays below 190 eV.

La/B(4)C multilayers have been fabricated by magnetron sputtering for use as x-ray mirrors at energies below 190 eV, particularly for detection of boron K and alpha x rays at 183 eV, their performance has been compared with that of Mo/B(4)C multilayers, which are currently the best-performing multilayers for this energy range. Transmission electron microscopy and synchrotron soft-x-ray reflectometry were used to study the structural quality of the multilayers and their performance as x-ray mirrors. The results show a significant improvement of the peak reflectivity and the spectral purity, indicating that La/B(4)C has a high potential to replace Mo/B(4)C in many x-ray optical applications below 190 eV.

Electron microscopy of interfaces in chemical vapour deposition diamond films on silicon

Abstract The structure of interfaces in diamond films grown on Si(100) has been investigated by transmission electron microscopy for the early stages of microwave-assisted chemical vapour deposition. Using conditions optimized for achieving so-called highly-oriented diamond films the depositions were performed in two steps, a bias-enhanced nucleation step and a subsequent growth step. Characteristic for the early deposition stages is the self-organized formation of regular arrays of predominantly {111}-facetted Si substrate surface grooves and islands elongated along [110] and [110] directions. Subsequently, an interlayer of nanocrystalline β-silicon carbide islands forms, followed by the formation of epitaxially oriented diamond nanocrystals with high fractions of {111} interfaces. High-resolution electron microscopy of the interface regions depicts arrays of terminating {111} diamond planes at an average ratio of five diamond to four SiC lattice planes which corresponds to a remaining lattice mismatch of 2.3%. The orientation relationships between the lattices may be described by a coincidence site lattice model if the elastic lattice distortions are taken into account. Only small fractions of amorphous inclusions are present near interfaces, essentially consisting of amorphous carbon as could be deduced from analyses of the C K edge fine structure in electron energy loss spectra. The observations are compared with cases for which diamond nucleation directly on silicon has been obtained.

Growth of SiGe quantum wires and dots on patterned Si substrates

SiGe/Si quantum well layers are selectively grown by low pressure chemical vapor deposition on patterned Si substrates. Transmission electron microscopy (TEM) shows that the growth rate of SiGe in convex corners between different surface planes is at least ten times higher than the growth rate observed on (001) planes. This high growth rate leads to the formation of quantum wires and dots between such facets. Photoluminescence (PL) spectra of square and rectangular patterns, bounded by quantum wires, ranging in size from 300μm down to 500nm are taken. The observed energy shifts of the (001) quantum well PL–peaks are explained by surface diffusion of Ge adatoms into the quantum wires. A surface diffusion model is used to obtain a Ge diffusion length of λ=2.5±0.6 μm at 700°C. Thus, a method for the determination of surface diffusion lengths in strained layer epitaxy is introduced. For SiGe layers grown above the Stranski–Krastanow critical thickness for three dimensional (3D) growth, a competition between t...

Current model considering oxide thickness non-uniformity in a MOS tunnel structure

Abstract A problem of oxide non-uniformity in MOS tunnel structures on n-type Si with a 2–3 nm nominal SiO2 thickness dn is investigated. Thickness distribution is described with Gauss law. Concepts of modeling of reverse current–voltage characteristics are formulated for metal-oxide-semiconductor (MOS) tunnel structures, regarding the effect of non-uniformity. The current crowding in relatively thin device sections is shown to result in a noticeable transformation of curves for electron and hole tunnel currents. Introduction of a mean effective thickness deff instead of the nominal one is possible only for the weak-inversion regime. The experimental part of this work includes the examination of samples by TEM and atomic force microscope methods, and their electrical characterization. Measured characteristics have been satisfactorily fitted by curves simulated for a standard thickness deviation of σ=0.3 nm.

Photoluminescence and microstructure of self‐ordered grown SiGe/Si quantum wires

Employing self‐ordered growth in convex corners of nonplanar Si substrates, SiGe quantum wires of approximately 30 nm lateral dimension were fabricated. Photoluminescence spectra of these structures are dominated by transitions originating from the quantum wires at measurement temperatures above 20 K. The energetic positions of the quantum wire transitions are in good agreement with Ge concentrations measured by spatially resolved energy dispersive x‐ray spectroscopy using a scanning transmission electron microscope. We find that the Ge concentration inside the wire is considerably lower than the nominal value for growth on planar parts of the substrate. In addition we find a pronounced gradient in the Ge concentration of the wire.

Reaction of iron and silicon during ion implantation

Using energy‐loss spectroscopy, energy dispersive x‐ray analysis, electron diffraction, and He+‐ion channeling the reaction of Fe during implantation into Si(111) has been investigated at various target temperatures and implantation doses. In samples implanted at 275 °C with 2.8×1017 Fe+ cm−2 a continuous α‐FeSi2 layer accompanied by α‐phase precipitates is formed. At 450 °C Fe agglomerates mostly in α‐phase precipitates with only a few being β‐FeSi2. At 350 °C 1×1017 Fe+ cm−2 produce precipitates electronically close to FeSi2 but crystallographically poorly defined. At 4×1017 Fe+ cm−2 a β‐FeSi2 layer is formed at the surface and a 20‐nm‐thick α‐FeSi2 one followed by α‐FeSi2 precipitates deeper in the volume. Channeling reveals a minimum yield decreasing with dose indicating improved α‐phase crystal quality. A sharp increase at 3.3×1017 cm−2 indicates an α–β phase transition. FeSi has not been detected. Precipitates of well defined silicide phases are formed already during implantation. Dose and temperatu...