Bernd Schröter

Low‐temperature growth of SiC thin films on Si and 6H–SiC by solid‐source molecular beam epitaxy

Epitaxial growth of stoichiometric SiC on Si(111) and 2°–5° off‐oriented 6H–SiC(0001) substrates was carried out at low temperatures (800–1000 °C) by means of solid‐source molecular beam epitaxy controlled by a quadrupole mass spectrometry based flux meter. The films were obtained on Si‐stabilized surfaces showing (3×3) and (2×2) superstructures in the case of SiC(0001). The reflection high‐energy diffraction (RHEED) patterns and damped RHEED‐oscillations during the growth on 6H–SiC(0001) at T≳900 °C indicate that two‐dimensional nucleation on terraces is the dominant growth process.

Epitaxial growth of SiC thin films on Si-stabilized α-SiC(0001) at low temperatures by solid-source molecular beam epitaxy

Abstract Epitaxial growth of SiC on α-SiC(0001) substrates was carried out at relatively low temperatures (900–1000°C) and high growth rates (about 1 nm/min), by means of solid-source molecular beam epitaxy controlled by a quadrupole mass spectrometry based flux meter. The films were obtained on silicon-stabilized surfaces showing (3 × 3) and (2 × 2) superstructures. The reflection high-energy electron diffraction (RHEED) and transmission electron microscopy (TEM) investigations show, independent of the surface orientation, that the film growth can be quantified in two steps: In the initial stage of growth, at first the Si-determined superstructure will be formed. Despite the low temperature, growth of SiC then proceeds in a layer-by-layer mode leading to flat film-substrate interfaces. This result demonstrates the significance of surface reconstruction for the growth process, corresponding to results recently obtained using gas source molecular beam epitaxy. The films grow by stacking of laminae of α-and β-SiC, respectively, which may be attributed to fluctuations in the Si adlayer thickness. Films grown on off-oriented substrates contain many defects, likely double-positioning boundaries, directly associated with surface steps. For increasing film thickness during the film growth this boundaries may be eliminated. Films grown on well-oriented substrates show only a few of these defects. In case of increasing Si excess flux during the growth, the TEM investigations, RHEED patterns and damped RHEED-oscillations indicate an abrupt change in the growth process. This is due to the formation of Si islands on the film surface and the SiC island growth by carbonization of these islands. Only β-SiC was found in this case.

Advances in the molecular-beam epitaxial growth of artificially layered heteropolytypic structures of SiC

The controlled growth of SiC heteropolytypic structures consisting of hexagonal and cubic polytypes has been performed by solid-source molecular-beam epitaxy. On on-axis substrates, 4H/3C/4H–SiC(0001) and 6H/3C/6H–SiC(0001) structures were obtained by first growing the 3C–SiC layer some nanometer thick at lower substrate temperatures (T=1550 K) and Si-rich conditions and a subsequent growth of α-SiC on top of the 3C–SiC layer at higher T (1600 K) under more C-rich conditions. On off-axis substrates, multiheterostructures consisting of 4H/3C- or 6H/3C-stacking sequences were also obtained by first nucleating selectively one-dimensional wire-like 3C–SiC on the terraces of well-prepared off-axis α-SiC(0001) substrates at low T(<1500 K). Next, SiC was grown further in a step-flow growth mode at higher T and Si-rich conditions. After the growth, many wire-like regions consisting of 3C–SiC were found also within the hexagonal layer material matrix indicating a simultaneous step-flow growth of both the cubic and...

Epitaxial relationship in the AlN/Si(001) heterosystem

The epitaxial growth of crystalline wurtzite AlN thin films on (001) Si substrates by plasma-assisted molecular-beam epitaxy is reported. The nucleation and the growth dynamics have been studied in situ by reflection high-energy electron diffraction. Cross-sectional transmission electron microscopy and x-ray diffraction investigations revealed a two-domain film structure (AlN1 and AlN2) with a 30° rotation between neighboring domain orientations and an epitaxial orientation relationship of [0001]AlN∥[001]Si and 〈0110〉AlN1∥〈2110〉AlN2∥[110]Si. A model for the nucleation and growth mechanism of 2H–AlN layers on Si(001) is proposed.

Hexagonal AlN films grown on nominal and off-axis Si(0 0 1) substrates

Nucleation and growth of wurtzite AlN layers on nominal and off-axis Si(0 0 1) substrates by plasma-assisted molecular beam epitaxy is reported. The nucleation and the growth dynamics have been studied in situ by reflection high-energy electron diffraction. For the films grown on the nominal Si(0 0 1) surface, cross-sectional transmission electron microscopy and X-ray diffraction investigations revealed a two-domain film structure (AlN 1 and AlN 2 ) with an epitaxial orientation relationship of [0 0 0 1]AlN || [0 0 1]Si and h 01 %

Growth of 6H–SiC on 6H–SiC(0001) by migration enhanced epitaxy controlled to an atomic level using surface superstructures

Epitaxial growth of 6H–SiC on 6H–SiC(0001) via two‐dimensional nucleation was realized at 930 °C by solid‐source molecular beam epitaxy using the alternate supply of Si and C. The deposition was controlled to an atomic level by surface superstructures. The growth was started on the (√3×√3)R30° surface which turns into the (1×1) phase upon deposition of about 1 monolayer silicon and recurs after subsequent deposition of about 1 monolayer carbon. Deviations from the monolayer deposition and, moreover, growth around substrate related defects result in the deposition of 3C–SiC.

13C High Resolution Solid State NMR Studies on Cellulose Samples of Different Physical Structure

Summary13C bigh resolution solid state MR spectra of cellulose samples differing in lattice type, crystallinity and gross morphology (pulp, filament, film, bead) are presented and discussed with regard to the above mentioned parameters of physical structure.

Si/Ge-nanocrystals on SiC(0001)

Abstract The growth and structure of Si- and Ge-nanocrystals was investigated using high resolution X-ray diffraction (HRXRD) and atomic force microscopy (AFM). AFM-images were used to determine the lateral and vertical dimensions of the nanocrystal. HRXRD measurements show clearly that Si- and Ge-nanocrystals grow on 6 H –SiC(0001) preferentially in two different orientations — 〈111〉 and 〈110〉 — with respect to the surface normal. The growth of Ge-nanocrystals on Si-rich 6 H –SiC(0001) surfaces leads to the formation of Si/Ge-alloy nanocrystals. Both types of nanocrystals grow coherently with respect to the substrate. Hence, due to the respective lattice mismatch, the degree of coherence was found to be much better for Si-nanocrystals.

The influence of surface preparation on the properties of SiC on Si(111)

The silicon (111) surface was converted into silicon carbide by using: 1. propane diluted in hydrogen and rapid thermal processing, 2. elemental carbon deposited onto the silicon surface by solid source molecular beam epitaxy and subsequent annealing, i.e. conversion in a hydrogen poor environment, 3. modification of the silicon surface by Ge predeposition prior to elemental carbon deposition. These methods were compared according to their influence on the structure, morphology and electronic properties of the SiC/Si(111) heteroepitaxial system. It was found that the conversion in a hydrogen rich environment leads to the formation of a carbon (111) silicon carbide face, whereas a silicon (111) silicon carbide face was formed under hydrogen poor conditions. Germanium predeposition led to an improvement of the structural morphological and electrical properties of the silicon carbide-silicon heteroepitaxial system.

Electron diffraction methods for the analysis of silicon carbide surfaces and the controlled growth of polytype heterostructures

The growth of different silicon carbide?(SiC) polytypes on each other is possible by control of the surface structure and the appropriate thermodynamic parameters. Special ultrahigh vacuum conditions, like those used in solid source molecular beam epitaxy, allow the determination of the species on the surface and also the in situ characterization of the growing polytype by electron diffraction methods. The surface reconstruction which favours the growth of a certain polytype can be controlled by reflection high energy electron diffraction. For a non-destructive determination of the polytype of a grown thin SiC film, methods like x-ray photoelectron diffraction?(XPD) and electron channelling can be used. The interaction length of electrons near 1?keV kinetic energy is in the range of 1?nm and therefore sensitive to the stacking sequence of the most common SiC polytypes 3C, 4H, 6H with c-axis dimensions between 0.75 and 1.5?nm. To prepare polytype heterostructures like 4H/3C/4H or 6H/3C/6H, untwinned 3C SiC films without double-positioning boundaries have to be grown. On-axis ?-SiC substrates with uniform surface stacking termination are a prerequisite for this. Such surfaces can be prepared using high temperature hydrogen etching, sublimation etching or step-flow growth. These equally terminated crystals with threefold surface symmetry are particularly suitable for detailed studies of the atomic-geometric structure and their changes during growth or after certain treatments. Results of surface-sensitive characterization methods like scanning tunnelling microscopy, XPD and low energy electron diffraction are presented.