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We demonstrate that variable angle spectroscopic ellipsometry (SE) is capable of determining both ordinary and extraordinary dielectric functions (DFs) in the transparent region of group III nitride wurtzite films even if the optical axis is oriented normal to the surface plane. In contrast to the so far used prism coupling technique, the SE method is neither restricted to layers deposited on lower refractive index substrates nor to available laser wavelengths. Application to AlN and GaN films grown on 6H-SiC substrates yields experimental data which can be represented in simple analytical form. The difference in energy dispersion for ordinary and extraordinary DFs is related to the effective optical band gap depending on the light polarization. Extrapolation of data to lower photon energies allows estimation of the ordinary and extraordinary high-frequency dielectric constants.

The self-organized growth of Ge islands on various hexagonal SiC{0001 surfaces is discussed. The formation of two (2-D) or three-dimensional (3-D) Ge islands depend on the polarity of the SiC substrate (silicon or carbon face), the SiC surface stoichiometry and reconstruction, the growth rate and temperature. Furthermore, the nucleation conditions can be modified by an oxidation of the Si-adlayer present on Si-rich SiC surfaces. On the Si face of SiC(0001)-(√3 × √3)R30°, 2-D Ge islands of lateral dimensions between 2 and 4 nm and a density of more than 10 12 cm - 2 are initially formed on a 0.3 nm thick Ge wetting layer. Similar to Si substrates, a further growth leads to the formation of equilibrium 3-D Ge nanocrystals of typically more than 20 nm in size and a density of about 10 10 cm 2 . A Ge deposition on the C face of SiC, as well as on the oxidized Si adlayer of a Si face of SiC results in small 3-D Ge islands of only a few nanometers in size and a number density between 10 and 10 12 cm -2 .

Multi-quantum well structures with 3C–SiC wells between α-SiC barriers were grown in a two-step procedure by solid-source molecular beam epitaxy. First, one-dimensional wire-like 3C–SiC was nucleated selectively on terraces of the well-prepared off-axis α-SiC(0 0 0 1) substrates at low temperature (T K ) . Next, 3C–SiC lamellae were incorporated into the hexagonal layer material via simultaneous step-flow growth mode of both the 3C–SiC nuclei and the hexagonal substrate material at higher T and Si-rich conditions. In comparison to homopolytypic SiC layers, photoluminescence investigations revealed additional signals, which can be explained by optical transitions within the thin cubic well layers.

Numerous stacking faults and dislocations (formed by intersection of stacking faults and dislocations limiting nonintersecting stacking faults) in the 3C-SiC films grown by molecular beam epitaxy on a silicon substrate were studied by electron microscopy with the use of weak beams. A procedure for determining any of possible Burgers vectors of the (1/6)〈116〉-type glide dislocations and the (1/6) 〈110〉-and (1/3)〈001〉-type sessile partial dislocations (in face-centered cubic lattices) is developed based on the criterion of the contrast value. The sessile dislocations formed by intersections of stacking faults were shown to have the (1/6)〈110〉-and (1/3)〈001〈-type Burgers vectors. The width of nonintersecting stacking faults corresponds to the stacking-fault energy ranging within 0.1–2 mJ/m2.

In the present work, we investigated the influence of step bunching on the optical properties of InGaAs/GaAs quantum wells (QWs) grown by molecular beam epitaxy (MBE) on vicinal surfaces. Photoluminescence (PL) measurements showed a larger full width at half maximum (FWHM) of the emission coming from the QWs grown on the vicinal surfaces with respect to the nominal sample. Transmission-electron-microscopy (TEM) measurements revealed the presence of step bunches that clearly roughen the interfaces of the heterostructures and worsen the optical properties of the samples.

The molecular beam epitaxial growth of Si on SiC(0 0 0 1), exhibiting the Stranski–Krastanov mode, was investigated by reflection high-energy electron diffraction (RHEED). In the initial stage some irregular RHEED oscillations were observed corresponding to both the occurrence of surface superstructures and monolayer coverages. After exceeding a critical coverage, Si island formation sets in. Two different kinds of islands were found with (1 1 1)/(0 0 0 1) and (1 1 0)/(0 0 0 1) epitaxial relationship. Under near equilibrium conditions, the critical coverage was found to be 1.4 monolayers and corresponds to the occurrence of a 3×3 superstructure remaining also as wetting layer after the island formation suggesting that the basic units (tetramers) forming the 3×3 superstructures act already as nucleation sites for the islands. Island formation at high deposition rates (R) and low temperatures (T) was found to be kinetically delayed, which can be described as a function of R and the diffusivity D by the relationship tc ∝ R/D . Si islands relatively uniform in size of some nanometers and a density of 1011/cm2 were obtained under these conditions. At lower R the critical thickness is only a function of T, indicating that the incorporation time of adatoms is the relevant time scale for surface diffusion. Ordered arrays of small dots were grown by decorating the steps on vicinal surfaces with Si islands.

Germanium nanocrystals as potential candidates for a future optoelectronics in group-IV semiconductors have been grown on SiC(0001). A monoatomic wetting layer is formed in a Stranski-Krastanov growth mode. The surface structure of this wetting layer studied by STM and LEED depends on the SiC surface preparation. 3x3 and 4x4 Ge superstructures are observed by growing on the silicon-rich SiC(0001)-3x3 surface. Surface structures with mainly two-fold periodicity as well as 6x6 are observed after Ge deposition on silicon-deficient SiC(0001)-(?3x?3)R30° or (6?3x6?3) surfaces. Two-dimensional Ge islands of lateral dimensions between 2 and 4 nm and a density of 3 * 10 cm are initially formed on the wetting layer to reduce strain. Further deposition results in the growth of nanocrystals of lateral dimensions between 40nm and 150nm and heights between 5 and 30nm. A maximum density of 10 cm and minimum size of these nanocrystals has been obtained for low deposition temperature of 470°C and high rate of 1.5nm/min. The epitaxial orientation of the nanocrystals has been determined as (111) and (220) by TEM and XRD. Introduction Low-dimensional semiconductor structures are investigated for their interesting properties for electronic and optical applications [1]. Nanometer scale structures are expected to show electron confinement the stronger the smaller the size. Quantum dots confining the electrons in three dimensions can be grown by Stranski-Krastanov growth mode. Due to the strain induced by a different lattice constant of substrate and deposited material, an initially two-dimensional (2D) epitaxial growth changes to 3D island growth on a thin wetting layer. This formation of selfassembled islands reduces the strain and lowers the total energy of the system. Germanium nanocrystals embedded in a semiconductor matrix are potential candidates for a future optoelectronics in group-IV semiconductors. Although an indirect semiconductor, high intensity luminescence is expected for germanium quantum dots due to carrier confinement. Many investigations have dealt with Ge on Si(001) or Si(111) [e.g., 2-5]. We have grown Ge on SiC(0001) by solid-source MBE. The wide-band gap semiconductor SiC may give the possibility to arrange quantum dots at a p-n junction for electroluminescent devices. A Stranski-Krastanov-like growth mode is observed due to the huge lattice mismatch of some 30% between silicon carbide and germanium. We investigated the surface structure of Ge on differently prepared SiC(0001) by Scanning Tunneling Microscopy (STM) and Low-Energy Electron Diffraction (LEED). The formation of Ge islands by Stranski-Krastanov growth mode is measured in situ by STM and ex situ by Atomic Force Microscopy (AFM), High-Resolution Transmission Electron Microscopy (TEM), and X-ray diffraction (XRD). Materials Science Forum 353-356 (2001) 247-250.

The photoluminescence of GaN films grown by molecular-beam epitaxy on 6H-SiC(0001) substrates was studied. Owing to the high structural perfection of the films, a series of narrow emission lines related to donor–acceptor recombination was detected in the blue spectral region. This finding lends support to the donor–acceptor origin of the blue emission in GaN. In the spectra of less perfect films, these lines overlap and merge into a broad band.

The molecular beam epitaxial growth of Si on SiC(0001), exhibiting a Stranski–Krastanov mode, was investigated by reflection high-energy electron diffraction. Several surface superstructures were observed in the initial stage of growth. After exceeding a critical coverage, Si island formation sets in. Under near-equilibrium conditions, the critical coverage was 1.4 monolayers and corresponds to the occurrence of a 3×3 superstructure remaining as a wetting layer after the island formation. Island formation at high deposition rates (R) and low temperatures (T) is kinetically delayed, which can be described as function of R and the diffusivity D by a relationship t c ∝ R/D . Si islands, which were relatively uniform size of several nm with a density of 1011 cm−2, were obtained under these conditions. At lower R values the critical thickness is only a function of T, indicating that the incorporation time of adatoms is the relevant time scale for surface diffusion. Ordered arrays of small dots were also grown on vicinal surfaces at higher T and lower R values, which can be attributed to a lower diffusivity at step edges, acting as perfect sinks for the Si adatoms. Furthermore, two different kinds of islands were found with a (111)/(0001) and (110)/(0001) epitaxial relationship.

Surface topographic (STM) and spectroscopic (STS) studies have been performed on the Si−terminated 6H-SiC(0001)(3 × 3) surface using a scanning tunneling microscope (STM) in ultrahigh vacuum. High-quality (3 × 3) overstructures have been prepared as observed by LEED and STM. The regular (3 × 3) surface sites revealed much weaker I(V) dependences as compared to the defect sites when measured using the constant tip-surface gap technique. The normalized (dI/dV)/(I/V) vs. V spectra exhibit distinct bands of empty and filled states, which are separated by 1.2 eV for both surface sites, respectively. The results thereby support a Mott-Hubbard-type model as used for the calculation of the density of states. However, the STS spectra become completely featureless in the range of small tip-surface distances and reveal a "metallic"-like Ohmic I-V dependence.

The phase composition and luminescent properties of GaN films grown by molecular beam epitaxy on (0001) sapphire and 6H-SiC substrates were studied. The films grown on SiC were found to consist only of the hexagonal phase and contain a lower concentration of impurities. Grains of cubic GaN, as well as donor and acceptor impurities, were found in the GaN film grown on sapphire. The formation of impurity centers is caused by the diffusion of oxygen and aluminum from the sapphire substrate during crystal growth.

The influence of heavily-doped substrates on the homoepitaxial growth kinetics of Si on Si(111) is studied in real time by means of reflection high-energy electron diffraction as function of temperature and growth rate. The growth experiments were performed by means of molecular beam epitaxy. The obtained results were compared with the growth kinetics on undoped substrates within the framework of nucleation theory. It was found that in comparison with undoped Si(111), the nucleus density in the case of Si(111):P is significantly increased at the same growth conditions that can be attributed to a longer lifetime of the smallest stable islands consisting of a Si pair on the Si(111):P surface. It can be concluded, therefore, that phosphorus is always present on the growing surface during high-temperature preparation of heavily P-doped Si(111). There, the phosphorus atoms saturate dangling bonds at the island edges. For further island growth, phosphorus atoms have to be displayed before silicon adatoms can be incorporated into the island edges, demanding an additional energy. In this sense, phosphorus acts as a surfactant.

Reflectance measurements under normal incidence of light and variable angle spectroscopic ellipsometry (SE) studies are applied for characterizing hexagonal GaN films grown by molecular beam epitaxy on both GaAs (111)B and 6H-SiC substrates. A comparison of the reflectance above the band gap with model calculations for a smooth film yields definitely the root mean square surface roughness. By analyzing the envelopes of the reflectance spectra, the influence of a buffer layer and/or a non-abrupt substrate/film interface is verified and qualitative conclusions about the interface properties are deduced which are emphasized by transmission electron microscopy studies. On this basis a suitable model for data fitting is established which contains only a small number of parameters to be adjusted. From the fit, the complex refractive index versus photon energy as well as the thicknesses of the active layer and the interlayers are obtained. A meticulous analysis of the SE data below the band gap shows that from those studies the extraordinary refractive index of hexagonal GaN can be determined for the first time.

The results of using molecular-beam epitaxy for growing piezoelectric AlN films on Si (111) substrates suitable for device applications are reported. The technological conditions for growth of stoichiometric AlN by controlling the surface reconstruction occurring under various thermodynamic conditions on the growth surface are determined. The films of the hexagonal polytype of AlN possess high crystalline perfection and an atomically smooth epitaxial surface. The mechanism for relaxation of the AlN crystal lattice over a distance of one monolayer from the heterojunction is found. It is demonstrated that the AlN film is piezoelectric. Investigations of the temporal characteristics of a SAW attest to a low level of scattering of the wave during propagation. The electromechanical coupling constant is measured in interdigital transducer geometry (λ=16 mm) and is found to be 0.07 % at a frequency f=286 MHz, in good agreement with the theoretical value for a 1.04-µm-thick AlN film.

The structure evolution characteristics of NdF 3 thin films has been analysed concerning texture and grain morphology. The structure could be classified according to the structure zone models, and the growth mode could be related to zone T and zone II. Contamination of the films by residual gases was found to inhibit the migration of grain boundaries and therefore, zone II does not appear even at high substrate temperatures in contaminated films. Stratification by MgF 2 interlayers may also result in a decrease of grain size and a smaller surface roughness. The correlation between deposition conditions, stratification, growth mode and optical properties is discussed.

A highly sensitive method for characterizing heteroepitaxial group III nitride films on various substrates is developed on the basis of reflectivity studies below and just above the fundamental absorption edge. By analyzing the envelopes of the reflectivity spectrum, the influence of a buffer layer, and/or a nonabrupt substrate/film interface is verified, the refractive index of the active layer can be calculated independently of both the active layer thickness and the interface properties, and finally, qualitative conclusions about the interface properties are deduced. On this basis a suitable model for data fitting is established which contains only a small number of adjusted parameters. Moreover, the correlation between the refractive index and the thickness of the active layer is removed. From the fit, the energy dispersion of the refractive index and the thickness for both the active layer and the interlayers are obtained. The ability and high accuracy of the method is demonstrated by applying it to ...

Epitaxial growth of SiC on α-SiC(0001) has been performed by means of solid-source molecular beam epitaxy (MBE). Low temperature (T 1200°C, with a step decrease of supersaturation, a step-flow growth mode and for the first time nucleation of both 4H- and 6H-SiC under C-rich conditions was obtained. Based on these results we have demonstrated the growth of a double-heterostructure by firstly growing a 3C-SiC film on 4H-SiC(0001) at low temperature and a subsequent growth of 4H-SiC at low supersaturation on a C-stabilized surface on top of this film. Moreover, we also propose a new model to explain quantitatively the occurrence of different growth features and polytypes under certain growth conditions.

3C–SiC thin films have been investigated by means of high resolution X-ray diffraction techniques ( ω /2Θ-scan, ω- scan) and X-ray topography. The SiC films were grown in a solid source molecular beam epitaxy (MBE) system on 6H–SiC substrate crystals. We investigate the lattice mismatch parallel and perpendicular to the [0001]-direction using the so-called reciprocal space mapping technique. When 3C–SiC films were grown on 6H–SiC substrates, double position boundaries (stacking sequence ACB instead of ABC) were frequently observed. For the visualization of this effect high resolution X-ray topographic measurements are investigated at 3C–SiC layers on 6H–SiC substrates. We find, that the domains of different stacking sequences are equally distributed in the 3C–SiC epilayer. This domains attain a size up to approximately 200×200 μm 2 .

Epitaxial wurtzite aluminum nitride (AlN) and gallium nitride (GaN) films have been grown on Si(1 1 1) by plasma-assisted molecular beam epitaxy (PA-MBE). Two-dimensional growth (2DG) of single-crystalline AlN films is achieved on Si(1 1 1) near stoichiometric supply of aluminum and atomic nitrogen. Two distinct categories of AlN-surface reconstructions have been observed. GaN films exhibit 1×1 high-energy electron diffraction (RHEED) pattern if grown between 650°C and 770°C substrate temperature on AlN buffers. Stable Ga- and N-adlayer-induced surface reconstructions have been studied below 600°C after the growth. The X-ray diffraction (XRD) pattern show sharp and well separated (0 0 0 l ) reflections of wurtzite GaN and AlN indicating complete texture with GaN[0 0 0 1]‖AlN[0 0 01]‖Si[1 1 1]. From the determined GaN lattice constant complete strain relaxation can be concluded which is further confirmed by the temperature-dependent photoluminescence (PL) investigations.

Epitaxial aluminum nitride AlN(0001) thin films have been grown by plasma-assisted molecular beam epitaxy (PA-MBE) on Si(111). The influence of the composition of the nitrogen plasma on the crystal quality, as judged by X-ray diffractometry (XRD) and atomic force microscopy (AFM), has been investigated. Under an Al/N vapor phase ratio close to unity atomically smooth AIN films have been grown at 850°C substrate temperature with maximum growth rates of 2.5 nm min - 1 . A √3 × √3 and a more Al-rich 2 × 6-surface reconstruction have been observed. Transmission electron microscopy (TEM) investigations show that these films are homogeneous 2H-AIN single crystals. Their defect structure consists of threading dislocations mostly. The hetero-interface is abrupt and flat. Processed high-resolution (HR) TEM images demonstrate a 4×d Si(T10) to 5 × d AlN(2110) coincidence between substrate and epilayer. The XRD FWHM of the (0002)-diffraction peak of 0.5 μm AIN is 0.06° in the ω/2 scan and 0.32° in the ω scan. Phonon modes of AIN have been detected by Raman and infra-red spectroscopy.

The effects of substrate cleaning, nitridation time, and substrate temperature in the range 800–1000 °C on the microstructure of AlN/Si(111) films grown by simultaneous plasma-assisted molecular beam epitaxy have been investigated. It has been demonstrated, using a combination of conventional and high-resolution transmission electron microscopy, that the interface structure, the film defect structure, and the film surface roughness are strongly related. The formation of single crystal 2H–AlN films with atomically flat surfaces occurs at 800 °C for conditions of 2.5 nm/min growth rate on very pure, atomically flat Si substrates.

The conditions for the growth of single-crystalline wurtzite aluminum nitride (AlN) films on Si(1 1 1) have been established by plasma-assisted molecular beam epitaxy. The yield of atomic nitrogen from the RF-source has been investigated by optical and by mass-spectrometry. AlN films grown under RF-source conditions with the highest yield of atomic nitrogen, as determined by mass-spectrometry, generally show narrowest AlN(0 0 0 2) rocking curve linewidths in X-ray diffraction. By supplying nitrogen and aluminum close to unity, AlN has been grown two-dimensionally with atomically smooth surfaces. Surface reconstructions have been investigated by reflection of high-energy electron diffraction as a function of the substrate temperature and the Al-overpressure. High-resolution transmission electron micrographs taken at the heterointerface show a coincidence between both materials with a ratio of 5·dAlN(2110) to 4·dSi(110). The misfit is accomodated within the very first monolayers on either side.

Epitaxial growth of SiC on hexagonal (or α)-SiC(0001) has been performed by means of solid-source molecular beam epitaxy (MBE). The solid-source MBE growth conditions have been analyzed concerning the supersaturation and the excess phase formation of silicon and carbon. In general, our results demonstrate that control of the Si/C ratio and the supersaturation (S) is essential for the growth mode and the kind of polytype grown. Low temperature (T 1450K under near surface equilibrium conditions, different growth modes, and conditions stabilizing the growth of certain polytypes have been found. With a step decrease of S, a step-flow growth mode of both 4H-and 6H-SiC was obtained and, for the first time in case of epitaxial SiC growth, a homogeneous nucleation of α-SiC at more C-rich conditions has been realized. Conditions stabilizing the growth of certain polytypes have been estimated by thermodynamic calculations considering the influence of polytype structure on the supersaturation and the surface energy. Based on these results, we have demonstrated the growth of a double-heterostructure by firstly growing a 3C-SiC layer on 4H-SiC(0001) at low temperature and a subsequent growth of 4H-SiC under near surface equilibrium conditions on a C-stabilized surface on top of this layer.

Tilt induced changes in HRTEM images obtained near [1 1 0] zone of cubic centrosymmetric Si and noncentrosymmetric SiC were studied by the use of multislice image simulations. The contrast of Si images was shown to change symmetrically with the tilt with low brightness variations while SiC demonstrates strong anisotropy along [0 0 1] direction and strong variations of contrast. The comparison of the brightness of crystals situated at twin orientation reveals that the above anisotropy leads to an anomalously high bright/dark (B/D) contrast of SiC twins appearing at extremely low misorientations from the exact [1 1 0] zone. It was also shown that the contrast of 6H–SiC can be treated to some extent as a sequence of independent twinned cubic modules. Limits of this representation were studied and the role of forbidden reflections in the image formation was clarified. A method was demonstrated to determine the polarity of the SiC crystal utilising the anisotropic tilt dependence of contrast.

Molecular beam epitaxial (MBE) experiments demonstrate that it is possible to achieve a controlled layer-by-layer growth of SiC polytypes at temperatures below 1000°C under surface-stabilized conditions. It is of substantial importance to stabilize characteristic Si-rich superstructures by an alternate supply of Si and C, switching periodically between more Si-rich and less Si-rich surface superstructures. The MBE growth of SiC happens in two basic steps: Firstly, Si overlayers are grown on SiC. Secondly, the Si overlayers are carbonized under supply of C, leading to growth of SiC. On the basis of first principles calculations within the framework of density functional theory in the local density approximation, we derive an equilibrium phase diagram for various structural models which provide a theoretical framework to rationalize some of the experimental findings qualitatively.

Due to the large misfit (20%), Si grows in Stranski–Krastanov mode on SiC(0001) leading to the formation of Si islands with quantum-size (quantum dot) dimensions in the range of 2–6 nm in height and 10–25 nm in diameter after the transition from two-dimensional (2D) to 3D growth mode at coverages >1.25 monolayers (ML). The growth mode transition and the post growth evolution of the system towards equilibrium surface conditions by dot formation was studied in real time at temperatures between 475 and 825°C and coverages in the range of 2 ML by recording the RHEED intensities of both the specular beam and the (222)-Si-bulk spot. From that we analysed the temperature-dependent dynamic processes leading to dot formation. Above 625°C, the formation of Si dots takes place within a sub-ML range above 1.25 ML. At these temperatures, the obtained activation energy of 0.6 eV for the dot growth indicates that surface diffusion is the dominant process. Below 625°C, the occurrence of RHEED specular beam intensity oscillations indicates a 2D growth mode also at higher coverages. However, after the deposition was finished, the specular beam intensity decreased, and simultaneously, a 3D Si diffraction pattern occurred due to the formation of Si islands. The activation energy for the island growth process at these lower temperatures was determined to be much higher (2.1 eV), indicating a more complex process due to the mass transfer out of the 2D layer towards the Si islands.

An atomically flat Si(111)/Ag(3×3)R30° surface has been modified using a scanning tunneling microscope in ultrahigh vacuum. Mesoscopic pits have been created by applying negative voltage pulses to the sample, while at opposite voltage polarity mounds were formed. Moreover, lines could be written by moving the scanner at elevated voltages. The threshold voltage for pit formation increases almost linearly with the distance of the tip to the surface and drops to a value below 2 V for the closest approach. At sufficiently high voltages the depth extends beyond the silver layer height. The lateral pit size is well below 8 nm and can be reduced to values between 2 nm and 5 nm for voltages slightly above the threshold. Even selective top layer Ag atom removal has been achieved.

Second-harmonic (SH) generation is a versatile method applicable to in-situ characterization of even non-centrosymmetric media like silicon carbide (SiC). In particular, the azimuthal rotational anisotropy of the SH response from SiC observed in reflection allows identification of various polytypes. The nonlinear-optical results are compared to X-ray diffraction data. The abundance of information obtained through the SH studies makes characteristic fingerprinting of the 6H, 15R, and 3C polytypes of SiC is possible. The spatial resolution of the optical sampling was about 50 μm in the lateral direction with a typical penetration depth of 100 nm for the fundamental radiation. Defect regions of different crystallographic structures in large SiC samples were identified by observing the spatially resolved dependence of the SH intensity.

Reflection high-energy electron diffraction has been used to study the molecular beam epitaxy-growth kinetics of Si on Si(111) and of SiC on SiC(100). By means of a simple kinematic scattering theory we have estimated diffusion barrier energy values for adatom migration and nucleation energies. Results clearly show that growth kinetics are different at low and high temperatures for both systems. At lower temperatures the nucleation can be explained by a single adatom diffusion process, whereas at higher temperatures much higher activation energies may be attributed to an island coarsening process.

The effect of in-chain and off-chain substitutions on one-dimensional (1D) spin fluctuations in the spin-Peierls compound ${\mathrm{CuGeO}}_{3}$ has been studied using Raman scattering in order to understand the interplay between defect induced states, enhanced spin-spin correlations, and the ground state of low-dimensional systems. In-chain and off-chain substitutions quench the spin-Peierls state and induce 3D antiferromagnetic order at T\ensuremath{\leqslant}5 K. Consequently the suppression of a 1D gap-induced mode as well as a constant intensity of a spinon continuum are observed at low temperatures. A 3D two-magnon density of states now gradually extends to higher temperatures T\ensuremath{\leqslant}60 K compared with pure ${\mathrm{CuGeO}}_{3}$. This effect is more pronounced in the case of off-chain substitutions (Si) for which a N\'eel state occurs over a larger substitution range, starting at very low concentrations. Besides, additional low energy excitations are induced. These effects, i.e., the shift of a dimensional crossover to higher temperatures are due to an enhancement of the spin-spin correlations induced by a small amount of substitutions. The results are compared with recent Monte Carlo studies on substituted spin ladders, pointing to a similar instability of coupled, dimerized spin chains and spin ladders upon substitution.

The effect of Si doping on the magnetic properties of the spin-Peierls SP system CuGeO3 was found to differ strongly between polycrystals PC’s and single crystals SC’s. In SC’s, the SP state is suppressed much more strongly, whereas the existence region of the antiferromagnetic AF state is enhanced. We investigated the origin of this difference by means of magnetic susceptibility, specific heat, thermal expansion, Raman scattering, elastic neutron scattering, and x-ray measurements on CuGe1xSixO3 samples prepared under different conditions. The partial oxygen pressure and the temperature during the synthesis were found to have a profound influence on the magnetic properties: preparation under reduced oxygen pressure leads to a stabilization of the AF state, whereas heating above the melting point results in a strong decrease of TSP in Si-doped samples. Therefore, both the AF stabilization and the TSP reduction observed in SC’s are not an intrinsic effect of Si doping PC samples, which can be prepared at lower temperatures and more oxidizing conditions, reflect much better the intrinsic properties of CuGe1xSixO3. We were able to prepare PC samples up to 50 at. % Si and found a continuous decrease of the one-dimensional character of the magnetic properties without pronounced changes in the structure. S0163-18299705321-6

We present a comparison of the effect of different dopants on both spin-Peierls and the Neel transition in CuGeO3. The composition dependence of the Neel-transition temperature TN can be scaled to a universal dependence for both in- and off-chain dopants taking into consideration the changes of the ionic radii. This suggests local distortions to play an important role for the effect of the doping. All in-chain dopants lead to a similar linear decrease of TSP; the slope differing only slightly within a factor of 1.5 for different dopands. The much weaker effect on TSP of the two off chain dopants Si and Ti scales to a common behaviour if the chemical pressure is taken into account. Furthermore, we found for in-chain dopants a clear correlation between the transition temperature TSP and the spin-Peierls gap.

Thin crystalline SiC films were grown on Si(111) using solid state evaporation at substrate temperatures between 780 and 900 °C. The growth rates were in the range between 30 and 120 nm h−1. The films were characterized by in situ reflection high-energy electron diffraction (RHEED) and ex situ transmission electron microscopy (TEM), scanning electron microscopy (SEM), infrared (IR) spectroscopy and X-ray diffraction (XRD). The films grown at high temperatures and low growth rates were found to be epitaxial. They mostly consist of twinned-cubic structure, but with increasing layer thickness hexagonal stacking sequences often were found. In the orientation distribution function full width at half maximum (FWHM) values of down to 1° were measured.

Using Raman scattering we studied the effect of substitutions on 1D spin fluctuations in CuGeO_3 observed as a spinon continuum in frustration induced exchange scattering. For temperatures below the spin-Peierls transition (T_{SP}=14K) the intensity of this continuum at 120-500 cm^{-1} is exponentially suppressed and transferred into a 3D two-magnon density of states. Besides a spin-Peierls gap-induced mode at 30 cm^{-1} and additional modes at 105 and 370 cm^{-1} are observed. Substitution of Zn on the Cu-site and Si on the Ge-site of CuGeO_3 quenches easily the spin-Peierls state. Consequently a suppression of the spin-Peierls gap observable below T_{SP}=14K as well as a change of the temperature dependence of the spinon continuum are observed. These effects are discussed in the context of a dimensional crossover of this compound below T_{SP} and strong spin-lattice interaction., 9 pages, 2 eps figures included

We investigated the effect of doping on the crystallographic, dynamic and magnetic properties of (Cu1−zZnz)GeO3 as well as Cu(Ge1−xAx)O3 with A  Al, Si, Ga, As, In, Sn, Sb and Ti. A large solubility was only observed for Zn, Si and Ti. For those elements, the lattice parameters show a linear dependence on doping within the whole solubility range. The spin-Peierls transition temperature TSP decreases linearly with doping, though the slope is strongly dependent on the dopant. For Zn-doping, a Neel-like ground state which exhibits spin-flop behavior appears for higher doping after the complete disappearance of the spin-Peierls ground state. Si- and Ti-doped samples exhibit a similar phase in a coexistence with the spin-Peierls ground state. The analysis of the susceptibility in the spin-Peierls phase for low Zn content indicates a reduction of the dimerization within the Cu chains and the energy gap upon doping. Whereas for Si and Ti all phonon frequencies changed linearly with the doping level, we observed in Zn-doped samples for some phonons a clear minimum at the composition z = 0.02 where the spin-Peierls transition disappears, suggesting a strong spin-phonon coupling for these modes. For higher Zn concentrations, no lattice dimerization can be observed by Raman scattering.

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.

Spectroscopic ellipsometry is used to study GaN films during growth by metalorganic vapor phase epitaxy (MOVPE) in correlation to well known results of plasma-assisted molecular beam epitaxy (PAMBE). Results for the PAMBE reveal clear differences between growth under Ga-rich and N-rich conditions, which are attributed to the presence of a Ga bilayer on the surface (also seen with low energy electron diffraction) in the Ga-rich case. Results for MOVPE surfaces during growth or for surfaces which are stabilized under NH3 are very similar to the N-rich PAMBE result. It is concluded that under normal growth conditions in MOVPE in contrast to PAMBE the surface is not terminated by a Ga bilayer. © 2003 American Institute of Physics.

The controlled growth of SiC heteropolytypic structures consisting of hexagonal (α-) and cubic (3C-) 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 some nanometers thick 3C-SiC layer at lower temperatures (1550 K) and Si-rich conditions, and subsequent growth of α-SiC on top of the 3C-SiC layer at higher temperatures (1600 K) under more C-rich conditions. On off-axis substrates, multi-heterostructures consisting of 4H/3C- or 6H/3C-stacking sequences were also obtained by first nucleating selectively wire-like 3C-SiC nuclei 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 condition. After the growth many wire-like regions consisting of 3C-SiC were found within the hexagonal SiC layer material matrix indicating a simultaneous step-flow growth of both the cubic and the hexagonal SiC material.

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.