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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.

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.

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.

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.

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.

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 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.

Ge x Se 1− x films were produced by means of quasi-simultaneous sputtering. The refractive index and the optical gap energy were determined by transmission measurements. Both parameters showed the expected dependence on film composition with an extermal value for GeSe 2 . After thermal annealing, a decrease of refractive index and an increase of gap energy up to bulk values was observed. This effect can be related to a reduction in the concentration of dangling bonds. The form of the absorption edge was found to depend on the film composition. Oxygen impurities produce an absorption between 700 and 800 cm −1 due to GeO vibrations, whereas GeSe 2 fundamental tetrahedral vibrations cause an absorption between 260 and 340 cm −1 .

PbF2 films evaporated on silicon and on aluminium at ambient and at elevated substrate temperatures have been investigated with respect to the influence of the deposition conditions on the film microstructure as well as on the water-induced IR absorption. Surface morphology and cross-sectional inspections were performed by scanning electron microscopy and transmission electron microscopy. X-ray diffraction was used for crystallographic investigations to determine phase, orientation, and grain sizes. From the analysis of light scattered from the surface as well as from the volume of the films, information about packing density and columnar film growth could be derived. IR spectroscopic studies were performed to show the effect of water sorption in the PbF2 films.

Sputtered and evaporated optical Ge films are investigated using electron microscopy and diffraction, total integrated scattering, infrared reflection absorption spectroscopy, Auger electron spectroscopy, and optical transmission measurements to observe differences of the chemical composition, atomic structure, morphology, optical constants, and degradation in relation to the parameters of technology during preparation. Gesputterte und gedampfte optische Ge-Schichten werden mit Elektronenmikroskopie und -beugung, der Methode des total integrierten Streulichts, Infrarot-Reflexions-Absorptionsspektroskopie, Auger-Elektronen-Spektroskopie und durch optische Transmissionsmessungen untersucht, um Unterschiede in der chemischen Zusammensetzung, Atomstruktur, Morphologie, den optischen Konstanten und dem Degradationsverhalten der Schichten in Abhangigkeit von der Herstellungstechnologie festzustellen.

BaF2, PbF2, and CaF2 films are investigated by IRAS (infrared reflection absorption spectroscopy), ATR (attenuated total reflection), IR transmission measurements, and IR ellipsometry. Interesting features of their spectra particularly occur near 300, 1500, and 3000 cm−1. These absorptions are due to intrinsic modes, hydroxyl, and methylen, respectively. The frequencies of the vibrational modes are related to the film material, but also to its history. The so called longitudinal optical frequencies roughly coincide with corresponding values for the cubic modifications of the fluorides, where the F ions are tetrahedrically surrounded by four nearest neighbour metal ions. Polarization dependent measurements hint at an anisotropic incorporation of water molecules in some cases. According to the lower melting point of PbF2 its water content can be reduced by heating above 373 K more quickly than it is possible for BaF2 films. This is proved by ATR measurements. BaF2-, PbF2- und CaF2-Schichten werden mittels IRAS (Infrarotreflexionsabsorptionsspektroskopie), ATR (gedampfte Totalreflexion), IR-Transmissionsmessungen und IR-Ellipsometrie untersucht. Interessante Einzelheiten ihrer Spektren treten insbesondere in der Nahe von 300, 1500 und 3000 cm−1 auf. Diese Absorptionen werden durch Eigenmoden, Hydroxyl bzw. Metylen verursacht. Die Frequenzen der Schwingungsmoden werden mit dem Schichtmaterial sowie mit dessen Vorbehandlung verknupft. Die sogenannten longitudinalen optischen Frequenzen koinzidieren naherungsweise mit entsprechenden Werten fur die kubischen Modifikationen der Fluoride, wo die F-Ionen tetraedrisch von vier nachsten Nachbarmetallionen umgeben sind. Polarisationsabhangige Messungen deuten auf einen anisotropen Einbau von Wassermolekulen in einigen Fallen hin. Entsprechend dem niedrigeren Schmelzpunkt von PbF2 kann sein Wassergehalt durch Aufheizen oberhalb 373 K schneller reduziert werden, als es fur BaF2-Schichten moglich ist. Dies wird durch ATR-Messungen bestatigt.

The growth structure of MgF 2 and NdF 3 films grown on polished CaF 2 (111) substrates deposited by molecular beam deposition has been investigated using transmission electron microscopy (TEM) of microfractographical and surface replications as well as cross-sectional TEM, atomic force microscopy, packing density, and absorption measurements. It has been shown that by taking advantage of ultrahigh vacuum environments and a special stratification property of MgF 2 and NdF 3 films, the preparation of nanocrystalline films of high packing density and low optical absorption is possible at a substrate temperature of 425 K.

Summary and Conclusion SiC hetero-polytypic structures have been investigated by conventional and high resolution TEM and the lattice parameters of a thin cubic stripe have been determined by CBED. For the case of the stepped substrate, nearly free of defects, about 2nm thick cubic stripes are formed within the hexagonal layer. For the case of the growth on on-axis substrates, it was demonstrated that 6H/3C as well as 4H/3C stacks are formed on 6H and 4H-SiC substrates, respectively, By CBED the lattice parameters of the cubic stripe were determined to a rhom =0.4360nm a rhom = (89.78±0.02)° demonstrating the rhombohedral distortion of the 3C stripes. It might be suggested that the result is valid also for the 2nm thick cubic stripes in the multi-quantum well structures grown on the stepped SiC substrate, although such layers are generally to thin for the lattice parameter determination by CBED. Acknowledgements The authors are indebted to Dr. A. Chuvilin for the generous scientific support during his stay as guest scientist at Jena University, thank Ch. Zaubitzer for sample preparation and acknowledge the financial support of the Deutsche Forschungsgemeinschaft, Sonderforschungsbereich 196.

The energy and current sensitivity of a flat thin film SQUID is increased by two types of transformers. The transformers are galvanically coupled to the SQUID and may be used down to zero frequency. A detailed design is given for the SQUID and the superconducting transformers. A practical transformer combination is described which has a current gain of 5.7 × 103 and an input inductance of 240 mH. The measured input current period of this transformer with the SQUID is 3.5 nA. Die Energie- und Stromempfindlichkeit eines ebenen Dunnschicht-SQUIDs wird mit zwei Arten von Transformatoren erhoht. Die Transformatoren sind galvanisch mit dem SQUID gekoppelt und konnen herunter bis zur Frequenz Null verwendet werden. Der Aufbau des SQUIDs und der supraleitenden Transformatoren wird ausfuhrlich erlautert. Eine praktische Kombination von Transformatoren mit einer Stromverstarkung von 5,7 × 103 und einer Eingangsinduktivitat von 240 mH wird beschrieben. Die experimentell bestimmte Eingangsstromperiode dieses Transformators mit dem SQUID betragt 3,5 nA.

Es wird uber experimentelle Untersuchungen an einem supraleitenden Quanteninterferometer (SQUID) berichtet, das aus dunnen, supraleitenden Schichten besteht. Als Josephson-Verbindungen werden Dayem-Brucken verwendet. Die Perioden bezuglich des Steuerstromes liegen zwischen 100 μA und 1 mA. Die Stromempfindlichkeit betragt 1 μA und die Spannungsempfindlichkeit eines mit solchen SQUID aufgebauten hochempfindlichen Galvanometers 10−14 V bei einer Zeitkonstante von 1 s. Es werden Formeln zur Berechnung der Periode und der Empfindlichkeit in Abhangigkeit von der Geometrie angegeben. A report is given on experimental studies of superconducting quantum interference devices (SQUID) consisting of thin superconducting layers. As Josephson junctions Dayem-bridges are used. The width of the current cycles caused by the control current was found to have values between 10−4 and 10−3 A. With a current sensitivity of 10−6 A and a time constant of 1 s the voltage sensitivity of a high-sensitivity galvanometer constructed with such a SQUID is 10−14 V. Formulae are given for calculating the current cycles and the sensitivity in dependence on the geometry.

The polytype, stacking sequence and polarity of SiC and AlN films grown by MBE can be determined by X-ray photoelectron diffraction and electron channeling patterns. The information depth of a diffraction pattern of electrons with medium kinetic energy of one or a few keV is 10 to 20 atomic layers and the pattern is hardly influenced by surface reconstruction and contamination. (orig.)