Search

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.

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.

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.

SiC thin films on Si(111) were grown using solid state evaporation at relatively low temperatures. Growth rates of 3–10 nm min −1 have been achieved at 750–900 °C. Results obtained from IR spectroscopy, Auger electron spectroscopy and X-ray diffraction indicate good crystalline growth at T > 800 °C, in the case of a stoichiometric composition. The crystallinity was found to be deteriorated in layers with excess Si or C. In addition, shifts in the IR absorption peak indicate that the non-stoichiometric layers were highly stressed. Annealing of non-stoichiometric layers shifts the peak to the position obtained for stoichiometric layers.

We have investigated the superconducting ${(T}_{c}{,J}_{c}{,H}_{c2}),$ electron transport, structural [x-ray diffraction (XRD), Rutherford backscattering spectrometry (RBS)], and surface [x-ray photoelectron spectroscopy (XPS), ultraviolet photoelectron spectroscopy (UPS)] properties of ${\mathrm{BaNbO}}_{3\ensuremath{-}x}$ and ${\mathrm{Ba}}_{2}{\mathrm{Nb}}_{5}{\mathrm{O}}_{x}$ epitaxial and oriented thin films grown on different substrates $({\mathrm{Al}}_{2}{\mathrm{O}}_{3},$ ${\mathrm{NdGaO}}_{3},$ ${\mathrm{SrTiO}}_{3},$ and YSZ). Superconducting films with ${T}_{c}=14 \mathrm{K}$ and unusually high ${H}_{c2}(0)=28 \mathrm{T}$ have been prepared on ${\mathrm{Al}}_{2}{\mathrm{O}}_{3}$ substrates. At the same time, films on ${\mathrm{NdGaO}}_{3}$ exhibit behavior similar to granular superconductors, while strong pure metal electron transport behavior of the films on ${\mathrm{SrTiO}}_{3}$ was observed. XPS and RBS investigations show that the composition of these films is almost the same, while UPS study evidenced the Fermi edge position corresponding to semimetallic behavior of the films for all substrates.

High-resolution transmission electron microscopy (HRTEM) images of the [1–10] zone of cubic SiC layers grown by molecular beam epitaxy (MBE) often reveal regions of material exhibiting an unusual threefold periodicity. The same contrast was found in earlier works of Jepps and Page, who attributed this contrast in HRTEM images of polycrystalline SiC to the 9R-SiC polytype. In this report we demonstrate by HRTEM image simulations that the model of the 9R polytype and an alternative twinning model can fit qualitatively the experimental HRTEM images. However, by comparing the fast Fourier transform (FFT) patterns of the experiments and the simulations, as well as by using dark-field imaging, we show unambiguously that only the model of overlapping twinned 3C-SiC crystals fully agrees with the experiments.

Epitaxial wurtzite aluminum nitride (AIN) films have been grown on (111) oriented silicon substrates by plasma-assisted solid source molecular beam epitaxy (MBE). The growth has been performed in the two-dimensional (2D) growth mode. Occuring surface reconstructions have been monitored by reflection of high-energy electron diffraction (RHEED). The films have been characterized by atomic force microscopy (AFM) and transmission electron microscopy (TEM). 2D grown films exhibit atomically smooth surfaces, are singlecrystalline and void of oriented domains. High-resolution (HR) TEM micrographs of the heterointerface show the presence of an interfacial atomic arrangement where (5:4) unit cells coincide along the AIN[2110]/Si[110] direction.Surface acoustic wave (SAW) properties of MBE-grown AIN(0001)/Si(ll1) have been studied for the first time. The AIN film exhibits very good piezoelectricity. The SAW time response indicates low scattering of the waves during their propagation. An electromechanical coupling coefficient (EMC) of 0.07% has been measured for the present interdigital transducer (IDT) geometry.

The structure evolution characteristics of single and stratified NdF3 optical thin films on amorphous quartz substrates have been investigated by cross sectional transmission electron microscopy, x-ray diffractometer measurements and atomic force microscopy. The films were deposited by physical vapor deposition under ultra high vacuum conditions. The morphology changes with substrate temperature from V shaped columnar (Zone T) to a columnar morphology corresponding to zone II. The zone boundary falls in the range above 300 °C. The surface roughness of the single films is lowest at 300 °C substrate temperature. The main texture components are

The structure evolution characteristics of MgF 2 and NdF 3 optical thin films on CaF 2 (111) substrates have been investigated by cross sectional transmission electron microscopy and atomic force microscopy and were related to their extinction coefficient. The films were deposited by physical vapor deposition under ultra high vacuum conditions. A series of experiments have been carried out at various substrate temperatures at 300 nm thickness and by the stratification of the two fluoride materials. The structure evolution of single and stratified films was investigated and discussed in relation to the structure zone models. The effect of stratification on the structure and extinction coefficients is discussed in comparison to the effect of segregating impurities. The development of morphology of stratified films is analyzed in relation to the thickness of the interlayers and temperature. The optimum conditions for preparing films of low extinction coefficients are found at parameters corresponding both to the transition between zone T and zone II growth and to the formation of nanocrystalline structures.

Raman experiments on the spin-Peierls compound CuGeO$_3$ and the substituted (Cu$_{1- x}$,Zn$_x$)GeO$_3$ and Cu(Ge$_{1-x}$,Ga$_x$)O$_3$ compounds were performed in order to investigate the response of specific magnetic excitations of the one-dimensional spin-1/2 chain to spin anisotropies and substitution-induced disorder. In pure CuGeO$_3$, in addition to normal phonon scattering which is not affected at all by the spin-Peierls transition, four types of magnetic scattering features were observed. Below T$_{SP}$=14 K a singlet-triplet excitation at 30 cm$^{-1}$, two-magnon scattering from 30 to 227 cm$^{-1}$ and folded phonon modes at 369 and 819 cm$^{-1}$ were identified. They were assigned by their temperature dependence and lineshape. For temperatures between the spin-Peierls transition T$_{SP}$ and approximately 100 K a broad intensity maximum centered at 300 cm$^{-1}$ is observed., 7 pages, LaTex2e, including 3 figures (eps) to be published in Physica B (1996)

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.

Cardiovascular risk factors depend decisively on living conditions and nutrition. The aim of the present study was to establish whether the different living conditions in the eastern and western parts of Germany were associated with differences in the frequency and severity of these risk factors.In Leipzig, 15,291 people (9,600 women and 5,691 men), in Nuremberg 8,387 (4,559 women and 3,828 men) were examined either after responding to a public appeal, or in the factory, etc.Clearly elevated cholesterol levels (250 mg/dl) were found in 33.8% of the men, and 35.9% of the women in Leipzig, and in 39.1% of the men and 50.7% of the women in Nuremberg. This larger incidence of treatment-requiring hypercholesterolemia in Nuremberg was distributed throughout all age groups up to the age of 60. Hypertension was significantly more common among the 30 to 80-year-old women, and the 20 to 40-year-old and 50 to 80-year-old men in Leipzig. the frequency of cigarette smoking, diabetes mellitus and overweight did not differ significantly between the two cities. In cases with known dyslipoproteinemia, drug treatment was rarely applied, and the therapeutic aim of a cholesterol level200 mg/dl, was achieved in only 2-6% of those treated. The most significant difference with respect to cardiovascular risk factors was serum cholesterol. It is possible that a higher ingestion of monounsaturated fatty acids and more physical activity may be responsible for the lower cholesterol levels in Leipzig.

BASICS: Drug treatment of the risk factor hypertension has not led to the expected reduction in morbidity and mortality of coronary arterial disease. A negative effect of antihypertensive drugs on the lipid metabolism is under discussion as a possible cause.Both the commonly employed thiazide diuretics and chlorthalidone and the loop diuretics furosemide, piretanide and xipamide lead, to varying extents, to an increase in total and LDL cholesterol as well as triglycerides in subjects with normal metabolism. In addition, some diuretics lower the levels of protective HDL cholesterol. These side effects can be avoided--at least in part--by a combination with such drugs as prazosin, pindolol or captopril, which have a favorable effect on metabolism.It is recommended to carefully monitor serum Lipoproteins during diuretic treatment so that possible adverse changes may be counteracted by appropriate measures.

Review of superconducting ( T c , H c 2 (T), J c ), electron transport Rs(T), surface (XPS, UPS) and structural (XRD, RBS) properties of thin films of a novel superconductor BaNbO3−x on different substrates is presented. Superconducting films have been obtained when grown on the Al2O3 with T c =14 K , J c ≈1×10 4 A cm −2 and large H c 2 (0)=28 T . At the same time, films on NdGaO3 exhibit behavior typical to granular superconductors.

Lipoprotein (a) is believed to be an independent risk factor for atherosclerosis. Fat modified diets or lipid lowering drugs seem to have little effect on Lp(a) serum levels. We tested the possibility of lowering Lp(a) by weight reduction and examined the correlation between Lp(a) serum levels, other lipoproteins, body weight and body fat distribution in 53 obese patients. Weight reduction by a 4000 kJ protein enriched diet for four weeks led to a mean 19 percent reduction of Lp(a) serum levels in men (P less than 0.01) and a mean 30 percent reduction in premenopausal women (P less than 0.001). Significant correlations could neither be demonstrated between Lp(a) and other lipoproteins nor between initial Lp(a), overweight and body fat distribution. This implies that an intraindividual interdependence between Lp(a) and body weight seems to be possible although an interindividual correlation can not be shown.

We investigated the effect of doping CuGeO3 with Zn, Ni, Mn and Mg on the Cu-site, and Al, Si, Ga, As, In, Sn, Sb and Ti on the Ge-site. The lattice parameters show a linear dependence on doping. The spin-Peierls (SP) transition temperature TSP decreases linearly with increasing substitution, the slope depending on the dopant. For increasing Zn-content, a Neel-like groundstate which exhibits spin-flop-behaviour appears after the disappearence of the SP-ground-state. Si-and Ti-doped samples exhibit a similar phase in co-existence with the SP-groundstate.

The substitution of Zn on the Cu-site and Si on the Ge-site of CuGeO3 quenches the spin-Peierls state. All substitutions lead to a suppression of the spin-Peierls gap seen below T SP =14K at 30 cm−1 and to a change of the temperature dependence of the spinon continuum at 200–350 cm−1 attributed to a frustration induced Heisenberg exchange scattering. A substitution of Zn on the Cu-site has the strongest effect. The change of the continuum results from a local depression of the spin-Peierls gap near the impurity.

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 optical properties of thin epitaxial AlN films have been determined in the 5‐10 eV energy range using a spectroscopic ellipsometer operating with synchrotron radiation. Measurements at low temperatures ~down to 120 K! were performed to enhance spectral features and to determine the temperature dependence of interband critical points. Our data are in qualitative agreement with recent reflectance experiments but show larger amplitudes and additional structures in the vicinity of the fundamental absorption edge. The possible origin of these structures is discussed on the basis of calculations of the electronic energy bands. @S0163-1829~99!01803-2#

We applied real-time spectroscopic ellipsometric (SE) measurements to assess the removal of overlayer material from 4H-SiC Si- and C-face surfaces in order to investigate the final step of an other ...

By conventional transmission electron microscopy (CTEM) investigations on 2H-AlN films grown by plasma-assisted molecular beam epitaxy (MBE) on Si(001) the influence of the offaxis angle of the substrate surface on the film structure was studied. Three types of Si(001) substrates were used: on-axis, ~1°, and ~5° off-axis Si(001) substrates. The AlN layer on an exact oriented Si(001) substrates consists of 3 AlN film domains: two main film domains, AlN1 and AlN2, and a small domain AlN3 at substrate surface defects. Their c-axis orientations are parallel to the caxis of the substrate: [0001]AlN1,2,3||[001]Si. The a-axes of AlN1 and AlN2 rotated by 30° to each other: [11 2 0]AlN1||[01 1 0]AlN2||[1 1 0]Si [3]. The a-axis orientation of AlN3 is [01 1 0]AlN3||[100]Si. In 2H-AlN films grown on off-axis Si(001) substrates (~1° and ~5°) the ratio between the AlN1 and AlN2 film domains changes dramatically as far as a single domain film structure consisting of only AlN1 is reached. The AlN c-axes of all domains on the off-axis substrates are not parallel to the Si c-axis but tilted by the off-axis angle of the Si(001) substrate (~1° respectively ~5°), i.e. [0001]AlN is parallel to the Si(001) substrate surface orientation.

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.)

The connection of body fat distribution (BFD) and the risk of developing mammary, cervical, endometrial or ovarian carcinoma was ascertained for 163 patients with carcinoma (mean age 49.9 [19-78] years) and 489 controls of comparable age and body-mass index. BFD was expressed as the ratio of waist and hip circumference (T/H ratio of 0.822 vs 0.781 and 0.826 vs 0.789, respectively; P less than 0.01). In premenopausal women with mammary or cervical carcinoma and in all postmenopausal women BFD was similar to that in the control subjects. A common cause of android obesity and ovarian or endometrial carcinoma may be a reduction of sex-hormone-binding globulins with an elevated serum level of free androgens and oestrogens.

It is speculated that endogenous opioid peptides are involved in glucose metabolism and that their homeostasis might be disturbed in obesity. Despite a different response of the pancreatic beta-cells after beta-endorphin and naloxone injections between obese patients and normal weight controls, there is little knowledge concerning the direct influence of a glucose load on beta-endorphin plasma levels, especially with respect to various nutrition states. During exploration of this topic we gained further insight on the difference of basal beta-endorphin plasma levels between normal and overweight persons. We compared beta-endorphin plasma levels during an oral glucose load in 60 obese, non-diabetic patients and in 20 normal weight controls. We also studied 40 of the obese patients after a weight reduction of 2.1 kg/m2. The following results were obtained: (1) Normal weight females have significantly lower (P less than 0.05) basal beta-endorphin levels compared to the male controls. This difference in gender is abolished in obesity where female and male patients do not differ in basal beta-endorphin plasma levels. Therefore, the difference between normal and overweight persons in beta-endorphin plasma levels was restricted to the subgroup of females. We suppose that former neglect of this difference in gender explains most of the so far reported discrepant results. (2) During the oral glucose tolerance test the beta-endorphin plasma values remained constant in the obese group. Despite improved insulin sensitivity after weight reduction there was still no change of beta-endorphin plasma levels both during the OGTT and when compared to the values before weight reduction.(ABSTRACT TRUNCATED AT 250 WORDS)