Search

The tin-enhanced growth of [Formula: see text] on (0001) [Formula: see text] by plasma-assisted molecular beam epitaxy using an ultrathin [Formula: see text]-layer of [Formula: see text] is demonstrated. It is shown that this growth method results in a significantly reduced incorporation of residual Sn in the [Formula: see text] film compared to the case of permanent Sn-supply. The ultrathin [Formula: see text] layer, pre-deposited on the substrate, is sufficient to initiate phase pure growth of [Formula: see text]-[Formula: see text] in metal-rich growth conditions where otherwise no growth occurs. The chemical and morphological properties of the [Formula: see text]-layer are analyzed and the presence of [Formula: see text] on the surface during the entire growth process is demonstrated. Furthermore, we show that this layer is stable during Ga-induced back-etching of a [Formula: see text] film. Its impact on the kinetics of [Formula: see text] growth is also discussed.

Cubic GaN/AlN short-period multiple quantum well structures were grown at 720 oC by plasma-assisted molecular beam epitaxy on free standing 3C-SiC substrates. The samples consist of 100 nm thick GaN buffer and 20 periods of GaN/AlN active regions. The thickness of the AlN barrier is 1.35 nm for all samples, while the thickness of the GaN well varies between 1.6 nm–2.10 nm depending on the samples. The periodicity of the GaN/AlN active regions was confirmed by the presence of several peaks in the high resolution x-ray diffraction (HRXRD) spectra. The thickness of the total period was estimated by fitting the HRXRD data using a dynamic scattering theory. Room temperature measurements of the optical absorption spectra of the intersubband transitions were obtained using a Bruker IFS-125HR spectrometer. The peak position wavelengths of these transitions were observed in the spectral region of 1.5–2.0 µm and confirmed theoretically by using a self-consistent Poisson-Schrodinger model. (© 2008 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

Ternary and quaternary cubic c-AlxIn1–xN/GaN and c-AlxGayIn1–x–yN/GaN heterostructures lattice-matched to c-GaN on freestanding 3C-SiC substrates were grown by plasma-assisted molecular beam epitaxy. The c-AlxGayIn1–x–yN alloy permits the independent control of band gap and lattice parameter. The ternary and quaternary films were grown at 620 °C. Different alloy compositions were obtained by varying the Al and Ga fluxes. The alloy composition was measured by Energy Dispersive X-ray Spectroscopy (EDX) and Rutherford Backscattering Spectrometry (RBS). X-ray reciprocal space map of asymmetric (-1-13) reflex were used to measure the lattice parameters and to verify the lattice match between the alloy and the c-GaN buffer. (© 2007 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

Cubic GaN, AlxGa1-xN/GaN and InyGa1-yN/GaN multiple quantum well (MQW) layers were grown by plasma assisted molecular beam epitaxy on 200 &m thick free standing 3C-SiC substrates. The influence of the surface roughness of the 3C-SiC substrates and the influence of metal coverage during growth are discussed. Optimum growth conditions of c-III nitrides exist, when a one monolayer Ga coverage is formed at the growing surface. The improvement of the structural properties of cubic III-nitride layers and multilayers grown on 3C-SiC substrates is demonstrated by 1 μm thick c-GaN layers with a minimum x-ray rocking curve width of 16 arcmin, and by c-AlGaN/GaN and c-InGaN/GaN MQWs which showed up to five satellite peaks in X-ray diffraction, respectively.

Phase pure cubic (c-) GaN/AlGaN heterostructures on 3C-SiC free standing (001) substrates have successfully been developed. Almost complete (100%) phase pure c-GaN films are achieved with 2-nm surface roughness on 3C-SiC substrate and stoichiometric growth conditions. The polarization effect in c-GaN/AlGaN has been evaluated, based on measuring the transition energy of GaN/AlGaN quantum wells (QWs). It is demonstrated that the polarization electric fields are negligible small in c-GaN/AIGaN/3C-SiC compared with those of hexagonal (h-)GaN/AlGaN, 710kV/cm for Al content x of 0.15, and 1.4MV/cm for x of 0.25. A sheet carrier concentration of c-GaN/AlGaN heterojunction interface is estimated to 1.6 x 10 12 cm -2 , one order of magnitude smaller than that of h-GaN/AlGaN. The band diagrams of c-GaN/AlGaN HEMTs have been simulated to demonstrate the normally-off mode operation. The blocking voltage capability of GaN films was demonstrated with C-V measurement of Schottky diode test vehicle, and extrapolated higher than 600 V in c-GaN films at a doping level below 5 x 10 15 cm -3 , to show the possibility for high power electronics applications.

We report on the optical and electrical properties of cubic AlxGa1–xN/GaN heterostructures. The optical properties were investigated by photoluminescence measurements. A luminescence band at 3.250 eV was observed with our cubic AlxGa1–xN/GaN heterostructures. The emission lies between the excitonic emission of c-GaN at 3.276 eV and the donor-acceptor transition at about 3.135 eV. With increasing excitation power we find a characteristic blue shift of 4.5 meV/decade. To confirm the existence of a two-dimensional electron gas the transition energy versus externally applied voltage has been investigated. A 1.3 meV/V shift was measured and quantitatively verified by a self consistent solution of the Schrodinger and Poisson equation. The electrical properties were determined by capacity-voltage (CV)-measurements. The measured carrier profile is in excellent agreement with theoretical simulations and showed a sheet carrier concentration up to n = 1.6 × 1012 cm–2 at the AlxGa1–xN/GaN interface. (© 2006 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

Cubic GaN, AlxGa1–xN/GaN multiple quantum wells and quaternary AlxGayIn1–x–yN layers were grown by plasma assisted molecular beam epitaxy on 3C-SiC substrates. Using the intensity of a reflected high energy electron beam as a probe optimum growth conditions of c-III nitrides were found, when a 1 monolayer Ga coverage is formed at the growing surface. Clear RHEED oscillations during the initial growth of AlxGa1–xN/GaN quantum wells were observed. X-ray diffraction measurements of these quantum well structures show clear satellite peaks indicating smooth interfaces. Growth of quaternary AlxGayIn1–x–yN lattice matched to GaN were demonstrated. (© 2006 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

In this contribution, we present results on the growth and characterization of c-InGaN/GaN MQWs and c-AlGaN/GaN DBRs, which may be used as building blocks of green (510 nm) resonant cavity light emitting diodes, which have a high potential as light sources for local area networks using plastic optical fibers. First, the impact of the indium and gallium flux on the growth of cubic-InGaN by plasma assisted molecular beam epitaxy has been studied. Indium is observed to incorporate into the c-GaInN films only when the gallium flux is reduced significantly below the value needed for stoichiometric c-GaN growth. A decrease of the surface roughness of the InGaN layers and an increase of their photoluminescence intensity per unit thickness at the transition from metal-flux limited to active nitrogen-limited growth is observed. High quality c-InGaN/GaN multi-quantum wells were grown with superlattice peaks clearly resolved in high resolution X-ray diffraction and a strong room temperature photoluminescence with a full width at half maximum of 240 meV. Cubic-AlGaN/GaN distributed Bragg reflectors with a maximum reflectivity of about 50% at 515 nm and a stop bandwidth of 33 nm have been realized. Enhanced 526 nm room temperature photoluminescence has been observed from a combined structure of a c-InGaN/GaN multi-quantum well and a c-AlGaN/GaN distributed Bragg reflector.

Planar semiconductor microcavities have found increasing attention since they allow to enhance and control the interaction between light and excitons. When the coupling between photon and exciton is strong enough, polaritons are formed which are observed in a pronounced Rabi-splitting in the cavity spectra. The strong exciton-light coupling regime, necessesary for polariton-based applications depends on the oscillator strength and the exciton binding energy. The wide-gap II–VI (CdTe,ZnSe and CdSe) as well as the group III-nitrides (GaN,InN and AlN) semiconductors excellently fulfil these conditions. We observed a large Rabi-splitting (about 44 meV) with ZnSe-based semiconductor microcavities containing four strained (Zn,Cd)Se quantum wells and ZnS/YF3 distributed Bragg-reflectors. Measurements of the reflectivity and of the photoluminescence revealed clear evidence of the strong coupling regime. We also report on first experiments to optimise the structural and optical properties of (Al,Ga)N/GaN quantum wells. The results obtained so far show that these structures may be used for the investigation of polariton Rabi-splitting at room temperature. (© 2004 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

An algorithm for obtaining the size dependent collection efficiency (or penetration) of ultrathin chrysotile fibers through nuclepore filters as a function of fiber diameter and aspect ratio is presented. Numerical simulations are used to provide estimates of the uncertainty in the calculated efficiencies. The algorithm is used to determine the variation of the efficiency (or penetration) for measurements with NPF filters of four different pore sizes.

The diffusion coefficient for chrysotile fibers was determined from measurements of the total penetration and the size distributions before and after a diffusion battery. A protocol was developed in which the fibers were produced by nebulizing a methanol suspension of chrysotile fibers. The fibers were collected on carbon-coated nucleopore filters. After dissolving the filter substrata, the fibers were verified as chrysotile and sized. Experimental variables were flow rate and total pressure. The fiber diffusion coefficients as a function of fiber length and diameter were obtained by first obtaining the size dependent penetrations, then calculating the diffusion coefficient from the Gormley-Kennedy equation. Four experiments were analyzed in depth. Although two of the experiments were at reduced pressure, the corrections due to increased Knudsen number were less than the variation among the experiments. p] The most significant experimental results were that the measured diffusion coefficients were larger than calculations from existing theory. Also, thicker fibers had surprisingly large diffusion coefficients. Theoretical calculations indicated that models based on hypothetical spherical particles gave similar results when either the hybrid, Asgharian-Yu-Gradon, or equivalent volume diameters were used.

Aerosols in a clean air region (remonte station Zugspitze, Germany) were sampled at a mountain observatory higher than 2900 m a.s.1 in June 1984.

Cubic InN films were grown on top of a c-GaN buffer layer by rf-plasma assisted MBE at different growth temperatures. X-Ray diffraction investigations show that the c-InN layers consist of a nearly phase-pure zinc blende (cubic) structure with a small fraction of the wurtzite (hexagonal) phase grown on the (111) facets of the cubic layer. The content of hexagonal inclusions is decreasing with decreasing growth temperature. The full-width at half-maximum (FWHM) of c-InN (002) rocking curve is about 50 arcmin. Low temperature photoluminescence measurements reveal a band gap of about 0.61eV for cubic InN., 3 pages with 3 figures; MRS Symposium, Fall Meeting 2006

Room temperature near-infrared intersubband transitions were observed in MBE grown non-polar cubic GaN/AlN superlattice structures. The peak wavelengths of these transitions were observed in the spectral region of 1.5–2.0 μm and were theoretically supported using a transfer matrix approach. All samples were unintentionally doped and grown on 3C-SiC substrates with a 100 nm GaN buffer. Each structure consisted of a 20 periods of GaN/AlN superlattice capped with 100nm of GaN. The thickness of the AlN barrier was fixed at 1.35, while the thickness of the GaN well was varied between 1.6 and 2.1nm. Electrochemical Capacitance Voltage (ECV) measurements allowed direct measurement of the intrinsic carrier concentration in a thick unintentionally doped cubic GaN layer, which confirmed sufficient population of the ground state energy level.

Zinc-blende GaN layers grown by molecular beam epitaxy were uniformly focused-ion-beam implanted with 300keV Gd3+ ions for doses ranging from 1×1012to1×1015cm−2, and their structural and magnetic properties were studied. The implanted samples were not subjected to any annealing treatment. Only Gd incorporation into zinc-blende GaN was observed by x-ray diffraction. Magnetic investigations using superconducting quantum interference device magnetometry reveal a (super)paramagneticlike behavior with an ordering temperature around 60K for the sample with the highest implantation dose. Our experimental studies indicate that the spontaneous electric polarization in wurtzite GaN is the crucial mechanism for its ferromagneticlike behavior upon Gd doping.

The complex dielectric function for cubic InN is determined by spectroscopic ellipsometry from the mid-infrared into the visible spectral region. Films were grown by molecular beam epitaxy on c-GaN/3C-SiC pseudo-substrates. The high electron densities above 1019 cm−3 cause pronounced Burstein–Moss shifts at the gap. Taking into account the non-parabolicity and the filling of the conduction band, data analysis yields renormalized band edges between 0.43 and 0.455 eV. Including carrier-induced band-gap renormalization, we estimate a zero-density band gap of ~0.595 eV for c-InN which is about 85 meV lower than for hexagonal InN. Values for the electron effective mass, the static and high-frequency dielectric constant are reported.

Cubic InGaN∕GaN multi-quantum-wells (MQWs) with high structural and optical quality are achieved by utilizing freestanding 3C-SiC (001) substrates and optimizing InGaN quantum well growth. Superlattice peaks up to fifth order are clearly resolved in x-ray diffraction. Bright green room temperature photoluminescence (PL) from c-InxGa1−xN∕GaN MQWs (x=0.16) is observed. The full width at half maximum of the PL emission is about 240meV at 300K. The PL intensity increases with well thickness, prooving that polarization fields which can limit the performance of the wurtzite III-nitride based devices are absent. The diffusion length of excess carriers is about 17nm.

Cubic InN layers were grown by plasma assisted molecular beam epitaxy on 3C-SiC (001) substrates at growth temperatures from 419to490°C. X-ray diffraction investigations show that the layers have zinc blende structure with only a small fraction of wurtzite phase inclusions on the (111) facets of the cubic layer. The full width at half maximum of the c-InN (002) x-ray rocking curve is less than 50arcmin. The lattice constant is 5.01±0.01A. Low temperature photoluminescence measurements yield a c-InN band gap of 0.61eV. At room temperature the band gap is about 0.56eV and the free electron concentration is about n∼1.7×1019cm−3.

Schottky diodes were fabricated by thermal evaporation of nickel on phase-pure cubic GaN(c-GaN) layers grown by plasma assisted molecular beam epitaxy on freestanding 3C-SiC. Detailed analysis of the I-V characteristics revealed the existence of a thin surface barrier at the Ni-semiconductor interface. Thermal annealing in air at 200°C alters the composition of this thin surface barrier, reduces the leakage current by three orders of magnitude, and increases the breakdown voltage. The dependence of the breakdown voltage on the doping density of the c-GaN layers is in good agreement with calculated values. We obtain a critical electric-field strength of Ecrit≅2.5×106V∕cm for c-GaN.

Experimental studies are reported dealing with the release of very fine chrysotile fibers from filter sheets containing chrysotile during the filtration of liquids and air. Using scanning electron microscopy as well as radioactivated chrysotile, concentrations of fibers released into aqueous filtrates, or into the air, were measured. It is shown that chrysotile fiber concentrations as high as 10 6 to 10 7 fibers/liter (∼ 1 to 10 μg of chrysotile/liter) in the case of liquid filtration, and concentrations of ∼ 0.1 fibers/ml (ca. 0.1 μg/m 3 ) in the case of air filtration could be found or estimated.

Luftschadstoffe anthropogener Herkunft reagieren mit den Baumaterialien und verursachen eine chemische Korrosion und Verwitterung von Bauten. Die Arbeit befasst sich mit dem Einfluss der aerosolfoermigen Luftschadstoffe. Ausserdem wird ein neues Verfahren zur nicht-destruktiver Probenahme der Reaktionsprodukte an Bauten beschrieben. (IUCT)

Chemical and physical microanalysis of asbestos fibers and man made mineral fibers recovered from samples of animal and human tissues showed chemical and crystalline changes in the fibers. Scanning electron microscopy, electron microprobe analysis, and mass spectroscopic analysis were used in these investigations. The degree of partial or total leaching could be assessed. The amount of leaching varied greatly among fibers which had undergone the same treatment. Certain fibers proved to be chemically resistant and were unchanged whereas for others all minerals but silica were partially or completely leached. The biological significance of the results of these measurements is discussed. In addition the physical and chemical changes of fibers resulting from milling and weathering in air and water are discussed.

The experimental design and results for the measurement of ultrafine asbestos fibers through Nuclepore (NPF) filters is described. The efficiencies were obtained by transmission electron microscopy. A special algorithm was developed for data analysis. For NPF with diameters 1.0 μm or less, the efficiency was dependent on fiber length. The fiber efficiencies were lower than would be expected for the fibers rotating within the pores.

Scanning (SEM) as well as transmission electron microscopy (TEM) have been proved to be suitable methods for the characterization of different aerosol filters. An equipment and a standardized procedure are described for a qualitative and quantitative evaluation of different characteristics for high efficiency aerosol filters. Solid aerosols, incl. radiolabeled aerosols, are used for this evaluation. The test aerosol is sampled in the up- and downstream on Nuclepore filters. By means of SEM- and TEM-methods aerosol concentrations, particle size distributions, particle forms and chemistry or radioactivity are measured on both Nuclepore filters. The total and the selective penetration and collection efficiencies are then estimated for particles of different sizes, forms, chemical composition and radioactivities. The SEM-procedure can also supply very useful information about deposition mechanisms of particles on the surface and inside the tested filter, about the clogging mechanism, about filter corrosion or destruction, etc.

Analysis of asbestos and glass fibers obtained by either environmental sampling or after longer periods of deposition in human and animal tissue revealed that the latter undergo changes of their chemical composition. Electron microscopy and LAMMA were used in these investigations. The extent of fiber dissolution could be assessed. Some fibers remained chemically unchanged. In certain fibers only some elements were partially leached out whereas in other fibers the majority of metallic elements were eluted.

To achieve size resolution with usual condensation nuclei counters (CNC's), a combination of a CNC and size discriminating devices like a diffusion battery or an electrostatic classifier is necessary. In the humidity gradient condensation nuclei counter (HGCNC) proposed here, the aerosol is dried before entering the CNC. The bottom of the expansion chamber is formed by a humidifier. Expansion takes place after a properly chosen time, which depends on the geometry of the expansion chamber. The humidity in the chamber is a function of the distance from the bottom and determines the maximum supersaturation during the expansion. Therefore the fraction of activated condensation nuclei and the droplet size to which they grow, decrease with increasing height above the humidifier. The condensation nuclei size spectrum can in principle be derived from the measured light extinction or scattering. Preliminary measurements show the feasibility of the proposed technique.

Sampling and measurements of mineral fiber concentrations were done on eight sites with different environmental fiber exposures in Lower Saxonian (West Germany) during 1985-1986. The measurements were organized in such a way, that the influence of traffic, of industrial emission sources, of residential areas, as well as of remote regions could be considered and evaluated. During the sampling also climatological data were collected.

Diffusion coefficients for chrysolite and crocidolite fibers were measured with a Sinclair collimated hole diffusion battery and a condensation nuclei counter. The diffusion coefficient for the chrysotile fibers was found to be ~ 4–5 × 10 −7 cm 2 s −1 and was independent of the frequency of the generator. The crocidolite fibers were 6 × 10 −7 cm 2 s −1 and show somewhat more scatter. Comparison of the measured diffusion coefficients with calculated values from scanning electron micrographs indicated good agreement for the crocidolite fibers; but that experimental values were twice that of the calculated values for chrysotile.

In the first of three studies regarding the effect of electrostatic forces on the collection efficiency of nuclepore filters, we describe the experimental design for quantitative measurements of the fractional penetration as a function of flow rate. When fully charged the fractional penetration was independent of the polarity of the charge. For superficial velocities below 1 cm/s there has been a significant decrease in penetration for charged particles. The velocities where significant differences between charged and uncharged particles occur increase with increasing particle size and decreasing pore size. This suggests collection primarily at the rim. Above superficial velocities of I cm/s there was no numerically significant difference between charged and uncharged particles.

An apparatus is described permitting the measurement of the momentum of single aerosol particles down to 1.8·10 −8 dyn s. The experimental arrangement accelerates particles up to velocities of the order of 150 m s −1 . Since the velocity is individually measured, the instrument, allows the detection of particle masses of 10 −12 g. This is equivalent to a spherical particle of unit density and 1.3 μm dia. The system utilizes an aerosol particle beam in a vacuum chamber. The impact of single particles on a thin elastomer membrane causes small vibrations. Since the membrane is one of the reflecting mirrors in a Mach-Zehnder interferometer, its vibrations are converted into vibrations of the interference pattern. A photodiode serves as a detector.

To achieve size resolution with usual condensation nuclei counters (CNC's), a combination of a CNC and size discriminating devices like a diffusion battery or an electrostatic classifier is necessary. In the humidity gradient condensation nuclei counter (HGCNC) proposed here, the aerosol is dried before entering the CNC. The bottom of the expansion chamber is formed by a humidifier. Expansion takes place after a properly chosen time, which depends on the geometry of the expansion chamber. The humidity in the chamber is a function of the distance from the bottom and determines the maximum supersaturation during the expansion. Therefore the fraction of activated condensation nuclei and the droplet size to which they grow, decrease with increasing height above the humidifier. The condensation nuclei size spectrum can in principle be derived from the measured light extinction or scattering. Preliminary measurements show the feasibility of the proposed technique.