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ABSTRACT: High-angle annular dark-field scanning transmission electron microscopy (HAADF STEM) at low energies (≤30 keV) was used to study quantitatively electron scattering in amorphous carbon and carbon-based materials. Experimental HAADF STEM intensities from samples with well-known composition and thickness are compared with results of Monte Carlo simulations and semiempirical equations describing multiple electron scattering. A well-defined relationship is found between the maximum HAADF STEM intensity and sample thickness which is exploited (a) to derive a quantitative description for the mean quadratic scattering angle and (b) to calculate the transmitted HAADF STEM intensity as a function of the relevant materials parameters and electron energy. The formalism can be also applied to determine TEM sample thicknesses by minimizing the contrast of the sample as a function of the electron energy.
Journal of Microscopy 12/2010; 243(1):31-9. · 1.63 Impact Factor
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ABSTRACT: High-angle annular dark-field (HAADF) scanning transmission electron microscopy (STEM) images of electron-transparent samples show dominant atomic number (Z-) contrast with a high lateral resolution. HAADF STEM at low electron energies <30 keV is applied in this work for quantitative composition analyses of InGaAs quantum wells. To determine the local composition, normalized experimental image intensities are compared with results of Monte Carlo simulations. For verification of the technique, InGaAs/GaAs quantum-well structures with known In concentration are used. Transmission electron microscopy samples with known thickness are prepared by the focused-ion-beam technique. The method can be extended to other material systems and is particularly promising for the analysis of materials that are sensitive toward knock-on damage.
Microscopy and Microanalysis 10/2010; 16(5):604-13. · 3.01 Impact Factor
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J. Cryst. Growth. 01/2005; 281(2--4):281-289.
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Thin Solid Films 01/2004; 459(1--2):37-40. · 1.89 Impact Factor
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ABSTRACT: Raman spectroscopy is used to study C and Ge diffusion in multilayers of C-induced Ge dots deposited on Si(100). The initial Ge content is fixed to 2 ML and the C precoverage varied from 0.1 to 0.3 ML. The resulting concentration of isolated substitutional C atoms depends on the C precoverage and the thermal annealing performed after growth. C atoms are mostly localized in the areas around the dots, due to the repulsive Ge–C interaction. When C is added, the interface around the burried dots becomes sharper, and less Ge alloying occurs. C mainly increases the strain contrast around the dots and induces a strain-enhanced Ge interdiffusion, even at 650 °C. At 800 °C, Ge and C interdiffuse simultaneously. © 2001 American Institute of Physics.
Applied Physics Letters 03/2001; 78(12):1742-1744. · 3.84 Impact Factor
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ABSTRACT: Self-assembled C-induced Ge dots are islands which are not formed by the Stranski–Krastanov mode of growth. They are formed by a three-dimensional mode originating from the undulating strain fields of the C alloyed Si (100) surface. This opens additional possibilities to control the size and the shape of these dots by modifying the strain fields of the C-alloyed Si surface. Here, we show that the amount of C deposited prior to the growth of the Ge islands strongly effects the diameter and height of the dots. Increasing the C coverage to 0.3 monolayer leads to the formation of comparably compact islands. Consequently, the photoluminescence of the dots is shifted to lower energies compared to dots grown with lower C coverages. © 2000 American Institute of Physics.
Applied Physics Letters 11/2000; 77(20):3218-3220. · 3.84 Impact Factor
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ABSTRACT: Molecular beam epitaxy has been utilized to grow small C-induced Ge islands in silicon. The evolution of C-induced Ge dots from nucleation to the formation of larger dots has been studied in detail. The Ge grows in a Vollmer-Weber mode of growth in areas between the C-rich patches on the Si surface. Using in situ STM analysis as well as transmission electron microscopy (TEM) it is shown that an increase in the Ge coverage leads to the overgrowth of the C-rich patches and to island coalescence, reducing the island density. The strongest luminescence intensity is found in this region where the Ge has buried the C-rich areas. The amount of C deposited on the Si(100) surface prior to the growth of Ge permits the control of the lateral size and the height of Ge quantum dots. Accordingly, intense photoluminescence (PL), with a stronger confinement shift dependending on the amount of Ge deposited, is observed for samples prepared with large C concentrations. The impact of the Si spacer layer width on the size of the dots has been studied by TEM and compared with PL data. The data give new insights into the structural peculiarities of C-induced Ge dots and their consequences on the electron and hole confinement.
Nanotechnology 11/2000; 11(4):298. · 3.98 Impact Factor
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ABSTRACT: Germanium quantum dots embedded in silicon have been used in the past to improve the opto-electronic properties of Si based materials. The idea is to overcome the limitation of the indirect band gap of Si by a strong localization of the carriers in quantum dots. However, the Ge quantum dots provide a strong carrier confinement only for the holes, the electrons are only weakly confined in the Si. In this study we embedded the Ge quantum dots in strained Si quantum wells grown on relaxed SiGe buffer layers. The strained Si quantum wells provide a confinement of the electrons in the vicinity of the Ge dots.The structures were deposited on planar as well as on patterned substrates by molecular beam epitaxy. The structural and optical properties of the samples were analyzed using high resolution cross sectional transmission electron microscopy (TEM) as well as low temperature photoluminescence. The size of the mesa structures have been used as experimental parameter. Relaxed buffer layers grown on line shaped mesa structures show a strongly reduced dislocation density. Consequently the deep luminescence attributed to dislocations in the buffer layers is strongly reduced and pronounced photoluminescence of the quantum structures grown on top of the buffer layers can be observed.
MRS Proceedings. 12/1999; 638.
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Microscopy and Microanalysis. 16(S2):612-613.
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S. Tsujino,
S. Mentese,
L. Diehl, E. Müller,
B. Haas,
D. Bächle,
S. Stutz,
D. Grützmacher,
Y. Campidelli,
O. Kermarrec,
D. Bensahel
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ABSTRACT: P-type Si–Si0.2Ge0.8 superlattices prepared on Si0.5Ge0.5-relaxed buffer substrates are promising structures for the development of SiGe quantum cascade lasers. To explore the resonant tunneling in this system, we studied the vertical transport in Si–Si0.2Ge0.8 superlattices. Low-temperature molecular beam epitaxy enables growth of highly uniform and relatively thick (∼0.5 μm) superlattices with strain symmetrized design. The sample with 8.3-nm thick Si0.2Ge0.8 quantum wells and 5-nm thick Si barriers exhibited a series of sharp resonant tunneling peaks and negative differential conductance. A signature of electric field domain formation was also found. By reducing the Si-barrier thickness to 3 nm and also reducing the quantum well thickness to 5 nm, only a single peak was observed, but the resonant tunneling peak is about a factor of 2 enhanced compared to the sample with thicker barriers.
Applied Surface Science 224:377-381. · 2.10 Impact Factor
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ABSTRACT: The principle of intersubband emission is applied to the Si/SiGe material system, using hole intersubband transitions in structures grown pseudo-morphically on Si substrate by molecular beam epitaxy. Cascade structures consisting of three times four repetitions of a five quantum well sequence are investigated. The design constraints are found to be imposed mainly by the total amount of strain, giving limitations to the number of wells per cascade and the total number of cascade periods. Despite the close approach to the critical thickness for misfit dislocations, requiring low temperature growth, these structures reveal intersubband electro-luminescence with a linewidth as narrow as 22meV. Peak energies between 125 and 154meV are obtained by tuning the well width and Ge content of the single active quantum well. By comparison with the emission from a III–V cascade structure, the non-radiative lifetime of the upper emission state is determined. It is found to depend strongly on the structure's design, but can reach values comparable to those in similar III–V cascade structures. A discussion of the importance of carrier escape to the continuum and the injection efficiency, as well as the relaxation via the light hole state is given.
Physica E Low-dimensional Systems and Nanostructures 11:240-244. · 1.53 Impact Factor
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ABSTRACT: We demonstrate mid-infrared emission from cascade structures based on InAs quantum dashes (QDashes) grown on tensile-strained AlInAs. The proposed design relies on a diagonal transition from the QDash ground state to the following quantum well. The measured electroluminescence is very broad and tunes strongly with the applied voltage. A comparison with an identical structure, where the dashes were replaced by a lattice-matched InGaAs quantum well (QW), shows quite different transport properties. A sharp alignment between the injector and the optical transition upper state is present in the QW sample, while the alignment is smoother in the QDash one. Moreover, the QW device exhibits a negative differential resistance not present in the QDash one. This can be attributed to the inhomogeneous nature of the QDashes. Decreasing the tunneling barrier between the QDashes and the following QW increases the injected current as does increasing the doping in the active region. A gain extrapolation from our electroluminescence measurements shows that these structures are promising for the development of quantum cascade lasers based on a 3D confined active region.
Journal of Crystal Growth 323(1):491-495. · 1.73 Impact Factor
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ABSTRACT: The metalorganic vapor-phase epitaxy growth of a highly reflective 24-pair AlGaAsSb/InP-distributed Bragg reflector (DBR) is reported for the first time. The influence of the growth parameters such as the V/III input ratio, the growth temperature and the pressure, the total H2 flow, the gas velocity and the switching sequence of the source gases at the interfaces has been deeply investigated and optimized to achieve stable growth conditions. The DBR achieves a reflectivity as high as 99.5% around 1.55 μm, a uniform stable composition, and an excellent crystal quality over the 2 inch wafer, with a surface free of crosshatch and a defect density below 1/cm2. For the optical characterizations, measurements of linear and nonlinear reflectivity, transmission, pump-probe and photoluminescence were done. The interfaces and bulk layers of InP/AlGaAsSb/InP heterostructures were analyzed by transmission electron microscopy. High resolution X-ray diffraction measurements were used to determine the composition shift in the growth plane of the DBR. The measurements show the high quality of the growth and demonstrate that thick AlGaAsSb/InP heterostructures can be grown by metalorganic vapor-phase epitaxy (MOVPE), and in particular DBRs above 1.31 μm.
Journal of Crystal Growth 286(2):247-254. · 1.73 Impact Factor
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ABSTRACT: Modal analysis and holography are two powerful tools of modern measuring techniques. Both methods may be used to test the behaviour of mechanical structures in order to detect and remove possible weak points. Due to the fully different physical principles of these methods, the measuring procedures, the representation of results and their evaluation are quite different.The first part of this contribution describes the typical operation of both systems. The resulting main differences in the measurement and analysis are illustrated with the help of several practical examples. The possibilities and the specific advantages and disadvantages of each instrument are then compared in detail. Corresponding fields of applications, as well as combined testing procedures, can be deduced there from. Some expected future improvements are also mentioned.
CIRP Annals - Manufacturing Technology 29(1):397-402. · 1.71 Impact Factor
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ABSTRACT: Pseudomorphically grown p-type Si/SiGe double barrier resonant tunneling diodes have been investigated. The main resonances are shown to be due to tunneling through heavy and light hole states in the well. However, temperature activated resonances and resonances arising in a B-field perpendicular to the current show the importance of the complicated emitter structure and its energy spectrum.
Thin Solid Films 369:390-393. · 1.89 Impact Factor
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ABSTRACT: The electrochemical corrosion resistance is one essential factor for determining the biocompatibility of metals. Although surgical alloys obtain an extraordinary high electrochemical corrosion resistance in the body environment, additional reduced metal ion release is expected to further improve the biocompatibility. In the following study, diamond-like carbon coatings with two different thickness were deposited on polished Co28Cr6Mo-samples (cast Protasul*-2, ASTM standard F75, ISO standard 5832/4) by a plasma deposition method (direct current discharge) using gaseous hydrocarbon as precursor. The electrochemical behaviour of DLC is investigated in 0.89 wt.% NaCl solution and compared with the behaviour of the uncoated medical grade CoCrMo. Potentiodynamic polarization tests were carried out starting from the rest potential into the anodic direction up to a potential of +1500 mV, and subsequently, into the cathodic direction. The polarization curves are evaluated and correlated to the thickness of the DLCs. Additional information is given by hardness measurements (loads 4 mN, 10 mN) as well as characterization of the nano- and microstructure (Raman spectroscopy). The shift of the break down potential to more positive values and the reduction of the current densities give evidence that DLC coatings provide an useful measure for reducing the corrosion attack of the cast Co28Cr6Mo-alloy in 89 wt.% NaCl. In this study, the 2.7-μm thick coating offers further improved protection in comparison with the 0.8-μm thick DLC.
Surface and Coatings Technology · 1.87 Impact Factor
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ABSTRACT: The rearrangement of SiGe islands during the deposition of Si was studied by a combination of scanning tunneling microscopy, transmission electron microscopy and high-resolution X-ray diffraction. With increasing silicon capping of the islands, an increasing flattening accompanied by a rising intermixing could be determined. Using a finite element calculation, which served as an input for X-ray simulations, the strain distribution within the islands was obtained.
Materials Science and Engineering: B. 101:71-76.
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ABSTRACT: Germanium islands were embedded in strained silicon quantum wells in order to provide an improved electron confinement in vicinity of the islands. Growth was performed on relaxed SiGe layers. Patterned substrates were used, favouring lattice relaxation as well permitting the fabrication of small Ge islands at deposition temperatures above 500 °C. Photoluminescence analysis reveals a strongly reduced dislocation related signal. The low temperature spectra are dominated by intense signals from the germanium islands. The origin of these signals were investigated by removing the islands by etching, analysing reference samples without a silicon quantum well, varying the germanium deposition and the growth temperature.
Microelectronics Journal 33(7):525-529. · 0.92 Impact Factor
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ABSTRACT: Low-energy plasma-enhanced chemical vapour deposition (LEPECVD) has been applied to the synthesis of SiGe relaxed buffer layers with Ge end concentrations between 35% and pure Ge. A growth rate of several nanometres per second for relaxed buffer layers is well above that obtainable by any other growth technique. The structural quality of SiGe buffers graded to pure Ge is compared with that of a Ge buffer of constant composition. The structural quality of the pure Ge buffer is remarkably good compared with the much more complicated graded buffer. Complete n-type Si-modulation doped field effect transistor structures have been synthesized by LEPECVD, and the electric properties have been characterized by magneto transport measurements.
Materials Science and Engineering: B. 74:113-117.
