Search Results - (Author, Cooperation:Lüth)

Lüth, Hans

Unknown

482 S.

978-3-540-71042-4

German

book

2007

Erziehung ; Bildungsgeschichte ; Zeitschrift ; Zeitung ; Kinderzeitschrift ; Geschichte (Histor) ; Sozialismus ; 20. Jahrhundert ; Kongress ; Deutschland-DDR

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1089-7550

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Physics

Schottky barrier height enhancement on n-InGaAs is studied on structures with thin surface layers of different compositions. Counter-doped p+-InGaAs layers, as well as layers of n- and p-InP, n-GaAs, and n-InGaP of different thicknesses and dopant densities, respectively, were used to enhance the barrier. Titanium was used as a barrier metal to prepare Schottky diodes of different areas and the barrier height is analyzed by current-voltage measurements. It is observed that the barrier height enhancement by p+-InGaAs layers increases with the layer thickness and dopant density, respectively, and effective barrier heights up to 0.63–0.68 eV, i.e., higher values than previously reported, have been measured. The barrier height enhancement by counter-doped p+-InGaAs layers on n-InGaAs can be described by the two-carrier model. Schottky diodes with extremely low reverse current densities have been prepared, JR(1 V) =4.5×10−6 A/cm2. It is shown that lattice-matched InP surface layers can be used as an alternative to enhance the barrier height on n-InGaAs. The barrier height increases with the layer thickness up to φB=0.53–0.55 eV, i.e., up to values previously reported as barrier heights on thick n-InP. Additional barrier enhancement can be achieved by counter doping of the InP surface layer and barrier heights of 0.66 eV have been obtained by p-InP surface layers on n-InGaAs. On structures with barrier-enhanced n-GaAs layers, a remarkable decrease of the reverse current density is observed if the layer thickness is reduced to the critical layer thickness, but the barrier height is very low due to the small n-GaAs thickness. For structures with slightly lattice-mismatched n-InGaP layers (xGaP=0.11) measured barrier heights are similar to those for n-InP enhancement layers of the same thicknesses.

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This paper presents a study of the electrical and structural properties of inverted modulation-doped GaInAs/InP heterostructures grown by low-pressure metalorganic vapor phase epitaxy. First, the thickness of the GaInAs layer was optimized in lattice-matched samples to find the smallest thickness in which high Hall mobility is observed. Next, in a section closest to the InP the In content was varied. A steady increase of mobility with indium composition was observed. A maximum of 450 000 and 15 500 cm2/V s was obtained for a 10-nm-thick Ga1−xInxAs layer with x=0.77 at 6 and 300 K, respectively. Channels with higher indium content exceed the critical thickness and mobility drops off sharply. The decreasing mobility correlates with the formation of misfit dislocations at the interface indicating increasing scattering processes of the GaInAs layer.

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Deep level transient spectroscopy (DLTS) was performed on p-isotype Si/SiGe/Si Schottky barrier diodes in order to obtain the valence band offset between Si and SiGe. A single strained Si0.7Ge0.3 layer was placed in such a depth in Si so as to be able to fill and empty the quantized SiGe well during the transient capacitance procedure. Broad capacitance transient peaks were obtained and interpreted as being due to the capture of holes by the quantum well. The broadness of the peaks was explained by thickness variations of the SiGe layer. From the dependence of the high temperature side of the DLTS peak on the rate window a valence band offset of 220±20 meV was evaluated.

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Ohmic contacts produced by high-energy pulsed laser beam alloying Au/Te/Au/n-GaAs are investigated by micro Raman spectroscopy. The results are compared to those from furnace annealed ohmic contacts. For the furnace as well as for the laser annealed ohmic contacts, no evidence for a doping of the contact region is found in the Raman spectra. The presence of a highly disordered GaAs surface layer is observed for both types of contacts. In addition, after furnace processing a Ga2Te3 layer is formed. These results are consistent with earlier Mössbauer studies. For the laser alloyed samples the results strengthen the role of a defective/disordered interface structure where conduction might occur by a resonant tunneling process involving localized gap states.

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A system of interdigital gates is used to create a periodic potential profile in a multilayer heterostructure. The electrostatic problem for the spatial distribution of the potential is solved and experimentally examined by measurements of current–voltage characteristics of resonant-tunnelling diodes embedded in the depletion region of the Schottky contact. It is shown that the position of the resonant peak voltage is sensitive to the spatial potential distribution and that with appropriate parameters of the heterostructure the sensitivity of the gates can be considerably enhanced. © 2001 American Institute of Physics.

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The two possible causes of depth inhomogeneities of the microstructure of porous silicon are changes in the HF concentration with depth and a varying chemical etching rate of the porous silicon layer. During anodization chemical etching will become important for microporous silicon — e.g. p-porous silicon — due to the large internal surface area, especially at long etching times. On the other hand, a considerable decrease of the HF concentration will occur during etching with high current densities to produce p+-porous silicon with high porosities. We have investigated the depth inhomogeneity of porous silicon layers by spectroscopic ellipsometry, Raman spectroscopy and photoluminescence measurements. From a line shape analysis of the Raman signal a size distribution of nanocrystals is deduced. For p-porous silicon smaller nanocrystals are found near the surface of the layer; for p+-porous silicon etched with high current densities smaller nanocrystals are found near the porous silicon/substrate interface. © 1996 American Institute of Physics.

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We report on optical and electrical properties of modulation doped InxGa1−xAs/InP single quantum wells in the composition range 0.56≤x≤0.79. Cyclotron resonance, contactless Shubnikov–de Haas and magnetophotoluminescence experiments are used to obtain two dimensional carrier densities, effective masses of electrons and holes and scattering times. We present data which give evidence for zero magnetic field spin splitting. The dispersion relation for electrons and holes is presented. © 1996 American Institute of Physics.

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We report on the determination of the valence band offset between strained Si1−xGex and unstrained Si layers by deep level transient spectroscopy (DLTS) on Si/Si1−xGex/Si quantum well (QW) structures. A problem of this technique is to store the holes long enough (≥1 ms) in the QW so that the thermal emission of holes is the dominating process. We achieved sufficiently long hole storage times by using two different structures. In the first ones, this is obtained by selective growth which leads to a lateral limitation of the smooth QW layer, and with good Schottky contacts. For the second ones, the localization of holes is due to the presence of Si1−xGex islands. For a sample containing a smooth QW with XGe=0.17 a valence band offset of 140±20 meV was obtained and for the island layer with XGe=0.3 a value of 258±20 meV was found. These results are in good agreement with theory. The DLTS measurements are compared to admittance spectroscopy results and photoluminescence measurements. © 1995 American Institute of Physics.

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SiGe/Si quantum well layers are selectively grown by low pressure chemical vapor deposition on patterned Si substrates. Transmission electron microscopy (TEM) shows that the growth rate of SiGe in convex corners between different surface planes is at least ten times higher than the growth rate observed on (001) planes. This high growth rate leads to the formation of quantum wires and dots between such facets. Photoluminescence (PL) spectra of square and rectangular patterns, bounded by quantum wires, ranging in size from 300μm down to 500nm are taken. The observed energy shifts of the (001) quantum well PL–peaks are explained by surface diffusion of Ge adatoms into the quantum wires. A surface diffusion model is used to obtain a Ge diffusion length of λ=2.5±0.6 μm at 700°C. Thus, a method for the determination of surface diffusion lengths in strained layer epitaxy is introduced. For SiGe layers grown above the Stranski–Krastanow critical thickness for three dimensional (3D) growth, a competition between the SiGe wires in the interfacet corners and the SK islands on the (001) planes is observed. In squares as large as 2×2 μm2 the SiGe wires lead to a suppression of 3D growth on the (001) plane altogether, as observed by TEM and PL. © 1995 American Institute of Physics.

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In this paper the fabrication and characterization of split-gate point contacts based on a pseudomorphic InGaAs/InP heterostructure with an indium content of 77% in the strained channel layer is described. Steps in the conductance were observed, which are due to quantized conductance through the quasi one-dimensional constriction formed by the split-gates. Deviations from the ideal quantization are studied by applying differing bias voltages on the two fingers forming the point contact. Since the channel layer of our structure consists of a ternary material it is argued that, beside impurity and interface roughness scattering, alloy scattering processes contribute significantly to the observed deviations of the ideal quantized conductance. © 1996 American Institute of Physics.

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We have determined the valence band discontinuity ΔEv of strained Si1−xGex on unstrained (100) Si using temperature-dependent current–voltage characteristics (I–V-T) of Si/Si0.83Ge0.17/Si heterostructures. In a first step, the measurements were performed on a Schottky diode, and in a second step, on a sample with ohmic contacts. The values of ΔEv obtained by these two different procedures are comparable. Moreover, they are in good agreement with the theoretical value of ΔEv=0.84x=143 meV predicted by Van de Walle measurements. © 1996 American Institute of Physics.

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The performance of the gaseous precursor diethyl-tellurium (DETe) for n-type doping of GaAs is studied. We report on both the properties of the molecular source DETe as well as on the suitability of the dopant element tellurium. Te is found to be an excellent dopant, comparable to Si in the lower-doping regime and superior at highest-doping levels. DETe reveals itself to be a convenient and reproducible doping source. At substrate temperatures above 540 °C, dopant desorption (most probably Te) is observed which limits its applications. Memory effects after doping up to the mid 1018 cm−3 range can be eliminated. At still higher-doping levels an optimized technique limits unintentional background carrier concentrations in subsequent layers to the 1014 cm−3 range.

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The transport properties of three p-type modulation-doped InxGa1−xAs/InP (0.73≤x≤0.82) single-quantum-well structures grown by metalorganic chemical-vapor deposition are reported. High carrier mobilities of μH=7800 cm2/V s coupled with total carrier concentrations of pS=2.1×1012 cm−2 were reached, for example, for x=0.73 at 5 K. Shubnikov–de Haas and quantum Hall-effect measurements at 50 mK showed the population of two spin-split V3/2 subbands. Using p-modulation-doped field-effect transistors with a gate length of LG=1 μm, fabricated on the same samples, the carrier transport at moderate and high fields was investigated at 77 K. Thereby, the population of the heavy-hole subband and, above a critical field, also the occupation of the light-hole subband were verified. With the help of dc transconductance (gmext-VGS) and magnetotransconductance measurements a decoupling between both subbands at cryogenic conditions and moderate fields was observed, resulting in two clearly defined conducting channels. Further analysis of the measured mobility-voltage (μ-VGS) and velocity-field (vavg-Eavg) profiles revealed that carrier transport in compressively strained two-dimensional hole gas (2DHG) systems is strongly affected by intersubband scattering and shows a nonlinear behavior at low fields, caused by the zone-center degeneracy of their E-k(parallel) distribution.

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1077-3118

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Physics

Erbium-substituted La1−xErxF3 lanthanum trifluoride epitaxial layers have been grown on Si(111) substrates by molecular beam epitaxy (MBE). Strong near-infrared luminescence, peaked at 1.54 μm, was observed from such films under electron beam excitation. This cathodoluminescence arises from the intra-4f-shell transitions 4I13/2→4I15/2 of Er3+(4f11). The infrared spectra reveal that MBE-grown LaF3 layers on Si(111) crystallize in the hexagonal tysonite structure, typical for bulk LaF3 single crystals.

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The supercurrent in a Nb–In0.53Ga0.47As/In0.77Ga0.23As/InP weak link structure is controlled by means of a current injected into the two-dimensional electron gas. For small injection currents the critical current to control current ratio is as large as 20. The measured features can be qualitively explained in terms of a modification of the Andreev level occupation by the injected carriers. © 1998 American Institute of Physics.

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Spin splitting of conduction band electrons in In0.53Ga0.47As/In0.77Ga0.23As/InP heterostructures due to spin-orbit coupling is studied by performing Shubnikov–de Haas measurements on nongated and gated Hall bars. From an analysis of the beating pattern in the Shubnikov–de Haas oscillations, the spin-orbit coupling constant is determined. For a symmetric sample no beating pattern and thus no spin splitting is observed. This demonstrates that the k3 contribution to the spin-orbit coupling constant can be neglected. By applying an envelope function theory it is shown that the major contribution to the Rashba spin-orbit coupling originates from the band offset at the interface of the quantum well. Using gated Hall bar structures it is possible to alter the spin-orbit coupling by application of an appropriate gate voltage. A more negative gate voltage leads to a more pronounced asymmetry of the quantum well, which gives rise to a stronger spin-orbit coupling. © 1998 American Institute of Physics.

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To investigate the transport properties of resonant tunneling diodes with dimensions in the submicron range, small area mesa diodes with surrounding Schottky gates have been processed. The gate turns out to provide excellent current control, which makes a resonant tunneling transistor operation mode feasible for our devices. In the single electron regime very distinct staircase-like features are observed in the current voltage characteristics. An accurate analysis of this staircase characteristic by means of magnetotransport measurements shows that tunneling through defect states can be ruled out as a reason for these current steps. Moreover, we show that the current steps are exclusively due to quantization effects of the gate potential. At high magnetic fields a saturation-like behavior of the step onset voltages occurs as a function of a magnetic field applied parallel to the direction of transport. This effect can be explained by boundary conditions for the electron number and the Fermi level in the electron supply layer next to the double barrier structure. © 1998 American Institute of Physics.

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We report on the observation of quantized conductance in split-gate In0.53Ga0.43As/Ga0.77In0.23As/InP point contacts. For the Schottky gates Au/Cr in combination with p-InP was used. As a result our split-gate point contacts show low pinch-off voltages and no measurable leakage current through the gates. Up to five conductance steps were observed at a temperature of 1.4 K. Our approach to fabricate split-gate Schottky contacts can be used for quantum point contacts operating at higher temperatures as well as for superconductive quantum point contacts. © 1998 American Institute of Physics.

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