B. Lengeler

Valency and Structure of Iridium in Anodic Iridium Oxide Films

The valence of iridium in anodic iridium oxide films has been determined by x-ray absorption spectroscopy and varies between 3.0 and 4. 8 in a voltammogram cycle. When the valency v starts to increase the first coordination shell around an iridium ion begins to shrink from 0.203 at v=3.0 to 0.191 nm at v=4.8. At the same time a strong spread in the interatomic distances in this shell is observed. Our data give support to the model proposed by Kotz et al. for the charge exchange reaction during a voltammogram cycle

Characterization of Interface Structure in GalnAs/lnP Superlattices by means of X-Ray Diffraction

Ga-In-As-P/InP superlattice structures grown by low-pressure metalorganic vapour phase epitaxy (LP-MOVPE) were analysed with high-resolution X-ray diffraction (HR-XRD). The XRD pattern of a Ga0.46In0.54As/InP multiple quantum well sample shows up to two orders in magnitude more intensity than is expected according to a simulation assuming abrupt transitions. The strong intensity of the high order satellite peaks is explained with a simple model based on discontinuity in the lattice constant at the interfaces themselves. Remaining deviations lead to the assumption of compositional grading near the interfaces. A carry-over of As into InP as well as of P into GaInAs on a scale of some nm can clearly be shown, whereas no hints for comparable problems with the group III elements are found. The results show the great sensitivity of X-ray diffraction to compositional changes when strain occurs.

Optimization of modulation‐doped Ga1−xInxAs/InP heterostructures towards extremely high mobilities

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.

Density, thickness and interface roughness of SiO2, TiO2 and Ta2O5 films on BK-7 glasses analyzed by x-ray reflection

BK-7 glass with high surface quality and thin oxidic films (SiO2, TiO2, Ta2O5) deposited by ion plating (IP) and reactive evaporation (RE) on BK-7 glass have been characterized by means of X-ray reflectivity. The quantities determined with great precision are the film thickness, the film density and the interface roughness. It turned out that the IP films are more dense and smoother than the corresponding RE films. The ion-plated layers have a thin surface layer on top of the oxide which is reduced in density, whereas for the RE films there is an intermediate layer with an enhanced density between the glass substrate and the oxide.

Surface analysis of floatglass by means of x‐ray absorption, reflection, and fluorescence analysis

The surface structure of the top and bottom sides of floatglass are investigated by x‐ray absorption, reflection, and fluorescence at grazing incidence. The data analysis includes the determination of the thickness, density, and interface roughness of surface layers and the depth profiling of different atomic species. Expressions for the reflectivity and the fluorescence from a substrate and from layers on a substrate are given and discussed in detail, with special emphasis on the influence of interface roughness on these quantities. It turns out that iron and tin are enriched on the bottom side of the floatglass. In addition, iron changes its valence with depth. Leaching the glass for 48 days in NaOH has a strong influence on the iron concentration, on the surface density, and on the surface roughness of the glass.

EFFECT OF ELECTRON-ELECTRON INTERACTION ON HOT BALLISTIC ELECTRON BEAMS

Electron‐electron scattering of ballistic electrons in a two‐dimensional electron gas was studied as a function of the electron excess energy above the Fermi energy and of temperature. At low temperatures of 1.4 K it is found that for excess energies of approximately 30% of the Fermi energy the electrons in a ballistic electron beam are already scattered significantly due to electron‐electron interaction. A very good agreement between our experimental data and theory was found, when the measured data were compared with numerical calculations based on a theory of Giuliani and Quinn [Phys. Rev. B 26, 4421 (1982)], while the agreement was only poor for the analytical approximation of the electron‐electron scattering rate.

Quantized conductance in a split‐gate point contact based on a pseudomorphic InGaAs/InP heterostructure

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.

Low‐ and high‐field transport properties of pseudomorphic InxGa1−xAs/InP (0.73≤x≤0.82) p‐type modulation‐doped single‐quantum‐well structures

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. Furt...

Quantum transport in quasi one-dimensional In/sub 0.77/Ga/sub 0.23/As/InP rings

We have fabricated mesoscopic rings in a strained high mobility In/sub 0.77/Ga/sub 0.23/As/InP heterostructure where periodic oscillations in the magnetoresistance due to the Aharonov-Bohm effect can be observed, although the one-dimensional transport regime with only one mode in the ring was not yet obtained. The influence of temperature and electron excess energy on the phase coherence length were investigated and could be explained by the phase breaking influence of electron-electron scattering events. We have shown that InGaAs/InP with its low electron mass is ideally suited for devices based on electron interference.<<ETX>>

Control of ballistic electrons in (AlGa)As/GaAs heterostructures by means of superconducting niobium gate structures

Abstract Superconducting gate structures on a modulation doped (AlGa)As/GaAs heterostructure were used to control a ballistic electron beam within the high mobility 2DEG. A local and variable magnetic field was produced by superconducting current loops. The magnetic field induced by the loops was analysed by means of an external magnetic field as well as by an electron interference experiment. The maximum field obtained by two superconducting loops was 0.4 mT. In addition, a gate structure containing a superconducting ring between two opposite split gates was prepared in order to detect magnetic flux quantisation.