Claus Schumacher

Claus Schumacher
University of Wuerzburg | JMU · Faculty of Physics and Astronomy

Doctor of Philosophy

About

46
Publications
2,629
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560
Citations
Citations since 2016
4 Research Items
277 Citations
2016201720182019202020212022010203040
2016201720182019202020212022010203040
2016201720182019202020212022010203040
2016201720182019202020212022010203040

Publications

Publications (46)
Preprint
A detailed study of the influence of molecular beam epitaxial growth conditions on the structural and magnetic characteristics of CuMnSb films on lattice matched GaSb is presented. For a set of nine 40~nm thick layers, the Mn and Sb fluxes are varied to produce material with different elemental compositions. It is found that the layers grown under...
Article
We report the growth of CuMnSb thin films by molecular beam epitaxy on InAs (001) substrates. The CuMnSb layers are compressively strained (0.6 %) due to lattice mismatch. The thin films have a ω full width at half-maximum of 7.7 arcsec according to high resolution x-ray diffraction, and a root-mean-square roughness of 0.14 nm as determined by atom...
Article
Full-text available
We present results on the growth of CdTe-HgTe core-shell nanowires, a realization of a quasi one-dimensional heterostructure of the topological insulator HgTe. The growth is a two step process consisting of the growth of single crystalline zinc blende CdTe nanowires with the vapor-liquid-solid method and the overgrowth of these wires with HgTe such...
Article
We study the chemical ordering in Bi2Te3-x Se x grown by molecular beam epitaxy on Si substrates. We produce films in the full composition range from x = 0 to 3, and determine their material properties using energy dispersive x-ray spectroscopy, x-ray diffraction and Raman spectroscopy. By fitting the parameters of a kinetic growth model to these r...
Article
The half-metal ferromagnet NiMnSb, with its high spin polarization, low magnetic damping and tunable magnetic anisotropy, is a promising material for applications in spin torque devices. We develop the epitaxial growth of NiMnSb/ZnTe/NiMnSb heterostructures, aiming towards the realization of an all-NiMnSb based magnetic tunneling junction (MTJ). La...
Article
Full-text available
We study nanoscale morphology of PbTe/CdTe multilayer heterostuctures grown by molecular beam epitaxy on hybrid GaAs/CdTe (100) substrates. Nominally, the structures consist of 25 repetitions of subsequently deposited CdTe and PbTe layers with comparable thicknesses of 21 and 8 nm, respectively. However, the morphology of the resulting structures c...
Article
We report on the optical characterization of single (Zn,Cd)Se quantum dots (QDs) embedded in vapor-liquid-solid-grown ZnSe nanowires (NWs). The temperature dependent quenching of the QD luminescence demonstrates that their electronic structure is comparable to that of self-assembled (Zn,Cd)Se QDs in ZnSe matrices. The photoluminescence excitation (...
Article
The microstructure of Bi2Se3 topological-insulator thin films grown by molecular beam epitaxy on InP(111)A and InP(111)B substrates that have different surface roughnesses has been studied in detail using X-ray diffraction, X-ray reflectivity, atomic force microscopy and probe-corrected scanning transmission electron microscopy. The use of a rough...
Article
NiMnSb is a ferromagnetic half-metal which, because of its rich anisotropy and very low Gilbert damping, is a promising candidate for applications in information technologies. We have investigated the in-plane anisotropy properties of thin, MBE-grown NiMnSb films as a function of their Mn concentration. Using ferromagnetic resonance (FMR) to determ...
Article
Full-text available
We report on a process that enables the removal of II-VI semiconductor epilayers from their GaAs growth substrate and their subsequent transfer to arbitrary host environments. The technique combines mechanical lapping and layer selective chemical wet etching and is generally applicable to any II-VI layer stack. We demonstrate the non-invasiveness o...
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Full-text available
The microstructure of Bi2Se3 thin films grown by molecular beam epitaxy on Si(111), InP(111)B and Fe-doped InP(111)B substrates has been studied in detail using scanning transmission electron microscopy. Films grown on Si(111) and InP(111)B substrates show the formation of twin domains: rotation twins (with the grain boundary perpendicular to the s...
Article
Full-text available
Epitaxial layers of the topological insulator Bi2Se3 have been grown by molecular beam epitaxy on laterally lattice-matched InP(111)B substrates. High resolution X-ray diffraction shows a significant improvement of Bi2Se3 crystal quality compared to layers deposited on other substrates. The measured full width at half maximum of the rocking curve i...
Article
We report on the electronic properties of n-doped (n ∼ 1018 cm-3) ZnSe/GaAs(001) heterostructures grown by molecular beam epitaxy with pre-deposition of different amounts of Zn or Se in the fractional monolayer range at ZnSe epilayer growth start. Special attention is spent on the depletion layers on both sides of the interface and to the conductio...
Article
Transmission electron microscopy (TEM) and scanning electron microscopy (SEM) have been carried out to clarify the microstructure of Bi2Se3 thin films grown by molecular beam epitaxy (MBE) on Si(111) and InP(111) substrates. The film grown on InP displays much better overall quality at the microstructural level than does the film grown on the Si su...
Article
We demonstrate the ability to release the growth-induced strain in (Ga,Mn)As layers and (In,Ga)As/(Ga,Mn) As bilayers by lifting them from the GaAs substrate. The lifted (bi)layers are then deposited back onto various substrates. The change in strain before and after processing has been studied by means of x-ray diffraction. Magnetic characterizati...
Article
We present a comprehensive study of band offsets and band bending at heterovalent semiconductor heterointerfaces. A perfectly abrupt heterovalent interface is usually thermodynamically unstable, and atomic intermixing of materials with different numbers of valence electrons causes large variations in band offsets and local doping density, depending...
Article
We show that important layer parameters of the double barrier active region in semimagnetic II–VI semiconductor resonant tunneling diodes (RTDs) can be accurately determined from high-resolution X-ray diffraction (HR-XRD) scans of complete structures. This is remarkable, since the scattering volume of the typically only few nm thin double barrier r...
Article
The temporal development of strain relaxation is monitored during and immediately after heteroepitaxy by x-ray diffraction with high resolution in strain and time. ZnSe layers on (001)GaAs with thicknesses just a little above the onset of plastic relaxation reveal inhomogeneous, partial relaxation which continues with a time constant of 50 s immedi...
Article
Formation and properties of quantum dots (QDs) and related nanostructures by conventional molecular beam epitaxy (MBE) and two other variants of the technique, have been systematically investigated. While MBE growth of CdSe on ZnSe at 300 °C results in the formation of rough CdSe quantum-well-like structures, by one of the variants, which combines...
Article
We developed a real-time in-situ X-ray (RIX) diffraction technique for the detailed layer characterisation during epitaxial deposition of ZnSe-based alloys on (001)GaAs substrates. A conventional molecular beam epitaxy chamber was equipped with a Cu X-ray tube. The 113 reflection of the deposited structure is detected in static geometry by a CCD an...
Article
The structural properties of two epitaxial model systems were investigated, NiMnSb thin films on a (In,Ga)As/InP(001) substrate and free-standing CdSe quantum dots on a ZnSe(001) surface. For the first system the influence of surface-oxidation on the relaxation behavior of the entire film was investigated by a comparison of capped and uncapped samp...
Article
The authors demonstrate in situ high-resolution x-ray diffraction applied during heteroepitaxy on (001)GaAs for instant layer characterization. The current thickness, composition, strain, and relaxation dynamics of pseudomorphic layers are precisely determined from qz scans at the (113) reflection measured at a molecular beam epitaxy chamber with a...
Article
Full-text available
We report on Au catalyst-assisted molecular beam epitaxy growth and properties of pure ZnSe and ZnSe/CdSe superlattice nanowires. In particular, we concentrate our attention on the morphological characterization by transmission and scanning electron microscopy of pure ZnSe NWs and we compare their optical properties with those of ZnSe/CdSe superlat...
Article
The morphology, composition, and optical properties of CdSe/ZnSe quantum dots (QDs), grown by conventional molecular beam epitaxy (MBE) and two other variants of the same (low temperature MBE and in-situ annealing), have been compared based on the results of high resolution X-ray diffraction (HRXRD), low temperature micro- and ensemble-photolumines...
Article
Full-text available
The formation of CdSe/ZnSe quantum dots by a method combining a low temperature MBE growth of a CdSe layer and its subsequent in-situ annealing at temperatures between 280-340ºC has been studied. The thermal treatment results in a re-organization of the surface from a nearly two-dimensional layer to an ensemble of three-dimensional dot-like feature...
Article
Full-text available
The structural and magnetic properties of NiMnSb films, 5–120 nm thick, grown on InGaAs/InP(001) substrates by molecular-beam epitaxy, were studied by x-ray diffraction, transmission electron microscopy (TEM), and ferromagnetic resonance (FMR) techniques. X-ray diffraction and TEM studies show that the NiMnSb films had the expected half-Heusler str...
Article
Warum eigentlich Schattenmasken als neues alternatives Verfahren zur lateralen Strukturierung? Alle bislang üblichen Verfahren zur Herstellung lateral begrenzter Halbleiter-Kristalle strukturieren die zuvor epitaktisch flächig aufgewachsenen Schichten nachträglich. Hierdurch können Probleme entstehen. Etwa erzeugen nach einem nasschemischen Ätzproz...
Article
In this contribution a model for the elastic relaxation of Al0.5Ga0.5As and ZnSe wire structures, respectively, is presented. The wire structures extend along [110] and were characterized by high resolution x-ray diffraction experiments. Based on Monte Carlo simulations, the wire structures, which have been simulated at atomic resolution, were rela...
Article
We investigate molecular beam epitaxy of II–VI materials through GaAs/AlGaAs epitaxial shadow masks on GaAs(001) substrate. Scanning electron and atomic force microscopy are employed to investigate the effects of surface diffusion, faceting, partial shadow (minimized in the experimental set-up), and material deposition on the mask (closing of apert...
Article
We present results on the elastic relaxation behaviour of II-VI and III-V wire structures. The structures were obtained by wet etching of extended layers as well as shadow mask assisted Selected Area Epitaxy (SAE). The SAE wire structures are especially interesting since they are grown directly, do not require treatment after the growth process, an...
Article
We developed a real-time in situ x-ray Bragg diffraction technique for monitoring epitaxial growth. In our setup, the x-ray diffraction requirement of an extremely exact sample adjustment and an angular scan of sample and detector are circumvented by using a slightly divergent x-ray beam and observing an extremely asymmetric Bragg reflection with a...
Article
Full-text available
An effective method for controlling the position and number of self-assembled quantum dots (QDs) grown by molecular-beam epitaxy has been developed. Epitaxially grown shadow masks are used to realize selective area growth, which exploits different incidence angles of the molecular beams. We applied this method to control the position and number of...
Article
We have investigated the underlying electronic and structural effects of the previously published influence of a Te substrate pretreatment on the heterovalent ZnSe/GaAs(1 0 0) interface [Appl. Phys. Lett. 78 (2001) 1867]. For this purpose, photoelectron spectroscopy and density functional theory calculations were employed. To determine the valence...
Article
We investigate the geometric deposition of compound semiconductors through shallow shadow masks as a means to obtain 3D nanostructures in situ. A high spatial resolution in the order of 10 nm and good material quality are achieved by epitaxy with directed molecular beams through stationary masks. Based on our study of growth dynamics new processes...
Article
We investigate molecular beam epitaxy (MBE) regrowth through shadow masks developed from AlGaAs/GaAs layers on GaAs [001] substrates. Adjusting the directions of the molecular beams relative to the masks results in in situ lateral structuring. This enables us to modify doping and composition within one layer, to shift it laterally, and even to spli...
Article
We discuss the possibilities of MBE regrowth through shadow masks for semiconductor technology. II–VI and III–V compound semiconductors have been deposited into fixed shadow masks developed from AlGaAs/GaAs layers on GaAs [0 0 1] substrates. Adjusting the directions of the molecular beams relative to the masks enables in situ lateral structuring by...
Article
Thin pseudomorphic ZnSe films can be grown by molecular beam epitaxy on GaAs(001) up to a layer thickness of about 100 nm. Above this critical thickness the topmost layers relax by forming large mosaic-domains which are tilted by ≈ 0.2° with respect to the macroscopic surface. This is evidenced by satellites arising in discrete directions close to...
Article
A selective dry etching technique for II–VI semiconductors based on ZnSe has been developed by using thermally assisted electron cyclotron resonance etching with a gas mixture of Ar, Cl2, and BCl3. While the etching process is found to be almost nonselective between ZnSe and MgZnSSe at low (≪100 °C) and high (≫220 °C) substrate temperatures, a stro...
Article
For the heterovalent system ZnSe/GaAs(100), we have investigated the influence of a Te pretreatment of the substrate on the electronic structure of the interface by photoelectron spectroscopy. We have paid special attention to correctly determine the valence band maximum in a k-resolved fashion, including the use of photon energies which enable exc...
Article
We present a novel method to grow sharply defined self-organized nanostructures into UHV-compatible Al50Ga50As-GaAs shadow masks. The lateral control of epitaxial growth is achieved by self-organized as well as the masks’geometry. We show that we obtain prism-shaped wire structures with triangular cross sections. These wires exhibit well-defined (1...
Article
This paper presents high-resolution x-ray diffraction measurements of shadow masks. Shadow masks are used for in situ growing of one- and zero-dimensional semiconductor heterostructures. The masks are produced by selective etching of AlxGa1-xAs-GaAs heterostructures. The lateral dimension of these structures is in the range 10-300 µm; the thickness...
Article
This contribution reports about the first in situ lateral structured II–VI semiconductors grown with molecular beam epitaxy into Al50Ga50As–GaAs shadow masks. The lateral control is achieved by tilting the sample manipulator with respect to the molecular beam emanated by the effusion cells. Thus wire structures can be grown whose width depends on t...
Article
We propose a new method to obtain well defined nanostructures in situ during MBE growth. The method is based on the MBE growth over UHV compatible shadow edges. These edges define a geometrical shadow region which can be modified by tilting the sample with respect to the direction of the molecular beams, so that layers with different lateral extens...
Article
The energy gap of zinc-blende epitaxial layers grown by molecular beam epitaxy has been determined over a wide range of composition using optical transmission and reflection and an empirical formula for the energy gap at room temperature is also given. In addition in situ spectroscopic ellipsometry was used in order to determine the temperature dep...
Article
ZnSe p-i-n-type photodiodes were grown on GaAs substrates using molecular beam epitaxy technique. ZnCl2 was used for n-doping and plasma-excited N2 for p-doping. The p-side contact consists of 200 nm Au in situ deposited on a 20 nm highly doped ZnTe layer on top of the p-doped ZnSe forming a well-defined Schottky barrier, as can be confirmed by cur...
Article
ZnTeZn(S,Te) short-period superlattices have been grown on (0 0 1) GaAs-substrates with very good structural quality. The growth conditions were found to be quite reproducible, leading to a series of samples with periods between 12 and 29 Å. Characterisation of the samples with high-resolution X-ray diffraction confirmed the high structural qualit...

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