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- ItemMultiple lobes in the far-field distribution of terahertz quantum-cascade lasers due to self-interference(New York : American Institute of Physics, 2016) Röben, B.; Wienold, M.; Schrottke, L.; Grahn, H.T.The far-field distribution of the emission intensity of terahertz (THz) quantumcascade lasers (QCLs) frequently exhibits multiple lobes instead of a single-lobed Gaussian distribution. We show that such multiple lobes can result from selfinterference related to the typically large beam divergence of THz QCLs and the presence of an inevitable cryogenic operation environment including optical windows. We develop a quantitative model to reproduce the multiple lobes. We also demonstrate how a single-lobed far-field distribution can be achieved.
- ItemThe influence of Mg doping on the nucleation of self-induced GaN nanowires(New York : American Institute of Physics, 2012) Limbach, F.; Caterino, R.; Gotschke, T.; Stoica, T.; Calarco, R.; Geelhaar, L.; Riechert, H.GaN nanowires were grown without any catalyst by plasma-assisted molecular beam epitaxy. Under supply of Mg, nanowire nucleation is faster, the areal density of wires increases to a higher value, and nanowire coalescence is more pronounced than without Mg. During nanowire nucleation the Ga desorption was monitored insitu by line-of-sight quadrupolemass spectrometry for various substrate temperatures. Nucleation energies of 4.0±0.3 eV and 3.2±0.3 eV without and with Mg supply were deduced, respectively. This effect has to be taken into account for the fabrication of nanowire devices and could be employed to tune the NW areal density.
- ItemThermal annealing studies of GeTe-Sb2Te3 alloys with multiple interfaces(New York : American Institute of Physics, 2017) Bragaglia, Valeria; Mio, Antonio M.; Calarco, RaffaellaA high degree of vacancy ordering is obtained by annealing amorphous GeTe-Sb2Te3 (GST) alloys deposited on a crystalline substrate, which acts as a template for the crystallization. Under annealing the material evolves from amorphous to disordered rocksalt, to ordered rocksalt with vacancies arranged into (111) oriented layers, and finally converts into the stable trigonal phase. The role of the interface in respect to the formation of an ordered crystalline phase is studied by comparing the transformation stages of crystalline GST with and without a capping layer. The capping layer offers another crystallization interface, which harms the overall crystalline quality.
- ItemSchottky contacts to In2O3(New York : American Institute of Physics, 2014) von Wenckstern, H.; Splith, D.; Schmidt, F.; Grundmann, M.; Bierwagen, O.; Speck, J.S.n-type binary compound semiconductors such as InN, InAs, or In2O3 are especial because the branch-point energy or charge neutrality level lies within the conduction band. Their tendency to form a surface electron accumulation layer prevents the formation of rectifying Schottky contacts. Utilizing a reactive sputtering process in an oxygen-containing atmosphere, we demonstrate Schottky barrier diodes on indium oxide thin films with rectifying properties being sufficient for space charge layer spectroscopy. Conventional non-reactive sputtering resulted in ohmic contacts. We compare the rectification of Pt, Pd, and Au Schottky contacts on In2O3 and discuss temperature-dependent current-voltage characteristics of Pt/In2O3 in detail. The results substantiate the picture of oxygen vacancies being the source of electrons accumulating at the surface, however, the position of the charge neutrality level and/or the prediction of Schottky barrier heights from it are questioned.
- ItemMagnetic properties of GaAs-Fe3Si core-shell nanowires — A comparison of ensemble and single nanowire investigation(New York : American Institute of Physics, 2017) Hilse, Maria; Jenichen, Bernd; Herfort, JensOn the basis of semiconductor-ferromagnet GaAs-Fe3Si core-shell nanowires (Nws) we compare the facilities of magnetic Nw ensemble measurements by superconducting quantum interference device magnetometry versus investigations on single Nws by magnetic force microscopy and computational micromagnetic modeling. Where a careful analysis of ensemble measurements backed up by transmission electron microscopy gave no insights on the properties of the Nw shells, single Nw investigation turned out to be absolutely essential.
- ItemKinetics versus thermodynamics of the metal incorporation in molecular beam epitaxy of (InxGa1−x)2O3(New York : American Institute of Physics, 2016) Vogt, Patrick; Bierwagen, OliverWe present a detailed study of the reaction kinetics and thermodynamics of the plasma-assisted oxide molecular beam epitaxy of the ternary compound (InxGa1−x)2O3 for 0 ≤ x ≤ 1. We measured the growth rate of the alloy in situ by laser reflectrometry as a function of growth temperature T G for different metal-to-oxygen flux ratios r Me, and nominal In concentrations x nom in the metal flux. We determined ex situ the In and Ga concentrations in the grown film by energy dispersive X-ray spectroscopy. The measured In concentration x shows a strong dependence on the growth parameters T G, r Me, and x nom whereas growth on different co-loaded substrates shows that in the macroscopic regime of ∼μm3 x does neither depend on the detailed layer crystallinity nor on crystal orientation. The data unveil that, in presence of In, Ga incorporation is kinetically limited by Ga2O desorption the same way as during Ga2O 3 growth. In contrast, In incorporation during ternary growth is thermodynamically suppressed by the presence of Ga due to stronger Ga–O bonds. Our experiments revealed that Ga adatoms decompose/etch the In–O bonds whereas In adatoms do not decompose/etch the Ga–O bonds. This result is supported by our thermochemical calculations. In addition we found that a low T G and/or excessively low r Me kinetically enables In incorporation into (InxGa1−x)2O3. This study may help growing high-quality ternary compounds (InxGa1−x)2O3 allowing band gap engineering over the range of 2.7–4.7 eV.
- ItemContinuous tuning of two-section, single-mode terahertz quantum-cascade lasers by fiber-coupled, near-infrared illumination(New York : American Institute of Physics, 2017) Hempel, Martin; Röben, Benjamin; Niehle, Michael; Schrottke, Lutz; Trampert, Achim; Grahn, Holger T.The dynamical tuning due to rear facet illumination of single-mode, terahertz (THz) quantum-cascade lasers (QCLs) which employ distributed feedback gratings are compared to the tuning of single-mode QCLs based on two-section cavities. The THz QCLs under investigation emit in the range of 3 to 4.7 THz. The tuning is achieved by illuminating the rear facet of the QCL with a fiber-coupled light source emitting at 777 nm. Tuning ranges of 5.0 and 11.9 GHz under continuous-wave and pulsed operation, respectively, are demonstrated for a single-mode, two-section cavity QCL emitting at about 3.1 THz, which exhibits a side-mode suppression ratio better than -25 dB.
- ItemTextured Sb2Te3 films and GeTe/Sb2Te3 superlattices grown on amorphous substrates by molecular beam epitaxy(New York : American Institute of Physics, 2017) Boschker, Jos E.; Tisbi, E.; Placidi, E.; Momand, Jamo; Redaelli, Andrea; Kooi, Bart J.; Arciprete, Fabrizio; Calarco, RaffaellaThe realization of textured films of 2-dimensionally (2D) bonded materials on amorphous substrates is important for the integration of this material class with silicon based technology. Here, we demonstrate the successful growth by molecular beam epitaxy of textured Sb2Te3 films and GeTe/Sb2Te3 superlattices on two types of amorphous substrates: carbon and SiO2. X-ray diffraction measurements reveal that the out-of-plane alignment of grains in the layers has a mosaic spread with a full width half maximum of 2.8°. We show that a good texture on SiO2 is only obtained for an appropriate surface preparation, which can be performed by ex situ exposure to Ar+ ions or by in situ exposure to an electron beam. X-ray photoelectron spectroscopy reveals that this surface preparation procedure results in reduced oxygen content. Finally, it is observed that film delamination can occur when a capping layer is deposited on top of a superlattice with a good texture. This is attributed to the stress in the capping layer and can be prevented by using optimized deposition conditions of the capping layer. The obtained results are also relevant to the growth of other 2D materials on amorphous substrates.
- ItemPerpendicular magnetic anisotropy in the Heusler alloy Co2TiSi/GaAs(001) hybrid structure(New York : American Institute of Physics, 2015) Dau, M.T.; Jenichen, B.; Herfort, J.Investigation of the thickness dependence of the magnetic anisotropy in B2-type Co2TiSi films on GaAs(001), shows a pronounced perpendicular magnetic anisotropy at 10 K for thicknesses up to 13.5 nm. We have evidenced that the interfacial anisotropy induced by interface clusters has a strong influence on the perpendicular magnetic anisotropy of this hybrid structure, especially at temperatures lower than the blocking temperature of the clusters (28 K). However, as this influence can be ruled out at higher temperatures, the perpendicular magnetic anisotropy which is found to persist up to room-temperature can be ascribed to the magnetic properties of the Co2TiSi films. For thicknesses larger than 15.0 nm, we observe an alignment of the magnetic easy axis parallel to the sample surface, which is most likely due to the shape anisotropy and the film structure.
- ItemDiffraction at GaAs/Fe3Si core/shell nanowires: The formation of nanofacets(Cambridge : arXiv, 2016) Jenichen, B.; Hanke, M.; Hilse, M.; Herfort, J.; Trampert, A.; Erwin, S.C.GaAs/Fe3Si core/shell nanowire structures were fabricated by molecular-beam epitaxy on oxidized Si(111) substrates and investigated by synchrotron x-ray diffraction. The surfaces of the Fe3Si shells exhibit nanofacets. These facets consist of well pronounced Fe3Si{111} planes. Density functional theory reveals that the Si-terminated Fe3Si{111} surface has the lowest energy in agreement with the experimental findings. We can analyze the x-ray diffuse scattering and diffraction of the ensemble of nanowires avoiding the signal of the substrate and poly-crystalline films located between the wires. Fe3Si nanofacets cause streaks in the x-ray reciprocal space map rotated by an azimuthal angle of 30° compared with those of bare GaAs nanowires. In the corresponding TEM micrograph the facets are revealed only if the incident electron beam is oriented along [1 1 ̄ 0] in accordance with the x-ray results. Additional maxima in the x-ray scans indicate the onset of chemical reactions between Fe3Si shells and GaAs cores occurring at increased growth temperatures.