Thomas Schäpers

Molecular beam epitaxy growth of GaAs/InAs core-shell nanowires and fabrication of InAs nanotubes.

We present results about the growth of GaAs/InAs core-shell nanowires (NWs) using molecular beam epitaxy. The core is grown via the Ga droplet-assisted growth mechanism. For a homogeneous growth of the InAs shell, the As(4) flux and substrate temperature are critical. The shell growth starts with InAs islands along the NW core, which increase in time and merge giving finally a continuous and smooth layer. At the top of the NWs, a small part of the core is free of InAs indicating a crystal phase selective growth. This allows a precise measurement of the shell thickness and the fabrication of InAs nanotubes by selective etching. The strain relaxation in the shell occurs mainly via the formation of misfit dislocations and saturates at ~80%. Additionally, other types of defects are observed, namely stacking faults transferred from the core or formed in the shell, and threading dislocations.

Flux Quantization Effects in InN Nanowires

InN nanowires, grown by plasma-enhanced molecular beam epitaxy, were investigated by means of magnetotransport. By performing temperature-dependent transport measurements and current measurements on a large number of nanowires of different dimensions, it is proven that the carrier transport mainly takes place in a tube-like surface electron gas. Measurements on three representative nanowires under an axially oriented magnetic field revealed pronounced magnetoconductance oscillations with a periodicity corresponding to a single magnetic flux quantum. The periodicity is explained by the effect of the magnetic flux penetrating the coherent circular quantum states in the InN nanowires, rather than by Aharonov-Bohm type interferences. The occurrence of the single magnetic flux quantum periodicity is attributed to the magnetic flux dependence of phase-coherent circular states with different angular momentum quantum numbers forming the one-dimensional transport channels. These phase coherent states can exist because of the almost ideal crystalline properties of the InN nanowires prepared by self-assembled growth.

Field effect transistor based on single crystalline InSb nanowire

InSb nanowires with zinc-blende crystal structure and precise stoichiometry are synthesized via pulsed-laser chemical vapor deposition. Raman spectroscopy shows Stokes and anti-Stokes peaks of transverse-optical mode with asymmetric broadening. The nanowire demonstrates n-type semiconductor behavior. Enhanced surface scattering due to size confinement leads to reduced electron mobility.

Realization of a vertical topological p-n junction in epitaxial Sb2Te3/Bi2Te3 heterostructures.

Three-dimensional (3D) topological insulators are a new state of quantum matter, which exhibits both a bulk band structure with an insulating energy gap as well as metallic spin-polarized Dirac fermion states when interfaced with a topologically trivial material. There have been various attempts to tune the Dirac point to a desired energetic position for exploring its unusual quantum properties. Here we show a direct experimental proof by angle-resolved photoemission of the realization of a vertical topological p–n junction made of a heterostructure of two different binary 3D TI materials Bi2Te3 and Sb2Te3 epitaxially grown on Si(111). We demonstrate that the chemical potential is tunable by about 200 meV when decreasing the upper Sb2Te3 layer thickness from 25 to 6 quintuple layers without applying any external bias. These results make it realistic to observe the topological exciton condensate and pave the way for exploring other exotic quantum phenomena in the near future.

Amphoteric nature of Sn in CdS nanowires.

High-quality CdS nanowires with uniform Sn doping were synthesized using a Sn-catalyzed chemical vapor deposition method. X-ray diffraction and transmission electron microscopy demonstrate the single crystalline wurtzite structure of the CdS/Sn nanowires. Both donor and acceptor levels, which originate from the amphoteric nature of Sn in II-VI semiconductors, are identified using low-temperature microphotoluminescence. This self-compensation effect was cross examined by gate modulation and temperature-dependent electrical transport measurement. They show an overall n-type behavior with relatively low carrier concentration and low carrier mobilities. Moreover, two different donor levels due to intrinsic and extrinsic doping could be distinguished. They agree well with both the electrical and optical data.

Manipulating InAs nanowires with submicrometer precision.

InAs nanowires are grown epitaxially by catalyst-free metal organic vapor phase epitaxy and are subsequently positioned with a lateral accuracy of less than 1 μm using simple adhesion forces between the nanowires and an indium tip. The technique, requiring only an optical microscope, is used to place individual nanowires onto the corner of a cleaved-edge wafer as well as across predefined holes in Si(3)N(4) membranes. The precision of the method is limited by the stability of the micromanipulators and the precision of the optical microscope.

Enhanced spin-orbit scattering length in narrow AlxGa1-xN/GaN wires

The magnetotransport in a set of identical parallel AlGaN/GaN quantum wire structures was investigated. The width of the wires was ranging between 1110 nm and 340 nm. For all sets of wires clear Shubnikov--de Haas oscillations are observed. We find that the electron concentration and mobility is approximately the same for all wires, confirming that the electron gas in the AlGaN/GaN heterostructure is not deteriorated by the fabrication procedure of the wire structures. For the wider quantum wires the weak antilocalization effect is clearly observed, indicating the presence of spin-orbit coupling. For narrow quantum wires with an effective electrical width below 250 nm the weak antilocalization effect is suppressed. By comparing the experimental data to a theoretical model for quasi one-dimensional structures we come to the conclusion that the spin-orbit scattering length is enhanced in narrow wires.

Optical and electrical properties of gold nanowires synthesized by electrochemical deposition

Gold nanowire arrays with different sizes were fabricated by electrochemical deposition in etched ion-track templates. The diameter of the gold nanowires between 30 and 130 nm could be well adjusted by pore sizes in the templates through etching time. Single-crystalline nanowires were achieved by changing the parameters of electrochemical deposition. The morphology and crystal structure of the fabricated gold nanowires were characterized by means of scanning electron microscopy and transmission electron microscopy. The optical properties of the gold nanowire arrays embodied in templates were systematically measured by absorption spectra with a UV/Vis/NIR spectrophotometer. Due to the surface plasmon resonance effect, the extinction peaks of gold nanowire arrays possessed a red-shift with increasing wires diameter and a blue-shift with decreasing angle between incident light and nanowire arrays. The failure current density of the single gold nanowire as a function of diameter was determined and the failure...

Weak antilocalization in high mobility Ga x In 1 − x As ∕ In P two-dimensional electron gases with strong spin-orbit coupling

We have studied the spin-orbit interaction in a high mobility two-dimensional electron gas in a

Crystal Phase Transformation in Self-Assembled InAs Nanowire Junctions on Patterned Si Substrates

\mathrm{Ga}\mathrm{In}\mathrm{As}∕\mathrm{In}\mathrm{P}