N. Demarina

Effect of Si-doping on InAs nanowire transport and morphology

The effect of Si-doping on the morphology, structure, and transport properties of nanowires was investigated. The nanowires were deposited by selective-area metal organic vapor phase epitaxy in an N2 ambient. It is observed that doping systematically affects the nanowire morphology but not the structure of the nanowires. However, the transport properties of the wires are greatly affected. Room-temperature four-terminal measurements show that with an increasing dopant supply the conductivity monotonously increases. For the highest doping level the conductivity is higher by a factor of 25 compared to only intrinsically doped reference nanowires. By means of back-gate field-effect transistor measurements it was confirmed that the doping results in an increased carrier concentration. Temperature dependent resistance measurements reveal, for lower doping concentrations, a thermally activated semiconductor-type increase of the conductivity. In contrast, the nanowires with the highest doping concentration show a...

Spin-orbit coupling and phase-coherence in InAs nanowires

We investigated the magnetotransport of InAs nanowires grown by selective area metal-organic vapor phase epitaxy. In the temperature range between 0.5 and 30 K reproducible fluctuations in the conductance upon variation of the magnetic field or the back-gate voltage are observed, which are attributed to electron interference effects in small disordered conductors. From the correlation field of the magnetoconductance fluctuations the phase-coherence length lis determined. At the lowest temperatures lis found to be at least 300 nm, while for temperatures exceeding 2 K a monotonous decrease of lwith temperature is observed. A direct observation of the weak antilocalization effect indicating the presence of spin-orbit coupling is masked by the strong magnetoconductance fluctua- tions. However, by averaging the magnetoconductance over a range of gate voltages a clear peak in the magnetoconductance due to the weak antilocalization effect was resolved. By comparison of the experimental data to simulations based on a recursive two-dimensional Greens function approach a spin-orbit scattering length of approximately 70 nm was extracted, indicating the presence of strong spin-orbit coupling.

Realization of nanoscaled tubular conductors by means of GaAs/InAs core/shell nanowires

We investigated the transport properties of GaAs/InAs core/shell nanowires grown by molecular beam epitaxy. Owing to the band alignment between GaAs and InAs, electrons are accumulated in the InAs shell as long as the shell thickness exceeds 12 nm. By performing simulations using a Schrödinger-Poisson solver, it is confirmed that confined states are present in the InAs shell, which are depleted if the shell thickness is below a threshold value. The existence of a tubular-shaped conductor is proved by performing magnetoconductance measurements at low temperatures. Here, flux periodic conductance oscillations are observed which can be attributed to transport in one-dimensional channels based on angular momentum states.

Preparation of Ohmic contacts to GaAs/AlGaAs-core/shell-nanowires

Ohmic contacts to GaAs/AlGaAs core/shell nanowires are prepared by using a Ni/AuGe/Ni/Au layer system. In contrast to Ohmic contacts to planar GaAs/AlGaAs layer systems here, relatively low alloying temperatures are used in cylindrical geometry. Lowest resistances are found for annealing temperatures of 320 °C and 340 °C. For annealing temperatures exceeding 360 °C, the nanowires degraded completely. Nanowires annealed under optimized conditions preserved their Ohmic characteristics even down to temperatures of 4 K.

Phase coherent transport in hollow InAs nanowires

Hollow InAs nanowires are produced from GaAs/InAs core/shell nanowires by wet chemical etching of the GaAs core. At room temperature, the resistivity of several nanowires is measured before and after removal of the GaAs core. The observed change in resistivity is explained by simulating the electronic states in both structures. At cryogenic temperatures, quantum transport in hollow InAs nanowires is studied. Flux periodic conductance oscillations are observed when the magnetic field is oriented parallel to the nanowire axis.

Quantum phenomena during electron transport in InAs nanowires

We investigated the quantum transport in InAs nanowires. From the universal conductance fluctuations at 0.5 K a phase-coherence length of 300 nm was extracted. By averaging the gate-dependent conductance fluctuations a magnetoconductance peak due to weak antilocalization was found indicating the presence of spin-orbit coupling. In a nanowire quantum dot formed by three gate fingers single electron tunneling was confirmed