Ch. Volk

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.

Low-temperature conductance of the weak junction in InAs nanowire in the field of AFM scanning gate

We have investigated the conductance of an InAs nanowire in the presence of an electrical potential created by an AFM scanning gate at liquid helium temperature. The influence of the direction of a local electrical field on the tunneling rate through a weak junction in the InAs wire is clearly observed. To explain this behavior, the redistribution of the electrons among conductive channels in the wire must be taken into account. We have confirmed that the pattern of Coulomb blockade diamonds gives the same result for the ratio of quantum dot sizes as that revealed by scanning gate imaging.

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.

LaLuO3 as a high-k gate dielectric for InAs nanowire structures

We investigated the suitability of lanthanum lutetium oxide (LaLuO3) as a gate dielectric by fabricating InAs nanowire field-effect transistors. The LaLuO3 layer was deposited by employing pulsed laser deposition. On transistors with a 1.6 µm long gate, a maximum transconductance of 11 µS at a source–drain bias voltage of 0.5 V was measured, while the threshold voltage had a value of −4.5 V. Owing to the complete coverage of the InAs nanowire by the LuLuO3 layer no significant leakage current was found. On a transistor with a 240 nm long gate short-channel effects were observed. The transfer characteristics showed a hysteretic behavior, which is attributed to charging of states at the InAs/LaLuO3 interface. We found that the threshold voltage gets reduced considerably when the temperature was decreased to 25 K. At this temperature the hysteresis in the transfer characteristics showed no dependence on the sweep rate.

The electronic transport of top subband and disordered sea in an InAs nanowire in the presence of a mobile gate

We performed measurements at helium temperatures of the electronic transport in an InAs quantum wire (R(wire) ∼ 30 kΩ) in the presence of a charged tip of an atomic force microscope serving as a mobile gate. The period and the amplitude of the observed quasi-periodic oscillations are investigated in detail as a function of electron concentration in the linear and non-linear regime. We demonstrate the influence of the tip-to-sample distance on the ability to locally affect the top subband electrons as well as the electrons in the disordered sea. Furthermore, we introduce a new method of detection of the subband occupation in an InAs wire, which allows us to evaluate the number of electrons in the conductive band of the wire.

Investigations of local electronic transport in InAs nanowires by scanning gate microscopy at liquid helium temperatures

A set of experiments dedicated to investigations of local electronic transport in undoped InAs nanowires at liquid helium temperatures in the presence of a charged atomic-force microscope tip has been presented. Both nanowires without defects and with internal tunneling barriers were studied. The measurements were performed at various carrier concentrations in the systems and opacity of contact-to-wire interfaces. The regime of Coulomb blockade is investigated in detail including negative differential conductivity of the whole system. The situation with open contacts with one tunneling barrier and undivided wire is also addressed. Special attention is devoted to recently observed quasi-periodic standing waves.

Distortions of the coulomb blockade conductance line in scanning gate measurements of inas nanowire based quantum dots

We performed measurements at helium temperatures of the electronic transport in the linear regime in an InAs quantum wire in the presence of a charged tip of an atomic force microscope (AFM) at low electron concentration. We show that at certain concentration of electrons, only two closely placed quantum dots, both in the Coulomb blockade regime, govern conductance of the whole wire. Under this condition, two types of peculiarities—wobbling and splitting—arise in the behavior of the lines of the conductance peaks of Coulomb blockade. These peculiarities are measured in quantum-wire-based structures for the first time. We explain both peculiarities as an interplay of the conductance of two quantum dots present in the wire. Detailed modeling of wobbling behavior made in the framework of the orthodox theory of Coulomb blockade demonstrates good agreement with the obtained experimental data.

Negative differential conductance in InAs wire based double quantum dot induced by a charged AFM tip

We investigate the conductance of an InAs nanowire in the nonlinear regime in the case of low electron density where the wire is split into quantum dots connected in series. The negative differential conductance in the wire is initiated by means of a charged atomic force microscope tip adjusting the transparency of the tunneling barrier between two adjoining quantum dots. We confirm that the negative differential conductance arises due to the resonant tunneling between these two adjoining quantum dots. The influence of the transparency of the blocking barriers and the relative position of energy states in the adjoining dots on a decrease of the negative differential conductance is investigated in detail.

Direct observation of standing electron waves in diffusively conducting inas nanowire

We investigate the disturbance of the InAs nanowire resistance by a conductive tip of a scanning probe micro-scope at helium temperature as a function of the tip position in close vicinity to the nanowire. At the tip displacement along the wire the resistance (Rwire ∼ 30 kΩ, what is typical for diffusive regime) demonstrates quasi-periodical oscillations with an amplitude about 3%. The period of the oscillations depends on the number of electrons in the nanowire and is consistent with expected for standing electron waves caused by ballistic electrons in the top subband of the InAs nanowire.