Th. Schäpers

Effect of the heterointerface on the spin splitting in modulation doped InxGa1−xAs/InP quantum wells for B→0

Spin splitting of conduction band electrons in In0.53Ga0.47As/In0.77Ga0.23As/InP heterostructures due to spin-orbit coupling is studied by performing Shubnikov–de Haas measurements on nongated and gated Hall bars. From an analysis of the beating pattern in the Shubnikov–de Haas oscillations, the spin-orbit coupling constant is determined. For a symmetric sample no beating pattern and thus no spin splitting is observed. This demonstrates that the k3 contribution to the spin-orbit coupling constant can be neglected. By applying an envelope function theory it is shown that the major contribution to the Rashba spin-orbit coupling originates from the band offset at the interface of the quantum well. Using gated Hall bar structures it is possible to alter the spin-orbit coupling by application of an appropriate gate voltage. A more negative gate voltage leads to a more pronounced asymmetry of the quantum well, which gives rise to a stronger spin-orbit coupling.

Hall effect measurements on InAs nanowires

We have processed Hall contacts on InAs nanowires grown by molecular beam epitaxy using an electron beam lithography process with an extremely high alignment accuracy. The carrier concentrations determined from the Hall effect measurements on these nanowires are lower by a factor of about 4 in comparison with those measured by the common field-effect technique. The results are used to evaluate quantitatively the charging effect of the interface and surface states.

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...

Electronic Phase Coherence in InAs Nanowires

Magnetotransport measurements at low temperatures have been performed on InAs nanowires grown by In-assisted molecular beam epitaxy. Information on the electron phase coherence is obtained from universal conductance fluctuations measured in a perpendicular magnetic field. By analysis of the universal conductance fluctuations pattern of a series of nanowires of different length, the phase-coherence length could be determined quantitatively. Furthermore, indications of a pronounced flux cancelation effect were found, which is attributed to the topology of the nanowire. Additionally, we present measurements in a parallel configuration between wire and magnetic field. In contrast to previous results on InN and InAs nanowires, we do not find periodic oscillations of the magnetoconductance in this configuration. An explanation of this behavior is suggested in terms of the high density of stacking faults present in our InAs wires.

Self-catalyzed VLS grown InAs nanowires with twinning superlattices.

We report on the self-catalyzed growth of InAs nanowires by molecular beam epitaxy on GaAs substrates covered by a thin silicon oxide layer. Clear evidence is presented to demonstrate that, under our experimental conditions, the growth takes place by the vapor-liquid-solid (VLS) mechanism via an In droplet. The nanowire growth rate is controlled by the arsenic pressure while the diameter depends mainly on the In rate. The contact angle of the In droplet is smaller than that of the Ga droplet involved in the growth of GaAs nanowires, resulting in much lower growth rates. The crystal structure of the VLS grown InAs nanowires is zinc blende with regularly spaced rotational twins forming a twinning superlattice.

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.

Electrical transport properties of single undoped and n-type doped InN nanowires

Electrical transport properties of undoped and n-type doped InN nanowires grown by molecular beam epitaxy were studied by current-voltage and back-gate field-effect transistor measurements. The current-voltage characteristics show ohmic behavior in the temperature range between 4 and 300 K. Down to about 120 K a linear decrease in resistance with temperature is observed. The investigation of a large number of nanowires revealed for undoped as well as doped wires an approximately linear relation between the normalized conductance and diameter for wires with a diameter below 100 nm. This shows that the main conduction takes place in the tubular surface accumulation layer of the wires. In contrast, for doped wires with a diameter larger than 100 nm a quadratic dependence of conduction on the diameter was found, which is attributed to bulk conductance as the main contribution. The successful doping of the wires is confirmed by an enhanced conduction and by the results of the back-gate field-effect transistor measurements.

Nanoimprint and selective-area MOVPE for growth of GaAs/InAs core/shell nanowires

We report on the technology and growth optimization of GaAs/InAs core/shell nanowires. The GaAs nanowire cores were grown selectively by metal organic vapor phase epitaxy (SA-MOVPE) on SiO(2) masked GaAs (111)B templates. These were structured by a complete thermal nanoimprint lithography process, which is presented in detail. The influence of the subsequent InAs shell growth temperature on the shell morphology and crystal structure was investigated by scanning and transmission electron microscopy in order to obtain the desired homogeneous and uniform InAs overgrowth. At the optimal growth temperature, the InAs shell adopted the morphology and crystal structure of the underlying GaAs core and was perfectly uniform.

Weak antilocalization in a polarization-doped AlxGa1−xN∕GaN heterostructure with single subband occupation

Spin-orbit scattering in a polarization-doped Al0.30Ga0.70N∕GaN two-dimensional electron gas with one occupied subband is studied at low temperatures. At low magnetic fields weak antilocalization is observed, which proves that spin-orbit scattering occurs in the two-dimensional electron gas. From measurements at various temperatures the elastic scattering time τtr, the dephasing time τϕ, and the spin-orbit scattering time τso are extracted. Measurements in tilted magnetic fields were performed, in order to separate spin and orbital effects.

Optimization of modulation‐doped Ga1−xInxAs/InP heterostructures towards extremely high mobilities

This paper presents a study of the electrical and structural properties of inverted modulation‐doped GaInAs/InP heterostructures grown by low‐pressure metalorganic vapor phase epitaxy. First, the thickness of the GaInAs layer was optimized in lattice‐matched samples to find the smallest thickness in which high Hall mobility is observed. Next, in a section closest to the InP the In content was varied. A steady increase of mobility with indium composition was observed. A maximum of 450 000 and 15 500 cm2/V s was obtained for a 10‐nm‐thick Ga1−xInxAs layer with x=0.77 at 6 and 300 K, respectively. Channels with higher indium content exceed the critical thickness and mobility drops off sharply. The decreasing mobility correlates with the formation of misfit dislocations at the interface indicating increasing scattering processes of the GaInAs layer.