F. Sfigakis

All-electric all-semiconductor spin field-effect transistors

The spin field-effect transistor envisioned by Datta and Das opens a gateway to spin information processing. Although the coherent manipulation of electron spins in semiconductors is now possible, the realization of a functional spin field-effect transistor for information processing has yet to be achieved, owing to several fundamental challenges such as the low spin-injection efficiency due to resistance mismatch, spin relaxation and the spread of spin precession angles. Alternative spin transistor designs have therefore been proposed, but these differ from the field-effect transistor concept and require the use of optical or magnetic elements, which pose difficulties for incorporation into integrated circuits. Here, we present an all-electric and all-semiconductor spin field-effect transistor in which these obstacles are overcome by using two quantum point contacts as spin injectors and detectors. Distinct engineering architectures of spin-orbit coupling are exploited for the quantum point contacts and the central semiconductor channel to achieve complete control of the electron spins (spin injection, manipulation and detection) in a purely electrical manner. Such a device is compatible with large-scale integration and holds promise for future spintronic devices for information processing.

Controlled electroplating and electromigration in nickel electrodes for nanogap formation

We report the fabrication of nickel nanospaced electrodes by electroplating and electromigration for nanoelectronic devices. Using a conventional electrochemical cell, nanogaps can be obtained by controlling the plating time alone and after a careful optimization of electrodeposition parameters such as electrolyte bath, applied potential, cleaning, etc. During the process, the gap width decreases exponentially with time until the electrode gaps are completely bridged. Once the bridge is formed, the ex situ electromigration technique can reopen the nanogap. When the gap is ∼ 1 nm, tunneling current-voltage characterization shows asymmetry which can be corrected by an external magnetic field. This suggests that charge transfer in the nickel electrodes depends on the orientation of magnetic moments.

Zero-bias anomaly in quantum wires

We use quantum wires fabricated on undoped GaAs/AlGaAs heterostructures in which the average impurity separation is greater than the device size to compare the behavior of the zero-bias anomaly against predictions from Kondo and spin-polarization models. Both theories display shortcomings, the most dramatic of which is the linear electron-density dependence of the zero-bias anomaly spin splitting at fixed magnetic field

Fabrication and characterization of ambipolar devices on an undoped AlGaAs/GaAs heterostructure

B

Quantum transport in In0.75Ga0.25As quantum wires

and the suppression of the Zeeman effect at pinch off.

Distinguishing impurity concentrations in GaAs and AlGaAs using very shallow undoped heterostructures

We have fabricated AlGaAs/GaAs heterostructure devices in which the conduction channel can be populated with either electrons or holes simply by changing the polarity of a gate bias. The heterostructures are entirely undoped, and carriers are, instead, induced electrostatically. We use these devices to perform a direct comparison of the scattering mechanisms of two-dimensional electrons (μpeak = 4 × 106 cm2/Vs) and holes (μpeak = 0.8 × 106 cm2/Vs) in the same conduction channel with nominally identical disorder potentials. We find significant discrepancies between electron and hole scattering, with the hole mobility being considerably lower than expected from simple theory.

Demonstration and characterization of an ambipolar high mobility transistor in an undoped GaAs/AlGaAs quantum well

In addition to quantized conductance plateaus at integer multiples of 2e2∕h, the differential conductance G=dI∕dV shows plateaus at 0.25(2e2∕h) and 0.75(2e2∕h) under applied source-drain bias in In0.75Ga0.25As quantum wires defined by insulated split gates. This observation is consistent with a spin-gap model for the 0.7 structure. Using a tilted magnetic field to induce Landau level crossings, the g factor was measured to be ∼9 by the coincidence method. This material, with a mobility of 1.8×105cm2∕Vs at a carrier density of 1.4×1011cm−2, may prove useful for further study of electron-electron interaction effects in quantum wires.

Switching between attractive and repulsive Coulomb-interaction-mediated drag in an ambipolar GaAs/AlGaAs bilayer device

We demonstrate a method of making a very shallow, gateable, undoped 2-dimensional electron gas. We have developed a method of making very low resistivity contacts to these structures and systematically studied the evolution of the mobility as a function of the depth of the 2DEG (from 300nm to 30nm). We demonstrate a way of extracting quantitative information about the background impurity concentration in GaAs and AlGaAs, the interface roughness and the charge in the surface states from the data. This information is very useful from the perspective of molecular beam epitaxy (MBE) growth. It is difficult to fabricate such shallow high-mobility 2DEGs using modulation doping due to the need to have a large enough spacer layer to reduce scattering and switching noise from remote ionsied dopants.

Spin effects in one-dimensional systems

We report an ambipolar device fabricated in undoped GaAs/AlGaAs quantum wells (widths 10 and 25 nm) with front and backgates that allow almost two orders of magnitude in density to be accessed in the same device (7×109cm−2 to 5×1011cm−2). By changing the well width, the relative electron and hole mobilities can be tuned, approaching similar velocities. We describe an approach to fully characterize the quantum well, including the impurity backgrounds and both the upper and lower interfaces, making use of the ability to control the carrier density and the position of the wavefunction independently over a wide range.

Linear non-hysteretic gating of a very high density 2DEG in an undoped metal–semiconductor–metal sandwich structure

We present measurements of Coulomb drag in an ambipolar GaAs/AlGaAs double quantum well structure that can be configured as both an electron-hole bilayer and a hole-hole bilayer, with an insulating barrier of only 10 nm between the two quantum wells. Coulomb drag resistivity is a direct measure of the strength of interlayer particle-particle interactions. We explore the strongly interacting regime of low carrier densities (2D interaction parameter rs up to 14). Our ambipolar device design allows a comparison between the effects of the attractive electron-hole and repulsive hole-hole interactions and also shows the effects of the different effective masses of electrons and holes in GaAs.