Cleaved Edge Overgrowth is employed to fabricate atomically precise single and coupled quantum wires. Single wires were investigated in the diffusive and ballistic transport regimes. In addition to a detailed understanding of the 1D sublevel structure in the complex CEO devices, clear indications of Luttinger liquid behavior of the one-dimensional electron systems were obtained. Coupled quantum wires were studied in the two limits of large and small separations. In the first case shallow ID potentials were chosen and an alternative description for commensurability oscillations in weakly modulated two-dimensional electron systems could be derived. For the coupled quantum wires with small separations and a large potential modulation the validity of the miniband transport picture leading to negative differential conductivity was demonstrated. (c) 2006 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.

Periodic modulation of a free two-dimensional electron system changes the bandstructure in which the electrons move. A weak modulation opens up small energy gaps in the free electron bandstructure while a strong, short-period modulation leads to the formation of well separated cosine-like minibands. We study transport in both cases using structures grown with the Cleaved-Edge-Overgrowth method. It allows to fabricate high quality two-dimensional electron systems with arbitrary modulation periods which are precise on an atomic scale. Perturbing the system only weakly with a long-period superlattice that is located some distance away from the electron layer leads to magneto-transport characteristics which exhibit each a number of different 1/B-periodic oscillations. These can be very well explained by semiclassical theory and allow the extraction of the Fermi surface of the system. The strong modulation with a short-period superlattice leads to the two-dimensional equivalent of a conventional superlattice. One very important difference lies in the stability of the expected negative differential conductance regime with respect to electric field instabilities. Geometrical considerations show that the observation of stable Bloch oscillations is possible in these systems. We will show that density dependent transport studies confirm the formation of a two-dimensional superlattice. Although negative diffential conductance, also observed in those samples, must be attributed to an inhomogeneous density distribution. (© 2004 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

Spin-related electronic phase transitions in the fractional quantum Hall regime are accompanied by a large change in resistance. Combined with their sensitivity to spin orientation of nuclei residing in the same plane as the 2D electrons, they offer a convenient electrical probe to carry out nuclear magnetometry. Despite conditions which should allow both electronic and nuclear-spin subsystems to approach thermodynamic equilibrium, we uncover for the nuclei a remarkable and strongly electronic filling-factor-dependent deviation from the anticipated thermal nuclear-spin polarization.

We have fabricated vertical field effect transistor devices using the cleaved-edge overgrowth method in the lattice-matched InGaAs/AlGaAs/GaAs-material system. These transistors contain a 20 nm thick InGaAs electron channel and have a source-drain distance of only 50 nm. The devices exhibit transistor operation at 77 K, which is obscured at room temperature by a bulk source-drain leakage current. The output and transfer characteristics are mildly influenced by short-channel effects, visible either as a loss of complete saturation in the I-V traces or as a shift of the threshold voltage in the transfer curves. The transconductance reveals a nearly constant value as high as 25 mS/mm in the quasi-saturated regime, which is interpreted in terms of a drift velocity saturation of the charge carriers.

The linear response theory is used to describe magnetoresistance oscillations of short-period unilateral superlattices with strong modulation (or alternatively arrays of coupled quantum wires). The semiclassical description of this system fails for strong magnetic fields (magnetic breakdown) and we employ a simple fully quantum-mechanical tight-binding model (owing to the fact that coupling between two neighboring wires is much smaller than the height of barrier between them) in conjunction with Kubo's formula instead. The resulting magnetoresistance data nicely compare to the experiments while the model opens good intuitive insight into the effects taking place in the system.

We present a brief survey of vertical transistor devices fabricated by the cleaved-edge overgrowth technique. Different device types are realized using different transistor substrates grown by molecular beam epitaxy. These substrates mainly vary in the layer sequence and thickness between the source/drain contacts. Common to all designs is the vertical gate structure overgrown on a cleavage plane of the substrates. By biasing the gate a two-dimensional electron system of tunable density is induced between source/drain. We study the DC transport properties of long channel (source–drain distance ∼1μm) as well as short-channel (source–drain distance ∼50nm) devices. Also the choice of the source/drain isolation (a superlattice or a p+-δ-doping or a heterobarrier) affects the characteristic device behavior.

Semiconductors are ubiquitous in device electronics, because their charge distributions can be conveniently manipulated with voltages to perform logic operations. Achieving a similar level of control over the spin degrees of freedom, either from electrons or nuclei, could provide intriguing prospects for both information processing and the study of fundamental solid-state physics issues. Here we report procedures that carry out the controlled transfer of spin angular momentum between electrons—confined to two dimensions and subjected to a perpendicular magnetic field—and the nuclei of the host semiconductor, using gate voltages only. We show that the spin transfer rate can be enhanced near a ferromagnetic ground state of the electron system, and that the induced nuclear spin polarization can be subsequently stored and ‘read out’. These techniques can also be combined into a spectroscopic tool to detect the low-energy collective excitations in the electron system that promote the spin transfer. The existence of such excitations is contingent on appropriate electron–electron correlations, and these can be tuned by changing, for example, the electron density via a gate voltage.

We present experimental investigations of tunneling between two quantum wells forming a T-shaped structure. At zero magnetic field we observe a nonlinear tunnel characteristic with clearly pronounced negative differential resistance. With magnetic field B, both quantum wells can independently be set in a quantum Hall state. We demonstrate spectroscopy of Quantum Hall bulk states in one B field orientation, and spectroscopy of edge states with the orthogonal orientation. The observed features can be explained assuming that transverse momentum is conserved during tunneling.

The purpose of this paper is twofold. First we introduce a system that gives independent control over the strength and period of an atomically precise one-dimensional potential modulation imposed on a two-dimensional electron system. Second, along with low temperature magnetotransport experiments on such a system, we employ a semiclassical framework to explain all observed magnetoresistance oscillation frequencies. In particular two distinct quantum interference processes are recognized. a) self-interference along closed orbits, partly rendered possible by magnetic breakdown b) mutual interference between open electron orbits. The notion of the latter mechanism bridges the gap between the known commensurability oscillations and the artificial bandstructure. We suggest a close relation also to quantum-interference effects observed in magnesium [1] and organic conductors [2].

Photoluminescence (PL) of near surface InAs quantum dots (QDs) has been studied as a function of the distance to the surface (30, 20, 10, 6 nm). We observe a strong decrease in the QD PL intensity with decreasing barrier thickness. Nevertheless, the QDs still show reasonably strong PL intensity even when they are only 10 nm beneath the surface. After the deposition of self-assembled monolayers of octadecylthiol, we observe an increase in PL intensity up to a factor of 1.87. Such an enhancement is attributed to a decrease in the density of surface states. This demonstrates that near surface InAs QDs are very sensitive to changes of the surface conditions and the deposition of octadecylthiol monolayer may be used to increase their sensitivity, which is promising toward future bio-sensor applications.

The non-equilibrium transport of electrons confined at an atomically sharp interface and subject to a periodic potential is studied. We find a pronounced negative differential resistance, the magnitude of which increases with increasing modulation strength. The data is qualitatively consistent with the Esaki–Tsu transport model. We emphasize the significance of the two-dimensionality of the electron system and the gate to inhibit domain formation. Additional features in the source–drain current are attributed to Bloch-phonon resonances.

We fabricate density-modulated two-dimensional electron systems by shallow compensation doping the donor layer of a modulation-doped heterostructure. Zinc acceptor atoms are diffused from the sample surface which is heated by a focused laser beam. Low-temperature magnetotransport experiments provide evidence that high-quality lateral surface superlattices can be fabricated. In weak periodic one-dimensional potentials, commensurability oscillations are recovered, whereas in strong periodic two-dimensional potentials the semiclassically expected antidot resistance resonances are found to dominate the low-field transport. Additionally, the homogeneity of the laser-induced doping is confirmed by magnetic focusing experiments.

In this letter we present a study of conductance fluctuations appearing in the transport of single mode quantum wires fabricated by cleaved edge overgrowth, The temperature behavior of the variance is discussed in the framework of the Luttinger liquid theory for a set of different quantum wire lengths. We observe an unexpected quenching of the Luttinger liquid behavior with increasing wire length.

The density driven quantum phase transition between the unpolarized and fully spin polarized nu = 2/3 fractional quantum Hall state is accompanied by hysteresis in accord with 2D Ising ferromagnetism and domain formation. The temporal behavior is reminiscent of the Barkhausen and time-logarithmic magnetic after-effects ubiquitous in familiar ferromagnets. It too suggests domain morphology and, in conjunction with NMR, intricate domain dynamics, which is partly mediated by the contact hyperfine interaction with nuclear spins of the host semiconductor.

This paper describes the simulations of vertical cleaved-edge overgrowth field effect transistors (CEO-FETs). For the simulation the device simulator SIMBA is used, which is capable to handle complex device geometries as well as several physical models represented by certain sets of partial differential equations. With a multidimensional solution of the Poisson equation the Schrodinger equation is solved either in one or in two dimensions according to the confinement of the electrons in the area where quantum mechanical effects are expected. As a new feature the involvement of a hydrodynamic (HD) transport model is implemented to include non-equilibrium transport phenomena in extremely short channels. The experimental results are compared with the simulated data of this device.

Magnetotransport experiments on two-dimensional electron systems with an atomically precise, one-dimensional potential modulation reveal striking quantum interference oscillations. Within a semiclassical framework, they are recognized either as self-interference along closed orbits, many of them rendered possible by magnetic breakdown between Fermi contour segments of the artificial band structure, or as interference-enhanced backscattering. The known commensurability oscillations appear as a special case of the latter mechanism.