Cleaved Edge Overgrowth Technique Research Articles

1D FFLO state in absence of time reversal symmetry breaking

We propose a route to a one-dimensional Fulde-Ferrell-Larkin-Ovchinnikov state in the absence of broken time-reversal symmetry. At present such a state may be encouraged in a clean (no disorder) AlAs quantum wire fabricated using the cleaved edge overgrowth technique. The fabrication technique captures two degenerate nonoverlapping bands separated in momentum-space by half an umklapp vector which leads to four Fermi points. Using field theoretic methods such as abelian bosonization and the renormalization group scheme we treat the important low energy long wavelength fermionic interaction terms for this one dimensional system. Due to the specific bandstructure arrangement of the quantum wire there is a new class of unique umklapp assisted interactions. These umklapp interactions are present at all electronic densities and are not related to the commensurability of the electron gas with the underlying lattice. We show that in the presence of the umklapp interactions and without any external perturbations such as a magnetic or electric field a singlet superconducting ground state is preferred with non-zero center-of-mass momentum for the Cooper pairs. The finite pairing momentum of the Cooper pairs is an indication of a Fulde-Ferrell-Larkin-Ovchinnikov state which is known to lead to inhomogeneous superconductivity.

The 0.7 structure in cleaved edge overgrowth wires

We study the conductance through long and ultra-clean one-dimensional conductors andfind a conductance anomaly at low charge densities—similar to the so-called 0.7 structurefound in short constrictions. Our wires, afforded by the cleaved edge overgrowth technique,allow a quantitative study of this phenomenon in long wires. We find that this anomalyoccurs whenever the kinetic energy provided to the carriers exceeds the Fermi energy. Inthis regime, the measured conductance is found to exceed the value naively expected fromnon-interacting models.

Current-induced heating in quantum well and quantum wire intersubband emitter structures

We discuss the influence of current-induced heating on the current-voltage (I-V) characteristics and the spectral behavior in quantum well and quantum wire intersubband emitter structures. A conventional quantum cascade laser structure in the AlGaAs∕GaAs material system with undoped cladding layers and an undoped active region is examined. This heterostructure serves as a first growth step for quantum wire intersubband emitters fabricated by the cleaved-edge overgrowth technique. We discuss the influence of electrons supplied by a remote δ-silicon doping. Duty-cycle dependent measurements on the quantum wire structures confirm the influence of current-induced heating on the I-V characteristics as well as on the emission spectra.

Guided Self Assembly of InAs Quantum Dots on a Cleaved Facet

ABSTRACTThe long range ordering of epitaxial semiconductor quantum dots (QDs) has been obtained by combing self assembly with the cleaved edge overgrowth technique. The introduction of nanometer thick AlAs stripes on a (110) oriented GaAs surface avoids the misfit dislocation growth mechanism of InAs on GaAs (110) and drives the formation of array of QDs. Atomic Force Microscopy (AFM) investigations highlight that InAs QDs only nucleate in chain like structure on Al-rich regions. Here, we present experimental results that demonstrate the ability to create ordered QDs lattices and discuss the conditions under which preferential growth of QDs on the AlAs stripes occurs.

Electric-field stabilization in a high-density surface superlattice

By application of the cleaved-edge overgrowth technique, we realize a two-channel superlattice (SL) device. The structure combines the parallel transport through a low-density SL under almost homogeneous electric field conditions with that through a surface SL (SSL) with large carrier density, which is, without parallel transport, subject to pronounced field instabilities. Direct control of the SSL density allows a separation of both transport contributions. With parallel transport through the low-density SL, the current carried by the SSL is characteristic for a SL with homogeneous field distribution. In particular, it exhibits negative differential conductivity over a wide range of applied electric fields. In contrast, for current only through the SSL clear electric-field instabilities, typical for SLs at high densities are observed. Thus, by means of the parallel transport channel, field instabilities are avoided and transport in high-density SLs with a homogeneous field distribution becomes accessible.

Strong charge carrier confinement in purely strain induced GaAs∕InAlAs single quantum wires

We report on micro-photoluminescence studies of single quantum wires which were grown by molecular beam epitaxy. Employing the cleaved edge overgrowth technique, quantum wires located in an overgrown (011) oriented GaAs quantum well originate purely from the tensile strain field of InAlAs layers grown along the [100] direction. These stressor layers are separated by 1-μm-wide AlGaAs barriers so that the photoluminescence signals of different quantum wires can be resolved individually. Their confinement energy varies systematically with the widths of the stressor and overgrown layers, reaching values as high as 51.5meV. The quantum wire signals are characterized by a smooth line shape even for the lowest excitation powers, indicating the absence of pronounced exciton localization.

Optical properties of low‐dimensional semiconductor systems fabricated by cleaved edge overgrowth

An overview of the cleaved edge overgrowth technique, which allows the realization of atomically precise quantum structures, is presented. We explain the origin of the formation of quantum wires, which were grown with this molecular beam epitaxy method. The emission of quantum wire lasers based on these intrinsic T-shaped structures shows a strong dependence on magnetic fields. Only weak band gap renormalization is observed for modulation doped wires when the electron density is varied. The introduction of strained layers results in an enhancement of the confinement energy, which is necessary to fully exploit the one-dimensional character of the examined quantum wires. Single as well as coupled quantum dots, and quantum dot superlattices have been successfully grown and are identified using micro-photoluminescence spectroscopy. By combining self-assembling of quantum dots due to Stranski-Krastanov growth with the cleaved edge overgrowth method we succeeded in fabricating arrays of long-range ordered InAs quantum dots. Both atomic force microscopy and micro-photoluminescence measurements performed on such samples show the ability to control size, position, and ordering of the quantum dots. (© 2004 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

Strongly confined quantum wire states in strained T-shaped GaAs/InAlAs structures

The confinement energy of T-shaped quantum wires (QWRs), which were fabricated by the cleaved edge overgrowth technique in a way that the QWRs form at the intersection of In 0.2Al 0.8As stressor layers and the overgrown (1 1 0) GaAs quantum well (QW), is examined using micro-photoluminescence spectroscopy. Photoluminescence (PL) signals from individual QWRs can be spatially resolved, since the strained films are separated by 1 μm wide Al 0.3Ga 0.7As layers. We find that due to the tensile strain being transmitted to the QW, the confinement energy of the QWRs rises systematically up to 40 meV with increasing thickness of the stressor layers. By reducing the excitation power to 0.1 μW the QWR PL emission occurs 48 meV redshifted with respect to the QW. All QWR peaks exhibit smooth lineshapes, indicating the absence of pronounced exciton localization.

Mid infrared emission of quantum wire cascade structures

A novel mid infrared emitting GaAs/AlGaAs quantum wire cascade device is presented that is fabricated using the cleaved edge overgrowth technique. The samples are pumped electrically. Radiative electronic transitions between discrete energy levels in coupled quantum wires are calculated for such a structure. Operation current dependent mid infrared emission is observed at a peak wave number of 1200 cm −1 with a full-width at half-maximum of 300 cm −1 at a heat sink temperature of 20 K . The presented sample is an experimental proposal for an unipolar quantum wire intersubband laser.

Tunneling Between Parallel Quantum Wires

Recent developments in fabrication techniques and experimental design have allowed the observation of tunneling conductance between parallel quantum wires of very high quality, in GaAs heterostructures, using the cleaved edge overgrowth technique [1]. To a first approximation momentum is conserved during the tunneling process, except for a momentum boost, ħQ = Bed/c, produced by an applied magnetic field B perpendicular to the plane containing the two wires, where d is the separation between the wires. The differential tunnel conductance G = dI/dV is proportional to the spectral density for creating a hole in one wire and and electron in the other, with total momentum Q and energy E = eV, where V is the voltage difference between the two wires.

Vertical field effect transistors realized by cleaved-edge overgrowth

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 ∼1 μm ) as well as short-channel (source–drain distance ∼50 nm ) devices. Also the choice of the source/drain isolation (a superlattice or a p +- δ-doping or a heterobarrier) affects the characteristic device behavior.

Miniband transport in vertical superlattice field-effect transistors

We study the nonequilibrium transport of two-dimensional electrons through a periodic potential. Our samples are fabricated using the cleaved-edge overgrowth technique to provide a vertical field-effect transistor with an undoped GaAs/AlGaAs superlattice channel orthogonal to the current flow. We find a pronounced negative differential resistance, the magnitude of which increases with increasing modulation strength. The data are qualitatively consistent with the Esaki–Tsu transport model in minibands, which we calculate for the given samples. We emphasize the significance of the two-dimensionality of the electron system and the gate to inhibit domain formation. Weak features in the source-drain current are attributed to Bloch-phonon resonances.

Effective mass enhancement of two-dimensional electrons in a one-dimensional superlattice potential

We report effective mass enhancement of two-dimensional (2D) electrons in an atomically precise one-dimensional AlGaAs/GaAs superlattice potential fabricated by the cleaved-edge overgrowth technique. Magnetotransport measurements reveal that the mobility of the 2D electrons increases with electron density n2D. At low densities (n2D∼1.3×1011 cm−2), the effective mass m* of the 2D electrons is 0.14 me, where me is the free-electron mass. This value of 0.14 me is twice the effective mass of electrons in GaAs (0.067 me). m* increases with n2D and is ∼0.16 me at 2.7×1011 cm−2. We explain these results based on the formation of energy minibands along the superlattice direction. The electron scattering time τ is calculated from the mobility and effective mass data. At 0.3 K, τ increases with n2D from 5 to 11 ps as n2D is varied from 1.1×1011 to 2.7×1011 cm−2.

Experimental evidence for resonant tunneling in a luttinger liquid

We have measured the low-temperature conductance of a one-dimensional island embedded in a single mode quantum wire. The quantum wire is fabricated using the cleaved edge overgrowth technique and the tunneling is through a single state of the island. Our results show that while the resonance line shape fits the derivative of the Fermi function the intrinsic linewidth decreases in a power law fashion as the temperature is reduced. This behavior agrees quantitatively with Furusaki's model for resonant tunneling in a Luttinger liquid.

Multi-terminal ballistic transport in one-dimensional wires

Recent experiments (Yacoby et al., Phys. Rev. Lett. 77 (1996) 4612; Solid State Commun. 101 (1997) 77; M. Rother et al., ICPS-24, Th-P137) have shown non-universal conductance quantization in one-dimensional wires that are fabricated using the cleaved edge overgrowth technique (L.N. Pfeiffer et al., Microelectronics J. 28 (1997) 817). In one of the papers (Yacoby et al., Phys. Rev. Lett. 77 (1996) 4612), it was speculated that the origin of the reduced conductance lies in the interface between the one-dimensional wire and the two-dimensional electron gas regions, which serve as ohmic contacts and thermal reservoirs. Here we report the results of a systematic study of such 2D–1D interfaces. By embedding a 2D–1D interaction section region of controllable length inside an otherwise isolated wire, we were able to study the properties of the coupling between these two subsystems. Our results show that 2D–1D interface is in fact the origin of the non-universal conductance.

New approach to the fabrication of CdSe/ZnSe quantum dots using a cleaved-edge overgrowth technique

Spontaneous formation of CdSe/ZnSe quantum dots (QDs) in ultra high vacuum (UHV) by molecular beam epitaxy (MBE) was investigated using cleavage-induced GaAs (1 1 0) surfaces as substrates. Stranski-Krastanow (SK)-type CdSe QDs were randomly distributed on the atomically flat surface. On the other hand, QD structures were abnormally formed at the macroscopic steps produced by the cleaving process of the substrates. These QDs were naturally arranged along the edge of the steps. Microscopic photoluminescence (PL) spectra of the QDs at step regions showed a few number of sharp peaks originating from the individual QDs. Therefore, the cleaved edge overgrowth technique is a novel method for fabrication of isolated CdSe QDs which is useful to investigate optical properties of individual QD structures.

New approach to the fabrication of CdSe/ZnSe quantum dots using a cleaved-edge overgrowth technique

New approach to the fabrication of CdSe/ZnSe quantum dots using a cleaved-edge overgrowth technique

Optimization of the confinement energy of quantum-wire states in T-shaped GaAs/AlxGa1-xAs structures.

We report on an optimization of the wire confinement energies of the confined electronic states at the T-shaped intersection of GaAs and ${\mathrm{Al}}_{\mathit{x}}$${\mathrm{Ga}}_{1\mathrm{\ensuremath{-}}\mathit{x}}$As quantum wells. These structures can be produced by the cleaved edge overgrowth technique. We present an analytical model for the confinement to give insight into the basic mechanism. The optimization of the confinement energy is done by calculations in a six-band k\ensuremath{\cdot}p approximation for the valence band and in an isotropic effective-mass approximation for the conduction band. The confined valence-band states are only weakly bound at the T-shaped intersection due to the large and anisotropic hole effective masses. Employing optimized sample parameters, confinement energies for the free-electron-hole pairs are nearly doubled compared to symmetric structures, and 34 meV are predicted for a 3-nm overgrown GaAs well. This is expected to be further enhanced by the Coulomb interaction, that is neglected in the numerical model. The experimental structures grown using the optimized geometry show wire confinement energies of up to 54 meV, which is significantly larger than KT at room temperature and larger than previously reported. \textcopyright{} 1996 The American Physical Society.

Magnetoresistance oscillations in 50-nm-wide GaAs/AlxGa1-xAs multiple-edge quantum wires.

We fabricated a 50-nm-wide GaAs/${\mathrm{Al}}_{\mathit{x}}$${\mathrm{Ga}}_{1\mathrm{\ensuremath{-}}\mathit{x}}$As multiple-quantum-wire structure with the cleaved-edge overgrowth technique and investigated the characteristics of its low-temperature magnetoresistances. A clear Shubnikov--de Haas oscillation was observed and found to contain unexpected oscillatory components. By analyzing the magnetoresistance and its temperature dependence, the formation of one-dimensional subbands was revealed. On the basis of a semiclassical model, the width of lateral confinement was estimated and found to be \ensuremath{\sim}50 nm.

Surface resonant tunneling transistor: A new negative transconductance device

A new three-terminal device, the surface resonant tunneling transistor, is realized by the molecular beam epitaxial cleaved edge overgrowth technique in the GaAs/AlGaAs system. The device exhibits negative transconductance as well as negative differential resistance. Some possibilities for future applications of the device to low-power logic circuits are discussed.