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We integrate resonant-cavity light-emitting diodes containing quantum dots onto substrates with giant piezoelectric response. Via strain, the energy of the photons emitted by the diode can be precisely controlled during electrical injection over a spectral range larger than 20 meV. Simultaneously, the exciton fine-structure-splitting and the biexciton binding energy can be tuned to the values required for entangled photon generation., (Copyright © 2012 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.)

The article gives a report on resistivity measurements on Mn-doped p-type FeSi2 single crystals and analyzes the data within the framework of different hopping conductivity models. Both the Mott [N. Mott and E. A. Davies, Electron Processes in Non-Crystalline Materials (Clarendon, Oxford, 1979)] and the Shklovskii–Efros [B. I. Shklovskii and A. L. Efros, Electronic Properties o0066 Doped Semiconductors (Springer, Berlin, 1984)] regimes of the variable-range hopping (VRH) conductivity are observed. It is shown that the temperature dependence of the resistivity of Mn-doped β-FeSi2 crystals, which follows a VRH conduction mechanism, can be expressed by a scaling expression of the form ln(ρ/ρ0)=Af(T/Tx). The characteristic and transition temperatures, as well as the complete set of parameters describing the properties of the localized holes (the localization radius, the dielectric permittivity, the width of the Coulomb gap Δ, and the values of the density of states at the Fermi level) are determined. The data above indicate existence of a rigid gap δ in the spectrum of the DOS in addition to Δ and point out to the polaronic nature of the charge carriers in the investigated compound. [ABSTRACT FROM AUTHOR]

Ultrathin Si and SiGe films on insulator are used as templates for the fabrication of hybrid metal/semiconductor rolled-up tubes. Structured Cr films are thermally evaporated on SiGe- and Si-on-insulator substrates. The metal/semiconductor bilayers are released from the substrate and formed into rolled-up tubes. The tube diameter is controlled by tuning the metal and the semiconductor layer properties. A cross section of a Cr/SiGe tube is investigated by transmission electron microscopy and electron energy loss spectroscopy. [ABSTRACT FROM AUTHOR]

Independent tuning of emission energy and decay time of neutral excitons confined in single selfassembled In(Ga)As/GaAs quantum dots is achieved by simultaneously employing vertical electric fields and lateral biaxial strain fields. By locking the emission energy via a closed-loop feedback on the piezoelectric actuator used to control the strain in the quantum dot, we continuously decrease the decay time of an exciton from 1.4 to 0.7 ns. Both perturbations are fully electrically controlled and their combination offers a promising route to engineer the indistinguishability of photons emitted from spatially separated single photon sources. [ABSTRACT FROM AUTHOR]

Semiconductor heteroepitaxy on top of thin compliant layers has been explored as a path to make inorganic electronics mechanically flexible as well as to integrate materials that cannot be grown directly on rigid substrates. Here, we show direct evidences of strain transfer for InAs islands on freestanding Si thin films (7 nm). Synchrotron X-ray diffraction measurements using a beam size of 300 x 700 nm² can directly probe the strain status of the compliant substrate underneath deposited islands. Using a recently developed diffraction mapping technique, three-dimensional reciprocal space maps were reconstructed around the Si (004) peak for specific illuminated positions of the sample. The strain retrieved was analyzed using continuous elasticity theory via Finite-element simulations. The comparison of experiment and simulations yields the amount of strain from the InAs islands, which is transferred to the compliant Si thin film. [ABSTRACT FROM AUTHOR]

The radiative biexciton-exciton decay in a semiconductor quantum dot (QD) has the potential of being a source of triggered polarization-entangled photon pairs. However, in most cases the anisotropy-induced exciton fine structure splitting destroys this entanglement. Here, we present measurements on improved QD structures, providing both significantly reduced inhomogeneous emission linewidths and near-zero fine structure splittings. A high-resolution detection technique is introduced which allows us to accurately determine the fine structure in the photoluminescence emission and therefore select appropriate QDs for quantum state tomography. We were able to verify the conditions of entangled or classically correlated photon pairs in full consistence with observed fine structure properties. Furthermore, we demonstrate reliable polarization-entanglement for elevated temperatures up to 30 K. The fidelity of the maximally entangled state decreases only a little from 72% at 4K to 68% at 30 K. This is especially encouraging for future implementations in practical devices. [ABSTRACT FROM AUTHOR]

The morphological evolution of both pits and SiGe islands on patterned Si(001) substrates is investigated. With increasing Si buffer layer thickness the patterned holes transform into multifaceted pits before evolving into inverted truncated pyramids. SiGe island formation and evolution are studied by systematically varying the Ge coverage and pit spacing and quantitative data on the influence of the pattern periodicity on the SiGe island volume are presented. The presence of pits allows the fabrication of uniform island arrays with any of their equilibrium shapes. [ABSTRACT FROM AUTHOR]

The quantum-confined Stark effect of excitonic states in self-assembled (In,Ga)As/GaAs quantum dots was studied by microphotoluminescence spectroscopy. A similar Stark-shift behavior for excitons, biexcitons, and a charged state was observed. Investigations suggest the absence of a permanent dipole moment in the lateral quantum dot plane. Values of the polarizability could be derived for all the investigated states. Furthermore, high-resolution Fabry-Pérot interferometry was applied to resolve the excitonic fine structure splitting and to investigate the influence of a lateral electric field. For a single dot, the splitting could be tuned to zero, thus affording the possibility to create electrically controlled entangled photon pairs. [ABSTRACT FROM AUTHOR]

We propose to improve a p-channel metal-oxide-semiconductor field-effect transistor using laterally closely spaced double self-assembled SiGe/Si islands as drain and source to create a high hole mobility channel. The strain distribution in and around the channel is calculated for two realistic island geometries with various distances between the islands. A compressive strain of more than 1% in the channel can be achieved for SiGe islands and small distance between these two islands. We demonstrate that the proposed double SiGe/Si island structure can be realized by epitaxial growth on patterned substrates designed for static random access memory cell. [ABSTRACT FROM AUTHOR]

Femtosecond carrier dynamics in biased AlxGa1-xAs heterostructure diodes is investigated tracing transient modifications of the Franz-Keldysh absorption spectrum. The nonlinear optical response is sensitive to the number of electron-hole pairs in the high-field region of the sample. As a result, the dynamical buildup of a nonequilibrium carrier avalanche due to impact ionization for electric fields F>=350 kV/cm is directly analyzed in the time domain. The time scale of the carrier multiplication is found to be in the order of 10 ps depending on the number of photoinjected carriers. Monte Carlo simulations in a simplified band structure agree well with the experiment. [ABSTRACT FROM AUTHOR]

We present microphotoluminescence investigations of InAs quantum dots (QDs) integrated into self-rolling InGaAs/GaAs strained layers. The emission signal from the QDs is redshifted due to strain relaxation and increased in intensity after the strained layers are released from the substrate and rolled up into tubes. We detect waveguided light at the tube ends, which originates from the QDs at the laser excitation spot. The possibility of integrating quantum emitters into the tube walls acting as waveguides represents a major step toward the realization of flexible high quality factor optical resonators based on rolled-up nano- and microtubes. [ABSTRACT FROM AUTHOR]

We investigate the photoluminescence (PL) properties of three-dimensional InAs/GaAs quantum-dot (QD) crystals grown on shallow modulated periodic hole arrays patterned on GaAs(001). We find that the PL spectra become narrower and more intense with increasing number of QD layers. A deconvoluted PL linewidth of 14.9 meV is obtained from a defect-free QD crystal consisting of 11 stacked QD layers. The PL spectra obtained for QD crystals containing QD vacancies show significantly broader spectra. The PL peak energy and linewidth of the QDs across the whole pattern (100×100 μm2) remain constant within 1.278±0.001 eV and 21.0±1.7 meV, respectively. From power-dependent PL measurement, we can resolve up to seven excited-state PL peaks confirming the remarkable size homogeneity of our QD crystals. This experimental result can be reasonably fitted by a calculation based on random population theory and on a simple model for the QD confinement potential. [ABSTRACT FROM AUTHOR]

By releasing strained epitaxial layers from their substrates, the layers roll up to form nanotubes. The structures of InAs/GaAs and SiGe/Si rolled-up nanotubes (RUNTs) are characterized by using transmission electron microscopy (TEM), electron diffraction, high-resolution TEM, and spatially resolved electron energy-loss spectroscopy. Freestanding RUNTs as well as their cross sections are investigated. It is found that the walls of the nanotubes are composed of alternating crystalline and noncrystalline oxide-containing layers. Defects may form in some nanotubes, where the rolling involves a misorientation. [ABSTRACT FROM AUTHOR]

We investigate the structure and thermal stability of freestanding SiGe/Si rolled-up microtubes by secondary ion mass spectroscopy, scanning and transmission electron microscopy, as well as micro-Raman spectroscopy. The tube walls show good crystal quality but become thinner during tube fabrication, leading to smaller diameters than expected. Si–Si vibration modes from the wall are consistent with Raman shifts induced by the strain distribution in the structure. Furthermore, the Raman shifts are used to monitor the tube temperature during laser annealing. At high temperatures, the tube structure irreversibly changes due to Si and Ge interdiffusion and, most remarkably, by Ge condensation. [ABSTRACT FROM AUTHOR]

We report on the formation of ordered and size homogeneous InAs quantum dot (QD) arrays on patterned GaAs(001) substrates. A material depletion region is observed around the patterned area while a long-range homogeneous distribution of QDs is found inside the patterned area. A sample with less InAs deposition shows a gradient in the material distribution over the patterned area. Based on these observations we propose a simple model to describe the QD formation in the patterned area. The QD shape evolution is also investigated and discussed. [ABSTRACT FROM AUTHOR]

Single quantum dots (QDs) reproduce into pairs of QDs with increasing separation distance during the growth of a three-dimensional QD crystal. Kinetic Monte Carlo simulations—that rely on strain profiles deduced from experiment—can describe this lateral replication process, which is triggered by a distinct ridge structure that evolves during the overgrowth of two-dimensional periodic QD arrays. [ABSTRACT FROM AUTHOR]

The model systems for self-organized quantum dots formed from elemental and compound semiconductors, namely Ge grown on Si(001) and InAs on GaAs(001), are comparatively studied by scanning tunneling microscopy. It is shown that in both material combinations only two well-defined families of faceted and defect-free nanocrystals exist (and coexist). These three-dimensional islands, pyramids, and domes show common morphological characteristics, independent of the specific material system. A universal behavior is further demonstrated in the capping-passivation process that turns the nanocrystals in true quantum dots. [ABSTRACT FROM AUTHOR]

This article presents information on quantum dot arrays. Researchers have reported on a technique to realize long-range ordered quantum dot (QD) arrays with pronounced size homogeneity. The technique comprises patterning of a GaAs substrate surface with an anodic aluminum oxide membrane and subsequent self-assembled InAs QD growth. The researchers developed this technique because they claimed that an effective means to synthesize three-dimensional confined nanostructures, which possess a high degree of spatial ordering did not exist before.

We develop an effective model to describe the statistical properties of exciton fine structure splitting (FSS) and polarization angle in quantum dot ensembles (QDEs) using only a few symmetry-related parameters. The connection between the effective model and the random matrix theory is established. Such effective model is verified both theoretically and experimentally using several rather different types of QDEs, each of which contains hundreds to thousands of QDs. The model naturally addresses three fundamental issues regarding the FSS and polarization angels of QDEs, which are frequently encountered in both theories and experiments. The answers to these fundamental questions yield an approach to characterize the optical properties of QDEs. Potential applications of the effective model are also discussed. [ABSTRACT FROM AUTHOR]

Spin-selective tunneling of holes in SiGe nanocrystals contacted by normal-metal leads is reported. The spin selectivity arises from an interplay of the orbital effect of the magnetic field with the strong spin-orbit interaction present in the valence band of the semiconductor. We demonstrate both experimentally and theoretically that spin-selective tunneling in semiconductor nanostructures can be achieved without the use of ferromagnetic contacts. The reported effect, which relies on mixing the light and heavy holes, should be observable in a broad class of quantum-dot systems formed in semiconductors with a degenerate valence band. [ABSTRACT FROM AUTHOR]