Optical absorption of composition-tunable InGaAs nanowire arrays.

InGaAs nanowire (NW) arrays have emerged as important active materials in future photovoltaic and photodetector applications, due to their excellent electronic properties and tunable band gap. Here, we report a systematic investigation of the optical absorption characteristics of composition-tunable vertical InGaAs NW arrays. Using finite-difference time-domain simulations we first study the effect of variable composition (Ga-molar fraction) and NW array geometry (NW diameter, period, fill factor) on the optical generation rate. NWs with typical diameters in the range of ∼100-250 nm lead to generation rates higher than the equivalent bulk case for moderate fill factors (NW period of ∼0.3-0.8 µm), while slightly smaller fill factors and increased diameters are required to maintain high generation rates at increased Ga-molar fraction. The optical absorption was further measured using spectrally resolved ultraviolet-visible-near-infrared (UV-vis-NIR) spectroscopy on NW arrays transferred to transparent substrates. Interestingly, large variations in Ga-molar fraction (0 < x(Ga) < 0.5) have a negligible influence, while minute changes in NW diameter of less than ±20 nm affect the absorption spectra very strongly, leading to pronounced shifts in the peak absorption energies by more than ∼700 meV. These results clearly highlight the much larger sensitivity of the optical absorption behavior to geometric parameters rather than to variations in the electronic band gap of the underlying NW array.

Monolithically Integrated High-ß Nanowire Lasers on Silicon.

Reliable technologies for the monolithic integration of lasers onto silicon represent the holy grail for chip-level optical interconnects. In this context, nanowires (NWs) fabricated using III-V semiconductors are of strong interest since they can be grown site-selectively on silicon using conventional epitaxial approaches. Their unique one-dimensional structure and high refractive index naturally facilitate low loss optical waveguiding and optical recirculation in the active NW-core region. However, lasing from NWs on silicon has not been achieved to date, due to the poor modal reflectivity at the NW-silicon interface. We demonstrate how, by inserting a tailored dielectric interlayer at the NW-Si interface, low-threshold single mode lasing can be achieved in vertical-cavity GaAs-AlGaAs core-shell NW lasers on silicon as measured at low temperature. By exploring the output characteristics along a detection direction parallel to the NW-axis, we measure very high spontaneous emission factors comparable to nanocavity lasers (ß = 0.2) and achieve ultralow threshold pump energies ≤11 pJ/pulse. Analysis of the input-output characteristics of the NW lasers and the power dependence of the lasing emission line width demonstrate the potential for high pulsation rates ≥250 GHz. Such highly efficient nanolasers grown monolithically on silicon are highly promising for the realization of chip-level optical interconnects.

Ultrafast Photodetection in the Quantum Wells of Single AlGaAs/GaAs-Based Nanowires.

We investigate the ultrafast optoelectronic properties of single Al0.3Ga0.7As/GaAs core-shell nanowires. The nanowires contain GaAs-based quantum wells. For a resonant excitation of the quantum wells, we find a picosecond photocurrent which is consistent with an ultrafast lateral expansion of the photogenerated charge carriers. This Dember-effect does not occur for an excitation of the GaAs-based core of the nanowires. Instead, the core exhibits an ultrafast displacement current and a photothermoelectric current at the metal Schottky contacts. Our results uncover the optoelectronic dynamics in semiconductor core-shell nanowires comprising quantum wells, and they demonstrate the possibility to use the low-dimensional quantum well states therein for ultrafast photoswitches and photodetectors.

Demonstration of Confined Electron Gas and Steep-Slope Behavior in Delta-Doped GaAs-AlGaAs Core-Shell Nanowire Transistors.

Strong surface and impurity scattering in III-V semiconductor-based nanowires (NW) degrade the performance of electronic devices, requiring refined concepts for controlling charge carrier conductivity. Here, we demonstrate remote Si delta (δ)-doping of radial GaAs-AlGaAs core-shell NWs that unambiguously exhibit a strongly confined electron gas with enhanced low-temperature field-effect mobilities up to 5 × 10(3) cm(2) V(-1) s(-1). The spatial separation between the high-mobility free electron gas at the NW core-shell interface and the Si dopants in the shell is directly verified by atom probe tomographic (APT) analysis, band-profile calculations, and transport characterization in advanced field-effect transistor (FET) geometries, demonstrating powerful control over the free electron gas density and conductivity. Multigated NW-FETs allow us to spatially resolve channel width- and crystal phase-dependent variations in electron gas density and mobility along single NW-FETs. Notably, dc output and transfer characteristics of these n-type depletion mode NW-FETs reveal excellent drain current saturation and record low subthreshold slopes of 70 mV/dec at on/off ratios >10(4)-10(5) at room temperature.

Electrical control of interdot electron tunneling in a double InGaAs quantum-dot nanostructure.

We employ ultrafast pump-probe spectroscopy to directly monitor electron tunneling between discrete orbital states in a pair of spatially separated quantum dots. Immediately after excitation, several peaks are observed in the pump-probe spectrum due to Coulomb interactions between the photogenerated charge carriers. By tuning the relative energy of the orbital states in the two dots and monitoring the temporal evolution of the pump-probe spectra the electron and hole tunneling times are separately measured and resonant tunneling between the two dots is shown to be mediated both by elastic and inelastic processes. Ultrafast (<5 ps) interdot tunneling is shown to occur over a surprisingly wide bandwidth, up to ∼8 meV, reflecting the spectrum of exciton-acoustic phonon coupling in the system.

Rate-limiting mechanisms in high-temperature growth of catalyst-free InAs nanowires with large thermal stability.

We identify the entire growth parameter space and rate-limiting mechanisms in non-catalytic InAs nanowires (NWs) grown by molecular beam epitaxy. Surprisingly huge growth temperature ranges are found with maximum temperatures close to ~600°C upon dramatic increase of V/III ratio, exceeding by far the typical growth temperature range for catalyst-assisted InAs NWs. Based on quantitative in situ line-of-sight quadrupole mass spectrometry, we determine the rate-limiting factors in high-temperature InAs NW growth by directly monitoring the critical desorption and thermal decomposition processes of InAs NWs. Both under dynamic (growth) and static (no growth, ultra-high vacuum) conditions the (111)-oriented InAs NWs evidence excellent thermal stability at elevated temperatures even under negligible supersaturation. The rate-limiting factor for InAs NW growth is hence dominated by In desorption from the substrate surface. Closer investigation of the group-III and group-V flux dependences on growth rate reveals two apparent growth regimes, an As-rich and an In-rich regime defined by the effective As/In flux ratio, and maximum achievable growth rates of > 6 µm h(-1). The unique features of high-T growth and excellent thermal stability provide the opportunity for operation of InAs-based NW materials under caustic environment and further allow access to temperature regimes suitable for alloying non-catalytic InAs NWs with GaAs.

Local modification of GaAs nanowires induced by laser heating.

GaAs nanowires were heated locally under ambient air conditions by a focused laser beam which led to oxidation and formation of crystalline arsenic on the nanowire surface. Atomic force microscopy, photoluminescence and Raman spectroscopy experiments were performed on the same single GaAs nanowires in order to correlate their structural and optical properties. We show that the local changes of the nanowires act as a barrier for thermal transport which is of interest for thermoelectric applications.

Quantum interference control of femtosecond, microA current bursts in single GaAs nanowires.

A phase-stable superposition of femtosecond pulses from a compact erbium-doped fiber source and their second harmonic is shown to induce ultrashort approximately microA current bursts in single unbiased GaAs nanowires. Current injection relies on a quantum interference of one- and two-photon absorption pathways. The vector direction of the current is solely dictated by the polarization and relative phase of the harmonically related light components while its power dependence is consistent with a third order optical nonlinearity.

Self-induced growth of vertical free-standing InAs nanowires on Si(111) by molecular beam epitaxy.

We report self-induced growth of vertically aligned (i.e. along the [111] direction), free-standing InAs nanowires on Si(111) substrates by solid-source molecular beam epitaxy. Implementation of an ultrathin amorphous SiO(x) mask on Si(111) facilitated epitaxial InAs nanowire growth, as confirmed by high-resolution x-ray diffraction 2theta-omega scans and transmission electron microscopy. Depending on growth temperature (in the range of 400-520 degrees C) substantial size variation of both nanowire length and diameter was found under preservation of uniform, non-tapered hexagon-shaped geometries. The majority of InAs nanowires exhibited phase-pure zinc blende crystal structure with few defective regions consisting of stacking faults. Photoluminescence spectroscopy at 20 K revealed peak emission of the InAs nanowires at 0.445 eV, which is approximately 30 meV blueshifted with respect to the emission of the bulk InAs reference due to radial quantum confinement effects. These results show a promising route towards integration of well-aligned, high structural quality InAs-based nanowires with the desired aspect ratio and tailored emission wavelengths on an Si platform.

An atomically resolved study of InGaAs quantum dot layers grown with an indium flush step.

In this cross-sectional scanning tunnelling microscopy study we investigate the indium flush method as a means to control the height of self-assembled InGaAs quantum dots and wetting layers. The results show that application of an indium flush step during growth results in flattened dots and a reduced wetting layer of which the height can be precisely controlled by varying the height of the first capping layer.

Ge(1-x) Mn(x) clusters: central structural and magnetic building blocks of nanoscale wire-like self-assembly in a magnetic semiconductor.

Controlled nanoscale self-assembly of magnetic entities in semiconductors opens novel perspectives for the tailoring of magnetic semiconductor films and nanostructures with room temperature functionality. We report that a strongly directional self-assembly in growth direction in Mn-alloyed Ge is due to a stacking of individual Ge(1-x)Mn(x) clusters. The clusters represent the relevant entities for the magnetization of the material. They are formed of a core-shell structure displaying a Mn concentration gradient. While the magnetic moments seem to be carried by the shells of the clusters, their core is magnetically inactive.

Catalyst-free nanowires with axial InxGa1-xAs/GaAs heterostructures.

Self-catalyzed growth of axial In(x)Ga(1-x)As/GaAs heterostructures has been realized by molecular beam epitaxy. The growth of the wires is achieved from gallium/indium alloy droplets that are nucleated in situ. By variation of the In/Ga beam flux during the growth it was possible to vary the effective indium content up to x = 5%, as deduced from photoluminescence measurements. We have analyzed the dependence of the alloy concentration on the growth conditions and present a simple model for the growth. The heterostructures grown with the method presented were spatially mapped along the wires with confocal microphotoluminescence and cathodoluminescence. It was found as expected that the emission of GaAs/In(x)Ga(1-x)As/GaAs heterostructures is localized. This work is important for the use of an external catalyst-free growth of complex axial heterostructures and related opto-electronic devices that facilitates its possible integration in the device or system fabrication processes.

Compositional analysis based on electron holography and a chemically sensitive reflection.

A method for compositional analysis of low-dimensional heterostructures is presented. The suggested procedure is based on electron holography and the exploitation of the chemically sensitive (0 0 2) reflection. We apply an off-axis imaging condition with the (0 0 2) beam strongly excited and centered on the optic axis. The first side band of the hologram is centered using an "empty" reference hologram obtained for a hole of the specimen. From the centered side band we use the phase of the central (0 0 0) and the amplitude of the (0 0 2) reflections to evaluate the local composition and the local specimen thickness in an iterative and self-consistent way. Delocalization effects that lead to a shift of the spatial information of (0 0 0) and (0 0 2) reflections are taken into account. The application of the procedure is demonstrated with an AlAs/GaAs(0 0 1) superlattice with a period of 5 nm. The concentration profiles obtained are discussed in relation to segregation. The measured segregation efficiency is R = 0.51 +/- 0.02.

All optical quantum control of a spin-quantum state and ultrafast transduction into an electric current.

The ability to control and exploit quantum coherence and entanglement drives research across many fields ranging from ultra-cold quantum gases to spin systems in condensed matter. Transcending different physical systems, optical approaches have proven themselves to be particularly powerful, since they profit from the established toolbox of quantum optical techniques, are state-selective, contact-less and can be extremely fast. Here, we demonstrate how a precisely timed sequence of monochromatic ultrafast (~ 2-5 ps) optical pulses, with a well defined polarisation can be used to prepare arbitrary superpositions of exciton spin states in a semiconductor quantum dot, achieve ultrafast control of the spin-wavefunction without an applied magnetic field and make high fidelity read-out the quantum state in an arbitrary basis simply by detecting a strong (~ 2-10 pA) electric current flowing in an external circuit. The results obtained show that the combined quantum state preparation, control and read-out can be performed with a near-unity (≥97%) fidelity.