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We investigated the interwire distance dependence on the growth kinetics of vertical, high-yield InAs nanowire arrays on Si(111) grown by catalyst-free selective area molecular beam epitaxy (MBE). Utilizing lithographically defined SiO2 nanomasks on Si(111) with regular hole patterns, catalyst-free and site-selective growth of vertically (111)-oriented InAs nanowires was achieved with very high yields of ∼90 percent. Interestingly, the yield of vertically ordered nanowires was independent of the interwire distance and the initial growth stages. Significant size variation in the nanowires was found to depend critically on the interwire distance and growth time. Two growth regimes were identified-(i) a competitive growth regime with shorter and thinner nanowires for narrow interwire distances and (ii) a diffusion-limited growth regime for wider distances, providing good estimates for the surface diffusion lengths. Surprisingly, despite these size-dependent effects the nanowire geometries remained unaltered with uniform, almost nontapered morphologies even over large variation in nanowire density (∼mid-106-109 cm-2 range). X-ray diffraction further confirmed the vertical (111) directionality with low crystal tilt by rocking curve widths (ω scans) as low as ∼0.6°. These findings demonstrate the capability to precisely tailor the position and size of well-oriented III-V semiconductor nanowires through noncatalytic MBE selective area growth and provide an important step toward fully integrated, uniform vertical III-V nanowire array-on-Si devices. [ABSTRACT FROM AUTHOR]

A corner-overgrown GaAs/AlGaAs heterostructure is investigated with transmission and scanning transmission electron microscopy, demonstrating self-limiting growth of an extremely sharp corner profile 3.5 nm wide. In the AlGaAs layers, we observe self-ordered diagonal stripes, precipitating exactly at the corner, which show increased Al content measured with x-ray spectroscopy. A quantitative model for self-limited growth is adapted to the present case of faceted molecular beam epitaxial growth, and the corner sharpness is discussed in relation to quantum confined structures. We note that corner overgrowth maintains nanometer sharpness after microns of growth, allowing corner-shaped nanostructures. [ABSTRACT FROM AUTHOR]

We report conductance measurements in quantum wires made of aluminum arsenide, a heavy-mass, multivalley one-dimensional (1D) system. Zero-bias conductance steps are observed as the electron density in the wire is lowered, with additional steps observable upon applying a finite dc bias. We attribute these steps to depopulation of successive 1D subbands. The quantum conductance is substantially reduced with respect to the anticipated value for a spin- and valley-degenerate 1D system. This reduction is consistent with disorder-induced, intrawire backscattering that suppresses the transmission of 1D modes. Calculations are presented to demonstrate the role of strain in the 1D states of this cleaved-edge structure. [ABSTRACT FROM AUTHOR]

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 In0.2Al0.8As 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 Al0.3Ga0.7As 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. [Copyright &y& Elsevier]

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. [Copyright &y& Elsevier]

Semiconductor nanostructures are realized by patterning AlGaAs/GaAs heterostructures with an atomic force microscope. Steep potential walls, precise pattern transfer and a combination of in-plane and top gates can be achieved with this technique. The electronic properties of nanostructures defined in this way are discussed on two examples, namely a quantum point contact and a single electron transistor. For the quantum point contact we demonstrate quantized conductance at temperatures of 4 K and above. This indicates the strong confinement energy in this system. For the single electron transistor, the realization of special potential shapes and the observation of high-quality Coulomb blockade is shown. [Copyright &y& Elsevier]