B. Grandidier

Faceting, composition and crystal phase evolution in III-V antimonide nanowire heterostructures revealed by combining microscopy techniques

III-V antimonide nanowires are among the most interesting semiconductors for transport physics, nanoelectronics and long-wavelength optoelectronic devices due to their optimal material properties. In order to investigate their complex crystal structure evolution, faceting and composition, we report a combined scanning electron microscopy (SEM), transmission electron microscopy (TEM), and scanning tunneling microscopy (STM) study of gold-nucleated ternary InAs/InAs(1-x)Sb(x) nanowire heterostructures grown by molecular beam epitaxy. SEM showed the general morphology and faceting, TEM revealed the internal crystal structure and ternary compositions, while STM was successfully applied to characterize the oxide-free nanowire sidewalls, in terms of nanofaceting morphology, atomic structure and surface composition. The complementary use of these techniques allows for correlation of the morphological and structural properties of the nanowires with the amount of Sb incorporated during growth. The addition of even a minute amount of Sb to InAs changes the crystal structure from perfect wurtzite to perfect zinc blende, via intermediate stacking fault and pseudo-periodic twinning regimes. Moreover, the addition of Sb during the axial growth of InAs/InAs(1-x)Sb(x) heterostructure nanowires causes a significant conformal lateral overgrowth on both segments, leading to the spontaneous formation of a core-shell structure, with an Sb-rich shell.

Scanning tunneling microscopy and scanning tunneling spectroscopy of self-assembled InAs quantum dots

We present cross-sectional scanning tunneling microscopy images and scanning tunneling spectroscopy results of InAs quantum dots grown on GaAs. The samples contain 12 arrays of quantum dots. The analysis of the scanning tunneling microscope images reveals the self-alignment of the dots as well as the different dot interfaces with the under- and overgrown GaAs layers. We measure the strain distribution along the [001] direction in the (110) plane. The roughness of the dot interfaces along the [110] direction is also estimated and local spectroscopy of the dots evidences the electronic confinement (measured gap of 1.25 eV compared with 0.4 eV for bulk InAs).

Atomic-scale study of GaMnAs/GaAs layers

Cross-sectional scanning tunneling microscopy was used to study GaMnAs diluted magnetic semiconductors grown by low temperature molecular beam epitaxy. The Ga1−xMnxAs layer, containing a concentration of x=0.005, shows that the dominant defect in the material is the arsenic antisite. Mn ions can also be resolved and show a signature distinct from the arsenic antisites. Spectroscopic measurements are perfomed to study the variation of the Fermi level between the Ga0.995Mn0.005As and GaAs layers. The Mn ions act as acceptor dopants. However, for x=0.005, the Mn concentration in comparison with the As antisite concentration is too small to induce a significant change of the Fermi level from the midgap position, preventing the layer from being ferromagnetic.

High charge mobility in two-dimensional percolative networks of PbSe quantum dots connected by atomic bonds

Two-dimensional networks of quantum dots connected by atomic bonds have an electronic structure that is distinct from that of arrays of quantum dots coupled by ligand molecules. We prepared atomically coherent two-dimensional percolative networks of PbSe quantum dots connected via atomic bonds. Here, we show that photoexcitation leads to generation of free charges that eventually decay via trapping. The charge mobility probed with an AC electric field increases with frequency from 150±15 cm2 V−1 s−1 at 0.2 terahertz to 260±15 cm2 V−1 s−1 at 0.6 terahertz. Gated four-probe measurements yield a DC electron mobility of 13±2 cm2 V−1 s−1. The terahertz mobilities are much higher than for arrays of quantum dots coupled via surface ligands and are similar to the highest DC mobilities reported for PbSe nanowires. The terahertz mobility increases only slightly with temperature in the range of 15–290 K. The extent of straight segments in the two-dimensional percolative networks limits the mobility, rather than charge scattering by phonons.

Boron distribution in the core of Si nanowire grown by chemical vapor deposition

The boron dopant distribution in Si nanowires grown by the Au-catalyzed chemical vapor deposition is characterized by laser-assisted atom probe tomography. A convenient and an effective method for performing the atom probe tomography of an individual nanowire is developed. Using this technique, we demonstrate that when Si nanowires are doped with boron at high silane partial pressure, the radial distribution of boron atoms is rather inhomogeneous. Much more boron atoms incorporate at the periphery than in the center, with the concentration increasing by an order of magnitude as the distance from the nanowire axis increases from zero to only 15 nm. A theoretical model is presented that is capable of describing the observed spatial inhomogeneity of boron dopant. We also consider different kinetic pathways of boron incorporation and discuss the values of diffusion length and diffusion coefficients obtained by fitting the experimental data.

Scanning tunneling spectroscopy of individual PbSe quantum dots and molecular aggregates stabilized in an inert nanocrystal matrix

The electronic local density of states (LDOS) of single PbSe quantum dots (QDs) and QD molecules is explored using low-temperature scanning tunneling microscopy (STM) and spectroscopy (STS). Both individual PbSe QDs and molecular aggregates of PbSe QDs (dimers, trimers, etc.) are mechanically stabilized in a two-dimensional superlattice of wide band gap CdSe QDs acting as an inert matrix. The LDOS measured at individual QDs dispersed in the matrix is identical to that of single isolated QDs chemically linked to a substrate. We investigate the degree of quantum mechanical coupling between the PbSe QDs in molecular aggregates by comparing the LDOS measured at each site in the aggregates to that of an individual PbSe QD. We observe a variable broadening of the resonances indicating a spatially dependent degree of electron delocalization in the molecular aggregates.

Can scanning tunnelling spectroscopy measure the density of states of semiconductor quantum dots

Molecules, supramolecular structures and semiconductor nanocrystals are increasingly used as the active components in prototype opto-electrical devices with miniaturized dimensions and novel functions. Therefore, there is a strong need to measure the electronic structure of such single, individual nano-objects. Here, we explore the potential of scanning tunnelling spectroscopy to obtain quantitative information on the energy levels and Coulomb interactions of semiconductor quantum dots. We discuss the conditions under which shell-tunnelling, shell-filling and bipolar spectroscopy can be performed, and illustrate this with spectra acquired on individual CdSe and PbSe quantum dots. We conclude that quantitative information on the energy levels and Coulomb interactions can be obtained if the physics of the tip/quantum dot/substrate double-barrier tunnel junction is well understood.

Quantum box size effect on vertical self-alignment studied using cross-sectional scanning tunneling microscopy

InAs quantum boxes separated by GaAs spacer layers are known to exhibit a vertical self-organization along the growth direction. The alignment probability between two sets of quantum boxes depends strongly on the spacer layer thickness Zs. In this letter, we study samples containing multiple arrays of quantum boxes separated by GaAs spacer layers of various thicknesses, using cross-sectional scanning tunneling microscopy. This work experimentally evidences that the spacer layer characteristic thickness Zs0 below which a vertical self-alignment occurs, depends on the size of the quantum boxes. These results are interpreted using a theoretical two-dimensional model.

Self-Equilibration of the Diameter of Ga-Catalyzed GaAs Nanowires

Designing strategies to reach monodispersity in fabrication of semiconductor nanowire ensembles is essential for numerous applications. When Ga-catalyzed GaAs nanowire arrays are grown by molecular beam epitaxy with help of droplet-engineering, we observe a significant narrowing of the diameter distribution of the final nanowire array with respect to the size distribution of the initial Ga droplets. Considering that the droplet serves as a nonequilibrium reservoir of a group III metal, we develop a model that demonstrates a self-equilibration effect on the droplet size in self-catalyzed III-V nanowires. This effect leads to arrays of nanowires with a high degree of uniformity regardless of the initial conditions, while the stationary diameter can be further finely tuned by varying the spacing of the array pitch on patterned Si substrates.

Growth of Si nanowires on micropillars for the study of their dopant distribution by atom probe tomography

This article reports on the growth of Au islands on the Si(111) surface as a function of the Au evaporation rate and the temperature of the surface in ultrahigh vacuum. By controlling the density of the Au islands and their size, it is possible to subsequently grow single vertically oriented Si nanowires on top of (111)-oriented silicon micropillar and analyze their chemical composition at the atomic scale with the femtosecond laser assisted tomographic atom probe. Three-dimensional images of the atom distribution in the nanowire, in particular, the distribution of boron impurities, are obtained and compared to the intended impurity concentration.