X. Wallart

Photoluminescence study of the interface in type II InAlAs–InP heterostructures

Spatially indirect radiative recombinations (type II) have been studied in InAlAs–InP heterostructures grown by gas source molecular beam epitaxy with emphasis on the direct (InAlAs grown on InP) or inverse (InP on InAlAs) interface composition profile. Based on the results of their injection-dependent energy, lifetime and polarization, a new transition scheme is proposed: type II transitions have a low injection limit between 1.27 and 1.28 eV, a long lifetime (τ>1 μs) and strongly shift towards higher energy when increasing the injection. The type II recombination is polarized, the direction of maximum intensity being correlated with the expected interface structure. Lower energy transitions (E⩽1.2 eV) indicate the presence of a well transition material at the interface: they should be better labeled as mixed type I–II. Previously published results are also reconsidered and seem to fit well within this model.

Kinetic model of element III segregation during molecular beam epitaxy of III‐III’‐V semiconductor compounds

Segregation of column III atoms during molecular beam epitaxy of III‐III’‐V semiconductor compounds causes nonabrupt interfaces and a surface composition different from the bulk one. To derive concentration profiles, a thermodynamical equilibrium model has been used for a long time. This model applies well to describe segregation processes at high growth temperatures, but fails in predicting concentration profile variations with substrate temperature. We have thus developed a kinetic model which correctly takes into account the evolution with the growth temperature. We apply this model to the case of indium segregation in the GaxIn1−xAs/GaAs system. The calculated indium concentration profiles are compared to those obtained with the thermodynamical equilibrium model. A kinetic limitation of segregation is shown to appear at low substrate temperatures and sufficiently high growth rates. This limitation is predicted to arise below 400 °C for a growth rate of 1 monolayer/s for In segregation in the GaxIn1−xA...

Organic monolayers on Si(111) by electrochemical method

Abstract This work details the formation of organic monolayers on Si(111) by electrochemical methods. We show that grafting of phenyl groups is possible by reduction of + N 2 –Ar–X cations where the substituent X may be Br, NO 2 , COOH, CN, C n H 2 n +1 ( n =1, 4, 12). Characterizations show that the electrochemical process is self stopped after completion of the first monolayer whose structure is (2×1) close packed in the case X=Br and CH 3 as observed by STM. The stability and passivating properties of films are also investigated.

Gold-free growth of GaAs nanowires on silicon: arrays and polytypism

We report growth by molecular beam epitaxy and structural characterization of gallium-nucleated GaAs nanowires on silicon. The influences of growth temperature and V/III ratio are investigated and compared in the case of oxide-covered and oxide-free substrates. We demonstrate a precise positioning process for Ga-nucleated GaAs nanowires using a hole array in a dielectric layer thermally grown on silicon. Crystal quality is analyzed by high resolution transmission electron microscopy. Crystal structure evolves from pure zinc blende to pure wurtzite along a single nanowire, with a transition region.

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.

Gold-free GaAs/GaAsSb heterostructure nanowires grown on silicon

Growth of GaAs/GaAsSb heterostructure nanowires on silicon without the need for gold seed particles is presented. A high vertical yield of GaAs nanowires is first obtained, and then GaAsxSb1-x segments are successfully grown axially in these nanowires. GaAsSb can also be integrated as a shell around the GaAs core. Finally, two GaAsSb segments are grown inside a GaAs nanowire and passivated using an AlxGa1-xAs shell. It is found that no stacking faults or twin planes occur in the GaAsSb segments.

Formation of platinum-based silicide contacts: Kinetics, stoichiometry, and current drive capabilities

A detailed analysis of the formation of Pt2Si and PtSi silicides is proposed, based on x-ray photoelectron spectroscopy (XPS), transmission electron microscopy (TEM), and electrical characterizations. Published kinetics of the Pt2Si and PtSi transformations under ultrahigh vacuum condition are consolidated on the basis of XPS measurements performed during an in situ annealing at a constant heating rate. At room temperature, an incomplete PtxSi reaction is clearly identified by XPS depth profiling. Using rapid thermal annealing at 300, 400, and 500 °C, the sequential Pt–Pt2Si–PtSi reaction chain is found to be completed within 2 min. Outdiffusion of silicon to the top surface is shown to be responsible for the formation of a thin SiO2 capping layer at 500 °C. Pileup of oxygen occurring at the Pt2Si/Pt reaction front is clearly identified as an inhibiting factor of the silicidation mechanism. Another incomplete reaction scheme limited to the unique formation of Pt2Si is exemplified in the case of ultra thin...

Gold-Free Ternary III-V Antimonide Nanowire Arrays on Silicon: Twin-Free down to the First Bilayer

With the continued maturation of III–V nanowire research, expectations of material quality should be concomitantly raised. Ideally, III–V nanowires integrated on silicon should be entirely free of extended planar defects such as twins, stacking faults, or polytypism, position-controlled for convenient device processing, and gold-free for compatibility with standard complementary metal–oxide–semiconductor (CMOS) processing tools. Here we demonstrate large area vertical GaAsxSb1–x nanowire arrays grown on silicon (111) by molecular beam epitaxy. The nanowires’ complex faceting, pure zinc blende crystal structure, and composition are mapped using characterization techniques both at the nanoscale and in large-area ensembles. We prove unambiguously that these gold-free nanowires are entirely twin-free down to the first bilayer and reveal their three-dimensional composition evolution, paving the way for novel infrared devices integrated directly on the cost-effective Si platform.

Exploring electronic structure of one-atom thick polycrystalline graphene films: A nano angle resolved photoemission study

The ability to produce large, continuous and defect free films of graphene is presently a major challenge for multiple applications. Even though the scalability of graphene films is closely associated to a manifest polycrystalline character, only a few numbers of experiments have explored so far the electronic structure down to single graphene grains. Here we report a high resolution angle and lateral resolved photoelectron spectroscopy (nano-ARPES) study of one-atom thick graphene films on thin copper foils synthesized by chemical vapor deposition. Our results show the robustness of the Dirac relativistic-like electronic spectrum as a function of the size, shape and orientation of the single-crystal pristine grains in the graphene films investigated. Moreover, by mapping grain by grain the electronic dynamics of this unique Dirac system, we show that the single-grain gap-size is 80% smaller than the multi-grain gap recently reported by classical ARPES.

Graphene growth by molecular beam epitaxy on the carbon-face of SiC

Graphene layers have been grown by molecular beam epitaxy (MBE) on the (0001¯) C-face of SiC and have been characterized by atomic force microscopy, low energy electron diffraction (LEED), and UV photoelectron spectroscopy. Contrary to the graphitization process, the step-terrace structure of SiC is fully preserved during the MBE growth. LEED patterns show multiple orientation domains which are characteristic of graphene on SiC (0001¯), indicating non-Bernal rotated graphene planes. Well-defined Dirac cones, typical of single-layer graphene, have been observed in the valence band for few graphene layers by synchrotron spectroscopy, confirming the electronic decoupling of graphene layers.