Frank Glas

Growth by molecular beam epitaxy and characterization of InAs/GaAs strained‐layer superlattices

InAs/GaAs superlattices with ultra‐thin InAs (few monolayer) were grown on GaAs substrates. Nucleation of InAs occurs in a two‐dimensional or a three‐dimensional way depending on the growth conditions. The physical properties: x ray, transmission electron microscopy, and photoluminescence were used to characterize the different growth processes.

Why does wurtzite form in nanowires of III-V zinc blende semiconductors?

We develop a nucleation-based model to explain the formation of the wurtzite phase during the catalyzed growth of freestanding nanowires of zinc blende semiconductors. We show that in vapor-liquid-solid nanowire growth, nucleation generally occurs preferentially at the triple phase line. This entails major differences between zinc blende and wurtzite nuclei. Depending on the pertinent interface energies, wurtzite nucleation is favored at high liquid supersaturation. This explains our systematic observation of zinc blende during early growth of gold-catalyzed GaAs nanowires.

Structural and optical properties of high quality zinc-blende/wurtzite GaAs nanowire heterostructures

The structural and optical properties of three different kinds of GaAs nanowires with 100% zinc-blende structure and with an average of 30% and 70% wurtzite are presented. A variety of shorter and longer segments of zinc-blende or wurtzite crystal phases are observed by transmission electron microscopy in the nanowires. Sharp photoluminescence lines are observed with emission energies tuned from 1.515 eV down to 1.43 eV when the percentage of wurtzite is increased. The downward shift of the emission peaks can be understood by carrier confinement at the interfaces, in quantum wells and in random short period superlattices existent in these nanowires, assuming a staggered band offset between wurtzite and zinc-blende GaAs. The latter is confirmed also by time-resolved measurements. The extremely local nature of these optical transitions is evidenced also by cathodoluminescence measurements. Raman spectroscopy on single wires shows different strain conditions, depending on the wurtzite content which affects also the band alignments. Finally, the occurrence of the two crystallographic phases is discussed in thermodynamic terms.

Analysis of vapor-liquid-solid mechanism in Au-assisted GaAs nanowire growth

GaAs nanowires were grown by molecular-beam epitaxy on (111)B oriented surfaces, after the deposition of Au nanoparticles. Different growth durations and different growth terminations were tested. After the growth of the nanowires, the structure and the composition of the metallic particles were analyzed by transmission electron microscopy and energy dispersive x-ray spectroscopy. We identified three different metallic compounds: the hexagonal β′Au7Ga2 structure, the orthorhombic AuGa structure, and an almost pure Au face centered cubic structure. We explain how these different solid phases are related to the growth history of the samples. It is concluded that during the wire growth, the metallic particles are liquid, in agreement with the generally accepted vapor-liquid-solid mechanism. In addition, the analysis of the wire morphology indicates that Ga adatoms migrate along the wire sidewalls with a mean length of about 3μm.

Elastic state and thermodynamical properties of inhomogeneous epitaxial layers: application to immiscible III-V alloys

The elastic strain and stress fields and the elastic energy of the system composed of a crystalline epitaxial layer of finite thickness coherently grown on a bulk substrate are calculated, when the intrinsic stress‐free lattice parameter of the layer is modulated along directions parallel to the substrate surface. When the modulation has components with spatial periods of the same order as the thickness of the layer, the elastic energy is considerably reduced with respect to the same modulation occurring in a bulk sample. There exists an optimal period of elementary sinusoidal modulation, proportional to the layer thickness. Consequently, for immiscible alloys where changes of composition induce changes of intrinsic lattice parameter, the critical temperature (below which they become thermodynamically unstable with respect to composition modulations) is much higher (and the domain of instability larger) if the material is in the epitaxial layer form than in the bulk form. This extends Cahn’s theory of spinodal decomposition to epitaxial layers. It is also pointed out that if a modulation has started to occur in a growing layer, the elastic deformation induced near the free surface should have important consequences on the subsequent growth of this layer. These results are applied to III‐V semiconductors immiscible alloys, such as In x Ga1−x As y P1−y , where such composition modulations are known to exist. New values of the critical temperatures for these alloys are calculated and compared with lower former estimates. The mode of development of these modulations is discussed in light of previous experimental results and these new calculations.

Au-assisted molecular beam epitaxy of InAs nanowires: Growth and theoretical analysis

The Au-assisted molecular beam epitaxial growth of InAs nanowires is discussed. In situ reflection high-energy electron diffraction observations of phase transitions of the catalyst particles indicate that they can be liquid below the eutectic point of the Au-In alloy. The temperature range where the catalyst can be liquid covers the range where we observed nanowire formation (380–430 °C). The variation of nanowire growth rate with temperature is investigated. Pure axial nanowire growth is observed at high temperature while mixed axial/lateral growth occurs at low temperature. The change of the InAs nanowire shape with growth duration is studied. It is shown that significant lateral growth of the lower part of the nanowire starts when its length exceeds a critical value, so that their shape presents a steplike profile along their axis. A theoretical model is proposed to explain the nanowire morphology as a result of the axial and lateral contributions of the nanowire growth.

Chemical potentials for Au-assisted vapor-liquid-solid growth of III-V nanowires

For use in quantitatively modeling the growth of gold-seeded semiconductor nanowires in the vapor-liquid-solid mode, we calculate the difference of chemical potential between a liquid melt formed of group III and group V atoms dissolved in gold and the corresponding solid III-V binary compound. Fits to our results are provided for seven compounds as simple polynomials of the concentrations in the III-V-Au liquid and temperature. We find that the difference of chemical potential increases with the group III and group V concentrations, decreases with increasing temperature, and can easily reach several hundreds of meV per III-V pair. We discuss these values and variations in the light of published experimental results, in particular as regards the crystalline structure adopted by the nanowires during growth.

Composition profiling of InAs∕GaAs quantum dots

We propose a simple and straightforward technique to measure the composition distribution of In(Ga)As∕GaAs quantum dots (QDs). This method is based on the quantitative analysis of 002 dark field image contrast in cross-sectional transmission electron microscopy. InAs∕GaAs QDs show strong InGaAs alloying resulting in the formation, prior to capping, of an inverted indium composition gradient, with higher In concentration at the top. Such a mechanism implies a large Ga mass transport from the substrate. The capping process is also investigated. It strongly affects the QD shape by dissolving the apex via surface migration of indium atoms away from the dot.

Advances in the theory of III–V nanowire growth dynamics

Nanowire (NW) crystal growth via the vapour–liquid–solid mechanism is a complex dynamic process involving interactions between many atoms of various thermodynamic states. With increasing speed over the last few decades many works have reported on various aspects of the growth mechanisms, both experimentally and theoretically. We will here propose a general continuum formalism for growth kinetics based on thermodynamic parameters and transition state kinetics. We use the formalism together with key elements of recent research to present a more overall treatment of III–V NW growth, which can serve as a basis to model and understand the dynamical mechanisms in terms of the basic control parameters, temperature and pressures/beam fluxes. Self-catalysed GaAs NW growth on Si substrates by molecular beam epitaxy is used as a model system.

Wurtzite to zinc blende phase transition in GaAs nanowires induced by epitaxial burying.

We bury vertical free-standing core-shell GaAs/AlGaAs nanowires by a planar GaAs overgrowth. As the nanowires get buried, their crystalline structure progressively transforms: whereas the upper emerging part retains its initial wurtzite structure, the buried part adopts the zinc blende structure of the burying layer. The burying process also suppresses all the stacking faults that existed in the wurtzite nanowires. We consider two possible mechanisms for the structural transition upon burying, examine how they can be discriminated from each other, and explain why the transition is favorable.