L. Travers

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.

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.

GaNAsSb: how does it compare with other dilute III–V-nitride alloys?

Growth and properties of GaNAsSb alloys are investigated and compared with those of other dilute III–N–V alloys. Similar properties are observed including very high bandgap bowing, carrier localization at low temperature, sensitivity to thermal annealing and passivation of N-related electronic states by hydrogen. On the other hand, we point out some features of this alloy system and evaluate its potential for device applications. Probably, GaNAsSb can achieve emission at longer wavelengths than GaInNAs alloys grown to date. Its conduction- and valence-band offsets can be independently tuned by adjusting the N and Sb composition, respectively. Since this compound has a single group III element, its electronic structure should be less dependent on alloy configuration than GaInNAs.

Epitaxial graphene on cubic SiC(111)/Si(111) substrate

Epitaxial graphene films grown on silicon carbide (SiC) substrate by solid state graphitization is of great interest for electronic and optoelectronic applications. In this paper, we explore the properties of epitaxial graphene films on 3C-SiC(111)Si(111) substrate. X-ray photoelectron spectroscopy and scanning tunneling microscopy were extensively used to characterize the quality of the few-layer graphene (FLG) surface. The Raman spectroscopy studies were useful in confirming the graphitic composition and measuring the thickness of the FLG samples.

GaInAs/GaAs quantum-well growth assisted by Sb surfactant: Toward 1.3 μm emission

The growth of highly strained GaInAs quantum wells on GaAs is investigated in the presence of Sb. Sb appears as an adequate isoelectronic surfactant: the lateral relaxation of strain is shown to be significantly delayed in comparison with a Sb-free growth. This effect is used to extend the emission wavelength of GaInAs quantum wells. We obtained a 9-nm-thick Ga0.59In0.41As0.986Sb0.008 quantum wells with smooth interfaces, emitting at 1.27 μm at room temperature.

Investigations on GaInNAsSb quinary alloy for 1.5 μm laser emission on GaAs

GaInNAsSb quantum wells grown by molecular-beam epitaxy on GaAs substrates were investigated. Intricate incorporation mechanisms of the constituents in this quinary alloy were seen. In highly strained indium-rich alloys, antimony incorporation is strongly reduced, and a beneficial surfactant effect is observed. Due to this effect, high structural quality is preserved even for an uncompensated 2.67% strained multiquantum-well structure. Narrow luminescence linewidth (35 meV) could be achieved near 1.55 μm wavelength with these quantum wells. Laser emission is demonstrated at 1.50 μm with threshold current density of 3.5 kA/cm2.

Morphology of self-catalyzed GaN nanowires and chronology of their formation by molecular beam epitaxy

GaN nanowires are synthesized by plasma-assisted molecular beam epitaxy on Si(111) substrates. The strong impact of the cell orientation relative to the substrate on the nanowire morphology is shown. To study the kinetics of growth, thin AlN markers are introduced periodically during NW growth. These markers are observed in single nanowires by transmission electron microscopy, giving access to the chronology of the nanowire formation and to the time evolution of the nanowire morphology. A long delay precedes the beginning of nanowire formation. Then, their elongation proceeds at a constant rate. Later, shells develop on the side-wall facets by ascending growth of layer bunches which first agglomerate at the nanowire foot.

Quantum-well saturable absorber at 1.55μm on GaAs substrate with a fast recombination rate

We propose and realize a structure designed for fast saturable absorber devices grown on GaAs substrate. The active region consists of a 1.55μm absorbing GaInNAsSb quantum well (QW) surrounded by two narrow QWs of GaAsN with a N concentration up to 13%. Photoexcited carriers in the GaInNAsSb QW are expected to recombine by tunneling into the wide distribution of subband gap states created in the GaAsN QW. An absorption study shows that edge energy and excitonic peak intensity of the GaInNAsSb QW are not affected by the proximity of the GaAsN QWs. Pump-probe measurements provide information on the carrier relaxation dynamics which is dependent on spacer thickness, as expected for a tunneling process. We show that this process can be enhanced by increasing the N content in the GaAsN layers. Using this design, we have realized a monolithic GaAs-based saturable absorber microcavity with a 1∕e recovery time of 12ps.

Strain control of the magnetic anisotropy in (Ga,Mn) (As,P) ferromagnetic semiconductor layers

A small fraction of phosphorus (up to 10%) was incorporated in ferromagnetic (Ga,Mn)As epilayers grown on a GaAs substrate. P incorporation allows reducing the epitaxial strain or even change its sign, resulting in strong modifications of the magnetic anisotropy. In particular a reorientation of the easy axis toward the growth direction is observed for high P concentration. It offers an interesting alternative to the metamorphic approach, in particular for magnetization reversal experiments where epitaxial defects strongly affect the domain wall propagation.

Piezo-generator integrating a vertical array of GaN nanowires

We demonstrate the first piezo-generator integrating a vertical array of GaN nanowires (NWs). We perform a systematic multi-scale analysis, going from single wire properties to macroscopic device fabrication and characterization, which allows us to establish for GaN NWs the relationship between the material properties and the piezo-generation, and to propose an efficient piezo-generator design. The piezo-conversion of individual MBE-grown p-doped GaN NWs in a dense array is assessed by atomic force microscopy (AFM) equipped with a Resiscope module yielding an average output voltage of 228 ± 120 mV and a maximum value of 350 mV generated per NW. In the case of p-doped GaN NWs, the piezo-generation is achieved when a positive piezo-potential is created inside the nanostructures, i.e. when the NWs are submitted to compressive deformation. The understanding of the piezo-generation mechanism in our GaN NWs, gained from AFM analyses, is applied to design a piezo-generator operated under compressive strain. The device consists of NW arrays of several square millimeters in size embedded into spin-on glass with a Schottky contact for rectification and collection of piezo-generated carriers. The generator delivers a maximum power density of ∼12.7 mW cm(-3). This value sets the new state of the art for piezo-generators based on GaN NWs and more generally on nitride NWs, and offers promising prospects for the use of GaN NWs as high-efficiency ultra-compact energy harvesters.