Lou-Fé Feiner

Twinning superlattices in indium phosphide nanowires

Semiconducting nanowires offer the possibility of nearly unlimited complex bottom-up design, which allows for new device concepts. However, essential parameters that determine the electronic quality of the wires, and which have not been controlled yet for the III–V compound semiconductors, are the wire crystal structure and the stacking fault density. In addition, a significant feature would be to have a constant spacing between rotational twins in the wires such that a twinning superlattice is formed, as this is predicted to induce a direct bandgap in normally indirect bandgap semiconductors, such as silicon and gallium phosphide. Optically active versions of these technologically relevant semiconductors could have a significant impact on the electronics and optics industry. Here we show first that we can control the crystal structure of indium phosphide (InP) nanowires by using impurity dopants. We have found that zinc decreases the activation barrier for two-dimensional nucleation growth of zinc-blende InP and therefore promotes crystallization of the InP nanowires in the zinc-blende, instead of the commonly found wurtzite, crystal structure. More importantly, we then demonstrate that we can, once we have enforced the zinc-blende crystal structure, induce twinning superlattices with long-range order in InP nanowires. We can tune the spacing of the superlattices by changing the wire diameter and the zinc concentration, and we present a model based on the distortion of the catalyst droplet in response to the evolution of the cross-sectional shape of the nanowires to quantitatively explain the formation of the periodic twinning.

A consistent interpretation of the magneto-optical spectra of spinel type ferrites (invited)

Through a systematic investigation of the complete dielectric tensor, between 0.5 and 5.0 eV, of Fe 3 O 4 and of related spinel ferrites, i.e., MgFe 2 O 4 , Li 0.5 Fe 2.5 O 4 , NiFe 2 O 4 , and CoFe 2 O 4 , we have established that intervalence charge transfer and intersublattice charge transfer transitions dominate the optical and magneto-optical spectrum (between 0.5 and 5.0 eV) of all spinel ferrites of the general composition Me x Fe 3−x O 4 . In all cases examined the same set of intersublattice charge transfer transitions was observed. These are the only transitions observed in the cases where Me is a nonmagnetic ion ( Mg 2+ , Li + ). In the cases where Me is a magnetic ion ( Fe 2+ , Ni 2+ , Co 2+ ) additional intervalence charge transfer transitions are observed. CoFe 2 O 4 is the only spinel ferrite with a major contribution of crystal field transitions to the magneto-optical spectrum. The observed presence of only two intense crystal field transitions in specifically CoFe 2 O 4 is explained. The observed relative strengths of these two transitions in CoFe 2 O 4 , in which remarkably the upper transition at 1.82 eV is more intense than the lower transition at 0.83 eV is also explained in a crystal field analysis.

The role of surface energies and chemical potential during nanowire growth

We present an approach to quantitatively determine the magnitudes and the variation of the chemical potential in the droplet (Δμ), the solid-liquid (γ(SL)) and the liquid-vapor (γ(LV)) interface energies upon variation of the group III partial pressure during vapor-liquid-solid-growth of nanowires. For this study, we use GaP twinning superlattice nanowires. We show that γ(LV) is the quantity that is most sensitive to the Ga partial pressure (p(Ga)), its dependence on p(Ga) being three to four times as strong as that of γ(SL) or Δμ, and that as a consequence the surface energies are as important in determining the twin density as the chemical potential. This unexpected result implies that surfactants could be used during nanowire growth to engineer the nanowire defect structure and crystal structure.

Electronic structure of piezoelectric double-barrier InAs/InP/InAs/InP/InAs (111) nanowires

We present a theoretical study of an n-type InAs nanowire with built-in InAs/InP heterojunctions in the effective-mass approximation via self-consistent Poisson–Schrodinger calculations in cylindrical coordinates. Rapid convergence and efficiency are achieved by (i) a suitable transformation of the radial part of the Hamiltonian matrix thereby maintaining symmetry (ii) using quantum mechanical perturbation theory to derive an expression for the change in electron density with electrostatic potential. We calculate the energy levels in a 150 A long InAs quantum dot surrounded by 50 A long InP barriers within an InAs quantum wire of radius 200 A, having a doping level of 3×1016 cm−3 and conduction-band discontinuities of ΔECB=0.6 eV. In equilibrium, the lowest quantum dot state is at 15 meV above the Fermi level and we find that upon variation of the applied collector–emitter voltage VCE, resonance occurs at VCE=88 mV. This is in good agreement with an experimental study of resonant tunneling in a nominally ...

Paired twins and [112] morphology in GaP nanowires.

Formation of random as well as periodic planar defects can occur during vapor-liquid-solid growth of nanowires with the zinc-blende crystal structure. Here we investigate the formation of pairs of twin planes in GaP nanowires. In such pairs, the first twin plane is formed at a random position, rapidly followed by the formation of a second twin plane of which the position is directly related to that of the first one. We show that the triangular [112] morphology of the nanowire is a key element in the formation of these twin pairs. We have extended our previous kinetic nucleation model and show that this describes the development of the nanowire morphology and its relation with the formation of single and paired twin planes.

Nanoelectronics: Crossing boundaries and borders

Collaborations between academic institutions and industrial companies are increasing across Europe, even though each measures progress on different time scales.

Advances in III-V Semiconductor Nanowires and Nanodevices

Description: Semiconductor nanowires exhibit novel electronic and optical properties due to their unique onedimensional structure and quantum confinement effects. In particular, III-V semiconductor nanowires have been of great scientific and technological interest for next generation optoelectronic devices including transistors, light emitting diodes, lasers, photodetectors, and solar cells. Advances in III-V Semiconductor Nanowires and Nanodevices is an account of recent progress in the synthesis, characterization, physical properties, device fabrication, and applications of binary compound and ternary alloy III-V semiconductor nanowires. Each chapter is prepared by renowned experts in the field, describing the current state of knowledge and key areas of research. The book is written at the expert level, but also serves as a guide for researchers or graduate students aiming to enter semiconductor research.

3-Dimensional Morphology of GaP-GaAs nanowires

Semiconductor nanowires are promising candidates for enabling integration of new functionalities. based on the advantageous properties of III–V semiconductors, such as high carrier mobility and optical activity into existing silicon technology. Because of the small dimensions and the consequently large ratio of surface to bulk atoms, the nanowire surface morphology and resulting chemistry can considerably affect the nanowire (opto-) electronic properties, such as carrier mobility and luminescence quantum yield. An elegant way to suppress such surface related effects is to cap the wires with a wide bandgap material to form core/shell nanowire structures. So far, only the vapour-liquid-solid (VLS), i.e. the axial growth mechanism and the zinc blende crystal structure has been considered in relation to the of III–V nanowire side faceting. In order to design and optimise nanowire properties for optical, electrical or mechanical performance, it is essential to take control over the nanowire surface morphology by fully understanding the parameters affecting radial growth.