Controlling the crystallographic phase purity of III-V nanowires is notoriously difficult, yet this is essential for future nanowire devices. Reported methods for controlling nanowire phase require dopant addition, or a restricted choice of nanowire diameter, and only rarely yield a pure phase. Here we demonstrate that phase-perfect nanowires, of arbitrary diameter, can be achieved simply by tailoring basic growth parameters: temperature and V/III ratio. Phase purity is achieved without sacrificing important specifications of diameter and dopant levels. Pure zinc blende nanowires, free of twin defects, were achieved using a low growth temperature coupled with a high V/III ratio. Conversely, a high growth temperature coupled with a low V/III ratio produced pure wurtzite nanowires free of stacking faults. We present a comprehensive nucleation model to explain the formation of these markedly different crystal phases under these growth conditions. Critical to achieving phase purity are changes in surface energy of the nanowire side facets, which in turn are controlled by the basic growth parameters of temperature and V/III ratio. This ability to tune crystal structure between twin-free zinc blende and stacking-fault-free wurtzite not only will enhance the performance of nanowire devices but also opens new possibilities for engineering nanowire devices, without restrictions on nanowire diameters or doping.

Phase Perfection in Zinc Blende and Wurtzite III-V Nanowires Using Basic Growth Parameters

Controlled growth of nanowires is an important, emerging research field with many applications in, for example, electronics, photonics, and life sciences. Nanowires of zinc blende crystal structure, grown in the 〈111〉B direction, which is the favoured direction of growth, usually have a large number of twin-plane defects. Such defects limit the performance of optoelectronic nanowire-based devices. To investigate this defect formation, we examine GaP nanowires grown by metal-organic vapour-phase epitaxy. We show that the nanowire segments between the twin planes are of octahedral shape and are terminated by {111} facets, resulting in a microfaceting of the nanowires. We discuss these findings in a nucleation context, where we present an idea on how the twin planes form. This investigation contributes to the understanding of defect formation in nanowires. One future prospect of such knowledge is to determine strategies on how to control the crystallinity of nanowires.

Structural properties of 〈111〉B -oriented III–V nanowires

Anisotropic II-VI semiconductor nanocrystals and nanoparticles have become important building blocks for (potential) nanotechnological applications. Even though a wide variety of differently shaped nanoparticles of this class can be prepared, the underlying mechanisms are mostly not fully understood. This Review article provides a brief overview of the currently studied shape-evolution mechanisms and the most prominent synthesis methods for such particles, with an aim to provide a fundamental understanding on how different morphologies evolve, and to function as a tool to aid in the preparation of specific nanocrystals.

Shape Control of II–VI Semiconductor Nanomaterials

Fundamental studies on Bridgman growth of CdTe

A novel application of the vertical gradient freeze method to the growth of high quality III–V crystals

Using weak beam electron microscopy the stacking fault energy (SFE) of III-V compounds is determined by measuring the dissociation width of edge dislocations. The SFE corrected for the lattice parameters is given in meV/atom for GaAs, GaP, GaSb, InAs, InP, and InSb. As expected there is a strong correlation of the SFE with the ionicity of the bond. The different plasticity of the compounds is traced back to different dislocation velocities.



Mit Weak-Beam-Elektronenmikroskopie wird die Stapelfehlerenergie (SFE) von III-V-Verbindungen durch Messung der Aufspaltungsweite von Stufenversetzungen bestimmt. Die SFE, korrigiert fur die Gitterparameter, wird in meV/Atom angegeben fur GaAs, GaP, GaSb, InAs, InP und InSb. Wie erwartet gibt es eine starke Abhangigkeit der SFE von der Ionizitat der Bindung. Die unterschiedliche Plastizitat der Verbindungen wird auf verschieden grose Versetzungsgeschwindigkeiten zuruckgefuhrt.

Stacking fault energy and ionicity of cubic III–V compounds

A new method of measuring dislocation velocities in the bulk by TEM is presented. Jogs on dislocations are strong obstacles against dislocation motion during a low temperature deformation. They can be used as markers for the initial course of the dislocation lines and allow to measure the paths travelled by the free dislocation segments between the jogs. First results for the activation energy of the motion of screws and 60° dislocations (2 eV) and for the edges (1.8 eV) are reported. An edge dislocation is assumed to be a series of geometrical kinks. Its motion is interpreted as the collective motion of kinks and the activation energy is tentatively attributed to the kink migration energy.



Eine neue Methode, Versetzungsgeschwindigkeiten im Inneren eines Kristalls mit TEM zu messen, wird vorgestellt. Jogs auf Versetzungen sind Hindernisse fur die Versetzungsbewegung bei einer Niedertemperatur-Verformung. Sie markieren die Ausgangslage der Versetzungen und ermoglichen, den Laufweg der freien Segmente zwischen den Jogs zu messen. Erste Ergebnisse fur die Aktivierungs-energien von Schrauben-, 60° - (2 eV), und Stufenversetzungen (1.8 eV) werden mitgeteilt. Es wird angenommen, eine Stufenversetzung sei eine Aneinanderreihung geometrischer Kinken und ihre Bewegung sei korrelierter Kinkenbewegung zuzuschreiben. Deshalb wird ihre Aktivierungsenergie versuchsweise der Kinkenwanderungsenergie zugeordnet.

Constricted dislocations and their use for TEM measurements of the velocities of edge and 60° dislocations in silicon. A new approach to the problem of kink migration

The dependence of the EPR spectrum of dislocations in deformed silicon on illumination with monochromatic light reveals the two EPR centers Si—K1 (S < 1/2) to be different ionization states of one and the same dislocation center. The energy level separating these ionization states lies near the middle of the gap. Photo-EPR of p- and n-Si demonstrates that extracting an electron from the single spin states Si—K1 transforms the dislocation center into a spin-free state. It is assumed that the transition K1 K2 is connected with a charge state stimulated structure change. In undoped silicon the relaxation of a light induced change of the dislocation charge is strongly inhibited below 140 K.



Die Anderungen des ESR-Spektrums der Versetzungen in verformtem Silizium bei Beleuchtung mit monochromatischem Licht (Photo-ESR) zeigen, das die ESR-Zentren Si—K1 (S = 1/2) und Si—K2 (S > 1/2) verschiedene Ionisationsstufen ein und desselben Versetzungszentrums sind. Die Grenzenergie zwischen den Ionisationsstufen liegt nahe der Mitte der Bandlucke. Photo-ESR von p-Si zeigt die Bildung des Zustands Si—K1 aus einem spinfreien Zustand. Fur den Ubergang K1 K2 wird ein durch die Anderung des Ladungszustandes induzierter Umbau des Zentrums vorgeschlagen. Lichtinduzierte Anderungen der Versetzungsladung sind in undotiertem Silizium fur lange Zeit persistent.

Photo-EPR of Dislocations in silicon

The stacking fault ribbon of dissociated 60 dislocations in silicon is imaged with resolution better than 0.33 nm. The dissociation width of the dislocations had been frozen in under high shear stress. It is relaxed by heating within the electron microscope. The dynamics of kinks on the partial dislocations can be analyzed in this way. We find the migration energy of kinks on 90 partials to be Wm  1:24 0:07 eV. The formation energy of a single kink is estimated to Fk  0:73 0:15 eV. Obstacles for kink motion are observed under the beam; they are thermally overcome with activation energy Eu  2:4 eV.

Kinks on Partials of 60° Dislocations in Silicon as Revealed by a Novel TEM Technique

Kink dynamics on dislocations in Si is studied in a wide stress range (up to 150 MPa) using the intermittent loading (IL) technique. At low stresses (0 to 7 MPa) a two-level IL is employed using selective etching to reveal the dislocations. Dependencies are obtained of the mean dislocation displacements on the pulse and pause durations, as well as on the sign and values of stresses acting during the pulse separation. Experimental data are analyzed in the framework of the kinetic model taking into account diffusion and drift of the kinks. A new method is employed for the measurements of the dynamical properties of geometrical kinks using a combination of IL and transmission electron microscopy (TEM). The displacements of the central parts of the pinned edge dislocations in dipoles are measured in the bulk of Si for different dipole widths in the temperature range 593 to 693 K. Assuming that the displacement of edge dislocation segments is determined by the motion of geometrical kinks, the quantitative parameters of kink motion are obtained by a comparison of the experimental data with computer simulations of the process. A correlation between the dynamical properties of geometrical kinks constituting an edge dislocation and the kink moving along 60° dislocations is discussed in the framework of the theories taking into account the influence of point defects and secondary Peierls relief on the processes of kink pair formation and spreading.



[Russian Text Ignored].

H. Alexander

Papers

Stacking fault energy and ionicity of cubic III–V compounds

Constricted dislocations and their use for TEM measurements of the velocities of edge and 60° dislocations in silicon. A new approach to the problem of kink migration

Photo-EPR of Dislocations in silicon

Kinks on Partials of 60° Dislocations in Silicon as Revealed by a Novel TEM Technique

Barriers for the kink motion on dislocations in Si