Probability of twin formation on self-catalyzed GaAs nanowires on Si substrate

Epitaxial growth of crystal phase quantum dots in III-V semiconductor nanowires

Crystal phase quantum dots (QDs) are formed during the axial growth of III-V semiconductor nanowires (NWs) by stacking different crystal phases of the same material. In III-V semiconductor NWs, both zinc blende (ZB) and wurtzite (WZ) crystal phases can coexist. The band structure difference between both crystal phases can lead to quantum confinement. Thanks to the precise control in III-V semiconductor NW growth conditions and the deep knowledge on the epitaxial growth mechanisms, it is nowadays possible to control, down to the atomic level, the switching between crystal phases in NWs forming the so-called crystal phase NW-based QDs (NWQDs). The shape and size of the NW bridge the gap between QDs and the macroscopic world. This review is focused on crystal phase NWQDs based on III-V NWs obtained by the bottom-up vapor-liquid-solid (VLS) method and their optical and electronic properties. Crystal phase switching can be achieved in the axial direction. In contrast, in the core/shell growth, the difference in surface energies between different polytypes can enable selective shell growth. One reason for the very intense research in this field is motivated by their excellent optical and electronic properties both appealing for applications in nanophotonics and quantum technologies.

Direct Observations of Twin Formation Dynamics in Binary Semiconductors

With the increased demand for controlled deterministic growth of III-V semiconductors at the nanoscale, the impact and interest of understanding defect formation and crystal structure switching becomes increasingly important. Vapor-liquid-solid (VLS) growth of semiconductor nanocrystals is an important mechanism for controlling and studying the formation of individual crystal layers and stacking defects. Using in situ studies, combining atomic resolution of transmission electron microscopy and controlled VLS crystal growth using metal organic chemical vapor deposition, we investigate the simplest achievable change in atomic layer stacking-single twinned layers formed in GaAs. Using Au-assisted GaAs nanowires of various diameters, we study the formation of individual layers with atomic resolution to reveal the growth difference in forming a twin defect. We determine that the formation of a twinned layer occurs significantly more slowly than that of a normal crystal layer. To understand this, we conduct thermodynamic modeling and determine that the propagation of a twin is limited by the energy cost of forming the twin interface. Finally, we determine that the slower propagation of twinned layers increases the probability of additional layers nucleating, such that multiple layers grow simultaneously. This observation challenges the current understanding that continuous uniform epitaxial growth, especially in the case of liquid-metal assisted nanowires, proceeds one single layer at a time and that its progression is limited by the nucleation rate.

GaAs Core/SrTiO3 Shell Nanowires Grown by Molecular Beam Epitaxy

We have studied the growth of a SrTiO3 shell on self-catalyzed GaAs nanowires grown by vapor-liquid-solid assisted molecular beam epitaxy on Si(111) substrates. To control the growth of the SrTiO3 shell, the GaAs nanowires were protected using an arsenic capping/decapping procedure in order to prevent uncontrolled oxidation and/or contamination of the nanowire facets. Reflection high energy electron diffraction, scanning electron microscopy, transmission electron microscopy, and X-ray photoelectron spectroscopy were performed to determine the structural, chemical, and morphological properties of the heterostructured nanowires. Using adapted oxide growth conditions, it is shown that most of the perovskite structure SrTiO3 shell appears to be oriented with respect to the GaAs lattice. These results are promising for achieving one-dimensional epitaxial semiconductor core/functional oxide shell nanostructures.

Crystal phase control in GaAs nanowires: opposing trends in the Ga- and As-limited growth regimes

Here we demonstrate the existence of two distinct regimes for tuning crystal structure in GaAs nanowires from zinc blende to wurtzite using a single process parameter: V/III-ratio, or variation of the group V precursor flow. Extensive previous studies have shown that crystal structure is sensitive to V/III-ratio, and even that it is possible to change structure entirely using this single parameter. However, an open question has remained about whether the observed dependencies are related to growth technique or types of precursors used. Specifically, opposite trends have been reported for molecular beam epitaxy (MBE) and metal organic vapour phase epitaxy (MOVPE): while wurtzite GaAs growth is reported for high nominal V/III-ratio in MBE, zinc blende GaAs is formed in MOVPE under apparently the same parameter change (increasing precursor V/III-ratio). Here we show that these observations are not necessarily contradictory, as it may first appear, by providing a consolidated picture covering all regimes in one MOVPE growth machine only. More precisely, we observe wurtzite formation for medium nominal V/III-ratios with a critical sensitivity to the balance between Ga and As supply. Slight deviations from wurtzite conditions will result in zinc blende formation for either low V/III-ratio in the As-limited regime or high V/III-ratio in the Ga-limited regime. Our observations strongly indicate that the applied growth conditions are the crucial ingredients for crystal structure control in GaAs nanowires rather than the growth technique or precursors used.

Evolution of polytypism in GaAs nanowires during growth revealed by time-resolved in situ x-ray diffraction

In III-V nanowires the energetic barriers for nucleation in the zinc blende or wurtzite arrangement are typically of a similar order of magnitude. As a result, both arrangements can occur in a single wire. Here, we investigate the evolution of this polytypism in self-catalyzed GaAs nanowires on Si(111) grown by molecular beam epitaxy with time-resolved in situ x-ray diffraction. We interpret our data in the framework of a height dependent Markov model for the stacking in the nanowires. In this way, we extract the mean sizes of faultless wurtzite and zinc blende segments-a key parameter of polytypic nanowires-and their temporal evolution during growth. Thereby, we infer quantitative information on the differences of the nucleation barriers including their evolution without requiring a model of the nucleus.

All zinc-blende GaAs/(Ga,Mn)As core-shell nanowires with ferromagnetic ordering

Combining self-catalyzed vapor-liquid-solid growth of GaAs nanowires and low-temperature molecular-beam epitaxy of (Ga,Mn)As, we successfully synthesized all zinc-blende (ZB) GaAs/(Ga,Mn)As core-shell nanowires on Si(111) substrates. The ZB GaAs nanowire cores are first fabricated at high temperature by utilizing the Ga droplets as the catalyst and controlling the triple phase line nucleation, then the (Ga,Mn)As shells are epitaxially grown on the side facets of the GaAs core at low temperature. The growth window for the pure phase GaAs/(Ga,Mn)As core-shell nanowires is found to be very narrow. Both high-resolution transmission electron microscopy and scanning electron microscopy observations confirm that all-ZB GaAs/(Ga,Mn)As core-shell nanowires with smooth side surface are obtained when the Mn concentration is not more than 2% and the growth temperature is 245 °C or below. Magnetic measurements with different applied field directions provide strong evidence for ferromagnetic ordering in the all-ZB GaAs/(Ga,Mn)As nanowires. The hybrid nanowires offer an attractive platform to explore spin transport and device concepts in fully epitaxial all-semiconductor nanospintronic structures.