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We present the first experimental study into how Au-seeded GaAs nanowire diameter affects the Ga-dependent incubation and As-dependent step-flow processes revealing a dramatic increase of step-flow process time for smaller diameter nanowires.

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.

We explore the energetics of microwaves interacting with a double quantum dot photodiode and show wave-particle aspects in photon-assisted tunneling. The experiments show that the single photon energy sets the relevant absorption energy in a weak-drive limit, which contrasts the strong drive limit where the wave amplitude determines the relevant-energy scale and opens up microwave induced bias triangles. The threshold condition between these two regimes is set by the fine-structure constant of the system. The energetics are determined here with the detuning conditions of the double dot system and stopping-potential measurements that constitute a microwave version of the photoelectric effect., 5 pages 4 figures

The world of environmental microscopy provides the possibility to study and analyze transformations and reactions during realistic conditions to understand the processes better. We report on the design and development of a metal-organic chemical vapor deposition (MOCVD) system integrated with an environmental transmission electron microscope intended for real-time investigations of crystal growth. We demonstrate methods for achieving a wide range of precisely controlled concentrations of precursor gas at the sample, as well as for calibrating the sample partial pressure using the pressure measured elsewhere in the microscope column. The influences of elevated temperature and reactive gas within the pole-piece gap are evaluated with respect to imaging and spectroscopy. We show that X-ray energy-dispersive spectroscopy can be strongly affected by temperatures beyond 500$^{\circ }$C, while the spatial resolution is largely unaffected by heat and microscope pressure for the relevant conditions. Finally, the influence of the electron beam on the investigated processes is discussed. With this work, we aim to provide crucial input in the development of advanced in situ electron microscopy systems for studies of complex reactions in real time under realistic conditions, for instance as used during formation of semiconductor crystals.

In this work we demonstrate a two-fold selectivity control of InAs shells grown on crystal phase and morphology engineered GaAs nanowire (NW) core templates. This selectivity occurs driven by differences in surface energies of the NW core facets. The occurrence of the different facets itself is controlled by either forming different crystal phases or additional tuning of the core NW morphology. First, in order to study the crystal phase selectivity, GaAs NW cores with an engineered crystal phase in the axial direction were employed. A crystal phase selective growth of InAs on GaAs was found for high growth rates and short growth times. Secondly, the facet-dependant selectivity of InAs growth was studied on crystal phase controlled GaAs cores which were additionally morphology-tuned by homoepitaxial overgrowth. Following this route, the original hexagonal cores with {110} sidewalls were converted into triangular truncated NWs with ridges and predominantly {112}

The interface between wurtzite and zinc blende InP has been identified as type-II, where electrons gather on the zinc blende side and holes on the wurtzite side of the interface. The photoluminescence resulting from recombination across the interface is expected to be long-lived and to exhibit non-exponential decay of emission intensity after pulsed excitation. We verify this prediction using time-resolved photoluminescence spectroscopy on nanowires containing a single heterostructure between a single segment of wurtzite and zinc blende. We find that a significant intensity of type-II emission remains even more than 30 ns after excitation. The decay of the emission intensity is also non-exponential and considerably longer than the exponential decay of the wurtzite InP segment (260 ps). Our results are consistent with the expected photoluminescence characteristics of a type-II interface between the two polytypes. We also find that the lifetime becomes shorter if we create an electron gas at the interface by n-type doping the entire wurtzite segment of the nanowire. This is expected since there are many electrons that a given hole can recombine with, in contrast to the undoped case.