Direct Observations of Twin Formation Dynamics in Binary Semiconductors

Abstract

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.

Observation of the Multilayer Growth Mode in Ternary InGaAs Nanowires

“…As was previously mentioned, it has been reported for binary nanowires that multilayer growth can occur in conjunction with changes in the crystal structure. 5 This is also true for ternary nanowires, as can be seen in Figure 1b, where the first layer in the multilayer stack nucleates as a twinned layer with regard to the already existing crystal. The following layers nucleate in the same twinned configuration as the first layer in the stack, creating a homogeneous segment.…”

“…However, recent in situ investigations have shown the presence of multilayer growth, where multiple layers nucleate and grow simultaneously. 5,6 So far, the presence of multilayers has been shown for a few binary compound nanowires. 5,6 Binary nanowires consist of a 1:1 combination of an element from group III and one from group V of the periodic table, forming compounds such as GaP and InAs.…”

“…After the layer finishes, there is a possibility for the nanoparticle to build up the supersaturation again by collecting more growth material, before the nucleation and growth of the next layer. , The process of a waiting period (incubation) followed by solidification of material (layer propagation) results in a cyclic layer-by-layer growth process. However, recent in situ investigations have shown the presence of multilayer growth, where multiple layers nucleate and grow simultaneously. , …”

Regulated Dynamics with Two Monolayer Steps in Vapor–Solid–Solid Growth of Nanowires

“…However, the (111) planes remain parallel in the presence of a 2 ML step (Figure a), an effect of the small value of the coincidence mismatch. This shows that the 2 ML step is something different from the piling up of two independent 1 ML steps, as observed if the propagation of a first 1 ML step is slow enough (for instance, if twins are formed) for a second one to nucleate and start propagating before completion of the first one.…”

“…It is widely understood that the growth of a nanowire takes place through layer-by-layer growth at the interface between the seed and the nanowire. , In situ transmission electron microscopy (TEM) studiesimages and movies recorded with a TEM equipped with a source of material allowing the growth of the nanowirehave confirmed this idea and provided details of step dynamics for various materials. The most studied are silicon − and III–V semiconductors, ,− including nitrides . The movies identify two periods which constitute the growth cycle: the nucleation of 1 ML steps at the liquid–solid interface and the expansion of the island due to the propagation of the step across the interface.…”