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Watch your steps while growing nanowires

Motivated by the emergence of new opto-electronics devices such as single-photon emitters, electron microscopes equipped with effusion cells have been developed in order to monitor in situ the molecular beam epitaxy of complex semiconductor nanostructures. Researchers at IRIG present the results on the growth of II-VI nanowires obtained on the Nanomax setup in Palaiseau, France.​

Published on 20 February 2023

​The development of semiconductor nanostructures has been achieved through a good control of their fabrication, for instance by Molecular Beam Epitaxy (MBE). For a long time, this technique has been limited to the growth of planar structures such as quantum wells, inducing a spatial confinement of charge carriers in the perpendicular direction. New challenges, such as the realization of single-photon emitters for quantum communications, require a stronger confinement in two or three directions, obtained by making nanowires and quantum dots.

The growth of nanowires is usually seeded by a liquid droplet of a few nanometers, which determines the diameter of the nanowire. During the growth, quantum dots can be inserted by changing abruptly the composition of the molecular beam. This “Vapor-Liquid-Solid” mechanism combines a vapor phase in the beam, a liquid phase in the droplet, and a solid phase in the nanowire. At the seed-nanowire interface, the growth was supposed to take place through the nucleation of a one-monolayer step and its propagation along this interface. For years, this was a mere assumption. The Nanomax setup at Centre for Nanoscience and Nanotechnology (C2N) Palaiseau, France, now allows to observe this process in-situ in a modified electron microscope.

Researchers at IRIG and at Neel Institute (Nano Physic and Semiconductors NPSC, Grenoble), are interested in II-VI semiconductors combining a metal (Zn or Cd column II of the Mendeleev table) and a chalcogen (Se or Te column VI). These materials are particularly interesting as active optical elements, for light emission or photovoltaics. II-VI nanowires can be grown with a gold seed which is solid and crystalline instead of liquid: this is the “Vapor-Solid-Solid” growth, which is expected to provide sharper interfaces when inserting a quantum dot. 
Observing the growth of ZnTe nanowires at Nanomax revealed two original aspects: the role of the lattice mismatch at the nanowire-seed interface, and a self-regulation of the step dynamics.

Gold and ZnTe crystals feature the same lattice, but the ZnTe unit cell is 3/2 larger than that of gold. When the gold monolayer located at the interface is progressively replaced by a ZnTe monolayer, a strong mismatch strain appears at the step, which creates a barrier against the formation of such step. 
Alternatively, a different step configuration, made of 2 ZnTe monolayers facing 3 gold monolayers, shows practically no mismatch making this configuration more favorable. And indeed, the Nanomax investigation revealed this growth proceeds for liquid nanoparticles  (Fig. 1 and movie 1). And when nanoparticle is liquid the propagation is made both with one or two monolayers steps (Movie 2).

Figure: (a) ZnTe nanowires grown in MBE cluster; (b) Nanomax image showing the gold seed at the top of the ZnTe nanowire, with a two-monolayer step at the interface; the step (indicated by the arrow) propagates from right to left; (c) idem, with a one-monolayer step. The schemes in-between display the Au atomic planes (in yellow) and the ZnTe planes (in green) for the two configurations.

VideoMovie 2-VLS.mp4

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