Probing the light hole/heavy hole switching with correlated magneto-optical spectroscopy and chemical analysis on a single quantum dot.

A whole series of complementary studies have been performed on the same single nanowire containing a quantum dot: cathodoluminescence spectroscopy and imaging, micro-photoluminescence spectroscopy under magnetic field and as a function of temperature, and energy-dispersive x-ray spectrometry and imaging. The ZnTe nanowire was deposited on a Si3N4 membrane with Ti/Al patterns. The complete set of data shows that the CdTe quantum dot features the heavy-hole state as a ground state, although the compressive mismatch strain promotes a light-hole ground state as soon as the aspect ratio is larger than unity (elongated dot). A numerical calculation of the whole structure shows that the transition from the heavy-hole to the light-hole configuration is pushed toward values of the aspect ratio much larger than unity by the presence of a (Zn, Mg)Te shell, and that the effect is further enhanced by a small valence band offset between the semiconductors in the dot and around it.

Ultrafast room temperature single-photon source from nanowire-quantum dots.

Epitaxial semiconductor quantum dots are particularly promising as realistic single-photon sources for their compatibility with manufacturing techniques and possibility to be implemented in compact devices. Here, we demonstrate for the first time single-photon emission up to room temperature from an epitaxial quantum dot inserted in a nanowire, namely a CdSe slice in a ZnSe nanowire. The exciton and biexciton lines can still be resolved at room temperature and the biexciton turns out to be the most appropriate transition for single-photon emission due to a large nonradiative decay of the bright exciton to dark exciton states. With an intrinsically short radiative decay time (≈300 ps) this system is the fastest room temperature single-photon emitter, allowing potentially gigahertz repetition rates.

The importance of the radial growth in the faceting of silicon nanowires.

The state of the lateral surface plays a great role in the physics of silicon nanowires. Surprisingly, little is known about the phenomena that occur during growth on the facets of the wires. We demonstrate here that the size and shape of the facets evolve with the exposure time and the radial growth speed. Depending on the chemistry of the surface, either passivated by chlorine or decorated by gold clusters, the radial growth speed varies and the evolution of the facets is enhanced or impeded. If the radial growth speed is high enough, the faceting of the wire can change from top to bottom due to the exposure time difference. Three types of faceting are exposed, dodecagonal, hexagonal, and triangular. An evolution model is introduced to link the different faceting structures and the possible transitions.

The morphology of silicon nanowires grown in the presence of trimethylaluminium.

The effects of trimethylaluminium (TMA) on silicon nanowires grown by chemical vapour deposition (CVD) were investigated in the 650-850 degrees C growth temperature range. Gold was used as the growth catalyst and SiH4 in H2 carrier gas as the Si precursor. Depending on substrate temperature and TMA partial pressure, the structure's morphology evolves from wires to tapered needles, pyramids or nanotrees. The TMA presence was linked to two specific growth modes: an enhanced surface growth which forms Si needles and a branched growth leading to Si nanotrees. We suggest that competition between these two specific growth modes and the usual Au-catalyzed VLS growth is responsible for the observed morphology changes.