RESUMEN

Surface diffusion is known to be of prime importance in the growth of semiconductor nanowires. In this work, we used ZnMgO layers as markers to analyze the growth mechanisms and kinetics during the deposition of ZnMgO/ZnO multilayered shells by molecular beam epitaxy on previously grown ZnO nanowire cores (so called core-shell heterostructures). Specifically, the influence of the O2 flow sent into the plasma cell on the adatom surface mobility was investigated. By carefully measuring the growth rate on the lateral facets as well as on the top of the nanowires, it is concluded that the surface diffusion length of adatoms, within the used MBE growth conditions, is very low. Such poor surface mobility explains why so few works can be found related to the spontaneous growth (without catalyst) of ZnO nanowires by MBE, contrary to other deposition techniques.

RESUMEN

Quantitative characterization of electrically active dopants and surface charges in nano-objects is challenging, since most characterization techniques using electrons [1-3], ions [4] or field ionization effects [5-7] study the chemical presence of dopants, which are not necessarily electrically active. We perform cathodoluminescence and voltage contrast experiments on a contacted and biased ZnO nanowire with a Schottky contact and measure the depletion length as a function of reverse bias. We compare these results with state-of-the-art off-axis electron holography in combination with electrical in situ biasing on the same nanowire. The extension of the depletion length under bias observed in scanning electron microscopy based techniques is unusual as it follows a linear rather than square root dependence, and is therefore difficult to model by bulk equations or finite element simulations. In contrast, the analysis of the axial depletion length observed by holography may be compared with three-dimensional simulations, which allows estimating an n-doping level of 1 × 1018 cm-3 and negative sidewall surface charge of 2.5 × 1012 cm-2 of the nanowire, resulting in a radial surface depletion to a depth of 36 nm. We found excellent agreement between the simulated diameter of the undepleted core and the active thickness observed in the experimental data. By combining TEM holography experiments and finite element simulation of the NW electrostatics, the bulk-like character of the nanowire core is revealed.

RESUMEN

The characterization of nanowires (NWs) often requires the use of scanning electron beam techniques because of their high spatial resolution. However, the impact of the high energetic electron beam on the physical parameters under investigation is rarely taken into account. In this work, a combination of optical and electrical techniques is involved for the measurement of the electron beam dose (EBD) dependence of cathodoluminescence intensity, exciton diffusion length and electrical resistance in ZnO NWs. Large EBD dependences of these key parameters are observed and their reversibility is investigated. The results are discussed in terms of bulk and surface reversible modifications. In particular, the behaviors of surface recombination velocity and surface space charge under electron beam exposure are determined and simulated. This study points out that caution must be taken and experimental protocols must be well defined when measuring physical parameters of NWs using electron beam techniques.

RESUMEN

Whereas nanowire (NW)-based devices offer numerous advantages compared to bulk ones, their performances are frequently limited by an incomplete understanding of their properties where surface effect should be carefully considered. Here, we demonstrate the ability to spatially map the electric field and determine the exciton diffusion length in NW by using an electron beam as the single excitation source. This approach is performed on numerous single ZnO NW Schottky diodes whose NW radius vary from 42.5 to 175 nm. The dominant impact of the surface on the NW properties is revealed through the comparison of three different physical quantities recorded on the same NW: electron-beam induced current, cathodoluminescence, and secondary electron signal. Indeed, the space charge region near the Schottky contact exhibits an unusual linear variation with reverse bias whatever the NW radius. On the contrary, the exciton diffusion length is shown to be controlled by the NW radius through surface recombination. This systematic comparison performed on a single ZnO NW demonstrates the power of these complementary techniques in understanding NW properties.

RESUMEN

Current-voltage and Kelvin probe force microscopy (KPFM) measurements were performed on single ZnO nanowires. Measurements are shown to be strongly correlated with the contact behavior, either Ohmic or diode-like. The ZnO nanowires were obtained by metallo-organic chemical vapor deposition (MOCVD) and contacted using electronic-beam lithography. Depending on the contact geometry, good quality Ohmic contacts (linear I-V behavior) or non-linear (diode-like) contacts were obtained. Current-voltage and KPFM measurements on both types of contacted ZnO nanowires were performed in order to investigate their behavior. A clear correlation could be established between the I-V curve, the electrical potential profile along the device and the nanowire geometry. Some arguments supporting this behavior are given based on technological issues and on depletion region extension. This work will help to better understand the electrical behavior of Ohmic contacts on single ZnO nanowires, for future applications in nanoscale field-effect transistors and nano-photodetectors.

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RESUMEN

We report on the efficient room-temperature photoluminescence (PL) quenching of ZnO in the presence of 2,4-dinitrotoluene (DNT) vapor and for concentration as low as 180 ppb. Compared to ZnO thin films, ZnO nanowires exhibit a strong (95%) and fast (41 s) quenching of the PL intensity in the presence of DNT vapor. Assuming that the PL quenching is due to a trapping of the ZnO excitons by adsorbed DNT molecules, Monte-Carlo calculations show that the nanometric dimensions as well as the better crystallographic quality (longer mean free path) of the ZnO nanowires result in an enhanced trapping process at the origin of the improved sensing properties of the nanowires. The results demonstrate the importance of nanostructures in improving the sensitivity of ZnO. The study also reveals the sensing capability of ZnO nanowires and paves the path towards the potential realization of low-cost sub-ppb nitroaromatic derivative sensors.

RESUMEN

We bury vertical free-standing core-shell GaAs/AlGaAs nanowires by a planar GaAs overgrowth. As the nanowires get buried, their crystalline structure progressively transforms: whereas the upper emerging part retains its initial wurtzite structure, the buried part adopts the zinc blende structure of the burying layer. The burying process also suppresses all the stacking faults that existed in the wurtzite nanowires. We consider two possible mechanisms for the structural transition upon burying, examine how they can be discriminated from each other, and explain why the transition is favorable.

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