Si Doping of Vapor-Liquid-Solid GaAs Nanowires: n-Type or p-Type?

The incorporation of Si into vapor-liquid-solid GaAs nanowires often leads to p-type doping, whereas it is routinely used as an n-dopant of planar layers. This property limits the applications of GaAs nanowires in electronic and optoelectronic devices. The strong amphoteric behavior of Si in nanowires is not yet fully understood. Here, we present the first attempt to quantify this behavior as a function of the droplet composition and temperature. It is shown that the doping type critically depends on the As/Ga ratio in the droplet. In sharp contrast to vapor-solid growth, the droplet contains very few As atoms, which enhance their reverse transfer from solid to liquid. As a result, Si atoms preferentially replace As in GaAs, leading to p-type doping in nanowires. Hydride vapor phase epitaxy provides the highest As concentrations in the catalyst droplets during their vapor-liquid-solid growth, resulting in n-type dopant behavior of Si. We present experimental data on n-doped Si-doped GaAs nanowires grown by this method and explain the doping within our model. These results give a clear route for obtaining n-type or p-type Si doping in GaAs nanowires and may be extended to other III-V nanowires.

Record pure zincblende phase in GaAs nanowires down to 5 nm in radius.

We report the Au catalyst-assisted synthesis of 20 µm long GaAs nanowires by the vapor-liquid-solid hydride vapor phase epitaxy (HVPE) exhibiting a polytypism-free zincblende phase for record radii lower than 15 nm down to 5 nm. HVPE makes use of GaCl gaseous growth precursors at high mass input of which fast dechlorination at the usual process temperature of 715 °C results in high planar growth rate (standard 30-40 µm/h). When it comes to the vapor-liquid-solid growth of nanowires, fast solidification at a rate higher than 100 µm/h is observed. Nanowire growth by HVPE only proceeds by introduction of precursors in the catalyst droplets from the vapor phase. This promotes almost pure axial growth leading to nanowires with a constant cylinder shape over unusual length. The question of the cubic zincblende structure observed in HVPE-grown GaAs nanowires regardless of their radius is at the heart of the paper. We demonstrate that the vapor-liquid-solid growth in our conditions takes place at high liquid chemical potential that originates from very high influxes of both As and Ga. This yields a Ga concentration systematically higher than 0.62 in the Au-Ga-As droplets. The high Ga concentration decreases the surface energy of the droplets, which disables nucleation at the triple phase line thus preventing the formation of wurtzite structure whatever the nanowire radius is.

Conduction Mechanisms in Au/0.8 nm-GaN/n-GaAs Schottky Contacts in a Wide Temperature Range.

Au/0.8 nm-GaN/n-GaAs Schottky diodes were manufactured and electrically characterized over a wide temperature range. As a result, the reverse current Iinv increments from 1 × 10-7 A at 80 K to about 1 × 10-5 A at 420 K. The ideality factor n shows low values, decreasing from 2 at 80 K to 1.01 at 420 K. The barrier height qϕb grows abnormally from 0.46 eV at 80 K to 0.83 eV at 420 K. The tunnel mechanism TFE effect is the responsible for the qϕb behavior. The series resistance Rs is very low, decreasing from 13.80 Ω at 80 K to 4.26 Ω at 420 K. These good results are due to the good quality of the interface treated by the nitridation process. However, the disadvantage of the nitridation treatment is the fact that the GaN thin layer causes an inhomogeneous barrier height.

Multi-Mode Elastic Peak Electron Microscopy (MM-EPEM): A new imaging technique with an ultimate in-depth resolution for surface analysis.

A non-destructive new imaging technique called Multi-Mode Elastic Peak Electron Microscopy (MM-EPEM), hypersensitive to surface chemistry and with an in-depth resolution of one atomic monolayer was developed. This method consists on performing several MM-EPEM images containing n × n pixels associated to an intensity of the elastic backscattered electrons by varying the incident electron energy in the range 200-2000 eV. This approach allows obtaining depth sampling information of the analyzed structures. Furthermore, MM-EPEM is associated with Monte-Carlo simulations describing the electron pathway in materials in order to obtain very precise quantitative information, for instance the growth mode and the organization of ultra-thin layers (2D materials) or nanoparticules. In this work, we used this new method to study the deposition of very small amount of gold down to one monolayer. Example of 3D reconstruction is also provided.