G. Monier

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.

Fast growth synthesis of GaAs nanowires with exceptional length.

We report the first synthesis of GaAs nanowires (NWs) by Au-assisted vapor-liquid-solid (VLS) growth in the novel hydride vapor phase epitaxy (HVPE) environment. Forty micrometer long rodlike <111> monocrystalline GaAs nanowires exhibiting a cubic zinc blende structure were grown in 15 min with a mean density of 10(6) cm(-2). The synthesis of such long figures in such a short duration could be explained by the growth physics of near-equilibrium HVPE. VLS-HVPE is mainly based on solidification after direct and continuous feeding of the arsenious and GaCl growth precursors through the Au-Ga liquid catalyst. Fast solidification (170 microm/h) is then assisted by the high decomposition frequency of GaCl. This predominant feeding through the liquid-solid interface with no mass and kinetic hindrance favors axial rather than radial growth, leading to twin-free nanowires with a constant cylinder shape over unusual length. The achievement of GaAs NWs several tens of micrometers long showing a high surface to volume ratio may open the field of III-V wires, as already addressed with ultralong Si nanowires.

Ultralong and defect-free GaN nanowires grown by the HVPE process.

GaN nanowires with exceptional lengths are synthesized by vapor-liquid-solid coupled with near-equilibrium hydride vapor phase epitaxy technique on c-plane sapphire substrates. Because of the high decomposition frequency of GaCl precursors and a direct supply of Ga through the catalyst particle, the growth of GaN nanowires with constant diameters takes place at an exceptional growth rate of 130 μm/h. The chemical composition of the catalyst droplet is analyzed by energy dispersive X-ray spectroscopy. High-resolution transmission electron microscopy and selective area diffraction show that the GaN nanowires crystallize in the hexagonal wurzite structure and are defect-free. GaN nanowires exhibit bare top facets without any droplet. Microphotoluminescence displays a narrow and intense emission line (1 meV line width) associated to the neutral-donor bound exciton revealing excellent optical properties of GaN nanowires.

Catalyst-assisted hydride vapor phase epitaxy of GaN nanowires: exceptional length and constant rod-like shape capability

The hydride vapor phase epitaxy (HVPE) process exhibits unexpected properties when growing GaN semiconductor nanowires (NWs). With respect to the classical well-known methods such as metal organic vapor phase epitaxy and molecular beam epitaxy, this near-equilibrium process based on hot wall reactor technology enables the synthesis of nanowires with a constant cylinder shape over unusual length. Catalyst-assisted HVPE shows a record short time process (less than 20 min) coupled to very low precursor consumption. NWs are grown at a fast solidification rate (50 μm h(-1)), facilitated by the high decomposition frequency of the chloride molecules involved in the HVPE process as element III precursors. In this work growth temperature and V/III ratio were investigated to determine the growth mechanism which led to such long NWs. Analysis based on the Ni-Ga phase diagram and the growth kinetics of near-equilibrium HVPE is proposed.

Vapor liquid solid-hydride vapor phase epitaxy (VLS-HVPE) growth of ultra-long defect-free GaAs nanowires: Ab initio simulations supporting center nucleation

High aspect ratio, rod-like and single crystal phase GaAs nanowires (NWs) were grown by gold catalyst-assisted hydride vapor phase epitaxy (HVPE). High resolution transmission electron microscopy and micro-Raman spectroscopy revealed polytypism-free zinc blende (ZB) NWs over lengths of several tens of micrometers for a mean diameter of 50 nm. Micro-photoluminescence studies of individual NWs showed linewidths smaller than those reported elsewhere which is consistent with the crystalline quality of the NWs. HVPE makes use of chloride growth precursors GaCl of which high decomposition frequency after adsorption onto the liquid droplet catalysts, favors a direct and rapid introduction of the Ga atoms from the vapor phase into the droplets. High influxes of Ga and As species then yield high axial growth rate of more than 100 μm/h. The diffusion of the Ga atoms in the liquid droplet towards the interface between the liquid and the solid nanowire was investigated by using density functional theory calculations. The diffusion coefficient of Ga atoms was estimated to be 3 × 10(-9) m(2)/s. The fast diffusion of Ga in the droplet favors nucleation at the liquid-solid line interface at the center of the NW. This is further evidence, provided by an alternative epitaxial method with respect to metal-organic vapor phase epitaxy and molecular beam epitaxy, of the current assumption which states that this type of nucleation should always lead to the formation of the ZB cubic phase.

Self-catalyzed GaAs nanowires on silicon by hydride vapor phase epitaxy

Gold-free GaAs nanowires on silicon substrates can pave the way for monolithic integration of photonic nanodevices with silicon electronic platforms. It is extensively documented that the self-catalyzed approach works well in molecular beam epitaxy but is much more difficult to implement in vapor phase epitaxies. Here, we report the first gallium-catalyzed hydride vapor phase epitaxy growth of long (more than 10 μm) GaAs nanowires on Si(111) substrates with a high integrated growth rate up to 60 μm h-1 and pure zincblende crystal structure. The growth is achieved by combining a low temperature of 600 °C with high gaseous GaCl/As flow ratios to enable dechlorination and formation of gallium droplets. GaAs nanowires exhibit an interesting bottle-like shape with strongly tapered bases, followed by straight tops with radii as small as 5 nm. We present a model that explains the peculiar growth mechanism in which the gallium droplets nucleate and rapidly swell on the silicon surface but then are gradually consumed to reach a stationary size. Our results unravel the necessary conditions for obtaining gallium-catalyzed GaAs nanowires by vapor phase epitaxy techniques.

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.

Hydride vapor phase epitaxy growth of III-V nanostructures for high performance devices

The fundamental performance limits of coherent optical transmission systems can be observed by a simple optimization between the linear noise and the nonlinear noise generated within the system. Optical Phase Conjugation (OPC) is considered to be one of the promising techniques to compensate for optical fiber’s dispersion and nonlinearity that cause crosstalk between signals traveling through long-haul optical transmission systems, nonlinearity compensation can lead to significant information capacity and distance reach expansion of optical fiber transmission links. To get the full benefit from the deployment of OPC in optical transmission systems, a few considerations must be taken into account, such as: power profile symmetry, fiber’s dispersion slope and Polarization Mode Dispersion (PMD). In this contribution, we will present our simplified theoretical predictions of optical fiber transmission systems performance that deploy mid-link OPC and multiOPC and we will show that the introduction of multi-OPC in an optical transmission system will minimize the impact of uncompensated/nondeterministic signal-signal nonlinear interactions due to fiber’s PMD and signal-noise interactions. We will show wide range of simulation and experimental results that validate the theoretical predictions of system’s performance for various types of links: dispersion managed, dispersion unmanaged, discretely amplified systems and distributed Raman amplified systems. Also, we will present an extensive experimental study shows that the deployment of mid-link OPC can provide a significant reach improvement in asymmetric lumped optical fiber links when optimizing the span length.III-V semiconductors have a direct bandgap that can be tuned through alloy engineering and therefore appear as very interesting for solar-cells, solid-state lighting and high power applications. The performances of current devices may be increased through the use of nanostructures and nanowires which look promising for the integration of high efficiency devices. Nanowires exhibit great properties such as efficient strain relieving capability and large specific area. Growth on silicon substrates and core-shell structures can be considered as well. Still, the production of nanowire-based devices faces material challenges related to morphological, structural, optical and electrical properties which are very linked to the synthesis process. This presentation will focus on Hydride Vapor Phase Epitaxy, which is a growth process implemented in a hot wall reactor using chloride precursors, and showing unique features regarding the growth of III-V and III-Nitride nanowires. For example, self-catalyzed GaAs nanowires were grown on silicon at a fast growth rate (60 µm.h-1) exhibiting a constant zinc-blende crystalline phase, for the potential fabrication of GaAs-based photonic devices on Si. For III-Nitride materials, InGaN nanowires demonstrating the entire composition range were grown by using a method compatible with the standard GaCl-based GaN growth process. Photoluminescence coupled with transmission electron microscopy measurements showed that these nanowires could overcome the so-called green gap and stretch the limits of solar cells efficiency. By taking advantage of the large growth rates anisotropy resulting from the use of chloride precursors, we could freely tuned the shape of GaN wires on masked substrates with (sub)-micrometric apertures.W the popularization of data centre and other bandwidth hungry inter-connect applications, the desired capacity of short reach optical network has exponentially increased to 400 Gbit/s or even more. Recent standardization efforts for 400 G intradata center connections specify link lengths of up to 2 km. 8×56 Gb/s or 4x100 Gb/s could enable such 400 G networks. Relative to coherent detection. Intensity modulation/direct detection (IM/DD) is a good candidate in inter-connect due to its low cost. For 56 and up to 100 Gb/s signal generation, a few modulation formats or schemes, such as pulse-amplitude-modulation (PAM4), discrete multitone (DMT), duobinary and chirp-managed laser (CML) are proposed and experimentally demonstrated. However, considering cost, size and power comsuption, the modulation format should be optimized for different networks to meet different requirements. In this talk, we will discuss this issue how to optimize the modulation formats for different optical networks?

Self-catalyzed growth of GaAs nanowires on silicon by HVPE

We report on the first self-catalyzed growth of GaAs nanowires on patterned and non-patterned silicon (111) wafers by hydride vapor phase epitaxy (HVPE) with a record elongation rate of 30 μm/h. The crystalline structure was analyzed using high resolution transmission electron microscopy (HRTEM). Self-catalyzed growth proceeds under gallium rich conditions at low-temperature (600 °C). Nanowires exhibit cylindrical rod-like shape morphology with a mean diameter of 50 nm and are randomly distributed.

Study of the surface state density and potential in MIS diode Schottky using the surface photovoltage method

ABSTRACT The effects of surface preparation and illumination on electric parameters of Au/GaN/GaAs Schottky diode were investigated. The thin GaN film is realized by nitridation of GaAs substrates with different thicknesses of GaN layers (0.7 – 2.2 nm). In order to study the electrical characteristics under illumination, we use an He-Ne laser of 632 nm wavelength. The I(V) current- voltage, the surface photovltage SPV measurement were plotted and analysed taking into consideration the influence of charge exchange between a continuum of the surface states and the semiconductor. The barrier height ФBn, the serial resistance Rs and the ideality factor n are respectively equal to 0.66 eV, 1980 Ω, 2.75 under dark and to 0.65 eV, 1160 Ω, 2.74 under illumination for simple 1 (GaN theckness of 0.7 nm). The interface states density Nss in the gap and the excess of concentration δn are determined by fitting the experimental curves of the surface photovltage SPV with the theoretical ones and are equal to 4.5×1012 eV−1 cm−2, 5×107 cm−3, respectively, for sample 1 and 3.5×1012 eV−1 cm−2, 7×108 cm−3 for sample 2 (GaN theckness of 2 nm). The results confirm that the surface photovoltage is an efficient method for optical and electrical characterizations.