A. Trassoudaine

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.

Thermodynamical and kinetic study of the GaN growth by HVPE under nitrogen

Abstract The growth of GaN by HVPE was analysed by means of a thermodynamical and kinetic study. The thermodynamical constants of the reactions involved in the GaN growth and the partition functions of the molecules used in the kinetic study were calculated. The kinetic coefficients and the activation energies of the reactions were tabulated. Good agreement was obtained between the model and the experimental results. The influence of the reactor geometry and of the parasitic nucleation on the glass walls of the reactor was demonstrated. After analysing the physical phenomena which might take place in the vapour phase, we concluded that the vapour phase was not homogeneous. The reaction of formation of GaCl 3 from HCl and GaCl appears to be incomplete at the GaCl inlet, and almost complete over the substrate. Both effects explain the difficulty of growing GaN layers without extraneous deposit.

Hydrogen and nitrogen ambient effects on epitaxial growth of GaN by hydride vapour phase epitaxy

This study presents the influence of the composition of the carrier gas on the growth of GaN by HVPE. Since no hydrogen is introduced in the vapour phase, the deposition is expected to be controlled by Cl desorption in the form of GaCl 3 , as has been proposed for GaAs. However, our published model predicts much lower growth rates than those observed. We can account for both the observed parasitic deposition and GaN growth rate if we assume that GaCl 3 is not at its equilibrium pressure in the deposition zone and where nucleation takes place on the walls as well as on the substrate. This yields a high rate of parasitic nucleation even though the nominal supersaturation is vanishing small. Very little growth takes place on the substrate where the equilibrium pressure of GaCl 3 is reached. We describe similar experiments performed with a H 2 /N 2 mixture as the carrier gas. In this case, we expect GaN deposition to be controlled by desorption of Cl as HCl, which is known as the H 2 mechanism. It is speculated that the results show the existence of a new growth mechanism.

Two-particle surface diffusion-reaction models of vapour-phase epitaxial growth on vicinal surfaces

A generalization of the model of Burton, Cabrera and Frank of step flow epitaxial growth on vicinal surfaces in multi-component systems is presented. In particular, the present model addresses chemical vapour epitaxial growth, where the atomic or molecular species composing the crystal, or growth units, are carried to the substrate inside more complex molecules, or precursors, as well as molecular beam epitaxial growth of compound semiconductors. Surface diffusion, chemical reactions, and incorporation at steps are included in the model, that allows for an analytic computation of the growth rate. Special attention is paid to the delicate problem of boundary conditions at steps in a two-component system.

Direct condensation modelling for a two-particle growth system: application to GaAs grown by hydride vapour phase epitaxy

Abstract A new phenomenological model for the growth of GaAs in the GaCl/AsH 3 /HCl/H 2 vapour phase system is developed. The surface growth kinetics are modelled by taking into account the mechanisms of As and GaCl adsorption and chlorine desorption by H 2 into HCl. Two ad-species AsGaCl and AsGa interact on the surface through a reversible reaction, which is described through a modified two-particle Burton, Cabrera and Frank model. Kinetics data are determined by synthesising experimental and computed results. It is shown that when surface diffusion limitations can be neglected, the growth rate is reduced to a one-particle-like direct condensation expression, weighted by a sticking coefficient which takes into account the desorption frequency of the precursor AsGaCl and its reversible transformation into the crystal particle AsGa. Variations of the growth rate are discussed as a function of the ad-species surface coverage ratios and of the supersaturation of the vapour phase.

Influence of the partial pressure of GaCl3 in the growth process of GaN by HVPE under nitrogen

Abstract This study presents the results of experiments performed in a conventional atmospheric horizontal HVPE reactor. The growth results are analysed with a model based on two desorption mechanisms of chlorine. The kinetic of the epitaxial film growth of GaN on sapphire by hydride vapour phase epitaxy is investigated under a variety of experimental conditions. The growth rate and the parasitical deposit on the quartz walls of the reactor upstream and above the substrate depend on the reactor geometry and on the composition of the vapour phase. The result of the experiment for a zero supersaturation over the substrate is discussed. Theoretical calculations compared with experimental results show the non-homogeneity of the gaseous species and the non-equilibrium of the GaCl 3 in the vapour phase.

Growth of gallium nitride by HVPE

The two mechanisms involved in the growth of (00.1) GaN HVPE have been deduced from the numerous experiments performed on (001) GaAs by the chloride method. They include a desorption of adsorbed Cl as HCl by H2 for the first mechanism and a desorption of two Cl as GaCl3 by GaCl for the second one. Theoretical curves have been computed by taking into account the interactions between the mass transfer, approximated by a simple model, the parasitic GaN deposition and the kinetics. They give a good approximation to the expected and observed growth rate values. A new domain of growth experimentally observed in conditions of expected fast etching by HCl ensures growth rates of 50-60 µm h-1 without parasitic GaN deposit by using a suitable temperature profile. This profile is computed by considering a combined mechanism of Cl desorption by GaCl as GaCl2 and of etching by HCl. The produced GaCl2 has to be supposed to be decomposed fast with respect to the mass transfer velocity. Its formation rate in the vapour phase has to be supposed to be very slow with respect to the reverse reaction. These assumptions are in agreement with the difficulty of observing this species and the well known doubt of its existence.