Comprehensive model toward optimization of SAG In-rich InGaN nanorods by hydride vapor phase epitaxy

Controlled growth of In-rich InGaN nanowires/nanorods (NRs) has long been considered as a very challenging task. Here, we present the first attempt to fabricate InGaN NRs by selective area growth using hydride vapor phase epitaxy. It is shown that InGaN NRs with different indium contents up to 90% can be grown by varying the In/Ga flow ratio. Furthermore, nanowires are observed on the surface of the grown NRs with a density that is proportional to the Ga content. The impact of varying the NH₃ partial pressure is investigated to suppress the growth of these nanowires. It is shown that the nanowire density is considerably reduced by increasing the NH₃ content in the vapor phase. We attribute the emergence of the nanowires to the final step of growth occurring after stopping the NH₃ flow and cooling down the substrate. This is supported by a theoretical model based on the calculation of the supersaturation of the ternary InGaN alloy in interaction with the vapor phase as a function of different parameters assessed at the end of growth. It is shown that the decomposition of the InGaN solid alloy indeed becomes favorable below a critical value of the NH₃ partial pressure. The time needed to reach this value increases with increasing the input flow of NH₃, and therefore the alloy decomposition leading to the formation of nanowires becomes less effective. These results should be useful for fundamental understanding of the growth of InGaN nanostructures and may help to control their morphology and chemical composition required for device applications.

Long catalyst-free InAs nanowires grown on silicon by HVPE

We report for the first time on the hydride vapor phase epitaxy (HVPE) growth of long (26 μm) InAs nanowires on Si(111) substrate. The thermodynamic and kinetic mechanisms involved during the growth of such long nanowires are identified.

Compositional control of homogeneous InGaN nanowires with the In content up to 90

Homogenous InGaN nanowires with a controlled indium composition up to 90% are grown on GaN/c-Al₂O₃ templates by catalyst-free hydride vapor phase epitaxy using InCl₃ and GaCl as group III element precursors. The influence of the partial pressures on the growth rate and composition of InGaN nanowires is investigated. It is shown how the InN mole fraction in nanowires can be finely tuned by changing the vapor phase composition. Thermodynamic calculations are presented that take into account different interconnected reactions in the vapor phase and show a good agreement with the compositional data. Energy dispersive x-ray spectroscopy profiles performed on single nanowires show a homogenous indium composition along the entire nanowire length. X-ray diffraction measurements performed on nanowires arrays confirm these data. High-resolution transmission electron microscopy analysis shows the wurtzite crystal structure with a reduced defect density for InGaN nanowires with the highest indium content.

Selective growth of ordered hexagonal InN nanorods

Well-ordered and vertically aligned InN nanorods with high aspect ratios are synthesized by hydride vapor phase epitaxy (HVPE) using the selective area growth (SAG) approach.

Circumventing the miscibility gap in InGaN nanowires emitting from blue to red

Widegap III-nitride alloys have enabled new classes of optoelectronic devices including light emitting diodes, lasers and solar cells, but it is admittedly challenging to extend their operating wavelength to the yellow-red band. This requires an increased In content x in In _x Ga_1-x N, prevented by the indium segregation within the miscibility gap. Beyond the known advantage of dislocation-free growth on dissimilar substrates, nanowires may help to extend the compositional range of InGaN. However, the necessary control over the material homogeneity is still lacking. Here, we present In _x Ga_1-x N nanowires grown by hydride vapor phase epitaxy on silicon substrates, showing rather homogeneous compositions and emitting from blue to red. The InN fraction in nanowires is tuned from x = 0.17 up to x = 0.7 by changing the growth temperature between 630 °C and 680 °C and adjusting some additional parameters. A dedicated model is presented, which attributes the wide compositional range of nanowires to the purely kinetic growth regime of self-catalyzed InGaN nanowires without macroscopic nucleation. These results may pave a new way for the controlled synthesis of indium-rich InGaN structures for optoelectronic applications in the extended spectral range.

Crystal engineering by tuning the growth kinetics of GaN 3-D microstructures in SAG-HVPE

The growth of GaN 3-D microstructures is investigated by SAG-HVPE.

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.

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.

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.

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.