Catalyst Droplet Research Articles

Механизм роста монослоя на верхней грани Ga-каталитических нитевидных нанокристаллов GaAs и GaP

The growth mechanism of monolayer on the top facet of Ga-catalyzed GaAs and GaP nanowires is investigated. Within the framework of a theoretical model, the maximal monolayer coverage due to the material in the catalyst droplet, the nanowire growth rate and the content of group V atoms in the droplet are found depending on the growth conditions. The estimates of the phosphorus re-evaporation coefficient from neighboring nanowires and substrate are obtained by comparing the theoretical and experimental growth rate of Ga-catalyzed GaP nanowires.

Growth mechanism of monolayer on the top facet of Ga-catalyzed GaAs and GaP nanowires

The growth mechanism of monolayer on the top facet of Ga-catalyzed GaAs and GaP nanowires is investigated. Within the framework of a theoretical model, the maximal monolayer coverage due to the material in the catalyst droplet, the nanowire growth rate and the content of group V atoms in the droplet are found depending on the growth conditions. The estimates of the phosphorus re-evaporation coefficient from neighboring nanowires and substrate are obtained by comparing the theoretical and experimental growth rate of Ga-catalyzed GaP nanowires. Keywords: III-V nanowires, vapor-liquid-solid growth mechanism, nucleation

Different III-V semiconductor nanowires with quantum dots on silicon: growth by molecular-beam epitaxy and properties

Recently, III-V semiconductor nanostructures of reduced dimension attract more and more interest of researchers for the new generation devices creation. Combinations of nanostructures with different dimensions are of special interest, among them, for example, quantum dots in the body of nanowires. Such quantum dotsʼ size and location control is strictly determined by the growth parameters. As a result of effective relaxation of mechanical stresses on the lateral faces of nanowires, the integration of hybrid nanostructures with silicon technology is possible. In this work, we have demonstrated the possibility of GaP nanowires with GaAs quantum dots and AlGaP nanowires with InGaP quantum dots growth on silicon by molecular-beam epitaxy. The physical properties of the selected nanowires have been investigated. Growth experiments were performed using Riber Compact 21 setup, which is equipped, in addition to the growth chamber, with a vacuum-aligned chamber for gold deposition (metallization chamber). The morphological properties of the obtained nanostructures were studied by scanning electron microscopy. The optical properties of the nanostructures were investigated by the photoluminescence method. The analyses of morphological properties showed that GaP nanowires with GaAs quantum dots were formed predominantly in the <111> direction, in contrast to AlGaP nanowires with InGaP quantum dots, which in some cases changed the growth direction. The reason for the change in the direction of growth of nanowires may be the participation of indium in the growth process. With a sufficient content of indium in the gold catalyst droplet, such mixed droplet can etch the facets at the top of the nanowires, thereby descending to the side of the nanowires and changing the direction of nanowires growth. The studies of the optical properties of the grown nanostructures showed that the photoluminescence signal from InGaP quantum dots in AlGaP nanowires is observed at a temperature of –263 °C with a peak maximum of around 550 nm. Thus, the synthesized nanostructures are promising for optoelectronic applications, in particular, for creating sources of single-photons.

Self-consistent modeling of MBE self-catalyzed GaAs nanowire growth

Self-catalyzed GaAs nanowires are synthesized by molecular beam epitaxy at various arsenic fluxes and growth temperatures. The growth of GaAs nanowires is simulated considering the kinetics of material transport inside the catalyst droplet. The re-evaporation coefficient of arsenic is estimated for the given growth conditions. Calculated nanowire growth rate is in satisfactory agreement with the experimental data.

Tin reduction from fluorine doped tin oxide for silicon nanowire-based solar energy harvesting and storage.

Hydrogen plasma reduction of fluorine doped tin oxide is a beneficial method to form tin nanodroplets on the sample surface directly in the plasma-enhanced chemical vapor deposition reactor. The formation of catalyst droplets is a crucial initial step for vapor-liquid-solid growth of silicon nanowires for radial junction solar cells and solar fuel cell technology. We present an original optical model which allows us to trace the formation process on fluorine doped tin oxide on soda-lime glass substrate from the in situ data and is in a good agreement with the spectroscopic ellipsometry data measured before and during the reduction process. The model reproduces well the phase shift introduced by a transition double layer in fluorine doped tin oxide which acts as a barrier against the sodium diffusion. Furthermore, we study the process of tin reduction from fluorine doped tin oxide in a real time and compare estimated amount of produced metallic tin with images from scanning electron microscopy.The proposed approach is very important for in situ real-time monitoring of the one-pump-down fabrication process used to grow nanowires and form radial junction devices.

Low Temperature Growth of One-Dimensional Al and Al2O3 Nano/Micro-Structures Using Al and SnO2 Powder Mixture

One-dimensional Al and Al2O3 nano/microstructures were fabricated via thermal oxidation of Al and SnO2 powder mixtures at temperatures below the melting point of Al (660 oC). Furthermore, the synthesis process was carried out in air at atmospheric pressure, which made the process very simple and easy. Sn metal particles with spherical shape were observed on the tips of the Al and Al2O3 nano/microstructures, suggesting that the nano/microstructures were grown via a catalyst-assisted growth mechanism. The Sn acted as a catalyst for growing the Al and Al2O3 nano/microstructures. The Sn with low melting point (232 oC) was produced via the reduction of SnO2 by Al, and formed catalyst droplets at the growth temperatures. Al atoms diffused and dissolved into the Sn liquid droplets, leading to the nucleation and then the growth of the Al and Al2O3 nano/microstructures. At 400 oC, the diffusion of Al atoms into the Sn liquid droplets was associated with high stress generated at the Al2O3/Al interface. At 600 oC close to the melting point of Al, Al atoms were diffused from the thin Al liquid layer, which was formed on the surface of the Al powder, to the Sn liquid droplets. Simultaneously, the Al atoms reacted with oxygen in air and formed solid Al2O3 nuclei. A relatively strong ultraviolet emission band centered at 330 nm was observed in the sample prepared at 600 oC.

Induced structural modifications in ZnS nanowires via physical state of catalyst: Highlights of 15R crystal phase

Peculiar and unique growth mechanisms involved in semiconductor nanowires (NWs) pave the way to the achievement of new crystallographic phases and remarkable material properties, and hence, studying polytypism in semiconductor NWs arouses a strong interest for the next generation of electronic and photonic applications. In this context, the growth of ZnS nanowires has been investigated, as bulk ZnS compound exhibits numerous unstable polytypes at high temperatures, but their stable occurrence is highly anticipated in a nanowire due to its special quasi-dimensional shape and growth modes. In this work, the idea is to provide a change in the growth mechanism via the physical state of catalyst droplet (liquid or solid) and hence, study the induced structural modifications in ZnS nanowires. The HRTEM images of VLS (via liquid alloyed catalyst) grown ZnS NWs show periodic stacking faults, which is precisely identified as a stacking sequence of cubic or hexagonal individual planes leading to an astonishing 15R crystal polymorph. This crystallographic phase is observed for the first time in nanowires. Contrastingly, NWs grown with VSS (via solid catalyst) show crystal polytypes of zinc blende and wurtzite. We calculate and discuss the role of cohesive energies in the formation of such ZnS polytypes. Further, we present the selection rules for the crystallization of such 15R structure in NWs and discuss the involved VLS and VSS growth mechanisms leading to the formation of different crystal phases.

On the kinetics of the early stage of growth of III-V nanowires

The Au-assisted III-V nanowire growth is often preceded by the formation of pyramid-like pedestals under catalyst droplets during pre-growth annealing. In the present paper, we propose a kinetic step-flow model of the transition from the growth of pedestals to the formation of nanowires with constant diameters. In our model an atomic step at the pedestal sidewall appears when the edge of the 2D island growing at the liquid-solid interface under the droplet attains the triple phase line. The steps move along the sidewall downward to the pedestal base at the expense of adatoms attaching to the step edges from adjacent terraces. The analytical expressions for the step velocities yielding a set of equations for time-dependent positions of steps are obtained. It is shown that the transition to nanowire growth might take place as the result of the step bunching at the pedestal top when the upper terrace becomes shorter than a certain critical value at which the step-step repulsion is significant. The numerical simulations of the pedestal growth demonstrate a rapid transition to nanowire growth at low temperatures and the absence of such a transition at high temperatures, which is in accordance with the experimental observations.

About a fundamental uncertainty of the contact angle of the catalyst drop on the top of the nanowire

In this paper, we aim to explain a fundamental uncertainty of the contact angle of the drops catalyzing vapor–liquid–solid growth of semiconductor nanowires (NWs). We developed the concepts about the crystallization processes occurring near the triple-phase boundary as well as the conditions of equilibrium wetting of NW top surface singular with a catalyst liquid drop characterized by the NW contact angle. We demonstrated that due to either dissolution of the crystallizable substance or its precipitation from the liquid catalyst drop on the NW tip, the wetting perimeter of the drop is not fixed and the drop can take an equilibrium shape with the NW contact angle, which does not satisfy Young's equation. The NW contact angle corresponds to the indifferent equilibrium of the catalyst droplet because of the wetting hysteresis. We proposed that the fundamental uncertainty of the NW contact angle of the catalyst drop saturated with a crystallizable substance at the NW tip is a result of the interaction of two opposing competing processes: the spheroidization of the catalytic liquid during the gravity neutralization and the wetting. We concluded that the steady-state NW contact angle of the drop depends on the free surface energies of the solid/liquid and liquid/vapor phase boundaries but does not depend on the free surface energy of the vertical sidewalls of the NWs. In this paper, we also proposed a new method to find the free surface energies of phase boundaries by the shape of the catalyst droplet on the NW tip as well as on the substrate. The paper also presents the results of obtaining the interphase free energy of the boundaries in Si-Au, Si-Cu, Ge-Au, GaAs-Au, InAs-Au, SiC-Fe, InP-Au, and so on systems.

Self‐Consistent Modeling of Nucleation and Growth of 2D Islands on the Top Facet of Self‐Catalyzed GaAs Nanowires

Self‐catalyzed GaAs nanowire (NW) growth via the vapor–liquid–solid mechanism is investigated by a theoretical model including the kinetics of material transport inside the catalyst droplet. The proposed model allows the description of nucleation and growth of 2D islands on the top facet of GaAs NWs. Analytical expressions for the growth rate of the disk‐shaped GaAs island due to the volume diffusion of species in the droplet and for the attachment rate of GaAs pairs to the critical island are derived. As a result, the duration of the droplet refilling stage and the island growth stage at typical growth conditions of self‐catalyzed GaAs NWs are obtained by a self‐consistent calculation. Also, the time evolution of the droplet composition and the island radius are found. The derived equations for the island growth rate can be applied for modeling of catalyst‐assisted growth of other III–V compounds. The results of the modeling are in good agreement with the experimental data on self‐catalyzed GaAs NW growth via molecular beam epitaxy and can be used for the optimization of the NW growth conditions.

Products characterization for fast in-situ catalytic pyrolysis of bio-oil over Ni/Al2O3

Pyrolysis is a promising technology for bio-oil utilization. However, bio-oil is thermally unstable and easily coking due to the complicated components and their interactions. This study developed an in-situ catalytic pyrolysis method for bio-oil over Ni/Al2O3. The results of product distribution and aromatic structures were investigated to elucidate the influence of catalyst participation and droplet sizes of bio-oil. The nickel catalyst can effectively decrease coke yield and increase gas yield at the studied condition (800 °C fast pyrolysis). Small droplets that lead to a higher heating rate would promote severer secondary reactions which lead to repolymerization. The catalyst would provide adsorptive points that help small fragments to be cracked and result in large fragments polymerizing.

Thermodynamics of the Vapor-Liquid-Solid Growth of Ternary III-V Nanowires in the Presence of Silicon.

Based on a thermodynamic model, we quantify the impact of adding silicon atoms to a catalyst droplet on the nucleation and growth of ternary III–V nanowires grown via the self-catalyzed vapor–liquid–solid process. Three technologically relevant ternaries are studied: InGaAs, AlGaAs and InGaN. For As-based alloys, it is shown that adding silicon atoms to the droplet increases the nanowire nucleation probability, which can increase by several orders magnitude depending on the initial chemical composition of the catalyst. Conversely, silicon atoms are found to suppress the nucleation rate of InGaN nanowires of different compositions. These results can be useful for understanding and controlling the vapor–liquid–solid growth of ternary III–V nanowires on silicon substrates as well as their intentional doping with Si.

Unexpected phosphorus doping routine of planar silicon nanowires for integrating CMOS logics.

Complementary doping control in silicon nanowire (SiNW) channels is crucial for the construction of high-performance CMOS logics. Though planar in-plane solid-liquid-solid (IPSLS) growth, with an amorphous Si (a-Si) thin film as a precursor, has demonstrated a precise and scalable integration of orderly SiNWs, a complementary and tunable n-type doping has not been accomplished. This has been hindered by the fact that the phosphorus (P) gas dopants will react with the indium (In) catalyst droplet to form insoluble InP precipitates. Nevertheless, we herein report on an unexpected discovery that the P dopants first incorporated into the a-Si matrix can easily diffuse over the In catalyst droplets, without forming an InP compound, and thus reverse continuously the initial p-type SiNWs into an n-type channel. Uniform and efficient doping effects have been confirmed by both atomic probe tomography mapping and the transfer properties of SiNW FETs, which demonstrate a steep subthreshold swing of 105 mV dec-1, an on/off ratio of >107 and an electron mobility of 142 cm2 V-1 s-1. Finally, true CMOS inverters are successfully demonstrated based on the closely-packed SiNW channels of distinct doping polarities, indicating a new convenient and highly efficient doping routine to construct more advanced SiNW logics and sensors.

Superfast Growth Dynamics of High-Quality Silicon Nanowires on Polymer Films via Self-Selected Laser-Droplet-Heating

Growing high quality silicon nanowires (SiNWs) at elevated temperature on cooler polymer films seems to be contradictive but highly desirable for building high performance flexible and wearable electronics. In this work, we demonstrate a superfast (vnw > 3.5 μm·s-1) growth of high quality SiNWs on polymer/glass substrates, powered by self-selected laser at 808 nm heating of indium catalyst droplets that absorb amorphous Si layer to produce SiNWs. Because of the tiny heat capacity of the nanodroplets, the SiNW growth can be quickly heated up and frozen via rapid laser ON/OFF switching, enabling a deterministic diameter modulation in the ultralong SiNWs. Finally, prototype field effect transistors are also fabricated upon the laser-droplet-heating grown SiNWs with a high Ion/Ioff ratio of >104 and reasonable subthreshold swing of 386 mV·dec-1, opening a generic new route to integrate high-quality NW channels directly upon large area and lightweight polymer substrates for developing high-performance flexible electronics.

Bismuth-catalyzed n-type doping and growth evolution of planar silicon nanowires

Guided growth of silicon nanowires (SiNWs) into precise locations, via an in-plane solid–liquid–solid (IPSLS) mechanism, is a key basis for scalable integration of SiNW-based electronics, but an effective n-type doping has not yet been accomplished. In this work, we report a bismuth (Bi) catalyzed and doped growth of IPSLS SiNWs, where the incorporation of Bi atoms gives rise to efficient n-type doping, as confirmed by electron dispersion analysis and transfer properties of SiNW transistors. Interestingly, a rich geometry evolution is observed during the Bi-catalyzed planar growth, which evolves from discrete islands to continuous island chains and to uniform segments, prior to a final droplet collapsing/flattening at the end. A growth model has been established to address this peculiar phenomenon, emphasizing the impact of surface/interface tensions on the stability of the leading catalyst droplet. These results provide a solid basis for the construction of more advanced complementary SiNW logics and electronics.

Catalyst-mediated doping in electrochemical growth of solar silicon

The direct electrochemical reduction of silicon dioxide in molten CaCl2 has been investigated as a promising candidate for alternative production process of solar silicon. Herein, we study a catalyst-mediated doping, which is carried out by introducing dopant precursors like Al2O3, In2O3, and Sb2O3 into the molten CaCl2 during the electrochemical growth of silicon on Ag catalyst. The photoelectrochemical measurement demonstrates controllable p-type or n-type doping, and the photoresponse of doped silicon reaches 1.1±0.4 mA/cm2 under 100 mW/cm2 irradiation. Doping densities, estimated from two-point micro-probe measurements and Mott–Schottky plot analyses range from 1015 to 1016/cm3. Dopant is incorporated through Si–Ag catalyst droplet, which is supported by a high dopant content in Ag catalyst revealed by the elemental mapping of secondary ion mass spectroscopy. The catalyst-mediated doping mechanism is proposed to elucidate the doping level. The liquid droplet is presumed to be under local equilibrium along with significant loss of the dopant from it whereby the doping level becomes a function of the segregation coefficient and the rate constant of the electrochemical reaction.

Growth Kinetics of Planar Nanowires

An approximate analytic equation is derived that describes the law of elongation of a semiconductor nanowire (NW) growing via the vapor–liquid–solid (VLS) mechanism in a substrate plane. Various growth regimes are theoretically analyzed as dependent on NW radius R and epitaxial deposition conditions. It is established that the growth rate of planar NWs can be controlled either by the Gibbs–Thomson effect (in the case of small catalyst droplet dimensions) or by the diffusion of adatoms from the substrate surface (for increasing radius of the crystal). Dependence of the diffusion-controlled growth rate on radius R obeys the R–m law, where the power exponent takes the values of 1, 3/2, or 2 depending on the character of surface diffusion.

Energetics and kinetics of monolayer formation in vapor-liquid-solid nanowire growth

The vapor-liquid-solid growth of semiconductor nanowires proceeds via the sequential addition of individual atomic or biatomic monolayers at the interface between the nanowire stem and a liquid catalyst droplet. Each monolayer growth cycle comprises a period of nucleation and extension of the monolayer followed by a waiting time before the next nucleation. The cyclic evolution of the liquid-solid system and the duration of these periods affect the properties of individual nanowires and the statistics of nanowire ensembles. Based on recent in situ growth experiments, we investigate theoretically the thermodynamics and kinetics of monolayer formation, with emphasis on nanowires of III-V materials. We first study the thermodynamics of the process in a quasistatic approximation. We calculate and minimize the work of formation of a partial monolayer, accounting for the depletion and refill of the liquid phase and all relevant interface energies. We focus on the regime where the liquid does not contain enough nanowire material at nucleation to form a full monolayer and where growth comprises a fast stage where a partial monolayer forms rapidly using the available material and a slower stage during droplet refill. We explore the effect of the nonmonotonous variation of the total monolayer edge energy on these two growth stages and determine the partition between liquid and monolayer of the atoms provided during refill. We then develop a kinetic model that accounts in simple terms for the competition between delivery and attachment of atoms to the monolayer and droplet refill. We perform fully analytical calculations that describe a continuum between the pulsed regime explored in our thermodynamic study and a smooth regime where the monolayer extends at the pace of refill. The various characteristic times of the monolayer cycle are calculated in presence of desorption of the volatile group V species from the liquid droplet. Simplified asymptotic formulas corresponding to fast attachment or low desorption are derived. Desorption is found to affect weakly the monolayer formation time but strongly the waiting time.

Catalytically mediated epitaxy of 3D semiconductors on van der Waals substrates

The formation of well-controlled interfaces between materials of different structure and bonding is a key requirement when developing new devices and functionalities. Of particular importance are epitaxial or low defect density interfaces between two-dimensional materials and three-dimensional semiconductors or metals, where an interfacial structure influences electrical conductivity in field effect and optoelectronic devices, charge transfer for spintronics and catalysis, and proximity-induced superconductivity. Epitaxy and hence well-defined interfacial structure has been demonstrated for several metals on van der Waals-bonded substrates. Semiconductor epitaxy on such substrates has been harder to control, for example during chemical vapor deposition of Si and Ge on graphene. Here, we demonstrate a catalytically mediated heteroepitaxy approach to achieve epitaxial growth of three-dimensional semiconductors such as Ge and Si on van der Waals-bonded materials such as graphene and hexagonal boron nitride. Epitaxy is “transferred” from the substrate to semiconductor nanocrystals via solid metal nanocrystals that readily align on the substrate and catalyze the formation of aligned nuclei of the semiconductor. In situ transmission electron microscopy allows us to elucidate the reaction pathway for this process and to show that solid metal nanocrystals can catalyze semiconductor growth at a significantly lower temperature than direct chemical vapor deposition or deposition mediated by liquid catalyst droplets. We discuss Ge and Si growth as a model system to explore the details of such hetero-interfacing and its applicability to a broader range of materials.

Impact of droplet composition on the nucleation rate and morphology of vapor-liquid-solid GeSn nanowires

It is well-known that the chemical potential which drives the vapor-liquid-solid growth of semiconductor nanowires is strongly affected by the liquid phase composition. Here, we investigate theoretically how the droplet composition influences the nucleation of Au-catalyzed GeSn nanowires on Ge(111) and Si(111) substrates. We compare the chemical potentials in an Au-Ge-Sn catalyst droplet before and after adding Ga and/or Si atoms. It is found that the presence of these atoms enhances the nucleation rate of nanowires on both substrates. Theoretical results are compared to experimental data on GeSn nanowires grown in a hot-wall reduced pressure chemical vapor deposition reactor. It is shown that the intentional addition of Ga in the de-wetting step improves the uniformity of the nanowire dimensions and yields higher density of nanowires over Ge(111) substrates. The nanowire growth on Si(111) substrate occurs only when Ga and/or Si are added to Au droplets. These results show that controlling the composition of the catalyst droplet is crucial for improving the quality of GeSn nanowires.