VLS mechanism

The VLS (VLS from English vapor liquid solid , dt. Vapor-liquid-solid method ) is a method for the production of one-dimensional structures such as nanowires (in English often whisker , dt. Barthaar called) using the catalyst-aided chemical Gas phase deposition (CVD).

history

CVD growth of silicon nanowires using gold particle catalysis

The VLS mechanism was proposed in 1964 as an explanation for the silicon nanowire growth from the gas phase in the presence of a liquid gold droplet on a silicon substrate. The explanation was stimulated by the lack of axial screw dislocations , which are in themselves a growth mechanism, in the nanowires, by the need for a gold droplet for growth, and the presence of the droplet at the tip of the nanowires during the entire growth process.

functionality

The growth of a crystal by direct adsorption of a gas phase on a solid surface is generally very slow. The VLS mechanism circumvents this fact by introducing a catalytically active liquid alloy phase, which is able to quickly adsorb molecules or atoms from a gas phase due to its supersaturation and from which the crystal growth of nuclei formed at the liquid-solid interface arises. The physical properties of nanowires grown in a controlled manner in this way depend on the size and physical properties of the liquid alloy.

Schematic representation of the nanowire growth catalyzed with the help of a metal alloy. The illustration shows the path of the starting materials through the droplets to the interface where the growth takes place.

The VLS mechanism is usually described in three phases:

Providing the liquid alloy droplets on the substrate on which the wire is to grow
Introduction of the reaction gases, which adsorb on the surface and diffuse in the droplets
Supersaturation and nucleation at the interface of the liquid drop and the solid base (first the substrate, then the wire itself), which leads to axial crystal growth

Typical features of the VLS mechanism are the activation or reaction energy that is greatly reduced through the use of a catalyst (compared to normal CVD processes). Furthermore, the nanostructures only grow in the areas of the metal catalyst and the size and position of the wires is determined by the metal catalysts. This enables the production of strongly anisotropic structures with a very high aspect ratio from a large number of materials.

Requirements for catalyst particles

The catalysts must meet the following requirements:

You need to form a liquid solution with the crystalline material.
The solubility of the catalyst must be low in the solid and liquid phase of the support material.
The equilibrium vapor pressure of the catalyst must be small across the liquid alloy, so that the drop cannot evaporate or shrink in the radius, otherwise inhomogeneous wires are created or growth is stopped prematurely.
The catalyst must not react with the reaction gases themselves.
The vapor-solid, vapor-liquid, and liquid-solid interface energies play a key role in droplet shape and must therefore be known before choosing a suitable catalyst. Small contact angles between the droplets and the solid are more suitable for large-area growth, while large contact angles lead to thinner structures.
The solid-liquid interface must be crystallographically well defined (single-crystal with a very good orientation towards the desired crystal surface) in order to enable the growth of well-aligned nanowires. At the same time, the interface must not be atomically smooth, since dislocations are required for growth. Otherwise it can happen that atoms from the solution do not find a defined place on the surface and randomly form nuclei.

Growth of silicon nanowires

Schematic representation of the silicon nanowire growth through the reaction of silicon tetrachloride (SiCl ₄ ) and molecular hydrogen (H ₂ ) from the gas phase. This reaction is catalyzed by gold- silicon droplets that were previously deposited on the wafer surface.

In the following, the growth of a silicon nanowire by the VLS mechanism will be described using an example. First, a 1 to 10 nm thin gold layer is deposited on a silicon wafer (the substrate) with the aid of sputter deposition or thermal evaporation. Then, the wafer at temperatures greater than the temperature of the eutectic point of Au-Si (about 363 ° C, at a ratio Au: Si of 4: 1) in a vacuum - coating machine (heated at such a eutectic melting temperature is significantly compared to the melting temperature of the pure materials). Droplets made of an Au-Si alloy are formed on the wafer surface; the thicker the Au layer, the larger the droplets. The diameter and position of the drops can also be checked with the aid of photolithographic structuring of the gold film.

Suitable reaction gases are now introduced into the reaction chamber of the vacuum coating system for the growth of nanowires. In the case of silicon, this is, for example, silicon tetrachloride (SiCl ₄ ) and molecular hydrogen (H ₂ ). The Au-Si droplets have a catalytic effect, i.e. the alloy lowers the activation energy required for a chemical reaction. SiCl ₄ reacts with H ₂ on the droplet surface at lower temperatures than the otherwise necessary temperatures higher than 800 ° C in a normal CVD process. Furthermore, no silicon nuclei form on the surface. However, at temperatures above 363 ° C., droplets of a eutectic Au-Si alloy can form and silicon can absorb silicon from the gas phase until it reaches a supersaturated state of silicon in gold. The advantage here is that gold forms up to 100% solid-state solutions with all silicon concentrations. The supersaturation of the alloy droplet with silicon atoms now leads to a part of the silicon joining together to form solid crystallites. If this happens at the interface with the solid silicon substrate, the aforementioned wire structures made of silicon arise.

Growth mechanism

Drop formation

Schematic representation of the three phases of growth in the VLS mechanism

The material system used, the presence of an oxide layer on the drop or the wafer surface and the cleanliness of the vacuum system (contamination) all have an impact on the magnitude of the forces acting on the drop. They in turn determine the shape of the droplets and thus the shape of the nanowires. The shape of the drop, for example the contact angle β ₀ , can be modeled mathematically, but the forces actually acting during growth are very difficult to measure experimentally. Nevertheless, the shape of a catalyst drop on the substrate surface is determined by an equilibrium of forces between the surface tension and the liquid-solid interfacial tension. The radius of the drop varies with the contact angle:

{\ displaystyle R = {\ frac {r _ {\ mathrm {o}}} {\ sin \ left (\ beta _ {\ mathrm {o}} \ right)}}}

,

where r _{0 is} the radius of the contact area and β _{0 is} the contact angle of the drop on the substrate surface . The contact angle results from the Young-Laplace equation :

{\ displaystyle \ sigma _ {\ mathrm {1}} \ cos (\ beta _ {\ mathrm {o}}) = \ sigma _ {\ mathrm {s}} - \ sigma _ {\ mathrm {ls}} - {\ frac {\ tau} {r _ {\ mathrm {o}}}}}

,

The contact angle depends on the surface (σ _s ) and liquid-solid interfacial tension (σ _ls ) as well as an additional line tension (τ), which is no longer negligible with the very small droplet radii. At the beginning of the growth, the drop height increases by the amount and the radius of the contact area decreases by the amount . With further growth, the angle of inclination α at the base of the nanowires and the contact angle β _{0 also increase} : ${\ displaystyle dh}$ ${\ displaystyle dr}$

{\ displaystyle \ sigma _ {\ mathrm {1}} \ cos (\ beta _ {\ mathrm {o}}) = \ sigma _ {\ mathrm {s}} \ cos (\ alpha) - \ sigma _ {\ mathrm {ls}} - {\ frac {\ tau} {r _ {\ mathrm {o}}}}}

.

The line tension therefore has a great influence on the contact area of the catalyst. The main finding from this conclusion is that different line voltages result in different growth modes. If the line tensions are too high, this results in the formation of nanometer-sized hillocks (material protruding from the surface in a pointed shape) and thus the end of growth.

Diameter of the nanowires

The diameter of the nanowires depends on the properties of the alloy droplets, so for wires with diameters in the nanometer range, correspondingly large alloy droplets are required on the substrate. In the case of equilibrium, this is not possible, since the minimum radius of such a metal drop is calculated as follows

{\ displaystyle R _ {\ mathrm {min}} = {\ frac {2V_ {l}} {RT \ ln (s)}} \ sigma _ {lv} \,}

where V _{l is} the molar volume of the drop, σ _{lv is} the surface energy of the liquid-gas interface, and s is the degree of supersaturation of the gas.

These equations restrict the minimum diameter of the droplet, and any crystal produced in this way, usually well above the nanometer range. Various techniques have been developed to create smaller droplets. Including the use of monodisperse nanoparticles, which were spread on the substrate in low dilution, and the laser ablation of a substrate-catalyst mixture.

Growth kinetics

During VLS nanowire growth, the growth rate depends on the diameter: the larger the diameter, the faster the nanowire grows axially. This is due to the fact that the supersaturation of the metal-alloy catalyst is the driving force for growth and also decreases with decreasing diameter (also known as the Gibbs-Thomson effect ): ${\ displaystyle \ Delta \ mu}$

{\ displaystyle \ Delta \ mu = \ Delta \ mu _ {\ mathrm {o}} - {\ frac {4 \ alpha \ Omega} {d}}}

.

Δμ ₀ is the difference between the chemical potential of the deposited material (in the example silicon) in the vapor phase and in the solid phase. Δμ is the initial difference in whisker growth (if ). Furthermore, is the atomic volume of silicon and the specific free energy of the wire surface. The equation above shows that smaller diameters (<100 nm) have smaller driving forces for growth than large wire diameters. ${\ displaystyle d \ rightarrow \ infty}$ ${\ displaystyle \ Omega}$ ${\ displaystyle \ alpha}$

literature

Florian Michael Kolb: Growth and characterization of silicon nanowires . 2005, p. 15–20 ( Abstract & PDF - Chapter 3: Growth of nanowires; Dissertation, Martin Luther University Halle-Wittenberg, 2005).

Web links

Growing Crystals in the Lab
Dear Research Group Home Page. Harvard University

Individual evidence

^ RS Wagner, WC Ellis: Vapor-liquid-solid mechanism of single crystal growth . In: Applied Physics Letters . tape 4 , no. 5 , 1964, pp. 89 , doi : 10.1063 / 1.1753975 .
↑ Yicheng Lu, Zhong, Jian: Todd Steiner (Ed.): Semiconductor Nanostructures for Optoelectronic Applications . Artech House, Inc., Norwood, MA 2004, ISBN 978-1-58053-751-3 , pp. 191-192.
^ RS Wagner, Albert P. Levitt: Whisker Technology . Wiley - Interscience - New York, 1975, ISBN 0-471-53150-2 .
^ MH Huang, Y. Wu, H. Feick, N. Tran, E. Weber, P. Yang: Catalytic Growth of Zinc Oxide Nanowires by Vapor Transport . In: Advanced Materials . tape 13 , no. 2 , January 2001, p. 113-116 , doi : 10.1002 / 1521-4095 (200101) 13: 2 <113 :: AID-ADMA113> 3.0.CO; 2-H .
^ Ji-Tao Wang: Nonequilibrium Nondissipative Thermodynamics: With Application to Low-pressure Diamond Synthesis . Springer Verlag, Berlin 2002, ISBN 978-3-540-42802-2 , p. 65.
↑ Bharat Bhushan: Springer Handbook of Nanotechnology . Springer-Verlag, Berlin, ISBN 3-540-01218-4 , p. 105.

[1] RS Wagner, WC Ellis: Vapor-liquid-solid mechanism of single crystal growth . In: Applied Physics Letters . tape 4 , no. 5 , 1964, pp. 89 , doi : 10.1063 / 1.1753975 .

[2] Yicheng Lu, Zhong, Jian: Todd Steiner (Ed.): Semiconductor Nanostructures for Optoelectronic Applications . Artech House, Inc., Norwood, MA 2004, ISBN 978-1-58053-751-3 , pp. 191-192.

[3] RS Wagner, Albert P. Levitt: Whisker Technology . Wiley - Interscience - New York, 1975, ISBN 0-471-53150-2 .

[4] MH Huang, Y. Wu, H. Feick, N. Tran, E. Weber, P. Yang: Catalytic Growth of Zinc Oxide Nanowires by Vapor Transport . In: Advanced Materials . tape 13 , no. 2 , January 2001, p. 113-116 , doi : 10.1002 / 1521-4095 (200101) 13: 2 <113 :: AID-ADMA113> 3.0.CO; 2-H .

[5] Ji-Tao Wang: Nonequilibrium Nondissipative Thermodynamics: With Application to Low-pressure Diamond Synthesis . Springer Verlag, Berlin 2002, ISBN 978-3-540-42802-2 , p. 65.

[6] Bharat Bhushan: Springer Handbook of Nanotechnology . Springer-Verlag, Berlin, ISBN 3-540-01218-4 , p. 105.