Vapor–liquid–solid method

The VLS mechanism (VLS of 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 mustache called ) using the catalyst- assisted chemical vapor deposition ( CVD).

History

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

Operation

The growth of a crystal by direct adsorption of a gas phase on a solid surface is very slow in general. The VLS mechanism avoids this situation by introducing a catalytic acting liquid alloy phase, which is due to supersaturation able to quickly adsorb molecules or atoms from a gas phase, and out of the crystal growth, then of nucleis formed at the liquid -solid interface arises. The physical properties of nanowires that have grown in this way controlled will depend on the size and physical properties of the liquid alloy.

The VLS mechanism will be described generally in three stages:

Typical features of the VLS mechanism are greatly reduced by the use of a catalyst or activation energy of reaction ( in comparison to normal CVD processes). Further growing the nanostructures only in the areas of the metal catalyst and the size and position of the wires is determined by the metal catalysts. Thus, the production of highly anisotropic structures with a very high aspect ratio from a wide variety of materials is possible.

Conditions for catalyst

The catalysts must meet the following requirements:

• You must form a liquid solution with the crystalline material.
• The solubility of the catalyst has to be low in the solid and liquid phase of the carrier material.
• The equilibrium vapor pressure of the catalyst needs to be small across over the liquid alloy, so that the droplets do not evaporate or shrink in the radius, because otherwise inhomogeneous wires emerge or growth is terminated at an early stage.
• , The catalyst must not react with the reaction gases themselves.
• The vapor -solid, vapor -liquid and liquid-solid GrenzflächeneEnergien play a key role in the droplet form and therefore must be known prior to the selection of a suitable catalyst. Small contact angle between the droplet and the solid state are more suitable for a large-area growth, while large contact angles lead to thinner structures.
• The solid-liquid interface must crystallographically well- defined ( single-crystalline with a very good orientation to the desired crystal face ) to allow growth of well-aligned nanowires. At the same time, the interface may not be atomically smooth because dislocations for growth are needed. Otherwise, it may happen that atoms from the solution find no defined place on the surface and randomly nucleate.

Growth of silicon nanowires

Next, the growth of a silicon nanowire is to be described by the VLS mechanism, by way of example. First, an about 1 to 10 nm thin layer of gold on a silicon wafer (substrate ) is deposited by means of sputter deposition or thermal evaporation. Then, the wafer is at a temperature greater than the temperature of the eutectic point of Au -Si (about 363 ° C, at a ratio of Au: Si of 4:1 ) was heated in a vacuum coating apparatus ( in such a eutectic melting temperature is distinctly compared with the melting temperature of the pure materials reduced). Thereby forming on the wafer surface droplets of an Au -Si alloy, the thicker the Au layer was, the larger the droplets. Diameter and position of the droplets can also be controlled by means of a photolithographic patterning of the gold film.

Now suitable reaction gases are 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 ( SiCl4 ) and molecular hydrogen (H2). The Au -Si droplets act catalytically, that is, the alloy lowers the necessary activation energy for a chemical reaction. So SiCl4 reacts with H2 at the drop 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 form of an eutectic Au -Si alloy and can absorb silicon from the gas phase to reach a supersaturated state of silicon in gold. It benefits that gold up to 100% solids solutions to all is silicon concentration. The supersaturation of the alloy droplet with silicon atoms now leads that part of the silicon combines them into solid crystallites, this is done at the interface of the solid silicon substrate, the result mentioned wire structures of silicon.

Growth mechanism

Drop formation

The material system being used, the presence of an oxide layer on the droplets or the wafer surface, as well as the cleanliness of the vacuum system ( contamination ) have an influence on the size of the forces acting on the drops of forces on the drops. In turn they define the shape of the droplet, and thus the shape of the nanowires. The shape of the droplet, for example, the contact angle β0, though can be modeled mathematically, but the forces actually acting during growth are very difficult to measure experimentally. However, the form of a catalyst droplet on the substrate surface is determined by a force equilibrium of surface tension and the liquid-solid interfacial tension. The radius of the drop will vary with the angle of contact:

Wherein r0 is the radius of the contact surface and the contact angle β0 of the drop on the substrate surface. The contact angle is obtained from the Young-Laplace equation:

The contact angle depends on the surface ( σs ) and liquid-solid interfacial tension ( σls ) and an additional line tension ( τ ), which is no longer negligible for very small drop radii. At the beginning of growth, the drop height is increased by the amount, and the radius of the contact surface is reduced by the amount. With further growth, and the inclination angle α is increased at the base of the nanowires as well as the contact angle β0:

Therefore, the line voltage has a great influence on the contact area of the catalyst. The most important result of this conclusion is that different Liniensspannungen result in different growth modes. If the line voltage is too large, this results in the formation of nanometer-sized hillocks (pointy shape of the surface outstanding material ) result and thus the end of growth.

Diameter of the nanowires

The diameter of the nanowires is dependent on the properties of the alloy drops, they will be suspended for wires having diameters in the nanometer range correspondingly large alloy droplets on the substrate are required. In the case of equilibrium, this is not possible, since the minimum radius of such a metal drop is calculated as follows

Vl wherein the molar volume of the drop, σlv the surface energy of the liquid-gas interface, and S is the degree of supersaturation of the gas.

These equations limits the minimum diameter of the droplet, and each crystal which is produced in this way, usually considerably higher than the nanometer range. Various techniques have been developed to produce smaller droplets. Including the use of monodisperse nanoparticles which have been spread on the substrate at low dilution, and laser ablation of a substrate-catalyst mixture.

Growth kinetics

During the 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 Gibbs -Thomson effect ):

Δμ0 is the difference between the chemical potential of the deposited material (in the example of silicon ) in the vapor phase and in the solid phase. Δμ is the difference in initial whisker (if ) Furthermore, the atomic volume of the silicon and the specific free energy of the wire surface. From the above equation it follows that smaller diameter (<100 nm) smaller driving forces for the growth have as large wire diameter.