Vapor–liquid–solid method

The vapor–liquid–solid method (VLS) is a mechanism for the growth of one-dimensional structures, such as nanowires, from chemical vapor deposition.

The growth of a crystal through direct adsorption of a gas phase on to a solid surface is generally very slow.

The VLS mechanism circumvents this by introducing a catalytic liquid alloy phase which can rapidly adsorb a vapor to supersaturation levels, and from which crystal growth can subsequently occur from nucleated seeds at the liquid–solid interface.

The VLS mechanism is typically described in three stages:[2] The VLS process takes place as follows: The requirements for catalysts are:[3] The materials system used, as well as the cleanliness of the vacuum system and therefore the amount of contamination and/or the presence of oxide layers at the droplet and wafer surface during the experiment, both greatly influence the absolute magnitude of the forces present at the droplet/surface interface and, in turn, determine the shape of the droplets.

The shape of the droplet, i.e. the contact angle (β0, see Figure 4) can, be modeled mathematically, however, the actual forces present during growth are extremely difficult to measure experimentally.

where r0 is the radius of the contact area and β0 is defined by a modified Young’s equation:

The diameter of the nanowire which is grown depends upon the properties of the alloy droplet.

This equations restricts the minimum diameter of the droplet, and of any crystals which can be grown from it, under typically conditions to well above the nanometer level.

Several techniques to generate smaller droplets have been developed, including the use of monodispersed nanoparticles spread in low dilution on the substrate, and the laser ablation of a substrate-catalyst mixture so to form a plasma which allows well-separated nanoclusters of the catalyst to form as the systems cools.

Again, Δμ is the main driving force for nanowhisker growth (the supersaturation of the metal droplet).

More specifically, Δμ0 is the difference between the chemical potential of the depositing species (Si in the above example) in the vapor and solid whisker phase.

Involves the removal of material from metal-containing solid targets by irradiating the surface with high-powered (~100 mJ/pulse) short (10 Hz) laser pulses, usually with wavelengths in the ultraviolet (UV) region of the light spectrum.

These particles are easily transferred to the substrate where they can nucleate and grow into nanowires.

Finally, the plasma formed during the laser absorption process allows for the deposition of charged particles as well as a catalytic means to lower the activation barrier of reactions between target constituents.

Some very interesting nanowires microstructures can be obtained by simply thermally evaporating solid materials.

This technique can be carried out in a relatively simple setup composed of a dual-zone vacuum furnace.

The hot end of the furnace contains the evaporating source material, while the evaporated particles are carrier downstream, (by way of a carrier gas) to the colder end of the furnace where they can absorb, nucleate, and grow on a desired substrate.

Molecular beam epitaxy (MBE) has been used since 2000 to create high-quality semiconductor wires based on the VLS growth mechanism.

This is because the chemical potential of the vapor can be drastically lowered by entering the liquid phase.

MBE is carried out under ultra-high vacuum (UHV) conditions where the mean-free-path (distance between collisions) of source atoms or molecules is on the order of meters.

Therefore, evaporated source atoms (from, say, an effusion cell) act as a beam of particles directed towards the substrate.

Figure 1: Schematic illustration of Si whisker growth from the reaction of SiCl 4 and H 2 vapor phases. This reaction is catalyzed by gold-silicon droplet deposited on the wafer surface prior to whisker growth.
Figure 2: CVD Growth of Si nanowires using Au particle catalysts
Figure 3: Schematic illustration of metal-alloy catalyzed whisker growth depicting the pathway of source materials through the droplet to the growing interface.
Figure 4: Schematic illustration of metal-alloy catalyzed whisker growth depicting the catalyst droplet formation during the early stages of whisker growth.
Figure 5: A plasma plume ejected from a target during pulsed laser deposition.
Figure 6: One possible configuration of a PLD deposition chamber.