Vapor–liquid–solid method


The vapor–liquid–solid method 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 physical characteristics of nanowires grown in this manner depend, in a controllable way, upon the size and physical properties of the liquid alloy.

Historical background

The VLS mechanism was proposed in 1964 as an explanation for silicon whisker growth from the gas phase in the presence of a liquid gold droplet placed upon a silicon substrate. The explanation was motivated by the absence of axial screw dislocations in the whiskers, the requirement of the gold droplet for growth, and the presence of the droplet at the tip of the whisker during the entire growth process.

Introduction

The VLS mechanism is typically described in three stages:
The VLS process takes place as follows:
  1. A thin Au film is deposited onto a silicon wafer substrate by sputter deposition or thermal evaporation.
  2. The wafer is annealed at temperatures higher than the Au-Si eutectic point, creating Au-Si alloy droplets on the wafer surface. Mixing Au with Si greatly reduces the melting temperature of the alloy as compared to the alloy constituents. The melting temperature of the Au:Si alloy reaches a minimum when the ratio of its constituents is 4:1 Au:Si, also known as the Au:Si eutectic point.
  3. Lithography techniques can also be used to controllably manipulate the diameter and position of the droplets.
  4. One-dimensional crystalline nanowires are then grown by a liquid metal-alloy droplet-catalyzed chemical or physical vapor deposition process, which takes place in a vacuum deposition system. Au-Si droplets on the surface of the substrate act to lower the activation energy of normal vapor-solid growth. For example, Si can be deposited by means of a SiCl4:H2 gaseous mixture reaction, only at temperatures above 800 °C, in normal vapor-solid growth. Moreover, below this temperature almost no Si is deposited on the growth surface. However, Au particles can form Au-Si eutectic droplets at temperatures above 363 °C and adsorb Si from the vapor state until reaching a supersaturated state of Si in Au. Furthermore, nanosized Au-Si droplets have much lower melting points because the surface area-to-volume ratio is increasing, becoming energetically unfavorable, and nanometer-sized particles act to minimize their surface energy by forming droplets.
  5. Si has a much higher melting point than that of the eutectic alloy, therefore Si atoms precipitate out of the supersaturated liquid-alloy droplet at the liquid-alloy/solid-Si interface, and the droplet rises from the surface. This process is illustrated in figure 1.

    Typical features of the VLS method

The requirements for catalysts are:

Catalyst droplet formation

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 can, be modeled mathematically, however, the actual forces present during growth are extremely difficult to measure experimentally. Nevertheless, the shape of a catalyst particle at the surface of a crystalline substrate is determined by a balance of the forces of surface tension and the liquid–solid interface tension. The radius of the droplet varies with the contact angle as:
where r0 is the radius of the contact area and β0 is defined by a modified Young’s equation:
It is dependent on the surface and liquid–solid interface tensions, as well as an additional line tension which comes into effect when the initial radius of the droplet is small. As a nanowire begins to grow, its height increases by an amount dh and the radius of the contact area decreases by an amount dr. As the growth continues, the inclination angle at the base of the nanowires increases, as does β0:
The line tension therefore greatly influences the catalyst contact area. The most import result from this conclusion is that different line tensions will result in different growth modes. If the line tensions are too large, nanohillock growth will result and thus stop the growth.

Nanowhisker diameter

The diameter of the nanowire which is grown depends upon the properties of the alloy droplet. The growth of nano-sized wires requires nano-size droplets to be prepared on the substrate. In an equilibrium situation this is not possible as the minimum radius of a metal droplet is given by
where Vl is the molar volume of the droplet, σlv the liquid-vapor surface energy, and s is the degree of supersaturation of the vapor. 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.

Whisker growth kinetics

During VLS whisker growth, the rate at which whiskers grow is dependent on the whisker diameter: the larger the whisker diameter, the faster the nanowire grows axially. This is because the supersaturation of the metal-alloy catalyst is the main driving force for nanowhisker growth and decreases with decreasing whisker diameter :
Again, Δµ is the main driving force for nanowhisker growth. More specifically, Δµ0 is the difference between the chemical potential of the depositing species in the vapor and solid whisker phase. Δµ0 is the initial difference proceeding whisker growth, while is the atomic volume of Si and the specific free energy of the wire surface. Examination of the above equation, indeed reveals that small diameters exhibit small driving forces for whisker growth while large wire diameters exhibit large driving forces.

Related growth techniques

Laser-assisted growth

Involves the removal of material from metal-containing solid targets by irradiating the surface with high-powered short laser pulses, usually with wavelengths in the ultraviolet region of the light spectrum. When such a laser pulse is adsorbed by a solid target, material from the surface region of the target absorbs the laser energy and either evaporates or sublimates from the surface or is converted into a plasma. These particles are easily transferred to the substrate where they can nucleate and grow into nanowires.
The laser-assisted growth technique is particularly useful for growing nanowires with high melting temperatures, multicomponent or doped nanowires, as well as nanowires with extremely high crystalline quality. The high intensity of the laser pulse incident at the target allows the deposition of high melting point materials, without having to try to evaporate the material using extremely high temperature resistive or electron bombardment heating. Furthermore, targets can simply be made from a mixture of materials or even a liquid. 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.

Thermal evaporation

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, to the colder end of the furnace where they can absorb, nucleate, and grow on a desired substrate.

Metal-catalyzed molecular beam epitaxy

has been used since 2000 to create high-quality semiconductor wires based on the VLS growth mechanism. However, in metal-catalyzed MBE the metal particles do not catalyze a reaction between precursors but rather adsorb vapor phase particles. 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 conditions where the mean-free-path of source atoms or molecules is on the order of meters. Therefore, evaporated source atoms act as a beam of particles directed towards the substrate. The growth rate of the process is very slow, the deposition conditions are very clean, and as a result four superior capabilities arise, when compared to other deposition methods: