Thursday, September 27, 2018

What is the mechanism of pulsed laser deposition?

Although the equipment of pulsed laser deposition (PLD) system is simple, its working mechanism is related to many complicated physical phenomena. It includes all physical interactions between the laser and the substance when the high-energy pulsed radiation strikes the solid target, the formation of plasma plumes and the transfer of the molten material through the plasma plume to the surface of the heated substrate. Therefore, PLD can generally be divided into the following three stages:

Interaction between laser radiation and the target
In this stage, the laser beam is focused on the surface of the target. When sufficient high energy flux and short pulse width are achieved, all elements of the target surface are rapidly heated to the evaporation temperature. At this point, the material in the target will be sputtered from the target. The instantaneous melting rate of the target is highly dependent on the flow of laser light onto the target. The melting mechanism involves many complex physical phenomena such as collisions, heat, excitation with electrons, delamination, and fluid mechanics.

Dynamics of molten matter
In the second stage, according to the law of aerodynamics, the sputtered particles have a tendency to move toward the substrate. The space thickness varies with the function cosn θ, and n>>1. The area of the laser spot and the temperature of the plasma have an important influence on the uniformity of the deposited film. The distance between the target and the substrate is another factor that affects the angular extent of the molten material. It has also been found that placing a baffle close to the substrate narrows the angular extent.

Deposition of molten material on the substrate
The third stage is the key to determining the quality of the film. The high-energy nuclides emitted hit the surface of the substrate and may cause various damages to the substrate. The high energy nuclide sputters some of the atoms on the surface, and a collision zone is established between the incident stream and the sputtered atoms. The film is formed immediately after the formation of this thermal energy zone (collision zone), which is the best place to condense particles. As long as the condensation rate is higher than the release rate of the sputtered particles, the heat balance condition can be quickly reached, and the film can be formed on the surface of the substrate due to the weakened flow of the molten particles.

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