7 PULSED LASER DEPOSITION

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The idea of PLD is straightforward. A pulsed laser beam is focused onto the surface of a strong solid target. The solid ingestion of the electromagnetic radiation by the strong surface prompts fast dissipation of the target materials. The dissipated materials comprise of exceedingly energized and ionized species. They introduced themselves as a plasma plume promptly before the target surface if the removal is completed in vacuum. Figure 1 demonstrates some common plasma observed during PLD process

PLD is straightforward to the point that only a couple of parameters, for example, laser energy density and pulse repetition rate, should be controlled during the procedure. The targets utilized as a part of PLD process are small as that required for other sputtering methods. It is very easy to deposit multi-layered films of different materials by sequential ablation of the used targets. Furthermore, by controlling the no of pulses, a fine control of film thickness down to nuclear monolayer can be accomplished. The most vital element of PLD is that the stoichiometry of the target can be achieved on the deposited films. This is the result of the extremely high heating rate of the target surface ( 108 K/s) due to pulsed laser irradiation.

 

It ensures the congruent dissipation of the target material independent of the evaporating point of the constituent elements or compounds of the target. And because of the high heating rate of ablated materials, laser deposition of crystalline film demands a much lower substrate temperature than other film growth techniques. For this reason, the semiconductor and the underlying integrated circuit can refrain from thermal degradation.

 

Disregarding the said focal points of PLD, a few weaknesses have been distinguished in utilizing this deposition technique. One of the significant issues is the sprinkling or the particulates affidavit on the films. The physical instruments prompting sprinkling incorporate sub-surface boiling, expulsion of the liquid layer by shock wave recoil pressure and exfoliation. The size of particulates might be as large as a couple of micrometers. Such particulates will significantly influence the development of the ensuing layers and also the electrical properties of the films. Another issue of PLD is the restricted narrow angular distribution of the ablated species, which is generated by the adiabatic expansion of laser, produced plasma plume and the pitting on the target surface. These highlights confine the handiness of PLD in delivering extensive region uniform thin movies, and PLD has not been completely conveyed in industry. As of late therapeutic measures have been proposed. Embeddings a shadow mask is powerful to close off the vast particulates. Pivoting both the target and substrate can create bigger uniform films.

 

Historial Development of PLD

 

Albert Einstein proposed the fortified discharge process in as right on time as 1916. The primary optical ace utilizing a bar of ruby as the lasing medium might have been, in any case, developed in 1960 by Theodore H. Maiman at Hughes Research Laboratories, a slip by of 44 years. Utilizing laser to remove material must be followed back to 1962 when Breech and Cross , utilized ruby laser to vaporize and energize particles from a strong surface. After three years, Smith and Turner utilized ruby laser to store thin films. This denoted the earliest reference point of the advancement of the pulsed laser deposition technique.

 

However, the development and investigations of pulsed laser deposition did not gather the expected momentum. In fact, the laser technology was immature at that time. The availability of the types of laser was limited; the stability output was poor and the laser repetition rate was too low for any realistic film growth processes. In this manner the advancement of PLD in thin film creation was moderate contrasting and different procedures, for example, MBE, which can deliver much better thin film quality.

 

The fast advance of the laser innovation, be that as it may, upgraded the intensity of PLD in the next decade. The lasers having a higher repetition rate than the early ruby lasers made the thin film development conceivable. Hence, dependable electronic Q-switches lasers wound up plainly accessible for age of short optical pulses. Hence PLD can be utilized to accomplish compatible vanishing of the target material and to store stoichiometric thin films. The absorption depth is shallower for UV radiation. Subsequent development led to lasers with high efficient harmonic generator and excimer lasers delivering powerful UV radiation. From then on, non-thermal laser ablation of the target material became highly efficient.

 

Pulsed laser deposition as a film growth technique has accomplished its presumed notoriety and has pulled in across the board enthusiasm after it has been utilized effectively to develop high-temperature superconducting films in 1987. Amid the most recent decade, pulsed laser deposition has been utilized to create crystalline thin films with epitaxy quality. Ceramic oxide, nitride films, metallic multilayers, and different superlattices developed by PLD have been illustrated. As of late, utilizing PLD to synthesis nanotubes, nanopowders and quantum dots have also been reported. Generation related issues concerning reproducibility, substantial zone scale-up and different level have started to be tended to. It might start up another period of thin film creation in industry

 

PROCESS:

 

Pulsed laser deposition (PLD) is a technique that has been used for several years now to synthesize discontinuous multilayers comprising metal nanoparticles dispersed oxide matrices. A high power density (10- 100 MW/cm2) pulsed laser beam is focussed on the surface of a material target. In order to synthesize multilayers, the targets are put successively in the beam. To avoid local damage, the targets are continuously rotated.

 

Usually, the excimer lasers are used to emit in the ultraviolet range with a pulse width in the order of 20 ns and pulse frequency of around a few tens of Hz. Under this impact, evaporation of target takes place locally and the matter is ejected in the form of plasma, which is deposited on a substrate held in place opposite to the target. There are several deposition parameters such as substrate- target distance, base pressure inside the PLD chamber, along with intrinsic characteristics of the laser that can be exploited for controlling the growth of thin film. PLD is a very flexible technique and it allows control over the composition of the deposited films. Hence, the stoichiometry of the target is conserved during deposition.

 

In addition, apart from its great flexibility, PLD allows very tight control over the composition of the deposited films. This is due to the fact that the stoichiometry of the target is conserved during transfer of matter onto the substrate. However, the homogeneous surface layer obtained by this process is not too wide.

 

Mechanisms of PLD

 

Unlike the easy system set-up, the working principle of pulsed laser deposition is a complex process. All the physical processes of laser-material interaction are involved during the impact of the high-power pulsed radiation on a solid target. Due to this, a plasma plume is formed of high energetic species through which subsequently the ablated material is transported onto the heated substrate surface and thus the thin film is deposited.

 

PLD can generally be divided into the following four stages.

 

1.  Interaction of Laser radiation with the target.

2.  Dynamic of the ablated materials.

3.  Deposition of the ablation materials onto the substrate.

4.  Nucleation and growth of a thin film on the surface of the substrate.

 

In the first stage, the laser beam is focused onto the surface of the target. When sufficient high energy density and short pulse duration are maintained, rapid heating of all th elements in the surface of target are rapidly heated up to their evaporation temperature. Materials are dissociated from the target and their ablation happens out with same stoichiometry as in the target. The instantaneous ablation rate is highly dependent on the fluence of the laser irradiating on the target. The ablation mechanisms involve many complex physical phenomena such as collisional, thermal and electronic excitation, exfoliation and hydrodynamics.

 

During the second stage the ejected materials tend to move towards the substrate according to the laws of gas-dynamic and show the forward peaking phenomenon. The spot size of the laser and the plasma temperature have significant effects on the uniformity of the deposited film. The target-to-substrate distance is another parameter that governs the angular spread of the ablated materials.

 

The nature of thin film is determined by the third stage. The shot out high-vitality species impinge onto the substrate surface and may prompt different kind of harm to the substrate. The mechanism of the interaction is illustrated in the following figure. These energetic species sputter a portion of the surface molecules and an impact locale is built up between the incident flow and the sputtered particles. Film develops promptly after this thermalized region is framed. The region serves as a source for condensation of particles. At the point when the buildup rate is higher than the rate of particles provided by the sputtering, warm harmony condition can be achieved rapidly and movie develops on the substrate surface to the detriment of the immediate stream of the removal particles.

 

Nucleation-and-growth of crystalline films depends on many factors such as the density, energy, degree of ionization, and the type of the condensing material, as well as the temperature and the physical-chemical properties of the substrate. The two main thermodynamic parameters for the growth mechanism are the substrate temperature T and the supersaturation ∆m They can be related by the following equation:

 

∆m= kT ln(R/ Re)

 

Here, k is the Boltzmann constant, R is the actual deposition rate, and Re is the equilibrium value at temperature T.

 

The nucleation procedure relies upon the interfacial energies between the three stages – substrate, the gathering material and the vapor. The minimum energy shape of a nucleus is like a cap. The basic size of the nucleus relies upon the main impetus, i.e. the statement rate and the substrate temperature. For the large nuclei, characteristics for little supersaturation, they make disconnect patches (islands) of the film on the substrates, which in this way develop and combine together. As the supersaturation builds, the basic core recoils until the point when its height achieves a nuclear distance across and its shape is that of a two-dimensional layer. For huge supersaturation, the layer-by-layer nucleation will occur for not completely wetted outside substrates.

 

The crystalline film development relies upon the surface versatility of the vapor particles. Ordinarily, the adatom will diffuse through a few nuclear separations previously adhering to a steady position inside the recently shaped film. The surface temperature of the substrate decides the adatom’s surface diffusion capacity. High temperature favors fast and imperfection free gem development, while low temperature or huge supersaturation precious stone development might be overpowered by fiery molecule impingement, bringing about disarranged or even amorphous structures.

 

In the PLD process, due to the short laser pulsed duration (~10 ns) and the small temporal spread (<10 µs) of the ablated materials, the deposition rate can be enormous (~10µm/ s ). Consequently a layer-by-layer nucleation is favoured and ultra-thin and smooth film can be produced. In addition the rapid deposition of the energetic ablation species helps to raise the substrate surface temperature. In this respect PLD tends to demand a lower substrate temperature for crystalline film growth.

 

ADVANTAGES OF PLD

 

PLD technique offers the following advantages over other deposition methods:

  1. Stoichiometric transfer of the target material to the substrate.
  2. Highly energetic species during deposition demands lower substrate temperature.
  3. Utilization of multi-targets ease out the multilayers deposition without breaking vacuum.
  4. Target fabrication is easy.

 

DISADVANTAGES OF PLD

 

The disadvantages of PLD technique are as:

  1. Uniform deposition over small area with high risk of substrate surface damage.
  2. Unsuitable for mass production.
  3. Ablation of material highly depends on the incident laser energy
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