The pulsed laser deposition (PLD) method is a widely known thin film growth technique for its potential in growing highly crystalline oxide thin films with near stoichiometry on an ultra-thin scale. Various thin film deposition techniques are used to prepare thin films and superlattices such as sputtering, molecular beam epitaxy, PLD, thermal evaporation method, chemical vapor deposition, electron beam evaporation, etc.
The PLD method is simple and suitable for the extremely controllable growth of a wide variety of materials has emerged as the most popular technique for the fabrication of oxide thin films as well as multilayer thin films in the ultrathin scale.
History of Pulsed Laser Deposition Technique
The postulate of Albert Einstein on the stimulated emission of radiation in 1917 got realised with the invention of the first ruby laser in 1960 by Theodore H. Maiman. The laser possesses distinct properties such as coherence, narrow frequency bandwidth, and high power density. These unique properties of laser are applied to ablate multi-component materials and deposit onto substrates to form stoichiometric thin films was named as pulsed laser deposition.
The PLD technique was introduced in the 1960s and progressed with the invention of low wavelength laser Technology (Smith and Turner, 1965) using Ruby laser. This unique Physical Vapor Deposition (PVD) process drawn considerable attention of the scientific community when in Dijkkamp et al. 1987 have published their work with the successful deposition of Yttrium Barium Copper Oxide (YBCO) films using PLD followed by the introduction of the excimer laser in 1985 and 1986 as a laser source to get shorter pulse duration and high pulse energy.
In the 1990s, the development of new laser technology with an improved repetition rate of laser and shorter pulse duration made PLD a competitive growth technique for the growth of thin films with complex stoichiometry. Device applications of PLD films of functional materials such as YBCO, BSTO, etc., in 2000 opened a receptive scope in this technique.
Principle of Pulsed Laser Deposition Technique
In general, the operational concept of PLD involves all physical processes induced by the laser-material interaction when the surface of a solid target interacts with a high power pulsed laser. The electromagnetic radiation gets absorbed by the solid surface of the material target, which in turn causes evaporation of the target material depending on its absorption properties. Sufficiently high energy is required for the ablation process, and the ejected material is directed towards the substrate in the form of a plume. The operational concepts of PLD include the transfer of ablated material from the target to the heated substrate through the plume, and, finally, the thin film grows on the substrate.
Stages of Pulsed Laser Deposition
In general, PLD includes the following stages,
- Interaction of laser radiation with the target and congruent evaporation of target material.
- Dynamics of the ablated materials.
- Decomposition of ablated material on target.
- Nucleation and growth of thin film on the substrate.
First Stage
In the first stage, the pulsed laser beam with short pulse duration and high energy density is focused onto the surface of the target. The transfer of energy from incident laser radiation to the atoms on the surface of the target heats up to their evaporation temperature and gets removed in a non-equilibrium state.
Second Stage
During the second stage, in accordance with the laws of gas-dynamics, the emitted materials tend to expand towards the substrate and show a forward peaking phenomenon with a reported trend of variation of spatial thickness as a function of nth power of cosine angular variation , where n is allowed to vary from 4 to 30 (Willmott and Huber, 2000).
The instantaneous ablation rate is determined by the fluence of laser striking on the target. The laser ablation mechanisms involve many complex physical phenomena such as thermal and electronic excitation, collision, exfoliation, and dynamics of the plume.
The deposited film uniformity is determined by the substrate temperature and laser spot size. In addition, the distance from the target to the substrate governs the angular spread of the ablated material.
Third Stage
The third stage is essential to ensure the quality of the film. The ejected species from the target may hit the substrate surface with high energy, which may cause damage to the substrate by sputtering off atoms from the surface. This establishes a collision region, serving as a source for condensation of particles between the incident flow and the sputtered atom.
When the rate of condensation is higher than the rate of particles supplied by the sputtering, a state of thermal equilibrium condition can be reached quickly. Then the film grows on the substrate surface at the expense of the direct flow of the ablated particles.
Fourth Stage
Finally, the fourth stage of PLD involves nucleation and growth of crystalline film on the surface of the substrate. This stage depends on many factors, such as the physical and chemical properties of the substrate, substrate temperature, laser parameters, and background pressure
Advantages and Disadvantages of PLD
The advantage of PLD compared to other thin film growth techniques are as follows:
- Simplicity in concept: A laser beam vaporizes the target surface and produces a film on the substrate with precise stoichiometry.
- Versatility: A wide range of materials covering metals, insulators, nitrides, oxides, etc., can be deposited using PLD. The flexibility of wavelength and power density realizes the growth of any materials.
- Congruent evaporation: PLD reproduces target stoichiometry in an oxidizing ambient, and this property makes PLD, the most appropriate method to grow thin films of functional oxide materials.
- High deposition rate: Thin films can be deposited with a deposition rate as high as 10 nm/minutes
- Cost-effective: The targets use for PLD are comparatively smaller (approximately 10 mm diameter) compared to the target diameter required for other techniques
However, there are a few cons associated with the evaporation mechanisms and technically inherited faults in PLD systems:
- Splashing, i.e., the formation of micron-sized particulates on the surface of thin films.
- The smaller area of uniformity (~ 1 cm2 ), highly directional nature of ablated material limits the angular range of uniform composition on the substrate surface.
- The high kinetic energy of the ablated material causes crystallographic defects in substrates as well as in thin films.
- Stoichiometry and thickness of the film can get affect by inhomogeneous flux and non-uniform energy distribution of the plume.
Endnote
The drawbacks can be solved to a larger extent by optimizing the various controllable growth parameters. The PLD offers different tunable parameters to control the deposition process to obtain high-quality thin films such as laser fluence, background gas used during deposition, the temperature of the substrate, ambient gas pressure inside the chamber, and target to substrate distance.