Energy Effects on Morphological Development

Many deposition techniques involve delivering the deposition flux at thermal energies that are generally less than 0.1eV. It was first recognized by Mattox(1963) that extra energy added to the surface in the form of energetic atomic fluxes improved film properties. An energetic deposition flux can greatly influence the mode of film growth, film density, adhesion, stress, crystal structure, and orientation.
The effects of collisions of energetic atoms with materials including ballistic collisions, ion mixing, and thermally stimulated exchange mixing produce effects similar to raising T_h of the substrate.Energy-stimulated atomic motion on film properties, such as reduced film stress, and change in crystal structure and preferred orientation are a secondary result of the fundamental atomic movements.
Molecular-dynamics (M-D) studies of the initial stages of thin-film growth have been shown to be particularly useful in understanding the effects of higher energy fluxes. In 1992, Gilmore and Sprague examined the effect of depositing Ag on Ag at 300K at energies of 0.1, 1, 10, 20 and 40 eV. Three monolayers consisting of 500 atoms were deposited at 0.5 and 1.0 ps intervals. Results showed that while the film growth was epitaxial at all energies, the morphology changed from island growth at 0.1 and 1 eV to layer by layer growth at 10, 20 and 40 eV.
The first conclusion is that changing the energy of the atom flux has a similar effect to increasing the temperature--namely, producing a more dense, homogeneous structure.
Another result is that there was little or no mixing of the substrate with the film atoms at 0.1, 1, and 10 eV, but for the 20 and 40 eV energies, some substrate atoms were found in the film and some film atoms were found in the substrate, with more atoms mixed with increasing energy.

Average energy per deposited atom (eV):
TE & CVD: 0.1 eV~1 eV
PLD: 1 eV ~ 10 eV

Temperature Effects on Film Morphological Development

Thick-Film Development

Substrate temperature not only determines the initial development of a film, but its subsequent thickening as well. A model for film morphology was given by Grovenor et al. (1984) derived from experiments on films grown in high vacuum. The gross microstructure changes as a function of the homologous temperature, T_h=T/T_m, where T is the temperature of the substrate. All temperatures are in degrees of Kelvin. Based on there model, small grains of 5-20 nm in diameter are observed at T_h<0.2.

When T_h is within the range from 0.2 to 0.3, it is suggested that grain boundaries of a single orientation are mobile so that favorably oriented boundaries grow and surface diffusion may contribute to a more dense morphology.

When T_h is within the range from 0.3 to 0.5, all grain boudaries become mobile and the formation of columnar grains requires surface recrystallization and diffusion.

When T_h is larger than 0.5, bulk diffusion, surface, and bulk recrystallization all occur leaving a larger grain structure.

In general, the 'best' value of the substrate temperature for high-quality thin film growth is from 0.3 to 0.5. In this regime, there is sufficient surface diffusion to allow surface atoms to minimize their surface energy.

For inital stages of film growth, please refer to this link

Influence of the Deposition Characteristics on the film properties

Influence of the Plasma Parameters:
Thermal and kinetic energy of the plasma particles can significantly influence some film properties. Depending on the energy of the particles.
1. The thermal energy of the evaporated particles is much less than the dissociation energy of the molecules of the target material. This regime can be realized with relatively long laser pulses (~10^-3 s) and low fluences q~10⁵ W.cm^-2. These conditions are suitable for obtaining stoichiometric films of polycomponent materials with high energy of dissociation, for example, oxides. This has been demonstrated with the deposition of dielectric films of SnO₂, TiO₂, ZrO₂, Al₂O₃, Nb₂O₅, BeO and their mixtures.

2. The thermal energy of the particles is of the order of the dissociation energy. In this case the target material is partially or completely dissociated and the dissociation energy is released on the substrate surface in the process of atom-to-molecule association. This regime concerns most of the polycomponent semiconductors and may also be realized with long laser pulses (~10^-3 s) and flux densities q~10⁵-10⁶W.cm^-2. In this regime stoichiometric polycomponent semiconductor films like GaAs, CdS, PbS,PbSe,PbTe and Pb_1-xCd_xSeof perfect structure, smooth surface, and high carrier mobility have been obtained.

3. The kinetic energy of the particles is of the order of the defect-formation energy (~20ev). Irradiation of monocrystalline substrates with particles of this energy produces a network of radiative defects, which are additional crystallization centers. This process allows the epitaxial growth of films at moderate mobility of the adsorbed atoms, that is, at lower temperatures as compared to other methods. This peculiarity has been used to prepare multilayer structures (up to 60 layers) of consecutively following heteroepitaxial semiconductor pairs of InSb-CdTe, InSb-PbTe and Bi-CdTe. Such structures, called superlattices, possess very interesting electrical and optical properties.

4. At high laser fluences (q~10⁸-10⁹) the laser-produced plasma contains a significant amount of high energetic particles (100-2000eV). Ions of this energy produce in a thin surface layer (~30 Å)individual vacancies, which then diffuse deep into the substrate and recombine over a time ~10^-8 s with alsmost no worsening of the crystal properties. As is well known, diffusion of adatoms in solids occurs mainly in the unoccupied nodes of the crystal lattice (vacancies), and the high temperature at which this diffusion takes place is needed to ensure that a maximum number of atoms escape from the lattice nodes. The number of plasma-produced vacancies exceeds the equilibrium value for a given temperature by many orders of magnitude. This results in an increase of the diffusion coefficient to the same extent. Radiation-stimulated diffusion ensures excellent adhesion of the deposited films, even at very low substrate temperatures.

Influence of the film growth mode
There's a critical substrate temperature, T_c, below which the structure of the films is not completely monocrystalline and the film composition deviates significantly from the stoichiometric one. Besides, the temperature changes when the target-substrate distances change. Take PbSe as an example, at E=3JT_c is 350K, 450K and 650K at target-substrate distances of 3cm,2cm and 1cm respectively.

Pulsed Laser Deposition Techniques

In general, PLD can be used to vaporize and to deposit thin films of any material if the abosrbed laser power density is high enough.

Origin of Splashing:
1. Subsurface boiling
It occurs if the time required to transfer laser energy into heat is shorter than that needed to evaporate a surface layer with a thickness on the order of skin depth. Under this condition, the subsurface layer is superheated before the surface itself has reached the vapor phase.
In general, the power densities used in most PLD-grown dielectric materials are lower than the maximum power density that a solid surface could absorb without causing splashing.
2.Expulsion of the Liquid layer by the shock wave recoil pressure
In this mechanism, the force comes from above the liquid layer in a form of recoil pressure exerted by the shock wave of the plume. The signature of this type of splashing is also the formation of micron-sized condensed globules, and therefore, indistinguishable from the true splashing. Its magniture can be reduced by lowering the laser power density at the expense of decreasing the deposition rate.
3. Exfoliation
For most materials, in particular sintered ceramic targets, the surface is eroded by repetitive laser ablation, forming long needle-shaped microstructures of only a few microns in dimension. These microdendritic structures point toward the direction of the incoming laser beam due to shadowing effect. Mechanically, they are very fragile and can be broken by the thermal shock induced during the intense laser irradiation. This process is termed "exfoliation" by Kelly and coworkers (1985). The loose debris are carried toward the substrate by the rapidly expanding plume and condense onto the thin film.

Solutions to Avoid Splashing
The splashing can be redued by adding a particle filter or manipulating plume, improving target surface, doing off-axis deposition.

Surface Segregation
There's a surface modification by pulsed laser on film composition. take YBCO system for example, it is severely modified by cumulative laser irradiation. After laser preconditioning of the target (tens of shots/site or longer), a steady-state composition can be reached.
The major effect of surface modification on PLD is a reduction of the deposition rate. This complicates film thickness control and slows the deposition process. Though both issues inhibits progress greatly, they are not an issure in a research lab because the target can be periodically resurfaced to its original level by sanding or scraping in a lab.

Penetration depth
For metals, the absorption coefficient α decreases with decreasing laser wavelength λ. Hence the laser penetration depth in metals is larger in the UV range than in the infrared (IR) range. For other materials, the variation of the absorption coefficient with the wavelength is more comlex since the various absorption mechanisms, such as lattice vibration, free carrier absorption, impurity centers, or bandgap transition, can take place. For the oxide semiconductor, the penetration depth appears to be larger in the near IR than in the UV, somewhat reverse to that of the metals.
The primary effect of the laser wavelength on particulate generation is most likely due to the difference in the absorption when different laser wavelengths are used.

Elimination of particulates
1.Reduce target surface roughness
2.Reduce the laser power density to below the threshold level that causes the splashing of the molten layer. Blank et al. observed that a simple relation holds for the deposited film thickness, d∝S²E, where S is the laser spot size on the target, and E is the laser energy density. Since the deposited film thickness is directly proportional to the spot size when the laser energy, SE, is fixed, increasing the spot size is the most efficient way to reduce the particulates as well as to increase the throughput of vapor species.

Effects of the high background gas pressure
The gas pressure commonly used for PLD of complex ceramic materials is approximately 300mT, which corresponds to approximately 1x10²⁰ gas atoms per square centimeter per second striking the substrate and the film. This arrival rate compares to typical peak arrival rates of film atoms of 10¹⁸-10¹⁹ atoms per square centimeter and typical average film atom arrival rates of 10¹⁵-10¹⁶ atoms per square centimeter per second. In addition to the obvious effect of promoting stoichiometric film formation for such systems as oxide deposition in a background gas of oxygen, such a high arrival rate of gas atoms could change film and substrate surface energies, possibly even changing the film growth mode.