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.