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Low temperature growth: From powders to nanoparticles

Figure 2 shows a schematic diagram of the genesis of powder formation. The evolution of the size of the particles, related to the time elapsed from the beginning of the discharge, is very fast. Detailed studies have shown that the time scale is in the range of seconds to minutes and that the increase in size is accompanied by a drastic decrease of the particle density, from 1011 cm-3 at the beginning down to 108 cm'3 after agglomeration [3\ 27]. A general review on particle formation, from monomers to macroscopic particles, has been given by J. Petrin and Ch. Hollenstein in the chapter II of the book « dusty plasmas » (Wiley & Sons, 1999).

Radicals Macro- Clusters, Molecules molecule Crystallites

Agglomeration Powder

Radicals Macro- Clusters, Molecules molecule Crystallites

Agglomeration Powder

Figure 2. Schematic representation of the genesis of the formation of powder.

Because of its simple implementation, we have used Mie scattering to determine the plasma conditions (pressure, power, temperature...) which are close to the limit of powder formation

[28]. Moreover, because powder formation is favoured at low temperatures, we have performed studies at 30 °C in silane-argon mixtures, a largely investigated system for which the kinetics are known in great detail [20], Figure 3 gives an example of the evolution of the scattered light intensity as a function of the plasma duration in a 7 % silane-in-argon discharge at room temperature with an RF power of 20 W. One can see that the increase of pressure results in a sharp transition between a pristine and a powdery plasma. Moreover, the formation of powder is an extremely fast process, as indicated by the strong increase of the signal after a few seconds at 67 Pa. Now, the fact that a scattered light is not detected does not necessarily imply the absence of particles in the plasma. However if there are any, they are present in a very low density or, rather, they are too small to produce a detectable signal. To prove the presence of particles at 20 Pa, we used modulated discharges with a microscope grid located on the substrate holder. As shown on figure 4, nanometer-size crystalline particles are evidenced on HRTEM micrographs in these conditions (silane-argon discharge at room temperature). Similar nanometric particles have been evidenced by other groups using different plasma conditions [29], [3I>], Although individual silicon particles with sizes in the nm range are not easily detected, this is precisely the range of sizes we are interested in, because their incorporation into the growing film leads to polymorphous films (with a medium-range order), intermediate between a-Si:H and pc-Si.

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