AVS 59th Annual International Symposium and Exhibition
    Spectroscopic Ellipsometry Focus Topic Monday Sessions
       Session EL+TF+AS+EM+SS+PS+EN+NM-MoM

Paper EL+TF+AS+EM+SS+PS+EN+NM-MoM3
Contribution of Plasma Generated Nanoparticles to the Growth of Microcrystalline Silicon Deposited from SiF4/H2/Argon Gas Mixtures

Monday, October 29, 2012, 9:00 am, Room 19

Session: Spectroscopic Ellipsometry for Photovoltaics and Semiconductor Manufacturing
Presenter: J.-C. Dornstetter, Total S.a, France
Authors: J.-C. Dornstetter, Total S.a, France
S. Kasouit, Total S.a, France
J.-F. Besnier, Total S.a, France
P. Roca i Cabarrocas, LPICM-CNRS, Ecole Polytechnique, France
Correspondent: Click to Email
Despite the low fabrication cost of thin film silicon solar modules, this type of technology remains non competitive in main stream markets because of the high BOS costs, due to the low energy conversion efficiency of this type of modules (~10%).We have recently shown that microcrystalline silicon films deposited using SiF4/H2/Argon RF capacitive plasmas have excellent structural and transport properties, compared to films deposited using conventional SiH4/H2 mixtures, allowing for a very good carrier collection, even for thick cells, and Voc values of 0.55 V, without device optimization, thus opening up the path for the realization of high performance solar cells. However, little is known so far about the growth mechanism of this type of materials and the reason for such interesting properties.Studies of silicon thin films deposition from SiF4/H2 mixes, under conditions different from ours, suggested that the growth is due to the deposition of SiF2 radicals, followed by the abstraction of fluorine by hydrogen. Previous work within our group has also shown that deposition occurs only when particles are present in the plasma, and that growth starts from crystallites without any amorphous phase.We present here a systematic study of the growth of microcrystalline films, together with the composition of nanoparticles attracted by thermophoresis to cold traps located both on the walls of the plasma chamber and in the fore line as a function of deposition conditions. The composition of the deposit on the traps is found to be amorphous at low power/ low hydrogen conditions and becomes crystalline when either of them increases. This correlates well with an increase in atomic hydrogen concentration in the plasma, as estimated by actinometry. The crystalline fraction of the deposited film was measured using in-situ ellipsometry and was found to correlate with the composition of the deposit on the cold traps. Deposition rate is drastically reduced when a water cooled trap is installed on the walls of the plasma chamber, and switches off at high H2 flow rates. Under these conditions, TEM and AFM images, show that at the initial stages of the growth the film is constituted of sparse, hexagonal crystalline particles, having sizes on the order of few tens of nanometers. We interpret the data above as a result of plasma-generated nanocrystals being a significant contribution to the deposited film. This may explain the excellent electronic properties of the films, as the particles are formed in the bulk of the plasma region, free from energetic ions bombardment. We will correlate the structural properties and the film growth mechanisms to the properties of solar cells.