This report summarizes results obtained in the projects MissionN and
Miracle, on the development of new deposition methods for microcrystalline
silicon (mc-Si). The research has been carried out on five different
locations in the Netherlands: TU-Eindhoven, Dimes-Delft, TAC-Delft,
Utrecht University and ECN. Two new deposition methods for growth of
intrinsic mc-Si layers for solar cell applications have been investigated:
Expanding Thermal Plasma (ETP) and Microwave Plasma Enhanced Chemical
Vapour Deposition MWPECVD. For both methods it is shown that fast growth
(> 1 nm/s) of mc-Si is possible. In this project however, it has
been found that these high growth rates are accompanied with high porosity
of the layers. As a result the required electronic quality of mc-Si
for solar cell application has not been obtained yet. Efficiencies of
cells with i-layers deposited by one of these methods did not meet the
project aim of 8% but were limited to 1.9 % and 0.5 % for respectively
ETP and MWPECVD. Cells made with MWPECVD grown i-layers suffered extra
from post oxidation resulting from vacuum interruption since different
layers of these cells had to be manufactured at different locations.
A new and more effective method for deposition of doped mc-Si layers
was developed successfully: the Layer-by-Layer method. With this method
doped layers can be made in a wide temperature regime (150 - 400 °C),
with a much higher doping efficiency than by the conventional continuous
deposition method.
Research on the growth mechanisms of mc-Si by ATR-FTIR and spectral
ellipsometry has shown that deposition of mc-Si always starts with an
amorphous incubation layer (even when a crystalline Si wafer is used
as substrate). The amorphous incubation layer continues into an a-Si/mc-Si
mixed layers and then continues into an almost full crystalline mc-Si
layer. Thicknesses of incubation and mixed layer can be controlled by
the ratio of SiH4 and H2. The substrate temperature has little or no
effect on the final crystal fraction of the layer.
Work on development of new and advanced characterization tools for mc-Si
has yielded some promising methods: (Transient Absorption Spectroscopy
and Time of Flight measurements) which will be applied in future for
analysis of mc-Si.