Optimisation of AIC Poly-Si Seed Layers
on Glass
Several of the poly-Si thin-film PV technologies
under development at the Centre use a thin crystalline “seed
layer” on the glass as a platform for the formation of the
solar cell’s absorber material. Important requirements that
such a seed layer must fulfill are: (i) a low surface roughness,
(ii) an oxide- and contamination-free growth interface, and (iii)
a good surface crystal quality. Past work at UNSW has shown that aluminium-induced
crystallisation (AIC) of amorphous Si is a promising method
for making a poly-Si seed layer on glass [1]. However, these baseline
AIC poly-Si films have a very rough surface due to Si precipitations
that adhere strongly to the surface.
In 2003, we developed and patented a method that
produces smooth AIC films on glass [2]. An optical transmission
microscope image of a smooth, 200 nm thick AIC poly-Si film formed
on SiN-coated glass is shown below.
Optical transmission microscope image of a
glass/SiN/AIC poly-Si sample. The displayed area is 370x296 µm2.
The dark spot-like features are associated with small
Al residues that are frequently observed trapped in the grain boundaries
of the AIC poly-Si film. Small voids (~100 nm) are also quite frequent
in the grain boundary region. The voids and Al residues form a
pattern that delineates a grain-like structure in the film. Selected-area
Raman microprobe measurements were performed on regions with different
amounts of voids/Al residues. The results indicate that the Al
residues/voids are primarily present in areas with lower crystal
quality. In addition, the intra-grain regions displayed
a bi-modal Raman response. The majority of the intra-grain regions
had a narrow and symmetrical Raman peak shape, whereas a few intra-grain
regions (~10% of total surface area) had an asymmetrical Raman
peak shape. An asymmetrical Raman peak broadening in the 490-510
cm-1 range is an indication of small-grained and/or
disordered material. It should be noted that the FWHM of the main
peak at ~518.5 cm-1 is not significantly different for
the two types of intra-grain region.
A powerful method to assess the crystalline quality
of a planar Si film is the measurement of the UV reflectance [3].
Using the “crystal quality figure of merit” proposed
in Ref. 3, it follows that this AIC seed layer has an excellent
crystal quality in the surface region of around 98 % (compared
to 100% for a polished high-quality Si wafer).
Plan-view transmission electron microscopy (TEM)
images were also taken. Our smooth AIC poly-Si films typically
look as shown below to the left. This image displays an essentially
defect-free poly-Si grain with a grain size well over 10 µm.
The film is continuous, apart from a few small inter-grain and
intra-grain voids. Only a relatively small fraction (total area
coverage <10 %) of the AIC poly-Si film consists of smaller-grained
and/or lower-quality material, such as shown below to the right.
Plan-view TEM images of a smooth AIC poly-Si
seed layer. High-quality region shown on left and lower-quality
region on the right(length markers 2.0 µm and 1.0 µm
respectively)
[1] P.I Widenborg and A.G. Aberle, “Surface morphology of
poly-Si films made by aluminium-induced crystallisation on glass
substrates”, Journal of Crystal Growth 242, 270-282 (2002).
[2] P.I. Widenborg, A.G. Aberle, A. Straub, N.-P. Harder, D.H. Neuhaus, O.
Nast-Hartley, Fabrication method for crystalline semiconductor films on foreign
substrates, International PCT patent application PCT/AU03/01313 (7 Oct 2003).
[3] A. Straub, P.I. Widenborg, A. Sproul, Y. Huang, N.-P. Harder, and A.G.
Aberle, “Fast and non-destructive assessment of epitaxial quality of
polycrystalline silicon films on glass by optical measure¬ments”,
Journal of Crystal Growth (in print). |
