Thin Film Group
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).