**Pathways to thin film polycrystalline silicon using amorphous silicon
precursors**

O. Ebil, S. Hegedus, M. Lu, R. Birkmire

Institute of Energy Conversion

Univeristy of Delaware

Newark DE USA 19716

#### ABSTRACT

Multiple pathways to producing large grain Si films on low cost
substrates have been investigated. A-Si films deposited by Hot Wire CVD
(HWCVD) on glass have been crystallized by in-situ Aluminum-induced
crystallization (AIC). The AIC occurs during the a-Si growth at 430°C
eliminating the need for a separate AIC step. Both the Si/Al thickness
ratio and Si thickness are critical to give optimum poly-Si films.
Maximum a-Si and Al thicknesses of 0.6 and 0.5 µm resulted in continuous
films with grains \~10 µm.

**1. Objectives**

The overall goals of this program are to expand the fundamental science
and engineering base for thin film photovoltaics needed to improve
module performance and manufacturing technology and to effectively
transfer these technologies to large-scale manufacturing. The objective
of the work reported here is to develop in-situ and/or post-deposition
methods to produce HWCVD deposited polycrystalline Si films and devices
with grains in the micrometer range.

**2. Technical Approach**

Thin-film poly-Si is a potential alternative to a-Si and c-Si based
photovoltaic devices. Poly-Si offers large area manufacturability at
temperatures below 600 ^o^C but typically has low minority carrier life
time limited by the nanocrystalline grain structure of poly-Si. AIC has
been used to prepare poly-Si thin-films with grain sizes comparable to
or greater than the film thickness at temperatures below 600 ^o^C. Most
AIC studies reported in the literature perform the crystallization
thermal step *ex-situ*, after the low-temperature deposition of both Si
and Al layers. Here, an *in-situ* AIC crystallization process is
presented where a-Si is deposited by HWCVD on Al coated glass substrates
at 430^o^C. This work builds on our results investigating AIC above and
below the eutectic point (577°C) with *in-situ* or *ex-situ* AIC ^1,2^.

**3. Results for *In-situ* AIC below the eutectic point**

Aluminum films 500 nm and 1000 nm thick were deposited on 7059 glass
substrates by electron beam evaporation at a fixed deposition rate of 4
nm/s. All Al coated glass substrates were exposed to air, at least 24
hours prior to Si deposition to ensure the formation of an oxide layer
which is critical for the layer exchange process^3^. Si deposition and
*in-situ* AIC experiments were performed using the HWCVD system. A-Si
films with 500, 600 and 1000 nm thicknesses were deposited from SiH~4~
at fixed substrate and filament temperatures of 430^o^C and 1850^o^C,
respectively, and a pressure of 25 mTorr. The deposition rate of Si was
either 1 or 5 µm/hour.

In a conventional AIC process where a-Si and Al layers are annealed at
temperatures below 500^o^C, the poly-Si film is formed on the glass
substrate, replacing the initial Al layer. Al is accumulated on top of
the newly formed poly-Si layer. If the thickness of the a-Si layer is
greater than the thickness of the initial Al layer, the excess Si is
accumulated on top of the newly formed poly-Si layer. The newly formed
poly-Si layer is covered with a surface layer of mixed Si and Al. This
mixture must be removed before the poly-Si can be used in a solar cell.

The *in-situ* AIC process can be divided into 3 steps^2^. First, Si
diffuses into the Al layer. A thin Si film might be formed on top of the
Al layer. Next, the Al layer is supersaturated with Si and dissolved Si
precipitates at Al grain boundaries. The grain boundaries are
preferential nucleation sites because supersaturation can be achieved
locally, although Al grains are not necessarily supersaturated. Finally,
Si grains grow both laterally and vertically until they reach the glass
substrates. After that, only lateral growth is possible, since the oxide
layer at the a-Si/Al interface acts as a physical barrier. The thickness
of the poly-Si layer is determined by the thickness of the Al layer.

In order to reveal the grain structure of poly-Si hidden by the excess
Si and Al, a new etching procedure was developed with several cycles of
XeF~2~ vapor followed by solution-based Al etching. The XeF~2~ gas
etches a-Si and nano-Si rapidly compared to c-Si. Figure 1 shows the SEM
image of a sample with an initial layer structure of glass/500 nm Al/
600 nm a-Si after partial etching, using the new etching process. The
sequential etching removes excess Si as well as the exposed and
unexposed Al trapped in the Si matrix, resulting in almost planar Si
surfaces. Since the etching is not complete, some excess non-c-Si is
still visible (textured areas) on top of smooth Si. Planar grains of
10-15 nm are visible.

Figure 2 shows the Raman spectrum taken from the Si grain from the
sample in Figure 1. For comparison, the profile of c-Si is also shown.
The AIC Raman profile reveals a high quality Si crystal without any
amorphous or nano-crystalline phase, which would be seen at 480 and 504
cm^-1^, respectively.

The effects of Si and Al layer thicknesses on the nucleation density and
the surface coverage of Si grains was evaluated using optical
microscopy. In reflection mode, Si crystals reaching to the glass
substrate are seen as dark spots, compared to a highly reflective Al
layer. The Si/Al thickness ratios, Al layer thicknesses, and fraction of
Si surface crystallization for 4 samples were; (a) 0.5, 1000 nm, 5% (b)
1.0, 1000 nm, 50% (c) 1.0, 500 nm, 90%, and (d) 1.2, 500 nm, 100%.
Increasing the Si/Al ratio increases the surface coverage of Si crystals
from 5% to 50%. Reducing the Al layer to 500 nm (c) increased the
surface coverage of the Si crystals to 90%. Finally, by increasing the
Si/Al ratio from 1.0 to 1.2 to provide sufficient Si for
crystallization, a continuous poly-Si film (d) was obtained (Fig. 1).
Using an Al layer greater than 500 nm resulted in non-continuous poly-Si
films with Al clusters between Si grains. This is the result of Si
grains coalescing and forming a diffusion barrier for Al atoms, as the
Si grains grow faster along Si/Al interface than on the glass surface.

Increasing the Si/Al ratio further does not improve either the quality
or the thickness of the poly-Si film. Increasing the Si layer thickness
results in a continuous poly-Si film with a thickness equal to the
thickness of the initial layer of Al, and on top of that layer, a
mixture of excess Si and Al. XRD analysis of the poly-Si layer revealed
preferential (400) or (422) orientations.

**4. Conclusions**

Continuous poly-Si films on glass substrates were obtained using an
*in-situ* aluminum-induced crystallization technique at 430^o^C. Poly-Si
films had an average grain size of 10-15 µm, corresponding to a grain
size/thickness ratio greater than 20. Poly-Si layers prepared by either
an in-situ or conventional AIC process can be used as seed layers for
epitaxial Si growth or for back contact formation in c-Si solar cell
fabrication at temperature below 500^o^C. As an alternative to
conventional AIC, the in-situ AIC process eliminates an additional
annealing step, which saves considerable time during processing.

# Acknowledgements

This work was supported by NREL under subcontract ADJ-1-30630-12.

### **References and Major FY 2005 Publications**

1\. O. Ebil, R. Aparicio, R. Birkmire "In-situ AIC of Si thin films on
glass above the eutectic point using HWCVD" MRS Symp. Proc. Vol. 808,
2004, 321-326.

2\. O. Ebil," Deposition and Characterization of Silicon Thin-Films by
Aluminum-Induced Crystallization", PhD dissertation, Department of
Material Science and Engineering, University of Delaware, 2005

3\. O. Ebil, R. Aparicio, R. Birkmire "Poly-Si Seed Layers Prepared by
Aluminum-Induced Crystallization During Deposition" submitted to
Progress in Photovoltaics.

### 

![](media/image1.jpeg){width="2.6993055555555556in"
height="2.7020833333333334in"}

Figure 1. SEM image of AIC Si film showing excess Si on top of
continuous Si grains after partial etching. Two-grain boundaries are
visible on the left hand side of the image. Average grain size was
estimated around 10-13 µm.

![](media/image2.wmf){width="3.05625in" height="2.890972222222222in"}

**Figure 2. Raman spectra of exposed poly-Si layer after etching. c-Si
shown for comparison.**