Applied Surface Science 258 (2012) 8371–8376
Contents lists available at SciVerse ScienceDirect
Applied Surface Science
j our nal ho me p age: www.elsevier.com/loc ate/apsusc
High-temperature stability of c-Si surface passivation by thick PECVD Al
2
O
3
with
and without hydrogenated capping layers
Pierre Saint-Cast
a,∗
, Daniel Kania
a,1
, René Heller
b
, Saskia Kuehnhold
a
, Marc Hofmann
a
,
Jochen Rentsch
a
, Ralf Preu
a
a
Fraunhofer Institute for Solar Energy Systems, Heidenhofstrasse 2, 79110 Freiburg, Germany
b
Helmholtz-Zentrum Dresden-Rossendorf, D-01314 Dresden, Germany
a r t i c l e i n f o
Article history:
Available online 14 April 2012
Keywords:
Passivation
Aluminum oxide
PECVD
a b s t r a c t
We are studying the thermal stability of thick hydrogenated amorphous aluminum oxide (Al
2
O
3
) lay-
ers (20–50 nm) prepared by a high-throughput plasma-enhanced chemical-vapor-deposition (PECVD)
technique for the electrical passivation of crystalline silicon surfaces. These passivation layers can
be applied alone or covered by a capping layer like amorphous hydrogenated silicon nitride (SiN
x
)
or amorphous hydrogenated silicon oxide (SiO
x
), also prepared by PECVD. After firing at 870
◦
C for
approximately 3 s, the layers show blistering for Al
2
O
3
of 30 nm or higher, independently from the cap-
ping layer. For thinner Al
2
O
3
, no blistering can be observed even using scanning electron microscope
(SEM).
Very long carrier lifetimes up to 900 s was obtained in passivated p-Si (1 cm) wafer after anneal-
ing and firing, without observing a strong influence of the layer thickness and the capping layer. All the
layer stacks, including the stacks with SiN
x
capping layer, show high negative charge densities in the
layer (1–4 × 10
12
cm
-2
). Additionally, low interface defect densities (∼10
11
cm
-2
eV
-1
), which could be
achieved with and without a hydrogenated capping layer, were measured even after firing. To explain
these phenomena, hydrogen concentration depth profiles were measured by nuclear reaction analysis.
These measurements have shown that, at the Al
2
O
3
–Si interface, hydrogen atomic concentration ranging
5–7% after annealing and 4% after firing are obtained independently from the capping hydrogen con-
centration. We conclude that PECVD Al
2
O
3
layers of 20 nm or thicker can provide enough hydrogen to
passivate the interface defects, even after a high temperature step. However, the layer thickness should
be limited to 30 nm in order to avoid the blistering.
© 2012 Elsevier B.V. All rights reserved.
1. Introduction
Aluminum oxide can provide outstanding passivation quality
on crystalline silicon surfaces. After efforts in the development of
high-quality deposition processes using atomic layer deposition
(ALD) [1], carrier lifetimes longer than the known Auger limit [2]
were measured [3]. However, the very low deposition rates are
the main drawback of classical ALD systems; which would limit
the spreading of this process in the photovoltaic industry, where
high throughput and low process cost are required. For this reason,
alternative processes based on plasma-enhanced chemical vapor
deposition (PECVD) [4,5], rf-sputerring [6] or spatial ALD [7] were
∗
Corresponding author.
E-mail address: pierre.saint-cast@ise.fraunhofer.de (P. Saint-Cast).
1
Now with Bosch Solar Energy AG, Wilhelm-Wolff-Strasse 23, 99099 Erfurt,
Germany.
developed, which can reach a passivation quality semi-equivalent
to classical ALD.
In this study, we investigate Al
2
O
3
layers deposited with a high-
throughput PECVD deposition system. If we would suppose that
for this technique the throughput is not a limiting factor, the use
of thick layers with thicknesses ranging 20–50 nm make sense
for industrial implementation of such a process. We have focused
specifically on the stability of these layers through short high-
temperature processes (firing), which are often necessary for the
formation of contacts of solar cells. We also focus on the cap-
ping dielectric layers that can be deposited on the Al
2
O
3
layer in
order to protect it from metal or to thicken the layer for optimiza-
tion of the optical properties (improved internal light reflection).
When the capping layer is intended to provide good insulation,
the mechanical stability after firing becomes very important, as the
layer could lose its resistivity by conduction though pinholes. The
capping layer can also have an influence on the surface-passivation
quality by, for example, hydrogenation of the layer interface
with silicon.
0169-4332/$ – see front matter © 2012 Elsevier B.V. All rights reserved.
http://dx.doi.org/10.1016/j.apsusc.2012.03.171