Charge Density in Atmospheric Pressure Chemical Vapor
Deposition TiO
2
on SiO
2
-Passivated Silicon
Keith R. McIntosh,
a,z
Simeon C. Baker-Finch,
a
Nicholas E. Grant,
a
Andrew F. Thomson,
a
Sonita Singh,
a
and Iain D. Baikie
b
a
Center for Sustainable Energy Systems, Australian National University, Canberra ACT 2600, Australia
b
KP Technology Limited, Wick KW1 5EH, United Kingdom
The charge density of a TiO
2
film deposited on a SiO
2
-passivated silicon wafer is determined. The TiO
2
is deposited by atmo-
spheric pressure chemical vapor deposition at 400°C, and the SiO
2
is grown thermally at 950°C. This TiO
2
–SiO
2
stack is a useful
coating for the front surface of a silicon solar cell, as it has a high optical transmission and a low density of interface states D
it
E
at the SiO
2
–Si interface. While these properties are beneficial to high efficiency solar cells, so too is a large charge density, as what
occurs in Si
3
N
4
–SiO
2
+10
12
cm
-2
and Al
2
O
3
–SiO
2
-10
13
cm
-2
stacks. The D
it
E and charge density of TiO
2
-coated and
SiO
2
-passivated silicon are evaluated by capacitance–voltage and Kelvin probe measurements. The charge density of the TiO
2
is
within the conservative limits of 8.5 and -1 10
11
cm
-2
after deposition and of 10 and +1 10
11
cm
-2
after a subsequent
800°C oxygen anneal. Photoconductance measurements suggest that the dangling-bond defects at the SiO
2
–Si interface are
predominantly donorlike and, hence, that the change density in the TiO
2
is closer to the upper limits less negative; this charge
is too small to benefit solar cells.
© 2009 The Electrochemical Society. DOI: 10.1149/1.3216029 All rights reserved.
Manuscript submitted January 16, 2009; revised manuscript received August 4, 2009. Published September 14, 2009.
The recombination rate at the interface between a semiconductor
and an insulator U
s
depends strongly on the insulator’s charge den-
sity Q
i
. Figure 1 plots U
s
Q
i
for a SiO
2
–Si interface over a range of
midgap interface-state densities D
it0
when all dangling-bond states
are either a donorlike or b acceptor-like. The figure shows that in
general, U
s
decreases with increasing Q
i
, U
s
decreases with de-
creasing D
it0
, and U
s
depends more strongly on Q
i
when D
it0
is
small. The first trend results from the suppression of recombination
via the limitation of either the hole for Q
i
0 or the electron for
Q
i
0 concentration; the second results from the suppression of
recombination via the reduction in recombination centers; and the
third results from less charge associated with the interface states Q
it
.
Exceptions to these trends arise when the interface-state density is
very large e.g., D
it0
=3 10
12
cm
-2
eV
-1
making the associated
interface charge Q
it
comparable to Q
i
. Similar plots are described in
more detail by Aberle et al.
1
The calculations and the variables
associated with Fig. 1 are described in the Appendixes.
The dependence of U
s
on D
it0
and Q
i
is important to the manu-
facture of high efficiency photovoltaic solar cells. Their performance
depends greatly on the recombination at the illuminated surface,
near which most electron–hole pairs are generated. In all high effi-
ciency designs,
2-6
a hydrogenated thermal SiO
2
is used as a passi-
vation layer nearest the silicon, providing a small D
it0
of
10
10
cm
-2
eV
-1
and a small Q
i
of 5–20 10
10
cm
-2
.
7
As shown
in Fig. 1, this Q
i
is insufficient to reduce U
s
substantially, and so a
surface diffusion is also used to reduce the minority carrier concen-
tration. In most cells, a surface diffusion is required to form the p-n
junction, but in either case, it helps to lower U
s
as Q
i
is insufficient.
However, with sufficient Q
i
a smaller U
s
can be attained without a
surface diffusion
8
presumably because the diffusion introduces ad-
ditional interface states leading to a higher D
it0
.
In addition to the SiO
2
layer, high efficiency cells incorporate
one or two more dielectrics for their antireflection properties. Such
dielectrics include titanium dioxide TiO
2
,
2,9-11
aluminum oxide
Al
2
O
3
,
11,12
silicon nitride Si
3
N
4
,
6
amorphous silicon nitride
SiN
x
,
7,13
magnesium fluoride MgF
2
,
3
and zinc sulfide ZnS.
3,9
Although not always considered, the charge density of these dielec-
trics also contributes to Q
i
and therefore influences U
s
and the cell
efficiency. The charge density is large and positive in SiN
x
and
Si
3
N
4
10
12
cm
-2
7,13
and large and negative in Al
2
O
3
–SiO
2
stacks
10
13
cm
-2
,
9
but the charge density of either TiO
2
or TiO
2
–SiO
2
is not accurately ascertained despite the application of TiO
2
as an
antireflection coating on solar cells since the 1970s
10
albeit in-
creasingly rarely.
To date, quantification of TiO
2
’s charge density has been made
by capacitance–voltage CV measurements,
14-16
a technique that
determines the sum of Q
i
and Q
it
at flatband
17
but not an indepen-
dent value of Q
i
. Because the Q
it
of such samples is not necessarily
negligible and the insulating surface of a solar cell does not operate
at a flatband potential, these studies do not provide a useful assess-
ment of Q for photovoltaic applications.
This paper concerns the measurement of Q
i
in a TiO
2
–SiO
2
–Si
structure, where TiO
2
has been deposited by atmospheric pressure
chemical vapor deposition APCVD. A combination of CV and
Kelvin probe KP techniques was applied to extract Q
i
, an approach
that is applicable to a semiconductor–insulator structure with a small
Q
i
. The results were supported by photoconductance PC measure-
ments.
Sample Preparation
Figure 2 presents diagrams of the samples used in this work. All
samples were fabricated from 100 float-zoned n-type silicon wa-
fers with nominal resistivities of 4.6–5.4 cm. Before thermal
oxidation, the samples received a 20 min etch in tetramethylammo-
nium hydroxide at 75–85°C to remove saw damage, a 3 min
hydrofluoric acid:HNO
3
chemical polish at room temperature, and
z
E-mail: KRMcIntosh@gmail.com
(b) (a)
Figure 1. The solid lines plot the surface recombination rate U
s
as a function
of insulator charge density Q
i
and midgap interface-state density D
it0
when
all dangling-bond states are either a donorlike or b acceptor-like. The
calculations were made for an n-type Si semiconductor with a donor concen-
tration of N
D
= 10
15
cm
-3
and an excess carrier density in the bulk of
n
b
= 10
15
cm
-3
; the remaining parameters are listed in Appendix B. The
dashed line plots U
s
of the experimental samples determined by PC decay, as
described in the PC Results section.
Journal of The Electrochemical Society, 156 11 G190-G195 2009
0013-4651/2009/15611/G190/6/$25.00 © The Electrochemical Society
G190
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