Surface Passivation Attained by Silicon Dioxide Grown at Low Temperature in Nitric Acid
Nicholas Grant
Keith McIntosh
Oxygen (O)
NO2
NO3-
1 Introduction
Some of the most efficient solar cells use a silicon dioxide (SiO2) layer to passivate
the silicon surface, where oxygen is used to bond with silicon (Si) to form an Si/SiO2
interface with minimal Si dangling bonds. Such a well passivating layer reduces
electron and hole recombination at the interface and therefore increases cell
efficiencies. However to achieve such a high quality passivating layer, the oxidation
is done at temperatures greater than 1000 °C for long periods of time making the
process expensive. In this project, we investigate a cheaper oxidation method by the
use of nitric acid (HNO3), and show that a chemical SiO2 layer can achieve surface
passivation to the same level as a thermal oxide under similar annealing conditions.
From Figure 3, it can be seen that after a high temperature anneal in nitrogen (N2)
and forming gas (FGA), the SRV can be reduced to ~40 cm/s, which is comparable
to a thermal oxide under similar annealing conditions. However prior to a high
temperature anneal, the two-step HNO3 oxidation provides poor passivation with an
SRV beyond 10 000 cm/s.
O-
3.2 Capacitance-Voltage and Density of Interface State Measurements
Capacitance-Voltage (C-V) measurements are a tool for probing the Si/SiO2 interface.
They can provide information on the density of interface states (i.e various types of
dangling bonds and their concentration) and also determine charge magnitude and
polarity.
14
1 nm SiO2
1.0
Figure 2. The NO3- molecules can be trapped by the nano-sized pores at the SiO2
surface which can contribute to the decomposition of the molecule, releasing
atomic/ionic oxygen, which can diffuse to the interface and form SiO2 [2]. Atomic
oxygen from the decomposition of HNO3 at 121 ºC is also a contributor to the
formation of SiO2.
3 Results
3.1 Surface Recombination Velocity (SRV)
2.2 Oxidation of Si in a 68 wt% HNO3 solution (Step-2)
After the 10 minute oxidation in the 40 wt% HNO3 solution, the Si wafer is then
immersed in a 68 wt% HNO3 solution at its boiling temperature of 121 ºC for 3
hours. At this point, the SiO2 layer continues to grow [1]. The oxidation of Si is likely
to proceed by the reaction with oxygen by the following chemical formulas [3], as
seen in Figure 2.
NO3-
NO2 + NO + H2O + 2O
NO2 +
O-
(1)
(2)
Due to the small size of the oxygen atoms/ions, they are able to diffuse through the
growing SiO2 layer and therefore continue to form a thicker SiO2 layer [3].
Maximum Surface Recombination Velocity (cm/s)
Figure 1. After a 10 minute oxidation in a 40 wt% HNO3 solution, an ultrathin SiO2
layer is formed. Immersion of the Si wafer beyond 10 minutes does not increase the
thickness of the SiO2 layer [1].
5
10
ο
1100 C N2 + FGA
ο
ο
800 C N2
ο
800 C N2 + FGA
13
10
12
0.6
10
0.4
11
ο
Shift in C-V curve is
caused by positive
charge
0.0
-1.5 -1.0 -0.5 0.0 0.5
Voltage (V)
1.0
1100 C N2 + FGA
10
ο
800 C N2 + FGA
10
0.2
0.4 0.6 0.8
EF-EV (eV)
1.0
10
1.2
Figure 4. (Left) C-V measurements showing that the improved passiavtion between
the 800 ºC and 1100 ºC N2 annealed samples is due to an increase in positive charge
at the Si/SiO2 interface. (Right) The density of interface states (Dit), showing that
both samples have very similar interfacial characteristics, further indicating that the
improved passivation is not caused by a decrease in interfacial dangling bonds.
ο
1100 C N2
(b)
0.8
0.2
The SRV is a direct measure of the surface passivation. A well passivated surface will
have an SRV of less than 100 cm/s, while a poorly passivated surface will have an
SRV greater than 1000 cm/s.
Silicon (Si)
Capacitance (C/Cox)
A silicon wafer is immersed in a 40 wt% HNO3 solution at its boiling temperature of
108 ºC for 10 mins. Over this period of time, ~1 nm of silicon dioxide is formed [1],
with an atomic density ~4% lower then that of thick thermal SiO2. The SiO2 layer
also contains a large number of nano-sized pores at its surface [2].
(a)
2
5 nm SiO2
2.1 Oxidation of Si in a 40 wt% HNO3 solution (Step-1)
2HNO3
10
ο
1100 C N2 + FGA
Dit (states/cm /eV)
2 Two-step Nitric Acid Oxidation Method
800 C N2 + FGA
4
10
4 Conclusion
Without an anneal, the two-step nitric oxide offered poor passivation, but after a high
temperature anneal in N2 and an FGA, an SRV of ~40 cm/s was attained, similar to that
of a thermal oxide. C-V and photoconductance measurements suggest the oxides contain
a high positive fixed charge—particularly after an 1100 ºC N2 anneal—which aids the
passivation of n-type and intrinsic silicon but harms the passivation of low-resistivity ptype silicon.
In this work, the high-temperature anneal was performed in a tube furnace, making the
procedure no less expensive than a conventional tube oxidation. Having demonstrated
the potential of the nitric oxidation, attention can now be paid to replacing the tube
anneal with a rapid-thermal anneal. If successful, the nitric oxidation could provide an
inexpensive means to attain good passivation for solar cells or lifetime test structures.
3
10
2
10
References
(b)
1
10
14
10
15
10
16
17
10
10
-3
Excess Carrier Concentration (cm )
Figure 3. SRV after various high temperature annealing as a function of excess hole
concentration within the silicon substrate.
[1] S. Imai, M. Takahashi, K. Matsuba, Asuha, Y. Ishikawa, and H. Kobayashi,
“Formation and
electrical characteristics of silicon dioxide layers by use of nitric acid oxidation method,” acta physica
slovaca, vol. 55, pp. 305-313, June 2005.
[2] Asuha, SS. Im, M. Tanaka, S. Imai, M. Takahashi and H. Kobayashi, “Formation of 10-30 nm
SiO2/Si structure with a uniform thickness at ~120 ºC by nitric acid oxidation method,” Surf. Sci., pp.
2523-2527, April 2006.
[3] M. Takahashi, SS Im, M. Madani and H. Kobayashi, “Nitric acid oxidation of 3C-SiC to fabricate
MOS diodes with a low leakage current density,” J. Electrochem. Soc., 155, pp. H47-H51, 2008.