Deposition of Al O thin films by sputtering for c-Si solar...
Transcript of Deposition of Al O thin films by sputtering for c-Si solar...
Deposition of Al2O3 thin films by sputtering for c-Si solar cells passivation Richard Rivera
- 1 -
Abstract—Passivation of silicon solar cells is an important issue
due to the importance of the photovoltaic industry. The present
work focuses on a possible material, Al2O3, to passivate such
devices by using sputtering, a technique easily accessible to the
industry. Characterization of the material and measurements of
its passivating features have been carried out, leading to
important conclusions.
Index Terms—5. Nanostructured Materials: Alumina,
Magnetron Sputtering, Photovoltaic Device, Silicon Solar Cells,
Surface Passivation.
I. INTRODUCTION
ILICON is the dominant material in the solar cell industry,
with more than 85% of the devices made in crystalline Si
wafers [1]. The efficiency of such devices is greatly reduced
by means of the electronic recombination losses at the wafer
surfaces, which can be reduced through a process called
surface passivation. This process involves the presence of an
outer-layer on the material to create a protective shell.
A solar cell, or photovoltaic cell, is an electrical device that
is able to transform light into electricity by means of the
photovoltaic effect: when photons strike the surface of a
semiconductor (like Si), electrons from valence band (VB) can
be excited to conduction band (CB) if they get sufficient
energy, where they can freely move within the semiconductor.
The hole that the electron has left in the VB is also able to
move: the absorbed photon has created a mobile electron-hole
pair, thus being able to produce an electrical current. The
electron in the CB is in a meta-stable state, and it will try to
stabilize to a lower energy level by filling any empty VB state
(and removing a hole), a process called recombination. So, in
order to extract a current, a charge separation process is
needed. This process requires a spatial asymmetry, like the one
produced by the presence of an electrical field [2], [3].
In order to stablish an electrical field, a p-n junction is
formed: an n-type layer of silicon (Si containing atoms with
extra electrons, like P) above a p-type silicon layer (Si
containing atoms with a lack of electrons, like B), which
greatly increases the number of charge carriers. Once the
layers are in contact, carriers accumulate in the interface
forming a depletion region: electrons recombine with holes,
leaving positive ions in the n-type Si, negative ones in the p-
type and no mobile charge carriers, which produce an
electrical field from the n-type to the p-type, which also
opposes the exchange of carriers. This electric field grows
until is able to arrest any further transfer of electrons and
holes, leading to an equilibrium in the depletion region. When
the light illuminates a solar cell surface the following can
occur: it can be reflected; it can pass through the material
(which is the case for the lower energy photons); or it can be
absorbed (if the energy is equal or higher than the band gap of
the material). If light is absorbed, a photon strikes an atom and
an electron-hole pair can be formed, but because of the electric
field the pair is unable to recombine: electrons are attracted to
the n-type side, and holes to the p-type. Metal contacts are
added to the layers, providing a way for them to recombine,
and at the same time extracting the electrical current to be used
[2], [3]. A schematic of the whole process can be seen in
Figure 1.
The presence of dangling bonds at the surface of the
semiconductor, which are left by the interruption of the
periodicity of the crystalline lattice, makes that the surface
becomes a site with a high rate of recombination processes.
These processes in the surroundings of the surface depletes the
region of carriers, which produces that the carriers from higher
concentration regions, by random motion, start flowing into
this localized region of low carrier concentration, through the
diffusion process. In order to reduce the number of dangling
bonds, and hence the surface recombination, a layer of a
material can be deposited on the top of the semiconductor
surface. This reduction in the number of dangling bonds is
known as surface passivation.
How could be possible to know that the surface passivation
is effective? As the surface is a place of high recombination
rate, a useful way to determine the level of passivation of a
surface is through the surface recombination velocity (SRV),
which measures the rate at which carriers move towards the
surface: if there is no recombination at the surface, the
movement of carriers towards it is zero. In a surface with
infinitely fast recombination, the speed of carrier towards the
surface is limited by the maximum velocity that they can
attain.
A wide variety of materials have been used to passivate the
surface of a Si solar device, such as thermally grown silicon
Deposition of Al2O3 thin films by sputtering for
c-Si solar cells passivation
Richard Rivera
Directors of Master’s Thesis: PhD. Jorge Alberto García Valenzuela and PhD. Joan Bertomeu
Balagueró. Departament de Física Aplicada i Òptica, Universitat de Barcelona
S
Deposition of Al2O3 thin films by sputtering for c-Si solar cells passivation Richard Rivera
- 2 -
oxide (SiO2), which relies on chemical passivation (by
reducing the number of dangling bonds by tying them with
hydrogen introduced during the annealing process) and has
achieved very low effective surface recombination velocities
(Seff < 10 cm/s) [1]; silicon nitride (a-SiNx:H), which uses field
effect passivation (reduction in the density of one type of
charge carrier at the surface due to the presence of positive
interface charges) [4], which has been able to achieve Seff
around 10 cm/s [5]; hydrogenated amorphous silicon (a-Si:H),
mainly used for thin film Si solar cells [6], achieving Seff as
low as 3 cm/s [7]; alumina (Al2O3), which is used to passivate
the rear side of a solar cell, achieving Seff as low as 6 cm/s [8];
etc.
The last mentioned passivating material, alumina, is the one
under study in the present work. There are some researches in
which it has been deposited through atomic layer deposition
(ALD) [8], which can offer a great control in the deposition
process but taking a considerable amount of time, which is not
suitable for industrial applications. With this in mind, it is of
our particular interest to study the effects and passivation
degree of Al2O3 deposited by means of a sputtering process,
which is more adequate for industrial implementation.
II. EXPERIMENTAL DETAILS
A. Deposition Technique
Radio Frequency (RF) Magnetron sputtering was the
technique used to deposit the passivation material. Sputtering
refers to the deposition of material by ejecting it from a solid
target because of the collision of high energy species [9]. In
this technique, a chamber is set to vacuum environment
conditions in order to reduce as much as possible any possible
contaminant, before introducing a noble gas as argon. Once the
Ar pressure has been set to a certain value, the deposition can
be started. Inside the chamber, a radiofrequency discharge is
activated between a cathode (which is the target) and an anode
(the substrate), thus producing the ionisation of the Ar atoms
(Ar+). Electrons are accelerated towards the anode and the
positively charged argon ions are accelerated towards the
cathode, which leads to an atmosphere consisting of ions,
electrons, and neutral gas atoms, thus producing plasma as
long as the pressure and electrical power are kept within an
appropriate range (the range varies depending on the type of
power source, RF or DC, and the target). If the ions striking
the cathode have the necessary momentum, atoms from the
Fig. 1. A schematic of the whole process in a solar device. First, (a) a photon illuminates the surface of a solar cell, (b) and a mobile electron-hole pair is
created. Due to the presence of the electrical field (c) from the n-type Si to the p-type, electrons are pushed upwards, were they are collected by the
metallic contacts; and the hole moves downwards. Finally, (d) both charge carriers found each other at the rear contact.
Deposition of Al2O3 thin films by sputtering for c-Si solar cells passivation Richard Rivera
- 3 -
target are ejected in vapour phase. As the environment has
been set at low pressures, condensation occurs under
concurrent bombardment by energetic species, promoting
nucleation, compound formation and film growth onto the
substrate [10].
The process was significantly improved through the
magnetic confinement of plasma particles near to the target:
magnetron sputtering [11]. With this process, the number of
collisions is increased, and more Ar ions appear, achieving
higher deposition rates.
In the present work, the equipment used was a commercial
ATC-ORION 8 HV system from AJA International, Inc, which
can be seen in Figure 2. Al2O3 thin films were deposited at
room temperature onto 5×5 cm2 Corning glass (1737F) and p-
type crystalline silicon wafers (high quality Float zone wafers,
10 cm in diameter, 280 µm in thickness and a resistivity
between 1-5 Ω×cm,) by RF magnetron sputtering. For this, an
Al2O3 target (3 inch diameter) of 99.9% purity was used. The
base pressure inside the chamber was always 2–3×10–6
Torr (1
Torr = 133.3 Pa). The target to substrate distance was fixed at
15 cm to achieve a better homogeneity. The working gas was
99.99% Ar, and the depositions were performed with the
substrate rotating at 50 rpm. In this work, we studied the
effect of the radio frequency power and the deposition
pressure on the alumina films deposited on glass substrates,
with the aim of selecting the best conditions for their later use
on the silicon wafers; the radio frequency power was varied
from 150 – 450 W and the deposition pressure was varied
from 1.0 – 5.0 mTorr.
B. Characterization Techniques
B.1. Thickness
A Dektak 3030 mechanical surface profiler, which is
equipped with a 25 µm diameter probe, has been used to
determine the thickness of the samples. In the deposited film,
some steps were created by means of a simple lift-off
technique employing ink before deposition and isopropanol for
cleaning it later. Thus, the depth of the steps could be
measured with the profiler, which has a vertical resolution of 1
nm [12]. The obtained results were corroborated by confocal
microscopy (which can produce a 3D profile, by scanning a
surface and eliminating the out-of-focus light), thanks to a
Sensofar PLµ 2300 optical imaging profiler device. A first set
of deposited films was measured to calculate the deposition
rate under the two studied parameters, assuming a constant
deposition rate, and the results are plotted in Figure 3. The
resulting values were used to select the deposition time
required to obtain 50 nm thick thin films, which is a common
value used to passivate Si wafers.
B.2 Transmittance - Reflectance
The percentage transmittance (%T) and percentage
reflectance (%R) of the deposited 50 nm Al2O3 thin films were
measured by using a Perking Elmer Lambda 950 device in the
250 – 2500 nm wavelength interval. This system is equipped
with an integrating sphere, which is able to distinguish
between the specular, total (T), and diffused (Td)
transmittance and reflectance (R) through the use of different
configurations. The system is equipped with a deuterium and a
halogen lamp, being able to take measures of transmitted and
reflected light between 200 and 2500 nm. The measurements
were carried out with the Al2O3 facing the incident light.
Fig. 3. Deposition rate as a function of deposition pressure and at
different RF power, as measured by profilometer and confocal microscopy.
Fig. 2. RF magnetron sputtering system as is installed in the
Laboratory of Micro/Nanotechnologies of the Physics Faculty of
Universitat de Barcelona.
Deposition of Al2O3 thin films by sputtering for c-Si solar cells passivation Richard Rivera
- 4 -
B.3 X-ray Diffraction
With the aim of finding some degree of crystallinity in the
deposited material, the films were also measured by X-ray
diffraction (XRD). In this technique, an X-ray beam is focused
in the sample with a certain θ angle, and measures of the
diffraction angles of the scattered beam are taken. As atoms
cause that the beam of X-rays scatter in many directions, an
intensity peak will appear due to constructive interference [13]
when Bragg’s law is fulfilled:
xhkl nd sin2 (1)
In the Bragg’s law, dhkl is the distance between the lattice
planes, λ corresponds to the X-ray wavelength, nx stands for
the diffraction order, and θ is the angle between the sample
surface and the X-ray beam. In the present study, a
PANalytical X'Pert PRO MRD diffractometer was used.
B.4 X-ray Photoelectron Spectroscopy
The X-ray photoelectron spectroscopy (XPS) technique is
based on counting the emitted electrons, as a function of their
kinetic energy, from the surface of a material when an X-ray
beam is focusing on it. These emitted electrons correspond to
the atoms located in the outer layers of the sample, and their
kinetic energy can be converted in their corresponding binding
energy. In this quantitative technique, the elemental
composition (species) and stoichiometry of a given material
can be determined. Furthermore, the kinetic energy of these
electrons can provide energy concerning the chemical state
and the bonded species, with the help of standard data sheets.
More information can be found in [14].
In the present research, the study of the chemical bonds in
the deposited materials through X-ray photoelectron
spectroscopy was performed by using a PHI 5500
Multitechnique system (from Physical Electronics), with a
monochromatic X-ray source from Al Kα line with an energy
of 1486.8 eV, at 350 W, and calibrated using the 3d5/2 line of
Ag. The binding energies have been considered by taking the
carbon 1s peak as a primary standard, whose binding energy
was taken as 284.8 eV [15].
B.5 Surface Recombination Velocity
In order to determine the effectiveness of the surface
passivation, it is necessary to find the surface recombination
velocity (SRV), which measures the rate at which carriers
move towards the wafer surface, where they recombine. In the
present work, a WCT120 Sinton lifetime tester has been used
to determine the effective lifetime of the minority charge
carriers (the average time which a carrier can spend in an
excited state after electron-hole generation before it
recombines). This effective lifetime can be related with the
effective surface recombination velocity through the equation
[1], [16]:
W
Seff
bulkeff
211
(2)
with τeff being the effective lifetime (which involves surface
and bulk recombination processes), τbulk is the lifetime in the
bulk, W is the wafer thickness and Seff is the effective surface
recombination velocity.
Lifetime has been measured through two techniques: the
first one is called photoconductance decay (PCD). In this
technique, very short pulses of light are shone on the sample,
which generates electron-hole pairs, thus enhancing the
conductivity. As the light pulse ceases, these pairs start to
recombine, which produces that the enhancement in the
conductivity fading over time [17]. The second technique is
the so-called quasi-steady state photoconductance (QSSPC)
[18], which relies on the number of charge carriers present
when a steady light has been shone on a sample. It implies that
the intensity of the light changes sufficient slowly so that the
charge carrier populations in the sample are always in steady
state.
Finally, for both techniques (PCD and QSSPC), the
illumination has been varied over a range of intensities, always
considering as the most important one, the equivalent to one
sun.
III. RESULTS AND DISCUSSION
A. Al2O3 Deposited on Glass Substrate
A.1 Deposition Rate
Figure 3 (already presented) shows that the deposition rate
increases with the applied power. It is due to a higher energy
introduced to the ions, which are able to eject more particles
from the target through the momentum transfer. It is also
possible to observe that the deposition rate is inversely
proportional to the deposition pressure: when higher pressures
are applied, it involves a shorter mean free path, because there
is a higher number of particles in the argon plasma, which act
as obstacles to the ejected target particles in their trajectory to
the substrate, which explains the decrease in the deposition
rate.
A.2 XPS
With the aim of corroborating that the deposited material
corresponds to the desired alumina, XPS analyses have been
carried out. The results shown in Table I correspond to the
binding energies found for the Al 2p and for the O 2s for a
different set of pressures and deposition power. As can be
seen, the value found for the Al 2p is practically identical to
TABLE I
BINDING ENERGIES FOR Al 2p AND O 1s
Sample Al 2p (eV) O 1s(eV)
150 W – 1.0 mTorr 74.16 531.04
150 W – 5.0 mTorr 74.15 531.13
300 W – 1.0 mTorr 74.14 531.16
300 W – 5.0 mTorr 74.12 531.29
450 W – 1.0 mTorr 74.14 530.99
450 W – 5.0 mTorr 74.10 530.97
Deposition of Al2O3 thin films by sputtering for c-Si solar cells passivation Richard Rivera
- 5 -
the reported values [15], whereas the O 2p is in very close
agreement to the reported ones [15].
Figure 4 shows the binding energies obtained for 300 W
(this power was chosen simply because it is the average) at
different deposition pressures. It is possible to appreciate that
for 1.0 mTorr there are argon peaks, which means that for
lower pressures there are higher probabilities that Ar atoms
from the plasma contaminate the films, being that the reason of
choosing 5.0 mTorr as the appropriate one for the sputtering
processes, along with the fact that at this pressure the
deposition process is less aggressive, due to reduced mobility
(velocity) of the ions. In the same Figure 4, it is possible to see
the amplified spectra for the deposited material, around the Ar
2p peak, where it is clear that for lower pressures there is
higher Ar incorporation. Also, Al 2p peak is showed in detail,
whereas the curves have been fit through a Voigt function to
see if they are formed by only one atomic species. The O 1s
for 1.0 mTorr is well fitted through the Voigt function,
whereas the other deposited film is slightly different. Figure 5
shows a comparison between the O 1s for the deposition at 1.0
mTorr and 5.0 mTorr. It can be seen that the origin of this
deviance is due the presence of not fully coordinated oxygen
(which is the cause for the second peak shown in the film
Fig. 4. (a) XPS spectra of alumina films deposited at 300 W and at different deposition pressures. Ar peaks are clearly visible in the film deposited at
1.0 mTorr. (b) Amplified spectra for the deposited material, around the Ar 2p peak. (c) Al 2p peak and fit in detail. (d) Detailed O 1s peak and fit.
Fig. 5. Differences in the fitted curves for the films deposited at (a) 1.0
mTorr and (b) 5.0 mTorr.
Deposition of Al2O3 thin films by sputtering for c-Si solar cells passivation Richard Rivera
- 6 -
deposited at 5.0 mTorr), which indicates the presence of
defects.
Figure 6 shows the spectra of the samples deposited at 5.0
mTorr at different deposition rates, along with the details
around the Ar 2p, Al 2p and O 1s peaks. It can be seen that the
aluminium and oxygen peaks are in close concordance with the
reported results [15], and there is no presence of argon species,
only noise. It is worth to mention that in both Figure 4 and
Figure 6 it is also appreciable a peak corresponding to C 1s,
which is a common contaminant, incorporated to the deposited
films from the atmosphere, usually as CO2.
Finally, Figure 7 shows the results in the deviation of the
stoichiometry of the deposited films. Alumina ideally shows an
O / Al ratio of 1.5 (2 aluminium atoms per 3 oxygen atoms),
and XPS spectra provide information about the concentration
of these atomic species by means of the intensity of its peaks
(actually, the area below them, which is integrated). The
intensity of these peaks has been approximated by means of
the Multipak Spectrum: ESCA software, and the change of this
ratio has been determined for a set of samples at different
deposition power and pressure, which is shown in Figure 7.
What this picture means is that there is an excess of oxygen in
the deposited films.
Fig. 6. a) XPS spectra of Al2O3 films deposited at 5.0 mTorr and different deposition power. (b) Amplified spectra for the deposited material, around
where the Ar 2p peak should be located. (c) Al 2p peak and fit in detail. (d) Detailed O 1s peak and fit.
Fig. 7. O/Al ratio versus the deposition power for the sample deposited
at different deposition rate and power.
Deposition of Al2O3 thin films by sputtering for c-Si solar cells passivation Richard Rivera
- 7 -
A.3 XRD
All the films were studied by means of X-ray diffraction in
order to determine any possible degree of crystallinity. Figure
8 shows the spectra for a film deposited at 5.0 mTorr and at a
different set of deposition power. As can be seen, there is no
presence of multiple peaks and no pattern is described by the
spectra, which implies that no crystallinity has been found in
the deposited alumina.
In an attempt to determine if the crystallinity is dependant
with the thickness of the film, another film with higher
thickness was deposited (200 nm), whose diffractogram can be
seen in Figure 9. In this figure it is possible to appreciate that
there is also no crystallinity, which means that the amorphous
state have remained independently of the film thickness.
A.4 R & T
At first sight, the deposited films are completely transparent,
very similar to the glass substrates, so reflection and
transmittance experiments have been carried out in order to
verify the absorption spectra of the deposited films. It is
important to state that all the deposited films, independently of
the RF power or deposition pressure, show very similar results,
as can be seen in Figure 10 (where the deposition pressure is
constant at 5.0 mTorr, and the deposition power varies) and
Figure 11 (where the deposition power is constant at 300 W
and the deposition pressure varies. In glass, the transmittance
is higher for shorter wavelengths, but as the wavelength
increases, all the transmittances seem to converge. In order to
quantify the change in the transmittance, the integrated
transmittance, which is shown in table II, has been calculated.
The interval of interest has been set from 400 nm to 1100 nm
Fig. 8. XRD diffractograms of 50 nm Al2O3 films deposited at 5.0
mTorr and different deposition power.
Fig. 9. XRD diffractogram of 200nm deposited Al2O3 film.
TABLE II
INTEGRATED TRANSMITTANCE
Deposition power Deposition pressure Integrated
transmitance (%)
150 W
1.0 mTorr 90.8
2.5 mTorr 91.5
5.0 mTorr 91.6
300 W
1.0 mTorr 91.1
2.5 mTorr 91.4
5.0 mTorr 91.8
450 W
1.0 mTorr 91.6
2.5 mTorr 91.8
5.0 mTorr 91.8
Corning - 94.1
Fig. 10. R & T spectra for the deposited samples at a constant
pressure of 5.0 mTorr and different deposition power.
Fig. 11. R & T spectra for the deposited samples at a constant power
of 300 W and different deposition pressures.
Deposition of Al2O3 thin films by sputtering for c-Si solar cells passivation Richard Rivera
- 8 -
wavelengths, which takes into account the visible and NIR
spectrum, with the band gap of silicon as upper limit (which is
1.1 eV).
As can be seen in table II, integrated transmittance does not
show large changes between the samples, but they have a little
lower transmittance in comparison with the glass substrate. It
implies that the passivating material is almost transparent,
allowing the pass of the incident light to the surface of the
passivated surface (which in the present case will be silicon).
The outcome slightly varies when the thickness of the
deposited film changes: the 200 nm alumina film shows
interference patterns, as can be appreciate in the Figure 12, but
even when the transmittance spectra looks a little different, the
integrated transmittance in the interval of interest is similar to
the 50 nm films.
B. Al2O3 Deposited on Silicon Substrate
After performing depositions on glass substrates, p type
crystalline silicon substrates have been used to study the
passivating effect of Al2O3. As it has been determined
previously, 5.0 mTorr has been considered the most
appropriate deposition pressure, so, all the samples have been
deposited at that pressure. The depositions have been
performed to obtain 50 nm thicknesses, assuming the same
deposition rates than the previous samples on glass substrates.
The obtained samples have been analysed through the XPS
technique, which does not show any significant difference
compared with the alumina deposited on glass substrate, as is
shown in Figure 13 (deposition power is 300 W). This means
that the substrate does not produce any change in the XPS
spectrum, which is not the case in the XRD diffractogram, as
can be seen in Figure 14. It this figure, it is possible to see
some changes in comparison with Figures 7 and 8 due to the
substrate, but the relevant fact is that the passivating layer
remains in an amorphous state, regardless that the Si substrate
is crystalline.
Fig. 13. a) XPS spectra of Al2O3 film deposited on p type silicon wafer at 300 W and 5.0 mTorr, and on glass substrate
Fig. 12. R & T spectra for the 200 nm thickness sample compared
with the glass substrate.
Fig. 14. XRD diffractigram for an alumina film deposited on silicon at
300 W.
Deposition of Al2O3 thin films by sputtering for c-Si solar cells passivation Richard Rivera
- 9 -
The most relevant measures on silicon substrates correspond
to the ones involving the passivating features of the alumina.
In order to determine these features, PCD and QSSPC
measures have been carried out in three different samples,
every one of them at different deposition power. In addition,
the measurements have been carried out before and after an
annealing at 350°C for 20 minutes. It has been found that the
results for PCD and QSSPC are quite similar, so only QSSPC
result are presented.
The mentioned techniques provided the effective carrier
lifetime (τeff). In a good quality wafer, as the ones used here,
the recombination in the bulk is negligible, which implies that
the bulk lifetime (τbulk) is very high in comparison with the
surface recombination, so the 1/τbulk term in equation 2 can be
neglected, which produces:
eff
eff
WS
2 (3)
Figure 15 shows the measured lifetime of the samples, and
Table III show the effective lifetime and effective SRV before
and after the annealing process, for two different
concentrations of light. For a Si solar cell device, the most
important is the 1 sun concentration, but the lifetime tester
device was unable to obtain it for the unpassivated Si wafer.
However, it has been obtained for a 5 suns concentration, so it
is possible to compare the degree of passivation achieved by
the alumina layer. The results show that, for both
concentrations, once the alumina is deposited, the effective
SRV decreases as the deposition power increases. Once the
samples have gone through an annealing process, the Seff
decreases by around two orders of magnitude in comparison
with the non-annealed samples, which shows the importance of
such a process. Also, it is worth to note that the tendency
relating the deposition power and Seff has been inverted: for
the higher deposition power, Seff is higher. The lowest Seff
found is 40 cm/s, which is showed for the sample deposited at
150 W. This result is one order of magnitude higher than
values reported through other techniques, such as ALD
(6cm/s) [8], but still it is a great improvement.
At the time of the measurements, it has been possible to
identify damages (“blisters”) on the surface of the annealed
samples. Observations performed through optical microscopy,
which can be seen in Figure 16, show that in the surface of the
300 W deposited film there are some circular regions of
different nature than the rest of the film, which have a diameter
around 22 µm. In the 450 W these blisters have also appeared
and have a higher area and quantity than those of the previous
TABLE III
SURFACE RECOMBINATION VELOCITIES AND LIFETIMES AT DIFFERENT
INCIDENT SUNLIGHTS AND DEPOSITION POWER
1 Sun No annealed samples Annealed samples
Power (W) τeff (µS) Seff (cm/s) τeff (µS) Seff (cm/s)
150 0.84 16.7×103 350 40
300 0.98 14.3×103 277 50.5
450 1.5 9.3×103 135 103.7
5 Suns
Si 0.98 14.3×103 - -
150 1.1 12.7×103 231 60.6
300 1.34 10.4×103 202 69.3
450 2.36 5.9×103 113 123.9
Fig. 15. Effective lifetime measured before and after the annealing,
for the three deposition power.
Fig. 16. Damage found in the (a) 300W and (b) 450W deposited films
after performing the annealing.
Deposition of Al2O3 thin films by sputtering for c-Si solar cells passivation Richard Rivera
- 10 -
case (being clearly visible at the naked eye, with a diameter
around 28 µm). The origin of these damages –blistering– is
unknown, but at the moment the available information is not
enough to find an appropriate explanation of this phenomenon,
which could be analysed in a future work. The film deposited
at 150 W does not show any damage, which could imply that
the deposition power could have some relevance in the
production of the blisters. It is worth to mention that this
blistering effect has also been reported in some other
researches, although an explanation of the cause remains
unknown [1], and we found an increase in the blisters
production with the increase of power deposition.
IV. CONCLUSIONS
RF magnetron sputtering has been used to successfully
deposit Al2O3 on p type silicon substrates in an attempt to
passivate their surfaces. First, properties of alumina films
deposited on glass substrates have been characterized by
means of XPS, to determine the chemical composition of
depositions; XRD, trying to find any possible degree of
crystallinity in the deposited films; R&T, to measure the
transmittance of the passivating layer, because it is an
important fact how much light can pass to the Si wafer. The
findings of these techniques show that the appropriate
deposition pressure is 5.0 mTorr, to avoid Ar incorporation
from the sputtering process; the deposited alumina is
amorphous; and the deposited films is almost completely
transparent to the spectrum of interest.
Once the material has been characterized, silicon substrates
have been used to measure any possible degree of passivation
capabilities in the alumina. The results show that the effective
surface recombination velocity decreases when the Al2O3 film
is deposited, but this velocity reduces by around two orders of
magnitude when the sample is annealed after deposition,
achieving a Seff as low as 40 cm/s, which means that the
material is a suitable alternative to passivate p sides of a
silicon solar cell. However, the degree of passivation is lower
compared with other techniques, as ALD, which can achieve
velocities as low as 6 cm/s. Even so, the results obtained are
very important because sputtering is a technique that can be
scalable and is more suitable for industrial applications.
Finally, the annealing process needs to be reviewed, because
in some cases damages on the surface of the samples may
appear, which could imply that this process should be
performed in stages, rather than putting the samples from
ambient conditions to high temperature in just one step.
ACKNOWLEDGMENTS
The author would like to thank to the FAO group for
allowing him to carry out his Master thesis and the use of their
equipments. Many thanks to Anna Belén Morales Vilches, for
her help relating with the passivated Si substrates. Thanks to
the directors of this Master thesis, Dr. Joan Bertomeu, and
especially to Dr. Jorge Alberto García Valenzuela, for his
continued encouragement and invaluable assistance in the
development of this research project.
REFERENCES
[1] G. Dingemans and W. M. M. Kessels, “Status and prospects of Al2O3-
based surface passivation schemes for silicon solar cells,” J. Vac. Sci.
Technol. A, vol. 30, no. 4, 040802, July 2012.
[2] M. A. Green, Solar cells: operating principles, technology, and system
applications, California: Prentice-Hall, 1982, ch. 3.
[3] J. Nelson, The Physics of Solar Cells, London: Imperial College Press,
2003, ch. 2
[4] M. Z. Rahman and S. I. Khan. (2012, October). Advances in surface
passivation of c-Si solar cells. Mater. Renew. Sustain. Energy [Online].
1(1). Available: http://link.springer.com/article/10.1007%2Fs40243-
012-0001-y
[5] S. Dauwe, “Low-temperature rear surface passivation of crystalline
silicon solar cells,” Ph.D. Thesis, ISFH, University of Hanover,
Germany 2003.
[6] J. Schmidt et al. “Progress in the surface passivation of silicon solar
cells,” in Proc. 23nd Eur. Photovolt. Sol. Energy Conf., Valencia,
Spain, 2008, pp. 974-981.
[7] S. Dauwe, J. Schmidt, and R. Hezel, “Very low surface recombination
velocities on p- and n-type silicon wafers passivated with hydrogenated
amorphous silicon films,” in Proc. 29th IEEE PVSEC., New Orleans,
USA, 2008, pp. 1246–1249.
[8] J. Schmidt et al. “Surface Passivation of High-efficiency Silicon Solar
Cells by Atomic-layer-deposited Al2O3,” Prog. Photovolt: Res. Appl.,
vol. 16. no. 6, pp. 461-466, March 2008.
[9] M. Ohring, The materials science of thin films. USA: Academic Press,
1992
[10] D. W. Hess, “Plasma material interactions,” J. Vac. Sci. Technol. A, vol.
8, no. 3, pp. 1677-1684, June 1990.
[11] J. S. Chapin, “Sputtering process and apparatus.” US Patent 4166018,
August 28, 1979.
[12] P. Carreras, “Doped and multi-compound ZnO-based transparent
conducting oxides for silicon thin film solar cells,” Ph.D Thesis, Dept.
de Física Aplicada I Ȯptica, Universitat de Barcelona, Barcelona, Spain,
2013.
[13] C. R. Brundle, C. A. Evans, and S. Wilson, Encyclopedia of materials
characterization: surfaces interfaces, thin films. USA: Butterworth-
Heinemann, 1992
[14] J. F. Moulder, W. F. Stickle, P. E. Sobol, and K. D. Bomben, Handbook
of X-ray photoelectron spectroscopy. Minnesota: Physical Electronics,
Inc., 1992
[15] K. T. Ng and D. M. Hercules. “Studies of nickel-tungsten-alumina
catalysts by x-ray photoelectron spectroscopy,” J. Phys. Chem., vol. 80,
no. 19, pp. 2094-2102, September 1976.
[16] L. Fesquet, S. Olibet, E. Vallat-Sauvain, A. Shah, C. Ballif. “High
quality surface passivation and heterojunction fabrication by VHF-
PECVD deposition of amorphous silicon on crystalline Si: theory and
experiments,” in Proc. 22nd Eur. Photovolt. Sol. Energy Conf., Milano,
Italy, 2007, pp. 1678-1781.
[17] D.T Stevenson and R.J Keyes, “Measurement of Carrier Lifetimes in
Germanium and Silicon,” J. Appl. Phys. Vol. 26, no. 2, pp. 190-195,
February 1955.
[18] R. Sinton and A. Cuevas, “Contactless determination of current–voltage
characteristics and minority‐carrier lifetimes in semiconductors from
quasi‐steady‐state photoconductance data,” Appl. Phys. Lett. Vol. 69,
no. 17, 2510, October, 1996.