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

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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


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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.


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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.


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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


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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.


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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.


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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.


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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.


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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.


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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.

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