WO2014080080A1

WO2014080080A1 - Method for fabricating a passivation film on a crystalline silicon surface

Info

Publication number: WO2014080080A1
Application number: PCT/FI2013/051092
Authority: WO
Inventors: Shuo Li
Original assignee: Beneq Oy
Priority date: 2012-11-22
Filing date: 2013-11-21
Publication date: 2014-05-30

Abstract

A method for fabricating a passivation film (1) on a crystalline silicon surface (2), the passivation film (1) comprising aluminium oxide A1O_x, comprises providing an existing crystalline silicon surface (2) in a reaction space; and depositing in the reaction space aluminium oxide A1O_x on the existing crystalline silicon surface, wherein the existing crystalline silicon surface with the deposit already formed thereon is alternately exposed to surface reactions of at least one precursor for aluminium and at least one precursor for oxygen, wherein the at the least one precursor for oxygen comprises ozone O3. According to the present invention, during exposing the existing crystalline silicon surface with the deposit already formed thereon to surface reactions of at least one precursor for oxygen, the existing crystalline silicon surface with the deposit already formed thereon is exposed to a deposition atmosphere comprising ozone O3 with a mole ratio of (kT+b)%, wherein k lies in the range of -0.09 to -0.10, b lies in the range of 21.0 to 22.5, and T is the deposition temperature in Celcius.

Description

METHOD FOR FABRICATING A PASSIVATION FILM ON A CRYSTALLINE SILICON SURFACE

FIELD OF THE INVENTION

The present invention relates generally to thin film surface passivation of semiconductor surfaces, in particular to passivation of crystalline silicon surfaces of e.g. photovoltaic cell structures.

BACKGROUND OF THE INVENTION

Photovoltaic (hereinafter abbreviated as "PV") cells are becoming more and more important means of generat^¬ ing electrical energy. Especially solar cells, i.e. photovoltaic cells designed to convert sunlight di^¬ rectly into electrical energy, are considered as one of the most promising candidates for renewable energy production .

Among the various PV and solar cell technologies which typically are classified according to the light ab- sorbing semiconductor material (s) of the cell, crys^¬ talline silicon, i.e. "c-Si" is one of the major mate^¬ rials in commercial PV modules and devices.

As in any energy converting device, the efficiency of conversion of the primary form of energy, i.e. light, into the secondary form of energy, i.e. electricity, is of crucial importance in commerce PV cells. There are numerous factors each having an impact to the overall conversion efficiency. These include e.g. the reflectance of the cell surface and the thermodynamic configuration of the cell structure affecting the ex^¬ ternal efficiency of the cell. On the other hand, there are also many factors related to the internal mechanisms within the cell structure. One particular internal mechanism related to the semiconductor mate^¬ rial as the active material, i.e. the material where the absorption of incident light and the generation of free charge carriers take place, is surface recombina^¬ tion. By surface recombination is meant undesired re^¬ combination of generated charge carriers at or in the vicinity of the surface of the active semiconductor material of the cell. Surface recombination in semi^¬ conductors is a result of many different mechanisms leading to trapping of charge carriers in specific en^¬ ergy states at or close to the surface of a semicon- ductor. These energy states, or surface states as they are often called, may originate from different sources, such as impurities at the surface or the in^¬ evitable disruption of periodicity of a semiconductor crystal at the surface. In a photovoltaic cell the quantum efficiency, and therefore the overall effi^¬ ciency, decreases as charge carriers generated by the absorption of photons in the semiconductor recombine with the surface states and therefore can not be col^¬ lected in the cell electrodes to contribute to the cell current.

As a measure of the passivation performance is typi^¬ cally used the minority charge carrier lifetime at the surface to be passivated. The higher the carrier life- time, the lower the surface recombination velocity, and thus the better the passivation performance. In practice, the actual parameter most often used as the measure of the passivation performance is the effec^¬ tive lifetime which is affected both by the sur-

face lifetime

at the passivated silicon surface and by the bulk lifetime in the bulk silicon.

In the case of c-Si PV cells, the surface recombina^¬ tion can be reduced by a particular passivation film on either or on both of the silicon surfaces. Nowadays, the most promising passivation film material used for commercial c-Si PV cells is aluminium oxide ΑΙΟχ . In the ALD ΑΙΟχ, the x typically lies in the range of 1.5...2.0, thus deviating from the stoichio^¬ metric composition AI₂O₃. One of the most common depo^¬ sition technologies is atomic layer deposition ALD and its different variations. ALD produces a very confor- mal and uniform passivation layer which can be produced by reasonable costs at moderate temperatures. According to the current common understanding in the art, the actual passivation function of an ALD alumin- ium oxide is based on at least two functions. First, hydrogen atoms present in the ALD aluminium oxide film passivate the Si dangling bonds, thereby reducing the interface state density D_it at the Si surface (chemical passivation) . Second, an oxygen-rich A10_x passivation film produces a high density of fixed negative charges at the Si surface, thereby creating a built-in elec^¬ trical field which reduces the minority charge carrier density at the Si surface (field effect passivation) . In addition to hydrogen H and oxygen 0, also other im- purities such as carbon C may affect the passivation performance, though the detailed mechanisms of differ^¬ ent impurities are not fully known.

As stated above, one key parameter affecting the pas- sivation performance is the amount of H atoms at and close to the Si/A10_x interface. This, as well as the composition of the A10_x passivation film in general, the composition also meaning the other impurities than H, is affected by the actual process type and the pro- cess parameters used. Those process parameters include e.g. the deposition temperature and the precursors, particularly the precursor (s) for oxygen.

One of the most promising precursors for oxygen is ozone O₃. One advantage of ozone is high excess oxygen content in the deposited film, affecting positively the field effect passivation via the high amount of fixed negative charges.

When using ozone O3 as a precursor for oxygen, accord- ing to the established understanding in the art, the optimal deposition temperature is believed to lie in the range of 150°C to 250°C, the optimum typically be^¬ ing about 200°C. The problem with too low temperatures is low density of the deposited A10_x, whereas a too high temperature, the hydrogen content is too small. In both cases, there is not sufficient hydrogen pre^¬ sent in the Si/AlOX interface to passivate the Si dan^¬ gling bonds. The deposition temperature of about 200°C can be con^¬ sidered as moderately low and causes no problem in R&D processes. However, in high-throughput industrial pro^¬ cesses, the time required to rise up the deposition chamber temperature and to heat the deposition sub- strate to the required temperature can be critical is^¬ sues also with such moderate deposition temperature. Lowering the deposition temperature would shorten the time required for heating up the deposition chamber and the deposition substrate, and thus would reduce the production costs via an increased throughput. How^¬ ever, said common understanding in the art does not allow any great deviation from the optimal temperature of about 200°C without deteriorating the passivation performance of the deposited passivation film.

SUMMARY OF THE INVENTION

The purpose of the present invention is to provide a novel method for fabricating a high quality A10_x passivation film on a crystalline silicon surface. The structure according to the present invention is characterized by what is presented in claim 1.

The present invention is focused on a method for fab- ricating a passivation film on a crystalline silicon surface, the passivation film comprising aluminium oxide ΑΙ Οχ , for reducing the surface recombination of charge carriers on the silicon surface. By passivation film is meant here a passivation coating in the form of a thin film. By thin film is meant here generally a film with a thickness of less than or equal to 200 nm. Typically, the total thickness of the thin film passivation film is clearly below this value, e.g. about 20-30 nm.

By crystalline silicon surface is meant here a surface of single crystal silicon, multicrystal silicon or mono-like (quasi mono) crystal silicon.

The passivation film, i.e. passivation material deposited on the silicon surface, comprises aluminium oxide ΑΙ Οχ , wherein the "x" denotes the possibly non- stoichiometry composition deviating from the stoichio- metric AI₂O₃. Said passivation material preferably con^¬ sists mainly of A10_x, though it may also contain some other material (s) . Aluminium oxide A10_x, in its turn, typically comprises some impurities such as hydrogen H, oxygen 0, and carbon C.

As described in the background section above, the main purpose of the passivation film is to reduce the undesired surface recombination of charge carriers on the silicon surface, which might cause deterioration of the optoelectronic device, to which said silicon surface belongs. The method of the present invention comprises provid^¬ ing an existing crystalline silicon surface in a reac^¬ tion space; and depositing in the reaction space aluminium oxide ΑΙ Οχ on the existing crystalline silicon surface, wherein the existing crystalline silicon surface with the deposit already formed thereon is alter^¬ nately exposed to surface reactions of at least one precursor for aluminium and at least one precursor for oxygen, wherein the at the least one precursor for ox- ygen comprises ozone O3.

The expression "with the deposit already formed there^¬ on", in the context of the "existing crystalline sili^¬ con surface" refers to the fact that only during the very first precursor pulse, the exposed deposition surface is actually the existing crystalline silicon surface. After this first pulse, the deposition sur^¬ face at issue comprises some deposit already formed thereon .

Said alternate exposure of the deposition surface, i.e. the surface on which the material is to be deposited, to the precursors for aluminium and oxygen refers to a chemical vapour deposition method often called atomic layer deposition ALD. Sometimes, also other names such as atomic layer epitaxy (ALE) , atomic layer chemical vapor deposition (ALCVD) , and corresponding plasma enhanced variants can be used to refer to this kind of "ALD-type" process. In an ALD or ALD-type process, the material is grown on the deposition surface, typically controlled by self- limiting surface reactions, by a monolayer basis, producing thus a very uniform, conformal film with very accurately controllable film thickness.

The alternate or sequential exposure of the deposition surface to different precursors can be carried out in different manners. In a batch type process, the precursors for the different elements of the composition to be deposited are supplied to the reaction space in gaseous form as separate pulses, between which the residuals of the precursor (s) for the previous element are removed from the reaction space during a separate purging step. When pulses of precursors for both elements, i.e. aluminium and oxygen have been supplied to the reaction chamber so as to expose the deposition surface to the precursors, the same cycle is usually started again and repeated as many time as required for the desired thickness of the deposit. As an alternative, in spatial atomic layer deposition systems, the exposure to different precursors is based on spatial separation of the precursor gases. The dif^¬ ferent precursors can be confined in specific process areas or zones within a reactor space, through which the deposition surface is passed. In this kind of con^¬ tinuous ALD-type process, constant gas flow zones sep^¬ arated in space and a moving substrate are thus used to provide the required sequential exposure to differ^¬ ent precursors. In continuous ALD-type process, the cycle time depends on the speed of movement of the substrate through and between the gas flow zones.

"At least one" precursor for aluminium or oxygen, respectively, means that in different cycles, different precursors can be possibly used for a single element. Also, two or more different precursors for a single element can be used during a single cycle, either simultaneously or sequentially. In the latter case, a purging step can be performed between the different precursors to remove the residuals of the previous precursor from the reaction space before supplying there the next precursor. In particular, the at least one precursor for oxygen comprising ozone can be implemented by using ozone, possibly supplied along one or more carrier/diluting gases, as the only precursor for oxygen. Alternatively, in the case of more than one precursor for oxygen, ozone is used as one of them.

The reaction space can be a part of any deposition chamber suitable for ALD-type deposition of aluminium oxide. The details of the reaction space or other equipment used in the deposition are not essential for the core principles of the present invention, and they can be selected according to principles as such known in the art. Suitable process equipment are commercially provided e.g. by Beneq.

As is clear for a person skilled in the art, the above description does not specify all details of an ALD- type deposition process. Naturally, there can be e.g. various pre-treatment steps of the deposition surfaces. Such details of the process can be selected according to the principles as such known in the art.

According to the present invention, during exposing the existing crystalline silicon surface with the deposit already formed thereon to surface reactions of at least one precursor for oxygen, the existing crystalline silicon surface with the deposit already formed thereon is exposed to a deposition atmosphere comprising ozone O3 with a mole ratio of (kT+b)%, wherein k lies in the range of -0.09 to -0.10, b lies in the range of 21.0 to 22.5, and T is the deposition temperature in Celcius (put in the formula as plain number without the unit thereof) .

By deposition atmosphere is meant here the gaseous atmosphere in the reaction space, with which atmosphere the deposition surface is in contact during the exposure to a precursor for oxygen. This deposition atmosphere can comprise e.g. nitrogen N₂ as a carrier gas, and ozone O3 and oxygen O₂ supplied by an ozone generator. Typically, precursor concentrations or concentrations are given in grams of ozone per normal cubic g/Nm³ produced by the ozone generator used, or supplied to the reaction space. However, this is actually not very informative unit because the actual concentration of ozone to which the deposition surface is exposed depends largely e.g. on the carrier gas flow with which the ozone flow is mixed, and also the specific reactor type used. Thus, here the ozone concentration is defined via the mole ratio, i.e. the molecular ratio of ozone of the overall deposition atmosphere, i.e. the gaseous atmosphere to which the deposition surface is exposed. In the case of O3+O₂+N₂ as the deposition atmosphere composition, this mole ratio is given by:

(0₃ mole/ (O3+O2+N2) mole) x 100.

It is a common practice in the art that when using ozone as a precursor for oxygen, the ozone concentration of the deposition atmosphere is kept high. As high ozone concentrations as 300 g/Nm³ and higher at the output of the ozone generator are often used. (With a typical case of N₂ as a carrier gas flow with 0.5 Standard Litre per Minute SLM, 300 g/Nm³ makes an ozone molar concentration of 16 % of the total flow.) However, the inventor has surprisingly observed that, in contrast to the established understanding in the art on the appropriate ozone concentration, such high ozone concentrations do not always result in a high passivation performance. Instead, surprisingly high passivation performance has been observed for passivation films deposited by using the lower ozone concentrations as defined above. Moreover, as a very significant advantage of the present invention, by using such ozone concentration, the deposition temperature can be decreased to as low as about 150°C and even lower without deteriorating the passivation performance of the film. Actually, as high passivation performance is achievable at about 150°C as with prior art processes using a deposition temperature of 200°C. This capability of using a lower temperature means great savings in the production time and costs of an industrial-scale manufacturing process .

In other words, the present invention could also be formulated as a use of said ozone concentration defined above in an ALD-type method for manufacturing an ΑΙ Οχ passivation film on crystalline silicon, main purposes of the use of said ozone concentration being to provide a high passivation performance and decrease the required deposition temperature level.

As can be seen in the in the formula of the ozone mole ratio above, the optimal ozone concentration depends on the deposition temperature T. This is due to the simple fact that also the deposition temperature affects the growth and the accurate composition of the deposited passivation film. As explained in the background section, it has been generally understood in the art that with the high ozone concentrations typically used, there is an optimal deposition temperature at about 200°. Deviating from this optimum decreases the carrier lifetime and thus the passivation performance. Actually, similar behaviour has been now found for the lower ozone concentrations according to the present invention also. Thus, for each deposition temperature, there is an optimal low ozone concentration defined by that formula above. Vice versa, for each such low ozone, there is an optimal deposition temperature. Thus, both the ozone concentration and the deposition temperature should be selected properly. For example, for T = 200°C, k = - 0.097, and b = 22.0, the ozone molar ratio according to said formula is 2.6 %. Changing the temperature to 150°C increases the ozone molar ratio to 7.45 %.

Even more preferably, the factors k and b of the formula above lie in the range of -0.097 to -0,098 and in the range of 21.9 to 22.1, respectively.

As was stated above, when using ozone O3 as a precursor for oxygen, according to the established understanding in the art, the optimal deposition temperature is believed to lie in the range of 150°C to 250°C, the optimum typically being about 200°C. However, as is clear for a skilled person on the basis of the above formula for the ozone mole ratio itself, the deposition temperature cannot be too high. For example, for deposition temperatures exceeding 250°C, said formula would produce a negative ozone mole ratio, which is mindless. On the other hand, a skilled person also knows that there is also some lower limit of the deposition temperature, below which the surface reactions of the ozone, and thus the deposition process, no more works properly. On the other hand, a skilled person also understands that when lowering the deposition temperature too much, the ozone mole ratio produced by said formula approaches the self-ignition limits of ozone concentration. Therefore, it is clear for a skilled person that the deposition temperature T during the exposure of the existing crystalline silicon surface with the deposit possibly already formed thereon to surface reactions of the at least one first precursor for oxygen comprising ozone can lie e.g. in the range of 150°C to 200°C. On the other hand, the inventors have found that by using the claimed ozone mole ratio according to the above formula, as low deposition temperature as 100°C can be used. Thus, an overall deposition temperature range in which the above formula can be used to determine the appropriate ozone mole ratio, and in which range the actual deposition temperature T during the exposure of the existing crystalline silicon surface with the deposit already formed thereon to a deposition atmosphere comprising ozone can lie, can be as wide as from about 100°C to about 200°C.

The advantages of the present invention become particular clear and useful if the deposition temperature T lies in the range of 100 to 190°C, preferably in the range of 120 to 150°C.

In addition to ozone, to enhance the adjustability of the properties of the passivation film, the at least one precursor for oxygen may further comprise e.g. water H₂O and/or dihydrogen dioxide H2O2 as (an) other precursor (s) for oxygen. For example, simultaneous or sequential supply of water and ozone can be used as the precursors supplied to the reaction space during a single deposition cycle. Alternatively, water and ozone can be used as precursors for oxygen in different deposition cycles. By use of different precursors, one can adjust the amount of hydrogen in the passivation film so as to achieve a high effective carrier lifetime x_eff.

Suitable compounds for use as the precursors for aluminium in the methods according to the present invention include e.g. trimethylaluminium TMA and dimethylaluminium hydride DMAH. Thus, the at least one precursor for aluminium may comprise at least one of TMA and DMAH. In addition to the precursors for oxygen, also the precursors for aluminium affect the deposited passivation film composition and thus the passivation performance.

The passivation film thickness can be selected according to the principles known in the art. For example, the passivation film can be fabricated so as to have a total thickness of 1 to 200 nm.

The basic principles of the present invention and the various embodiments thereof apply to passivation of any crystalline silicon surface. Passivation enhances, via the decreased surface recombination, the operational performance of any optoelectronic device. The advantages of the present invention and the embodiments thereof are particularly clear when the crystalline silicon surface, on which the passivation film is deposited, is a surface in a photovoltaic cell structure. The method can be used to passivate both the rear and the front surface of a photovoltaic cell. "Cell structure" refers here to the fact that a passivation film is typically deposited on a solar cell at an intermediate stage of its manufacturing process. In this sense, the cell is not a complete device at that stage, but rather a "cell structure" to form a part of the final completed PV cell.

The embodiments of the invention described hereinbe- fore may be used in any combination with each other. Several of the embodiments may be combined together to form a further embodiment of the invention. In general, a method according to the present invention may comprise one or more of the preferred features of the embodiments above in any combination.

BRIEF DESCRIPTION OF THE DRAWINGS In the following, some embodiments of the present in^¬ vention are explained with reference to the accompany^¬ ing drawings, wherein

Fig. 1 is a flow chart illustrating the steps of a method according to an embodiment of the present in^¬ vention,

Fig. 2 is a block diagram illustrating a passivation film deposited according to an embodiment of the pre^¬ sent invention, and

Fig. 3 illustrates the passivation performance of a passivation film deposited according to an embodiment of the present invention.

DETAILED DESCRIPTION OF THE INVENTION

Figure 1 illustrates a method for fabricating a thin film ΑΙ Ο_x passivation film on a crystalline silicon surface .

This exemplary embodiment of Figure 1 begins by bringing an existing crystalline silicon surface into the reaction space of an ALD system (step A) . The existing crystalline silicon surface can be e.g. a rear or back surface of a prefabricated silicon solar cell structure. The ALD reactor can be a part of an ALD system configured for a batch type or a spatial process. The reaction space is subsequently pumped down to a pressure suitable for forming a passivation film, using e.g. a mechanical vacuum pump. Alternatively, in the case of atmospheric pressure ALD systems and/or processes, protection flows are typically set to protect the deposition zone from the ambient atmosphere. The substrate is heated to a temperature suitable for forming the passivation film by the selected process.

After the existing silicon surface and the reaction space have reached the targeted temperature, e.g. 150 - 200°C, and other conditions suitable for deposition, e.g. a deposition pressure of about 1 mbar, the surface of the silicon substrate is typically pre- treated so as to condition it such that the passivation film may be deposited thereon. This conditioning of the silicon surface typically includes chemical purification of the surface from impurities and/or oxidation. Especially removal of oxide is beneficial when the silicon surface has been imported into the reaction space via an oxidizing environment, e.g. when transporting the exposed silicon surface from one deposition tool to another. In some alternative embodiments, the conditioning can be done ex-situ, i.e. outside the reaction space before introducing the existing silicon surface therein. An example of an ex-situ conditioning process is etching for 1 min in 1 % HF solution followed by rinsing in Dl-water . After the possible pre-treatment of the crystalline silicon surface, in step B, an alternating exposure of the deposition surface to gaseous precursors for aluminium and oxygen, wherein the precursor (s) for oxygen comprises ozone, is started. Said alternating exposure means that the deposition surface is exposed alternately to one or more precursors for aluminium and to one or more precursors of oxygen, the latter one(s) comprising ozone. Subsequent exposures to precursors for aluminium and oxygen determines one deposition cycle. During exposure to one or more precursors for oxygen, the deposition surface is exposed to a deposition atmosphere comprising ozone O3 with a mole ratio (kT+b) %, wherein k lies in the range of -0.09 to -0.10, b lies in the range of 21.0 to 22.5, and T is the deposition temperature in Celcius. In other words, the mole fraction of ozone, i.e. the proportion of ozone, in moles, of the total amount of all constituents, in moles, in the deposition atmosphere lies within the values according to said formula . In each deposition cycle, there can be also more than only one precursor for oxygen, at least in some cycles. Using also some other precursors than ozone may help to adjust the accurate amount and distribution of impurities in the passivation film and at the A10_x/Si interface.

TMA and DMAH are examples of the suitable precursors for aluminium. As usual in ALD, the reaction space may be purged between the subsequent exposures of the deposition surface to different precursors and between the subsequent deposition cycles.

Cycles of exposure of the deposition surface to the alternating precursors for aluminium and oxygen are repeated until the desired thickness of the passivation film, which can lie e.g. between 1 and 200 nm, is achieved.

In the above exemplary method, all the details of the equipment as well as the process concerning the general arrangements and process steps can be selected and arranged according to the principles known in the art. The equipment used in the method can be based on a batch type reactor, a single wafer reactor, or a spatial reactor.

Figure 2 shows an example of a passivation film 1 deposited on a crystalline silicon surface 2, e.g. a rear or back surface of a prefabricated silicon solar cell structure. A passivation film as that illustrated in Figure 2 can be manufactured, for example, according to the principles illustrated and explained in Figure 1 and the associated description above.

To test the feasibility of the present invention, five samples were manufactured, each comprising an A10_x passivation film on a crystalline silicon surface of a double side polished Czochralski (CZ) p-type (boron doped) silicon wafer with a thickness of 400 ym. The resistivity of the wafers was 2.4 - 3.6 ohm cm. TMA and ozone were used as precursors for aluminium and oxygen, respectively. Three different ALD reactors by Beneq (TFS500, P400, and P800) with different ozone generators were used to manufacture the samples. The deposition pressure of about 1 mbar and a deposition temperature of 200°C were used in all processes. Different N₂ flows and different O3 output concentrations from the ozone generators were used.

The table in Figure 3 shows the used ozone O3 concentrations in those five cases. As shown in the table, the molecular ratio of ozone in the deposition atmosphere varied from 0.85 % to 5.4 %. The other constituents of the deposition atmosphere were nitrogen N₂ used as a carrier gas and oxygen O₂ produced by the ozone generators in addition to ozone. After deposition, the samples were annealed at 400°C for 30 min to activate the passivation film and to simulate the heat treatments a passivation film on a silicon solar cell surface typically experienced during the manufacture of the solar cell.

The graph of Figure 3 shows the minority carrier effective lifetimes of the passivated silicon of the samples, measured by quasi steady state photoconductivity (QSSPC) . The graph shows a clear optimum of the lifetime somewhere slightly below 3 % mole ratio. Thus, the ozone concentration affects the lifetime similarly as the temperature, i.e. having a clear optimum value, around which the lifetime decreases. An optimized O3 concentration makes the film to include enough H and 0 impurities to achieve the high chemical and field effect passivation. As high minority carrier effective lifetime as over 3 ms was achieved for ozone molar ratio of 2.94%. As clear for those skilled in the art, lifetimes exceeding 2 ms are excellent results for a CZ wafer. The results shown in Figure 3 represent one specific temperature (200°C) only. However, also other tests have been performed, based on which the basic principle of the invention as defined earlier in this description, and the advantages thereof have been confirmed. In addition to the high passivation performance in general, on the other hand, the tests have shown that with an ozone concentration selected according to the present invention, also clearly lower deposition temperatures, e.g. about 150°C, even as low as about 100°C, can be used without substantially reducing the carrier lifetime from that level typically achieved in prior art with a deposition temperature of 200°C. As is obvious to a person skilled in the art, with the advancement of technology, the basic idea of the invention may be implemented in various ways. The invention is thus not limited to the examples described above but may vary within the scope of the claims. In particular, it is to be noted that, except for the basic principle of the at least one precursor for oxygen comprising ozone, the other precursors used in methods according to the present invention may deviate from the examples above. The precursor (s) for aluminium and the precursor (s) other than ozone for oxygen can be selected e.g. from the table 1 below [Puurunen: Surface chemistry of atomic layer deposition: a case study for the trimethylaluminum/water process, J. Appl . Phys . 97, 121301] . In table 1, "reactant A" and "reactant B" refer to precursors for aluminium and oxygen, respectively .

Claims

1. A method for fabricating a passivation film (1) on a crystalline silicon surface (2), the passivation film (1) comprising aluminium oxide A10_x, for reducing the surface recombination of charge carriers on the silicon surface (2), wherein the method comprises

- providing an existing crystalline silicon surface (2) in a reaction space; and

- depositing in the reaction space aluminium oxide A10_x on the existing crystalline silicon sur^¬ face, wherein the existing crystalline silicon surface with the deposit already formed thereon is alternately exposed to surface reactions of at least one precursor for aluminium and at least one precursor for oxygen, wherein the at the least one precursor for oxygen comprises ozone O3;

characteri zed in that during exposing the ex^¬ isting crystalline silicon surface with the deposit already formed thereon to surface reactions of at least one precursor for oxygen, the existing crystalline silicon surface with the deposit already formed thereon is exposed to a deposition atmosphere compris^¬ ing ozone O3 with a mole ratio of (kT+b)%, wherein k lies in the range of -0.09 to -0.10, b lies in the range of 21.0 to 22.5, and T is the deposition temper^¬ ature in Celcius.

2. A method as defined in claim 1, wherein k lies in the range of -0.096 to -0.098, and b lies in the range of 21.9 to 22.1.

3. A method as defined in claim 1 or 2, wherein the deposition temperature T lies in the range of 100 to 200°C.

4. A method as defined in claim 1 or 2, wherein the deposition temperature T lies in the range of 100 to 190°C, preferably 120 to 150°C.

5. A method as defined in any of claims 1 to 4, where^¬ in the at least one precursor for oxygen further comprises at least one of water H₂O and dihydrogen diox^¬ ide H₂O₂.

6. A method as defined in any of claims 1 to 5, where^¬ in the at least one precursor for aluminium comprises at least one of trimethylaluminium TMA and dimethylal- uminium hydride DMAH.

7. A method as defined in any of claims 1 to 6, where^¬ in the existing silicon surface is a surface in a pho^¬ tovoltaic cell structure.