Surface Passivation and Antireflection Behavior of ALD TiO2 on n-Type Silicon for Solar Cells

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Volume 2013, Article ID 431614,7pages http://dx.doi.org/10.1155/2013/431614

Research Article

Surface Passivation and Antireflection Behavior of ALD TiO

₂

on

n-Type Silicon for Solar Cells

Ing-Song Yu,

1

Yu-Wun Wang,

2

Hsyi-En Cheng,

2

Zu-Po Yang,

3

and Chun-Tin Lin

3

1_{Department of Materials Science and Engineering, National Dong Hwa University, Hualien 97401, Taiwan}

2_{Department of Electro-Optical Engineering, Southern Taiwan University of Science and Technology, Tainan 710, Taiwan} 3_{Institute of Photonic System, National Chiao Tung University, Tainan 71150, Taiwan}

Correspondence should be addressed to Ing-Song Yu; [email protected] Received 16 September 2013; Accepted 30 October 2013

Academic Editor: Jimmy Yu

Copyright © 2013 Ing-Song Yu et al. This is an open access article distributed under the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited. Atomic layer deposition, a method of excellent step coverage and conformal deposition, was used to deposit TiO₂thin films for the surface passivation and antireflection coating of silicon solar cells. TiO₂thin films deposited at different temperatures (200∘C, 300∘C, 400∘C, and 500∘C) on FZ n-type silicon wafers are in the thickness of 66.4 nm± 1.1 nm and in the form of self-limiting growth. For the properties of surface passivation, Si surface is effectively passivated by the 200∘C deposition TiO₂thin film. Its effective minority carrier lifetime, measured by the photoconductance decay method, is improved 133% at the injection level of 1 × 1015_cm−3_{. Depending on different deposition parameters and annealing processes, we can control the crystallinity of TiO}

2 and find low-temperature TiO₂phase (anatase) better passivation performance than the high-temperature one (rutile), which is consistent with the results of work function measured by Kelvin probe. In addition, TiO₂thin films on polished Si wafer serve as good ARC layers with refractive index between 2.13 and 2.44 at 632.8 nm. Weighted average reflectance at AM1.5G reduces more than half after the deposition of TiO₂. Finally, surface passivation and antireflection properties of TiO₂are stable after the cofire process of conventional crystalline Si solar cells.

1. Introduction

Most commercially available solar cells are from silicon, and p-type crystalline silicon solar cells are the mainstream now. In order to increase the efficiency of crystalline silicon solar cells, people in industry try to find the better integration processes of conventional solar cells, to improve the quality of materials, and to cost down the fabrication. Besides, new structures of silicon solar cells with higher efficiency are studied. Passivated emitter and rear, locally diffused cell (PERL), proposed by University of New South Wales, Australia, in 1994, is also one of the designs, which performs

very high efficiency up to 25% [1]. In industry, the conversion

efficiency of 6 inch× 6 inch p-type crystalline silicon solar

cells can be over 20% using the surface passivation technology

with Al₂O₃ thin films [2]. Unfortunately, the light induced

degradation (LID) makes p-type crystalline silicon solar cells drop around 1% efficiency due to the formation of

boron-oxygen clusters after light exposure [3]. Therefore, PERL

with n-type silicon wafers attracts researchers’ great attention recently and will dominate the crystalline Si solar cell in the

near future [4,5].

For the surface passivation and antireflection coating of

p-type Si solar cells, amorphous SiN_𝑥films deposed by the

technique of plasma-enhanced chemical vapor deposition

(PECVD) were well developed in the early 1990s. SiN_𝑥thin

films can reduce surface recombination and light reflection and additionally provide very efficient passivation for bulk

defects of low cost Si materials [6, 7]. However, for the

p-type diffused surface of n-p-type Si solar cells, SiN_𝑥dielectric

coatings are not suitable for passivation, because its positive charge will decrease the properties of surface passivation. Thermal silicon oxide could be good for passivation

ini-tially, but the Si-SiO₂ interface on boron-diffused and

un-diffused surfaces degrades slowly over three months at room

temperature [8, 9]. Besides, amorphous silicon (a-Si) and

Al₂O₃ are good passivation layers but are not suitable for

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oxide (TiO₂) reattracts researchers’ attentions, which has been used in the photovoltaic industry as an antireflection coating for many years since the 1980s due to its low growth temperature, a nontoxic and noncorrosive liquid precursor, excellent chemical resistance, an optimal reflective index, and low absorbance at wavelengths relevant to silicon solar cells.

Surface passivation properties of spray pyrolysis TiO₂ on

silicon wafer were enhanced by the formation of a SiO₂layer

at TiO₂/Si interface after being annealed at 950∘C without

degradation [10,11]. Nonstoichiometric TiO_𝑥films grown by

pulsed laser deposition (PLD) had some degree of passivation for nondiffused p-type Si surface through the substrate placed

at the edge of plasma [12]. Recently, Thomson et al. proposed

that the TiO₂thin films deposited by atmospheric pressure

chemical vapor deposition (APCVD) can effectively passivate n-type silicon and Boron-diffused surfaces as better as the

passivation performance of SiO₂. These films were annealed

at 300∘C in N₂ambient and light soaked by halogen lamp to

create negative charges for better surface passivation [8].

Many technologies can be employed to grow TiO₂thin

films, such as PECVD, APCVD, PLD, spray pyrolysis, reactive

sputtering, and sol-gel [13]. In this paper, we grew TiO₂thin

films on Si wafers by atomic layer deposition (ALD) which performs excellent uniformity, accurate thickness control,

and almost 100% step coverage on substrate surface [14].

Some characterization of TiO₂ thin films by ALD will be

shown. An investigation of the surface passivation and antire-flection coating on silicon relative to the growth temperatures of ALD will be discussed. The degradation and influence of cofire process for the metallization of Si solar cells will also be studied. This work is essential for the applications of ALD

TiO₂thin films in n-type crystalline silicon solar cells.

2. Experiment

In the experiment, we used n-type double-polished FZ silicon

wafers with resistivity 1000Ω-cm, thickness 500 𝜇m, and

orientation (100). Before the deposition of TiO₂, the cleanness

of Si wafers was conducted by acetone to remove the organics and 5% hydrofluoric acid to remove the native oxide. Then,

wafer was rinsed in deionized water and dried with N₂

gas. Right after the cleaning, Si substrate was placed in the reaction chamber of our ALD system which is shown in

Figure 1. Until the base pressure of chamber reached 2 ×

10−2 torr, we set the deposition temperature as 200∘C, 300∘C,

400∘C, and 500∘C, respectively. For the deposition process of

TiO₂ thin films, we employed TiCl₄ and H₂O as reactants

and Ar (99.999%) as purging gas. The injection volume of

TiCl₄and H₂O was 0.10 cc and 0.06 cc, respectively, for each

step according to the ideal gas law. One cycle of a monolayer

deposition included eight steps (TiCl₄reactant, pump-down,

Ar purge, pump-down, H₂O reactant, pump-down, Ar purge,

and pump-down), and it took eight seconds. After 1000-cycle deposition and cool-down to room temperature of the

chamber, n-type FZ Si wafer with double-side TiO₂coating

was done for further analysis [15]. First, the characterization

of TiO₂ films was made by scanning electron microscopy

(SEM) and X-ray diffraction (XRD). Second, the study of

Table 1: Estimation of grain size of TiO2 thin films by SEM and XRD. ALD 200∘C ALD 300∘C ALD 400∘C ALD 500∘C Grain size from SEM (nm) 110 70 45 70 Grain size from XRD (nm) 39 31 22 25

surface passivation for ALD TiO₂on n-type FZ silicon wafers

was carried out by Sinton’s quasi-steady-state photoconduc-tance (QSSPC) method and Kelvin probe for work function

measurement [16, 17]. Finally, optical properties of TiO₂

thin films were decided by the spectroscopic ellipsometry measurement and reflection spectroscopy in the wavelength range of 300 nm–1200 nm.

3. Results and Discussions

3.1. The Characterization of ALD TiO₂ Thin Films. The

surface morphology and cross-section of our TiO₂ thin

films were observed by a JSM-6700F SEM with accelerating

voltage 10 kV. In Figure 2, SEM images of TiO₂ thin films

deposited at different temperatures show that these films are polycrystalline and have grain sizes in the range of 45 nm and

110 nm. Cross-section SEM images, in the inset ofFigure 3,

were used to decide the thickness of TiO₂ thin films. The

thickness of TiO₂thin films is 66.4 nm± 1.1 nm for all growth

temperatures shown inFigure 3. Here, we can find that the

growth rate is independent of the substrate’s temperature, which indicates that the reaction is self-limited by the saturated surface adsorption of reactants. The adsorption thickness, 0.066 nm, is almost the same for each cycle of ALD process. Our ALD system demonstrates very large growth

windows for the TiO₂thin films deposited on Si wafers.

The crystallinity of ALD TiO₂ thin films at different

temperatures was studied by Bruker X-ray diffractometry. InFigure 4, XRD patterns demonstrate that TiO₂thin films deposited at low temperatures are primarily with anatase

phase. As the deposition temperature increased up to 500∘C,

rutile phase began to be obtained in TiO₂thin films. When

the TiO₂ films are annealed with the parameter of 900∘C,

60 seconds, and O₂ ambient, rutile phase dominated in

the films. Rapid thermal annealing (RTA) induces phase

transformation of TiO₂films from low-temperature anatase

phase to high-temperature rutile phase. The higher RTA temperature is, the more rutile phase we observe. From this

work, we can control the phase of TiO₂ thin films for the

following studies of surface passivation and antireflection coating on n-type Si.

In addition, grain size of ALD TiO₂ can be estimated

by SEM images and Scherrer equation of XRD, which is

summarized inTable 1. We get smaller grain size by XRD than

the one by SEM, but their trends are the same at different

deposition temperatures. For TiO₂ films only with anatase

phase (at deposition temperatures 200∘C, 300∘C, and 400∘C),

the lower deposition temperature we set, the larger grain size

we observe. According to the growth mechanism of TiO₂thin

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Chamber TiCl₄ H₂O Ar Val.1 Val.2 Val.₃ Val.4 Val.₇ Waste gas treatment Automatic Pressure gauge Manual valve Pump MFC

Figure 1: Schematic of the ALD system.

(a) (b)

(c) (d)

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200 300 400 500 0.00 0.02 0.04 0.06 0.08 G ro w th ra te (nm/c yc le) Deposition temperature (∘C)

Figure 3: ALD TiO₂ growth rate at temperature 200∘C, 300∘C, 400∘C, and 500∘C is 0.066 nm per cycle due to self-limiting growth. Inset is the cross-section SEM image.

15 20 25 30 35 40 45 50 55 60 A(101) R(211) R(101) ALD200∘_C ALD300∘_C ALD400∘_C ALD500∘_C RTA900∘_C ALD500∘_C R(₁₁₀₎ R: rutile A: anatse In te n si ty (a.u .) 2𝜃 (deg)

Figure 4: XRD patterns of TiO₂ thin films deposited at 200∘C, 300∘C, 400∘C, 500∘C, and RTA 900∘C.

grain refinement, due to an increased density of sites for

nucleation of crystallization [18]. As the deposition

tempera-ture increases up to 500∘C, we find the phase transformation

to rutile and the effect of grain growth, which could influence

the surface passivation of TiO₂thin film on n-type Si.

3.2. Surface Passivation Properties of ALD TiO₂. Sinton’s

WCT-120, a photoconductance decay method, was employed to measure the effective minority carrier lifetime of samples.

The effective lifetime (𝜏eff) is a combination of bulk lifetime

(𝜏bulk) and surface lifetime (𝜏sur) as follows:

1 𝜏eff = 1 𝜏bulk + 1 𝜏sur , 1 𝜏sur =2𝑆 𝑊, (1)

where𝑊 is the sample thickness and 𝑆 is surface

recombi-nation velocity. In order to study the surface passivation of

ALD TiO₂thin films, we used FZ n-type Si substrate which

5E + 014 1.5E + 015 2E + 015 0 5 10 15 20 25 30 35 40 Eff ec ti ve lif etime ( 𝜇 s)

Minority carrier density (cm−3)

ALD200∘C (21.5 𝜇s) ALD₃₀₀∘C (_{11.5 𝜇s)} ALD400∘C (9.2 𝜇s) ALD500∘C (5.5 𝜇s) FZ wafer (_{9.2 𝜇s)} 1E + 015

Figure 5: Effective minority carrier lifetime as function of carrier density at different deposition temperatures and their lifetime values at injection level 1× 1015cm−3.

has very large bulk lifetime. Therefore, the effective lifetime measured is close to surface lifetime to calculate the surface

recombination velocity.Figure 5shows the effective lifetime

as function of minority carrier density by WCT-120. The

effective lifetime without ALD TiO₂ passivation is 9.2𝜇s at

the injection level of 1× 1015cm−3(𝑆 = 2717 cm/s). After the

deposition of TiO₂at 200∘C, the effective lifetime is 21.5𝜇s

at the injection level of 1× 1015cm−3 (𝑆 = 1163 cm/s). Si

surface is effectively passivated by the ALD TiO₂thin films,

especially for the one deposited at the low temperature. At

the deposition temperature 500∘C, when rutile phase can be

observed, the degree of surface passivation does not exist

anymore. For the samples with RTA 900∘C, their effective

lifetimes are very low (not shown). From the results of the

effective lifetime measurement, we can find that TiO₂ thin

film deposited at 200∘C has the best performance for surface

passivation on n-type Si.

Moreover, we study the degradation and influence of

metallization process in crystalline solar cells. InFigure 6,

the effective lifetime of 30 nm TiO₂ thin film grown at

temperature 200∘C slightly increased after five months of

deposition. After cofire process of belt-type furnace at peak

temperature 800∘C, the effective lifetime decreased a little

but is still good enough for surface passivation. We can

summarize that ALD TiO₂thin films have high stability for

the applications in crystalline silicon solar cells.

In the study of solar cells, work function is sensitive to the voltage across the barrier of p-n junction and the surface traps in the passivation emitter interface. In this work, nonscanning ambient Kelvin probe, KP 020, was used to

measure the work function of TiO₂ thin films at different

deposition temperatures. InFigure 7, the higher deposition

temperature we use, the larger work function of TiO₂ we

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100 90 80 70 60 50 40 30 20 10 0

Minority carrier density (cm−3)

ALD200∘C (after deposition)15.7 𝜇s

ALD200∘C (after five months)17.8 𝜇s

ALD200∘C (after cofire)16.6 𝜇s

Eff ec ti ve lif etime ( 𝜇 s) 5E + 014 1E + 015 1.5E + 015 2E + 015

Figure 6: Effective minority carrier lifetime of 30 nm TiO₂ thin films deposited at 200∘C in three cases: after deposition, five months, and cofire. 200 300 400 500 0 5 10 15 20 25 30 ₅₄₀₀ 5200 5000 4800 4600 4400 4200 W o rk f u nc tio n (meV) Eff ec ti ve lif etime ( 𝜇 s) Deposition temperature (∘C)

Figure 7: Work function (◼) and effective lifetime (󳵳) of TiO2thin films at different deposition temperatures.

lifetimes at different deposition temperatures. To explain this,

we can see the energy band diagram of TiO₂ and Si in

Figure 8. TiO₂/Si heterojunction makes energy band bending

at the interface of TiO₂and n-type Si [19]. According to the

work function of TiO₂ thin films we measured, more band

bending occurred in the case of TiO₂film deposited at 500∘C,

which induce more recombination at the interface and lower effective lifetime of samples.

3.3. Optical Properties of ALD TiO₂. Now we will

demon-strate that our ALD TiO₂thin films are an excellent

antire-flection coating layer for crystalline silicon solar cells. For the conventional quarter-wavelength antireflection coating (ARC), the refractive index of coating layer has to be chosen

as √𝑛𝑠, where 𝑛𝑠 is the refractive index of substrate, that

is, Si in this case. For example, considering the refractive

index of Si in the spectral ranges of 400 nm–800 nm [20], the

refractive index of quarter-wavelength ARC thin films should

be in the range of∼2.36–1.92. To verify this, we performed the

spectroscopic ellipsometry measurements and extracted the

refractive index of our ALD TiO₂thin films.Figure 9shows

the refractive index as function of wavelength for our TiO₂

thin film deposited at temperature 200∘C. Its refractive index

decreases monotonically from 2.65 to 2.25 as wavelength decreases from 375 nm to 900 nm, showing a similar trend as

anatase-phase bulk TiO₂[21] but having a little lower value

of index, which probably is because of polycrystal structure

in our films (see SEM images inFigure 2). Most importantly,

the results show that our ALD TiO₂thin films are an excellent

choice of quarter-wavelength ARC layer for Si solar cells. To further prove this, we used Hitachi U-4100 to measure

the reflectance spectra of our ALD TiO₂ thin films and a Si

wafer as a reference, shown inFigure 10. Firstly, all of TiO₂

deposited Si wafers show a much lower reflectance than those of a bare Si wafer over the measured spectral range. Secondly,

a minimum reflectance of TiO₂deposed Si wafers is observed

at∼550 nm, which is consistent with the requirement of film

thickness for quarter-wavelength ARC; that is,𝑑 = 𝜆/4𝑛TiO2.

Taking𝜆 = 550 nm and 𝑛TiO2 ≈ 2.15, we have 𝑑 = 64 nm,

which agrees with our SEM results. Again, this indicates

that our TiO₂thin films serve as an ARC layer. Thirdly, the

reflectance of all TiO₂deposited Si increases monotonically

from∼550 nm to ∼1000 nm. This is because the thickness

(66.4 nm) of our TiO₂film deviates the𝑑 = 𝜆/4𝑛TiO₂ further

and further as wavelength increases. Fourthly, a bump rising at around 1000 nm is observed for all samples. This is because the reflectance from Si backside surface starts to contribute the measured reflectance as wavelength approaching the band gap wavelength of Si.

To overall justify the ARC performance of our TiO₂

thin films for solar cells, we calculate the weight average reflectance (WAR) at AM1.5G in the range of 300 nm to

1200 nm. The results show that all of our ALD TiO₂ thin

films have WAR (16.68%–18.92%) less than the half of WAR

(38.36%) of bare Si. The TiO₂ thin film grown at 200∘C

shows the lowest value of WAR because it has the lowest refractive index deduced from ellipsometry. A lower WAR value can be achieved with optimized growth conditions and film thickness. Moreover, the reflectance spectra as well as the WAR are almost the same after cofire process by

belt-type furnace at peak temperature 800∘C (not shown), which

indicates that our ALD TiO₂ thin films are suitable for the

fabrication processes of crystalline silicon solar cells.

4. Conclusion

Growth window of self-limiting can be from 200∘C to 500∘C

in our TiO₂ ALD system with the growth rate 0.066 nm

per cycle. All these films are excellent antireflection coating

layers for Si. For lower deposition temperature of TiO₂thin

films, we find smaller energy band bending in the interface of

TiO₂/Si and better surface passivation performance. The best

surface passivation and antireflection coating on Si wafers

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VB CB Si Si CB VB ALD200∘CE_F ALD₅₀₀∘C_EF EF TiO₂ TiO₂ Contact EF EC EV q𝜒 = 4.2 eV q𝜒 = 4.05 eV

Figure 8: The left hand side is energy band diagrams of TiO₂and Si before contact. After contact, band bending occurs at the interface of TiO₂and Si.

300 400 500 600 700 800 900 2.10 2.15 2.20 2.25 2.30 2.35 2.40 2.45 2.50 2.55 2.60 2.65 2.70 Ref rac ti ve index Wavelength (nm) ALD200∘C

Figure 9: Refractive index as function of wavelength for TiO₂thin film grown at 200∘C. 200 400 600 800 1000 1200 0 10 20 30 40 50 60 70 80 90 100 Reﬂec ta n ce (%) Wavelength (nm) FZ wafer (WAR38.36%) ALD₂₀₀∘C (WAR_16.68%) ALD300∘C (WAR17.38%) ALD400∘C (WAR18.92%) ALD500∘C (WAR18.67%)

Figure 10: Reflectance of bare Si wafer and Si with TiO₂thin films grown at different temperatures as function of wavelength from 300 nm to 1200 nm.

anatase phase in TiO₂film. Once the rutile appears via

high-temperature depositions or annealing processes, the degree of surface passivation will not exist. After the cofire process of conventional crystalline Si solar cells, we prove that their surface passivation and antireflection properties are very stable, which can be applied for n-type crystalline silicon solar cells.

Conflict of Interests

The authors declare that there is no conflict of interests regarding the publication of this paper.

Acknowledgments

The authors acknowledge National Science Council of Taiwan (NSC 101-2218-E-259-001) for financially supporting this study and E-Ton Solar Tech. Co., Ltd., for the measurement of Sinton WCT-120 and Hitachi U-4100.

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