Study on Surface Passivation by YZO/ AIOx Stacking Double Layer

for Crystalline Si Solar Cells

T. Katsumatal,2,4, N. Ikenol, S. Satoh3,4, H. Yoshida3,4, K. Arafune3,4, T. Chikyow2,4, and A. Ogural,4*

I Meiji University, 1-1-1 Higashimita, Tama-ku, Kawasaki, 214-8571, Japan

2 NIMS, 1-1 Namiki, Tsukuba, Ibaraki 305-0044, Japan

3 University of Hyogo, 2167 Shosha, Himeji, Hyogo 671-2280, Japan

4 JST-CREST, 4-1-8 Honchou, Kawaguchi, Saitama 332-0012, Japan

*E-mail: a_ogura@isc.meiji.ac.jp

Abstract - We investigated stacking double layer structure, the Y 203-Zr02 composite film (YZO) on AIO" for the field effect passivation with high negative fixed charge densities on p-type Si. The composition spread YZO films were deposited at room temperature by using combinatorial sputtering technique. The fixed charge densities were extracted from the flat band voltage shift in the capacitance-voltage characteristics. The as-deposited Zr02 film incorporated with 15% Y 203 stacking on the ALD AIOx structure showed the highest negative fixed charge of -1.9 x 1012

cm·2

. The field effect passivation can be controlled by the negative fixed charges in the YZO film depending on the composition and dipole uniformly formed at AIOx/Si interface. After annealing in the oxygen atmosphere, passivation properties deteriorated caused by the AI diffusion at the YZO/AIOx interface.

Index Terms - stacking double layer, combinatorial technique, capacitance-voltage characteristics, field effect passivation, negative fixed charge density.

I. INTRODUCTION

In the crystalline Si solar cells, traditional full Al rear contacts on p-type solar cells is going to be replaced by the partial rear metallization contacts through the dielectric passivation layer such as PERC structure, whose cell efficiencies are reported beyond 20 % [1-2]. In the structure, it is essential to have superior passivation properties at the rear surface as well as front one. The surface passivation properties can be improved by the effects of the chemical passivation and field effect passivation to suppress the minority carrier recombination at the Si surface [3]. Recently the use of the thin silicon substrate is recommended to decrease the material cost. On the other hands, surface recombination increases because the surface area ratio to the volume also increases. Moreover, in order to improve the current density, suppression of the surface optical reflection with stacking structure such as SiN/Ab03 has much attention to achieve high cell performance [4].

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The passivation layer for p-type Si is desired to have high density negative fixed charges, which can induce the effective field effect passivation[5-7]. We have investigated the Y20r Zr02 composite film (YZO) for the field effect passivation and found significantly large negative fixed charge density [8]. In the case, we also verified the negative fixed charges were originated in the film, therefore they increased with increasing the film thickness [9]. On the other hand, the AIO" which is well known as the superior passivation film for p-type Si, has electrical dipole at the interface between Si and provides the similar desired effects as negative fixed charges. Thus, the effect is independent of the film thickness [10]. Therefore, in this study, we investigate the YZO/Ab03 stacking double layer structure to enjoy the both benefits of the dipole at the AIOxiSi interface and negative fixed charges in the YZO films.

II. EXPERIMENTAL

In the experiment, p-type silicon of 15-30 Qcm (MCZ, 770 J.U11, (100)) was used as substrates. IO-nm-thick AI203 passivation films were symmetrically deposited on the both front and rear surfaces of the silicon substrate by batch-type ALD (atomic layer deposition) technique. Trimethyl aluminum (AI(CH3)3) and ozone were alternatively supplied as the aluminum source and oxidant, respectively. The deposition was carried out at room temperature. After AIOx deposition, post-deposition annealing (PDA) was performed at 450 °C for 30 min in nitrogen atmosphere to fabricate interface dipole [10]. Then, the 20-nm-thick Y 20rZr02 composition spread film was deposited on the AIOx layer by the combinatorial magnetron sputtering system [8]. After the deposition, rapid thermal annealing was performed at 400-800 °c for 30 min in an oxygen atmosphere. For the capacitance-voltage (C-V) measurements, the Pt gate electrodes were fabricated on the

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front YZO film, and the rear AIOx film was removed to form an ohmic contact with the silicon substrate.

III. RESULTS AND DISCUSSION

The total fixed charge densities, including interface dipole effect, depending on the YZO composition in the as-deposited and annealed samples are shown in Fig. I. The negative fixed charge densities (Qf) were extracted from the flat band shift of the C-V characteristics using following equation [10],

(1)

where q, Co, S, V FB, and <DMS are elementary charge, capacitance of oxide layer, planer dimension of gate electrode, flat band shit and metal-semiconductor work function difference, respectively. The interfacial dipole is supposed to exist uniformly over the entire AIOxlSi02 interface, nevertheless the position composed 85% Zr02 with 15% Y203 shows the highest negative fixed charge density because the YZO film possessed the negative fixed charge density varied according to composition ratio of Y203 and Zr02. The varied negative fixed charge densities depending on the YZO film composition proved that the charges in the YZO film can influence through the uniform dipole at the AIOxiSi interface. After annealing, the fixed charge density decreased probably caused by the interdiffusion between AIOx and YZO films.

0.5 c:------.----,---,---,-------.---,-----,------, • as-depo. • 400°c • 800°c

N5 0.0 - -..... ... - - - - - - - - - - - - - - - - - - - -"" ....... . � -0 •

= .... >< = -1.0

d -1.5

•

.. • ................

••••

.... ••

.... ••

••••••••

-2.0 �-............:=---....L----'-----'L---'---...L-----'--.....:::l Zr02

Fig. I Fixed charge densities depending on the film composition.Y 203 content increases from left to right.

Fig.2 shows the fixed charge densities with the single AIOx layer passivation film depending on the annealing temperature in the oxygen atmosphere. The sample before O2 annealing showed the highest fixed charge density, and decreased by the annealing. This should also affect the total fixed charge densities described in Fig. 1.

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§ � '" ... .s :: '" .... :: o U

.. = � N

-c ..... ;.< ::: ..

0

0.0 -0.5 -1.0 -1.5

450"(; in N2 450"(; in N2 + 400"(; in O2

450"(; in N2 + 800"(; in O2

Fig.2 Fixed charge densities with single AlOx passivation layer

depending on annealing process.

Fig.3 shows the film composition changes at the surface measured by the X-ray photoelectron spectroscopy (XPS), whose detection depth is estimated as approximately 8 nm . It is apparent that the aluminum concentration increased with annealing temperature. Thus, we verified the Al diffusion at the YZO/ AIOx interface and consider this should be one of the causes of the decrease in the total negative fixed charge densities after the annealing process. Especially, the Al diffusion at YZO/ AIOx interface occurred for the relatively high Y composition ratio.

100

80

60

40

20

Al Si Y

D Zr D 0

O �==�==�==� F==��F=� r��==�� "'KIt; BOot; as-d_ 4aJt; 8oot; BS-tiepo. .foot: 800t: ZrOx(90%)

+ YOx(lO%) ZrOx(50%) ZrOx(lO%)

+YOx(50%) +YOx(90%)

Fig.3 Film composition changes by the annealing at the surface of YZO with (a) 10%, (b) 50%, (c) 90% Zr02 incorporation in Y203.

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To verify the diffusion at the interface, we fabricated double layer structure samples, in which Y203 or Zr02 layer was deposited on the AIOx layer. After deposition, samples were annealed under the same oxygen annealing conditions at 400-800 °C for 30 min. Thus, we measured the depth composition profiles to evaluate the interdiffusion at (Y 203 or Zr02)/ AIOx interfaces by XPS. Fig.4 and Fig.S shows the composition depth profiles at the surface in the as-deposited films and after the annealing at various temperatures. Even in the as-deposited sample, AI diffusion to the surface seems to occur in Y203/AIOx structure. After the annealing, the AI composition increases while Y decreases implying apparent interdiffusion. On the contrary, the compositions at the surface of Zr20/AIOx sample were stable with little AI even after the annealing. Thus,

70rr------,------,------,------,-----,-------n a) Y20/ALO/Si as-de po. • Al • Y A Si ,. 0

60 ,. -t " " "I' ,. ,. ,. ,. ,. ,. ,. "I' ,. "I' ,. .... 50 c: '" '" 40 ... '" Co 30 '" . 'E 20 s � 10·

o

20 -•

10 -

o

• • • •

• • • •

5

"I' "I' "I' ,.

• • • • • • •

• • • • •

• • • • : . . • • • • •

10 15 20 25

Etch time [s]

,. ,. "I' ,. "I' ,. ,. "I' ,. "I'

• • • • • • • •

• • • •

• • • • • •

10 15 20 25

Etch time [s]

30

30

70rr------,------,------,------,-----,-------n c) YP/ALO/Si at 800 t • Al • Y A Si ,. 0

-t 60"1' ,. "I' "I' ,. ,. "I' ,. ,. ,. ,. ,. ,. "I' ,.

.... 50 c: '" '" 40 ... '" Co 30 '"

I· · 'E 20 • •

S � 10

0 + A o 5

• • • •

• • • • • •

• • • • • •

10 15 20

Etch time [s]

• •

• •

25

• •

• •

30

Fig.4 Depth profiles at the surface of Y 2031 AIOxiSi structure in a) as-deposited film and after annealing at b) 400°C and c) 800°C

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we can conclude almost no interdiffusion occurred in this structure. Consequently, the deterioration in the passivation properties in YZOI AIOx structure should be attributed to the Y and AI interdiffusion, which might affect negatively on both fixed charge density in YZO and dipole at the AIOxiSi interface.

On the other hands, samples with Zr02 for top layer after annealing were stable. From these XPS profiles, we believe the Zr diffusion does not occur at YZO/AIOx interface instead the Y diffusion occurs. The Y diffusion affects not only the YZO film but also the AIOx film and causes the change of the composition ratio. Therefore, we conclude the decrease of the fixed charge density after annealing caused by the interdiffusion between AIOx and YZO films.

70 rr------,------,------,------,-----,-------n d) ZrO/ALO/Si as-depo. � 60 '" ,. ,. ,. ,. ,. ,.

� ,.

• Al • Zr A Si "I' 0 ,. ,. ,. ,. ,. ,. ,.

.... 50 c: '" '" 40 ... '" . . Co 30 '" 'E 20 o � 10

• • 0 '" A

o

• • • • • • • • • • • • •

• • • • • • • • • • • • • A A A ... A A A • ... • • • A

5 10 15 20 25

Etch time [s] 30

70 ,I, ,I_ e) ZrO/ALO/Si at 400 t

I • All. Zr •

I Si "I' 0 +

-t 60 % "I' ,. "I' ,. ,. ,. .... 50 c: '" '" 40

"I' ,. ,. "I' ,. "I' ,. ,.

� . . . . . . . . . . . . . . . Co 30 '" 'E 20 s � 10

• • • • • • • • • • • • • • 0 : • AI. A ... . . 1. A ... A AI. A t

o 10 15 20 25 30

Etch time [s] 70

f) ZrO/AIZO./Si at 800 t

I • All. Zr •

I Si "I' 0 +

� 60 .1: ' ,. "I' "I' "I' ,. ,. ,. ,. ,. ,. ,. "I' ,. ,. � .... 50 c: '" '" 40 ... � 30· '" 'E 20 o � 10

• 0 "

o

• • •

• • • A A •

• • • • • • • • • • •

• • • · ... .

10

• A

15

• A

• • • • ... A

20

Etch time [s]

• A

25

• •

• A

30

Fig.S Depth profiles at the surface of Zr02/AIOxiSi structure in a) as-deposited film and after annealing at b) 400°C and c) 800°C

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IV. SUMMARY

We fabricated and investigated the stacking double layer structure for the field effect passivation, Y 20rZr02 binary system on AIOx for p-type Si wafer. The negative fixed charge density depended on the composition ratio, however the passivation performance was deteriorated after the annealing in the oxygen atmosphere due to the Y and Al interdiffusion. The fixed charge variation depending on the YZO film composition thorough the AIOx showed the possibility to realize the combination of the both benefits from the interface dipole due to the AIOx and high density negative fixed charges from the YZO film. The interdiffusion in the stacking double layer, however, should be controlled to realize the maximum potential.

ACKNOWLEDGEMENT This work was supported in part by Core Research for

Evolutional Science and Technology (CREST), Japan Science and Technology Agency (JST).

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