Silicon Electrodeposition in a Water-Soluble KF–KCl Molten Salt: Properties of Si Films on Graphite Substrates

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Figure Captions

Fig. 1

Fig. 2

Fig. 3

Fig. 4

Fig. 5

Fig. 6

Fig. 7

Fig. 8

Cyclic voltammograms at (a) a Ag flag electrode and (b) a graphite plate electrode in

molten KF–KCl before and after addition of K2SiF6 (0.10 mol%) at 1073 K. Scan

rate: 0.50 V s−1. (c) Current–potential curve calculated from the difference for the

voltammograms obtained at a graphite plate electrode in blank melt and molten KF–

KCl−K2SiF6 (0.10 mol%).

Surface SEM images of the samples obtained by galvanostatic electrolysis of

graphite plate electrodes at various current densities and K2SiF6 concentrations in

molten KF–KCl at 1073 K. The charge density was at 180 C cm−2.

Cross-sectional SEM images of the samples obtained by galvanostatic electrolysis of

graphite plate electrodes at various current densities and K2SiF6 concentrations in

molten KF–KCl at 1073 K. The charge density was 180 C cm−2.

Relationship between electrolysis conditions and morphology of Si deposits on

graphite electrode and the optimum electrolysis conditions for Ag substrate [20].

(a) XRD pattern, (b) cross-sectional SEM image, and (c) a crystal grain map from

electron backscatter diffraction (EBSD) analysis of the deposits obtained by

galvanostatic electrolysis of a graphite plate electrode at 100 mA cm−2 for 30 min in

molten KF–KCl–K2SiF6 (3.5 mol%) at 1073 K.

(a) Open-circuit potentiogram and (b) linear sweep voltammograms at an

electrodeposited Si in CH3CN–TBAPF6(0.1 M)–Fc(0.05 M) at room temperature.

Scan rate: 0.05 V s−1.

Surface and cross-sectional SEM image of the samples obtained by galvanostatic

electrolysis of graphite plate electrodes. The Si layer was electrodeposited (a-1)(a-2)

at 100 mA cm−2 for 30 min in molten KF–KCl (45:55 mol%) at 1073 K after the

introduction of SiCl4 (2.79mol%) and (b-1)(b-2) at 45.6 mA cm−2 for 60 min in

molten KF–KCl (60:40 mol%) at 1073 K after the introduction of SiCl4 (2.37

mol%).

Linear sweep voltammograms at the electrodeposited Si in CH3CN–TBAClO4 (0.3

M)–EVBr2 (0.05 M) at room temperature. Scan rate: 0.05 V s−1.

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498

499

500

501

Table 1 Acceptable impurity levels for SOG-Si [31–34] and impurity contents determined by GD-MS for the Si film samples. The samples were

obtained by galvanostatic electrolysis of a Ag plate at 100 mA cm−2 for 50 min in molten KF–KCl–K2SiF6 (2.0 mol%) at 923 K [18], a graphite

plate at 100 mA cm−2 for 30 min in molten KF–KCl–K2SiF6 (3.5 mol%) at 1073 K, and a graphite plate at 45.6 mA cm−2 for 60 min in molten KF–

KCl at 1073 K after the introduction of 2.37 mol% SiCl4.

Acceptable levels for SOG-Si

/ ppm

Impurity content in electrodeposited Si film

/ ppm

[31]

[32]

[33, 34]

Ag plate (GC

rod anode)

[18]

K2SiF6

Graphite plate

(Si rod anode)

[This study]

K2SiF6

Graphite plate

(GC plate anode)

[This study]

SiCl4

0.1–0.3

< 0.06

< 0.1

< 4×10−5

< 0.007

0.1–10

0.005–0.05

0.02–2

<2

<1

<1

< 1×10−4

< 4×10−3

< 8×10−3

< 0.02

< 0.3

< 20

< 7×10−5

3.2

0.8

2.7

<2

<1

< 0.1

5.1

1.8

5.0

11

1.8

0.6

76

< 0.5

<1

11

10

<1

<1

<1

<1

<1

<1

<1

<1

23

12

12

78

1.0

0.13

< 0.01

< 0.01

0.04

0.14

0.33

< 0.01

0.82

< 0.01

Element

Al

Ca

Ti

Cr

Mn

Fe

Ni

Cu

Mo

Ag

Pt

502

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(a)

Current density / A cm−2

0.3

0.2

0.1

−0.1

−0.2

−0.3

Blank

0.10 mol% K2SiF6

0.2

0.4

0.6

0.8

1.0

Potential vs. K+/K / V

1.2

1.4

(b)

Current density / A cm−2

0.3

0.2

0.1

−0.1

−0.2

−0.3

Blank

0.10 mol% K2SiF6

0.2

0.4

0.6

0.8

1.0

Potential vs. K /K / V

1.2

1.4

(c)

Current density / A cm−2

0.05

−0.05

−0.10

−0.15

−0.20

0.4

0.6

0.8

1.0

1.2

Potential vs. K+/K / V

Fig. 1 Cyclic voltammograms at (a) a Ag flag electrode and (b) a graphite plate electrode in

molten KF–KCl before and after addition of K2SiF6 (0.10 mol%) at 1073 K. Scan rate: 0.50 V s−1.

(c) Current–potential curve calculated from the difference for the voltammograms obtained at a

graphite plate electrode in blank melt and molten KF–KCl−K2SiF6 (0.10 mol%).

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0.50 mol%

2.0 mol%

3.5 mol%

5.0 mol%

300

mA cm−2

200

mA cm−2

100

mA cm−2

25

mA cm−2

100 µm

Fig. 2 Surface SEM images of the samples obtained by galvanostatic

electrolysis of graphite plate electrodes at various current densities

and K2SiF6 concentrations in molten KF–KCl at 1073 K. The charge

density was 180 C cm−2.

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Resin Si Graphite

0.50 mol%

2.0 mol%

3.5 mol%

5.0 mol%

300

mA cm−2

200

mA cm−2

100

mA cm−2

25

mA cm−2

100 µm

Fig. 3 Cross-sectional SEM images of the samples obtained by

galvanostatic electrolysis of graphite plate electrodes at various

current densities and K2SiF6 concentrations in molten KF–KCl at

1073 K. The charge density was 180 C cm−2.

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Compact and smooth Si was

obtained on Ag substrate

500

Current density / mA cm−2

Porous & nodular Si

400

300

Peeled

200

100

Nodular ★

Si

Nonuniform

K2SiF6 concentration / mol%

Compact &

smooth Si

Fig. 4 Relationship between electrolysis conditions and morphology

of Si deposits on graphite electrode and the optimum electrolysis

conditions for Ag substrate [20].

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(a)

(#00-005-0565)

● Si

× Graphite(#00-001-0640)

Intensity / a.u.

20

(b)

30

Resin

40

50

60

70

2θ / deg. (Cu-Kα)

Si

Graphite

× ×

80

90

(c)

100 µm

Fig. 5 (a) XRD pattern, (b) cross-sectional SEM image, and (c) a crystal grain

map from electron backscatter diffraction (EBSD) analysis of the deposits

obtained by galvanostatic electrolysis of a graphite plate electrode at 100 mA

cm−2 for 30 min in molten KF–KCl–K2SiF6 (3.5 mol%) at 1073 K.

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(a)

Potential vs. Ag+/Ag / V

−0.1

In the dark

−0.2

−0.3

−0.4

Under illumination

−0.5

−0.6

10

20

30

40

Time / s

50

60

70

(b)

Current density / mA cm−2

Photocurrent

Dark current

−1

−0.2

0.2

0.4

0.6

Potential vs. Ag /Ag / V

0.8

Fig. 6 (a) Open-circuit potentiogram and (b) linear sweep voltammograms

at an electrodeposited Si in CH3CN–TBAPF6(0.1 M)–Fc(0.05 M) at room

temperature. Scan rate: 0.05 V s−1.

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(b-1)

(a-1)

100 µm

100 µm

(b-2)

(a-2)

Resin

Graphite

Si

Resin

Graphite

Si

100 µm

100 µm

Fig. 7 Surface and cross-sectional SEM image of the samples obtained by galvanostatic

electrolysis of graphite plate electrodes. The Si layer was electrodeposited (a-1)(a-2) at 100

mA cm−2 for 30 min in molten KF–KCl (45:55 mol%) at 1073 K after the introduction of

SiCl4 (2.79mol%) and (b-1)(b-2) at 45.6 mA cm−2 for 60 min in molten KF–KCl (60:40

mol%) at 1073 K after the introduction of SiCl4 (2.37 mol%).

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Current density / mA cm−2

0.5

−0.5

−1.0

−1.5

Photocurrent

Dark current

−2.0

−0.8

−0.7

−0.6 −0.5 −0.4 −0.3

Potential vs. Ag+/Ag / V

−0.2

Fig. 8 Linear sweep voltammograms at the electrodeposited Si in

CH3CN–TBAClO4 (0.3 M)–EVBr2 (0.05 M) at room temperature.

Scan rate: 0.05 V s−1.

...