Effect of annealing on the thermoelectricity of indium tin oxide thin film thermocouples

Journal Pre-proof Effect of annealing on the thermoelectricity of indium tin oxide thin film thermocouples Xinhang Jin, Binghe Ma, Keli Zhao, Zexun Zhang, Jinjun Deng, Jian Luo, Weizheng Yuan PII:

S0272-8842(19)33053-6

DOI:

https://doi.org/10.1016/j.ceramint.2019.10.190

Reference:

CERI 23258

To appear in:

Ceramics International

Received Date: 20 May 2019 Revised Date:

3 October 2019

Accepted Date: 20 October 2019

Please cite this article as: X. Jin, B. Ma, K. Zhao, Z. Zhang, J. Deng, J. Luo, W. Yuan, Effect of annealing on the thermoelectricity of indium tin oxide thin film thermocouples, Ceramics International (2019), doi: https://doi.org/10.1016/j.ceramint.2019.10.190. This is a PDF file of an article that has undergone enhancements after acceptance, such as the addition of a cover page and metadata, and formatting for readability, but it is not yet the definitive version of record. This version will undergo additional copyediting, typesetting and review before it is published in its final form, but we are providing this version to give early visibility of the article. Please note that, during the production process, errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain. © 2019 Published by Elsevier Ltd.

Effect of annealing on the thermoelectricity of indium tin oxide thin film thermocouples Xinhang Jin, Binghe Ma∗, Keli Zhao, Zexun Zhang, Jinjun Deng, Jian Luo, Weizheng Yuan a

Northwestern Polytechnical University, Xian 710068, China

Abstract The effect of annealing on the thermoelectricity of indium tin oxide (ITO) thin film thermocouples (TFTCs) was studied. A microstructure characterisation indicate the influence of two different annealing mechanisms on the thermoelectric properties, namely, recrystallisation and carrier concentration. The ITO thin films recrystallised when the annealing temperature reached 1100◦ C in air, which improved the thermoelectric reproducibility, linearity, and stability of the TFTCs. More oxygen vacancies were produced in the ITO thin films after annealing at 400◦ C in vacuum, but were reduced after annealing at 1100◦ C in air. The Sn2+ ions transformed into Sn4+ ions in the ITO 90/10 thin films after annealing, providing additional free electrons in the films. These microstructural transformations are related to changes in the thermoelectric properties of the ITO TFTCs, as described by the thermoelectric theory of degenerated semiconductors. A decrease in the Seebeck coefficient after annealing was attributed to increases in the carrier concentrations and recrystallisation. Additionally, after annealing at 1100◦ C in air, the Seebeck coefficient of the ITO TFTCs agreed with the theoretical value. These results can be used to control thin film thermoelectric properties by different annealing processes. Keywords: ITO, Thin film, Thermocouples, Annealing, Seebeck coefficient

∗

Northwestern Polytechnical University, Xian 710068, China Email address: [email protected] (Binghe Ma)

Preprint submitted to Ceramics International

October 23, 2019

1. Introduction With the dramatic development of the aero-engine industry, direct measurements of the dynamic surface temperature with high frequency is of great significance for the evaluation of combustion states and monitoring health conditions.[1][2] Compared with traditional temperature measuring instruments, such as wire thermocouples, infrared thermometer, and thermal paint, thin film thermocouples (TFTCs) have a rapid response, high accuracy, and simple structure. Therefore, TFTCs are considered ideal sensors for measuring the transient temperature.[3][4] Melt thin films are not reliable materials for TFTCs because they oxidise at temperatures greater than 1000◦ C in air.[5] Ceramic TFTCs with high melting points have been widely studied in recent years, such as indium tin oxide (ITO) TFTCs.[6][7][8] ITO thin films have high Seebeck coefficients, excellent chemical stabilities and oxidation resistances under hightemperatures. Therefore, ITO-based TFTCs provide a high sensitivity and thermoelectric stability. [9][10][11][12] Annealing is an important process during the fabrication of oxide semiconductor thin films, and the properties of thermoelectricity thin films can be improved by optimising annealing processes. For example, the Zn2GeO4 thin film will generate the secondary phases by annealing at 900◦ C, which can improve the Seebeck coefficient obviously.[13][14] Also, the Seebeck coefficient of Cu2 InO4 thin film was found to be increased with annealing temperature.[15] Annealed ZnO thin films will exhibit better thermoelectric performance than unannealed thin films.[16] ITO is a typical example of and oxide semiconductor. Therefore, its thermoelectric properties (key parameters that describe the performance of TFTCs) can be significantly influenced by annealing, which is particularly the case for the Seebeck coefficient and thermoelectric stability.[17][18][19]. However, very few studies have addressed the influence of annealing mechanism on the thermoelectric properties of ITO TFTCs. In the present study, ITO TFTCs were deposited onto sapphire substrates with different annealing conditions. The influence of annealing at 400◦ C in vacuum and 1100◦ C in air on the microstructure, chemical composition, and morphology of ITO thin films were investigated. According to the experimental results, the Seebeck coefficient decreased after annealing owing increases in the carrier concentrations and recrystallisation. Changes in the number of oxygen vacancies and doping ions produced a change in the carrier concentrations after the annealing process. since previous stud2

ies have demonstrated that oxygen vacancies and doping ions are the source of carrier concentrations in ITO thin films. In addition, the changes in the carrier concentration can explain the effect of annealing on the Seebeck coefficient of ITO thin films.[20][21][22] Recrystallisation can improve the thermoelectric reproducibility, linearity and stability of ITO TFTCs under high temperatures, and the Seebeck coefficient of ITO TFTCs agrees with the thermoelectric theory of semiconductor materials. 2. Experimental procedures In this study, ITO TFTCs were consisted by In2 O3 and ITO 90/10 (In2 O3 :SnO2 =90:10 wt%) thin films on sapphire substrates (length: 200 mm and width: 15 mm), produced by radio frequency(RF) magnetron sputtering using In2 O3 and ITO 90/10 ceramic targets (99.999 wt% purity, diameter: 152 mm, and thickness: 30 mm ). The base pressure of the deposition chamber was less than 8.0×10 −4 Pa, and an RF power of 300 W was used for the In2 O3 and ITO 90/10 targets. The targets were sputtered in 1.5 Pa argon atmospheres for 2 hr to form the In2 O3 and ITO 90/10 films. The legs and junctions of the thermocouples were patterned by photolithography. The crystal structure of the In2 O3 and ITO 90/10 films annealed under different conditions were analyzed by X-ray diffraction (XRD), and the chemical compositions were characterised by X-ray photoelectron spectroscopy (XPS). The surface and cross-sectional morphologies were obtained by a scanning electron microscope (SEM). The carrier concentration of the thin films was measured using the Hall measurement system (CRYOGENIC-CFMS14T). A test bed was built to study the thermoelectric properties of ITO TFTCs under high-temperatures, as shown in Fig.1. A furnace was heated to create a uniform high temperature environment. The hot junction of the TFTCs was inserted 100 mm into the furnace, and the cold junction was kept outside in air. A heat shield was applied to increase the temperature difference load on the thermocouples. To continuously measure the temperature difference loaded onto the ITO TFTCs, the hot and cold junction temperatures were monitored by S and K-type thermocouples, respectively. The thermoelectric voltage was monitored by a high precision multimeter (Keithley 2002).

3

Figure 1: Schematic of the thermoelectric calibration system.

3. Results and discussion 3.1. Effect of annealing on micro-structure The In2 O3 and ITO 90/10 thin films were characterised by their structural, chemical compositions, structural and morphological properties. The XRD patterns of the In2 O3 and ITO 90/10 thin films under different annealing conditions are shown in Fig.2. All the peaks of the ITO 90/10 films were attributed to the In2 O3 phase (JCPDS #89-4597). No X-ray diffraction peaks of crystalline SnO or SnO2 were observed for any of the samples indicating that Sn2 + or Sn4 + were introduced into the In2 O3 lattice and formed solid solutions without any impurities. It was observed that the In2 O3 and ITO 90/10 thin films were polycrystalline, with different preferential growths. For the In2 O3 thin films, the strong peaks were < 222 > at 30.5◦ and < 622 > at 60.5◦ . However, for the ITO 90/10 thin films, the strong peaks were < 400 > at 35.5◦ and < 622 > at 60.5◦ . The Sn2 + and Sn4 + ions in the ITO 90/10 films were largely responsible for the difference in their preferred orientation. As both the In2 O3 and ITO 90/10 films have a crystal orientation of < 622 >, the diffraction peaks of the In2 O3 and ITO 90/10 thin films in the direction < 622 > were analysed, as shown in Fig.3. Both films shifted to a higher diffraction angle after annealing in vacuum at 400◦ C. These shifts were mainly caused by additional oxygen vacancies. Oxygen atoms escape from the lattice and form oxygen vacancies during vacuum annealing. An increase in the number of oxygen vacancies in ITO thin films can reduce 4

400

Vacuum

(622)

(125)

Air

(136)

(400)

Vacuum

&1100

ITO 90/10

(411)

400

3

Intensity (a.u.)

(622)

(136)

Intensity (a.u.)

(125)

(222)

2

(222)

In O

400

400

As grown

30

40

50

60

Vcuum

1100

Air

Vacuum

As grown

30

70

40

2-theta(° )

50

60

70

2-theta(° )

Figure 2: XRD patterns of the In2 O3 and ITO 90/10 films under different annealing conditions. 2

400

Intensity (a.u.)

1100

400

ITO 90/10

3

Intensity (a.u.)

In O

Vacuum Air

Vacuum

400 1100

400

As grown

59.5

60.0

60.5

61.0

Vcuum Air

Vacuum

As grown

59.5

61.5

2-theta(° )

60.0

60.5

61.0

61.5

2-theta(° )

Figure 3: X-ray diffraction peak h622i for In2 O3 and ITO 90/10 thin films under different annealing conditions.

the lattice parameter and cause the diffraction angle to shift to a higher angle.[23][24][25] However, the < 622 > peak in the ITO 90/10 thin films continued to shift after annealing in air at 1100◦ C, whereas in the In2 O3 thin film was unchanged. Based on Vergards law, different radii of solute and solvent ions will influence the lattice parameter.[26] Combining the formula for Bragg’s formula gives: 2dSinθ = Kλ

(1)

where d is the interplanar distance and 2θ is the diffraction angle. We can infer that the Sn2+ ions with bigger ionic radii (0.093nm) transferred into Sn4+ ions (0.069nm), causing lattice contraction and the peak to diffract to a higher angle.[27] The oxygen vacancies and Sn ions in ITO 90/10 thin films under differ5

As grown

529.0 eV

531.1 eV

A(I) A(II)

O1s =0.86

2vacanices O

(II)

Intensity (arb. units)

(I)

400

O in Lattice

in vacuum A(I) A(II)

(II)

(I)

400 &1100

in vacuum A(I)

in air

536

534

A(II)

(II)

(I)

538

=1.87

532

530

528

=1.19

526

524

Binding Energy (eV)

Figure 4: O 1s photoelectron peaks in the XPS profiles of ITO 90/10 films under different annealing conditions.

ent annealing conditions were analysed by XPS. The original carbon spectrum was calibrated by the carbon peak. The spectrums of tin and oxygen on the surfaces of the ITO 90/10 films were calibrated. The O 1s core level spectrums of ITO 90/10 are shown in Fig.4. The O 1S spectra of the ITO 90/10 thin films are divided into two Gaussian fitted curves.[28][29] The binding energies at approximately 529 eV and 531eV correspond to the In2 O3 crystal oxygen attributed to the strong In-O bond and oxygen vacancies, respectively.[30] The under area ratio of the O1s (I) to O1s (II) peaks increased after annealing in vacuum environment at 400◦ C, as the oxygen escaped from the films and produced oxygen vacancies.[31][32] After annealing in air at 1100◦ C, there were more oxygen atoms into the film, and the number of oxygen vacancies decreased. The spectrum of Sn 3 d5/2 in the ITO thin films is shown in Fig.5. The peaks of Sn 3d 5/2 at 485 eV were attributed to Sn2+ and the peak at 487 eV was attributed to Sn4+ .[33] The ratio of the area under the Sn 3d (I) and Sn 3d (II) peaks decreased when the thin film was annealed in vacuum at 400◦ C and in air at 1100◦ C. These results verify that Sn2+ ions transfer to Sn4+ ions after annealing processes. The carrier concentrations of In2 O3 and ITO 90/10 thin films under dif6

As grown

Sn 3d 5/2 Sn

Intensity (arb. units)

Sn

4+

2+

A(I)

(II)

A(II)

=0.38

(I)

400

in vacuum

A(I) A(II)

(I)

400

in vacuum

& 1100

A(I)

in air

489

488

A(II)

(II)

(I)

490

=0.65

(II)

487

486

485

=0.82

484

483

Bingding Energy (eV)

Figure 5: Sn 3d 5/2 photoelectron peaks in the XPS profiles of ITO 90/10 films under different annealing conditions.

ferent annealing conditions were probed by Hall effect measurements at room temperature, as shown in Table I. After annealing at 400◦ C in vacuum, the carrier concentrations of the In2 O3 and ITO 90/10 thin films increased as additional oxygen vacancies were produced. After annealing at 1100◦ C in air, the oxygen atoms entered the oxygen vacancies, leading to a decrease in the carrier concentration of the In2 O3 thin film. It is worth noting that the carrier concentrations of thin films after annealing at 1100◦ C in air are higher than that of as-grown thin films, which is mainly due to the recrystallisation processes that reduce the defects in thin films. The decrease in the carrier concentration of the ITO 90/10 thin films after the annealing processes was less than that of the In2 O3 thin films, owing to the conversion of Sn2 + ions to Sn4 + ions, which provide a higher carrier concentration in the ITO 90/10 films. Figs.6 and 7 show the representative surface and cross-sectional micrographs of ITO 90/10 thin films under different annealing conditions: the as-grown thin film is denoted by C1, annealing at 400◦ C in vacuum is denoted by C2, and annealing at 1100◦ C in air is denoted by C3. The Thornton model can be used to explain the formation of columnar crystal structures 7

Table 1: Carrier concentration of thin films probed by Hall effect measurements Thin film type Annealing conditions

In2 O3

ITO 90/10

As-grown 400◦ C in vacuum 1100◦ C in air 400◦ C in vacuum &1100◦ C in air As-grown 400◦ C in vacuum 1100◦ C in air 400◦ C in vacuum &400◦ C in vacuum

Thickness µm 4.8 4.9 4.8

Hall coefficient m3 /C 2.40×10−4 3.38×10−5 1.13×10−4

Carrier concentration m3 5.41×1023 3.76×1024 1.13×1024

4.8

1.02×10−4

1.25×1024

5.2 5.2 5.2

1.54×10−5 1.89×10−6 3.21×10−6

7.77×1024 6.34×1025 3.74×1025

5.1

5.60×10−6

2.14×1025

Figure 6: SEM image of the surface morphologies of ITO thin films under different annealing conditions. C1: as grown, C2: 400◦ C in vacuum and C3: 1100◦ C in air.

in as-grown thin films by deposition under low temperatures.[34] No gaps or pore appeared on the ITO thin films, indicating that the ITO thin films were uniformly and compactly deposited by RF sputtering. All the thin films had thicknesses of 5.6 µm and no expansion occurred after annealing processes. After annealing at 400◦ C in vacuum, more distinct grain boundaries were formed (C2), and several crystal nuclei appeared inside the thin films. This transformation was more dramatic after annealing in air at 1100◦ C. The thin film crystal structures aggregated into cubic crystals on the surface, and the column crystals changed into cellular crystals (C3). These results indicate that recrystallisation in ITO thin films occurred after annealing at 1100◦ C in air.

8

Figure 7: Change in the cross-sectional of microstructure of the ITO thin films’ under different annealing conditions. C1: as grown, C2: 400◦ C in vacuum and C3: 1100◦ C in air.

3.2. Effect of annealing on the thermoelectric properties The Seebeck coefficient is the basic parameter to describe the performance of ITO TFTCs, and can be calibrated by the apparatus shown in Fig.1. The Seebeck coefficient is given by S=

V V = ∆T Th − Tc

(2)

where S is the Seebeck coefficient (mv/◦ C), V is the thermoelectric voltage of the TFTCs, and ∆T is the temperature difference between the temperature of the hot and cold junctions (Th and Tc , respectively). To study the influence of recrystallisation on the thermoelectric stability of ITO thin films, the as-grown TFTCs were heated to 1300◦ C with a heating ratio of 5◦ C/min for 5 hr. After the furnace temperature decreased to room temperature, the thermocouple was retested. The hot junction temperature, cold junction temperature, and associated thermoelectric output voltage were collected every minute and plotted as a function of time. As shown in Figs.8 and 9, the thermoelectric voltage and Seebeck coefficient exhibited strong fluctuations when the temperature reached 1100◦ C the first time, becoming stable the second time. These results imply that recrystallisation processes occur in ITO thin films at 1100◦ C in air. The Seebeck coefficient trends of two tests at 1300◦ C (shown in Fig.10) were used to describe the thermoelectric stability. The Seebeck coefficient deviation rates for the ITO TFTCs in the first and second tests were ±10% and ±2.5%, respectively. It is noteworthy that the initial Seebeck coefficient in the first test was unstable, but was similar to the second test with increasing time. These results demonstrated that recrystallisation can improve the thermoelectric stability of ITO TFTCs. 9

T hot

First test

T cold

Second test

250

1000

Temperature (

200

800 150 600

100

(mV)

)

1200

Thermoelectric voltage

300

1400

400

50

200

0

0 0

100

200

300

400

500

600

Time (min)

Figure 8: Thermoelectric voltage of ITO TFTCs FOR two test processes.

0.25

First test

Seebeck coefficient (mV/

)

Second test 0.20

0.15

0.10

1100 0.05

0.00 400

600

800

Temperature (

1000

1200

)

Figure 9: Seebeck coefficient of ITO TFTCs for two test processes.

10

0.20

First test Second test

Seebeck coefficient

mV/

0.18

0.16

0.14

0.12

0.10 0

50

100

150

200

250

300

350

Time (min)

Figure 10: Seebeck coefficient stability of ITO TFTCs for two test processes.

To study the effect of annealing on the thermoelectricity of ITO TFTCs, the influence of recrystallisation should be eliminated. Therefore, all TFTCs were firstly heated up 1300◦ C, maintained at this temperature for 5 h, and then cooled to room temperature. By this process, the hot junction parts of the TFTCs inside the furnace acted as the recrystallisation areas, and the legs of the TFTCs outside the furnace maintained the initial thermoelectricity under different annealing conditions. After this process, the TFTCs were tested three times to study the influence of annealing on the reproducibility, linearity, and stability of the thermoelectric voltage. The thermoelectric voltages of ITO TFTCs under different annealing conditions are shown in Fig.11. When maintained at 1100 and 1200◦ C, the thermoelectric voltage of the as-grown ITO TFTCs decreased to a value similar to that of the ITO TFTCs after annealing at 1100◦ C in air. The thermoelectric voltages of the as-grown ITO TFTCs were not stable during the three testing cycles, implying a poor reproducibility. Reproducibility is one of the most important requirements of thermocouples. To clearly characterise the reproducibility of the thermoelectricity of ITO TFTCs, the changes in the Seebeck coefficient during the cooling processes of three testing cycles were

11

400

300

T cold

As grown

1100

in vacuum & 1100

in air

400

in vacuum

in air

250

Temperature(

)

1000

200

800

600

150

400

100

200 50

Thermoelectric Voltage (mV)

T hot

1200

0 0 0

200

400

600

800

1000

1200

1400

1600

Time (min)

Figure 11: Thermoelectric voltage of ITO TFTCs under different annealing conditions during three testing cycles.

analysed, as shown in Fig.12. Both the as-grown and annealed at 400◦ C in a vacuum ITO TFTCs exhibited irreproducibility during the tests. However, after annealing in air at 1100◦ C, the Seebeck coefficients of the ITO TFTCs had small standard deviations during the three tests. These results demonstrate that annealing at 1100◦ C in air improves the thermoelectric reproducibility of ITO TFTCs. The linearities of the thermoelectric voltages during each cooling process were also analysed. Linear fits of the thermoelectric voltage versus temperature difference for different annealing conditions are shown in Fig.13. Annealing at 1100◦ C in air greatly improved the linearity of the ITO TFTC thermoelectric voltage. Additionally, after calculating the margin fitting error rate (Fig.14) and R-squared value of the linear fit results, it was concluded that annealing at 400◦ C in a vacuum also improves the linearity. Table 2 provides the linear fitting results. The thermoelectricity stability is another key property of ITO TFTCs. The Seebeck coefficient of ITO TFTCs at 1200◦ C for 2 hr was analysed, as shown in Fig.15. The root mean square errors (RMSEs) were used to describe the stability of the ITO TFTCs and are given in Table 3. The ITO TFTCs 12

0.25

As grown

400

0.155

in Vacuum

)

0.24

Seebeck coeffcient (mV/

)

Seebeck coeffcient (mV/

0.23

0.22

0.21

0.20

0.19

0.18

First time

0.17

Second time 0.16

0.150

0.145

0.140

0.135

First time Second time

0.130

Third time

Third time

0.15

0.125

1300

1200

1100

1000

900

800

Temperature ( 0.155

700

600

500

1300

1100

1000

)

900

800

Temperature ( 0.110

in Air

400

in Vacuum & 1100

700

600

500

700

600

500

)

in Air

)

)

1100

1200

Seebeck coeffcient (mV/

Seebeck coeffcient (mV/

0.150

0.145

0.140

0.135

First time

0.105

0.100

0.095

First time

Second time

Second time

Third time

0.130

Third time 0.090

0.125 1300

1200

1100

1000

900

800

Temperature (

700

600

500

1300

1200

1100

1000

)

900

800

Temperature (

)

Figure 12: Seebeck coefficient of ITO TFTCs under different annealing conditions for three cooling processes

160

As-grown

400 Thermoelectric Voltage (mV)

220

Thermoelectric Voltage (mV)

200

180

160

140

Fitting results 120

First testing Second testing

100

Third testing

1200

120

100

Fitting results

80

First testing Second testing Third testing

60

80

1100

1000

900

800

700

600

Temperature difference (

500

400

300

in Vacuum

140

1200

1100

1000

)

900

800

700

600

Temperature difference (

500

400

300

400

300

)

180

1100

400

in Air

in Vacuum & 1100

in Air

120

Thermoelectric Voltage (mV)

Thermoelectric Voltage (mV)

160

140

120

100

80

Fitting results First testing

60

Second testing Third testing

100

80

Fitting results First testing

60

Second testing Third testing

40

40 1200

1100

1000

900

800

700

Temperature difference (

600

500

400

300

1200

1100

1000

900

800

700

600

Temperature difference (

)

500

)

Figure 13: Linear fits of the ITO TFTCs thermoelectric voltage versus temperature difference

13

40

15

400

First time

As-grown

30

Second time

25

Third time

in Vacuum

First time Second time

10

Fitting error rate (%)

Fitting error rate (%)

35

20 15 10 5 0 -5 -10 -15

Third time

5

0

-5

-20 -25

-10

-30 -35 -40 1200

-15 1100

1000

900

800

700

600

Temperature difference (

500

400

300

1200

1100

1000

900

800

700

600

Temperature difference (

)

2.0

400

in Air

First time

in Vacuum & 1100

in Air

300

First time

1.5

Second time

Second time

0.5

0.0

-0.5

Third time

Fitting error rate (%)

Third time

1.0

Fitting error rate (%)

400

)

2.0

1100 1.5

1.0

0.5

0.0

-0.5

-1.0

-1.0

-1.5

-1.5

-2.0 1200

500

-2.0 1100

1000

900

800

700

600

Temperature difference (

500

400

300

)

1200

1100

1000

900

800

700

600

Temperature difference (

500

400

300

)

Figure 14: Linear fits error rates of the ITO TFTCs thermoelectric voltage versus temperature difference

Table 2: Linear fits of the thermoelectric voltage versus temperature difference

U=A∆T +B Margin of fitting error A B As-grown 0.210 -14.7 ±30% ◦ 400 C in vacuum 0.134 1.085 ±12% ◦ 1100 C in air 0.130 -15.576 ±1.5% ◦ 400 C in vacuum 0.082 -13.369 ±1% & 1100◦ C in air Samples

14

R-square 0.9481 0.9590 0.9995 0.9996

0.24

As grown

Seebeck coefficient (mV/

)

400 1100

0.20

400

in Vacuum in Air in Vacuum & 1100

in Air

0.16

0.12

0.08

-20

0

20

40

60

80

100

120

140

160

Time (min)

Figure 15: Stability of ITO TFTCs under different annealing conditions.

annealed at 400◦ C in a vacuum and 1100◦ C in air improved the stability of the thermoelectricity, with the most significant improvements observed for annealing at 1100◦ C in air. The order of the ITO TFTCs’ Seebeck coefficients under different annealing conditions is: as-grown, 1100◦ C in air, 400◦ C in vacuum, and 400◦ C in vacuum and 1100◦ C in air. To explain this phenomenon, the theoretical Seebeck coefficient values of the In2 O3 and ITO 90/10 thin films were calculated, and the thermoelectric theory of semi-conductors can be used to explain the effects of different annealing conditions on the thermoelectricity

Table 3: Seebeck coefficient of ITO TFTCs under different annealing conditions Annealing process As-grown 400◦ C in vacuum 1100◦ C in air 400◦ C in vacuum & 1100◦ C in air

Experiment Seebeck coefficient mv/◦ C 0.184 0.138 0.152

Theoretical Seebeck coefficient mv/◦ C 0.244 0.068 0.162

0.108

0.143

15

RMES of experiment Seebeck coefficient 1.948×10−3 0.822×10−3 0.268×10−3 0.308×10−3

of ITO TFTCs. In2 O3 and ITO 90/10 are both degenerated semiconductors, and their Seebeck coefficients are given by Eq.(3).[35][36]  2  π  23 8k 2 m∗ T  3 1 3 S=− A+ 3 3eh2 2 ND

(3)

here S is the Seebeck coefficient of In2 O3 and ITO 90/10 films, respectively, k is the Boltzmann constant, h is Plancks constant, ND is the carrier concentration of the In2 O3 and ITO 90/10 films, e is the electron charge, m∗ is the effective mass (0.18 m0 ), and A is a fitting parameter in the range -0.5 to 0.5 (A=-0.5 in this study).[37] The Seebeck coefficient of ITO TFTCs is the difference between the In2 O3 and ITO 90/10 thin films, and the carrier concentration of the In2 O3 thin film is less than that of the ITO 90/10 thin film. Therefore, the Seebeck coefficient of the ITO TFTCs areg given by Eq.(4).  $  23   23 %  π  23 8k 2 m∗ T  1 3 1 − S = SIT O90/10 − SIn2 O3 = A+ 3 3eh2 2 ND1 ND2 (4) here ND1 and ND2 are the carrier concentrations of the In2 O3 and ITO 90/10 films, respectively. The theoretical Seebeck coefficients of the ITO TFTCs under different annealing conditions can be calculated by Eq.(4) using the carrier concentrations of Table 1. The calculations are compared with the experiment results in Table 3. From the result comparisons in Table 3, it can be concluded that the Seebeck coefficients of the ITO TFTCs are described by the thermoelectric theory of degenerated semiconductors. The theory also describes the effect of the annealing conditions on the Seebeck coefficients of ITO TFTCs. The increases in the Seebeck coefficients after the first test in Figs.8 and 9 are due to decreases in the carrier concentrations of the In2 O3 and ITO 90/10 thin films, as the oxygen atoms enters the oxygen vacancies. After annealing at 400◦ C in vacuum, the Seebeck coefficient of the ITO TFTCs decreased, owing to increases in the carrier concentrations of the In2 O3 and ITO 90/10 thin films. However, the theoretical values obtained for these annealing processes do not agree with the experiments. The main reason for this is that the hot junctions of the ITO TFTCs were heated to 1200◦ C during the tests, 16

and therefore the carrier concentrations of the hot junction areas decreased as oxygen atoms entered the oxygen vacancies. The effect of annealing in vacuum can also be demonstrated by comparing the Seebeck coefficients of the ITO TFTCs annealed at 1100◦ C in air and 400◦ C in vacuum & 1100◦ C in air. The Seebeck coefficients of the ITO TFTCs were lower for the annealing processes at 400◦ C in vacuum. For the as-grown thin film, the Seebeck coefficient was lower for annealing at 1100◦ C in air as the carrier concentrations in the In2 O3 and ITO 90/10 thin film were less, due to the recrystallisation processes that reduce the defects in the films. As the deviation rate of the ITO TFTCs annealed at 1100◦ C in air is lowest, annealing in air reduces the defects in the In2 O3 and ITO 90/10 thin films and the thermoelectric properties of thin films agree with the theoretical Seebeck coefficient. 4. Conclusion The effect of different annealing conditions on thermoelectric properties of ITO TFTCs were investigated. The thin film microstructural transformations by annealing are thought to be largely responsible for changes in the thermoelectric properties. The XRD and XPS results show that the number of oxygen vacancies increased after annealing at 400◦ C in vacuum, and annealing at 1100◦ C in air forced the oxygen atoms to enter the vacancies, thus reducing the number of oxygen vacancies. The Sn2+ ions in the ITO 90/10 thin films transformed into Sn4+ ions after annealing at 400◦ C in vacuum and 1100◦ C in air, which provided additional free electrons in the ITO 90/10 thin films. In additional, the In2 O3 and ITO 90/10 thin films recrystallised at temperature up to 1100◦ C in air. These changes affected the carrier concentrations in the In2 O3 and ITO 90/10 thin films and finally led to differences in the thermoelectricity of the ITO TFTCs. Both of annealing at 400◦ C in vacuum and 1100◦ C in air was beneficial for improving the thermoelectric reproducibility, linearity and stability of the ITO TFTCs, and 1100◦ C in air was more significant due to the recrystallisation. Moreover, the thermoelectric theory of degenerated semiconductors was used to interpret the effect mechanism of annealing on thin film thermoelectric properties. The Seebeck coefficients of the ITO TFTCs after annealing at 400◦ C in a vacuum and 1100◦ C in air agreed with the theoretical values, which can be used as a method for the design and fabrication of high-performance ITO TFTCs in the future.

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Conflict of interest statement: We declare that we have no financial and personal relationships with other people or organizations that can inappropriately influence our work, there is no professional or other personal interest of any nature or kind in any product, service and/or company that could be construed as influencing the position presented in, or the review of, the manuscript entitled, “Effect of annealing on the thermoelectricity of indium tin oxide thin film thermocouples”. Name: Xinhang Jin E-mail: [email protected] Corresponding author: Name: Binghe Ma E-mail: [email protected]