High-Performance Zinc Tin Oxide Semiconductor Grown by

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High Performance Zinc Tin Oxide Semiconductor grown by Atmospheric Pressure Mist-CVD and the Associated Thin Film Transistor Properties Jozeph Park, Keun-Tae Oh, Dong-Hyun Kim, Hyun-Jun Jeong, Yun Chang Park, Hyun-Suk Kim, and Jin-Seong Park ACS Appl. Mater. Interfaces, Just Accepted Manuscript • Publication Date (Web): 29 May 2017 Downloaded from http://pubs.acs.org on May 29, 2017

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ACS Applied Materials & Interfaces

High Performance Zinc Tin Oxide Semiconductor grown by Atmospheric Pressure Mist-CVD and the Associated Thin Film Transistor Properties Jozeph Park1†, Keun-Tae Oh2, Dong-Hyun Kim3, Hyun-Jun Jeong3, Yun Chang Park4, HyunSuk Kim5* & Jin-Seong Park2,3* 1

Department of Material Science and Engineering, Korea Advanced Institute of Science and Technology, Daejeon 305-338, Republic of Korea

2

Department of Information Display and Engineering, Hanyang University, 222 Wangsimni-ro, Seoul 133-719, Republic of Korea

3

Department of Materials Science and Engineering, Hanyang University, 222 Wangsimni-ro, Seoul 133-719, Republic of Korea

4

National Nano Fab Center, Daejeon 305-806, Republic of Korea

5

Department of Materials Science and Engineering, Chungnam National University, Daejeon 305-764, Republic of Korea

1

ACS Paragon Plus Environment


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ABSTRACT

Zinc tin oxide (Zn-Sn-O or ZTO) semiconductor layers were synthesized based on solution processes, of which one type involves the conventional spin coating method and the other is grown by mist chemical vapor deposition (Mist-CVD). Liquid precursor solutions are used in each case, with tin chloride and zinc chloride (1:1) as solutes in solvent mixtures of acetone and deionized water. Mist-CVD ZTO films are mostly polycrystalline, while those synthesized by spin-coating are amorphous. Thin-film transistors based on Mist-CVD ZTO active layers exhibit excellent electron transport properties with a saturation mobility of 14.6 cm2/Vs, which is superior to that (6.88 cm2/Vs) of their spin-coated counterparts. X-ray photoelectron spectroscopy (XPS) analyses suggest that the Mist-CVD ZTO films contain relatively small amounts of oxygen vacancies, hence lower free carrier concentrations. The enhanced electron mobility of Mist-CVD ZTO is thus anticipated to be associated with the electronic band structure, which is examined by X-ray absorption near-edge structure (XANES) analyses, rather than the density of electron carriers.

Keywords : Mist –CVD, Sol-gel process, Zinc tin oxide, Tin Films Transistors(TFTs), Solution process, Atmospheric pressure

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Introduction The high optical transparency and excellent charge transport characteristics, combined with high chemical stability and mechanical tolerance make metal oxide semiconductors attractive for thin-film transistor (TFT) applications in large area optoelectronics1-3. Despite their short history, devices based on oxide semiconductors have proven to exhibit high carrier mobility values comparable to those of polycrystalline silicon TFTs4.5. The deposition of oxide semiconductors can be done by several different techniques, namely radio frequency magnetron sputtering6, chemical vapor deposition (CVD) 7, and spin-coating followed by post-annealing8. Although the vacuum-based methods such as sputtering and CVD are well known to result in high quality devices with reasonable performance and reliability, the processes are far from being cost-effective and require the use of quite complex instruments. In this regard, many research groups have recently focused on the development of alternative deposition methods for oxide semiconductors, based on solution-processes9-15.

Because solution-based methods usually consist of sol-gel processes involving hydrocarbon complexes, the major drawback is the fact that carbon-related impurities remain in the synthesized films. The transport of charge carriers may be affected to a large extent by the incorporation of such undesired contamination. The amount of carbon content may be lowered by annealing the layers at relatively high temperatures (> 400 °C), which allows the use of heat-resistant rigid substrates such as silicon or glass only16. To expand their range of applications into fields such as flexible electronics on polymer substrates, one must be able to deposit high purity oxide semiconductors at relatively low temperatures. In the present work, zinc tin oxide (Zn-Sn-O or ZTO) semiconductor films are grown using a conventional spin coating method and a novel technique known as mist chemical vapor deposition (mist-CVD). 3



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The latter is a solution-based technique that can be performed under atmospheric pressure with a relatively simple, inexpensive configuration. Sol-gel techniques have also been studied to synthesize thin films, however they generally require the use of high temperature and toxic solvents such as 2-methoxyethanol. On the other hand, spin-coating processes involve repetitive procedures to control the resulting layer’s thickness. Compared to previous solution processes, mist-CVD allows relatively convenient thickness modulation and involve the application of environment friendly solvents such as water and acetone17,18. Recently, quite a few studies based on oxide films such as InOx, SnOx, ZnOx deposited by mist-CVD have been reported19-21.The films synthesized by the two methods are comparatively examined using X-ray diffraction (XRD), transmission electron microscopy (TEM), spectroscopic ellipsometry (SE), atomic force microscopy (AFM), X-ray photoelectron spectroscopy (XPS), Auger electron spectroscopy (AES), and X-ray absorption near-edge structure (XANES) analyses. TFTs based on the two types of ZTO semiconductors are evaluated, and the mistCVD ZTO devices are found to exhibit superior electrical performance. The film characterization results indicate that the microstructure, chemical composition, and electronic band structure may influence the electrical properties of the ZTO layers considerably.

Experimental Section The ZTO films were grown onto silicon substrates using the conventional spin-coating and mist-CVD methods. The liquid precursor solution for mist-CVD was prepared by dissolving zinc chloride (ZnCl2, Aldrich) and tin chloride (SnCl2, Aldrich) in a solvent mixture of deionized water and acetone to a concentration of 0.02 M. The precursor solution for spincoating was prepared the same way, except for the concentration, which is 0.2 M. The atomic ratio of zinc chloride and tin chloride was 1:1 in both solutions, which were each stirred at a temperature of 60 °C on a hot plate for 1 hour. The silicon substrates were exposed to oxygen 4


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plasma at 60 W for 10 minutes, in order to render their surface hydrophilic prior to the spin coating process. The spin-coated ZTO film was formed first by coating the solution with a spin-coater at 3000 rpm for 20 sec on the substrate, followed by annealing on a hot plate at temperatures between 250 and 400 °C for 1 hour in air. On the other hand, mist-CVD was processed with the following steps. The mist of the liquid source was generated by an ultrasonic atomizer (frequency: 1.5 MHz) and then carried into a chamber by the infiltration of N2 carrier gas (5000 sccm). The mist was next vaporized within the chamber and deposited on the substrate under ambient atmosphere. The chamber was designed to form a laminar flow. The substrate temperature was controlled to vary between 250 and 400 °C. The entire process was carried out at a pressure of 1 atm. The thicknesses of the ZTO films grown for TFT fabrication and film analyses were approximately 20 nm and 40 nm, respectively. The crystal structure was examined by X-ray diffraction (XRD) using a Cu Kα radiation. The chemical states and compositions of the films were analyzed by X-ray photoelectron spectroscopy (XPS) and Auger electron spectroscopy (AES). The optical properties of the films were measured using a spectroscopic ellipsometer. Height profile images of the film surfaces were obtained by atomic force microscopy (AFM). The microstructure and the crystallinity of ZTO thin films were examined by transmission electron microscopy (TEM) using a JEOL, JEM-ARM200F instrument operated at 200 keV. In order to examine the electronic configuration near the conduction band, X-ray absorption near-edge structure (XANES) analyses were performed using a total electron yield mode at the BL-10D beamline of the Pohang Accelerator Laboratory, Korea. For the fabrication of TFT devices, highly doped Si substrates were used as gate electrodes, and thermally grown SiO2 (100 nm) films were used as gate insulators. After growing the ZTO films by spin-coating and mist-CVD, indium tin oxide (ITO) layers were deposited by sputtering at room temperature to form the source/drain electrodes. The 5



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electrical properties of the TFT devices were analyzed using a HP 4155A semiconductor parameter analyzer in a dark room under ambient conditions. The channel width and length of each TFT device were 800 and 200 µm, respectively.

Results and Discussion The XRD diffractograms of the spin-coated and mist-CVD ZTO films are shown in figures 1 (a) and (b), respectively. The spin-coated ZTO films appear to be amorphous at all annealing temperatures, without the presence of any particular diffraction peak. On the other hand, the mist-CVD ZTO films exhibit some crystallinity at all growth temperatures, indicating the presence of tetragonal SnO2. Further structural characterization was performed by cross sectional TEM imaging, as shown in figures 2 (a) through (d). Figures 2 (a) and (b) are bright field images of the spin-coated film annealed at 350 °C, which confirm the amorphous nature of the layer observed in the XRD patterns. Figures 2 (c) and (d) are bright field images of the mist-CVD ZTO deposited at 350 °C. An interesting feature is the presence of an initial amorphous phase approximately 5 nm-thick, followed by the growth of a nanocrystalline layer on top of it. A higher magnification of this film is shown in figure 3 (a), with a fast Fourier transform (FFT) of the observed area shown in figure 3 (b). The distinct peaks in the FFT pattern correspond mainly to SnO2 microstructure, which implies that a phase separation into ZnO and SnO2 may have occurred during growth.

In the case of spin-coated ZTO layers, it may be conjectured that the carbon remnants prevented the crystallization of the films even at temperatures as high as 400 °C. The relative amounts of Sn, Zn, O, C and Cl detected by AES are listed in tables 1 and 2, for the spincoated and mist-CVD ZTO, respectively. The presence of carbon and chlorine elements is 6


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well observed in the former, while comparatively small or negligible amounts are identified in the mist-CVD films. This suggests that the impurities remaining in the spin-coated films may have inhibited the crystallization of ZTO by inducing structural deformation within the lattice22. On the other hand, the origin of the initial amorphous layer in the mist-CVD ZTO may be attributed to the amorphous nature of the underlying native silicon oxide, which does not provide any nucleation site for crystal growth. As the thickness increases, small nucleation sites may randomly appear on the surface of the initially amorphous ZTO film, onto which nanocrystals may successively form. The mist-CVD ZTO films are relatively rich in Sn content as indicated in table 2. In contrast to spin-coating, where the precursor is coated onto the substrate first and annealed to evaporate the solvent, the different chemical composition of the mist-CVD films may have resulted from the in situ evaporation of Zn element, along with Cl and C during the mist-CVD process. This is highly likely to be the reason why only SnO2 phases are identified in the TEM images. The mist-CVD ZTO is thus suggested to consist of a polycrystalline phase with SnO2 crystallites formed therein. The surface topography examined by AFM are shown in figures 4 (a) and (b) for the spincoated and mist-CVD ZTO, respectively. The overall root-mean-square (RMS) roughness values of the mist-CVD films are slightly higher than their spin-coated counterparts, which comply well with the formation of crystalline phases. The optical bandgap values were extracted by spectroscopic ellipsometry, as shown in figures 5 (a) and (b), for the spin-coated and mist-CVD ZTO, respectively. As the synthesis temperature increases, the bandgap values of spin-coated ZTO films approach ~3.3 eV, which is typical of ZTO films with Zn/Sn ~1.23 On the other hand, the bandgap of mist-CVD films decreases to below 3.1 eV, as the SnO2 phase contributes a relatively low bandgap of ~2.9 eV. 24

The typical values are listed in table 3.

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The transfer characteristics of the TFTs are shown in figures 6 (a) through (d) for the devices incorporating spin-coated ZTO active layers, and in figures 7 (a) through (d) for those based on mist-CVD ZTO semiconductors. The drain current (Id) versus gate voltage (Vg) plots are shown with the Id values on a logarithmic scale and the Vg values on a linear scale. The drain voltages (Vd) were kept at 0.1 and 20.1 V for each set of measurements. The field effect mobility (µFE) values along with the threshold voltage (Vth) and subthreshold swing (S.S.) values were extracted in compliance with the gradual channel approximation25, as listed in tables 4 and 5 for the spin-coated and mist-CVD ZTO devices, respectively. For each type of ZTO film, the device performance increases with increasing synthesis temperature, reaching peak mobility values at 350 °C. Further increase in synthesis temperature deteriorates the device performance, which is highly likely to be due to the sudden increase in Zn/Sn ratio26,27. Although the carbon content of the spin-coated ZTO films may be of concern, mist-CVD ZTO film annealed at 300 °C results in lower field effect mobility than the spincoated ZTO layer annealed at 350 °C. However, the carbon content of the latter film is higher, therefore at temperatures below 350 °C it may be assumed that the residual carbon does not have a direct influence on the relatively low field effect mobility of TFTs incorporating spincoated ZTO active layers.

XPS analyses of the O 1s peak are shown in figures 8 (a) and (b), for the spin-coated and mist-CVD ZTO films, respectively. The peaks could be deconvoluted into three different subpeaks A, B, and C28. Sub-peak A corresponds to photoelectrons originating from Zn-O or SnO bonds, while sub-peak B corresponds to signals arising from oxygen-deficient regions within the ZTO films. Sub-peak C is generally attributed to –OH groups, which are in turn often correlated to the presence of moisture or oxygen bonds with impurities such as carbon. The relative sub-peak intensities with respect to the synthesis temperatures are listed in table 8


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6 and 7, for the spin-coated and mist-CVD films, respectively. It is worth noticing that the spin-coated ZTO films appear to contain a larger portion of oxygen-related defects (sub-peak B) than the mist-CVD ZTO layers grown at the same temperature. Here it is suggested that while part of the oxygen deficient sites that contribute to sub-peak B may act as sources of free carriers, a considerable portion act as trap centers that deteriorate the charge transport properties of the spin-coated ZTO. Also, the presence of contaminants that give rise to subpeak C may not be neglected, as they may also act as charge scattering centers that inhibit the charge transport further. The sub-peak C ratio in the spin-coated ZTO films is larger compared to that in the mist-CVD ZTO.

X-ray absorption near edge spectroscopy (XANES) analyses were carried out in order to obtain information on the electronic structure of the ZTO films. The oxygen K edge structure spectra of spin-coated and mist-CVD ZTO films synthesized at 350 °C are shown in figure 9. The relative intensities of the peaks reflect the ordering of either Sn-O or Zn-O hybrid orbitals

29,30

. More pronounced ordering is observed in the mist-CVD ZTO, which suggests

that high conductivity paths for electrons have formed. Fast carrier transport may thus be interpreted in terms of molecular orbital ordering. Also, because the mist-CVD ZTO films are relatively rich in Sn, more abundant overlapping of the spatially larger Sn 5s orbitals is consistent with the above results.

In this regard, spin-coated ZTO films with Zn/Sn ratio similar to that of mist-CVD ZTO films were prepared and the associated TFT devices were analyzed (Table S1 and figure S1~S3). While the increase in tin content resulted in relatively high field effect mobility (~ 11.2 cm2/Vs), the devices based on mist-CVD ZTO still exhibit superior performance (~ 14.6 cm2/Vs). The latter results suggest that the impurity content may be a dominant factor that 9



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conveys higher carrier mobility in films grown by mist-CVD.

Conclusions In this work, spin-coated and mist-CVD ZTO films were synthesized at 250, 300, 350, and 400 °C. While the former films were annealed after spin-coating the precursor solutions onto the substrates at room temperature, the latter were grown onto substrates already heated up to the synthesis temperatures. All spin-coated ZTO films were amorphous, while the mist-CVD layers exhibit crystalline structures, apparently with the presence of phase separation into ZnO and SnO2. The TFT devices adopting the mist-CVD ZTO semiconductors exhibit higher field effect mobility than the ones incorporating spin-coated ZTO. XPS studies suggest that the amount of oxygen vacant sites is not critical in the determination of charge transport properties. Instead XANES analyses strongly imply that molecular ordering in the semiconductor influences the electron mobility, with the mist-CVD ZTO films exhibiting more pronounced crystal field splitting than the spin-coated ZTO. The mist-CVD process can thus be considered as a relatively simple and cost-effective synthesis method, which is very promising regarding the synthesis of high performance semiconductor devices for a myriad of applications.

Supporting Information Chemical compositions determined by Auger electron spectroscopy, which shows the similar ratio of Sn:Zn = 3.88:1 to the mist-CVD ZTO films (Sn:Zn = 3.51:1) (Table S1). X-ray diffraction patterns of spin-coated ZTO films (Sn:Zn = 3.88:1) with SnO2 phases (Figure S1). Topographical image collected by atomic force microscopy (AFM) of the spin-coated ZTO films (Sn:Zn = 3.88:1) (Figure S2). Transfer characteristics (Id-Vg) of the TFTs incorporating spin-coated ZTO active layers (Figure S3). 10


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AUTHOR INFORMATION Corresponding Authors E-mail: [email protected] (Jin-Seong Park) E-mail: [email protected] (Hyun-Suk Km) *

These authors contributed equally to this work

†

Current affiliation: R&D Center, Samsung Display, Yongin-Si, Giheung-gu, Republic of

Korea

AUTHOR CONTRIBUTIONS J.P., J.-S.P. and H.-S.K. designed this work. J.P., J.-S.P. and H.-S.K. wrote the manuscript. D.H.K., K.-T.O. and H.-J.J. carried out the experiments and electrical measurements. Y.C.P. performed TEM analysis.

Acknowledgements This work was supported by the Industry Technology R&D program of MOTIE/KEIT [10051080 and 10051403] and KDRC (Korea Display Research Corporation).

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Electron‐beam‐evaporated Indium Tin Oxide Films as Studied by X‐ray Photoelectron Spectroscopy and Work‐function Measurements. J. Appl. Phys. 1993, 73, 4344. [29] Ekicibil, A.; Ozkendir, O. M.; Farha, A. H.; Ufuktepe, Y. Study of the Electronic Properties of Zn 0.8–4x Ho x O y (0.05≤ x≤ 0.09) by X-ray Absorption and 15



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Photoemission Spectroscopy. J. Electron. Spect. Rel. Phen. 2015, 202, 56-67. [30] Domashevskaya, E. P., Chuvenkova, O. A., Kashkarov, V. M., Kushev, S. B., Ryabtsev, S. V., Turishchev, S. Y., & Yurakov, Y. A. TEM and XANES Investigations and Optical Properties of SnO Nanolayers. Surf. Interface. Anal. 2006, 38.4, 514-517.

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Figure Captions Figure. 1. X-ray diffraction patterns of (a) spin-coated and (b) mist-CVD ZTO films, each synthesized at 250, 300, 350 and 400 °C. The spin-coated films exhibit an amorphous structure, while SnO2 phases are observed in the mist-CVD ZTO.

Figure. 2. (a), (c) Bright field TEM images and (b), (d) high resolution TEM images of spincoated and mist-CVD ZTO films, both synthesized at 350 °C. Note the presence of an initial amorphous layer in the mist-CVD film.

Figure 3. (a) High resolution TEM image of the mist-CVD ZTO film synthesized at 350 °C. (b) Fast Fourier transform (FFT) of the images area in (a), where most of the identified phases correspond to those of tetragonal SnO2.

Figure 4. Topographical images collected by atomic force microscopy (AFM), of the (a) spin-coated ZTO films and (b) the mist-CVD ZTO films. The overall root-mean-square (RMS) roughness is higher in the mist-CVD ZTO layers, which is in accordance with the formation of nanocrystalline phases.

Figure 5. Optical bandgap measurements by spectroscopic ellipsometry (SE), for the (a) spin-coated ZTO films and (b) the mist-CVD ZTO films. The Tauc method is used to estimate the bandgap as the x-axis intercept for each plot.

Figure 6. Transfer characteristics (Id-Vg) and saturation mobility curves of the TFTs incorporating spin-coated ZTO active layers, which were annealed at (a) 250 °C, (b) 300 °C, 17



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(c) 350 °C and (d) 400 °C.

Figure 7. Transfer characteristics (Id-Vg) and saturation mobility curves of the TFTs incorporating mist-CVD ZTO active layers, which were grown at (a) 250 °C, (b) 300 °C, (c) 350 °C and (d) 400 °C.

Figure 8. XPS O 1s peaks of the (a) spin-coated films and (b) mist-CVD layers. Each peak is deconvoluted into three sub-peaks labeled as A, B, and C. Sub-peak A corresponds to signals arising from Zn-O or Sn-O bonds. Sub-peak B is related to the photoelectrons emitted from oxygen-deficient regions, and sub-peak C is often interpreted to originate from -OH groups, which are in turn suggested to occur from moisture.

Figure 9. XANES oxygen K-edge structure of the spin-coated and mist-CVD ZTO films synthesized at 350 °C. More pronounced molecular orbital ordering is manifested in the mistCVD ZTO.

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List of Tables Table 1. Chemical compositions determined by Auger electron spectroscopy, for the spincoated ZTO films annealed at 300, 350 and 400 °C. Spin-coated

Zn

Sn

O

C

Cl

300 °C

24.9

20.7

46.5

5.2

2.7

350 °C

20.9

20.8

50.4

5.4

2.5

400 °C

27.9

19.4

46.1

5.2

1.3

Table 2. Chemical compositions determined by Auger electron spectroscopy, for the mistCVD ZTO films grown at 300, 350 and 400 °C. Mist-CVD

Zn

Sn

O

C

Cl

300 °C

10.4

36.5

50.9

2.2

N.D.

350 °C

9.50

33.3

57.2

N.D.

N.D.

400 °C

20.4

28.4

51.2

N.D.

N.D.

Table 3. Optical bandgap values of the spin-coated and mist-CVD ZTO films, measured by spectroscopic ellipsometry. Band Gap (eV)

250℃

300℃

350℃

400℃

Spin-coated

3.86

3.39

3.22

3.30

Mist-CVD

3.79

3.22

3.01

3.12

19



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Page 20 of 31

Table 4. Representative device parameters of the TFTs incorporating spin-coated ZTO active layers, with respect to the synthesis temperatures. Spin-coated

µFE (cm2/Vs)

Vth (V)

S.S. (V/dec)

250℃

0.02

25.9

5.14

300℃

0.24

6.29

0.77

350℃

6.88

3.33

0.47

400℃

1.80

4.54

0.45

Table 5. Representative device parameters of the TFTs incorporating mist-CVD ZTO active layers, with respect to the synthesis temperatures. Mist-CVD

µFE (cm2/Vs)

Vth (V)

S.S. (V/dec)

250℃

0.25

2.49

1.42

300℃

4.05

2.30

1.29

350℃

14.6

3.28

0.99

400℃

2.70

4.54

0.66

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Table 6. Relative sub-peak intensities of the XPS O 1s peaks in the spin-coated ZTO films, with respect to the synthesis temperatures. Spin-coated

A

B

C

250℃

65.45

22.35

12.2

300℃

66.35

22.55

11.1

350℃

73.03

17.67

9.3

400℃

83.75

13.55

2.7

Table 7. Relative sub-peak intensities of the XPS O 1s peaks in the mist-CVD ZTO films, with respect to the synthesis temperatures. Mist-CVD

A

B

C

250℃

78.74

15.34

5.92

300℃

85.73

11.84

2.43

350℃

88.19

9.28

2.53

400℃

91.12

8.08

0.8

21



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Figure 1.

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Figure 2.

23



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Figure 3.

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Figure 4.

25



10

9.0x10

(a)

10

6.0x10

o

250 C o 300 C o 350 C o 400 C

2

-2

2

(ahv) (cm eV )

Spin-coated

10

3.0x10

0.0 1

2

3

4

5

6

Band gap (eV) 10

9.0x10

(b)

-2

2

(ahv) (cm eV )

Mist-CVD

10

6.0x10

o

250 C o 300 C o 350 C o 400 C

2

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 26 of 31

10

3.0x10

0.0 1

2

3

4

Band gap (eV) Figure 5.

26


5

6

Page 27 of 31

(a)

-2

0.4

10

-3

10

VDS= 0.1 V VDS= 20.1 V

Spin-coated o 250 C

-2

(b)

10

-5

10

-6

10

-7

0.2

10

-8

10

-9

10

VDS= 20.1 V

-6

10

-7

-8

10

-9

10 10

Saturation mobility

-11

10

0

20

-12

10

0.0

40

60

-20

-2

Spin-coated o 350 C VDS= 20.1 V

10

VDS= 0.1 V -3

10

-4

-3

10

10

Spin-coated o 400 C

VDS= 0.1 V VDS= 20.1 V

8

10

-5

-5

-6

Drain Current (A)

Saturation mobility

10

6

-7

10

-8

4

10

-9

10

-10

10

-6

6

10

-7

10

-8

4

10

Saturation mobility

-9

10

-10

2

10

2

10

-11

-11

10

10

-12

-20

0 60

40

-4

10

10

20

-2

(d) 10

8

10

0

Gate Voltage (V)

Gate Voltage (V) 10

Saturation mobility

-11

10

-12

(c)

2

10

-10

-10

10

-20

Spin-coated o 300 C

VDS= 0.1 V

-5

10

Drain Current (A)

Drain Current (A)

-3

10

-4

10

4

10

-4

10

Drain Current (A)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


0

20

40

-12

0 60

10

-20

Gate Voltage (V)

0

20

40

Gate Voltage (V)

Figure 6.

27


0 60


(a)

-2

-3

-2

2

10 10

VDS= 0.1 V VDS= 20.1 V

(b) 10

Mist-CVD o 250 C

-3

10

-4

-6

10

-7

1

10

-8

10

-9

10

Saturation mobility

-10

10

10

-6

-7

-8

-9

10

0

20

40

VDS= 0.1 V Mist-CVD

0 60

-12

10

-20

25

o

(d)

20

40

0 60

-2

10

10

-3

10

VDS= 0.1 V VDS= 20.1 V

Mist-CVD o 400 C

-4

20

10

0

Gate Voltage (V)

VDS= 20.1 V 350 C

-4

8

10

-5

Saturation mobility

-6

10

15

-7

10

-8

10

10

-9

10

-10

Drain Current (A)

-5

10

10

-6

6

10

-7

10

-8

10

Saturation mobility

-9

10

-10

5

10

-11

4

2

10

-11

10

10

-12

-20

2

-11

-2

10

4

10

Gate Voltage (V)

-3

Saturation mobility

10

10

-12

10

6

10

-10

-11

(c)

8

10

10

10

o

300 C

-5

Drain Current (A)

Drain Current (A)

-5

-20

VDS= 20.1 V

10

Mist-CVD

10

10

10

VDS= 0.1 V

-4

10

Drain Current (A)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 28 of 31

0

20

40

0 60

-12

10

-20

Gate Voltage (V)

0

20

40

Gate Voltage (V)

Figure 7.

28


0 60

Page 29 of 31

Intensity (Arb. unit)

(a)

Spin-coated o 250 C

o

o

o

300 C

400 C

350 C

A O 1s B C

534

531

528 534

531

528 534

531

528 534

531

528

Binding energy (eV)

(b) Mist-CVD250 C o

Intensity (Arb. unit)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


o

o

o

300 C

400 C

350 C

A O 1s

B C

534

531

528 534

531

528 534

531

Binding energy (eV)

Figure 8.

29


528 534

531

528

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Intensity (arb.unit)


O K-edge

Page 30 of 31

Sn 5p/ Sn 5s Zn 4s

o

Spin-coated (350 C) o

Mist CVD (350 C)

520

530

540

550

Photon Energy (eV) Figure 9.

30


560

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Table of Contents

31


High-Performance Zinc Tin Oxide Semiconductor Grown by

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