Improving Thermal Stability of Solution Process Indium Zinc Oxide

Subscriber access provided by UNIV OF DURHAM

Functional Inorganic Materials and Devices

Improving Thermal Stability of Solution Process Indium Zinc Oxide Thin Film Transistors by Praseodymium Oxide Doping Min Li, Wei Zhang, Weifeng Chen, Meiling Li, Weijing Wu, Hua Xu, Jianhua Zou, Hong Tao, Lei Wang, Miao Xu, and Junbiao Peng ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.8b07612 • Publication Date (Web): 03 Aug 2018 Downloaded from http://pubs.acs.org on August 4, 2018

Just Accepted “Just Accepted” manuscripts have been peer-reviewed and accepted for publication. They are posted online prior to technical editing, formatting for publication and author proofing. The American Chemical Society provides “Just Accepted” as a service to the research community to expedite the dissemination of scientific material as soon as possible after acceptance. “Just Accepted” manuscripts appear in full in PDF format accompanied by an HTML abstract. “Just Accepted” manuscripts have been fully peer reviewed, but should not be considered the official version of record. They are citable by the Digital Object Identifier (DOI®). “Just Accepted” is an optional service offered to authors. Therefore, the “Just Accepted” Web site may not include all articles that will be published in the journal. After a manuscript is technically edited and formatted, it will be removed from the “Just Accepted” Web site and published as an ASAP article. Note that technical editing may introduce minor changes to the manuscript text and/or graphics which could affect content, and all legal disclaimers and ethical guidelines that apply to the journal pertain. ACS cannot be held responsible for errors or consequences arising from the use of information contained in these “Just Accepted” manuscripts.

is published by the American Chemical Society. 1155 Sixteenth Street N.W., Washington, DC 20036 Published by American Chemical Society. Copyright © American Chemical Society. However, no copyright claim is made to original U.S. Government works, or works produced by employees of any Commonwealth realm Crown government in the course of their duties.

Page 1 of 19 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

ACS Applied Materials & Interfaces

Improving Thermal Stability of Solution Process Indium Zinc Oxide Thin Film Transistors by Praseodymium Oxide Doping Min Li1,2, Wei Zhang2, Weifeng Chen1, Meiling Li1, Weijing Wu1*, Hua Xu1,2, Jianhua Zou1,2, Hong Tao1,2, Lei Wang1,2, Miao Xu1,2* and Junbiao Peng1 1 Institute of Polymer Optoelectronic Materials and Devices, State Key Laboratory of Luminescent Materials and Devices, South China University of Technology, Guangzhou 510640, PR China 2 Guangzhou New Vision Optoelectronic Co., Ltd., Guangzhou 510530, PR China Corresponding author: [email protected]; [email protected];

Abstract: Praseodymium-doped indium zinc oxide (PrIZO) channel materials have been fabricated by a solution process with conventional chemical precursor. The PrIZO based thin film transistors (TFTs) exhibited a field effect mobility of 10.10 cm2/Vs, a sub-threshold swing (SS) value of 0.25 V/decade, and an Ion/Ioff ratio of 108. The as-fabricated PrIZO-TFTs showed improved device performance against positive bias temperature stress (PBTS shift of 1.97 V for 7200 s), which was evidently better than the undoped IZO-TFTs (PBTS shift of 9.52 V). This result indicates that the organic residual (-OCH3 and -CH2-) in metal oxide semiconductor, which is confirmed to be dominant effect on the performance of PBTS, can be passivated by the rare earth of praseodymium element. The residual is intended to be oxidized with a more stable ester group with the assistant of PrOx, weakening the electron-withdrawing characteristic during thermal bias stress. Keywords：：Praseodymium doping, solution process, organic residual, ester group, electron-withdrawing, positive bias temperature stress

1. Introduction Conventional sputtering produced amorphous silicon and oxide thin-film transistors (TFTs) had been gradually replaced by solution process for the backplane in active-matrix organic light-emitting displays (AMOLED) and liquid-crystal displays (LCD), due to its merits of low cost, simple process and easy control of the compounds composition.1–3 Solution process can be doped uniformly and ACS Paragon Plus Environment

ACS Applied Materials & Interfaces 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 2 of 19

quantitatively at molecular level to meet the needs in R&D incubation. Particularly, indium gallium zinc oxide (IGZO), indium zinc oxide (IZO) and amorphous zinc tin oxide (ZTO) are usually investigated and considered as channel materials, 4–6 because it exhibits high mobility, transparency and high switching ratio.7–10 But one of the obstacles in applications of solution-process amorphous oxide TFTs is the electrical stability under variable operating conditions, such as gate bias stress, light and thermal.11,12 Research progress includes Liu et al. reported that nitrogenation of amorphous IGZO-TFTs could improve gate-bias stability by reducing oxygen reaction.13 Juang et al.

has

investigated the

effects

of

ultraviolet (UV)-ozone

treatment on

solution-processed amorphous IGZO.14 The enhancement of UV-ozone-treated TFTs is mostly attributed to the increased film density. However, most of these researches in device stability have merely focused on bias stress performance, such as positive or negative bias stress (PBS/NBS). Seldom of them have involved thermal shock in bias stress to examine the durative in more realistic or harsh environments. Herein, there is a strong need for comprehensive investigation of thermal and bias stress in solution process devices, which would be the evaluation criteria for commercial uses as compared with the state-of-art sputtering technique. In this work, we have mainly studied the mechanism of thermal stability affecting PBTS performance by doping rare earth element of praseodymium (Pr) in the solution IZO-TFTs. The metal-oxide bond becomes enhanced with a lower standard electrode potential of Pr (SEP, -2.35 eV) and ultimately improves PBTS performance.15 2. Experiment 2.1. Materials Preparation The precursor solutions for IZO and PrIZO (0.1M) were prepared by sol-gel method using a mixture of zinc nitrate hexahydrate, indium nitrate hydrate, praseodymium nitrate tetrahydrate, and 2-methoxyethanol (Aladdin). For the 2-ME based solution, the monoethanolamine and acetic acid were added as a stabilizer. As a comparison, the precursor for aqueous route were also prepared by dissolving raw materials and deionized water in order. The molar ratio of In:Zn was fixed at 2.5:1, ACS Paragon Plus Environment

Page 3 of 19 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


and Pr was added at x.%/Zn (i.e., atomic ratio of 0.02/0.08/0.15). The mixed solution was stirred for 12 hours and filtered using a 0.44 µm syringe filter. 2.2. Device Fabrication An aluminum (Al) gate electrode was formed by magnetron sputtering with thickness of 300 nm and then anodized to produce a 200-nm-thick layer of Al2O3 as dielectric. The measured capacitance (Cox) of Al2O3 was ~41 nF/cm2 with a dielectric constant of ~9.16 Bottom-contacted indium tin oxide (ITO) as source/drain was formed by sputtering. Then, these substrates were cleaned using an UV-ozone treatment (420 nm) to increase adhesion. The IZO or PrIZO precursor was spin-coated and pre-baked, finally post-annealed at 450 °C. The channel width/length (W/L) as 500/250 µm was patterned by commercial acid etchant. All the devices were passivated with 300 nm thick SiO2 layer by plasma-enhanced chemical vapor deposition (PECVD) and were annealed at 350 °C. For each experimental condition, 20 samples were fabricated to verify the process repeatability. The performance of specified devices have a good representation of this unique process, because it takes a variety of statistics from all the samples with excellent uniformity and small standard deviation. 2.3. Electrical, and Structural Characterization The electrical characteristics were measured by using probe station and semiconductor parameter analyzer (Agilent B1500A). The structural properties and thickness of IZO/PrIZO films (deposited on glass substrates) were examined by X-ray diffractometer (XRD, Bruker, D8 Advance) using Cu Kα radiation. The chemical state of the IZO/PrIZO films was evaluated by X-ray photoelectron spectroscopy (XPS, Thermo SCIENTIFIC

ESCALAB 250Xi). The Fourier transform infrared

spectroscopy (FTIR, SHIMADZU, IRPrestige-21) was used to investigate the chemical compositions variation in IZO and PrIZO films. The surface morphology was observed by atomic force microscopy (AFM). 3. Results and Discussion 3.1 Measurement of TFT performance Figure 1a shows the output characteristics (IDS vs VDS) obtained from IZO-TFTs

ACS Paragon Plus Environment


based on conventional method of 2-ME alcoholysis process. The device exhibits classical n-channel behavior with no any other current crowding phenomenon. The transfer characteristic was also shown in Figure 1b. It can be seen that the on/off current ratio was larger than 107, and the threshold voltage is only 1.34±0.02 V, indicating that solution process thin film transistors has favorable performance. The operating stability of TFTs is one of the other criterion for the commercialization of oxide semiconductor backplane. In this experiment, NBS/PBS were performed with VGS/VDS as −20/0 V and 20/0 V with the stress time of 7200 s. As shown in Figure 1c, the Von shift in air ambient for NBS and PBS was −0.81 V and 1.84 V, which were quite familiar to the published results.17 As for the NBTS/PBTS, the Von shift was −1.23 V and 9.52 V respectively, while the constant temperature was fixed at 60℃. It means that the NBTS performance is reasonable, but the thermal induced bias stress is more severely in the case of PBTS performance in solution process devices. The PBTS measurement was also carried out under vacuum condition, because the solution-processed thin film was easily influenced by the ambient species (O2 or water molecule), owing to its inferior film density as compared with the routine sputtering process. Figure 1c shows the Von shift was still obvious with value of 6.90 V in the vacuum chamber of 4.61×10-5 torr, indicating that the issue of PBTS stability was very serious no whether in vacuum or atmospheric environment, which will dramatically hamper the application of solution process in mass production. Actually in sputtering IZO-TFTs, this severe degradation of PBTS stability was not as common.18 As a result, the deterioration induced by PBTS is highly dependent on the

4 10

-5

3 10

-5

2 10

(a) W/L=500/250um V = G 0.1V 4.1V 8.1V 12.1V 16.1V

-5

-4

10

-6

10

14

(b) W/L=500/250um

12

V =0.1 V

Von shift (V)

-5

IDS (A)

5 10

DS

(A)

preparation method of semiconductor channel with solution process.

I

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 4 of 19

DS

V =1.1 V DS

-8

10

V =5.1 V DS

V =10.1 V DS

1 10

-10

-5

10

0

10

V =20.1 V

(c) NBS PBS NBTS PBTS PBTS@Test in Vacuum

10 8 6 4 2

DS

0 -12

0

2

4 V

DS

6 (V)

8

10

-30 -20 -10 0 10 VG (V)

20

30

-2

0

1500 3000 4500 6000 7500 Stress time (s)

Figure 1. The output (a) and transfer (b) characteristic of solution process 2-ME based IZO-TFTs;


Page 5 of 19

(c) Bias stability test of alcoholysis processed IZO-TFTs.

Residual organic solvent solutions are considered to be the major contributors to the poor thermal stability of solution process TFTs.19 Decades of research have shown that the solution with hydrolysis process can effectively remove organic residual impurities,20,21 but the storage continuity and inhomogeneous distribution of aqueous solution have constrained its commercial uses. In our experiment, hydrolysis process was used as a probe to testify the effect of organic carbon residual on the PBTS performance. The TFT devices using the method of hydrolysis was fabricated as comparison to alcoholysis process. Figure 2a and 2b show a decent performance of the output and transfer characteristics for the IZO-TFTs based on hydrolysis process, which was a little superior than the conventional alcoholysis process. What is more, the PBTS of IZO-TFTs using this method can be significantly improved, as shown in Table 1. The Von shift for PBTS at air and vacuum ambient were 4.12 V and 3.01 V. The homogeneous 2-ME precursor is stable for storage exceeding 6 months without attenuation of the fabricated TFTs. In this experiment, we have shed light on the reasons triggering the poor thermal positive bias stress for alcoholysis solution TFTs by the consideration of removal or passivation of these organic residuals.

-5

6 10

-5

4 10

-5

2 10

-5

DS

8 10

14

(b) W/L=500/250um

(a) W/L=500/250um V = G 0.1V 4.1V 8.1V 12.1V 16.1V

-4

10

12

V =0.1 V DS

-6

10

Von shift (V)

1 10

-4

IDS (A)

-4

(A)

1.2 10

I

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


V =1.1 V DS

V =5.1 V DS

-8

10

V =10.1 V DS

V =20.1 V -10

DS

10 8 6

(c) NBS PBS NBTS PBTS PBTS@Test in Vacuum

4 2

10

0

0

-12

0

2

4 V

DS

6 (V)

8

10

10

-30 -20 -10 0 10 VG (V)

20

30

-2

0 1500 3000 4500 6000 7500 Stress time (s)

Figure 2. The output (a) and transfer (b) characteristic of solution process water based IZO-TFTs; (c) Bias stability test of the hydrolysis processed IZO-TFTs.

Table 1. Summarized properties of the alcoholysis and hydrolysis IZO-TFTs. The basic parameters are average values of the designated 20 samples with good uniformity. InZnO

Vth SS Ion/Ioff NBS NBTS PBS PBTS PBTS µsat 2 (V) (cm /Vs) (V/dec) ×107 shift shift shift (air) (vac.) alcoholysis 1.34 11.21 0.22 2.1 ̶ 0.81 ̶ 1.23 1.84 9.52 6.90 hydrolysis 1.05 12.35 0.36 1.5 ̶ 1.56 ̶ 0.81 1.20 4.12 3.01

For the following discussion, praseodymium doping was used to tackle the



organic residual and investigate its relevance with PBTS stability. To begin with, Figure 3 showed the transfer property of the PrIZO-TFTs with various ration of praseodymium doping. It was found that almost no hysteresis was found between forward/reverse sweeps of the transfer curves in PrIZO-TFTs with lower concentration of Pr doping, indicating low interface trap density between channel and gate dielectric. The devices saturation mobility (µsat) and sub-threshold swing (SS) were 10.10±0.20 cm2/Vs and 0.25±0.01 V/dec for the PrIZO-TFTs with atomic ratio of In:Zn:Pr as 2.5:1:0.02. The saturated µsat is calculated from a linear fitting to the plot of the square root IDS versus the VDS in saturated operation region. The sub-threshold swing (SS) is taken as the minimum value of (dlog(IDS)/dVGS)-1. The device performance has decreased severely with mobility and SS as 6.02±0.21 cm2/Vs and 0.31±0.02 V/dec while the atomic In:Zn:Pr was fixed at 2.5:1:0.08, and the Von voltage was gradually increased as elevating the concentration of praseodymium doping, as shown in Table 2. In metal oxide semiconductors, the percolation conduction model was systematically used to expound the carrier transport mechanism, as reported by Werner and co-workers,22,23 for thin films of c-IGZO and a-IGZO.24,25 So this serious degradation of PrIZO-TFTs seemed to have correlation to tail states near conduction band when doping with exceeding amount of Pr. The metal-oxide bond of Pr-O becomes enhanced and the number of oxygen vacancies decreases owning to lower SEP value. The PrIZO may has lower carrier concentration than IZO due to carrier suppression. 0.030

10

-10

10

-12

0.010

-20

-10

1/2

(A )

-8

0.020

1/2

10

2.5:1:0 2.5:1:0.02 2.5:1:0.08 2.5:1:0.15

0

V (V)

10

DS

10

-6

In/Zn/Pr

I

10

-4

DS

I (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 6 of 19

0.0 20

GS

Figure 3. Transfer characteristics of the PrIZO-TFTs.


Page 7 of 19

Table 2. Summarized properties of the doping PrIZO-TFTs. The basic parameters are average values of the designated 20 samples with good uniformity. In:Zn:Pr ratio 2.5:1:0 2.5:1:0.02 2.5:1:0.08 2.5:1:0.15

Vth (V) 1.37 1.56 2.12 3.05

Von (V) -1.94 -1.84 -0.79 0.36

µsat (cm2/Vs) 11.20 10.10 6.02 3.31

µlin (cm2/Vs) 18.60 14.40 9.82 6.32

SS (V/dec.) 0.22 0.25 0.31 0.36

As for the exploration of biased stress performance, herein we have mainly focused on PrIZO-TFTs with lower concentration in solution process film (0.02 Pr), which has a decent saturation mobility and SS. Figure 4 shows the transfer characteristics of PrIZO-TFTs under stress based on 2-ME organic solvents. The Von shift for NBS, NBTS, PBS and PBTS was −0.24 V, −0.71 V, 1.10 V, and 1.97 V in air ambient, as shown in Figure 4a-d. Moreover, the Von shift for PBTS under vacuum was only 0.72 V, as shown in Figure 4e. The stability of PrIZO-TFTs (especially the PBTS) was greatly improved. It was implied that rare earth Pr doping has a significant effect for performance modulation, which will be also discussed broadly in the

-7

10

-8

10

-9

-4

10

-5

10

-6

10

-7

10

-8

10

-9

I

10

-0.24 V

(b)

10

-4

10

-5

10

-6

10

-7

10

-8

10

-9

PBS at RT 0s 100s 600s 1200s 3600s 7200s

(A)

-6

10

NBS at RT 0s 100s 600s 1200s 3600s 7200s

DS

10

(a)

I

-5

(A)

-4

10

DS

10

DS

(A)

following.

I

+1.10 V

-10

10

-10

10

-10

10

-11

10

-11

10

-11

10

-12

10

-12

10

-12

-20 -15 -10 -5 0 5 V (V)

10 15 20

G

-6

10

-7

10

-8

10

-9

D

10

10

-10

10

-11

10

-12

NBTS at 60 C 0s 100s 600s 1200s 3600s 7200s -0.71 V

-20 -15 -10 -5 0 5 V (V)

10 15 20

G

-4

(d)

10

o

PBTS at 60 C

-5

10

0s 100s 600s 1200s 3600s 7200s

-6

10

-7

10

(A)

10

-5

-8

DS

-4

10 15 20

G

10

I

10

-20 -15 -10 -5 0 5 V (V)

o

(c)

10

I (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


+1.97 V

(e) PBTS at 60 oC, vacuum 0s 100 s 600 s 1200 s 3600 s 7200 s

-9

10 10

-10

10

-11

+0.72 V

-12

-20 -15 -10 -5 0 5 V (V) G

10 15 20

10 -20 -15 -10 -5 0 5 V (V)

10 15 20

G

Figure 4. Bias stability test of PrIZO-TFT: (a) NBS and (b) PBS at room temperature, (c) NBTS and (d) PBTS at 60℃ in air ambient; (e) PBTS in vacuum ambient.

3.2 The Origin of PBTS The crystallinity of solution process films was firstly investigated using XRD



measurement. The IZO and PrIZO film showed no diffraction peaks indicative of an amorphous state, as shown in Figure 5a, despite of post-annealed as 450℃. Therefore the crystal state of the film would not be the main factor for the thermal stability. The ingredients and chemical states of IZO and PrIZO films were examined by XPS measurement. The back surface region (∼ 2 nm) was etched off by in-situ plasma to remove contamination. Figure 5b shows the C1s XPS spectra of IZO films by hydrolysis and alcoholysis process. The binding energy was calibrated to the C1s peak for the C-C bonds at 285.0 eV and 289.5 eV. It was found that carbon residual barely existed in hydrolysis reacted film as compared with alcoholysis process. It seems possible that carbon residual may be the main factor for poor thermal stability of IZO-TFTs. But there was no distinctive difference of this residual in IZO and PrIZO with alcoholysis process, illustrating that carbon residual status may have a significant effect on thermal property of solution processing film. It was deduced that the Pr may passivate residual carbon to improve thermal stability. 10800 (b)

(a) 2-ME-IZO H2O-IZO PrIZO-0.02 PrIZO-0.08 PrIZO-0.15

160 120

H2O-IZO

600

10200 Counts

200

Counts

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 8 of 19

80

9600

2-ME-IZO PrIZO-0.02 PrIZO-0.08 PrIZO-0.15

400

9000

284

288

292

40

8400 0 10

20

30 40 50 60 Degree (2 therta)

70

80

282 284 286 288 290 292 B.E. (eV)

Figure 5. (a) XRD and (b) XPS spectra of C1s in the films by hydrolysis and alcoholysis.

The O1s XPS spectra of different films were also present in Figure 6. The peaks were deconvoluted into three regions being relevant to stoichiometric M-O bonding (OI, 529.5 eV), oxygen-deficient (OII, 531.0 eV) and loosely bond oxygen or hydroxyl (OIII, 532.1 eV). 26 The ration of oxygen related defects OII/(OI+OII+OIII) existed in IZO film was 32.14%. But the proportion was decreased from 27.41% (0.02 Pr) to 20.87% (0.08 Pr), owning to stronger binding energy of Pr-O than In-O. Maybe that is the reason of decreased mobility in PrIZO-TFTs, which shows less carrier concentration that correlated to oxygen vacancy. Figure 6b and 6c show In3d and Zn2p XPS spectra of alcoholysis films,27,28 which did not show any shift of binding energy regardless of IZO or PrIZO film. Figure 6d shows Pr3d spectra of PrIZO films


Page 9 of 19

with different doping ratios. There are two distinct bonds corresponding with the spin-orbit doublet Pr3d3/2 and Pr3d5/2, and a well-defined shoulder satellite peak at side of lower binding energy.29,30 These peaks were precisely in agreement with

0.6

2-ME-IZO H O-IZO 2 PrIZO-0.02 PrIZO-0.08 PrIZO-0.15

0.4

1 0.8 0.6

(b) In3d

2-ME-IZO H O-IZO 2 PrIZO-0.02 PrIZO-0.08 PrIZO-0.15

0.4 0.2

0.2 0 527 528 529 530 531 532 533 534 535 B.E. (eV)

0 440 442 444 446 448 450 452 454 B.E. (eV)

1 0.9 0.8 0.7 0.6 0.5

(c)

1.1

Zn2p 2-ME-IZO H O-IZO 2 PrIZO-0.02 PrIZO-0.08 PrIZO-0.15

0.4 0.3 0.2 1015 1020 1025 1030 1035 1040 1045 1050 B.E. (eV)

(d) 1 Pr3d

Intensity (a. u.)

0.8

(a) O 1s

Intensity (a. u.)

1

Intensity (a. u.)

previous reports,31 implying a Pr3+ oxidation state and a lack of valence variation. Intensity (a. u.)

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


PrIZO-0.02 PrIZO-0.08 PrIZO-0.15

0.9 0.8 0.7 0.6 0.5 920 925 930 935 940 945 950 955 960 B.E. (eV)

Figure 6. XPS spectra of (a) O1s, (b) In3d, (c) Zn2p and (d) Pr3d in IZO/PrIZO films.

More in-depth analysis was necessary to conduct on the thermal stability of PrIZO-TFTs, which was suggested to relate with many factors. It was speculated that the physical degradation of PBTS-induced shift in IZO-TFTs was originated from the following reasons: (1) field-induced adsorption/desorption process; (2) deep energy level; (3) thermal-generated traps. Metal oxide semiconductor materials were quite sensitive to the oxygen and water molecules in the ambient atmosphere32,33, which were believed to affect the transistor properties, the adsorption/desorption dynamics of these molecules onto the exposed back-channel region of the oxide transistor might play an important role during bias stress, All the devices were encapsulated with a dense film of SiO2 that can weaken the back-channel adsorption/desorption phenomenon. Moreover, the PBTS measurement executed under vacuum condition, just exhibits a modest improvement, but far away from the optimum. It was indicated that adsorption/desorption process was not the main reason for instable PBTS performance for undoped IZO-TFTs. Figure 7a and 7b show the recovery status of the being biased IZO-TFTs with alcoholysis and hydrolysis process. In the recovery characteristics, the ∆Von lowers quickly at the early stage of recovery and saturates to a certain value, finally regaining their initial state over time. This fast recovery component may include several mechanisms and requires further decomposition, but it seems not likely that deep energy level will dominate the PBTS-induced shift. In other words, the shallow tail state is of more concern than the deep state that leading to unstable carrier traps. Therefore, the ∆Von contribution caused by the ACS Paragon Plus Environment


PBTS-induced shift in density of state (DOS) of channel layer was measured to further clarify the mechanism. The DOS was calculated by using P. M method,34 which expounded that the trap states relevant with the flat belt voltage Vfb and free electron density n0 could be extracted according to the I-V and C-V measurement.35 Figure 7c and 7d show the C-V measurement of the devices before and after PBTS test at the frequency of 10 kHz. It can be seen in Figure 7e and 7f that DOS was increased dramatically in alcoholysis process IZO-TFTs after biased stress, while there was a slight change in hydrolysis devices. As considering the energy distribution nearby the conduction band (E-EC=0 eV), it can be seen that DOS was increased dramatically in alcoholysis process IZO-TFTs after biased stress (from 2.68×1016 cm-3eV-1 to 3.69×1017), while there was a slight change in hydrolysis devices (from 2.74×1017 cm-3eV-1 to 1.70×1017 cm-3eV-1). As a result, the thermal enhancement of solution process thin film transistors was likely ascribed to thermal generated traps. 10 (b)

II 2-ME-IZO

8

I: Stress VG/VD=20/0

6

II: Recovery VG/VD=0/0

4

Delta Vth (V)

Delta Vth (V)

10 (a) I

0

H O-IZO 2

8 6 II

I 4

I: Stress VG/VD=20/0

2

2 0

3 10

0

1000020000300004000050000 Time (s) -10

-10

10

-10

0

3 10

2-ME-IZO

W/L=500/250um Freq.=10K

2 10

II: Recovery VG/VD=0/0 1000020000300004000050000 Time (s)

-10

(c)

Capacitance (F)

Capacitance (F)

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 10 of 19

before stress after stress

(d)

H O-IZO

W/L=500/250um Freq.=10K -10

2 10

2

before stress after stress

-10

-20 -15 -10 -5 0 5 V (V) G

10 15 20

10 -20 -15 -10 -5 0 5 V (V)


G

10 15 20

Page 11 of 19

2-ME-IZO

(e)

10 DOS (cm-3eV-1)

18

10 DOS (cm-3eV-1)

Before Stress After Stress

17

10

16

10

15

10

-2

-1.5

-1 E-E (eV)

-0.5

0

(f)

18

H O-IZO 2

Before stress After stress 10

17

10

16

10

15

-2

-1.5

C

-1 E-E (eV)

-0.5

0

C

Figure 7. (a-b) The recovery of the being biased IZO-TFTs; (c-d) The C-V curves

and (e-f) calculated DOS of the alcoholysis and hydrolysis IZO-TFTs during PBTS test. To further confirm the statues of thermal-generated traps during PBTS measurement, the carbon residual in IZO and PrIZO film was characterized using FTIR. As shown of FTIR spectra in Figure 8a, the solvent is not fully volatized after pre-bake process (120 oC). Some of the organic residues could be observed: the peaks of 1080 cm-1 and 1400 cm-1 were C-O bond of the solvent; the peaks of 1520 cm-1 and 1620 cm-1 were the C=O bond. But the solvent was completely volatilized once being annealed at high temperature, as shown in Figure 8b. It was found that the peak 1080 cm-1 was disappeared, while the intensity of 1400 cm-1 was decreased. It was worth noting that C=O bond at 1520 cm-1 in alcoholysis IZO shifts towards higher wavelengths (~1580 cm-1) due to a stronger electron-withdrawing groups as compared PrIZO film. (b)

0.15Pr 0.08Pr 0.02Pr IZO

1400 1620

1080

1520

1050 1200 1350 1500 1650 1800 -1 Wavelength (cm )

Transmittance (a.u.)

(a) Transmittance (a.u.)

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


0.15Pr 0.08Pr 0.02Pr 1520 1080

1400

1580

IZO

1050 1200 1350 1500 1650 1800 -1 Wavelength (cm )

Figure 8. FTIR spectra of IZO/PrIZO film annealed at (a) 120 oC and (b) 450 oC.

During the formation of IZO and PrIZO film in solution process, the following reaction as shown in Figure. 9 was proposed with respect to external heat supply: (Ⅰ)at 130℃, the alcoholic hydroxyl group would substitute part of the nitrate and



then polymerized, with the formation of alkoxy metal polymer. (Ⅱ)at 200 °C, the alcoholic hydroxyl substitute all the nitrate to produce fully-substituted metal polymers with alkoxy. (Ⅲ)at 400 °C, metal-hydroxy compounds were produced during the alcoholysis process of the metal alkoxide. But the alkoxy groups cannot be completely alcoholysised due to a large steric hindrance, with the residues of -CH2CH2OCH3 embedded. (Ⅳ)at 450 ℃, metal oxides were generated in the dehydration and condensation of metal-hydroxy compounds.

Figure 9. The reaction of metal oxide molecular precursors with respect to external heat supply.

In the IZO film with alcoholysis process, the -OCH3 and -CH2- (linked to -OCH3) with strong electron-donor properties were easily oxidized to carboxylic acid groups in high temperature, as manifested in the result of FTIR, which shows a higher wavelengths of C=O bond; These carboxy groups with strong electron-withdrawing property would trap electron during thermal stress, finally generating a thermodynamically-stable 5-member metal ring, as shown in Figure 10a. During the recovery process, the steric hindrance and tension of this 5-member ring will lead to the rupture and produce carboxylic acid. But as for the PrIZO film, the -OCH3 and


Page 12 of 19

Page 13 of 19 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


-CH2- as shown in Figure. 10b is scarcely oxidized to carboxylic acid groups, replaced it, being intended to be oxidized with a more stable ester group under the passivation effect of PrOx.36,37 That is the reason C=O bond in PrIZO film has relatively lower wavelength with weakened electron-withdrawing characteristic, resulting into more stable PBTS performance. O2

M

(a) IZO: M-OCH2 CH2 OCH3

O

HO

(b) PrIZO: M-OCH2 CH2 OCH 3

O2 PrOx

stress O

M

O

HO

M O

-

O

O

HO

M O-

O

O

M-OCH 2COOCH3

Figure 10. The electron-withdrawing characteristic during PBTS measurement of (a) IZO and (b) PrIZO based thin film transistors.

The last thing to notice, thermal-induced stability of IZO-TFTs can be improved by hydrolysis process, but it was still inferior to the performance of alcoholysis processed PrIZO-TFTs. It was speculated that dispersion of organic solution and film roughness should be the factor for deteriorated PBTS. The surface roughness root mean square (RMS) of IZO film was 0.491±0.003 and 0.433±0.001 for the hydrolysis and alcoholysis respectively. In order to more accurately describe the AFM measurement, the errors during the AFM test have been added to the RMS values, which picks up five different points for a sample. But as for the doping PrIZO film, the RMS was relatively improved to be 0.394±0.002 nm, as shown of the AFM images in Figure 11. It was indicated that there are more carrier capture centers in IZO film than PrIZO owning to more rough surface morphology, which might play an important role during the thermal stress. Therefore the doping of PrIZO was considered the most effective method to improve PBTS in solution process thin film transistors.

Figure 11. AFM image of (a) 2-ME IZO, (b) H2O-IZO, and (c) PrIZO film.



4. Conclusion In summary, the solution-process praseodymium doped InZnO thin film transistors was successfully fabricated with improved thermal stability. The praseodymium dopants could well control the carrier concentration with a decent transfer property. The as fabricated PrIZO-TFTs showed enhanced device performance under positive bias temperature stress as comparison of undoped IZO-TFTs, with PBTS shift of 1.97 V and 9.52 V for 7200 s. It was indicated that carbon residual will dominate the drift mechanism against thermal-induced instability, but this organic remains can be well retarded by the rare earth of praseodymium. It is feasible for AOS-TFTs and has the potential to reduce the manufacturing cost of oxide semiconductor-based TFT backplanes by this simple and easy solution process.

Acknowledgements The authors are grateful to the National Basic Research Program of China (Grant No. 2015CB655000) the National Natural Science Foundation of China (Grant No.51502093), Science and Technology Program of Guangdong Province (2016B09090-6002, 2017B090901006), Pearl River S&T Nova Program of Guangzhou (No. 201610010052, 201710010066, 201806010090) and Tip-top Scientific and Technical Innovative Youth Talents of Guangdong Special Support Program (No. 2015TQ01C777, 2016TQ03C331). The Fundamental Research Funds for the Central Universities (2017MS042, 2017ZD059, 2017MS008). Industrial Technology Research and Development Funds for Science and Technology Program of Guangzhou (No. 201802020036).

References: (1) Nomura, K.; Ohta, H.; Takagi, A.; Kamiya, T.; Hirano, M.; Hosono, H. Room-Temperature Fabrication of Transparent Flexible Thin-Film Transistors Using Amorphous Oxide Semiconductors. Nature 2004, 432 (7016), 488–492.


Page 14 of 19

Page 15 of 19 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


(2) Rim, Y. S.; Lim, H. S.; Kim, H. J. Low-Temperature Metal-Oxide Thin-Film Transistors Formed by Directly Photopatternable and Combustible Solution Synthesis. ACS Appl. Mater.

Interfaces 2013, 5 (9), 3565–3571. (3) Banger, K. K.; Yamashita, Y.; Mori, K.; Peterson, R. L.; Leedham, T.; Rickard, J.; Sirringhaus, H. Low-Temperature, High-Performance Solution-Processed Metal Oxide Thin-Film Transistors Formed by a 'Sol-Gel on Chip' Process. Nature Materials 2011, 10, 45–50. (4) Olziersky, A.; Barquinha, P.; Vilà, A.; Magaña, C.; Fortunato, E.; Morante, J. R.; Martins, R. Role of Ga2O3-In2O3-ZnO Channel Composition on the Electrical Performance of Thin-Film Transistors. Mater. Chem. Phys. 2011, 131(1–2), 512–518. (5) Nayak, P. K.; Busani, T.; Elamurugu, E.; Barquinha, P.; Martins, R.; Hong, Y.; Fortunato, E. Zinc Concentration Dependence Study of Solution Processed Amorphous Indium Gallium Zinc Oxide Thin Film Transistors Using High-k Dielectric. Appl. Phys. Lett. 2010, 97(18), 183504. (6) Liu, A.; Guo, Z. D.; Liu, G. X; Zhu, C. D.; Zhu, H. H.; Shin, B.; Fortunato, E.; Martins, R.; Shan, F. K. Redox Chloride Elimination Reaction: Facile Solution Route for Indium-Free, Low Voltage, and High-Performance Transistors. Adv. Electron. Mater. 2017, 3(3), 1600513. (7) Jeong, S.; Lee, J.-Y.; Lee, S. S.; Seo, Y.-H.; Kim, S.-Y.; Park, J.-U.; Ryu, B.-H.; Yang, W.; Moon, J.; Choi, Y. Metal Salt-Derived In–Ga–Zn–O Semiconductors Incorporating Formamide as a Novel Co-Solvent for Producing Solution-Processed, Electrohydrodynamic-Jet Printed, High Performance Oxide Transistors. J. Mater. Chem. C 2013, 1, 4236–4243. (8) Yoon, J.-Y.; Kim, Y. H.; Ka, J.-W.; Hong, S.-K.; Yi, M. H.; Jang, K.-S. A High-Temperature Resistant Polyimide Gate Insulator Surface-Modified with a YOx Interlayer for High-Performance, Solution-Processed Li-doped ZnO Thin-Film Transistors. J. Mater. Chem. C 2014, 2, 2191–2197.



(9) Martins, R.; Almeida, P.; Barquinha, P.; Pereira, L.; Pimentel, A.; Ferreria, I.; Fortunato, E. Electron Transport and Optical Characteristics in Amorphous Indium Zinc Oxide Films. J.

Non-Cryst. Solids 2006, 352 (9–20), 1471–1474. (10) Fortunato, E.; Barquinha, P.; Goncalves, G.; Pereira, L.; Martins, R. High Mobility and Low Threshold Voltage Transparent Thin Film Transistors Based on Amorphous Indium Zinc Oxide Semiconductors. Solid-State Electron. 2008, 52 (3), 443–448. (11) Suresh, A.; Muth, J. Bias Stress Stability of Indium Gallium Zinc Oxide Channel Based Transparent Thin Film Transistors. Appl. Phys. Lett. 2008, 92 (3), 033502. (12) Barquinha, P.; Pimentel, A.; Marques, A.; Pereira, L.; Martins, R.; Fortunato, E. Effect of UV and Visible Light Radiation on the Electrical Performances of Transparent TFTs Based on Amorphous Indium Zinc Oxide. J. Non-Cryst. Solids. 2006, 352 (9–20), 1756–1760. (13) Liu, P.; Chou, Y.; Teng, L.; Li, F.; Shieh, H. Nitrogenated Amorphous InGaZnO Thin Film Transistor. Appl. Phys. Lett. 2011, 98 (5), 052102. (14) Su, B.; Cheng, A.; Wu, J.; Lin, C.; Tang, J.; Chu, S.; Juang, Y. UV-Ozone Process for Film Densification of Solution-Processed InGaZnO Thin-Film Transistors. J. Disp. Technol. 2015, 11 (1), 6–12. (15) Milazzo, G.; Caroli, S. Tables of Standard Electrode Potentials 1978. (16) Lan, L. F.; Peng, J. B. High-Performance Indium–Gallium–Zinc Oxide Thin-Film Transistors Based on Anodic Aluminum Oxide. IEEE Trans. Electron Devices 2011, 58 (5), 1452–1455. (17) Kiazadeh, A.; Salgueiro, D.; Branquinho, R.; Pinto, J.; Gomes, H. L.; Barquinha, P.; Martins, R.; Fortunato, E. Operational Stability of Solution Based Zinc Tin Oxide/SiO2 Thin Film Transistors under Gate Bias Stress. APL MATERIALS 2015, 3 (6), 062804.


Page 16 of 19

Page 17 of 19 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


(18) Chen, T.-C.; Chang, T.-C.; Tsai, C.-T.; Hsieh, T.-Y.; Chen, S.-C.; Lin, C.-S.; Hung, M.-C.; Tu, C.-H.; Chang, J.-J.; Chen, P.-L. Behaviors of InGaZnO Thin Film Transistor under Illuminated Positive Gate-Bias Stress. Appl. Phys. Lett. 2010, 97 (11), 112104. (19) Choi, C.-H.; Han, S.-Y.; Su, Y.-W.; Fang, Z.; Lin, L.-Y.; Cheng, C.-C.; Chang, C.-H. Fabrication of High-Performance, Low-Temperature Solution Processed Amorphous Indium Oxide Thin-Film Transistors Using a Volatile Nitrate Precursor. J. Mater. Chem. C 2015, 3, 854–860. (20) Liu, A.; Liu, G. X.; Zhu, H. H.; Xu, F.; Fortunato, E.; Martins, R.; Shan, F. K. Fully Solution-Processed Low-Voltage Aqueous In2O3 Thin-Film Transistors Using an Ultrathin ZrOx Dielectric. ACS Appl. Mater. Interfaces 2014, 6 (20), 17364–17369. (21) Huang, G.; Duan, L.; Zhao, Y.; Dong, G.; Zhang, D.; Qiu, Y. Enhanced Mobility of Solution-Processed Polycrystalline Zinc Tin Oxide Thin-Film Transistors via Direct Incorporation of Water into Precursor Solution. Appl. Phys. Lett. 2014, 105 (12), 122105. (22) Werner, J. H. Origin of Curved Arrhenius Plots for the Conductivity of Polycrystalline Semiconductors. Sol. Stat. Phenomena 1994, 37–38, 213–218. (23) Werner, J. H.; Güttier, H. H. Barrier Inhomogeneities at Schottky Contacts. J. Appl. Phys. 1991, 69, 1522–1533. (24) Kamiya, T.; Nomura, K.; Hosono, H. Electronic Structures Above Mobility Edges inCrystalline and Amorphous In-Ga-Zn-O: Percolation Conduction Examined by Analytical Model. J. Disp. Technol. 2009, 5(12), 462–467. (25) Kamiya, T.; Nomura, K.; Hosono, H. Origins of High Mobility and Low Operation Voltage of Amorphous Oxide TFTs: Electronic Structure, Electron Transport, Defects and Doping. J. Disp.

Technol. 2009, 5(7), 273–288.



Page 18 of 19

(26) Haming, M.; Issanin, A.; Walker, D.; Seggern, H. von; Jagermann, W.; Bonrad, K. Interrelation

between

Chemical,

Electronic,

and

Charge

Transport

Properties

of

Solution-Processed Indium–Zinc Oxide Semiconductor Thin Films. J. Phys. Chem. C 2014, 118 (24), 12826–12836. (27) Szorenyi, T.; Laude, L.; Bertoti, I.; Geretovszky, Z.; Kantor, Z. Low-Fluence Excimer Laser Irradiation-Induced Defect Formation in Indium-Tin Oxide Film. Appl. Surf. Sci. 1996, 96–98, 363–369. (28) Kim, G.; Kim, H.; Shin, H.; Ahn, B.; Kim, K.; Kim, H. Inkjet-Printed InGaZnO Thin Film Transistor. Thin Solid Films 2009, 517 (14), 4007–4010. (29) Schmeisser, D. The Pr2O3/Si(0 0 1) interface. Mater. Sci. Semicond. Process. 2003, 6 (1–3), 59–70. (30) Ogasawara, H.; Kotani, A.; Potze, R.; Sawatzky, G.; Thole, B. Praseodymium 3d- and 4d-Core Photoemission Spectra of Pr2O3. Phys. Rev. B 1991, 44 (11), 5465. (31) Mekki, A.; Ziq, K.; Holland, D.; McConville, C. Magnetic Properties of Praseodymium Ions in Na2O–Pr2O3–SiO2 Glasses. J. Magn. Magn. Mater. 2003, 260 (1–2), 60–69. (32)

An, S.;

Mativenga, M.;

Kim,

Y.;

Jang, J.;

Improvement

of

Bias-Stability

in

Amorphous-Indium-Gallium-Zinc-Oxide Thin-Film Transistors by Using Solution-Processed Y2O3 Passivation. Appl. Phys. Lett. 2014, 105 (05), 053507. (33) Jeong, J. K.; Yang, H. W.; Jeong, J. H.; Mo, Y.-G.; Kim, H. D. Origin of Threshold Voltage Instability in Indium-Gallium-Zinc Oxide Thin Film Transistors. Appl. Phys. Lett. 2008, 93 (12), 123508. (34) Migliorato, P.; Seok, M.; Jang, J. Determination of Flat Band Voltage in Thin Film Transistors:


Page 19 of 19 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


The Case of Amorphous-Indium Gallium Zinc Oxide. Appl. Phys. Lett. 2012, 100 (7), 073506. (35) Wu, W.; Chen, C.; Hu, X.; Xia, X.; Zhou, L.; Xu, M.; Wang, L.; Peng, J. Analytical Extraction Method for Density of States in Metal Oxide Thin-Film Transistors by Using Low-Frequency Capacitance–Voltage Characteristics. J. Disp. Technol. 2016, 12 (9), 888–891. (36) Ocaña, M.; Caballero, A.; González-Elıṕ e, A. R.; Tartaj, P.; Serna, C. J. Valence and Localization of Praseodymium in Pr-Doped Zircon. J. Solid State Chem. 1998, 139 (2), 412–415. (37) Wang, R. Y.; Lu, G. M.; Qiao, W. M.; Sun, Z.; Zhuang, H. Z.; Yu, J. G. Catalytic Effect of Praseodymium Oxide Additive on the Microstructure and Electrical Property of Graphite Anode. Carbon 2015, 95, 940–948.

Graphical abstract IZO: M-OCH 2CH2 OCH3

O2

M

O

HO

stress

M

O

PrIZO: M-OCH2 CH2 OCH3

O

HO

O2 PrOx

M O-

O

O

HO

M O-

M-OCH2 COOCH3


O

O

Improving Thermal Stability of Solution Process Indium Zinc Oxide

Recommend Documents