Improving Thermal Stability of Solution Process Indium Zinc Oxide

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Cite This: ACS Appl. Mater. Interfaces 2018, 10, 28764−28771

Improving Thermal Stability of Solution-Processed 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† †

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Institute of Polymer Optoelectronic Materials and Devices, State Key Laboratory of Luminescent Materials and Devices, South China University of Technology, Guangzhou 510640, PR China ‡ Guangzhou New Vision Optoelectronic Co., Ltd., Guangzhou 510530, PR China 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/V s, a subthreshold swing value of 0.25 V/decade, and an Ion/Ioff ratio of 108. The as-fabricated PrIZO-TFTs showed an 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 a 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 the 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 because of its merits of low cost, simple process, and easy control of the compounds’ composition.1−3 Solution process can be doped uniformly and 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 are usually investigated and considered as channel materials4−6 because it exhibits high mobility, transparency, and high switching ratio.7−10 However, one of the obstacles in the applications of solution-processed amorphous oxide TFTs is the electrical stability under variable operating conditions, such as gate bias stress, light, and thermal stress.11,12 Research progress includes the report of Liu et al. stating that nitrogenation of amorphous IGZO-TFTs could improve gate-bias stability by reducing oxygen reaction.13 Juang et al. have 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 research studies 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 © 2018 American Chemical Society

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-the-art sputtering technique. In this work, we have mainly studied the mechanism of thermal stability affecting positive bias temperature stress (PBTS) performance by doping rare-earth elements 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 praseodymium-doped IZO (PrIZO) (0.1 M) 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-MEbased solution, the monoethanolamine and acetic acid were added as a stabilizer. As a comparison, the precursor for aqueous route was also prepared by dissolving raw materials and deionized water in order. The molar ratio of In/Zn was Received: May 10, 2018 Accepted: August 3, 2018 Published: August 3, 2018 28764

DOI: 10.1021/acsami.8b07612 ACS Appl. Mater. Interfaces 2018, 10, 28764−28771

Research Article

ACS Applied Materials & Interfaces

Figure 1. Output (a) and transfer (b) characteristics of solution-processed 2-ME-based IZO-TFTs; (c) bias stability test of alcoholysis-processed IZO-TFTs.

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

fixed at 2.5:1, 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 h and filtered using a 0.44 μm syringe filter. 2.2. Device Fabrication. An aluminum (Al) gate electrode was formed by magnetron sputtering with a thickness of 300 nm and then anodized to produce a 200 nm thick layer of Al2O3 as a dielectric. The measured capacitance (Cox) of Al2O3 was ∼41 nF/cm2 with a dielectric constant of ∼9.16 Bottomcontacted indium tin oxide as source/drain was formed by sputtering. Then, these substrates were cleaned using a UV− ozone treatment (420 nm) to increase adhesion. The IZO or PrIZO precursor was spin-coated and prebaked and finally postannealed at 450 °C. The channel width/length (W/L) as 500/250 μm was patterned by commercial acid etchant. All of the devices were passivated with 300 nm thick SiO2 layer by plasma-enhanced chemical vapor deposition and were annealed at 350 °C. For each experimental condition, 20 samples were fabricated to verify the process repeatability. The performance of specified devices has a good representation of this unique process because it takes a variety of statistics from all of 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 an 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 (Shimadzu, IRPrestige-21) was used to investigate the chemical composition 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 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 is 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 solutionprocessed TFTs have 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 was performed with VGS/VDS as −20/0 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 and 1.84 V, respectively, which were quite familiar to the published results.17 As for the NBTS/PBTS, the Von shift was −1.23 and 9.52 V, respectively, whereas the constant temperature was fixed at 60 °C. It means that the NBTS performance is reasonable, but the thermal-induced bias stress is more severe in the case of PBTS performance in solution process devices. The PBTS measurement was also carried out under vacuum conditions 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 that the Von shift was still obvious with a 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 irrespective of 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 preparation method of semiconductor channel with solution process. 28765


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ACS Applied Materials & Interfaces Table 1. Summarized Properties of the Alcoholysis and Hydrolysis IZO-TFTsa InZnO

Vth (V)

μsat (cm2/V s)

SS (V/dec)

Ion/Ioff × 107

NBS shift

NBTS shift

PBS shift

PBTS (air)

PBTS (vac.)

alcoholysis hydrolysis

1.34 1.05

11.21 12.35

0.22 0.36

2.1 1.5

−0.81 −1.56

−1.23 −0.81

1.84 1.20

9.52 4.12

6.90 3.01

a

The basic parameters are average values of the designated 20 samples with good uniformity.

Residual organic solvent solutions are considered to be major contributors to the poor thermal stability of solutionprocessed 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 were fabricated in comparison to alcoholysis process. Figure 2a,b shows 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 was 4.12 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 PBS for alcoholysis solution TFTs by the consideration of removal or passivation of these organic residuals. 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

Table 2. Summarized Properties of the Doping PrIZOTFTsa In/Zn/Pr ratio

Vth (V)

Von (V)

μsat (cm2/V s)

μlin (cm2/V s)

SS (V/dec)

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

1.37 1.56 2.12 3.05

−1.94 −1.84 −0.79 0.36

11.20 10.10 6.02 3.31

18.60 14.40 9.82 6.32

0.22 0.25 0.31 0.36

a

The basic parameters are average values of the designated 20 samples with good uniformity.

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 Therefore, 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 have lower carrier concentration than that of IZO because of carrier suppression. As for the exploration of the biased stress performance, herein, we have mainly focused on PrIZO-TFTs with lower concentrations in solution-processed 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, −0.71, 1.10, and 1.97 V, respectively, 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 also be discussed broadly in the following. 3.2. Origin of PBTS. The crystallinity of solution-processed films was first investigated using XRD measurement. The IZO and PrIZO film showed no diffraction peaks indicative of an amorphous state, as shown in Figure 5a, despite postannealed as 450 °C. Therefore, the crystal state of the film would not be the main factor for 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 C 1s XPS spectra of IZO films by hydrolysis and alcoholysis process. The binding energy was calibrated to the C 1s peak for the C−C bonds at 285.0 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. However, there was no distinctive difference of this residual in IZO and PrIZO with alcoholysis process, illustrating that the 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.

Figure 3. Transfer characteristics of the PrIZO-TFTs.

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 subthreshold swing (SS) were 10.10 ± 0.20 cm2/V s 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 of IDS versus the VDS in a saturated operation region. The 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/V s and 0.31 ± 0.02 V/dec, whereas 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 28766


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

shoulder satellite peak at the side of lower binding energy.29,30 These peaks were precisely in agreement with previous reports,31 implying a Pr3+ oxidation state and a lack of valence variation. 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; and (3) thermal-generated traps. Metal−oxide semiconductor materials were quite sensitive to the oxygen and water molecules in the ambient atmosphere,32,33 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. But all of the fabricated devices in our experiment 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,b shows the recovery status of the being-biased IZO-TFTs with alcoholysis and hydrolysis processes. In the recovery characteristics, the ΔVon lowers quickly at the early stage of recovery and saturates to a certain value, finally regaining their initial state over time.

Figure 5. (a) XRD and (b) XPS spectra of C 1s in the films by hydrolysis and alcoholysis.

The O 1s XPS spectra of different films are 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 bound oxygen or hydroxyl (OIII, 532.1 eV).26 The ratio of oxygen-related defects OII/(OI + OII + OIII) existing in IZO film was 32.14%. However, the proportion was decreased from 27.41% (0.02 Pr) to 20.87% (0.08 Pr), owing 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 with oxygen vacancy. Figure 6b,c shows In 3d and Zn 2p 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 Pr 3d spectra of PrIZO films with different doping ratios. There are two distinct bonds corresponding with the spin−orbit doublet Pr 3d3/2 and Pr 3d5/2 and a well-defined

Figure 6. XPS spectra of (a) O 1s, (b) In 3d, (c) Zn 2p, and (d) Pr 3d in IZO/PrIZO films. 28767


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

CO bond. However, 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 disappeared, whereas the intensity of 1400 cm−1 was decreased. It was worth noting that CO bond at 1520 cm−1 in alcoholysis IZO shifts toward higher wavelengths (∼1580 cm−1) because of a stronger electron-withdrawing groups as compared to PrIZO film. 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 °C, the alcoholic hydroxyl group would substitute a part of the nitrate and then polymerized, with the formation of alkoxy metal polymer. (II) At 200 °C, the alcoholic hydroxyl substitutes all of 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. However, the alkoxy groups cannot be completely alcoholized because of a large steric hindrance, with the residues of −CH2CH2OCH3 embedded. (IV) At 450 °C, metal oxides were generated in the dehydration and condensation of metal-hydroxy compounds. 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 temperatures, as manifested in the result of FTIR, which shows 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 five-member metal ring, as shown in Figure 10a. During the recovery process, the steric hindrance and tension of this five-member ring will lead to the rupture and produce carboxylic acid. However, as for the PrIZO film, the −OCH3 and −CH2−, as shown in Figure 10b, are 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 a relatively lower wavelength with weakened electron-withdrawing characteristic, resulting into more stable PBTS performance. The last thing to notice, thermal-induced stability of IZOTFTs can be improved by hydrolysis process, but it was still inferior to the performance of alcoholysis-processed PrIZOTFTs. It was speculated that the 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. 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. However, as for the doping PrIZO film, the rms was relatively improved to be 0.394 ± 0.002 nm, as shown in the

Figure 7. (a,b) Recovery of the being-biased IZO-TFTs; (c,d) C−V curves and (e,f) calculated DOS of the alcoholysis and hydrolysis IZO-TFTs during PBTS test.

This fast recovery component may include several mechanisms and require further decomposition, but it does not seem likely that the 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 leads to unstable carrier traps. Therefore, the ΔVon contribution caused by the PBTS-induced shift in the density of state (DOS) of the channel layer was measured to further clarify the mechanism. The DOS was calculated by using the PM 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,d shows 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,f that DOS was increased dramatically in alcoholysis-processed IZO-TFTs after biased stress, whereas there was a slight change in hydrolysis devices. Considering the energy distribution nearby the conduction band (E − EC = 0 eV), it can be seen that DOS was increased dramatically in alcoholysis-processed IZO-TFTs after biased stress (from 2.68 × 1016 to 3.69 × 1017 cm−3 eV-1), whereas there was a slight change in hydrolysis devices (from 2.74 × 1017 to 1.70 × 1017 cm−3 eV−1). As a result, the thermal enhancement of solution-processed TFTs was likely ascribed to thermal-generated traps. To further confirm the status 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 volatilized after prebake process (120 °C). Some of the organic residues could be observed: the peaks of 1080 and 1400 cm−1 were C−O bond of the solvent; the peaks of 1520 and 1620 cm−1 were the 28768


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Figure 9. Reaction of metal−oxide molecular precursors with respect to external heat supply.

Figure 10. Electron-withdrawing characteristics during PBTS measurement of (a) IZO- and (b) PrIZO-based TFTs.

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

AFM images in Figure 11. It was indicated that there are more carrier capture centers in IZO film than PrIZO owing 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-processed TFTs.

potential to reduce the manufacturing cost of oxide semiconductor-based TFT backplanes by this simple and easy solution process.

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected] (W.W.). *E-mail: [email protected] (M.X.).

4. CONCLUSIONS In summary, the solution-processed praseodymium-doped InZnO TFTs 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 PBTS in comparison with that of undoped IZO-TFTs, with PBTS shift of 1.97 and 9.52 V for 7200 s. It was indicated that the 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

ORCID

Jianhua Zou: 0000-0002-5262-7062 Miao Xu: 0000-0001-6227-9287 Junbiao Peng: 0000-0003-1671-2750 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS 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), 28769


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Science and Technology Program of Guangdong Province (2016B09090-6002 and 2017B090901006), Pearl River S&T Nova Program of Guangzhou (nos. 201610010052, 201710010066, and 201806010090) and Tip-top Scientific and Technical Innovative Youth Talents of Guangdong Special Support Program (nos. 2015TQ01C777 and 2016TQ03C331). The Fundamental Research Funds for the Central Universities (2017MS042, 2017ZD059, and 2017MS008). Industrial Technology Research and Development Funds for Science and Technology Program of Guangzhou (no. 201802020036).

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Improving Thermal Stability of Solution Process Indium Zinc Oxide

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