Improvement of Electrical Characteristics and ... - ACS Publications

Subscriber access provided by Fudan University

Article

Improvement of electrical characteristics and stability of amorphous indium gallium zinc oxide thin film transistors using nitrocellulose passivation layer Kwan Yup Shin, Young Jun Tak, Won-Gi Kim, Seonghwan Hong, and Hyun Jae Kim ACS Appl. Mater. Interfaces, Just Accepted Manuscript • Publication Date (Web): 16 Mar 2017 Downloaded from http://pubs.acs.org on March 16, 2017

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 free 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 accessible to all readers and 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.

ACS Applied Materials & Interfaces 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 26

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

Improvement of electrical characteristics and stability of amorphous indium gallium zinc oxide thin

film

transistors

using

nitrocellulose

passivation layer

Kwan Yup Shin, Young Jun Tak, Won-Gi Kim, Seonghwan Hong, and Hyun Jae Kim*

School of Electrical and Electronic Engineering, Yonsei University, 50 Yonsei-ro, Seodaemun-gu, Seoul 120-749, Republic of Korea

KEYWORDS: Oxide semiconductor, Thin-film transistors, a-IGZO, Passivation, Nitrocellulose

CORESPONDING AUTHORS: [email protected]

ACS Paragon Plus Environment 1


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

Abstract In this research, nitrocellulose is proposed as a new material for the passivation layers of amorphous indium gallium zinc oxide thin film transistors (a-IGZO TFTs). The a-IGZO TFTs with nitrocellulose passivation layers (NC-PVLs) demonstrate improved electrical characteristics and stability. The a-IGZO TFTs with NC-PVLs exhibit improvements in fieldeffect mobility (µFE) from 11.72 ± 1.14 to 20.68 ± 1.94 cm2/Vs, threshold voltage (Vth) from 1.85 ± 1.19 to 0.56 ± 0.35 V, and on/off current ratio (Ion/off) from (5.31 ± 2.19) x 107 to (4.79 ± 1.54) x 108 compared to a-IGZO TFTs without PVLs, respectively. The Vth shifts of aIGZO TFTs without PVLs, with poly (methyl methacrylate) (PMMA) PVLs, and with NCPVLs under positive bias stress (PBS) test for 10,000 s represented 5.08 V, 3.94 V, and 2.35 V, respectively. These improvements were induced by nitrogen diffusion from NC-PVLs to aIGZO TFTs. The lone-pair electrons of diffused nitrogen attract weakly bonded oxygen serving as defect sites in a-IGZO TFTs. Consequently, the electrical characteristics are improved by an increase of carrier concentration in a-IGZO TFTs, and a decrease of defects in the back channel layer. Also, NC-PVLs have an excellent property as a barrier against ambient gases. Therefore, the NC-PVL is a promising passivation layer for next-generation display devices that simultaneously can improve electrical characteristics and stability against ambient gases.


Page 2 of 26

Page 3 of 26

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. Introduction Amorphous oxide semiconductor thin-film transistors (AOS-TFTs) have been extensively researched and developed as an alternative for amorphous silicon (a-Si) TFTs. Especially, amorphous indium-gallium-zinc-oxide (a-IGZO) based TFTs among AOS-TFTs exhibit many advantages such as high field-effect mobility (µFE), a low off current, a low subthreshold swing (S.S), good uniformity, and a high transparency for visible light.1-6 However, a-IGZO TFTs have critical instability issues against light illumination, temperature, bias stress, and ambient gases. Among these instability issues, a-IGZO TFTs have the vulnerability for the threshold voltage (Vth) shifts caused by O2 and H2O, when they are exposed to the atmosphere.7,8 The adsorbed O2 and H2O acted as donor-like states or acceptor-like states shifting Vth of a-IGZO TFTs to a positive or negative direction, respectively. For these reasons, a-IGZO TFTs need passivation layers (PVLs) to prevent adsorption of ambient O2 and H2O. To address these issues, many studies have intensively researched inorganic and organic material PVLs to prevent O2 or H2O adsorption.9 For inorganic materials, SiO2, Al2O3, and Y2O3 have usually been used as PVLs.10-13 However, most PVLs using these inorganic materials are fabricated at a high fabrication temperature using an expensive vacuum process. These fabrication conditions are inadaptable to flexible display devices because they are fabricated on a polymer substrate at low fabrication temperature (< 200oC).9 And, their low flexibility results in a limitation of flexible display devices due to brittle properties. Furthermore, the vacuum process (plasma-based process) causes performance degradation of the TFTs, complicated process, and a high unit cost of production.14,15 In contrast, for PVLs using conventional organic materials such as polydimethylsiloxane (PDMS),16 poly (methyl methacrylate) (PMMA),17,18 and Cytop,9 there have been studies to overcome the above-mentioned limitations of inorganic PVLs. This is because they can be



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

fabricated at a low temperature under 200oC without the vacuum process. However, barrier performances against ambient gas were lower than inorganic PVLs due to a high permeability against gases caused by the intrinsic material properties.15 For these reasons, to overcome the disadvantages of conventional inorganic and organic materials, we suggest nitrocellulose (NC) as a new material for PVLs. NC is made by a nitration process of cellulose [C6H10O5] and its chemical formula is [C6H7(NO2)3O5]. The nitration process means hydroxyl groups (-OH) in cellulose are exchanged by nitro-ester groups (-ONO2-).19 In early research, NC was used for different purposes; the first artificial billiard ball was invented by John Wesley Hyatt using NC in 1865 when billiard balls were being made of elephant tusks, and NC was also used as the ingredients of dynamite invented by Alfred Nobel in 1867. In recent research, NC was employed as membrane substrates for the fabrication of protein chips.20 In this research, we used NC as a PVLs material for a-IGZO TFTs using a solution process at a low fabrication temperature of 120oC. NC-PVLs improved the stability of a-IGZO TFTs as the passivation layer to prevent ambient gases. The nitrogen, which was diffused from NCPVLs to the a-IGZO active layer in the fabrication process, improved the electrical characteristics of a-IGZO TFTs.

2. Experimental methods 2.1 Fabrication process of a-IGZO TFTs Here, a-IGZO TFTs were fabricated with an inverted-staggered structure. For a-IGZO TFTs, heavily boron-doped silicon (p+-Si) was used as the gate electrode and a SiO2 (1200 Å) layer as a gate insulator was grown by thermal oxidation on p+-Si. The a-IGZO thin films (40 nm) on SiO2/p+-Si were deposited using a In2O3:Ga2O3:ZnO target with the composition ratio of 1:1:1. The a-IGZO thin films were deposited by radio frequency (RF) magnetron sputtering at


Page 4 of 26

Page 5 of 26

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


room temperature. The RF power, time, operation pressure, and oxygen partial pressure ([O2]/[Ar+O2]) of RF sputtering were fixed to 150 W, 5 min, 5.0 x 10-3 Torr, and 0%, respectively. For the activation process, a-IGZO thin films were annealed at 300oC for 1 hr on a hot plate in air. Then, source and drain electrodes were deposited using Al sputtering via a shadow mask. The width (W) and length (L) channel were defined as 1,000 µm, and 150 µm, respectively.

2.2 Fabrication process of NC-PVLs To make PVLs using the spin coating method on a-IGZO TFTs, collodion was used as the NC solution, which is a viscous solution of dissolved NC powder in solvent. However, diluted NC solution had to be additionally synthesized because of the high viscosity of collodion. Therefore, the diluted NC solution was synthesized with collodion and ethanol to dilute the viscosity. The volumetric ratios of diluted NC solution were 1:1, 1:2, 1:3, 1:4, 1:5, and 1:10 (collodion : ethanol). The NC-PVLs were deposited by the spin coating method (3,000 rpm for 30 s) with diluted NC solutions. Finally, the post-annealing at 120oC for 5 min in air was conducted to form a NC thin film as a PVLs. Figure 1 shows both the schematic structure and the fabrication process of a-IGZO TFTs with NC-PVLs of inverted-staggered structure. The optimization results for post-annealing condition of NC-PVLs are shown in Figure S1 and Table S1.

2.3 Measurement and analysis method The thickness, an optical transparency at various wavelengths, I-V transfer curves, and a carrier concentration (Nc) of the NC-PVLs were measured by a field emission scanning electron microscopy (FESEM) (JEOL Ltd., JEOL-7800F), a UV-Visible spectrophotometer (JASCO Corp., V-650), a semiconductor parameter analyzer (KEYSIGHT Technologies, HP



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

4156), and Hall measurement (ISTECH, HMS 3000), respectively. Also, the positive bias stability (PBS), positive bias temperature stability (PBTS), and negative bias temperature stability (NBTS) were measured under VGS=20 or -20 V and VDS=10.1 V for 10,000 s at 50oC using a HP 4156 semiconductor parameter analyzer. A change of chemical composition in aIGZO TFTs with NC-PVLs was analyzed by X-ray photoelectron spectroscopy (XPS) (Thermo U. K. K-alpha) and Fourier transform infrared spectrometer (FT-IR) (Bruker, Vertex 70).

3. Results and discussion 3.1 Optimization of NC solution for NC-PVLs Figure 2 represents the formation process of NC by nitration using cellulose and nitrating acid.19 The chemical structure of NC as shown in Figure 2 has a nitro-ester group composed of oxygen and nitrogen. However, due to the solid state of NC at room temperature, NC solution needed to be synthesized to fabricate NC thin film using spin coating. It was determined to use collodion, which is a mixture of NC and ethanol. After synthesizing the NC solution of various dilution ratios between collodion and ethanol as shown in Figure 3a, evaluations were conducted to find the optimized condition of the NC solution for spin coating. The volumetric ratios of the diluted NC solutions were 1:1, 1:2, 1:3, 1:4, 1:5, and 1:10 (collodion:ethanol), respectively. Figure 3b shows surface images of NC thin films coated by spin coating for the several diluted NC solutions. Imaging Uniformity Measurement (IUM) was used to evaluate uniformity of the NC thin films with these images. IUM as programming coded by Open source Computer Vision (OpenCV) is a new method to simply estimate film uniformity. When compiling surface images using this program, the images are converted to images of achromatic color and the dots of these images are digitized as gray levels from 0 to 255. The gray values of the dots in an image are calculated as max,


Page 6 of 26

Page 7 of 26

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


min, mean, and standard deviation. Figure 3c represents schematic graphs for the digitized values of surface images. As shown in the surface images, the NC thin films using the solutions with the volumetric ratios of 1:1, 1:2, 1:3, and 1:4 showed a poor uniformity indicating two or more colors due to the cohesion by the high viscosity, while the NC thin films using the solution with 1:10 volumetric ratio were hardly deposited on the a-IGZO thin films due to the low concentration. However, the NC thin film of 1:5 volumetric ratio has the high uniformity without a cohesion. IUM digitized the uniformity of surface calculating the color distribution in these surface images. Thus, the low standard deviation means the color distribution in surface image is concentrated on the specific gray level. As a result, the surface image of 1:5 volumetric ratio indicated the lowest standard deviation in gray level of surface image. It means that the film deposited by this volumetric ratio is the best uniform. Therefore, the NC-solution of 1:5 volumetric ratio was used to fabricate NC-PVLs using spin coating. Figure 3d shows a cross sectional SEM image of the NC thin film coated by the NC solution with a 1:5 volumetric ratio. The thickness of NC thin film coated by spin coating was 25 nm. Thus, it was found that the NC thin film can be controlled in nanometer scale by controlling the volumetric ratio and spin coating.

3.2 Chemical and optical properties of NC-PVLs Figure 4a shows the XPS surface spectra for NC-PVLs. It was verified that NC-PVLs fabricated in this study were composed of oxygen, nitrogen, and carbon as shown in Figure 1.21 Figure 4b shows the results of UV-visible measurements to analyze transparency of NCPVLs. The transparency of NC-PVLs was compared with PMMA-PVL, which is commonly used for material of organic PVLs. The transmittance of NC-PVLs was 91% in the visible range (350-780 nm) and was similar to that of PMMA-PVLs. Therefore, it was found that NC is capable for materials of transparent devices.22



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

3.3 I-V Transfer curve and electrical parameters measurement To investigate the influence of NC-PVLs, the measurement of electrical characteristics was conducted for a-IGZO TFTs without PVLs and with NC-PVLs. Figure 5 shows the I-V transfer curves of a-IGZO TFTs without PVLs and with NC-PVLs. Table 1 is a representation of the electrical parameters with error range for a-IGZO TFTs without PVLs and with NCPVLs. The a-IGZO TFTs with NC-PVLs demonstrate improvements of the electrical parameters, such as µFE from 11.72 ± 1.14 to 20.68 ± 1.94 cm2/Vs, Vth from 1.85 ± 1.19 to 0.56 ± 0.35 V, and Ion/off from (5.31 ± 2.19) x 107 to (4.79 ± 1.54) x 108. And, Figure S2 shows the error bars of these electrical parameters.

3.4 Hall measurement Hall measurement was conducted to verify whether ethanol, post-annealing, or nitrocellulose in the NC solution is the cause of the improved electrical performance of aIGZO TFTs. Samples were prepared with a-IGZO thin films untreated, dipped in ethanol, annealed after dipped in ethanol, and annealed after passivated by NC. The ethanol on aIGZO thin films dipped in ethanol was removed using N2 gas blowing and the NC on a-IGZO thin films with NC-PVLs was etched using ethanol and N2 gas blowing. Figure 6 shows the results of Hall measurement according to these samples. The inset pictures of Figure 6 show the before and after of dipping in ethanol for a-IGZO thin films and etching for a-IGZO thin films with NC-PVLs. As a result, the Nc of a-IGZO thin films represented a similar level compared to that of a-IGZO thin films dipped in ethanol, because ethanol is a solvent that does not affect the Nc of the a-IGZO thin films.23 And a-IGZO thin films annealed after dipped in ethanol also exhibited the similar results with a-IGZO thin films dipped in ethanol. Even though, in previous research, it was reported that the thermal energy increases a Nc in


Page 8 of 26

Page 9 of 26

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 channel layer, the post-annealing temperature of 120oC used in this research was not enough to induce the carriers like Vos.24 However, the Nc of a-IGZO thin films with NCPVLs increased from (6.08 ± 3.52) x 1015 to (5.44 ± 0.98) x 1017 cm-3 compared to that of aIGZO thin films. Therefore, it was determined that the ethanol in the NC solution and annealing after dipped in ethanol did not have an influence on the Nc of a-IGZO thin films, while NC-PVLs increased the Nc of a-IGZO thin films. Table S2 and Figure S3 show the Nc with error range of these samples and the error bar for the values, respectively. Therefore, improvement of µFE with increased Nc can be explained by the percolation model,25 and the reduction of Vth with an increase of µFE can be interpreted in the following equation (1).26 Especially, the µFE and Vth were computed by fitting straight lines to the plots of the square root of drain current (Id) vs gate-source voltage (Vgs), while the Id values in the saturation region (Vd > Vgs-Vth, Vgs > Vth) are retracted from the following equation (1); Ci is capacitance per unit area of the gate insulator layer.

=

( − ) , ( 2

> − , > )

(1)

Furthermore, as confirmed in previous studies, it was demonstrated here that the improvement of Ion/off was caused by a decrease of defects in the back channel layer and an increase of Nc in the front channel layer.14,26 To investigate this claim specifically, the XPS depth and FT-IR measurement for a-IGZO TFTs without PVLs and with NC-PVLs was implemented.

3.5 XPS analysis Figure 7a, b, and c show N 1s spectra of the XPS depth analysis at back, bulk, and front channel layers of a-IGZO TFTs without PVLs, respectively. Figure 7d, e, and f represent



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

them at back, bulk, and front channel layers of a-IGZO TFTs with NC-PVLs, respectively. The N 1s spectra of a-IGZO TFTs without PVLs and with NC-PVLs were deconvoluted by two peaks, which were represented to Ga auger peak at 396.7 eV and nitrogen-gallium (N-Ga) peak at 397.8 eV based on Gaussian distribution, respectively.27 As a result, the peak of N-Ga of a-IGZO TFTs with NC-PVLs was higher than that of a-IGZO TFTs without PVLs near the back channel layer of a-IGZO TFTs. Therefore, it was discovered that nitrogen in NC-PVLs was gradually diffused into a-IGZO TFTs. The FT-IR analysis was also conducted to verify nitrogen diffusion and the change of chemical composition for a-IGZO TFTs without PVLs and with NC-PVLs.

3.6 FT-IR analysis Figure 8 shows the FT-IR spectra of a-IGZO thin films without PVLs and with NC-PVLs. The FT-IR spectra of a-IGZO thin films with NC-PVLs have the higher peak at wavenumbers of 400-500 cm-1, which is related with nitrogen species, than those of a-IGZO thin films without PVLs.28-32 Therefore, the difference of the FT-IR spectra at wavenumbers of 400-500 cm-1 can be explained by a diffusion of nitrogen from NC-PVLs and indicates the formation of species related with nitrogen such as –MN– and –NMN–; M and N represent metal and nitrogen, respectively. Consequently, from the results of the XPS and FT-IR analyses, it should be noted that nitrogen is diffused from NC-PVLs to IGZO films.

3.7 Improvement of electrical characteristics Nitrogen was diffused from NC-PVLs to a-IGZO TFTs through the XPS depth and FT-IR analyses. Then, as shown in Figure 9a and b, O 1s peaks at back channel layer of a-IGZO without PVLs and with NC-PVLs was investigated to establish a correlation between the electrical improvements and the diffused nitrogen. The O 1s peaks were deconvoluted to 3


Page 10 of 26

Page 11 of 26

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


individual peaks of metal-oxygen (M-O), oxygen vacancy (Vo), and -OH groups, which were represented as a Gaussian distribution at 530 ± 0.4, 531 ± 0.4, and 532 ± 0.4 eV, respectively.6 An increase of M-O bonding and a decrease of Vos in a-IGZO TFTs with NCPVLs was ascertained compared to a-IGZO TFTs without PVLs. These results consequently can be interpreted with the attraction of nitrogen diffused from NC-PVLs to the back channel layer. The lone-pair electrons of nitrogen to bond with metal atoms attract weakly bonded oxygen from the front channel layer to the back channel layer.33 As a result, the increase of Vos in a-IGZO thin films can be explained by the increase of Nc mentioned in the Hall measurement analysis and the on current is improved by the increase of Nc. Also, oxygen attracted by the lone-pair electrons reduces defects such as Vos at the back channel and the off current is decreased by the reduction of defects at back channel.34 Therefore, from the analyses of these results, it was found that Ion/off is improved.

3.8 Stability Test The PBS was measured to confirm the performance of the gas barrier of NC-PVLs. Generally, the oxygen molecules adsorbed on a-IGZO film from ambient air combine with electrons under positive bias stress like the equation (2) and decrease the Nc of a-IGZO TFTs.7,8 As a result, Vth shifts to a positive direction and stability of a-IGZO TFTs is also degraded.

O2 (gas) + e- ↔ 2O- (solid)

(2)

Therefore, samples were supplemented of a-IGZO TFTs with PMMA-PVLs as typical organic PVLs for the verification of NC-PVLs. Figure 10 shows the results of the PBS test for 10,000 s of respective samples, such as a-IGZO TFTs without PVLs, with PMMA-PVLs, and with NC-PVLs. The Vth shifts of a-IGZO TFTs without PVLs, with PMMA-PVLs, and



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

with NC-PVLs were 5.08 V, 3.94 V, and 2.35 V, respectively. The stability of a-IGZO TFTs with NC-PVLs was improved up to 53% and 40% compared to PVLs ability of a-IGZO TFTs without PVLs and with PMMA-PVLs, respectively. Consequently, these results show NCPVLs have excellent barrier properties to prevent adsorption of oxygen and demonstrate an improved stability of a-IGZO TFTs. PBTS and NBTS tests at 50oC were also conducted to verify the influence of temperature for organic PVLs and moisture in ambient air, respectively.35,36 As shown in Figure 11, in the results of the PBTS test for a-IGZO TFTs without PVLs, with PMMA-PVLs, and with NC-PVLs, the values of positive Vth shifts were 5.74 V, 4.77 V, and 3.88 V, respectively. In the results of the PBTS test, the stability of aIGZO TFTs with NC-PVLs was improved up to 32% and 19% compared to PVLs ability of a-IGZO TFTs without PVLs and with PMMA-PVLs, respectively. The a-IGZO TFTs with NC-PVLs have superior stability characteristics under PBTS conditions compared to the other devices. And, Figure S4 shows that the Vth shift of a-IGZO TFTs with NC-PVLs is comparable with that of a-IGZO TFTs with SiO2-PVLs of the same thickness for the PBTS test. Figure 12 shows the results of the NBTS test of a-IGZO TFTs without PVLs and with NC-PVLs. The Vth shifts of a-IGZO TFTs without PVLs and with NC-PVLs were -1.98 V and -1.90 V, respectively. As reported in previous research,36 it was confirmed that the Vth of a-IGZO TFTs is scarcely affected by negative bias stress because a-IGZO TFTs is n-type. Therefore, in this research, influence for negative bias voltage was negligible.

3.9 Mechanism illustration of doping and passivation effect Figure 13 shows the mechanisms for the electrical performance enhanced by nitrogen diffusion and the barrier effect of NC-PVLs. Firstly, for the nitrogen diffusion effect, the nitrogen diffusion from NC-PVLs to a-IGZO films can improve electrical performances of aIGZO TFTs due to the reduction of weakly bonded oxygen and defects related to oxygen


Page 12 of 26

Page 13 of 26

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


species near the back channel layer. Furthermore, nitrogen diffusion induces an increase of Vos near the front channel layer by attracting weakly bonded oxygen due to nitrogen’s lonepair electron properties. Consequently, the reduction of weakly bonded oxygen and defects near the back channel induced a decrease of off-current, and an increase of Vos near the front channel induced increase of Nc resulting in higher mobility compared to a-IGZO TFTs without NC-PVLs. Secondly, for the barrier effect, the NC-PVLs can effectively act as a barrier against adsorption of ambient gases such as O2 and H2O on the back channel layer resulting in lower Vth shift under various bias temperature stresses.

4. Conclusion In this research, NC is proposed as the new PVL material for a-IGZO TFTs, which can make up for the vulnerabilities of conventional organic and inorganic PVLs. NC-PVLs were deposited by spin coating at the low temperature of 120oC without any vacuum processes. As a result, a-IGZO TFTs with NC-PVLs showed outstanding electrical performance. The nitrogen diffusion was verified through N 1s peak of the XPS depth and FT-IR analyses to investigate the cause for the improvement of electrical characteristics. Also, from the increase of M-O bonding and decrease of Vos at the back channel layer verified in O 1s peak, it was confirmed that diffused nitrogen attracts weakly bonded oxygen from the front channel layer to back channel layer. Furthermore, the Vth shifts of a-IGZO TFTs with NC-PVLs under bias stress tests represented the lowest values. So far, it has been confirmed that NC is a useful and excellent material to enhance the electrical performances of a-IGZO TFTs and has excellent properties as passivation to prevent external environment.



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

Figure

Figure 1.

Process to fabricate a-IGZO TFTs with NC-PVLs.

Figure 2. Esterification and nitration reaction of NC by cellulose and nitrating acid.


Page 14 of 26

Page 15 of 26

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


Figure 3. (a) Synthesis process of diluted NC solution. (b) Deposition results of diluted NC solution using spin coating. (c) Comparison for the uniformity of NC thin films spin-coated with NC-solution of the various volumetric ratios using IUM analysis. (d) Result of thickness measurement for 1:5 NC-PVLs.

Figure 4. (a) Measurement result of XPS surface for NC-PVLs. (b) Measurement results of transmittance for glass, PMMA, and NC.



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

Figure 5. I-V transfer curves of a-IGZO TFT without PVLs and with NC-PVLs.

Figure 6. Results of Hall measurement according to a-IGZO TFTs untreated, dipped in ethanol, annealed after dipped in ethanol, and annealed after passivated by NC.


Page 16 of 26

Page 17 of 26

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


Figure 7. N 1s spectra of XPS depth analysis for (a) back channel, (b) bulk channel, and (c) front channel of a-IGZO TFTs without PVLs, and (d) back channel, (e) bulk channel, and (f) front channel of a-IGZO TFTs with NC-PVLs.

Figure 8. FT-IR spectra of a-IGZO TFTs without PVLs and with NC-PVLs.



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

Figure 9. O 1s spectra of XPS depth analysis at back channel layers of a-IGZO TFTs (a) without PVLs and (b) with NC-PVLs.

Figure 10. PBS test results of a-IGZO TFTs (a) without PVLs, (b) with PMMA-PVLs, (c) with NC-PVLs, and (d) comparison of PBS test results.


Page 18 of 26

Page 19 of 26

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


Figure 11. PBTS test results of a-IGZO TFTs (a) without PVLs, (b) with PMMA-PVLs, (c) with NC-PVLs, and (d) comparison of PBTS test results.

Figure 12. NBTS test results of a-IGZO TFTs (a) without PVLs and (b) with NC-PVLs.



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 20 of 26

Figure 13. Mechanism for nitrogen diffusion and barrier effect of NC-PVLs.

Table 1. Electrical parameters with error range of a-IGZO TFTs without PVLs and with NCPVLs.

µFE

Vth

On/off

S.S

2

(cm /Vs)

(V)

ratio

(V/decade)

w/o PVLs

11.72 ± 1.14

1.85 ± 1.19

(5.31 ± 2.19) x 107

0.42 ± 0.03

w/ NC-PVLs

20.68 ± 1.94

0.56 ± 0.35

(4.79 ± 1.54) x 108

0.38 ± 0.04

Samples


Page 21 of 26

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


Author Information Corresponding Authors Hyun Jae Kim: [email protected]

Acknowledgement This work was supported by the Industrial Strategic Technology Development Program (10063038, Development of sub-micro in-situ light patterning to minimize damage on flexible substrates) funded by the Ministry of Trade, Industry & Energy (MOTIE, Korea) and Samsung Display.

Supporting Information Comparison of electrical parameters according to annealing conditions of NC-PVLs; error limits for electrical parameters of a-IGZO TFTs without PVLs and with NC-PVLs; error limits for carrier concentration of untreated, dipped in ethanol, annealed after ethanol dipping, and annealed after NC-PVLs a-IGZO TFTs; PBTS test results of a-IGZO TFTs with SiO2PVLs and with NC-PVLs



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

Reference (1) Hosono, H.; Kikuchi, N.; Ueda, N.; Kawazoe, H. Working Hypothesis to Explore Novel Wide Band Gap Electrically Conducting Amorphous Oxides and Examples. J. Non-Cryst.

Solids 1996, 198-200, 165-169. (2) Nomura, K.; Ohta, H.; Ueda, K.; Kamiya, T.; Hirano, M.; Hosono, H. Thin-Film Transistor Fabricated in Single-Crystalline Transparent Oxide Semiconductor. Science 2003, 300, 1269-1272. (3) Kim, S. J.; Yoon, S.; Kim, H. J. Review of Solution-Processed Oxide Thin-Film Transistors. Jpn. J. Appl. Phys. 2014, 53, 02BA02. (4) Ahn, B. D.; Shin, H. S.; Kim, G. H.; Park, J.-S.; Kim, H. J. A Novel Amorphous InGaZnO Thin Film Transistor Structure without Source/Drain Layer Deposition. Jpn. J. Appl. Phys. 2009, 48, 03B019. (5) Kim, D. H.; Choi, S.-H.; Cho, N. G.; Chang, Y.; Kim, H.-G.; Hong, J.-M.; Kim, I-D. High Stability InGaZnO4 Thin-Film Transistors Using Sputter-Deposited PMMA Gate Insulators and PMMA Passivation Layers. J. Electrochem. Soc. 2009, 12, H296–H298. (6) Tak, Y. J.; Park, S. P.; Jung, T. S.; Lee, H.; Kim, W.-G.; Park, J. W.; Kim, H. J. Reduction of Activation Temperature at 150°C for IGZO Films with Improved Electrical Performance via UV-Thermal Treatment. J. Inf. Disp. 2016, 17, 73-78. (7) 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, 123508. (8) Liu, P.-T.; Chou, Y.-T.; Teng, L.-F. Environment-Dependent Metastability of PassivationFree Indium Zinc Oxide Thin Film Transistor after Gate Bias Stress. Appl. Phys. Lett. 2009, 95, 233504. (9) Choi, S.-H.; Jang, J.-H.; Kim, J.-J.; Han, M.-K. Low-Temperature Organic (CYTOP)


Page 22 of 26

Page 23 of 26

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


Passivation for Improvement of Electric Characteristics and Reliability in IGZO TFTs. IEEE

Electron Device Lett. 2012, 33, 381-383. (10) Wu, J.; Chen, Y.; Zhou, D.; Hu, Z.; Xie, H.; Dong, C. Sputtered Oxides Used for Passivation Layers of Amorphous InGaZnO Thin Film Transistors. Mater. Sci. Semicond.

Process 2015, 29, 277–282. (11) Kang, D. H.; Kang, In.; Ryu, S. H.; Ahn, Y. S.; Jang, J. Effect of SiO2 and/or SiNx Passivation Layer on Thermal Stability of Self-Aligned Coplanar Amorphous Indium– Gallium–Zinc–Oxide Thin-Film Transistors. J. Disp. Technol. 2013, 9, 699-703. (12) Han, D.-S.; Park, J.-H.; Kang, M.-S.; Shin, S.-R.; Jung, Y.-J.; Choi, D.-K.; Park, J.-W. Effect of an Al2O3/TiO2 Passivation Layer on the Performance of Amorphous Zinc–Tin Oxide Thin-Film Transistors. J. Electron. Mater. 2015, 44, 651-657. (13) An, S.; Mativenga, M.; Kim, Y.; Jang, J. Improvement of Bias-Stability in AmorphousIndium-Gallium-Zinc-Oxide Thin-Film Transistors by Using Solution-Processed Y2O3 Passivation. Appl. Phys. Lett.2014, 105, 053507. (14) Dong, C.; Shi, J.; Wu, J.; Chen, Y.; Zhou, D.; Hu, Z.; Xie, H.; Zhan, R.; Zou, Z. Improvements in Passivation Effect of Amorphous InGaZnO Thin Film Transistors. Mater.

Sci. Semicond. Process 2014, 20, 7-11. (15) Seo, S.-J.; Yang, S.; Ko, J.-H.; Bae, B.-S. Effects of Sol-Gel Organic-Inorganic Hybrid Passivation on Stability of Solution-Processed Zinc Tin Oxide Thin Film Transistors. J.

Electrochem. Soc. 2011, 14, H375-H379. (16) Xu, X.; Feng, L.; He, S.; Jin, Y.; Guo, X. Solution-Processed Zinc Oxide Thin-Film Transistors with a Low-Temperature Polymer Passivation Layer. IEEE Electron Device Lett. 2012, 33, 1420-1422. (17) Kim, K. H.; Kim, Y.-H.; Kim, H. J.; Han, J.-I.; Park, S. K. Fast and Stable SolutionProcessed Transparent Oxide Thin-Film Transistor Circuits. IEEE Electron Device Lett. 2011,



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

32, 524-526. (18) Park, S. K.; Kim, Y.-H.; Kim, H.-S.; Han, J.-I. High Performance Solution-Processed and Lithographically Patterned Zinc–Tin Oxide Thin-Film Transistors with Good Operational Stability. J. Electrochem. Soc. 2009, 12, H256-H258. (19) Yetisen, A. K.; Akram, M. S.; Lowe, C. Paper-Based Microfluidic Point-of-Care Diagnostic Devices. Lab Chip. 2013, 13, 2210-2251. (20) Yin, L.-T.; Hu, C.-Y.; Chang, C.-H. A Single Layer Nitrocellulose Substrate for Fabricating Protein Chips. Sens. Actuator B-Chem. 2008, 130, 374–378. (21) Beard, B. C. Cellulose Nitrate as a Binding Energy Reference in N(ls) XPS Studies of Nitrogen-Containing Organic Molecules. Appl. Surf. Sci. 1990, 45, 221-227. (22) Seo, S.-J.; Choi, C. G.; Hwang, Y. H.; Bae, B.-S. High Performance Solution-Processed Amorphous Zinc Tin Oxide Thin Film Transistor. J. Phys. D-Appl. Phys. 2009, 42, 035106. (23) Kim, S. J.; Jung, J.; Yoon, D. H.; Kim, H. J. The Effect of Various Solvents on the Back Channel of Solution-Processed In–Ga–Zn–O Thin-Film Transistors Intended for Biosensor Applications. J. Phys. D-Appl. Phys. 2013, 46, 035012. (24) Hwang, S. Y.; Lee, J. H.; Woo, C. H.; Lee, J. Y.; Cho, H. K. Effect of Annealing Temperature on the Electrical Performances of Solution-processed InGaZnO Thin Film Transistors. Thin Solid Films 2011, 519, 5146-5149. (25) Takagi, A.; Nomura, K.; Ohta, H.; Yanagia, H.; Kamiya, T.; Hirano, M.; Hosono, H. Carrier Transport and Electronic Structure in Amorphous Oxide Semiconductor, a-InGaZnO4.

Thin Solid Films. 2005, 486, 38-41. (26) Masuda, S.; Kitamura, K.; Okumura, Y.; Miyatake, S. Transparent Thin Film Transistors Using ZnO as an Active Channel Layer and Their Electrical Properties. J. Phys. D-Appl. Phys. 2003, 93, 1624-1630. (27) Huang, X.; Wu, C.; Lu, H.; Ren, F.; Chen, D.; Zhang, R.; Zheng, Y. Enhanced Bias


Page 24 of 26

Page 25 of 26

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


Stress Stability of a-InGaZnO Thin Film Transistors by Inserting an Ultra-Thin Interfacial InGaZnO:N Layer. Appl. Phys. Lett. 2013, 102, 193505. (28) Nakamoto, K. Infrared Spectra of Inorganic Coordination Compounds, 2nd ed; WileyInterscience: New York, 1970. (29) Dolphin, D. Tabulation of Infrared Spectral Data, Wiley-Interscience: New York, 1977. (30) Drago, R. S. Physical Methods in Chemistry W. B., Saunders: Philadelphia, 1977. (31) Ebsworth, E. A. V.; Rankin, D. W. H.; Cradock, S. Structural Methods in Inorganic

Chemistry, 2nd ed; Blackwell Scientific Publications: Boston, 1991. (32) Wilson, Jr., E. B.; Decius, J. C.; Cross, P. C. Molecular Vibrations: The Theory of

Infrared and Raman Vibrational Spectra, Dover: New York, 1955. (33) Syu, Y.-E.; Zhang, R.; Chang, T.-C.; Tsai, T.-M.; Chang, K.-C.; Lou, J.-C.; Young, T.-F.; Chen, J.-H.; Chen, M.-C.; Yang, Y.-L.; Shih, C.-C.; Chu, T.-J.; Chen, J.-U; Pan, C.-H.; Su, Y.T; Huang, H.-C.; Gan, D.-S.; Sze, S. M. Endurance Improvement Technology with Nitrogen Implanted in the Interface of WSiOx Resistance Switching Device. IEEE Electron Device

Lett. 2013, 34, 864-866. (34) Liu, P.-T.; Chou, Y.-T.; Teng, L.-F.; Li, F.-H.; Shieh, H.-P. Nitrogenated Amorphous InGaZnO Thin Film Transistor. Appl. Phys. Lett. 2011, 98, 052102. (35) Lee, J.-M.; Cho, I.-T.; Lee, J.-H.; Kwon, H.-I. Bias-Stress-Induced StretchedExponential Time Dependence of Threshold Voltage Shift in InGaZnO Thin Film Transistors.

Appl. Phys. Lett. 2008, 93, 093504. (36) Fan, C.-L.; Tseng, F.-P.; Li, B.-J.; Lin, Y.-Z.; Wang, S.-J.; Lee, W.-D.; Huang, B.-R. Improvement in Reliability of Amorphous Indium–Gallium–Zinc Oxide Thin-Film Transistors with Teflon/SiO2 Bilayer Passivation under Gate Bias Stress. Jpn. J. Appl. Phys. 2016, 55, 02BC17.



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

Table of contents


Page 26 of 26

Improvement of Electrical Characteristics and ... - ACS Publications

Recommend Documents