High Mobility Flexible Amorphous IGZO Thin-Film Transistors with a

Subscriber access provided by FONDREN LIBRARY, RICE UNIVERSITY

Article

High Mobility Flexible Amorphous IGZO Thin-Film Transistors with a Low Thermal Budget Ultra-Violet Pulsed Light Process. Mohammed Benwadih, Romain Coppard, Klaus Bonrad, Andreas Klyszcz, and Dominique Vuillaume ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.6b09990 • Publication Date (Web): 22 Nov 2016 Downloaded from http://pubs.acs.org on November 23, 2016

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 31

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

High Mobility Flexible Amorphous IGZO ThinFilm Transistors with a Low Thermal Budget Ultra-Violet Pulsed Light Process. M. Benwadih1*, R. Coppard1, K. Bonrad2, A. Klyszcz2, D. Vuillaume3 1 : Univ. Grenoble Alpes, CEA, F-38000 Grenoble, France 2: Merck TU-Darmstadt Laboratories, 64287 Darmstadt, Germany 3 : Institute for Electronics Microelectronics and Nanotechnology, CNRS, University of Lille, 59652, Villeneuve d'Ascq, France *corresponding author: [email protected]

Keywords: IGZO, UV annealing, sol-gel, plastic substrate, semiconductors.

Abstract

Amorphous, sol-gel processed, indium gallium zinc oxide (IGZO) transistors on plastic substrate with a printable gate dielectric and an electron mobility of 4.5 cm2/Vs, as well as a mobility of 7 cm2/Vs on solid substrate (Si/SiO2) are reported. These performances are obtained using a low temperature pulsed light annealing technique. Ultra-violet (UV) pulsed light system is an innovative technique compared to conventional (furnace or hot-plate) annealing process that we successfully implemented on sol-gel IGZO thin film transistors (TFTs) made on plastic substrate. The photonic annealing treatment has been optimized to obtain IGZO TFTs with significant electrical properties. Organic gate dielectric layers deposited on this pulsed UV light annealed films have also been optimized. This technique is very promising for the development of amorphous IGZO TFTs on plastic substrates.

1 Environment ACS Paragon Plus


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

Metal oxide semiconducting compounds have attracted considerable interest as the active layer for thin film transistors (TFTs) in large area displays due to their high electron mobility, good uniformity, and high transparency. Zinc oxide (ZnO), indium gallium zinc oxide (IGZO), and zinc tin oxide (ZTO) are representative of such metal oxide semiconductors compounds.1-3 In addition to their good electrical properties, the metal oxides may be prepared using various techniques. In most studies on these materials, they are deposited by physical deposition, either evaporation or sputtering. However, these techniques need high vacuum, are relatively expensive and difficult to implement. A more innovative technique is the solution-processed material approach, which is less expensive and allows a better control of the composition of the material, while having good homogeneity.4 In addition, solution processes facilitate deposition over large areas. Both the semiconductors and the dielectric layers of the transistors can be synthesized by the sol-gel route. In sol–gel processes, annealing at high temperatures (>450°C) is needed to turn the gel film into an oxide compound through chemical reactions,5 which may be a drawback for devices on plastic substrates. To overcome this problem, the ultra-violet (UV) low temperature annealing process begins to give very satisfactory results. For example, functional transistors (with electron mobility of 7 cm2/Vs) containing an IGZO layer annealed by continuous exposition to UV lamp have been reported.6 The obtained electron mobilities are on a par with performances of transistors (on solid substrates) produced by conventional thermal anneal (10-20 cm2/Vs).6 However, the procedure is relatively restrictive, especially with regard to its sensitivity to humidity. The annealing must be performed under argon.6 In addition, the annealing time required with this type of lamp is relatively long, around 90 minutes but it is nevertheless a step forward in the research on innovative annealing. UV annealing is a


Page 2 of 31

Page 3 of 31

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


solution to be focused on, especially deep UV radiation, which breaks the metal alkoxide bonds and promotes the formation of metal-oxide-metal network. 6-8 Here, we report on a new type of photonic annealing, using short-time light pulses generated in the UV range by a xenon lamp. Such pulsed radiation rapidly heats exposed materials. Unlike conventional technologies, which use hot plates or furnaces, the pulsed nature of this heating brings thin films to a high temperature (˃800°C) for a short time without thermal damage to their substrates (such as polymers).9 It is a very versatile system. Parameters such as pulse duration, power, frequency and number of flashes, can be tuned for process optimization. In this study, these parameters have been optimized to process low thermal budget sol-gel IGZO TFTs on plastic substrate (Kapton®) with a high electron mobility values (4.5 cm2/Vs). This value is on a par with, or even slightly better than those reported for plastic IGZO fabricated by other techniques.6-14

2. Experimental Section. Fabrication of IGZO transistors on Si/SiO2: We started with a n-type doped silicon substrate (ND ∼ 3 × 1017 cm−3) covered with a 100 nm thick, thermally grown, silicon dioxide (SiO2) layer. Source and drain electrodes (10 nm Ti/30 nm Au) were patterned by optical lithography and lift off technique. Channel length (L) and width (W) are L=20 µm and W/L=500. The substrates were cleaned by ultrasonication in acetone and propane-diol for 10 min each to obtain a clean and residue free surface. The substrates were dried and UV ozone treated for 5 min, which significantly improves the wetting of the sol-gel precursor formulation onto the substrate. The IGZO precursor solutions (from Merck KGaA) were prepared by dissolving organometallic zinc oxymates,15 indium oxymates,16 and gallium oxymates,17 in 2-methoxyethanol. The details of the chemical composition of these precursors are given in Ref. 15-17. The chemical composition ratio of IGZO precursor solution was In:Ga:Zn = 3:0.4:2 at a single 3 Environment ACS Paragon Plus


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

layer concentration of 30 mg of precursor in a total of 1 g of solvent (2-methoxyethanol). This formulation was spin coated onto the substrate at 2000 rpm for 30s, which was subsequently placed onto a hot plate in air at temperatures in the range of 100°C to 350°C for 1min in order to evaporate the solvent, and then submitted to UV annealing in order to form the metal-oxide layer. The typical IGZO layer was 5 nm. Several layers are successively deposited by the same method. The IGZO TFTs measured in this work have 4 layers with a total thickness of the IGZO film around 20 nm (measured from SEM on a cross-section, see Figure S7 in the supporting information).18 Pulsed UV light annealing: We used a photonic annealing equipment "Sinteron 2000", from XENON Company, to anneal the thin films of sol-gel IGZO. This technology of flash lamp can provide adjustable light pulses, giving energies between 500 and 2070 J for a pulse time between 200µs and 2ms. The principle of this technique is based on charging an electric capacitance with a continuous voltage (several kV), which is then discharged in the lamp containing xenon gas. This discharge can generate a high light intensity with wavelength ranging from UV to the near infrared (see Figure S1 in supporting information). To check the good match between the UV lamp spectrum and the IGZO optical properties, we also deposited IGZO solution directly onto a quartz substrate to measure the optical properties of the IGZO film (see Figure S1 in Supporting Information). Optical UV-Vis Transmittance spectra analysis: IGZO films deposited on glass substrates were used for optical measurements. A baseline measurement, of the substrate transmittance was carried out using an ultraviolet–visible (UV-Vis) spectrophotometer Perkin Elmer LAMBDA 950. We analyzed the optical properties of the solution processed IGZO and IGZO films in the 250–1500 nm wavelength range (see Figure S2 in Supporting Information). Electrical measurements: Transistor characteristics were measured using an Agilent 4156 semiconductor parameter analyzer. The saturation field-effect mobility µ is commonly evaluated from the slope of the square root of the drain current against the gate voltage √(IDS4 Environment ACS Paragon Plus

Page 4 of 31

Page 5 of 31

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


VG) and the threshold voltage Vth from the intercept of the extrapolated √(IDS-VG) plot with the horizontal, VG axis. SEM measurement: The microstructure of the IGZO on flat Si/SiO2 substrates was analyzed by Field-Effect Scanning Electron Microscopy (FE-SEM) on a Hitachi 5500. XPS measurement: X-ray photoelectron spectroscopy (XPS) measurements were carried out to establish the chemical and local bonding configuration and structural details of the studied ternary oxides. XPS was performed on a Thermo Electron Scientific ESCALAB 250 spectrometer (the source Al Kα 1486.6 eV) at pressure of 4.5x10-10 Torr (UHV), XPS spectra were obtained immediately after film fabrication in order to minimize surface contamination. TGA/DSC measurement: Thermogravimetric analysis (TGA/DSC) on the metal oxide solution to check the behavior of the Kapton® in temperature (see Figure S3 in Supporting Information). TGA were performed on 40–45 mg of materials by using a Pegasus Netzsch thermal analysis instrument under air with a flow rate of 10 mL/min and a heating ramp rate of 10 °C/min from 25 to 500°C. Interferometric profilometry: The surface roughness of the Kapton® and Kapton®/PI was measured with a WYKO NT8000 optical profiler (see Figure S4 in Supporting Information). Fabrication of IGZO transistors on Kapton®: Polyimide substrates (PI) "DuPont Kapton®" and derivatives from polyarylate are described in the literature.6,19,20 These plastic substrates have viscoelastic properties depending of the temperature, and if the annealing temperature is higher than the glass transition Tg, the polymer becomes rubbery and non-usable. We used Kapton® KJ from DuPont. We carried out a thermogravimetric analysis (TGA/DSC) to determine the behavior of Kapton with temperature and to identify maximum annealing conditions. A maximum of 350°C during 1h was determined (see Supporting Information, Fig. S3). The surface of Kapton® is indeed very rough, and has many defaults with very large peak heights (typically 2 µm in height) and large areas (diameters larger than 5 µm). This feature makes this substrate not suitable for deposition of a thin (20 nm) IGZO films. The 5 Environment ACS Paragon Plus


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

deposition of a smoothing polyimide layer PI2611 from “HD MicroSystems” covered the major defects and greatly reduced the rms roughness to 11 nm (see Supporting Information, Fig S4). Source and drain electrodes (10 nm Ti/30 nm Au) were fabricated on Kapton and were patterned by optical lithography and lift-off technique defining channel length L=20 µm and W=10000µm. IGZO films were deposited as previously for TFTs on solid substrate, followed by a hot-plate annealing (1 min) and pulsed UV light annealing. Vinyl ether polyperfluoroalkenyl (CYTOP), polystyrene (PS) and polyvinylphenol (PVP) were tested as gate dielectrics. Solutions of the three dielectrics were deposited by spin coating on top of the annealed IGZO layers and cured at 100°C (15 min) to eliminate the solvents. To limit the occurrence of leakage current densities below 0.1 µA/cm2 (see Figure S6 in supporting information), we used a dielectric thickness of 400 nm. Finally, silver gate electrodes (70 nm) were deposited trough a shadow mask by evaporation. (Ti/Au)/organic dielectric/metal (Au) capacitors were also fabricated. Different devices were fabricated to study the electrical properties of the organic dielectrics. These devices a surface areas(S =0,785 mm2. A 30 nm gold layer (Au) was deposited by physical vapor deposition (PVD) on a Kapton substrate (125 µm thick), the different polymers were deposited by spin coating on top of the gold electrode. All dielectric films were annealed at 60°C during 5 min and then at 100°C during 15 min. The layer thickness (same as for TFTs, 400 nm) was measured by a Dektak profilomètre. The dielectric characteristic were first analyzed for the different dielectric polymers by measuring the capacitance of the fabricated devices as function of the oscillating voltage frequency between 20 Hz and 2 MHz. Measurement were performed with an Agilent E4980A LCR meter under an AC voltage level of 0.1 V. We have investigated two samples for each polymers. The dielectric constant were extracted using the definition of a plane capacitance as

follow:

where ε0: is the permittivity of the vacuum taken at 8.85x 10-12 F/m, C: the


Page 6 of 31

Page 7 of 31

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


electrical capacitance,

ε r:

the relative permittivity of the polymer, S: the area of the

capacitance and e: the polymer film thickness (400 nm).

3. IGZO transistors on solid substrates. First, the parameters of the photonic annealing were optimized on sol-gel IGZO TFTs fabricated on Si/SiO2 substrates (bottom gate, bottom contacts configuration, see details in the Experimental Section). Typical parameters for the TFTs are SiO2 thickness 100 nm, channel length L = 20 µm, channel width W = 10,000 µm. IGZO films, with a thickness of 20 nm were deposited from metal-oxymates precursors (from Merck KGaA) according a sol-gel process described elsewhere15-17 and summarized in the Experimental Section. The films were pre-annealed on a hot plate in air at temperatures in the range 100°C to 350°C for 1 min in order to evaporate the solvent, and then submitted to UV annealing in order to form the metaloxide layer (see Experimental Section). Figure 1 shows the typical IDS-VG curves in the saturation region of IGZO TFTs on Si/SiO2 substrate and the evolution of the corresponding measured electron mobility (see Experimental Section for details) for an UV photonic annealing of 50 pulses (pulse duration 200µs) of 2000J, with different pre-annealing temperatures. The IGZO TFTs cured with a thermal pre-annealing step higher than 200°C exhibit a clear transistor effect (Figure 1-a), we clearly observe two current regions (‘on’ with current up to 10-2 A and ‘off’ with current below 10-8 A) for positive and negative gate voltages, respectively.



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

a)

b)

Figure 1. (a) Transfer characteristic curves IDS-VG (channel length L=20 µm, VDS=20V) for TFTs submitted to pre-annealing (hot plate, 1 min) at various temperatures and 50 UV pulses (200 µs) at 2000J (inset: IDS-VG in linear scale). (b) Evolution of the average electron mobility (averaged on 8 TFTs).

The TFTs having undergone a pre-anneal step at 325°C and 350°C have a mobility of 6.5 and 7 cm2/Vs (Figure 1-b). To obtain the same mobility (i.e. 7 cm2/Vs), the hot-plate annealing process requires a temperature of 450°C,4 which it is not compatible with plastic substrates. The TFTs pre-annealed at 300°C have a mobility of 4 cm2/Vs, four times higher


Page 8 of 31

Page 9 of 31

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


than sol-gel processed IGZO transistors only annealed on hot plate at the same temperature but for a longer time (1h).4 4. IGZO films: physico-chemical characterization. To gain more insights and establish correlations between the electrical properties of the IGZO transistors and the physico-chemical properties of the films, we did some scanning electron microscope (SEM) and X-ray photoelectron spectroscopy (XPS) measurements on sol-gel IGZO films deposited on flat Si/SiO2 substrates (i.e. without source/drain electrodes) (see Experimental Section). It was shown that solution processed IGZO transistors with a high mobility of 8 cm2/Vs,4 having undergone a thermal annealing at 450°C for 1h, present a microstrutured IGZO film.21 SEM images revealed a dense network of pores (Figure 2-a, and Ref. 21). These pores come from the evaporation of the solvents and the self-organization of nanoclusters into the films as revealed elsewhere by GISAXS (grazing incidence small-angle X-ray scattering).21 For comparison, Figure 2-b shows the SEM image, of IGZO film after the pre-annealing (300°C/1min) and the photonic annealing (50 UV 200µs pulses at 2000J). The average diameter of the pores are distributed between 2 and 5 nm for both the 450°C/1h and photonic (350°C/1min, 50 UV pulses) and the surface occupancies are 26-28% in both cases. On the contrary, a very small density of pores (about 2% of surface coverage) is observed for IZGO film having undergone only the pre-annealing step (Figure 2-c).



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

a)

100nm

450°C_1h

b)

100nm

300°C_1min_50 Flashes

c)

100nm

300°C_1min 300°C_1min

Figure 2: SEM images of IGZO thin films annealed at (a) 450°C / 1h, (b) 300 °C/1 min and with 50 UV pulses at 2000J, and (c) pre-annealed only 300°C / 1 min.

These results are consistent with the fact that both the 450°C/1h annealed IGZO TFTs and 300°C/1min/50 UV flashes) have a high mobility (8 cm2/Vs 4 and 4 cm2/Vs, Figure 1-b), while no transistor behavior is observed with the pre-annealing step alone. In order to investigate the effect of the annealing process on chemical and structural changes in the films after annealing. XPS was used to analyze the metal-oxygen (M-O)


Page 10 of 31

Page 11 of 31

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


bonding in IGZO films. Figure 3 shows the results of a deconvolution of the oxygen O1s peaks of IGZO films to estimate the different forms of O2− ions in the films.

a)

b)

Figure 3. XPS spectra of oxygen 1s for IGZO films annealed at (a) 300 °C/1min, (b) 300 °C during 1 min and 50 UV pulses (200µs) at 2000 J.

The O1s peaks were fitted by three nearly Gaussian curves attributed to: (i) a peak centered at 530.2 eV which represent the O2− ion bonded with metal ions (In, Ga and Zn) with oxygen vacancies (M-O-M); (ii) a peak at 531.5 eV attributed to bulk and surface metal 11 Environment ACS Paragon Plus


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

hydroxide (M-OH) and containing non-stoichiometric oxide species and finally, (iii) the weakly bound (M-OR) species such as surface adsorbed carbon species, at 532.5 eV.11 The main feature is that the hydroxide M-OH and M-OR peaks still have a significant contribution after the pre-annealing at 300°C/1min (Figure 3-a), while they are strongly reduced with the pulsed UV light annealing (Figure 3-b). Note that the XPS spectrum shown in Figure 3-b is similar to the one for a solution processed IGZO film classically annealing at 450°C for 1h (see Figure 2-c in Ref. 4). For instance, the ratios of the peak maximum amplitudes [MO]/[M-OH] and [M-O]/[M-OR] are∼ 2.7 and ∼3.3 for IGZO 450°C/1h4 and ∼2.8 and ∼3.4 in the present case. Since the electron mobility is related to the IGZO stoichiometry (oxygen vacancy),6 these XPS results are consistent with the fact that both devices have almost the same mobility (8 cm2/V.s for IGZO 450°C/1h)4 and 4 cm2/V.s for the pulsed UV light annealed IGZO TFT (see Figure 1-b) and that the device only pre-annealed at 300°C/1min, which contains more hydroxide and carbon-related species does not exhibit any transistor behavior.

5. IGZO transistors on plastic substrate To take advantage of the low-thermal budget photo-activation of IGZO by pulsed UV light annealing, we fabricated IGZO TFTs on commercially available polyimide (DuPont Kapton® KJ Polyimide Film) substrates. The UV flash anneal process optimized above on Si/SiO2 substrate was transferred to sol-gel IGZO TFTs made on plastic substrates. The IGZO TFTs on Kapton® have bottom contacts, top gate configuration as described in the Experimental Section. Before the IGZO deposition, the surface of Kapton® is planarized using a smoothing polyimide layer (see Figure S4 in supporting information). IGZO films were deposited from solution and annealed as previously described for Si/SiO2 substrates, since Kapton can sustain annealing temperatures up to 350°C (see Figure S3 in Supporting Information).22 Then, on


Page 12 of 31

Page 13 of 31

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


top of annealed IGZO films, we deposited solution processed gate dielectrics, which can be printed. Several polymers, such as vinyl ether polyperfluoroalkenyl (CYTOP), polystyrene (PS) and polyvinylphenol (PVP), were tested to determine the most suitable to be used with the solution IGZO annealed by pulsed UV light. It is known that the electrical properties of TFTs, such as mobility, threshold voltage, and the subthreshold slope, do not only depend on the nature of the organic semiconductor but also on chemical and physical properties of the organic gate insulator.23 We started with a set of measurements to define the best gate dielectric. Figure 4-a shows the values of the relative dielectric permittivity as function of the frequency obtained from capacitance measurements on metal (Ti/Au)/organic dielectric/metal (Au) capacitors (see Experimental Section). CYTOP and polystyrene films have a stable frequency behavior and quite similar relative permittivity values (εr ∼ 2.2), while PVP shows a permittivity decreasing with frequency. The permittivity of the bi-layer PS/PVP remains stable as frequency changes, and is approximately εr ∼ 3. For all these gate dielectrics, the leakage current densities are lower than 0.1 µA/cm2 (supporting information, Fig. S6) in the gate voltage of interest, i.e. Ioff of the IZGO transistor is lowest (Figure 4-b). Figure 4-b shows the characteristic curves IDS-VG of the IGZO transistors on Kapton® with the different gate insulators and processed with a thermal annealing of IGZO (hot plate) step only, at 350°C for 1 h. These curves are significantly influenced by the nature of the insulating layer. The presence of dipoles at the interface between the dielectric and the IGZO is critical and can induce differences in the mobility depending on the nature of the organic dielectrics.24 Polystyrene (PS), an insulating polymer with the less reactive chemical structure, provides low off-current Ioff (~10-10 A) and a sub-threshold slope (1.7 V/decade) better than those of PVP and CYTOP (3.2 and 2.9 V/decade, respectively).



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

a)

b)

Figure 4. a) Relative dielectric permittivity of gate insulators PS, CYTOP, PVP, PS/PVP. b) IDS-VG curves (channel length L = 20 µm, VDS= 20 V) of IGZO TFTs on Kapton® substrate with various dielectrics.

In the case of PVP, the presence of hydroxyl groups and adsorbed water molecules can create traps, thereby degrading the sub-threshold slope of the IDS-VG curves. Inserting a thin layer of 50 nm-thick polystyrene (PS) between the IGZO and PVP layers improves the interface between the semiconductor and the organic dielectric, and therefore allows to obtain 14 Environment ACS Paragon Plus

Page 14 of 31

Page 15 of 31

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


transfer curves IDS-VG with a smaller threshold voltage (5V, while values > 12.5V -Fig. 5- are observed for lower pre-annealing temperatures), the lowest subthreshold slope (1.1 V/decade), and a low off-current Ioff (2x10-10 A). The main properties of IGZO transistors fabricated on a plastic substrate with these different gate dielectrics are summarized in Table 1: the highest mobility (3.8 cm2/Vs) is obtained (for a thermal annealing at 350°C during 1h) for transistors with a bilayer dielectric PS/PVP, thanks to the good quality of the PS/IGZO interface.

Table 1. Electrical parameters of IGZO transistors fabricated on plastic substrate: electron mobility µ, threshold voltage Vth, subthreshold slope (SS) and off current (Ioff). µ (cm2/Vs)

Vth (V)

SS (V/decade)

Ioff (A)

PS 350°C (1h)

1.9

6

1.7

2x10-10

PVP 350°C (1h)

2.7

10

3.2

6x10-8

CYTOP 350°C (1h)

0.1

8

2.9

2x10-8

PS/PVP 350°C (1h)

3.8

5

1.1

2x10-10

PS/PVP 350°C(1min)+UV Flash

4.5

4

1.2

~10-9

PS/PVP 300°C(1min)+UV Flash

1

7

2.3

~10-10

We therefore tested UV photonic annealing with this optimized gate dielectric configuration (PVP/PS) with various pre-annealing steps. Figure 5 shows the measured IDSVG curves, and the extracted transistor parameters (electron mobility, threshold voltage, subthreshold slope) are given in Table 1. An electron mobility around 4.5 cm2/Vs is obtained in the case of a pre-annealing at 350°C for 1 minute, followed by 50 flashes (200µs) at 2000J. This result is slightly higher than the one obtained for conventional “hot plate” thermal



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

annealing (Table 1) for 1h at the same temperature. The threshold voltage and the subthreshold slope are comparable in both cases. The combination of the pre-annealing process step and UV flash annealing step also provides a marked improvement of the mobility (1 cm2/Vs) of the transistors pre-annealed at 300°C, while a purely “hot plate” thermal annealing at 300°C/1h is insufficient to obtain a transistor effect (not shown). However, decreasing the pre-annealing step below 300°C does not allow us to obtain a transistor behavior (Figure 5, see pre-annealing at 250°C). Note that we have observed a larger dispersion of the mobility for the TFTs on Kapton®, compared to those on solid substrates. For 16 samples (pre-anneal 350°C/1min + 50 UV flashes) the mobility is in the range 1 to 4.5 cm2/Vs. Out of these 16 measured samples, 10 have a mobility higher than 2 cm2/Vs and 2 showed the maximum mobility of 4.5 cm2/Vs (see Figure S5 in Supporting Information).

Figure 5. Characteristic curves IDS-VG of IGZO TFTs (channel length L = 20 µm, VDS= 20V) on plastic substrate with PS/PVP gate dielectric and the photonic annealing.

It is likely that this larger dispersion is due to the more important roughness of the smoothed Kapton® substrate (about 11 nm, see Figure S4 Supporting Information), compare 16 Environment ACS Paragon Plus

Page 16 of 31

Page 17 of 31

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


to the very flat SiO2 surface (rms roughness of about 0.1 nm), which makes more difficult obtaining a homogeneous IGZO film. Further improvement of the surface treatment of Kapton® will probably allow reaching higher mobility and/or a lower dispersion of the IGZO TFTs properties. These results are comparable or even slightly better than those reported in the literature for various amorphous oxide semiconductor transistors (IGZO, ZnO, In2O3, etc..) on plastic substrates, as well as on solid substrates with vacuum-deposited inorganic high k gate dielectric.6-14,18-19,25-28

Conclusion In conclusion, using the combination of a short time (1min) thermal pre-annealing step at 350°C and a pulsed UV light anneal, we have developed high-performance, solution-process IGZO transistors on both a solid substrate (Si/SiO2) with an electron mobility of 7 cm2/Vs, and on a plastic substrate, incorporating a printable gate dielectric, with an electron mobility of 4.5 cm2/Vs. These results represent a real progress for an integration IGZO transistors on plastic substrate. This performance is all the more interesting that the deposition on our substrate is still rough, and there is room to improve these interfaces. We note that a combination of this flash photonic annealing and a vacuum pumping which can help evaporation of solvents and organic precursor molecules need for IGZO deposition as well as all other volatile species is likely to allow decreasing the temperature (or even suppressing) of the thermal pre-annealing step. This work is under optimization and will be reported elsewhere. By pursuing the development of plastic electronics based on IGZO, the performance improvements, especially in terms of power and speed, will reinforce its potential in various applications, such as sensor control electronics (pressure, temperature,..), plastic display systems, or radio frequency identification (RFID).



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

Acknowledgments. We thank Dr A. Aliane for helpful discussions about UV annealing techniques and Tournon Laurent. Dr Gareth P. Keeley is also acknowledged for proof-reading the manuscript.

Abstract graphic

Ultra-Violet

Ultra-Violet Pulsed Light

Abstract graphic: Pulsed UV light annealing allows the fabrication of flexible IGZO TFTs with mobility up to 4.5 cm2/Vs without damaging the plastic substrates.


Page 18 of 31

Page 19 of 31

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


References (1) Nomura, K.; Ohta, H.; Takagi, A.; Kamiya, T.; Hirano, M.; Hosono, H. RoomTemperature Fabrication of Transparent Flexible Thin-Film Transistors Using Amorphous Oxide Semiconductors. Nature. 2004, 432, 488-492. (2) Hoffman, R. L.; Norris, B. J.; Wager J. F. ZnO-Based Transparent Thin-Film Transistors. Appl. Phys. Lett. 2003, 82, 733-735. (3) Jeong, J. K.; Jeong, J. H.; Yang, H. W.; Park, J. S.; Mo, Y. G.; Kim, H. D. High Performance Thin Film Transistors with Cosputtered Amorphous Indium Gallium Zinc Oxide Channel. Appl. Phys. Lett. 2007, 91, 113505. (4) Benwadih, M.; Chroboczek, J. A.; Ghibaudo, G.; Coppard, R.; Vuillaume, D. Impact of Dopant Species on the Interfacial Trap Density and Mobility in Amorphous In-X-Zn-O Solution-Processed Thin-Film Transistors. J. Appl. Phys. 2014, 115, 214501. (5) Jeon, H.; Song, J.; Na, S.; Moon, M.; Lim, J.; Joo, J.; Jung, D.; Kim, H.; Noh, J.; Lee, H. J. A Study on the Microstructural and Chemical Evolution of In–Ga–Zn–O Sol–Gel Films and the Effects on the Electrical Properties. Thin Solid Films. 2013, 540, 31-35. (6) Kim, Y. H.; Heo, J. S.; Kim, T. H.; Park, S.; Yoon, M. H.; Kim, J.; Oh, M. S.; Yi, G. R.; Noh, Y. Y.; Park, S. K. Flexible Metal-Oxide Devices Made by RoomTemperature Photochemical Activation of Sol–Gel Films. Nature. 2012, 489, 128– 132. (7) Kang, C. M; Kim, H.; Oh, Y. W.; Baek, K. H.; Do, L. M. High-Performance, Solution-Processed Indium-Oxide TFTs Using Rapid Flash Lamp Annealing. IEEE ELECTRON DEVICE LETTERS. 2016, 37, 595–598. (8) Kim, D. W.; Park, J.; Hwang, J.; Kim, H. D.; Ryu, J. H.; Lee, K. B.; Baek, K. H.; Do, L. M.; Choi, J. S. Rapid Curing of Solution-Processed Zinc Oxide Films by Pulse19 Environment ACS Paragon Plus


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

Light Annealing for Thin-Film Transistor Applications. Electron. Mater. Lett. 2015, 11, 82-87. (9) Kim, H. S.; Dhage, S. R.; Shim, D. E.; Hahn, H. T. Intense Pulsed Light Sintering of Copper Nanoink for Printed Electronics. Appl. Phys. A, 2009, 97, 791–798. (10) Kim, K. M.; Kim, C. W.; Heo, J. S.; Na, H.; Lee, J. E.; Park, C. B.; Bae, J. U.; Kim, C. D.; Jun, M.; Hwang, Y. K.; Meyers, S. T.; Grenville, A.; Keszler, D. A. Competitive Device Performance of Low-Temperature and All-Solution-Processed Metal-Oxide Thin-Film Transistors. Appl. Phys. Lett. 2011, 99, 242109. (11) Kim, M. G.; Kanatzidis, M. G.; Facchetti, A.; Marks, T. J. Low-Temperature Fabrication of High-Performance Metal Oxide Thin-Film Electronics Via Combustion Processing. Nature Materials. 2011, 10, 382–388. (12) Meyers, S. T.; Anderson, J. T.; Hung, C. M.; Thompson, J.; Wager, J. F.; Keszler, D. A. Aqueous Inorganic Inks for Low-Temperature Fabrication of ZnO TFTs. J. Am. Chem. Soc. 2008, 130, 17603–17609. (13) Jeong, S.; Lee, J. Y.; Lee, S. S.; Oh, S. W.; Lee, H. H.; Seo, Y. H.; Ryu, B. H.; Choi, Y. Chemically Improved High Performance Printed Indium Gallium Zinc Oxide Thin-Film Transistors. J. Mater. Chem. 2011, 21, 17066-17070. (14) Su, B. Y.; Chu, S. Y.; Juang, Y. D.; Chen, H. C, High-Performance LowTemperature Solution-Processed InGaZnO Thin-Film Transistors via UltravioletOzone Photo-Annealing. Appl. Phys. Lett. 2013, 102, 192101–192104. (15) Schneider, J. J.; Hoffmann, R. C.; Engstler, J.; Dilfer, S.; Klyszcz, A.; Erdem, E.; Jakes, P.; Eichel, R. A. Zinc Oxide Derived from Single Source Precursor Chemistry under Chimie Douce Conditions: Formation Pathway, Defect Chemistry and Possible Applications in Thin Film Printing. J. Mater. Chem. 2009, 19, 1449-1457. (16) Kuegler, R.; Schneider, J. J; Hoffmann, R. Organometallic Zinc Compound for Preparing Zinc Oxide Films, (Merck) Patent No. WO/2009/010142, 2009. 20 Environment ACS Paragon Plus

Page 20 of 31

Page 21 of 31

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


(17) Hoffmann, R. C.; Kaloumenos, M.; Heinschke, S.; Erdem, E.; Jakes, P.; Eichel, R. A.; Schneider, J. J. Molecular Precursor Derived and Solution Processed Indium– Zinc Oxide as a Semiconductor in a Field-Effect Transistor Device. Towards an Improved Understanding of Semiconductor Film Composition. J. Mater. Chem. C, 2013, 1, 2577-2584. (18) Daniel E. Walker, D. E.; Major, M.; Yazdi, M.B.; Klyszcz, A.; Haeming, M.; Bonrad, K.; Melzer, C.; Donner, W.; Seggern, H.V. High Mobility Indium Zinc Oxide Thin Film Field-Effect Transistors by Semiconductor Layer Engineering. ACS Appl. Mater. Interfaces, 2012, 4, 6835−6841. (19) Jeong, S.; Moon, J. Low-Temperature, Solution-Processed Metal Oxide Thin Film Transistors. J. Mater. Chem. 2012, 22, 1243-1250. (20) Chien, C. W.; Wu, C. H.; Tsai, Y. T.; Kung, Y. C.; Lin, C. Y.; Hsu, P. C.; Hsieh, H. H.; Wu, C. C.; Yeh, Y. H; Leu, C. M.; Lee, T. M. High-Performance Flexible aIGZO TFTs Adopting Stacked Electrodes and Transparent Polyimide-Based Nanocomposite Substrate. IEEE Transactions on Electron Devices. 2011, 58, 1440– 1446. (21)

Revenant, C.; Benwadih, M.; Maret, M. Self-Organized Nanoclusters in

Solution-Processed Mesoporous In–Ga–Zn–O Thin Films. Chem. Commun., 2015, 51, 1218-1221. (22) Jang, J.; Choi, M. H.; Kim, B. S.; Lee, W. G.; Seok, M. J.; Ryu, D. S. Robust TFT Backplane for Flexible AMOLED. SID Symposium Digest of Technical Papers, 2012, 43, 260–263. (23) Liu, C.; Xu, Y.; Li, Y.; Scheideler, W.; Minari, T. Critical Impact of Gate Dielectric Interfaces on the Contact Resistance of High-Performance Organic FieldEffect Transistors. J. Phys. Chem C. 2013, 117, 12337−12345.



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

(24)

Boudinet, D.; Benwadih, M.; Altazin, S.; Verilhac, J. M.; DeVito, E.;

Serbutoviez, C.; Horowitz, G.; Facchetti, A. Influence of Substrate Surface Chemistry on the Performance of Top-Gate Organic Thin-Film Transistors. J. Am. Chem. Soc. 2011, 133, 9968–9971. (25) 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. (26) Cobb, B.; Rodriguez, F. G.; Maas, J.; Ellis, T.; van der Steen, J. L.; Myny, K.; Smout, S.; Vicca, P.; Bhoolokam, A.; Rockelé, M.; Steudel, S.; Heremans, P.; Marinkovic, M.; Pham, D. V.; Hoppe, A.; Steiger, J.; Anselman, R.; Gelinck, G. Flexible Low Temperature Solution Processed Oxide Semiconductor TFT Backplanes for Use in AMOLED Displays. SID Symposium Digest of Technical Papers. 2014, 45, 161–163. (27) Rockelé, M.; Pham, D. V.; Hoppe, A.; Steiger, J. ; Botnaras, S.; Nag, M.; Steudel, S.; Myny, K.; Schols, S.; Müller, R.; van der Putten, B.; Genoe, J.; Heremans, P. Low-Temperature and Scalable Complementary Thin-Film Technology Based on Solution-Processed Metal Oxide n-TFTs and Pentacene p-TFTs. Organic Electronics. 2011, 12, 1909-1913. (28) Xu, X.; Cui, Q.; Jin, Y.; Guo, X. Low-Voltage Zinc Oxide Thin-Film Transistors with Solution-Processed Channel and Dielectric Layers below 150 °C. Appl. Phys. Lett. 2012, 101, 222114.


Page 22 of 31

Page 23 of 31

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 graphic: Pulsed UV light annealing allows the fabrication of flexible IGZO TFTs with mobility up to 4.5 cm2/Vs without damaging the plastic substrates. 153x85mm (96 x 96 DPI)

ACS Paragon Plus 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

(a) Transfer characteristic curves IDS-VG (channel length L=20 µm, VDS=20V) for TFTs submitted to preannealing (hot plate, 1 min) at various temperatures and 50 UV pulses (200 µs) at 2000J (inset: IDS-VG in linear scale) 201x141mm (300 x 300 DPI)


Page 24 of 31

Page 25 of 31

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


Evolution of the average electron mobility (averaged on 8 TFTs). 199x139mm (300 x 300 DPI)



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

SEM images of IGZO thin films annealed at (a) 450°C / 1h, (b) 300 °C/1 min and with 50 UV pulses at 2000J, and (c) pre-annealed only 300°C / 1 min.


Page 26 of 31

Page 27 of 31

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


XPS spectra of oxygen 1s for IGZO films annealed at 300 °C/1min 199x139mm (300 x 300 DPI)



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

XPS spectra of oxygen 1s for IGZO films annealed at 300 °C during 1 min and 50 UV pulses (200µs) at 2000 J. 199x139mm (300 x 300 DPI)


Page 28 of 31

Page 29 of 31

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


Relative dielectric permittivity of gate insulators PS, CYTOP, PVP, PS/PVP 201x141mm (300 x 300 DPI)



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

IDS-VG curves (channel length L = 20 µm, VDS= 20 V) of IGZO TFTs on Kapton® substrate with various dielectrics 201x141mm (300 x 300 DPI)


Page 30 of 31

Page 31 of 31

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


Characteristic curves IDS-VG of IGZO TFTs (channel length L = 20 µm, VDS= 20V) on plastic substrate with PS/PVP gate dielectric and the photonic annealing 201x141mm (300 x 300 DPI)


High Mobility Flexible Amorphous IGZO Thin-Film Transistors with a

Recommend Documents