High-Throughput Open-Air Plasma Activation of Metal-Oxide Thin

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Functional Inorganic Materials and Devices

High-throughput open-air plasma activation of metal-oxide thin films with low thermal budget Young Jun Tak, Florian Hilt, Scott Tom Keene, Won-Gi Kim, Reinhold H. Dauskardt, Alberto Salleo, and Hyun Jae Kim ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.8b12373 • Publication Date (Web): 05 Oct 2018 Downloaded from http://pubs.acs.org on October 6, 2018

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ACS Applied Materials & Interfaces

High-throughput open-air plasma activation of metal-oxide thin films with low thermal budget Young Jun Tak1, Florian Hilt2, Scott Keene2, Won-Gi Kim1, Reinhold H. Dauskardt2,*, Alberto Salleo2,*, Hyun Jae Kim1,*

1

Y. J. Tak, W. –G. Kim, Prof. H. J. Kim

School of Electrical and Electronic Engineering, Yonsei University, 50 Yonsei-ro, Seodaemun-gu, Seoul 120749, Republic of Korea

2

F. Hilt, S. Keene, Prof. A. Salleo, Prof. R. H. Dauskardt

Materials Science and Engineering, Stanford University, Stanford, CA 94305, USA

*

E-mail: [email protected]; [email protected]; [email protected]

KEYWORDS: activation, open-air plasma, indium-gallium-zinc-oxide, metal-oxide network, thin-film transistor

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ABSTRACT Sputter processed oxide films are typically annealed at high temperature (activation process) to achieve stable electrical characteristics through the formation of strong metaloxide chemical bonds. For instance, indium-gallium-zinc oxide (IGZO) films typically need a thermal treatment at 300°C for ≥1 hour as an activation process. We propose an open-air plasma treatment (OPT) to rapidly and effectively activate sputter processed IGZO films. OPT effectively induces metal-oxide chemical bonds in IGZO films at temperatures as low as 240oC with a dwell time on the order of a second. Furthermore, by controlling the plasma processing conditions (scan speed, distance a between plasma nozzle and samples, and gas flow rate), the electrical characteristics and the microstructure of the IGZO films can be easily tuned. Finally, OPT can be utilized to implement a selective activation process. Plasmatreated IGZO thin film transistors (TFTs) exhibit comparable electrical characteristics to those of conventionally thermal treated IGZO TFTs. Through in-depth optical, chemical, and physical characterizations, we confirm that OPT simultaneously dissociates weak chemical bonds by UV radiation and ion bombardment, and reestablishes the metal-oxide network by radical reaction and OPT-induced heat.

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1. INTRODUCTION Oxide films have been widely used in various kinds of electronic components such as conducting electrodes,1,2 semiconductors,3,4 and insulating layers.5,6 In particular, oxide semiconductors, including ZnO, InGaZnO, InGaO, ZnSnO, etc., have been candidates for channel layer in thin-film transistors (TFTs),3,4 which are essential for sensor applications7,8 and display backplanes.9,10 This interest is attributed to their low off-current (10 cm2/Vs), and high transparency over the visible region (>90 %), all properties that make them attractive compared to amorphous Si and organic-based TFTs.11 While solution processes have been explored as a way to fabricate oxide semiconductor films at low temperature and low cost,12,13 sputter processed oxide semiconductors exhibit superior electrical characteristics compared to solution processed materials due to a higher degree of metal-oxide network formation combined with a low concentration of uncoordinated chemical species (interstitial cation and anions, diatomic bonds, oxygen vacancies, etc) and residual carbon species.4 Sputter processed oxide films, however, and in particular InGaZnO, are generally annealed at high temperatures (≥300oC) for ≥1 hour as an activation process to achieve stable semiconducting properties, even though occasionally some lower-temperature processes have been reported.14 This annealing process is needed because as-deposited IGZO films have many weak chemical bonds and uncoordinated oxygen species such as oxygen vacancies and hydroxyl groups which impart metallic characteristics to the material.15,16 Therefore, the IGZO films must be activated to establish metal-oxide networks and decrease the concentration of uncoordinated oxygen species to achieve stable semiconductor behavior. Such high temperature, however, causes unwanted inter-diffusions: formation of intermixed interface layer between oxide channel layer and gate insulator,17 metal contact oxidation at the 3



interface between oxide channel layer and S/D electrodes,18 and hydrogen diffusion from the adjacent layers with oxide channel layer.19 Furthermore, the use of flexible substrates that have low glass transition temperatures is limited by high processing temperature. To address these challenges, many researchers have reported on the use of various external energy sources such as electrical current,20 high pressure,21 UV irradiation,15 and microwave irradiation22 to reduce the activation temperature. Although these processes can effectively reduce the annealing temperature, several issues are noted: (1) they require prolonged treatment time (≥1 hour) which lowers the device throughput and makes it difficult to achieve competitive fabrication costs; (2) they need an additional chamber which would disrupt existing industrial manufacturing processes; (3) they can only be applied over the entire substrate, therefore, making it impossible to tune the electrical characteristics over selective areas of a large substrate. In this study, we describe an open-air plasma treatment (OPT) to activate sputter processed IGZO films and address aforementioned issues. The OPT generates a plasma in air without the need for any vacuum system. Also, we show that OPT enables selective treatment over isolated regions of the substrate due to its lateral resolution (ca. 3 mm currently). Electrical, optical, chemical, and physical characterizations were conducted to investigate and to understand the influence of reactive species in plasma and thermal energy on the metal-oxide network of IGZO films.

2. EXPERIMENTAL DESIGN Fabrication of IGZO TFTs: To fabricate bottom gate IGZO TFTs, we use a heavily doped ptype Si wafer (p+-Si) with a thermally grown 120 nm-thick SiO2 coating. The heavily doped Si wafer and SiO2 act as gate electrode and gate insulator, respectively. Then, we immerse the 4


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substrate in acetone and methanol for 10 min in an ultrasonic bath for cleaning in sequence. Following the cleaning process, IGZO films are deposited on the substrate using a radio frequency (RF) sputter system using an IGZO target composed of In2O3:Ga2O3:ZnO at 1:1:1 mol%. The RF power, working pressure, and oxygen pressure were set to 150 W, 5 mTorr, and 0 Torr, respectively. The IGZO films are either annealed at 300°C (conventional treatment) or receive OPT for activation of the films in air. Finally, we deposit Al source and drain electrodes with a shadow mask using a thermal evaporator. The length and width of channel layers were 150 and 1000 um, respectively. For fabrication of flexible IGZO TFTs, we applied the same fabrication process on polyimide substrates coated with a SiOx/SiNx buffer layers, Mo gate electrode, and SiO2 gate insulator. Open-air plasma system: Plasma treatment was performed using an open-air plasma system provided by Plasmatreat GmbH (Hayward, CA, USA). The open-air pressure plasma jet system is based on the blown-arc discharge configuration. An arc discharge is produced between two coaxial electrodes and is blown out of a nozzle by the main process gas flow (air) at different flow rates. The resulting arc discharge was driven by a DC power supply at an excitation frequency of 21 kHz and the lateral dimension of the plasma is ca. 3 mm. The overall procedure was carried out in an open-air enclosure at ca. 25°C and 40% R.H.

Electrical, chemical, and optical analysis: The transfer characteristics of IGZO TFTs were measured using an Keithley 2400 semiconductor analyzer with a drain voltage (VDS) of 10.1 V in air and in the dark. Positive bias stress (PBS) test was measured by applying a constant gate voltage (VGS) of 20 V and VDS of 10.1 V for 3600 s in air and negative bias illumination stress (NBIS) test was evaluated by applying a constant VGS of -20 V and VDS of 0.1 V for 3600 s under white light emitting diode (LED) illumination with 5700 lux. Sheet resistances

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(RS) were measured using a four point probe and transient temperatures at surface of IGZO films were measured using in-situ thermocouple while OPT. Surface morphology was characterized using atomic force microscopy (AFM) and scanning electron microscopy (SEM). For quantitative investigation of chemical stoichiometry and atomic concentration, we used x-ray photoelectron spectroscopy (XPS) with depth profiling and auger electron spectroscopy (AES). Spectroscopic ellipsometry (SE) was used to determine the optical bandgap (Eg) and oxygen related defect sites near the conduction band of IGZO films. We used x-ray diffraction (XRD) to examine the crystallinity of the treated IGZO films.

3. RESULTS AND DISCUSSION Figure 1(a-b) present an experimental steps of OPT treated IGZO TFTs and equipment components of OPT system, respectively. Figure S1(a) shows a cross-sectional SEM image of an as-deposited IGZO film on a p+-Si substrate with a SiO2 layer, revealing IGZO films that were evenly deposited with thickness of ca. 40 nm. Figure 1(c-d) show transfer characteristics of conventionally-treated IGZO TFTs as a function of treatment temperature (Figure 1(c)) and treatment time at 300°C (Figure 1(d)). Figure 1(e) shows the corresponding change of IGZO films’ sheet resistance (RS) with treatment time, where Rs increases from 2.00kΩ/□ to 728MΩ/□ with increasing treatment time. These results reveal that high temperature (≥300°C) and long treatment time (≥1 hour) are necessary to entirely promote the change of IGZO films’ properties from metallic to semiconducting. Indeed, 30 min annealed IGZO TFTs exhibit non-uniform properties and pronounced electrical instability. Figure S1(b) shows a cross-section SEM image of IGZO films after conventional treatment, indicating that the conventional thermal treatment did not induce any structural or physical change of the IGZO films. 6


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We optimized OPT activation of IGZO films by probing the effect of scan speed (Figure 2(a)), distance a between the plasma nozzle and the sample (Figure 2(d)), and gas flow rate (Figure 2(g)). Scan speed influenced the electrical characteristics of IGZO TFTs; as the scan speed decreases, the electrical properties of IGZO TFTs changed from metallic to semiconducting (Figure 2(b)). A lower scan speed increases the dwell time of the OPT and forms higher amount of metal-oxide bonds on the IGZO film. The changes of RS and OPTinduced temperature for each condition are summarized in Figure 2(c) and indicate that decreasing the scan speed induced an increase of both RS and OPT-induced temperature, which coincide with the change of IGZO TFTs’ electrical characteristics from metallic to semiconducting. The electrical characteristics of IGZO TFTs can also be tailored by OPT by modulating the distance a between the plasma nozzle and the samples. Figure 2(e) indicates that the electrical characteristics of IGZO TFTs change from metallic to insulating as distance a decreases with increase of metal-oxide bonds formation. Hence, the same IGZO film can be made a metal, an insulator or a semiconductor when OPT are performed by controlling distance a. Figure 2(f) shows the changes of Rs and OPT-induced temperatures as a function of the distances a. Figure 2(h) shows the transfer characteristics obtained for IGZO TFTs treated with different gas flow rates. Increasing the gas flow rate leads to incomplete activation of IGZO films. It means as the gas flow rate increases, the amount of radical species formed by the OPT decreases with reducing mean free path. The corresponding RS and OPT-induced temperature (Figure 2(i)) decreased with increasing gas flow rate. Based on the overall electrical results, we determined the optimal OPT conditions for activation of IGZO films (a = 2 cm, scan speed = 1 mm/s, and gas flow rate = 35 slm). These conditions effectively reduced the treatment time from 1 hour needed for the conventional

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treatment to ~3 seconds for OPT. The temperature can also be reduced from 300°C (for the conventional treatment) to 240oC for OPT. In addition, we could figure out that OPT accompanied several chemical reactions along with OPT-induced heating. Indeed, temperature alone does not explain the difference of electrical characteristics since there are clear distinctions in RS between gas flow rates of 35 and 40 slm whereas the temperature difference of the two processes is relatively small (~20oC). Transfer characteristics of conventionally-treated and OPT IGZO TFTs, and the statistical distribution of electrical parameters including mobility, on/off ratio, and subthreshold swing (S.S) are presented in Figure 3(a) and (b), respectively. OPT IGZO TFTs exhibited comparable mobility and S.S to conventionally-treated IGZO TFTs, and showed a hundredfold increase in on/off ratio. The mobility, S.S, and on/off ratio of OPT IGZO TFTs are 14.72 ± 1.02 cm2/(V.s), 0.55 ± 0.10 V/decade, and (3.45 ± 0.9) x 109, respectively. The slight difference in mobility and S.S. may be caused by a decrease in carrier concentrations3,23,24 and the generation of defects and trap sites25 due to OPT, respectively, however the data are insufficient to conclude whether the difference is statistically significant. Figure 3(c) show threshold voltage (Vth) shifts as a function of time under PBS and NBIS for conventionally-treated and OPT IGZO TFTs. For PBS, Vth shifts are similar for both conventionally-treated and OPT IGZO TFTs, which means that the generation of defects and trap sites responsible for bias stress via OPT is negligible. Indeed, the Vth shift under PBS typically originates from trapped electrons in defect sites at the interface between the channel and the gate insulator.26,27 In contrast, for NBIS, Vth shifts in OPT IGZO TFTs are significantly reduced than those in conventionally-treated ones, which implied that OPT can decrease in amount of oxygen vacancies (VO) within IGZO films. It is attributed that Vth shifts under NBIS are dominantly affected under light illumination because extra electrons can be 8


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supplied into IGZO films by ionizing VO due to its low formation energy (~2 eV).28–30 Nevertheless, OPT may cause a different surface morphology on IGZO films compared to a conventional treatment,31,32 which will be further discussed later. XPS depth-profile analysis was used to investigate the chemical stoichiometry as a function of distance from the IGZO surface (Figure 4(a) and (b)). The IGZO samples (IGZO/SiO2/p+Si substrate) were prepared for XPS analysis. Figure 4(a) shows the surface O 1s core-level spectra of untreated, conventionally-treated, and OPT IGZO films. Three contributions are noted and attributed to metal-oxide bonds (M-O), uncoordinated oxygen species of VO, and hydroxyl groups (M-OH) at 530 ± 0.3 eV, 531 ± 0.2 eV, and 532 ± 0.1 eV, respectively.33 As expected, conventionally-treated and OPT IGZO films have a higher concentration of MO bonds and lower concentration of VO and OH groups compared to those of the untreated films. This result indicates that oxygen molecules and oxidative reactive species can diffuse into the IGZO films and induce oxidation of the IGZO films during both the conventional treatment and OPT. However, since reactive species in a plasma are more reactive than ambient gas molecules,34,35 OPT IGZO films exhibited higher M-O and lower VO concentration compared to those of the conventionally-treated IGZO films. Figure 4(b) shows that the concentration of M-O and VO for untreated, conventionallytreated, and OPT IGZO films vary through the film thickness. (Figure S2 shows summarization of numerical area percentages of M-O, VO, and OH for all IGZO films according to depth profile) Interestingly, OPT’s oxidation processes occurred predominately at the back-channel region (IGZO surface) where oxidative reactive species are generated resulting in a higher M-O concentration than the front channel region (IGZO/SiO2 interface). In contrast, oxidation processes occurred uniformly throughout the IGZO thickness with the conventional treatment. Therefore, we could figure out that the low off-current of OPT IGZO

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TFTs is caused by the increased oxidation in the back-channel region of the IGZO film compared to the conventional treatment, suppressing residual back-channel conductivity.36 In addition, OPT IGZO films had lower VO throughout the film, which resulted in reduced carrier concentration, possibly causing a slight decrease in mobility since in IGZO, it has been found that mobility increases with carrier concentration.24 From these results, fine-tuning of the OPT process might enable a complete control of defect concentration throughout the thickness of the IGZO films. In, Ga, and Zn XPS core-level spectra (Figure S3) revealed that OPT oxidized all the metal species in the IGZO films, primarily oxidizing Ga atoms, as demonstrated by the higher intensity peak of Ga 3d oxide at 21.2 eV compared to untreated and conventionally-treated films (Figure S4). Such complete oxidation is due to the low standard electrode potential and high ionic potential of Ga compared to In and Zn.37,38 As the gas flow rate increased, the oxygen stoichiometry of the OPT IGZO films became closer to the untreated IGZO films causing them to exhibit metallic characteristics, as shown by the increase in the shoulder peak attributed to oxygen vacancies (Figure 4(c)). Furthermore, carbon concentrations increased with increasing gas flow rate as shown in Figure S5. This trend is attributed to the fact that carbon species such as CO and CO2 with high diffusion lengths have a dominant effect on IGZO films relative to other chemical species due to lower mean free path at high gas flow rates. The optical bandgap (Eg) and electronic structure around conduction band of IGZO films were investigated by spectroscopic ellipsometry (SE) (Figure S6 and 4(d), respectively). Untreated, conventionally-treated, and OPT IGZO films had an Eg of 3.36, 3.41, and 3.46 eV, respectively. Oxidation with oxidative reactive species in OPT effectively increases the optical Eg of the IGZO films compared to that of the conventionally-treated IGZO films.39 Furthermore, as shown in Figure 4(d), these radical reactions can more effectively quench

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shallow (D2) oxygen defect sites below the conduction band edge of IGZO films, while deep (D1) oxygen defect sites are almost identical. D2 and D1 are strongly correlated with carrier concentration

and

charge

scattering

centers,

respectively.40,41

Thus,

the

optical

characterization corroborates the hypothesis that the OPT IGZO TFTs have slightly lower carrier mobility compared to the conventional IGZO TFTs because of a lower carrier concentration caused by the reduction of electron-donating shallow oxygen defect sites. Furthermore, Figure S7 shows a qualitative comparison of relative ratio of Vo (from XPS analysis) and band edge states near conduction band (from SE analysis) for untreated, conventionally thermal, and OPT IGZO films. From this result, we indeed confirmed that these band edge states near conduction band are closely related to Vo, which qualitatively agreed with XPS O1s core-level spectra. We also probed the influence of plasma reactive species on the crystallographic phase of IGZO films (Figure 4(e)); the IGZO films did not show any peaks suggesting an amorphous film structure3 implying that the OPT energy is too low to induce the solid-state crystallization of the IGZO films. Figure 5 illustrates the proposed mechanism of OPT for the activation of IGZO films. In OPT, there are four effects that collectively activate the IGZO films: 1) generation of ultraviolet (UV) radiation, 2) ion bombardment, 3) chemical reaction with radical species in plasma, and 4) plasma-induced temperature increase. Among these effects, UV radiation and ion bombardment can provide enough dissociation energy to break the weak chemical bonds in the IGZO films and induce the formation of metastable IGZO states by ionizing uncoordinated oxygen species. We measured the UV radiation in OPT using in-situ optical emission spectroscopy (Figure S8). Among the generated radiation, wavelength of UV light from 200 to 300 nm is sufficient to break weak chemical bonds (≤2.0 eV) and diatomic bonds (In-O: 1.7 eV Zn-O: 1.5 eV Ga-O: 2.0 eV) in IGZO films42 due to higher energy of UV light.

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Furthermore, UV radiation also enables the formation of metastable states in the IGZO films from the ionization of oxygen vacancies due to their low ionization energy (~2 eV).28 Ion bombardment can also displace atomic bonds by transferring kinetic energy to the atoms at the IGZO surface.43 Simultaneously, radical species and OPT-induced heat can give rise to the reestablishment and rearrangement of the metal-oxide network by promoting oxidation and chemical reactions in IGZO films, respectively. Generated oxidative radical species such as NO•, O•, and OH• are confirmed using in-situ gas phase mass spectrometry (Figure S9). These radicals have been reported as strong oxidants,15,34,35 leading to a more effective oxidation of the IGZO films. Among these radical species, we also confirmed that O• and OH• are dominantly involved for activation of IGZO films than NO• because N 1s core-level spectra (Figure S10) is negligible changes, while Zn 3d (Figure S11) and O 1s core-level spectra are remarkably modified by OPT.44 Furthermore, the OPT-induced heat can also facilitate the formation of rigid metal-oxide networks and decrease the concentration of uncoordinated oxygen species by providing energy to overcome reaction activation barriers. As a result, OPT activates IGZO films by simultaneous decomposition of weak chemical bonds (by UV radiation and ion bombardment) and reestablishment of the metal-oxide network (oxidative radical reactions and OPT-induced heat). We also observed that OPT can modify the surface topography of IGZO films using AFM (Figure 6(a-b)) and SEM analyses (Figure S12(a-c)). Figure 6(a) shows the OPT IGZO film surface roughness as a function of the plasma nozzle-substrate distance a. We found that OPT can change the microstructure of IGZO films by inducing dome structures on the surface. These features increased in dome size with decreasing distance a, but the thickness of IGZO films did not change (Figure S12(d-f)). In contrast, for different gas flow rates with a fixed distance a = 2 cm, the change in surface roughness is negligible (Figure S13). AES analysis

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was performed to measure the composition of the dome structure (Figure 6(c-d)). Interestingly, the concentrations of oxygen on the domes are higher than other areas of the film, in contrast with conventionally-treated IGZO films, which show uniform compositional distribution (Figure S14). These results confirmed that dome structures were grown during OPT. Thus, the IGZO film with large amount and size of dome structure has higher metaloxide and lower oxygen vacancies concentration compared to other region. Thus, it has low carrier concentration and high resistance. For this reason, the electrical characteristics of OPT-processed IGZO TFTs change toward the insulating characteristics as the size and amount of the dome structure increases by controlling OPT conditions, as shown in Figure 2(e). Increasing the OPT reactive species density can decompose more chemical bonds and as a result the dome structure becomes larger as distance a decreases from 3 to 1 cm. It noted that controlling distance a more carefully may allow fabrication of an oxide based sensor with high specific surface area. Lastly, we confirmed that OPT can be used for selective activation and flexible applications. Figure 7(a) illustrates a schematic of selective activation of IGZO TFTs using OPT. We conducted OPT on specific IGZO regions and compared the electrical characteristics of IGZO TFTs with and without OPT on the same samples. IGZO regions treated by OPT showed semiconductor characteristics (Figure 7(b)), while the regions without OPT retained metallic characteristics of the as-deposited IGZO (Figure 7(c)). Flexible IGZO TFTs were fabricated based on the architecture depicted in Figure 7(d) and transfer characteristics were measured (Figure 7(e)). Even though the transfer characteristics of IGZO TFTs on flexible substrates were slightly different compared to those measured on p+-Si substrates due to the use of different gate electrode materials and gate insulator thickness, OPT is suitable to activate IGZO TFTs without any degradation of the temperature-sensitive flexible polyimide substrate.

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From these results, we conclude that OPT can be regarded as a promising activation method for sputter processed oxide films that is amenable for selective treatment and flexible applications.

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4. CONCLUSION We demonstrated OPT for the activation of sputter-processed IGZO films for TFT devices. While conventional treatment requires high temperature (300°C) with long treatment time (≥1 hour) to achieve semiconductor properties, OPT significantly reduces the thermal budget and increase the throughput of the IGZO activation process (240oC, 3 seconds). Furthermore, despite the low thermal budget, OPT IGZO TFTs exhibited improved on/off ratio, and similar mobility and S.S. compared to conventionally-treated IGZO TFTs. The OPT activation mechanism is subdivided in two simultaneous effects. UV radiation and plasma ion bombardment first dissociate weak chemical bonds and induce metastable states of IGZO films due to their high energy (decomposition of weak chemical bonds). At the same time, radical species (NO•, O•, and OH•) and OPT-induced heat can promote reconstruction and rearrangement of metal-oxide network effectively (reestablishment of the metal-oxide network). Finally, we confirmed that OPT can be used for selective activation and activation on flexible substrates.

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FIGURES

Figure 1. Scheme of OPT treatment and electrical characteristics of conventionally-treated IGZO TFTs. (a) experimental steps of OPT treated IGZO TFTs. (b) illustration and photo image of OPT system. Transfer characteristics of IGZO TFTs as a function of (c) activation temperatures and (d) treatment times (at 300oC). (e) Change of sheet resistances for IGZO films with increased treatment times (at 300oC).

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Figure 2. Electrical characteristics of OPT IGZO films as a function of scan speed, distance a, and gas flow rate. Illustration of OPT IGZO films treated with different (a) scan speeds, (d) distances a, and (g) gas flow rates. Transfer characteristics for OPT IGZO films as a function of (b) scan speed, (e) distance a, and (h) gas flow rates. Sheet resistance and OPT-induced temperature for OPT of IGZO films as a function of (c) scan speed, (f) distance a, and (i) gas flow rates.

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Figure 3. Comparison of electrical characteristics for conventionally-treated and OPT IGZO TFTs. Comparison of (a) transfer characteristics, (b) electrical parameters such as mobility, on/off ratio, and S.S, and (c) threshold voltage shift under PBS test (VGS = 20 V and VDS = 10.1 V) and NBIS test (VGS= -20 V and VDS= 0.1 V under 5700 lux) for conventionallytreated and OPT IGZO TFTs.

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Figure 4. Chemical and Optical analysis of un-treated, conventionally-treated, and OPT IGZO films. (a) deconvoluted XPS O 1s spectra and (b) area fraction of M-O and oxygen vacancies in the spectra for untreated, conventionally-treated, and OPT IGZO films with respect to depth. Deconvoluted O 1s spectra of OPT IGZO films with (c) gas flow rate of 40 and 45 slm. Comparison of (d) band edge states below the conduction band and (e) XRD patterns of untreated, conventionally-treated, and OPT IGZO films. The XRD peaks near 53o and 56o are substrate peaks.

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Figure 5. Illustration of physical and chemical mechanism for activation of IGZO films using OPT. OPT IGZO films simultaneously underwent 1) decomposition of weak chemical bonds (generation of UV and ion bombardment) and 2) formation of metal-oxide bonds (radical reaction and OPTinduced heat). 20


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Figure 6. Structural changes upon OPT of IGZO films. (a) AFM images of OPT IGZO films as a function of different distance a between nozzle and IGZO films. (b) IGZO surface roughness for various conditions of OPT. (c) compositional mapping images and (d) intensity of dome structure and surface on OPT IGZO films.

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Figure 7. Various utilizations of OPT. (a) illustration of selective IGZO activation using OPT. Transfer characteristics of IGZO TFTs treated by (b) OPT and (c) untreated. (d) illustration of structure and photo image and (e) transfer characteristics of flexible OPT IGZO TFTs

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ASSOCIATED CONTENT Supporting information Cross section image of SEM for untreated and conventionally-treated IGZO films; Summarized XPS O 1s spectral area percentages; XPS In, Ga, and Zn metal spectra for untreated, conventionally-treated, and OPT IGZO films; Deconvoluted Ga spectrum of untreated, conventionally-treated, and OPT IGZO films; Comparison of carbon concentration with increasing gas flow rate; Optical band gap energy for untreated, conventionally-treated, and OPT IGZO films; Qualitative comparison between XPS and SE analyses; Spectrum of plasma-generated radiation; Chemical species generated in optimized OPT conditions; XPS N 1s core-level spectrum of untreated, conventionally-treated, and OPT IGZO films; XPS Zn 3d core-level spectrum of untreated, conventionally-treated, and OPT IGZO films; SEM image of OPT IGZO surface and cross section as a function of distance; AFM image of OPT IGZO with different gas flow rate; AES mapping image of conventionally-treated IGZO films AUTHOR INFORMATION Corresponding Authors Reinhold H. Dauskardt: [email protected] Alberto Salleo: [email protected] Hyun Jae Kim: [email protected]

ACKNOWLEDGMENTS This work was supported by the National Research Foundation of Korea (NRF) grant funded by the Korea government (MSIP) (No. 2017R1A2B3008719) and Department of Energy through the Bay Area Photovoltaics Consortium under Award Number DE-

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EE0004946. Part of this work was performed at the Stanford Nano Shared Facilities (SNSF), supported by the National Science Foundation under award ECCS-1542152.

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High-Throughput Open-Air Plasma Activation of Metal-Oxide Thin

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