Scalable, High-Performance Printed InOx Transistors Enabled by UV

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Functional Inorganic Materials and Devices x

Scalable, High-Performance Printed InO Transistors Enabled by UV-Annealed Printed High-k AlO Gate Dielectrics x

William J. Scheideler, Matthew McPhail, Rajan Kumar, Jeremy Smith, and Vivek Subramanian ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.8b12895 • Publication Date (Web): 09 Oct 2018 Downloaded from http://pubs.acs.org on October 13, 2018

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ACS Applied Materials & Interfaces

Scalable, High-Performance Printed InOx Transistors Enabled by UV-Annealed Printed High-k AlOx Gate Dielectrics William J. Scheideler†, Matthew W. McPhail, Rajan Kumar, Jeremy Smith, and Vivek Subramanian*

William J. Scheideler, Matthew W. McPhail, Jeremy Smith, Vivek Subramanian* Department of Electrical Engineering and Computer Sciences University of California, Berkeley, CA, 94720, USA E-mail: [email protected]

R. Kumar Department of Materials Science and Engineering University of California, Berkeley, CA, 94720, USA

Keywords: transparent metal oxide transistors, UV-Annealing, high-speed inkjet printing, high-k dielectrics, bias-stress stability

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ABSTRACT Inorganic transparent metal oxides represent one of the highest performing material systems for thin film flexible electronics. Integrating these materials with low-temperature processing and printing technologies could fuel the next generation of ubiquitous transparent devices. In this work, we investigate the integration of UV-annealing with inkjet printing, demonstrating how UV-annealing of high-k AlOx dielectrics facilitates the fabrication of high-performance InOx transistors at low processing temperatures and improves bias-stress stability of devices with all-printed dielectrics, semiconductors, and source/drain electrodes. First, the influence of UV-annealing on printed MIM capacitors is explored, illustrating the effects of UV-annealing on the electrical, chemical and morphological properties of the printed gate dielectrics. Utilizing these dielectrics, printed InOx transistors were fabricated which achieved exceptional performance at low process temperatures (< 250 ºC), with linear mobility µlin ~ 12 ±1.6 cm2/Vs, subthreshold slope < 150mV/dec, Ion / Ioff > 107, and minimal hysteresis (< 50 mV). Importantly, detailed characterization of these UV-annealed printed devices reveals enhanced operational stability, with reduced threshold voltage (Vt) shifts and more stable on-current. This work, therefore, highlights a unique, synergistic interaction between low-temperature-processed high-k dielectrics

and

printed

metal

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semiconductors.

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1. Introduction Thin film transistors (TFTs) are important devices for evolving applications of ubiquitous electronic systems in the Internet of Things (IoT)1,2, wearables3, flexible electronics4, and biomedical devices5,6, particularly in applications involving information display7. Metal oxide semiconductors are leading materials to drive these thin film electronic systems since they offer significantly higher performance than competing materials such as amorphous silicon8 and organic semiconductors9, as evidenced by the shift to oxide TFTs for use in commercial flat panel display backplanes10. Metal oxide TFTs have excellent switching behavior including high mobility and steep subthreshold swing11, as well as good performance under flexion afforded by their amorphous or nanocrystalline nature.12 Additionally, metal oxides have high visible range transparency, which makes them attractive for optoelectronic applications involving light emission and absorption13. However, the relatively high cost of processing metal oxide materials over large areas and on flexible substrates14 currently limits their viability for widespread use in thin film devices. Scalable, low-cost fabrication of such transparent electronics could be possible via the scalability of printing and solution-processed metal oxides. As demonstrated by the graphic arts industry, printing technologies can efficiently pattern inks at extremely high-throughput on flexible substrates. Leveraging these methods in printed electronics requires precursor inks which facilitate both high electronic performance and high-resolution printing. Spin-coated metal oxides have recently reached performance benchmarks comparable to that of state-of-theart TFTs used in modern display technology15,16 using materials such as InOx, ZnOx, and SnOx, as well as alloys thereof. Additionally, techniques such as inkjet17, flexography18, and gravure 3 ACS Paragon Plus Environment


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printing19 have been developed to allow high-throughput fabrication of metal oxide thin films. However, multiple issues must be solved before printed metal oxides can overcome incumbent technologies. First, high temperature annealing of printed metal oxide TFTs17 limits the use of polymer substrates and thereby increases manufacturing costs and limiting flexibility. Secondly, the lack of printable formulations of high-quality metal oxide dielectrics must be addressed to replace expensive vacuum-processed dielectrics in printed metal oxide transistors. Finally, there is a need for high-speed, scalable printing methods compatible with roll-to-roll (R2R) manufacturing. Ultraviolet (UV)-annealing is a R2R-compatible method for well-suited for processing printed films. UV curing systems are vital to the coating industry due to their high-speed and low temperatures for thermally sensitive materials and substrates. In solution-processed sol-gel metal oxides, UV irradiation during fabrication can improve low-temperature conversion of metal oxide semiconductors to functional films16. Current understanding of the effects of UV on metal oxide precursors is that high energy photons break coordination complexes between metal cations and organic ligands incorporated into the sol-gel films. The net effect of UV, as illustrated by Park et al.20 and Jo et al.21, is rapid densification of the metal oxide matrix and an increase in formation of M-O-M bonds as observed by surface sensitive techniques such as XPS22. These observations suggest UV-annealing could address major technological barriers to printed transistors by lowering process temperatures and allowing R2R processing. Performance and reliability of solution-processed metal oxide transistors is closely tied to processing of the dielectric.

Low-temperature

processed

transistors,

in

particular,

exhibit

considerable

enhancement in mobility and current when integrated with low-temperature processed high-k dielectrics23. To date, UV-annealing has been the predominant route to high-performance, metal

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oxide semiconductors16,24 and dielectrics20 at low process temperatures below 300 °C. The majority of this work focused on the impact of UV-annealing in inert atmospheres on ultrathin spincoated films (thickness < 20 nm). However, ultrathin dielectrics21 and inert atmosphere steps present problems for integration into printed TFTs. Printed devices have unique requirements due to their reliance on nanoparticle-based electrodes with higher surface roughness and the presence of artifacts such as coffee-rings. Thicker dielectrics are needed to compensate for these printingspecific features and ensure reliability.

Figure 1. (a) Scheme illustrating inline integration of high-speed inkjet printing, open-air UVannealing, and low-temperature thermal annealing for printed transistor fabrication. (b) Photograph of a 75mm x 30mm array of printed AlOx dielectrics. (c) UV-annealing lamp spectrum. (d) InOx thin film transistor structure including all-printed dielectrics, conductors, and conductors. This work addresses these challenges these challenges by integrating UV-processing and inkjet to develop low-temperature-processed printed metal oxide transistors with UV-annealed high-k AlOx dielectrics. The design of AlOx inks as well as multi-nozzle printing methodologies are engineered to enhance uniformity and speed. The influence of UV-annealing on low5 ACS Paragon Plus Environment


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temperature-processed high-k dielectric properties and the characteristics of printed InOx TFTs is studied, as well as the role of UV in improving bias-stress stability. By exploiting these techniques, we achieve high device performance at low temperatures and accomplish integration of dielectrics, semiconductor, and source / drain electrodes via additive inkjet printing. A scheme illustrating the combination of these methods is shown in Figure 1. In this study, all three layers are deposited via inkjet printing without vacuum processing or photolithography which otherwise

limit

manufacturing

throughput.

2. Experimental Methods 2.1 Precursor Ink Preparation AlOx dielectric precursor inks were prepared from Al(NO3)3 (Sigma 229415, 99.997 %, 200 mM-800 mM) in 2-methoxyethanol. Dielectric inks were mixed fresh prior to printing, sonicated for 10 minutes, and filtered through a 0.45 µm PTFE filter before loading into 1 pL Dimatix DMP (DMC-11601) printing cartridge. InOx semiconductor inks were prepared from In(NO3)3 (Sigma 326127, 99.99 %, 200 mM) in DI water and loaded into 1 pL Dimatix cartridges (DMC11601). Finally, transparent conductive CdO:Al precursor inks were prepared from aqueous Cd(NO3)2 (Sigma 229520, 99.997 %, 200 mM) with a 3% molar ratio of Al(NO3)3 and filtered before loading into 10 pL Dimatix cartridges (DMC-11610). All solvents were obtained from Sigma Aldrich. 2.2 Printed Film Deposition: Printed high-k AlOx dielectrics (600 µm x 1200 µm) were deposited on n++ silicon at 40 µm drop spacing and a platten temperature of 36 ºC using a Fujifilm Dimatix DMP-2850 inkjet printer. Prior to printing, a 2 minute UV-ozone treatment (Jelight 42) was used to achieve 6 ACS Paragon Plus Environment

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favorable wetting. To print the AlOx ink, the Dimatix Model Fluid 2 waveform was used, with a jetting voltage of 11.0 V and a frequency of 5 kHz (Figure S1). To build thicker wet films, multiple consecutive layers (four, unless otherwise specified) were deposited consecutively without delay. AlOx films were dried on a hot plate for 5 minutes at 150 ºC and exposed to UV in air using a 400 W metal halide lamp (Loctite ZETA 7401) with a parabolic reflector and an intensity of 1 W/cm2 for 5 – 20 minutes. The spectral intensity of the UV source is shown in Figure 1c. We note that although the UV-annealing lamp housing was vented, a high volume of ozone was generated during exposure. Following UV, the dielectrics underwent a thermal postanneal (2 hrs) at 250 ºC on a hot plate in air (Relative Humidity ~ 40-55%). Separate sets of Metal-Insulator-Metal (MIM) test capacitors (Si / AlOx / ITO) were postannealed at 150 °C, 200 °C, and 300 °C for 2 hrs to examine the impact of the thermal post-annealing temperature. ITO nanoparticle (NP) (15 weight % in 1:1 IPA: Ethylene glycol, Aldrich 700460) electrodes were printed (100 µm x 300 µm, thickness 400 nm) to achieve a low-temperature (150 °C) processed contact for comparing leakage and breakdown of AlOx capacitors. Printed capacitors with Si / AlOx / CdO:Al structures were fabricated by equivalent dielectric printing and electrode printing procedures for use in stress-induced leakage (SILC) measurements. MIMs with metallic Ag electrodes were printed (ANP DG-40LT-15C, 20 µm DS, 1 pL nozzle) for higher frequency (100 kHz – 1 MHz) capacitance evaluation due to the limited conductivity of ITO NP printed electrodes. 2.3 Transistor Fabrication and Device Measurements Substrate-gated InOx transistors were printed with UV-annealed AlOx dielectrics on n++ Si. Printing conditions, ink parameters, and annealing conditions are listed in supporting information table S1. Further details of the printed source / drain fabrication and semiconductor printing are



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listed in a previous work25. The printed devices had a channel width of approximately 400 µm and a channel length of approximately 250 µm. Control TFTs for bias-stress measurements were fabricated on Si wafers with 100 nm thermally grown SiO2 using printed semiconductor layers and electrodes by an equivalent process to devices with AlOx. A 20 minutes UV ozone treatment was performed before printing the InOx semiconductor on SiO2. Printed films were characterized with stylus profilometry (Dektak Veeco 6M) measurements of film thickness and optical microscopy. Transistor and dielectric IV characteristics were measured at room temperature, in air, with a semiconductor parameter analyzer (Agilent 4156). Capacitance vs frequency of the printed capacitors were measured with an LCR meter (Agilent E4980). 3. Results and Discussion 3.1 Inkjet Printing High-k AlOx Dielectrics To design optimal high-k dielectrics and study the impact of UV-annealing on dielectric properties and TFT performance, multilayer printing and varying solute concentrations were used to deposit sol-gel films with a wide range of thicknesses from 15 nm to 200 nm. Highly uniform, insulating dielectric films were printed by utilizing multi-nozzle jetting and by tuning the concentration of the inks. Increasing the concentration of the sol-gel inks from 200 mM to 800 mM resulted in approximately a four-fold enhancement in the viscosity (Figure 2a), resulting in suitable range (8-12 cP) for reliable inkjet printing of both 1 pL and 10 pL nominal droplet sizes. This study also highlights an interesting feature of inkjet printing, which is that it easily allows the printing of multiple film thicknesses on the same substrate. As shown in Figure 2b, a wide range of film thicknesses were printed with a 200 mM ink by varying the drop spacing and depositing additional layers. This offers an interesting capability for printed circuit design to 8 ACS Paragon Plus Environment

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easily fabricate capacitors of varying areal capacitance or circuits with high and low operating voltages. The design of a printing process for high-k gate dielectrics is influenced by many factors such as ink viscosity, surface tension, wetting, jetting, and drying kinetics. Figure 2c summarizes the two types of inkjet printing processes used to deposit gate dielectrics in this work, highlighting the difference between single nozzle and multi-nozzle printing. In single nozzle printing of nanoscale films, the sequential and intermittent drying of the film during print head raster cycles results in thickness non-uniformity due to capillary flows. In cases when droplets merge to form a continuous film free of pinholes, the surface profiles (Figure 2d,e) inherit a roughness with a wavelength approximately equal to the drop-spacing of the pattern. Multi-nozzle printing, in contrast, can deposit an entire film in a single pass, eliminating intermittent drying and enhancing film uniformity. In Figure 2f, cases (ii) and (iv) and Figure 2g illustrate the results of using 16 nozzles at a time to print a single film. The thickness profiles lack the drop-spacing determined roughness, achieving a relatively flat surface (RMS variation < 5 % of thickness) as measured with a stylus profilometer in the horizontal direction. The reduction in intermittent drying in these multinozzle printed films also eliminates significant thickness variation in the perpendicular direction (Figure 2g), which may otherwise result from the incomplete merging, coffee-ring deposits, and leveling of consecutively printed lines with a single nozzle (Figure S2a). An important characteristic of these dielectric profiles is that they lack the considerable coffeering deposits that are typically left by low-boiling point printed sol-gels26. Coffee-rings typically result from outward convective flows that deposit excess material near the contact line of printed features27. However, in this case, the coffee-ring effect was mitigated by using a thinner initial



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fluid film thickness and a higher solute concentration. Switching from the standard 10 pL nominal drop volume to a 1 pL nominal drop volume considerably reduced the coffee-ring edge deposits formed at the edge of the films (Figure S2b) and improved overall film uniformity. Similarly, higher concentration inks (400 mM – 1200 mM) resulted in elimination of the coffeering effect, as shown in Figure 2. These observations are consistent with an explanation that considers the shorter drying time of thinner films. Reduced drying time limits convective outward flows, ensuring the dried film will more closely resemble the initial profile of the wet film. Similarly, the increased viscosity of concentrated inks retards outward convective flows. Therefore, minimizing the individual drop volume and increasing the concentration of the solute are two keys to printing uniform dielectrics. Dielectric thickness uniformity is important for device operation because it establishes a tradeoff between capacitance, operating voltage, and reliability. Films with large thickness variations sustain higher peak electric fields in the thinnest regions, resulting in a lower breakdown field for equivalent capacitance. Thus, for a given thickness, uniform films sustain higher operating voltages without suffering reliability issues. Processes that enhance dielectric uniformity can therefore impact performance and reliability. Additionally, these strategies are essential for scaling a process for R2R manufacturing. The fact that industrial inkjet technologies utilize large format print heads with over 1,000 nozzles28, makes multi-nozzle printing essential and demands higher throughput annealing methods such as UV.


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Figure 2. a) Viscosity vs concentration of AlOx precursor inks. b) Contour plot of average high-k dielectric film thickness printed with 1-7 layers and variable drop spacing. c) Schematic illustrating single nozzle and multinozzle printing processes. d) Thickness profiles of 200 mM concentration printed films with single and multiple nozzles. e) Thickness profiles of 800 mM concentration printed films with single and multiple nozzles. f) Four layer printed high-k dielectric films micrographs using 200mM concentration (i,ii) and 800 mM concentration (iii, iv) inks printed using single (i, iii) or multiple nozzles (ii, iv), with uniform scale bars of length 300 µm.

g)

Thickness profiles measured perpendicular to print head scanning direction.

3.2 UV-Annealing Printed Dielectrics UV-annealing offers a route to mitigate the tradeoff between thermal budget and electrical performance of sol-gel films. In this study, UV-annealing was applied to the printed dielectric films prior to low-temperature thermal annealing, yielding a significantly improved dielectric



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response. As shown in Figure 3a and 3b, UV-annealing yielded printed capacitors with flatter frequency response and dielectric constants of 6-8, similar to that of high-temperature processed AlOx sol-gel dielectrics29. This suggests that residual organics incorporated during the film drying process are decomposed by UV radiation20 and can be volatilized during low-temperature thermal annealing. This is consistent with previous literature on this sol-gel material system, which showed that increasing UV-dose could enhance the dielectric response30. However, a unique result in this work is that a high intensity (1 W/cm2) broadband source can induce conversion in air rather than an inert environment20,31, relying on the absorption of the AlOx precursor 250 nm to 400 nm (Figure S3c). The current results showing the improvement of UVannealed films with increased post-annealing temperature up to 250 °C (Figure 3a), indicate the synergy between these two methods, necessary for achieving high-performance devices. By comparison, the dielectric response of the thermal-only annealed AlOx printed dielectrics (Figure 3b) showed significant low-frequency dispersion and excessively high dielectric constants. Even after annealing at 300 °C, the films showed dielectric constants of nearly 30 measured at 20 Hz. Additionally, samples processed by thermal annealing showed higher variability in their dielectric response, particularly at low-frequencies. One reason for the poor characteristics of thicker films could be the presence of residual organic ligands left in the film after thermal annealing, originating from the coordination of 2-ME solvent molecules with the sol-gel precursors. Thermal gravimetric analysis (Figure S3a,b) of the aluminum nitrate ink indicates that decomposition is not completed until the sol-gel has been heated to beyond 300 ºC. Indeed, in other sol-gel systems, film quality and performance tends to decrease above a critical film thickness19, as precursor compounds are trapped in the bulk of the film rather than being


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volatilized. These mechanisms could explain the need for UV-annealing to achieve well-behaved printed dielectrics at plastic compatible temperatures. The enhancement of dielectric properties with the combination of UV and thermal annealing is evident in the leakage current density (Figure 3c,d) and breakdown fields (Figure 3e) of the printed metal-insulator-metal (MIM) capacitors (final thickness of 100 nm). UV-treated AlOx films post-annealed at 250 ºC achieve low-leakage currents (5 ∙ 10-6 A/cm2), considerably improved beyond UV-treated films post-annealed at 150 ºC and 200 ºC as well as those treated with only thermal annealing at 250 ºC. This comparison is illustrated below in a Weibull plot of the leakage current measured at a field 1 MV/cm for each annealing condition. The improved films display higher breakdown fields of approximately 5.1 MV/cm, as shown in the Weibull plot of Figure 3e and the J-V characteristic in Figure 3c. These results indicate the necessity of coupling UV and thermal annealing to achieve ideal dielectric characteristics for use in printed transistors. Importantly, these characteristics extend to MIMs printed with varying AlOx thickness, which exhibited similar leakage and breakdown (Figure S4). Furthermore, the thickness of the printed films (Figure 3f), show that UV-annealing as short as 5 minutes rapidly induces significant thickness reduction, consistent with the decomposition and conversion of the precursor film. The effect is particularly noticeable for the thicker precursor films, which exhibit a 50 % volume reduction following a 20-minute UV-anneal. After long exposures (20 - 40 minutes), the film approaches the thickness otherwise achieved with thermal annealing at 250 ºC for two hours. This suggests the role of UV-annealing in converting the film to a functional dielectric. Indeed, XPS measurements of the surface of the printed dielectric films (Figure S5) confirm that the extent of M-O-M bonding is enhanced by UV-annealing compared to other reports of low-temperature processed AlOx films32, and is lower than that of the control



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samples which received only thermal annealing at 250 °C, as summarized in Table S2. Indeed, combination of UV-annealing with low-temperature thermal annealing lead to a 20 % improvement in the fraction of AlOx observed by XPS. We expect that additional, higher energy DUV annealing33 or high-temperature thermal annealing could further convert the film and improve M-O-M bonding. Given the dramatic thickness reduction and chemical conversion, atomic-force-microscopy (AFM) was performed on the printed AlOx films to examine their surface morphology, uniformity, and suitability for subsequent film deposition. The AlOx films (4 layers) processed at 250 °C showed ideal low RMS surface roughness of approximately 0.2 - 0.3 nm for films with and without UV-annealing, respectively over both small (2 µm x 2 µm) as well as large area scans (50 µm x 50 µm) (Figure S6). Combined, these factors, namely the smoothness and uniformity of these films and enhanced M-O-M bonding, indicate that UV-annealing can play a vital role in enhancing the film quality of low-temperature-processed, printed high-k dielectrics.


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(a)

(b) UV + Thermal Annealing

(c) 150 200 250 250

Only Thermal Annealing

(e)

(d)

°C °C °C °C (NO UV)

(f)

150 C 200 C 250 C

150 C 200 C 250 C

a

Figure 3. a) Dielectric constant vs frequency for UV-annealed AlOx with varying thermal postannealing. b) Dielectric constant vs frequency for thermal-only annealed printed AlOx dielectrics. c) Leakage current density vs electric field for printed AlOx capacitors with ITO electrodes. d) Weibull distribution and fits of leakage current density measured at 1 MV/cm and breakdown field (e) of printed AlOx dielectrics with UV-annealing. f) Average thickness vs UV-annealing time for AlOx printed capacitors printed with 3 layers of 800 mM Al(NO3)3 ink at 25 µm dropspacing.

3.3 Low-Temperature Printed Transistors with UV-Annealed High-k AlOx Dielectrics InOx transistors fabricated with UV-annealing show excellent switching behavior while utilizing an inkjet-printed structure consisting of AlOx high-k dielectrics, aqueous printed InOx semiconductor layers, and printed aluminum-doped cadmium oxide source / drain electrodes, all 15 ACS Paragon Plus Environment


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processed at low-temperatures (< 250 °C). As shown below in Figure 4a, the linear and saturation transfer characteristics of printed TFTs on 200 nm thick AlOx dielectrics have steep turn on (subthreshold slope < 150 mV/dec) and show minimal counter-clockwise hysteresis (< 50 mV). Additionally, the well-matched turn on voltage (Von) for the linear and saturation regimes indicates the lack of any DIBL-like behavior34. These characteristics suggest that the integration of InOx with UV-annealed AlOx is a favorable combination for producing a superior semiconductor to dielectric interface, leading to well-behaved, stable device characteristics. Control devices fabricated on thermally-grown SiO2 dielectrics of thickness 100 nm (Figure 4b) with equivalent capacitance to printed AlOx films show considerably larger hysteresis windows and worse subthreshold slope. The comparison illustrates the advantages of using high-k dielectrics with solution-processed metal oxide semiconductors and demonstrates the highquality devices which can be produced by using UV-annealing. We note that the ozone generated during UV-annealing and UV-ozone surface pretreatment before the printing of the semiconductor layer have also been reported to improve the AlOx / semiconductor interface, enhance subthreshold slope and reduce hysteresis35. Additionally, unlike many other solution-processed and printed metal oxide TFTs (InOx36, SnOx17, etc) which operate in depletion mode, the printed transistors demonstrated in this study operate with only a small negative turn-on voltage (Von). As a result of the lower gate-field used to turn off these devices, the off current is relatively low (less than 10 pA) in most cases, improving the Ion / Ioff ratio to greater than 107, a key metric for applications such as power efficient high-resolution emissive displays and low-power sensors. The distribution of the linear mobility for devices built on printed AlOx dielectrics is shown below in Figure 4c, with an average µlin = 12 ± 1.6 cm2/Vs. This performance, achieved with only low-temperature


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processing, is comparable to recent reports of all-inkjet printed transistors with printed dielectrics fabricated at significantly higher processing temperatures of 450 - 500 °C17,37. The principal benefits of leveraging UV-annealing printed AlOx dielectric films are the improved device stability and flexible control over TFT operating voltages that can be achieved by varying the film thickness. Without the integration of UV-annealing, AlOx films above 100 nm in thickness often become cracked following thermal annealing at 250 ºC, leading to nonfunctional transistor structures with very high gate leakage. With UV-annealing, however, thicker high-quality printed dielectrics can be formed in a single deposition step without cracking. Using higher concentration AlOx precursor solutions (400 - 800 mM), films anywhere from 15 nm to 200 nm in thickness were printed using multiple passes and were UV-annealed to form robust dielectrics. Different thicknesses of AlOx allowed operation at voltages low as ± 2 V or up to ± 30 V. The output characteristics of these printed devices (Figure 5a-c) show efficient current injection at low drain bias and good current saturation at high drain bias for each AlOx thickness. This is also reflected in the saturation regime transfer characteristics (Figure S7), which closely fit the square-law and exhibit saturation mobility of approximately 10 cm2/Vs with gate leakage in the range of 1-50 nA.



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Figure 4. a) Transfer characteristics for printed InOx devices with 200 nm thick UV-annealed AlOx dielectrics and with 100 nm thick thermally grown SiO2 dielectrics (b). Distribution of peak linear mobility (c) for printed InOx transistors on AlOx dielectrics. (a)

(b)

(c)

Figure 5. a) Output characteristics of printed InOx TFTs with AlOx dielectrics of tox = 15 nm, 30 nm (b), and 200 nm (c).

3.4 Impact of UV-Annealing on TFT Bias-Stress Stability Bias-stress stability is an essential property of thin film transistors which can potentially limit their viability in various applications. Bias-stress describes the results of applying a gate bias in the on or off-state over an extended period of time, resulting in shifts in device properties such as the threshold-voltage (Vt)14. Transistors with high bias-stress stability retain their properties and maintain consistent drive current over time, a necessary feature for many circuit-level applications. The bias-stress stability of the printed InOx transistors was investigated to understand the influence of the UV-annealed AlOx dielectric film properties. To distinguish the influence of bias-stress on the dielectric and the semiconductor layers, control devices utilizing a high-quality thermally grown SiO2 (100 nm) as a gate dielectric were fabricated by similar printing methods 18 ACS Paragon Plus Environment

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for the semiconductor layers and electrodes. These control devices were subjected to positive bias-stress (PBS) and negative bias-stress-under white light illumination (NBIS) at VG = ± 50 V for durations up to 103 s. Figure 6a illustrates the results of (PBS) on these control devices. These devices showed moderate bias-stress stability with Vt shifts of approximately 10-15 % of the operating voltage range, consistent with the high quality typically observed of aqueousprocessed printed InOx channel layers38. PBS induces positive Vt shifts and NBIS induces negative Vt shifts, consistent with electron trapping in the InOx semiconductor layers and at the SiO2 / InOx interface39. In both cases of NBIS and PBS, the subthreshold swing is only slightly degraded (Figure S8) in these control devices and the Von shifts by a similar amount to the threshold voltage. The stressing characteristics of the TFTs with thin (15 – 30 nm) AlOx show similar general trends to the control devices on SiO2, but have larger Vt shifts relative to the operating voltage, resulting in worse degradation to on-current during stress measurements (Figure 6b). TFTs with thicker printed dielectrics, however, show superior stressing characteristics. The PBS and NBIS characteristics of InOx TFTs with 200 nm thick AlOx dielectrics show degraded subthreshold swing after stressing, resulting in negative shifts to Von (Figure 6c-d). However, in both cases the Vt is relatively stable. Interestingly, the devices with thicker dielectrics have inverted stressing with respect to Vt, showing negative Vt shifts for positive stress (PBS) and positive Vt shifts due to negative stress (NBIS). This change in bias-stress polarity suggests an alternative mechanism is responsible for the stressing characteristics of thicker printed dielectrics which partially compensates the typical shifts observed due to interface trapping in the InOx semiconductor layer. Indeed, a similar trend was recently observed by Carlos, et. al., for sputtered Indium Gallium Zinc Oxide (IGZO) TFTs



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fabricated using UV-processed sol-gel AlOx dielectrics31, suggesting that this compensation effect may be a more general feature of low-temperature high-k dielectric films. This behavior has been observed before in vacuum-processed devices with low-temperature ALD-processed AlOx dielectrics40. One explanation for these observations is electron emission from donor traps in the bulk of the AlOx dielectric. The ionization of these donor traps would create positive charge in the dielectric, resulting in the observed negative Vt shifts. The enhanced density of donor-like traps in solution-processed high-k dielectrics may, in fact, explain their tendency to compensate the electron trapping in the semiconductor bulk and interface states that normally drives the PBS behavior of these devices. Another viable explanation for inverted stressing suggested by Chang, et al. for ALD processed AlOx dielectrics is a two-process model which involves the release of hydrogen from the bulk of the dielectric due to breakage of AlO-H bonds by energetic carriers during bias-stress. This mechanism is consistent with the relatively high percentage of AlO-H bonds exhibited in x-ray photoelectron spectroscopy of the surface of the printed dielectrics (Figure S5). Future development of the ink chemistries and annealing methods for these high-k dielectric materials will be required to control this effect and achieve optimal

bias-stress

compensation.


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Figure 6. a) Inkjet printed InOx TFTs under PBS with 100 nm thermal SiO2 and 15 nm AlOx (b) dielectrics. c) InOx TFTs on 200 nm printed AlOx dielectrics under NBIS and PBS (d).

The Vt shifts (normalized by the operating voltage, VDD) exhibited over the course of biasstress tests are shown below in Table 1 for AlOx dielectrics of varying thickness and SiO2 control dielectrics. The devices were biased such that they maintained an equivalent product of gate capacitance and operating voltage. Devices with ultrathin dielectrics have larger Vt shifts relative to their operating voltage, amounting to greater than 25 % of VDD over the course of a 103 s 21 ACS Paragon Plus Environment


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period of stress. However, devices with thicker 200 nm AlOx show Vt shifts roughly on par with the control devices built with highly stable thermally grown SiO2, a dielectric which has been shown to lead to excellent bias-stress stability and low interface defect densities for IGZO TFTs41,42. The result of this improved stability, as well as the inverted stressing characteristics of thicker AlOx dielectrics, is that the devices fabricated with these printed dielectrics maintain a greater fraction of their original on current after lengthy positive bias-stress. Devices on 200 nm AlOx dielectrics remain within 10% of their original Ion throughout negative bias-stress measurements. In comparison, devices on thin AlOx (15 nm), have severely degraded Ion which changes by more than 50 % of its original value from induced Vt shifts. In the case of very thin dielectrics, the large Vt shifts also decrease the peak mobility of the TFTs (Figure S9), contributing to decreases in Ion. Table 1. Bias-Stressing Characteristic Summary of Printed InOx TFTs under 103 s stress Dielectric

Thickness [nm]

Bias-Stress

∆ Vt / VDD [%]

∆ IDSmax [%]

Thermal SiO2

100

PBS

+ 13.4

- 21.8%

100

NBIS

- 12.1

+22.0

15

PBS

+ 30.0

-78.0

15

NBIS

- 24.4

+56.8

200

PBS

-20.9

+26.0

200

NBIS

+26.0

-12.3

Printed AlOx

Printed AlOx


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Additionally, we investigated the ways in which the steep subthreshold turn on of these printed TFTs can allow operation at a relatively lower gate bias to improve long-term operational stability. As shown in Figure 7a, the linear mobility demonstrated by InOx TFTs with printed AlOx dielectrics exhibits the same sharp transition as the transfer characteristics, reaching its peak value at a relatively low applied gate voltage compared to the devices on SiO2. The result of this is that the devices may be operated at a lower gate voltage without sacrificing large amounts of drive current, and the shifts during bias-stress can be reduced by an order of magnitude to the ~ 100 mV range (Figure 7b). To understand the stressing characteristics of thicker printed dielectrics, stress-induced leakage current (SILC) measurements were performed on MIM capacitors of varying thicknesses, using a stepped method of charge injection (Figure S10). After moderate injected charge (100 µC to 100 mC), the thicker dielectrics showed lower stress-induced leakage than thinner dielectrics (Figure 7c). This result is to be expected for thicker dielectrics, since higher areal defect concentrations are required to induce percolative leakage paths through the film. In addition, the non-monotonic trend in the leakage current in thicker dielectrics may suggest a complementary mechanism for how AlOx dielectrics compensate Vt shifts in InOx. During PBS, for example, electron trapping at the channel interface could initially be compensated by electron emission from donor traps in AlOx. Thinner dielectrics, however, showed higher, monotonically increasing leakage current with the application of constant-current stress, suggesting faster accumulation of the critical density of bulk defects in the AlOx. Collectively, these observations indicate that the limits of these printed transistors’ operational stability are driven by the quality and thickness of the printed dielectric.



(a)

(b)

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(c)

PBS

Figure 7. a) Extracted linear mobility vs applied gate voltage for InOx TFTs with printed AlOx and thermal SiO2 gate dielectrics. b) Normalized Vt shifts for TFTs with printed AlOx dielectrics (200 nm) under PBS at varying gate voltages. c) Stress-induced leakage current for AlOx dielectrics

of

varying

thickness.

4. Conclusion In conclusion, we have shown the benefit of integrating UV-annealing with low-temperatureprocessed printed transistors to achieve improved operational stability and high-performance. UV-annealing offers a route to forming high-quality printed dielectrics with low leakage and suppressed dispersion that are not limited by tradeoffs between thickness and performance. These AlOx dielectrics are leveraged to fabricate printed InOx transistors with high mobility (µlin = 12 ±1.6 cm2/Vs), minimal hysteresis, and excellent subthreshold behavior at a low processing temperature of only 250 °C. The bias-stress characteristics of these devices are also analyzed to understand critical relationship between printed dielectrics and threshold voltage stability. The improved stability of InOx transistors with thicker printed AlOx dielectrics highlights

the

unique

synergy

between

solution-processed

high-k

dielectrics

and

semiconductors such as InOx. We expect that this study can help enable fabrication of printed


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transistors with higher operational stability for ubiquitous applications in sensing and display technology. ASSOCIATED CONTENT Supporting Information Further information about the profilometry of printed AlOx films, thermal gravimetry of the AlOx inks, dielectric breakdown of printed capacitors, large area AFM studies of printed dielectric surface morphology, X-Ray Photoelectron Spectroscopy analysis of UV-annealed AlOx films, the subthreshold swing, and stress-induced leakage is available in the attached electronic supporting information. The Supporting Information is available free of charge on the ACS Publications website. ACKNOWLEDGMENT The authors wish to thank Andre Zeumault for valuable discussions. William Scheideler was supported by a National Science Foundation Graduate Research Fellowship. AUTHOR INFORMATION Corresponding Author

Vivek Subramanian Department of Electrical Engineering and Computer Sciences University of California, Berkeley, CA, 94720, USA E-mail: [email protected] Present Addresses

† William Scheideler



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Durand Building, Room 111 Department of Materials Science and Engineering Stanford University 496 Lomita Mall, Stanford, CA 94305

Author Contributions

The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. Funding Sources

This work is based upon work supported primarily by the National Science Foundation under Cooperative Agreement No. EEC-1160494. Any opinions, findings, and conclusions or recommendations expressed in this material are those of the author(s) and do not necessarily reflect the views of the National Science Foundation. REFERENCES (1) Zhan, Y.; Mei, Y.; Zheng, L. Materials Capability and Device Performance in Flexible Electronics for the Internet of Things. J. Mater. Chem. C 2014, 2 (7), 1220–1232. (2) Street, R. A.; Ng, T. N.; Schwartz, D. E.; Whiting, G. L.; Lu, J. P.; Bringans, R. D.; Veres, J. From Printed Transistors to Printed Smart Systems. Proc. IEEE 2015, 103 (4), 607–618. (3) Lochner, C. M.; Khan, Y.; Pierre, A.; Arias, A. C. All-Organic Optoelectronic Sensor for Pulse Oximetry. Nat. Commun. 2014, 5, 5745. (4) Sekitani, T.; Takamiya, M.; Noguchi, Y.; Nakano, S.; Kato, Y.; Sakurai, T.; Someya, T. A Large-Area Wireless Power-Transmission Sheet Using Printed Organic Transistors and Plastic MEMS Switches. Nat. Mater. 2007, 6 (6), 413–417. (5) Khan, Y.; Ostfeld, A. E.; Lochner, C. M.; Pierre, A.; Arias, A. C. Monitoring of Vital Signs with Flexible and Wearable Medical Devices. Adv. Mater. 2016, 28 (22), 4373–4395. (6) Fukuda, K.; Takeda, Y.; Yoshimura, Y.; Shiwaku, R.; Tran, L. T.; Sekine, T.; Mizukami, M.; Kumaki, D.; Tokito, S. Fully-Printed High-Performance Organic Thin-Film Transistors and Circuitry on One-Micron-Thick Polymer Films. Nat. Commun. 2014, 5, ncomms5147. (7) Amegadze, P. S. K.; Noh, Y.-Y. Development of High-Performance Printed Polymer FieldEffect Transistors for Flexible Display. J. Inf. Disp. 2014, 15 (4), 213–229.


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Table of Contents Figure


Scalable, High-Performance Printed InOx Transistors Enabled by UV

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