Research Article www.acsami.org
Modulating Cationic Ratios for High-Performance Transparent Solution-Processed Electronics Rohit Abraham John,† Anh Chien Nguyen,† Yuxin Chen,† Sudhanshu Shukla,†,§ Shi Chen,∥ and Nripan Mathews*,†,‡ †
School of Materials Science and Engineering, Nanyang Technological University, Singapore 637553 Energy Research Institute@NTU (ERI@N), Nanyang Technological University, Singapore 637553 § Energy Research Institute@NTU (ERI@N), Interdisciplinary Graduate School, Singapore 637553 ∥ Division of Physics and Applied Physics, School of Physical and Mathematical Sciences, Singapore 637371 ‡
S Supporting Information *
ABSTRACT: Amorphous oxide semiconductors such as indium zinc tin oxide (IZTO) are considered favorites to serve as channel materials for thin film transistors (TFTs) because they combine high charge carrier mobility with high optical transmittance, allowing for the development of transparent electronics. Although the influence of relative cationic concentrations in determining the electronic properties have been studied in sputtered and PLD films, the development of printed transparent electronics hinges on such dependencies being explored for solution-processed systems. Here, we study solution-processed indium zinc tin oxide thin film transistors (TFTs) to investigate variation in their electrical properties with change in cationic composition. Charge transport mobility ranging from 0.3 to 20.3 cm2/(V s), subthreshold swing ranging from 1.2 to 8.4 V/dec, threshold voltage ranging from −50 to 5 V, and drain current on−off ratio ranging from 3 to 6 orders of magnitude were obtained by examining different compositions of the semiconductor films. Mobility was found to increase with the incorporation of large cations such as In3+ and Sn4+ due to the vast s-orbital overlap they can achieve when compared to the intercationic distance. Subthreshold swing decreased with an increase in Zn2+ concentration due to reduced interfacial state formation between the semiconductor and dielectric. The optimized transistor obtained at a compositional ratio of In/Zn/Sn = 1:1:1, exhibited a high field-effect mobility of 8.62 cm2/(V s), subthreshold swing of 1.75 V/dec, and current on− off ratio of 106. Such impressive performances reaffirm the promise of amorphous metal oxide semiconductors for printed electronics. KEYWORDS: printed electronics, indium zinc tin oxide, thin film transistors, mobility, on−off ratio, activation energy
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thin film transistors (TFTs), memories and solar cells.7−11 Amorphous semiconducting films devoid of grain boundaries are good candidates for flexible devices because of the large area deposition uniformity and because their performance is not affected by grain boundary scattering.1,3,4 Although primarily explored in sputtered systems, their compatibility with various solution based printing techniques makes large scale fabrication plausible at reduced costs.12 Amorphous oxide semiconductors of post-transition metals like indium (In), gallium (Ga), tin (Sn), and zinc (Zn) hold great promise for a variety of electronic applications, including drivers for LCD and LED displays because of their high mobility, stability in air, transparency, and compositional uniformity.13 A variety of semiconductors like zinc oxide, indium zinc oxide (IZO), zinc
INTRODUCTION Printed electronics enable us to envision a future in which electronic devices can be produced through simple solution processable methodologies, enabling cost-reductions while maintaining high performance.1,2 Organic semiconductors show promising characteristics of low processing temperature and compatibility with cheap, facile solution processing; but they are limited by their performance, reliability and stability in air.1,3−5 Transistors made from graphene, CNTs, and MoS2 are also being explored for their high mobility and stability, but impractical material extraction methods, expensive vacuum deposition techniques, incompatibility with large scale fabrication methods, and low on−off ratio limits their application regime.6 The on−off ratio is especially an important characteristic for transistors because it determines the applicability of the material for digital electronics. Amorphous metal oxide semiconductors (AMOS) have emerged as promising materials for novel applications such as transparent © 2015 American Chemical Society
Received: September 20, 2015 Accepted: December 23, 2015 Published: December 23, 2015 1139
DOI: 10.1021/acsami.5b08880 ACS Appl. Mater. Interfaces 2016, 8, 1139−1146
Research Article
ACS Applied Materials & Interfaces
Figure 1. (a) Magnified GIAXRD pattern of IZTO thin film (In/Zn/Sn = 1:1:1) on Si/SiO2 (the small hump near 20° indicates the SiO2). Figure S1 shows XRD spectra for all the tested compositions. (b) Optical transmission spectrum of a 30 nm IZTO (In/Zn/Sn = 1:1:1) thin film on quartz substrate. (c) Wide-scan XPS spectra for various compositions depicting In 3d, Zn 2p, Sn 3d, and O 1s signals of a 30 nm IZTO thin film annealed at 400 °C for 1 h (711 denotes In/Zn/Sn = 7:1:1 and so on).
In this work, we fabricate bottom-gate top-contact indium zinc tin oxide (IZTO) TFTs with varying cationic concentrations to discern the individual and combined contribution of these cations toward the final TFT parameters like saturation mobility (μsat), subthreshold swing (S), threshold voltage (Vth) and on−off ratio. Such a combinatorial approach helps to audit the compositional landscape rapidly to efficiently pursue materials having improved performances, thereby allowing a more reliable and systematic study on TFT characteristics. Such combinatorial approaches have been previously undertaken for IGZO,15 IZO,21 and ZTO22 films and for vacuum deposited IZTO films,23 but has not been done so far for solutionprocessed IZTO thin films. Research on IZTO has been focused toward semiconducting thin films,24−30 transparent electrodes,31,32 and memories,33 mostly based on vacuum deposition techniques, but a comprehensive study on solution processed TFTs with regards to the precursor design and composition is rarely discussed. We attempt to fill this gap with this study.
tin oxide (ZTO), and indium gallium zinc oxide (IGZO)3,4,14−18 have been studied as the channel layer in thin film transistors. Such multicomponent materials offer a large flexibility to tune their properties. However, finding the best chemical composition in such a large parameter space is challenging. Therefore, it is important to identify general trends in material composition for tuning the electrical properties in the active layers of the TFTs. The charge carrier transport in amorphous post-transition metal oxide semiconductors is primarily through highly dispersed vacant s-orbitals of cations with additional 2p oxygen overlap. Overlap between these vacant s-orbitals determine the nature of the conduction band minimum (CBM) in these oxides. The contribution of oxygen 2p-orbitals, dominant at the valence band maximum (VBM), was reported to be small.3,4 Conduction pathways through such isotropic s-orbitals make these devices insensitive to the bond angle distortion that exists in amorphous materials. Direct overlap between s-orbitals of these neighboring cations become possible in heavy metal oxides of post-transition metals such as In3+, Sn4+, and Zn2+, thereby resulting in a smaller electron effective mass and, hence, higher mobility.3,14,19,20 Thus, high electron mobility can be achieved by exploiting large cation-rich compositions for large s-orbital overlap. The mixture of indium, zinc, and tin is chosen on the basis of this size criterion.
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EXPERIMENTAL SECTION
Synthesis of IZTO Films. The IZTO solution was prepared by dissolving chloride salts of indium (In), zinc (Zn), and tin (Sn) (Sigma-Aldrich) in 2-methoxyethanol mixed with monoethanolamine. In all cases, the total metal cation concentration was kept constant at 0.1 M, and one of the cationic components was varied while keeping 1140
DOI: 10.1021/acsami.5b08880 ACS Appl. Mater. Interfaces 2016, 8, 1139−1146
Research Article
ACS Applied Materials & Interfaces
Figure 2. Saturation mobility, subthreshold swing, and threshold voltage plots as a function of (a) In%, (b) Zn%, and (c) Sn%; (d) transfer characteristics depicting shift in Vth with varying Zn%.
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the other two constant. For example, to examine the effects of Indium concentration, zinc and tin molar fractions were kept constant at 1, and indium concentration was varied from 1 to 7, that is, In/Zn/Sn = 1:1:1 to 7:1:1 films were studied. 2-Methoxyethanol was used as the solvent due to its good properties, as previously reported, while monoethanolamine acts as a stabilizer and solubility enhancer.19 The OH groups of the monoethanolamine form weak bonds with the metal cations, prevents precipitation and even inhibits crystallization in the solution.34,35 The solutions were kept stirring for 12 h to allow complete dissolution. Device Fabrication. p-Type Silicon wafer (Silicon Valley Microelectronics) with 300 nm thick silicon dioxide (SiO2) was used as the substrate. These were precleaned by ultrasonicating in deionized (DI) water, acetone and ethanol for 10 min each with nitrogen drying steps in between followed by plasma cleaning for 10 min. Solutions were then spin-coated onto the substrate at a speed of 3000 rpm for 1 min to get a film thickness of 30 nm, as measured by surface profilometry. The spin-coated films were baked on a hot plate at 110 °C for 5 min to remove excess solvent followed by annealing in furnace at 400 °C for 1 h. 100 nm thick aluminum was thermally evaporated to form source and drain electrodes (W = 4000 μm/L = 75 μm-200 μm) using shadow masks to complete the TFT architecture. Characterization. Structural properties, chemical composition and optical properties of the solution-processed IZTO films were examined by grazing incidence X-ray diffraction (GIXRD) (Bruker D8 Advance Diffractometer), scanning electron microscopy (SEM), X-ray photoelectron spectroscopy (XPS), UV photoelectron spectroscopy (UPS), and ultraviolet−visible spectrophotometer (UV−vis) (Shimadzu 3600 UV−vis Spectrophotometer), respectively. Electrical properties were measured using a Keithley 4200-SCS semiconductor characterization system. Temperature-dependent conductivity measurements were carried out in Lakeshore-1200 cryogenic probe station. A total of 25 TFT devices were tested for each composition.
RESULTS AND DISCUSSION Figure 1a and Figure S1 (Supporting Information) show GIAXRD scans of IZTO films with various compositions. All the films are found to be amorphous, as previously reported.19 Such amorphous oxides are devoid of grain boundary scattering mechanisms, which deteriorate mobility, and exhibit excellent short-range uniformity and surface flatness, both of which can be utilized as advantages for large-area processing. Figure 1b shows the UV−vis spectra of IZTO thin film with In/Zn/Sn = 1:1:1 referenced to quartz substrate. Transmittance around 90% for the entire visible range makes this class of material ideal candidates for applications in transparent electronics. From the UPS measurements shown in Figure S2, the valence band maximum (VBM) is determined by linear extrapolation of valence band onset to be around 3.3 eV for all the compositions. Because no conduction-band-related feature is shown above the valence band onset, Fermi levels are still within the energy band gap. This, in turn, means that the band gap is greater than 3.3 eV, as reported in literature.36 Such a high band gap makes them ideal materials for transparent electronic applications. XPS and UPS measurements are performed on the samples in a home-built UHV multichamber system with base pressure better than 1 × 10−9 Torr. The XPS source is monochromatic Al Kα with photon energy at 1486.7 eV. The UPS source is from a helium discharge lamp (hν = 21.2 eV). The photoelectrons were measured by an electron analyzer (Omicron EA125). Wide-scan XPS spectra shown in Figure 1c show the presence of all the three cations, indium (In), zinc (Zn), and tin (Sn), together with oxygen. Atomic ratios of the various compositions were confirmed to be equivalent to the 1141
DOI: 10.1021/acsami.5b08880 ACS Appl. Mater. Interfaces 2016, 8, 1139−1146
Research Article
ACS Applied Materials & Interfaces
the increased addition of indium had been noted previously for sputtered IZO and IGZO transistors.15,21 All TFT measurements were performed in ambient air and the transfer characteristics (Id−Vg) and output characteristics (Id−Vd) were plotted out. All the data shown in the table are from the same batch of devices. To assess the control over channel formation and leakage, we analyzed subthreshold swing as a function of the composition. Subthreshold swing (S) and interfacial state density (Dit) were calculated using eq 2:19
molar ratios of the individual components in the precursor solution by monitoring the strongest peaks of In 3d, Zn 2p, and Sn 3d located at 445.4, 1022.7, and 487.5 eV, respectively (Figure S3). For example, for In/Zn/Sn = 1:1:1, the atomic ratios calculated from XPS were In/Zn/Sn = 0.85:1.2:1. This slight change in composition after annealing can be attributed to slight variation in vapor pressure of precursors and processing conditions.15 The detailed XPS scan of all the elements for various compositions is shown in Figure S3. They depict signals for Zn 2p, In 3d, Sn 3d and O 1s, respectively. Sn2+ formation was ruled out as only one peak is visible for Sn 3d at 487.5 eV which originated from Sn4+, thereby ruling out the coexistence of p-type SnO and n-type SnO2.37 Figure S2 shows position of the valence band maximum (VBM) of the various compositions measured via UV photoelectron spectroscopy (UPS). The VBM in such mixed transition metal oxides are mainly composed of O 2p orbitals mixed with some s and p orbitals of the metal cations, In3+, Sn3+ and Zn2+. It is speculated that the O 2s states do not contribute to any bonding.38,39 Optical band gap of the oxide thin films were found to be close to 3.3 eV consistent with the UV−vis measurements and other reports.36,40 Morphology studies were also performed on the samples using SEM, as shown in Figure S4. All compositions depict nanosized island (maximum size 5 cm2/(V s)), low subthreshold swing (105). It exhibits a high field-effect mobility of 8.62 cm2/(V s), subthreshold swing of 1.75 V/ decade and on−off ratio of 106 with a W/L = 4000 μm/200 μm. Transfer and output characteristics of the transistor are shown above (Figure 4a,b).
associated with the cationic imperfections in the structure. Incorporation of larger cations like In3+ and Sn4+, which are loosely bound with oxygen, could result in the creation of more oxygen vacancies and hence, more charge carriers.19 Increased concentration of In3+ cations is seen to have greater effects on reducing activation energy as compared to the same ratio of Sn cations despite having similar electronic orbital formation, and only difference of a single electron in the outmost orbital in neutral state (Table 1, Figure 3b,d). We attribute this phenomenon to the difference in hybridization forming in each element. For In’s outmost orbital, hybridization takes place between 5s2 and 5p1 to form three sp2 orbitals that align in trigonal planar structure. In the case of Sn, four hybridization orbitals of sp3 are created by 5s2 and 5p2, which forms tetrahedral structure. On the basis of differences of space organization, In is expected to have shorter intercationic overlapping, resulting in improved charge transport. For compositions with varying Zn content, activation energy linearly increases with higher ratio of Zn and well exceeds those from compounds with high In and Sn (Figure 3c). Multiple trapping and release (MTR) model has been reported to dominate in charge transport of ZnO devices.20 With the increase of Zn concentration, it is expected that this same model governs the transport mechanism. In and Sn rich compositions with lower activation energy fits well with higher mobility and negative threshold voltages observed in such compositions, while Zn-rich compositions having higher activation energies, depict low mobility and threshold voltages shifted to the positive side. The activation energy values derived for IZTO compositions (76−406 meV) resemble those extracted from sputtered IGZO45,46 layers (50−700 meV) although no specific compositional variations were for the IGZO films.
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ASSOCIATED CONTENT
* Supporting Information S
The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsami.5b08880. XRD pattern, SEM images, XPS Nanoscan and UPS spectra of the various compositions. (PDF)
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AUTHOR INFORMATION
Corresponding Author
* E-mail:
[email protected]. Funding
NTU-A*STAR Silicon Technologies Centre of Excellence under the program grant No. 11235100003, MOE Tier 1 grant RG184/14. Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS The authors would like to acknowledge the funding support from NTU-A*STAR Silicon Technologies Centre of Excellence under the program grant No. 11235100003 and MOE Tier 1 grant RG184/14. We acknowledge Dr. Natalia Yantara for help with imaging.
CONCLUSION Therefore, it is seen that TFT characteristics rely heavily on the chemical composition of these films. A trade-off between the relative percentages of these cations is thus required to fabricate a TFT with desirable properties. It is also observed that the TFT characteristics are deeply affected by the ambient conditions under which the measurements are carried out. This is probably due to the high level of traps present in the solution processed film. For example, devices measured under vacuum show decreased off-currents (
