Performance Enhancement for Tungsten-Doped Indium Oxide Thin

Research Article Cite This: ACS Appl. Mater. Interfaces 2019, 11, 22521−22530

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Performance Enhancement for Tungsten-Doped Indium Oxide Thin Film Transistor by Hydrogen Peroxide as Cosolvent in RoomTemperature Supercritical Fluid Systems Dun-Bao Ruan,† Po-Tsun Liu,*,‡ Min-Chin Yu,‡ Ta-Chun Chien,‡ Yu-Chuan Chiu,‡ Kai-Jhih Gan,† and Simon M. Sze† Department of Electronics Engineering and ‡Department of Photonics and Institute of Electro-Optical Engineering, National Chiao Tung University, Hsinchu 30010, Taiwan

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S Supporting Information *

ABSTRACT: In this study, hydrogen peroxide (H2O2) cosolvent, which was dissolved into supercritical-phase carbon dioxide fluid (SCCO2), is employed to passivate excessive oxygen vacancies of the high-mobility tungsten-doped indium oxide without any essential thermal process. With the detailed material analysis, the internal physical mechanism of the cosolvent effect or the interaction between the cosolvent solution and supercritical-phase fluid is well discussed. In addition, the optimized result has been applied for the thin film transistor device fabrication. As a result, the device with SCCO2 + H2O2 treatment exhibits the lowest subthreshold swing of 82 mV/dec, the lowest interface trap density of 8.76 × 1011 eV−1 cm−2, the lowest hysteresis of 47 mV, and an excellent reliability and uniformity characteristic compared with any other control groups. Besides, an extremely high field-effect mobility of 98.91 cm2/V s can also be observed, while there is even a desirable positive shift for the threshold voltage. Notably, compared with the untreated sample, the highest on/off current ratio of 5.11 × 107 can be achieved with at least four orders of magnitude enhancement by this unique treatment. KEYWORDS: high-mobility thin film transistor, tungsten-doped indium oxide, cosolvent effect, room-temperature supercritical CO2 fluid, H2O2 interface treatment, multilayer high κ insulator

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INTRODUCTION Driven by the demand for high-resolution, high-yield, low-cost, and flexible electronics, there has been an explosive growth trend of investment on transparent amorphous oxide semiconductor thin film transistors (TFTs) for both the display industry and academia research.1−3 Unlike the conventional technology based on low-temperature polycrystalline silicon devices with poor uniformity over a large area4 or amorphous silicon devices with poor carrier transport properties,5 amorphous oxide semiconductor TFTs can even be fabricated without any essential annealing process and succeed in eluding the disadvantages mentioned above simultaneously.6 Properly, that may be desirable for next-generation foldable electronic devices or even stacking three-dimensional integrated circuit (3D-IC) application. Since amorphous indium gallium zinc oxide (a-IGZO), the most archetypical and potential candidate for all the amorphous oxide semiconductor channel materials, was first proposed by Hosono and co-workers, the material © 2019 American Chemical Society

structure analysis of a-IGZO and the physical theory model of the dopant system have been studied amply.7−9 According to those discussions, indium ions may act as a matrix to provide high carrier mobility due to the unoccupied s orbital (4d105s0) of the heavy transition-metal cation, which may originate from an edge-sharing polyhedral structure.10 Hence, the mobility and carrier concentration of devices have been strongly associated with the proportion of indium oxide, which would be more likely to form the oxygen vacancy and be irreplaceable in this doping system at present.11 Incidentally, higher carrier mobility is also the most important issue for transistors to decrease the RC delay in signal lines and the charging time for each pixel as the requirement of panel size and resolution increases, especially for the thermal-budget-limited flexible Received: March 8, 2019 Accepted: June 3, 2019 Published: June 3, 2019 22521

DOI: 10.1021/acsami.9b04257 ACS Appl. Mater. Interfaces 2019, 11, 22521−22530

Research Article

ACS Applied Materials & Interfaces display.12 However, it is reported that there is a trade-off between device stability and high mobility.13 It also means that the pure indium oxide active layer with high conductivity and carrier density would be anomalously difficult to control or deplete under a reasonable gate bias. Therefore, many research efforts, like oxygen microwave plasma14 or high-bonddissociation-energy dopants,15 are spent on lowering the concentration of deep oxygen vacancies (VO) in indium oxide films without sacrificing their mobility or the number of donor-like positively charged states (VO2+). In the previous research, a novel type of dopant element, tungsten (W), has been proposed to replace the expensive and rare gallium (Ga) element for its lower price and higher oxygen bond dissociation energy. It may be easier to make the highly conductive channel controllable and improve device instability without serious carrier mobility degradation.16−21 On the other hand, many interface technology or oxygen-involved processes were also applied for reducing the oxygen vacancy, but most of the treatments were based on an inescapable high transition temperature.22,23 Consequently, finding a low-temperature or thermal-budget-free interface engineering method to compensate for the oxygen vacancy in indium-based oxide films without mobility degradation is of great importance and worthy of further investigation. In general, supercritical-phase fluid (SCF) treatment was used to improve the performance of dielectric properties in resistive random-access memory or passivate defect states in the hydrogenated amorphous silicon TFT without any process damage.24−26 With a special high-pressure system, the supercritical-phase carbon dioxide (SCCO2) can even be achieved around room temperature, which is unique due to the characteristics of high liquid-like solubility and a strong gas-like penetration ability.27,28 In addition, it is reported that the tiny liquid water can be dissolved as cosolvent in the SCCO2 fluid and also reach the supercritical phase under a relatively simple condition, which may exhibit an unanticipated oxidation ability.29 However, most of the SCF applications applied for the oxide TFTs are focusing on the improvement of device reliability,30,31 and this oxidant and cosolvent effect of highmobility materials has not been discussed yet. In this work, the oxygen-rich hydrogen peroxide (H2O2) is introduced into the room-temperature SCCO2 system as a special cosolvent and forceful oxidant for the thin film modification and interface engineering of a novel high-mobility material, tungsten-doped indium oxide (IWO). With detailed physical property and material analyses, the influences of the oxygen vacancy and oxygen-hydroxide bond distribution for the SCF treatment have been well discussed. Besides, in order to comprehend the internal physical mechanism and support those variations induced by the interaction between SCCO2 and cosolvent, the optimized thin films have been used as the active layer of TFT devices for further investigation. Exhilaratingly, the significant improvement of electrical performance and reliability characteristics can be achieved without any essential thermal process. Moreover, a multilayer high κ gate insulator stack was applied to enhance the ability of the high-conductivity channel to gate-control and adjust the negative threshold voltage into a reasonable gate bias operation.

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frequency (RF) magnetron sputtering using an InWO target (with an optimized In2O3/WO3 content of 96:4 wt % in 99.99% purity) at room temperature. Briefly, the sputtering system was initially evacuated to 3 × 10−4 Pa and followed by the optimized case of IWO thin film deposition at an RF power of 50 W, a chamber pressure of 0.4 Pa, and a mixed Ar/O2 (29 sccm/1 sccm) gas flow rate. Afterward, without any thermal process, one of the IWO thin film samples was set aside and named “without treatment” as the first control group without any SCF or cosolvent treatment. Some of those IWO thin film samples were sent into the SCF generation and highpressure cosolvent system, while the remaining samples were prepared for the vaporization system. On the other hand, two different types of the cosolvent solution were prepared in a wet bench and marked “Solution H2O cosolvent” and “Solution H2O2 cosolvent”. “Solution H2O cosolvent” was made with 50 vol % ethanol and 50 vol % pure water, while “Solution H2O2 cosolvent” was made with 50 vol % ethanol, 40 vol % pure water, and 10 vol % hydrogen peroxide. Subsequently, ∼5 mL of “Solution H2O cosolvent” was injected into the SCF system and mixed with the SCCO2 fluid under 2.41 × 107 Pa at room temperature, while it was labeled “with SCCO2 + H2O treatment”. With similar process conditions, the sample with “Solution H2O2 cosolvent” injection was named “with SCCO2 + H2O2 treatment”. Besides, the sample treated solely with the SCCO2 fluid without any cosolvent solution in the SCF generation system was named “with SCCO2 treatment”. It was chosen as the second control group to investigate the interaction between the cosolvent and SCF system. It is worth noting that the complete SCF treatment needs the samples to be immersed in the specific SCCO2 fluid (with or without cosolvent solution) with a fixed flow rate of 0.2 mL/s for 30 min. Furthermore, generation of vapor by the vaporization system with particular cosolvent solution was also performed on the remaining IWO samples for 30 min. Those samples acted as the third control group to confirm the necessity of the SCF system and were marked “with H2O vapor treatment” and “with H2O2 vapor treatment”. After those different post-treatments, material analysis by X-ray photoelectron spectroscopy (XPS) was performed to examine the evolution and depth distribution of the chemical bonding for IWO thin films. Meanwhile, elemental concentration analysis by time-of-flight secondary ion mass spectroscopy (TOF-SIMS) was also applied for investigating the depth distribution of the involved chemical elements such as indium, oxygen, tungsten, and even hydrogen. Moreover, the optimized IWO thin film has been used as the active layer of TFT devices with a staggered bottom-gate structure for further discussion. In detail, the devices were prepared on a highly doped n-type Si wafer with a 100 nm-thick thermal oxide layer grown on top. Then, a 60 nm-thick TaN layer was deposited and patterned as the metal gate electrode by direct current (DC) sputtering with a 100 sccm/10 sccm Ar/N2 gas flow and a power of 800 W. After the bottom-gate formation, the multilayer high κ gate dielectrics, 5 nm-thick SiO2, 25 nm-thick TiO2, 30 nm-thick HfO2, and 5 nm-thick SiO2, were stacked as the gate insulator (GI) by electron gun evaporation. It was reported that the multilayer GI structure was conducive to reducing the gate leakage current and increasing the effective dielectric constant without any annealing process.32 Sequentially, the IWO active layer was formed with the optimized condition and followed by the different post-treatments (SCF or vapor) mentioned above. Afterward, in order to achieve high-conductivity metal contact and a relatively low work function, the drain (D) and source (S) electrodes were deposited with a 300 nm-thick aluminum layer using a thermal coater. All the devices were patterned using a shadow mask with a channel length (L) of 200 μm and width (W) of 500 μm, while the TFTs can be completely fabricated by a whole room-temperature process (even without soft baking or hard baking in the lithography process). Electrical characteristics were measured at room temperature in a dark chamber with a 4284A precision LCR meter and an Agilent 4156C semiconductor parameter analyzer. Furthermore, the cross-sectional transmission electron microscope (TEM) image was taken using a JEM-2010F, while the result of energy-dispersive spectrometer (EDS) line scan was also obtained. In addition, X-ray diffraction (XRD) and ultraviolet−visible spectroscopy were used for determining the

EXPERIMENTAL METHODS

In order to study the cosolvent effect of the SCF system, a 10 nmthick IWO thin film was deposited on a blank Si wafer by radio 22522


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Figure 1. Schematic configuration diagram of the supercritical-phase fluid generation and high pressure cosolvent systems, including the mechanism of water nanodroplet formation in SCCO2 fluid with the alcohol surfactant by its nonpolarized hydrophobic hydrocarbon group and polarized hydrophilic hydroxyl group. material and optical characteristics of the IWO thin film sample with or without the specific treatment under the same process condition.

fitting software XPSPEAK41 with three nearly Gaussian distributions. Those peaks are approximately centered at 531.8, 531.1, and 529.8 eV.34 In detail, the signal peak located at the lowest binding energy (529.8 eV) with a full width at half-maximum (FWHM) of 1.6 eV can be attributed to an oxygen-lattice bond. It may be related to the O2− ions combined with the In or W atoms in the complicated IWO compound system. Besides, for the highest binding energy peak (531.8 eV) with an FWHM of 1.5 eV, the signal can be assigned to the oxygen-hydroxide bond. The peak may be associated with the loosely bonded oxygen gathering at the surface of the IWO film, which can be terminated with specific chemisorbed oxygen, such as absorbed H2O or even absorbed CO2. Moreover, the last peak located at the middle binding energy (531.1 eV) with an FWHM of 1.5 eV can be attributed to the oxygen vacancy bond. The oxygen vacancy peak can be attributed to O2− ions, which are in an oxygen-deficient region of the a-IWO matrix. According to the XPS depth distribution analysis results, it can be found that the sample without any treatment may exhibit the highest proportion of oxygen vacancy bonds (41.05%) at the surface of the back channel. Those oxygen vacancies are easily excited into the charged states in the form of a donor-like positively charged state, which may introduce the uncontrollable high conductivity or the excess carrier concentration. A typical U-shape depth distribution of the proportion of oxygen vacancy bonds can be observed for the sample without any treatment or the samples with cosolvent vapor treatment, which has been marked in the inset image of Figure 2. It may be caused by the phenomenon of oxygen desorption from both front and back channel interfaces, while similar results were well discussed in the previous work.19,34 Furthermore, the schematic diagrams of the a-IWO channel material are exhibited in Figure S2 of the Supporting Information, which may conceptually depict the mechanism of the phenomenon of oxygen desorption. In brief, some oxygen atoms in the channel material might be desorbed from the front channel, while more oxygen vacancies and oxygen-hydroxide bonds might form at the back channel by the moisture adsorption from the environment or vapor process. Besides, the proportion of oxygen-hydroxide bonds for the

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RESULTS AND DISCUSSION Figure 1 shows the schematic configuration diagram of the supercritical-phase fluid generation and high-pressure cosolvent system. During the SCF treatment, the sample was placed in a pressure-proof stainless steel chamber full of the SCCO2 fluid, which was generated in a high-pressure syringe pump. Besides, the cosolvent solution was also pressurized in another high-pressure syringe pump and injected into the reaction chamber. In order to maximize the solubility of cosolvent in the SCCO2 fluid, the alcohol in the cosolvent solution may act as a surfactant, which is capable of enhancing the polarization of the SCCO2 fluid. Specifically, the alcohol molecule has both a polarized hydrophilic hydroxyl group and a nonpolarized hydrophobic hydrocarbon group, which can easily attract polarized H2O/H2O2 cosolvent molecules and spontaneously associate with nonpolarized SCCO2 molecules, respectively.33 As a result, a self-assembled nanodroplet of cosolvent, which is uniformly distributed in the SCCO2 fluid, can easily diffuse through the narrow spaces between micro- or nanostructure surfaces and terminate trap states in the deep depth of the IWO sample. To further confirm the influence of the oxidant and cosolvent on the high-conductivity material treated by the SCF process, the XPS spectra of the O1s signal’s depth distribution analysis profile, as shown in Figure 2, were extracted to examine the inner relationship between different oxygen binding states for the a-IWO films (a) without treatment, (b) with SCCO2 treatment, (c) with SCCO2 + H2O treatment, (d) with SCCO2 + H2O2 treatment, (e) with H2O vapor treatment, and (f) with H2O2 vapor treatment, while the inset image summarizes the proportion of the different oxygen binding states. In general, all the data have been calibrated by tacking the peak of C1s (∼284.8 eV) as the reference, while the complete XPS C1s spectrum for each aIWO sample has been exhibited in Figure S1 of the Supporting Information. Besides, the peak located around 530 eV corresponding to the O1s core level was fitted by the peak 22523


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Figure 2. Depth distribution analysis profile of X-ray photoelectron spectroscopy (XPS) O1s spectrum for the a-IWO films (a) without treatment, (b) with SCCO2 treatment, (c) with SCCO2 + H2O treatment, (d) with SCCO2 + H2O2 treatment, (e) with H2O vapor treatment, and (f) with H2O2 vapor treatment, while the inset image summarizes the proportion of different oxygen binding states.

(∼21.08%) only by the SCCO2 fluid treatment with H2O2 cosolvent, which may imply that the excess oxygen vacancies in the indium-based oxide film can be effectively compensated for without any essential thermal treatment. Meanwhile, compared with the sample treated solely with the SCCO2 fluid, the other samples with H2O cosolvent in the SCF system also shows an exhilarative oxidation ability (∼25.15%). However, a higher proportion of oxygen-hydroxide bonds, caused by the specific loosely bonded chemisorbed oxygen (like CO2 or H2O), can also be obtained for either the sample treated solely with the SCCO2 fluid (∼13.85%) or the sample treated with the SCCO2 fluid and H2O cosolvent (∼17.12%). The higher proportion of oxygen-hydroxide bonds may also result in some serious side effects (like reliability issue, shallow trap increment, and so on) for the IWO film and can be compulsively suppressed by the forceful H2O2 cosolvent and

samples without SCF-based treatment shows its maximum value at the surface of the back channel and decreases with the depth increasing, which also supports this hypothesis. Moreover, it is interesting to note that the proportions of oxygen vacancy bonds (around 27.68 to 32.07%) or oxygen-hydroxide bonds (below 5%) show similar values in the middle, or even deeper, of the deposited IWO layer for the sample without treatment, with H2O vapor treatment, and with H2O2 vapor treatment. It means that the oxidant effect or interface engineering achieved by the vapor processes only occurs at the surface of the back channel. On the contrary, all the samples with SCF-based treatment may exhibit a relatively uniform depth distribution of the proportion of different oxygen bonds, which may be attributed to the strong gas-like penetration ability of the SCF fluid. Notably, the proportion of oxygen vacancy bonds can be significantly lowered down 22524


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even without any essential high-temperature annealing process for quality improvement, the multilayer gate stack still exhibits an excellent dielectric characteristic. In addition, the crosssectional transmission electron microscopy image and result of energy-dispersive spectrometer line scan for the a-IWO TFT are shown in Figure 3b,c, respectively. Incidentally, both the IWO channel and multilayer gate dielectric exhibit amorphous phases from the TFM image. Furthermore, the TFT device structure with a TaN metal gate at the bottom, a 5 nm-thick SiO2, 25 nm-thick TiO2, 30 nm-thick HfO2, and 5 nm-thick SiO2 multilayer gate dielectric stack, a 10 nm-thick IWO channel, and an Al metal layer on top as drain/source electrodes can also be clearly observed from Figure 3b. Without considering the ignorable outward diffusion (lowmelting-point material) induced by inevitable damage during the sample preparation (focused ion beam system), the EDS line scan profile of the relational elemental distribution with depth may also be consistent with the observed structure of the IWO device. Figure 4a shows the transfer characteristics of the a-IWO TFT device with or without different cosolvent treatments in the SCF or vapor system, while all the electrical parameters for each sample including threshold voltage (VTH), on/off current ratio (ION/IOFF), subthreshold swing (SS), interface trap density (DIT), and field-effect mobility (μFE) are summarized in Table S1 of the Supporting Information. Generally, the threshold voltage can be defined by the VGS at constant normalized drain current in the linear region (ID/(W/L)) of 10 nA. The subthreshold swing can be determined by an increasing decade in the drain current (ID) from 1010 to 108 A. The value of field-effect mobility can be extracted by transconductance in the linear region (VD = 0.1 V). The trap density at the interface between the multilayer gate stack and aIWO channel can be calculated by eq 136

Figure 3. (a) Schematic device diagram of the bottom-gate TFT structure with the IWO channel film in this work; (b) cross-sectional transmission electron microscopy image and (c) energy-dispersive spectrometer line scan profile for the a-IWO TFT device with the optimized condition.

oxidant (below 5%). In addition, the depth profiles of the involved chemical element concentration are shown in Figure S3 of the Supporting Information, which have been extracted from the transparent IWO thin film samples by TOF-SIMS material analysis. The SIMS results are also consistent with the observed XPS data (especially for hydrogen in the oxygenhydroxide bond). Based on those analyses of the novel high-conductivity material modification, the optimized thin film has been used as the active layer of TFT devices to experimentally support those variations introduced by the special cosolvent in SCF treatment. Hence, a three-dimensional schematic device diagram of the bottom-gate IWO TFT is shown in Figure 3a. The TFT devices could be easily fabricated on flexible display panels with a similar room-temperature process.12,22 Besides, it is reported that the high κ material can effectively control the conductive channel with high carrier concentration and high electron mobility.35 Nevertheless, all those conductive channel materials are always sacrificed or abandoned for the poor controllability of the traditional SiO2 gate insulator. Therefore, in order to achieve a qualitatively ideal IWO TFT with high mobility and a large on/off current ratio simultaneously, a multilayer high κ dielectric stack (SiO2/ HfO2/TiO2/SiO2) was chosen as the gate insulator. Moreover,

ij SS· lg e yz C DIT = jjj − 1zzz· OX j kT /q z q2 k {

(1)

where T is the absolute temperature, k is the Boltzmann constant, COX is the capacitance of the gate dielectric, and q is the unit electron charge. As a result, it is found that the basic electrical characteristics of the IWO TFT devices without any treatment or with vapor cosolvent treatment are relatively poor, especially for the on/off current ratio. It means that the high-conductivity and high-carrier-density indium oxide active layer without any essential thermal process would be anomalously difficult to be controlled under a reasonable gate bias in the depletion-mode operation. Hence, it is also

Figure 4. (a) Transfer characteristics (IDS−VGS) of the a-IWO TFT device with or without different cosolvent treatments; (b) oxide capacitance− voltage (C−V) and current density−voltage (J−V) characteristics of the Al/SiO2/HfO2/TiO2/SiO2/TaN MIM capacitor fabricated on the same wafer during the TFT process and cosolvent treatment. 22525


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Table 1. Comparison of the Device Performance for the Present a-IWO TFT with SCCO2 + H2O2 Treatment and Some Reported Previous Works with Different Fabrication Processes in Recent Years sample 37

IWO/SiO2 TFT IWO (precursor solution)/AlOX TFT38 IWO (nanosheet)/HfO2 TFT39 IWO/Al2O3 flexible TFT40 IWZO/Al2O3 TFT (glass)41 this work

SS (V/decade)

μEF (cm2/V·s)

0.5 0.068 0.063 0.30 0.31 0.082

∼26.5 ∼15.3 ∼25.3 ∼25.86 ∼11.1 ∼98.91

ION/IOFF ∼1 ∼5 ∼1 ∼5.6 ∼1 ∼5.11

× × × × × ×

107 107 108 105 107 107

VTH (V) −2.5 −0.15 −0.25 −1.5 4 0.209

Figure 5. Hysteresis characteristics and gate leakage current versus gate voltage (IG−VG) of the a-IWO TFT (a) without treatment, (b) with SCCO2 treatment, (c) with SCCO2 + H2O treatment, (d) with SCCO2 + H2O2 treatment, (e) with H2O vapor treatment, and (f) with H2O2 vapor treatment.

previous work on the high-performance TFT devices with different fabrication processes in recent years.37−41 Considering the overall performance with process complexity and temperature limitation, the special cosolvent treatment based on the SCF system indeed holds potential to realize the nextgeneration flexible TFT devices in a more economic and simple approach. Besides, Figure 4b shows the capacitance− voltage (C−V) and the current density−voltage (J−V) characteristics of the Al/SiO2/HfO2/TiO2/SiO2/TaN MIM capacitor fabricated on the same wafer during the TFT process and cosolvent treatment. The capacitance of all the samples, which was measured at a fixed frequency of 100 kHz, exhibits a similar value. It is ∼0.375 μF/cm2, while the effective dielectric constant of multilayer gate stacks is ∼27.54. It reveals that the influence on the capacitance of the different cosolvent SCF or vapor treatments can be ignored. In addition, all the samples exhibit a low-level leakage current, which means that the highquality multilayer high κ dielectric stacks can provide excellent gate control indeed even without any essential annealing

indicated that indeed, there is a close relation between the total carrier concentration and the oxygen vacancy distribution with depth of the IWO channel, as shown in Figure 2. Moreover, it is believed that the influence achieved by the vapor processes can be negligible, while the oxidant effect only occurs at the surface of the back channel. Notably, the IWO TFT device with SCCO2 + H2O2 treatment exhibits the lowest SS of 82 mV/dec, the highest ION/IOFF of 5.11 × 107 and the lowest DIT of 8.76 × 1011 eV−1 cm−2 compared to any other control groups, while there is even a desirable positive shift of the threshold voltage and a sufficiently high field-effect mobility of 98.91 cm2/V s. Compared with the sample treated solely with the SCCO2 fluid or the sample treated with H2O cosolvent in the SCF system, the remarkable enhancement of electrical performance can be attributed to the more forceful oxidation ability of the oxygen-rich hydrogen peroxide cosolvent. It can more effectively passivate the oxygen vacancy and repair the dangling bonds for the high-mobility channel material even at room temperature. Furthermore, Table 1 compares some 22526


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applied with the same gate insulator stacks, which also show similar capacitance values. Therefore, the sample with SCCO2 + H2O2 treatment exhibits an extremely low value of hysteresis (

Performance Enhancement for Tungsten-Doped Indium Oxide Thin

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