ZnO-based Thin-film Transistors Research Articles

Subthreshold Conduction of Disordered ZnO-Based Thin-Film Transistors.

This study presents the disorderedness effects on the subthreshold characteristics of atomically deposited ZnO thin-film transistors (TFTs). Bottom-gate ZnO TFTs show n-type enhancement-mode transfer characteristics but a gate-voltage-dependent, degradable subthreshold swing. The charge-transport characteristics of the disordered semiconductor TFTs are severely affected by the localized trap states. Thus, we posit that the disorderedness factors, which are the interface trap capacitance and the diffusion coefficient of electrons, would result in the degradation. Considering the factors as gate-dependent power laws, we derive the subthreshold current-voltage relationship for disordered semiconductors. Notably, the gate-dependent disorderedness parameters are successfully deduced and consistent with those obtained by the gm/Ids method, which was for the FinFETs. In addition, temperature-dependent current-voltage analyses reveal that the gate-dependent interface traps limit the subthreshold conduction, leading to the diffusion current. Thus, we conclude that the disorderedness factors of the ZnO films lead to the indefinable subthreshold swing of the ZnO TFTs.

Hf-doped ZnO transistor with high bias stability and high field-effect mobility by modulation of oxygen vacancies and interfaces

ZnO-based thin film transistors (TFTs) with high bias stability are challenging due to the intrinsic defects and overhigh interface trap density. In this work, we fabricated Hf-doped ZnO films with different cycle ratio of Zn/Hf via atomic layer deposition and subsequent annealing treatment. The results show that the cycle ratio of Zn/Hf is optimized to be 10:1, and the corresponding atomic ratio is 2.24%. The threshold voltage and subthreshold swing of the devices are improved as the annealing temperature increases, owning to the decrease of the oxygen vacancies and interface trap density. Furthermore, we developed Hf-doped ZnO TFT with high bias stability by introducing HfO2 intermediate layer between the active layer and SiO2 dielectric layer, and the shift of threshold voltage is as low as -0.273 V, showing high bias stability. Also, the device has the high field-effective mobility of 52.4 cm2/Vs, low subthreshold swing of 0.68 V/dec and high Ion/Ioff of 3.6 × 108. The results indicate a promising fabrication method for high-performance ZnO-based TFTs, which may be applied in logic circuits, radio frequency identification and so on.

High-Performance Thin-Film Transistors with ZnO:H/ZnO Double Active Layers Fabricated at Room Temperature.

H doping can enhance the performance of ZnO thin-film transistors (TFTs) to a certain extent, and the design of double active layers is an effective way to further improve a device's performance. However, there are few studies on the combination of these two strategies. We fabricated TFTs with ZnO:H (4 nm)/ZnO (20 nm) double active layers by magnetron sputtering at room temperature, and studied the effect of the hydrogen flow ratio on the devices' performance. ZnO:H/ZnO-TFT has the best overall performance when H2/(Ar + H2) = 0.13% with a mobility of 12.10 cm2/Vs, an on/off current ratio of 2.32 × 107, a subthreshold swing of 0.67 V/Dec, and a threshold voltage of 1.68 V, which is significantly better than the performance of single active layer ZnO:H-TFTs. This exhibits that the transport mechanism of carriers in double active layer devices is more complicated. On one hand, increasing the hydrogen flow ratio can more effectively suppress the oxygen-related defect states, thus reducing the carrier scattering and increasing the carrier concentration. On the other hand, the energy band analysis shows that electrons accumulate at the interface of the ZnO layer close to the ZnO:H layer, providing an additional path for carrier transport. Our research exhibits that the combination of a simple hydrogen doping process and double active layer construction can achieve the fabrication of high-performance ZnO-based TFTs, and that the whole room temperature process also provides important reference value for the subsequent development of flexible devices.

Enhanced Electrical and Mechanical Performance of InSnZnO TFTs With Multifunctional Laminated Organic Passivation Layer

To realize high-performance and stable electrical characteristics for ZnO-based thin-film transistors (TFTs) under various environments, we propose InSnZnO (ITZO) TFTs with multifunctional laminated organic (MLO) passivation layers (PVLs). The MLO PVLs were constructed from self-assembled monolayers (SAMs) and an ultraviolet (UV) light absorber-modified polydimethylsiloxane (PDMS) layer. The SAMs can passivate the ITZO surface and suppress the chemical reaction during the spin-coating process, and the modified PDMS layer can block UV light. The MLO-treated ITZO TFTs exhibited a high-performance and robust stability with high mobility (~21.18 cm2 <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$\cdot $ </tex-math></inline-formula> V−1 <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$\cdot $ </tex-math></inline-formula> s−1), a considerable ON– OFF current ratio (~ <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$6.65\times10$ </tex-math></inline-formula> 9), and a steep subthreshold slope (~100 mV/dec). The shift of the threshold voltage ( <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">${\textit V}_{{\mathrm {th}}}$ </tex-math></inline-formula> ) under gate bias stress and UV light illumination stress was investigated for the devices with different PVLs. The ITZO TFTs with MLO PVLs showed improved stability under positive gate bias stress (PBS), positive gate bias illumination stress (PBIS), and negative gate bias illumination stress (NBIS) compared to untreated and SAM-treated ITZO TFTs. Furthermore, the MLO-treated ITZO TFTs achieved a weak response to 365-nm UV light and showed superb stability with the negligible change of electrical performance, even after a 1-h water soaking test and a 100-day storing test. In addition, the MLO PVL exhibited high flexibility without signs of cracking even after 10 000 bending cycles and was proven suitable for fabricating flexible oxide TFTs.

Fabrication of high-performance ZnO-based thin-film transistors by Mg/H co-doping at room temperature

Fabrication of high-performance ZnO-based thin-film transistors by Mg/H co-doping at room temperature

Facile fabrication of MgZnO/ZnO composites for high performance thin film transistor

The disadvantages of the low on/off current ratio (Ion/off) and high leakage current in ZnO-based thin-film transistors (TFTs) impede its development to realize industrial production for replacing Si-TFTs. In this work, we fabricated a high performance MgZnO/ZnO dual-active-layer TFTs (DAL-TFTs) through systematic optimize the synthesis conditions. The optimized Mg0.2Zn0.8O/ZnO TFTs presents high mobility of 21.1 cm2V−1s−1 with a high Ion/off of 1.5 × 108, which is much higher than that of ZnO-based TFTs. The results indicate that high carrier concentration ZnO close to the insulation layer ensures the excellent mobility of MgZnO/ZnO DAL-TFTs and low carrier concentration MgZnO layer can reduce the off-state current of the device, and increase the switching ratio of the device. Our results indicate the MgZnO/ZnO composite is a promising candidate for high performance TFTs and other electronic devices.

The influence of low indium composition ratio on sol–gel solution-deposited amorphous zinc oxide thin film transistors

In this work, we examined the influence of In doping on the morphological and structural characteristics of ZnO active layers grown by the spin coating and the electrical performance of ZnO-based thin film transistors. XRD results indicated that the active layers were amorphous due to the absence of any sharp peaks. XPS analysis was carried out to determine indium amounts as an atomic percentage in ZnO and oxidation state of In. AFM images indicated that the roughness of the active layers decreased with increasing indium concentration in ZnO. The indium doping has dramatically improved the electrical parameters of ZnO-based transistors. The field-effect mobility of undoped TFT increased ~ 157 times by adding %3 In content. The highest field-effect mobility (μsat) of 12.9 cm2V−1 s−1 was obtained for %3 In-doped ZnO TFT (IZO3). Also IZO3 has a 6.96 V threshold voltage (Vth), 106Ion/Ioff ratio, 1.98 V/dec subthreshold slope (SS), and a high on-current of 4.6 mA. We ascribed the performance enhancement of devices with In doping due to increasing carrier concentration of channel. These results show that the low concentration of indium incorporation is very crucial to change the morphological properties of ZnO active layers and to obtain high-performance solution-processed TFTs.

Highly Transparent and Surface-Plasmon-Enhanced Visible-Photodetector Based on Zinc Oxide Thin-Film Transistors with Heterojunction Structure.

Highly transparent zinc oxide (ZnO)-based thin-film transistors (TFTs) with gold nanoparticles (AuNPs) capable of detecting visible light were fabricated through spray pyrolysis on a fluorine-doped tin oxide substrate. The spray-deposited channel layer of ZnO had a thickness of approximately 15 nm, and the thickness exhibited a linear increase with an increasing number of sprays. Furthermore, the ZnO thin-film exhibited a markedly smoother channel layer with a significantly lower surface roughness of 1.84 nm when the substrate was 20 cm from the spray nozzle compared with when it was 10 cm away. Finally, a ZnO and Au-NP heterojunction nanohybrid structure using plasmonic energy detection as an electrical signal, constitutes an ideal combination for a visible-light photodetector. The ZnO-based TFTs convert localized surface plasmon energy into an electrical signal, thereby extending the wide band-gap of materials used for photodetectors to achieve visible-light wavelength detection. The photo-transistors demonstrate an elevated on-current with an increase of the AuNP density in the concentration of 1.26, 12.6, and 126 pM and reach values of 3.75, 5.18, and 9.79 × 10−7 A with applied gate and drain voltages. Moreover, the threshold voltage (Vth) also drifts to negative values as the AuNP density increases.

Printed Water-Based ITO Nanoparticle via Electrohydrodynamic (EHD) Jet Printing and Its Application of ZnO Transistors

An electrohydrodynamic jet (EHD) printing process was optimized for the patterning of an indium tin oxide (ITO) nanoparticles ink by manipulating its surface tension through the addition of a nonionic surfactant, Triton X-100 (TX-100). As a result, a stable cone-jet mode was performed, which could provide printed drops with the smallest diameter and printing fidelity among EHD jet printing modes, and lead to a line width ranging from 230 µm to 30 µm in addition to well-define patterns with various shapes for ITO transparent conducting electrodes (TCEs). The multi-printing technique and subsequent thermal annealing for printed ITO TCEs allows improvement in its electrical conductivity. In addition, the evolution of the chemical composition and crystalline structure of the printed ITO TCE according to the annealing temperature was investigated to determine the optimal conditions to utilize printed ITO nanoparticles for ZnO-based thin-film transistors.

Improved Structural and Electrical Properties of ZnO-Based Thin Film Transistors by Using Pulsed KrF Excimer Laser Irradiation

We report on the characteristics of ZnO-based thin film transistors (TFT) fabricated by radio frequency magnetron sputtering. The surface of the ZnO channel layers were treated by pulsed KrF excimer laser irradiation (ELI) to improve the structural and electrical properties. The ZnO TFT device is of bottom gate type, which consists of SiO2 as a gate insulator and indium tin oxide as a gate deposited onto Corning glass substrates. The root-mean-square surface roughness and structural property of ZnO channel layer was significantly improved by the ELI treatment. The laser-treated ZnO TFT exhibited a saturation mobility of 19.27 cm2/V s, an on/off ratio greater than 105, the off current of less than 10−7 A, and a threshold voltage of 1.1 V. These results revealed the significant improvement of device characteristics and demonstrated that the pulsed KrF ELI treatment is an effective way to improve the ZnO TFT device performance.

Advanced Oxide Materials − Growth, Application, Characterization

In the present issue of Physica Status Solidi A readers will find a set of papers submitted by the participants of the 2017 E-MRS Fall Meeting Symposium N: Advanced Oxide Materials − Growth, Application, Characterization. The 2017 E-MRS Fall Meeting was held in Warsaw, September 18th – 21st. The Symposium N covered current topics of multiple functional and multifunctional oxides and comprised new trends in experimental as well as theoretical research. It was devoted to bulk and surface properties, films, nanocrystals, electronic, optical and magnetic properties with special attention paid to structure − property relationships. The presented papers demonstrate that physicochemical properties of oxides can be tuned by multitude of parameters, such as composition, temperature, pressure, strain, external fields, defects, film orientation and nanoparticle size. Detailed structure-property analysis and understanding of the physicochemical properties of the oxides are prerequisites to improve materials properties, and to spur development of new oxide materials. The Symposium N drew great attention − it was the second largest symposium among the 23 E-MRS Fall 2017 symposia. 178 abstracts were received from 36 countries with attendances from Poland (26), France (16), Germany (15), Korea (14) and Japan (11) (see Figure 1). A total number of 165 presentations were given, 20 as invited lectures, 45 as orals and over 100 as poster presentations. Among these, 13 oral and 26 poster presentations were given by students. Most of them were at an extraordinary high scientific level, so the competition for the best student oral and the best student poster presentation was a challenging task. Finally, Johanna Jochum from KU Leuven, Belgium received the best student oral Symposium N award for her excellent talk on Improved magnetoelectric coupling strength in BaTiO3-BiFeO3 superlatices. The Symposium N Scientific Committee decided to highlight two more excellent talks: Maximilian Schaube from MPI for Solid State Research, Stuttgart, Germany (Influencing the Oxygen Surface Exchange Reaction on Ceria by Dopants) and Esteban Rucavado from École Polytechnique Fédérale de Lausanne, Switzerland (Unraveling and tuning defects in indium-free transparent conductive oxides). The best student poster Symposium N award was given to Chethana Gadiyar from École Polytechnique Fédérale de Lausanne, Switzerland, for her poster Nanocrystal seeded growth for nanostructured multinary metal oxides as photoanodes for the water splitting reaction. Symposium N organizers also decided to highlight another excellent poster p-type Dye Sensitized Solar Cells Based on Mg-doped CuCrO2 Nanofiber presented by Ismail Cihan Kaya from Ismail University in Turkey. The scope of the symposium was very broad and, accordingly, the proceeding papers represent varying subjects ranging from synthesis (electrospinning, atomic layer deposition, magnetron sputtering, electrodeposition) to different applications (ZnO-based thin-film transistors, high-k dielectric charge-trapping memories, RE doped high-temperature superconductors, ZnO:Al nanosensors and nanodetectors, ZnO:RE light emitters). The high attendance at the Symposium N reflects a growing interest in advanced oxides, which are becoming increasingly important materials in new generation of advanced solid state devices. Consequently, it is worth considering organizing related symposia in the forthcoming E-MRS meetings. Elżbieta Guziewicz, Albena Paskaleva, Mato Knez, and Lars Österlund

Dual functionality of metal-tetraphenylporphyrin in ZnO-based hybrid thin film transistors

Complementary hybridization of ZnO thin films with aluminum tetraphenylporphyrin (Al(III)TPP), zinc tetraphenylporphyrin (Zn(II)TPP) and H2TPP (tetraphenylporphyrin) was adopted for tuning the hybrid thin film transistor (TFT) performance and improving their flexibility. After the hybridization with the organic layers, the chemical and structural features of ZnO thin films were well-preserved as compared with those of solely ZnO thin films. The existence of organic layers was monitored by X-ray photoelectron spectroscopy depth profiling. We fabricated the TFT based on ZnO/organic layers, resulting in the on-off ratio and threshold voltage of the devices manipulated by selecting the organic layers. These results can be understood by the performance tuning mechanisms related with the electron charge transfer induced by a work function difference. Remarkably, a significant improvement of the flexibility in the hybrid films was achieved without any significant loss in optical transmittance, which will be high demand in transparent and flexible electronics.

Sol-Gel PMMA-ZrO2 Hybrid Layers as Gate Dielectric for Low-Temperature ZnO-Based Thin-Film Transistors.

We report a simple sol–gel process for the deposition of poly(methyl methacrylate) (PMMA)–ZrO2 organic–inorganic hybrid films at low temperature and studied their properties as a function of the molar ratios of the precursors in the hybrid sol–gel solution, which included zirconium propoxide as the inorganic (zirconia) source, methyl methacrylate as the organic source, and 3-trimethoxy-silyl-propyl-methacrylate (TMSPM) as the coupling agent to enhance the compatibility between the organic and inorganic phases. The hybrid thin-film deposition was done on glass slide substrates by the dip-coating method. After deposition, the films were heat-treated at 100 °C for 24 h. The analysis of the hybrid films included Fourier transform infrared spectroscopy to identify their chemical groups and thermogravimetric analysis to determine the content of their organic and inorganic components. In addition, capacitance–voltage (C–V) and current–voltage (I–V) curves in metal–insulator–metal structures, using gold as metal contacts, were measured to find the dielectric constant and leakage current of the PMMA–ZrO2 hybrid films. Finally, because of their adequate dielectric characteristics, single hybrid layers were deposited on indium tin oxide-coated glass substrates and were tested as gate dielectric in thin-film transistors (TFTs), using sputtered ZnO layers as the semiconductor active channel. We measured the output electrical response and transfer characteristics of these hybrid dielectric gate-based devices and determined their main electrical parameters as a function of the TMSPM content in the hybrid dielectric gate layer. The better TFT electrical behavior presents field effect mobility of 0.48 cm2/V s, low threshold voltage of 3.3 V, and on/off current ratio of 105, and it was obtained by using PMMA–ZrO2 with 0.3 TMSPM content as the gate dielectric layer. The values obtained for the electrical parameters show that PMMA–ZrO2 hybrid films are quite suitable for dielectric gate applications in TFTs

Effects of Nitrogen and Hydrogen Codoping on the Electrical Performance and Reliability of InGaZnO Thin-Film Transistors.

Despite intensive research on improvement in electrical performances of ZnO-based thin-film transistors (TFTs), the instability issues have limited their applications for complementary electronics. Herein, we have investigated the effect of nitrogen and hydrogen (N/H) codoping on the electrical performance and reliability of amorphous InGaZnO (α-IGZO) TFTs. The performance and bias stress stability of α-IGZO device were simultaneously improved by N/H plasma treatment with a high field-effect mobility of 45.3 cm2/(V s) and small shifts of threshold voltage (Vth). On the basis of X-ray photoelectron spectroscopy analysis, the improved electrical performances of α-IGZO TFT should be attributed to the appropriate amount of N/H codoping, which could not only control the Vth and carrier concentration efficiently, but also passivate the defects such as oxygen vacancy due to the formation of stable Zn-N and N-H bonds. Meanwhile, low-frequency noise analysis indicates that the average trap density near the α-IGZO/SiO2 interface is reduced by the nitrogen and hydrogen plasma treatment. This method could provide a step toward the development of α-IGZO TFTs for potential applications in next-generation high-definition optoelectronic displays.

Microwave Annealing as a Low Thermal Budget Technique for ZnO Thin-Film Transistors Fabricated Using Atomic Layer Deposition

Microwave annealing (MWA) and furnace annealing are compared for their low thermal budget capability to improve the characteristics of ZnO-based thin-film transistors (TFTs). Both the ZnO channel and the Al2O3 gate dielectric are fabricated using atomic layer deposition. Using Si-wafer-susceptor assisted MWA with a substantial reduction of both annealing temperature and duration, significant improvements of the characteristics of the ZnO TFTs can be attained. A multi-step MWA process is found to further improve the characteristics of the TFTs. For the same microwave power and total duration, the field-effect mobility of the multi-step MWA TFT is 42% greater than that of the one-step MWA TFT with a similar sub-threshold swing. The multi-step MWA process can serve the purpose at temperatures as low as 220 °C.

RETRACTED: Effect of thickness of Bi1.5Zn1.0Nb1.5O7 gate insulator on performance of ZnO based thin film transistors

s The bottom-gate structure ZnO based thin film transistors (ZnO-TFTs) using Bi1.5Zn1.0Nb1.5O7 (BZN) thin films as gate insulator were fabricated on Pt/SiO2/Si substrate by radio frequency magnetic sputtering at room temperature. We investigated the effect of thicknesses (100 nm, 170 nm, 250 nm, and 330 nm) of BZN thin films on the performance of BZN thin films and ZnO-TFTs. The results show that 170-nm-thick BZN thin film obtained high dielectric constant of 71, low roughness of 3.51 nm and low leakage current density of 10−7 A/cm2, which is approximately one order magnitude lower than that of 100-nm-thick BZN thin film. Also, we concluded that the electrical performance of ZnO-TFTs using 170-nm-thick BZN thin film as gate insulator is enhanced remarkably. On/off current ratio (1.1 × 105) and field effect mobility (0.22 cm2/Vs) of ZnO-TFTs are approximately one order magnitude and seven times larger, respectively than that of ZnO-TFTs using 100 nm-thick BZN thin film as gate insulator. Meanwhile, surface states density (3.21 × 1012) of ZnO-TFTs is reduced three times.

Effect of annealing temperature of Bi1.5Zn1.0Nb1.5O7 gate insulator on performance of ZnO based thin film transistors**Project supported by the National Natural Science Foundation of China (Nos. 51332003, 51202184), the International Science & Technology Cooperation Program of China (Nos. 2010DFB13640, 2011DFA51880), and the “111 Project” of China (No. B14040).

The bottom-gate structure ZnO based thin film transistors (ZnO-TFTs) using Bi1.5Zn1.0Nb1.5O7 (BZN) thin films as gate insulator were fabricated on Pt/SiO2/Si substrate by radio frequency magnetic sputtering. We investigated the effect of annealing temperature at 300, 400, and 500 °C on the performance of BZN thin films and ZnO-TFTs. XRD measurement confirmed that BZN thin films were amorphous in nature. BZN thin films annealed at 400 °C obtain the high capacitance density of 249 nF/cm2, high dielectric constant of 71, and low leakage current density of 10−7 A/cm2 on/off current ratio and field effect mobility of ZnO-TFTs annealed at 400 °C are approximately one order of magnitude and two times, respectively higher than that of ZnO-TFTs annealed at 300 °C. When the annealing temperature is 400 °C, the electrical performance of ZnO-TFTs is enhanced remarkably. Devices obtain a low sub-threshold swing of 470 mV/dec and surface states density of 3.21 × 1012cm−2.

Rational Design of ZnO:H/ZnO Bilayer Structure for High-Performance Thin-Film Transistors.

The intriguing properties of zinc oxide-based semiconductors are being extensively studied as they are attractive alternatives to current silicon-based semiconductors for applications in transparent and flexible electronics. Although they have promising properties, significant improvements on performance and electrical reliability of ZnO-based thin film transistors (TFTs) should be achieved before they can be applied widely in practical applications. This work demonstrates a rational and elegant design of TFT, composed of poly crystalline ZnO:H/ZnO bilayer structure without using other metal elements for doping. The field-effect mobility and gate bias stability of the bilayer structured devices have been improved. In this device structure, the hydrogenated ultrathin ZnO:H active layer (∼3 nm) could provide suitable carrier concentration and decrease the interface trap density, while thick pure-ZnO layer could control channel conductance. Based on this novel structure, a high field-effect mobility of 42.6 cm(2) V(-1) s(-1), a high on/off current ratio of 10(8) and a small subthreshold swing of 0.13 V dec(-1) have been achieved. Additionally, the bias stress stability of the bilayer structured devices is enhanced compared to the simple single channel layer ZnO device. These results suggest that the bilayer ZnO:H/ZnO TFTs have a great potential for low-cost thin-film electronics.

Rational Hydrogenation for Enhanced Mobility and High Reliability on ZnO-based Thin Film Transistors: From Simulation to Experiment.

Hydrogenation is one of the effective methods for improving the performance of ZnO thin film transistors (TFTs), which originate from the fact that hydrogen (H) acts as a defect passivator and a shallow n-type dopant in ZnO materials. However, passivation accompanied by an excessive H doping of the channel region of a ZnO TFT is undesirable because high carrier density leads to negative threshold voltages. Herein, we report that Mg/H codoping could overcome the trade-off between performance and reliability in the ZnO TFTs. The theoretical calculation suggests that the incorporation of Mg in hydrogenated ZnO decrease the formation energy of interstitial H and increase formation energy of O-vacancy (VO). The experimental results demonstrate that the existence of the diluted Mg in hydrogenated ZnO TFTs could be sufficient to boost up mobility from 10 to 32.2 cm(2)/(V s) at a low carrier density (∼2.0 × 10(18) cm(-3)), which can be attributed to the decreased electron effective mass by surface band bending. The all results verified that the Mg/H codoping can significantly passivate the VO to improve device reliability and enhance mobility. Thus, this finding clearly points the way to realize high-performance metal oxide TFTs for low-cost, large-volume, flexible electronics.

Chemically Functionalized, Well-Dispersed Carbon Nanotubes in Lithium-Doped Zinc Oxide for Low-Cost, High-Performance Thin-Film Transistors.

Surface-functionalized carbon nanotubes (CNTs) are introduced into lithium-doped ZnO thin-film transistors (TFTs) as an alternative to the conventional incorporation of an expensive element, indium. The crucial role of surface functionalization of CNTs is clarified with the demonstration of indium-free ZnO-based TFTs with a field-effect mobility of 28.6 cm(2) V(-1) s(-1) and an on/off current ratio of 9 × 10(6) for low-cost, high-performance electronics.