Zinc Tin Oxide Thin-film Transistors Research Articles

Effects of Ga:N addition on the electrical performance of zinc tin oxide thin film transistor by solution-processing.

This paper addressed the effect of gallium nitrate hydrate addition on thin film transistor (TFT) performance and positive bias stability of amorphous zinc tin oxide (ZTO) TFTs by solution processing, Further, the mechanisms responsible for chemical properties and electronic band structure are explored. A broad exothermic peak accompanied by weight loss appeared in the range from about 350 to 570 °C for the ZTO solution; the thermal reaction of the Ga-ZTO:N solution was completed at 520 °C. This is because the gallium nitrate hydrate precursor promoted the decomposition and dehydroxylation reaction for Zn(CH3COO)2·2H2O and/or SnCl2·2H2O precursors. The concentrations of carbon and chloride in gallium nitrate hydrate added ZTO films annealed at 400 °C have a lower value (C 0.65, Cl 0.65 at. %) compared with those of ZTO films (C 3.15, Cl 0.82 at. %). Absorption bands at 416, 1550, and 1350 cm(-1) for GaZTO:N films indicated the presence of ZnGa2O4, N-H, and N═O groups by Fourier transform infrared spectroscopy measurement, respectively. As a result, an inverted staggered Ga-ZTO:N TFT exhibited a mobility of 4.84 cm(2) V(-1) s(-1) in the saturation region, a subthreshold swing of 0.35 V/decade, and a threshold gate voltage (Vth) of 0.04 V. In addition, the instability of Vth values of the ZTO TFTs under positive bias stress conditions was suppressed by adding Ga and N from 13.6 to 3.17 V, which caused a reduction in the oxygen-related defects located near the conduction band.

The Anomalous Effect of Oxygen Ratio on the Mobility and Photobias Stability of Sputtered Zinc–Tin–Oxide Transistors

The oxygen ratio-dependent device performance and reliability of zinc tin oxide (ZTO) thin-film transistors (TFTs) were examined. ZTO TFTs fabricated under pure Ar conditions exhibited a high saturation mobility of 21.5 cm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> /Vs, low subthreshold gate swing of 0.34 V/decade, and high I <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">ON/OFF</sub> ratio (10 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">9</sup> ), whereas modest mobility of 7.3 cm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> /Vs was obtained for the ZTO TFTs prepared at an oxygen ratio [R = O <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> /(Ar + O <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> )] of 0.3. The photobias stability (AV <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">th</sub> ≈-2.0 V) of the ZTO TFTs under the Ar only condition was much better than that (AV <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">th</sub> ≈-5.9 V) of the device at R of 0.3, even though their channel layers contained the larger density of oxygen vacancies. This abnormal behavior was attributed to the compressive stress of ZTO films retarding the phototransition of oxygen vacancies.

Molybdenum as a contact material in zinc tin oxide thin film transistors

Amorphous oxide semiconductors are of increasing interest for a variety of thin film electronics applications. Here, the contact properties of different source/drain electrode materials to solution-processed amorphous zinc tin oxide (ZTO) thin-film transistors are studied using the transmission line method. The width-normalized contact resistance between ZTO and sputtered molybdenum is measured to be 8.7 Ω-cm, which is 10, 20, and 600 times smaller than that of gold/titanium, indium tin oxide, and evaporated molybdenum electrodes, respectively. The superior contact formed using sputtered molybdenum is due to a favorable work function lineup, an insulator-free interface, bombardment of ZTO during molybdenum sputtering, and trap-assisted tunneling. The transfer length of the sputtered molybdenum/ZTO contact is 0.34 μm, opening the door to future radio-frequency sub-micron molybdenum/ZTO thin film transistors.

Effects of contact resistance on the evaluation of charge carrier mobilities and transport parameters in amorphous zinc tin oxide thin-film transistors

Accurate determination of the charge transport characteristics of amorphous metal-oxide transistors requires the mitigation of the effects of contact resistance. The use of additional electrodes as voltage probes can overcome contact resistance-related limitations and yields accurate charge carrier mobility values, trap depths and temperature and carrier density dependencies of mobility as well as trap depths. We show that large differences in measured charge carrier mobility values are obtained when such contact resistances are not factored out. Upon exclusion of the contact resistance, the true temperature dependence of charge carrier mobility appears in the form of two clearly distinct mobility regimes. Analyzing these revealed mobility regions leads to a more accurate determination of the underlying transport physics, which shows that contact resistance-related artefacts yield incorrect trends of trap depth with gate voltage, potentially leading to a misconstruction of the charge transport picture. Furthermore, a comparison of low- and high-mobility samples indicates that the observed effects are more general.

Nonhydrolytic alkyl halide elimination reaction and its application in solution-processed zinc tin oxide thin film transistors

In this study, we fabricated zinc tin oxide (ZTO) thin-film transistors (TFTs) using a sol–gel solution at an annealing temperature of 350 °C. We used a precursor combination of alkoxide and metal chloride to utilize the alkyl halide elimination reaction. Compared with transistor using chloride-only precursors, the resulting ZTO transistor showed improved performance. Solution-processed ZTO-TFTs prepared at 350 °C using an alkoxide–chloride precursor combination showed a field-effect mobility of 4.17 cm2 V−1 s−1, whereas that prepared using a chloride-only solution showed a mobility of 0.98 cm2 V−1 s−1. Thermal analysis showed that the alkoxide–chloride precursor was decomposed well at a given annealing temperature and formed oxide with few residual impurities compared with chloride-only precursors.

Parabolic behavior of solution processed ZnSnO device performances depending on Zn/Sn ratios

Thin film transistors (TFTs) with amorphous zinc tin oxide (ZTO) channel layer were fabricated by a simple and low-cost solution process, prepared by dissolving 0.2 M of zinc acetate dihydrate and tin chloride dihydrate in 20 mL of 2-methoxyethanol. All ZTO thin films showed amorphous phases and no impurities (no carbon and chlorine content) even at process temperature of 350 °C. As the Sn ratio in ZTO films increased, the values of saturated mobility (usat) and subthreshold gate swing (SS) exhibited a parabolic behavior in ZTO TFTs, depicting that the μsat and SS values were a maximum (3.4 cm2/V.s) and minimum (0.38 V/decade) at Zn/Sn = 1 ratio. Interestingly, the x-ray absorption and X-ray photoemission spectroscopy revealed the origin of parabolic behavior, indicating not only to improve a charge transport in conduction bands but also to increase the Sn4+/Sn2+ ratio at the peak values (Sn/(Zn + Sn) = 1).

Investigation of zinc-tin-oxide thin-film transistors with varying SnO2 contents

Zinc tin oxide (ZTO) thin-film transistors (TFTs) were prepared on a glass substrate by deposition using radio frequency (RF) magnetron co-sputtering, followed by annealing at 300°C for 20 min. The properties of ZTO thin films were found to be dependent on the atomic compositional ratio of Zn:Sn; the device performance and operational stability of the fabricated ZTO TFTs, including the mobility, on-off current ratio, threshold voltage, and subthreshold slope, were strongly influenced by the Sn content. Better TFT performance was achieved when the Sn content in the ZTO thin film was low, and the optimal mobility was 18 cm2 V−1 s−1, threshold voltage was 0.5 V, and subthreshold slope was 0.227 V·dec−1. Notably, the device performance and operational stability of the RF magnetron co-sputtered ZTO TFTs could be improved by optimizing the Zn:Sn atomic compositional ratio in the films.

Tuning Electrical Properties in Amorphous Zinc Tin Oxide Thin Films for Solution Processed Electronics

Solution processed zinc tin oxide (ZTO) thin film transistors (TFTs) were fabricated by varying the Zn/Sn composition. The addition of Sn to the zinc oxide (ZnO) films resulted in improved electrical characteristics, with devices of Zn0.7Sn0.3O composition showing the highest mobility of 7.7 cm(2)/(V s). An improvement in subthreshold swings was also observed, indicative of a reduction of the interfacial trap densities. Mobility studies at low temperature have been carried out, which indicated that the activation energy was reduced with Sn incorporation. Kelvin probe force microscopy was performed on the films to evaluate work function and correlated to the metal-semiconductor barrier indicating Zn0.7Sn0.3O films had the smallest barrier for charge injection. Organic-inorganic hybrid complementary inverters with a maximum gain of 10 were fabricated by integrating ZTO TFTs with poly-3-hexylthiophene (P3HT) transistors.

Effects of annealing process on characteristics of fully transparent zinc tin oxide thin-film transistor

Annealing effect on the performance of fully transparent thin-film transistor (TTFT), in which zinc tin oxide (ZnSnO) is used as the channel material and SiO2 as the gate insulator, is investigated. The ZnSnO active layer is deposited by radio frequency magnetron sputtering while a SiO2 gate insulator is formed by plasma-enhanced chemical vapor deposition. The saturation field-effect mobility and on/off ratio of the TTFT are improved by low temperature annealing in vacuum. Maximum saturation field-effect mobility and on/off ratio of 56.2 cm2/(V·s) and 3 × 105 are obtained, respectively. The transfer characteristics of the ZnSnO TFT are simulated using an analytical model and good agreement between measured and the calculated transfer characteristics is demonstrated.

Improvement in the Photo-Bias Stability of Zinc Tin Oxide Thin-Film Transistors by Introducing a Thermal Oxidized ${\rm TiO}_{2}$ Film as a Hole Carrier Blocking Layer

This paper examined the morphological, structural, and electrical properties of thermal titanium oxide (TiOx) films as a function of the physical thickness. All the thermal TiOx films were assigned to a TiO2 chemical state irrespective of the film thickness. The thinner TiO2 films (≤ 5 nm) showed an amorphous phase, whereas the thicker TiO2 film (≥ 7 nm) had a nanocrystalline structure. This intriguing thickness-dependent crystallization behavior can be explained by the dimensional effect. The mobility of the resulting zinc tin oxide (ZTO) thin-film transistors (TFTs) with a gate-stack of silicon nitride (SiNx) and TiO2/SiNx was monotonously reduced with increasing TiO2 film thickness, which can be attributed to the enhanced Columbic scattering effect of TiO2 films. On the other hand, the negative bias illumination stress instability of the ZTO TFTs can be suppressed significantly to -2.4 V by the adoption of a 5-nm-thick TiO2 film compared with that (-14.4 V) of the ZTO device without a TiO2 film, which is discussed based on the valence band-off structure and the amorphous nature of thermal TiO2 films.

Recovery characteristics of solution-processed ZTO thin-film transistors

We investigated the negative-bias illumination stress (NBIS) and the recovery characteristics of solution-processed amorphous zinc-tin-oxide (ZTO)-based thin-film transistors (TFTs). We developed TFTs with a bottom gate structure that used SiO2 films for the gate insulator. The developed TFTs were under NBIS in air at room temperature (RT) with a gate-to-source voltage of −10 V and a white-light power density of 6.2 mW/cm2. For the devices under NBIS, the NBIS-induced instability is attributed to the combined effects of 1) the creation of a low density of subgap states such as ionized oxygen vacancies (V 2+ ) near the Fermi level in ZTO, 2) a fast and large increase in the photo-conductivity at the off-state due to photo-induced band-to-band electron-hole pair generation, and 3) the hole trapping near the ZTO/SiO2 interface. The devices under NBIS also exhibit fast exponential recovery characteristics at RT, which are attributed dominantly to detrapping of holes near the ZTO/SiO2 interface and partially to the weak stabilization of V 2+ near the interface.

Effects of channel thickness on electrical properties and stability of zinc tin oxide thin-film transistors

We investigated the effects of channel thickness (tch) on the electrical properties and the bias stress-induced instability of zinc tin oxide (ZTO, Zn : Sn = 4 : 1) thin-film transistors (TFTs). With increasing tch, the electrical properties of the TFTs showed a trend: threshold voltage (Vth) decreased and field-effect mobility (μFE) increased; on the other hand, subthreshold swing and hysteresis characteristics did not show any significant change. The positive bias stress (PBS) test and the following analysis by the stretched-exponential equation revealed that the PBS-induced threshold voltage shift (ΔVth) decreased and the extracted characteristic trapping time (τ) increased with increasing tch. This thickness-dependence of PBS instability cannot be explained by charge trapping/injection model or defect creation model but can be well explained by less ambient effect at the backchannel due to less gate field through a thicker film.

Performances of Microcrystalline Zinc Tin Oxide Thin-Film Transistors Processed by Spray Pyrolysis

In this work, we report results concerning the performances of thin-film transistors (TFTs) where the channel layer is based on microcrystalline zinc tin oxide (ZTO) processed by spray pyrolysis technique. TFTs made with ~30 nm thick ZTO channel layer deposited at a substrate temperature of 400°C and 300°C exhibited, respectively, a saturation mobility of ~2.9 cm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> ·V <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">-1</sup> ·s <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">-1</sup> and 1.45 cm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> ·V <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">-1</sup> ·s <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">-1</sup> ; VON voltage of ~0.15 V, and 0.2 V; a sub-threshold swing of ~400 mV/dec and 500 mV/dec; ON/OFF ratio at the onset of hard saturation current of ~3.5×10 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">5</sup> and 6×10 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sup> , for a drain to source voltage of 10 V (close to or below the gate to source voltage). This indicates that the substrate temperature is relevant in determining the devices' electronic performances.

Impact of humidity on the electrical performance of amorphous oxide semiconductor thin-film transistors

An indium–gallium–zinc oxide or a zinc–tin oxide thin-film transistor (TFT) fabricated when the relative humidity in the laboratory is less than 50% is found to exhibit good electrical performance, with an abrupt, distortion-free transfer curve and a turn-on voltage close to 0 V. In contrast, when such an amorphous oxide semiconductor (AOS) TFT is fabricated at a relative humidity greater than 50%, its “as-fabricated” electrical performance is very poor, typically characterized by a large amount of hysteresis, a strongly negative turn-on voltage, and a kink-like distortion in the subthreshold region of its transfer curve. However, the electrical performance of such a poor-quality TFT is observed to improve over time, if it is simply stored in the dark at room temperature without being subjected to electrical stress. This recovery usually requires weeks (months) for an unpassivated (passivated) AOS TFT. Recovery is tentatively ascribed to the gradual removal of moisture from the AOS TFT channel layer.

Effect of the interlayer for source-drain electrodes on a solution-processed zinc-tin-oxide TFT

An interlayer was introduced between the source and drain (S-D) electrodes and the semiconductor in a solution-processed zinc tin oxide (ZTO) thin-film transistor (TFT). The ZTO TFT with a MoO3 interlayer of 0.5 nm between the ZTO and the S-D electrodes displayed better properties: a mobility of 5.6 cm2/V s, a threshold voltage (Vth) of −1.3 V and a subthreshold slope of 0.9 V/dec with better bias stability and hysteresis. This improvement is attributed to the MoO3’s reduction to a conductor, MoO2, and to the inter-diffusion of the interlayer between the active channel and the S-D layers, as confirmed by the XPS spectra and by a TEM-EDX analysis, respectively.

Improvement in the Bias Stability of Zinc-Tin Oxide Thin-Film Transistors by Hafnium Doping

The influence of hafnium (Hf) doping on negative-bias temperature instability in zinc-tin oxide thin-film transistors was studied. Hafnium-zinc-tin oxide TFTs exhibited a turn-on voltage (V ON) that shifted from 0 V to −1 V with negligible changes in the subthreshold swing and field-effect mobility after 3 h of total stresses. The enhanced improvement of the V ON shift (ΔV ON) was attributed to the reduction in the interface trap density, which may result from the suppression of oxygen-vacancy-related defects by the Hf ions.

Effects of film thickness and Sn concentration on electrical properties of solution-processed zinc tin oxide thin film transistors

This paper investigates the effect of Sn concentration and film thickness on properties of zinc tin oxide (ZTO) thin film transistors (TFTs) fabricated using a solution process. ZTO solution was synthesized using zinc acetate dehydrate and tin chloride dehydrate dissolved in a solvent composed of 2-methoxyethanol and mono-ethanolamine. A ZTO film was obtained for an active channel on a gate oxide layer by spin-coating the solution at room temperature, drying at 300°C for 10min, and annealing at 550°C for 120min. The thickness and Sn concentration affected the material structure and electrical properties of ZTO film. The best solution processed ZTO TFT was obtained at film thickness of 35nm and Sn concentration of 30at.%. The fabricated ZTO TFT exhibited an on/off ratio of 1.88×107, a field effect mobility of 17.02cm2/Vs, a subthreshold swing of 0.77V/decade, and a threshold voltage of 5.01V.

Impact of Soft Annealing on the Performance of Solution-Processed Amorphous Zinc Tin Oxide Thin-Film Transistors

It is demonstrated that soft annealing duration strongly affects the performance of solution-processed amorphous zinc tin oxide thin-film transistors. Prolonged soft annealing times are found to induce two important changes in the device: (i) a decrease in zinc tin oxide film thickness, and (ii) an increase in oxygen vacancy concentration. The devices prepared without soft annealing exhibited inferior transistor performances, in comparison to devices in which the active channel layer (zinc tin oxide) was subjected to soft annealing. The highest saturation field-effect mobility-5.6 cm(2) V(-1) s(-1) with a drain-to-source on-off current ratio (Ion/Ioff) of 2 × 10(8)-was achieved in the case of devices with 10-min soft-annealed zinc tin oxide thin films as the channel layer. The findings of this work identify soft annealing as a critical parameter for the processing of chemically derived thin-film transistors, and it correlates device performance to the changes in material structure induced by soft annealing.

Combined effect of the large ionic radius and low electronegativity of lanthanum additive on solution-processed zinc–tin–oxide thin-film transistors

Solution-processed lanthanum–zinc–tin–oxide thin films were fabricated with varying La content. A peak shift to small angles was observed in X-ray diffractometer spectra due to the expansion of the zinc–tin–oxide lattice originated from the larger ionic radius of La3+, while oxygen vacancies first increased, and then decreased with increasing La molar ratio. These results indicate that La, which has both low electronegativity and a large ionic radius, plays the role of either a generator or a suppressor of oxygen vacancies. Therefore, when choosing an additive to control the electrical properties of an oxide semiconductor, the ionic radius of the added atom should be carefully considered along with its electronegativity.

Photobias Instability of High Performance Solution Processed Amorphous Zinc Tin Oxide Transistors

The effects of the annealing temperature on the structural and chemical properties of soluble-processed zinc-tin-oxide (ZTO) films were examined by transmission electron microscopy, atomic force microscopy, high resolution X-ray reflectivity, and X-ray photoelectron spectroscopy. The density and purity of the resulting ZTO channel layer increased with increasing annealing temperature, whereas the oxygen vacancy defect density decreased. As a result, the device performance of soluble ZTO thin film transistors (TFTs) was improved at higher annealing temperature. Although the 300 °C-annealed ZTO TFT showed a marginal field-effect mobility (μFE) and high threshold voltage (Vth) of 0.1 cm(2)/(V s) and 7.3 V, respectively, the 500 °C-annealed device exhibited a reasonably high μFE, low subthreshold gate swing (SS), Vth, and Ion/off of 6.0 cm(2)/(V s), 0.28 V/decade, 0.58 V, and 4.0 × 10(7), respectively. The effects of dark negative bias stress (NBS) and negative bias illumination stress (NBIS) on the degradation of transfer characteristics of ZTO TFTs were also investigated. The instability of Vth values of the ZTO TFTs under NBS and NBIS conditions was suppressed with increasing annealing temperature. To better understand the charge trapping mechanism, the dynamics of Vth shift with NBS and NBIS time for all ZTO TFTs was analyzed on the basis of the stretched exponential relaxation. The negative Vth shift for each transistor was accelerated under NBIS conditions compared to NBS, which resulted in a higher dispersion parameter and smaller relaxation time for NBIS degradation. The relaxation time for NBS and NBIS instability increased with increasing annealing temperature, which is discussed on the basis of the transition mechanism of oxygen vacancy defects.