Transparent Oxide Thin-film Transistors Research Articles

Surface engineering of Al2O3 dielectric layer for all-sputtered-oxide transparent and flexible a-IGZO thin film transistor

Transparent flexible display devices have attracted massive attention in recent years owing to their practical applications that can be compatibly adapted to human demand and technological development. Worldwide researchers have studied flexible transparent thin film transistors (TFTs) to qualify them for the backplane component of future transparent and flexible displays. Simplifying the fabrication process of a TFT can be considered a critical need for reducing consumed cost, time, toxic chemicals. In this study, sputtering was utilized as a major fabrication method for manufacturing a transparent oxide TFT device, which demonstrated good mechanical flexibility. A simple surface treatment process was applied to enhance the susceptibility of the sputtered Al2O3 dielectric through multiple coatings of solution processed Al2O3. It is noteworthy that the transparent oxide TFT device exhibits a high optical transparency of 81.12% and good electrical switchability at low operating voltage (3 V) with a decent mobility of 4.37 cm2 V−1 s−1 and proven mechanical durability.

Hydrogen Barrier Properties of Atomic Layer Deposited Al2O3 with Different Oxidants for Ingazno Thin Film Transistor

Amorphous In-Ga-Zn-O (a-IGZO) semiconductors have been used as an active channel material in high mobility, flexible, and transparent thin film transistors (TFTs), but it is highly influenced by the external environment. Many factors could affect TFT performance, such as moisture, oxygen, and hydrogen, etc. Among them, hydrogen incorporation into the oxide semiconductor is unavoidable due to the high electronegativity of oxygen. Hydrogen incorporated into a-IGZO acts as a shallow donor by ionizing and bonding with oxygen to form hydroxyl groups, so that it makes the oxide TFT conductive and causes it not to show an on/off property.[1] Hydrogen can be incorporated through various processes, such as diffusion from ambient atmosphere, incorporation during the deposition of encapsulation layer, and diffusion from adjacent layer during post-annealing, etc. In particular, it has been observed that a large amount of hydrogen atoms produced by SiH4 plasma is incorporated into the a-IGZO layer, creating a shallow electron donor, when PE-CVD SiNx layer is deposited for encapsulation. [2] Thus, an appropriate hydrogen barrier is essential to prevent the inflow of hydrogen to the a-IGZO layer during the subsequent process. Therefore, a method of preventing hydrogen incorporation by chemical combination should also be performed. However, there is no systematic study to prevent hydrogen incorporation. The analysis of hydrogen atomic behavior in thin films with different properties have not been clearly examined so far, thus further studies for this are necessary. Techniques commonly used for the deposition of gas diffusion barrier deposition are sputtering, chemical vapor deposition (CVD), and atomic layer deposition (ALD). Among them, ALD is considered as the most promising method to produce the gas diffusion barrier for the encapsulation due to its special features, such as conformal deposition, defect-free, high-quality layer and excellent uniformity.[3], [4] Since the ALD process is based on sequential self-limiting surface reaction where growth occurs by depositing the material layer by layer, thin films with various properties can be obtained depending on the materials (precursor and reactant) used for deposition as well as the thermodynamic conditions.[5] In this study, Al2O3 was deposited on the a-IGZO TFT by atomic layer deposition (ALD) using trimethylaluminum (TMA) with water or O3, as the precursor and oxidant, respectively, at low temperature (about 60 °C). First, we fundamentally investigated the characteristics of Al2O3 according to the oxidant. The chemical composition of the Al2O3 was different depending on the oxidant used in the ALD process (XPS, FT-IR measurement). Based on this, we analyzed the effect of these characteristics on hydrogen barrier properties by using transfer curve (I-V measurement) and stress test of device. As a result, the device in which the Al2O3 was deposited exhibited excellent hydrogen barrier properties as compared with the bare device. There was no device degradation after the hydrogen treatment, which suggested the possibility of enhancing the stability by effectively blocking hydrogen in various fields and device reliability in mass production in the future. Reference [1] S. I. Oh, G. Choi, H. Hwang, W. Lu, and J. H. Jang, “Hydrogenated IGZO thin-film transistors using high-pressure hydrogen annealing,” IEEE Trans. Electron Devices, vol. 60, no. 8, pp. 2537–2541, 2013. [2] A. Sato et al., “Amorphous In-Ga-Zn-O thin-film transistor with coplanar homojunction structure,” Thin Solid Films, vol. 518, no. 4, pp. 1309–1313, 2009. [3] J. Meyer et al., “Al2O3/ZrO2Nanolaminates as ultrahigh gas-diffusion barriersa strategy for reliable encapsulation of organic electronics,” Adv. Mater., vol. 21, no. 18, pp. 1845–1849, 2009. [4] A. Dameron, S. Davidson, B. Burton, P. Carcia, R. McLean, and S. George, “Gas diffusion barriers on polymers using multilayers fabricated by Al2O3 and rapid SiO2 atomic layer deposition,” J. Phys. Chem. C, vol. 112, pp. 4573–4580, 2008. [5] S. M. George, A. W. Ott, and J. W. Klaus, “Surface Chemistry for Atomic Layer Growth,” J. Phys. Chem., vol. 100, no. 31, pp. 13121–13131, 1996. Figure 1

Regulation of precursor solution concentration for In-Zn oxide thin film transistors

The tunable electronic performance of the solution-processed semiconductor metal oxide is of great significance for the printing electronics. In current work, transparent thin-film transistors (TFTs) with indium-zinc oxide (IZO) were fabricated as active layer by a simple eco-friendly aqueous route. The aqueous precursor solution is composed of water without any other organic additives and the IZO films are amorphous revealed by the X-ray diffraction (XRD). With systematic studies of atomic force microscopy (AFM), X-ray photoemission spectroscopy (XPS) and the semiconductor property characterizations, it was revealed that the electrical performance of the IZO TFTs is dependent on the concentration of precursor solution. As well, the optimum preparation process was obtained. The concentrations induced the regulation of the electronic performance was clearly demonstrated with a proposed mechanism. The results are expected to be beneficial for development of solution-processed metal oxide TFTs.

Fabrication of Tin Oxide Thin Film Transistors by RF Magnetron Sputtering Using Sn/SnO Composite Target

P-type tin oxide thin film deposited by RF sputtering for transparent thin film transistor (TFT) applications is the subject of this study. P-type tin oxide thin film can be made by doping a cation with a lower valence into n-type SnO 2 as an acceptor impurity or by fabrication with tin monoxide (SnO). The later method was investigated in this study by RF magnetron sputtering process, which has a high deposition rate, uniform thickness control, simple stoichiometry control, and reproducibility. A Sn/SnO composite target was used in RF sputtering to utilize the benefits of both metallic and ceramic sputtering targets. A Sn/SnO composite target is expected to provide p-type tin oxide thin films that are easier to manufacture than metallic target or ceramic target. The metallic Sn element provides an excellent control of structural defects, while the ceramic SnO element provides stable stoichiometry. The p-type tin oxide thin film with a Sn/SnO composite target showed very good transparency of ~95%. and excellent electrical properties. The p-type tin oxide thin film of 15nm thickness had a carrier concentration of 8.03X10 15 cm -3 and a mobility of 15.2cm 2 /Vs. With these ultrathin tin oxide films, a p-channel TFT was fabricated with a staggered bottom-gate structure, and the effect of different channel thicknesses and different distances between the two electrodes were evaluated. The tin oxide TFT with a thick p-channel layer showed a significant increase in current measured than the tin oxide with a thin p-channel layer. The current measured tended to increase greatly as the distance between electrodes decreased. The IV output curves of the tin oxide TFTs exhibited characteristics of bipolar transistors potentially due to the partial creation of the oxygen-deficient SnO 2 like structure, but the mechanism of bipolar transistor characteristics from the p-channel tin oxide TFT is still unclear. The fabrication of highly transparent tin oxide thin film with bipolar transistor characteristics is demonstrated herein.

Thermal analysis techniques and the degradation mechanism of transparent oxide thin film transistors

Thermal analysis techniques and the degradation mechanism of transparent oxide thin film transistors

Low-Resistive High-Work-Function Gate Electrode for Transparent a-IGZO TFTs

Highly transparent and low-resistive multilayered gate electrodes, MoO3/indium–tin oxide (ITO)/Ag/ZnS (MIAZ) playing as the high-work-function layer, the nonreactive interface layer, the lateral conduction layer, and the index-matching layer, respectively, have been investigated for the application to the transparent oxide thin-film transistors (TFTs). The transmittance of the optimized MIAZ electrode is 92.46% and the sheet resistance is $7.77~\Omega / \Box $ . The top gate InGaZnO TFT with this gate electrode shows the mobility of 11.57 cm2/( $\text {V}\cdot ~\text {s}$ ) and positive $V_{\text {th}}$ of 0.210 V compared with that with single ITO gate electrode of which $V_{\text {th}}$ is −0.086 V.

Capping Ti-Doped GaZnO on InGaZnO Layer As the Composite-Channel Structure for Enhancing the Device Performances and Stability of Thin-Film Transistors

The device performances of an amorphous indium gallium zinc oxide transparent thin-film transistor (a-IGZO TFT) were improved by using a Ti-doped GaZnO (GTZO) as the channel capping layer in this study. The GTZO thin film exhibited a high carrier concentration of $\text{1.5} \times \text{10} ^{21} \text{cm} ^{- 3}$ and served as both a carrier supplement and passivation layer for reducing the absorption of oxygen and moisture as well as the back-channel charge trapping in the composite-channel TFT. An enhanced carrier mobility with high electron concentration in the composite GTZO/IGZO heterostructure was verified in the Hall measurement. The GTZO/IGZO composite-channel TFT was demonstrated to show an improved carrier mobility of 15.5 cm2/V-s, subthreshold swing of 0.35 V/decade, off current of $1.4 \times 10^{- 11}$ A, threshold voltage of 1.4 V, and on/off current ratio of $1.2 \times 10 ^{7}$ . Stable operation with unchanged device characteristics was observed following a bias-stress test. Thus, the fabricated GTZO/IGZO TFTs exhibited advantageous device performance and improved device operation stability.

Light-adaptable display for the future advertising service

ABSTRACTIn this paper, a new light-adaptable display (LAD) structure with minimum power consumption is proposed for the future advertising service, and the demonstrated results are reported. An organic light-emitting diode with color reflection (colored OLED) was applied for the reflective- and emissive-mode device, and a guest-host liquid-crystal device (GH-LC) was adopted for the light shutter device. The current efficiency and reflectance of the colored OLED were 35.15 cd/A at 457 cd/m2 luminance and 63% for the yellow color, respectively. The measured contrast ratio of GH-LC was 15.5:1 at dark-room conditions, respectively. Transparent oxide thin-film transistors were used for the backplane, and their average mobility was 9.08 cm2/V s, with a 0.5 standard deviation. Through the optimization of the fabrication process and the structures of each device, the LAD adaptively operating according to the environmental illuminance from dark to 10,000 nits was successfully demonstrated. Moreover, a new LAD driving method was proposed for minimizing the power consumption.

Zinc Tin Oxide 투명 박막트랜지스터의 특성에 미치는 소스/드레인 전극의 영향

Zinc tin oxide transparent thin film transistors (ZTO TTFTs) were fabricated by using Si wafers as gate electrodes. Indium (In), aluminum (Al), indium tin oxide (ITO), silver (Ag), and gold (Au) were employed for source and drain electrodes, and the mobility and the threshold voltage of ZTO TTFTs were observed as a function of electrode. The ZTO TTFTs adopting In as electrodes showed the highest mobility and the lowest threshold voltage. It was shown that Ag and Au are not suitable for the electrodes of ZTO TTFTs. As the results of this study, it is considered that the interface properties of electrode/ZTO are more influential in the properties of ZTO TTFTs than the conductivity of electrode.

게이트절연막의 열처리가 Zinc Tin Oxide 투명 박막트랜지스터의 특성에 미치는 영향

Zinc tin oxide transparent thin film transistors (ZTO TTFTs) were fabricated on oxidized Si wafers. The thickness of ~30 nm films were deposited on the oxidized Si wafers by atomic layer deposition, which acted as the gate insulators of ZTO TTFTs. The films were rapid-annealed at , , , and , respectively. Active layers of ZTO films were deposited on the coated Si wafers by rf magnetron sputtering. Mobility and threshold voltage were measured as a function of the rapid-annealing temperature. X-ray photoelectron spectroscopy (XPS) were carried out to observe the chemical bindings of films. The annealing effects of gate-insulator on the properties of TTFTs were analyzed based on the results of XPS.

High Precision Active-Matrix Self-Capacitive Touch Panel Based on Fluorinated ZnO Thin-Film Transistor

Matrix-addressed capacitive sensing is popularly employed in advanced multi-touch panels. While the number of simultaneous touch points of a passively addressed matrix is limited, the simultaneous touch points of an actively addressed matrix are constrained only by the size of the matrix and can be more densely packed. Since the touch panel is typically inserted between a display panel and the user, it would be the best if the transistors in an active-matrix touch panel are transparent. Reported presently is such a panel realized using a fluorinated zinc oxide transparent thin-film transistor technology. The technology can be applied to construct panels requiring high touch resolution, such as precision stylus pen-based writing, fingerprint sensing and sensing of the area of the touch, etc.

Thin film complementary metal oxide semiconductor (CMOS) device using a single-step deposition of the channel layer.

We report, for the first time, the use of a single step deposition of semiconductor channel layer to simultaneously achieve both n- and p-type transport in transparent oxide thin film transistors (TFTs). This effect is achieved by controlling the concentration of hydroxyl groups (OH-groups) in the underlying gate dielectrics. The semiconducting tin oxide layer was deposited at room temperature, and the maximum device fabrication temperature was 350°C. Both n and p-type TFTs showed fairly comparable performance. A functional CMOS inverter was fabricated using this novel scheme, indicating the potential use of our approach for various practical applications.

Effects of an Aluminum Contact on the Carrier Mobility and Threshold Voltage of Zinc Tin Oxide Transparent Thin Film Transistors

We fabricated amorphous zinc tin oxide (ZTO) transparent thin-film transistors (TTFTs). The effects of Al electrode on the mobility and threshold voltage of the ZTO TTFTs were investigated. It was found that the aluminum (Al)-ZTO contact decreased the mobility and increased the threshold voltage. Traps, originating from <TEX>$AlO_x$</TEX>, were assumed to be the cause of degradation. An indium tin oxide film was inserted between Al and ZTO as a buffer layer, forming an ohmic contact, which was revealed to improve the performance of ZTO TTFTs.

Unusual instability mode of transparent all oxide thin film transistor under dynamic bias condition

We report a degradation behavior of fully transparent oxide thin film transistor under dynamic bias stress which is the condition similar to actual pixel switching operation in active matrix display. After the stress test, drain current increased while the threshold voltage was almost unchanged. We found that shortening of effective channel length is leading cause of increase in drain current. Electrons activate the neutral donor defects by colliding with them during short gate-on period. These ionized donors are stabilized during the subsequent gate-off period due to electron depletion. This local increase in doping density reduces the channel length.

Bias-stability improvement using Al2O3 interfacial dielectrics in a-InSnZnO thin-film transistors

We report amorphous-indium–tin–zinc-oxide (a-ITZO) thin-film transistors (TFTs) obtained using an aluminum-oxide (Al2O3) interfacial dielectric using atomic layer deposition between a silicon-nitride (SiNX) gate dielectric and an a-ITZO active channel layer. The effect of the Al2O3 interfacial layer on the suppression of charge trapping in a-ITZO TFTs is presented. In transparent oxide TFTs, reducing the shift in threshold voltage by stress-including negative-bias stress (NBS) is one of the key issues in improving the stability performance of TFTs. The NBS stability of an a-ITZO TFT using an Al2O3/SiNX double-layered dielectric is superior to that using an SiNX single-gate dielectric, because of the smooth surface with a root-mean-square roughness of 0.147 nm and a low defect density of less than 3×1011 eV−1 cm−2, which increases hydrophobicity. The a-ITZO TFTs using the Al2O3 interfacial dielectric show little change in the threshold voltage (∼0 V), and a long trapping time of ∼5000 s when a gate voltage of −25 V and drain voltage of 1 V are applied for 10 000 s. We show that the gate dielectric has a profound effect on the electrical stability, and suggest a way of improving the stability of a-ITZO TFTs.

An ‘aqueous route’ for the fabrication of low-temperature-processable oxide flexible transparent thin-film transistors on plastic substrates

Metal-oxide semiconductors have attracted considerable attention as next-generation circuitry for displays and energy devices because of their unique transparency and high performance. We propose a simple, novel and inexpensive ‘aqueous route’ for the fabrication of oxide thin-film transistors (TFTs) at low annealing temperatures (that is, <200 °C). These results provide substantial progress toward solution-processed metal-oxide TFTs through naturally formed, unique indium complex and post annealing. The fabricated TFTs exhibited acceptable electrical performance with good large-area uniformity at low temperatures. Additional vacuum annealing facilitated the condensation reaction by effectively removing byproduct water molecules and resulted in the activation of the In2O3 TFT at low annealing temperatures, even temperatures as low as 100 °C. In addition, we have demonstrated that the flexible and transparent oxide TFTs on plastic substrates exhibit good resistance to external gate bias stress.

Low temperature and solution-processed Na-doped zinc oxide transparent thin film transistors with reliable electrical performance using methanol developing and surface engineering

A transparent thin film transistor (TTFT), including zinc oxide (ZnO), has come into the spotlight as an innovative TFT that has the potential to drive the future of the information technology industry. Herein, we developed a new direct patterning method, drop-casting with a new developing method, through the combination of an aqueous ammonia–ZnO process with the doping of Na ions and surface engineering for high n-type semiconducting performance with good operational stability at low temperature. In particular, the effective decomposition and removal of the residual ammonia compounds using methanol have a successful effect on both intrinsic and Na doped ZnO precursor processes for TFTs and they showed the extensive possibility of ammonia based metal oxide precursor solutions. In this method, the Na doped ZnO TTFTs showed good operational stability even with the process of low temperature sintering. The mobility μ = 0.80 cm2 V−1 s−1 was obtained at 200 °C sintering and the mobility μ = 0.10 cm2 V−1 s−1 at 100 °C sintering. In addition, in ambient conditions, the patterned Na doped ZnO TTFT exhibited high electron mobility μ = 1.84 cm2 V−1 s−1 with excellent device operational stability and scant hysteresis with sintering at 300 °C. This method is not only simple as compared with photolithography and inkjet printing, but is also a sophisticated patterning process with good fidelity for solution-processed ZnO TFTs. Moreover, the proposed method can be extended to plastic substrates on a large scale because of the low temperature development process of the ammonia–ZnO precursor using methanol and continuous patterning at ambient conditions. We believe that this method can be adapted to the advanced process toward future printed transparent electronic devices.

Room temperature fabricated transparent amorphous indium zinc oxide based thin film transistor using high-κ HfO2 as gate insulator

We report here the performance of amorphous oxide transparent thin film transistors (TFTs) fabricated using DC magnetron sputter techniques at room temperature. Amorphous indium zinc oxide (a-IZO, In2O3:ZnO, 90:10wt.%) and high dielectric constant hafnium oxide (HfO2) films were deposited for the semiconducting channel and gate insulator under a mixture of ambient argon and oxygen gas, respectively. The reported transparent TFT structure was a bottom-gate type, consisting of indium-tin-oxide and aluminium as gate electrode and source/drain electrodes, respectively. The optical transmittance of a-IZO films in the visible region was greater than 84%. The a-IZO TFTs showed that the field effect saturation mobility, Ion/Ioff ratio, sub-threshold swing, and threshold voltage were extracted to give 13.09cm2/V-s, 4×105, 0.42V/decade, and 3.54V, respectively. The reported a-IZO TFT with high dielectric constant HfO2 gate dielectric sputtered at room temperature presents high quality characteristics, which can be used as good candidate for the application of low-temperature fabricated optoelectronic devices on organic flexible substrates.

Improvements in the device characteristics of IZO-based transparent thin-film transistors with co-sputtered HfO 2–Al 2O 3 gate dielectrics

Transparent oxide thin-film transistors (TFTs) have been fabricated by RF magnetron sputtering at room temperature using amorphous indium zinc oxide (IZO) as both active channel and source/drain electrodes and co-sputtered HfO 2–Al 2O 3 (HfAlO) as gate dielectric. In spite of its high dielectric constant, HfO 2 has some drawbacks including a high leakage current and rough surface morphologies due to its small band gap (5.31 eV) and microcrystalline structure. In this work, the incorporation of Al 2O 3 into hafnium (Hf) was achieved by co-sputtering of HfO 2 and Al 2O 3 without any intentional substrate heating, and its structural and electrical properties were investigated by X-ray diffraction (XRD), atomic force microscopy (AFM), and spectroscopic ellipsometry (SE) analyses, depending on the Hf:Al compositional ratio. The XRD studies confirmed that the microcrystalline structures of HfO 2 were transformed to amorphous structures of HfAlO at approximately Hf:Al = 3:1 at% ratio. According to the AFM analysis, the HfAlO films (0.54 nm) were considerably smoother than the HfO 2 films (4.27 nm) due to their amorphous structure. The band gap ( E g) deduced by spectroscopic ellipsometry was increased from 5.31 eV (HfO 2) to 5.58 eV (HfAlO). The electrical performances of the TFTs, which were made of well-controlled active/electrode IZO materials and co-sputtered HfAlO dielectric material, exhibited a field-effect mobility of more than 10 cm 2/V s, a threshold voltage of about 1.5 V, a sub-threshold swing of about 0.5 V/decade, and an I on/off ratio of about 10 5.

Transparent Oxide Thin-Film Transistors Using n-(In2O3)0.9(SnO2)0.1/InGaZnO4 Modulation-Doped Heterostructures

We investigated transparent oxide thin-film transistors (TFTs) using n-(In2O3)0.9(SnO2)0.1/InGaZnO4 (n-ITO/IGZO) modulation-doped heterostructures, which are effective in achieving high carrier mobilities. From transmittance measurements and UV photoemission spectroscopy, n-ITO/IGZO modulation-doped heterostructures are expected to realize the type-II energy band lineup, in which both the conduction band minimum and the valence band maximum of n-ITO are higher in energy than those of IGZO. Van der Pauw Hall measurements revealed Hall mobility enhancement and two-dimensional behavior of electrons at the n-ITO/IGZO interface. Using the n-ITO/IGZO modulation-doped heterostructures, we obtained TFTs with higher electron mobility than that of IGZO TFTs. We consider that modulation doping is a promising method for performance improvements of TFTs using transparent oxide semiconductors.