Yi-Teh Chou

Environment-dependent metastability of passivation-free indium zinc oxide thin film transistor after gate bias stress

We investigated the effects of bias stress on a passivation-free InZnO thin-film transistors (a-IZO TFTs) exposed to either the atmosphere or a vacuum. The magnitude of threshold voltage shift increased with the application duration of bias stress, to an extent that was much larger in the atmosphere than in the vacuum. The threshold voltage recovered slowly to its nearly initial value when the gate bias stress was removed. The electrical metastability was attributed to the interaction between the exposed a-IZO backchannel and oxygen/moisture from the atmosphere, and a dynamic equilibrium was finally achieved, regardless of the polarity of stress voltage.

Nitrogenated amorphous InGaZnO thin film transistor

This work presents the electrical characteristics of the nitrogenated amorphous InGaZnO thin film transistor (a-IGZO:N TFT). The a-IGZO:N film acting as a channel layer of a thin film transistor (TFT) device was prepared by dc reactive sputter with a nitrogen and argon gas mixture at room temperature. Experimental results show that the in situ nitrogen incorporation to IGZO film can properly adjust the threshold voltage and enhance the ambient stability of a TFT device. Furthermore, the a-IGZO:N TFT has a 44% increase in the carrier mobility and electrical reliability and uniformity also progress obviously while comparing with those not implementing a nitrogen doping process.

Charge pumping method for photosensor application by using amorphous indium-zinc oxide thin film transistors

The study investigated the photoreaction behavior of amorphous indium-zinc oxide thin film transistor (a-IZO TFT), which was thought to be insensitive to visible light. The obvious threshold voltage shift was observed after light illumination, and it exhibited slow recovery while returning to initial status. The photoreaction mechanism is well explained by the dynamic equilibrium of charge exchange reaction between O2(g) and O2− in a-IZO layer. A charge pumping technique is used to confirm the mechanism and accelerate recoverability. Using knowledge of photoreaction behavior, an operation scheme of photosensing elements consist of a-IZO TFT is also demonstrated in this work.

Ambient Stability Enhancement of Thin-Film Transistor With InGaZnO Capped With InGaZnO:N Bilayer Stack Channel Layers

A thin-film transistor (TFT) with bilayer stack structure of amorphous nitrogenated InGaZnO (IGZO) (a-IGZO:N) on an IGZO channel is proposed to enhance device stability. The a-IGZO:N acting as a back-channel passivation (BCP) is formed sequentially just after the sputter-deposited amorphous IGZO (a-IGZO) film with in situ nitrogen incorporation process. The a-IGZO:N can effectively prevent the a-IGZO channel from exposing to the atmosphere and retarding interactions with ambient oxygen species. Also, the optical energy bandgap of the a-IGZO:N film is decreased due to the addition of nitrogen. This causes the a-IGZO TFT with a-IGZO:N BCP to exhibit high immunity to the ultraviolet-radiation impact.

High-gain complementary inverter with InGaZnO/pentacene hybrid ambipolar thin film transistors

Ambipolar thin film transistors (TFTs) with InGaZnO/pentacene heterostructure channels are demonstrated for a high-voltage-gain complementary metal oxide semiconductor (CMOS) inverter. The ambipolar TFT exhibits a electron mobility of 23.8 cm2/V s and hole mobility of 0.15 cm2/V s for the InGaZnO and pentacene, respectively. The thermal annealing process was also studied to adjust electron concentration reducing operating voltage of the CMOS inverter. The voltage gain achieves as high as 60 obtained in the first and third quadrants of the voltage transfer characteristic. The high performance and simple manufacture of the heterostructure CMOS inverter show promise as critical components in various electrical applications.

Improvement of Electron-Gun Evaporated Aluminum Oxide for Pentacene Thin-Film Transistor

Supercritical fluid (SCF) technology was proposed to improve the dielectric properties of electron-gun evaporated aluminum oxide (AlO x ) film in this work. The leakage current of AlO x film deposited at room temperature was suppressed significantly from 10 -4 to 10 -10 A at a bias voltage of -20 V after the SCF treatment mixed with water and propyl alcohol. The evolution of the leakage conduction mechanism was confirmed theoretically from trap-assisted quantum tunneling to the Schottky emission process due to the reduction of electric traps in the AlO x dielectric film. In addition, our work demonstrated the application of SCF-treated AlO x gate dielectric to a pentacene-based thin-film transistor.

P-15: Retarded Photoreaction Reversibility of a-IZO TFT for Light Sensor Applications

The retarded photoreaction reversibility of a-IZO TFT has been discussed in this work. with a small DC voltage bias, the reaction was speeded up and the electrical characteristics were recovered back to the fresh state. The Vth shifting behavior possesses the potential for the light sensor applications in display technologies.

Stabilization of oxide-based thin-film transistors

As development of flat-panel displays continues to grow, thinfilm-transistor (TFT) technologies have been extensively used as switching devices or peripheral drivers in active-matrix LCDs (AMLCDs) and as pixel drivers for organic LEDs (AMOLEDs). However, amorphous silicon, which is conventionally used as the channel layer in TFT devices, faces development limitations because of physical drawbacks, such as low electron mobility, high photosensitivity, and Staebler-Wronski effects. For example, high-current-driving capabilities have made lowtemperature polycrystalline silicon TFTs suitable candidates for pixel elements in AMOLEDs, despite issues related to device uniformity and high manufacturing costs. To reduce the problems related to conventional silicon-based materials, novel semiconductors have been considered. The latter use amorphous oxide semiconductors as channel layers in TFT devices. They have attracted attention because of their high carrier mobility, low-temperature deposition, and optical transparency. Amorphous indium gallium zinc oxide films (a-IGZO) are the most obvious candidates.1 They have electrons as majority carriers, mainly because of the oxygen vacancies and interstitials created during the deposition process.2, 3 The bonding structure of a-IGZO films causes the resultant TFT devices to exhibit high field-effect mobility.4, 5 Nevertheless, a-IGZO TFT sensitivity to the environment hinders their performance under realistic operating conditions.6 This dependence is caused by absorption and desorption of oxygen species present in ambient air into the a-IGZO channel layer. This interaction changes the concentration of oxygen vacancies in the a-IGZO films and results in a shift of the TFT threshold voltage (Vth). Over time, this leads to a nonuniformity problem.7 To achieve reliable device operation, passivation methods have been developed to protect the a-IGZO channel from ambient-air interference.8 However, these methods require extrinsic channel-passivation Figure 1. Scanning-electron-microscopy images of (a) an amorphous indium gallium zinc oxide thin film (IGZO) and (b) an amorphous nitrogenated IGZO thin film (IGZON). Both films were sputterdeposited under a nitrogen/argon (N2/Ar) gas-flow ratio of 0.2.

P‐64: Using Electroless Plating Technology for Copper Metallization in AMLCD Application

The feasibility of using electro-less plating (ELP) technology to manufacture copper (Cu) gate electrodes in TFTs is investigated. The poor adhesion between Cu and glass substrates is overcome by introducing ELP nickel-phosphorus layers. Self-aligning characteristics also omits the Cu-etching process. The similar electrical performance verifies the compatibility of this technology.

Photosensor application of amorphous InZnO-based thin film transistor

Thin film transistor (TFT) device structure with transparent conductive oxide semiconductor is proposed for the photosensor application. The adoption of TFT-based photosensor device also is promising to be integrated with pixel-array circuits in a flat panel display and realize the system-on-panel (SoP) concept. The photosensitive TFT device can be applied to sense the ambient light brightness and then give the feedback to the backlight system adjusting the backlight intensity for the power-saving green displays. In this work, we studied the photosensitivity of amorphous indium zinc oxide (a-IZO) TFT to ultraviolet light. The a-IZO-based semiconductors have been paid much attention due to their uniform amorphous phase and high field-effect carrier mobility characteristics. The obvious threshold voltage shift was observed after light illumination, and exhibited slow recovery while returning to initial status after removing the light source. This mechanism for the photoreaction is well explained by the dynamic equilibrium of charge exchange reaction between O2(g) and O2- in the backchannel region of IZO-based films. An electrical trigger using charge pumping method is used to confirm the proposed mechanism and accelerate photoreaction recoverability for the first time. Using knowledge of photoreaction behavior, an operation scheme of photosensing elements consist of a-IZO TFTs is also demonstrated in this paper.