Li-Feng Teng

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.

Effects of microwave annealing on electrical enhancement of amorphous oxide semiconductor thin film transistor

By using microwave annealing technology instead of thermal furnace annealing, this work elucidates the electrical characteristics of amorphous InGaZnO thin film transistor (a-IGZO TFT) with a carrier mobility of 13.5 cm2/Vs, threshold voltage of 3.28 V, and subthreshold swing of 0.43 V/decade. This TFT performance with microwave annealing of 100 s is well competitive with its counterpart with furnace annealing at 450 °C for 1 h. A physical mechanism for the electrical improvement is also deduced. Owing to its low thermal budget and selective heating to materials of interest, microwave annealing is highly promising for amorphous oxide in semiconductor TFT manufacturing.

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.

Bipolar resistive switching characteristics of Al-doped zinc tin oxide for nonvolatile memory applications

Resistive random access memory using Al-doped zinc tin oxide (AZTO) as resistive switching layer was prepared by radio-frequency magnetron sputtering at room temperature. The Ti/AZTO/Pt device exhibits reversible and robust bi-stable resistance switching behavior over hundreds of switching cycles within 2 V sweep voltage. The Ti/AZTO/Pt device showed stable retention characteristics for over 104 s under read disturb stress condition. Besides, the electrical conduction mechanism was dominated by ohmic conduction in low resistance state, while the current transport behavior followed a trap-controlled space-charge-limited conduction process in high resistance state.

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.

Electrical Performance Enhancement of Al–Zn-Sn–O Thin Film Transistor by Supercritical Fluid Treatment

In this letter, a low-temperature supercritical fluid (SCF) treatment was employed to enhance the electrical and optical properties of amorphous Al-Zn-Sn-O thin film transistors (a-AZTO TFTs) for flat-panel displays. The carrier mobility and threshold voltage of a-AZTO TFT were improved significantly after SCF process because of the reduction of trap density in the a-AZTO active layer. In addition, the SCF-treated a-AZTO TFT exhibited superior electrical reliability and less degradation after negative gate bias illumination stress. X-ray photoelectron spectroscopy analysis confirmed that the proposed SCF treatment could effectively oxidize a-AZTO film and change the oxidation states of Sn, resulting in the improvement of a-AZTO TFT device characteristics.

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.