Investigation On Aspects Of Oxide Semiconductor Based Thin-film Transistors

Recently, thin-film transistors (TFTs) based on oxide semiconductors have attracted considerable attention because of their potential application in the next generation flat panel displays, which are characterized by large screen, high frame rate and high resolution. Compared to amorphous silicon TFTs that is currently used in active-matrix liquid crystal display (AMLCD) and active-matrix organic light-emitting-diode display (AMOLED), oxide semiconductors TFTs have numerous advantages including high mobility, high optical transparency and low processing temperature, making them suitable to be used in transparent and flexible display.In this thesis, our work focused on three different oxide semiconductors:zinc oxide (ZnO), indium gallium zinc oxide (InGaZnO, IGZO) and indium aluminum zinc oxide (InAlZnO, IAZO). The major contents of this thesis are summarized as follows:1. ZnO TFTs with ITO or Cr as Drain/Source electrodes were fabricated. The effect of contact with different electrodes on the performance was investigated. ITO electrodes can form a better contact with the ZnO channel, thus TFTs with ITO electrodes showed superior performance than TFTs with Cr electrodes.2. Annealing treatment was used to control the carrier concentration in ZnO films and achieve the operation of ZnO TFTs. For as-deposited ZnO TFTs, low-temperature annealing under N2is preferable. Because the low-temperature can not only increase the mobility and reduce the defects density, but also keep a low off-current. The disadvantage of low-temperature annealing is its disability to remove the defects that only high-temperature annealing can do.3. We employed a TFT device to investigate the persistent photoconductivity (PPC) in ZnO films. The phenomenon that the rate of photocurrent decay can be controlled by the gate bias was observed. We proposed a plausible model to explain the PPC controlled by gate-bias. Under the gate-bias, the electric field will drive electrons away from the electropositive oxygen vacancies (VO2+) and holes, leading to the reduction of recombination rate between electrons and VO2+or holes.4. The IGZO TFTs were fabricated and their stability was investigated. Two different instability mechanisms were observed in IGZO TFTs. The first one is shallow trap mechanism that happens to as-deposited IGZO TFTs when the transfer characteristic curves were continuously swept. These shallow traps are formed when an oxygen vacancy site is adjacent to a large open space in the film. And high-temperature annealing with O2can annihilate these shallow traps. After high-temperature annealing, IGZO TFTs only suffer from gate-bias stress instability, characterized by parallel positive threshold voltage shift with no change of sub-threshold swing. The interface trapping mechanism is responsible for gate-bias stress instability.5. Amorphous IAZO films were deposited at room temperature and annealed at different temperatures. The effects of annealing temperature on the structural, morphology, optical and electrical performance of the films were investigated. The amorphous structure is stable even at high-temperature annealing. The annealing treatment would lead to redistribute of atom in the film while show no significant influence on the morphology. The annealing treatment also increased the mobility and reduced the resistivity of the film. Red shift was observed for the absorption edge for the film and the optical band gap narrowed as the annealing temperature increase.6. IAZO TFTs were fabricated, showing comparable electrical performance to IGZO TFTs. IAZO TFTs annealed at low temperature had the positive shift of transfer characteristic curve under continuous sweeping, as same as as-deposited IGZO TFTs. XPS results showed the number of oxygen vacancies reduced as the annealing temperature increased, indicating the shallow traps are related to oxygen vacancies. This instability disappear after high-temperature annealing, only left the gate-bias stress instability cause by interface trapping site.7. The impact of ambient on stability of IAZO TFTs was investigated and how the ingredients of ambient affect the TFT performances was confirmed. It was observed that dry O2and N2had little influence on the TFT performances, while the relative humidity of O2had significant impact on the TFT performances. Firstly, it induced the back surface conductivity of the channel layer. Secondly, it led to positive shift of threshold voltage. Further study on the gate-bias stressing showed the larger of humidity of O2, the large shift of threshold voltage and the less stable for the TFT devices. A water-assisted oxygen absorption model was proposed to account for the impact of wet O2on stability of oxide TFT.