Stability Of Metal-oxide Thin-film Transistors

Thin-Film Transistors (TFT), as a kind of field-effect transistors, are fabricated on thesubastrate with depositing variable functional film continuously, such as semiconductor active,dielectric, and metal electrodes. As a core component in liquid-crystal displays and organiclight-emitting diodes, the performance of TFT is of vial importance to the dislay quality. Thesupporting backplane technology for the TFT, recently, is diversified. In particular, it is a goodprospect to predict the metal oxide semiconductor could provide a solution to the intrinsicproblems including the inferior mobility of amorphous silicon TFTs, non-uniformity of lowtemperature poly-silicon, and bad processing-repeatability of micro-silicon. The rise oforganic TFT represents a potential direction, but the poor mobility and reliability stillbarricade its further development. However, like the other TFTs’ technologies before beingintegrated into commercial products, metal oxide TFTs also suffer from the materials’ intrinsiccapabilities and limiting factors, such as material fabrication and device stability. Nowadays,the investigation of metal oxide TFTs is all in the early stages, and the physical mechanismsare not yet clear. What is more, the restraint of the developing metal oxide TFTs intoutility-type is complicated and variable. From this viewpoint, it is urgent to deepen theunderstanding and argumentation of metal oxide TFTs when targeted as the mainstream ofbackplane technology. So in this doctoral dissertation, we focus on the developments ofhighly-stable metal oxide thin-films transistor and its backplane technology.Firstly, we shed light on the device performance and stability of metal oxide TFTs basedon anodic aluminum oxide gate dielectrics. Via the comparison of the negative biasillumination stress (NBIS) stability of metal oxide TFTs based on PECVD-deposited SiO2andanodic Al2O3gate dielectric, it was proved that the negative Vonshift for SiO2-based TFTduring NBIS was due to the hole trapping phenomenon, and the positive Vonshift for anodicAl2O3-based TFT was due to the electron injection from gate to dielectric. Thus, threemethods have been proposed to enhance the stability of TFT with Al2O3gate dielectric:(1)insert a SiO2layer between Al2O3and IZO, the NBIS stability of IZO-TFTs was greatlyimproved, accompanying with a high stability under PBIS;(2) fabricate dual-gate structuremetal oxide TFTs, barricading the illumination lighting in to the IZO film via layout design oflight shielding unit by top gate;(3) modify the band structure of anodic Al2O3dielectric, usingthe doping effect of anodic Al2O3to improve the NBIS stability, and the work is on.Highly-stable metal oxide thin-films transistor based on anodic Al2O3dielectric has beensuccessfully fabricated, making a hint in the potential applications in LCDs and OLEDs. Secondly, dual gate indium zinc oxide thin-film transistors have already provided a goodsolution to the NBIS stability, and from another point of view, dual structure TFTs have greatadvantages in threshold voltage modulation. It is shown that the threshold voltage of TFTscould been linearly modulated with respect to the applied top gate voltage due to themodification of vertical electric field distribution between the bottom and top gate. Owning toits linear controllability of threshold voltage, it is believed that the dual gate structure will betentatively introduced in the application of compensation pixel circuit, making an accessiblepath for developing highly-efficient AMOLEDs with TFT backplane, which has highimmunity to the threshold voltage variation.Thirdly, highly-stable IZO-TFTs using a two-step-annealing method were fabricated. Itwas shown that the pre-annealing step before ESL deposition in O2atmosphere can greatlyimprove the uniformity of IZO-TFTs. Furthermore, the electrical stability against PBS, NBIS,and TS was greatly improved for TFTs with pre-annealing step, owing to the fewer trap statesat the gate insulator/channel interface or in the semiconductor film. The calculated FRsconfirmed that the pre-annealing step can reduce the trap states. In addition, annealingtemperature and ambient in the pre-annealing step make a great impact on the performance ofmetal oxide TFTs. Therefore, the pre-annealing step is essential and indispensable in oxideTFT fabrication.Fourthly, we comprehensively study the effect of passivation technology on theperformance and stability of metal oxide TFTs. IZO-TFTs with spin-coating organic film aspassivation hardly show switchable TFT performance, but with SiO2deposited by PECVD aspassivation, the performance of IZO-TFTs has greatly improved. In this work, the influence ofthe deposition temperature of the SiO2passivation on the active layer was investigated. It isfound that the TFT passivated by210°C-SiO2has the lowest conductivity, lowest turn-onvoltage (|Von|), and highest mobility. However, the IZO-TFTs with SiO2deposited at lowertemperature are less stable under positive bias stress (PBS). By using X-ray PhotoelectronSpectroscopy (XPS) analysis, it is found that the deposition temperature of the SiO2passivation will influence on not only the hydrogen content, but also the amount of oxygenvacancies and loosely bound oxygen impurities in the bulk IZO film. The loosely boundoxygen impurities in IZO act as an acceptor-type trap owning to its lower migration barriersheight, resulting in positive Vonshift under PBS. Meanwhile, we have found that thedeposition of SiO2passivation is a multi-factors related and influenced process, such as thepower, gas pressure, and SiH4/N2O gas ratio will deeply affect the performance of IZO-TFTs.TFT backplane technology based on metal oxide semiconductor has shown us a flourishing vitality in the application of flat panel displays, as a result, the fundamentalinvestigation of metal oxide TFTs is urgent and significant. At the end of this doctoraldissertation, we try to emphasize on the novel materials exploitation, device reliability, lowcost, high resolution, and driving IC design, which leads to the further research with thedesitination of highly-performance metal oxide thin-film transistors.