Fabrication and characterization of ZnO-based oxide thin film transistors by solution process

Abstract

Nowadays, all commercial AMOLED displays utilize LTPS (low temperature poly silicon) TFTs. LTPS TFTs have high mobility so they seem to be good for AMOLED. However, poor uniformity and high cost of LTPS limit to applications and it is considered that 40 inch is the upper limit. Other materials such as organics, oxides for TFTs have been researched severely. Properties of oxide TFTs are especially excellent for future displays: high mobility, transparency, processibility to low temperature, solution process capability, and flexibility. Various methods such as RF sputtering, ALD, MOCVD, PLD and solution method are employed in depositing oxide thin film. Among them, solution method is the easiest, most economic, and most favorable for printable technique and future display. In this work, ZnO based TFTs were fabricated by solution process. ZnO and InGaZnO thin film were prepared by spin coating on p-Si wafer with 200 nm $SiO_2$. Thermal annealing was followed after spin coating. Si and silicon dioxide played role to gate and gate dielectric, respectively. Effect of annealing temperature on ZnO TFT was investigated. Grain size of ZnO was larger as annealing temperature got higher. Bigger grain size was favorable for high mobility because scattering at grain boundary was reduced. Threshold voltage was shifted negatively as annealing temperature went higher due to oxygen vacancy formation. InGaZnO TFTs were influenced by atomic ratio among In, Ga, and Zn. Between In:Ga:Zn=3:1:2 and 5:1:2 samples, 5:1:2 sample had better device performance than 3:1:2 sample. Indium is considered to act as mobility enhancer and to hinder the crystallization of thin film. InGaZnO TFTs were also sensitive to annealing temperature. Higher temperature made active layer more conductive and device had higher mobility. Threshold voltage shift also happened like ZnO TFTs. InGaZnO thin films had amorphous structure; however, very small crystalline particle was formed inside thin film at ...