Fabrication And Characterization Of ZnO/BNT/LNO/Si Ferroelectric Thin Film Transistors

Ferroelectric-gate thin-film transistor (FGT) has become a promisingferroelectric memory structure due to its non-volatility, simple device structure andfabrication process, direct ferroelectric-semiconductor contact with excellent interfaceproperties and being easy to realize large-area integration, all epitaxial structure andflexible devices. The various types of FGTs composed of different stacked structureshave been investigated. Nevertheless, the gate insulators of most of these FGTs areepitaxial ferroelectric thin films grown on some special substrates (e.g., SrTiO3). Theexpensive substrates and the incompatibility with the well-established Si technologywill hinder the progress of the FGTs applications. Thus, developing Si-based thin-filmtransistors with high-quality ferroelectric thin films as gate insulators is significant forpromoting the FGTs applications. In this thesis, we use LaNiO3(LNO) as the bufferlayer, Bi3.15Nd0.85Ti3O12(BNT) as ferroelectric-gate insulator and ZnO as channellayer to fabricate Si-based FGTs. The main research contents and obtained results aresummarized as follows.1. Preparation and characterization of LNO thin filmThe conductive oxide LNO thin films were fabricated by pulsed laser deposition(PLD) on Si (100) substrates. The effects of substrate temperature and oxygenpressure on the structure and performance of the LNO thin films were investigated.The results indicate that both substrate temperature and oxygen partial pressure havegreat influence on the LNO thin film. With increasing substrate temperature, thecrystallinity of LNO film improves. The crystallinity is best when the substratetemperature reaches600oC. As the substrate temperature further increases, a smallamount of impurity phase generates in the film. With the increase of oxygen partialpressure, the (110) orientation of LNO films increases while the resistivity decreases.The LNO film fabricated under600oC and200mTorr has a smoother surface and asmaller resistivity of1.26m·cm, which is benificial to serves as bottom electrodeand buffer layer.2. Preparation and characterization of BNT/LNO/Si heterojunctionThe BNT ferroelectric thin films were fabricated on the Pt/Ti/SiO2/Si,LNO/exact-Si and LNO/miscut-Si substrates respectively. The effects of bottomelectrode and substrate on the structure and performance of the BNT thin films wereinvestigated. The BNT thin films on Pt electrode mainly show a c-axis crystalline orientation, while BNT thin films on LNO electrode mainly show (117) and (200)orientations and display larger remnant polarization and dielectric constant. Using themiscut Si (100) substrate, a-axis preferential orientation BNT films with smoothersurface and a significant enhancement of the polarization and dielectric constant wereobtained.3. Preparation and characterization of ZnO/BNT/LNO/miscut-Si FGTBased on above results, we chose BNT thin film as ferroelectric-gate layer andLNO thin film as buffer/bottom electrode layer to fabricate ZnO/BNT/LNO FGTs onmiscut Si (100) substrates. The electrical properties of the fabricated FGTs wereinvestigated. The prepared FGT device shows n-type enhancement mode transistorbehavior with good electrical properties. The threshold voltage, memory window,on/off ratio and subthreshold slope are measured to be1.1V,3.0V,1.8×106and240mV/decade, respectively. The results suggest that ZnO/BNT/LNO/miscut-Sistructures are well suited for thin-film transistors for future nonvolatile memoryapplications.