Preparation Of Ferroelectric Thin Films And New Ferroelectric Memory

The ferroelectric random access memory (FRAM) as one type of non-volatile memories makes use of bipolar orientations of ferroelectric polarization under an external electrical field to store the "0" and "1" binary information. FRAM is considered as a candidate for the next generation of emerging memories with non-volatile information storage, high speed operation, high density integration, high airspace anti-radiation, and so on. The studies in this Ph.D thesis are focused on two main aspects:One is the investigation of the interfacial passive layers between the ferroelectric thin film and top and bottom electrodes characterterized by our novel pulse characterization technique, as well as Al-doped ZnO thin-film transistors using the above lead zirconate titanate (PZT) back gate insulator; and another aspect is the study of the properties of bismuth ferrite (BFO) ferroelectric diode memory with switchable resistivity modulated by bipolar polarization orientations.In the first part, the PZT thin films were prepared by a Sol-Gel method. The metal/ferroelectric/metal (MFM) capacitor structure as well as Al-doped ZnO thin film transistors with PZT as a back gate insulator was fabricated based on semiconductor standard processes. Furthermore, the properties of individual PZT ferroelectric thin films, the interfacial passive layers, thin-film transistor with ferroelectric gate insulator (FeFET) were investigated, as summarized in follows:(1) The PZT thin films prepared by Sol-Gel method were deposited on Pt/Ti/SiO2/Si substrates. The effect of different Pb excess contents on PZT thin film properties was investigated and the Pb excess in the film composition was optimized. The research proves the preferred (111) orientation of PZT thin film due to the epitaxial lattice matching of the bottom Pt (111) orientation with the film. The30mol%Pb excess is enough to compensate for the Pb loss resulted from the thermal annealing during the film preparation process. The film with the30mol%Pb excess shows a symmetric rectangular shape of P-V hysteresis loops with good fatigue endurance, typical butterfly shaped C-V loops, and low leakage currents, and so on.(2) We gave the relationship between the coercive voltage of PZT thin film and frequency, in consistence with Ishibashi’s power law. It shows that the frequency coefficient (βf) is closely relative to the interfacial passive layers. The interfacial layers thickened with PbO volatilization during the film processing can increase the coercive voltage, which results in the variation of coercive voltage with the frequency.(3) We investigated the fatigue model of PZT ferroelectric thin films with the assumption of concurrent pinning and depinning of domain walls during fatigue cycling. Our results confirm that the high temperature accelerates the mobile charge injection into ferroelectric thin films. The injected charges within deeply trap levels of a ferroelectric, especially within the interfacial layers, screen the boundary charges of the positive domains and suppress seed nucleation in their embryonic states. It’s found that the Schottky emission is more reasonable to describe the inhomogeneous leakage current of the interfacial passive layers during the charge injection, where the pinning coefficients (F1,F2) to describe the injected charge density within each fatigue cycle are extracted from our of fatigue model. This provides a new method in investigating the dielectric constant and barrier height of the interfacial passive layers of ferroelectric thin films.(4) We estimated the coercive voltage from domain switching currents under various short voltage pulses directly. This measurement also proves the presence of the interfacial passive layers. With the aid of the domain switching currents under different voltage pulses, we estimated the interfacial current-voltage (Ii,-Vi) dependence besides the intrinsic coercive voltage of the domains. It is also confirmed that the Schottky emission mechanism can describe the interfical current-voltage relationship from which we calculated the relative coefficient βs,. Moreover, our results show the relation between domain switching current and intrinsic coercive voltage (Isw-Ec) consistent with the Merz law. From this dependence, we obtained the intrinsic domain wall activation field (δ=1.4kV/cm).(5) Al-doped zinc oxide thin films (AZO) with a single (002) orientation were successfully deposited on SiO2/Si substrates at room temperature by a magnetron sputtering method. The investigation proves that the optimized conductivity of the AZO thin film can be obtained at400℃in N2atmosphere. Al-doped zinc oxide thin film transistors were fabricated with PZT as a back gate insulator (FeFET) for the application of novolatile random access memory. This memory exhibits a soruce-drain current modulation with an ON/OFF current ratio close to 1000.In the second part, SrRuO3(SRO) conductive thin films and bismuth ferrite (BFO) ferroelectric thin films were both prepared by pulse laser deposition method (PLD). The metal/ferroelectric/metal (MFM) capacitor structure demonstrates the one-way current conduction of a ferroelectric diode with the p-n junction switchable with ferroelectric domain reorientation. This resistive switching behaviour and its conduction mechanism were analyized. The main results are as follows:(1) SRO conductive thin films were deposited on STO single crystal substrates with different orientations by PLD, whose resistivity is around359.7μΩ·cm and367.5μΩ·cmfor (100) and (111) orientations, respectively. The AFM photographs show the flat film surfaces with the profile roughness below1nm and the RMS roughness within0.25nm.(2) BFO ferroelectric thin films were deposited on STO and SRO/STO substrates with different orientations by PLD. It seems that an optimized substrate temperature is necessary to promote the formation of the flat surface of BFO ferroelectric thin films at each O2pressure. In other words, the general tendency for preparing a flat thin film is either at a higher O2pressure with a higher substrate temperature or at a lower O2pressure with a lower substrate temperature. Moreover, the XRD analysis indicates the preferred orientation of BFO ferroelectric thin film to match with the STO crystal lattice regardless of interpolatory thin SRO layer.(3) We investigated the film thickness effect on the BFO domain patterns. The domain patterns for a thinner (100) BFO ferroelectric thin film (below100nm) are strongly affected by the effect of substrate lattice mismatch stresses with the domains clamped in a specific orientation; while it doesn’t occur in a thicker (above100nm) film due to the stress release above a critical film thickness.(4) The Au/BFO/SRO/STO capacitor structure to demonstrate a switchable ferroelectric diode current was fabricated on (111) STO crystal substrates. The large ferroelectric diode current around550mA/cm2, which can be modulated by bipolar domain orientations, has been clearly observed. The retention time for the On and Off diode currents is over104s, and the On/Off current ratio is around5:1.(5) With the survey of present current leakage models, it is found that the space-charge limited current (SCLC) dominates the conduction mechanism. For the intrinsic physical understanding, the rectification of diode currents near domain coercive fields is attributed to the gradient distribution of trapped charges between top and bottom electrode/ferroelectric interfaces, where the distributed charges within these layers can be reversed upon polarization reversal.