Investigation of the electroforming and resistive switching mechanisms in Fe-doped SrTiO 3 thin films (2009)

To overcome the physical limits of todays memory technologies new concepts are needed. The resistive random access memory (RRAM), which bases on a nonvolatile and repeatable change of the resistance by external electrical stimuli, seems to be one promising candidate. Within the scope of this work, the model system Strontiumtitanate (SrTiO3) has been investigated to get a deeper understanding of the underlying physical mechanism related to the resistance change. The electrical properties of SrTiO3 (STO) can be modulated from a band insulator to metallic conduction by a self-doping with oxygen vacancies which act as shallow donors. A local accumulation or depletion of oxygen vacancies at the vicinity of the surface will lead to a local redox process which is responsible for the resistance change. To study the influence of the interfaces on the switching properties of SrTiO3 thin films, epitaxial films of Fe-doped SrTiO3 were grown on different bottom electrodes (SrRuO3, LaNiO3 und Nb:STO) by a “Pulsed Laser Deposition” technique. An atomic force microscope equipped with a conductive tip (LC-AFM) allowed studying the conductivity of the deposited films on the nanometer scale. Resistive switching of lateral structures smaller than ~5nm could be realized which represents the potential of this material for a further downscaling of RRAM devices. The deposition of topelectrodes, made of Platinum or Titanium, allowed the electrical characterization of metal-insulator-metal (MIM) structures. An extensive investigation of pristine MIM-devices by impedance spectroscopy showed the big impact of the metal-insulator interface on the overall device resistance. Furthermore, a chemical polarization was studied by dynamical current sweeps and identified as a volatile resistance variation. Usually a forming procedure is needed to “enable” the resistive switching properties in MIM devices. The electroforming of these devices was extensively studied and could be assigned, for samples exhibiting a Schottky contact, to a local bypassing of the Schottky depletion layer. Additional, annealing experiments under defined atmospheric conditions led to the result that the electroforming process can be identified as a loss of oxygen from the device. The investigations allowed drawing a detailed model of the electroformation of the prepared devices. Furthermore, two different switching mechanisms (interface related, bulk related) were identified and characterized. By variation of the voltage amplitude applied, a repeatable switching between both mechanisms could be demonstrated. Both mechanisms have been separately and extensively characterized (e.g. by impedance spectroscopy), which allowed drawing models for both types of switching effects. The interface related switching mechanism was attributed to the oxygen related opening and closing of conducting channels at the upper or lower interface and follows therewith the conventional explanations. In case of the bulk related switching effect, the resistance change will be explained by the electrically induced internal redistribution of mobile donors leading to a p-n doping profile. The main features of this switching mechanism are its improved switching stability and large resistance ratios achieved. In case of the bulk related switching mechanism, switching times of 30ns, retention times longer than 9 hours and the storage of two bits per cell (multibit capability) could be realized. These results demonstrate the great potential of SrTiO3 as possible material for a future RRAM cell.