Resistive switching in polymer-metal oxide diodes for electronic memory applications

Resistive switching in metal-insulator-metal (MIM) structures is an intriguing phenomenon in which the electrical resistance can be altered reversibly, and permanent in case of a non-volatile memory, by applying a voltage. It is being investigated intensely, partly because of its potential application in data storage. The mechanism of this switching is largely unknown. This thesis describes the design, construction and electrical characterization of diodes with an active layer consisting of organic-inorganic hybrid materials that show resistive switching. In the realization of the memory diodes, two different approaches were followed. Chapter 2 describes the first approach in which the active layer is made of a block copolymer consisting of a semiconducting part (sexithiophene) and a part facilitating migration of added inorganic ions (ethylene oxide). Inorganic salt is added as a dopant. From an analysis of the electro-optical behavior of these diodes, it is concluded that the resistive switching and the associated memory effect is due to electric field induced migration of the inorganic ions in the active layer. The switching allows for storage of information and rewritable memory operation is demonstrated for the diodes although the retention time of the information is still very short (~ 10 s). In this system, the energetic barrier for injection of a mobile carrier into the semiconducting block can be lowered by the presence of an inorganic ion of opposite sign in a neighboring ion transporting block. Hence, the resistivity of the diode can be modulated by changing the concentration of ions at the electrode. The second approach involves metal oxides in combination with polymers as active layer. In Chapter 3 to 6, zinc oxide nanoparticles have been used as one of the active components. After a forming reaction, resistive switching could be established within 50 ms and retention of the memory was increased to hours. The forming process itself is interpreted in terms of desorption of molecular oxygen from the ZnO nanoparticle surface, induced by injection of holes via the PEDOT:PSS contact, leading to a higher ntype conductivity via interparticle ZnO contacts. The forming can also be induced by ultraviolet light and the process is studied with electron paramagnetic resonance, photoinduced absorption spectroscopy, and field-effect measurements. By varying the content of ZnO and the type of polymer in the active layer the memory effects can by influenced and a data storage with lifetime >14 hours has been achieved. Chapter 5 shows that the electronic properties can be altered by capping the ZnO nanoparticles with various ligands. Capping with propylamine gives hysteresis and resistive switching after application of –5 V and +5 V bias pulses, while retaining the rectifying behavior of the device. This is a crucial requirement when these devices are used in passive matrix arrays. In Chapter 6 it is shown that with increasing surface coverage of the ZnO nanoparticles with a thiol ligand, the electrical resistance, associated with electron transport via percolating networks of ZnO particles in the matrix, increases, due to deterioration of the ZnO interparticle contacts. Just before reaching the percolation limit obtained by adding ~0.05 mol thiol per mol Zn to the ZnO nanoparticles, where the electrical current is supported by just a few percolation paths, the electrical characteristics change. For unmodified ZnO particles, voltage pulses of opposite polarity (bipolar) bring the diode to a low and a high resistive state. With the ZnO particles modified with octane thiol, the diodes can be switched between low and high resistance with unipolar voltage pulses of 1 µs. Using write and erase pulses of 10 ms, an ON/OFF ratio of 103 can be achieved with good cycle endurance. This change is observed with Al or Pd top electrodes, and is interpreted in terms of the conduction taking place via essentially a single, narrow channel of ZnO particles that can be blocked by trapping of a single charged species. In Chapter 7, it is concluded that the switching function of the polymer–aluminum oxide diodes mainly reflects a property of the aluminum oxide. The yield in switching of solid state-memories can be increased to about unity by deliberately adding a thin sputtered Al2O3 layer to organic diodes. Before memory operation, the devices have to be formed at an electric field of 109 V/m, corresponding to soft-breakdown of Al2O3. After forming, the structures show pronounced negative differential resistance and the local maximum in the current scales with the thickness of the oxide layer. After the forming, switching in hundreds of nanoseconds can be achieved. Repeated pulse sequence measurements of Chapter 8, show the occurrence of a ‘dead time’, that is the time after programming in which a next switch is inhibited, of about 3 ms. The dead time, which is known for bulk oxides, explains the huge variation in the reported switching times. Resistive switching is demonstrated in diodes based on spin coated layers of various metal oxide nanoparticles and a semiconducting polymer, sandwiched between two electrodes in Chapter 9. Inclusion of the oxide nanoparticles results in non-volatile electronic memory characteristics that are similar to those observed for the corresponding ‘bulk’ oxide. A major difference is that the nanoparticule layers do not require a forming step. In the absence of oxygen, resistive switching is observed in many metal oxides. Among the various oxides, there are differences in switching behavior (unipolar/bipolar) that may be related to the different electronic structure of the materials. In the case of ZnO particles modification of the layer morphology, leads to change in the memory properties (Chapter 7). This indicates that besides electronic structure, also the geometry and number of the filaments formed is important in determining the type of resistive switching of the memory. In Chapter 10, negative differential resistance (NDR) in polymer-Al2O3 diodes is investigated using time and frequency domain electrical measurements and thermal imaging. After a forming step, the diodes show a time dependent NDR. In the bias voltage range where NDR is observed, the capacitance at low frequency is significantly larger than for the unformed diode. Conduction in the formed diodes is filamentary in nature. Assuming different metastable ionization states of defects in the oxide and space charge limited conduction, some fundamental aspects of the switching can be accounted for. In conclusion, this thesis shows resistive switching for diodes containing various materials. The combination of metal oxides and polymers emerges as appealing because it exhibits a range of intriguing resistive switching effects that are potentially of interest for future nonvolatile memory element applications. A first step is made to rationalize the origin, mechanism, and magnitude of the resistive switching in terms of a quantitative model.