Integration and characterization of atomic layer deposited TiO 2 thin films for resistive switching applications

摘要

In the last decades the commercialization of computer and multimedia applications for consumer electronics increased the desire for faster, denser, and non-volatile memory. At present, FLASH memory is the standard non-volatile memory based on complementary metal oxide semiconductor (CMOS) technology. But actual research is already dealing with concepts for the next era ’aside FLASH’ or ’beyond FLASH’. Resistive random access memory (ReRAM) is one of the new candidates which has the potential candidate to replace FLASH in future. The concept of ReRAM is based on the change of the resistance state of a passive device by an electrical stimulus. Typical devices are built from chalcogenide thin films sandwiched between metallic conducting electrodes. In general, transition metal oxide based ReRAM needs an electroforming process to enable resistance switching. This is an obstacle if ready-to-use devices are required. The focus of this thesis adresses the question if it is possible to design ’forming-free’ ReRAM devices by a control of TMO thin film defect structure. As a material which is intensively investigated for ReRAM applications, titanium oxide is used. TiO2 in ready-to-use ReRAM should be oxygen deficient, and should contain a certain amount of well-conducting Magnéli-phases within a crystalline TiO2 matrix. In addition, semiconductor industry requires a deposition method which enables precise, defect-free, shadow-free and 3D coverage by the functional layer for a stacked ReRAM architecture. These requirements are fulfilled by the atomic layer deposition (ALD) technique. For the integration of TiO2 into ReRAM an ALD process was investigated and further optimized to achieve the desired properties. The process parameters were elaborated to grow amorphous and crystalline TiO2 thin films in order to study the effect of the films’ morphology and structure on the resistive switching behavior. Detailed studies on the crystallization of TiO2 while the ALD thin film growth reveal that the crystallization kinetics allow to explain the control of the phase composition of TiO2 by the growth temperature, the thickness, and the process time. The deeper understanding of the crystallization of TiO2 into different phases additionally revealed how to suppress the surface roughening for thicker TiO2 layers which is an important aspect for extremely thin films. The structural investigations on crystalline grown TiO2 reveal, that corundum Ti2O3 could be deposited within a matrix of rutile and anatase TiO2. From these studies, the presence of Magnéli-type phases could deduced by thermodynamical stability considerations. ALD TiO2 thin films of amorphous and crystalline state were integrated into nano cross-point devices to systematically study their resistive switching properties. The comparison of the transport, the electroforming, and the resistive switching measurements clearly revealed that as-deposited crystalline TiO2 films which contain Magnéli-phases are advantageous of functioning TiO2 based ReRAM. Crystalline TiO2 films exhibited soft forming characteristic at low voltages which were in the range of the SET voltages of the subsequent switching hysteresis. In contraction to that, amorphous TiO2 showed abrupt forming at higher voltages resulting in a strongly linear ON state after electroforming as compared to crystalline TiO2. The gained knowledge on the correlation of the electrical transport properties of the pristine device state, the resistive switching properties, and the material properties of the crystalline TiO2 was utilized to develop a new promising concept for the design of forming-free TiO2 ReRAM. This concept involves the elimination of parasitic current paths which are linked to the wellconducting crystalline phase. By the change of the device process flow from a lift-off to a top down approach for the structuring of the top electrodes, the parasitic current paths aside the device stack are eliminated. Parasitic paths within the device stack are actively circumvented by their destruction by an initial reset sweep instead of an electroforming step. The newly developed ReRAM concept exhibits low switching voltages, a non-linear characteristic, and a memory window greater than 10.