Integration and Characterization of Atomic Layer Deposited TiO2 Thin Films for Resistive Switching Applications
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Beschreibung
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 Magn´eli-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 eect 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 dierent 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 Magn´eli-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 Magn´eli-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-o 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.
Autor*in
Marcel Reiners