Kyung Jean Yoon

Highly Improved Uniformity in the Resistive Switching Parameters of TiO2 Thin Films by Inserting Ru Nanodots

Limiting the location where electron injection occurs at the cathode interface to a narrower region is the key factor for achieving a highly improved RS performance, which can be achieved by including Ru Nanodots. The development of a memory cell structure truly at the nanoscale with such a limiting factor for the electric-field distribution can solve the non-uniformity issue of future ReRAM.

Pt/Ta2O5/HfO2−x/Ti Resistive Switching Memory Competing with Multilevel NAND Flash

Pt/Ta2 O5 /HfO2- x /Ti resistive switching memory with a new circuit design is presented as a feasible candidate to succeed multilevel-cell (MLC) NAND flash memory. This device has the following characteristics: 3 bit MLC, electroforming-free, self-rectifying, much higher cell resistance than interconnection wire resistance, low voltage operation, low power consumption, long-term reliability, and only an electronic switching mechanism, without an ionic-motion-related mechanism.

Memristive tri-stable resistive switching at ruptured conducting filaments of a Pt/TiO2/Pt cell

A tri-stable memristive switching was demonstrated on a Pt/TiO₂/Pt device and its underlying mechanism was suggested through a series of electrical measurements. Tri-stable switching could be initiated from a device in unipolar reset status. The unipolar reset status was obtained by performing an electroforming step on a pristine cell which was then followed by unipolar reset switching. It was postulated that tri-stable switching occurred at the location where the conductive filament (initially formed by the electroforming step) was ruptured by a subsequent unipolar reset process. The mechanism of the tri-stable memristive switching presented in this article was attributed to the migration of oxygen ions through the ruptured filament region and the resulting modulation of the Schottky-like interfaces. The assertion was further supported by a comparison study performed on a Pt/TiO₂/TiO(2-x)/Pt cell.

Improved endurance of resistive switching TiO2 thin film by hourglass shaped Magnéli filaments

A modified biasing scheme was adopted to improve the electrical endurance characteristics of conducting filamentary resistive switching (RS) in a Pt/TiO2/Pt RS cell. The modified bias scheme included the application of bias voltages with alternating polarity, even though RS proceeds in non-polar mode, which results in the stable distribution of each resistance states as well as improved endurance. This was attributed to the minimized consumption of oxygen ions in the TiO2 film, which can be induced by the formation of hourglass-shaped conducting filament (HSCF). The presence of a HSCF was confirmed by high-resolution transmission electron microscopy.

Schottky diode with excellent performance for large integration density of crossbar resistive memory

A Schottky diode (SD) with Au/Pt/TiO2/Ti/Pt stacked structure were fabricated for its application to crossbar type resistive switching (RS) memory. The SDs showed a highly promising rectification ratio (∼2.4 × 106 @ ±2 V) between forward and reverse state currents and a high forward current density (∼3 × 105 A/cm2 @ 2 V), which is useful for highly integrated crossbar RS memory. The SD has local forward current conduction paths, which provides extremely scaled devices with an advantage. The minimization of interconnection line resistance is also important to provide sufficient current to achieve stable operation of RS memory.

Ionic bipolar resistive switching modes determined by the preceding unipolar resistive switching reset behavior in Pt/TiO2/Pt.

Various types of bipolar resistive switching (BRS) at the filament ruptured region by the unipolar resistive switching (URS) reset in the structure Pt/TiO2/Pt were categorized in terms of operation polarity and switching parameters. The differences in BRS behavior, even under identical current-voltage switching, are closely related to the previously performed URS reset parameter, especially the power consumed during the reset process. Various modes of BRS from the URS reset status in the structure Pt/TiO2/Pt are reported, and interpreted in terms of a distinct oxygen vacancy configuration in the ruptured region of a Magnéli filament.

Two-step reset in the resistance switching of the Al/TiOx/Cu structure.

Two-step reset behaviors in the resistance switching properties of the top Al/TiOx/bottom Cu structure were studied. During the electroforming and set steps, two types of conducting filaments composed of Cu and oxygen vacancies (Cu-CF and V(O)-CF) were simultaneously (or sequentially) formed when Al was negatively biased. In the subsequent reset step with the opposite bias polarity, the Cu-CFs ruptured first at ~0.5 V, and formed an intermediate state. The trap-filled V(O)-CFs were transformed into a trap-empty state, resulting in a high-resistance state at ~1 V. Matrix phase in the electrochemical metallization cell can play an active role in resistance switching.

Resistance switching behavior of atomic layer deposited SrTiO3 film through possible formation of Sr2Ti6O13 or Sr1Ti11O20 phases.

Identification of microstructural evolution of nanoscale conducting phase, such as conducting filament (CF), in many resistance switching (RS) devices is a crucial factor to unambiguously understand the electrical behaviours of the RS-based electronic devices. Among the diverse RS material systems, oxide-based redox system comprises the major category of these intriguing electronic devices, where the local, along both lateral and vertical directions of thin films, changes in oxygen chemistry has been suggested to be the main RS mechanism. However, there are systems which involve distinctive crystallographic phases as CF; the Magnéli phase in TiO2 is one of the very well-known examples. The current research reports the possible presence of distinctive local conducting phase in atomic layer deposited SrTiO3 RS thin film. The conducting phase was identified through extensive transmission electron microscopy studies, which indicated that oxygen-deficient Sr2Ti6O13 or Sr1Ti11O20 phase was presumably present mainly along the grain boundaries of SrTiO3 after the unipolar set switching in Pt/TiN/SrTiO3/Pt structure. A detailed electrical characterization revealed that the samples showed typical bipolar and complementary RS after the memory cell was unipolar reset.

Interface engineering for improving reliability of resistance switching in Cu/HfO2/TiO2/Pt structure

Reliability and uniformity in resistance switching behaviours in top electrode Cu-sputtered TiO2-bottom electrode Pt memory structure were greatly improved by inserting an interface layer of 5 nm-thick HfO2 between Cu and 50 nm-thick TiO2. The thin HfO2 layer, with much smaller cluster size than TiO2, limited the Cu migration appropriately and induced more uniform Cu conducting filament distribution. The repeated rejuvenation and rupture of Cu filament was limited within the HfO2 layer, thereby improving the switching reliability and uniformity. This also greatly decreased operation power compared to a memory cell without the thin HfO2 layer.

Optimization of chemical structure of Schottky-type selection diode for crossbar resistive memory.

The electrical performances of Pt/TiO(2)/Ti/Pt stacked Schottky-type diode (SD) was systematically examined, and this performance is dependent on the chemical structures of the each layer and their interfaces. The Ti layers containing a tolerable amount of oxygen showed metallic electrical conduction characteristics, which was confirmed by sheet resistance measurement with elevating the temperature, transmission line measurement (TLM), and Auger electron spectroscopy (AES) analysis. However, the chemical structure of SD stack and resulting electrical properties were crucially affected by the dissolved oxygen concentration in the Ti layers. The lower oxidation potential of the Ti layer with initially higher oxygen concentration suppressed the oxygen deficiency of the overlying TiO(2) layer induced by consumption of the oxygen from TiO(2) layer. This structure results in the lower reverse current of SDs without significant degradation of forward-state current. Conductive atomic force microscopy (CAFM) analysis showed the current conduction through the local conduction paths in the presented SDs, which guarantees a sufficient forward-current density as a selection device for highly integrated crossbar array resistive memory.