Debanjan Jana

TaOx-based resistive switching memories: prospective and challenges.

Resistive switching memories (RRAMs) are attractive for replacement of conventional flash in the future. Although different switching materials have been reported; however, low-current operated devices (<100 μA) are necessary for productive RRAM applications. Therefore, TaOx is one of the prospective switching materials because of two stable phases of TaO2 and Ta2O5, which can also control the stable low- and high-resistance states. Long program/erase endurance and data retention at high temperature under low-current operation are also reported in published literature. So far, bilayered TaOx with inert electrodes (Pt and/or Ir) or single layer TaOx with semi-reactive electrodes (W and Ti/W or Ta/Pt) is proposed for real RRAM applications. It is found that the memory characteristics at current compliance (CC) of 80 μA is acceptable for real application; however, data are becoming worst at CC of 10 μA. Therefore, it is very challenging to reduce the operation current (few microampere) of the RRAM devices. This study investigates the switching mode, mechanism, and performance of low-current operated TaOx-based devices as compared to other RRAM devices. This topical review will not only help for application of TaOx-based nanoscale RRAM devices but also encourage researcher to overcome the challenges in the future production.

Evolution of complementary resistive switching characteristics using IrOx/GdOx/Al2O3/TiN structure

The complementary resistive switching (CRS) characteristics using an IrOx/GdOx/Al2O3/TiN single cell are observed whereas the bipolar resistive switching (BRS) characteristics are observed for the IrOx/GdOx/TiN structure. Transmission electron microscope and energy dispersive X-ray spectroscopy depth profile show crystalline GdOx film and the presence of higher amount of oxygen at both IrOx/GdOx interface and Al2O3 layer. Inserting thin Al2O3 layer, the BRS is changed to CRS. This CRS has hopping distance of 0.58 nm and Poole-Frenkel current conductions for the “0” and “1” states, respectively. A schematic model using oxygen vacancy filament formation/rupture at the TE/GdOx interface and Al2O3 layer has been illustrated. This CRS device has good endurance of 1000 cycles with a pulse width of 1 μs, which is very useful for future crossbar architecture.

Impact of electrically formed interfacial layer and improved memory characteristics of IrOx/high-κx/W structures containing AlOx, GdOx, HfOx, and TaOx switching materials

Improved switching characteristics were obtained from high-κ oxides AlOx, GdOx, HfOx, and TaOx in IrOx/high-κx/W structures because of a layer that formed at the IrOx/high-κx interface under external positive bias. The surface roughness and morphology of the bottom electrode in these devices were observed by atomic force microscopy. Device size was investigated using high-resolution transmission electron microscopy. More than 100 repeatable consecutive switching cycles were observed for positive-formatted memory devices compared with that of the negative-formatted devices (only five unstable cycles) because it contained an electrically formed interfacial layer that controlled ‘SET/RESET’ current overshoot. This phenomenon was independent of the switching material in the device. The electrically formed oxygen-rich interfacial layer at the IrOx/high-κx interface improved switching in both via-hole and cross-point structures. The switching mechanism was attributed to filamentary conduction and oxygen ion migration. Using the positive-formatted design approach, cross-point memory in an IrOx/AlOx/W structure was fabricated. This cross-point memory exhibited forming-free, uniform switching for >1,000 consecutive dc cycles with a small voltage/current operation of ±2 V/200 μA and high yield of >95% switchable with a large resistance ratio of >100. These properties make this cross-point memory particularly promising for high-density applications. Furthermore, this memory device also showed multilevel capability with a switching current as low as 10 μA and a RESET current of 137 μA, good pulse read endurance of each level (>105 cycles), and data retention of >104 s at a low current compliance of 50 μA at 85°C. Our improvement of the switching characteristics of this resistive memory device will aid in the design of memory stacks for practical applications.

Formation-Polarity-Dependent Improved Resistive Switching Memory Performance Using IrOx/GdOx/WOx/W Structure

The formation-polarity-dependent improved resistive switching memory performance using a new IrOx/GdOx/WOx/W structure has been investigated. The memory device has been observed by both high-resolution transmission electron microscopy and energy dispersive X-ray spectroscopy. The thicknesses of the GdOx and WOx layers are observed to be approximately 15 and 5.5 nm, respectively. All layers are also analyzed by X-ray photoelectron spectroscopy. The resistive switching mechanism is filament formation/rupture in the high-κ GdOx layer, which is controlled by the oxygen ion migration in bilayer GdOx/WOx films under negative and positive formation polarities. Excellent uniformity of SET/RESET voltages, low/high resistance states, and switching cycles have been observed under positive formation polarity owing to the charge trapping/detrapping in the high-κ GdOx switching layer. The memory device shows a long endurance of >104 times, and extrapolated 10-year data retention at 85 °C. This device shows great potential for future nonvolatile memory (NVM) applications.

Impact of device size and thickness of Al2O3 film on the Cu pillar and resistive switching characteristics for 3D cross-point memory application

Impact of the device size and thickness of Al2O3 film on the Cu pillars and resistive switching memory characteristics of the Al/Cu/Al2O3/TiN structures have been investigated for the first time. The memory device size and thickness of Al2O3 of 18 nm are observed by transmission electron microscope image. The 20-nm-thick Al2O3 films have been used for the Cu pillar formation (i.e., stronger Cu filaments) in the Al/Cu/Al2O3/TiN structures, which can be used for three-dimensional (3D) cross-point architecture as reported previously Nanoscale Res. Lett.9:366, 2014. Fifty randomly picked devices with sizes ranging from 8 × 8 to 0.4 × 0.4 μm2 have been measured. The 8-μm devices show 100% yield of Cu pillars, whereas only 74% successful is observed for the 0.4-μm devices, because smaller size devices have higher Joule heating effect and larger size devices show long read endurance of 105 cycles at a high read voltage of -1.5 V. On the other hand, the resistive switching memory characteristics of the 0.4-μm devices with a 2-nm-thick Al2O3 film show superior as compared to those of both the larger device sizes and thicker (10 nm) Al2O3 film, owing to higher Cu diffusion rate for the larger size and thicker Al2O3 film. In consequence, higher device-to-device uniformity of 88% and lower average RESET current of approximately 328 μA are observed for the 0.4-μm devices with a 2-nm-thick Al2O3 film. Data retention capability of our memory device of >48 h makes it a promising one for future nanoscale nonvolatile application. This conductive bridging resistive random access memory (CBRAM) device is forming free at a current compliance (CC) of 30 μA (even at a lowest CC of 0.1 μA) and operation voltage of ±3 V at a high resistance ratio of >104.

Impact of AlO x interfacial layer and switching mechanism in W/AlO x /TaO x /TiN RRAMs

Effects of AlO_x interfacial layer in the W/AlO_x/TaO_x/TiN structures have been investigated for the first time. This RRAM device shows long endurance of 10⁶ cycles and good retention at 85°C for a low current compliance of 100 μA. A physics based simulation is studied to understand the set and reset mechanism of RRAM. The nature of ions migration, potential profile and temperature of filament during switching is explained using numerical simulation done by MATLAB.

Low current cross-point memory using gadolinium-oxide switching material

Low current cross-point memory using gadolinium-oxide switching material in an IrOx/GdOx/W structure has been investigated for the first time. Memory device shows low current bipolar resistive switching phenomena and self-compliance phenomena as well, repeatable switching cycles, good uniformity, long program/erase endurance of >60k every cycles, and good data retention of >104 s at a low CC of 50 μA.

Rough surface improved formation-free low power resistive switching memory using IrO x /GdO x /W structure

Promising resistive switching memory characteristics in a IrO_x/GdO_x/W structure have been investigated. The surface roughness of bottom electrode plays a major role for forming-free and low current resistive switching, due to electric field enhancement. Memory device shows repeatable switching cycles with a small compliance current of 20 μA (60μW), long program/erase endurance of >10⁴ cycles, and excellent data retention of >10⁴ s at 85°C.