Ijaz Talib

Forming-free bipolar resistive switching in nonstoichiometric ceria films

The mechanism of forming-free bipolar resistive switching in a Zr/CeOx/Pt device was investigated. High-resolution transmission electron microscopy and energy-dispersive spectroscopy analysis indicated the formation of a ZrOy layer at the Zr/CeOx interface. X-ray diffraction studies of CeOx films revealed that they consist of nano-polycrystals embedded in a disordered lattice. The observed resistive switching was suggested to be linked with the formation and rupture of conductive filaments constituted by oxygen vacancies in the CeOx film and in the nonstoichiometric ZrOy interfacial layer. X-ray photoelectron spectroscopy study confirmed the presence of oxygen vacancies in both of the said regions. In the low-resistance ON state, the electrical conduction was found to be of ohmic nature, while the high-resistance OFF state was governed by trap-controlled space charge-limited mechanism. The stable resistive switching behavior and long retention times with an acceptable resistance ratio enable the device for its application in future nonvolatile resistive random access memory (RRAM).

Improved Endurance and Resistive Switching Stability in Ceria Thin Films Due to Charge Transfer Ability of Al Dopant

An improvement in resistive switching (RS) characteristics of CeO2-based devices has been reported by charge transfer through Al metal as a dopant. Moreover, density functional theory (DFT) calculations have been performed to investigate the role of Al-layer sandwiched between CeO2 layers by the Vienna ab initio simulation package (VASP). Total density of states (TDOS) and partial electron density of states (PDOS) have been calculated and analyzed with respect to resistive switching. It is established that the oxygen vacancy based conductive filaments are formed and ruptured in the upper region of CeO2 layer, because of the fact that maximum transport of charge takes place in this region by Al and Ti (top electrode), while the lower region revealed less capability to generate conductive filaments because minimum charge transfer takes place in this region by Al and/or Pt (bottom electrode). The effect of Al and Al2O3 on both the electronic charge transfer from valence to conduction bands and the formation stability of oxygen vacancies in conductive filament have been discussed in detail. Experimental results demonstrated that the Ti/CeO2:Al/Pt sandwich structure exhibits significantly better switching characteristics including lower forming voltage, improved and stable SET/RESET voltages, enhanced endurance of more than 10(4) repetitive switching cycles and large memory window (ROFF/RON > 10(2)) as compared to undoped Ti/CeOx/Pt device. This improvement in memory switching behavior has been attributed to a significant decrease in the formation energy of oxygen vacancies and to the enhanced oxygen vacancies generation within the CeO2 layers owing to charge transferring and oxygen gettering ability of Al-dopant.

Performance stability and functional reliability in bipolar resistive switching of bilayer ceria based resistive random access memory devices

Memory devices based on Ti/CeO2-x:CeO2/ITO stacks with bilayer structure fabricated by rf-magnetron sputtering demonstrate promising bipolar resistive switching behavior with relatively low-voltage operation and small distribution of switching parameters. These devices show much reliable repeatability and good endurance (>104 switching cycles) without any significant degradation in their performance. The cycle-to-cycle and device-to-device distribution of resistance switching parameters, such as resistances in the low and high resistance states, set and reset voltages have been investigated and discussed. Resistive switching behavior in our devices has been proposed to originate from the electric field induced drift of defects (specifically oxygen vacancies) preferably along grain boundaries in the bilayer structure of active dielectric layer.

Resistive switching characteristics of Pt/CeOx/TiN memory device

The resistive switching characteristics of Pt/CeOx/TiN memory devices are investigated for potential applications in nonvolatile resistive random access memory (RRAM). The X-ray diffraction characteristics of the sputtered CeOx layer indicate the formation of nanocrystalline single-phase CeO2 with a cubic fluorite structure. The existence of oxygen vacancies in the Pt/CeOx/TiN memory device was determined by X-ray photoelectron spectroscopic studies, while the presence of an interfacial layer between CeOx and the TiN bottom electrode was investigated by X-ray diffraction and high resolution transmission electron microscopy. The TiON layer formed at the TiN/CeOx interface seems to play a key role in the resistive switching mechanism of the device. The present CeOx-based device shows excellent bipolar resistive switching characteristics, including a low operation current (100 ?A), high ON/OFF resistance ratio (?105), and good retention/stress characteristics at both room temperature and 85 ?C.

Endurance and Cycle-to-cycle Uniformity Improvement in Tri-Layered CeO 2 /Ti/CeO 2 Resistive Switching Devices by Changing Top Electrode Material

Resistance switching characteristics of CeO2/Ti/CeO2 tri-layered films sandwiched between Pt bottom electrode and two different top electrodes (Ti and TaN) with different work functions have been investigated. RRAM memory cells composed of TaN/CeO2/Ti/CeO2/Pt reveal better resistive switching performance instead of Ti/CeO2/Ti/CeO2/Pt memory stacks. As compared to the Ti/CeO2 interface, much better ability of TaN/CeO2 interface to store and exchange plays a key role in the RS performance improvement, including lower forming/SET voltages, large memory window (~102) and no significant data degradation during endurance test of >104 switching cycles. The formation of TaON thinner interfacial layer between TaN TE and CeO2 film is found to be accountable for improved resistance switching behavior. Partial charge density of states is analyzed using density functional theory. It is found that the conductive filaments formed in CeO2 based devices is assisted by interstitial Ti dopant. Better stability and reproducibility in cycle-to-cycle (C2C) resistance distribution and Vset/Vreset uniformity were achieved due to the modulation of current conduction mechanism from Ohmic in low field region to Schottky emission in high field region.

Bipolar tri-state resistive switching characteristics in Ti/CeOx/Pt memory device

Highly repeatable multilevel bipolar resistive switching in Ti/CeOx/Pt nonvolatile memory device has been demonstrated. X-ray diffraction studies of CeO2 films reveal the formation of weak polycrystalline structure. The observed good memory performance, including stable cycling endurance and long data retention times (> 104 s) with an acceptable resistance ratio (~102), enables the device for its applications in future non-volatile resistive random access memories (RRAMs). Based on the unique distribution characteristics of oxygen vacancies in CeOx films, the possible mechanism of multilevel resistive switching in CeOx RRAM devices has been discussed. The conduction mechanism in low resistance state is found to be Ohmic due to conductive filamentary paths, while that in the high resistance state was identified as Ohmic for low applied voltages and a space-charge-limited conduction dominated by Schottky emission at high applied voltages.

Effect of Bilayer CeO 2−x /ZnO and ZnO/CeO 2−x Heterostructures and Electroforming Polarity on Switching Properties of Non-volatile Memory

Memory devices with bilayer CeO2−x/ZnO and ZnO/CeO2−x heterostructures sandwiched between Ti top and Pt bottom electrodes were fabricated by RF-magnetron sputtering at room temperature. N-type semiconductor materials were used in both device heterostructures, but interestingly, change in heterostructure and electroforming polarity caused significant variations in resistive switching (RS) properties. Results have revealed that the electroforming polarity has great influence on both CeO2−x/ZnO and ZnO/CeO2−x heterostructure performance such as electroforming voltage, good switching cycle-to-cycle endurance (~ 102), and ON/OFF ratio. A device with CeO2−x/ZnO heterostructure reveals good RS performance due to the formation of Schottky barrier at top and bottom interfaces. Dominant conduction mechanism of high resistance state (HRS) was Schottky emission in high field region. Nature of the temperature dependence of low resistance state and HRS confirmed that RS is caused by the formation and rupture of conductive filaments composed of oxygen vacancies.