Umesh Chand

Overview of emerging nonvolatile memory technologies

Nonvolatile memory technologies in Si-based electronics date back to the 1990s. Ferroelectric field-effect transistor (FeFET) was one of the most promising devices replacing the conventional Flash memory facing physical scaling limitations at those times. A variant of charge storage memory referred to as Flash memory is widely used in consumer electronic products such as cell phones and music players while NAND Flash-based solid-state disks (SSDs) are increasingly displacing hard disk drives as the primary storage device in laptops, desktops, and even data centers. The integration limit of Flash memories is approaching, and many new types of memory to replace conventional Flash memories have been proposed. Emerging memory technologies promise new memories to store more data at less cost than the expensive-to-build silicon chips used by popular consumer gadgets including digital cameras, cell phones and portable music players. They are being investigated and lead to the future as potential alternatives to existing memories in future computing systems. Emerging nonvolatile memory technologies such as magnetic random-access memory (MRAM), spin-transfer torque random-access memory (STT-RAM), ferroelectric random-access memory (FeRAM), phase-change memory (PCM), and resistive random-access memory (RRAM) combine the speed of static random-access memory (SRAM), the density of dynamic random-access memory (DRAM), and the nonvolatility of Flash memory and so become very attractive as another possibility for future memory hierarchies. Many other new classes of emerging memory technologies such as transparent and plastic, three-dimensional (3-D), and quantum dot memory technologies have also gained tremendous popularity in recent years. Subsequently, not an exaggeration to say that computer memory could soon earn the ultimate commercial validation for commercial scale-up and production the cheap plastic knockoff. Therefore, this review is devoted to the rapidly developing new class of memory technologies and scaling of scientific procedures based on an investigation of recent progress in advanced Flash memory devices.

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).

Suppression of endurance degradation by utilizing oxygen plasma treatment in HfO2 resistive switching memory

Based on the phenomenon of endurance degradation problem caused by no sufficient oxygen ions for resistive switching, we use the oxygen plasma treatment in HfO2 layer to increase the extra available oxygen ions in resistive random access memory devices. To avoid the Ti top electrode directly absorbing the additional oxygen ions from HfO2 layer with oxygen plasma treatment, a thin HfO2 film is inserted to separate them. Therefore, the endurance degradation can be suppressed in the present structure. High speed (30 ns) and large endurance cycles (up to 1010 cycles) are achieved in this device structure for next generation nonvolatile memory application.

Mechanism of Nonlinear Switching in HfO 2 -Based Crossbar RRAM With Inserting Large Bandgap Tunneling Barrier Layer

In this paper, the nonlinear switching mechanism of the Ti/HfO₂/Al₂O₃/TiN crossbar structure resistive random access memory device with good reliability is investigated. The nonlinearity of the device can be revealed by inserting a large bandgap of an Al₂O₃ thin layer between the TiN bottom electrode and the HfO₂ switching film. The nonlinear switching mechanism caused by Flower-Nordheim tunneling involves the tunneling barrier of the Al₂O₃ layer. Besides, the nonlinear behavior is also sensitive to the thickness of the inserting Al₂O₃ layer. A high nonlinear factor of 37, large endurance more than 10⁴, and good retention properties are achieved in the Ti/HfO₂/Al₂O₃ (1-nm)/TiN structure.

Investigation of thermal stability and reliability of HfO2 based resistive random access memory devices with cross-bar structure

The effect of the annealing treatment of a HfO2 resistive switching layer and the memory performance of a HfO2-based resistive random access memory (cross-bar structure) device were investigated. Oxygen is released from HfO2 resistive switching layers during vacuum annealing, leading to unstable resistive switching properties. This oxygen release problem can be suppressed by inserting an Al2O3 thin film, which has a lower Gibbs free energy, between the HfO2 layer and top electrode to form a Ti/Al2O3/HfO2/TiN structure. This device structure exhibited good reliability after high temperature vacuum annealing and post metal annealing (PMA) treatments. Moreover, the endurance and retention properties of the device were also improved after the PMA treatment.

Mechanism of High Temperature Retention Property (up to 200 °C) in ZrO 2 -Based Memory Device With Inserting a ZnO Thin Layer

In this letter, the switching mechanism of the ZrO2-based RRAM devices with the uniform and reliable reliability properties is investigated. The stability of memory window can be improved by inserting a ZnO thin film with the nonstoichiometric property between Ti top electrode and ZrO2 layer. Compared with ZrO2 film, the ZnO one can easily perform the oxidation-reduction reaction for stable endurance properties. In addition, through Gibbs free energy comparison of TiOx, ZrO2, and ZnO, we can demonstrate that the Gibbs free energy contributes to the retention performance. The device with thin ZnO layer has stable retention performance at high temperature (200 °C) due to its higher Gibbs free energy value than TiOx.

Metal induced crystallized poly-Si-based conductive bridge resistive switching memory device with one transistor and one resistor architecture

In this letter, the metal induced crystallization (MIC) process is used in the Si-based conductive bridging resistive random access memory (CBRAM) application. The amorphous Si (a-Si) is transformed to crystallized poly-silicon (poly-Si) at a low temperature by using Ni metal for inducing poly-Si to provide the resistive switching. The MIC process can produce a highly preferred orientation poly-Si film, which can create the exact paths or grain boundaries through the top and down electrodes in the present CBRAM device. The grain boundary in MIC poly-Si layer can confine the conductive filament of metal bridging growth in it, which can improve the switching fluctuation behavior in the nonvolatile memory application. Compared with the a-Si based device, a significant improvement in terms of resistive switching parameters such as stability and resistance distribution is demonstrated in the MIC poly-Si CBRAM device. Moreover, the well-behaved memory performance, such as high ON/OFF resistance ratio (4 order),...

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.

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.

Enhancement of resistive switching properties in nitride based CBRAM device by inserting an Al2O3 thin layer

In this letter, we propose a method to enhance resistive switching properties in SiCN-based conductive-bridge resistive switching memory (CBRAM) devices by inserting a thin Al2O3 layer between the SiCN resistive switching layer and the TiN bottom electrode. Compared with the Cu/Ta/SiCN/TiN single-layer device, the Cu/Ta/SiCN/Al2O3/TiN double layer device exhibits uniform resistive switching, long stable endurance cycles (>1.6 × 104), and stable retention (104 s) at 125 °C. These substantial improvements in the resistive switching properties are attributed to the location of the formation and rupture of conductive filaments that can be precisely controlled in the device after introducing the Al2O3 layer. Moreover, a multilevel resistive switching characteristic is observed in the Cu/Ta/SiCN/Al2O3/TiN double layer CBRAM device. The distinct six-level resistance states are obtained in double layer devices by varying the compliance current. The highly stable retention characteristics (>104) of the Cu/Ta/SiCN/...