The next-generation nonvolatile memories (NVMs) has recently received much attention because the conventional memories based on charge are considered to approach their scaling limits. Among several types of NVMs, resistive switching random access memory (ReRAM) is the most promising candidate for NVMs applications due to its excellent characteristics such as high scalability, fast switching speed, easy fabrication, high density, and low power consumption.
Fig. 1. Evaluaion of ReRAM, ITRS 2011
l Simple cell structure (Metal-Insulator-Metal) ¡æ 3D stack friendly
l Small cell size of < 4F2 possibility ( if 3D stack structure is used)
l Fast switching (On ¡ê OFF) speed
l Easy fabrication process and CMOS compatibility
l Low power consumption
2. Properties of ReRAM
ReRAM has two types of switching modes, bipolar and unipolar switching mode, as shown in Fig. 2. In bipolar switching, the voltage polarity between switching from the low resistance state (LRS) to the high resistance state (HRS) [RESET operation] is reversed compared to the switching from HRS to LRS [SET operation]. On the contrary, in unipolar switching, the RESET and SET operations are achieved by applying the voltage in same polarity, but different magnitude.
The resistive switching mechanism is still unclear, but there are the several kinds of proposed switching mechanisms.
- Formation/rupture of filaments (oxygen vacancies or metal)
- Formation/rupture of cation metal filament
- Formation of interfacial oxide between electrode and insulator
- SCLC (filled trap/detrap)
- Modification of a Schottky-like barrier width or height
As mentioned earlier, diverse conduction mechanisms of ReRAM has been proposed. However, the its exact mechanism is not yet fully understood. To improve the performance of ReRAM, an understanding of the switching mechanism is necessary.
A number of materials consisting of the metal-insulator-metal structure, such as perovskite, binary metal oxide, and metal sulfide, have been reported as the resistive switching layer of ReRAM, as shown in Fig. 3. To accelerate the mass production of ReRAM devices, the discovery of new materials with good resistive characteristics should be accomplished. In addition, continuous material and process engineering should be performed for practical ReRAM application.
Fig. 2. Properties of ReRAM
3. Our research
Our laboratory is focusing on filamentary-type ReRAM because it has advantages in terms of scalability and switching speed. Currently, in our laboratory, studies for filamentary-type bipolar ReRAM are actively underway using the new material and process approaches to improve resistive characteristics of our ReRAM.
Fig 3. I-V characteristics of our ReRAM device
<Our research interests>
- ReRAM for higher density, scalability
- Improvement of resistive switching characteristics
- Selection device for 3D cross-point ReRAM array
- ReRAM with non-linear I-V characteristics
- Understanding of the resistive switching and current conduction mechanism