Internal resistor of multi-functional tunnel barrier for selectivity and switching uniformity in resistive random access memory
© Lee et al.; licensee Springer. 2014
Received: 20 May 2014
Accepted: 15 July 2014
Published: 25 July 2014
In this research, we analyzed the multi-functional role of a tunnel barrier that can be integrated in devices. This tunnel barrier, acting as an internal resistor, changes its resistance with applied bias. Therefore, the current flow in the devices can be controlled by a tunneling mechanism that modifies the tunnel barrier thickness for non-linearity and switching uniformity of devices. When a device is in a low-resistance state, the tunnel barrier controls the current behavior of the device because most of the bias is applied to the tunnel barrier owing to its higher resistance. Furthermore, the tunnel barrier induces uniform filament formation during set operation with the tunnel barrier controlling the current flow.
Various new types of memories, such as phase change memory, spin-torque-transfer magnetic memory, and resistive random access memory (ReRAM), have been considered to replace conventional memory owing to their improved scaling limit and low power operation [1, 2]. ReRAM is the most promising candidate memory for next-generation non-volatile memory owing to the simple structure of the two-terminal type device and the fact that its cross-point array (4 F2) structure can be significantly scaled down. However, ReRAM exhibits large resistive-switching fluctuation and suffers from leakage current in cross-point array operation.
To mitigate the resistive switching fluctuation in ReRAM, various analyses of switching behaviors and structural solutions have been suggested [3–8]. The resistive switching uniformity is highly affected by oxide states and filament formation properties. Although various ReRAM structures have been investigated and the switching variability has been improved, ReRAMs still retain non-uniform resistive switching parameters of resistance state and voltage when the devices operate with low currents (approximately 50 μA) of devices. In addition, the currents flowing through unselected cells during the read operations are a severe problem in cross-point array ReRAMs. When a high-resistance state (HRS) cell is read, it is biased with VRead, while the unselected neighboring low-resistance state (LRS) cells are biased with ½VRead. Although LRS cells are biased with a lower voltage than the HRS cell, most current flows through the unselected LRS cells because of their very low resistance values. To prevent this sneak path current, various selection devices are introduced. Selection devices have very a high resistance at low voltage levels (VLow) and low resistance at high voltage levels (VHigh). Therefore, the use of a selection device and ReRAM integration can reduce the leakage current in cross-point array operation. However, they are structurally and compositionally complex for one-selector one-ReRAM (1S1R) integration [9, 10]. Therefore, selector-less ReRAMs with non-linear ILRS behavior and without complex compositional and structural integration have been investigated [11, 12]. However, the origin of the selector-less ReRAM has not been investigated, and its switching reliability has not been considered for cross-point array operation. Most researches have focused only on the selectivity of the selector-less ReRAM.
In this research, the multi-functional role of the TiOx tunnel barrier which can be integrated with ReRAM was analyzed. We significantly improved the selectivity and switching uniformity by designing the device with a simple triple-layer structure of a tunnel-barrier-layer-inserted ReRAM. The tunnel barrier can act as an internal resistor whose resistance changes with the applied bias. Direct tunneling (DT) of the tunnel barrier shows high resistance at VLow, whereas Fowler-Nordheim tunneling (FNT) shows low resistance at VHigh. DT of the tunnel barrier reduces the sneak-path current of the ReRAM and controls the filament formation in the HfO2 switching layer for selectivity and uniformity. Thus, the multi-functional tunnel barrier plays an important role in the selectivity and switching uniformity of ReRAMs.
We fabricated Ti/HfO2/multi-layer TiOy-TiOx/Pt devices in a 250-nm via-hole structure. For the isolation layer, a 100-nm-thick SiO2 sidewall layer was deposited on a Pt bottom electrode (BE)/Ti/SiO2/Si substrate by plasma-enhanced chemical vapor deposition. Subsequently, a 250-nm via-hole was formed by a KrF lithography process, followed by reactive-ion etching. First, a 6-nm TiOx tunnel barrier was deposited in an Ar-and-O2 mixed plasma (Ar/O2 = 30:1 sccm) by radio frequency (RF) sputtering (working pressure 5 mTorr, RF power 100 W). To form the multi-layer TiOy/TiOx (y > x), a tunnel barrier was annealed in O2 ambient by rapid thermal annealing at 300°C. We varied the thermal oxidation time to evaluate the role of the tunnel barrier in the ReRAM (0 to 10 min). Then, a switching layer of 4-nm-thick HfO2 was deposited using an atomic layer deposition system using TEMAH as a precursor and H2O as an oxidizer at 250°C. The Ti oxygen reservoir and a top electrode (TE) of 50 μm were deposited using direct current (DC) sputtering and a shadow mask.
In the device structure shown in Figure 1a, Ti/HfO2 acts as a memory with filament formation and dissolution with set and reset operations. The integrated multi-layer TiOy/TiOx acts as an internal resistor for the non-linear ILRS and the filament formation control. Accordingly, the memory and multi-layer tunnel barrier can be considered as serially connected resistors. Thus, if the operating current of the ReRAM is higher than that of the internal resistor (RReRAM < Rinternal resistor), the current of the ReRAM is mainly determined by the internal resistor. In serially connected resistors, most of the bias is applied to the higher resistance, and the same current flows through the lower resistance. Therefore, we analyzed the behaviors of the selector-less ReRAM, which is integrated with the internal resistor of the TiOx tunnel barrier.
The role of a multi-functional tunnel barrier was investigated. The main concern areas of selectivity and switching uniformity were significantly improved. This is attributed to the tunnel barrier acting as an internal resistor that controls electron transfer owing to its variable resistance. In addition, the effect of the tunnel barrier on selectivity and switching uniformity was stronger in a multi-layer TiOy/TiOx than in a single-layer TiOx owing to the greater suppression of the VLow current flow.
SL is currently a 2nd-year Ph.D. candidate at the Materials Science and Engineering of POSTECH, and his research field is ReRAM process and integration for high density memory.
This work was supported by the R&D MOTIE/KEIT (10039191) and Brain Korea 21 PLUS project for Center for Creative Industrial Materials.
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