Jun 1, 2010
Schottky diodes: the 'door' to future memory devices
Innovations in modern information technology are critically dependent on the development of denser, faster and less energy-consuming non-volatile memories (NVMs). Resistance-switching random access memory, or RRAM, is a hot prospect in these terms compared with its charge-based counterparts, such as dynamic random access memory (DRAM) and flash memory, which suffer from performance degradation as the scaling limit is approached. RRAM not only has advantages in fabrication and performance, but also stands out from all of its competitors because it can realize a three-dimensional memory cell, which is the ultimate challenge for denser memories. The cross-bar type array memory device, where the resistance switching material is interposed between inter-crossing word- and bit-lines, is the most promising structure in this regard. However, the extreme parallel geometry of the device requires the implementation of a certain selection device to prevent read disturbance problems. Implementing a Schottky type diode with each resistance switching element can be a solution to this problem. The diode in question will require a very high current rectification ratio and a very low forward resistance, in addition to a low fabrication temperature (<~400 °C), which is necessary to preserve the resistance switching material.
Prof. Cheol Seong Hwang's research group at Seoul National University, Korea, has used a state-of-the-art atomic layer deposition technique to fabricate a Schottky-type oxide diode composed of a Pt/TiO2/Ti stacked structure. The TiO2 layer acts as an n-type wide band gap semiconductor, so that the Ti/TiO2 interface constitutes an electron injecting junction whereas the TiO2/Pt interface works as a blocking junction. The n-type TiO2 layer was deposited by atomic layer deposition and Pt and Ti layers were deposited by sputtering. All the processes were performed at temperatures <200 °C.
To the researchers' surprise, the diode showed a current rectification ratio as high as 109 at ~1 V even when the TiO2 film thickness was as thin as 19 nm. This is more than enough to be used in the cross-bar array mentioned above. Turning their attention to the forward resistance, the scientists found that the value was not low enough and began investigating the device using conductive atomic force microscopy. As you can see in the top image, the group found that the forward current flows quite locally at specific areas of the junction. Further improvement is necessary concerning this matter, but it is likely that smaller device sizes will have a favourable influence because the ratio of local current flowing areas will increase.
Memory or diode?
It is interesting to note that a structure similar to the diode, namely Pt/TiO2/Pt, offers repeatable resistance switching with a resistance contrast of >100. Therefore, simply changing one of the electrodes transforms the system from an active memory element to a passive selection device or vice versa. This could benefit memory fabrication at the mass-production level.
The researchers presented their results in the journal Nanotechnology.
About the author
The study was conducted by research teams of the Dielectric Thin Film Research Laboratory at the Department of Materials Science and Engineering, Seoul National University. The team is funded by the National Research Program for Nano Semiconductor Apparatus Development and World Class University programme through the National Research Foundation of Korea, funded by the Ministry of Education, Science and Technology (R31-2008-000-10075-0). Woo Young Park was a PhD student and fabricated the sample and analysed the performance of the device. He was supported by Gun Hwan Kim, Jun Yeong Seok, Kyung Min Kim, Seul Ji Song and Min Hwan Lee. Min Hwan Lee performed the conductive atomic force microscopy. Prof. Cheol Seong Hwang is head of the laboratory. He and his 30 group members are devoted to developing new devices, novel materials and innovative fabrication processes for semiconductor memory devices.