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.