Cross-point memory devices are a promising structure to increase memory density as we reach the physical limitations of current memory technologies. However, the stacked nature of the components used in such structures does not permit high-temperature processing. Thus the inability to use silicon p-n diodes has led to research on high performance devices that could be produced at low temperatures without loss of performance.

Here, Bo Soo Kang and colleagues1 report the fabrication of oxide-based p-n heterojunctions, which were grown at room temperature, and showed high performance.

The researchers used copper oxide—a p-type conductor—combined with indium zinc oxide (IZO)—an n-type conductor—for their devices. All the films were amorphous, thus enabling low-temperature deposition without concerns about the lattice mismatch between materials in the stacked heterostructures.

The initial, relatively large 10μm × 10μm devices, had a current density under forward bias of 3.5×104A cm-2—the highest value reported for oxide thin-film diodes.

Fig.1: Scanning electron microscopy image of the cross point cell.

In order to test the scalability of the device properties, electron-beam lithography was used to pattern the materials, and platinum electrodes were added to provide an active area of just 50nm × 50nm (Fig. 1). The current density of these devices was still over 1.5×104 A cm-2, a remarkably high density in-spite of the large reduction in size.

The researchers also tested the response of the diodes to pulse signals, and found that the turn-on and turn-off transient behaviours were stabilised within a few tens of nanoseconds—encouraging results for fabricating high-density, high-speed cross-point memory devices.

Kang says that the main achievements of the work are “primarily the low-temperature deposition and secondly the high performance.” He adds that if a silicon diode is made at a low temperature of 300°C, then the current density is at least an order of magnitude less than the oxide device the authors prepared. To obtain higher current densities with silicon, much higher temperatures are required.

The researchers are optimistic about the future of the project. “When we finally solve the issues such as controlling defects in oxides, we believe this will lead to another world of electronics.”