Current-voltage of a device with metal/insulator/semiconductor structure: Ni/HfO2/SiO2/Si-diode. Note that the reverse current is very low. The inset shows the schematic diagram for RRAM intergration with a Si-subsatrte as selector. The device shows good switching performance after 500 DC cycles (red lines).
Initial step of nanofilament formation. (a) The device has been stressed to soft breakdown regime at 4 V with a compliance current of 10 μA. (b)&(c) Real time TEM micrographs show no obvious physical change when soft breakdown occurred. The times indicated in the micrographs correspond to 0 and 22 s, respectively. (d) High resolution TEM micrograph of the virgin Ni/HfO2/SiO2/Si structure, with Ni thickness of 100 nm, HfO2 thickness 3.1 nm, and SiO2 0.7 nm. (e) High resolution TEM micrograph of the soft breakdown sample. Si atoms from substrate protrude into the oxide layer forming hillocks, which is known as DBIE (as indicated by white arrows). This was not observed in (b) and (c) due to a lower resolution setting in the real-time TEM analysis.
Nanofilament rupture in RESET. (a) The device has been subjected to a constant voltage stress higher than the SET voltage, which was 2.6 V with higher compliance current of 100 μA. Unipolar RESET of the device where the ON/OFF ratio is more than three orders of magnitude. (b) and (c) Real time TEM micrographs showing the morphology of the metal filament. The times indicated in the micrographs correspond to 0 and 5 s, respectively, upon electrical stressing. No changes can be seen in the nano-structural defect. (d) High resolution TEM, (e) HAADF STEM micrographs, (f) EELS mapping, and (g) EDX result of the electrically “switched-off” device shown in (c). The filament has been disconnected between the Ni-based top electrode and the bottom electrode. It was “burned-off” in RESET due to Joule heating. There are still some Ni residuals embedded in the insulator layer. But the fragments are now again an insulator rather than a metal conductor.
Schematics of the evolution of the metal filament formation and rupture, and represent positive and negative voltage biases, respectively. (a) Pristine state. (b) DBIE forming process upon initial breakdown. (c) SET process, where the metal nanofilament of Ni-rich punches through the oxide and migrates along the 〈111〉 direction in the silicon substrate. (d) and (e) RESET process, where the Ni-rich filament ruptures under Joule heating. The voltage polarity signs “+” and “−” are in lower size to indicate lower voltages.
Bandgap calculations for HfOxNiy structures. Different stoichiometries are indicated, where (a) x = 2 and y = 0 represent perfect HfO2, (b) x +y = 2 represents SET state, and (c) x + y < 2 represents RESET state. The vertical dashed lines show the top of the valence band Ev and the bottom of the conduction band Ec . The zero of the energy scale corresponds to 0 eV. Sub-bands are labeled. The bandgap value of perfect HfO2 is 4.6 eV calculated by first-principles simulation using GGA + U method. Upon SET, where Ni diffuses into the HfO2, and occupies the oxygen vacancy sites, the bandgap collapses to zero. For the RESET, the bandgap opens a little bit when Ni atoms are dislodged from the oxygen vacancy sites.
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