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Cross-sectional TEM micrograph of multilayer RRAM devices with (a) low magnification and (b) high magnification. The thickness of the oxide layer was measured by averaging the whole layer based on a full-width-at-half maximum method. It is clear that distinctive multiplayer oxides have been achieved in this work.
(Color online) (a) Forming process followed by five successive bipolar switching cycles (i.e., SET/RESET) for single layer HfOx device. The forming voltage is as large as 1.7 V, which is 0.6 V larger than the SET voltage (1.1 V). The ON/OFF ratio is 10. The inset is the high resolution cross-sectional TEM micrograph of the single layer HfOx RRAM device. The thickness of the HfOx layer is 9.70 nm. (b) Initial SET/RESET in the forming-step free device followed by five successive bipolar switching cycles. No forming step is needed in the multiple oxides layer RRAM device. The voltage difference between the first “forming” sweep and the SET voltage is less than 0.1 V. The ON/OFF ratio has been improved from 10 to 100 as compared with the single layer HfOx device shown in (a). (c) SET voltage distribution comparison of devices between single layer and multilayer RRAM in 500 dc cycles.
(Color online) EELS results of the fresh multilayer oxide RRAM device showing N K-edge at 401 eV, Ti L2,3-edge at 453 eV, as well as O K-edge at 532 eV. The sharp rise in the Ti L2,3 edge can been seen in every oxide layer, indicting Ti has diffused into both HfOx layers. The point to point distance is 3.0 Å in the EELS line scan. The N K-edge can be clearly observed in the TiN top electrode layer, while no signal was found in the multiple oxides layer. Oxygen atoms counts clearly show up in every oxide layer, while negligible count in the TiN top electrode layer.
(Color online) ELNES of the TiN top electrode, first HfOx layer, and second HfOx layer shown in Fig. 3, respectively. The different Ti states are labeled. The peak shifts towards a lower energy which indicates that a change of Ti ionic state. For the TiN TE, in all the samples, we measured the Ti L2-edge onsets at 453 eV. In both HfOx layers, a shift of the L2 line to higher energy by 1.0 eV is clearly visible with also an increase in L3 line intensity. Based on these results, Ti in the HfO2 has a higher valence state than Ti in the TiN gate.
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