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An experimental investigation on the switching reliability of a phase change memory device with an oxidized TiN electrode

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Fluctuations (or drifts) in switching voltages such as programming set/reset voltages and threshold voltage pose serious obstacles to the reliable operation of electrical phase change memory devices. Using a phase change memory device having a GeSb2Te4 phase change material and TiN electrode, these fluctuations are demonstrated to result from device resistances varying with programming cycles. Fluctuating resistances appear to stem primarily from large contact resistances at the interface between the phase change material and the TiN electrode and from inhomogeneous phase distribution across the GeSb2Te4 layer due to unsuccessful heat confinement near the interface with TiN. Oxidation of a TiN electrode surface (via thermal annealing at 350 °C under an atmospheric gas mixture of 97.9 vol % N2 and 2.1 vol % O2) is very effective in the reduction of fluctuations in device resistances and switching voltages hence the resulting increase in the programming cycles by two orders of magnitude. From a high resolution transmission electron microscopy, the oxidized surface was shown to consist of a titanium oxide layer primarily with Ti2O3 crystallites which is presumed to yield enhanced stability of the device by the following two effects. Firstly, Ge, Sb, and Te atoms would have stronger bonds to oxygen atoms than to nitrogen atoms by about 0.5 eV, thereby producing more robust interface. Accordingly, the magnitude of contact resistance and its variation are red- uced significantly so as to have little influence on the device resistances and their fluctuations. Secondly, thermally and electrically more resistive nature of the oxide layer would tend to yield, by enhanced generation and confinement of Joule heat, more uniform temperature distribution across the phase change material layer, rendering possibly a more homogeneous single phase material hence steadier sheet resistances with programming cycles.

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Journal of Applied Physics  (Volume:100 ,  Issue: 5 )