TEM image of a phase change memory device. The active contact area is in diameter. The process flow used for fabrication of the device is given in the inset.
(a) Normalized resistance against set current for programming pulse width of 300 ns. (b) Contact resistances between amorphous nitrogen-doped and TiW bottom electrode. The addition of nitrogen into the film increases the contact resistance between the phase change material and the metal contact substantially, which ameliorates the current required to set the PCRAM device. (c) Change in resistance as a function of temperature for films with nitrogen concentration from 0 to 8.4 at. %. Crystallization leads to an abrupt change in the resistance in with low nitrogen content. Incorporation of a high nitrogen concentration leads to a less abrupt change in resistance during crystallization.
(a) Normalized resistance against reset current at pulse width of 10 ns. XRD plots for films with atomic nitrogen concentration of (b) 0%, (c) 3.5%, and (d) 6.2%. The films were separately annealed at temperatures of 150, 250, and in ambient for 10 min. Diffraction peaks appear at higher crystallization temperatures for the nitrogen-doped samples.
Sheet resistance measurements as a function of annealing temperature for various nitrogen doping concentration. There appears to be two regimes of crystallization depending on the nitrogen content in the phase change film. Direct transformation from the amorphous to the hcp phase occurs for atomic nitrogen concentration greater than 3.5 %.
Dependence of set current on pulse width and nitrogen doping concentration in . As compared to the device with undoped , the set current is generally reduced upon doping with nitrogen.
Plot of reset current against pulse width as a function of nitrogen doping concentration in . Reduction in the reset current is observed only for nitrogen content of 3.5 at. %, while higher current values ensue when nitrogen doping concentrations increases further.
(a) Cell resistances in set and reset states on the number of set/reset cycles, with an initial resistance ratio of at least two orders. The square and circle symbols represent the set and reset resistances, while the 0 and 3.5 at. % nitrogen-doped devices are denoted by the solid and open symbols, respectively. A one order increase in set/reset cycles is achieved for the 3.5 at. % nitrogen-doped device over the undoped one. (b) Endurance characteristics as a function of nitrogen doping content. PCRAM devices with 3.5 at. % nitrogen content exhibits the best endurance behavior, while the endurance capability subsequently degrades on further addition of nitrogen.
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