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Use of the Nudged Elastic Band Method to Calculate Diffusion Paths in Metals and Semiconductors

Roland Stumpf, Motorola, Predictive Engineering Laboratory;
Clarence Tracy, Motorola, Materials Research and Strategic Technologies.

A serious problem preventing the use of new high dielectric oxide materials (e.g. Barium-Strontium-Titanate) for capacitors as part of future DRAM is the oxidation of or the O diffusion through the Pt electrode.[1] Polycrystalline Pt does not oxidize but it allows for O interdiffusion while the dielectric is deposited and annealed.

Using first principles electronic structure calculations[2] we first determine the O diffusion mechanism in polycrystalline Pt. We find that O diffuses as interstitial O along Pt grain boundaries (GBs) with a barrier of 0.68 eV in agreement with experiment. Conventional mapping of the potential energy surface and the nudged elastic band (NEB) method is used to determine the diffusion path.[3abstract][3postscript]We screen nine elements for their potential to retard O diffusion if added to the Pt in small amounts. Be is the most promising candidate. Be segregates to Pt GBs at interstitial and substitutional sites. As GB interstitial Be diffuses at a rate comparable to that of O and it repels O. This leads to a stuffing of the GB. As substitutionally absorbed Be, Be has a high diffusion barrier (about 2 eV) and it forms strong bonds to O. Preliminary experimental results confirm our theoretical predictions.

 In a second application of the NEB we investigate dopant diffusion in bulk Si. Our results show that a large number of paths has to be investigated. The NEB allows this routinely for the first time.

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