Electromigration behaviors of Ge2Sb2Te5 chalcogenide thin films under DC bias

Abstract

Electromigration (EM) behaviors of pristine Ge2Sb2Te5 (GST), nitrogen-doped GST (N-GST) and cerium-doped GST (Ce-GST) thin-film strips under DC bias are presented. The mean-time-to-failure (MTTF) analysis based on the Black equation found that the EM failure times at room temperature are 1.2 x 10(4), 40 and 9.2 x 10(2) years and the activation energies (E-a) of EM process are 1.07, 0.57 and 0.68 eV for GST, N-GST and Ce-GST, respectively. Moreover, the calibration of the current density exponent, n, of Black's equation found n values are close to 2 for all samples, implying the dominance of grain boundary diffusion during the mass transport of EM process. For doped GSTs, the inferior EM failure lifespans and smaller E-a values were ascribed to the grain refinement effect which increases the number of grain boundaries in such samples. It consequently promoted the short-circuit diffusion and accelerated the EM failure in doped GSTs. The Blech-type tests on GSTs found that the threshold product, i.e., the product of current density and sample length ((j.L)(th)), is 200 A/cm for GST, 50 A/cm for N-GST and 66.67 A/cm for Ce-GST. Moreover, the product of diffusivity and effective charge number (i.e., DZ*) for GST, N-GST and Ce-GST was 2.0 x 10(-7), 4.5 x 10(-6) and 3.8 x 10(-6) cm(2)/sec, respectively. Analytical results illustrated that the electrostatic force effect dominates the EM failure in samples with short strip lengths while the electron-wind force effect dominates the EM failure in samples with long strip lengths. Doping might alleviate the mass segregation in GST; however, its effect was moderate. (C) 2013 Elsevier B.V. All rights reserved.