Coexistence of filamentary and homogeneous resistive switching in graded WO x thin films

Reversible clockwise and counter‐clockwise resistive switching in a Pt/graded WO x /W stack is reported. Two stable switching modes with opposite switching polarity and different electrical characteristics are found to coexist in the same memory cell. Clockwise switching shows filamentary characteristics that lead to relatively faster switching with excellent retention at high temperature. Counter‐clockwise switching exhibits homogeneous conduction with slower switching and moderate retention. The field‐induced switching reversal might be due to inhomogeneous expansion of W during thermal oxidation. Our results provide a clue to modulate the switching type in Pt/WO x /W memory cells. (© 2011 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)