  Electric-field-induced resistance switching (RS) phenomena have been studied
for over 60 years in metal/dielectrics/metal structures. In these experiments a
wide range of dielectrics have been studied including binary transition metal
oxides, perovskite oxides, chalcogenides, carbon- and silicon-based materials,
as well as organic materials. RS phenomena can be used to store information and
offer an attractive performance, which encompasses fast switching speeds, high
scalability, and the desirable compatibility with Si-based
complementary-metal-oxide-semiconductor fabrication. This is promising for
nonvolatile memory technology, i.e. resistance random access memory (RRAM).
However, a comprehensive understanding of the underlying mechanism is still
lacking. This impedes a faster product development as well as an accurate
assessment of the device performance potential. Generally speaking, RS occurs
not in the entire dielectric but only a small, confined region, which results
from the local variation of conductivity in dielectrics. In this review, we
focus on the RS in oxides with such an inhomogeneous conductivity. According to
the origin of the conductivity inhomogeneity, the RS phenomena and their
working mechanism are reviewed by dividing them into two aspects: interface RS,
based on the change of contact resistance at metal/oxide interface due to the
change of Schottky barrier and interface chemical layer, and bulk RS, realized
by the formation, connection, and disconnection of conductive channels in the
oxides. Finally the current challenges of RS investigation and the potential
improvement of the RS performance for the nonvolatile memories are discussed.
