In a novel nonvolatile
memory cell formed above a substrate, a
diode is paired with a reversible resistance-switching material, preferably a
metal oxide or
nitride such as, for example, NixOy, NbxOy, TixOy, HFxOy, AlxOy, MgxOy, CoxOy, CrxOy, VxOy, ZnxOy, ZrxOy, BxNy, and AlxNy. In preferred embodiments, the
diode is formed as a vertical pillar disposed between conductors. Multiple memory levels can be stacked to form a monolithic three dimensional
memory array. In some embodiments, the
diode comprises
germanium or a
germanium alloy, which can be deposited and crystallized at relatively low temperatures, allowing use of aluminum or
copper in the conductors. The
memory cell of the present invention can be used as a rewriteable
memory cell or a one-time-programmable memory
cell, and can store two or more data states.