  Over the past years, transition metal dichalcogenides (TMDs) have attracted
attention as potential building blocks for various electronic applications due
to their atomically thin nature. An exciting development is the recent success
in 'engineering' crystal phases of TMD compounds during the growth due to their
polymorphic character. Here, we report an electric field induced reversible
engineered phase transition in vertical 2H-MoTe2 devices, a crucial
experimental finding that enables electrical phase switching for these
ultra-thin layered materials. Scanning tunneling microscopy (STM) was utilized
to analyze the TMD crystalline structure after applying an electric field, and
scanning tunneling spectroscopy (STS) was employed to map a
semiconductor-to-metal phase transition on the nanoscale. In addition, direct
confirmation of a phase transition from 2H semiconductor to a distorted 2H'
metallic phase was obtained by scanning transmission electron microscopy
(STEM). MoTe2 and Mo1-xWxTe2 alloy based vertical resistive random access
memory (RRAM) cells were fabricated to demonstrate clear reproducible and
controlled switching with programming voltages that are tunable by the layer
thickness and that show a distinctly different trend for the binary compound if
compared to the ternary materials.
