Supplementary materials for
Single-Crystalline CuO Nanowire for Resistive Random
Access Memory Applications
Yi-Siang Hong,１ Jui-Yuan Chen,１ Chun-Wei Huang,１ Chung-Hua Chiu,１ Yu-Ting
Huang,１Ting Kai Huang,１Jo Hsuan Ho,１ Wen-Wei Wu.１,a)
1
Department of Materials Science and Engineering, National Chiao Tung University,
Hsinchu 300, Taiwan
a)
E-mail: WWWu@mail.nctu.edu.tw
FIG. S1. The schematic diagrams for device fabrication. (a) The CuO nanowires was
dripped on the blank specimen by using alcohol as transfer liquid. The center of the
blank specimen was electron transparent used for TEM observation. (b) The MMA
and PMMA A5 was coated sequentially on the specimen as photoresist, followed by
150°C baking for 2minutes. (c) The electron beam lithography was used to connect
the NWs with outer electrodes. (d) After the development and fixing, Ti (20nm)/Au
(180nm) wasdepositedas the top and bottom electrodes by electron evaporation
system. Finally, the device was finished after the lift-of process.
FIG. S2. The forming process and first RESET. The operation voltage was higher
than SET in Fig. 1(a) resulting from the energy requirement for filament formation.
FIG. S3. The EDS analysis for CuO NW RRAM device at LRS. The results showed
that the atomic percentage of oxygen at anode was 61.96% and gradually reduced to
60.05, 57.20, 51.43% along cathode. The concentration of oxygen decreased to 7.51%
at cathode which revealed a number of oxygen vacancies accumulated. The unusual
amount
of
oxygen
vacancies
were
contributed
from
redox
reaction
( O0  V0..  1 / 2O2  2e  ). However, the accumulation of oxygen vacancies was not
the main reason to resistive switching. The detail analysis was in Fig. S3.
FIG. S4. The SEM image of CuO NWs. The inhomogeneous of radial distribution
(Fig. 2) resulted from the morphology of nanowire. The non-uniform radial
distribution in mapping resulted from the inhomogeneous morphology of the
nanowire (Fig. S4) instead of a ratio difference in composition, both copper and
oxygen signal decreased/ increased, as shown in Fig. 2.
FIG. S5. The in situ TEM and EDS mapping for CuONW RRAM device. The signal
of oxygen ions was reduced when the external voltage enhanced from 0 to 5V,while
the device remain at HRS. This result demonstrated the accumulation of oxygen
vacancies at local cathodecannot lead to the SET or forming process. The
accumulation of oxygen vacancies at cathode was also observed at LRS. Therefore,
we consider that was a step in forming process. At the beginning of forming process,
the oxygen vacancies was generated and attracted at anode which resulted in the
followed reaction.
Cathode
1 nm
2 nm
FIG. S6. The TEM observation for conducting filament. (a) The low mag TEM image
for CuONW RRAM device at cathode. The red and green marks correspond to the
followed figures. (b) The HRTEM image of the left side of CuO nanowire showed the
Cu2O phase while (c) the HRTEM image of right side showed the nanowire remain
CuO structure.The sufficient concentration of oxygen vacancies at left side and the
charges are balanced by electrons from applied bias, the oxygen vacancies rearrange


(CuO)  Vo..  2e   2CuCu
(Cu 2 O) ).
to form Cu2O phase( 2CuCu
FIG. S7. The fitting for I-V measurement of CuO NW RRAM device at LRS. The
fitting result showed the conducting method was SCLC (space-charge-limited current)
characteristic, which revealed a number of electron passed through the CuO nanowire
by trap and detrap from oxygen vacancies.22
FIG. S8. The EDS mapping for CuO-NW RRAM device after applied voltage
(V<Vforming) and released voltage. The mapping diagrams showed that the signal of
oxygen at cathode was recovered after the device was departed from electrical
measurement, indicated the oxygen ions migrating back to the previous distribution.
The driven force for recovery was resulted from the concentration gradient of oxygen
ions.
FIG. S9. The multi-temperature electrical measurement for CuO-NW RRAM device
at LRS. The resistance decreased when the temperature increased, indicated that the
conducting filament was a semiconductor characteristic.