Formation of
metallic Tantalum (Ta) filament within Ta/TaO(x)/Pt ReRAM memory cell.
Positively charged
Ta(5+)-ions and oxygen vacancies (V(O)) contribute to the process.
Copyright:
Forschungszentrum Jülich / RWTH Aachen / Pössinger
(September 29, 2015) Resistive
memory cells or ReRAMs for short are deemed to be the new super
information-storage solution of the future. At present, two basic concepts are
being pursued, which, up to now, were associated with different types of active
ions. But this is not quite correct, as Jülich researchers working together
with their Korean, Japanese and American colleagues were surprised to discover.
In valence change memory (VCM) cells, not only are negatively charged oxygen
ions active, but – akin to electrochemical metallization memory (ECM) cells –
so too are positively charged metal ions. The effect enables switching
characteristics to be modified as required and makes it possible to move back
and forth from one concept to the other, as reported by the researchers in the
journals Nature Nanotechnology and Advanced Materials.
ReRAM cells have a unique characteristic: their electrical
resistance can be altered by applying an electric voltage. The cells behave
like a magnetic material that can be magnetized and demagnetized again. In
other words, they have an ON and an OFF state. This enables digital information
to be stored, i.e. information that distinguishes between "1" and
"0". The most important advantages of ReRAMs are that they can be
switched rapidly, consume little energy, and maintain their state even after
long periods of time with no external voltage.
A look into the
Oxide Cluster at Forschungszentrum Jülich in which resistive cells
and other layers
of material are produced and examined in an ultrahigh vacuum.
Copyright:
Forschungszentrum Jülich
The memristive behaviour of ReRAMs relay on mobile ions.
These ions move in a similar manner to in a battery, flowing back and forth
between two electrodes in a metal oxide layer no more than a few nanometres
thick. For a long time, researchers believed that VCMs and ECMs functioned very
differently. In ECMs, the ON and OFF states are achieved when metal ions move
and form whisker-like filaments. This happens when an electric voltage is
applied, causing such filaments to grow between the two electrodes of the cell.
The cell is practically short-circuited and the resistance decreases abruptly.
When the process is carefully controlled, information can be stored. The
switching behaviour of VCMs, in contrast, were primarily associated with the
displacement of oxygen ions. Contrary to metal ions, they are negatively
charged. When a voltage is applied, the ions move out of an oxygen-containing
metal compound. The material abruptly becomes more conductive. In this case as
well, the process needs to be more carefully controlled.