1. Physics of Conductive Bridging-Random Access Memory
Next Generation Memory Technology
Mayur Sundararajan, Martin Kordesch and Gang Chen, Department of Physics and Astronomy, Ohio University
Background Experimental
Results
Optical Observation of Filament
Conclusion
Inert Electrode
Solid Electrolyte
Active Electrode
Inert Electrode
Active Electrode
Inert Electrode
Active Electrode
STATE ‘0’ STATE ‘1’
Conductive Bridge
(Filament)
Advantages of Conductive Bridging
Random Access Memory(CBRAM):
• Better scalability than
DRAM,NAND.
• Faster than FLASH
• Tunable speed
• Low power consumption
• High Endurance
Device Structure:
Solid Electrolyte sandwiched between two metal electrodes
• Active electrode: Ag,Cu, Ni
• Solid Electrolyte: GeSe2, GeS2, SiO2, GST
• Inert electrode:Pt,W,Ir
Device Operation:
Filament formation
Positive bias on active electrode oxidizes the metal and the
ions migrates to the inert electrode where the ion reduces to
the metal. This process builds the filament from the inert
electrode and reach the active electrode. This is the low
resistance state(LRS).
M  M+ + e-
M+ + e-  M
Filament dissolution
Negative bias on active electrode, reverses the formation
process and dissolves the filament. This is the high resistance
state(HRS).
Read
Applying a voltage below the switching voltage is used to read
the state.
Switching Mechanism
1
Hypothesis & Objective
• We believe the filament to be a semiconductor because
• The solid electrolyte cannot move at the same rate as the filament growth(1m/s)
• The voids in the electrolyte are not large enough for a thick filament equivalent to the observed resistance.
• Understand the origin of the conductive filament
• Understand the switching kinetics
Ag/Agx(GeSe2)1-x/Au
• OFF Voltage is dependent on the ON voltage
• Increasing ON voltage increases the stability of the state
• LRS and HRS contrast depends on ON voltage
• Stability of the filament strongly depends on the ON voltage
• The contrast between LRS and HRS can be tuned by ON voltage.
• The conductive bridge filament may be observed to grow laterally by suitable
substrate such as Ta. If it can be characterized with suitable techniques it would
reveal more information of the conductive bridge filament.
Ag/Agx(GeSe2)1-x/Ta
• The Tantalum(Ta) substrate is different from Au because it has natural dielectric
oxide layer. This porous oxide layer cover the Ta layer underneath the Ag pad and
forces the filament to grow sideways which was optically observed as seen in the
inset pictures above.
• These dendrites form and dissolve around the same time as the switching
happens in the IV curve which strongly suggests that this might be same as the
conductive bridge filament.
1.Nanotechnology, vol. 22, no.25, p. 254003, 2011
2.Phys. Chem. Chem. Phys., vol.17, no.14, pp8627-8632,2015
3.Phys. Chem. Chem. Phys., vol 11, no.17, p.3010,2009
Research is funded by NSF and DMR
This switching mechanism is still a matter of debate, there has been no direct
evidence of a continuous bridge filament.
Postulated schematic of CBRAM switching mechanism(1)
Au/Ta
GeSe2/Agx(GeSe2)1-x
Ag
WE
CE
CE WE
SAMPLE
CAMERA
1mm
WE
100µm
WE
Home built Thermal
Evaporation System
0.0 0.2 0.4 0.6 0.8 1.0
-0.020
-0.015
-0.010
-0.005
0.000
Current(A)
Potential (V)
Filament Building
References
ON Voltage vs OFF Voltage
-2.0 -1.8 -1.6 -1.4 -1.2 -1.0 -0.8 -0.6 -0.4
0.0
0.2
0.4
0.6
0.8
1.0
1.2
1.4
1.6
1.8
2.0
OFFVoltage
ON Voltage
ON vs OFF Potential
-2 -1 0 1 2
-0.4
-0.2
0.0
0.2
0.4
0.6
Current(A)
Potential (V)
-2V
-1.9V
-1.8V
-1.7V
-1.6V
-1.5V
-1.4V
-1.3V
-1.2V
-1.1V
-1.0V
-0.9V
-0.8V
-0.7V
-0.6V
-0.5V
-0.4V
ON Voltage vs OFF Voltage
ON
OFF
Switching
Voltage
-2.2 -2.0 -1.8 -1.6 -1.4 -1.2 -1.0 -0.8 -0.6 -0.4
4.0
4.5
5.0
5.5
6.0
6.5
LRS
HRS
On Voltage(V)
LRS(Ohm)
ON Voltage vs LRS,HRS
10
20
30
40
50
60
HRS(Ohm)
Ag/GeSe2/ITO
The observed switching is different from the postulated switching mechanism.
A fresh memory cell does not switch readily so Ag has to be pumped into GeSe2
-0.3 -0.2 -0.1 0.0 0.1
-0.001
0.000
0.001
0.002
0.003
0.004
0.005
Current(A)
Potential (V)
Typical Switching I-V curve
HRS
RESET
SET
Slow scan(50mV/s)
Fast scan(1000mV/s)
40s
0.2s
0.5s
ON OFF