Appendix 1: Memristive Devices: Materials and Complex Physics
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(electric potential gradient); (b) electromigration (electron kinetic energy); (c) Fick
diffusion (concentration gradient); (d) thermophoresis (temperature gradient). Any
of the four factors, or a combination of them, may occur to produce different
idealized type of resistance modulation or switching (field/thermal dominated
unipolar/bipolar) in real-world memristor (switching) devices.
• Material structures
– Oxide based memristor: Similar to the HP memristor, almost all memristors
exploit a capacitor like structure with metal electrodes and a switching
oxide layer. Switching layers are usually made up of dielectric materials.
In particular, binary oxides such as titanium oxide TiO 2 , tantalum oxide
Ta 2 O 5 , silicon oxide SiO 2 , copper oxide CuO, nickel oxide NiO, zinc oxide
ZnO, hafnium oxide HfO 2 , and aluminum oxide Al 2 O 3 are mainly used.
Memristors based on these binary oxide present a good resistive switching
performance. The resistive switching properties are typically either a sharp
resistance change between two resistive states R on and R off (suitable for
binary storage memories) or a progressive resistance adaptation (useful in
analog memories for neuromorphic computing). The operation of resistance
change is usually referred to as “write operation” and it is performed by
applying voltage/current pulses with appropriate amplitude, duration, and
polarity on the memristor electrodes. Input pulses with low amplitude are
used in “read operation” in order to sense the resistive state of the memristor
without inducing any resistance change. Voltage pulses trigger Joule-heating
effects that play a crucial role in the formation and destruction of localized
conducting paths in the oxide. Although many switching oxide layers are plausible for realizing memristors, a few of them result to be CMOS compatible
and lead to superior device performance. For instance, Ta 2 O 5 , HfO 2 , SiO 2 ,
and Al 2 O 3 are used in crossbar configurations to implement arrays with crosspoint devices embedding a selector device and a memristor (1T1R). Oxide
based memristors are also referred to as Resistive Random Access Memories
(RRAMs) or Resistive Switching Devices.
– Phase Change Memory (PCM): Memristor based on phase change materials
are mainly assembled by means of a chalcogenide. Ge 2 Sb 2 Te 5 (GST)
material is one of the most used chalcogenide to build PCM devices.
Voltage/current pulses are applied to change the phase of the chalcogenide
between crystalline and amorphous due to Joule heating. Read-from and
write-to PCM devices are operations that involve quite different current and
voltage levels. Although PCM is one of the most promising technology for
memristor devices, there are still many technological issues.
– Other memristors: Many studies explore other switching materials besides
binary oxides. Such materials include: (a) heterogeneous materials (e.g.,
cerium oxide CeO 2 and strontium titanate SrTiO 3 ); (b) organic materials
[44, 45]; (c) electrolyte materials such as copper sulfide (CuS). Unconventional fabrication methods, CMOS compatibility, and unreliable physical
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(electric potential gradient); (b) electromigration (electron kinetic energy); (c) Fick
diffusion (concentration gradient); (d) thermophoresis (temperature gradient). Any
of the four factors, or a combination of them, may occur to produce different
idealized type of resistance modulation or switching (field/thermal dominated
unipolar/bipolar) in real-world memristor (switching) devices.
• Material structures
– Oxide based memristor: Similar to the HP memristor, almost all memristors
exploit a capacitor like structure with metal electrodes and a switching
oxide layer. Switching layers are usually made up of dielectric materials.
In particular, binary oxides such as titanium oxide TiO 2 , tantalum oxide
Ta 2 O 5 , silicon oxide SiO 2 , copper oxide CuO, nickel oxide NiO, zinc oxide
ZnO, hafnium oxide HfO 2 , and aluminum oxide Al 2 O 3 are mainly used.
Memristors based on these binary oxide present a good resistive switching
performance. The resistive switching properties are typically either a sharp
resistance change between two resistive states R on and R off (suitable for
binary storage memories) or a progressive resistance adaptation (useful in
analog memories for neuromorphic computing). The operation of resistance
change is usually referred to as “write operation” and it is performed by
applying voltage/current pulses with appropriate amplitude, duration, and
polarity on the memristor electrodes. Input pulses with low amplitude are
used in “read operation” in order to sense the resistive state of the memristor
without inducing any resistance change. Voltage pulses trigger Joule-heating
effects that play a crucial role in the formation and destruction of localized
conducting paths in the oxide. Although many switching oxide layers are plausible for realizing memristors, a few of them result to be CMOS compatible
and lead to superior device performance. For instance, Ta 2 O 5 , HfO 2 , SiO 2 ,
and Al 2 O 3 are used in crossbar configurations to implement arrays with crosspoint devices embedding a selector device and a memristor (1T1R). Oxide
based memristors are also referred to as Resistive Random Access Memories
(RRAMs) or Resistive Switching Devices.
– Phase Change Memory (PCM): Memristor based on phase change materials
are mainly assembled by means of a chalcogenide. Ge 2 Sb 2 Te 5 (GST)
material is one of the most used chalcogenide to build PCM devices.
Voltage/current pulses are applied to change the phase of the chalcogenide
between crystalline and amorphous due to Joule heating. Read-from and
write-to PCM devices are operations that involve quite different current and
voltage levels. Although PCM is one of the most promising technology for
memristor devices, there are still many technological issues.
– Other memristors: Many studies explore other switching materials besides
binary oxides. Such materials include: (a) heterogeneous materials (e.g.,
cerium oxide CeO 2 and strontium titanate SrTiO 3 ); (b) organic materials
[44, 45]; (c) electrolyte materials such as copper sulfide (CuS). Unconventional fabrication methods, CMOS compatibility, and unreliable physical