82
2 Fundamental Properties of Mem-Elements
characteristics limit the use of these materials for memristor devices. Looking
beyond the electrical domain, it is not difficult to find physical systems whose
characteristics may be conveniently represented by a memristor. Especially
interesting is the discussion of mechanical [46, 47], hydraulic [48], and
thermal [49] physical arrangements acting as a memristor.
The physical mechanisms underlying the change between different resistive
states of switching materials can be summarized as follows:
• Physical mechanisms [50, Figure 1], [51]
– Schottky emission: thermally activated electrons injected over the barrier into
the conduction band
– Fowler–Nordheim tunneling: electrons tunnel from the cathode into the
conduction band; usually occurs at high electric field
– Direct tunneling: electrons tunnel from cathode to anode directly; only when
the oxide is thin enough. When the insulator has localized states (traps) caused
by disorder, off-stoichiometry or impurities, trap-assisted transport contributes
to additional conduction, including:
(a) tunneling from cathode to traps;
(b) emission from traps to the conduction band (Poole-Frenkel emission);
(c) tunneling from trap to conduction band;
(d) trap-to-trap hopping or tunneling, ranging from Mott hopping between
localized states to metallic conduction through extended states;
(e) tunneling from traps to anode.
The lists above are far from being complete and are intended to provide a
minimum basis of materials and physics for memristors.
Manufacturing processes compatible with CMOS technology, sustainable scaling, and superior computer efficiency are the key factors for the use of memristor
devices in information technology markets. The material challenges and integration
strategies for memristor-based computing systems include the accurate control
of device-to-device and cycle-to-cycle variability, electrical programming (e.g.,
analog/digital resistive memory, R on /R off ratio, etc.), and switching properties (e.g.,
endurance, retention, speed, etc.). Reliability is among the most important concerns
for RRAM devices. Frequent migration of ions/atoms under a high local field,
high current density, high power dissipation, and high temperature can degrade
the electrodes and the active material. Another high priority is cycling endurance,
especially for memory applications where the memory is accessed multiple times
from the CPU for in-memory applications. As a nonvolatile memory, memristors
must demonstrate data retention at both room temperature and elevated temperature,
which is mandatory for meeting specifications for embedded memory and/ or
automotive applications. The breakthrough for achieving a complete control of the
peculiar memristor properties are complete understanding of the complex physics
in the devices and a suitable process to model the electrical characteristics.
2 Fundamental Properties of Mem-Elements
characteristics limit the use of these materials for memristor devices. Looking
beyond the electrical domain, it is not difficult to find physical systems whose
characteristics may be conveniently represented by a memristor. Especially
interesting is the discussion of mechanical [46, 47], hydraulic [48], and
thermal [49] physical arrangements acting as a memristor.
The physical mechanisms underlying the change between different resistive
states of switching materials can be summarized as follows:
• Physical mechanisms [50, Figure 1], [51]
– Schottky emission: thermally activated electrons injected over the barrier into
the conduction band
– Fowler–Nordheim tunneling: electrons tunnel from the cathode into the
conduction band; usually occurs at high electric field
– Direct tunneling: electrons tunnel from cathode to anode directly; only when
the oxide is thin enough. When the insulator has localized states (traps) caused
by disorder, off-stoichiometry or impurities, trap-assisted transport contributes
to additional conduction, including:
(a) tunneling from cathode to traps;
(b) emission from traps to the conduction band (Poole-Frenkel emission);
(c) tunneling from trap to conduction band;
(d) trap-to-trap hopping or tunneling, ranging from Mott hopping between
localized states to metallic conduction through extended states;
(e) tunneling from traps to anode.
The lists above are far from being complete and are intended to provide a
minimum basis of materials and physics for memristors.
Manufacturing processes compatible with CMOS technology, sustainable scaling, and superior computer efficiency are the key factors for the use of memristor
devices in information technology markets. The material challenges and integration
strategies for memristor-based computing systems include the accurate control
of device-to-device and cycle-to-cycle variability, electrical programming (e.g.,
analog/digital resistive memory, R on /R off ratio, etc.), and switching properties (e.g.,
endurance, retention, speed, etc.). Reliability is among the most important concerns
for RRAM devices. Frequent migration of ions/atoms under a high local field,
high current density, high power dissipation, and high temperature can degrade
the electrodes and the active material. Another high priority is cycling endurance,
especially for memory applications where the memory is accessed multiple times
from the CPU for in-memory applications. As a nonvolatile memory, memristors
must demonstrate data retention at both room temperature and elevated temperature,
which is mandatory for meeting specifications for embedded memory and/ or
automotive applications. The breakthrough for achieving a complete control of the
peculiar memristor properties are complete understanding of the complex physics
in the devices and a suitable process to model the electrical characteristics.