Abstract
Resistive switching in metal-insulator-metal (MIM) structures is an intriguing
phenomenon in which the electrical resistance can be altered reversibly, and permanent
in case of a non-volatile memory, by applying a voltage. It is being investigated
intensely, partly because of its potential application in data storage. The mechanism of
this switching is largely unknown. This thesis describes the design, construction and
electrical characterization of diodes with an active layer consisting of organic-inorganic
hybrid materials that show resistive switching.
In the realization of the memory diodes, two different approaches were followed.
Chapter 2 describes the first approach in which the active layer is made of a block
copolymer consisting of a semiconducting part (sexithiophene) and a part facilitating
migration of added inorganic ions (ethylene oxide). Inorganic salt is added as a dopant.
From an analysis of the electro-optical behavior of these diodes, it is concluded that the
resistive switching and the associated memory effect is due to electric field induced
migration of the inorganic ions in the active layer. The switching allows for storage of
information and rewritable memory operation is demonstrated for the diodes although
the retention time of the information is still very short (~ 10 s). In this system, the
energetic barrier for injection of a mobile carrier into the semiconducting block can be
lowered by the presence of an inorganic ion of opposite sign in a neighboring ion
transporting block. Hence, the resistivity of the diode can be modulated by changing the
concentration of ions at the electrode.
The second approach involves metal oxides in combination with polymers as
active layer. In Chapter 3 to 6, zinc oxide nanoparticles have been used as one of the
active components. After a forming reaction, resistive switching could be established
within 50 ms and retention of the memory was increased to hours. The forming process
itself is interpreted in terms of desorption of molecular oxygen from the ZnO nanoparticle
surface, induced by injection of holes via the PEDOT:PSS contact, leading to a higher ntype
conductivity via interparticle ZnO contacts. The forming can also be induced by
ultraviolet light and the process is studied with electron paramagnetic resonance,
photoinduced absorption spectroscopy, and field-effect measurements. By varying the
content of ZnO and the type of polymer in the active layer the memory effects can by
influenced and a data storage with lifetime >14 hours has been achieved.
Chapter 5 shows that the electronic properties can be altered by capping the ZnO
nanoparticles with various ligands. Capping with propylamine gives hysteresis and
resistive switching after application of –5 V and +5 V bias pulses, while retaining the
rectifying behavior of the device. This is a crucial requirement when these devices are
used in passive matrix arrays. In Chapter 6 it is shown that with increasing surface
coverage of the ZnO nanoparticles with a thiol ligand, the electrical resistance, associated
with electron transport via percolating networks of ZnO particles in the matrix, increases,
due to deterioration of the ZnO interparticle contacts. Just before reaching the
percolation limit obtained by adding ~0.05 mol thiol per mol Zn to the ZnO nanoparticles,
where the electrical current is supported by just a few percolation paths, the electrical
characteristics change. For unmodified ZnO particles, voltage pulses of opposite polarity
(bipolar) bring the diode to a low and a high resistive state. With the ZnO particles
modified with octane thiol, the diodes can be switched between low and high resistance
with unipolar voltage pulses of 1 µs. Using write and erase pulses of 10 ms, an ON/OFF
ratio of 103 can be achieved with good cycle endurance. This change is observed with Al
or Pd top electrodes, and is interpreted in terms of the conduction taking place via
essentially a single, narrow channel of ZnO particles that can be blocked by trapping of a
single charged species.
In Chapter 7, it is concluded that the switching function of the polymer–aluminum
oxide diodes mainly reflects a property of the aluminum oxide. The yield in switching of
solid state-memories can be increased to about unity by deliberately adding a thin
sputtered Al2O3 layer to organic diodes. Before memory operation, the devices have to be
formed at an electric field of 109 V/m, corresponding to soft-breakdown of Al2O3. After
forming, the structures show pronounced negative differential resistance and the local
maximum in the current scales with the thickness of the oxide layer. After the forming,
switching in hundreds of nanoseconds can be achieved. Repeated pulse sequence
measurements of Chapter 8, show the occurrence of a ‘dead time’, that is the time after
programming in which a next switch is inhibited, of about 3 ms. The dead time, which is
known for bulk oxides, explains the huge variation in the reported switching times.
Resistive switching is demonstrated in diodes based on spin coated layers of
various metal oxide nanoparticles and a semiconducting polymer, sandwiched between
two electrodes in Chapter 9. Inclusion of the oxide nanoparticles results in non-volatile
electronic memory characteristics that are similar to those observed for the
corresponding ‘bulk’ oxide. A major difference is that the nanoparticule layers do not
require a forming step. In the absence of oxygen, resistive switching is observed in many
metal oxides. Among the various oxides, there are differences in switching behavior
(unipolar/bipolar) that may be related to the different electronic structure of the
materials. In the case of ZnO particles modification of the layer morphology, leads to
change in the memory properties (Chapter 7). This indicates that besides electronic
structure, also the geometry and number of the filaments formed is important in
determining the type of resistive switching of the memory.
In Chapter 10, negative differential resistance (NDR) in polymer-Al2O3 diodes is
investigated using time and frequency domain electrical measurements and thermal
imaging. After a forming step, the diodes show a time dependent NDR. In the bias
voltage range where NDR is observed, the capacitance at low frequency is significantly
larger than for the unformed diode. Conduction in the formed diodes is filamentary in
nature. Assuming different metastable ionization states of defects in the oxide and space
charge limited conduction, some fundamental aspects of the switching can be accounted
for.
In conclusion, this thesis shows resistive switching for diodes containing various
materials. The combination of metal oxides and polymers emerges as appealing because
it exhibits a range of intriguing resistive switching effects that are potentially of interest
for future nonvolatile memory element applications. A first step is made to rationalize the
origin, mechanism, and magnitude of the resistive switching in terms of a quantitative
model.
Original language | English |
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Qualification | Doctor of Philosophy |
Awarding Institution |
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Supervisors/Advisors |
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Award date | 16 Apr 2008 |
Place of Publication | Eindhoven |
Publisher | |
Print ISBNs | 978-90-386-1238-6 |
DOIs | |
Publication status | Published - 2008 |