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Recently there has been an increased interest in investigating
various sorts of random access memory (RAM) that retain their
memory state after the electrical power is removed from the device.
This is known as nonvolatile memory [1]. One of the most mature
nonvolatile memory technologies is so-called flash memory, which
is based on a field effect transistor with an additional floating
gate. These memory cells, however, are rather slow in operation.
A possible replacement technology for flash memory is magnetic
random access memory (MRAM), which is based on magnetic tunnel
junctions. Further exploratory research is being done in the field
of memory effects in ferroelectrics, chalcogenides, polymers, and
transition-metal oxides.
We are currently investigating transition-metal oxides for nonvolatile
memory applications. Certain transition-metal oxides exhibit a charge-induced
insulator-to-metal transition with a resistive memory effect. Exposed
to an electrical field, the resistance of the transition-metal oxides
is reduced by several orders of magnitude and a conductor is obtained.
Consecutive electrical current pulses (typically 100 ns) of opposite
polarity switch the resistance of the transition-metal oxides reversibly
between a high-resistance and a low-resistance state. These two
different states persist after removal of the applied electrical
bias. Owing to their very long retention time, these compounds are
interesting candidates for nonvolatile memory applications. In addition,
by fine-tuning the amplitude of the write voltage pulses, it is
possible to generate several memory levels. This multilevel switching
implies that several bits can be stored in one memory cell.
All these phenomena have been studied in thin films of various
transition-metal oxides, for example Cr-doped SrZrO3 and Cr-doped
SrTiO3 [2-6]. Moreover bulk single crystals of Cr-doped SrTiO3
can be used as a model system for this class of materials to study
the drastic resistivity changes under applied electrical field
and the memory effect [3,7-9].
References
[1] G. I. Meijer, Who wins the nonvolatile memory race, Science
319, 1625 (2008). [Full
text]
[2] A. Beck, J.G. Bednorz, Ch. Gerber, C. Rossel, and D. Widmer,
Reproducible switching effect in thin oxide films for memory applications,
Appl. Phys. Lett. 77, 139 (2000).
[3] Y. Watanabe, J.G. Bednorz, A. Bietsch, Ch. Gerber, D. Widmer,
A. Beck, and S.J. Wind, Current-driven insulator-conductor transition
and nonvolatile memory in chromium-doped SrTiO3 single crystals,
Appl. Phys. Lett. 78, 3738 (2001).
[4] C. Rossel, G. I. Meijer, D. Brémaud, and D. Widmer,
Electrical current distribution across a metal–insulator–metal
structure during bistable switching, J. Appl. Phys. 90, 2892
(2001).
[5] S. Karg, G. I. Meijer, D. Widmer, and J. G. Bednorz, Electrical-stress-induced
conductivity increase in SrTiO3 films, Appl. Phys. Lett. 89, 072106
(2006).
[6] S. Karg, G. I. Meijer, D. Widmer, R. Stutz, J. G. Bednorz,
and Ch. Rettner, Nanoscale resistive memory device using SrTiO3
films, IEEE NVSMW Monterey, California, pp. 68 (2007).
[7] G.
I. Meijer, U. Staub, M. Janousch, S. L. Johnson, B. Delley, and
T. Neisius, Valence states of Cr and the insulator-to-metal transition
in Cr-doped SrTiO3, Phys. Rev. B 72, 155102 (2005).
[8] S. F. Alvarado, F. La Mattina, and J. G. Bednorz, Electroluminescence
in SrTiO3:Cr single-crystal nonvolatile memory cells, Applied Physics
A, 89, 85 (2007).
[9] M. Janousch, G. I. Meijer, U. Staub, B. Delley, S. F. Karg,
and B. P. Andreasson, Role of oxygen vacancies in Cr-doped SrTiO3
for resistance-change memory, Adv. Mater. 19, 2232 (2007).
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