Active solid-state devices (e.g. – transistors – solid-state diode – Organic semiconductor material
Reexamination Certificate
2001-10-26
2003-09-16
Flynn, Nathan J. (Department: 2826)
Active solid-state devices (e.g., transistors, solid-state diode
Organic semiconductor material
C257S295000, C257S422000, C257S425000, C257S427000, C360S324100, C360S324200, C365S171000, C365S173000
Reexamination Certificate
active
06621100
ABSTRACT:
TECHNICAL FIELD OF THE INVENTION
This invention relates to organic based spintronic devices, and electronic devices comprising them, such as spin valves (such as those shown in FIGS.
1
and
3
), spin tunnel junctions, spin transistors and spin light-emitting devices that use the arrangements of the present inventions.
BACKGROUND AND SUMMARY OF THE INVENTION
1. Innovation and Concepts
New polymer-, organic- and molecular-based electronic devices in which the electron spin degree of freedom controls the electric current to enhance device performance provide a key basis for a broad range of technologies. Polymer-, organic-, and molecular-based spintronic devices have enhanced functionality, ease of manufacture, are less costly than inorganic ones. The long spin coherence times due to the weak spin-orbit interaction of carbon and other low atomic number atoms that comprise organic materials make them ideal for exploiting the concepts of spin quantum devices. The hopping mechanism of charge transport that dominates in semiconducting polymers (vs. band transport in crystalline inorganic semiconductors) enhances spin-magneto sensitivity and reduces the expected power loss.
Introduction
In the past decade there has been extensive progress in inorganic multilayer based spin valves.
1
Giant magnetic resistance and spin valves based on this effect recently enabled 100% per year growth of the areal density in the hard drive disk industry.
2
Recent extensions to magnetic semiconductor-based spin valves
3
and related spin-LEDs
4
have been shown to be promising embodiments of spintronics; however, there are substantial problems due to interfaces as well as low Curie temperatures (T
c
) for present magnetic semiconductors.
5
There is growing interest in replacing inorganic electronic materials with inexpensive, easier-to-fabricate polymers akin to the interest in using conducting polymers in a myriad of electronic applications.
6
Many polymer/molecular/organic materials can be dissolved in solution and spun into thin films or readily evaporated onto many substrates. This is anticipated to lead to cost efficiency and ease of manufacture in devices, especially for large area use and flexible substrates. Also, electronic polymers are known to be radiation hard. Another advantage of molecular/organic materials is the richness of chemistry which enables the synthesis of materials with very specific properties. Recently it was demonstrated
7
that organic molecules can be used to create nano-sized heterojunctions and as building blocks for a molecular computer.
8
Today, selected polymers readily conduct electric charge.
9
The room-temperature conductivity &sgr;
RT
of conjugated polymers, such as polyacetylene, polyaniline, and polypyrrole, can be controlled over 15 decades by doping and structural order and attain a value that is only an order of magnitude below the record conductivity of superpure Cu (&sgr;
RT
=6×10
5
S/cm).
Since the report in 1990 of electroluminescence
10
in the light emitting polymer (LEP) PPV, there have been many advances in polymer light-emitting diodes. Polymer/molecule based LEDs in all wavelengths of light
11
(from ir to uv) and with a wide variety of output parameters are known
12
and commercialization of polymer LEDs has begun.
13
Molecule-based magnetism started with the discovery
14
in the mid 1980's of ferromagnetism at 4.8 K in the linear chain of electron transfer salt [FeCp*
2
][TCNE] (Cp*=pentamethylcyclopendienide; TCNE=tetracyanoethylene) by Miller and Epstein. Today there is “polymeric” material such as V[TCNE]
x
(x~2) that is room temperature magnet and which is processable at room temperature by using conventional organic chemistry, common solvents, and low temperature chemical vapor deposition (CVD).
15
V[TCNE]
x
is a ‘soft’ magnet with magnetic ordering temperature T
c
=400 K, a small coercive field H
c
=4.5 Oe (varying by more that one order of magnitude with details of preparation/processsing) at 300 K, and semiconducting conductivity &sgr;
RT
~10
−5
S/sm. Other examples of polymer/molcule/organgic based magnets include Prussian Blue(s). Examples of Prussian Blue(s) structure-containing compounds include V[Cr(CN)
6
]
x
·YH
2
O, where x is between 0.5 and 1.5; preferably 0.8 to 1.2, and Y is between 0 and 4; preferably 1.5 and 2.8.
Approach/innovative Concepts
We propose the concept of spintronics in polymer devices as all the mandatory elements can be achieved with polymers. A key argument for exploiting polymer spintronics is the very long spin coherence time (&tgr;
s
) in polymers. Analysis of EPR data yields 10
−7
s for poorly conducting polymers
16
and microseconds for well-conducting samples.
17
Also, &tgr;
s
is 10
−8
s for V[TCNE]
x
18
, which is longer than that of conventional inorganic semiconductors ~10
−9
s. Polymers enable us to overcome difficulties that inorganic spintronics faces such as poor spin injection through the interfaces, low Curie temperature, and low sensitivity. Preliminary measurements yield low barriers to charge injection between magnetic and conducting polymers. Table 1 is a comparative summary of semiconductor and polymer-based magnet parameters.
TABLE 1
Example comparison of inorganic
and polymer magnetic semiconductors
parameters
Polymer-
Semiconduct-
based
or Magnets
Magnets
T
C
90 K
400 K
H
C
40 Oe at 6 K
4.5 Oe at 300
K
&sgr;
RT
10
−5
S/cm
10
−5
S/cm
&Dgr;G/G of
0.1% at 300
40% at 300 K
spin-valve
K
(anticipated)
0.3% at 77 K
&tgr;
s
10
−9
S
10
−8
S
Interfaces
Rigid
Flexible, inter-
penetrating
Processing
High (typically
Low (<40° C.)
T
>600° C.)
The schematic layout of a vertical polymer-based magneto-spin gate (spin-valve) is shown in FIG.
1
. The resistance of the sandwich structure (Hard Magnet—Conductor—Soft Magnet) strongly depends on the relative orientation of the magnetization of hard and soft magnets. The hard magnet aligns spins of electrons injected from the metal contact. The transit time for electrons across the central conductor is shorter than the spin coherence time; therefore, if the magnetization of the soft magnet is aligned with that of the hard magnet, electrons easily continue their path to reach another electrode. For opposite magnetization of the soft magnet, transit of the polarized electrons is forbidden unless the spins reorient. The conducting layer in the center serves as a spacer to separate soft and hard magnets, thus, enabling the soft magnet to be tunable by an external magnetic field.
Estimates of the parameters and characteristics of a polymer spin-valve vary depending on mechanism of charge transport. In highly conducting polymers (&sgr;
RT
≦100 S/cm) the room-temperature conductivity is provided by metallic band-like motion, with charge hopping over nearest neighbor states.
19
Then the hopping rate &ohgr;
h
is given by the typical phonon frequency 10
12
s
−1
and the length of hopping L
hop
is the localization radius &xgr;, which is given by scale of inhomogenieties, typically 2 nm. Taking the spin coherence time &tgr;
s
as 10
−7
s we find that the electron makes N
h
~10
5
hops before the electron loses its spin orientation. Hence, the spin coherence length can be estimated as L
coherence
≈L
hop
×N
h
~1 &mgr;m.
If the poorly conducting polymers (&sgr;
RT
≦10
−1
S/cm) are used, the Mott variable range hopping (VRH) mechanism of transport dominates. In this case the hopping rate is essentially decreased, but simultaneously the length of hopping increases; therefore, on the whole the coherence length L
coherence
remains at the micron scale at least for room temperature. It is important to have L
coherence
large, as this parameter controls the effectiveness of the device and the spin coherence length also limits the allowed thickness of conducting layer L
C
<L
coherence
. For a hard magnet, a metallic ferromagnetic or ferrimagnetic or similar film m
Epstein Arthur J.
Prigodin Vladimir N.
Flynn Nathan J.
Greene Pershelle
The Ohio State University
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