U.S. patent application number 14/695632 was filed with the patent office on 2015-08-13 for organic molecular memory.
This patent application is currently assigned to KABUSHIKI KAISHA TOSHIBA. The applicant listed for this patent is KABUSHIKI KAISHA TOSHIBA. Invention is credited to Koji ASAKAWA, Shigeki HATTORI, Satoshi MIKOSHIBA, Hideyuki NISHIZAWA, Tsukasa TADA, Masaya TERAI, Reiko YOSHIMURA.
Application Number | 20150228335 14/695632 |
Document ID | / |
Family ID | 46876563 |
Filed Date | 2015-08-13 |
United States Patent
Application |
20150228335 |
Kind Code |
A1 |
NISHIZAWA; Hideyuki ; et
al. |
August 13, 2015 |
ORGANIC MOLECULAR MEMORY
Abstract
An organic molecular memory of an embodiment includes a first
conductive layer, a second conductive layer, and an organic
molecular layer interposed between the first conductive layer and
the second conductive layer, the organic molecular layer including
charge-storage molecular chains or variable-resistance molecular
chains, the charge-storage molecular chains or the
variable-resistance molecular chains including fused polycyclic
groups.
Inventors: |
NISHIZAWA; Hideyuki; (Tokyo,
JP) ; YOSHIMURA; Reiko; (Kanagawa, JP) ; TADA;
Tsukasa; (Tokyo, JP) ; HATTORI; Shigeki;
(Kanagawa, JP) ; TERAI; Masaya; (Tokyo, JP)
; MIKOSHIBA; Satoshi; (Kanagawa, JP) ; ASAKAWA;
Koji; (Kanagawa, JP) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
KABUSHIKI KAISHA TOSHIBA |
Tokyo |
|
JP |
|
|
Assignee: |
KABUSHIKI KAISHA TOSHIBA
Tokyo
JP
|
Family ID: |
46876563 |
Appl. No.: |
14/695632 |
Filed: |
April 24, 2015 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
|
13425796 |
Mar 21, 2012 |
9047941 |
|
|
14695632 |
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Current U.S.
Class: |
257/4 |
Current CPC
Class: |
H01L 51/0092 20130101;
G11C 2213/53 20130101; G11C 13/0016 20130101; H01L 51/0078
20130101; G11C 13/0014 20130101; H01L 45/1253 20130101; B82Y 10/00
20130101; H01L 51/0098 20130101; H01L 45/1233 20130101; H01L
51/0595 20130101 |
International
Class: |
G11C 13/00 20060101
G11C013/00; H01L 45/00 20060101 H01L045/00 |
Foreign Application Data
Date |
Code |
Application Number |
Mar 24, 2011 |
JP |
2011-065295 |
Claims
1. An organic molecular memory comprising: a first conductive
layer; a second conductive layer; and an organic molecular layer
interposed between the first conductive layer and the second
conductive layer, the organic molecular layer including a first
molecule having a variable-resistance molecular chain with a fused
polycyclic group.
2. The memory according to claim 1, wherein the organic molecular
layer includes a second organic molecule having a
variable-resistance molecular chain without a fused polycyclic
group.
3. The memory according to claim 2, wherein the variable-resistance
molecular chain of the first organic molecule and the
variable-resistance molecular chain of the second organic molecule
have same carbon skeleton.
4. The memory according to claim 1, wherein relative permittivity
of the organic molecular layer is 5.5 or higher.
5. The memory according to claim 1, wherein an electron-withdrawing
substituent bonds to the fused polycyclic group.
6. The memory according to claim 1, wherein the fused polycyclic
group is anthracene.
7. The memory according to claim 5, wherein the
electron-withdrawing substituent is a fluorine atom, a chlorine
atom, or a cyano group.
8. The memory according to claim 1, wherein the first conductive
layer and the second conductive layer are electrode interconnects
intersecting with each other, and the organic molecular layer is
located at an intersection portion between the first conductive
layer and the second conductive layer.
Description
CROSS-REFERENCE TO RELATED APPLICATION
[0001] This application is a Continuation of U.S. application Ser.
No. 13/425,796, filed Mar. 21, 2012, now allowed. This application
is also based upon and claims the benefit of priority from Japanese
Patent Application No. 2011-065295, filed on Mar. 24, 2011, the
entire contents of which are incorporated herein by reference.
FIELD
[0002] Embodiments described herein relate generally to organic
molecular memories.
BACKGROUND
[0003] When organic molecules are used in memory cells, the memory
cells can be made smaller in size, because organic molecules
themselves are small in size. As a result, storage density of
memory using molecules can be increased. The operation of memory
cell can be achieving the change between the low resistance state
and the high resistance state, and the change is corresponding to
the change of electric current. For this purpose, molecules having
a function to change its resistance depending on the applied
electric field or injected charges are introduced between upper and
lower electrodes. To change the state, the voltage is applied
between the upper and lower electrodes. The difference of state can
be detected by the electric current. Such attempts have been made
to form memory cells. Another operation of memory cell can be
achieving the change of the stored charges in molecule between the
channel (electrode) and the gate electrode of FET, and the change
is corresponding to the change of drain current. For this purpose,
molecules having a function to storing injected charges are formed
on an electrode, and the charges injected from the electrode are
stored in the molecule. To change the stored charges, the voltage
is applied between the upper and lower electrodes. The
charge-stored state can be detected by the drain current. Such
attempts have also been made to form memory cells.
[0004] In a small memory cell, however, the distances between the
charges in the molecules and the surrounding electrodes are short.
Therefore, charges are easily cleared from molecules due to
movement of charges between the molecules and the electrodes. As a
result, the charge retention time (the life or data retention time)
of the organic molecular memory becomes shorter.
BRIEF DESCRIPTION OF THE DRAWINGS
[0005] FIG. 1 is a schematic cross-sectional view of a memory cell
portion of an organic molecular memory according to a first
embodiment;
[0006] FIG. 2 is a schematic cross-sectional view of the organic
molecular memory according to the first embodiment;
[0007] FIG. 3 is a diagram showing a molecular structure of a
charge-storage molecular chain without fused polycyclic groups;
[0008] FIG. 4 shows graphs each representing the relationship
between the relative permittivity and life of an organic molecular
layer;
[0009] FIGS. 5A and 5B are diagrams showing the molecular
structures of the organic molecular layers of FIG. 4;
[0010] FIG. 6 is a diagram for explaining an effect of the first
embodiment;
[0011] FIG. 7 is a diagram for explaining another effect of the
first embodiment;
[0012] FIG. 8 is a diagram for explaining yet another effect of the
first embodiment;
[0013] FIGS. 9A and 9B are diagrams showing example of molecular
structures of the charge-storage molecular chains according to the
first embodiment;
[0014] FIGS. 10A through 10F are diagrams showing examples of fused
polycyclic groups bound to the charge-storage molecular chains
according to the first embodiment;
[0015] FIG. 11 is a schematic cross-sectional view of a memory cell
portion of an organic molecular memory according to a second
embodiment;
[0016] FIG. 12 is a schematic cross-sectional view of a memory cell
portion of an organic molecular memory according to a third
embodiment;
[0017] FIG. 13 is a schematic cross-sectional view of a memory cell
portion of an organic molecular memory according to a fourth
embodiment;
[0018] FIG. 14A is a diagram showing a molecular structure of an
organic molecule in a memory cell portion according to a fifth
embodiment; FIG. 14B is a diagram showing a molecular structure of
an organic molecule in a memory cell portion in a case where no
fused polycyclic groups are provided;
[0019] FIG. 15 is a schematic perspective view of an organic
molecular memory according to the fifth embodiment;
[0020] FIG. 16 is a schematic cross-sectional view of a memory cell
portion of the organic molecular memory according to the fifth
embodiment;
[0021] FIGS. 17A through 17F show examples of molecular units that
can form molecules each having the n-conjugated system extending in
a one-dimensional direction in the fifth embodiment;
[0022] FIG. 18 is a schematic cross-sectional view of a memory cell
portion of an organic molecular memory according to a sixth
embodiment; and
[0023] FIG. 19 is a schematic cross-sectional view of a memory cell
portion of an organic molecular memory according to a seventh
embodiment.
DETAILED DESCRIPTION
[0024] An organic molecular memory of an embodiment includes a
first conductive layer, a second conductive layer, and an organic
molecular layer interposed between the first conductive layer and
the second conductive layer, the organic molecular layer including
charge-storage molecular chains or variable-resistance molecular
chains, the charge-storage molecular chains or the
variable-resistance molecular chains including fused polycyclic
groups.
[0025] The following is a description of embodiments, with
reference to the accompanying drawings.
[0026] It should be noted that, in this embodiment, "charge-storage
molecular chain" means a molecular chain that has a function to
store charges therein, and can switch between a state to store the
charges and a state not to store the charges through application
and removal of external voltage.
[0027] It should also be noted that, in this specification,
"variable-resistance molecular chain" means a molecule chain having
a function to change its resistance, depending on whether an
electric field exists or whether charges are injected
thereinto.
[0028] Also, in this specification, "chemical bond" is a concept
indicating covalent bond, ion bond, or metallic bond, but is not a
concept indicting hydrogen bond or bond by van der Waals'
forces.
First Embodiment
[0029] An organic molecular memory of this embodiment includes a
first conductive layer, a second conductive layer, and an organic
molecular layer interposed between the first conductive layer and
the second conductive layer. The organic molecular layer includes
charge-storage molecular chains. The charge-storage molecular
chains include fused polycyclic groups.
[0030] According to this embodiment, the fused polycyclic groups
are introduced into the organic molecular layer, to increase the
relative permittivity of the organic molecular layer. Therefore,
charges stored in the organic molecular layer are hardly pulled out
of the organic molecular layer, and the charge retention properties
of the organic molecular memory (hereinafter also referred to
simply as the molecular memory) are improved.
[0031] FIG. 1 is a schematic cross-sectional view of a memory cell
portion of the organic molecular memory according to this
embodiment. FIG. 2 is a schematic cross-sectional view of the
organic molecular memory according to this embodiment. The organic
molecular memory of this embodiment is a nonvolatile organic
molecular memory of a stacked-gate type.
[0032] In the organic molecular memory of this embodiment, an
organic molecular layer 16, a block insulating film (insulating
layer) 18, and a gate electrode (a second conductive layer) 20 are
formed on a silicon substrate (a first conductive layer or
semiconductor layer) 10, for example. Source and drain regions 22
formed by diffusing an impurity are provided in portions of the
silicon substrate 10 located on both sides of the stack
structure.
[0033] The organic molecular layer 16 formed above the silicon
substrate (semiconductor layer) 10, the block insulating film
(insulating layer) 18 formed above the organic molecular layer 16
and the gate electrode 20 formed above he block insulating film
(insulating layer) 18.
[0034] The organic molecular layer 16 is formed with charge-storage
molecular chains 16a and the molecular chains bond to the silicon
substrate (semiconductor layer) 10. The charge-storage molecular
chains 16a have a function to store charges in the molecular
chains, and can switch between a state to store the charges and a
state not to store the charges through application and removal of
external voltage. The organic molecular layer 16 functions as a
charge-storage electrode. The thickness of the organic molecular
layer is 2 to 20 nm, for example.
[0035] The block insulating film 18 is a film stack of a silicon
oxide film and a silicon nitride film, or a high-permittivity film,
for example. The block insulating film 18 has a function to hinder
movement of charges between the organic molecular layer 16 and the
gate electrode 20.
[0036] In the organic molecular memory of this embodiment, a
voltage is applied between the gate electrode 20 and the silicon
substrate 10, to store charges into the organic molecular layer 16
or pull out the charges from the organic molecular layer 16. A
memory cell functions, using changes in transistor threshold value
depending on whether charges exist in the organic molecular layer
16.
[0037] FIG. 3 is a diagram showing a molecular structure of a
charge-storage molecular chain without fused polycyclic groups. In
this embodiment, the organic molecular layer 16 of each memory cell
portion contains organic molecules to which fused polycyclic groups
are bound.
[0038] The charge-storage molecular chains 16a forming the organic
molecular layer 16 of this embodiment have the molecular structures
shown in FIG. 1, for example. The charge-storage molecular chains
of FIG. 1 are derivatives of zinc porphyrin, which forms a
charge-storage molecular chain as shown in FIG. 3.
[0039] The oxygen atom (O) at one end of each of the charge-storage
molecular chains of FIGS. 1 and 3 is chemically bound to a silicon
atom (Si) of the silicon substrate 10. Silicon atoms in the surface
of the silicon substrate 10 and oxygen atoms (O) are bound together
in this manner, to form the organic molecular layer 16 that is a
so-called self-assembled monolayer (SAM). Meanwhile, the other end
of each of the charge-storage molecular chains 16a is not
chemically bound to the block insulating film 18.
[0040] Further, in the charge-storage molecular chains of FIG. 1,
anthracene that is fused polycyclic groups are bound to the zinc
porphyrin.
[0041] The fused polycyclic groups include pi-electrons that are
free electrons basically scattered in a two-dimensional direction.
As shown in FIG. 1, the organic molecules 16a secured at one point
to the electrode by a linker can rotate about the linker serving as
the rotational axis. Therefore, the pi-electrons scattered in the
two-dimensional direction rotate so that the organic molecules 16a
can cause apparent three-dimensional electronic polarization. The
electronic polarization of the adjacent organic molecules 16a
cancel the electric field formed by charges delocalized in the
organic molecules 16a. Accordingly, the charge retention properties
of the organic molecular memory are improved. It should be noted
that, if the fused polycyclic groups are oriented in both the
longitudinal and width direction of the molecular chains as shown
in FIG. 1, the three-dimensional electronic polarization is
strengthened, which is preferable.
[0042] As described above, in this embodiment, the charge-storage
molecular chains include organic molecules with fused polycyclic
groups as shown in FIG. 1, so that electronic polarization
canceling the electric field is induced. In other words, the
relative permittivity of the organic molecular layer 16 becomes
higher. Because of this, the organic molecular layer 16 has higher
relative permittivity than that achieved in a case where the
charge-storage molecular chain of FIG. 3 is used, for example. As a
result, the charge retention properties of the organic molecular
memory are improved. It should be noted that the relative
permittivity of the organic molecular layer using the
charge-storage molecular chain of FIG. 3 is approximately 3.0.
[0043] Here, the relative permittivity of the organic molecular
layer 16 can be appropriately set by adjusting the molecular
structures, placement density, and the like of the charge-storage
molecular chains 16a in the organic molecular layer 16.
[0044] FIG. 4 shows graphs each representing the relationship
between the relative permittivity and life (charge retention time)
of an organic molecular layer. FIGS. 5A and 5B are diagrams showing
the molecular structures of the organic molecular layers used in
the measurement illustrated in FIG. 4. The graphs shown in FIG. 4
were calculated by using a later shown equation (2) based on the
results obtained from the later described measurement (Measurement
1 and Measurement 2). It should be noted that the optical phonon
frequency, 10.sup.15 (s.sup.-1), which is the theoretical upper
limit, is used as the constant term P.sub.0 of the equation (2).
Therefore, the constant term represents the lower limit of the
retention time.
[0045] FIG. 4 shows the charge retention times of the two organic
molecular layers of FIGS. 5A and 5B where the relative permittivity
of each of the organic molecular layers is changed. Specifically,
the two samples are: the organic molecular layer of FIG. 5A (an
organic molecular layer A in FIG. 4) that is formed with
p-terphenylthiol, which is variable-resistance molecular chains,
and fluoroalkylthiol with electron-withdrawing substituents; and
the organic molecular layer of FIG. 5B (an organic molecular layer
B in FIG. 4) that is formed with
4-[2-amino-5-nitro-4-(phenylethynyl)phenylethynyl]benzenethiol,
which is variable-resistance molecular chains with
electron-withdrawing substituents, and fluoroalkylthiol with
electron-withdrawing substituents. Each of the two samples is
interposed between gold as the lower electrode and tungsten as the
upper electrode. The quantitative ratio between the
variable-resistance molecular chains and the fluoroalkylthiol with
electron-withdrawing substituents is varied, to change the relative
permittivity of each of the organic molecular layers.
[0046] As shown in FIG. 4, when the relative permittivity of an
organic molecular layer is 5.5 or higher, the charge retention time
exceeds approximately 1 second, and preferred characteristics of a
memory can be achieved. Further, when the relative permittivity
becomes 6.0 or higher, the charge retention time exceeds one hour,
and a more preferable charge retention time for a memory to be used
can be realized. The charge retention time here is the period of
time in which 37% of the initial characteristics change, and the
measurement temperature is room temperature (300 K).
[0047] The relative permittivity of the organic molecular layer
forming an organic molecular memory can be evaluated by applying an
AC bias between the substrate and the gate electrode, and measuring
the capacitance. At this point, the organic molecular layer
thickness and the block insulating film thickness required for the
relative permittivity calculation can be determined by observation
with a TEM (Transmission Electron Microscope).
[0048] In the following, effects of this embodiment are described.
It is considered that charges from the organic molecules in a
memory cell interposed between electrodes (conductive layers) are
cleared due to the following two mechanisms: 1) tunneling injection
of charges of the opposite sign from the electrodes; and 2) hopping
of charges from the molecules into the electrodes.
[0049] In this embodiment, the charge-storage molecular chains
forming an organic molecular layer has fused polycyclic groups.
Accordingly, the relative permittivity of the organic molecular
layer becomes higher, and clearing of charges due to the above
described mechanisms 1) and 2) is restrained.
[0050] First, the mechanism 1) is described. Charges are easily
cleared by tunneling injection of charges of the opposite sign from
the electrodes, because the electric field induced by the charges
in the organic molecules is strong. Due to the strong electric
field, the energy barrier between the molecules and the electrodes
becomes lower, and the tunneling probability becomes higher.
Therefore, charges are easily cleared from the organic molecular
layer.
[0051] FIG. 6 is a diagram for explaining an effect of this
embodiment. As shown in FIG. 6, when charges (holes in FIG. 6)
exist in an organic molecular layer, carriers (electrons in the
drawing) having charges of the opposite sign in the electrodes and
the charges in the organic molecular layer attract each other.
Accordingly, the potential barrier between the organic molecular
layer and the electrodes becomes lower.
[0052] Where the potential barrier is represented by U(x), the
probability that charges with an energy E in the electrode tunnel
into the organic molecular layer is expressed by the following
equation (1):
T .varies. exp ( - 4 .pi. h .intg. A B 2 m ( U ( x ) - E ) x ) [
Equation 1 ] ##EQU00001##
[0053] Here, .pi. represents the circumference ratio, h represents
the Planck's constant, m represents the effective mass, and A and B
are the two points where the potential U(x) has the value of the
energy E and serve as the start point (A) and the end point (B) of
the tunneling.
[0054] As can be seen from the equation (1), the tunneling
probability becomes higher, as the distance between A and B (the
potential width) becomes shorter and the difference between the
potential and the energy (U(x)-E) becomes smaller. The distance
between A and B becomes shorter as the change in U(x) becomes
larger. Since the change in U(x) corresponds to the electric field,
the distance between A and B becomes longer, and the tunneling
probability becomes lower as the electric field becomes weaker.
[0055] Therefore, to restrain clearing of charges due to tunneling
and facilitate charge retention, weakening the electric field is
essential. As can be seen from the Maxwell's equations (the flux
density conservation law), the electric field can be weakened by
increasing the relative permittivity between the charges in the
organic molecular layer and the electrodes. The portion between the
charges in the organic molecular layer and the electrodes is the
organic molecular layer. Therefore, by increasing the relative
permittivity of the organic molecular layer, clearing of charges
due to tunneling can be restrained.
[0056] In this embodiment, the charge-storage molecular chains are
designed to include organic molecules with fused polycyclic groups.
Accordingly, the relative permittivity of the organic molecular
layer is made higher.
[0057] FIG. 7 is a diagram for explaining another effect of this
embodiment. FIG. 7 is a diagram showing a change in the potential
barrier in a case where an organic molecular layer has higher
relative permittivity than that of FIG. 6.
[0058] In a case where charges having the same energy E as that of
FIG. 6 tunnel from the electrodes into the organic molecular layer,
the charges need to pass through the start point C and the end
point D of the tunneling. The distance between C and D is longer
than the distance between A and B of FIG. 6, and the difference
between the potential and the energy (U(x)-E) is larger than that
of FIG. 6. Therefore, the tunneling probability is lower than that
of FIG. 6. Accordingly, the charge retention time becomes
longer.
[0059] Next, the mechanism 2) is described. Where there is
electronic polarization, the potential barrier becomes lower, and
tunneling is restrained, as described above. Not only that, the
polarization energy becomes larger. Accordingly, clearing due to
escape of charges from the molecules to the electrodes by hopping
is restrained.
[0060] FIG. 8 is a diagram for explaining yet another effect of
this embodiment.
[0061] Making an electric field smaller by electric dipoles is
storing the energy of the electric field in the form of the
polarization energy W of a dielectric material. The polarization
energy W is the energy to be scattered around when charges are
removed, and is equivalent to the difference between the HOMO
(Highest Occupied Molecular Orbital) energy level and the SOMO
(Singly Occupied Molecular Orbital) energy level from which one
electron has been pulled out.
[0062] Therefore, the activation energy A required for the charges
in an organic molecular layer to hop is equal to a half of the
polarization energy W. Accordingly, by increasing the polarization
energy W, the probability of outflow of charges hopping from
molecules can be made lower. The outflow probability P can be
expressed by the following equation (2):
P = P 0 exp ( - .DELTA. kT ) = P 0 exp ( - W 2 kT ) [ Equation 2 ]
##EQU00002##
[0063] Here, P.sub.0 represents the constant, and .DELTA.
represents the activation energy for removing charges.
[0064] In determining the polarization energy W, a local electric
field, not a macroscopic electric field, needs to be used, with
fluctuation of polarization at the molecular level being taken into
account. Where E.sub.0 represents the electric field without
polarization, the local field E is expressed by the following
equation (3):
E = + 2 3 E 0 [ Equation 3 ] ##EQU00003##
[0065] Accordingly, the polarization energy W is expressed by the
following equation (4):
W = .intg. 0 D E D = r 0 .intg. 0 D E E = r 0 2 ( r + 2 3 ) 2 E 0 2
[ Equation 4 ] ##EQU00004##
[0066] As can be seen from the equation (4), the polarization
energy W becomes larger as the relative permittivity becomes
higher. As can be seen from the equation (2), the outflow
probability P becomes lower as the polarization energy W becomes
larger. Accordingly, hopping is restrained by increasing the
relative permittivity. Thus, the charge retention time becomes
longer.
[0067] In the following, the results of measurement of polarization
energy and relative permittivity are described.
[0068] (Measurement 1)
[0069] A sample having a self-assembled film of terphenylthiol
formed on a gold substrate is observed with a scanning tunneling
microscope. As the top end of each molecule can be observed, a
needle probe of the scanning tunneling microscope is put close to
the top end of a molecule, and a bias is applied between the
substrate and the needle probe. In this manner, the electric
properties of a single molecule can be measured. Calculated from
the result of measurement of current, the polarization energy (the
activation energy) W.sub.1 is 0.36 eV. The relative permittivity
.epsilon..sub.1 of the terphenylthiol molecular group is 3.1 (a
literature-based value).
[0070] (Measurement 2)
[0071] A sample having a self-assembled film formed on a gold
substrate is observed with a scanning tunneling microscope. In the
self-assembled film, the compound ratio by weight of terphenylthiol
is 5% while the compound ratio by weight of hexanethiol is 95%.
Since the molecular chains of terphenylthiol are longer than those
of hexanethiol, a structure having the top end of a terphenylthiol
molecule protruding from the sample is observed. A needle probe of
the scanning tunneling microscope is put close to the top end of
the molecule, and a bias is applied between the substrate and the
needle probe. In this manner, the electric properties of a single
molecule can be measured. Calculated from the result of measurement
of current, the polarization energy (the activation energy) W.sub.2
is 0.22 eV. The relative permittivity .epsilon..sub.2 of the
hexanethiol is 2.3.
[0072] The effects of polarization energy can be confirmed by
Measurement 1 and Measurement 2. The following equation (5) is
established from the above measurement results:
W 1 W 2 = 0.36 0.22 = 1.64 . [ Equation 5 ] ##EQU00005##
[0073] Meanwhile, according to the equation (4), the following
equation (6) is established:
W 1 W 2 = 1 ( 1 + 2 ) 2 2 ( 2 + 2 ) 2 = 3.1 ( 3.1 + 2 ) 2 2.3 ( 2.3
+ 2 ) 2 = 80.631 48.668 = 1.66 [ Equation 6 ] ##EQU00006##
[0074] The equation (5) and the equation (6) are the same within
the margin of measurement error. In this manner, experiments
confirm that the polarization energy (the activation energy)
becomes larger as the relative permittivity is made higher.
[0075] As described above, according to this embodiment, clearing
of charges due to movement of charges by tunneling and hopping is
restrained. Accordingly, an organic molecular memory with excellent
charge retention properties can be realized.
[0076] The charge-storage molecular chains of this embodiment are
not limited to the molecular structures illustrated in FIG. 1, as
long as fused polycyclic groups are bound to the charge-storage
molecular chains.
[0077] FIGS. 9A and 9B are diagrams showing examples of the
molecular structures of the charge-storage molecular chains
according to this embodiment. FIG. 9A shows metalloporphyrin and
derivatives thereof. In the drawing, M represents a metal atom or a
metallic compound, such as iron (Fe), cobalt (Co), nickel (Ni), or
copper (Cu). Also, in the drawing, X and Y represent, independently
of each other, fused polycyclic groups such as hydrogen atoms or
anthracene, or electron-withdrawing substituents such as halogen
atoms, cyano groups, carbonyl groups, or carboxyl groups. However,
at least some of them are fused polycyclic groups.
[0078] FIG. 9B shows metallophthalocyanine and derivatives thereof.
In the drawing, M represents a metal atom or a metallic compound,
such as copper (Cu), cobalt (Co), iron (Fe), nickel (Ni), titanium
oxide (TiO), or aluminum chloride (AlCl). Also, in the drawing, X
and Y represent, independently of each other, fused polycyclic
groups such as hydrogen atoms or anthracene, or
electron-withdrawing substituents such as halogen atoms, cyano
groups, carbonyl groups, or carboxyl groups. However, at least some
of them are fused polycyclic groups.
[0079] Appropriate linkers in accordance with the material of the
conductive layer to be subjected to the binding are bound to part
of the charge-storage molecular chains of FIGS. 9A and 9B.
[0080] FIGS. 10A through 10F are diagrams showing examples of the
fused polycyclic groups bound to the charge-storage molecular
chains according to this embodiment. In the drawings, X, Y, Z, U,
V, and W represent, independently of each other, hydrogen atoms, or
electron-withdrawing substituents such as halogen atoms, cyano
groups, carbonyl groups, or carboxyl groups. M represents a binding
site with a charge-storage molecular chain.
[0081] As the fused polycyclic groups bound to the charge-storage
molecular chains, anthracene or derivatives thereof illustrated in
FIG. 10B are preferable, because anthracene or derivatives thereof
easily form symmetrical molecules, and increase the stability of
the functions of the organic molecular memory.
Second Embodiment
[0082] An organic molecular memory of this embodiment is the same
as the organic molecular memory of the first embodiment, except
that electron-withdrawing substituents are further bound to the
fused polycyclic groups bound to the charge-storage molecular
chains. In the following, the same explanations as those of the
substrate, electrodes, charge-storage molecular chains, fused
polycyclic groups, and the like of the first embodiment will not be
repeated.
[0083] FIG. 11 is a schematic cross-sectional view of a memory cell
portion of the organic molecular memory according to this
embodiment. In this embodiment, electron-withdrawing substituents
are further bound to the charge-storage molecular chains to which
fused polycyclic groups are bound in the organic molecular layer 16
of each memory cell portion.
[0084] As shown in FIG. 11, anthracene as fused polycyclic groups
is bound to zinc porphyrin, and cyano groups as
electron-withdrawing substituents are bound to the anthracene, for
example.
[0085] As the electron-withdrawing substituents are provided,
electric dipoles are formed in the charge-storage molecular chains
in this embodiment. The electric dipoles weaken the electric field
induced by the charges in the charge-storage molecular chains 16a.
Accordingly, the charge retention properties of the organic
molecular memory are further improved by the same effects as the
effects of the electronic polarization induced by the fused
polycyclic groups described in the first embodiment.
[0086] In other words, having electron-withdrawing substituents in
the molecules, the charge-storage molecular chains of this
embodiment have a flexible, large electric dipole moment. Because
of this, the relative permittivity of the organic molecular layer
16 can be made even higher than that achieved in a case where the
charge-storage molecular chains of FIG. 1 are used, for example. As
a result, the charge retention properties of the organic molecular
memory are further improved by the same effects as those described
in the first embodiment.
[0087] It should be noted that the electron-withdrawing
substituents are not limited to the above described cyano groups.
The electron-withdrawing substituents may be fluorine atoms (F),
chlorine atoms (Cl), bromine atoms (Br), iodine atoms (I), cyano
groups, nitro groups, amino groups, hydroxyl groups, carbonyl
groups, or carboxyl groups, for example. Highly anionic groups can
form large electric dipoles. Therefore, to increase the relative
permittivity, it is preferable to use fluorine atoms, chlorine
atoms, or cyano groups.
Third Embodiment
[0088] An organic molecular memory of this embodiment includes a
first conductive layer, a second conductive layer, and an organic
molecular layer interposed between the first conductive layer and
the second conductive layer. The organic molecular layer includes
first organic molecules with charge-storage molecular chains, and
second organic molecules with fused polycyclic groups.
[0089] In the organic molecular memory of the first embodiment, the
charge-storage molecular chains serving as memory elements have the
fused polycyclic groups. On the other hand, the organic molecular
memory of this embodiment includes organic molecules having the
fused polycyclic groups in the organic molecular layer, as well as
the charge-storage molecular chains serving as the memory elements
in the organic molecular layer. In this aspect, this embodiment
differs from the first embodiment. In the following, the same
explanations as those of the substrate, electrodes, charge-storage
molecular chains, electron-withdrawing substituents, and the like
of the first embodiment will not be repeated.
[0090] FIG. 12 is a schematic cross-sectional view of a memory cell
(molecular cell) portion of the organic molecular memory according
to this embodiment.
[0091] The organic molecular layer 16 is formed with charge-storage
molecular chains (the first organic molecules) 16a and organic
molecules (the second organic molecules) 16b with fused polycyclic
groups.
[0092] As shown in FIG. 12, the charge-storage molecular chains 16a
are zinc porphyrin, for example. The organic molecules 16b with
fused polycyclic groups are zinc porphyrin derivatives to which
anthracene is bound, for example.
[0093] In this embodiment, a memory cell is realized by using
changes in the charge-stored state of the charge-storage molecular
chains 16a. The electronic polarization in the organic molecules
16b with fused polycyclic groups weaken the electric field induced
by the charges in the charge-storage molecular chains 16a. In other
words, the relative permittivity of the organic molecular layer
becomes higher than that achieved in a case where the organic
molecular layer is formed only with the charge-storage molecular
chains 16a. As a result, the charge retention properties of the
organic molecular memory are improved by the same effects as those
described in the first embodiment.
[0094] To improve the charge retention properties of the organic
molecular memory, the relative permittivity is preferably 5.5 or
higher, or more preferably, 6.0 or higher, as in the first
embodiment.
[0095] The relative permittivity of the organic molecular layer 16
can be appropriately set by adjusting the molecular structures,
placement densities, and the like of the charge-storage molecular
chains 16a and the organic molecules 16b in the organic molecular
layer 16.
[0096] The charge-storage molecular chains 16a and the organic
molecules 16b of this embodiment are not limited to the above
described structures. Any molecular chains that have a function to
store charges in the molecular chains and can switch between a
state to store the charges and a state not to store the charges by
application and removal of external voltage suffice as the
charge-storage molecular chains 16a.
[0097] For example, organic molecules each having either of the
molecular structures illustrated in FIGS. 9A and 9B can be used. As
the fused polycyclic groups, any of the molecular structures
illustrated in FIGS. 10A through 10F can be used.
[0098] It should be noted that, in this embodiment, fused
polycyclic groups may be or may not be bound to the charge-storage
molecular chains 16a functioning as memory elements. Also, the
organic molecules 16b with fused polycyclic groups may not be used
as the molecules to realize the memory functions in cooperation
with the charge-storage molecular chains 16a.
[0099] In the example case described above, derivatives of the
charge-storage molecular chains 16a as the first organic molecules
are used as the second organic molecules to which fused polycyclic
groups are bound. As derivatives of the charge-storage molecular
chains 16a are used as the organic molecules 16b with fused
polycyclic groups, it is easy to form the organic molecular layer
16 as a self-assembled film having two kinds of organic molecules
mixed therein. However, the second organic molecules may not be
derivatives of the first organic molecules. The second organic
molecules may be any organic molecules to which fused polycyclic
groups are bound, other than charge-storage molecular chains.
Fourth Embodiment
[0100] An organic molecular memory of this embodiment is the same
as the organic molecular memory of the third embodiment, except
that electron-withdrawing substituents are further bound to the
fused polycyclic groups bound to the charge-storage molecular
chains. In the following, the same explanations as those of the
substrate, electrodes, charge-storage molecular chains, fused
polycyclic groups, and the like of the third embodiment will not be
repeated.
[0101] FIG. 13 is a schematic cross-sectional view of a memory cell
portion of the organic molecular memory according to this
embodiment. In this embodiment, electron-withdrawing substituents
are further bound to the organic molecules (the second organic
molecules) 16b with fused polycyclic groups in the organic
molecular layer 16 of each memory cell portion.
[0102] As shown in FIG. 13, anthracene as fused polycyclic groups
is bound to zinc porphyrin, and cyano groups as
electron-withdrawing substituents are bound to the anthracene, for
example.
[0103] As the electron-withdrawing substituents are provided,
electric dipoles are formed in the organic molecules 16b with fused
polycyclic groups in this embodiment. The electric dipoles weaken
the electric field induced by the charges in the charge-storage
molecular chains (the first organic molecules) 16a functioning as
memory elements. Accordingly, the charge retention properties of
the organic molecular memory are further improved by the same
effects as the effects of the electronic polarization induced by
the fused polycyclic groups described in the first embodiment.
[0104] As the second organic molecules have electron-withdrawing
substituents, the energy level is changed for the first organic
molecules having the memory functions, so that movement of charges
from the first organic molecules to the second organic molecules
can be restrained. Accordingly, the charge retention properties of
the organic molecular memory are also improved in this aspect.
Fifth Embodiment
[0105] An organic molecular memory of this embodiment is a
cross-point organic molecular memory using variable-resistance
molecular chains, unlike the organic molecular memory of the first
embodiment, which is a stacked-gate organic molecular memory using
charge-storage molecular chains. In the following, the same
explanations as those of the actions and effects to be achieved by
electronic polarization, the actions and effects to be achieved by
increasing relative permittivity, and the like described in the
first embodiment will not be repeated.
[0106] FIG. 15 is a schematic perspective view of the organic
molecular memory according to this embodiment. FIG. 16 is a
schematic cross-sectional view of a memory cell (molecular cell)
portion of the organic molecular memory.
[0107] The molecular memory of this embodiment is a cross-point
molecular memory. As shown in FIGS. 15 and 16, a lower electrode
interconnect (a first conductive layer) 22 is provided on an upper
portion of a substrate (not shown), for example. An upper electrode
interconnect (a second conductive layer) 24 is positioned so as to
intersect with the lower electrode interconnect 22. The rules in
design of the electrode interconnects specify 5 to 20 nm, for
example.
[0108] As shown in FIGS. 15 and 16, an organic molecular layer 26
is provided at an intersection portion between the lower electrode
interconnect 22 and the upper electrode interconnect 24, and in
between the lower electrode interconnect 22 and the upper electrode
interconnect 24. Variable-resistance molecular chains 26a form the
organic molecular layer 26. The thickness of the organic molecular
layer is 1 to 20 nm, for example.
[0109] The organic molecular layer 26 is provided at each of the
intersection points between lower electrode interconnects 22 and
upper electrode interconnects 24 as shown in FIG. 15, for example,
to form memory cells. With this arrangement, a memory cell array
formed with memory cells is realized.
[0110] In this embodiment, each organic molecular layer 26 is
designed to include organic molecules with fused polycyclic groups.
Accordingly, the relative permittivity of each organic molecular
layer 26 becomes higher. To improve the charge retention properties
of the organic molecular memory, the relative permittivity of each
organic molecular layer 26 is preferably 5.5 or higher, or more
preferably, 6.0 or higher.
[0111] As shown in FIG. 16, each organic molecular layer 26 of this
embodiment is formed with the variable-resistance molecular chains
26a. One end of each variable-resistance molecular chain 26a is
chemically bound to the lower electrode interconnect 22.
[0112] The lower electrode interconnect 22 is formed on a silicon
(Si) substrate (not shown) having the (110) plane as a surface, for
example. The lower electrode interconnect 22 is made of a metallic
material such as gold (Au). The face of the lower electrode
interconnect 22 in contact with the organic molecular layers 26 is
the (111) plane, for example. The upper electrode interconnect 24
is made of a metallic material such as molybdenum (Mo).
[0113] FIGS. 14A and 14B are diagrams showing molecular structures
of organic molecules in memory cell portions. FIG. 14A shows
organic molecules of this embodiment, and FIG. 14B shows organic
molecules in a case where fused polycyclic groups are not provided.
In this embodiment, the organic molecular layer 26 of each memory
cell portion contains organic molecules to which fused polycyclic
groups are bound.
[0114] Each of the variable-resistance molecular chains 26a forming
the organic molecular layers 26 of this embodiment has the
molecular structure shown in FIG. 14A, for example. The
variable-resistance molecular chain of FIG. 14A is a derivative of
the variable-resistance molecular chain without fused polycyclic
groups as shown in FIG. 14B, which is
4-[2-nitro-5-amino-4-(phenylethynyl)phenylethynyl]benzenethiol. A
variable-resistance molecular chain having the molecule structure
shown in FIG. 14B is also called a "tour wire".
[0115] A thiol group exists as a linker at one end of each of the
variable-resistance molecular chains of FIGS. 14A and 14B, and a
sulfur atom (S) and a gold atom (Au) in the surface of the lower
electrode interconnect 22 are chemically bound together. Here,
"linker" means a site that secures a molecule to an electrode (a
conductive layer) through chemical bond.
[0116] Gold atoms in the surface of the lower electrode 22 and
thiol groups are bound together in this manner, to form each
organic molecular layer 26 that is a so-called self-assembled
monolayer (SAM). Meanwhile, the other end of each
variable-resistance molecular chain 26a is not chemically bound to
molybdenum (Mo) atoms in the surface of the upper electrode 24.
[0117] Further, anthracene as a fused polycyclic group is bound to
the variable-resistance molecular chain 26a of FIG. 14A.
[0118] Here, each variable-resistance molecular chain 26a is a
molecule chain having a function to change its resistance,
depending on whether an electric field exists or whether charges
are injected thereinto. For example, each variable-resistance
molecular chain having the molecular structure shown in FIG. 14A or
14B can switch between a low-resistance state and a high-resistance
state through voltage application between both ends. Such changes
in the resistance state are used to realize a memory cell.
[0119] In this embodiment, the variable-resistance molecular chains
contain fused polycyclic groups, as shown in FIG. 14A. As the fused
polycyclic groups are provided in this manner, the
variable-resistance molecular chains of this embodiment have
electronic polarization. Because of this, the relative permittivity
of each organic molecular layer 26 becomes higher than that
achieved in a case where the variable-resistance molecular chain of
FIG. 14B is used, for example. As a result, the charge retention
properties of the organic molecular memory are improved as
described above. The relative permittivity of each organic
molecular layer in the case where the variable-resistance molecular
chain of FIG. 14B is approximately 3.0.
[0120] The relative permittivity of each organic molecular layer 26
can be appropriately set by adjusting the molecular structures,
placement density, and the like of the variable-resistance
molecular chains 26a in the organic molecular layer 26.
[0121] The variable-resistance molecular chains of this embodiment
do not necessarily have the molecular structure illustrated in FIG.
14A, as long as fused polycyclic group are bound to the
variable-resistance molecular chains. First, as variable-resistance
molecular chains to which fused polycyclic groups can be bound,
molecules having a .pi.(pi)-conjugated system extending in a
one-dimensional direction can be used. For example, it is possible
to use
4-[2-nitro-5-amino-4-(phenylethynyl)phenylethynyl]benzenethiol
illustrated in FIG. 14B or derivatives thereof, or paraphenylene
derivatives, oligothiophene derivatives, oligopyrrole derivatives,
oligofuran derivatives, paraphenylenevinylene derivatives, or the
like.
[0122] FIGS. 17A through 17F show examples of molecular units that
can form molecules each having the n-conjugated system extending in
a one-dimensional direction. FIG. 17A shows paraphenylene, FIG. 17B
shows thiophene, FIG. 17C shows pyrrole, FIG. 17D shows furan, FIG.
17E shows vinylene, and FIG. 17F shows alkyne.
[0123] In a case where the length of the n-conjugated system is
short, electrons injected from electrodes do not stay on the
molecules, and the electrons simply pass by the molecules.
Therefore, to store charges, each molecule preferably has a certain
length. The number of --CH.dbd.CH-- unit in the molecule in a
one-dimensional direction is preferably 5 or more. This is
equivalent to 3 or more in the case of benzene rings
(paraphenylene).
[0124] In a case where the .pi.-conjugated system is long, a
voltage drop due to charge conduction among the molecules becomes a
problem. Therefore, the number of a --CH.dbd.CH-- unit in the
molecule in a one-dimensional direction is preferably 20 or less
(ten benzene rings=twice the spread width of polaron as the carrier
of the n-conjugated system).
[0125] The materials of the above described electrodes (the
conductive layers) forming the organic molecular memory are not
particularly limited to the above described gold and molybdenum. In
the electrode (the lower electrode interconnect 22 in this
embodiment) to which the linker at one end of each
variable-resistance molecular chain 26a is chemically bound, at
least the regions to which the variable-resistance molecular chains
26a are chemically bound are preferably made of a material with
which the one end of each of the variable-resistance molecular
chains 26a easily forms a chemical bond, so as to form a
self-assembled film. Also, in the electrode (the upper electrode
interconnect 24 in this embodiment) on the side of the other end of
each of the variable-resistance molecular chains 26a, at least the
regions facing the variable-resistance molecular chains 26a are
preferably made of a material that does not easily form chemical
bonds with one ends of the variable-resistance molecular chains
26a, so as to form an organic molecular layer by using a
self-organizing process after the electrode formation.
[0126] The preferred electrode material varies depending on the
structure of the linker at the one end of each variable-resistance
molecular chain 26a. For example, in a case where the one end is a
thiol group as shown in FIGS. 14A and 14B, the electrode on the
chemical bond side is preferably gold (Au), silver (Ag), copper
(Cu), tungsten (W), tungsten nitride (WN), tantalum nitride (TaN),
or titanium nitride (TiN). Among those materials, gold (Au), silver
(Ag), and tungsten (W), which easily form chemical bonds, are
particularly preferable. Meanwhile, the electrode at the other end
is preferably tantalum (Ta), molybdenum (Mo), molybdenum nitride
(MoN), or silicon (Si).
[0127] In a case where the one end is an alcohol group or a
carboxyl group, for example, the electrode on the chemical bond
side is preferably tungsten (W), tungsten nitride (WN), tantalum
(Ta), tantalum nitride (TaN), molybdenum (Mo), molybdenum nitride
(MoN), or titanium nitride (TiN). Among those materials, tantalum
(Ta), tantalum nitride (TaN), molybdenum nitride (MoN), and
titanium nitride (TiN), which easily form chemical bonds, are
particularly preferable. Meanwhile, the electrode at the other end
is preferably gold (Au), silver (Ag), copper (Cu) , or silicon
(Si).
[0128] In a case where the one end is a silanol group, for example,
the electrode on the chemical bond side is preferably silicon (Si)
or a metal oxide. Meanwhile, the electrode at the other end is
preferably gold (Au), silver (Ag), copper (Cu), tungsten (W),
tungsten nitride (WN), tantalum (Ta), tantalum nitride (TaN),
molybdenum (Mo), molybdenum nitride (MoN), or titanium nitride
(TiN).
[0129] Alternatively, graphene or carbon nanotube can be used as
the electrode material.
Sixth Embodiment
[0130] An organic molecular memory of this embodiment includes a
first conductive layer, a second conductive layer, and an organic
molecular layer interposed between the first conductive layer and
the second conductive layer. The organic molecular layer includes
second organic molecules with variable-resistance molecular chains,
and first organic molecules with fused polycyclic groups.
[0131] While the organic molecular memory of the fifth embodiment
includes variable-resistance molecular chains as memory elements
with fused polycyclic groups, the organic molecular memory of this
embodiment includes organic molecules with fused polycyclic groups
in each organic molecular layer, as well as the variable-resistance
molecular chains as the memory elements in each organic molecular
layer. In this aspect, this embodiment differs from the fifth
embodiment. In the following, the same explanations as those of the
substrate, electrodes, variable-resistance molecular chains, fused
polycyclic groups, and the like of the fifth embodiment will not be
repeated.
[0132] FIG. 18 is a schematic cross-sectional view of a memory cell
(molecular cell) portion of the organic molecular memory according
to this embodiment.
[0133] Each organic molecular layer 26 is formed with
variable-resistance molecular chains (the second organic molecules)
26a and organic molecules (the first organic molecules) 26b with
fused polycyclic groups.
[0134] The variable-resistance molecular chains 26a are
4-[2-nitro-5-amino-4-(phenylethynyl)phenylethynyl]benzenethiol
illustrated in FIG. 14B, for example. The organic molecules 26b
with fused polycyclic groups are derivatives of
4-[2-nitro-5-amino-4-(phenylethynyl)phenylethynyl]benzenethiol
having anthracene bound thereto, as shown in FIG. 14A, for
example.
[0135] In this embodiment, changes in the resistance states of the
variable-resistance molecular chains 26a are used to realize memory
cells. The electronic polarization in the organic molecules 26b
with fused polycyclic groups weaken the electric field induced by
the charges in the variable-resistance molecular chains 26a.
Accordingly, the charge retention properties of the organic
molecular memory are improved by the same effects as those
described in the first embodiment.
[0136] In this embodiment, the organic molecules 26b include fused
polycyclic groups. Because of this, the relative permittivity of
each organic molecular layer becomes higher than that in a case
where each organic molecular layer is formed only with the
variable-resistance molecular chains 26a, for example. As a result,
the charge retention properties of the organic molecular memory are
improved by the same effects as those described in the first
embodiment.
[0137] To improve the charge retention properties of the organic
molecular memory, the relative permittivity is preferably 5.5 or
higher, or more preferably, 6.0 or higher, as in the first
embodiment.
[0138] The relative permittivity of each organic molecular layer 26
can be appropriately set by adjusting the molecular structures,
placement densities, and the like of the variable-resistance
molecular chains 26a and the organic molecules 26b in each organic
molecular layer 26.
[0139] The variable-resistance molecular chains 26a and the organic
molecules 26b of this embodiment are not limited to the above
described structures. Any molecular chains that have a function to
change their resistance depending on the existence of an electric
field or injection of charges suffice as the variable-resistance
molecular chains 26a.
[0140] It should be noted that, in this embodiment, fused
polycyclic groups may be or may not be bound to the
variable-resistance molecular chains 26a. Also, the organic
molecules 26b with fused polycyclic groups may not be used as the
molecules to realize the memory functions in cooperation with the
variable-resistance molecular chains 26a.
[0141] In the example case described above, derivatives of the
variable-resistance molecular chains 26a as the second organic
molecules are used as the first organic molecules to which fused
polycyclic groups are bound. In other words first and second
organic molecules have the variable-resistance molecular chains of
same carbon skeleton. As derivative structures of the
variable-resistance molecular chains 26a are used as the organic
molecules 26b with fused polycyclic groups as described above, it
is easy to form each organic molecular layer 26 as a self-assembled
film having two kinds of organic molecules mixed therein. However,
the first organic molecules may not be derivatives of the second
organic molecules. The first organic molecules may be any organic
molecules to which fused polycyclic groups are bound, other than
variable-resistance molecular chains.
Seventh Embodiment
[0142] An organic molecular memory of this embodiment is the same
as the organic molecular memory of the sixth embodiment, except
that electron-withdrawing substituents are further bound to the
fused polycyclic groups of the first organic molecules having the
fused polycyclic groups. In the following, the same explanations as
those of the substrate, electrodes, variable-resistance molecular
chains, fused polycyclic groups, and the like of the sixth
embodiment will not be repeated.
[0143] FIG. 19 is a schematic cross-sectional view of a memory cell
portion of the organic molecular memory according to this
embodiment. In this embodiment, electron-withdrawing substituents
are further bound to the organic molecules (the first organic
molecules) 26b with fused polycyclic groups in the organic
molecular layer 26 of each memory cell portion.
[0144] As shown in FIG. 19, anthracene as fused polycyclic groups
is bound to the organic molecules 26b, and cyano groups as
electron-withdrawing substituents are bound to the anthracene, for
example.
[0145] As the electron-withdrawing substituents are provided,
electric dipoles are formed in the organic molecules 26b with fused
polycyclic groups in this embodiment. The electric dipoles weaken
the electric field induced by the charges in the
variable-resistance molecular chains (the second organic molecules)
26a functioning as memory elements. Accordingly, the charge
retention properties of the organic molecular memory are further
improved.
[0146] As the sfirst organic molecules have electron-withdrawing
substituents, the energy level is changed for the second organic
molecules having the memory functions, so that movement of charges
from the second organic molecules to the first organic molecules
can be restrained. Accordingly, the charge retention properties of
the organic molecular memory are also improved in this aspect.
EXAMPLES
[0147] In the following, examples are described.
Example
[0148] As shown in FIG. 13, a self-assembled film of molecules in
which anthracene derivatives as the fused polycyclic groups for
causing the porphyrin to contribute to electronic polarization
(molecular polarization) are bound to porphyrin derivatives serving
to store charges, is formed on a silicon substrate. Cyano groups
are bound to the anthracene, to adjust the energy level and prevent
charge transfers between the anthracene and the porphyrin
derivatives serving to store charges.
[0149] A 5-nm thick silicon oxide film is formed on the
self-assembled film. A gold electrode is deposited on the silicon
oxide film, to form memory elements.
[0150] The bias voltage dependence of the capacitance between the
silicon substrate and the gold electrode is measured. With the
silicon substrate being the reference, the capacitance-bias voltage
dependence in the 0 to -5 V region is measured before and after a
voltage of -15 V is applied to the gold electrode. As charges are
stored in the molecules, the capacitance-bias voltage dependence
shifts 0.9 V, which is equivalent to the potential generated by the
charges.
[0151] The dependence of the shift on the time that has elapsed
since writing is measured. In this manner, the charge retention
time of the molecules can be estimated. In this case, the charges
decrease by half in approximately one hour.
Comparative Example
[0152] A self-assembled film of the porphyrin derivative shown in
FIG. 3 is formed on a silicon substrate, and a 5-nm thick silicon
oxide film is formed on the self-assembled film. Further, a gold
electrode is deposited on the silicon oxide film, to form memory
elements.
[0153] After that, the bias voltage dependence of the capacitance
between the silicon substrate and the gold electrode is measured.
With the silicon substrate being the reference, the
capacitance-bias voltage dependence in the 0 to -5 V region is
evaluated before and after a voltage of -15 V is applied to the
gold electrode. As charges are stored in the molecules, the
capacitance-bias voltage dependence shifts 1.1 V, which is
equivalent to the potential generated by the charges.
[0154] The dependence of the shift on the time that has elapsed
since writing is measured. In this manner, the charge retention
time of the molecules can be estimated. In this case, the charges
decrease by half in approximately ten seconds, though the precision
is not necessarily high because the time is short.
[0155] As can be seen from a comparison between Example and
Comparative Example, the charge retention time is improved by
introducing fused polycyclic groups into the organic molecules in
the organic molecular layer.
[0156] In the above described embodiments and Example, the organic
molecules forming organic molecular layers include charge-storage
molecular chains or variable-resistance molecular chains, and
organic molecules with fused polycyclic groups. However, it should
be noted that organic molecular layers may contain other organic
molecules as well as charge-storage molecular chains or
variable-resistance molecular chains, and organic molecules with
fused polycyclic group.
[0157] Also, each organic molecular memory is not necessarily of a
stacked-gate type or a cross-point type, but may have any other
structure such as a three-dimensional structure.
[0158] While certain embodiments have been described, these
embodiments have been presented by way of example only, and are not
intended to limit the scope of the inventions. Indeed, organic
molecular memories described herein may be embodied in a variety of
other forms; furthermore, various omissions, substitutions and
changes in the form of the devices and methods described herein may
be made without departing from the spirit of the inventions. The
accompanying claims and their equivalents are intended to cover
such forms or modifications as would fall within the scope and
spirit of the inventions.
* * * * *