This application is based upon and claims the benefit of priority from Japanese Patent Application No. 2014-193009, filed on Sep. 22, 2014, the entire contents of which are incorporated herein by reference.
Embodiments described herein relate generally to organic molecular memories.
Using organic molecules in a memory cell reduces the size of the memory cell because the size of the organic molecules themselves is small. Consequently, the memory density can be increased. Therefore, it has been attempted to interpose, between upper and lower electrodes, molecules having the capability of being changed in resistance by the presence or absence of an electric field or the injection of charges, and thereby to make a memory cell. The molecules changes the resistance by a voltage applied across the upper and lower electrodes. By utilizing the change of resistance, the memory cell stores a data. And, by detecting a difference in flowing currents, a data stored in the memory cell can be read. It is important in an organic molecular memory to reduce characteristics variations between a plurality of memory cells in realizing an organic molecular memory with high operational stability.
An organic molecular memory in an embodiment includes a first electrode having a first work function; a second electrode having a second work function; and an organic molecular layer provided between the first electrode and the second electrode, the organic molecular layer containing a first organic molecule chemically bonded to the first electrode, the first organic molecule having a resistance-change type molecular chain, and the first organic molecule having a first energy level higher than the first work function, and a second organic molecule chemically bonded to the second electrode and the second organic molecule having a second energy level higher than the second work function and lower than the first energy level.
In the description, the same or similar members are denoted by the same reference numerals, and redundant descriptions of them will not be made.
In the description, the words “upper” and “lower” are used to express a relative positional relationship between components or the like. In the description, the concept of the words “upper” and “lower” does not necessarily refer to a relationship with respect to the direction of gravity.
In the description, the word “resistance-change type molecular chain” means a molecular chain having the capability of being changed in resistance by the presence or absence of an electric field or the injection of charges.
In the description, the word “chemical bond” is a concept referring to one of a covalent bond, an ionic bond, and a metallic bond, and is a concept excluding a hydrogen bond and a bond by a Van der Waals force.
Hereinafter, an embodiment will be described with reference to the drawings.
An organic molecular memory in this embodiment includes a first electrode having a first work function, a second electrode having a second work function, and an organic molecular layer provided between the first electrode and the second electrode and containing a first organic molecule chemically bonded to the first electrode, having a resistance-change type molecular chain, and having a first energy level higher than the first work function, and a second organic molecule chemically bonded to the second electrode and having a second energy level higher than the second work function and lower than the first energy level.
In the organic molecular memory in this embodiment, the difference in energy that needs to be equated between the first organic molecule and the second electrode to pass current through the organic molecular layer is substantially the energy difference between the first organic molecule and the second organic molecule. Consequently, the energy difference between the first organic molecule and the second electrode side is prevented from being affected by the surface state of the second electrode. Consequently, current flowing through the organic molecular layer is stabilized. Therefore, an organic molecular memory with high operational stability is realized.
The organic molecular memory in this embodiment is a resistance-change type organic molecular memory of a cross-point type. As shown in
As shown in
The organic molecular layer 16 is provided at each of the intersections of lower electrode wires 12 and upper electrode wires 14 as shown in
The organic molecular layer 16 in this embodiment includes a first organic molecule 16a chemically bonded to the first electrode 12, and a second organic molecule 16b chemically bonded to the second electrode 14. The first organic molecule 16a has a resistance-change type molecular chain chemically bonded to the first electrode 12. The first organic molecule 16a and the second organic molecule 16b are not chemically bonded.
The first electrode 12 has a first work function φ1, and the second electrode 14 has a second work function φ2. The work function is an energy difference between the vacuum level and the Fermi level.
The first organic molecule 16a has a first energy level e1, and the second organic molecule 16b has a second energy level e2. Here, when holes are held in a resistance-change type molecular chain, for example, the energy level of the organic molecule is an energy difference between the vacuum level and the highest occupied molecular orbital (HOMO) level. When the energy level of an organic molecule is a difference between the vacuum level and the HOMO level, it is referred to as an ionization potential. When electrons are held in a resistance-change type molecular chain, for example, the energy level of the organic molecule is an energy difference between the vacuum level and the lowest unoccupied molecular orbital (LUMO) level. When the energy level of an organic molecule is an energy difference between the vacuum level and the LUMO level, it is also referred to as an electron withdrawing force.
In this embodiment, the first organic molecule 16a and the second organic molecule 16b are not chemically bonded, so that different organic molecules can have independent energy levels.
The first energy level e1 is higher than the first work function φ1. The second energy level e2 is higher than the second work function φ2, and lower than the first energy level e1.
One ends of the first organic molecules 16a are chemically bonded to the lower electrode wire 12. The first organic molecules 16a extend from the lower electrode wire 12 toward the upper electrode wire 14.
One ends of the second organic molecules 16b are chemically bonded to the upper electrode wire 14. The second organic molecules 16b extend from the upper electrode wire 14 toward the lower electrode wire 12.
The lower electrode wire 12 is formed on a silicon (Si) substrate (not shown), for example. The lower electrode wire 12 is tungsten (W), a metallic material, for example. The upper electrode wire 14 is molybdenum (Mo), a metallic material, for example.
The first organic molecule 16a has 4-[2-nitro-5-amino-4-(phenylethynyl)phenylethynyl]benzenethiol as a resistance-change type molecular chain. The resistance-change type molecular chain shown in
The first organic molecule 16a has a thioether group (—S—) as a linker at one end. A sulfur atom (S) is chemically bonded to a surface of the lower electrode wire 12. Here, a linker means a chemically modifying group having a function of fixing a molecule to an electrode (conductive layer) by chemical bonding. The first organic molecules 16a form a so-called self-assembled monolayer (SAM).
The second organic molecule 16b has an alkyl chain (—(CH2)n-1—(CH3)) in which n is from 3 to 11, and a thioether group (—S—) as a linker at one end. A sulfur atom (S) is chemically bonded to a surface of the upper electrode wire 14. The second organic molecules 16b form a so-called self-assembled monolayer (SAM). The linker of the first organic molecule 16a and the linker of the second organic molecule 16b may be the same or different. In terms of selectively bonding different organic molecules to electrodes on different sides, the linker of the first organic molecule 16a and the linker of the second organic molecule 16b are desirably different.
Next, functions and effects of this embodiment will be described.
An organic molecular layer 16 in the comparative embodiment is different from that in the embodiment in that it does not include the second organic molecules 16b in the embodiment.
The current-voltage characteristics show a similar tendency such as that of current rises in a region where the absolute value of voltage is 1[V] or more. However, for example, current at 2[V] is greatly different between them, and is inconsistent. Further, variations with time are seen in measurements of the same organic molecule.
The cause of the above-described variations in initial characteristics of the different organic molecules and variations with time of the same organic molecule is considered to lie in the surface state of the second electrode 14. Specifically, the cause is considered to lie in a change in the Fermi level due to surface oxidation of the second electrode 14, or a change in the Fermi level due to polycrystallization in the second electrode 14 during the production of the second electrode 14. The same crystal has different Fermi levels, depending on the orientation.
When current is passed through the organic molecular layer 16, a voltage is applied across the first electrode 12 and the second electrode 14 to shift the Fermi levels of the electrodes. For example, the first HOMO (first energy level ed of the first organic molecule 16a is brought into agreement with the second Fermi level (Ef2) of the second electrode 14 in
Therefore, when the Fermi level of the second electrode 14 varies, the rise of current across the first organic molecule 16a and the second electrode 14 varies, resulting in variations in the current-voltage characteristics. Consequently, the operating current of the memory cell varies.
In this embodiment, the second organic molecules 16b chemically bonded to the second electrode 14 are provided between the first organic molecules 16a and the second electrode 14. Consequently, the energy difference between the first organic molecules 16a and the second electrode 14 when current is passed through the organic molecular layer 16 is effectively the energy difference between the first organic molecules 16a and the second organic molecules 16b.
The second energy level e2 of the second organic molecules 16b is not affected by the surface state of the second electrode 14, for example. Further, unlike the Fermi level in the surface of the second electrode 14, the second energy level e2 of the second organic molecules 16b has no variation factors.
Consequently, the energy difference between the first organic molecules 16a and the second electrode 14 side is stabilized. Thus, current flowing through the organic molecular layer 16 and the operating current of the memory cell are stabilized.
Further, in this embodiment, the second organic molecules 16b also function as an electrical resistance, increasing the resistance of the organic molecular layer 16. Consequently, leak current of the memory cell is reduced, and the signal/noise (S/N) ratio of the operating current of the memory cell is increased.
In terms of reducing a barrier between the second organic molecules 16b and the second electrode 14, the second energy level e2 is desirably higher than the second work function φ2. In terms of reducing a barrier between the first organic molecules 16a and the second organic molecules 16b, the second energy level e2 is desirably lower than the first energy level e1. In terms of preventing the memory cell from being normally on, the first energy level e1 is desirably higher than the first work function φ1.
It is desirable that a second blocking coefficient p2 expressed by expression (2) wherein φ2 is a second work function, e2 is a second energy level, and L2 is the length of a second organic molecule, is lower than a first blocking coefficient p1 expressed by expression (1) wherein φ1 is a first work function, e1 is a first energy level, and L1 is the length of a first organic molecule. L1 and L2 are lengths including linkers. In other words, the first blocking coefficient p1 is higher than a second blocking coefficient p2.
p1=(φ1−e1)1/2×L1 (1)
p2=(φ2−e2)1/2×L2 (2)
By the second blocking coefficient p2 being lower than the first blocking coefficient p1, the electrical resistance of the first organic molecules 16a becomes greater than the electrical resistance of the second organic molecules 16b. Consequently, the value of current flowing through the memory cell is mainly controlled by the first organic molecules 16a having the resistance-change type molecular chains. Consequently, the data detection accuracy of the memory cell is increased.
Further, according to this embodiment, the second organic molecules 16b also function as a surface protective layer of the second electrode 14, reducing a change in the state of the surface of the second electrode 14 due to oxidation or the like. Consequently, for example, a change in the resistance of the second electrode 14 itself, or in the resistance between the second organic molecules 16b and the second electrode 14 due to the surface oxidation of the second electrode 14 or the like is reduced.
The second organic molecules 16b desirably contain an alkyl chain with a carbon number of three or more to eleven or less. Exceeding the above range can cause too great an electrical resistance of the second organic molecules 16b, causing too small an operating current of the memory cell. Falling below the above range can deteriorate the function as a surface protective layer of the second electrode 14. Further, it can cause too large a leak current. The second organic molecules 16b more desirably contain an alkyl chain with a carbon number of five or more to eight or less.
In terms of having the resistance change characteristics, the first organic molecules 16a desirably contain a one-dimensional or pseudo-one-dimensional π-conjugated system chain to which an electron withdrawing group or an electron donating group is bonded in a direction other than a straight chain axis direction. The π-conjugated system chain contains either of a carbon compound selected from a group consisting of an acetylene skeleton, a diethylene skeleton, and a phenylene ring, or a heterocyclic compound selected from a thiophene ring, a pyrrole ring, and a furan ring. The carbon number of the π-conjugated system chain is desirably six or more to forty or less.
The electron withdrawing group bonded to the π-conjugated system chain is, for example, a nitro group (—NO2), halogen (—F, —Cl, —Br, —I), a cyano group (—C≡N), a carbonyl group (—C(═O)—), a sulfonyl group (—S(═O)2—), or a trialkylamino group (—N−R3).
The electron donating group bonded to the π-conjugated system chain is, for example, an alkoxy group (—OR), a hydroxyl group (—OH), an amino group (—NH2), an alkylamino group (—NHR), a dialkylamino group (—NR2), or an amide group (—NHCOR).
In terms of having rectification, the first organic molecule 16a is desirably described by molecular formula (1). Having rectification, resistance-change type molecular chains can also have a diode function required for each memory cell to realize a cross-point type memory cell. Thus, further scaling-down of a memory cell can be achieved.
wherein, P is a one-dimensional or a pseudo-one-dimensional π-conjugated system chain, the π-conjugated system chain having an electron withdrawing group or an electron donating group bonded in a direction other than a straight chain axis direction, R1 is an amino group (—NH2) or a nitro group (—NO2), R2 to R4 are hydrogen atoms or methyl groups (—CH3), and L is a chemically modifying group chemically bonding a first organic molecule to a first electrode.
In terms of having rectification, the first organic molecule 16a is desirably described by molecular formula (2):
wherein P is a one-dimensional or pseudo-one-dimensional π-conjugated system chain, the π-conjugated system chain having an electron withdrawing group or an electron donating group bonded in a direction other than a straight chain axis direction, the π-conjugated system chain having an electron withdrawing group or an electron donating group bonded in a direction other than a straight chain axis direction. The one-dimensional or pseudo-one-dimensional π-conjugated system chain may be an oligophenylene ethynylene skeleton or an oligophenylenevinylene skeleton. L is a chemically modifying group chemically bonding a first organic molecule to a first electrode.
In terms of having rectification, the first organic molecule 16a is desirably described by molecular formula (3):
wherein P is a one-dimensional or pseudo-one-dimensional π-conjugated system chain, the π-conjugated system chain having an electron withdrawing group or an electron donating group bonded in a direction other than a straight chain axis direction. The one-dimensional or pseudo-one-dimensional π-conjugated system chain may be an oligophenyleneethynylene skeleton or an oligophenylenevinylene skeleton. L is a chemically modifying group chemically bonding a first organic molecule to a first electrode.
Further, L in molecular formulae (1), (2), and (3) is desirably a chemically modifying group selected from a group consisting of a thioether group (—S—), a dialkylsilylether group (—SiR2O—), an ether group (—O—), a phosphonate ester group (—PO3—), an ester group (—COO—), and an azo group (—N2—), in terms of stably chemically bonding the first organic molecule 16a to the first electrode 12.
The first electrode 12 or the second electrode 14 desirably contains a metal selected from a group consisting of gold (Au), silver (Ag), copper (Cu), platinum (Pt), palladium (Pd), iron (Fe), tungsten (W), tungsten nitride (WN2), tantalum (Ta), tantalum nitride (TaN), molybdenum (Mo), molybdenum nitride (MoN), and titanium nitride (TiN).
In terms of facilitating the manufacturing of the organic molecular memory, the first electrode 12 and the second electrode 14 desirably contain different metals.
As above, according to this embodiment, the current-voltage characteristics are less likely to be affected by the surface state of the second electrode 14. Consequently, current flowing through an organic molecular layer, that is, the operating current of a memory cell is stabilized. Thus, an organic molecular memory with high operational stability can be realized.
Hereinafter, examples will be described.
A first organic molecule was fixed onto a gold (Au) substrate with a (111) surface by bonding gold and a sulfur atom using the organic molecule shown in
Other than using the organic molecules shown in
Other than not fixing an octaalkyl chain to a tungsten probe, sample preparations and measurements similar to those in Example 1 were performed.
Other than using the organic molecules shown in
In any of Examples 1 to 9, it was confirmed that the current-voltage characteristics have hysteresis to function as a resistance-change type memory. Further, in any of Examples 1 to 9, a noise reduction and a leak current reduction were confirmed, compared to Comparative Examples 1 to 9 as the respective comparative examples.
In the graphs of current-voltage characteristics in
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, the organic molecular memory 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.
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