JP4674912B2 - Memory element - Google Patents

Description

  The present invention relates to a memory device.

  Memory elements include a dynamic random access memory (DRAM) that is a volatile memory and a magnetic random access memory (MRAM) that is a nonvolatile memory. The advantages of MRAM are that it is non-volatile, consumes less energy and has higher read and write speeds than DRAM. A disadvantage of MRAM is that the smaller the size of the MRAM cell, the greater the magnetic field required to switch the magnetization of the free layer, ie the greater the power consumption of the element.

  In recent years, a non-volatile memory element using a carbon nanotube as a channel, which has attracted much attention as a new material, is known (for example, see Patent Document 1).

JP 2006-210910 A (Claim 1, Claim 11 etc.)

  However, a nonvolatile memory element using the carbon nanotube as a channel has not been put into practical use.

  Incidentally, as one of the special structures of carbon nanotubes, there is a so-called carbon nanopeapod in which fullerene molecules are encapsulated in the internal space of single-walled carbon nanotubes, but the electrical properties of this carbon nanopeapod have not been clarified yet. This is because it is difficult to produce high-quality carbon nanopeapods and the electronic structure is extremely complicated because it contains many fullerene molecules. In addition, when fabricating an FET structure using carbon nanopeapods as a channel in order to investigate the electrical characteristics of carbon nanopeapods, the characteristics of carbon nanopeapods are extremely sensitive to process damage during FET structure formation. The fact that the properties of carbon nanopeapods cannot be investigated is one reason why their physical properties have not been elucidated. Therefore, for these reasons, quantum devices using carbon nanopeapods as channels are difficult to produce and have not yet been realized.

  An object of the present invention is to solve the above-mentioned problems of the prior art, and an object of the present invention is to provide a small-sized and large-capacity memory element using a carbon nanopeapod.

The memory element of the present invention is a memory element having carbon nanopeapods composed of single-walled carbon nanotubes containing fullerene molecules, and the carbon nanopeapods are carbon nanopeapods in the form of bundles of 2 to 50 in number. , Placed on an insulating layer stacked on a back gate electrode, and connected to a source electrode and a drain electrode provided at a distance of 500 nm to 1000 nm, and the source electrode, the drain electrode, and the carbon nanopeapod The interfacial resistance between them is greater than 25.6 kΩ and 1 MΩ or less, and single electrons are injected into the fullerene molecules encapsulated by applying 1 to 5 V from the back gate electrode to the carbon nanopeapod, The fullerene molecule retains an injected single electron Characterized in that it is configured such that the memory cell holding the memory information.

  According to such a configuration, the memory element of the present invention can be very downsized as compared with the conventional memory element. Further, with this configuration, when a voltage is applied from the back gate electrode, single electrons are injected into the fullerene molecule in a region where the back gate voltage is higher than a threshold value, and single electrons are injected in a region where the back gate voltage is lower than the threshold value. Is injected into the single-walled carbon nanotube. Therefore, by controlling the back gate voltage, single electrons can be injected into the fullerene molecule as a memory cell, and the “0” and “1” states can be expressed by the spin moment of this single electron. Can be memory cells.

Here, the source electrode and the drain electrode are provided apart from each other by 500 nm or more and less than 1000 nm . When the predetermined distance is 500 nm or more and less than 1000 nm, it is possible to inject a single electron into the carbon nanopeapod as a channel.

Further, the interface resistance between the metal electrode and the carbon nanopeapod is greater than 25.6 kΩ and 1 MΩ or less. When the interface resistance is greater than 25.6 kΩ and 1 MΩ or less, it is possible to inject single electrons into the carbon nanopeapod as a channel of the memory element. On the other hand, when the interfacial resistance is 25.6 kΩ or less, the resistance value is too low, so that one or more electrons flow into the carbon nanopeapod and it is impossible to inject only a single electron into the carbon nanopeapod.

Also, that it is structured as a single electron is injected into the fullerene molecule contained by. 1 to 5 V is applied from the back gate electrode. By applying a voltage from the back gate electrode in this range, single electrons can be injected into the fullerene molecules as described above.
In addition, as a preferred embodiment of the present invention, it is mentioned that the single-walled carbon nanotube of the carbon nanopeapod exhibits metallic conduction.
The carbon nano peapod is Ru bundle-like carbon nano peapod der below 50 or more two. This is because a single electron can be injected into the carbon nanopeapod by being a bundle-like carbon nanopeapod composed of two or more and 50 or less carbon nanopeapods. On the other hand, if the number is more than 50, the number of carbon nanopeapods is too large, and the single electronic properties are averaged and disappear.

  According to the memory device of the present invention, since fullerene molecules are used as memory cells in the carbon nanopeapod, the entire device is small and easy to integrate. Further, by injecting single electrons into fullerene molecules and using them as memory cells, it is possible to achieve an excellent effect that the capacity can be increased and the power consumption is small.

  An embodiment of a memory element of the present invention will be described with reference to FIG. FIG. 1 is a schematic cross-sectional view of a memory element.

  The memory element 1 is formed by laminating a metal layer 11, an Si layer 12, a carbon nanopeapod 13, and metal electrodes 14a and 14b in this order.

  The metal layer 11 functions as a back gate electrode in the memory element 1. The metal layer 11 is made of Au. In addition to Au, the metal layer 11 may be formed of titanium (Ti), for example. The thickness of the metal layer 11 may be about 100 nm to 500 nm, for example.

  A Si layer 12 is laminated on the metal layer 11, and a Ti layer 111 is formed between the metal layer 11 and the Si layer 12 in order to improve adhesion between the metal layer 11 and the Si layer 12. ing. The Ti layer 111 may have any thickness as long as adhesion is secured, and is formed, for example, with a thickness of about 20 nm to 50 nm.

  As the Si layer 12, a known p-type Si substrate (having a thickness of about 400 nm to 1000 nm, for example) itself can be used.

An SiO ₂ layer 121 as an insulating layer is formed on the Si layer 12, and the carbon nanopeapod 13 is disposed on the SiO ₂ layer 121. The thickness of the SiO ₂ layer 121 may be about 100 nm to ₂₀₀ nm, for example.

The carbon nanopeapod 13 functions as a transistor channel in the memory element 1. The carbon nanopeapod 13 is a single-walled carbon nanotube 131 in which fullerene (not shown) is included. The single-walled carbon nanotube 131 is a single-walled carbon nanotube, and may be a metal nanotube or a semiconductor nanotube. Examples of the metal nanotube include those having a chirality of (10, 10). The fullerene functions as a memory cell in the memory element 1, and examples thereof include C ₆₀ and C ₈₂ . The carbon nanopeapod 13 is preferably a high quality one having as few defects as possible (for example, a five-membered ring or the like is mixed). The carbon nanopeapod 13 may be a single carbon nanopeapod, or may be a bundle of a plurality of carbon nanopeapods. However, in this case, those composed of 2 or more and 50 or less carbon nanopeapods 13 are preferable. If the number is more than 50, the single electronic characteristics described later cannot be obtained, and the cell does not function.

Two metal electrodes 14a and 14b are formed apart from each other so as to cover the end portion of the carbon nanopeapod 13 and the SiO ₂ layer 121. The metal electrode 14 a functions as a source electrode in the memory element 1, and the metal electrode 14 b functions as a drain electrode in the memory element 1. The distance H between the end of the metal electrode 14a and the end of the metal electrode 14b is about 600 nm. This distance H is preferably 500 nm to less than 1000 nm, more preferably 500 nm to 600 nm. When the distance H is 500 nm to 1000 nm, the carbon nanopeapod 13 functions as a zero-dimensional conductor between the metal electrodes 14a-14b, and a single electron can be injected into the carbon nanopeapod by the applied back gate voltage. It is. On the other hand, if it is 1000 nm or more, electrons cannot be injected one by one into the carbon nanopeapod, and if it is less than 500 nm, the number of fullerene molecules as memory cells existing between the source electrode and the drain electrode is small. Since the memory capacity is small, it is not preferable.

The metal electrodes 14a and 14b are composed of Au electrode portions 141a and 141b made of Au and Ti electrode portions 142a and 142b made of Ti. The metal electrodes 14a and 14b may be made of Au alone. The Ti electrode portions 142a and 142b are provided to maintain adhesion between the Au electrode portions 141a and 141b, the SiO ₂ layer 121, and the carbon nanopeapod 13, and therefore the thickness thereof is about 20 nm to 50 nm. That's fine. The interface resistance between the metal electrodes 14a and 14b and the carbon nanopeapod 13 is preferably higher than the theoretical quantum resistance (25.6 kΩ). When the interface resistance is sufficiently high, single electrons can be injected into the carbon nanopeapod 13 as a channel. On the other hand, if it is higher than 1 MΩ, it is difficult to inject single electrons.

That is, the memory element 1 includes a metal layer 11 as a back gate electrode, a metal electrode 14a as a source electrode, a metal electrode 14b as a drain electrode, a SiO ₂ layer as an insulating layer, and a carbon nanopeapod as a channel. 13 is an FET structure. Since the carbon nanopeapod 13 behaves as a zero-dimensional conductor because the source electrode and the drain electrode are separated from each other by a predetermined distance, the voltage from the back gate electrode is adjusted so that electrons are integrated into the carbon nanopeapod 13. Can be injected one by one.

  In this case, when a voltage is applied from the back gate electrode (metal layer 11), the memory device 1 can inject a single electron into the single-walled carbon nanotube in a region where the back gate voltage is lower than a threshold value. In the region, single electrons can be injected into the fullerene molecule. As described above, in the memory element 1, it is possible to easily change which part of the carbon nanopeapod is injected with a single electron by controlling the back gate voltage. In many cases, the threshold value in this case is about 1.5 V to 2 V. However, since this threshold value is considered to depend on the interface resistance between the single-walled carbon nanotube and the fullerene molecule, the interface between the single-walled carbon nanotube and the fullerene molecule is considered. It can be changed to a desired value by controlling the resistance.

  Each fullerene molecule functions as a memory cell by injecting single electrons into the fullerene molecule in a region where the back gate voltage is higher than the threshold value. In this case, in the region where the back gate voltage is higher than the threshold value, one electron is injected into each fullerene molecule, that is, each electron is injected into a different fullerene molecule. Control charging energy. This charging energy is determined by the number of endohedral fullerenes. Therefore, it is possible to control the charging energy by changing the number. By controlling in this way, only one electron can be injected into each fullerene molecule, and each electron has a spin moment of either “0” or “1”. By utilizing the spin moment of electrons, the fullerene molecule can be a memory cell in which “0” and “1” are recorded.

  In such a memory device 1, at the time of writing, electrons are injected one by one into each endohedral fullerene by an applied back gate voltage, and an external magnetic field synchronized with the injection voltage is applied or laser light is irradiated. Thus, the spin moment of the injected electrons is controlled and “0” or “1” is written. At the time of reading, it is possible to read “0” or “1” by irradiating each fullerene with a laser beam and monitoring the absorbed / reflected light.

  A method for manufacturing the memory element will be described below.

  The carbon nanopeapod 13 can be obtained by a known production method. For example, single-walled carbon nanotubes with open ends are placed in a volatilized fullerene molecule atmosphere and fullerene molecules are inserted into the carbon nanotubes to produce carbon nanopeapods, or when the double-walled tube is heated, only the inner-walled nanotubes A method of producing a carbon nanopeapod utilizing the fact that becomes a fullerene molecule can be used. These are detailed in B. W. Smith et al., Nature 393, 323 (1998), M. Yudasaka, S. Saito, and A. Oshiyama, Phys. Rev. Lett. 86, 3835 (2001), respectively.

Next, the memory element is manufactured using the carbon nanopeapod. As a manufacturing method of such a memory element, a known FET structure manufacturing method can be applied. For example, first, an SiO ₂ layer 121 is formed on one surface of a p-type Si substrate (for example, a thickness of 600 nm) as the Si layer 12 by a plasma CVD method. This formation condition is, for example, a known condition. In the plasma CVD apparatus, an organic silane gas made of TEOS (tetraethoxysilane) and an oxidizing gas made of O ₂ (oxygen) are introduced (for example, the amount of TEOS introduced). 90 sccm, O ₂ introduction amount 3000 sccm), and a predetermined pressure condition. Note that a Si substrate on which the SiO ₂ layer 121 is already formed may be used.

Thereafter, the carbon nanopeapod 13 is placed on the SiO ₂ layer.

After the carbon nanopeapod 13 is placed, the Ti electrode portions 142a and 142b are produced using a lift-off method. That is, a resist is applied to the surface on which the carbon nanopeapod 13 is placed, and a film is formed by spin coating. After forming the resist film, the resist film is exposed, developed and washed. Then, the Ti electrode portions 142a and 142b are formed by depositing the Ti film on the exposed mask at a degree of vacuum of ^˜10 −7 Torr so that the Ti film has a predetermined thickness and lifting off the mask. As a vapor deposition method, a known vapor deposition method such as an EB vapor deposition method can be used.

  Next, Au electrode parts 141a and 141b are produced in the same procedure as the Ti electrode parts 142a and 142b.

Finally, the back gate electrode 11 is formed on the back surface of the Si substrate (the surface opposite to the carbon nanopeapod mounting side) by a vapor deposition method at a degree of vacuum of 10 ⁻⁷ Torr so as to have a predetermined thickness.

  As described above, the memory element 1 using the carbon nanopeapod 13 as a channel can be manufactured. In manufacturing the memory element 1, it is important not to perform an annealing process so as not to reduce the interface resistance when the metal electrodes 14a and 14b are manufactured. This is because when the interface resistance decreases, electrons cannot be injected into the carbon nanopeapod one by one. In particular, when the metal electrodes 14a and 14b are made of only Au, it is important not to perform the annealing process.

  Hereinafter, the present invention will be described in more detail based on examples.

In this example, the memory element 1 was produced. The manufacturing conditions of the memory element 1 were as follows, and the annealing process was not performed.
SiO ₂ layer: previously formed on the Si substrate, thickness: 100 nm
Au electrode part: production method: vapor deposition method, thickness 100 nm
Ti electrode part: production method: vapor deposition method, thickness 50 nm
Metal layer: Preparation method: Vapor deposition method, thickness 300nm
Ti layer: preparation method: vapor deposition method, thickness 30 nm
Distance between metal electrodes 14a and 14b: 500 nm

FIG. 2A shows a top SEM photograph of the memory element 1. Carbon nanopeapods are arranged between the metal electrodes. FIG. 2B is an upper surface SEM photograph of the memory element 1 before the production of the Ti electrode portions 142a and 142b to show this. A bundle of two carbon nanopeapods 13 is arranged on the SiO ₂ film, and two metal electrodes are formed at positions shown in the drawing.

The carbon nanopeapod has a tube diameter of ˜1.6 nm identified by AFM and TEM, and further shows metallic conduction, so it is highly likely that it has (10, 10) chirality. Further, from the tube diameter and the bundle diameter, the number of carbon nanopeapods 13 in each bundle was about 20. As channel conductivity, G = 40 × [4 (2e ² / h) = 640] μS was predicted, but the actual measured value was about 10 μS. Therefore, it means that the contact resistance between the metal electrode and the carbon nanopeapod is large, and it was confirmed that the carbon nanopeapod as a channel between the electrodes can be configured to function as a zero dimension.

  For the memory device 1 obtained in Example 1, the back gate applied voltage Vbg is changed from −4 V to 3 V and the source-drain voltage Vsd is changed, and the channel conductivity is examined at T = 1.5K. It was. The results are shown in FIG. In FIG. 3, the horizontal axis represents the voltage Vbg applied to the back gate, and the vertical axis represents the source-drain voltage Vsd. The Z axis shows the differential value of channel conductivity as shown in detail in the margin.

As shown in FIG. 3, the electrical characteristics were diamond-shaped. The inside of the diamond shape is a voltage range where the conductivity is minimized by the Coulomb blockade. Coulomb blockade is a phenomenon in which the tunneling of electrons to the carbon nanopeapod 13 is limited to one by one due to the charging energy E _c = e ² / 2C _eff of the carbon nanopeapod 13 having the minute effective capacity C _eff of the memory element 1. is there. The source-drain voltage Vsd in this diamond shape, that is, the magnitude in the vertical axis direction corresponds to E _c . Furthermore, when quantization energy levels are present in the carbon nano-peapod 13, the energy gap is larger than the size of the diamond is added to E _c appears.

  According to FIG. 3, the electrical characteristics (Coulomb blockade characteristics) of the memory device 1 manufactured in Example 1 seemed to be smaller than the electrical characteristics (Coulomb blockade characteristics) reported for semiconductor quantum dots and high-quality carbon nanotubes. The diamond shape looks irregular. However, from this result, it will be explained below. (1) Diamond size and overlap change dramatically with Vbg = ± 1.7V as a boundary. (2) Asymmetric and polar with respect to Vbg. Two points are clear.

  Regarding (1), in the region I (see FIG. 3) where −1.7 V <Vbg <1.7 V (see FIG. 3), the size of the diamond shape (in the vertical axis direction) is relatively small at 10 mV or less, and there is little overlap. On the other hand, in the region II (see FIG. 3) of −1.7 V> Vbg, Vbg,> 1.7 V, the size of the diamond shape reaches a maximum of 40 mV or more, and the overlap becomes extremely large.

  Regarding (2), in region I where Vbg is positive, three diamonds of about 8 mV in size are arranged from Vbg = 0 V, one diamond of less than 20 mV exists, and a diamond of less than 8 mV appears. . On the other hand, in the region I in the case where Vbg is negative, the diamond gradually becomes smaller after about three diamonds of less than 8 mV are arranged. In region II, diamonds of about 10 mV slightly overlap on the negative Vbg side, and large diamonds of about 40 mV appear on the positive Vbg side.

  Conventionally, a diamond shape with irregular size and large overlapping part is a stochastic Coulomb diamond, and one carbon nanotube with many defects and impurities is divided into many small dots by these defects. It has often been reported with multiple bonded structures. However, there is no clear example of a sudden change in diamond size at a certain gate voltage.

In such a tendency clarified in FIG. 3, the characteristics of the region I in the drawing, that is, the low Vbg region, substantially coincided with E _c of the single-walled carbon nanotube not including fullerene. That is, in the low Vbg region, single electrons flow only through single-walled carbon nanotubes, and it is considered that fullerene molecules do not contribute at least as effective capacity C _eff of E _c .

On the other hand, in the region II, that is, the high Vbg region, a single electron traveling only in the single-walled carbon nanotube in the low Vbg region can be attracted to the fullerene molecule by increasing the applied back gate voltage Vbg. That is, by increasing the applied back gate voltage Vbg, single electrons can be injected into the fullerene molecule. In this case, electrons flow into the fullerene molecule through a chemical bond resulting from the bond between the fullerene molecular orbital and the quasi-free electron orbital. The capacity of fullerene molecules contributing to E _c estimated from the diamond size in region II is about the same as the capacity of single-walled carbon nanotubes in low Vbg region I and about 1/3 in high Vbg region II. In the Vbg region, more fullerene molecules are electrostatically coupled in series. When a single electron flows into only one fullerene molecule, the number of bonds is small, so the Coulomb diamond characteristic does not increase as shown in the figure, so single electrons are injected into different fullerene molecules in this memory device. It has been confirmed.

  Therefore, the memory device 1 manufactured in Example 1 can inject electrons into the single-walled carbon nanotubes or inject electrons into the fullerene molecules one by one by changing the back gate voltage. The threshold for changing these characteristics was ± 1.7V. Note that such a Coulomb diamond characteristic is also obtained in a memory element using other carbon nanopeapods manufactured by the same manufacturing method as in this example, and the threshold values in each memory element are all between 1V and 5V. Yes, most were between 1.5V and 2V. Moreover, it was found from the Coulomb diamond characteristic that each electron was injected into different fullerene molecules.

  In this case, since the included fullerene molecules can function as a cell that confines a single electron, that is, a single spin, the memory element 1 can be configured as a large-capacity molecular magnetic spin memory. In particular, since it is considered that one carbon nanopeapod 13 contains about 600 fullerene molecules between the metal electrodes 14a and 14b, the bundle-like carbon nanopeapod is used as a source. If a structure in which many drain electrodes are arranged in parallel is manufactured, a spin memory having a very high memory capacity can be manufactured.

(Comparative Example 1)
Two memory elements were produced under the same conditions as in Example 1 except that the number of carbon nanopeapods in the bundle was different. The number of carbon nanopeapods in a bundle in one memory device was estimated to be 60, and the other was estimated to be about 100. However, even when a voltage was applied under the same conditions as in Example 2, single-electron characteristics were obtained. Was not obtained.

(Comparative Example 2)
The memory element was manufactured under the same conditions as in Example 1 except that the distance between the metal electrode 14a as the source electrode and the metal electrode 14b as the drain electrode was 1 μm. Even in this case, even when a voltage was applied under the same conditions as in Example 2, no single electronic characteristics were obtained.

(Comparative Example 3)
In Example 1, two memory devices were manufactured under the same conditions except that the Au electrode portions 141a and 141b were manufactured and then annealed. The annealing conditions are annealing temperature: 800 ° C. and atmosphere: nitrogen atmosphere. In this case, the resistance of each memory element dropped to about 5 kΩ or less, and as a result, single electronic characteristics could not be obtained.

  The memory element of the present invention can be used as a large capacity memory as described above. Therefore, it can be used in the semiconductor field.

It is a cross-sectional schematic diagram of the memory element for demonstrating a memory element. FIG. 3A is an enlarged view of a top surface SEM photograph of the memory element fabricated in Example 1, and FIG. 3 is a graph showing electrical characteristics of the memory element manufactured in Example 1.

Explanation of symbols

11 Metal layer 12 Si layer 13 Carbon nano peapod 14a, 14b Metal electrode

Claims (2)

A memory device having a carbon nanopeapod composed of single-walled carbon nanotubes containing fullerene molecules,
The carbon nanopeapod is a bundle of carbon nanopeapods of 2 or more and 50 or less, and is placed on an insulating layer stacked on a back gate electrode and provided with a spacing of 500 nm to 1000 nm Connected to the electrode and drain electrode,
The interface resistance between the source and drain electrodes and the carbon nanopeapod is greater than 25.6 kΩ and less than or equal to 1 MΩ,
A single electron is injected into the fullerene molecule contained in the carbon nanopeapod by applying 1 to 5 V from the back gate electrode , and the fullerene molecule holds the injected single electron. And a memory cell configured to be a memory cell that holds memory information. The memory device according to claim 1, wherein the single-walled carbon nanotube of the carbon nanopeapod exhibits metallic conduction .

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