Spin polarization emitter

The present invention relates to a spin-polarized emitter that emits spin-polarized electrons.

In recent years, devices have been developed that utilize the properties of spin in addition to the charge of conduction electrons (spin electronics). Since materials such as magnetic semiconductors and magnetic metals are used in spin electronics, it is necessary to evaluate the magnetic properties of the minute region. One of the promising evaluation methods is a method using a spin-polarized current, but the development of a minute and highly efficient spin-polarized emitter is indispensable for its realization.

Conventionally, a method using a ferromagnetic probe or a method of irradiating a probe of a III-V compound such as GaAs with circularly polarized light is known to emit a spin polarization current. However, it is difficult to obtain a current with a high polarization rate by the method using a ferromagnetic deep needle, and the method using a deep needle of III-V compound requires light irradiation at the time of emission, resulting in various problems. The point is pointed out.

Further, in the conventional emitter, a strong electric field is concentrated on the tip of the probe to emit electrons, but the tip is deteriorated by the application of the strong electric field, so that the emission efficiency is lowered. In recent years, metal-insulator-metal (MIM) type electron emitting elements that emit electrons at a relatively low electric field have been developed (see Patent Document 1), but the electrical withstand voltage of tunnel junctions that generate hot electrons is high. It has the disadvantage of low electron emission and low electron emission efficiency.<patcit num="1"><text>U.S. Patent Application Publication No. 2003/71555</text></patcit>

<p> An object of the present invention is to provide a spin-polarized emitter capable of emitting electrons having a high spin polarization rate with high efficiency and having a good electrical withstand voltage.</p>

<p> In order to solve the above problems, according to the first aspect of the present invention, a metal film, a tunnel insulating film laminated on the metal film, and a tunnel insulating film laminated and formed, and fixed magnetization in one direction is imparted. It is characterized by comprising a ferromagnetic laminated film including a plurality of ferromagnetic metal layers and a non-magnetic metal layer formed between a plurality of ferromagnetic metal layers, and a non-magnetic metal film laminated and formed on the ferromagnetic laminated film. Provide a spin-polarized emitter.</p><p> Further, according to the second aspect of the present invention, a metal film, an insulating film laminated on the metal film, and a plurality of ferromagnetic particles having unidirectional fixed magnetization formed dispersed in the insulating layer are provided. Provided is a spin polarization emitter including a provided tunnel insulating film and a non-magnetic metal film laminated on the tunnel insulating film and facing the metal film via the tunnel insulating film.</p><p> Such a spin-polarized emitter can be used in a spin-polarized scanning probe microscope, a spin-polarized electron diffractometer, or the like to enable highly accurate analysis.</p>

<p> According to the present invention, it is possible to obtain a spin-polarized emitter having a high spin polarization rate and high electron emission efficiency by providing a spin filter unit made of ferromagnetism.</p>

Hereinafter, embodiments of the present invention will be described with reference to the drawings. It should be noted that the same reference numerals are given to common configurations throughout the embodiments and examples, and duplicate description will be omitted. In addition, each of the referenced figures is a schematic diagram for explaining the invention and promoting its understanding, and for convenience of drawing display, the shape, dimensions, ratio, etc. may differ from those of the actual device. The design can be changed as appropriate by taking into consideration known techniques.

(First Embodiment) First, the first embodiment relating to the spin-polarized emitter of the present invention will be described.

FIG. 1 is a schematic cross-sectional view of a spin-polarized emitter according to the first embodiment.

The spin-polarized emitters are the first electrode 1, the tunnel insulating film 3 formed on the first electrode 1, the ferromagnetic laminated film 5 formed on the tunnel insulating film 3, and the tunnel insulating film 3 and ferromagnetic. A second electrode 7 facing the first electrode 1 is provided via the laminated film 5.

The first electrode 1 and the ferromagnetic laminated film 5 are made of metal, and the first electrode 1, the tunnel insulating film 3 of the insulator, and the ferromagnetic laminated film 5 form an MIM structure. This MIM structure forms a hot electron generator, and hot electrons 11 can be injected from one metal to the other metal by applying a voltage between the first electrode 1 and the ferromagnetic laminated film 5.

The first electrode 1 may be a non-magnetic metal film or a ferromagnetic metal film. A ferromagnetic metal film made of Fe, Co, Ni, or an alloy or compound containing any of these is used for the first electrode 1, and the ferromagnetic metal film is fixed in the same direction as the magnetization of the ferromagnetic laminated film 5. When magnetized, the spin directions of the hot electrons to be injected can be aligned, and the emission efficiency can be increased up to about twice as much as that of an emitter using a non-magnetic metal film.

The ferromagnetic laminated film 5 is a film in which a plurality of ferromagnetic metal layers and a non-magnetic metal layer are alternately laminated, and by aligning the magnetization directions of the plurality of ferromagnetic metal layers, the hot electron 11 can be spun. It can be selectively transparent (filtered) depending on the orientation. That is, only hot electrons having spins in the direction parallel to the magnetization direction of the ferromagnetic metal layer can pass through the film 5 and be transmitted to the second electrode 7. Such a ferromagnetic laminated film 5 is called a spin filter film.

For the ferromagnetic metal layer of the ferromagnetic laminated film 5, a ferromagnetic metal material such as Fe, Co, Ni, and an alloy or compound containing at least one of these metals can be used. Further, the thickness in the stacking direction can be about 1 nm or more and about 10 nm or less.

For the non-magnetic metal layer in the ferromagnetic laminated film, a non-magnetic metal material such as Au, Cu, Ag, Al, and an alloy or compound containing at least one of these metals can be used. Further, the thickness in the stacking direction can be about 1 nm or more and about 30 nm or less.

The ferromagnetic metal layers adjacent to each other via the non-magnetic metal layer in the ferromagnetic laminated film 5 are ferromagnetically coupled to each other, and their magnetizations are substantially parallel to each other. The number of times the magnetic metal layer and the non-magnetic metal layer in the ferromagnetic laminated film 5 are laminated shall be 1 or more and 5 or less.

For the first electrode 1, a non-magnetic metal film such as Au, Ag, Cu, Al, Pt, Ti, Cr, V, Ta, W, Nb, Mo can be used, and the thickness thereof is about. It should be 10 nm or more and about 200 nm or less. Further, as described above, the emission efficiency can be improved by using a magnetic material for the first electrode 1.

A non-magnetic metal film such as Au, Pt, Pd, or carbon can be used for the second electrode 7, and the thickness thereof can be about 1 nm or more and about 20 nm or less.

AlO, HfO, AlN, HfN, AlHfO, AlHfN, MgO and the like can be used for the tunnel insulating film 3. The thickness can be about 1 nm or more and about 2 nm or less.

The shape of the spin-polarized emitter shall be, for example, a quadrangular prism having a width (horizontal direction of the paper surface in FIG. 1) of about 1 nm or more and about 10 nm or less and a depth (vertical direction of the paper surface of FIG. 1) of about 1 μm or more and about 10 μm. Can be done.

Next, the operation of this spin-polarized emitter will be described.

When a voltage is applied between the first and second electrodes 1 and 7 by the voltage source 9, the hot electron 11 can be injected from the first electrode 1 into the second magnetic laminated film 5. The injected hot electrons are selectively transmitted / impermeable in the magnetic laminated film 5 depending on the spin direction. A part of the hot electrons 11 that have passed through the magnetic laminated film 5, that is, the hot electrons having a predetermined spin, are emitted into a vacuum as spin-polarized electrons 13 from the emission surface of the second electrode 7.

Next, the electron emission efficiency and the spin polarization rate of this spin-polarized emitter will be described.

The electron emission efficiency of such a spin-polarized emitter is the emission current I._EAnd the diode current flowing through the device I₀As a ratio of, it is approximately given by Eq. (1).

<maths num="1"><img file="JP2005190878A_D0001.tif" /></maths>

Here T_M→_SIs the current transmission at the interface between the second electrode and the vacuum, T_M→_MIs the current transmittance at the interface between the magnetic metal layer and the non-magnetic metal layer in the magnetic laminated film 5, D is the thickness of the magnetic laminated film 5, and λ is the attenuation length as well.

The spin-polarized emitter of the present embodiment can emit a current having a high spin polarization ratio. This is because the decay length λ of the hot electron current in the magnetic laminated film 5 largely depends on the spin direction.

Here, Table 1 shows the spin-dependent attenuation lengths in the magnetic laminated film 5 measured by the method described in Examples described later.

<tables num="1"><img file="JP2005190878A_D0002.tif" /></tables>

As can be seen from Table 1, the decay length in the magnetic laminated film 5 largely depends on the direction of electron spin, and the decay length λ of upward spin electrons is about 50 angstroms, but the decay length λ of downward spin electrons. Is only about 10 angstroms.

The spin polarization of the emitted hot electron is expressed by Eq. (4) using the current I due to the upspin hot electron and the current I due to the downspin hot electron.

<maths num="2"><img file="JP2005190878A_D0003.tif" /></maths>

Here, the current I and the current I are expressed by Eqs. (2) and (3) by the thickness d of the ferromagnetic layer in the magnetic laminated film 5 and the decay length λ of the hot electron depending on the spin. To.

For example, a hot electron with an energy of about 1.5 eV has a thickness d of about 20 angstroms.₈₄Fe₁₆Make the layer transparent. From Table 1, Co₈₄Fe₁₆Since the attenuation length λ of the upspin electrons in the layer is 50 angstroms and the attenuation length λ of the downspin electrons is 8 ongstroems, the spin polarization rate is as high as about 90% from Eq. (4). It can be seen that the current can be obtained.

On the other hand, it is known that the current transmittance at the interface between the ferromagnetic metal layer and the non-magnetic metal layer in the ferromagnetic laminated film 5 is generally as high as several tens of percent. In addition, the damping length in the non-magnetic metal layer is generally long, and is about 200 angstroms in precious metals such as Au and Cu.

Therefore, by using the magnetic laminated film 5, it is possible to realize a spin filter having a current transmittance of several tens of percent and a spin polarization ratio of about 90% with respect to the hot electron current.

By increasing the number of magnetic metal layers in the ferromagnetic laminated film 5, the spin polarization rate of emitted electrons can be further increased. However, the electron emission efficiency is reduced.

Next, a modified example of the present embodiment will be described.

In the spin-polarized emitter of Modification 1, an alkali metal such as Li or Na having a small work function, an alkaline earth metal such as Mg or Ba, or an alloy or compound containing these metals is placed on the electron emitting surface of the second electrode 7. Provide. Alkali metals such as Li and Na, alkaline earth metals such as Mg and Ba, and alloys and compounds containing these metals are effective in improving the efficiency of photoelectron emission and thermionic emission, and have a current transmission rate of T._M→_SCan be improved.

Strong field electron emission using a probe has a drawback in terms of material strength, and it is difficult to use these metals. However, since the spin-polarized emitter according to the present embodiment is a hot electron type that can emit without using a strong electric field, these materials can be used for improving emission efficiency.

This improvement in emission efficiency will be described using the energy diagram of FIG.

E in Figure 2_FIndicates the Fermi energy level. When a metal having a small work function such as an alkali metal is used for the surface, the barrier is lowered as shown by the solid line, and the transmittance of the hot electron 11 into vacuum can be increased. Such metals having a small work function are alkali metals such as Li and Na, alkaline earth metals such as Mg and Ba, and compounds containing these metals.

On the other hand, in the case of a metal having a large work function or its compound, as shown by the dotted line in FIG. 2, a high barrier exists at the interface with the vacuum, so that the transmittance of hot electrons is lowered and the emission efficiency is lowered. That is, according to this modification 1, the current transmittance T from the second electrode 7 to the vacuum_M→_SCan be improved.

Next, the spin-polarized emitter according to the second modification of the first embodiment will be described.

In this modification 2, the electrical withstand voltage of the tunnel junction can be improved by using the multiple tunnel junction or the fine particle multiple tunnel junction.

The weak point of the hot electron type emitter is that the electrical withstand voltage of the tunnel junction is low, and it is expected that the tunnel junction will be destroyed by external electrical noise or static electricity. However, if the tunnel insulating film is thickened in order to increase the electrical withstand voltage, the tunnel junction resistance increases and the emission current decreases.

Therefore, instead of the tunnel insulating film 3 of FIG. 1, the multiple tunnel junction film 3A shown in the schematic cross section of FIG. 3 (a) or the fine particle multiple tunnel junction film 3B shown in the schematic cross section of FIG. 3 (b) is used. Use. According to such a multiple tunnel junction membrane 3A and a fine particle multiple tunnel junction membrane 3B, the voltage applied from the outside can be dispersed in each junction, and the withstand voltage of the tunnel junction can be remarkably increased.

The multi-tunnel junction film 3A includes a metal layer 31 and a plurality of insulating layers 33 laminated so as to sandwich the metal layer 31. As shown in FIG. 3A, a plurality of metal layers 31 and a plurality of insulating layers 33 may be alternately laminated. Al, Cu, or the like can be used for the metal layer 31, and the thickness thereof can be, for example, about 1 nm or more and about 5 nm or less. AlO, AlN, and AlCuO can be used for the insulating layer 33, and the thickness thereof can be, for example, about 1 nm or more and about 2 nm or less.

The fine particle multiple tunnel junction film 3B is a laminated film of a metal layer 35 and a fine particle multiple tunnel layer 37. The metal layer 35 is made of, for example, Al, Cu, Ag, Pt, etc., and has a thickness of about 10 n or more and about 200 nm. The fine particle multiple tunnel layer 37 is a layer in which conductive fine particles having a diameter of about 1 nm or more and about 10 nm or less are dispersed in a non-magnetic insulating film such as AlO, BN, AlN, and diamond-like carbon. The thickness of the fine particle multiple tunnel layer 37 can be, for example, about 20 nm or more and about 100 nm or less.

Since the fine particle multiple tunnel layer 37 of the fine particle multiple tunnel bonding film 3B is formed by dispersing metal fine particles in an insulator, even if the tunnel junction between two metal fine particles is broken and a short circuit occurs, the fine particles are caused by this. The entire multiple tunnel layer 37 is not destroyed and the metal layer 35 and the second electrode 7 are not short-circuited. Therefore, the resistance to external electrical noise, static electricity, and the like can be significantly improved.

Next, a modified example 3 of the first embodiment will be described with reference to the schematic cross-sectional view of FIG.

In the third modification, ferromagnetic fine particles are used for the metal fine particles of the fine particle multiple tunnel junction film 3B shown in FIG. 3 (b), and fixed magnetization is applied to the ferromagnetic fine particles of the entire film. By using a multi-tunnel junction membrane to which such a fixed magnetization is applied, spin filtering and hot electron generation can be performed at the same time.

In this fine particle multiple tunnel junction film 3B, an alloy or compound containing Fe, Co, NI, or any of these metals is used as the material of the ferromagnetic fine particles, and the average diameter of each particle is about 5 nm or more and about 20 nm. It is as follows. Further, the thickness of the fine particle multiple tunnel junction film can be about 50 nm.

As shown in the schematic cross-sectional view of FIG. 4, the spin-polarized emitter can be formed only by the first and second electrodes of the non-magnetic metal and the three layers of the fine particle multiple tunnel film 39. Although the spin polarization rate of this three-layered spin-polarized emitter is as low as about 20% or more and about 30% or less, it is not necessary to use ferromagnetic materials for the first and second electrodes 1 and 7, so current transmission occurs. The rate can be increased, and the current emission efficiency can be further improved.

(Example 1) Measurement of attenuation length of hot electrons traveling in a magnetic material Example 1 will be described with reference to a schematic cross-sectional view of FIG.

CoSi with a thickness of about 1 nm on the Si layer₂CoFe layer, Cu layer, base layer composed of CoFe layer, Al₂O₃A tunnel insulating film was formed. Each layer of the base layer is made with a multi-chamber MBE device (2 x 10).^-10torr) was used.

Introduction n in the first chamber⁺The Si (111) substrate was heated at about 500 ° C. for about 2 hours, followed by about 700 ° C. for about 0.5 hours to remove the adsorbed gas on the surface. Subsequently, the Si substrate was heated to about 840 ° C in a weak Si flux to remove the surface oxide film. At this stage, the 7x7 structure of the Si surface was confirmed by RHEED.

Next, non-doped Si was formed on this surface as a buffer layer at 700 ° C. to a thickness of 1 μm. After that, Co and Si with stoichiometric composition are simultaneously vapor-deposited and annealed at about 600 ° C for about 10 minutes to make CoSi with a thickness of about 1 nm.₂A film was formed.

The base laminated film (CoFe layer, Cu layer, CoFe layer) was formed by the ion beam sputtering method in the second chamber of the MBE apparatus. The thickness of the Cu layer was fixed at about 2 nm, and the film thicknesses of the two CoFe were changed between about 1 nm and more and about 10 nm or less, respectively. CoFe was formed while applying a magnetic field of about 1000 Oe to impart magnetic uniaxial anisotropy.

Then, in the third chamber, Al₂O₃A tunnel insulating film was formed. O₂Partial pressure 10^-5Under torr, Al with Al source₂O₃Was formed to a thickness of about 1.5 nm.

After that, CaF with a thickness of about 200 nm₂An interlayer insulating film was formed to make the area of the base / emitter tunnel junction about 50 × 50 μm, and then an Al (10 nm) / Au (100 nm) laminated film was formed as an emitter.

For each transistor with different CoFe film thickness, apply a magnetic field in the plane and measure the current / voltage characteristics using a voltmeter 9 and an ammeter 53 to measure the current transmission coefficient (collector current / emitter-current ratio). ) Was asked. From the CoFe film thickness dependence of this current transmittance, the decay length of the hot electron current depending on the spin in the CoFe film was obtained. As shown in Table 1, at an emitter voltage of about 1.5 V, the decay length of upspin electrons is λ = about 50 angstroms, the decay length of downspin atoms is λ = about 8 angstroms, and at about 2.0 V, λ. = 43 angstroms, λ = 8 angstroms.

From this result, it can be seen that the decay length of the hot electron transmitted through the ferromagnetic laminated film differs depending on the direction of the electron spin, and the upspin electron can obtain a higher transmittance.

(Example 2) Spin-polarized emitter using a Cu / CoFe / Au / Cs laminated film for the spin filter section A schematic cross-sectional view is shown in Fig. 6 on a Si substrate using the manufacturing equipment used in Example 1. A spin-polarized emitter was prepared.

First, an Al film with a thickness of about 200 nm is formed as a lower electrode by a vapor deposition method, and the surface is oxidized in an oxygen atmosphere to form an Al with a thickness of about 2 nm.₂O₃Was formed to form a tunnel insulating film. Subsequently, a laminated film of a Cu layer (about 10 nm thick), a CoFe layer (about 2 nm thick), and an Au layer (about 1 nm thick) of the spin filter portion was formed on the tunnel insulating film by an ion beam sputtering method. The CoFe layer was magnetized in the same manner as in Example 1.

The film on the obtained Si substrate was 50 × 50 μm using an optical lithography method and an ion milling device.²A spin-polarized emitter was produced by processing to the junction size of. After attaching the current reed to this emitter, it was set in the vacuum chamber again and the electron emission characteristics were measured. When a voltage of 40V is applied, the emission current is about 0.1μA and the electron emission efficiency is 10 for a diode current of about 10mA.^-5It was about.

After depositing Cs on the upper electrode Au of the same emitter at about 0.5 nm in a vacuum chamber, the electron emission characteristics were measured again. At an applied voltage of about 40V, the emission current is about 0.7μA for a diode current of about 10mA, and the electron emission efficiency is about 7 × 10-5. By depositing Cs on the surface, the electron emission efficiency is improved. Was done.

The spin polarization was measured using a Mott detector. The Mott detector utilizes the fact that when spin-polarized electrons are scattered by heavy atoms, their spatial distribution becomes asymmetric due to spin-orbit interaction. The spin polarization of the emission current at an applied voltage of about 40 V was about 60%. There was no significant difference in spin polarization between the device with Cs vapor-deposited on the surface and the device with only Au. This device suffered dielectric breakdown when about 60 V was applied. (Example 3) Spin-polarized emitter using a fine-grained multi-tunnel layer A spin-polarized emitter was prepared to multiple-tunnel conduct a fine-grained layer dispersed in an insulating layer as shown in the schematic cross-sectional view of FIG.

After forming a lower electrode Cu layer (about 100 nm thick) on a Si substrate by ion beam sputtering, Al₂O₃And CoFe are sputtered alternately, and Al₂O₃A fine particle layer (about 40 nm thick) in which CoFe fine particles were dispersed was prepared in the insulating layer.

Then, the CoFe layer (about 1 nm thick) and the Au layer (about 10 nm thick) were sputtered to form an upper electrode. Uniaxial anisotropy was imparted to the CoFe layer by the same method as in Example 1, the bonding size was processed to about 50 μm × 50 μm, and the electron emission characteristics were measured.

The emission current was 50 nA and the electron emission efficiency was 10-5, while the diode current was 5 mA when a voltage of 50 V was applied. When a voltage of 100 V was applied, the diode current was 20 mA, the emission current was 2 μA, and the electron emission efficiency was 10-4. This device did not cause dielectric breakdown even when a voltage of 120 V was applied. When the spin polarization rate was measured by the Mott detector, the spin polarization rate of the emission current when 100V was applied was about 40%.

In this Example 3, although the spin polarization rate is lower than that of the emitter of Example 2, the withstand voltage is high, so that the emission current can be increased.

<figref num="1">FIG. 5 is a schematic cross-sectional view for explaining a spin-polarized emitter according to the first embodiment of the present invention.</figref><figref num="2">An energy diagram for explaining a modification 1 of the first embodiment.</figref><figref num="3">Schematic cross-sectional view for explaining the modified example 2 of the first embodiment</figref><figref num="4">Schematic cross-sectional view for explaining the modified example 3 of the first embodiment</figref><figref num="5">Schematic cross-sectional view for explaining Example 1.</figref><figref num="6">Schematic cross-sectional view for explaining Example 2.</figref><figref num="7">Schematic cross-sectional view for explaining Example 3</figref>

Code description

1 ... 1st electrode 3 ... Tunnel insulation film 5 ... Ferromagnetic laminated film 7 ... 2nd electrode 9 ... Voltage source 11 ... Hot electron 13 ... Spin polarization Electron 3A Multiple tunnel junction film 3B Fine particle multiple tunnel junction film 31 Metal film 33 Insulation film 35 Metal film 37 Fine particle multiple tunnel film 39 Ferromagnetism Fine particle multiple tunnel junction film 53 Current meter