WO2002093660A2

WO2002093660A2 - Tmr layer system with diode characteristics

Info

Publication number: WO2002093660A2
Application number: PCT/DE2002/001672
Authority: WO
Inventors: Joachim Bangert
Original assignee: Infineon Technologies Ag
Priority date: 2001-05-16
Filing date: 2002-05-08
Publication date: 2002-11-21
Also published as: DE10123820A1; DE10123820C2; WO2002093660A3

Abstract

The invention relates to a TMR layer system consisting of a first ferromagnetic electrode layer (11), a dielectric barrier layer (10) and a second ferromagnetic layer (12) which are placed on top of each other in the above-mentioned sequence or which are arranged horizontally next to each other. The electric resistance of the layer system (10,11,12) depends upon the relative magnetic polarization direction of the first and second ferromagnetic electrode layer. The inventive TMR layer system is characterized in that at least one of the ferromagnetic electrode layers (11,12) has a voltage-dependent state charge carrier density, whereby the electric current voltage characteristic of the TMR layer system has diode behavior, or the dielectric barrier layer is embodied in such a way that it has a strongly asymmetrical tunnel behavior.

Description

description

TMR layer system with diode characteristics

The invention relates to a TMR layer system (tunnel magnetoresistive layer system) comprising a first ferromagnetic electrode layer, a dielectric barrier layer and a second ferromagnetic layer, which are stacked one above the other in this order or arranged horizontally next to one another, the electrical resistance of the layer system being below depends on the relative magnetic polarization direction of the first and second ferromagnetic electrode layers.

TMR elements or TMR layer systems are favored in the MRAM development of the latest generation, since they show a high relative change in resistance and their absolute resistance value can be set in a favorable range. (MRAM: Magnetoresistive Random Access Memory.)

A TMR component that has the above-mentioned effect based on the tunnel magnetoresistance effect (TMR effect) that the electrical resistance of the component depends on the relative magnetic polarization direction of the first and second ferromagnetic electrode layers has a current-voltage characteristic curve that for positive and negative voltages is generally almost symmetrical but not linear.

For the application of a TMR component in MRAM memories, a strongly asymmetrical current-voltage characteristic, ideally a diode characteristic, has proven to be favorable. As a result, a larger number of memory cells can then be connected in a memory field, it being possible at the same time to reduce the electrical losses caused by parasitic currents. The change in magnetic polarization direction of a ferromagnetic layer is carried out by currents or by the associated magnetic fields on the contacting conductors. The layer stack should be as thin as possible in order to avoid an increase in the current and thus an enlarged magnetic stray field on adjacent, unselected MRAM cells.

In order to achieve a diode-like behavior of the current-voltage characteristic of TMR elements, several proposals have so far been made. For example, there is the proposal to integrate an additional, as thin as possible diode in the layer stack, for example a MIM (aluminum, aluminum oxide, platinum), MIS or MS (Schottky) diode. It should be difficult to manufacture the very thin semiconductor layers required to build an integrated diode in such a way that the layer stack is not thickened. Furthermore, Sousa et al. in a technical article "Vertical Integration of a spin dependent tunnel junction with an amorphous Si diode", Applied Physics Letters 74, 25 (1999), 3893 describes the series connection of a diode with a TMR layer system. Furthermore, the series connection of an external diode and a TMR component in various technologies has been proposed on various occasions (see, for example, Boeve et al .: "Integration of spin valves and GaAs diodes in magnetoresistive random access memory cells"; Journal of Applied Physics 85, 8: 4779 (1999).

It should be noted that high integration of TMR components, as is necessary, for example, for MRAM semiconductor memory arrangements, is only feasible if the diode is in the layer stack. The thickness of the diode is decisive for the achievable density of the TMR components, in particular in memory arrays, since the external magnetic field required for aligning the polarization of the magnetic memory layer must be generated by the higher currents, the thicker the layer stack. Limit the stray fields, which are becoming stronger, affect neighboring memory cells. Other problems are the permissible current density in the conductors and the heat development.

In view of the above, it is an object of the invention to enable a TMR layer system with a diode behavior that does not require additional functional layers in the TMR layer system and can avoid thickening of the stack of the layer system or external components.

According to one aspect of the invention, the above object is achieved by a generic TMR layer system in which at least one of the ferromagnetic electrode layers has a voltage-dependent density of states of the charge carriers, so that the current-voltage characteristic of the TMR

Layer system shows diode behavior. One or both ferromagnetic electrode layers can have a semimetal. The semimetal can, for example, consist of chromium dioxide, Fe ₃ 0 ₄ or a Heusler 'see or Halb-Heusler' see alloy. As a result, at least one of the ferromagnetic electrode layers receives a voltage-dependent density of states, which can bring about the desired diode behavior of the TMR layer system without additional functional layers, a thickening of the stack or external components being required. The saturation current of the diode can advantageously be set by the properties of the tunnel barrier and the area of the component containing the TMR layer system.

The magnetic materials can perform two functions at the same time, which means that no thickening of the TMR layer system (in the case of a vertical arrangement) is required.

According to a second aspect, the dielectric bamer layer of a generic TMR layer system is designed such that it exhibits a strongly asymmetrical tunnel behavior. has. Such asymmetrical tunnel behavior can be achieved by a corresponding doping profile of the dielectric barrier layer or by the dielectric barrier layer consisting of several material layers in such a way that these material layers together produce the asymmetrical tunnel behavior. Here too, the saturation current of the diode can advantageously be set by the properties of the tunnel barrier and the area of the component containing the TMR layer system.

Here, too, the magnetic materials can perform two functions at the same time, which means that there is no need to thicken the TMR layer system (in the case of a vertical arrangement).

The measures corresponding to the first aspect of the invention can advantageously also be combined with the measures corresponding to the second aspect of the invention, in order in this way to further strengthen the diode behavior of the TMR layer system.

A TMR layer system according to the invention can be used particularly advantageously in an MRAM memory arrangement in which the individual memory cells then consist of a TMR layer system according to the invention.

The following description describes exemplary embodiments of the above two aspects of the invention with reference to the drawing figures.

The figures show in detail:

1 graphically shows a band structure model which illustrates the density of states of the charge carriers of a ferromagnetic semimetal above and below the Ferminev level; 2 shows an idealized band structure model of an ideal counter electrode (eg made of cobalt);

3 shows a diagram which illustrates differently high barriers on both sides of the dielectric barrier layer in the de-energized state;

Fig. 4 is a similar diagram as in Fig. 3, the

Relationships when applying a voltage to the two ferromagnetic electrode layers shows and

Fig. 5 current-voltage characteristics of a conventional TMR layer system (b) and a TMR layer system (a) according to the invention.

First aspect of the invention

According to the first aspect of the invention, a TMR layer system is characterized in that at least one of the ferromagnetic electrode layers has a voltage-dependent density of states of the charge carriers, so that the current-voltage characteristic curve of the TMR layer system shows diode behavior. An embodiment of a TMR layer system implemented according to the first aspect of the invention is described below, in which one or both ferromagnetic electrode layers have a semimetal. For the purposes of this invention, a semimetal is a material in which, depending on the spin, half of the electrons are conducted and the other half are not conducted, that is to say that a ferromagnetic semimetal has a 100% preference for spin alignment (at least for d electrons) Has. A metal is characterized, among other things, by the fact that free electrons and also free electron states are present above the Fermini level (W _F ). A metallic ferromagnet is characterized, among other things, by the fact that the state densities for spin-up and spin-down are different above the Fermi level (W _F ), especially in the d-electron bands. Spin alignment sees preferred propagation conditions (majorities). For these spins there are many freely occupied states (metallic line), while the other spin orientation finds fewer occupied states (insulator behavior for minorities). Depending on the magnetization, this asymmetry "exchanges".

1) semi-metal

As the band structure model in FIG. 1 illustrates, there are only free states 1 for spin-up charge carriers, which are designated by the reference number 2, above the Fermini level (W _F ) for a ferromagnetic semimetal, and there are also below the Fermini level W _F an area where only spin-up charge carriers 2 are present. The spin-down charge carriers, which are designated by the reference number 3, are located far below the Fermini level W _F. Tunneling into such a semimetal is only permitted for the preferred spins, that is to say the spin-up charge carriers. Therefore, the flowing current is an exact representation of the density of states in the counter electrode. The target area when tunneling into the semi-metal is area 1. When tunneling out of the semi-metal, area 1 is the source area. When conducting electricity, the line takes place in area 1. Will in one

TMR layer system caused by the barrier (e.g. from Al ₂ 0 ₃ ) no spin scattering, only spin-up electrons are expected at the counter electrode (with negative voltage at the counter electrode) or only spin-up Electrons can leave the counter electrode (with positive voltage at the counter electrode). Applying a voltage to the TMR barrier shifts the density of states. By choosing suitable counter-electrode materials with a suitable electron configuration, a shift of the occupied states towards more or fewer states is possible.

2) Ferromanget

2 shows the example of a ferromagnetic metal, for example cobalt, in which the state densities 1 and 4 for spin-up and spin-down are different above the Fermini level W _F , but one spin orientation, namely spin-up, is preferred Direction of propagation sees. In principle, however, currents of both polarizations are possible.

The band structures, that is to say the state density distribution according to FIG. 2, show that if the Fermini level W _{F were} shifted upward by an electrical voltage, the state density would be significantly reduced. This increases the resistance of the metal. The likelihood of tunneling is reduced for a tunnel section. A downward shift results in a significantly increased number of free states. This reduces the resistance of the tunnel section, ie tunneling is more likely. This is a diode-like behavior.

Second aspect of the invention

According to the second aspect of the invention, a TMR layer system is characterized in that the dielectric barrier layer is designed in such a way that it exhibits a highly asymmetrical tunnel behavior. This is explained in the description below with reference to FIGS. 3 and 4. 3 and 4, the dielectric barrier layer is designated by 10, and the ferromagnetic electrode layers adjoining on both sides are designated by 11 and 12. The Barrier 10 is designed in such a way that a highly asymmetrical tunnel behavior results, which, as FIG. 4 illustrates, is achieved by barriers of different heights from both sides. Applying a voltage to the barrier 10 changes the slope of the barrier height (Fig. 5). Electrons e ^" from left to right see a lower barrier height than in the reverse (electron) current direction. FIG. 5 illustrates that an applied voltage can shorten the barrier length t. This effect does not occur in the reverse current direction. The asymmetry in the barrier 10 leads to a shift in the characteristic curve branches: one branch receives a higher resistance, the other branch a lower one.

FIG. 5 graphically shows a comparison of the current dependence on the voltage for calculations with assumed asymmetry (curve a) and with a symmetrical barrier (curve b). The amount is shown to illustrate the effect.

If realistic values are used for barrier materials currently in use, current differences of 30% are realistic for a constant voltage across the barrier. The effect depends somewhat on the voltage applied to the barrier. For constant current you can therefore expect somewhat larger effects (50%). Larger resistance differences can also be achieved with new materials. The theory gives differences of up to 10000% for gold and aluminum electrodes.

Exemplary embodiments for realizing the first aspect according to the invention are given below. In the first exemplary embodiment, a ferromagnetic semimetal is proposed for a ferromagnetic electrode, since the observed diode effect is greatest in this. However, materials with a comparable effect for a barrier side (for example, due to an inverse dependence of the density of states on the voltage) can also be used. The basic behavior was demonstrated with the manganese perovskite (LaO, 7Sr0, 3Mn0 ₃ ) against a cobalt electrode for extremely low temperatures (cf. De Teresa et al .: "Role of Metal-Oxide Interface in Determining the Spin Polarization of Magnetic Tunnel Junctions, Science 286: 507, 1999.

A semi-metal suitable for the room temperature range is chromium dioxide (Cr0 ₂ ). Another proposed semimetal is Fe ₃ 0 ₄ .

Other possibilities are Heusler's alloys (Pd ₂ MnSn; Co ₂ TiSn and Co ₂ TiAl) or semi-Heusler's alloys (NiMnSb) (the majority are metallically conductive, the minorities are semiconducting).

Further usable material combinations (eg permalloy versus cobalt-platinum) can be found by evaluating the tables for the electron configurations of ferromagnetic materials. The fundamental influence of work functions on a barrier has already been described in theory, but is not applied to a diode (cf. John G. Simmons: "Electric Tunnel Effect between Dissimilar Electrodes separated by a Thin Insulating Film" in J. of Appl. Physics , Vol. 34, N. 9, Sept. 1963).

Furthermore, exemplary embodiments for realizing an asymmetrical barrier corresponding to the second aspect of the invention are shown. There are basically two options for realizing an asymmetrical barrier:

- a corresponding doping profile in the barrier. The electronic structure is disturbed by doping the barrier with foreign atoms. Doping can be introduced or implanted during growth or also Structural disorders (for example due to ion bombardment in crystalline barriers).

- Compositions of the barrier from different material layers. The dielectric layer required for the barrier 10 can be produced using a very large number of methods. Therefore, only a few examples need to be listed for clarification:

a) partial oxidation of the barrier material, nitriding of the remaining material;

- Application of the barrier material in two steps

(for example twice 1 nm aluminum);

Through oxidation of the barrier material, production of oxynitride with a lower material depth;

b) growth of a first barrier material (for example iron), onto this a second one (for example aluminum);

- oxidation / nitration between growth, - through-oxidation after growth

The intermediate layers formed can also be magnetic functional materials in the barrier, for example the ferromagnets Fe ₂ 0 ₃ or Fe ₃ 0. This can also increase the TMR effect. Furthermore, a thin insulating antiferromagnet, e.g. B. nickel oxide or a manganite, conceivable, which supports the properties of the reference layer (for example cobalt single or artificial antiferromagnet -> Exchanged Bias System). LIST OF REFERENCE NUMBERS

1 free states with spin-up

2 charge carriers with spin-up below the Fermi edge 3 charge carriers with spin-down below the Fermi edge

4 free states with spin down

10 barrier layer

11 first ferromagnetic electrode layer

12 second ferromagnetic electrode layer W energy _f Fermi level t barrier length e ^~ electrons eV electron volts

Claims

claims

1. TMR layer system comprising a first ferromagnetic electrode layer (11), a dielectric barrier layer (10) and a second ferromagnetic layer

(12) which are stacked one above the other in this order or arranged horizontally next to one another, the electrical resistance of the layer system (10, 11, 12) depending on the relative magnetic polarization direction of the first and second ferromagnetic electrode layers, characterized in that at least one of the ferromagnetic electrode layers (11, 12) has a voltage-dependent density of states of the charge carriers, so that the current-voltage characteristic of the TMR layer system shows a diode behavior.

2. The TMR layer system according to claim 1, which also has one of the ferromagnetic electrode layers (11, 12) having semi-metallic properties.

3. TMR layer system according to claim 1, so that the two ferromagnetic electrode layers (11, 12) have semi-metallic properties.

4. TMR layer system according to claim 2 or 3, d a d u r c h g e k e n n z e i c h n e t that the semimetal is a Heusler 'see alloy or a semi-Heusler' see alloy.

5. TMR layer system according to claim 1, characterized in that the electrode layers (11, 12) on both sides of the dielectric barrier layer (10) are formed from different materials with different work functions.

6. TMR layer system consisting of a first ferromagnetic electrode layer (11), a dielectric barrier layer (10) and a second ferromagnetic electrode layer (12), which are stacked one above the other in this order or arranged horizontally next to one another, the electrical resistance of the layer system ( 10, 11, 12) depends on the relative magnetic polarization direction of the first and second ferromagnetic electrode layers (11, 12), characterized in that the dielectric barrier layer (10) is designed such that it exhibits a highly asymmetrical tunnel behavior.

7. The TMR layer system as claimed in claim 6, so that the dielectric barrier layer (10) has a doping profile which causes an asymmetrical tunnel behavior.

8. TMR layer system according to claim 6, so that the dielectric barrier layer (10) consists of several material layers which together cause the asymmetrical tunnel behavior.

9. TMR layer system according to one or more of the preceding claims, g e k e n n z e i c h n e t d u r c h combination of one or both electrode layers, which have a voltage-dependent density of states of the charge carriers, with an asymmetrical tunnel behavior of the barrier layer.

10. MRAM memory arrangement, characterized by TMR memory cells containing a TMR layer system according to one or more of the preceding claims.