JP2007005641A - Spin transistor - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To control the drain-current value of a spin transistor by its gate electrode by so using a spatial nonuniform Rashba spin orbit interaction as to separate from each other the upward and downward spins of its carrier, and further, by using a superconductive junction wherein its drain current flows dependently on the spin polarization degree of its carrier. <P>SOLUTION: The spin transistor has a normally conductive electrode (source) 101 constituted out of a normal conductor, i.e., non-ferromagnetic body, a superconductive electrode (drain) 102, an electrode 120, a gate electrode 103, a gate contact layer 104, a channel layer 105 having a formed two-dimensional electron gas, a spacer layer 106, a carrier feeding layer 107, a buffer layer 108, a substrate 109, and a gate insulating layer 110. Hereupon, there are used its gate electrode capable of separating from each other the upward and downward spins of its carrier by a spatial nonuniform Rashba spin orbit interaction, and used its drain electrode so formed out of a superconductor as to make controllable its drain current by the spin polarization degree of its carrier. As a result, the spin transistor is formed by using entirely no magnetic field and no magnetic body. <P>COPYRIGHT: (C)2007,JPO&INPIT

Description

  The present invention relates to a spin transistor that controls drain current by controlling upward and downward carrier spin.

  FIG. 4 shows a cross-sectional structure diagram of a conventional spin transistor. In this cross-sectional structure diagram, a magnetic electrode (source) 201, a magnetic electrode (drain) 202, a metal gate electrode 203, a metal gate electrode 203, a gate contact layer 204, and a two-dimensional electron gas are formed. A channel layer 205, a spacer layer 206, a carrier supply layer 207, a buffer layer 208, a substrate 209, and an electron 210 schematically showing spin are also shown.

  Normally, Ni—Fe (permalloy) or Co is used as the electrode material for the magnetic electrode (source) 201 and the magnetic electrode (drain) 202. The operating principle of this device is as follows.

  The spin-polarized electrons are injected from the magnetic electrode (source) 201 into the semiconductor two-dimensional electron gas, and the direction of this spin is rotated by an effective magnetic field due to the Rashba spin-orbit interaction. The drain current is controlled by making it parallel or antiparallel to the spin direction.

For example, when the spin in the semiconductor two-dimensional electron gas is reversed, the magnetic current electrode (drain) 202 does not have a density of states for accepting the reversed electron spin, so that the drain current does not flow. On the other hand, when the spin direction is rotated 360 degrees by the gate voltage, the direction becomes the same as that of the magnetic electrode (drain) 202, and a current flows. That is, on / off of the current can be controlled by the rotation angle of the spin (Non-Patent Document 1).
S. Datta and B. Das, Appl. Phys. Lett., Vol. 56, p. 665, 1990.

In order to realize this spin transistor, it is necessary to efficiently inject spin-polarized electrons from the ferromagnetic electrode (drain electrode) represented by the above-mentioned permalloy or Co into the channel layer. However, it has been pointed out theoretically that spin injection from a ferromagnetic electrode into a semiconductor is difficult to obtain a large degree of spin polarization in the semiconductor due to the difference in conductance between the metal and the semiconductor. It has been difficult to realize a spin transistor having (Reference: G. Schmidt, D. Ferrand, LWMolenkamp, ATFilip, and BJvan Wees, Phys. Rev. B, vol. 62, p. R4790, 2000.)
The present invention has been made in view of such a problem, and the object of the present invention is to use an inhomogeneous Rashba spin-orbit interaction so that the spin of the carrier is upward without using any magnetic field. A spin transistor that can be separated downward and has a sufficient switching characteristic by controlling the magnitude of the drain current with a gate electrode by using a superconducting junction in which the drain current flows depending on the degree of spin polarization. Is to realize.

  In order to solve the above-mentioned problems, the present invention according to claim 1 is directed to carrier injection means for injecting carriers, a channel layer for propagating carriers, and a direction perpendicular to the propagation direction of carriers in the channel layer. An electric field control means for generating a spatially non-uniform electric field and an electrode made of a superconductor are connected to the channel layer.

  According to a second aspect of the present invention, in the first aspect, the channel layer is branched in at least two directions in a carrier path through which carriers propagate.

  According to a third aspect of the present invention, in the first or second aspect, the carrier injection means is constituted by a normal conductor.

  According to a fourth aspect of the present invention, in any one of the first to third aspects, a layer having a different thickness is inserted on the channel layer.

  According to the present invention, by using spatial non-uniform Rashba spin-orbit interaction, it is possible to separate the upward and downward of the spin of the carrier without using any magnetic field, and further depending on the spin polarization, By using a superconducting junction through which a drain current flows, the magnitude of the drain current can be controlled by the gate electrode, and a spin transistor having sufficient switching characteristics can be realized.

  FIGS. 1A and 1B are structural diagrams for explaining the element structure of the embodiment of the spin transistor. The structure diagram (a) of the spin transistor includes a normal electrode (source) 101, a superconductive electrode (drain) 102, an electrode 120, a gate electrode 103, and a normal conductor instead of a ferromagnetic material. ,It is shown. FIG. 5B shows a cross-sectional structure of the gate electrode 103 taken along the line AB, the gate electrode 103, the gate contact layer 104, the channel layer 105 in which a two-dimensional electron gas is formed, the spacer layer 106, and the carrier supply. A layer 107, a buffer layer 108, a substrate 109, and a gate insulating layer 110 are shown.

A superconductor suitable for the superconducting electrode (drain) 102 constituting the drain electrode is Nb, NbN, MgB ₂ or an oxide high-temperature superconductor. This superconductor is in contact with a two-dimensional electron gas layer that is a channel layer, and has a thickness of about 100 nm.

The operating temperature of these superconductors is below the superconducting transition temperature and depends on the superconductor material, Nb is about 9K or less, NbN is about 16K, MgB ₂ is about 40K, and the oxide high temperature superconductor is about 130K. It becomes.

  As shown in FIG. 1B, since an effective gate voltage applied to the two-dimensional electron gas 105 is inclined by inserting gate insulating layers 110 having different thicknesses in the y-axis direction, Similarly, the intensity of the spatial non-uniform Rashba spin-orbit interaction of the underlying two-dimensional electron gas 105 causes a spatial gradient in the y-axis direction.

  Therefore, while spin-bearing carriers propagate in this space in the x-axis direction, spin-up carriers and spin-down carriers receive reverse forces, and spin-bearing carriers are spatially separated. For example, if the Y-type branch shown in FIG. 1A is provided behind the gate electrode 103, the upper branch has carriers that have upward spins, and the lower branch has carriers that have downward spins. Flows in.

  In addition, as an electric field control means for generating a spatially non-uniform electric field, a spin gradient orbital interaction is given a spatial gradient by inserting gate insulating layers 110 having different thicknesses in the y-axis direction. However, instead of this, a plurality of gate electrodes (not shown) having a plurality of different gate voltages in the y-axis direction may be provided so that the spin-orbit interaction has a spatial gradient.

  Further, the gate insulating layer 110 may be omitted as a configuration for modulating the thickness of the gate contact layer 104 in the channel direction.

Furthermore, in order to modulate the dielectric constant of the material of the gate insulating layer 110, for example, the thickness of the gate insulating layer 110 is made uniform, and the constituent materials are composed of SiO ₂ to Si ₃ N ₄ and arranged side by side. The rate may be modulated.

  Further, the shape of the gate electrode 103 is described in, for example, literature: J.-I.Ohe, M. Yamamoto, T. Ohtsuki and J. Nitta, “Spin filterring effect in nonuniform spin-orbit interaction,” International Workshop Nanoscale Dynamics and Quantum Coherence. , Sep. 19-23, 2004 (Hamburg).

  When a Y-type branch is provided, when a carrier having an upward spin flows into the upper branch and a carrier having a downward spin flows into the lower branch, the superconducting electrode 102 is placed at the end of the upper branch. As a result of the arrangement, the following phenomenon occurs.

  Generally, at the superconductor / normal conductor interface, when an electron having energy equal to or less than the energy gap Δ of the superconductor is incident from the normal conductor to the superconductor, a Cooper pair is generated on the superconductor side. In order to generate this Cooper pair, another electron is required, but this part is generated as a hole on the normal conductor side.

  That is, incident electrons are reflected as holes. This is called Andreaf reflection, and is a phenomenon that occurs at the superconductor / normal conductor interface. In this Andreff reflection, one electron is incident and one Cooper pair consisting of two electrons is generated, so two net charges are conducted, and the conductance of the system including the interface is a normal normal conductor. This doubles the electrode conductance.

  What is important at this time is that the Cooper pair is composed of carriers having upward spins and carriers having downward spins. That is, in order to cause Andreff reflection, electrons having a spin opposite to that of incident electrons (carriers) are required on the normal conductor side. If the superconducting electrode 102 is polarized so that the spin of the electrons on the upper branch is 100% upward, electrons incident from the two-dimensional electron gas 105 cannot be Andreff-reflected, and superconductivity No current flows through the electrode 102.

  Note that by providing the electrode 120, a potential is applied to the drain electrode that is the end of the Y-shaped branch, and electrons are extracted so that accumulation of electrons does not occur.

  FIG. 2 shows a circuit diagram for measuring the characteristics of the spin transistor of the embodiment. A voltage is applied by placing a power supply 10 between the normal electrode (source) 101 and the superconductive electrode (drain) 102 of the spin transistor. In order to measure the current flowing through the superconducting electrode (drain) 102, an ammeter 11 is disposed between the superconducting electrode (drain) 102 and the power supply 10. The electrode 120 is connected to the power supply 10.

  FIG. 3 shows the dependence of the current-voltage characteristic between the source and drain electrodes of the spin transistor on the applied voltage to the gate electrode 103 as a measurement result based on the circuit diagram shown in FIG. When no gate voltage is applied to the gate electrode 103 and carriers having upward spins and carriers having downward spins are not separated, the conductance is doubled below the gap voltage Δ of the superconductor.

  On the other hand, when a gate voltage is applied and 100% separation of carriers having an upward spin and a downward spin is realized, the conductance becomes zero. That is, the drain current can be controlled by the gate voltage. This is the operating principle of the spin transistor of this embodiment.

  As described above, according to the present embodiment, by using the spatially non-uniform Rashba spin orbit interaction, the gate electrode that can separate the spin upward and downward of the carrier, and the superconductor By using the formed drain electrode, the drain current can be controlled by the degree of spin polarization, and a spin transistor can be formed without using any magnetic field or magnetic substance.

  In addition, when materials are combined so that Andreff reflection is close to 100%, it is not necessary to apply the gate voltage to a voltage that causes pinch-off of the channel, and the drain current can be set to “0”. Power and speed can be made possible.

BRIEF DESCRIPTION OF THE DRAWINGS It is a block diagram of the spin transistor of embodiment, Comprising: (a) shows the whole structure, (b) shows a cross-sectional block diagram. A circuit diagram of characteristic measurement of a spin transistor of an embodiment is shown. The applied voltage characteristic to the gate electrode of the current-voltage characteristic between the source and drain electrodes of the spin transistor of the embodiment is shown. An example of the structure of a conventional spin transistor is shown.

Explanation of symbols

101 Normal electrode (source)
102 Superconducting electrode (drain)
103 Gate electrode 104, 204 Gate contact layer 105, 205 Channel layer 106, 206 Spacer layer 107, 207 Carrier supply layer 108, 208 Buffer layer 109, 209 Substrate 120 Electrode 110 Gate insulating layer 201 Magnetic electrode (source)
202 Magnetic electrode (drain)
203 Metal gate electrode 210 Electron

Claims (4)

Carrier injection means for injecting carriers;
A channel layer that propagates carriers;
An electric field control means for generating a spatially non-uniform electric field in a direction perpendicular to the propagation direction of carriers in the channel layer;
A spin transistor, wherein an electrode made of a superconductor is connected to the channel layer. The channel layer is
The spin transistor according to claim 1, wherein a carrier path through which carriers propagate is branched in at least two directions. The spin transistor according to claim 1 or 2, wherein the carrier injection means is made of a normal conductor. 4. The spin transistor according to claim 1, wherein layers having different thicknesses are inserted on the channel layer.

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