JP5286502B2 - Ferromagnetic semiconductor device - Google Patents

Description

  The present invention relates to a ferromagnetic semiconductor device that changes the properties of a ferromagnetic semiconductor by an electric field.

  In a conventional semiconductor element, electric conductivity is changed by applying a voltage to a nonmagnetic semiconductor to control the carrier concentration of the semiconductor. The rewriting of information for a semiconductor element utilizes electrical conductivity due to a field effect. Since voltage can be applied even with a minute electrode, integration of semiconductor elements can be easily realized. On the other hand, it is necessary to keep electric conductivity constant when reading information from a semiconductor element. Therefore, electric power is always required, and it is not suitable for power saving in the situation of reading information using a semiconductor element.

  On the other hand, in the conventional ferromagnetic element, the micro domain structure of the ferromagnetic substance is associated with information. Therefore, no power is required for holding information, that is, holding the magnetic domain structure, which is suitable for power saving. On the other hand, in order to rewrite information, that is, to change the magnetic domain structure, it is necessary to apply a magnetic field from the outside or to pass a large current. Since a magnet is required to generate an external magnetic field, integration is not easy and a large current flows, so it is not suitable for power saving.

  Therefore, ferromagnetic semiconductor materials having both properties of ferromagnetism and semiconductor are attracting attention. For example, in (In, Mn) As and (Ga, Mn) As, ferromagnetic electric field control has been demonstrated. (Non-Patent Documents 1 to 3).

JP 2005-206890 A

H. Ohno et al., Nature 408,944 (2000) D. Chiba et al., Science 301,943 (2003) D. Chiba et al., Nature 455,5155 (2008)

  However, in the ferromagnetic semiconductors of Mn-doped InAs and Mn-doped GaAs, the Curie temperatures Tc are 60K and 170K, respectively, and these ferromagnetic semiconductors lose their ferromagnetism at a sufficiently low temperature compared to room temperature. It was impossible to operate a device using a ferromagnetic semiconductor at room temperature.

  An object of the present invention is to provide a ferromagnetic semiconductor device capable of operating at room temperature.

  As a result of diligent research, the present inventors have noticed that a ferromagnetic semiconductor made of transition element-doped titanium dioxide has a Curie temperature sufficiently higher than room temperature, and by applying a gate voltage to the ferromagnetic semiconductor. Focusing on the fact that ferromagnetism can be controlled, the present invention has been completed.

In order to achieve the above object, the first configuration of the ferromagnetic semiconductor device in the present invention includes a substrate, a transition element-doped titanium dioxide layer provided on the substrate, and an electrolysis provided on the transition element-doped titanium dioxide layer. And a gate electrode provided in contact with the electrolytic solution, the transition element is Co or Fe, and the electrolytic solution is CsClO ₄ , Sr (ClO ₄ ) ₂ , KClO ₄ , NaClO ₄ , LiClO _4. One or more electrolytes are dissolved in a solvent.
The second configuration of the ferromagnetic semiconductor element in the present invention includes a substrate, a titanium dioxide layer provided on the substrate, a transition element-doped titanium dioxide layer provided on the titanium dioxide layer, and a transition element-doped titanium dioxide layer. And a gate electrode provided in contact with the electrolytic solution, the transition element is Co or Fe, and the electrolytic solution is CsClO ₄ , Sr (ClO ₄ ) ₂ , KClO _4. It is characterized in that one or more electrolytes of NaClO ₄ and LiClO ₄ are dissolved in a solvent .

  According to the present invention, a ferromagnetic semiconductor made of transition element-doped titanium dioxide can be controlled by an electric field at room temperature, and the carrier concentration of the ferromagnetic semiconductor can be controlled. Thus, for example, information reading and writing processes can be performed at room temperature.

It is sectional drawing of the ferromagnetic semiconductor element which concerns on 1st embodiment of this invention. It is sectional drawing of the ferromagnetic semiconductor element which concerns on 2nd embodiment of this invention. In the ferromagnetic semiconductor element in 1st and 2nd embodiment, it is a schematic diagram when a gate voltage is applied to a gate electrode. It is sectional drawing of the ferromagnetic semiconductor element which concerns on 3rd embodiment of this invention. It is sectional drawing of the ferromagnetic semiconductor element which concerns on 4th embodiment of this invention. It is a conceptual diagram of the laser ablation apparatus used when forming a titanium dioxide layer and a transition element dope titanium dioxide layer in each Example. In Example 1, it is a figure which shows the result at the time of not including an annealing process process, (A) is the intensity | strength of RHEED of a titanium dioxide layer, (B) is a RHEED image of a titanium dioxide layer, (C) is cobalt. The intensity | strength of the RHEED of a dope titanium dioxide layer is observed with the AFM of (D). In Example 1, it is a figure which shows the result at the time of including an annealing process process, (A) is the intensity | strength of RHEED of a titanium dioxide layer, (B) is a RHEED image of a titanium dioxide layer, (C) is cobalt. The intensity | strength of the RHEED of a dope titanium dioxide layer is observed with the AFM of (D). In Example 2, it is a figure which shows typically the formation process of the hole bar by fine processing in order to perform the electrical measurement of a cobalt dope titanium dioxide film. It is a graph which shows the temperature dependence of the resistivity of a cobalt dope titanium dioxide layer regarding Example 2. FIG. It is a graph which shows the magnetic field dependence of the anomalous hole conductivity of the cobalt dope titanium dioxide layer created in different oxygen atmosphere regarding Example 2. FIG. It is a graph which shows the result of the magnetization measurement of the cobalt dope titanium dioxide layer created in different oxygen atmosphere regarding Example 2. FIG. It is a graph which shows the relationship between the carrier density | concentration of the cobalt dope titanium dioxide layer produced in different oxygen atmosphere, and anomalous hole conductivity regarding Example 2. FIG. It is a graph which shows the relationship between the longitudinal conductivity and abnormal hole conductivity of the cobalt dope titanium dioxide layer created in different atmosphere regarding Example 2. FIG. FIG. 6 shows the results of Hall measurement in the state where no gate voltage is applied in Example 3, where (A) shows the magnetic field dependence of the anomalous hole conductivity σ _AH and (B) shows the magnetic field dependence of the Hall resistance R _xy . It is a graph which shows. It is a graph which shows the time dependence of resistance at the time of applying a gate voltage regarding Example 3. FIG. FIG. 9 shows the results of Hall measurement with and without applying a gate voltage for Example 3, where (A) shows the magnetic field dependence of anomalous hole conductivity σ _AH and (B) shows the Hall resistance R _xy. It is a graph which shows the magnetic field dependence of. It is a graph which shows the magnetic field dependence of abnormal hole conductivity (sigma) _AH about Example 4 about the result of the hole measurement in the state which applied the state which does not apply a gate voltage. (A) is a graph showing a magnetic phase diagram of a Ti _0.9 Co _0.1 O _2-δ film and a change in carrier concentration and anomalous hole conductivity due to gate voltage application, and (B) is a partially enlarged view thereof. FIG.

Hereinafter, some embodiments of the present invention will be described in detail with reference to the drawings. In addition, the same code | symbol is attached | subjected to the same or corresponding thing, and description is abbreviate | omitted.
FIG. 1 is a sectional view of a ferromagnetic semiconductor device 1 according to the first embodiment of the present invention. The ferromagnetic semiconductor device 1 according to the first embodiment includes a substrate 11, a transition element-doped titanium dioxide layer 12 provided on the substrate 11, and an electrolyte solution 13 provided on the transition element-doped titanium dioxide layer 12. And a gate electrode 14 provided on the electrolytic solution 13.

  FIG. 2 is a cross-sectional view of the ferromagnetic semiconductor device 2 according to the second embodiment of the present invention. The ferromagnetic semiconductor element 2 according to the second embodiment is different from the ferromagnetic semiconductor element 1 according to the first embodiment in that a titanium dioxide layer 15 as a buffer layer is provided on the substrate 11. This is because the transition element-doped titanium dioxide layer 12 is provided with high quality, as will be described in detail in the examples described later. Since the first embodiment and the second embodiment differ depending on the presence or absence of the titanium dioxide layer 15, the following description will focus on the second embodiment.

In the ferromagnetic semiconductor device 2 shown in FIG. 1 and FIG. 2, materials such as LaAlO ₃ , SrTiO ₃ , (La, Sr) (Al, Ta) O ₃ , sapphire, TiO ₂ , glass, etc. are applied to any substrate 11. Can be used. By using the substrate 11 made of any one of LaAlO ₃ , SrTiO ₃ , (La, Sr) (Al, Ta) O ₃ , anatase-type titanium dioxide and anatase-type transition element-doped titanium dioxide can be epitaxially grown. . By using the substrate 11 made of sapphire and rutile-type TiO ₂ , rutile-type titanium dioxide and rutile-type transition element-doped titanium dioxide can be epitaxially grown. When the substrate 11 made of glass is used, a mixture of the rutile type and the anatase type may grow, but the rutile type or the anatase type grows selectively depending on the setting of manufacturing conditions.
Cobalt or iron can be used as the transition element in the transition element-doped titanium dioxide layer 12. A ferromagnetic material is obtained by doping transition elements into titanium dioxide. When cobalt is doped, in Ti _1-x Co _x O _2-δ , x is preferably less than the oxygen deficiency δ in a range where 0 <x <0.1. When the amount of doping is less than Co, the conductivity is lowered and ferromagnetism does not appear.
As the electrolytic solution 13, a polymer solvent such as a polyethylene oxide solvent and a solute such as CsClO ₄ , Sr (ClO ₄ ) ₂ , KClO ₄ , NaClO ₄ , LiClO ₄ or the like can be used. . The gate electrode 14 is made of a metal such as Pt, Au, Ag, Al, or Ti, and may have a layered shape, a wire shape, or a spiral shape. Even if the gate electrode 14 has a single layer structure of an electrode layer made of Au or the like or two or more multilayer structures, a two-layer structure of an adhesion layer made of Ti or the like and an electrode layer made of Au or the like or three or more multilayer structures It may be.

  FIG. 3 is a schematic diagram when a gate voltage is applied to the gate electrode 14 in the ferromagnetic semiconductor elements 1 and 2 in the first and second embodiments. As shown in FIG. 3, when a positive (or negative) gate voltage is applied to the gate electrode 14 with respect to the transition element-doped titanium dioxide layer 12, the cations and anions in the electrolyte 13 are attracted to the electric field and move. An ultrathin electric double layer 23 composed of electrons 21 and ions 22 is formed at the boundary surface between the transition element-doped titanium dioxide layer 12 and the electrolytic solution 13. Thereby, the carrier of a high concentration electron (or hole) increases. Thereby, as will be described later, when a positive gate voltage is applied to the gate electrode 14, the electron concentration increases, and as a result, the Hall resistance decreases. Further, the component contributing to the ferromagnetic magnetization of the hole conductivity, that is, the anomalous hole conductivity increases. Since the transition element-doped titanium dioxide has a Curie temperature higher than the room temperature of 600 K, the ferromagnetic strength is controlled by applying a gate voltage to the gate electrode 14 even at a room temperature of 320 K, for example. Can do.

FIG. 4 is a cross-sectional view of the ferromagnetic semiconductor device 3 according to the third embodiment of the present invention.
In the ferromagnetic semiconductor device 3 according to the third embodiment, in the second embodiment, the insulating layer 18 is formed on the transition element-doped titanium dioxide layer 12 so as to surround the electrolytic solution 13, and further, the insulating layer A source electrode 16 and a drain electrode 17 are provided on the transition element-doped titanium dioxide layer 12 outside the element 18 to form a three-terminal field effect transistor. In this embodiment, by applying a gate voltage to the gate electrode 14, the source electrode is changed by a change in carrier concentration in the channel of the transition element-doped titanium dioxide layer 12 formed between the source electrode 16 and the drain electrode 17. The current flowing between 16 and the drain electrode 17 changes.

  The ferromagnetic semiconductor elements 1 to 3 according to the first to third embodiments of the present invention are field effect transistors in which the electrolyte solution 13 is sandwiched between the transition element-doped titanium dioxide layer 12 and the gate electrode 14. Instead of the electrolytic solution 13, an oxide, that is, a gate oxide may be used. Hereinafter, although the case corresponding to 2nd embodiment is demonstrated, the case of other embodiments is also the same.

FIG. 5 is a cross-sectional view of the ferromagnetic semiconductor device 4 according to the fourth embodiment of the present invention. The ferromagnetic semiconductor device 4 according to the fourth embodiment includes a substrate 11, a titanium dioxide layer 15 provided on the substrate 11, a transition element-doped titanium dioxide layer 12 provided on the titanium dioxide layer 15, and a transition. An oxide layer 19 provided on the element-doped titanium dioxide layer 12 and a gate electrode 14 provided on the oxide layer 19 are included. The oxide layer 19 may be made of any one of AlO _x , HfO ₂ , ZrO ₂ , and MgO.

  Also in the case of the ferromagnetic semiconductor device 4 according to the fourth embodiment, a positive (or negative) gate voltage is applied to the gate electrode 14 with respect to the transition element-doped titanium dioxide layer 12 as in the case described with reference to FIG. Is applied, an ultrathin electric double layer composed of electrons and ions is formed at the interface between the transition element-doped titanium dioxide layer 12 and the oxide layer 19, and high-concentration electron (or hole) carriers are generated. To increase. Thus, when a positive gate voltage is applied to the gate electrode 14, the electron concentration increases, and as a result, the Hall resistance decreases. Further, the component contributing to the ferromagnetic magnetization of the hole conductivity, that is, the anomalous hole conductivity increases. Since the transition element-doped titanium dioxide has a Curie temperature higher than the room temperature of 600 K, the ferromagnetic strength is controlled by applying a gate voltage to the gate electrode 14 even at a room temperature of 320 K, for example. Can do.

Hereinafter, embodiments of the present invention will be described in detail by way of examples.
As a premise for producing the ferromagnetic semiconductor device of the present invention, a method for producing a high-quality titanium dioxide thin film having an anatase structure on a LaAlO ₃ substrate 11 was examined. FIG. 6 is a conceptual diagram of a laser ablation apparatus used when forming the titanium dioxide layer 15 and the transition element-doped titanium dioxide layer 12 in each example.

As shown in FIG. 6, in the laser ablation apparatus 30, a substrate holder 32 is installed in a vacuum chamber 31 that can be evacuated to a high vacuum, and a semiconductor infrared is irradiated so that a laser beam is irradiated toward the back surface of the substrate holder 32. A laser device 33 is provided, and the substrate holder 32 can be heated by a semiconductor infrared laser from the back surface. A target holder 34 is disposed so as to face the substrate holder 32. The KrF excimer laser 35 can be irradiated to the target 34a from outside the vacuum chamber. The vacuum chamber 31 is provided with an introduction pipe 36 for introducing oxygen gas. In the laser ablation apparatus 30, an RHEED (reflection high energy electron diffraction) screen 37 for irradiating the substrate 11 with the electron beam e ⁻ and detecting a diffraction image thereof is provided in the vacuum chamber 31.

The LaAlO ₃ (100) substrate 11 was introduced into the vacuum chamber 31 and heated to 650 ° C. by the semiconductor infrared laser device 33. Thereafter, oxygen gas is introduced into the vacuum chamber 31, and a target 34a made of pure titanium dioxide is ablated with a laser in an oxidizing atmosphere having an oxygen partial pressure of 1 × 10 ⁻³ Torr. Layer 15 was deposited to a thickness of 5 nm. Next, the amount of oxygen vacancies in the material was controlled by controlling the amount of oxygen introduced from the introduction pipe 36, and the cobalt-doped titanium dioxide layer 12 was deposited at 400 ° C.

  FIG. 7 is a diagram showing the results when the annealing process is not included in Example 1, where (A) shows the RHEED intensity of the titanium dioxide layer 15, (B) shows the RHEED image of the titanium dioxide layer 15, ( (C) shows the intensity of RHEED of the cobalt-doped titanium dioxide layer 12, and (D) shows an image observed with an AFM (Atomic force microscope). 7A and 7C, the horizontal axis represents time (minutes), and the vertical axis represents RHEED intensity (arbitrary scale). Under the above conditions, vibration is observed in the RHEED intensity in the buffer layer, that is, the titanium dioxide layer 15 as shown in FIG. 7 (A), whereas in the cobalt-doped titanium dioxide layer as shown in FIG. 7 (C). In No. 12, no vibration of RHEED intensity was observed. When the surface of the prepared thin film was observed by AFM, three-dimensional growth was observed because there was a structure in which crystal grains of about several tens of nanometers gathered as shown in FIG.

Therefore, in order to optimize the cobalt-doped titanium dioxide layer 12, the titanium dioxide layer 15 is produced at an oxygen partial pressure of 1 × 10 ⁻³ Torr and 550 ° C., and then the substrate 11 is heated to 900 ° C. and annealed for 10 minutes. went. Next, the substrate temperature was kept at 250 ° C. on the titanium dioxide layer 15 to form a cobalt-doped titanium dioxide layer 12.

  FIG. 8 is a diagram showing the results when an annealing process is included in Example 1, where (A) shows the RHEED intensity of the titanium dioxide layer 15, (B) shows the RHEED image of the titanium dioxide layer 15, ( (C) shows the intensity of RHEED of the cobalt-doped titanium dioxide layer 12, and (D) shows an image observed by AFM. 8A and 8C, the horizontal axis represents time (minutes), and the vertical axis represents RHEED intensity (arbitrary scale). By this annealing treatment, spots are observed on the Laue sphere in the image on the RHEED screen that was streak immediately after the film formation (see FIGS. 8A and 8B, FIG. 7B), and is flatter. It was found to be a good buffer layer, ie titanium dioxide layer 15. When the cobalt-doped titanium dioxide layer 12 was formed on the planarized buffer layer while maintaining the substrate temperature at 250 ° C., a vibration of the RHEED diffraction intensity indicating two-dimensional growth as monoatomic layer growth was observed (FIG. 8). (See (C)). When the AFM image is observed after producing the cobalt-doped titanium dioxide layer 12, as shown in FIG. 8D, the planarization is improved and a cobalt-doped titanium dioxide film having a good quality anatase structure can be produced. I found out.

  From the above, the substrate 11 is heated to produce the titanium dioxide layer 15 as a buffer layer, and then the substrate 11 is heated and annealed to planarize the titanium dioxide layer 15. It was found that a cobalt-doped titanium dioxide film of anatase can be produced.

In Example 2, the oxygen partial pressure during thin film production was adjusted, the amount of oxygen deficiency as a dopant was changed, and the carrier concentration of the cobalt-doped titanium dioxide layer 12 was controlled. Oxygen vacancies in oxides are accompanied by the generation of oxygen vacancies or interstitial metals and the release of free electrons into the lattice. This electron emission causes n-type electrical conductivity in the oxide semiconductor. Here, the state of electron emission is expressed by the following reaction formula.
Here, O ²⁻ is an oxygen ion at a normal position in the lattice, V _O ″ is an oxygen vacancy having a divalent positive charge, M ²⁺ is a metal ion at a normal position in the lattice, M _i ²⁺ Therefore, the carrier concentration can be changed by modulating the oxygen deficiency δ, so that δ in the Ti _1-x Co _x O _2-δ film This was modulated by changing the oxygen partial pressure _P02 .

  In Example 2, in order to perform electrical measurement of the cobalt-doped titanium dioxide film, a hole bar was formed by fine processing. FIG. 9 is a diagram schematically showing a hole bar forming step by microfabrication in order to perform electrical measurement of a cobalt-doped titanium dioxide film in Example 2.

First, a titanium dioxide layer 15 and a cobalt-doped titanium dioxide layer 12 were sequentially formed on the LaAlO ₃ substrate 11 in the same procedure as in Example 1. When the cobalt-doped titanium dioxide layer 12 was produced, it was produced at different oxygen partial pressures, that is, 1 × 10 ⁻⁶ to 8 × 10 ⁻⁶ Torr.
Next, an electrode pattern was formed on the cobalt-doped titanium dioxide layer 12 as follows. First, the sample was subjected to ultrasonic cleaning with an organic solvent of acetone and ethanol, and dried on a hot plate. Next, a photoresist 41 (SHIPLEY S9912NX) was applied to the sample and spin-coated at 5000 rpm for 30 seconds. The sample was pre-baked on a hot plate at a temperature of 90 ° C. for 15 minutes. It exposed using the photomask for electrode patterns drawn with chromium on the glass plate which is not illustrated separately, developed with the developing solution, and wash | cleaned with the ultrapure water (refer FIG. 9 (A)).
Subsequently, Ti as an adhesion layer and Au as an electrode layer are sequentially deposited on the cobalt-doped titanium dioxide layer 12 using an electron beam deposition method, lifted off with acetone, and an electrode portion 42 having an electrode pattern is formed as shown in FIG. As shown in FIG.
Thereafter, in the same manner as the electrode pattern formation, a mesa pattern is formed on the photoresist 43 as shown in FIG. 9C, and a titanium dioxide layer is formed by dry etching using Ar ions as shown in FIG. 9D. 15 and a mesa pattern were cut out in the cobalt-doped titanium dioxide layer 12.
Finally, Au wire was wired by wire bonding.

As shown in FIG. 9D, the hole bar 50 is formed on the LaAlO ₃ substrate 11 with a rod-shaped portion 51 serving as a channel portion, and a first current terminal 52 and a second current terminal on both sides of the rod-shaped portion 51. The pads 52a and 53a of 53, and the pads 54a, 55a and 56a of the first voltage terminal 54, the second voltage terminal 55 and the third voltage terminal 56 are arranged. The pads 52a and 53a of the first current terminal 52 and the second current terminal 53 are extended by tapered portions 52b and 53b to connect the side surfaces of the rod-shaped portion 51, that is, the first and second current terminals 52 and 53. Touching a surface perpendicular to the imaginary line. The pads 54a and 55a of the first voltage terminal 54 and the second voltage terminal 55 are disposed adjacent to the pad 52a with the pad 52a of the first current terminal 52 interposed therebetween. The first voltage terminal 54 and the second voltage terminal 55 are in contact with the side surface of the rod-shaped portion 51 obliquely along the rod-shaped portion 51 by means of narrow extending portions 54b and 55b from the pads 54a and 55a, respectively. The extending portion 54 b of the first voltage terminal 54 and the extending portion 55 b of the second voltage terminal 55 are arranged symmetrically with respect to the rod-shaped portion 51. The third voltage terminal 56 is composed of a pad 56 a and an extending portion 56 b, and the extending portion 56 b of the third voltage terminal 56 is connected to the extending portion 54 b of the first voltage terminal 54 and the second voltage terminal 55. It is in contact with the side surface of the rod-shaped part 51 at a position different from the extending part 55b. Here, the rod-shaped portion 51 has a laminated structure of the titanium dioxide layer 15 and the cobalt-doped titanium dioxide layer 12. The first current terminal 52, the second current terminal 53, the first voltage terminal 54, the second voltage terminal 55, and the third voltage terminal 56 are composed of the titanium dioxide layer 15, the cobalt-doped titanium dioxide layer 12, and the adhesion layer. And an electrode layer.

  Using the created hole bar 50, the resistivity was determined by four-terminal measurement. At that time, the first current terminal 52, the second current terminal 53, the first voltage terminal 54, and the third voltage terminal 56 were used. For the measurement, an electrical property measuring device with a superconducting magnet (PPMS: Physical Property Measurement System, manufactured by Quantum Design) was used.

FIG. 10 shows the temperature dependence of the resistivity of the cobalt-doped titanium dioxide layer 12. The horizontal axis in FIG. 10 is the absolute temperature T (K), and the vertical axis is the resistivity ρ _xx (Ωcm). Using the first current terminal 52, the second current terminal 53, the first voltage terminal 54, and the third voltage terminal 56, the voltage difference was measured in a direction parallel to the current. PPMS was used for the measurement. The ratio of Ti to Co in the cobalt-doped titanium dioxide layer 12 is 9: 1, that is, Ti _0.9 Co _0.1 O _2−δ (x = 0.1). From the graph shown in FIG. 10, cobalt-doped titanium dioxide having an anatase structure can widely change electrical characteristics from insulating properties to conductive properties.

Hole measurement was performed using the produced hole bar 50. At that time, the first current terminal 52, the second current terminal 53, the first voltage terminal 54, and the second voltage terminal 55 were used. PPMS was used for the measurement. The measurement temperature was 300K. In a ferromagnetic semiconductor, a Hall electromotive force corresponding to magnetization is generated, and the Hall electromotive force actually measured by Hall measurement is a combination of a linear component and a component corresponding to magnetization. FIG. 11 shows the magnetic field dependence of the anomalous hole conductivity of the cobalt-doped titanium dioxide layer 12 prepared in different oxygen atmospheres (1 × 10 ⁻⁶ to 8 × 10 ⁻⁶ Torr). The horizontal axis represents the magnetic field μ ₀ H (T), and the vertical axis represents the anomalous Hall conductivity σ _AH (mΩ ⁻¹ cm ⁻¹ ). When the carrier concentration n is 5 × 10 ¹⁸ cm ⁻³ and 1 × 10 ¹⁹ cm ⁻³ , there is no hysteresis and paramagnetism, but when the carrier concentration n is 4 × 10 ¹⁹ cm ⁻³ and 2 × 10 ²⁰ cm ⁻³ , hysteresis is achieved. It turns out that it is ferromagnetic.

Magnetization measurement was performed using the created hole bar 50. For the measurement, a magnetic property measurement system (MPMS: Magnetic Property Measurement System, manufactured by Quantum Design) was used. The measurement temperature was 300K.
FIG. 12 is a diagram showing the results of magnetization measurement of cobalt-doped titanium dioxide layers prepared in different oxygen atmospheres. The horizontal axis is the magnetic field μ ₀ H (T), and the vertical axis is the magnetization M (μ _B / Co). Since the cobalt doping amount is 10% (x = 0.1), the value represented by (μ _B / Co) is multiplied by about 27.2 / 10000 to obtain a value in (T) units. Magnetic hysteresis began to be observed as the carrier concentration increased, and it was found that the sample with a high carrier concentration showed ferromagnetism and the sample with a small sample was paramagnetic, similar to the result of electrical measurement. That is, it was found that the magnetic characteristics are modulated by the carrier concentration.

FIG. 13 is a magnetic phase diagram showing the relationship between the carrier concentration and the anomalous hole conductivity of cobalt-doped titanium dioxide layers prepared in different oxygen atmospheres. The measurement temperature was 300K. The horizontal axis is the carrier concentration n (cm ⁻³ ), and the vertical axis is the anomalous hole conductivity σ _AH (mΩ ⁻¹ cm ⁻¹ ). In FIG. 13, the black circle (●) plot is the case of the anatase type which is the result of Example 2, and the white square (□) plot is the rutile type shown for reference, which is obtained by multiplying the actual value by 5 times. Show. The anatase type Ti _0.9 Co _0.1 O _2-δ shown in the plot shows ferromagnetism when the carrier concentration is 2 × 10 ¹⁹ cm ⁻³ or more, and the anomalous hole conductivity corresponding to the magnitude of magnetization is the carrier concentration. It turned out that it is increasing. This suggests that ferromagnetism is induced by carriers. In addition, in the cobalt-doped titanium dioxide film, it was found that in the case of the anatase type as compared with the case of the rutile type shown by the □ plot, the magnitude of the carrier concentration at which ferromagnetism appears was about 1/10.

FIG. 14 is a diagram showing the relationship between the longitudinal conductivity and the anomalous hole conductivity of cobalt-doped titanium dioxide layers prepared in different atmospheres. The horizontal axis is the longitudinal conductivity σ _xx (Ω ⁻¹ cm ⁻¹ ), and the vertical axis is the anomalous hole conductivity σ _AH (mΩ ⁻¹ cm ⁻¹ ). From FIG. 14, in the Ti _1-x Co _x O _2-δ film having the anatase structure, the abnormal hole conductivity σ _AH is proportional to the 1.6th power of the longitudinal conductivity σ _xx . Therefore, it is suggested that the Ti _1-x Co _x O _2-δ film having the anatase structure originates from the intrinsic abnormal Hall effect.

A ferromagnetic semiconductor device according to an embodiment of the present invention was produced. First, the titanium dioxide layer 15 is formed on the LaAlO ₃ substrate 11 by the pulse laser deposition method, the cobalt-doped titanium dioxide layer 12 is formed on the titanium dioxide layer 15, and then the hole bar 50 is formed as in the second embodiment. It was created. The hole bar 50 was shaped to immerse the channel portion which is the rod-like portion 51 in the electrolytic solution 13. The electrolytic solution used was a solution of cesium chlorate dissolved in polyethylene oxide at 0.3 mol / liter. Thereafter, a platinum wire was wound in a coil shape as the gate electrode 14 to increase the surface area and immersed in the electrolytic solution, and the electrolytic solution 13 was brought into contact with the gate electrode 14. Then, it put in PPMS at 350K for 1 hour, evacuated, and the water | moisture content in the electrolyte solution 13 was removed. Next, the temperature of the measurement environment was maintained so that the electrolytic solution 13 was in a liquid state of 320 K, and the electrical characteristics were measured as in Example 2.

FIGS. 15A and 15B are diagrams showing the results of Hall measurement in a state where no gate voltage is applied. FIG. 15A shows the magnetic field dependence of the anomalous Hall conductivity σ _AH , and FIG. 15B shows the magnetic field dependence of the Hall resistance R _xy . It is sex. The horizontal axis is the magnetic field μ ₀ H (T), the vertical axis of (A) is the anomalous Hall conductivity σ _AH (× 10 ⁻³ Ω ⁻¹ cm ⁻¹ ), and the vertical axis of (B) is the Hall resistance R _xy. (Ω). The measurement environment was 320K.
When the gate voltage _{V G} is 0V, the results of Hall measurements, carrier concentration n of cobalt-doped titanium dioxide layer 12 is 3.1 × ¹⁰ 19 ^{cm -3,} the hysteresis in the magnetic field dependence of the anomalous Hall conductivity sigma _AH Was observed, but it can be seen that it is very small.
Next, a gate voltage V _G of +2 V was applied. The measurement environment was 320K. Figure 16 is a diagram showing the time dependence of the resistance at the time of applying a gate voltage V _G, the horizontal axis represents time (minutes), which is the vertical axis resistor ^{_{R xx (× 10 3 Ω)}} . When applying the gate voltage V _G, the resistance will not change with decreased after 30 minutes, the resistance as compared with before the application of the gate voltage becomes approximately 1/3. This, by applying a gate voltage V _G to the electrolyte 13, positive and negative ions in the electrolyte 13 is an electric double layer is formed by assembling the gate electrode 14 and the channel surface, the carrier in the channel is induced. Therefore, the resistance decreased. When the resistance R _xx reaches a constant value, the movement of ions is almost completed.

Thereafter, Hall measurement and four-terminal resistance measurement were performed to determine carrier concentration and conductivity. FIG. 17 is a graph showing the results of Hall measurement with and without the gate voltage V _G applied, where (A) shows the magnetic field dependence of the anomalous Hall conductivity σ _AH and (B) shows the Hall resistance. The magnetic field dependence of _Rxy is shown. The horizontal axis is the magnetic field μ ₀ H, the vertical axis is the abnormal hole conductivity σ _AH for (A), and R _xy for (B). The measurement temperature is 320K.
The carrier concentration of the cobalt-doped titanium dioxide layer 12, by applying a gate voltage _{V G,} was increased approximately 3-fold from 3.1 × ¹⁰ 19 ^{cm -3} to 9.0 × ¹⁰ 19 ^{cm -3.} This carrier concentration was derived from the slope of the normal hole term shown in FIG. As described above, the resistance application of the gate voltage V _G becomes 1/3 times has increased carrier concentration and factor.
Anomalous Hall conductivity sigma _AH, as shown in FIG. 17 (A), was increased approximately 2-fold by application of the gate voltage V _G. The abnormal hole conductivity σ _AH was derived by subtracting the normal hole term. It was found that ferromagnetism is enhanced by inducing carriers in the film by applying the gate voltage V _G.

In place of the electrolyte CsClO ₄ in the electrolytic solution 13 used in Example 3, the electrolyte Sr (ClO ₄ ) ₂ was used, and the electrical characteristics were measured in the same manner as in Example 3.
FIG. 18 is a diagram showing the magnetic field dependence of the anomalous hole conductivity σ _AH with respect to the results of hole measurement with and without the gate voltage applied. The horizontal axis is the magnetic field μ ₀ H (T), and the vertical axis is the anomalous Hall conductivity σ _AH (mΩ ⁻¹ cm ⁻¹ ). The measurement temperature is 320K.
The carrier concentration of the cobalt-doped titanium dioxide layer 12, by applying a gate voltage _{V G,} increased about 6 times from 7.0 × ¹⁰ 18 ^{cm -3} to 4.1 × ¹⁰ 19 ^{cm -3.} Anomalous Hall conductivity sigma _AH, as shown in FIG. 18, in a state where a gate voltage is not applied _{V G} hysteresis is very small, 0.1mΩ ^-1 cm ^-1 following values. Hysteresis is the clarity of the gate voltage _{V G} by 2V applied was increased to about 0.3mΩ ^-1 cm ^-1. Thus, by applying a gate voltage V _G, by inducing the carriers in the cobalt-doped titanium dioxide layer 12, it was found that the ferromagnetic is enhanced.

FIG. 19A is a magnetic phase diagram of a Ti _0.9 Co _0.1 O _2-δ film, and shows changes in carrier concentration and anomalous hole conductivity due to application of a gate voltage, and FIG. FIG. 20 is an enlarged view of the data of Sr (ClO ₄ ) ₂ in FIG. The horizontal axis is the carrier concentration n (cm ⁻³ ), and the vertical axis is the abnormal hole conductivity σ _AH (mΩ ⁻¹ cm ⁻¹ ). The dotted line shows the case where a cobalt-doped titanium dioxide film was produced by changing the oxygen partial pressure. In FIG. 19A, the black square (■) plot is a film having the same chemical composition, and CsClO ₄ was used as the electrolyte. a case of applying the gate voltage V _G, a film of black circles (●) plot the same chemical composition, a case of applying the gate voltage V _G with Sr (ClO _{4) 2} in the electrolyte. In FIG. 19B, the ● plot is data when the gate voltage is increased, and the white triangle (Δ) plot is data when the gate voltage is decreased.
From FIG. 19, in the same chemical composition Ti _0.9 Co _0.1 O _2-δ film, the carrier concentration and the anomalous hole conductivity are changed without changing the chemical composition depending on whether or not the gate voltage is applied. I found out that

  As described above, the magnetism of the transition element-doped titanium dioxide can be controlled at room temperature by applying a gate voltage to the transition element-doped titanium dioxide. For various ferromagnetic semiconductor devices that have been demonstrated only at a low temperature compared to room temperature, the transition element-doped titanium dioxide can be used even at room temperature as a ferromagnetic semiconductor.

  According to the present invention, the ferromagnetic control of a ferromagnetic semiconductor at room temperature can be controlled by the gate voltage. Therefore, it can be used as a ubiquitous ferromagnetic semiconductor element such as a magnetic sensor or a spin transport element as a field effect device. In particular, titanium dioxide used as a ferromagnetic semiconductor is chemically stable and can be used as a transparent electrode, and a transition element-doped titanium dioxide film can be produced on a glass substrate by sputtering, so that an automobile. The device can be mounted on the windshield.

1, 2, 3, 4: Ferromagnetic semiconductor device (field effect device)
11: Substrate 12: Transition element doped titanium dioxide layer (cobalt doped titanium dioxide layer)
13: Electrolytic solution 14: Gate electrode 15: Titanium dioxide layer 16: Source electrode 17: Drain electrode 18: Insulating layer 19: Oxide layer (gate oxide layer)
21: Electron 22: Ion 23: Electric double layer 30: Laser ablation device 31: Vacuum chamber 32: Substrate holder 33: Semiconductor infrared laser device 34: Target holder 34a: Target 35: Excimer laser 36: Introduction tube 37: RHEED screen 41, 43: Photoresist 42: Electrode portion 50: Hall bar 51: Bar-shaped portion 52: First current terminal 53: Second current terminal 54: First voltage terminal 55: Second voltage terminal 56: Third Voltage terminals 52a, 53a, 54a, 55a, 56a: pads 52b, 53b: taper portions 54b, 55b, 56b: extension portions

Claims (4)

A substrate,
A transition element-doped titanium dioxide layer provided on the substrate;
An electrolyte provided on the transition element-doped titanium dioxide layer;
A gate electrode provided in contact with the electrolytic solution;
Including
The transition element is Co or Fe;
A ferromagnetic semiconductor device, wherein the electrolytic solution is obtained by dissolving one or more electrolytes of CsClO ₄ , Sr (ClO ₄ ) ₂ , KClO ₄ , NaClO ₄ , LiClO _{4 in} a solvent. A substrate,
A titanium dioxide layer provided on the substrate;
A transition element-doped titanium dioxide layer provided on the titanium dioxide layer;
An electrolyte provided on the transition element-doped titanium dioxide layer;
A gate electrode provided in contact with the electrolytic solution;
Including
The transition element is Co or Fe;
A ferromagnetic semiconductor device, wherein the electrolytic solution is obtained by dissolving one or more electrolytes of CsClO ₄ , Sr (ClO ₄ ) ₂ , KClO ₄ , NaClO ₄ , LiClO _{4 in} a solvent. Wherein in response to the presence or absence of a voltage applied to the gate electrode, the strength of the ferromagnetic changes of the transition element-doped titanium dioxide layer, the ferromagnetic semiconductor device according to claim 1 or 2. The transition element-doped titanium dioxide layer and the source and drain electrodes to isolate across the gate electrode is provided, the ferromagnetic semiconductor device according to claim 1 or 2.