WO2018131532A1 - Thermoelectric conversion element and method for manufacturing same - Google Patents

Thermoelectric conversion element and method for manufacturing same Download PDF

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Publication number
WO2018131532A1
WO2018131532A1 PCT/JP2018/000046 JP2018000046W WO2018131532A1 WO 2018131532 A1 WO2018131532 A1 WO 2018131532A1 JP 2018000046 W JP2018000046 W JP 2018000046W WO 2018131532 A1 WO2018131532 A1 WO 2018131532A1
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thermoelectric conversion
conversion layer
conversion element
iron
fese
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PCT/JP2018/000046
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French (fr)
Japanese (ja)
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清水　直
義宏 岩佐
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国立研究開発法人理化学研究所
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    • C—CHEMISTRY; METALLURGY
    • C01—INORGANIC CHEMISTRY
    • C01B—NON-METALLIC ELEMENTS; COMPOUNDS THEREOF; METALLOIDS OR COMPOUNDS THEREOF NOT COVERED BY SUBCLASS C01C
    • C01B19/00—Selenium; Tellurium; Compounds thereof
    • C01B19/04—Binary compounds including binary selenium-tellurium compounds
    • C—CHEMISTRY; METALLURGY
    • C01—INORGANIC CHEMISTRY
    • C01G—COMPOUNDS CONTAINING METALS NOT COVERED BY SUBCLASSES C01D OR C01F
    • C01G49/00—Compounds of iron
    • C01G49/12—Sulfides
    • H—ELECTRICITY
    • H10—SEMICONDUCTOR DEVICES; ELECTRIC SOLID-STATE DEVICES NOT OTHERWISE PROVIDED FOR
    • H10N—ELECTRIC SOLID-STATE DEVICES NOT OTHERWISE PROVIDED FOR
    • H10N10/00—Thermoelectric devices comprising a junction of dissimilar materials, i.e. devices exhibiting Seebeck or Peltier effects
    • H10N10/10—Thermoelectric devices comprising a junction of dissimilar materials, i.e. devices exhibiting Seebeck or Peltier effects operating with only the Peltier or Seebeck effects
    • H10N10/17—Thermoelectric devices comprising a junction of dissimilar materials, i.e. devices exhibiting Seebeck or Peltier effects operating with only the Peltier or Seebeck effects characterised by the structure or configuration of the cell or thermocouple forming the device
    • H—ELECTRICITY
    • H10—SEMICONDUCTOR DEVICES; ELECTRIC SOLID-STATE DEVICES NOT OTHERWISE PROVIDED FOR
    • H10N—ELECTRIC SOLID-STATE DEVICES NOT OTHERWISE PROVIDED FOR
    • H10N10/00—Thermoelectric devices comprising a junction of dissimilar materials, i.e. devices exhibiting Seebeck or Peltier effects
    • H10N10/80—Constructional details
    • H10N10/85—Thermoelectric active materials
    • H10N10/851—Thermoelectric active materials comprising inorganic compositions
    • H10N10/852—Thermoelectric active materials comprising inorganic compositions comprising tellurium, selenium or sulfur

Definitions

  • the present invention relates to a thermoelectric conversion element and a manufacturing method thereof.
  • this invention relates to the thermoelectric conversion element excellent in the efficiency which produces
  • thermoelectric power generation using a thermoelectric conversion element is expected as a promising candidate because of the advantage that local heat can be used by a small and simple device.
  • the physical phenomenon that is the principle is also called a thermoelectric conversion phenomenon, and has long been known as the Seebeck effect, Peltier effect, and Thomson effect.
  • Non-Patent Documents 1 and 2 This suggests that the reduced carrier conduction, which is easy to move only in the two-dimensional or one-dimensional direction, is more advantageous for the thermoelectric effect in the semiconductor than the three-dimensional carrier conduction.
  • the present inventor also has different thermoelectric effects in three-dimensional conduction and two-dimensional conduction in the electronic conduction of zinc oxide (ZnO), specifically, the efficiency of generating thermoelectromotive force by two-dimensional conduction. (Non-patent Document 3).
  • a value (ZT) called a dimensionless figure of merit is used as a standard for practical use of thermoelectric conversion materials.
  • the dimensionless figure of merit ZT is the product of the figure of merit Z of the thermoelectric conversion material and the absolute temperature T (unit: Kelvin).
  • S the Seebeck coefficient
  • ⁇ the electrical conductivity
  • a practical thermoelectric conversion performance can be expected for a substance having a dimensionless figure of merit ZT of approximately 1 or a value exceeding it.
  • the factor S 2 ⁇ which is a part of the figure of merit Z, is also called a power factor and is one of the indices of the thermoelectric conversion performance of the material.
  • thermoelectric conversion materials Although it has been suggested that lowering the conduction carrier is advantageous for improving the performance of thermoelectric conversion materials, specific methodologies and guidelines for increasing the thermoelectric effect have not necessarily been fully clarified.
  • the present invention clarifies a methodology for realizing a larger thermoelectric effect, and clarifies a guideline for selecting a material capable of realizing high-performance thermoelectric conversion by accumulating knowledge about the thermoelectric conversion effect thereby. Therefore, it contributes to the improvement of the performance of any device that uses thermoelectric conversion as its operating principle.
  • thermoelectric conversion material has succeeded in identifying the properties that affect the performance of the thermoelectric conversion material, and has completed the thermoelectric conversion element based on a new methodology for designing the thermoelectric conversion material.
  • thermoelectric conversion layer having a certain thickness
  • the material forming the thermoelectric conversion layer has a conductive carrier distribution that extends in a two-dimensional direction along the thermoelectric conversion layer
  • a thermoelectric conversion layer in which the effective mass of the conduction carrier is larger than that of free electrons in the direction along the thermoelectric conversion layer, and the electric conductivity of the conduction carrier is larger than the value of the Bi 2 Te 3 material.
  • a thermoelectric conversion element is provided.
  • the material of the thermoelectric conversion layer is represented by FeS 1-xy Se x Te y (where x and y are both 0 or more and 1 or less and x + y is 0 or more and 1 or less).
  • Iron chalcogenide is a compound of iron (Fe) and at least one element selected from the chalcogen element group consisting of sulfur (S), selenium (Se), or tellurium (Te). It is.
  • thermoelectric conversion element a step of preparing a substrate of SrTiO 3 and a chalcogen element group consisting of sulfur (S), selenium (Se), or tellurium (Te) in contact with the surface of the substrate are selected.
  • the manufacturing method of the thermoelectric conversion element including the process to perform is provided.
  • the terminology used in the field to which the present invention belongs and the measuring method thereof are used.
  • the particle responsible for electrical conduction is an electron
  • the value of the effective mass of the conduction carrier can be obtained from cyclotron resonance or a determined band structure at a typical use temperature.
  • generates a thermoelectromotive force by this invention, and its manufacturing method are provided.
  • thermoelectric conversion element 1 is a schematic diagram showing a crystal structure of iron chalcogenide containing FeSe having a crystal structure called a PbO structure, which is employed in an embodiment of the present invention. It is a flowchart which shows the manufacturing method of the thermoelectric conversion element in embodiment of this invention. In the Example of this invention, it is a schematic diagram which shows a mode that the performance of the thermoelectric effect with respect to a film thickness is confirmed, and the structure for adjusting the film thickness of a FeSe thin film by electrochemical etching is shown.
  • FIG. 6 is a schematic diagram showing a specific setup of measurement that obtained the result shown in FIG. 5 in the example of the present invention.
  • the graph (FIG. 7A) showing the film thickness dependence of the power factor S 2 ⁇ determined from the film thickness dependence of the thermoelectromotive force S and the electrical conductivity ⁇ of the FeSe thin film obtained in the example of the present invention, and the FeSe thin film it is a graph of the power factor S 2 sigma of comparison with values of conventional thermoelectric conversion material (FIG. 7B).
  • thermoelectric conversion element an embodiment of the thermoelectric conversion element
  • FIG. 1 shows a configuration of a thermoelectric conversion element in the present embodiment.
  • the thermoelectric conversion element 100 includes a thermoelectric conversion layer 10 formed on the surface of the substrate 2 such as SrTiO 3 (abbreviated as STO) as an example.
  • An exemplary material of the thermoelectric conversion layer 10 is preferably an iron chalcogenide represented by, for example, FeS 1-xy Se x Te y (where x and y are both 0 or more and 1 or less and x + y is 0 or more and 1 or less). (Details will be described later).
  • Each of the pair of electrodes 22 and 24 is attached to each of the first portion 12 and the second portion 14 that are separated from each other in the spread of the thermoelectric conversion layer 10.
  • thermoelectric conversion element 100 the first portion 12 and the second portion 14 that are separated from each other in the thermoelectric conversion layer 10 are set to different temperatures in the stage of the operation of extracting the thermoelectromotive force through the pair of electrodes 22 and 24.
  • heat is transmitted between the heat source 32 and the heat sink 34 via the substrate 2, for example.
  • the specific arrangement of the heat source 32 and the heat sink 34 with respect to the thermoelectric conversion layer 10 is appropriately set according to the situation of implementation.
  • the c-axis of the crystal can be more preferably oriented in the direction of the thickness d.
  • Such crystal orientation can be realized by using the crystal of the substrate 2 as a template whose plane orientation is appropriately set for crystal growth of the thermoelectric conversion layer 10.
  • STO is used for the substrate 2
  • such orientation can be realized by setting the STO of the substrate 2 to have a (001) plane orientation. Note that the substrate 2 is not necessarily required for the operation of the thermoelectric conversion element 100.
  • the dimensionless figure of merit ZT is used to evaluate the efficiency of generating the thermoelectromotive force in the thermoelectric conversion element 100 having the above-described configuration.
  • Z is a figure of merit and T is an absolute temperature (unit: Kelvin).
  • the dimensionless figure of merit ZT is desirably a large value, and a practical thermoelectric effect can be expected for a substance having a value of approximately 1 or more.
  • a bismuth tellurium (Bi 2 Te 3 ) -based material the value is about 0.8 at a room temperature of 300K.
  • thermoelectric conversion layer 10 Furthermore, based on the analysis of the free electron model using the density of states of electrons and the distribution function, the effect of the dimensionality of the spatial extent of conduction carriers in materials such as semiconductors on the Seebeck effect has already been elucidated (non-patented References 1, 2).
  • One of the findings is that lower-order carrier conduction that is easier to move in two-dimensional and one-dimensional directions is more advantageous for the thermoelectric effect than three-dimensional carrier conduction.
  • the conduction carrier distribution spreading in a two-dimensional direction means at least one of the following: (1) The relative electrical conductivity by direction is high in the plane of the two-dimensional plane and low in the direction perpendicular to the plane; (2) When the spatial expansion of the conduction carriers is viewed relative to each direction, the conduction carriers are spread within the plane of the two-dimensional plane and have no overlap, and do not have an overlap spread in the direction perpendicular to the plane.
  • One of these conditions is satisfied in the electron distribution (two-dimensional electron gas) in which the spatial spread is limited to one direction perpendicular to the plane and spreads in the other two directions (in-plane).
  • a conductive carrier that satisfies any of the above conditions is intended, and a two-dimensional electron gas is taken as an example thereof, and a three-dimensional electron gas is taken as an example of a conductive carrier having a three-dimensional distribution in order to contrast it. Also mention.
  • ZT ⁇ m * 3/2 ( ⁇ / ⁇ ) T (About 3D electron gas) ⁇ m * ( ⁇ / ⁇ ) T (About 2D electron gas) It is shown that the relationship is established.
  • m * is the effective mass of conduction carriers (electrons)
  • ⁇ is the mobility (m 2 / V / s)
  • ⁇ is a symbol indicating that both sides are in a proportional relationship.
  • the bismuth tellurium (Bi 2 Te 3 ) -based material which is a known thermoelectric conversion material having a value of about 0.8 as ZT, is a case of a three-dimensional electron gas.
  • the desirable properties for the efficiency of thermoelectric conversion are, firstly, that the material forming the thermoelectric conversion layer has a conduction carrier distribution that extends in a two-dimensional direction along the thermoelectric conversion layer, Secondly, the effective mass of the conductive carrier is large, and thirdly, the mobility of the conductive carrier is large and the electric conductivity is large.
  • the effective mass of the conductive carrier is preferably larger than the mass of free electrons (static mass).
  • the electric conductivity of the conductive carrier is the electric conductivity of Bi 2 Te 3 material (for example, 5000 S / cm, VA Kulbachinskii et al., “Thermoelectric properties of Bi 2 Te 3 , Sb 2 Te 3 and Bi 2 Se 3. larger than single crystals with magnetic impurities, "J. Solid State Chem. 193, 47 (2012).).
  • the chalcogen element is sulfur, selenium, or tellurium
  • they are sequentially called iron (II) sulfide (II), iron selenide (or iron selenium) FeSe, and iron telluride (or iron tellurium) FeTe.
  • iron selenium FeSe has attracted attention due to several unique properties.
  • 9K approximately at bulk, in the thin film is a superconductor having a superconducting transition temperature T C in excess of 50K.
  • the superconducting transition temperature T C is added or pressure, also increased by intercalation of other atoms (K, Rb, Cs, etc.), ammonia molecule to iron atoms interlayer.
  • K, Rb, Cs, etc. other atoms
  • ammonia molecule to iron atoms interlayer Although it is an iron compound, it does not show magnetic order. It has a nematic order associated with a structural phase transition near 90K.
  • FIG. 2 is a schematic diagram showing a crystal structure of iron chalcogenide containing FeSe having a crystal structure called a PbO structure.
  • the iron Fe atoms are spread and arranged in a plane perpendicular to the c-axis, and the iron Fe atoms in different planes are arranged at positions overlapping in the c-axis direction.
  • Selenium Se is arranged at the position of the chalcogen element Ch in FIG. 2, and is arranged at a position deviating from the plane of the iron atom by shifting in the c-axis direction in the middle of each iron atom.
  • the effective mass m * has about three times the value of almost free electron mass (electron rest mass) m e (Non-Patent Document 5).
  • the crystal structure and basic physical and chemical properties of FeSe are represented by the general formula FeS 1-xy Se x Te y (where x and y are both 0 or more and 1 or less, and x + y is 0 or more and 1 or less). It is common to iron chalcogenides having a composition. For this reason, it can be said that the iron chalcogenide in which the chalcogen element is selected from sulfur (S), selenium (Se), or tellurium (Te) is suitable for the method for the thermoelectric conversion element proposed by the present inventor.
  • FIG. 3 is a flowchart showing a manufacturing method of the thermoelectric conversion element 100 of the present embodiment.
  • the thermoelectric conversion element 100 is roughly divided into three steps.
  • the first is a substrate preparation step (S02).
  • a substrate suitable for crystal growth such as SrTiO 3 prepared so as to have an appropriate plane orientation such as the (001) plane orientation is prepared and mounted on an appropriate apparatus such as a film forming apparatus.
  • a step of cleaning the surface can be added if necessary.
  • the second is a thermoelectric conversion layer forming step (S04).
  • a thermoelectric conversion layer is formed on the surface of the prepared substrate by any film forming method.
  • the MBE (Molecular Beam Epitaxy) method and the PLD (pulsed laser deposition) method can be employed as the film forming method.
  • thermoelectric conversion layer to be formed is an iron chalcogenide crystal film in this embodiment, and at least one element selected from a chalcogen element group consisting of sulfur (S), selenium (Se), or tellurium (Te). It is a compound with iron, and a typical FeSe thin film.
  • an electrode forming step is performed in which each of the paired electrodes is formed on each of the first portion and the second portion that are separated from each other in the thermoelectric conversion layer (S06). This step is a step of forming an appropriate metal film in a necessary range pattern that realizes ohmic connection with the thermoelectric conversion layer and is easily connected to external wiring.
  • an optional step that is useful in implementation as an optional step can be added at an arbitrary timing.
  • the step of removing the substrate when the substrate is unnecessary, the step of collecting and accumulating a large number of formed thermoelectric conversion layers, the step of adjusting the thickness of the thermoelectric conversion layer afterwards, and shaping the outer shape of the thermoelectric conversion layer The process of performing etc. can be implemented as an option.
  • thermoelectric conversion element of the present embodiment examples for confirming the performance of the thermoelectric conversion element of the present embodiment will be described.
  • the materials, amounts used, ratios, processing contents, processing procedures, directions of elements or members, specific arrangements, and the like shown in the following examples can be appropriately changed without departing from the gist of the present invention. Therefore, the scope of the present invention is not limited to the following specific examples.
  • thermoelectric conversion element of the present embodiment can be implemented by forming a FeSe thin film having an appropriate thickness on one surface of an appropriate substrate by a general film forming method.
  • (001) -oriented SrTiO 3 is used as an example of the substrate
  • a known PLD method is used as an example of the film forming method
  • the film thickness of the FeSe thin film is first set to a thickness of about 20 nm.
  • a FeSe thin film sample for the thermoelectric conversion element 100 was prepared by the manufacturing method shown in FIG. The produced FeSe thin film sample can be used as it is as the thermoelectric conversion element 100, but here, the thickness of the FeSe thin film was sequentially changed so as to decrease later, and the relationship between the thickness and the characteristics was investigated.
  • the thickness of the FeSe thin film as the thermoelectric conversion layer 10 was adjusted as follows. First, an ionic liquid is sandwiched between a sample FeSe thin film and a platinum electrode, and a voltage of 5 V is applied while maintaining a temperature near 245K. This schematic diagram is shown in FIG. At the temperature, atoms on the outermost surface of the thermoelectric conversion layer 10 which is an FeSe thin film disposed in contact with the substrate 2 are dissolved in the ionic liquid and subjected to electrochemical etching. Thereby, the film thickness of the FeSe thin film can be reduced.
  • the film thickness is sequentially reduced with high controllability from the as-deposited 20 nm to the single layer (0.6 nm) which is the minimum film thickness, and the thermoelectric effect is measured at each film thickness in the middle.
  • DEME-TFSI N-diethyl-N- (2-methoxyethyl) -N-methylammoniumbis- (trifluoromethylsulfonyl) -imide
  • thermoelectric effect Measurement Thermoelectric Effect was measured while the electrochemical etching was performed and the film thickness of the FeSe thin film was gradually reduced. The measurement was performed at a temperature (200K) at which etching stops.
  • FIG. 5 is a graph showing the results. As shown in FIG. 5, a phenomenon was observed in which the Seebeck effect increased as the film thickness decreased.
  • FIG. 6 is a schematic diagram showing a specific setup for this measurement. In order to measure the thermoelectric effect characteristics by applying the structure of the thermoelectric conversion element 100 shown in FIG. 1, this setup uses a platinum electrode via an ionic liquid on the side opposite to the substrate 2 of the thermoelectric conversion layer 10.
  • the electrodes 22 and 24 are added so as to face each other, and the electrodes 22 and 24 are respectively configured as a source electrode and a drain electrode, and the added platinum electrode is used as a gate electrode to have an electrode configuration similar to an FET (field effect transistor).
  • a temperature difference is generated at both ends of the sample using the heater of FIG.
  • the resulting temperature difference ⁇ T was measured using a thermocouple.
  • thermoelectromotive force Using a device structure as shown in FIG. 6, it is possible to measure not only the thermoelectromotive force but also the electrical conductivity at the same time. Specifically, a current was passed by application of a voltage V DS between the electrodes spaced apart in FeSe film. Then, if the voltage drop of the sample is measured using a thermocouple, the electrical resistance can be obtained by Ohm's law. The electrical conductivity can be obtained from this value and the film thickness. By measuring the electrical conductivity while changing the film thickness by etching, the film thickness dependence of the electrical conductivity was measured.
  • FIG. 7B compares the power factor S 2 ⁇ of this FeSe thin film with the value obtained with the conventional bulk material of the thermoelectric conversion material. As described above, it was confirmed that the FeSe thin film realizes a power factor larger than that of the conventionally known substances.
  • thermoelectric conversion layer 10 PbO-structured iron selenium (FeSe) is formed by forming the thermoelectric conversion layer 10 with the c-axis oriented in the thickness direction with respect to the (001) -oriented STO substrate.
  • This is an example suitable for the use of the thermoelectric conversion element 100 in which a temperature difference is generated at different positions in the surface of the thermoelectric conversion layer 10 in order to form a two-dimensional electron gas that spreads in the plane including the atoms.
  • the preferable thermoelectric conversion layer 10 has a thickness of 12 nm or less, more preferably 9 nm or less.
  • the PbO structure iron selenium (FeSe) has a thickness of 12 nm or less, a power factor value larger than that of NaCoO 2 showing a large power factor value among conventional thermoelectric conversion materials is realized, and when the thickness is 9 nm or less, NaCoO 2 This is because a significantly large power factor value is realized.
  • thermoelectric conversion layer 10 high performance can be expected by any means for promoting the reduction in the order of conduction carriers (electrons) in the thermoelectric conversion layer 10.
  • any means capable of suppressing the three-dimensional expansion and helping the two-dimensional property to appear or avoiding the weakening of the two-dimensional property can be employed.
  • techniques such as intercalation in which atoms that are neither iron nor chalcogen elements intervene in correlation, selective carrier doping on the surface of FeSe, particularly delta doping that realizes a two-dimensional carrier distribution, and the like are high performance of the thermoelectric conversion layer 10. It is useful for
  • thermoelectric conversion element based on the methodology proposed in the present embodiment and the selection of a material that embodies the methodology, an innovative thermoelectric conversion element can be realized.
  • thermoelectric conversion element of the present invention can be used for any device that uses thermoelectromotive force.

Abstract

In order to provide a thermoelectric conversion element that generates thermoelectromotive force with high efficiency, an embodiment of the present invention provides a thermoelectric conversion element 100 having a thermoelectric conversion layer 10, in which a charge carrier distribution extends in directions along two dimensions, the effective mass of the charge carriers is larger than that of free electrons, and the electrical conductivity due to the charge carriers is larger than that of a Bi2Te3-based material. Preferably, the thermoelectric conversion layer 10 is made of iron chalcogenide represented by FeS1-x-ySexTey (where, x and y both are 0-1, and x+y is 0-1). An embodiment of the present invention also provides a method for manufacturing the thermoelectric conversion element 100.

Description

熱電変換素子およびその製造方法Thermoelectric conversion element and manufacturing method thereof
　本発明は熱電変換素子およびその製造方法に関する。さらに詳細には本発明は熱起電力を生成する効率に優れた熱電変換素子およびその製造方法に関する。 The present invention relates to a thermoelectric conversion element and a manufacturing method thereof. In more detail, this invention relates to the thermoelectric conversion element excellent in the efficiency which produces | generates a thermoelectromotive force, and its manufacturing method.
　近年、ＩｏＴ（Internet of Things）などの用途のために分散配置される機器の電源のために、環境に存在する熱、振動などの散逸されるのみであったエネルギーを利用するエナジーハーベスティングが精力的に研究されている。局所的な熱を小型で簡便な装置により利用できる利点のためにその有力な候補として期待されているのが、熱電変換素子を利用する熱電発電である。その原理となる物理現象は、熱電変換現象とも呼ばれ、ゼーベック効果やペルチエ効果、トムソン効果として古くから知られている。 In recent years, energy harvesting that uses energy that has only been dissipated, such as heat and vibration existing in the environment, for the power supply of equipment that is distributed for applications such as IoT (Internet） of 精 Things) Has been studied. Thermoelectric power generation using a thermoelectric conversion element is expected as a promising candidate because of the advantage that local heat can be used by a small and simple device. The physical phenomenon that is the principle is also called a thermoelectric conversion phenomenon, and has long been known as the Seebeck effect, Peltier effect, and Thomson effect.
　熱電変換素子に使用される材料すなわち熱電変換材料の性能向上が求められてきたところ、１９９０年代になると低次元系における量子力学的側面の解析が進み、伝導キャリアの空間的次元性とゼーベック効果との関係が解き明かされた（非特許文献１、２）。そこで示唆されるのは、３次元的なキャリア伝導に比べ、２次元や１次元の方向にのみ動きやすいという低次元化されたキャリア伝導こそが半導体での熱電効果に有利、というものである。本発明者も、酸化亜鉛（ＺｎＯ）の電子伝導において、３次元的伝導と２次元的伝導とにおける熱電効果が相異すること、具体的には２次元的伝導により熱起電力を生成する効率が高まることを確認している（非特許文献３）。 Improvements in the performance of materials used for thermoelectric conversion elements, ie thermoelectric conversion materials, have been demanded, but in the 1990s, analysis of quantum mechanical aspects in low-dimensional systems advanced, and the spatial dimensionality of conduction carriers and the Seebeck effect The relationship was solved (Non-Patent Documents 1 and 2). This suggests that the reduced carrier conduction, which is easy to move only in the two-dimensional or one-dimensional direction, is more advantageous for the thermoelectric effect in the semiconductor than the three-dimensional carrier conduction. The present inventor also has different thermoelectric effects in three-dimensional conduction and two-dimensional conduction in the electronic conduction of zinc oxide (ZnO), specifically, the efficiency of generating thermoelectromotive force by two-dimensional conduction. (Non-patent Document 3).
　熱電変換材料の実用化の目安として無次元性能指数と呼ばれる値（ＺＴ）が用いられる。この無次元性能指数ＺＴは、熱電変換材料の性能指数（figure of merit）Ｚと絶対温度Ｔ（単位：ケルビン）との積である。ここでの性能指数Ｚは、Ｚ＝Ｓ２σ／κ、ただしＳはゼーベック係数（μＶ／Ｋ）、σは電気伝導率（Ω－１ｃｍ－１）、κは熱伝導率（Ｗ／（ｍ・Ｋ））、と表される。無次元性能指数ＺＴが概ね１またはそれを越す値を示す物質には実用的な熱電変換性能を期待することができる。また性能指数Ｚの一部である因子Ｓ２σはパワーファクターとも呼ばれ、材質の熱電変換性能の指標の一つである。 A value (ZT) called a dimensionless figure of merit is used as a standard for practical use of thermoelectric conversion materials. The dimensionless figure of merit ZT is the product of the figure of merit Z of the thermoelectric conversion material and the absolute temperature T (unit: Kelvin). The figure of merit Z here is Z = S 2 σ / κ, where S is the Seebeck coefficient (μV / K), σ is the electrical conductivity (Ω −1 cm −1 ), and κ is the thermal conductivity (W / ( m · K)). A practical thermoelectric conversion performance can be expected for a substance having a dimensionless figure of merit ZT of approximately 1 or a value exceeding it. The factor S 2 σ, which is a part of the figure of merit Z, is also called a power factor and is one of the indices of the thermoelectric conversion performance of the material.
　熱電変換材料の高性能化には伝導キャリアの低次元化が有利であることが示唆されてきたものの、熱電効果を増大させる具体的な方法論や指針は必ずしも十分に明らかにはされてこなかった。本発明は、より大きな熱電効果を実現するための方法論を明らかにすること、およびそれによる熱電変換効果についての知見を蓄積することにより高性能な熱電変換を実現しうる材料選択の指針を明かにし、もって熱電変換を動作原理とする任意の装置の高性能化に寄与するものである。 Although it has been suggested that lowering the conduction carrier is advantageous for improving the performance of thermoelectric conversion materials, specific methodologies and guidelines for increasing the thermoelectric effect have not necessarily been fully clarified. The present invention clarifies a methodology for realizing a larger thermoelectric effect, and clarifies a guideline for selecting a material capable of realizing high-performance thermoelectric conversion by accumulating knowledge about the thermoelectric conversion effect thereby. Therefore, it contributes to the improvement of the performance of any device that uses thermoelectric conversion as its operating principle.
　本発明者は、熱電変換材料において性能を左右する要素となる性質を特定することに成功し、熱電変換材料の設計の新たな方法論に基づいて熱電変換素子を完成させた。 The present inventor has succeeded in identifying the properties that affect the performance of the thermoelectric conversion material, and has completed the thermoelectric conversion element based on a new methodology for designing the thermoelectric conversion material.
　すなわち、本発明のある態様においては、ある厚みの熱電変換層であって、該熱電変換層をなす材質が、該熱電変換層に沿った２次元的な方向に拡がる伝導キャリア分布を持ち、該熱電変換層に沿う向きにおいて、該伝導キャリアの有効質量が自由電子のものよりも大きく、該伝導キャリアによる電気伝導率がＢｉ２Ｔｅ３系材料の値よりも大きいものである、熱電変換層を備えている熱電変換素子が提供される。 That is, in one aspect of the present invention, a thermoelectric conversion layer having a certain thickness, the material forming the thermoelectric conversion layer has a conductive carrier distribution that extends in a two-dimensional direction along the thermoelectric conversion layer, and A thermoelectric conversion layer in which the effective mass of the conduction carrier is larger than that of free electrons in the direction along the thermoelectric conversion layer, and the electric conductivity of the conduction carrier is larger than the value of the Bi 2 Te 3 material. A thermoelectric conversion element is provided.
　上記態様において、好ましくは、前記熱電変換層の前記材質が、ＦｅＳ１－ｘ－ｙＳｅｘＴｅｙ（ただしｘ、ｙはともに０以上１以下、かつｘ＋ｙは０以上１以下）により表される鉄カルコゲナイドである。ここで、上記組成式にて特定される鉄カルコゲナイドは、硫黄（Ｓ）、セレン（Ｓｅ）またはテルル（Ｔｅ）からなるカルコゲン元素群から選択される少なくとも一の元素と鉄（Ｆｅ）との化合物である。 In the above aspect, preferably, the material of the thermoelectric conversion layer is represented by FeS 1-xy Se x Te y (where x and y are both 0 or more and 1 or less and x + y is 0 or more and 1 or less). Iron chalcogenide. Here, the iron chalcogenide specified by the above composition formula is a compound of iron (Fe) and at least one element selected from the chalcogen element group consisting of sulfur (S), selenium (Se), or tellurium (Te). It is.
　さらに本発明者は、熱電変換素子の作製方法も創出した。すなわち、本発明のある態様では、ＳｒＴｉＯ３の基板を準備する工程と、該基板の表面に接して硫黄（Ｓ）、セレン（Ｓｅ）、またはテルル（Ｔｅ）からなるカルコゲン元素群から選択される少なくとも一の元素と鉄との化合物である鉄カルコゲナイドの熱電変換層を形成する工程と、該熱電変換層の広がりにおいて互いに離間した第１部分および第２部分それぞれに対をなす電極の各々を接続する工程とを含む熱電変換素子の製造方法が提供される。 Furthermore, the inventor has also created a method for producing a thermoelectric conversion element. That is, in one aspect of the present invention, a step of preparing a substrate of SrTiO 3 and a chalcogen element group consisting of sulfur (S), selenium (Se), or tellurium (Te) in contact with the surface of the substrate are selected. A step of forming a thermoelectric conversion layer of iron chalcogenide, which is a compound of at least one element and iron, and connecting each of the pair of electrodes to the first portion and the second portion separated from each other in the extent of the thermoelectric conversion layer The manufacturing method of the thermoelectric conversion element including the process to perform is provided.
　なお、本出願においては不明瞭にならない限り本発明の属する分野における慣用に従った用語法やその測定方法を利用する。例えば伝導キャリアの有効質量の値は、電気伝導を担う粒子が電子であれば、典型的な使用温度でサイクロトロン共鳴や、決定したバンド構造から求めることができる。 In this application, unless otherwise obscured, the terminology used in the field to which the present invention belongs and the measuring method thereof are used. For example, if the particle responsible for electrical conduction is an electron, the value of the effective mass of the conduction carrier can be obtained from cyclotron resonance or a determined band structure at a typical use temperature.
　本発明により熱起電力を生成する効率に優れた熱電変換素子およびその製造方法が提供される。
The thermoelectric conversion element excellent in the efficiency which produces | generates a thermoelectromotive force by this invention, and its manufacturing method are provided.
本発明の実施形態における熱電変換素子の構成を示す斜視模式図である。It is a perspective schematic diagram which shows the structure of the thermoelectric conversion element in embodiment of this invention. 本発明の実施形態に採用される、ＰｂＯ構造と呼ばれる結晶構造のＦｅＳｅを含む鉄カルコゲナイドの結晶構造を示す模式図である1 is a schematic diagram showing a crystal structure of iron chalcogenide containing FeSe having a crystal structure called a PbO structure, which is employed in an embodiment of the present invention. 本発明の実施形態における熱電変換素子の製造方法を示すフローチャートである。It is a flowchart which shows the manufacturing method of the thermoelectric conversion element in embodiment of this invention. 本発明の実施例において、膜厚に対する熱電効果の性能を確認する様子を示す模式図であり、電気化学エッチングによりＦｅＳｅ薄膜の膜厚を調整するための構成を示す。In the Example of this invention, it is a schematic diagram which shows a mode that the performance of the thermoelectric effect with respect to a film thickness is confirmed, and the structure for adjusting the film thickness of a FeSe thin film by electrochemical etching is shown. 本発明の実施例において電気化学エッチングを行いＦｅＳｅ薄膜の膜厚を徐々に減少させながら熱電効果を測定した結果を示すグラフである。It is a graph which shows the result of having measured the thermoelectric effect, performing electrochemical etching in the Example of this invention, and reducing the film thickness of a FeSe thin film gradually. 本発明の実施例において図５に示した結果を得た測定の具体的なセットアップを示す模式図である。FIG. 6 is a schematic diagram showing a specific setup of measurement that obtained the result shown in FIG. 5 in the example of the present invention. 本発明の実施例において得られたＦｅＳｅ薄膜の熱起電力Ｓと電気伝導率σの膜厚依存性から求めたパワーファクターＳ２σの膜厚依存性を示すグラフ（図７Ａ）および同ＦｅＳｅ薄膜のパワーファクターＳ２σを従来の熱電変換材料の値と比較したグラフ（図７Ｂ）である。The graph (FIG. 7A) showing the film thickness dependence of the power factor S 2 σ determined from the film thickness dependence of the thermoelectromotive force S and the electrical conductivity σ of the FeSe thin film obtained in the example of the present invention, and the FeSe thin film it is a graph of the power factor S 2 sigma of comparison with values of conventional thermoelectric conversion material (FIG. 7B).
　以下、図面を参照して本発明に係る熱電変換現象の改良の原理を説明し、さらに熱電変換素子の実施形態を説明する。当該説明に際し特に言及がない限り、共通する部分または要素には共通する参照符号が付されている。 Hereinafter, the principle of improving the thermoelectric conversion phenomenon according to the present invention will be described with reference to the drawings, and an embodiment of the thermoelectric conversion element will be described. Unless otherwise stated in the description, common portions or elements are denoted by common reference symbols.
１．原理
１－１．構造
　図１に本実施形態における熱電変換素子の構成を示す。熱電変換素子１００は、一例としてＳｒＴｉＯ３（ＳＴＯと略記する）などの基板２の表面に形成された、熱電変換層１０を備えている。熱電変換層１０の例示の材質は、好ましくは、例えばＦｅＳ１－ｘ－ｙＳｅｘＴｅｙ（ただしｘ、ｙはともに０以上１以下、かつｘ＋ｙは０以上１以下）により表される鉄カルコゲナイドである（詳細は後述する）。熱電変換層１０の広がりにおいて互いに離間した第１部分１２および第２部分１４それぞれには、対をなす電極２２、２４の各々が取り付けられている。熱電変換素子１００は、対をなす電極２２、２４を通じて熱起電力を取り出す動作の段階において、熱電変換層１０のうち、互いに離間した第１部分１２と第２部分１４とが互いに異なる温度とされる。このために、図に例示したように、例えば基板２を介して熱源３２およびヒートシンク３４との間で熱が伝達するようになっている。熱源３２およびヒートシンク３４の熱電変換層１０に対する具体的配置は実施の状況に合わせ適宜設定される。 1. Principle 1-1. Structure FIG. 1 shows a configuration of a thermoelectric conversion element in the present embodiment. The thermoelectric conversion element 100 includes a thermoelectric conversion layer 10 formed on the surface of the substrate 2 such as SrTiO 3 (abbreviated as STO) as an example. An exemplary material of the thermoelectric conversion layer 10 is preferably an iron chalcogenide represented by, for example, FeS 1-xy Se x Te y (where x and y are both 0 or more and 1 or less and x + y is 0 or more and 1 or less). (Details will be described later). Each of the pair of electrodes 22 and 24 is attached to each of the first portion 12 and the second portion 14 that are separated from each other in the spread of the thermoelectric conversion layer 10. In the thermoelectric conversion element 100, the first portion 12 and the second portion 14 that are separated from each other in the thermoelectric conversion layer 10 are set to different temperatures in the stage of the operation of extracting the thermoelectromotive force through the pair of electrodes 22 and 24. The For this reason, as illustrated in the figure, heat is transmitted between the heat source 32 and the heat sink 34 via the substrate 2, for example. The specific arrangement of the heat source 32 and the heat sink 34 with respect to the thermoelectric conversion layer 10 is appropriately set according to the situation of implementation.
　熱電変換層１０の材質が好ましい例であるＦｅＳ１－ｘ－ｙＳｅｘＴｅｙであるとき、さらに好ましくは、厚みｄの方向にその結晶のｃ軸を向けることができる。このような結晶配向は、基板２の結晶を熱電変換層１０の結晶成長のために面方位が適切に設定されたテンプレートとすることにより実現できる。例えばＳＴＯを基板２に採用する場合、このような配向は基板２のＳＴＯを（００１）面方位のものとすることにより実現できる。なお、熱電変換素子１００の動作のために基板２は必ずしも必要ではない。 When the material of the thermoelectric conversion layer 10 is FeS 1-xy Se x Te y which is a preferred example, the c-axis of the crystal can be more preferably oriented in the direction of the thickness d. Such crystal orientation can be realized by using the crystal of the substrate 2 as a template whose plane orientation is appropriately set for crystal growth of the thermoelectric conversion layer 10. For example, when STO is used for the substrate 2, such orientation can be realized by setting the STO of the substrate 2 to have a (001) plane orientation. Note that the substrate 2 is not necessarily required for the operation of the thermoelectric conversion element 100.
１－２．方法論
　上述した構成の熱電変換素子１００において熱起電力を生成する効率の評価のためには、無次元性能指数ＺＴが用いられる。ただし、Ｚは性能指数（figure of merit）、Ｔは絶対温度（単位：ケルビン）である。無次元性能指数ＺＴは、
　ＺＴ＝Ｓ２σＴ／κ、
ただしＳはゼーベック係数（μＶ／Ｋ）、σは電気伝導率（Ω－１ｃｍ－１）、κは熱伝導率（Ｗ／（ｍ・Ｋ））、と表される。この無次元性能指数ＺＴは大きな値が望ましく、概ね１またはそれを越す値を示す物質には実用的な熱電効果を期待することができる。公知の熱電変換材料の一例として、ビスマステルル（Ｂｉ２Ｔｅ３）系材料で無次元性能指数ＺＴを求めると、室温３００Ｋにて約０．８程度の値となる。 1-2. Methodology The dimensionless figure of merit ZT is used to evaluate the efficiency of generating the thermoelectromotive force in the thermoelectric conversion element 100 having the above-described configuration. However, Z is a figure of merit and T is an absolute temperature (unit: Kelvin). The dimensionless figure of merit ZT is
ZT = S 2 σT / κ,
Where S is the Seebeck coefficient (μV / K), σ is the electrical conductivity (Ω −1 cm −1 ), and κ is the thermal conductivity (W / (m · K)). The dimensionless figure of merit ZT is desirably a large value, and a practical thermoelectric effect can be expected for a substance having a value of approximately 1 or more. As an example of a known thermoelectric conversion material, when the dimensionless figure of merit ZT is obtained with a bismuth tellurium (Bi 2 Te 3 ) -based material, the value is about 0.8 at a room temperature of 300K.
　さらに、電子の状態密度および分布関数を利用した自由電子モデルの解析に基づいて、半導体などの物質における伝導キャリアの空間的広がりの次元性のゼーベック効果への影響がすでに解き明かされている（非特許文献１、２）。その知見の一つが、３次元的なキャリア伝導に比べ２次元や１次元の方向に動きやすいような低次元化されたキャリア伝導こそが熱電効果に有利、というものである。図１に示した熱電変換素子１００において生じる上記温度差を利用する場合、熱電変換層１０の層の広がりに沿う向きの２次元的な方向に拡がる伝導キャリア分布を持つことが有利である。なお、２次元的な方向に拡がる伝導キャリア分布とは、次の少なくともいずれかを意味している：
　（１）方向別の相対的な電気伝導率が２次元平面の面内で高くその平面に垂直な面直向きで低いこと、
　（２）伝導キャリアの空間的広がりが、方向別に相対的にみたとき、２次元平面の面内で広がっていて重なりを持ちその平面に垂直な面直向きで広がった重なりを持たないこと。
空間的広がりが面直の１方向には制限され他の２方向（面内）に広がっている電子分布（２次元電子ガス）においてはこれらの条件のいずれかを満たす。このため、以下の説明では上記条件のいずれかを満たす伝導キャリアを意図しその一例として２次元電子ガスを取り上げ、それと対照させるために３次元的に拡がる分布の伝導キャリアの一例として３次元電子ガスにも言及する。 Furthermore, based on the analysis of the free electron model using the density of states of electrons and the distribution function, the effect of the dimensionality of the spatial extent of conduction carriers in materials such as semiconductors on the Seebeck effect has already been elucidated (non-patented References 1, 2). One of the findings is that lower-order carrier conduction that is easier to move in two-dimensional and one-dimensional directions is more advantageous for the thermoelectric effect than three-dimensional carrier conduction. When the temperature difference generated in the thermoelectric conversion element 100 shown in FIG. 1 is used, it is advantageous to have a conductive carrier distribution that extends in a two-dimensional direction along the expansion of the thermoelectric conversion layer 10. The conduction carrier distribution spreading in a two-dimensional direction means at least one of the following:
(1) The relative electrical conductivity by direction is high in the plane of the two-dimensional plane and low in the direction perpendicular to the plane;
(2) When the spatial expansion of the conduction carriers is viewed relative to each direction, the conduction carriers are spread within the plane of the two-dimensional plane and have no overlap, and do not have an overlap spread in the direction perpendicular to the plane.
One of these conditions is satisfied in the electron distribution (two-dimensional electron gas) in which the spatial spread is limited to one direction perpendicular to the plane and spreads in the other two directions (in-plane). For this reason, in the following description, a conductive carrier that satisfies any of the above conditions is intended, and a two-dimensional electron gas is taken as an example thereof, and a three-dimensional electron gas is taken as an example of a conductive carrier having a three-dimensional distribution in order to contrast it. Also mention.
　上述した空間の次元性も反映させて上記無次元性能指数ＺＴをさらに詳しく解析すると、
　ＺＴ ∝ ｍ*３／２（μ／κ）Ｔ　（３次元電子ガスについて）
　　　 ∝ ｍ*（μ／κ）Ｔ　　　（２次元電子ガスについて）
との関係が成り立つことが示される。ただし、ｍ*は伝導キャリア（電子）の有効質量、μは移動度（ｍ２／Ｖ／ｓ）であり、「∝」の記号は両辺が比例関係にあることを示す記号である。なお、上述したＺＴとして０．８程度の値を持つ公知の熱電変換材料であるビスマステルル（Ｂｉ２Ｔｅ３）系材料は、３次元電子ガスの場合である。 When the dimensionless figure of merit ZT is analyzed in more detail, reflecting the dimensionality of the space described above,
ZT ∝ m * 3/2 (μ / κ) T (About 3D electron gas)
∝ m * (μ / κ) T (About 2D electron gas)
It is shown that the relationship is established. However, m * is the effective mass of conduction carriers (electrons), μ is the mobility (m 2 / V / s), and the symbol “∝” is a symbol indicating that both sides are in a proportional relationship. The bismuth tellurium (Bi 2 Te 3 ) -based material, which is a known thermoelectric conversion material having a value of about 0.8 as ZT, is a case of a three-dimensional electron gas.
　これらの解析結果から、熱電変換の効率のために好ましい性質は、第１に、熱電変換層をなす材質が、熱電変換層に沿った２次元的な方向に拡がる伝導キャリア分布を持つこと、第２に、伝導キャリアの有効質量が大きいこと、第３に、伝導キャリアの移動度が大きく電気伝導率が大きいこと、である。上記伝導キャリアの有効質量は、特に自由電子の質量（静止質量）よりも大きいことが好ましい。また上記伝導キャリアの電気伝導率は、Ｂｉ２Ｔｅ３系材料の電気伝導率（例えば５０００Ｓ／ｃｍ、V.A. Kulbachinskii et al., "Thermoelectric properties of Bi2Te3, Sb2Te3 and Bi2Se3 single crystals with magnetic impurities," J. Solid State Chem. 193, 47 (2012).）よりも大きいことが好ましい。 From these analysis results, the desirable properties for the efficiency of thermoelectric conversion are, firstly, that the material forming the thermoelectric conversion layer has a conduction carrier distribution that extends in a two-dimensional direction along the thermoelectric conversion layer, Secondly, the effective mass of the conductive carrier is large, and thirdly, the mobility of the conductive carrier is large and the electric conductivity is large. The effective mass of the conductive carrier is preferably larger than the mass of free electrons (static mass). The electric conductivity of the conductive carrier is the electric conductivity of Bi 2 Te 3 material (for example, 5000 S / cm, VA Kulbachinskii et al., “Thermoelectric properties of Bi 2 Te 3 , Sb 2 Te 3 and Bi 2 Se 3. larger than single crystals with magnetic impurities, "J. Solid State Chem. 193, 47 (2012).).
２．熱電変換層の材質
　本実施形態では、上述した原理に応じた材料の典型例として、周期律表１６族の酸素（Ｏ）、硫黄（Ｓ）、セレン（Ｓｅ）、テルル（Ｔｅ）、ポロニウム（Ｐｏ)であるカルコゲン元素と鉄との化合物（鉄カルコゲナイド）のうち、カルコゲン元素をＳ、Ｓｅ、Ｔｅから選択する鉄カルコゲナイドに着目した。すなわち、本実施形態で注目する化合物は、一般式ＦｅＳ１－ｘ－ｙＳｅｘＴｅｙ（ただしｘ、ｙはともに０以上１以下、かつｘ＋ｙは０以上１以下）により表される組成をもつ。なお、カルコゲン元素が硫黄、セレン、テルルである場合は、順に硫化鉄（ＩＩ）ＦｅＳ(ＩＩ)、セレン化鉄（または鉄セレン）ＦｅＳｅ、およびテルル化鉄（または鉄テルル）ＦｅＴｅと呼ばれる。 2. Material of Thermoelectric Conversion Layer In this embodiment, as a typical example of the material according to the above-described principle, oxygen (O), sulfur (S), selenium (Se), tellurium (Te), polonium (group 16 of the periodic table) Of the compound (iron chalcogenide) of chalcogen element and iron which is Po), attention was paid to iron chalcogenide in which the chalcogen element is selected from S, Se, and Te. That is, the compound of interest in the present embodiment has a composition represented by the general formula FeS 1-xy Se x Te y (where x and y are both 0 or more and 1 or less and x + y is 0 or more and 1 or less). . When the chalcogen element is sulfur, selenium, or tellurium, they are sequentially called iron (II) sulfide (II), iron selenide (or iron selenium) FeSe, and iron telluride (or iron tellurium) FeTe.
　なかでも鉄セレンＦｅＳｅは、いくつかのユニークな性質から注目を集めている。まず、比較的簡単な結晶構造を有している（後述）。また、バルクにて９Ｋ程度、薄膜では５０Ｋを超す超伝導転移温度ＴＣを持つ超伝導体である。この超伝導転移温度ＴＣは、圧力を加えたり、また、鉄原子層間への他の原子（Ｋ、Ｒｂ、Ｃｓなど）やアンモニア分子のインターカレーションにより上昇する。鉄化合物であるが磁気秩序を示さない。９０Ｋ付近での構造相転移に伴うネマティック秩序を持つ。 Among them, iron selenium FeSe has attracted attention due to several unique properties. First, it has a relatively simple crystal structure (described later). Further, 9K approximately at bulk, in the thin film is a superconductor having a superconducting transition temperature T C in excess of 50K. The superconducting transition temperature T C is added or pressure, also increased by intercalation of other atoms (K, Rb, Cs, etc.), ammonia molecule to iron atoms interlayer. Although it is an iron compound, it does not show magnetic order. It has a nematic order associated with a structural phase transition near 90K.
　これらの興味ある性質を持つ鉄セレンＦｅＳｅは、層状化合物半導体であり、図２に示す結晶構造を持つ。図２は、ＰｂＯ構造と呼ばれる結晶構造のＦｅＳｅを含む鉄カルコゲナイドの結晶構造を示す模式図である。鉄Ｆｅ原子は、ｃ軸に垂直な平面内に広がって配列し、異なる平面の鉄Ｆｅ原子はｃ軸方向に重なる位置に配置される。セレンＳｅは、図２においてカルコゲン元素Ｃｈの位置に配置され、各鉄原子の中間でｃ軸方向に互い違いにずれて鉄原子の平面内から外れた位置に配置される。図２で図示した４つの四角柱の範囲に、鉄原子、カルコゲン元素原子はともに４つ分含まれる。また、各原子の中心をみると、セレンは鉄を取り囲む四面体の頂点４カ所に配置される。図２では鉄原子一つを囲むセレンの配置の四面体の例を一点鎖線により示している。このような構造をもつ鉄セレンＦｅＳｅは、熱電変換に対し有利な性質を示す。例えば、電気伝導率σは１ユニットセル分の厚みのＦｅＳｅで約５０００Ω-1ｃｍ-1と大きい（非特許文献４）。また、有効質量ｍ＊は、自由電子の質量（電子の静止質量）ｍｅの約３倍近い値を持つ（非特許文献５）。ＦｅＳｅの結晶構造や基本的な物理的・化学的性質は、一般式ＦｅＳ１－ｘ－ｙＳｅｘＴｅｙ（ただしｘ、ｙはともに０以上１以下、かつｘ＋ｙは０以上１以下）により表される組成をもつ鉄カルコゲナイドに共通している。このため、カルコゲン元素を硫黄（Ｓ）、セレン（Ｓｅ）またはテルル（Ｔｅ）から選択する鉄カルコゲナイドは本発明者が提案する熱電変換素子のための方法に適うものといえる。 Iron selenium FeSe having these interesting properties is a layered compound semiconductor and has a crystal structure shown in FIG. FIG. 2 is a schematic diagram showing a crystal structure of iron chalcogenide containing FeSe having a crystal structure called a PbO structure. The iron Fe atoms are spread and arranged in a plane perpendicular to the c-axis, and the iron Fe atoms in different planes are arranged at positions overlapping in the c-axis direction. Selenium Se is arranged at the position of the chalcogen element Ch in FIG. 2, and is arranged at a position deviating from the plane of the iron atom by shifting in the c-axis direction in the middle of each iron atom. In the range of the four quadrangular prisms shown in FIG. 2, four iron atoms and four chalcogen element atoms are included. Looking at the center of each atom, selenium is arranged at the four apexes of the tetrahedron surrounding iron. In FIG. 2, the example of the tetrahedron of the arrangement | positioning of the selenium surrounding one iron atom is shown with the dashed-dotted line. Iron selenium FeSe having such a structure exhibits advantageous properties for thermoelectric conversion. For example, the electrical conductivity σ is as large as about 5000Ω −1 cm −1 for FeSe having a thickness of one unit cell (Non-patent Document 4). The effective mass m * has about three times the value of almost free electron mass (electron rest mass) m e (Non-Patent Document 5). The crystal structure and basic physical and chemical properties of FeSe are represented by the general formula FeS 1-xy Se x Te y (where x and y are both 0 or more and 1 or less, and x + y is 0 or more and 1 or less). It is common to iron chalcogenides having a composition. For this reason, it can be said that the iron chalcogenide in which the chalcogen element is selected from sulfur (S), selenium (Se), or tellurium (Te) is suitable for the method for the thermoelectric conversion element proposed by the present inventor.
３．製造方法
　図３は、本実施形態の熱電変換素子１００の製造方法を示すフローチャートである。熱電変換素子１００は大別すると３工程にて製造される。第１は基板準備工程である（Ｓ０２）。例えば（００１）面方位など適当な面方位になるように作製されたＳｒＴｉＯ３などの結晶成長に適する基板を準備して、成膜装置など適切な装置に搭載する。必要に応じてその表面を清浄にする工程を加えることもできる。第２は、熱電変換層形成工程である（Ｓ０４）。準備した基板の表面に対して、任意の成膜方法によって熱電変換層を形成する。成膜方法として採用できるのは、ＭＢＥ（Molecular Beam Epitaxy）法やＰＬＤ（pulsed laser deposition）法である。この際、熱電変換層を目的の厚みとなるように成膜するために、あらかじめ決定しておいた各種の条件が採用される。成膜される熱電変換層は、本実施形態では鉄カルコゲナイドの結晶膜であり、硫黄（Ｓ）、セレン（Ｓｅ）、またはテルル（Ｔｅ）からなるカルコゲン元素群から選択される少なくとも一の元素と鉄との化合物であり、ＦｅＳｅ薄膜が代表的なものである。その後、熱電変換層に互いに離間した第１部分および第２部分それぞれに対をなす電極の各々を形成する電極形成工程を行う（Ｓ０６）。この工程は、熱電変換層との間でオーミック接続を実現し、外部の配線とも接続しやすい適切な金属膜を必要な範囲のパターンに形成する工程である。なお、任意選択の工程として実施の上で役立つ任意の工程を任意のタイミングに追加して実施することもできる。たとえば、基板が不要である場合に基板を除去する工程、成膜した熱電変換層を多数集めて集積する工程、事後的に熱電変換層の厚みを調整する工程、熱電変換層の外形形状を整形する工程、などを任意選択として実施できる。 3. Manufacturing Method FIG. 3 is a flowchart showing a manufacturing method of the thermoelectric conversion element 100 of the present embodiment. The thermoelectric conversion element 100 is roughly divided into three steps. The first is a substrate preparation step (S02). For example, a substrate suitable for crystal growth such as SrTiO 3 prepared so as to have an appropriate plane orientation such as the (001) plane orientation is prepared and mounted on an appropriate apparatus such as a film forming apparatus. A step of cleaning the surface can be added if necessary. The second is a thermoelectric conversion layer forming step (S04). A thermoelectric conversion layer is formed on the surface of the prepared substrate by any film forming method. The MBE (Molecular Beam Epitaxy) method and the PLD (pulsed laser deposition) method can be employed as the film forming method. Under the present circumstances, in order to form the thermoelectric conversion layer into the target thickness, various conditions determined in advance are employed. The thermoelectric conversion layer to be formed is an iron chalcogenide crystal film in this embodiment, and at least one element selected from a chalcogen element group consisting of sulfur (S), selenium (Se), or tellurium (Te). It is a compound with iron, and a typical FeSe thin film. Thereafter, an electrode forming step is performed in which each of the paired electrodes is formed on each of the first portion and the second portion that are separated from each other in the thermoelectric conversion layer (S06). This step is a step of forming an appropriate metal film in a necessary range pattern that realizes ohmic connection with the thermoelectric conversion layer and is easily connected to external wiring. It should be noted that an optional step that is useful in implementation as an optional step can be added at an arbitrary timing. For example, the step of removing the substrate when the substrate is unnecessary, the step of collecting and accumulating a large number of formed thermoelectric conversion layers, the step of adjusting the thickness of the thermoelectric conversion layer afterwards, and shaping the outer shape of the thermoelectric conversion layer The process of performing etc. can be implemented as an option.
４．実施例
　以下、本実施形態の熱電変換素子の性能確認のための実施例を説明する。以下の実施例に示す材料、使用量、割合、処理内容、処理手順、要素または部材の向きや具体的配置等は本発明の趣旨を逸脱しない限り適宜変更することかできる。したがって、本発明の範囲は以下の具体例に限定されるものではない。 4). Examples Hereinafter, examples for confirming the performance of the thermoelectric conversion element of the present embodiment will be described. The materials, amounts used, ratios, processing contents, processing procedures, directions of elements or members, specific arrangements, and the like shown in the following examples can be appropriately changed without departing from the gist of the present invention. Therefore, the scope of the present invention is not limited to the following specific examples.
４－１．ＦｅＳｅ薄膜の作製
　本実施形態の熱電変換素子は適当な基板の一方の表面に適当な厚さのＦｅＳｅ薄膜を一般的な成膜方法により形成することにより実施できる。実施例では、基板の一例として（００１）面方位のＳｒＴｉＯ３を採用し、成膜方法の一例として公知のＰＬＤ法、そしてＦｅＳｅ薄膜の膜厚を最初に２０ｎｍ程度の厚さとすることにより、図３に示した製造方法にて熱電変換素子１００のためのＦｅＳｅ薄膜サンプルを作製した。作製したＦｅＳｅ薄膜サンプルは、そのまま熱電変換素子１００として使用することもできるが、ここでは、ＦｅＳｅ薄膜の膜厚は後に減じるよう順次変化させ、厚みと特性の関係を調査することとした。 4-1. Production of FeSe Thin Film The thermoelectric conversion element of the present embodiment can be implemented by forming a FeSe thin film having an appropriate thickness on one surface of an appropriate substrate by a general film forming method. In the embodiment, (001) -oriented SrTiO 3 is used as an example of the substrate, a known PLD method is used as an example of the film forming method, and the film thickness of the FeSe thin film is first set to a thickness of about 20 nm. A FeSe thin film sample for the thermoelectric conversion element 100 was prepared by the manufacturing method shown in FIG. The produced FeSe thin film sample can be used as it is as the thermoelectric conversion element 100, but here, the thickness of the FeSe thin film was sequentially changed so as to decrease later, and the relationship between the thickness and the characteristics was investigated.
４－２．電気化学エッチングによるＦｅＳｅ薄膜の膜厚の調整
　熱電変換層１０であるＦｅＳｅ薄膜の厚みの調整は次のようにして行った。まずサンプルのＦｅＳｅ薄膜と白金電極でイオン液体を挟み、２４５Ｋ付近の温度に保ち５Ｖの電圧を印加する。この模式図を図４に示す。基板２に接して配置されたＦｅＳｅ薄膜である熱電変換層１０の最表面の原子はこの温度でイオン液体中に溶け出して電気化学エッチングされる。これによりＦｅＳｅ薄膜の膜厚を減少させることができる。この手法を利用すれば、膜厚を成膜したままの２０ｎｍから最小膜厚である単層（０．６ｎｍ）まで高い制御性で順次に減少させ、その途中の各膜厚において熱電効果を測定することもできる。エッチング剤には、ＤＥＭＥ－ＴＦＳＩ（N,N-diethyl-N-(2-methoxyethyl)-N-methylammoniumbis-(trifluoromethylsulfonyl)-imide）を用いた。このエッチングの詳細は既に報告されている手法を採用した（非特許文献６：J. Shiogai et al., Nature Phys. 12, 42 (2016)）。 4-2. Adjustment of the thickness of the FeSe thin film by electrochemical etching The thickness of the FeSe thin film as the thermoelectric conversion layer 10 was adjusted as follows. First, an ionic liquid is sandwiched between a sample FeSe thin film and a platinum electrode, and a voltage of 5 V is applied while maintaining a temperature near 245K. This schematic diagram is shown in FIG. At the temperature, atoms on the outermost surface of the thermoelectric conversion layer 10 which is an FeSe thin film disposed in contact with the substrate 2 are dissolved in the ionic liquid and subjected to electrochemical etching. Thereby, the film thickness of the FeSe thin film can be reduced. If this method is used, the film thickness is sequentially reduced with high controllability from the as-deposited 20 nm to the single layer (0.6 nm) which is the minimum film thickness, and the thermoelectric effect is measured at each film thickness in the middle. You can also As the etching agent, DEME-TFSI (N, N-diethyl-N- (2-methoxyethyl) -N-methylammoniumbis- (trifluoromethylsulfonyl) -imide) was used. For the details of this etching, a previously reported method was adopted (Non-patent Document 6: J. Shiogai et al., Nature Phys. 12, 42 (2016)).
４－３．熱電効果測定
　上記電気化学エッチングを行いＦｅＳｅ薄膜の膜厚を徐々に減少させながら、熱電効果の測定を行った。測定はエッチングが停止する温度（２００Ｋ）にて行った。図５はその結果を示すグラフである。図５に示すように、膜厚の減少に応じ、ゼーベック効果が増大する現象が観察された。図６は、この測定の具体的なセットアップを示す模式図である。このセットアップは、図１に示した熱電変換素子１００の構造を適用して熱電効果の特性を測定するために、イオン液体を介した白金の電極を熱電変換層１０の基板２とは逆側の面に対向させるように追加することによって、電極２２、２４をそれぞれソース電極およびドレイン電極、追加した白金の電極をゲート電極としてＦＥＴ（電界効果トランジスタ）に類似した電極構成とした例である。ＦeＳe膜は２０ｎｍの厚みとしたままこのようなセットアップを組上げた状態で、まず温度を２５０Ｋ付近としておいてゲート電圧ＶＧ＝５Ｖを印加し１ｎｍ程度のエッチングを行い、その後温度を２００Ｋに下げる。温度を下げるのはエッチングをストップさせるためである。そして図６のヒーターを用いて試料の両端に温度差を発生させる。生じた温度差ΔＴは熱電対を用いて測定した。また、同じ熱電対を用いて熱起電力ΔＶも測定し、ゼーベック係数ＳをＳ＝－ΔＶ／ΔＴとして求めた。これを繰り返すことでゼーベック係数の膜厚依存性が得られた。 4-3. Thermoelectric Effect Measurement The thermoelectric effect was measured while the electrochemical etching was performed and the film thickness of the FeSe thin film was gradually reduced. The measurement was performed at a temperature (200K) at which etching stops. FIG. 5 is a graph showing the results. As shown in FIG. 5, a phenomenon was observed in which the Seebeck effect increased as the film thickness decreased. FIG. 6 is a schematic diagram showing a specific setup for this measurement. In order to measure the thermoelectric effect characteristics by applying the structure of the thermoelectric conversion element 100 shown in FIG. 1, this setup uses a platinum electrode via an ionic liquid on the side opposite to the substrate 2 of the thermoelectric conversion layer 10. In this example, the electrodes 22 and 24 are added so as to face each other, and the electrodes 22 and 24 are respectively configured as a source electrode and a drain electrode, and the added platinum electrode is used as a gate electrode to have an electrode configuration similar to an FET (field effect transistor). In the state where such a setup is assembled with the FeSe film having a thickness of 20 nm, first, the gate voltage V G = 5 V is applied at a temperature of about 250 K, and etching is performed to about 1 nm, and then the temperature is lowered to 200 K. The reason for lowering the temperature is to stop the etching. Then, a temperature difference is generated at both ends of the sample using the heater of FIG. The resulting temperature difference ΔT was measured using a thermocouple. Further, the thermoelectromotive force ΔV was also measured using the same thermocouple, and the Seebeck coefficient S was determined as S = −ΔV / ΔT. By repeating this, the film thickness dependence of the Seebeck coefficient was obtained.
４－４．電気伝導率の測定
　図６のようなデバイス構造を用いると、熱起電力だけでなく、電気伝導率を同時に測定することも可能となる。具体的には、ＦｅＳｅ薄膜において互いに離間された電極間に電圧ＶＤＳを印加し電流を流した。そして、熱電対を用いて試料の電圧降下を測定すれば、オームの法則により電気抵抗が求まる。この値と膜厚から電気伝導率を求めることができる。この電気伝導率の測定をエッチングにより膜厚を変化させながら行うことにより、電気伝導率の膜厚依存性を測定した。 4-4. Measurement of electrical conductivity Using a device structure as shown in FIG. 6, it is possible to measure not only the thermoelectromotive force but also the electrical conductivity at the same time. Specifically, a current was passed by application of a voltage V DS between the electrodes spaced apart in FeSe film. Then, if the voltage drop of the sample is measured using a thermocouple, the electrical resistance can be obtained by Ohm's law. The electrical conductivity can be obtained from this value and the film thickness. By measuring the electrical conductivity while changing the film thickness by etching, the film thickness dependence of the electrical conductivity was measured.
　測定した結果の熱起電力Ｓと電気伝導率σの膜厚依存性から、パワーファクターＳ２σの膜厚依存性が測定できることになる。これをグラフにしたものが図７Ａである。このようにしてＰｂＯ構造ＦｅＳｅ薄膜が厚み１ｎｍのときにＳ２σ＝約１５００もの大きな値が得られることを確認した。この際、ゲート電圧ＶＧは印加したまま測定を実行した。 The film thickness dependence of the power factor S 2 σ can be measured from the film thickness dependence of the thermoelectromotive force S and the electrical conductivity σ as a result of the measurement. This is shown in a graph in FIG. 7A. In this way, it was confirmed that a large value of S 2 σ = about 1500 was obtained when the PbO structure FeSe thin film had a thickness of 1 nm. At this time, the gate voltage V G was run the measurement while applying.
　さらに、このＦｅＳｅ薄膜のパワーファクターＳ２σを従来の熱電変換材料のバルク材料にて得られた値と比較したものが図７Ｂである。このように、ＦｅＳｅ薄膜では、これまでに知られている物質よりも大きなパワーファクターが実現することを確認した。 Further, FIG. 7B compares the power factor S 2 σ of this FeSe thin film with the value obtained with the conventional bulk material of the thermoelectric conversion material. As described above, it was confirmed that the FeSe thin film realizes a power factor larger than that of the conventionally known substances.
５．熱電変換層の最適化
　再び図１を参照すると、ＰｂＯ構造鉄セレン（ＦｅＳｅ）は、（００１）面配向ＳＴＯ基板に対しｃ軸を厚み方向に向けた熱電変換層１０を形成することにより、鉄原子を含む面内に対し広がる２次元電子ガスを形成するため、熱電変換層１０の面内の異なる位置で温度差を生じるような熱電変換素子１００の用途に適する一例となる。 5). Optimization of Thermoelectric Conversion Layer Referring again to FIG. 1, PbO-structured iron selenium (FeSe) is formed by forming the thermoelectric conversion layer 10 with the c-axis oriented in the thickness direction with respect to the (001) -oriented STO substrate. This is an example suitable for the use of the thermoelectric conversion element 100 in which a temperature difference is generated at different positions in the surface of the thermoelectric conversion layer 10 in order to form a two-dimensional electron gas that spreads in the plane including the atoms.
　熱電変換層のためにＰｂＯ構造鉄セレン（ＦｅＳｅ）を採用する場合、好ましい熱電変換層１０では、その厚みは１２ｎｍ以下とされ、さらに好ましくは９ｎｍ以下とされる。ＰｂＯ構造鉄セレン（ＦｅＳｅ）で１２ｎｍ以下の厚み以下となると、従来の熱電変換材料のうち大きなパワーファクター値を示すＮａＣｏＯ２よりもさらに大きなパワーファクター値が実現し、さらに９ｎｍ以下となるとＮａＣｏＯ２からみて有意に大きなパワーファクター値が実現するためである。 When PbO structure iron selenium (FeSe) is employed for the thermoelectric conversion layer, the preferable thermoelectric conversion layer 10 has a thickness of 12 nm or less, more preferably 9 nm or less. When the PbO structure iron selenium (FeSe) has a thickness of 12 nm or less, a power factor value larger than that of NaCoO 2 showing a large power factor value among conventional thermoelectric conversion materials is realized, and when the thickness is 9 nm or less, NaCoO 2 This is because a significantly large power factor value is realized.
　さらに、本実施形態では、熱電変換層１０の伝導キャリア（電子）の低次元化を促進する任意の手段による高性能化も期待できる。このために例えば、特に３次元的拡がりを抑制して２次元的な性質が現われることを助けたり、２次元的性質が弱まることを回避できる任意の手段を採用することができる。例えば、鉄でもカルコゲン元素でもない原子を相関に介在させるインターカレーションや、ＦｅＳｅの表面への選択的キャリアドーピング、特に２次元的なキャリア分布を実現するデルタドープなど手法は熱電変換層１０の高性能化のために有用である。 Furthermore, in the present embodiment, high performance can be expected by any means for promoting the reduction in the order of conduction carriers (electrons) in the thermoelectric conversion layer 10. For this purpose, for example, any means capable of suppressing the three-dimensional expansion and helping the two-dimensional property to appear or avoiding the weakening of the two-dimensional property can be employed. For example, techniques such as intercalation in which atoms that are neither iron nor chalcogen elements intervene in correlation, selective carrier doping on the surface of FeSe, particularly delta doping that realizes a two-dimensional carrier distribution, and the like are high performance of the thermoelectric conversion layer 10. It is useful for
　以上、本発明の実施形態を具体的に説明した。本実施形態において提案された方法論に基づく熱電変換素子の設計やその方法論を具現化する材料の選択の結果、革新的な熱電変換素子を実現することが可能となる。上述の各実施形態および構成例は、発明を説明するために記載されたものであり、本出願の発明の範囲は、請求の範囲の記載に基づいて定められるべきものである。また、各実施形態の他の組合せを含む本発明の範囲内に存在する変形例もまた請求の範囲に含まれるものである。 The embodiment of the present invention has been specifically described above. As a result of the design of the thermoelectric conversion element based on the methodology proposed in the present embodiment and the selection of a material that embodies the methodology, an innovative thermoelectric conversion element can be realized. Each of the above-described embodiments and configuration examples are described for explaining the invention, and the scope of the invention of the present application should be determined based on the description of the claims. In addition, modifications that exist within the scope of the present invention including other combinations of the embodiments are also included in the scope of the claims.
　本発明の熱電変換素子は熱起電力を利用する任意の機器に利用可能である。 The thermoelectric conversion element of the present invention can be used for any device that uses thermoelectromotive force.
　１００　熱電変換素子
　　　２　基板
　　１０　熱電変換層
　　１２　第１部分
　　１４　第２部分
　　２２、２４　電極
　　３２　熱源
　　３４　ヒートシンク DESCRIPTION OF SYMBOLS 100 Thermoelectric conversion element 2 Board | substrate 10 Thermoelectric conversion layer 12 1st part 14 2nd part 22, 24 Electrode 32 Heat source 34 Heat sink

Claims (7)

  1. 　ある厚みの熱電変換層であって、該熱電変換層をなす材質が、該熱電変換層に沿った２次元的な方向に拡がる伝導キャリア分布を持ち、該熱電変換層に沿う向きにおいて、該伝導キャリアの有効質量が自由電子のものよりも大きく、該伝導キャリアによる電気伝導率がＢｉ２Ｔｅ３系材料の値よりも大きいものである、熱電変換層
    　を備えている熱電変換素子。     A thermoelectric conversion layer having a certain thickness, wherein the material forming the thermoelectric conversion layer has a distribution of conduction carriers that extends in a two-dimensional direction along the thermoelectric conversion layer, and the conduction in the direction along the thermoelectric conversion layer. greater than the effective mass of a free electron carrier, an electric conductivity by the conduction carriers is greater than the value of Bi 2 Te 3 based materials, thermoelectric conversion element and a thermoelectric conversion layer.
  2. 　前記熱電変換層の前記材質が、ＦｅＳ１－ｘ－ｙＳｅｘＴｅｙ（ただしｘ、ｙはともに０以上１以下、かつｘ＋ｙは０以上１以下）により表される鉄カルコゲナイドである、請求項１に記載の熱電変換素子。 The material of the thermoelectric conversion layer is an iron chalcogenide represented by FeS 1-xy Se x Te y (where x and y are both 0 or more and 1 or less and x + y is 0 or more and 1 or less). 1. The thermoelectric conversion element according to 1.
  3. 　前記熱電変換層の前記材質がＰｂＯ構造鉄セレン（ＦｅＳｅ）である、請求項１に記載の熱電変換素子。 The thermoelectric conversion element according to claim 1, wherein the material of the thermoelectric conversion layer is PbO structure iron selenium (FeSe).
  4. 　前記熱電変換層が（００１）面方位のＳｒＴｉＯ３基板の表面に接して配置され、前記熱電変換層の結晶構造のｃ軸が前記厚み方向に向いている、請求項１に記載の熱電変換素子。 2. The thermoelectric conversion element according to claim 1, wherein the thermoelectric conversion layer is disposed in contact with a surface of an SrTiO 3 substrate having a (001) orientation, and a c-axis of a crystal structure of the thermoelectric conversion layer is oriented in the thickness direction. .
  5. 　前記熱電変換層は、当該熱電変換層の広がりにおいて互いに離間した第１部分および第２部分それぞれに対をなす電極の各々が取り付けられており、
    　前記熱電変換層のうち前記第１部分と前記第２部分とが互いに異なる温度となりうるように配置して、前記対をなす電極から電力が取り出される、請求項１に記載の熱電変換素子。     Each of the thermoelectric conversion layer is provided with a pair of electrodes on each of the first part and the second part that are separated from each other in the spread of the thermoelectric conversion layer,
    2. The thermoelectric conversion element according to claim 1, wherein the first portion and the second portion of the thermoelectric conversion layer are arranged so as to have different temperatures, and electric power is extracted from the paired electrodes.
  6. 　前記熱電変換層が１２ｎｍ以下の厚みをもつ、請求項３に記載の熱電変換素子。 The thermoelectric conversion element according to claim 3, wherein the thermoelectric conversion layer has a thickness of 12 nm or less.
  7. 　ＳｒＴｉＯ３の基板を準備する工程と、
    　該基板の表面に接して硫黄（Ｓ）、セレン（Ｓｅ）、またはテルル（Ｔｅ）からなるカルコゲン元素群から選択される少なくとも一の元素と鉄との化合物である鉄カルコゲナイドの熱電変換層を形成する工程と、
    　該熱電変換層の広がりにおいて互いに離間した第１部分および第２部分それぞれに対をなす電極の各々を形成する工程と
    　を含む熱電変換素子の製造方法。     Preparing a substrate of SrTiO 3 ;
    A thermoelectric conversion layer of iron chalcogenide that is a compound of iron and at least one element selected from the chalcogen element group consisting of sulfur (S), selenium (Se), or tellurium (Te) is formed in contact with the surface of the substrate. And a process of
    Forming a pair of electrodes on each of the first part and the second part that are spaced apart from each other in the spread of the thermoelectric conversion layer.
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