JP5834871B2 - Thermoelectric conversion element and manufacturing method thereof - Google Patents

Description

  The present application relates to a thermoelectric conversion element and a manufacturing method thereof.

  In recent years, thermoelectric conversion elements that convert thermal energy into electrical energy have attracted attention from the viewpoint of reducing carbon dioxide and protecting the environment. By using the thermoelectric conversion element, it is possible to convert the thermal energy that has been discarded until now into electric energy and reuse it.

  Since one such thermoelectric conversion element has a low output voltage, usually, a plurality of thermoelectric conversion elements are connected in series and used as a thermoelectric conversion module. A general thermoelectric conversion module includes a large number of semiconductor blocks (p-type semiconductor blocks) made of a p-type thermoelectric conversion material between two heat transfer plates and a large number of semiconductor blocks (n-type semiconductor blocks) made of an n-type thermoelectric conversion material. ). The two types of semiconductor blocks are alternately arranged in the in-plane direction of the heat transfer plate, and are connected in series by metal terminals arranged between the semiconductor blocks. Lead electrodes are connected to both ends of the semiconductor blocks connected in series.

  In such a thermoelectric conversion module, when a temperature difference is given to two heat transfer plates, a potential difference is generated inside each of the p-type semiconductor block and the n-type semiconductor block due to the Seebeck effect, and electric power is taken out from the extraction electrode. Can do. The thermoelectric conversion module is expected to be applied as a power source for, for example, a micro power electronic device. Further, Patent Document 1 discloses a power generation device in which a heat conversion module is combined with a solar cell.

JP 2009-16812 A

The thermoelectric conversion element can be made using SrTiO ₃ (strontium titanate: hereinafter referred to as STO) which is a thermoelectric conversion material having a small environmental load. However, although it has been reported that the STO thin film shows high thermoelectric properties, a single crystal substrate such as STO is required because the manufacturing method requires high crystallinity. Since the single crystal substrate has a very high thermal conductivity, the figure of merit of the thermoelectric conversion element is relatively low, and the temperature gradient is difficult to be applied. The figure of merit Z is expressed as Z = σS ² / K, where σ is electrical conductivity, S is Seebeck coefficient, and K is thermal conductivity.

  In view of the problems of the related art described above, the present application has characteristics of higher electromotive force, higher electrical conductivity, and lower thermal conductivity than the thermoelectric conversion elements described in the related art described above. An object of the present invention is to provide a thermoelectric conversion element having a high figure of merit and a method for manufacturing the same.

  According to one aspect of the embodiment, a strontium titanate thin film buffer layer having a non-conductive cationic non-stoichiometric composition doped with niobium or lanthanum is provided on a strontium titanate single crystal substrate. A thermoelectric conversion element comprising a strontium titanate thermoelectric thin film having a stoichiometric composition doped with niobium and lanthanum, or a strontium titanate thermoelectric thin film having a non-stoichiometric composition doped with niobium and lanthanum. provide.

  According to another aspect of the embodiment, a non-conductive cation non-stoichiometric strontium titanate thin film buffer layer doped with niobium or lanthanum is formed on a strontium titanate single crystal substrate. A strontium titanate thermoelectric thin film with a niobium and lanthanum doped composition is formed on the layer, or a non-stoichiometric strontium titanate thermoelectric thin film with a niobium and lanthanum doped composition is formed on the thermoelectric conversion element. A method of manufacturing a thermoelectric conversion element is provided.

It is a block diagram which shows the structure of the apparatus for making the thermoelectric conversion element of this application. 1A and 1B show a manufacturing process of a thermoelectric conversion element of the present application, in which FIG. 1A is a sectional view of an STO substrate placed on the substrate holder of FIG. 1, and FIG. 2B is an intermediate layer formed on the STO substrate of FIG. FIG. 6C is a cross-sectional view showing a state in which a thermoelectric thin film layer is formed on the intermediate layer of FIG. It is sectional drawing which shows the movement of a heat | fever when a high temperature part and a low temperature part are connected to the thermoelectric conversion element manufactured by the manufacturing process of FIG. (A) is a diagram showing the number of niobium chips placed on the STO substrate and changes in thin film composition, (b) is a characteristic diagram showing the number of niobium chips placed on the STO substrate and changes in the A / B ratio, and (c) It is a characteristic view showing the number of lantern chips placed on the STO substrate and the change in A / B ratio. (A) is a characteristic figure which shows the X-ray-analysis result of the buffer layer produced by the method of this application, (b) is a characteristic figure which shows the relationship between A / B ratio and a Hall coefficient. It is a characteristic view which compares and shows the temperature rise with and without the intermediate | middle layer of this application when a thermoelectric conversion element is excited by 3omega method.

  Hereinafter, embodiments of the present application will be described in detail based on specific examples with reference to the accompanying drawings.

  FIG. 1 shows a configuration of a film forming chamber 1 which is an apparatus for making a thermoelectric conversion element of the present application. Inside the film forming chamber 1, there are a target holder 4 for holding the target 7 and a substrate holder 5 for holding the substrate 9. The target holder 4 and the substrate holder 5 are provided outside the film forming chamber 1. A DC high voltage or a high frequency high voltage is applied from the power source 6. Although not shown, the substrate holder 5 includes a heater. A vacuum pump 2 is connected to the film forming chamber 1 and an argon gas source (indicated as Ar gas source in the drawing) 3 for introducing an inert gas, in this example, argon gas, into the film forming chamber 1 is provided. It is connected.

In this application, in order to make a thermoelectric conversion element, a polycrystalline Sr (Ti _0.98 Nb _0.02 ) O ₃ is used as a target 7, a metal niobium chip 8 is disposed on the target 7, and an STO (crystal Azimuth) (100) substrate is used. Here, the chip is a shape that can be placed on the target, for example, an ingot that is separated into pieces. Then, the inside of the film forming chamber 1 is evacuated by the vacuum pump 2, the STO (100) substrate 9 is heated to 600 ° C. by the substrate holder 5, and then argon gas is introduced into the film forming chamber 1 from the argon gas source 3. To do. In this state, a niobium-doped STO thin film having a thickness of 300 to 400 nm was produced by a sputtering method in which a high-frequency voltage was applied from the power source 6 to cause the argon gas ions 10 to collide with the target 7 at a pressure of 10 mTorr.

  FIG. 2A is a cross-sectional view of the STO substrate 9 placed on the substrate holder 5 in FIG. 1, and FIG. 2B shows a niobium-doped STO thin film 11 formed on the STO substrate 9 in FIG. This shows the state. Thereafter, the niobium-doped STO thin film 11 is annealed in an oxygen atmosphere.

  An STO having a perovskite structure is composed of strontium Sr and lanthanum La arranged at the A site, and titanium Ti and niobium Nb arranged at the B site to form a crystal. Therefore, when the molar ratio A / B = 1, the stoichiometric structure is obtained. It becomes a metric composition. Therefore, the state where A / B = 1 is not the stoichiometric composition.

  On the other hand, when the number of metal niobium chips 8 arranged on the target 7 is increased, the thin film composition of titanium Ti, strontium Sr, and niobium Nb changes as shown in FIG. The change in the A / B ratio at this time is shown in FIG. 4B, and it can be seen that the A / B ratio changes from 0.9 to 0.5. When the number of metal niobium chips 8 arranged on the target 7 is further increased, epitaxial growth cannot be performed. For this reason, the lower limit of the A / B ratio is considered to be 0.5.

  When lantern chips are arranged on the target 7 shown in FIG. 1, the change in the A / B ratio when the number of lantern chips is increased is as shown in FIG. FIG. 4C shows that the A / B ratio changes to about 1.1 by increasing the number of lantern chips arranged on the target 7.

  It has been found this time that the niobium-doped STO thin film produced by the above-described sputtering method cannot exist as a bulk crystal if the A / B ratio is large and deviates from stoichiometry A / B = 1. It is considered that niobium-doped STO having a non-stoichiometric composition is difficult to grow as a bulk crystal. However, it has now been found that epitaxial growth can be achieved by forming a thin film on an STO single crystal substrate. Furthermore, it was found that a further epitaxially grown thermoelectric thin film layer can be formed on the niobium-doped STO thin film having the non-stoichiometric composition produced as described above.

  FIG. 2C is a cross-sectional view showing a state where the thermoelectric conversion element 20 of the present application is formed by forming the thermoelectric thin film layer 12 on the niobium-doped STO thin film 11 shown in FIG. In such a three-layer structure, the niobium-doped STO thin film 11 is called an intermediate layer or a buffer. The thermoelectric thin film layer 12 can be formed to a film thickness of 300 nm by performing sputtering in the film forming chamber 1 shown in FIG. 1 for 3 hours at a substrate temperature of 600 ° C. and a pressure of 2.5 mTorr of argon.

  FIG. 5A shows the result of X-ray diffraction of the niobium-doped STO thin film 11 which is the buffer layer configured as described above, and is the XRD result obtained by examining the crystal structure of the thin film. The peak P1 in the vicinity of the angle of 45 degrees is the (002) X-ray peak of the STO substrate. Lower peaks P2 to P4 indicate the peaks of the niobium-doped STO thin film 11 when the niobium content is 1.3%, 18%, and 27%, respectively. As the content of niobium Nb increases, the lattice constant changes. However, even if the niobium Nb content is different, the fact that the peaks of the niobium-doped STO thin film 11 are located at substantially the same angle indicates that the crystal structures of the STO substrate and the niobium-doped STO thin film 11 are approximate. ing. Thereby, it can be confirmed that the niobium-doped STO thin film 11 is epitaxially grown.

  In FIG. 3, the high temperature portion 13 and the low temperature portion 14 are provided on the thermoelectric thin film layer 12 of the thermoelectric conversion element 20 configured as described above, and the STO substrate 9 and the intermediate layer 11 from the high temperature portion 13 to the low temperature portion 14 are provided. The structure which investigates the heat-transfer characteristic of is shown. The STO substrate 9 had a thermal conductivity K of 11 to 12 W / Km, and the intermediate layer 11 composed of the STO layer having a non-stoichiometric composition had a thermal conductivity K of 1.3 W / Km.

Thus, these cationic STO thin films having a non-stoichiometric composition showed a very low thermal conductivity K. Here, the cation is, for example, the above-mentioned lanthanum La or niobium Nb. Furthermore, when measuring the hole mobility using the Van der Pauw method, it was found that the hole mobility was as low as 0.6 cm ² / Vs or less as shown in FIG. Note that the hole mobility of the single crystal bulk is 5.8 cm ² / Vs. This is thought to be due to the fact that many defects exist in the thin film due to the non-stoichiometric composition, so that the movement of electrons is remarkably inhibited. In addition, when these samples were annealed at 400 ° C. in an oxygen atmosphere, a high resistance value of 1 MΩ or higher was exhibited, and the conductivity could not be confirmed.

  As described above, the thermal conductivity K of the STO thin film doped with niobium Nb or lanthanum La was measured using the 3ω method, and as a result, the thermal conductivity K was found to be as small as 1.3 W / mK. It was. This is about 1/10 of the thermal conductivity of 11.2 W / mK of the STO single crystal, and is very small compared to the reported value of 3 to 4 W / mK of the STO thin film.

FIG. 6 shows the temperature rise when the thermoelectric conversion element manufactured in the present application is excited by the 3ω method. The horizontal axis indicates the frequency (the position of 0 indicates the frequency 1 Hz), and the vertical axis indicates the temperature rise ΔT. Yes. The characteristic indicated by the broken line indicates the temperature rise when only the STO single crystal substrate is used, and the straight line indicates the case where a buffer layer having a non-stoichiometric composition doped with lanthanum La and niobium Nb is provided on the STO single crystal substrate. The temperature rise ΔT on the thin film surface can be calculated using the following formula:

Here, T is temperature, R represents the resistance value of the wiring pattern, V is the applied voltage, V ₃ is the voltage 3ω detected by the lock-in amplifier which is highly sensitive AC voltmeter.

The thermal conductivity of the buffer layer having a non-stoichiometric composition doped with lanthanum La and niobium Nb can be calculated from the following equation.

Where d _F is the thickness of the thin film, v _3ω is the voltage of 3ω detected by the lock-in amplifier, b is the wiring width, l is the wiring length, p is the applied power, C _rt is the heat capacity, and v _lω is the applied voltage. It is.

  From the above, it can be seen that a temperature increase effect of about 0.7 ° C. is observed when there is a buffer layer. This effect is due to the fact that many holes are formed in the thin film due to the non-stoichiometric composition, resulting in a drastic decrease in hole mobility. It is thought that it was done.

  Thus, in this application, by using STO having a cationic non-stoichiometric composition doped with niobium Nb or lanthanum La as a buffer layer on the STO substrate, high electromotive force, high electrical conductivity, and heat Since an excellent intermediate layer with low conduction characteristics can be formed and a thermoelectric conversion element with an excellent figure of merit can be produced, a thermoelectric conversion module with an excellent figure of merit can be produced using this.

  The present application has been described in detail with particular reference to preferred embodiments thereof. For easy understanding of the present application, specific forms of the present application are appended below.

(Supplementary note 1) A strontium titanate thin film buffer layer having a non-conductive cationic non-stoichiometric composition doped with niobium or lanthanum is provided on a strontium titanate single crystal substrate. A thermoelectric conversion element comprising a strontium titanate thermoelectric thin film having a stoichiometric composition doped with lanthanum.
(Supplementary Note 2) A strontium titanate thin film buffer layer having a non-conductive cationic non-stoichiometric composition doped with niobium or lanthanum is provided on a single crystal substrate of strontium titanate. A thermoelectric conversion element comprising a strontium titanate thermoelectric thin film having a non-stoichiometric composition doped with lanthanum.
(Supplementary note 3) In the strontium titanate thermoelectric thin film having a perovskite structure and having the non-stoichiometric composition, when the strontium and lanthanum are arranged at the A site and the titanium and niobium are arranged at the B site, the A / B site The thermoelectric conversion element according to appendix 2, wherein the niobium and lanthanum are doped so that the ratio is 0.5 to 0.9 or 1.05 to 1.2.
(Additional remark 4) The thermoelectric conversion element of Additional remark 2 or 3 characterized by the doping amount of the said niobium and the lanthanum in the strontium titanate thermoelectric thin film of the said nonstoichiometric composition being 10 at% or more and less than 30 at%.
(Supplementary Note 5) The strontium titanate thermoelectric thin film having the stoichiometric composition or the strontium titanate thermoelectric thin film having the non-stoichiometric composition has strontium titanate as a target, and metallic niobium or lanthanum is locally disposed on the target. The thermoelectric conversion element according to any one of appendices 1 to 4, wherein the thermoelectric conversion element is formed by sputtering in an argon gas.

(Supplementary Note 6) The supplementary note 1 is characterized in that the non-conductive cation non-stoichiometric strontium titanate thin film buffer layer doped with niobium or lanthanum is made nonconductive by annealing in an oxygen atmosphere after fabrication. The thermoelectric conversion element in any one of 5-5.
(Appendix 7) A strontium titanate thin film buffer layer having a non-conductive cationic non-stoichiometric composition doped with niobium or lanthanum is formed on a single crystal substrate of strontium titanate,
A method of manufacturing a thermoelectric conversion element, comprising forming a thermoelectric conversion element by forming a strontium titanate thermoelectric thin film having a stoichiometric composition doped with niobium and lanthanum on the thin film buffer layer.
(Appendix 8) Forming a non-conductive cation non-stoichiometric thin film buffer layer of strontium titanate doped with niobium or lanthanum on a single crystal substrate of strontium titanate,
A thermoelectric conversion element manufacturing method comprising forming a thermoelectric conversion element by forming a non-stoichiometric composition strontium titanate thermoelectric thin film doped with niobium and lanthanum on the thin film buffer layer.
(Supplementary Note 9) In the strontium titanate thermoelectric thin film having a perovskite structure and having a non-stoichiometric composition, when the strontium and lanthanum are arranged at the A site and the titanium and niobium are arranged at the B site, the A / B site The method for producing a thermoelectric conversion element according to appendix 8, wherein the niobium and lanthanum are doped so that the ratio is 0.5 to 0.9 or 1.05 to 1.2.
(Additional remark 10) Manufacture of the thermoelectric conversion element of Additional remark 8 or 9 characterized by the dope amount of the said niobium and lanthanum in the strontium titanate thermoelectric thin film of the said nonstoichiometric composition being 10 at% or more and less than 30 at% Method.

(Supplementary Note 11) The strontium titanate thermoelectric thin film having the stoichiometric composition or the strontium titanate thermoelectric thin film having the non-stoichiometric composition has strontium titanate as a target, and metallic niobium or lanthanum is locally disposed on the target. 11. The method for producing a thermoelectric conversion element according to any one of appendices 7 to 10, wherein the thermoelectric conversion element is formed by sputtering in an argon gas.
(Supplementary Note 12) The non-conductive strontium titanate thin film buffer layer doped with niobium or lanthanum having a nonionic cationic nonstoichiometric composition is made nonconductive by annealing in an oxygen atmosphere after fabrication. The manufacturing method of the thermoelectric conversion element in any one of 11 to 11.

DESCRIPTION OF SYMBOLS 1 Deposition chamber 2 Vacuum pump 3 Argon gas source 4 Target holder 5 Substrate holder 6 Power source 7 Target 8 Niobium chip 9 Substrate (STO substrate)
10 Argon gas ions 11 Intermediate layer (buffer layer, non-stoichiometric STO layer)
12 Thermoelectric thin film layer (stoichiometric STO layer)

Claims (5)

  A strontium titanate thin film buffer layer having a non-conductive cationic non-stoichiometric composition doped with niobium or lanthanum is provided on a single crystal substrate of strontium titanate, and niobium and lanthanum are doped on the thin film buffer layer. A thermoelectric conversion element comprising a strontium titanate thermoelectric thin film having a stoichiometric composition.   A strontium titanate thin film buffer layer having a non-conductive cationic non-stoichiometric composition doped with niobium or lanthanum is provided on a single crystal substrate of strontium titanate, and niobium and lanthanum are doped on the thin film buffer layer. A thermoelectric conversion element comprising a strontium titanate thermoelectric thin film having a non-stoichiometric composition.   In the non-stoichiometric strontium titanate thermoelectric thin film having a perovskite structure, when the strontium and lanthanum are arranged at the A site and the titanium and niobium are arranged at the B site, the A / B site ratio is 0. The thermoelectric conversion element according to claim 2, wherein the niobium and lanthanum are doped so as to be 5 to 0.9 or 1.05 to 1.2. On a single crystal substrate of strontium titanate, a strontium titanate thin film buffer layer having a non-conductive cationic non-stoichiometric composition doped with niobium or lanthanum is formed.
A method of manufacturing a thermoelectric conversion element, comprising forming a thermoelectric conversion element by forming a strontium titanate thermoelectric thin film having a stoichiometric composition doped with niobium and lanthanum on the thin film buffer layer. On a single crystal substrate of strontium titanate, a strontium titanate thin film buffer layer having a non-conductive cationic non-stoichiometric composition doped with niobium or lanthanum is formed.
A thermoelectric conversion element manufacturing method comprising forming a thermoelectric conversion element by forming a non-stoichiometric composition strontium titanate thermoelectric thin film doped with niobium and lanthanum on the thin film buffer layer.

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