JP6465397B2 - Group IV clathrate production method - Google Patents

Description

  The present invention relates to a method for efficiently producing a group IV clathrate using a group IV element such as germanium or silicon as a host atom.

  A clathrate (inclusion compound) is a compound in which guest atoms are encapsulated in a three-dimensional cage-like structure formed by host atoms, and has characteristics different from those of conventional crystal structures. The clathrate is classified from type I (structure 1, also referred to as Type I) to type VIII (also referred to as structure 8, Type VIII), depending on its three-dimensional structure.

For example, when a silicon clathrate whose host atom is silicon contains sodium as a guest atom, a silicon clathrate containing a large amount of sodium has metallic properties, but a silicon clathrate with a reduced sodium content Is known to clearly show band gap characteristics as a semiconductor. When a silicon clathrate containing sodium is produced, a clathrate mainly composed of a mixture of type I and type II (structure 2, also referred to as Type II) is generated. The I-type silicon clathrate has a cubic structure composed of silicon ₂₀ having a dodecahedron structure and silicon ₂₄ having a tetrahedral structure. The type II silicon clathrate has a cubic structure composed of silicon ₂₀ having a dodecahedron structure and silicon ₂₈ having a hexahedral structure. Type I silicon clathrate has a characteristic that once sodium atoms are included, sodium atoms are hardly removed in subsequent steps. On the other hand, it is known that the type II silicon clathrate can remove sodium and reduce the content by heat treatment or the like. Therefore, in order to improve convenience as a semiconductor, a technique for manufacturing a II-type silicon clathrate in a film shape with high efficiency is required.

  In addition, since the clathrate in which the host atom is composed of germanium exhibits characteristics close to that of gallium arsenide, application to a light absorbing material for solar cells is expected. In addition, some type II group IV clathrates function as direct transition semiconductors and can change light absorption characteristics from infrared to visible light by alloying. To do. Therefore, a highly versatile technique that can efficiently produce a film-like group IV clathrate is required.

  The inventor has previously disclosed a novel method for producing a film-like silicon clathrate in Patent Document 1. Patent Document 2 discloses a novel method for producing a type II clathrate having silicon and germanium as host atoms. Furthermore, Patent Document 3 discloses a novel method for producing germanium clathrate.

Japanese Patent No. 5626896 Japanese Patent No. 5641481 JP 2014-43599 A

  With the techniques disclosed in the above patents and patent applications, it has become possible to produce film-like silicon clathrate and germanium clathrate. However, these conventional film-like clathrate production methods are methods for producing clathrate by supplying guest atoms to the surface of a substrate made of high-purity crystals of host atoms and heating them. For this reason, when it was going to be used for various devices in a state where the film-like clathrate was integrated with the substrate, there was a risk of being restricted by the characteristics of the substrate.

  The present invention has been made in view of the problems of such a conventional clathrate production method, and is a novel production method capable of producing a homogeneous film-like clathrate on a substrate made of a material different from host atoms. Is a problem to be solved.

The present invention relates to a method for producing a clathrate. The clathrate production method of the present invention includes a step of laminating a group IV element serving as a host atom in a film form on a substrate, and the substrate having a group IV element stack and a crystal of a guest atom are spaced from each other. And a first heating step of heating the reaction vessel at a temperature of 350 ° C. or more and 650 ° C. or less for a heating time of 0.5 hours or more and 24 hours or less in an inert gas atmosphere, A cooling step of cooling the reaction vessel containing the guest atoms and the substrate, and a heating time of 1 hour to 24 hours at a temperature of 250 ° C. to 450 ° C. under a negative pressure of 10 ⁻² Pa or less. And a second heating step for heating.

  In the laminating step of the clathrate production method of the present invention, a group IV element that becomes a host atom is preferably laminated on the substrate by sputtering.

  Furthermore, in the method for producing a clathrate according to the present invention, it is preferable to produce a germanium clathrate using germanium as a host atom and sodium as a guest atom. Germanium can produce a clathrate by heat treatment at a relatively low temperature among group IV elements, and can easily obtain a film-like clathrate.

  The clathrate production method of the present invention makes it possible to produce a film-like clathrate by laminating a group IV element serving as a host atom on a substrate in a film shape and heating it together with guest atoms. Compared to the conventional manufacturing method in which the substance forming the substrate and the host atoms are basically the same, it is possible to manufacture a film-like clathrate on an arbitrary substrate. Can be expanded. Various device structures can be manufactured. In addition, the options for evaluating the physical properties of the manufactured clathrate are widened, and precise electronic physical properties can be evaluated.

  The clathrate production method of the present invention can use a substrate formed of a material different from the host atom, so that a clathrate using an expensive host atom can be produced at a lower cost than conventional ones. In addition, since it becomes easy to control the composition ratio of the materials to be laminated, the production or doping of clathrate with an alloy becomes easier than before.

  On the other hand, the method for producing a clathrate of the present invention can produce an unprecedented clathrate by performing epitaxial growth affected by the substrate by selecting a specific combination of the substrate and the host atom. It becomes possible.

  Furthermore, the method for producing a clathrate of the present invention can produce a suitable germanium clathrate by using germanium as the host atom. While germanium is expensive, among group IV elements, heat treatment can be performed at a relatively low temperature, and a film-like clathrate can be produced by the present invention more easily and at a lower cost.

FIG. 1 is a flowchart showing a preferred method for producing a clathrate according to the present invention. FIG. 2 is a diagram showing an X-ray diffraction pattern of the germanium film after completion of the cooling process of Example 1. FIG. 3 is a drawing-substituting photograph of the germanium film after completion of the cooling step of Example 1. 4 is a diagram showing an X-ray diffraction pattern of the film-like germanium clathrate produced in Example 1. FIG. FIG. 5 is a drawing-substituting photograph of the film-like germanium clathrate produced in Example 1. 6 is a diagram showing an X-ray diffraction pattern of a film-like germanium clathrate produced in Example 2. FIG. FIG. 7 is a drawing-substituting photograph of the germanium film after completion of the cooling step of Example 2. FIG. 8 is a drawing-substituting photograph of the film-like germanium clathrate produced in Example 2. FIG. 9 is a drawing-substituting photograph of the germanium film deposited on the surface of the sapphire substrate in the laminating process of Example 3. FIG. 10 is a diagram showing an X-ray diffraction pattern of a film-like germanium clathrate produced in Example 3. FIG. 11 is a drawing-substituting photograph of the cross section of the film-like germanium clathrate produced in Example 3 using an electron microscope. FIG. 12 is a diagram showing an X-ray diffraction pattern of a film-like germanium clathrate produced in a comparative example. FIG. 13 is a drawing-substituting photograph of the film-like germanium clathrate produced in the comparative example.

  Hereinafter, an embodiment in which the present invention is embodied as a manufacturing method for manufacturing a film-like germanium clathrate using germanium as a host atom and sodium as a guest atom will be described with reference to the drawings. FIG. 1 is a flowchart showing a preferred manufacturing method for manufacturing a film-like silicon clathrate. As shown in FIG. 1, the preferred clathrate manufacturing method of the present embodiment includes a laminating process, a pretreatment process, a first heating process, a cooling process, and a second heating process.

  In the laminating process shown in Step 1 (S1) of FIG. 1, germanium, which is a host atom, is laminated in a thin film on a substrate, preferably by sputtering. Suitable sputtering conditions are an input power of 200 W, an argon flow rate of 50 sccm, and a pressure of 1.0 Pa. The film formation time is set to 15 minutes or more and 60 minutes or less according to the required film thickness.

  As a method for stacking germanium on the substrate, a chemical vapor deposition method (CVD) or a vapor deposition method can be applied in addition to the sputtering method.

  The type of the substrate on which germanium is laminated is that the substrate is a substrate that has little alteration in the first heating step and the second heating step and has low reactivity with sodium as a guest atom. In the case of epitaxial growth, it is preferable that the atomic spacing and arrangement are close at the interface between the manufactured group IV clathrate and the substrate. Examples of suitable substrates for laminating germanium include various glass substrates including silicon substrates, sapphire substrates, or anhydrous synthetic quartz substrates.

  In the pretreatment step shown in step 2 (S2) of FIG. 1, a substrate on which germanium is laminated and a crystal of sodium are placed in a reaction container with an interval therebetween. A suitable distance between the substrate and the sodium crystal is 5 mm or more and 15 mm or less. The germanium substrate is preferably arranged horizontally.

  In the first heating step shown in Step 3 (S3) of FIG. 1, the silicon substrate 2 and the sodium-containing vessel 3 accommodated in the reaction vessel are heated at a temperature of 350 ° C. or higher and 650 ° C. or lower for 0.5 hour or longer and 24 hours or shorter. Heat during. In the first heating process of this embodiment, sodium vapor is supplied to the surface of germanium on the substrate. It has been confirmed that the heating at a temperature of less than 350 ° C. does not generate a germanium clathrate even if the subsequent steps are performed. In the first heating step, it is necessary to perform a treatment at a temperature of 350 ° C. or higher.

  In the cooling step shown in Step 4 (S4) of FIG. 1, preferably slow cooling is performed. When performing slow cooling, it is preferable to cool the whole reaction vessel at a rate of 5 ° C./hr or more and 20 ° C./hr or less, or naturally cool the substrate while the substrate is accommodated. This cooling process is a process in which it is estimated that the sodium which has become vapor in the first heating process is condensed and reacts with the substrate-like germanium.

In the second heating step shown in Step 5 (S5) of FIG. 1, the substrate on which the germanium clathrate precursor has been formed in the steps up to here is 250 ° C. or higher and 450 ° C. or lower under a negative pressure of 10 ⁻² Pa or lower. Heat to the temperature of. The heating time is set within 1 hour and within 24 hours. During the second heating step, the germanium clathrate precursor on the substrate changes to a germanium clathrate containing sodium.

  In the method for producing a germanium clathrate according to the present embodiment, a germanium clathrate is produced by laminating a germanium thin film on the surface of a substrate by sputtering and reacting the thin film with sodium at a high temperature. With this manufacturing method, a thin film-like germanium clathrate can be manufactured on various substrates, so that it can be applied to an unprecedented device.

  Hereinafter, Examples 1 to 3 in which the present embodiment is further embodied and germanium clathrate is formed on various substrates, and a comparative example in which germanium clathrate is manufactured on a germanium substrate by a conventional manufacturing method are shown.

Example 1
In this example, a germanium clathrate was manufactured using a silicon substrate having a (111) plane appearing on the surface. First, in Step 1 (S1), a germanium film was formed by sputtering. The sputtering conditions in this example are as follows: input power is 200 W, argon flow rate is 50 sccm, pressure is 1.0 Pa, and film formation time is 30 minutes. X-ray diffraction confirmed that the germanium films on these substrates had an amorphous structure.

In the pretreatment step of Step 2 (S2), the substrate on which the germanium film was formed by the sputtering method was accommodated in the reaction vessel together with the metal sodium piece. Next, in the first heating step of Step 3 (S3), heat treatment was performed in an argon atmosphere at a pressure of 10 ⁵ Pa, a treatment temperature of 400 ° C., and a treatment time of 3 hours. In the reaction vessel, the germanium film on the substrate and sodium vapor reacted. Further, in the cooling process of Step 4 (S4), the reaction vessel was cooled by natural temperature drop. FIG. 2 shows an X-ray diffraction pattern of the germanium film formed on the silicon substrate when the cooling process is completed. At this time, the germanium film reacts with sodium to become a precursor of germanium clathrate. FIG. 3 shows an optical photograph of a germanium clathrate precursor on a silicon substrate analyzed by X-ray diffraction.

As the second heating step of Step 5 (S5), the substrate on which the germanium clathrate precursor was formed was heated for 12 hours under the conditions of a pressure of 10 ⁻² Pa and a temperature of 300 ° C. Then, it cooled by natural temperature fall. FIG. 4 shows an X-ray diffraction pattern of germanium clathrate formed on the silicon substrate after cooling. From this analysis result, it was confirmed that the formed germanium clathrate film was of type II and had (111) orientation and (022) orientation. FIG. 5 shows an optical photograph of germanium clathrate on the produced silicon substrate. As shown in FIG. 5, a film-like germanium clathrate was produced.

  According to the manufacturing method of the present embodiment, germanium clathrate can be manufactured on a silicon substrate. The silicon substrate is cheaper than the germanium substrate that has been conventionally used for producing germanium clathrate, and the production method of this embodiment makes it possible to produce germanium clathrate at a lower cost than before. . The (111) orientation and (022) orientation are unique structures obtained by a clathrate manufacturing method using a silicon substrate with the (111) plane appearing on the surface, and are epitaxially grown under the influence of the substrate. Germanium clathrate could be produced.

(Example 2)
In this example, a germanium clathrate was produced using a glass substrate. The substrates used were a Corning Incorporated EAGLE XG (registered trademark) glass substrate and an anhydrous synthetic quartz substrate (hereinafter also simply referred to as a quartz substrate).

  In step 1 (S1), a germanium film was formed by sputtering. The sputtering conditions are an input power of 200 W, an argon flow rate of 50 sccm, and a pressure of 1.0 Pa. In this example, the characteristics of the manufactured germanium clathrate were verified by changing the film formation time from 15 minutes to 60 minutes and changing the thickness of the deposited germanium film. As in Example 1, it was confirmed by X-ray diffraction that an amorphous germanium film was formed on the substrate.

  In this example, heating and cooling were performed under the same conditions as in Example 1 from the pretreatment process of Step 2 (S2) to the second heating process of Step 5 (S5). FIG. 6 shows an X-ray diffraction pattern of germanium clathrate formed on a quartz substrate. From this analysis result, it was confirmed that the formed germanium clathrate film was type II. Similarly, Raman spectroscopy detected a peak corresponding to type II germanium clathrate and verified that the germanium clathrate was formed on the substrate. FIG. 7 shows an optical photograph of the quartz substrate on which the germanium clathrate precursor is formed after the cooling step. FIG. 8 shows an optical photograph of germanium clathrate on a quartz substrate used for X-ray diffraction. As shown in FIG. 8, a film-like germanium clathrate was produced. Similarly, a film-like germanium clathrate could be produced using an EAGLE XG (registered trademark) glass substrate.

  In this example, the thickness of the germanium film to be stacked was changed by changing the sputtering time in the stacking process of Step 1 from 15 minutes to 60 minutes. Then, an attempt was made to improve the yield of germanium clathrate by changing the heating time of the first heating step corresponding to the thickness of the germanium film. However, the quartz substrate is reactive with sodium, and discoloration at the interface between the germanium film and the substrate starts when the heat treatment for 24 hours or longer is performed in the first heating step. For this reason, when the glass substrate was used, it became clear that the preferable processing time of a 1st heating process is less than 24 hours.

  In the clathrate production method of the present example, germanium clathrate can be produced on a transparent substrate by using a glass substrate. The clathrate on a transparent substrate has wide application to optical devices such as solar cells.

(Example 3)
In this example, a germanium clathrate was manufactured using a sapphire substrate. The manufacturing method and the manufacturing conditions from Step 1 to Step 5 are the same as those in the first embodiment, and a duplicate description is omitted.
FIG. 9 shows an optical photograph of a state in which germanium deposited on the surface of the sapphire substrate in the layering process in step 2 is a film. FIG. 10 shows an X-ray diffraction pattern of germanium clathrate formed on the sapphire substrate. From this analysis result, it was confirmed that the formed germanium clathrate film was type II. Similarly, Raman spectroscopy detected a peak corresponding to type II germanium clathrate and verified that the germanium clathrate was formed on the substrate. FIG. 11 shows a cross-sectional electron micrograph of germanium clathrate on the produced sapphire substrate. As shown in FIG. 11, a film-like germanium clathrate was produced.

  The sapphire substrate used in this example is less reactive with sodium than the glass substrate, and even when the heat treatment is performed for 24 hours in the first heating step, discoloration of the interface between the germanium film and the substrate is recognized. I couldn't. By using a sapphire substrate, germanium clathrate having various film thicknesses can be manufactured.

(Comparative example)
As a comparative example, the characteristics of a germanium clathrate manufactured by applying the conventional technique disclosed in Patent Document 3 by the inventors using a germanium substrate and sodium as raw materials are shown. In this comparative example, a germanium substrate and a sodium crystal were placed in a reaction vessel at an interval. In the first heating step, the temperature was naturally lowered by heating at a pressure of 10 ⁵ Pa in an argon atmosphere at a temperature of 400 ° C. for 3 hours. . Further, in the second heating step, the germanium substrate on which the precursor is formed is heated at 10 ⁻² Pa or less and a temperature of 300 ° C. for 12 hours, and is naturally cooled to produce a germanium clathrate.

  The lower part of FIG. 12 shows an X-ray diffraction pattern of a precursor composed of germanium and sodium formed on a germanium substrate at the end of the first heating step of the comparative example. The upper part of FIG. 12 shows an X-ray diffraction pattern of a germanium clathrate formed on a germanium substrate at the end of the second heating step in the comparative example. From this analysis result, it was confirmed that sodium germanide was (101) and (200) oriented in the precursor, and the formed germanium clathrate film was (111) oriented. FIG. 13 shows an optical photograph of germanium clathrate on a germanium substrate. There is no significant difference in appearance between the germanium clathrate of this comparative example and the germanium clathrate of Example 1 to Example 3, and a uniform film-like clathrate is formed in both cases.

  The germanium clathrate film of the comparative example is (111) oriented, whereas the germanium clathrate produced on the silicon substrate of Example 1 has (111) orientation and (022) orientation as shown in FIG. And have. It is also clear from the comparison between this comparative example and Example 1 that the clathrate manufacturing method of the present invention can manufacture various clathrates corresponding to the structure of the substrate on which the host atoms are deposited. .

  As mentioned above, although the specific example of this invention was described in detail in the Example, these are only illustrations and do not limit a claim. The technology described in the claims includes various modifications and changes of the specific examples illustrated above. For example, in the examples, a detailed description was given of the case where the temperature of the second heating step was 300 ° C., but the germanium and sodium compound once formed with the precursor has a temperature of 250 ° C. or higher and 450 ° C. or lower The clathrate can be produced by performing the heat treatment at. Further, in the examples, the clathrate was produced using germanium and sodium, but the clathrate can be produced by the method of this example also using other group IV elements as host atoms.

  The clathrate production method of the present invention can produce a suitable film-like germanium clathrate and a clathrate having another group IV element as a host atom. The resulting clathrate can be used as a solar electric field, a thermoelectric element, or a superconducting material.

Claims (2)

A method of manufacturing a clathrate,
A laminating step of laminating a group IV element serving as a host atom on a substrate;
A pretreatment step of accommodating the substrate having a stack of group IV elements and a crystal of a guest atom in a reaction container at an interval;
A first heating step of heating the reaction vessel at a temperature of 350 ° C. or more and 650 ° C. or less in an inert gas atmosphere for a heating time of 0.5 hours or more and 24 hours or less;
A cooling step of cooling a reaction vessel in which guest atoms and the substrate are accommodated;
A second heating step of heating the substrate under a negative pressure of 10 ⁻² Pa or less at a temperature of 250 ° C. or more and 450 ° C. or less for a heating time of 1 hour or more and 24 hours or less;
Equipped with a,
A method for producing a clathrate, wherein the host atom is germanium and the guest atom is sodium .   2. The method for producing a clathrate according to claim 1, wherein in the laminating step, a group IV element serving as a host atom is laminated on the substrate by a sputtering method.