JP2000256093A - Silicon material having clathrate crystal structure and its production - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a clathrate crystal consisting of only silicon atoms without containing other elements and to provide a method for its production. SOLUTION: This silicon material having a composite crystal structure consists of a mixture of Si46 single crystal lattices as inclusion lattices of silicon atoms and Si136 single crystal lattices as clathrate lattices. The silicon material is produced by forming an amorphous silicon layer on a silicon clathrate compd. crystal and heating to epitaxially grow the layer. Or, the silicon material is produced by forming amorphous silicon by ion beam sputtering at <=450 deg.C on a compd. clathrate crystal expressed by BaxRySi46-y (wherein x is 1 to 8, y is 1 to 6, and R is Ag or Au) and heating the amorphous silicon in vacuum or inert atmosphere at a temp. over 450 deg.C and <=1300 deg.C to epitaxially grow.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

The present invention relates to a silicon material used as a material for forming a semiconductor element such as an electronic circuit or an integrated circuit.

[0002]

2. Description of the Related Art Currently, various semiconductor devices are manufactured using materials such as germanium, silicon, and gallium arsenide. Among these, silicon single crystals are especially the most widely produced and used, because of their excellent stability, uniformity, wide operating temperature range, good electrical characteristics, and reasonable prices. It is also a material that has been thoroughly studied technically. However, the band gap of a normal silicon single crystal semiconductor is about 1.1 eV,
The application is limited depending on the application because it is regulated by the value of the band gap. For example, demand for light emitting devices and semiconductor lasers is expected to increase in response to higher signal speeds and higher density. However, the band gap of silicon crystal is low for these optical devices, making it difficult to apply them. is there.

An ordinary silicon single crystal or polycrystal has a diamond type crystal structure, and the value of the band gap is derived from this crystal structure. If the crystal structure is changed, the value of the band gap also changes. However, currently known crystal structures of silicon other than the diamond type include amorphous and clathrate. Amorphous silicon can be manufactured by sputtering at a relatively low temperature, but is extremely unstable to heating, so that the device manufacturing technology cultivated with conventional silicon single crystals cannot be applied.

[0004] A clathrate compound is generally called an inclusion compound. When two kinds of molecules crystallize together under appropriate conditions, one molecule forms a three-dimensional network structure and the other molecule forms a three-dimensional network structure. Is in the shape of the mesh. The three-dimensional network structure is called the inclusion grid,
Strong intermolecular forces do not act between the molecules that make up the inclusion lattice and the molecules that go into it, and it is important that the size of the space and the size of the molecules that go into it are just appropriate. It is supposed to be a condition. For this reason, a certain ratio may not be shown between the two molecules.

It has long been known that silicon clathrate compounds have a crystal structure consisting of an inclusion lattice in which an alkali metal atom has entered.
However, it has recently been found that a silicon clathrate compound having a composition represented by Ba ₆ Na ₂ Si ₄₆ has a superconducting phenomenon (H. Kawaji, et al .: PHYSICAL REVIEW LETTER, 74 (19)
95), p.1427)). Furthermore, in this case, the results estimated by the first principle calculation show that, if silicon crystals comprising an inclusion lattice were produced, the band gap was about 0.7 eV higher than that of silicon having a diamond-type crystal structure. Was. In addition, it was presumed that the silicon crystal of this inclusion lattice had the same level of stability as the diamond type crystal in terms of energy.

This silicon clathrate compound crystal has two crystal structures. One is that all 5
A dodecahedral Si ₂₀ inclusion lattice composed of membered ring surfaces, and a tetrahedral S surface composed of 12 five-membered ring surfaces and two six-membered ring surfaces
Si ₄₆ , which is connected to the i ₂₄ clathrate lattice by sharing a plane, forms one unit cell. The other is Si ₁₃₆ in which a Si ₂₀ clathrate lattice and a hexahedral Si ₂₈ clathrate lattice composed of 12 five-membered rings and four six-membered rings are connected in a shared manner.
Constitute one unit cell. FIG. 1 shows a part of the crystal structure of Si ₄₆ containing no inclusion atoms, and FIG.
shows part of the crystal structure of i ₁₃₆ respectively.

A silicon clathrate compound crystal containing an alkali metal atom such as Na, K, or Rb in the clathrate lattice is, for example, Na ₈ Si ₄₆ or Na _x Si ₁₃₆ (1.5 <x
In the case of the crystal represented by <24), Na and Si are mixed and heated to produce NaSi, which is heated under reduced pressure in a vacuum or an inert atmosphere to remove Na. It is obtained by going. On the other hand, attempts have been made to include atoms other than alkali metals, and the aforementioned Ba ₆ Na ₂
Si ₄₆ was synthesized (S. Yamanaka, et al .: FULLERENE SCIE
NCE & TECHNOLOGY, 3 (1995), 21). In this case, NaSi
And BaSi ₂ were mixed by mixing the respective raw materials, and heated and melted. After synthesizing, a required amount was blended and heated to obtain a solid solution.
It is made by heating in vacuum at 0 ° C. to exclude Na.

Japanese Patent Application Laid-Open No. 9-183607 discloses an invention of a silicon clathrate compound crystal by a similar manufacturing method. This is because, in addition to Na and K, the alkali metal element including even Li is A, and in addition to Ba, Sr and C
When the alkaline earth metal element including a is expressed as Ae, a solid solution having a composition of A ₂ AeSi ₄ is produced as a precursor, and this is heated under reduced pressure to remove the alkali metal, thereby obtaining A _x Ae It is intended to obtain a clathrate compound crystal of ₆ Si ₄₆ (where 0 ≦ x ≦ 2). This Japanese patent
9-183607 discloses the electrical conductivity of the obtained clathrate compound crystal, and depending on the composition, a superconductor, a semiconductor,
Or it shows the properties of metal. In the case of semiconductor characteristics, the band gap is relatively small at 0.2 eV, and the high band gap expected from a silicon crystal comprising an inclusion lattice has not been obtained. It is considered that this is because the alkali metal is inevitably doped from the manufacturing method of using a solid solution of the alkali metal as a raw material and removing the alkali metal therefrom.

Recently, instead of using such a solid solution containing an alkali metal as a precursor, Ag having an ionic radius almost equal to that of Na is used, and a laser ablation method is used.
A thin film of a crust compound crystal of silicon having a composition of a ₈ Ag _y Si _46-y has been prepared (Yamanaka: Proceedings of the Second Symposium on “Physical Phenomena such as Quantum Effects” (December 1998)
21-22 days), p.83). The obtained thin film shows superconductivity in a cryogenic temperature range, but shows electrical properties similar to ordinary silicon metal at room temperature, and includes metal elements such as Ag and Ba in its inclusion lattice. .

As described above, a clathrate crystal of silicon is expected as a semiconductor material having characteristics different from those of a conventional silicon single crystal having a diamond-type crystal structure. No silicon-only clathrate crystal containing elements has been produced.

[0011]

The ordinary silicon crystal as a semiconductor material has a band gap value of about 1.1 eV, but the band gap can be increased by changing the diamond type crystal structure to a clathrate crystal structure. It can be easily applied to optical devices and the like. However, the silicon clathrate compound crystals thus far produced have only a smaller band gap than ordinary silicon crystals. This is because the clathrate inclusion lattice contains an alkali metal element. The purpose of the present invention is
It is an object of the present invention to provide a clathrate crystal comprising only silicon atoms containing no other element and a method for producing the same.

[0012]

Means for Solving the Problems The present inventors have conducted various studies on silicon crystal as a semiconductor material, which has a possibility of increasing the band gap, as a material that can be used for actual devices. Was done. First, a simulation calculation called a molecular dynamics method was performed on a crystal composed of only silicon which does not include other elements and having Si ₄₆ or Si ₁₃₆ as a unit cell.

As a result of obtaining the cohesion energy of Si constituting the crystal, as compared with the case of the diamond crystal structure, about 0.06 eV for Si _{46 and} about 0.04 eV for Si ₁₃₆ , respectively.
It was only eV big. On the other hand, when the cohesive energy of amorphous silicon was similarly estimated, it was found that the cohesive energy was 0.3 eV larger than that of the diamond crystal structure. This indicates that the clathrate crystal structure is much more stable in terms of energy than the amorphous one. It was speculated that there is a possibility of conversion to a clathrate crystal structure. Therefore, an attempt was made to convert amorphous silicon into a clathrate crystal based on a thin layer of a clathrate compound of Ba ₈ Ag _y Si _46-y whose production method is already known.

Ba-Ag-S produced by the arc melting method
Using an i-alloy as a target, a thin film was formed on a silicon wafer used for a semiconductor substrate by a laser ablation method. In this case, if the target alloy composition is selected and the film is formed under the highest possible vacuum,
It was confirmed that a crystal film of a silicon clathrate compound of Ba ₈ Ag _y Si _46-y was obtained. As a result of forming an amorphous silicon layer on this film at a low temperature and heating it to a high temperature, it is possible to obtain a polycrystalline clathrate crystal of Si ₄₆ and Si ₁₃₆ which does not contain other elements consisting only of Si. It was found. As described above, the reason why the clathrate silicon crystal formed _therefrom has a mixed crystal structure of Si ₄₆ and Si ₁₃₆ although the crystal structure of the base seed crystal is only the Si ₄₆ type is apparent. is not. However, Si _{46 in which} a 6-membered ring is added to a 5-membered ring
And Si ₁₃₆ have very similar structures, and due to the interaction between the growth nuclei during the epitaxial growth process,
This is probably because the mixed structure is more stable. It was also confirmed that if an amorphous silicon layer was further formed on the once formed silicon clathrate crystal and heated, the silicon layer would also be a similar silicon clathrate crystal.

As described above, a clathrate crystal containing only silicon and containing no other element was obtained. Therefore, in order to stably obtain this crystal, the composition range of a target alloy for laser ablation for producing a clathrate compound film to be a seed crystal film, a method of forming amorphous silicon on the seed crystal film and its conditions, The heating conditions for epitaxial growth were further studied. Based on these results, the manufacturing conditions were clarified to complete the present invention. The gist of the present invention is as follows.

(1) Si ₂₀ which is an inclusion lattice of silicon atoms
A unit crystal lattice of Si ₄₆ composed of Si and Si ₂₄ and an Si lattice composed of Si ₂₀ and Si ₂₈ which are also inclusion lattices.
A silicon material with a composite crystal structure consisting of only silicon atoms and a mixture of ₁₃₆ unit crystal lattices.

(2) The method for producing a silicon material having a composite crystal structure according to the above (1), wherein an amorphous silicon layer is formed on a silicon clathrate crystal, and the amorphous silicon layer is heated and epitaxially grown.

(3) Ba _x R _y Si _46-y (where x = 1 to
8, y = 1 to 6, R is Ag or Au) Clathrate compound crystal or clathrate crystal consisting of silicon atoms only, amorphous silicon film is formed by ion beam sputtering at a temperature of 450 ° C. or less After doing
(2) The method for producing a silicon material having a composite crystal structure according to the above (2), wherein the epitaxial growth is performed by heating to a temperature of 500 to 1200 ° C. in a vacuum or an inert atmosphere.

[0019]

DESCRIPTION OF THE PREFERRED EMBODIMENTS A conventional silicon clathrate compound crystal contains an alkali metal,
Or, an alkaline earth metal or another atom is included. On the other hand, the silicon clathrate crystal of the present invention is a crystal having a complex structure in which a unit cell of Si ₄₆ and Si ₁₃₆ consisting only of the silicon inclusion lattice is present without these inclusion atoms. However, since the crystal having this composite structure is grown from a clathrate compound that includes other atoms, the clathrate compound crystal that includes other atoms is closely attached to the clathrate crystal composed of only silicon. It doesn't matter.

The clathrate crystal composed of only silicon is a crystal having a complex structure in which unit cells of Si ₄₆ and Si ₁₃₆ are mixed, and has a band gap of 1.8 to 1.9 eV, which is a conventional silicon crystal having a diamond type crystal structure. The value is much higher than that. Also, similar to normal silicon single crystal,
The semiconductor can be used as a semiconductor by doping with a Group 3 or Group 5 element by a technique such as diffusion or ion implantation.

The clathrate crystal composed of only silicon is prepared by using a clathrate compound in the form of Ba _x R _y Si _46-y as a seed crystal and epitaxially growing only silicon from the crystal. In this composition formula, x is 1 to 8, y is 1 to 6, and R is Ag or Au. x or y
The value of is limited to 1 to 8 or 1 to 6 because if the value is out of this range, a clathrate compound crystal serving as a seed crystal cannot be obtained. R is Ag or Au. By using these elements, a stable seed crystal can be obtained.

The method of manufacturing a clathrate crystal composed of only silicon is as follows, taking the case of a flat plate as an example. The substrate to be used is preferably a silicon wafer used for an LSI or the like, and may be made of glass or ceramic. First, Ba as a raw material of a clathrate compound crystal,
Ag, Au and Si are blended in a required composition, and an alloy is melted and produced by an arc melting method or the like. Using this alloy as a target, heating the substrate to 600 to 800 ° C under high vacuum and depositing a thin film on the substrate by laser ablation method,
Clathrate compound crystals are formed. In this case, in order to stably form the clathrate compound, it is desirable that the atmosphere during the film formation be as high as possible, and 10 ^-6 Torr
It is preferable to perform the process while evacuating to the following pressure.

After confirming that the obtained film is a clathrate compound crystal, amorphous silicon is formed thereon. Although clathrate crystals can be obtained by direct epitaxial growth from the gas phase by CVD using SiH ₄ as a source gas on this seed crystal film, silicon having a diamond-type crystal structure is likely to be generated. Therefore, it is better to stack amorphous silicon once and heat it to grow it. Amorphous silicon adjusts the temperature of the substrate on which the seed crystal film is formed,
The film is formed by a sputtering method at a temperature of 450 ° C. or lower, preferably 100 ° C. or lower, preferably at about room temperature. In this case, the substrate is crystallized when the substrate temperature rises to form a diamond-type crystal structure. Therefore, an ion beam sputtering method in which the temperature does not easily rise is preferable. After the amorphous silicon film is formed, in a vacuum at a pressure of 10 ^-5 Torr or less, or in an inert gas such as high-purity argon with a dew point of -45 ° C. Heat to form a clathrate crystal by epitaxial growth. The reason why the vacuum or the high-purity inert atmosphere is used is that slight oxidation may hinder epitaxial growth, so that oxidation is sufficiently prevented. If the heating temperature is lower than 500 ° C., the rate of epitaxial growth becomes extremely slow, and it takes a long time.
If the temperature exceeds 1200 ° C., a diamond-type crystal structure is likely to be formed.

[0024]

EXAMPLE An alloy having a composition of Ba ₈ Ag ₂ Si _{44 was produced} by the arc melting method. Using this alloy as a target, S
A thin film was formed on the (001) substrate i by a laser ablation method. In that case, the ultimate vacuum degree is 10 ^-9 Torr,
Substrate heating temperature 650 ° C, laser source is ArF (wavelength 193n)
m), a pulse width of 20 ns, and a laser energy range of 60 to 100 mJ, and a film was formed for 30 minutes.

As a result of analyzing the obtained film by X-ray diffraction,
It was confirmed that the film was a polycrystalline film of a compound clathrate of Ba _x Ag _y Si _46-y . On the substrate on which the polycrystalline film was formed, silicon was formed by an ion beam sputtering method. The film formation conditions were as follows: Ar was used as ions, the substrate temperature was maintained at room temperature, the degree of vacuum was 5 × 10 ⁻⁸ Torr, the acceleration voltage was 300 V, the current was 60 mA, and the film formation time was 2 hours. After film formation 1
Heating was performed at 650 ° C. for 3 hours in a vacuum of 0 ⁻⁶ Torr or less. The thickness of this film was 1 μm. As a result of X-ray crystal structure analysis, the film was amorphous silicon immediately after the film formation, but after heating, the diffraction pattern corresponding to the amorphous silicon disappeared, and the silicon clathrate crystal having a crystal structure of a mixture of Si ₄₆ and Si ₁₃₆ I found it.

Next, using the substrate on which the silicon clathrate crystal film having a thickness of 1 μm was formed, amorphous silicon was further formed thereon by ion beam sputtering under exactly the same conditions as above, At 650
The operation of heating at 3 ° C. for 3 hours was repeated to make the total silicon film thickness 2 μm. Employment and the results was carried out X-ray analysis of the obtained film, the whole film is Si ₄₆ and S
it was confirmed i ₁₃₆ is a mixed silicon clathrate crystals of the crystal structure. Further, as a result of measurement by ultraviolet photoelectron spectroscopy, the band gap was 1.85 eV.

[0027]

The present invention provides a silicon crystal composed of only silicon and having a clathrate structure containing no other element and a method for producing the same. The silicon clathrate crystal has a larger band gap than silicon having a normal diamond crystal structure, and is expected to be used for semiconductors and other electronic materials such as optical devices that could not be applied conventionally.

[Brief description of the drawings]

FIG. 1 is a view showing a part of a crystal structure of a silicon clathrate of Si ₄₆ .

FIG. 2 is a view showing a part of a crystal structure of a silicon clathrate of Si ₁₃₆ .

 ──────────────────────────────────────────────────続 き Continuing on the front page (72) Inventor Koji Yonemura 1-8 Fuso-cho, Amagasaki-shi, Hyogo Prefecture Inside Sumitomo Metal Industries, Ltd. Electronics Technology Research Laboratories (72) Tatsuo Sawazaki 1-8 Fuso-cho, Amagasaki-shi, Hyogo Sumitomo Metal Industries, Ltd. Electronics Technology Research Laboratories (72) Inventor Shigeki Shibaki 1-8 Fuso-cho, Amagasaki City, Hyogo Prefecture Sumitomo Metal Industries Co., Ltd. Electronics Technology Research Laboratory (72) Inventor Yoshinori Shirakawa 1st Fuso-cho, Amagasaki City, Hyogo Prefecture No. 8 Sumitomo Metal Industries, Ltd. Electronics Technology Research Laboratories (72) Inventor Naoki Ikeda 1-8 Fuso-cho, Amagasaki City, Hyogo Prefecture Sumitomo Metal Industries, Ltd. Electronics Technology Research Laboratories F-term (reference) HH02 NN11 NN24 UU01 4G077 AA03 BA04 DA0 3 DA11 DA15 ED01 HA06 4K029 AA06 BA35 BB02 CA08

Claims (3)

[Claims] 1. An inclusion lattice of silicon atoms, Si ₂₀ and S
A silicon material having a complex crystal structure in which a unit crystal lattice of Si ₄₆ composed of i ₂₄ and a unit crystal lattice of Si ₁₃₆ composed of Si ₂₀ and Si ₂₈ which are also inclusion lattices are mixed. 2. The method according to claim 1, wherein amorphous silicon is formed on the clathrate crystal of silicon, and the amorphous silicon is heated and epitaxially grown. Wherein _{Ba x R y Si 46-y} ( where, x = 1~8, y
= 1 to 6, R is an Ag or Au) clathrate compound crystal or an amorphous silicon film formed by ion beam sputtering at a temperature of 450 ° C or less on a clathrate crystal composed of only silicon atoms. 3. An epitaxial growth by heating to a temperature of 500 to 1200 [deg.] C. in a vacuum or inert atmosphere.
3. The method for producing a silicon material having a composite crystal structure according to 1.