Method for producing single crystal diamond film

Abstract: A single crystal diamond film having good electri­ cal characteristics is produced by a method which comprises steps of decomposing a raw material gas comprising a hydro­ gen gas and a carbon-containing compound and epitaxially growing a single crystal diamond film on a single crystal substrate in a vapor phase, wherein a molar ratio of the carbon atoms in the carbon-containing compound to the hydro­ gen is from 2:100 to 10:100 and a lattice constant of the single crystal substrate satisfies the following relation: |(a - a₀)/a| x 100 ≦ 20 (I) wherein a₀ is the lattice constant of diamond (3.567 Å) and a is a lattice constant of the single crystal substrate.

Description: BACKGROUND OF THE INVENTION Field of the Invention The present invention relates to a method for producing a single crystal diamond film. In particular, the present invention relates to a method for producing a single crystal diamond film having good crystallinity, a smooth surface, and good electrical and optical characteristics. Description of the Related Art Diamond has a large forbidden band width, a large electron mobility, a large hole mobility, a large saturation electron mobility, a large thermal conductivity, a low di-electric constant and high clarity. Therefore, the diamond is highly expected as a material for semiconductors, for example, semiconductive elements having heat resistance and radiation resistance and high speed high output semiconductive elements, and as an element for emitting blue light or UV light. Recently, R. Mania et al proposed a method for synthesizing diamond in a vapor phase which comprises decomposing and exciting a raw material comprising a hydrocarbon gas such as methane with radiofrequency discharge (R. Mania et al, Crystal Research and Technology, 16 , 785 (1981)). For example, when methane and hydrogen gas are supplied as raw materials under such condition that the methane concentration is 1 % or less, a good quality single diamond film containing no graphite component can be grown on a silicon substrate. However, in case where the single diamond film is epitaxially grown on a single crystal diamond substrate, when the thickness of the single diamond film exceeds 1 µm, the film is cracked or abnormally grows (Kagakugijutsu-cho, Muki-zairyo Kenkyusho Kenkyuu Houkokusho, Vol. 39, 39 (1984)). Patent Abstracts of Japan, vol. 13, no. 61 (C-567) [3409], 1989 (corresponding to JP-A-63 252 997) discloses a method of growing epitaxial single crystal diamond films onto silicon carbide single crystal substrates by plasma CVD of a gas mixture of hydrogen and methane, wherein the concentration of methane is 0.5 %. EP-A-0 288 065 describes a method for synthesizing a diamond onto a substrate by contacting the substrate with a gas obtained by excitation of carbon monoxide or carbon dioxide and hydrogen at a ratio of CO being at least 1 mol% per total CO and hydrogen or at a ratio of CO₂ being 0.1-20 mol% per hydrogen respectively. According to EP-A-0 348 026 the growth of an epitaxial crystalline diamond onto diamond or C-BN substrates is disclosed wherein a gas mixture of hydrogen and less than 5 vol% of a carbon compound using microwave energy is decomposed. SUMMARY OF THE INVENTION An object of the present invention is to provide a method for producing a single crystal diamond film having a smooth surface, good crystallinity, and improved electrical characteristics. Accordingly, the present invention provides a method for producing a single crystal diamond film, which comprises steps of decomposing a raw material gas comprising a hydrogen gas and a carbon-containing compound and epitaxially growing a single crystal diamond film on a single crystal substrate in a vapor phase, wherein said carbon containing compound is a hydrocarbon, the single crystal film is deposited on a (100) plane of a single crystal substrate selected from a single crystal diamond, Ni, Cu, Ni/Cu alloy, MgO, β-SiC, α-SiC, AlN, PbTiO₃, Ba₂TiO₄, GaN, ZnO, ZnS, ZrO₂, Tl₂Ba₂Ca₂Cu₃O₁₀ and c-BN, and a molar ratio of the carbon atom in the hydrocarbon to the hydrogen is from 5:100 to 7:100. BRIEF DESCRIPTION OF THE DRAWING Figure schematically shows a preferred embodiment of an apparatus for carrying out the method of the present invention. DETAILED DESCRIPTION OF THE DRAWING The raw material to be used according to the present invention comprises a hydrogen gas, a carbon-containing compound and optionally a dopant. The raw material is supplied to a reaction chamber in a vapor phase. The raw material is preferably diluted with an inert gas. Specific examples of the carbon-containing compound are paraffinic hydrocarbons (e.g. methane, ethane, propane, butane, etc.), olefinic hydrocarbons (e.g. ethylene, propylene, butylene, etc.), acetylene type hydrocarbons (e.g. acetylene, allylene, etc.), dienes (e.g. butadiene, etc.), alicyclic hydrocarbons (e.g. cyclopropane, cyclobutane, cyclopentane, cyclohexane, etc.), aromatic hydrocarbons (e.g. cyclobutadiene, benzene, toluene, xylene, naphthalene, etc.), and mixtures thereof. The inert gas includes argon, helium, neon, krypton, xenon and radon. As the dopant, elemental boron, lithium, nitrogen, phosphorus, sulfur, chlorine, arsenic or selenium and their compounds can be used. In the present invention, the molar ratio of the carbon-containing compound to the hydrogen gas is larger than in the conventional method. The molar ratio of the carbon atoms in the carbon containing compound to the hydrogen is from 5:100 to 7:100. When this ratio is less than 5:100, a polycrystal film tends to grow. When this ratio is larger than 7:100, the graphite component tends to deposit. As the substrate a single crystal substrate selected from single crystal diamond, Ni (a: 352.4 pm (3.524 Å)), Cu (a: 361.47 pm (3.6147 Å)), Ni/Cu alloy, MgO (a: 421.3 pm (4.213 Å)), β-SiC (a: 436 pm (4.36 Å), α-SiC (a: 308.07 pm (3.0807 Å)), ARN (a: 310.4 pm (3.104 Å)), PbTiO₃ (a: 390 pm (3.90 Å)), Ba₂TiO₄ (a: 399.4 pm (3.994 Å)), GaN (a: 318 pm (3.180 Å)), ZnO (324.07 pm (3.2407 Å)), ZnS (a: 382 pm (3.820 Å)), ZrO₂ (a: 364 pm (3.64 Å), Tl₂Ba₂Ca₂Cu₃O₁₀ (a: 385 pm (3.85 Å)) and C-BN (361,5 pm (3.615 Å)) is used (a is the lattice constant of the substrate). All substrates according to the present invention satisfy the relation |(a - a₀)/a| x 100 ≦ 20 (a o (= 356.7 pm (3.567 Å)) is the lattice constant of the single crystal diamond). When a substrate having a lattice constant which greatly differs from that of the diamond such as a silicon substrate, a polycrystal diamond grows on the substrate. Since the polycrystal diamond has many grain boundaries, deposition of graphite cannot be avoided when the carbon-containing compound is under supersaturated condition in the reaction vessel. Because of this, in the conventional method, the amount of the carbon-containing compound is reduced and the diamond is grown while etching graphite with a large amount of the hydrogen gas. When the single crystal diamond or the single crystal material having the lattice constant close to that of the single crystal diamond is used as the substrate, nucleation of the diamond is easily achieved, so that the high quality single crystal diamond film can be epitaxially grown with supplying a large amount of the carbon-containing compound in a state in which a degree of supersaturation of the carbon-containing compound in the vapor phase is large and which is close to the thermodynamic equilibrium. The vapor deposition of diamond can be carried out by a conventional method such as microwave plasma CVD (chemical vapor deposition), thermal filament CVD, RF plasma CVD, DC plasma CVD, plasma jet, chemical transportation, and the like. Among them, the plasma CVD using alternating current of 1 KHz or larger, particularly microwave plasma CVD is preferred, since the deposited diamond is less contaminated with impurities from a reaction chamber. The pressure in the reaction chamber is usually from 13.33 mPa to 133.3 kPa (10⁻⁴ to 10³) Torr. The temperature of the substrate depends on the kind of substrate. Preferably, it is from 700 to 1000°C. The method of the present invention will be illustrated by making reference to the accompanying drawing. Figure schematically shows a preferred embodiment of an apparatus for carrying out the method of the present invention. Hydrogen gas 1, methane gas 2 as the carbon-containing compound and diborane gas 3 are metered with respective mass flow meters 5, mixed and then supplied in a reaction chamber 6. An inert gas 4 is metered with a mass flow meter 5 and then supplied in the reaction chamber 6. The gas mixture is decomposed in the chamber 6 with microwave 7. Carbon atoms generated through decomposition of methane is heated to several hundred degree (°C) and reaches a single crystal substrate 8 to form a diamond layer and a graphite layer. The graphite layer is removed through reaction of carbon with hydrogen atoms generated by the decomposition of hydrogen. The reaction chamber 6 is evacuated with a vacuum pump 9. PREFERRED EMBODIMENTS OF THE INVENTION The present invention will be explained further in detail by following examples. Examples 1-9 Comparative Examples 1 and 2 A single crystal diamond film was grown by using the apparatus of Fig. 1. As a substrate, a diamond single crystal of 2 mm in length, 1.5 mm in width and 0.3 mm in thickness was used with the (100) plane forming a deposition surface. As the raw material, methane, hydrogen and diborane (in Example 5 only) or argon (in Example 6 only) were supplied at flow rates of Table 1 for 2 hours under pressure of 5.33 kPa (40 Torr) at a substrate temperature of 830°C (in Examples 1-5 and 7-9 and Comparative Examples 1 and 2) or 850°C (in Example 6). The microwave having a frequency of 2.45 GHz and power of 300 W was used. The deposited diamond film had a thickness of about 1 µm. The diamond film was analyzed with electron-ray diffraction, a scanning type electron microscope and Raman spectroscopic analysis. The results are shown in Table 1. When the percentage of the mole number (B) of the carbon atoms in the carbon-containing compound per mole number (A) of the hydrogen is from 2 to 10 %, the single crystal diamond was confirmed with the electron-ray diffraction. When B/A is from 2 to 7 %, in the Raman spectroscopic analysis of the diamond films, no broad peak appeared around 1500cm⁻¹ which indicates presence of amorphous diamond. This means that the diamond film was single crystal. When B/A is from 5 to 7 %, the single crystal diamond films had completely smooth surfaces. When the diamond was grown on the silicon substrate having the lattice constant of 543 pm (5.43 Å) under the same conditions, the amorphous carbon was partly contained in the diamond film. The surface morphology of the deposited film largely depends on the plane orientation, and the smoothness of the deposited film was in the following order: (100) > (111) > (110). As understood from the results of Example 5, when B₂H₆ was used as a dopant, the deposited diamond film had a property of a p-type semiconductor, which had a hole mobility of 1000 cm²/V.sec. Such high hole mobility cannot be achieved by the diamond film deposited under the conventional condition, namely B/A being less than 2 %. The results of Example 6 indicate that the inert gas stabilizes the plasma. Examples 10-17 and Comparative Examples 3 and 4 In the same manner as in Example 1 but supplying the hydrogen gas and methane at flow rates of 100 sccm and 6 sccm, respectively and carrying out the deposition reaction at a substrate temperature of 830°C under pressure of 5.33 kPa (40 Torr). for 2 hours with a microwave power of 300 W, a single crystal diamond film was deposited on the (100) plane of the diamond substrate. The thickness of the deposited diamond film was 2 µm. Kinds of the substrate used in these Examples are shown in Table 2. The results are also shown in Table 2. When the lattice constant of the substrate satisfies the above equation (I), the good quality single crystal diamond film can be deposited.

Claims: A method for producing a single crystal diamond film which comprises steps of decomposing a raw material gas comprising a hydrogen gas and a carbon containing compound and epitaxially growing a single crystal diamond film on a single crystal substrate in a vapor phase, wherein said carbon containing compound is a hydrocarbon, the single crystal film is deposited on a (100) plane of a single crystal substrate selected from a single crystal diamond, Ni, Cu, Ni/Cu alloy, MgO, β-SiC, α-SiC, AlN, PbTiO₃, Ba₂TiO₄, GaN, ZnO, ZnS, ZrO₂, Tl₂Ba₂Ca₂Cu₃O₁₀ and C-BN, and a molar ratio of the carbon atom in the hydrocarbon to the hydrogen is from 5:100 to 7:100. The method according to claim 1, wherein said hydrocarbon is methane.

Inventors: FUJIMORI NAOJI C O ITAMI WORKS, SHIOMI HIROMU C O ITAMI WORKS, FUJIMORI, NAOJI, SHIOMI, HIROMU, FUJIMORI, NAOJI, C/O ITAMI WORKS OF, SHIOMI, HIROMU, C/O ITAMI WORKS OF SUMITOMO
Applicants: SUMITOMO ELECTRIC INDUSTRIES, SUMITOMO ELECTRIC INDUSTRIES, LTD.
Application year: 1990
Main classification: C30B25
IPC codes: C30B29, C30B25