Thin film single crystal substrate

Abstract: A thin film single crystal substrate useful in the production of a semiconductor, comprising a base substrate made of a single crystal diamond having the (100) or (110) plane orientation and at least one thin film of silicon.

Description: BACKGROUND OF THE INVENTION Field of the Invention The present invention relates to a thin film single crystal substrate which is useful in the production of a semiconductor. More particularly, it relates to a thin film single crystal substrate comprising a single crystal diamond base and at least one film of a single crystal material formed thereon. Description of the Prior Art Some of materials having a crystalline structure is useful as materials of electric elements. For example, silicon carbide has been studied as a material for a heat and environment resistant element or a light-emitting element since it has several crystal structures and band gap of 2.2 to 3.3 eV depending on the crystal structure, its heat conductivity is as large as 4.9 W/cm.K, its maximum electron mobility is confirmed to be 1,000 cm 2 /V.sec., and it is possible to control p-type and n-type valence electrons. The crystal structures of silicon carbide are roughly classified into an α-type and a β-type. Since silicon carbide of the α-type hexagonal system (6H) has a large band gap of about 2.9 eV, it is believed that it can be used as a material for a blue light-emitting element. Silicon carbide of the β-type cubic system (3C) has a large electron mobility, it is investigated as a material for an environment resistant element. Since a silicon carbide single crystal has a high melting point and is chemically stable, it is difficult to produce a large single crystal with good quality which can be used as a semiconductive material. Therefore, the single crystal of silicon carbide is formed by growing it on a silicon single crystal or sapphire in a gas or liquid phase (cf. Japanese Patent Kokai Publication Nos. 83588/1978 and 146299/1978). However, the silicon single crystal has various drawbacks. For example, since it has a band gap of 1.1 eV which is smaller than that of the silicon carbide single crystal, it has smaller resistivity at high temperature. It has a low melting point and a small thermal conductivity of 1.5 W/cm.K). Because of these characteristics of silicon carbide, when the silicon carbide single crystal is grown on the silicon single crystal, the advantages of the latter as the heat resistant semiconductive material are greatly impaired. Therefore, the silicon carbide single crystal grown on the silicon single crystal is used by removing the latter with an acid, but the remaining thin film of the silicon carbide has a thickness of several ten µm and is easily cracked or strained, which deteriorates its handling properties. When the silicon carbide single crystal is used without removing the silicon single crystal, elements should be separated by applying reverse bias at a p-n junction formed in the silicon carbide layer. This makes the production method complicate. Although sapphire is an insulating material up to high temperature and thermally stable, it has some drawbacks such as a small coefficient of thermal conductivity of 0.11 W/cm.K and a large coefficient of thermal expansion of 7.8 x 10 -6 /K. In addition to the above proposals, it may be contemplated to grow a thin film single crystal of GaAs on the silicon single crystal or to grow the silicon single crystal on sapphire. However, these techniques have the same problems as above. JP-A-61-236687 relates to machine parts made of diamond having a high adhesion to a coating film and a high abrasion resistance being suitable for video disc players. The electroconductive coating film layer is distinguished by a boundary layer between a diamond base body and an outermost layer being constituted of a superhard semiconductive material such as B, BN, B 4 C, Si, SiC etc. and an intermediate layer made of an electroconductive metal such as Ti, Ta, Nb, Hf, W, Zr, Cr, Ni etc. The intermediate layer may also be constituted of alternately formed electroconductive metal layers and superhard semiconductive material layers. A paper by A.G. Cullis and G. R. Booker published in "Thin Solid Films", 31, (1976), 53-67 is concerned with an electron microscope study of epitaxial silicon films on sapphire and diamond substrates. For Si deposited on diamond the thicker films showed evidence for partial retention of the original (111) substrate orientation. However, the films were highly defective, containing microtwins and regions of Si exhibiting general crystallographic rotations. JP-A-59-213126 relates to a diamond semiconductor element. The substrate material is a P or As-added N type diamond single crystal having face 100 or face 111 on the upper surface. A thin film of the same type of diamond is formed thereon. SUMMARY OF THE INVENTION One object of the present invention is to provide a substrate having a large coefficient of thermal conductivity. Another object of the present invention is to provide a substrate having a small coefficient of thermal expansion. Further object of the present invention is to provide a substrate having good resistance against heat and/or environment. These and other object of the present invention are achieved by a thin film single crystal substrate which comprises a base substrate made of single crystal diamond having the (100) plane orientation and at least one thin film of a single crystal of silicon. BRIEF DESCRIPTION OF THE DRAWINGS Figs. 1 and 2 are cross sections of typical structures of the thin film single crystal substrate according to the present invention, Fig. 3 is a photograph of reflection electron beam diffraction pattern of the thin film single crystal formed in Example 1, Fig. 4 is a schematic cross section of an apparatus for forming a thin film single crystal used in reference example 1, Fig. 5 is a reflection electron beam diffraction pattern of the thin film silicon carbide single crystal formed in reference example 1. DETAILED DESCRIPTION OF THE INVENTION The diamond single crystal has a larger coefficient of thermal conductivity and a smaller relative dielectric constant than other materials as shown in Table 1. When the diamond is used as a base material for a semi-conductive substrate, it is possible to produce an element which resists a large amount of heat such as a high power high speed element. Since the diamond has a wide band gap, a highly pure single crystal of diamond is a good insulator up to about 1,000°C and thermally and chemically stable up to about 1,000°C, it is possible to make use of its advantages when it is used in the heat and environment resistant element in combination with silicon carbide. Band gap (eV) Coefficient of thermal conductivity (W/cm.K) Relative dielectric constant Diamond 5.5 20 5.7 Silicon carbide 2.2-3.3 4.9 10 Silicon 1.1 1.5 12 Sapphire --- 0.11 10.5 According to the present invention, the base substrate may be made of natural or artificial diamond. Nowadays, an artificial single crystal diamond of several millimeter square having less impurities (particularly, nitrogen) is produced by the extra-high pressure method. According to the extra-high pressure method, it is possible to produce a p-type semiconductive single crystal diamond, namely the IIb type, which is suitable for the production of a high power element. When the high power element is produced, an electrode is provided on the single crystal diamond. By a thin film method or an ion-implantation method, either of p-type and n-type diamond can be produced. Preferably, the single crystal diamond is a p-type semiconductive diamond which contains at least one of boron and aluminum as an impurity. The plane of the single crystal diamond of the base on which the single crystal thin film of silicon and/or the intermediate layer is grown has the orientation (100). The thickness of the diamond base is usually from 0.05 mm to 5 mm, preferably from 0.1 mm to 0.5 mm. Apart from at least one thin film of a single crystal of silicon the substrate of the invention may further comprise at least one single crystal of a material selected from silicon carbide, silicon, boron nitride, gallium nitride, indium nitride, aluminum nitride, boron phosphide, cadmium selenide, germanium, gallium arsenide, gallium phosphide, indium phosphide, gallium antimonide, indium arsenide, indium antimonide, aluminum phosphide, aluminum arsenide, aluminum antimonide, cadmium telluride, mercury sulfide, zinc oxide, zinc sulfide, zinc selenide and zinc telluride. The single crystal may be a mixed crystal. Further, two or more thin films of the single crystal may be formed. The thickness of the thin film single crystal is usually from 10 to 10000 nm, preferably from 50 to 2000 nm. Since silicon carbide has similar bond status to that of diamond and a close lattice constant to that of diamond, a further single crystal thin layer is preferably made of cubic system or hexagonal system silicon carbide single crystal. Two or more of such thin layers can be laminated. In addition, the single crystal thin layer preferably made of p-type silicon carbide single crystal which contains at least one impurity element selected from the group consisting of boron, aluminum, gallium and indium or n-type silicon carbide single crystal which contains at least one impurity element selected from the group consisting of nitrogen, phosphorus, arsenic, antimony and lithium. Two or more of the p-type single crystal thin films and/or the n-type single crystal thin films may be laminated. The thin film single crystal silicon may be formed on the diamond base by any of conventional methods such as gas phase methods (e.g. the sublimation method, the thermal CVD method, the plasma CVD method, the reaction deposition method and the MBE method) and liquid phase methods. When the lattice constant of the single crystal to be grown is very different from that of the single crystal diamond, the thin film single crystal substrate of the present invention may comprise an intermediate layer between the diamond base and the thin film of single crystal. For example, the single crystal diamond has a lattice constant of 3.5667 Å, while the silicon single crystal has a lattice constant of 5.4301 Å. In such case, flexibility of the conditions in growing the single crystal on the diamond single crystal is very limited, and it is difficult to grow a single crystal having good quality. The intermediate layer may be made of single crystal silicon carbide or single crystal diamond. The single crystal silicon carbide can be formed by the CVD (chemical vapor phase deposition) method, the modified CVD (MCVD) method, the MBE (molecular beam epitaxy) method. The single crystal diamond can be formed by the thermal CVD method, the plasma CVD method, the electron beam CVD method, the ion sputtering method, the optical CVD method and the like. As described above, the thin film single crystal substrate consists of the single crystal diamond base 1 and the single crystal thin layer 2 as shown in Fig. 1 or consists of the single crystal diamond base 1, the intermediate layer 3 and the 'single crystal thin film 2 as shown in Fig. 2. PREFERRED EMBODIMENT OF THE INVENTION The present invention will be illustrated in the following examples. Example 1 On the (100) plane of a base made of IIa type single crystal diamond (2 x 2 x 0.3 mm) produced by the extra-high pressure method, a thin film of silicon having a thickness of 500 nm was formed by plasma CVD from a mixture of monosilane (SiH 4 ) and hydrogen (molar ratio of 1:50) at 1,050°C under reduced pressure of 0.67 kPa (5 Torr). After heating the produced substrate at 900°C under reduced pressure of 1.3 · 10 -6 Pa (1 x 10 -8 Torr), the substrate was observed by reflection electron beam diffraction to give a photograph of Fig. 3. The diffraction pattern had the Kikuchi Line, which indicated that the silicon thin film consisted of single crystal with good quality. Example 2 On the same plane of the same single crystal diamond as used in Example 1, as an intermediate layer, a silicon carbide thin film having a thickness of 500 µm was formed by CVD from a mixture of monosilane, propane and hydrogen (a molar ratio of 1:4:95) at 1,350°C under reduced pressure of 53.3 kPa (400 Torr) for 20 minutes. On the silicon carbide intermediate layer, an n-type phosphorus-doped silicon film was grown by CVD from a mixture of monosilane, PH 3 and hydrogen (a molar ratio of 10 5 :1:10 7 ) at 1,050°C under reduced pressure of 13.3 kPa (100 Torr). for 30 minutes. The reflection electron beam diffraction pattern of the formed silicon film had the Kikuchi Line, which indicated that the silicon thin film consisted of single crystal. According to the measurement of the Hall effect, the n-type silicon film had a carrier density of 3 x 10 17 /cm 3 and electron mobility of 900 cm 2 /V.sec. Reference example 1 A thin film single crystal was formed by using an apparatus shown in Fig. 4, which comprises a double-walled quartz tube 4, a supporting plate 6 made of graphite covered with silicon carbide, a 400 KHz high-frequency oscillator 7, an inlet 8 and an outlet 9 for cooling water, a (water cooled) work coil 10, and an inlet 11 and an outlet 2 for gasses. A piece 5 of single crystal of IIb type natural diamond (2 x 2 x 0.5 mm) was placed on the supporting plate 6. After evacuating the interior of the quartz tube.4, an AC current of 400 KHz and 2 KW was applied to the work coil 10 with supplying hydrogen gas in the tube to keep the pressure at 10.7 kPa (80 Torr). While keeping the supporting plate 6 at 1,340°C by induction heating from the coil 10, nitrogen, monosilane, propane and hydrogen were supplied at a flow ratio of 1:50:100:2,000 into the tube for 20 minutes under reduced pressure of 10.7 kPa (80 Torr) to grow n-type silicon carbide single crystal. Then, diboran, monosilane, propane and hydrogen were supplied at the flow ratio of 0.2:50:100:2,000 for 30 minutes under the same conditions to grow p-type silicon carbide single crystal. The reflective electron beam diffraction analysis of the surface of grown silicon carbide layer gave a diffraction image as shown in Fig. 5. From this pattern, the lattice constant calculated to be 4.35 Å, which indicated that the crystal was the cubic system (3C) silicon carbide. Reference example 2 A piece of Ib type single crystal artificial diamond was placed in a graphite crucible together with hexagonal system silicon carbide powder and heated to 1,750°C under reduced pressure of 66.7 Pa (0.5 Torr) while supplying argon gas to grow silicon carbide crystal on the surface of the diamond piece for 15 minutes by sublimation of silicon carbide. The reflection electron beam diffraction analysis of the surface of grown silicon carbide layer revealed that the crystal was the hexagonal system (6H) silicon carbide. The reference examples 1 and 2 do not form part of the invention.

Claims: A thin film single crystal substrate which comprises a base substrate made of single crystal diamond having the (100) plane orientation and at least one thin film of a single crystal of silicon. A thin film single crystal substrate according to claim 1, wherein the diamond single crystal base is made of a p-type diamond containing at least one impurity element selected from the group consisting of boron and aluminum.

Inventors: FUJIMORI NAOJI, IMAI TAKAHIRO, NAKAHATA HIDEAKI, FUJIMORI, NAOJI, IMAI, TAKAHIRO, NAKAHATA, HIDEAKI, FUJIMORI, NAOJI, C/O ITAMI WORKS OF, IMAI, TAKAHIRO, C/O ITAMI WORKS OF, NAKAHATA, HIDEAKI, C/O ITAMI WORKS OF
Applicants: SUMITOMO ELECTRIC INDUSTRIES, SUMITOMO ELECTRIC INDUSTRIES, LTD.
Application year: 1988
Main classification: H01L21
IPC codes: H01L21