Patent Publication Number: US-2005115642-A1

Title: Semiconductor substrate and method for fabricating the same

Description:
CROSS-REFERENCE TO RELATED APPLICATION  
      This application is based upon and claims priority of Japanese Patent Application No. 2002-192133, filed on Jul. 1, 2002, the contents being incorporated herein by reference.  
     BACKGROUND OF THE INVENTION  
      The present invention relates to a semiconductor substrate and a method for fabricating the semiconductor substrate, more specifically a semiconductor substrate which can improve heat radiation and a method for fabricating the semiconductor substrate.  
      Improvements for higher speed have been continuously made on MOSFETs (Metal Oxide Semiconductor-Field Effect Transistors), etc. by micronizing elements, typically reducing the gate length.  
      A propagation delay time τ of a signal in a MOSFET is expressed by the following formula. 
 
τ=C load   ·V   dd   /[{W·μ·ε)/(   L·T   OX )}×( V   dd   −V   t ) 2 ]  (1) 
 
 wherein C load  represents a load capacitance; V dd  represents a source voltage; W represents a gate width of the MOSFET; L represents a gate length of the MOSFET; μ represents a carrier mobility; ε represents a dielectric constant of the gate insulation film; T OX  represents a thickness of the gate insulation film; and V t  represents a threshold voltage. 
 
      Based on the above-described formula, it can be seen that a transistor is micronized, specifically a gate length L is shortened, whereby higher speed can be realized. However, in order to fabricate an ultra micronized transistor having a below 70 nm-gate length, an optical aligner having a light source of a 157 nm-wavelength F 2  excimer laser is necessary. One optical aligner using an F 2  excimer laser light source is as expensive as 2-3 billions. A plurality of such expensive aligners are necessary to constitute a fabrication line, which requires vast investment.  
      Based on the above-described formula, it can be seen that higher speed of the MOSFET can be realized by increasing a source voltage V dd . However, the electric power consumption of the MOSFET increases in proportion of a square of a source voltage (Reference 1: T. Tsuchiya, Oyo Butsuri 66, 1191 (1997)). In consideration of high integration, it is not preferable to increase a source voltage.  
      Based on the above-described formula, it can be seen that higher speed of the MOSFET can be realized also by decreasing a film thickness T OX  of the gate insulation film. However, the thermal oxide film of a 1.5 nm-thickness has been already developed, and it is very difficult to further thin the gate insulation film.  
      Based on the above-described formula, it can be seen that higher speed of the MOSFET can be realized also by increasing a dielectric constant ε of the gate insulation film. However, the gate insulation film of high dielectric constant ε has a number of problems for the practical use and takes much time to be practically used.  
      Based on the above-described formula, it can be seen that higher speed of the MOSFET can be realized also by decreasing a load capacitance C load . It is not easy to further improve values of the above-described other parameters, and techniques of decreasing a value of the load capacitance C load  are noted.  
      As a technique of decreasing a load capacitance of the MOSFET, SOI (Silicon On Insulator) substrates are proposed. The SOI substrates have the structure that a silicon crystal layer for semiconductor elements to be fabricated on is spaced from a silicon crystal substrate by a buried oxide film. In a case that the MOSFET is fabricated, using an SOI substrate, a junction capacitance between the source and the drain is decreased by about {fraction (1/10)} in comparison with a case that the MOSFET is fabricated using a usual CZ wafer (see Reference 1), and the wiring capacitance is decreased by several dozen % (see Reference 2: Y. Yamaguchi, etal., IEEE Trans. Electron Devices 40, 179 (1993)). The parasitic capacitance C load  of the MOSFET can be decreased by using an SOI substrate. Accordingly, the operational speed of the MOSFET can be increased by using the SOI substrate.  
      It is reported that a band structure of a silicon crystal layer is changed when a silicon crystal layer is crystal strained, and a mobility of electrons and holes in the silicon crystal layer is increased (see Reference 3: G. Abstreiter, et. al., Phys. Rev. Lett. 54, 2441 (1985) and Reference 4: D. K. Nayak, et al., Appl. Phys. Lett. 62, 2853 (1993)).  
      Recently, a semiconductor substrate of the strained Si/SiGe structure is proposed. The semiconductor substrate of the strained Si/SiGe structure comprises on a silicon crystal substrate a silicon germanium crystal layer of, e.g., a 10-30% Ge concentration and a several dozen—several hundred nm-thickness silicon germanium crystal layer, and a silicon crystal layer formed on the silicon germanium crystal layer (Reference 5: K. K. Linder, et al., Appl. Phys. Lett. 70, 3224 (1997)). Because Si and Ge, which have properties of solid solution of all composition ratio, a silicon genermanium crystal layer is an alloy even in any ratio of Si to Ge. Because the covalent radius of a Ge atom is larger by some percentages than that of an Si atom, the interstitial mean distance of a silicon germanium crystal layer is larger than that of a silicon crystal layer. Accordingly, when the silicon crystal layer is formed on the silicon germanium crystal layer, crystal strains are caused in the silicon crystal layer.  
      A technique that a Ge concentration in the silicon germanium crystal layer is decreased gradually to the side of the silicon crystal substrate to thereby decrease a dislocation density of the silicon germanium crystal layer is also proposed (Reference 6: E. A. Fitzgerald, et al., Appl. Phys. Lett. 59, 811 (1991)).  
      Thus, a semiconductor substrate of the strained Si/SiGe structure is used to thereby increase a carrier mobility of the silicon crystal layer, whereby the MOSFET can have higher operation speed.  
      Recently, semiconductor substrates of the strained Si/SiGeOI structure are proposed (Reference  7 : A. R. Powell, et al., Appl. Phys. Lett. 64, 1856 (1994) and Reference 8: Y. Ishikawa, et al., Appl. Phys. Lett. 75, 983 (1999)). The semiconductor substrate of the strained Si/SiGeOI structure comprises a layer of a silicon germanium crystal layer and a silicon crystal layer spaced from a silicon crystal substrate by a buried oxide film.  
      A semiconductor substrate of the strained Si/SiGeOI structure is fabricated as exemplified below.  
      That is, first, a silicon germanium crystal layer of, e.g., a 1 μm-thickness Si 0.9 Ge 0.1  is epitaxially grown on a silicon crystal substrate. At this time, a Ge concentration in the 850 nm-region of the silicon germanium layer, which is nearer to the lower layer is set to lower gradually to the side of the silicon crystal substrate.  
      Next, oxygen ions are implanted at a 170 keV acceleration energy and a 3×10 17  cm −2  dose. Then, thermal processing is performed at an above −1300° C. temperature and for 6 hours. Then, the buried oxide film of a 110 nm-thickness is formed in the silicon germanium crystal layer.  
      Then, the oxide film formed on the silicon germanium crystal layer is removed by using a hydrofluoric acid solution while the surface of the silicon germanium crystal layer is terminated with hydrogen.  
      Next, a silicon germanium crystal layer of, e.g., 150 nm-thickness Si 0.9 Ge 0.1 , and a silicon crystal layer of, e.g., a 15 nm-thickness are sequentially epitaxially grown. A thickness of the silicon germanium crystal layer on the buried oxide film and a thickness of the silicon crystal layer is totally about, e.g., 600 nm.  
      Thus, the semiconductor substrate of the strained Si/SiGeOI is fabricated.  
      The semiconductor substrate of the thus-fabricated semiconductor structure has, in comparison with the usual SOI substrate, the electron mobility increased about 60% (see Reference 9: T. Mizuno, etal., IEEE Electron Device Lett. EDL-21, 230 (2000) and the hole mobility by about 18% (see Reference 10: T. Mizuno et al., Tech. Dig. Int. Electron, Devices Meet., Washington, 1999, p. 934). Accordingly, the use of the semiconductor substrate of the strained Si/SiGeOI structure can further increase the operation speed of MOS transistors.  
      Here, it is very important in micronized integrated circuits that Joule&#39;s heat generated during their operation is effectively scattered. Then, this will be explained by means of a microprocessor, which is a typical high-end ultra fast device.  
      Millions—ten millions MOSFETs are formed in the core part of the microporcessor. When the MOSFETs are operated, the drain current causes Joule&#39;s heat. The core part is concentrated in only 5-10% of regionofa semiconductor chip, anda temperature of the core part of a most advanced microprocessor rises to above 100° C. (see Reference 11: S. J. Burden, SEMICONDUCTOR FABTECH, March, 2001, 13th Edition, p. 297). Thus, it is very important to scatter Joule&#39;s heat generated in the core part.  
      As integrated circuits are more micronized, size deviations of the source/drains, dopant concentration deviations, contact resistance deviations, etc. are conspicuous due to deviations of processing for etching, ion implantation, etc. Topography of the surfaces of semiconductor substrates are a factor for deviations in the miconization. Such deviations cause parts where Joule&#39;s heat is much generated, in locations called hot spots (see Reference 11). The temperature rise is a factor for low reliability of the integrated circuits. It is very important to effectively scatter Joule&#39;s heat generated in the hot spots.  
      It is very important to effectively scatter Joule&#39;s heat generated in the core part or the hot spot, etc., in order to improve the operation speed of a integrated circuit without causing a decline of reliability.  
      However, the above-described SOI substrate, strained Si/SiGe semiconductor substrate and strained Si/SiGeOI semiconductor substrate lower the heat radiation in comparison with the usual CZ wafer and epitaxial wafer as will be described below.  
      Generally, the thermal conductivity of Si crystals is about 150 W/mK at a 300 K temperature (Reference 12: Y. S. Touloukian, et al., Thermophysical Properties of Matter 2, Thermal Conductivity, Nonmetallic Solids, Plenum (1970)). The thermal conductivity of Ge crystals is about 60-77 W at a 300 K temperature (see Reference 13: M. A. Palmer, et al., Phy. Rev. B 56, 9431 (1997)). The thermal conductivity of silicon oxide film is about 1.38 at a 300 K temperature (see Reference 13). However, because the thermal conductivity of silicon oxide film is unknown, the thermal conductivity of quartz glass is shown here. The thermal conductivity of quartz glass is the generally used approximation.  
      Based on the above, the thermal conductivity of silicon oxide film is about {fraction (1/100)} of that of the silicon crystals. Accordingly, in the SOI substrate, a heat radiation amount of Joule s heat generated in the silicon crystal layer and radiated in the direction of depth of the SOI substrate will be about {fraction (1/100)} in comparison with heat radiation amounts of the CZ wafer, etc.  
      Based on the above, the thermal conductivity of Ge crystals is about ½ of that of Si crystals, based on which the thermal conductivity of SiGe crystals will be lower than that of Si crystals. Accordingly, in the strained Si/SiGe structure semiconductor substrate, a heat radiation amount of Joule&#39;s heat generated in the silicon crystal layer and radiated in the direction of depth of the strained Si/SiGe structure will be smaller in comparison with heat radiation amount of the CZ wafer, etc.  
      In the strained Si/SiGeOI structure semiconductor substrate, three layers, a silicon germanium crystal layer, a buried oxide film and a silicon germanium crystal layer are formed below a silicon crystal layer. It is evident that in the strained Si/SiGeOI structure semiconductor substrate, a heat radiation amount of Joule&#39;s heat generated in the silicon crystal layer and radiated in the direction of depth of the strained Si/SiGeOI structure semiconductor substrate is further smaller than that of the SOI substrate described above.  
      As described above, the heat radiation of the SOI substrate, the strained Si/SiGe structure semiconductor substrate and the strained Si/SiGeOI structure semiconductor substrate is lower in comparison with the usual CZ wafer, etc. Techniques which can improve the heat radiation of the SOI substrate, the strained Si/SiGe structure semiconductor substrate, and the strained Si/SiGeOI structure semiconductor substrate have been expected.  
     SUMMARY OF THE INVENTION  
      An object of the present invention is to provide a semiconductor substrate which can improve the heat radiation and a method for fabricating the semiconductor substrate.  
      According to one aspect of the present invention, there is provided a semiconductor substrate comprising a silicon substrate; a silicon germanium layer formed on the silicon substrate; and a silicon layer formed on the silicon germanium layer, at least one of an isotope composition ratio of one Si isotope and an isotope composition ratio of one Ge isotope of at least one of the silicon substrate, the silicon germanium layer and the silicon layer being above 95%.  
      According to another aspect of the present invention, there is provided a semiconductor substrate comprising a silicon germanium substrate; and a silicon layer formed on the silicon germanium substrate, at lest one of an isotope composition ratio of one Si isotope and an isotope composition ratio of one Ge isotope of at least one of the silicon germanium substrate and the silicon layer being above 95%.  
      According to further another aspect of the present invention, there is provided a semiconductor substrate comprising a base substrate and a silicon layer bonded to each other with an insulation film formed therebetween, an isotope composition ratio of one Si isotope of at least one of the base substrate and the silicon layer being above 95%.  
      According to further another aspect of the present invention, there is provided a semiconductor substrate comprising a base substrate; a silicon germanium layer formed on the base substrate with an insulation film formed therebetween; and a silicon layer formed on the silicon germanium layer, one of an isotope composition ratio of one Si isotope and an isotope composition ratio of one Ge isotope of at least one of the silicon germanium layer and the silicon layer being above 95%.  
      According to further another aspect of the present invention, there is provided a method for fabricating a semiconductor substrate comprising the steps of: forming a silicon germanium layer on a silicon substrate, and forming a silicon layer on the silicon germanium layer, the silicon germanium layer or the silicon layer being formed by using a raw material gas in which at least one of an isotope composition ratio of one Si isotope and an isotope composition ratio of one Ge isotope is above 95% in at least one of the step of forming a silicon germanium layer and the step of forming a silicon layer.  
      According to further another aspect of the present invention, there is provided a method for fabricating a semiconductor substrate comprising the step of forming a silicon layer on a silicon germanium substrate, the silicon layer being formed by using a raw material gas having an above 95% isotope composition ratio of one Si isotope in the step of forming the silicon layer.  
      According to further another aspect of the present invention, there is provided a method for fabricating a semiconductor substrate comprising the step of: forming an insulation film on one surface of a silicon substrate having an above 95% isotope composition ratio of one Si isotope; bonding the insulation film to abase substrate; and thinning the silicon substrate at the other surface of the silicon substrate.  
      According to further another aspect of the present invention, there is provided a method for fabricating a semiconductor substrate comprising the steps of: forming a silicon layer on one surface of a silicon substrate by using a raw material gas having an 95% isotope composition ratio of one Si isotope; forming an insulation film on the silicon layer; bonding a base substrate to the insulation film; and thinning the silicon substrate at the other surface of the silicon substrate.  
      According to further another aspect of the present invention, there is provided a method for fabricating a semiconductor substrate comprising the steps of: bonding a silicon substrate to a base substrate with an insulation film formed therebetween, thinning the silicon substrate on the side of the silicon substrate; forming a silicon germanium layer on said thinned silicon substrate; and the step of forming a silicon layer on the silicon germanium layer, the silicon germanium layer or the silicon layer being formed by using a raw material gas in which at least one of an isotope composition ratio of one Si isotope and an isotope composition ratio of one Ge isotope is above 95% in at least one of the step of forming a silicon germanium layer and the step of forming a silicon layer.  
      According to further another aspect of the present invention, there is provided a method for fabricating a semiconductor substrate comprising the steps of: burying an insulation film in a silicon substrate; forming a silicon germanium layer on the silicon substrate with the insulation film buried in; and forming a silicon layer on the silicon germanium layer, the silicon germanium layer or the silicon layer being formed by using a raw material gas in which at least an isotope composition ratio of one Si isotope and an isotope composition ratio of one Ge isotope is above 95% in at least one of the step of forming a silicon germanium layer and the step of forming a silicon layer.  
      According to further another aspect of the present invention, there is provided a method for fabricating a semiconductor substrate comprising the steps of: forming a silicon germanium layer on a silicon substrate; forming a silicon layer on the silicon germanium layer; and burying an insulation film in the silicon substrate, the silicon germanium layer or the silicon layer being formed by using a raw material gas in which at least one of an isotope composition ratio of an Si isotope and an isotope composition ratio of a Ge isotope is above 95% in at lest one of the step of forming a silicon germanium layer and the step of forming a silicon layer.  
      According to further another aspect of the present invention, there is provided a method for fabricating a semiconductor substrate comprising the steps of: forming a silicon germanium layer on a silicon substrate; burying an insulation film in the silicon substrate; and forming a silicon layer on the silicon germanium layer, the silicon germanium layer or the silicon layer being formed by using a raw material gas in which at least one of an isotope composition ratio of an Si isotope and an isotope composition ratio of a Ge isotope is above 95% in at least one of the step of forming a silicon germanium layer and the step of forming a silicon layer.  
      According to the present invention, in the silicon crystal layer, the silicon germanium crystal layer, the silicon crystal substrate, etc., the isotope composition ratio of any one of the Si isotopes and the isotope composition ratio of any one of the Ge isotopes are set very high, whereby the thermal conductivities of the silicon crystal layer, the silicon germanium crystal layer, the silicon crystal substrate, etc. can be higher. Thus, according to the present invention, the heat radiation can be enhanced in the direction horizontal to the substrate plane. Accordingly, in the present invention, the heat generated from the core parts, hot spots, etc. of microprocessors can be effectively radiated. The present invention can provide a semiconductor substrate having the heat radiation improved, and accordingly can contribute to faster operations and higher reliability of high end ultra fast devices, etc. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       FIG. 1  is a sectional view of the semiconductor substrate according to a first embodiment of the present invention.  
       FIGS. 2A  to  2 C are sectional views of the semiconductor substrate according to the first embodiment in the steps of the method for fabricating the semiconductor substrate, which explain the method.  
       FIG. 3  is a sectional view of the semiconductor substrate according to a second embodiment of the present invention.  
       FIGS. 4A  to  4 C are sectional views of the semiconductor substrate according to the second embodiment of the present invention in the steps of the method for fabricating the semiconductor substrate, which explain the method.  
       FIG. 5  is a sectional view of the semiconductor substrate according to a third embodiment of the present invention.  
       FIGS. 6A  to  6 C are sectional views of the semiconductor substrate according to the third embodiment in the steps of the method for fabricating the semiconductor substrate, which explain the method.  
       FIG. 7  is a sectional view of the semiconductor substrate according to a fourth embodiment of the present invention.  
       FIGS. 8A and 8B  are sectional views of the semiconductor substrate according to the fourth embodiment in the steps of the method for fabricating the semiconductor substrate, which explain the method.  
       FIG. 9  is a sectional view of the semiconductor substrate according to a fifth embodiment of the present embodiment.  
       FIGS. 10A  to  10 C are sectional views of the semiconductor substrate according to the fifth embodiment of the present invention in the steps of the method for fabricating the semiconductor substrate, which explain the method (Part 1).  
       FIGS. 11A and 11B  are sectional views of the semiconductor substrate according to the fifth embodiment of the present invention in the steps of the method for fabricating the semiconductor substrate, which explain the method (Part 2).  
       FIGS. 12A  to  12 C are sectional views of the semiconductor substrate in the steps of method for fabricating the semiconductor substrate according to a modification of the fifth embodiment of the present invention, which explain the method (Part 1).  
       FIGS. 13A and 13B  are sectional views of the semiconductor substrate in the steps of method for fabricating the semiconductor substrate according to a modification of the fifth embodiment of the present invention, which explain the method (Part 2).  
       FIG. 14  is a sectional view of the semiconductor substrate according to a sixth embodiment of the present invention.  
       FIGS. 15A  to  15 C are sectional views of the semiconductor substrate according to the sixth embodiment in the steps of the method for fabricating the semiconductor substrate, which show the method (Part 1).  
       FIGS. 16A  to  16 C are sectional views of the semiconductor substrate according to the sixth embodiment in the steps of the method for fabricating the semiconductor substrate, which show the method (Part 2).  
       FIGS. 17A  to  17 C are sectional views of the semiconductor substrate in the steps of method for fabricating the semiconductor substrate according to Modification  1  of the sixth embodiment of the present invention, which explain the method (Part 1).  
       FIGS. 18A  to  18 C are sectional views of the semiconductor substrate in the steps of method for fabricating the semiconductor substrate according to Modification 1 of the sixth embodiment of the present invention, which explain the method (Part 2).  
       FIGS. 19A  to  19 C are sectional views of the semiconductor substrate in the steps of method for fabricating the semiconductor substrate according to Modification 2 of the sixth embodiment of the present invention, which explain the method.  
       FIGS. 20A  to  20 D are sectional views of the semiconductor substrate in the steps of method for fabricating the semiconductor substrate according to Modification 3 of the sixth embodiment of the present invention, which explain the method.  
       FIGS. 21A  to  21 C are sectional views of the semiconductor substrate in the steps of method for fabricating the semiconductor substrate according to Modification 4 of the sixth embodiment of the present invention, which explain the method (Part 1).  
       FIGS. 22A and 22B  are sectional views of the semiconductor substrate in the steps of method for fabricating the semiconductor substrate according to Modification 4 of the sixth embodiment of the present invention, which explain the method (Part 2).  
       FIGS. 23A  to  23 C are sectional views of the semiconductor substrate in the steps of method for fabricating the semiconductor substrate according to Modification 5 of the sixth embodiment of the present invention, which explain the method  
       FIG. 24  is a sectional view of the semiconductor substrate according to a seventh embodiment of the present invention.  
       FIGS. 25A  to  25 C are sectional views of the semiconductor substrate according to the seventh embodiment in the steps of the method for fabricating the semiconductor substrate, which show the method (Part 1).  
       FIGS. 26A  to  26 C are sectional views of the semiconductor substrate according to the seventh embodiment in the steps of the method for fabricating the semiconductor substrate, which show the method (Part 2).  
       FIG. 27  is a sectional view of the semiconductor substrate according to an eighth embodiment of the present invention.  
       FIGS. 28A  to  28 C are sectional views of the semiconductor substrate according to the eighth embodiment in the steps of the method for fabricating the semiconductor substrate, which show the method.  
       FIG. 29  is a sectional view of the semiconductor substrate according to a ninth embodiment of the present invention.  
       FIGS. 30A  to  30 C are sectional views of the semiconductor substrate according to the ninth embodiment in the steps of the method for fabricating the semiconductor substrate, which show the method.  
       FIG. 31  is a sectional view of the semiconductor substrate according to a tenth embodiment of the present invention.  
       FIGS. 32A  to  32 C are sectional views of the semiconductor substrate according to the tenth embodiment in the steps of the method for fabricating the semiconductor substrate, which show the method (Part 1).  
       FIGS. 33A  to  33 C are sectional views of the semiconductor substrate according to the tenth embodiment in the steps of the method for fabricating the semiconductor substrate, which show the method (Part 2).  
       FIGS. 34A  to  34 C are sectional views of the semiconductor substrate in the steps of method for fabricating the semiconductor substrate according to Modification 1 of the tenth embodiment of the present invention, which explain the method (Part 1).  
       FIGS. 35A  to  35 C are sectional views of the semiconductor substrate in the steps of method for fabricating the semiconductor substrate according to Modification 1 of the tenth embodiment of the present invention, which explain the method (Part 2).  
       FIGS. 36A  to  36 C are sectional views of the semiconductor substrate in the steps of method for fabricating the semiconductor substrate according to Modification 2 of the tenth embodiment of the present invention, which explain the method (Part 1).  
       FIGS. 37A  to  37 C are sectional views of the semiconductor substrate in the steps of method for fabricating the semiconductor substrate according to Modification 2 of the tenth embodiment of the present invention, which explain the method (Part 2).  
       FIG. 38  is a sectional view of the semiconductor substrate according to an eleventh embodiment of the present invention.  
       FIGS. 39A  to  39 C are sectional views of the semiconductor substrate according to the eleventh embodiment in the steps of the method for fabricating the semiconductor substrate, which show the method.  
       FIGS. 40A  to  40 C are sectional views of the semiconductor substrate in the steps of method for fabricating the semiconductor substrate according to Modification 1 of the eleventh embodiment of the present invention, which explain the method (Part 1).  
       FIGS. 41A  to  41 C are sectional views of the semiconductor substrate in the steps of method for fabricating the semiconductor substrate according to Modification 1 of the eleventh embodiment of the present invention, which explain the method (Part 2).  
       FIGS. 42A  to  42 C are sectional views of the semiconductor substrate in the steps of method for fabricating the semiconductor substrate according to Modification 2 of the eleventh embodiment of the present invention, which explain the method (Part 1).  
       FIGS. 43A  to  43 C are sectional views of the semiconductor substrate in the steps of method for fabricating the semiconductor substrate according to Modification 2 of the eleventh embodiment of the present invention, which explain the method (Part 2).  
       FIG. 44  is a sectional view of the semiconductor substrate according to a twelfth embodiment of the present invention.  
       FIGS. 45A  to  45 C are sectional views of the semiconductor substrate according to the twelfth embodiment in the steps of the method for fabricating the semiconductor substrate, which show the method.  
       FIGS. 46A  to  46 C are sectional views of the semiconductor substrate in the steps of method for fabricating the semiconductor substrate according to Modification 1 of the twelfth embodiment of the present invention, which explain the method (Part 1).  
       FIGS. 47A  to  47 C are sectional views of the semiconductor substrate in the steps of method for fabricating the semiconductor substrate according to Modification 1 of the twelfth embodiment of the present invention, which explain the method (Part 2).  
       FIGS. 48A  to  48 C are sectional views of the semiconductor substrate in the steps of method for fabricating the semiconductor substrate according to Modification 2 of the twelfth embodiment of the present invention, which explain the method (Part 1).  
       FIGS. 49A  to  49 C are sectional views of the semiconductor substrate in the steps of method for fabricating the semiconductor substrate according to Modification 2 of the twelfth embodiment of the present invention, which explain the method (Part 2).  
    
    
     DETAILED DESCRIPTION OF THE INVENTION  
      [The principle of the Invention] 
      The principle of the present invention will be explained before embodiments of the present invention are explained.  
      As described above, improvement of the heat radiation is a problem of the SOI substrate, semiconductor substrates of the strained Si/SiGe structure and semiconductor substrates of the strained Si/SiGeOI structure.  
      The inventors of the present invention have made earnest studies and got an idea that an isotope composition ratio of any one of  28 Si,  29 Si,  30 Si and an isotope ratio of  70 Ge,  72 Ge,  73 Ge,  74 Ge and  76 Ge of a silicon crystal layer, a silicon germanium crystal layer, etc., are set to be high, whereby the heat radiation of a semiconductor substrate can be improved.  
      The usual silicon crystal is composed of three kinds of isotopes,  28 Si,  29 Si and  30 Si.  28 Si is Si whose mass number is 28;  29 Si is Si whose mass number is 29; and  30 Si is Si whose mass number is 30. The isotope abundance ratios of Si in nature is 92.2% of  28 Si, 4.7% of  29 Si and 3.1% of  30 Si, and are always constant (Reference 14: W.S. Capinski et al., Appl. Phys. Lett. 71, 2109 (1997)).  
      The thermal conductivity of such usual silicon crystal is, as described above, about 150 W/mK at, e.g., 300 K (see Reference 12).  
      In contrast to this, when an isotope composition ratio of, e.g.,  28 Si is set as high as 99.86%, the thermal conductivity of the silicon crystal is about 237 W/mK at, e.g., 300 K (see Reference 15: T. Ruf, et al., Solid State Commum., 115, 243 (2000)).  
      Based on this, it can be seen that an isotope composition ratio of  28 Si is set very high, whereby the thermal conductivity of the silicon crystal can be raised by about 58%.  
      In a silicon crystal, an isotope composition ratio of any one of the Si isotopes is set high, whereby the thermal conductivity of the usual silicon crystal can be higher. The mechanism for this will be as follows.  
      That is, the thermal conduction is a phenomena that lattice vibrations (phonons) excited by heat propagate in waves from a higher-temperature part to a lower temperature part. When all the atoms in a crystal lattice have the same mass, an idealistic progressive waves can be formed. In contrast to this, in a system where a plurality of isotopes are present, progressive waves are scattered, and the thermal conductivity is lowered.  
      The increase of the thermal conductivity will be due to such mechanism. Not only when an isotope composition ratio of  28 Si set high, but also when isotope ratios of  29 Si and  30 Si are set high, the thermal conductivity will be similarly increased.  
      A usual Ge crystal is composed of five kinds of isotope elements,  70 Ge,  72 Ge,  73 Ge,  74 Ge and  76 Ge.  70 Ge is Ge h number is 70;  72 Ge is Ge whose mass number is 72;  74 Ge is Ge whose mass number is 74; and  76 Ge is Ge whose mass number is 76. The isotope abundance ratios of  70 Ge,  72 Ge,  73 Ge,  74 Ge and  76 Ge are always constantly 20.5%, 27.4%, 7.8%, 36.5% and 7.8% respectively.  
      The thermal conductivity of such usual Ge crystal is about 60-77 W/mK at, e.g., a temperature of 300K (see Reference 13).  
      In contrast to this, when an isotope composition ratio of, e.g.,  70 Ge is set to be 99.99%, the thermal conductivity of Germanium crystal is about 100 W/mK at 300K (see Reference 13).  
      Based on the above, an isotope composition ratio of  70 Ge is set very high, whereby the thermal conductivity of germanium crystal can be increased by about 30-67%.  
      The mechanism that in germanium crystal, an isotope composition ratio of any one of the Ge isotopes is set high, whereby the thermal conductivity is increased than that of the usual germanium crystal will be the same as that of silicon crystal described above.  
      Thus, not only when an isotope abundance ratio of  70 Ge is set high, but also when an isotope abundance ratio of  72 Ge,  73 Ge,  74 Ge or  76 Ge is set high, the thermal conductivity will be able to be similarly increased.  
      The inventors of the present application, based on such results of their studies, have got an idea that an isotope composition ratio of any one of the Si isotopes and an isotope composition ratio of any one of the Ge isotopes of a silicon crystal layer and a silicon germanium crystal layer are set high, whereby thermal conductivities of the silicon crystal layer and the silicon germanium layer can be increased, and the heat can be scattered more in the direction horizontal to the substrate plane, which enables a semiconductor substrate having good heat radiation to be provided.  
     A FIRST EMBODIMENT  
      The semiconductor substrate according to a first embodiment of the present invention and the method for fabricating the semiconductor substrate will be explained with reference to FIGS.  1  to  2 C.  FIG. 1  is a sectional view of the semiconductor substrate according to the present embodiment.  
      (The Semiconductor Substrate)  
      First, the structure of the semiconductor substrate according to the present embodiment will be explained with reference to  FIG. 1 .  
      The semiconductor substrate according to the present embodiment is characterized mainly by a strained Si/SiGe structure having an isotope composition ratio of  28 Si of a silicon crystal layer  14  set high.  
      As shown in  FIG. 1 , a 200 nm-thickness silicon germanium crystal layer  12  is epitaxially grown on the silicon crystal substrate  10 . The silicon germanium crystal layer  12  has a composition of, e.g., Si 0.7 Ge 0.3 .  
      A 200 nm-thickness silicon crystal layer  14  is epitaxially grown on the silicon germanium crystal layer  12 . The  28 Si isotope composition ratio of the silicon crystal layer  14  is, e.g., 99.9%. Crystal strains are introduced into the silicon crystal layer  14  because of a lattice constant of the silicon crystal layer  14  different from that of the silicon germanium crystal layer  12 . In the present embodiment, since the crystal strains are introduced into the silicon crystal layer  14 , it is possible to obtain high carrier mobility.  
      A plane orientation of the surface of the silicon crystal layer  14  is, e.g., {100}.  
      It is reported that the usual &lt;110&gt; direction of the channel of a MOSFET is changed to &lt;100&gt;, which increases the driving capacity of the p-channel MOSFET by about 15% (see Reference 16: G. Ottaviani, et al., Phys. Rev. B12, 3318 (1975)).  
      In order to make a channel direction of a MOSFET &lt; 100 &gt;, an orientation flat or a notch is set to be &lt;011&gt;+45° or &lt;011&gt;−45°. This permits a MOSFET to have &lt;100&gt; channel direction.  
      The substrate is aligned to have &lt;100&gt; channel direction in the exposure step, whereby a MOSFET can have &lt;100&gt; channel direction.  
      A plane orientation of the surface of the silicon crystal layer may be {113}.  
      It has been confirmed by TDDB (Time Dependent Dielectric Breakdown) that a silicon oxide film formed on a semiconductor wafer a plane orientation of the surface of which is {113} has better insulation than silicon oxide film formed on a semiconductor wafer a plane orientation of the surface of which is {100} (see Reference 17: H-J. Mussig, et al., Proc. 3rd Int&#39;l Symp. of Advanced Sci. and Tech. of Si Mat., The Jpn Soc. Prom. Sci., 2000, p. 374). Reasons for the better insulation of the silicon oxide film formed on a {113} semiconductor wafer are that a stress in the interface between the silicon oxide film and the silicon crystal layer is smaller than that of the silicon oxide film formed on a {100} semiconductor wafer, and that roughness of the {113} surface is about ½ of that of the {100} surface (see Reference 17). Based on this, the plane orientation of the silicon crystal layer  14  is set to be {113}, whereby MOSFETs, etc. of high reliability will be able to be fabricated.  
      A plane orientation of the silicon crystal layer  14  may be {011}.  
      A p-channel MOSFET fabricated on a semiconductor wafer the plane orientation of the surface of which is {011} has higher hole mobility in comparison with a p-channel MOSFET fabricated on a semiconductor wafer the plane orientation of the surface of which is {100} (see Reference 18: T. Sato et al., Phys. Rev. B4, 1950 (1971) ). However, when an n-channel MOSFET is fabricated on a semiconductor wafer the plane orientation of the surface of which is {011}, the electron mobility is decreased by about 20% in comparison with that of an n-channel MOSFET the plane orientation of the surface of which is {100}.  
      However, an operational speed of a CMOS circuit is determined by a mobility of holes, whose mobility is lower than electrons. To increase the operational speed of the CMOS circuit it is important to increase the mobility of holes in the p-channel MOSFETs.  
      As described above, the plane orientation of the silicon crystal layer  14  is set to be {011}, whereby the operational speed of the p-channel MOSFET can be further improved.  
      As described above, according to the present embodiment, the isotope composition ratio of  28 Si of the silicon crystal layer  14  is set to be so high as 99.9%, whereby the thermal conductivity of the silicon crystal layer  14  can be increased. Thus, according to the present embodiment, scattering of the heat in the direction horizontal to the substrate surface can be accelerated. According to the present embodiment, the heat generated in the core part, hot spots, etc. of the microprocessor can be effectively radiated. The semiconductor substrate according to the present embodiment can have heat radiation improved, and can contribute to higher operational speed and higher reliability of high end ultra fast devices.  
      (The Method for Fabricating the Semiconductor Substrate)  
      Next, the method for fabricating the semiconductor substrate according to the present embodiment will be explained with reference to  FIGS. 2A  to  2 C.  FIGS. 2A  to  2 C are sectional views of the semiconductor substrate according to the present embodiment in the steps of the method for fabricating the semiconductor substrate, which explained the method.  
      First, as shown in  FIG. 2A , a silicon crystal substrate  10  is prepared.  
      Then, as shown in  FIG. 2B , the 200 nm-thickness silicon germanium crystal layer  12  is epitaxially grown on the silicon crystal substrate  10  by, e.g., CVD (Chemical Vapor Deposition). A composition of the silicon germanium crystal layer  12  is, e.g., Si 0.7 Ge 0.3 . As a raw material gas for the Si, monosilane (SiH 4 ), for example, is used. As a raw material gas for the Ge, germane (GeH 4 ) is used. These raw material gases are usual raw material gases whose isotope abundance ratios are not specifically controlled. Because usual raw material gases whose isotope abundance ratios are not specified are used, isotope abundance ratios of the Si and the Ge of the silicon germanium crystal layer  12  are the same as those of Si and Ge in nature.  
      Then, as shown in  FIG. 2C , the 200 nm-thickness silicon crystal layer  14  is epitaxially grown on the silicon germanium crystal layer  12  by, e.g., CVD. As a raw material gas for the Si, monosilane ( 28 SiH 4 ) having, e.g., a 99.9% isotope composition ratio of  28 Si is used. Thus, the silicon crystal layer  14  having, e.g., a 99.9% isotope composition ratio of  28 Sican be formed. Because of a difference in the lattice constant between the silicon crystal layer  14  and the silicon germanium crystal layer  12 , crystal strains are introduced into the silicon crystal layer  14 .  
      When the silicon crystal layer  14  is formed, in order to control a specific resistance of the silicon crystal layer  14 , raw material gas, hydrogen (H 2 ) and raw material gas for boron are used together with the raw material gas for the Si. As the raw material gas for the boron, diborane (B 2 H 6 ), for example, is used. However, to control a specific resistance of the silicon crystal layer  14  is not directly involved in the contents of the present invention and will not be explained below.  
      Thus, the semiconductor substrate according to the present embodiment is fabricated.  
     A SECOND EMBODIMENT  
      The semiconductor substrate according to a second embodiment of the present invention and the method for fabricating the semiconductor substrate will be explained with reference to FIGS.  3  to  4 C.  FIG. 3  is a sectional view of the semiconductor substrate according to the present embodiment. The same members of the present embodiment as those of the semiconductor substrate and the method for fabricating the semiconductor substrate shown in FIGS.  1  to  2 C are represented by the same reference numbers not to repeat or to simplify their explanation.  
      (The Semiconductor Substrate)  
      First, the semiconductor substrate according to the present embodiment will be explained with reference to  FIG. 3 .  
      The semiconductor substrate according to the present embodiment is characterized mainly by a strained Si/SiGe structure having  70 Ge isotope composition ratio of a silicon germanium crystal layer  12   a  set high.  
      As shown in  FIG. 3 , the silicon germanium crystal layer  12   a  of a 200 nm-thickness is formed on a silicon crystal substrate  10 . A composition of the silicon germanium crystal layer  12   a  is, e.g., Si 0.7 Ge 0.3 . An isotope composition ratio of  70 Ge of the silicon germanium crystal layer  12   a  is, e.g., 99.9%. Theisotope abundance ratio of the Si of the silicon germanium crystal layer  12   a  is the same as that of Si in nature.  
      In the present embodiment, the isotope abundance ratio of the Si of the silicon germanium crystal layer  12   a  is the same as that of Si in nature, but the isotope composition ratio of  28 Si of the silicon germanium crystal layer  12   a  may be set higher. That is, the isotope composition ratios of both  70 Ge and  28 Si of the silicon germanium crystal layer  12   a  may be set higher.  
      A 200 nm-thickness silicon crystal layer  14   a  is epitaxially grown on the silicon germanium crystal layer  12   a . Because of a difference in the lattice constant between the silicon crystal layer  14   a  and the silicon germanium crystal layer  12   a , crystal strains are introduced into the silicon crystal layer  14   a . The isotope abundance ratio of Si of the silicon crystal layer  14   a  is the same as that of Si in nature. The plane orientation of the surface of the silicon crystal layer  14   a  is, e.g., {100}, {113} or {011}.  
      The semiconductor substrate according to the present embodiment is characterized mainly in that an isotope composition ratio of  70 Ge of the silicon germanium crystal layer  12   a  is set as high as 99.9% as described above.  
      According to the present embodiment, an isotope composition ratio of  70 Ge of the silicon germanium crystal layer  12   a  is set high, whereby the thermal conductivity of the silicon germanium crystal layer  12   a  can be increased. Thus, the semiconductor substrate according to the present embodiment can efficiently radiate heat.  
      (The Method for Fabricating the Semiconductor Device)  
      Then, the method for fabricating the semiconductor substrate according to the present embodiment will be explained with reference to  FIGS. 4A  to  4 C.  FIGS. 4A  to  4 C are sectional views of the semiconductor substrate according to the present embodiment in the steps of the method for fabricating the semiconductor substrate, which explain the method.  
      First, as shown in  FIG. 4A , the silicon crystal substrate  10  is prepared.  
      Then, as shown in  FIG. 4B , the 200 nm-thickness silicon germanium crystal layer  12   a  is epitaxially grown on the silicon crystal substrate  10 . A composition of the silicon germanium crystal layer  12   a  is, e.g., Si 0.7 Ge 0.3 . As a raw material gas of the Ge, a raw material gas having, e.g., a  70 Ge isotope composition ratio of, e.g., 99.9% is used. As a raw material gas of the Si, a usual raw material gas having the isotope abundance ratio of Si specifically not controlled is used. Thus, the silicon germanium crystal layer having a 99.9% isotope composition ratio of  70 Ge is formed.  
      Next, as shown in  FIG. 4C , the 20 nm-thickness silicon crystal layer  14   a  of silicon crystals is epitaxially grown on the silicon germanium crystal layer  12   a  by, e.g., CVD. As a raw material gas, a usual raw material gas an Si isotope abundance ratio of which is not specifically controlled is used.  
      Thus, the semiconductor substrate according to the present embodiment is fabricated.  
     A THIRD EMBODIMENT  
      The semiconductor substrate according to a third embodiment of the present invention and the method for fabricating the semiconductor substrate will be explained with reference to FIGS.  5  to  6 C.  FIG. 5  is a sectional view of the semiconductor substrate according to the present embodiment. The same members of the present embodiment as those of the semiconductor substrate and the method for fabricating the semiconductor substrate according to the first or the second embodiment shown in FIGS.  1  to  4 C are represented by the same reference numbers not to repeat or to simplify their explanation  
      (The Semiconductor Substrate)  
      The semiconductor substrate according to the present embodiment will be explained with reference to  FIG. 5 .  
      The semiconductor substrate according to the present embodiment is characterized mainly by a strained Si/SiGe structure having a  70 Ge isotope composition ratio of a silicon germanium crystal layer  12   a  set high and a  28 Si isotope composition ratio of a silicon crystal layer  14  set high.  
      As shown in  FIG. 5 , the silicon germanium crystal layer  12   a  of a 200 nm thickness is epitaxially grown on a silicon crystal substrate  10 . An isotope composition ratio of  70 Ge of the silicon germanium crystal layer  12   a  is set as high as, e.g., 99.9%. The isotope abundance ratio of Si of the silicon germanium crystal layer  12   a  is the same as that of Si in nature.  
      In the present embodiment, the isotope abundance ratio of Si of the silicon germanium crystal layer  12   a  is the same as that of Si in nature, but an isotope composition ratio of  28 Si of the silicon germanium crystal layer  12   a  may be set higher. That is, isotope composition ratios of both  70 Ge and  28 Si of the silicon germanium crystal layer  12   a  may be set higher.  
      The silicon crystal layer  14  of a 20 nm thickness is grown on the silicon germanium crystal layer  12   a . Crystal strains are introduced into the silicon crystal layer  14 . An isotope composition ratio of  28 Si of the silicon crystal layer  14  is set as high as, e.g., 99.9%. A plane orientation of the surface of the silicon crystal layer  14  is, e.g., {100}, {113} or {011}.  
      The semiconductor substrate according to the present embodiment is characterized mainly in that, as described above, an isotope composition ratio of  70 Ge of the silicon germanium crystal layer  12   a  is set high, and an isotope composition ratio of  28 Si of the silicon crystal layer  14  is set high.  
      According to the present embodiment, an isotope composition ratio of  70 Ge of the silicon germanium crystal layer  12   a  is set high, and an isotope composition ratio of  28 Si of the silicon crystal layer  14  is set high, whereby the thermal conductivity of both the silicon germanium crystal layer  12   a  and the silicon layer  14  can be increased. Thus, the semiconductor substrate according to the present embodiment can more effectively radiate the heat.  
      (The Method for Fabricating the Semiconductor Substrate)  
      Next, the method for fabricating the semiconductor substrate according to the present embodiment will be explained with reference to  FIGS. 6A  to  6 C.  FIGS. 6A  to  6 C are sectional views of the semiconductor substrate according to the present embodiment in the steps of the method for fabricating the semiconductor substrate, which explain the method.  
      First, as shown in  FIG. 6A , the silicon crystal substrate  10  is prepared.  
      Next, as shown in  FIG. 6B , the 200 nm-thickness silicon germanium crystal layer  12   a  is epitaxially grown on the silicon crystal substrate  10  by, e.g., CVD. The composition of the silicon germanium crystal layer  12   a  is, e.g., Si 0.7 Ge 0.3 . As a raw material gas of the Ge, a raw material gas a  70 Ge isotope composition ratio of which is, e.g., 99.9% is used. As a raw material gas of the Si, a raw material gas having the isotope abundance ratio of the Si not specifically controlled is used. Thus, the silicon germanium crystal layer  12   a  a  70 Ge isotope composition ratio of which is, e.g., 99.9% is formed.  
      Then, the 20 nm-thickness silicon crystal layer  14  is epitaxially grown on the silicon germanium crystal layer  12   a  by, e.g., CVD. As a raw material of the Si, a raw material gas an  28 Si isotope composition ratio of which is 99.9% is used.  
      Thus, the semiconductor substrate according to the present embodiment is fabricated.  
     A FOURTH EMBODIMENT  
      The semiconductor substrate according to a fourth embodiment of the present invention and the method for fabricating the semiconductor substrate will be explained with reference to FIGS.  7  to  8 B.  FIG. 7  is a sectional view of the semiconductor substrate according to the present embodiment. The same members of the present embodiment as those of the semiconductor substrate according to the first to the third embodiments and the method for fabricating the semiconductor substrate shown in FIGS.  1  to  6 C are represented by the same reference numbers not to repeat or to simplify their explanation.  
      (The Semiconductor Substrate)  
      The semiconductor substrate according to the present embodiment is characterized mainly in that a silicon germanium crystal substrate is used as a base substrate, and a silicon crystal layer a  28 Si isotope composition ratio of which is high is formed on the silicon germanium crystal substrate.  
      As shown in  FIG. 7 , in the present embodiment, the silicon germanium crystal substrate  10   a  is used as the base substrate. The composition of the silicon germanium crystal substrate  10   a  is, e.g., Si 0.7 Ge 0.3 .  
      The silicon crystal layer  14  of a 20 nm-thickness is formed on the silicon germanium crystal substrate  10 . Crystal strains are introduced into the silicon crystal layer  14 . A 28 Si isotope composition ratio of the silicon crystal layer  14  is, e.g., 99.9%. The plane orientation of the surface of the silicon crystal layer  14  is, e.g., {100}, {113} or {011}.  
      As described above, the semiconductor substrate according to the present embodiment is characterized mainly in that the silicon germanium crystal substrate  10   a  is used as a base substrate, and the silicon crystal layer  14  a  28 Si isotope composition ratio of which is high is formed on the silicon germanium crystal substrate  10   a.    
      According to the present embodiment, the silicon germanium crystal substrate  10   a  is used as the base substrate, which permits the strained silicon crystal layer  14  to be formed directly on the base substrate. Thus, the semiconductor device according to the present embodiment can effectively radiate the heat and can be fabricated by simpler steps.  
      (The Method for Fabricating the Semiconductor Substrate)  
      Next, the method for fabricating the semiconductor substrate according to the present embodiment will be explained with reference to  FIGS. 8A and 8B .  FIGS. 8A and 8B  are sectional views of the semiconductor substrate according to the present embodiment in the steps of the method for fabricating the semiconductor substrate, which explain the method.  
      First, as shown in  FIG. 8A , the silicon germanium crystal substrate  10   a  is prepared. A composition of the silicon germanium crystal substrate  10   a  is, e.g., Si 0.7 Ge 0.3 .  
      Then, the silicon crystal layer  14  of a 20 nm-thickness is epitaxially grown on the silicon germanium crystal substrate  10   a  by, e.g., CVD. As a raw material gas of the Si, monosilane ( 28 SiH 4 ) a  28 Si isotope composition ratio of which is, e.g., 99.9% is used. Thus, the silicon crystal layer  14  a  28 Si isotope composition ratio of which is, e.g., 99.9% is fabricated. A lattice constant difference between the silicon crystal layer  14  and the silicon germanium crystal substrate  10   a  introduce crystal strains into the silicon crystal layer  14 .  
      Thus, the semiconductor substrate according to the present embodiment is fabricated.  
     A FIFTH EMBODIMENT  
      The semiconductor substrate according to a fifth embodiment of the present invention and the method for fabricating the semiconductor substrate will be explained with reference to FIGS.  9  to  11 B.  FIG. 9  is a sectional view of the semiconductor substrate according to the present embodiment. The same members of the present embodiment as those of the semiconductor substrate according to the first to the fourth embodiments and the method for fabricating the semiconductor substrate shown in FIGS.  1  to  8 B are represented by the same reference numbers not to repeat or to simplify their explanation.  
      (The semiconductor Substrate)  
      The semiconductor substrate according to the present embodiment will be explained with reference to  FIG. 9 .  
      The semiconductor substrate according to the present embodiment is firstly characterized mainly by an SOI substrate fabricated by bonding, which has a  28 Si isotope composition ratio of a silicon crystal layer  14   b  set high.  
      As shown in  FIG. 9 , in the present embodiment, a base substrate  10   b  of silicon crystals and the silicon crystal layer  14   b  are bonded with an insulation film  16  formed therebetween. The insulation film  16  is formed of SiO 2  of a 200 nm-thickness. A  28 Si isotope composition ratio of the silicon crystal layer  14   b  is, e.g., 99.9%. The base substrate  10   b  is a usual silicon crystal substrate having the isotope abundance ratio not specifically controlled. In the semiconductor substrate according to the present embodiment, which is the SOI substrate fabricated by bonding, an oxygen concentration profile in the interface between the base substrate  10   b  and the insulation film  16 , and an oxygen concentration profile in the interface between the insulation film and the silicon crystal layer  14   b  are steeper than an oxygen concentration profile in the SOI substrate fabricated by SIMOX (Separation by IMplantation of OXygen).  
      The plane orientation of the surface of the silicon crystal layer  14   b  is, e.g., {100}.  
      The specification of Japanese Patent Laid-Open Publication No. Hei9-246505/1997 describes an SOI substrate bonded with the &lt;011&gt; axis of the silicon crystal layer and the &lt;011&gt; axis of the base substrate forming an angle of 10-45°. By using such SOI substrate, even when a MOSFET is fabricated by the usual exposure, the channel direction of the transistors can be along &lt;100&gt;. Accordingly, by using the bonding described in the specification of Japanese Patent Laid-Open Publication No. Hei9-246505/1997 in fabricating the semiconductor substrate according to the present embodiment, even when a MOSFET is fabricated by the usual exposure, the channel direction can be along &lt;100&gt;.  
      The plane orientation of the silicon crystal layer may be {113} or {011}.  
      As described above, the semiconductor substrate according to the present embodiment is firstly characterized mainly in that a  28 Si isotope composition ratio of the silicon crystal layer  14   b  is set so high as 99.9%.  
      According to the present embodiment, a  28 Si isotope composition ratio of the silicon crystal layer  14   b  is set so high as 99.9%, whereby the thermal conductivity of the silicon crystal layer  14   b  can be increased. Thus, the semiconductor substrate according to the present embodiment can effectively radiate Joule&#39;s heat generated in transistors (not shown), etc. fabricated on the silicon crystal layer  14   b , etc.  
      The semiconductor substrate according to the present embodiment is secondly characterized mainly by an SOI substrate fabricated by bonding as described above.  
      Reference 11 describes an SOI substrate including a silicon crystal layer a  28 Si isotope composition ratio of which is set to be above 92.2%. However, the SOI substrate described in Reference 11 will not be able to have good heat radiation for the following reason.  
      In Reference 11, the buried oxide film is formed by SIMOX. In forming a buried oxide film by SIMOX, oxygen ions (16O + ) are implanted at the surface of a silicon crystal substrate at a 180 keV acceleration energy and a 4×10 17  cm −2  dose, for example, and then thermal processing is performed in an atmosphere of a mixed gas of argon and oxygen, at 1350° C. and for several hours, for example, to thereby form the buried oxide film (Reference 19: S. Nakashima. et al., J. Electrochem. Soc. 143, 244 (1996)).  
      When the buried oxide film is thus formed by SIMOX, a large number of interstitial Si atoms are implanted into the silicon crystal layer both at the interface between the silicon oxide film formed on the surface of the silicon crystal layer, and the silicon crystal layer, and at the interface between the buried oxide film and the silicon crystal layer. Because the thermal processing temperature, 1350° C., for forming the buried oxide film is very near 1400° C., which is the dissolution temperature of silicon crystals, an enormous number of interstitial Si atoms are implanted into the silicon crystal layer. The interstitial Si atoms implanted into the silicon crystal layer are trapped and reside in the silicon crystal layer, which is sandwiched between the buried oxide film and the silicon oxide film.  
      In the high end ultra fast device using the SOI substrate, which uses full depleted MOSFETs, the thickness of the silicon crystal layer has a very small thickness of, e.g., below 50 nm. In the SOI substrate, a thickness of the silicon crystal layer of which is set small for higher operational speed of the MOSFETs, a concentration of interstitial Si atoms in the silicon crystal layer is very high. When heat processing of about 1000° C. is performed in fabricating the MOSFETS, the interstitial Si atoms are deposited to resultantly form stacking faults.  
      It is reported that, in the SOI substrate including the buried oxide film by SIMOX, dislocations caused by implantation of oxygen ions are present in the silicon crystal layer in a density of several hundred dislocations/cm 2  (see Reference 20: S. Nakashima, et al., Electron, Lett. 26, 1647 (1990)).  
      Thermal conduction is propagation of lattice vibrations thermally excited in waves. When an enormous number of crystal defects and dislocations in the silicon crystal layer, progressive waves of lattice vibrations are scattered. Accordingly, the SOI substrate described in Reference 11 cannot provide good heat radiation.  
      In contrast to this, according to the present embodiment, the SOI substrate is fabricated by bonding, whereby the generation of stacking faults and dislocations in the silicon crystal layer  14   b  can be prevented. Thus, the semiconductor substrate according to the present embodiment can have higher thermal conductivity of the silicon crystal layer  14   b  and can effectively radiate the heat.  
      (The Method for Fabricating the Semiconductor Substrate)  
      Next, the semiconductor substrate according to the present embodiment and the method for fabricating the semiconductor substrate will be explained with reference to  FIGS. 10A  to  11 B.  FIGS. 10A  to  11 B are sectional views of the semiconductor substrate according to the present embodiment in the steps of the method for fabricating the semiconductor substrate, which explain the method.  
      As shown in  FIG. 10A , the silicon crystal substrate  18  a  28 Si isotope concentration of which is, e.g., 99.9% is prepared. The silicon crystal substrate  18  is to be thinned in a later step to be the silicon crystal layer  14   b  of the SOI substrate.  
      Next, as shown in  FIG. 10B , the insulation film  16  of SiO 2  is formed on the surface of the silicon crystal substrate  14   b  by thermal oxidation or CVD.  
      As shown in  FIG. 10C , the base substrate  10   b  of silicon crystal is prepared.  
      Next, as shown in  FIG. 11A , the silicon crystal substrate  18  and the base substrate  10   b  are bonded to each other with the insulation film  16  formed therebetween.  
      Then, as shown in  FIG. 11B , the silicon crystal substrate  18  is thinned by mechanical processing or chemical etching. Thus, the silicon crystal layer  14   b  is formed of the thinned silicon crystal substrate  18 .  
      Thus, the semiconductor substrate according to the present embodiment is fabricated.  
      (Modification)  
      Next, a modification of the method for fabricating the semiconductor substrate according to the present embodiment will be explained with reference to  FIG. 12A  to  13 B.  FIGS. 12A  to  13 B are sectional views of the semiconductor substrate in the steps of method for fabricating the semiconductor substrate according to the present modification, which explain the method.  
      The modification of the method for fabricating the semiconductor substrate according to the present embodiment is characterized mainly in that the silicon crystal substrate  18  is thinned by cleavage to thereby form the silicon crystal layer  14   b  of the silicon crystal substrate  18 .  
      First, the steps of the present modification up to the step of forming the insulation film  16  on the surface of the silicon crystal substrate  18  including the insulation film forming step are the same as those of the method for fabricating the semiconductor substrate described above with reference to  FIG. 10A and 10B , and their explanation will not be repeated (see  FIGS. 12A and 12B ).  
      Then, as shown in  FIG. 12C , hydrogen ions are implanted into the silicon crystal substrate  18  through the insulation film  16 . In the drawing, the region  20  with the hydrogen ions implanted in is indicated by crosses (x).  
      Next, as shown in  FIG. 13A , the silicon crystal substrate  18  and the base substrate  10   b  are bonded to each other with the insulation film  16  therebetween.  
      Then, as shown in  FIG. 13B , the silicon crystal substrate  18  is separated along the region  20  with the hydrogen ions implanted in. The silicon crystal substrate  18  is thus thinned by cleavage to form the silicon crystal layer  14   b  of the silicon crystal substrate  18 . In a case that the surface of the silicon crystal layer  14   b  has to be further planarized, the surface of the silicon crystal layer  14   b  is polished by CMP (Chemical Mechanical Polishing) (not shown).  
      As described above, the silicon crystal layer  14   b  may be formed of the silicon crystal substrate  18  by thinning the silicon crystal substrate  18  by cleavage.  
     A SIXTH EMBODIMENT  
      The semiconductor substrate according to a sixth embodiment of the present invention and the method for fabricating the semiconductor substrate will be explained with reference to FIGS.  14  to  16 C.  FIG. 14  is a sectional view of the semiconductor substrate according to the present embodiment. The same members of the present embodiment as those of the semiconductor substrate and the method for fabricating the semiconductor substrate shown in FIGS.  1  to  13 B are represented by the same reference numbers not to repeat or to simplify their explanation.  
      (The Semiconductor Substrate)  
      First, the semiconductor substrate according to the present embodiment will be explained with reference to  FIG. 14 .  
      The semiconductor substrate according to the present embodiment is characterized mainly by an SOI substrate fabricated by bonding with carbon (C) atoms implanted in a region of a silicon crystal layer  14   b , which is nearer to the interface between an insulation film  16  and the silicon crystal layer  14   b.    
      As shown in  FIG. 14 , carbon atoms are implanted in the region of the silicon crystal layer  14   b , which is nearer to the interface between the silicon crystal layer  14   b  and the insulation film  16 . In the drawing, the region with the carbon atoms implanted is indicated by dots. In the drawing, denser dots indicate more heavily implanted carbon atoms. A carbon concentration in the region nearer to the interface between the silicon crystal layer  14   b  and the insulation film  16  is, e.g., about 5×10 20  cm −3  at the most heavily implanted part.  
      In the present embodiment, carbon atoms are implanted in a region of the silicon crystal layer  14   b , which is nearer to the interface between the silicon crystal layer  14   b  and the insulation film  16  so as to mitigate a tensile strain exerted to the silicon crystal layer  14   b  in the region thereof nearer to the interface between the insulation film  16  and the silicon crystal layer  14   b.    
      It is known that generally strains are present in the interface between silicon crystals and silicon oxide film. It is considered that strains are generated also in the interface between the silicon crystal layer and the buried insulation film of the SOI substrate. Because of the thermal expansion coefficient of the silicon crystal layer, which is higher than that of the silicon oxide film, in a case that the silicon oxide film is formed directly on the silicon crystals, tensile strains are generated in the silicon crystal layer nearer to the interface between the silicon crystal layer and the silicon oxide film, and compression strains are generated in the silicon oxide film nearer to the interface of the silicon crystal layer and the silicon oxide film. A tensile stress generated in the silicon crystal layer nearer to the interface between the silicon crystal layer and the silicon oxide film is about 1×10 9 -4×10 9  dyn/cm 2 , and a tensile strain generated in the silicon crystal layer nearer to the interface between the silicon crystal layer and the silicon oxide film is about 1×10 −3 −4×10 −3  (see Reference 21: R. J. Jaccodine, et al., J. Appl. Phys. 37, 2429 (1966); Reference 22: G. Lucovsky et al., The Physics and Chemistry of SiO 2  and the SiO 2  Interface, edited by C. R. Helms, et al., Plenum Press, NY, 1988, p. 139).  
      The stress thus generated in the interface between the silicon crystal layer and the silicon oxide film will be similarly generated even in a case that an isotope composition ratio of any one of the isotopes of silicon crystal layer is set high. The strain generated in the region nearer to the interface between the silicon crystal layer and the insulation film scatters lattice vibration waves propagating in the silicon crystal layer, and will be a cause for lowering the thermal conductivity.  
      Then, in the present embodiment, carbon atoms are implanted in the silicon crystal layer  14   b  nearer to the interface between the silicon crystal layer  14   b  and the insulation film  16 . Because of the covalent radius of Si atom, which is smaller than that of carbon atom, carbon atoms are implanted to thereby contract the crystal lattices expanded in the silicon crystal layer  14   b , and the tensile strain can be mitigated (principle of strain compensation).  
      Carbon atoms, which are electrically neutral in the silicon crystal  14   b , are implanted in the silicon crystal layer  14   b  without affecting electric characteristics of MOSFETs, etc. to be fabricated on the silicon crystal layer  14   b.    
      According to the present embodiment, crystal strains of the silicon crystal layer  14   b  nearer to the interface between the silicon crystal layer  14   b  and the insulation film  16  can be mitigated, whereby the thermal conductivity of the silicon crystal layer  14   b  can be higher, and the semiconductor substrate can more effectively radiate the heat.  
      In the present embodiment, a concentration of carbon atoms implanted in the silicon crystal layer  14   b  nearer to the interface between the insulation film  16  and the silicon crystal layer  14   b  is 5×10 20  cm −3  but is not essentially limited to 5×10 20  cm −3 .  
      A concentration of carbon atoms to be implanted may be set suitably so as to mitigate the tensile strain exerted to the silicon crystal layer  14   b  nearer to the interface between the insulation film  16  and the silicon crystal layer  14   b . A suitable concentration of carbon atoms to be implanted in the silicon crystal layer  14   b  nearer to the interface between the insulation film  16  and the silicon crystal layer  14   b  can be given as follows.  
      A strain ε generated in a crystal lattice by the implantation of an impurity can be given by the following formula (see Reference 23: H. J. Herzog, et al., J. Electrochem. Soc., 131, 2969 (1984)). 
 
ε=α i   ×N   c  ( i=L, V )   (2) 
 
α L =[1−( R   c   /R   si )]× D   −1    (3) 
 
α V =[1−(R c /R si ) 3 ]×(3 D ) −1    (4) 
 
 wherein α i  represents a lattice contraction coefficient; N c ; a concentration of implanted carbon atoms; R si  represents a covalent radius of Si; R c  represents a covalent radius of carbon; D represents an atom density of an Si crystal lattice; α L  represents a lattice contraction coefficiency of a linear model; and α v  represents a lattice contraction coefficiency of a volume model. 
 
      When an Si covalent radius, a carbon covlanet radius, and an Si crystal atom density are substituted respectively 0.117nm (see Reference 23), 0.077 nm (see Reference 24: Bunichi Tamamushi et al., Rikagakujiten, 3rd supplemented edition, 1983, Iwanami Shoten, p. 324) and 5×10 22  cm −2  in Formula 3 and Formula 4, α L  and α v  are as follows. 
 
α L =6.84×10 −24    (5) 
 
α L =4.77×10 −24    (6) 
 
      Then, when Formulae 2, 5 and 6 are used, and ε=1×10 −3 , a carbon atom concentration N c  for mitigating a tensile strain of the silicon crystal layer  14   b  is 
 
 N   c =1.46×10 20 −2.10×10 20  cm −3 . 
 
      When Formulae 2, 5 and 6 are used, and ε=4×10 −3 , a carbon atom concentration N c  for mitigating a tensile strain of the silicon crystal layer  14   b  is 
 
 N   c =5.85×10 20 −8.39×10 20  cm −3 . 
 
      Here, a difference between the values of N c  for the one and the same strain is made by computing the values by using both the linear model and volume model.  
      Based on the above, in order to make the mitigation of a tensile strain of the silicon crystal layer nearer to the interface between the insulation film  16  and the silicon crystal layer  14   b  practically effective, it is suitable to set a concentration of carbon atoms to be implanted to be 1×10 20 −1×10 21  cm −3 .  
      The concentration of carbon atoms to be implanted in a region of the silicon crystal layer  14   b  nearer to the interface between the insulation film  16  and the silicon crystal layer  14   b  is not limited to 1×10 20 −1×10 21  cm −3 ; when a carbon concentration is below 1×10 20  cm −3  or when a carbon concentration is above 1×10 2  cm −3 , a tensile strain in the region of the silicon crystal layer  14   b  nearer to the interface between the insulation film  16  and the silicon crystal layer  14   b  can be mitigated to some extent. That is, as long as carbon atoms are implanted in a region of the silicon crystal layer  14   b  nearer to the interface between the insulation film  16  and the silicon crystal layer  14   b , a tensile strain in the region of the silicon crystal layer  14   b  nearer to the interface between the insulation film  16  and the silicon crystal layer  14   b  can be mitigated to some extent.  
      As carbon atoms to be implanted,  12 C may be used, or  13 C may be used. However, the isotope abundance ratio of  12 C in nature is so high as 98.89% (see Reference 25: Bunichi Tamamushi et al., Rikagakujiten, 3rd supplemented edition, 1983, Iwanami Shoten, p. 1560). That is, the  12 C isotope composition ratio is so high without specifically controlling the abundance ratio of an isotope of carbon to be implanted. Accordingly, even by implanting the usual carbon atoms without controlling an isotope abundance ratio, the same effect as that produced by controlling an isotope composition ratio of  12 C will be produced.  
      As described above, according to the present embodiment, the SOI substrate fabricated by bonding has carbon atoms implanted in the region of the silicon crystal layer  14   b  nearer to the interface between the insulation film  16  and the silicon crystal layer  14   b , whereby the tensile strain of the silicon crystal layer  14   b  can be mitigated. According to the present embodiment, the tensile strain of the silicon crystal layer  14   b  can be mitigated, whereby the thermal conductivity of the silicon crystal layer  14   b  can be higher, and the semiconductor substrate can have good heat radiation.  
      In many of high end ultra fast devices using the SOI substrate, full depletion type-MOSFETs are fabricated. In the full depletion type-MOSFETs, in operation, the depletion layer reaches the interface between the silicon crystal layer  14   b  and the insulation film  16 . Accordingly, the electric characteristics of the MOSFETs are susceptible to strains of the interface between the silicon crystal layer  14   b  and the insulation film  16 . According to the present embodiment, as described above, the strain in the interface between the silicon crystal layer  14   b  and the insulation film  16  can be mitigated, and accordingly the electric characteristics of the MOSFETs can be also better.  
      As described above, according to the present embodiment, the thermal conductivity of the silicon crystal layer  14   b  can be increased, and the electric characteristics of MOSFETs, etc. to be fabricated on the silicon crystal layer  14   b  can be also better.  
      In the present embodiment, the SOI substrate including the silicon crystal layer  14   b  having a  28 Si isotope composition ratio set high has carbon atoms implanted in the region of the silicon crystal layer  14   b  nearer to the interface between the silicon crystal layer  14   b  and the insulation film  16 . The usual SOI substrate including the silicon crystal layer  14   b  having an isotope abundance ratio of Si not specifically controlled has carbon atoms implanted in the region of the silicon crystal layer  14   b  nearer to the interface between the silicon crystal layer  14   b  and the insulation film  16 , whereby the electric characteristics of MOSFETs, etc. to be fabricated on the silicon crystal layer  14   b  can be also better.  
      (The Method for Fabricating the Semiconductor Substrate)  
      Next, the method for fabricating the semiconductor substrate according to the present embodiment will be explained with reference to  FIGS. 15A  to  16 C.  FIGS. 15A  to  16 C are sectional views of the semiconductor substrate according to the present embodiment in the steps of the method for fabricating the substrate, which explain the method.  
      First, as shown in  FIG. 15A , the silicon crystal substrate  18  having an isotope concentration of  28 Si is, e.g., 99.9% is prepared.  
      Then, as shown in  FIG. 15B , carbon atoms are implanted by ion implantation in a region of the silicon crystal substrate  18  which is nearer to the surface thereof. At this time, carbon atoms are implanted so that a concentration of the carbon atoms in the silicon crystal substrate  18  nearer to the surface is, e.g., 5×10 20  cm −3 . Carbon atoms are implanted so that a concentration of the carbon atoms lowers gradually from the surface of the silicon crystal substrate  18  to the inside of the silicon crystal substrate  18 . In the drawings, the carbon atoms are indicated by dots. In the drawings, higher densities of the dots indicate higher carbon concentrations.  
      Next, as shown in  FIG. 15C , the insulation film  16  of SiO 2  is formed on the surface of the silicon crystal substrate  18  by thermal oxidation or CVD.  
      Next, as shown in  FIG. 16A , hydrogen ions are implanted in the silicon crystal substrate  18  on the side of the insulation film  16 . In the drawing, the region  20  where the hydrogen ions have been implanted is indicated by crosses.  
      Then, as shown in  FIG. 16B , the silicon crystal substrate  18  and the base substrate  10   b  are bonded to each other with the insulation film  16  formed therebetween.  
      Next, as shown in  FIG. 16C , the silicon crystal substrate  18  is separated by cleavage in the region  20  with the hydrogen ions implanted in, whereby the silicon crystal layer  14  is formed of the thinned silicon crystal substrate  18 .  
      Thus, the semiconductor substrate according to the present embodiment is fabricated.  
      (Modification 1)  
      Next, Modification 1 of the method for fabricating the semiconductor substrate according to the present embodiment will be explained with reference to  FIGS. 17A  to  18 C.  FIGS. 17A  to  18 C are sectional views of the semiconductor substrate in the steps of method for fabricating the semiconductor substrate according to the present modification, which explain the method.  
      The semiconductor substrate fabricating method according to the present modification is characterized mainly in that the SOI substrate is fabricated by forming a silicon crystal layer  14   c  having a  28 Si isotope concentration of, e.g., 99.9% on a usual silicon crystal substrate  22  having the isotope abundance ratio not specifically controlled and bonding the silicon crystal layer  14   c  and the base substrate  10   b  with the insulation film  16  formed therebetween.  
      First, as shown in  FIG. 17A , a usual silicon crystal substrate  22  having the isotope abundance ratio not controlled is prepared.  
      Next, as shown in  FIG. 17B , the silicon crystal layer  14   c  having a 99.9%  28 Si isotope composition ratio is epitaxially grown by, e.g., CVD. The thickness of the silicon crystal layer  14   c  is, e.g., 500 nm. As a raw material gas, a raw material gas of a 99.9%  28 Si isotope composition ratio is used. Thus, the silicon crystal layer  14   c  of, e.g., a 99.9%  28 Si isotope composition ratio is formed.  
      Next, as shown in  FIG. 17C , carbon atoms are implanted in the region of the silicon crystal layer  14   c  nearer to the surface thereof by ion implantation. At this time, carbon atoms are implanted so that a carbon concentration of in the region of the silicon crystal layer  14   c  nearer to the surface thereof is, e.g., 5×10 20  cm −3 . The carbon atoms are implanted so that the carbon concentration is decreased gradually from the surface of the silicon crystal layer  14   c  to the inside of the silicon crystal layer  14   c .  
      Then, as shown in  FIG. 17   d , the insulation film  16  of SiO 2  is formed on the surface of the silicon crystal layer  14   c  by, e.g., thermal oxidation.  
      Next, as shown in  FIG. 18A , hydrogen ions are implanted into the silicon crystal layer  14   c  through the insulation film  16 . At this time, the hydrogen ions are implanted into a region which is deeper than the region with carbon atoms implanted in. In the drawing, the region  20  with the hydrogen ions implanted in is indicated by crosses.  
      Then, as shown in  FIG. 18B , the silicon crystal substrate  22  and the base substrate  10   b  are bonded to each other through the silicon crystal layer  14   c  and the insulation film  16 .  
      Next, as shown in  FIG. 18C , the silicon crystal layer  14   c  is separated by cleavage in the region  20  with the hydrogen ions implanted in.  
      Thus, the semiconductor substrate is fabricated by the present modification.  
      As described above, the SOI substrate may be fabricated by forming the silicon crystal layer  14   c  of, e.g., a 99.9%  28 Si isotope concentration on the usual silicon crystal substrate  22  with the isotope abundance ratio not controlled, and bonding the silicon crystal layer  14   c  to the base substrate  10   b  through the insulation film  16 .  
      (Modification 2)  
      Next, Modification 2 of the method for fabricating the semiconductor substrate according to the present embodiment will be explained with reference to  FIGS. 19A  to  19 C.  FIGS. 19A  to  19 C are sectional views of the semiconductor substrate in the steps of method for fabricating the semiconductor substrate according to the present modification, which explain the method.  
      The semiconductor substrate fabricating method according to the present modification is characterized mainly in that the insulation film  16  is formed on the silicon crystal substrate  18  of a e.g., 99.9%  28 Si isotope concentration, and then carbon atoms are implanted in a region of the silicon crystal layer  18  nearer to the interface between the silicon crystal substrate  18  and the insulation film  16 .  
      The steps up to the step of forming the insulation film  16  of SiO 2  on the surface of, e.g., a 99.9%  28 Si isotope concentration including the insulation film  16  forming step are the same as those of the semiconductor substrate fabricating method described above with reference to  FIGS. 10A and 10B , and their explanation will not be repeated (see  FIGS. 19A and 19B ).  
      Next, as shown in  FIG. 19C , carbon atoms are implanted in a region of the silicon crystal substrate  18  nearer to the interface between the silicon crystal substrate  18  and the insulation film  16  through the insulation film  16 . A carbon concentration is, e.g., 5×10 20  cm 3 . Carbon atoms are implanted so that the carbon concentration is decreased gradually from the interface between the silicon crystal substrate  18  and the insulation film  16  to the inside of the silicon crystal substrate  18 .  
      The following steps are the same as those of the semiconductor substrate fabricating method described above with reference to  FIGS. 16A  to  16 C, and their explanation will not be repeated.  
      Thus, the semiconductor substrate is fabricated by the present modification.  
      As described above, it is possible that the insulation film  16  is formed on the silicon crystal substrate  18  of, e.g., a 99.9%  28 Si isotope concentration, and then carbon atoms are implanted in the silicon crystal substrate  18  through the insulation film  16 .  
      (Modification 3)  
      Next, Modification 3 of the method for fabricating the semiconductor substrate according to the present embodiment will be explained with reference to  FIGS. 20A  to  20 D.  FIGS. 20A  to  20 D are sectional views of the semiconductor substrate in the steps of method for fabricating the semiconductor substrate according to the present modification, which explain the method.  
      The semiconductor substrate fabricating method according to the present modification is characterized mainly in that the silicon crystal layer  14   c  of, e.g., a 99.9%  28 Si isotope concentration is formed on the silicon crystal substrate  22  having the Si isotope concentration not controlled, and after the insulation film  16  is formed on the silicon crystal layer  14   c , carbon atoms are implanted in the silicon crystal layer  14   c  through the insulation film  16 .  
      The steps up to the step of epitaxially growing the silicon crystal layer of a 99.9%  28 Si isotopoe composition ratio on the usual silicon crystal substrate  22  having the isotope abundance ratio not specifically controlled including the silicon crystal layer epitaxially growing step are the same as those of the semiconductor substrate fabricating method shown in  FIG. 17A and 17B , and their explanation will not be repeated (see  FIG. 20A and 20B ).  
      Next, as shown in  FIG. 20C , the insulation film  16  of SiO 2  is formed on the surface of the silicon crystal layer  14   c  by, e.g., thermal oxidation.  
      Next, as shown in  FIG. 20D , carbon atoms are implanted by ion implantation into a region of the silicon crystal layer  14   c  nearer to the interface between the silicon crystal layer  14   c  and the insulation film  16  through the insulation film  16 . At this time, carbon atoms are implanted so that a carbon concentration in the region of the silicon crystal layer nearer to the interface between the silicon crystal layer  14   c  and the insulation film  16  is, e.g., 5×10 20  cm −3 . The carbon atoms are implanted so that the carbon concentration decreases gradually from the interface between the silicon crystal layer  14   c  and the insulation film  16  to the inside of the silicon crystal layer  14   c.    
      The following steps are the same as those of the semiconductor substrate fabricating method described above with reference to  FIGS. 18A  to  18 C, and their explanation will not be repeated.  
      Thus, the semiconductor substrate according to the present modification is fabricated.  
      As described above, it is possible that the silicon crystal layer  14   c  of a 99.9%  28 Si isotope concentration is formed on the silicon crystal substrate  22  having concentration of the Si isotopes not controlled, and after the insulation film  16  has been formed on the silicon crystal layer  14   c , carbon atoms are implanted in the silicon crystal layer  14   c  through the insulation film  16 .  
      (Modification 4)  
      Next, Modification 4 of the method for fabricating the semiconductor substrate according to the present embodiment will be explained with reference to  FIGS. 21A  to  22 B.  FIGS. 21A  to  22 B are sectional views of the semiconductor substrate in the steps of method for fabricating the semiconductor substrate according to the present modification, which explain the method.  
      The semiconductor substrate fabricating method according to the present modification is characterized mainly in that the silicon crystal substrate  18  is thinned, and the silicon crystal layer  14   b  is formed of the thinned silicon crystal substrate  18 , and then carbon atoms are implanted in a region of the silicon crystal layer  14   b  nearer to the interface between the silicon crystal layer  14   b  and the insulation film  16 .  
      First, the steps up to the step of forming the insulation film  16  of SiO 2  on the surface of the silicon crystal substrate  18  of, e.g., a 99.9%  28 Si isotope concentration including the insulation film forming step are the same as those of the semiconductor substrate fabricating method described above with reference to  FIGS. 10A and 10B , and their explanation will not be repeated (see  FIGS. 21A and 21B ).  
      Next, as shown in  FIG. 21C , the silicon crystal substrate  18  and the base substrate  10   b  are bonded to each other with the insulation film  16  formed therebetween.  
      Then, as shown in  FIG. 22A , the silicon crystal substrate  18  is thinned by mechanical processing or chemical etching. Thus, the silicon crystal layer  14   b  is formed of the thinned silicon crystal substrate  18 .  
      Next, as shown in  FIG. 22B , carbon atoms are implanted in the region of the silicon crystal layer  14   b  nearer to the interface between the silicon crystal layer  14   b  and the insulation film  16  by ion implantation. The carbon concentration is, e.g., 5×10 20  cm −3 . The carbon atoms are implanted so that the carbon concentration decreases gradually from the interface between the silicon crystal layer  14   b  and the insulation film  16  to the inside of the silicon crystal layer  14   b.    
      Thus, the semiconductor substrate according to the present modification is fabricated.  
      As described above, it is possible that the silicon crystal substrate  18  is thinned, and after the silicon crystal layer  14   b  has been formed of the thinned silicon crystal substrate  18 , carbon atoms are implanted into the region of the silicon crystal layer  14   b  nearer to the interface between the silicon crystal layer  14   b  and the insulation film  16 .  
      (Modification 5)  
      Next, Modification 5 of the method for fabricating the semiconductor substrate according to the present embodiment will be explained with reference to  FIGS. 23A  to  23 C.  FIGS. 23A  to  23 C are sectional views of the semiconductor substrate in the steps of method for fabricating the semiconductor substrate according to the present modification, which explain the method.  
      The semiconductor substrate fabricating method according to the present modification is characterized mainly in that the silicon crystal layer  14   c  of a high  28 Si isotope composition ratio on the silicon crystal substrate  22  of abundance ratio of the Si isotopes not controlled, and after the silicon crystal layer  14   c  is bonded to the base substrate  10   b  with the insulation film  16  formed therebetween, carbon atoms are implanted into the region of the silicon crystal layer  14   c  nearer to the interface between the silicon crystal layer  14   c  and the insulation film  16 .  
      The steps up to the step of forming the insulation film  16  of SiO 2  on the surface of the silicon crystal layer  14   c  including the insulation film forming step are the same as those of the method for fabricating the semiconductor substrate described above with reference to  FIG. 20A  to  FIG. 20C , and their explanation will not be repeated.  
      Then, as shown in  FIG. 23A , the silicon crystal substrate  22  and the base substrate  10   b  are bonded to each other with the insulation film  16  and the silicon crystal layer  14   c  formed therebetween.  
      Next, as shown in  FIG. 23B , the silicon crystal substrate  22  is removed by mechanical processing or chemical etching.  
      Then, as shown in  FIG. 23C , carbon atoms are implanted into the region of the silicon crystal layer  14   c  nearer to the interface between the silicon crystal layer  14   c  and the insulation film  16  by ion implantation. The carbon concentration is, e.g., 5×10 20  cm −3 . The carbon atoms are implanted so that the carbon concentration is decreased gradually from the interface between the silicon crystal layer  14   c  and the insulation film  16  to the surface of the silicon crystal layer  14   c.    
      Thus, the semiconductor substrate according to the present modification is fabricated.  
      As described above, it is possible that the silicon crystal layer  14   c  of a high  28 Si isotope composition ratio is formed on the silicon crystal substrate  22  with abundance ratio of the Si isotopes not controlled, and after the silicon crystal layer  14   c  and the base substrate  10   b  are bonded to each other with the insulation film  16  formed therebetween, carbon atoms are implanted into the region of the silicon crystal layer  14   b  nearer to the interface between the silicon crystal layer  14   b  and the insulation film  16 .  
     A SEVENTH EMBODIMENT  
      The semiconductor substrate according to a seventh embodiment of the present invention and the method for fabricating the semiconductor substrate will be explained with reference to FIGS.  24  to  26 C.  FIG. 24  is a sectional view of the semiconductor substrate according to the present embodiment. The same members of the present embodiment as those of the semiconductor substrate according to the first to the sixth embodiments and the method for fabricating the semiconductor substrate shown in FIGS.  1  to  23   c  are represented by the same reference numbers not to repeat or to simplify their explanation.  
      (The Semiconductor Substrate)  
      First, the semiconductor substrate according to the present embodiment will be explained with reference to  FIG. 24 .  
      The semiconductor substrate according to the present embodiment is characterized mainly by a strained Si/SiGeOI structure having an isotope composition ratio of  28 Si of a silicon crystal layer  14   e  set high.  
      As shown in  FIG. 24 , a silicon crystal layer  14   d  of, e.g., a 30 nm-thickness is formed on a silicon crystal substrate  10   b  with an insulation film  16  of SiO 2  formed therebetween. A Si isotope abundance ratio of the silicon crystal layer  14   d  is the same as that of Si in nature.  
      A silicon germanium crystal layer  12   b  is formed on the silicon crystal layer  14   d . A thickness of the silicon germanium crystal layer  12   b  is, e.g., 200 nm. A Ge composition of the silicon germanium crystal layer  12   b  is set to increase gradually from the lower surface to the upper surface. That is, the composition of the silicon germanium crystal layer  12   b  is a graded composition. A concentration of the Ge near the lower surface of the silicon germanium crystal layer  12   b  is, e.g., 0%. A concentration of the Ge near the upper surface of the silicon germanium crystal layer  12   b  is, e.g., 30%. The Ge composition of the silicon germanium crystal layer  12   b  is set to increase gradually from the lower surface to the upper surface so that the silicon germanium crystal layer  12   b  can be epitaxially grown on the silicon crystal layer  14   d , and a composition of the Ge in the upper surface of the silicon germanium crystal layer  12   b  can be set higher. The isotope abundance ratio of the Si and the Ge of the silicon germanium crystal layer  12   b  is substantially equal to that of Si and Ge in nature.  
      The silicon crystal layer  14   e  of, e.g., a 20 nm-thickness is epitaxially grown on the silicon germanium crystal layer  12   b . An  28 Si isotope composition ratio of the silicon crystal layer  14   e  is, e.g., 99.9%. Because of different lattice constants between the upper surface of the silicon germanium crystal layer  12   b  and the silicon crystal layer  14   e , the silicon crystal layer  14   e  is crystal strained.  
      A plane orientation of the silicon crystal layer  14   e  is, e.g., {100} {113} or {011}.  
      The semiconductor substrate according to the present embodiment is characterized mainly, as described above, by the strained Si/SiGeOI structure having a  28 Si isotope composition ratio of the silicon crystal layer  14   e  set high.  
      The semiconductor substrate of such strained Si/SiGeOI structure can effectively radiate the heat because of the  28 Si isotope composition ratio of the silicon crystal layer  14   e , which is set high.  
      (The Method for Fabricating the Semiconductor Substrate)  
      Next, the method for fabricating the semiconductor substrate according to the present embodiment will be explained with reference to  FIGS. 25A  to  26 C.  FIGS. 25A  to  26 C are sectional views of the semiconductor substrate according to the present embodiment in the steps of the method for fabricating the semiconductor substrate, which explain the method.  
      First, as shown in  FIG. 25A , the usual silicon crystal substrate  22  of silicon crystals having the isotope abundance ratio not specifically controlled is prepared. The silicon crystal substrate  22  is to be thinned in a later step to form the silicon crystal layer  14   d.    
      Next, as shown in  FIG. 25B , the insulation film  16  of SiO 2  is formed on the surface of the silicon crystal substrate  22  by thermal oxidation or CVD.  
      As shown in  FIG. 25C , the base substrate  10   b  of usual silicon crystals having the isotope abundance ratio not specifically controlled is prepared.  
      Next, the silicon crystal substrate  22  and the base substrate  10   b  are bonded to each other with the insulation film  16  therebetween.  
      Next, as shown in  FIG. 26A , the silicon crystal substrate  22  is thinned by mechanical processing or chemical etching. Thus, the silicon crystal layer  14   d  is formed of the thinned silicon crystal substrate  22 .  
      Next, as shown in  FIG. 26B , the silicon germanium crystal layer  12   b  is epitaxially grown by, e.g., CVD. As a raw material gas, a raw material gas having the isotope abundance ratio of Si and Ge not specifically controlled is used. A thickness of the silicon germanium crystal layer  12   b  is, e.g., 200 nm. A composition of the Ge of the silicon germanium crystal layer  12   b  is set so as to increase gradually from the lower surface to the upper surface. A Ge concentration near the lower surface of the silicon germanium crystal layer  12   b  is, e.g., 0%, and a Ge concentration near the upper surface of the silicon germanium crystal layer  12   b  is, e.g., 30%.  
      Next, as shown in  FIG. 26C , the silicon crystal layer  14   e  of, e.g., a 20 nm-thickness is epitaxially grown on the silicon germanium crystal layer  12   b  by, e.g., CVD. As a raw material gas, a raw material gas of a  28 Si isotope composition ratio of, e.g., 99.9% is used, whereby the silicon crystal layer  14   e  of a  28 Si isotope composition ratio of, e.g., 99.9% is formed. Because of different lattice constants between the surface of the silicon germanium crystal layer  12   b  and the silicon crystal layer  14   e , crystal strains are introduced into the silicon crystal layer  14   e.    
      Thus, the semiconductor substrate according to the present embodiment can be fabricated.  
     AN EIGHTH EMBODIMENT  
      The semiconductor substrate according to an eighth embodiment of the present invention and the method for fabricating the semiconductor substrate will be explained with reference to FIGS.  27  to  28 C.  FIG. 27  is a sectional view of the semiconductor substrate according to the present embodiment. The same members of the present embodiment as those of the semiconductor substrate according to the first to the seventh embodiments and the method for fabricating the semiconductor substrate shown in FIGS.  1  to  26 C are represented by the same reference numbers not to repeat or to simplify their explanation.  
      (The Semiconductor Substrate)  
      First, the semiconductor substrate according to the present embodiment will be explained with reference to  FIG. 27 .  
      The semiconductor substrate according to the present embodiment is characterized mainly by a strained Si/SiGeOI structure having a  70 Ge isotope composition ratio of a silicon germanium crystal layer  12   c  set high.  
      As shown in  FIG. 27 , a silicon crystal layer  14   d  is formed on a base substrate  10   b  with an insulation film  16  formed therebetween.  
      A silicon germanium crystal layer  12   c  is formed on the silicon crystal layer  14   d . A  70 Ge isotope composition ratio of the silicon germanium crystal layer  12   c  is, e.g., 99.9%. An Si isotope abundance ratio of the silicon germanium crystal layer  12   c  is the same as that of Si in nature. A Ge composition of the silicon germanium crystal layer  12   c  is set to increase gradually from the lower surface to the upper surface. A Ge concentration near the lower surface of the silicon germanium crystal layer  12   c  is, e.g., 0%, and a Ge concentration near the upper surface of the silicon germanium crystal layer  12   c  is, e.g., 30%.  
      A silicon crystal layer  14   f  of, e.g., a 20 nm-thickness is formed on the silicon germanium crystal layer  12   c . Crystal strains are introduced into the silicon crystal layer  14   f . A Si isotope abundance ratio of the silicon crystal layer  14   f  is the same as that of the isotope abundance ratio of Si in nature.  
      As described above, the semiconductor substrate of the strained Si/SiGeOI structure has the  70 Ge isotope composition ratio of the silicon germanium crystal layer  12   c  set high, whereby the semiconductor substrate can effectively radiate the heat.  
      (The Method for Fabricating the Semiconductor Substrate)  
      Next, the method for fabricating the semiconductor substrate according to the present embodiment will be explained with reference to  FIGS. 28A  to  28 C.  FIGS. 28A  to  28 C are sectional views of the semiconductor substrate according to the present embodiment in the steps of the method for fabricating the semiconductor substrate, which explain the method.  
      The steps up to the step of thinning the silicon crystal substrate  22  to form the silicon crystal layer  14   d  of the thinned silicon crystal substrate  22  including the silicon crystal layer forming step are the same as those of the method for fabricating the semiconductor substrate shown in  FIGS. 25A  to  26 A, and their explanation will not be repeated (see  FIG. 28A ).  
      Next, as shown in  FIG. 28B , the silicon germanium crystal layer  12   c  is epitaxially grown by, e.g., CVD. As a raw material of the Ge, a raw material gas of a  70 Ge isotope concentration of, e.g., 99.9% is used. As a raw material gas of the Si, a usual raw material gas having the isotope abundance ratio not specifically controlled is used. A thickness of the silicon germanium crystal layer  12   c  is, e.g., 200 nm. A Ge composition of the silicon germanium crystal layer  12   c  is set to increase gradually from the lower surface to the upper surface. A Ge concentration near the lower surface is, e.g., 0%, and a Ge concentration near the upper surface of the silicon germanium crystal layer  12   c  is, e.g., 30%.  
      Next, as shown in  FIG. 28C , the silicon crystal layer  14   f  of, e.g., a 20 nm-thickness is epitaxially grown on the silicon germanium crystal layer  12   c  by, e.g., CVD. As a raw material gas, a usual raw material gas having the isotope abundance ratio not specifically controlled is used.  
      Thus, the semiconductor substrate according to the present embodiment is fabricated.  
     A NINTH EMBODIMENT  
      The semiconductor substrate according to a ninth embodiment of the present invention and the method for fabricating the semiconductor substrate will be explained with reference to FIGS.  29  to  30 C.  FIG. 29  is a sectional view of the semiconductor substrate according to the present embodiment. The same members of the present embodiment as those of the semiconductor substrate according to the first to the eighth embodiments and the method for fabricating the semiconductor substrate shown in FIGS.  1  to  28 C are represented by the same reference numbers not to repeat or to simplify their explanation.  
      (The Semiconductor Substrate)  
      First, the semiconductor substrate according to the present embodiment will be explained with reference to  FIG. 29 .  
      The semiconductor substrate according to the present embodiment is characterized mainly by a strained Si/SiGeOI structure having isotope composition ratios of both  28 Si and  70 Ge of a silicon germanium crystal layer  12   d  set high, and a  28 Si isotope composition ratio of a silicon crystal layer  14   e  set high.  
      As shown in  FIG. 29 , the silicon crystal layer  14   d  is formed on a base substrate  10   b  with an insulation film  16  therebetween. An isotope abundance ratio of the silicon crystal layer  14   d  is the same as that of Si in nature.  
      The silicon germanium crystal layer  12   d  is epitaxially grown on the silicon crystal layer  14   d . A  28 Si isotope abundance ratio of the silicon germanium crystal layer  12   d  is set to be, e.g., 99.9%. A  70 Ge isotope composition ratio of the silicon germanium crystal layer  12   d  is set to be, e.g., 99.9%.  
      The silicon crystal layer  14   e  is epitaxially grown on the silicon germanium crystal layer  12   d . A  28 Si isotope composition ratio of the silicon crystal layer  14   e  is set to be, e.g., 99.9%.  
      The semiconductor substrate according to the present embodiment is characterized in that, as described above, isotope composition ratios of  2   8 Si and  70 Ge of the silicon germanium crystal layer  12   d  are set high, and a  28 Si isotope composition ratio of the silicon crystal layer  14   e  is set high.  
      According to the present embodiment, an isotope composition ratios of  28 Si and  70 Ge of the silicon germanium crystal layer  12   d  are set high, and a  28 Si isotope composition ratio of the silicon crystal layer  14   e  is set high, whereby thermal conductivities of both the silicon germanium crystal layer  12   d  and the silicon crystal layer  14   e  can be high. Thus, the semiconductor substrate according to the present embodiment can have the strained Si/SiGeOI structure which can effectively radiate the heat.  
      (The Method for Fabricating the Semiconductor Substrate)  
      Next, the method for fabricating the semiconductor substrate according to the present embodiment will be explained with reference to  FIGS. 30A  to  30 C.  FIGS. 30A  to  30 C are sectional views of the semiconductor substrate according to the present embodiment in the steps of the method for fabricating the semiconductor substrate, which explain the method.  
      First, the steps up to the step of thinning the silicon crystal substrate  22  by mechanical processing or chemical processing to form the silicon crystal substrate  22  of the thinned silicon crystal substrate  22  including the silicon crystal layer forming step are the same as those of the semiconductor substrate fabricating method shown in  FIGS. 25A  to  26 A, and their explanation will not be repeated (see  FIG. 30A ).  
      Then, as shown in  FIG. 30B , the silicon germanium crystal layer  12   d  is epitaxially grown by, e.g., CVD. As a raw material gas of the Ge, a raw material gas having a  70 Ge isotope concentration of, e.g., 99.9% is used. As a raw material gas of the Si, a raw material gas having a  28 Si isotope composition ratio of, e.g., 99.9% is used. A thickness of the silicon germanium crystal layer  12   d  is, e.g., 200 nm. A composition of the Ge of the silicon germanium crystal layer  12   d  is set to increase gradually from the lower surface to the upper surface. A Ge concentration near the lower surface of the silicon germanium crystal layer  12   d  is, e.g., 0%, and a Ge concentration near the upper surface of the silicon germanium crystal layer  12   c  is, e.g., 30%.  
      Then, as shown in  FIG. 30C , the silicon crystal layer  14   e  of, e.g., a 20 nm-thickness is epitaxially grown on the silicon germanium crystal layer  12   c  by, e.g., CVD. As a raw material gas, a raw material gas having a  28 Si isotope composition ratio of, e.g., 99.9 % is used.  
      Thus, the semiconductor substrate according to the present embodiment is fabricated.  
     A TENTH EMBODIMENT  
      The semiconductor substrate according to a tenth embodiment of the present invention and the method for fabricating the semiconductor substrate will be explained with reference to FIGS.  31  to  33 C.  FIG. 31  is a sectional view of the semiconductor substrate according to the present embodiment. The same members of the present embodiment as those of the semiconductor substrate according to the first to the ninth embodiments and the method for fabricating the substrate shown in FIGS.  1  to  30 C are represented by the same reference numbers not to repeat or to simplify their explanation.  
      (The semiconductor Substrate)  
      First, the semiconductor substrate according to the present embodiment will be explained with reference to  FIG. 31 .  
      The semiconductor substrate according to the present embodiment is characterized mainly by a strained Si/SiGeOI structure including an insulation film  16   a  formed by SIMOX (Separation by IMplantation OXygen).  
      As shown in  FIG. 31 , the insulation film  16   a  of SiO 2  is buried in the base substrate  10   b  of usual silicon crystals having the isotope abundance ratio not controlled. The insulation film  16   a  is buried in a region which is, e.g., 150 nm deep from the surface of the base substrate  10   b . The insulation film  16   a  is formed by SIMOX. That is, the insulation film  16   a  is formed by implanting oxygen ions into the base substrate  10   b  and thermally processing the base substrate  10   b . Accordingly, the concentration distribution of the oxygen near the interface between the insulation film  16   a  and the base substrate  10   b  is smoother than that of the semiconductor substrate fabricated by the bonding described above.  
      A silicon crystal layer  14   g  is formed on the insulation film  16   a .  
      A silicon germanium crystal layer  12   e  of, e.g., a 200 nm-thickness is formed on the silicon crystal layer  14   g . A  28 Si isotope composition ratio of the silicon germanium crystal layer  12   e  is, e.g., 99.9 %. An isotope abundance ratio of the Ge of the silicon germanium crystal layer  12   e  is the same as that of Ge in nature. A Ge composition of the silicon germanium crystal layer  12   e  is set to increase gradually from the lower surface to the upper surface. A concentration of the Ge near the lower surface of the silicon germanium crystal layer  12   e  is, e.g., 0%, and a concentration of the Ge near the upper surface of the silicon germanium crystal layer  12   e  is, e.g., 30%.  
      The silicon crystal layer  14   e  of, e.g., a 20 nm-thickness is epitaxially grown on the silicon germanium crystal layer  12   e . A  28 Si isotope composition ratio of the silicon crystal layer  14   e  is, e.g., 99.9%. Crystal strains are introduced into the silicon crystal layer  14   e.    
      The semiconductor substrate according to the present embodiment is characterized mainly, as described above, by the strained Si/SiGeOI structure whose insulation film  16   a  is formed by SIMOX.  
      Even in a case that the insulation film  16   a  is formed by SIMOX, because of the silicon germanium crystal layer  12   e  and the silicon crystal layer  14   e  having the  28 Si isotope composition ratios set high, the thermal conductivities of the silicon germanium crystal layer  12   e  and the silicon crystal layer  14   e  can be high. Thus, the semiconductor substrate according to the present embodiment can effectively radiate the heat.  
      (The Method for Fabricating the Semiconductor Substrate)  
      Next, the method for fabricating the semiconductor substrate according to the present embodiment will be explained with reference to  FIGS. 32A  to  33 C.  FIGS. 32A  to  33 C are sectional views of the semiconductor substrate according to the present embodiment in the steps of the method for fabricating the semiconductor substrate, which explain the method.  
      As shown in  FIG. 32A , the base substrate  10   b  of usual silicon crystals having the isotope abundance ratio not controlled is prepared.  
      Next, as shown in  FIG. 32B , oxygen ions are implanted into the entire surface at the surface of the base substrate  10   b  by ion implantation. Ion implantation conditions are, e.g., 180 keV acceleration energy and a 4×10 17  cm −2  dose. In the drawing, the region  24  with the oxygen ions implanted in is indicated by circles.  
      Then, as shown in  FIG. 32C , thermal processing is performed in an atmosphere of argon gas and oxygen gas, at 1350° C. and for 5 hours. Thus, the insulation film  16   a  of SiO 2  is formed in the region  24  with oxygen ions implanted in. The insulation film  16   a  isolates the silicon crystal layer  14   a  and the base substrate  10   b  from each other. A silicon oxide film  26  is formed on the surface of the silicon crystal layer  14   a.    
      Next, as shown in  FIG. 33A , the silicon oxide film  26  on the surface of the silicon crystal layer  14   a  is etched off.  
      Thus, the SOI substrate  10   c  with the insulation film  16   a  buried in is formed by SIMOX.  
      Then, as shown in  FIG. 33B , the silicon germanium crystal layer  12   e  is epitaxially grown on the entire surface by, e.g., CVD. As a raw material gas of the Si, a raw material gas of a  28 Si isotope composition ratio of, e.g., 99.9% is used. As a raw material gas of the Ge, a usual raw material gas having the isotope abundance ratio of Ge not specifically controlled is used. Thus, the silicon germanium crystal layer  12   e  having a  28 Si isotope composition ratio of, e.g., 99.9% is formed. A thickness of the silicon germanium crystal layer is, e.g., 200 nm. A germanium composition of the silicon germanium crystal layer is set to increase gradually from the lower surface to the upper surface. A concentration of the Ge near the lower surface of the silicon germanium crystal layer is, e.g., 0%, and a concentration of the Ge near the upper surface of the silicon germanium crystal layer is, e.g., 30%.  
      Next, as shown in  FIG. 33C , the silicon crystal layer  14   e  of, e.g., a 20 nm-thickness is epitaxially grown on the silicon germanium crystal layer  12   e  by, e.g., CVD. As a raw material gas, a raw material gas of a  28 Si isotope composition ratio of, e.g., 99.9% is used. Thus, the silicon crystal layer  14   e  of a  28 Si composition ratio of, e.g., 99.9% is formed. Because of the lattice constant difference between the surface of the silicon germanium crystal layer  12   e , crystal strains are introduced into the silicon crystal layer  14   e.    
      Thus, the semiconductor substrate according to the present embodiment is fabricated.  
      (Modification 1)  
      Next, Modification 1 of the method for fabricating the semiconductor substrate according to the present embodiment will be explained with reference to  FIGS. 34A  to  35 C.  FIGS. 34A  to  35 C are sectional views of the semiconductor substrate in the steps of method for fabricating the semiconductor substrate according to the present modification, which explain the method.  
      The semiconductor substrate fabricating method according to the present modification is characterized mainly in that after a silicon germanium crystal layer  12   e  and a silicon crystal layer  14   e  have been formed on a base substrate  10   b , an insulation film  16   a  is formed in the base substrate  12   b  by SIMOX.  
      First, as shown in  FIG. 34A , the base substrate  10   b  of usual silicon crystals having the isotope abundance ratio not controlled is prepared.  
      Next, in the same way as in the method for fabricating the semiconductor substrate described above with reference to  FIG. 33B , the silicon germanium crystal layer  12   e  is epitaxially grown on the base substrate  10   b  (see  FIG. 34B ).  
      Then, in the same way as in the method for fabricating the semiconductor device described above with reference to  FIG. 33C , the silicon crystal layer  14   e  is epitaxially grown on the silicon germanium crystal layer  12   e  (see  FIG. 34C ).  
      Then, as shown in  FIG. 35A , oxygen ions are implanted into the base substrate  10   b  through the silicon crystal layer  14   e  and the silicon germanium crystal layer  12   e . The oxygen ions are implanted in a region which is about 400 nm deep from the surface of the silicon crystal layer  14   e . Ion implantation conditions are, e.g., a 180 keV acceleration energy and a 4×10 17  cm −2  dose. The region  24  with the oxygen ions implanted in is indicated by circles.  
      Then, as shown in  FIG. 35B , thermal processing is performed in an atmosphere containing argon gas and oxygen gas, at 1350° C. and for 5 hours. Thus, the insulation film  16   a  of SiO 2  is formed in the region  24  with the oxygen ions implanted in. A silicon oxide film  28  is formed on the silicon germanium crystal layer  12   e.    
      Next, as shown in  FIG. 35   c , the silicon oxide film  28  formed on the silicon germanium crystal layer  12   e  is etched off.  
      Thus, the semiconductor substrate according to the present modification is fabricated.  
      As described above, it is possible that the insulation film  16   a  is buried in the base substrate  10   b  by SIMOX after the silicon germanium layer  12   e  and the silicon crystal layer  14   e  have been formed on the base substrate  10   b.    
      (Modification 2)  
      Next, Modification 2 of the method for fabricating the semiconductor substrate according to the present embodiment will be explained with reference to  FIGS. 36A  to  37 C.  FIGS. 36A  to  37 C are sectional views of the semiconductor substrate in the steps of method for fabricating the semiconductor substrate according to the present modification, which explain the method.  
      The semiconductor substrate fabricating method according to the present modification is characterized mainly in that after the silicon germanium crystal layer  12   e  has been formed on the base substrate  10   b , the insulation film  16   b  is buried in the base substrate  10   b  by SIMOX, and then the silicon crystal layer  14   e  is formed on the silicon germanium crystal layer  12   e.    
      First, as shown in  FIG. 36A , the base substrate  10   b  of usual silicon crystals having the isotope abundance ratio not controlled is prepared.  
      Next, in the same way as in the semiconductor substrate fabricating method described above with reference to  FIG. 33B , the silicon germanium crystal layer  12   e  is formed (see  FIG. 36B ).  
      Next, as shown in  FIG. 36C , oxygen ions are implanted in the base substrate  10   b  over the entire surface through the silicon germanium crystal layer  12   e  by ion implantation. The oxygen ions are implanted in a region which is, e.g., 400 nm deep from the surface of the silicon germanium crystal layer  12   e . Ion implantation conditions are, e.g., a 180 keV acceleration energy, and a 4×10 17  cm −2  dose. In the drawing, the region  24  with the oxygen ions implanted in is indicated by circles.  
      Next, as shown in  FIG. 37A , thermal processing is performed in an atmosphere containing argon gas and oxygen gas, at 1350° C. and a 4×10 17  cm −2 . Thus the insulation film  16   a  of SiO 2  is formed in the base substrate  10   b  with the oxygen ions implanted in. A silicon oxide film  30  is formed on the silicon germanium crystal layer  12   e.    
      Next, as shown in  FIG. 37B , the silicon oxide film  30  formed on the silicon germanium crystal layer  12   e  is etched off.  
      Next, in the same way as in the method for fabricating the semiconductor substrate described above with reference to  FIG. 33C , the silicon crystal layer  14   e  is epitaxially grown on the silicon germanium crystal layer  12   e  (see  FIG. 37C ).  
      Thus, the semiconductor substrate according to the present modification is fabricated.  
      As described above, it is possible that after the silicon germanium crystal layer  12   e  has been formed on the base substrate  10   b , the insulation film  16   a  is buried in the base substrate  10   b  by SIMOX, and then the silicon crystal layer  14   e  is formed on the silicon germanium crystal layer  12   e.    
     AN ELEVENTH EMBODIMENT  
      The semiconductor substrate according to an eleventh embodiment of the present invention and the method for fabricating the semiconductor substrate will be explained with reference to FIGS.  38  to  39 C.  FIG. 38  is a sectional view of the semiconductor substrate according to the present embodiment. The same members of the present embodiment as those of the semiconductor substrate according to the first to the tenth embodiments shown in FIGS.  1  to  37 C will be represented by the same reference numbers not to repeat or to simplify their explanation.  
      (The Semiconductor Substrate)  
      First, the semiconductor substrate according to the present embodiment will be explained with reference to  FIG. 38 .  
      The semiconductor substrate according to the present embodiment is characterized mainly by a strained Si/SiGe structure including a insulation film  16   a  formed by SIMOX, and a 70 Ge isotope composition ratio of a silicon germanium crystal layer  12   c  is set high.  
      As shown in  FIG. 38 , the silicon germanium crystal layer  12   c  is formed on a silicon crystal layer  14   g . A  70 Ge isotope composition ratio of the silicon germanium crystal layer  12   c  is, e.g., 99.9%. An isotope abundance ratio of the Si of the silicon germanium crystal layer  12   c  is the same as that of Si in nature. A Ge composition of the silicon germanium crystal layer  12   c  is set to increase gradually from the lower surface to the upper surface. A concentration of the Ge near the lower surface of silicon germanium crystal layer  12   c  is, e.g., 0%, and that of the Ge near the upper surface of the silicon germanium crystal layer  12   c  is, e.g., 30%.  
      A silicon crystal layer  14   f  is formed on the silicon germanium crystal layer  12   c . An isotope abundance ratio of the Si of the silicon crystal layer  14   f  is the same as that of Si in nature.  
      The semiconductor substrate according to the present embodiment is characterized mainly, as described above, by the strained Si/SiGe structure including the insulation film  16   a  formed by SIMOX and having a  70 Ge isotope composition ratio of the silicon germanium crystal layer  12   c  set high.  
      In the present embodiment, because of a  70 Ge isotope composition ratio of the silicon germanium crystal layer  12   c  set high, the silicon germanium crystal layer  12   c  can have high thermal conductivity. As described above, in the semiconductor substrate of the strained Si/SiGe structure, the semiconductor substrate can have high thermal conductivity by setting a  70 Ge isotope composition ratio high.  
      (The Method for Fabricating the Semiconductor Substrate)  
      Next, the method for fabricating the semiconductor substrate according to the present embodiment will be explained with reference to  FIGS. 39A  to  39 C.  FIGS. 39A  to  39 C are sectional views of the semiconductor substrate according to the present embodiment in the steps of the method for fabricating the semiconductor substrate.  
      The steps up to the step of fabricating the SOI substrate  10   c  with the insulation film  16   a  formed by SIMOX buried in including the SOI substrate  10   c  fabricating step are the same as those of the semiconductor substrate fabricating method described above with reference to  FIGS. 32A  to  33 A, and their explanation will be not be repeated (see  FIG. 39A ).  
      Then, in the same way as in the method for fabricating the semiconductor substrate described above with reference to  FIG. 28B , the silicon germanium crystal layer  12   c  is epitaxially grown on the silicon crystal layer  14   g.    
      Next, in the same way as in the method for fabricating the semiconductor substrate described above with reference to  FIG. 28C , the silicon crystal layer  14   f  is epitaxially grown on the silicon germanium crystal layer  12   c.    
      Thus, the semiconductor substrate according to the present embodiment can be fabricated.  
      (Modification 1)  
      Next, Modification 1 of the method for fabricating the semiconductor substrate according to the present embodiment will be explained with reference to  FIGS. 40A  to  41 C.  FIGS. 40A  to  41 C are sectional views of the semiconductor substrate in the steps of method for fabricating the semiconductor substrate according to the present modification, which explain the method.  
      The semiconductor substrate fabricating method according to the present modification is characterized mainly in that after the silicon germanium crystal layer  12   c  and the silicon crystal layer  14   f  are formed on the base substrate, the insulation film  16   a  is buried in the base substrate  10   b  by SIMOX.  
      First, as shown in  FIG. 40A , the base substrate  10   b  of usual silicon crystals having the isotope abundance ratio not controlled is prepared.  
      Then, in the same way as in the method fabricating the semiconductor substrate described above with reference to  FIG. 28B , the silicon germanium crystal layer  12   c  is epitaxially grown on the base substrate  10   b  (see  FIG. 40B ).  
      Next, in the same way as in the method for fabricating the semiconductor substrate described above with reference to  FIG. 28C , the silicon crystal layer  14   f  is epitaxially grown on the silicon germanium crystal layer  12   c  (see  FIG. 40C ).  
      Next, as shown in  FIG. 41A , oxygen ions are implanted in the entire surface at the surface of the silicon crystal layer  14   f  by ion implantation. The oxygen ions are implanted in a region which is, e.g., about 400 nm deep from the surface of the silicon crystal layer  14   f . Ion implantation conditions are, e.g., a 180 keV acceleration energy and a 4×10 17  cm −2  dose. In the drawing, the region  24  with the oxygen ions implanted in is indicated by circles.  
      Next, as shown in  FIG. 41B , thermal processing is performed in an atmosphere containing argon gas and oxygen gas, at 1350° C. and 5 hours. Thus, the insulation film  16   a  of SiO 2  is formed in the region  24  with the oxygen ions implanted in. A silicon oxide film  32  is formed on the silicon germanium crystal layer  14   f.    
      Then, as shown in  FIG. 41C , the silicon oxide film  32  formed on the silicon germanium crystal layer  14   f  is etched off.  
      Thus, the semiconductor substrate according to the present modification is fabricated.  
      As described above, it is possible that after the silicon germanium crystal layer  12   c  and the silicon crystal layer  14   f  have been formed on the base substrate  10   b , the insulation film  16   a  may be buried in the base substrate  10   b  by SIMOX.  
      (Modification 2)  
      Next, Modification 2 of the method for fabricating the semiconductor substrate according to the present embodiment will be explained with reference to  FIGS. 42A  to  43 C.  FIGS. 42A  to  43 C are sectional views of the semiconductor substrate in the steps of method for fabricating the semiconductor substrate according to the present modification, which explain the method.  
      The semiconductor fabricating method according to the present modification is characterized mainly in that after the silicon germanium crystal layer  12   c  has been formed on the base substrate  10   b , the insulation film  16   a  is buried in the base substrate  10   b  by SIMOX, and then the silicon crystal layer  14   f  is formed on the silicon germanium crystal layer  12   c.    
      First, as shown in  FIG. 42A , the base substrate  10   b  of usual silicon crystals having the isotope abundance ratio not controlled is prepared.  
      Next, in the same way as in the semiconductor fabricating method described above with reference to  FIG. 28B , the silicon germanium crystal layer  12   c  is epitaxially formed on the base substrate  10   b  (see  FIG. 42B ).  
      Next, as shown in  FIG. 42C , oxygen ions are implanted in the base substrate  10   b  over the entire surface through the silicon germanium crystal layer  12   c  by ion implantation. The oxygen ions are implanted in a region which is, e.g., 400 nm deep from the surface of the silicon germanium crystal layer  12   c . Ion implantation conditions are, e.g., a 180 keV acceleration energy and a 4×10 17  cm −2  dose. In the drawing, the region  24  with the oxygen ions implanted in is indicated by circles.  
      Then, as shown in  FIG. 43A , thermal processing is performed in an atmosphere containing argon gas and oxygen gas, at 1350° C. and for 5 hours. Thus, the insulation film  16   a  of SiO 2  is formed in the base substrate  10   b  with the oxygen ions implanted in. A silicon oxide film  30  is formed on the silicon germanium crystal layer  12   c.    
      Next, as shown in  FIG. 43B , the silicon oxide film  30  formed on the silicon germanium crystal layer  12   c  is etched off.  
      Then, in the same way as in the semiconductor substrate fabricating method described above with reference to  FIG. 28C , the silicon crystal layer  14   f  is epitaxially grown on the silicon germanium crystal layer  12   c.    
      Thus, the semiconductor substrate according to the present modification can be fabricated.  
      As described above, it is possible that after the silicon germanium crystal layer  12   c  has been formed on the base substrate  10   b , the insulation film  16   a  is buried in the base substrate  10   b  by SIMOX, and then the silicon crystal layer  14   f  is formed on the silicon germanium crystal layer  12   c.    
     A TWELFTH EMBODIMENT  
      The semiconductor substrate according to a twelfth embodiment of the present invention and the method for fabricating the semiconductor substrate will be explained with reference to FIGS.  44  to  45 C.  FIG. 44  is a sectional view of the semiconductor substrate according to the present embodiment. The same members of the present embodiment as those of the semiconductor substrate according to the first to the eleventh embodiments and the method for fabricating the semiconductor device shown in FIGS.  1  to  43 C are represented by the same reference numbers not to repeat or to simplify their explanation.  
      (The Semiconductor Substrate)  
      First, the semiconductor substrate according to the present embodiment will be explained with reference to  FIG. 44 .  
      The semiconductor substrate according to the present embodiment is characterized mainly by a strained Si/SiGe structure having an insulation film  16   a  buried in a base substrate  10   b  by SIMOX, having a  28 Si isotope composition ratio and a  70 Ge isotope composition ratio of a silicon germanium crystal layer  12   d  set high, and having a  28 Si isotope composition ratio of a silicon crystal layer  14   e  set high.  
      Then, as shown in  FIG. 44 , the silicon germanium crystal layer  12   d  is formed on the silicon crystal layer  14   g . A  70 Ge isotope composition ratio of the silicon germanium crystal layer  12   d  is, e.g., 99.9%. A  28 Si isotope composition ratio of the silicon germanium crystal layer  12   d  is, e.g., 99.9%. A Ge composition of the silicon germanium crystal layer  12   d  is set to increase gradually from the lower surface to the upper surface. A Ge concentration near the lower surface of the silicon germanium crystal layer  12   d  is, e.g., 0%, and a Ge concentration near the upper surface of the silicon germanium crystal layer  12   d  is, e.g., 30%.  
      The silicon crystal layer  14   e  is formed on the silicon germanium crystal layer  12   d . A  28 Si isotope composition ratio of the silicon crystal layer  14   e  is, e.g., 99.9%. Crystal strains are introduced into the silicon crystal layer  14   e.    
      The semiconductor substrate according to the present embodiment is characterized mainly, as described above, by the strained Si/SiGe structure having the insulation film  16   a  buried in the base substrate  10   b  by SIMOX, and having both a  28 Si isotope composition ratio and a  70 Ge isotope composition ratio of the silicon germanium crystal layer  12   d  set high and having a  28 Si isotope composition ratio of the silicon crystal layer  14   e  set high.  
      According to the present embodiment, both a  28 Si isotope composition ratio and a  70 Ge isotope composition ratio of the silicon germanium crystal layer  12   d  are set high, and a  28 Si isotope composition ratio of the silicon crystal layer  14   e  is set high, whereby both the silicon germanium crystal layer  12   d  and the silicon crystal layer  14   e  can have high thermal conductivity. Accordingly, the semiconductor substrate according to the present embodiment can effectively radiate the heat.  
      (The Method for Fabricating the Semiconductor Substrate)  
      Then, the method for fabricating the semiconductor substrate according to the present embodiment will be explained with reference to  FIGS. 45A  to  45 C.  FIGS. 45A  to  45 C are sectional views of the semiconductor substrate according to the present embodiment in the steps of the method for fabricating the semiconductor substrate, which explain the method.  
      First, the steps up to the step of fabricating by SIMOX the SOI substrate  10   c  with the insulation film  16   a  buried in including the SOI substrate fabricating step are the same as those of the semiconductor fabricating method described above with reference to  FIGS. 32A  to  33 A, and their explanation will not be repeated (see  FIG. 45A ).  
      Then, in the same way as in the semiconductor substrate fabricating method described above with reference to  FIG. 30B , the silicon germanium crystal layer  12   d  is epitaxially grown on the silicon crystal layer  14   g.    
      Then, in the same way as in the semiconductor substrate fabricating method described above with reference to  FIG. 30C , the silicon crystal layer  14   e  is epitaxially grown on the silicon germanium crystal layer  12   d.    
      Thus, the semiconductor substrate according to the present embodiment is fabricated.  
      (Modification 1)  
      Next, Modification 1 of the method for fabricating the semiconductor substrate according to the present embodiment will be explained with reference to  FIGS. 46A  to  47 C.  FIGS. 46A  to  47 C are sectional views of the semiconductor substrate in the steps of method for fabricating the semiconductor substrate according to the present modification, which explain the method.  
      The semiconductor fabricating method according to the present modification is characterized mainly in that after the silicon germanium crystal layer  12   d  and the silicon crystal layer  14   e  have been formed on the base substrate  10   b , the insulation film  16   a  is buried in the base substrate  10   b  by SIMOX.  
      First, as shown in  FIG. 46A , the base substrate  10   b  of usual silicon crystals having the isotope abundance ratio not controlled is prepared.  
      Then, in the same way as in the semiconductor substrate fabricating method described above with reference to  FIG. 30B , the silicon germanium crystal layer  12   d  is epitaxially grown on the base substrate  10   b.    
      Next, in the same way as in the semiconductor substrate fabricating method described above with reference to  FIG. 30C , the silicon crystal layer  14   e  is epitaxially grown on the silicon germanium crystal layer  12   d.    
      Next, as shown in  FIG. 47A , oxygen ions are implanted into the base substrate  10   b  over the entire surface through the silicon crystal layer  14   e  and the silicon germanium crystal layer  14   e  by ion implantation. The oxygen ions are implanted in a region which is about 400 nm deep from the surface of the silicon crystal layer  14   e . Ion implantation conditions are, e.g., a 180 keV acceleration energy and a 4×10 17  cm −2  dose. The region  24  with the oxygen ions implanted in is indicated by circles.  
      Next, as shown in  FIG. 47B , thermal processing is performed in an atmosphere containing argon gas and oxygen gas, at 1350° C. and for 5 hours. Thus, the insulation film  16   a  of SiO 2  is formed in the base substrate  10   b  with the oxygen ions implanted in. A silicon oxide film  32  is formed on the silicon crystal layer  14   e.    
      The following semiconductor fabricating process is the same as that of the semiconductor substrate fabricating method described above with reference to  FIG. 41C , and its explanation will not be repeated (see  FIG. 47C ).  
      Thus, the semiconductor substrate according to the present modification is fabricated.  
      As described above, it is possible that after the silicon germanium crystal layer  12   d  and the silicon layer  14   e  have been formed on the base substrate  10   b , the insulation film  16   a  is buried in the base substrate  12   b  by SIMOX.  
      (Modification 2)  
      Next, Modification 2 of the method for fabricating the semiconductor substrate according to the present embodiment will be explained with reference to  FIGS. 48A  to  49 C.  FIGS. 48A  to  49 C are sectional views of the semiconductor substrate in the steps of method for fabricating the semiconductor substrate according to the present modification, which explain the method (Part 2).  
      The semiconductor substrate fabricating method according to the present modification is characterized mainly in that after the silicon germanium crystal layer  12   d  has been formed on the base substrate  10   b , the insulation film  16   a  is formed in the base substrate  10   b  by SIMOX, and then the silicon crystal layer  14   e  is formed on the silicon germanium crystal layer  12   d.    
      First, as shown in  FIG. 48A , the base substrate  10   b  of usual silicon crystals having the isotope abundance ratio not controlled is prepared.  
      Next, in the same way as in the semiconductor substrate fabricating method described above with reference to  FIG. 30B , the silicon germanium crystal layer  12   d  is epitaxially grown on the base substrate  10   b.    
      Next, as shown in  FIG. 30C , oxygen ions are implanted in the base substrate  10   b  over the entire surface through the silicon germanium crystal layer  12   d  by ion implantation. The oxygen ions are implanted in a region which is, e.g., 400 nm deep from the surface of the silicon germanium crystal layer  12   d . Ion implantation conditions are, e.g., a 180 keV acceleration energy and a 4×10 17  cm −2  dose. In the drawing, the region  24  with the oxygen ions implanted in is indicated by circles.  
      Then, as shown in  FIG. 49A , thermal processing is performed in an atmosphere containing argon gas and oxygen gas, at 1350° C. and for 5 hours. Thus, the insulation film  16   a  of SiO 2  is formed in the base substrate  10   b  with the oxygen ions implanted in. A silicon oxide film  30  is formed on the silicon germanium crystal layer  12   d.    
      Next, as shown in  FIG. 49B , the silicon oxide film  30  formed on the silicon germanium crystal layer  12   d  is etched off.  
      Then, as shown in  FIG. 49C , the silicon crystal layer  14   e  is epitaxially grown on the silicon germanium crystal layer  12   d,    
      Thus, the semiconductor substrate according to the present modification can be fabricated.  
      As described above, it is possible that after the silicon germanium crystal layer  12   d  has been formed on the base substrate  10   b , the insulation film  16  is buried in the base substrate  10   b  by SIMOX, and then the silicon crystal layer  14   e  is formed on the silicon germanium crystal layer  12   d.    
     MODIFIED EMBODIMENTS  
      The present invention is not limited to the above-described embodiments and can cover other various modifications.  
      For example, in the above-described embodiments, the  28 Si isotope composition ratio is set high but is not essentially set high. The  29 Si or  30 Si isotope composition ratios may be set high. That is, the isotope composition ratio of any one of  28 Si,  29 Si and  3 Si is set high, whereby the thermal conductivity can be higher.  
      In the above-described embodiments, the isotope composition of  70 Ge is set high but is not essentially set high. The isotope composition ratios of  72 Ge,  73 Ge,  74 Ge or  76 Ge may be set high. The isotope composition ratio of any one of  70 Ge,  72 Ge,  73 Ge,  74 Ge and  76 Ge is set high, whereby the thermal conductivity can be higher.  
      In the above-described embodiments, the isotope composition ratio of any one of the Si isotopes and the isotope composition ratio of any one of the Ge isotopes are set to be 99.9% but are not essentially set to be 99.9%. They may be set sothatarequiredthermalconductivitycanbeobtained. However, the isotope composition ratios are set to be above 95%, whereby the thermal conductivity can be much increased. It is preferable to set the isotope compositions ratios to be above 95%. The isotope composition ratios are set to be above 98%, whereby the thermal conductivity can be more increased. It is preferable to set the isotope composition ratios to be above 98%.  
      In the above-described embodiments, the composition of the silicon germanium crystal layers  12 ,  12   a  is Si 0.7 Ge 0.3  but is not essentially Si 0.7 Ge 0.3 . The compositions of the silicon germanium crystal layers  12 ,  12   a  are suitably set so that the silicon crystal layers  14 ,  14   a  formed on the silicon germanium layers  14 ,  14   a  are suitably strained.  
      In the above-described embodiments, the plane orientation of the surface of the silicon crystal layer is {100}, {113} or {011}. The plane orientation of the silicon crystal layer is not essentially {100}, {113} or {011} and may be suitably set.  
      In the above-described embodiments, in the silicon crystal layer and the silicon germanium crystal layer, the isotope composition ratio of any one of the Si isotopes and the isotope composition ratio of any one of the Ge isotopes are set high. However, in the silicon crystal substrate and the silicon germanium crystal substrate, the isotope composition ratio of any one of the Si isotope composition ratios and the isotope composition ratio of any one of the Ge isotope composition ratios may be set high. That is, in at least any one of the silicon crystal layer, the silicon germanium crystal layer, the silicon crystal substrate and the silicon germanium crystal substrate, the isotope composition ratio of any one of the Si isotopes, the isotope composition ratio of any one of the Ge isotopes are set high, whereby the semiconductor substrate can effectively radiate the heat. In all of the silicon crystal layer, the silicon germanium crystal layer, the silicon crystal substrate, etc., the isotope composition ratio of any one of the Si isotopes and the isotope composition ratio of any one of the Ge isotopes may be set high, whereby the semiconductor substrate can more effectively radiate the heat.  
      In the seventh and the eleventh embodiments, the insulation film is formed below the silicon germanium crystal layer but is not essentially formed below the silicon germanium crystal layer. For example, the insulation film may be buried in the silicon germanium crystal layer.