Abstract:
The invention is intended to form, on an insulating layer, a thin SiGe layer serving as an underlying layer for obtaining a strained silicon layer, and to provide a satisfactory strained Si layer. A SiGe layer  13  is formed on a Si substrate  11  and an oxygen ion implantation is effected with ensuring the detainment within the layer thickness of the SiGe layer  13 . The SiGe layer  13  is lattice-relaxed by a heat treatment and a buried insulating layer  15  is formed simultaneously in the SiGe layer  13 . A strained Si layer  17  is re-grown on the lattice-relaxed SiGe layer  13.

Description:
FIELD OF THE INVENTION 
     The present invention relates to a semiconductor device having a strained Si layer and a method of producing a semiconductor device having a strained Si layer. 
     BACKGROUND ART 
     Various semiconductor devices employing silicon crystals are used widely. To enhance mobility of an electron that runs through a silicon crystal make performance of such a semiconductor device enhance effectively. 
     However, since the upper limit of the mobility of an electron that runs through a silicon crystal depends on the physical characteristic of a silicon crystal, any change in the structure of a semiconductor device can&#39;t change the proper mobility that the silicon crystal has own. Nevertheless, it is reported that the mobility of an electron can be enhanced in a strained silicon crystal that is obtained by straining a usual silicon crystal. 
     Generally the strained Si layer is produced by growing a thin silicon crystal layer, whose thickness is thinner than the thickness that the crystal is lattice relaxed, on a crystal having a lattice constant that is slightly different from the lattice constant of the silicon crystal. Typically, a SiGe alloy crystal layer whose Ge content is about 20% (where the lattice constant of the SiGe crystal is larger by about 0.8% than the lattice constant of the silicon crystal) is provided as an underlying layer, and then the thin silicon layer having thickness of 100 nm or less on the SiGe layer. 
     However, because it is difficult to obtain an SiGe crystal substrate which is produced at an industrial large scale and which is not expensive but of a high quality, a silicon wafer is usually employed as a substrate and an SiGe layer is grown by a vapor phase growth on the silicon wafer to a thickness lattice-relaxed (critical film thickness). So, it can obtain a lattice-relaxed SiGe underlying layer. 
     Nevertheless, Since in this method the SiGe crystal layer whose Ge content is about 20% grows on the Si substrate directly, a lot of defects such as the dislocation that is yielded when the SiGe crystal layer is lattice-relaxed become nucleuses that make the dislocation penetrate to a strained silicon layer growing thereon. 
     In an attempt to prevent the defects in a SiGe layer upon the lattice relaxation, a buffer layer is formed between a silicon substrate and a lattice-relaxed SiGe layer. Such buffer layer is generally a sufficiently thick SiGe layer having the composition similar to the lattice-relaxed SiGe layer (similar lattice constant) or a gradient SiGe layer having the composition Ge to gradually increase to the lattice-relaxed SiGe layer. 
     However, since total thickness of the buffer layer and the lattice-relaxed SiGe layer is extremely thick layer, it may make following process difficult. For example, in a case where the devices are integrated, fine devices should be separated from each other, but a SiGe layer having a thickness of 1 μm or more is too thick to separate the devices from each other. Also in an SOI (silicon-on-insulator) technology expected to be capable of reducing the junction capacity, since a SiGe layer (combined with the buffer layer) having a thickness of 1 μm or more is too thick, it makes junction capacity of a device increase. 
     Thus, it is difficult to obtain a satisfactory strained Si layer unless a thick lattice-relaxed SiGe layer is formed in combination with a buffer layer, so it is difficult to separate devices and to decrease the junction capacity of a device. 
     SUMMARY OF THE INVENTION 
     An object of the invention is to provide a semiconductor device and a method of producing a semiconductor device capable of forming a thin lattice-relaxed SiGe layer on an oxide layer and also capable of forming on this lattice-relaxed SiGe layer a satisfactory strained Si layer. 
     Another object of the invention is to provide a method of producing a semiconductor device capable of re-growing a satisfactory strained Si layer on a lattice-relaxed SiGe layer. 
     Accordingly, the first aspect of the invention provides a method of producing a semiconductor device comprising the steps of: 
     forming a strained SiGe layer on a substrate; 
     introducing oxygen into said strained SiGe layer; 
     forming an oxide layer by a heat treatment at the position where the oxygen is introduced, so as to make a lattice-relaxed SiGe layer located over said oxide layer; and 
     growing a strained Si layer over said lattice-relaxed SiGe layer. 
     In this aspect, it is preferable further comprising a step of forming a Si cap layer over said strained SiGe layer, wherein the surface of said strained SiGe layer is protected during said heat treatment. 
     Also preferred is that a step of growing an SiGe growing layer over said lattice-relaxed SiGe layer is further comprised, and then growing said strained Si layer over said SiGe growing layer. 
     Also preferred is that a step of etching the surface of said lattice-relaxed SiGe layer is further comprised, and then growing said strained Si layer. 
     Also preferred is that said oxide layer divides said strained SiGe layer into a SiGe upper layer that formed over said oxide layer and a SiGe lower layer that formed under said oxide layer. 
     Also preferred is that said introducing oxygen by an oxygen ion implantation under a condition that the oxygen ion is in said strained SiGe layer. 
     Also preferred is that a step of a HF treatment for terminating the surface of said lattice-relaxed SiGe layer by hydrogen is further comprised, and then growing said strained Si layer. 
     Also preferred is that said hydrogen on the surface of said lattice-relaxed SiGe layer is removed by a heat treatment, and then growing said strained Si layer. 
     Also preferred is that the steps of forming an oxide layer on the surface of said lattice-relaxed SiGe layer and removing said oxide layer by a heat treatment under vacuum are further comprised, and then growing said strained Si layer. 
     Also preferred is that a step of forming a SiGe buffer layer on said substrate is further comprised, and then forming said strained SiGe layer on said SiGe buffer layer. 
     Also preferred is that said substrate is a Si substrate. 
     Also preferred is that said substrate is a silicon-on-insulator substrate. 
     According to the first aspect of the invention, oxygen atoms are introduced into the strained SiGe layer and then the oxide layer is formed in the strained SiGe layer by heat treatment. The oxide layer divides the strained SiGe layer into a lattice-relaxed SiGe upper layer and a lattice-relaxed SiGe lower layer. A layer thickness of the lattice-relaxed SiGe upper layer becomes thicker by controlling the projection range of the oxygen ion implantation. Since the strain of the strained SiGe layer is relaxed into the oxide layer during the heat treatment, a defect such as a dislocation does not occur upon the lattice relaxation of the SiGe layer. 
     A second aspect of the invention provides a method of producing a semiconductor device comprising the steps of: 
     forming a lattice-relaxed SiGe layer on an insulating layer; 
     a HF treatment for terminating the surface of said lattice-relaxed SiGe layer by hydrogen; and 
     growing a strained Si layer on the surface of said lattice-relaxed SiGe layer. 
     In this aspect, it is preferred further comprising a step of removing a part of the surface of said lattice-relaxed SiGe layer, and then terminating said hydrogen on the surface of said lattice-relaxed SiGe layer. 
     Also preferred is that a step of removing said hydrogen on the surface of said lattice-relaxed SiGe layer by a heat treatment is further comprised, and then growing said strained Si layer. 
     Also preferred is that a step of growing a SiGe growing layer on said lattice-relaxed SiGe layer is further comprised, and then growing said strained Si layer on said SiGe growing layer. 
     According to the second aspect of the invention, since terminating the hydrogen protects the surface of the lattice-relaxed SiGe layer and then the strained Si layer grows on the surface of the lattice-relaxed SiGe after removing the hydrogen, the strained Si layer can grow in the better condition. 
     The third aspect of the invention provides a method of producing a semiconductor device comprising the steps of: 
     forming a lattice-relaxed SiGe layer on an insulating layer; 
     forming an oxide layer on the surface of said lattice-relaxed SiGe layer; 
     removing said oxide layer by a heat treatment under vacuum; and 
     growing a strained Si layer on the surface of said lattice-relaxed SiGe layer. 
     In this aspect, it is preferred further comprising a step of removing a part of the surface of said lattice-relaxed SiGe layer, and then forming said oxide layer on the surface of said lattice-relaxed SiGe layer. 
     Also preferred is that a step of growing a SiGe growing layer on said lattice-relaxed SiGe layer after removing said oxide layer is further comprised, and then growing said strained Si layer on said SiGe growing layer. 
     According to the third aspect of the invention, since the oxide layer protects the surface of the lattice-relaxed SiGe layer and then the strained Si layer grows on the surface of the lattice-relaxed SiGe after removing the oxide layer, the strained Si layer can grows in the better condition. 
     The fourth aspect of the invention provides a semiconductor device comprising: 
     a substrate; 
     a first SiGe layer formed on the substrate; 
     an oxide layer formed on said first SiGe layer; 
     a second SiGe layer formed on said oxide layer, wherein said second SiGe layer is lattice-relaxed and has a thickness of 200 nm or less; 
     a strained Si layer formed on said second SiGe layer. 
     In this aspect, it is preferred that said second SiGe layer has a thickness of 20 nm or less. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 shows a sectional view illustrating a method of producing a multilayered structure of strained Si/lattice-relaxed SiGe/insulating layers according to the invention. 
     FIG. 2 shows a sectional view illustrating a method of producing a multilayered structure of strained Si/lattice-relaxed SiGe/insulating layers according to the invention. 
     FIG. 3 shows a sectional view illustrating a method of producing a multilayered structure of strained Si/lattice-relaxed SiGe/insulating layers according to the invention. 
     FIG. 4 shows a sectional view illustrating a method of producing a multilayered structure of strained Si/lattice-relaxed SiGe/insulating layers according to the invention. 
     FIG. 5 is a table showing the condition of the lattice-relaxed SiGe layer surface treatment upon producing a strained Si/lattice-relaxed SiGe/insulating layer structure according to the invention. 
     FIG. 6 is a sectional view of MOSFET using a multilayered structure of strained Si/lattice-relaxed SiGe/insulating layers according to the invention. 
     FIG. 7 shows a sectional view in each process illustrating a method of producing a multilayered structure of strained Si/lattice-relaxed SiGe/insulating layers according to the invention. 
     FIG. 8 shows a sectional view in each process illustrating a method of producing an MOSFET using a multilayered structure of strained Si/lattice-relaxed SiGe/insulating layers according to the invention. 
     FIG. 9 shows a sectional view in each process illustrating a method of producing an MOSFET using a multilayered structure of strained Si/lattice-relaxed SiGe/insulating layers according to the invention. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Preferred embodiments of the invention is detailed below with referring to the drawings 
     (Embodiment 1) 
     As shown in FIG. 1, an ultra high vacuum CVD (chemical vapor deposition) apparatus is used to grow a Si 1-x Ge x  gradient composition layer  12  on a p-type Si substrate  11 . This p-type Si substrate  11  has a specific resistance of 4.5 Ωcm to 6 Ωcm and its main surface is a ( 100 ) surface. The film thickness of Si 1-x Ge x  gradient composition layer  12  is 1800 nm, the Ge content X is increased 0 to 2 gradually from the start to the end of growth. This Si1−xGex gradient composition layer  12  serves as a buffer layer. 
     The source gases for the Si 1-x Ge x  gradient composition layer  12  are Si 2 H 6  and GeH 4 , with no added dopant. The substrate temperature is at 650° C., the Si 2 H 6  source gas partial pressure is at 30 mPa and the GeH 4  source gas partial pressure gradually increases up to 60 mPa to obtain a gradient composition. Increasing the flow rate setting stepwise can increase the GeH 4  source gas partial pressure. On the other hands, 200 nm thickness of the Si 1-x Ge x  layers differing in the Ge content X stepwise by 2% from 2% to 18% may be laminated to form a Si 1-x Ge x  gradient composition layer  12  having thickness of approximately 1800 nm. 
     A strained Si 1-x Ge x  layer  13  is then grown on the Si 1-x Ge x  (X:0→0.2) gradient composition layer  12  using the ultra high vacuum CVD device. The Ge content X of the strained Si 1-x Ge x  layer  13  is constant at 0.2 from the beginning to the end of growth and the layer thickness is 1000 nm. While the strained Si 0.8 Ge 0.2  layer  13  is partly strained due to the layer thickness thereof and also due to the layer thickness of the underlying Si 1-x Ge x  (S:0→0.2) gradient composition layer  12 , it may partly be relaxed. The Si 1-x Ge x  (X:0→0.2) gradient composition layer  12  serves as a buffer layer to suppress a penetrating dislocation in the strained Si 0.8 Ge 0.2  layer  13 . 
     The source gases for the strained Si 0.8 Ge 0.2 layer  13  are Si 2 H 6  and GeH 4 , with no added dopant. The substrate temperature is at 650° C. with the Si 2 H 6  source gas partial pressure of 30 mPa and the GeH 4  source gas partial pressure of 60 mPa. 
     A Si cap layer  14  is then formed to the layer thickness of 30 nm continuously on the strained Si 0.8 Ge 0.2  layer  13  using the ultra high vacuum CVD apparatus. 
     The source gas for the Si cap layer  14  is Si 2 H 6 , with no added dopant. The substrate temperature is at 650° C. with the Si 2 H 6  source gas partial pressure of 30 mPa. 
     As shown in FIG. 2, the substrate is then transferred from the ultra high vacuum CVD apparatus to an ion implantation apparatus, where the oxygen ion is implanted. In this process, the oxygen ion is implanted under a condition to locate the oxygen in the strained Si 0.8 Ge 0.2  layer  13  (1000 nm) to ensure that the oxygen ion is retained in the strained Si 0.8 Ge 0.2  layer  13 . For this purpose, the acceleration energy is 180 keV and the implantation dose is 4×10 17  cm −2 . This energy gives the projection range of 400 nm with the fluctuation of ±100 nm. 
     The depth at which the buried oxide layer is formed can be adjusted by altering the accelerating energy. For example, a higher accelerating energy gives a deeper position of the buried oxidizing layer. On the other hand, a lower accelerating energy gives a shallower position of the buried oxidizing layer. It should be noted that a too lower accelerating energy makes the oxygen distributed toward the surface of the strained Si 0.8 Ge 0.2  layer  13  since the projection range fluctuates ±100 nm. Typically, the accelerating energy is preferably 25 keV or higher. It prefers that the range of depth that the oxygen ion located is 150 nm or more and 600 nm or less from the surface of the strained Si 0.8 Ge 0.2  layer  13 . 
     As shown in FIG. 3, the substrate is then taken out from the ion implantation apparatus and subjected to a heat treatment at 1350° C. for 4 hours. As a result of this heat treatment, a buried oxide layer  15  having thickness of 100 nm is formed mainly at the depth of 400 nm from the surface. This buried oxide layer  5  divides the strained Si 0.8 Ge 0.2  layer  13  into a Si 1-x Ge x  lower layer  13   a  and a Si 1-x Ge x  upper layer  13   b . Also as a result of the heat treatment, the Si 1-x Ge x  upper layer  13   b  is lattice-relaxed. 
     The most important parameter in this heat treatment is the temperature setting. A SiGe layer should be ion-implanted and annealed to lattice-relax at a somewhat low temperature because it deteriorate the surface of the SiGe layer such as rough surface by a high heat load process. For example, a temperature of 1200° C. to 1350° C. is preferable. 
     During this heat treatment, the surface condition of the strained Si 0.8 Ge 0.2  layer  13  can be kept satisfactory by changing the crystal surface of the Si cap layer  14  to a thin oxidizing layer  18 . Accordingly, it is effective to add a small amount of an oxygen gas into an atmosphere of the heat treatment. 
     For example, when a heat treatment atmosphere may employ an inert gas such as an argon gas to which an oxygen gas is introduced at about 0.5%, the heat treatment simultaneously forms a thin oxide layer on the surface of the Si cap layer  14 . The inert gas employed here may also be a rare gas or nitrogen instead of argon. 
     While the layer thickness of the Si cap layer  14  employed here is 30 nm, it is acceptable that it annealed under the condition that the surface oxide layer  18  is formed to a thickness less than 30 nm. The non-oxidized Si layer, which is lower layer of the Si layer  14 , changes a SiGe layer, into which Ge atoms are diffused from the underlying Si 0.8 Ge 0.2  layer  13 . The SiGe layer is lattice-relaxed. So, it&#39;s no problem. 
     Also when the heat treatment is given without forming the Si cap layer  14 , it is preferable to oxidize the surface of the Si 0.8 Ge 0.2  layer  13  slightly in an atmosphere with small amount of oxygen gas to ensure a satisfactory surface condition of the Si 0.8 Ge 0.2  layer  13 . This oxide layer is removed by an etching in a subsequent process. 
     The buried oxide layer  15  formed in this heat treatment contains almost no Ge content, which is diffused into the Si 1-x Ge x  lower layer  13   a  and the Si1−xGex upper layer  13   b . As a result, the buried oxide layer  15  becomes a SiO x . 
     On the other hand, this heat treatment makes the Si 1-x Ge x  lower layer  13   a  have a Ge content X which is lower slightly than 0.2 as a result of the diffusion of Ge to the Si 1-x Ge x  (X:0 →0.2) buffer layer  12 . 
     Also upon the lattice relaxation of the Si 1-x Ge x  upper layer  13   b , since the strain energy is released toward the amorphous buried oxide layer  15  instead of the Si 1-x Ge x  lower layer  13   a , a thin lattice-relaxed Si 1-x Ge x  upper layer  13   b  can be obtained without any new dislocation. 
     The silicon oxide layer  18  formed on the surface of the Si cap layer  14  is then etched off by hydrofluoric acid or ammonium fluoride. 
     A non-oxidized portion of the Si cap layer  14  and the surface of the Si 1-x Ge x  upper layer  13   b  are then etched off by an HF+HNO 3 -based etchant. As a result, a satisfactory surface of the lattice-relaxed Si 1-x Ge x  upper layer  13   b  can be obtained. 
     In this process, the HF+HNO 3 -based etchant has the composition of HF:H 2 O:HNO 3 =1:20:50, and the etching rate at room temperature is 600 nm/min for Si and 1300 nm/min for Si 0.8 Ge 0.2 . Reducing the concentration of hydrofluoric acid or nitric acid can reduce the etching rate further. For example, Si 0.8 Ge 0.2  is etched with HF:H 2 O:HNO 3 =1:100:500 at 70 nm/min. 
     Although etching the surface of the lattice-relaxed Si 1-x Ge x  upper layer  13   b  is not necessary always, it is desirable for the purpose of a thinner SiGe layer formed on the buried oxide layer  15 . This etching can reduce the thickness of the lattice-relaxed Si 1-x Ge x  upper layer  13   b  to 100 nm or less, more preferably as thin as 5 nm to 10 nm. 
     The etched surface of the lattice-relaxed Si 1-x Ge x  upper layer  13   b  is hydrogen-terminated by hydrofluoric acid (HF) solution treatment. 
     Since the surface of the lattice-relaxed Si 1-x Ge x  upper layer  13   b  once etched here is now exposed to an atmosphere, the surface of the lattice-relaxed Si 1-x Ge x  upper layer  13   b  is oxidized with atmospheric moisture or oxygen or tends to be contaminated unless hydrogen-terminating. Accordingly, a protecting layer is formed by terminating hydrogen on the surface of the lattice-relaxed Si 1-x Ge x  upper layer  13   b  to ensure to protect from the oxidation or contamination. As a result, a satisfactory strained Si layer can be formed on the lattice-relaxed Si 1-x Ge x  upper layer  13   b  upon re-growing the strained Si layer subsequently. 
     As shown in FIG. 4, the substrate is then placed again in the ultra high vacuum CVD apparatus, and hydrogen and impurities on the surface of the lattice-relaxed Si 1-x Ge x  upper layer  13   b  terminated by hydrogen are removed by a heat treatment. 
     A lattice-relaxed Si 0.8 Ge 0.2  layer  16  is then grown to the layer thickness of 100 nm on lattice-relaxed Si 1−x  Ge x  upper layer  13   b  using the ultra high vacuum CVD apparatus. The source gases for the lattice-relaxed Si 0.8 Ge 0.2  layer  16  were Si 2 H 6  and GeH 4 . The substrate temperature is at 650° C. with the Si 2 H 6  source gas partial pressure of 30 mPa and the GeH 4  source gas partial pressure of 60 mPa. 
     A strained Si layer  17  is then formed to the layer thickness of 20 nm on the lattice-relaxed Si 0.8 Ge 0.2  layer  16  using the ultra high vacuum CVD apparatus. The source gas for the strained Si layer  17  is Si 2 H 6 . The substrate temperature is at 650° C. with the Si 2 H 6  source gas partial pressure of 30 mPa. 
     In this process, a strained Si layer  17  having a further satisfactory crystal structure can be formed on the Si 0.8 Ge 0.2  buffer layer  16  instead of forming a strained Si layer  17  directly on the lattice-relaxed Si 1-x Ge x  upper layer  13   b . It is a matter of course that a strained Si layer  17  may be formed directly on the lattice-relaxed Si 1-x Ge x  upper layer  13   b.    
     It is desirable that the total layer thickness of the lattice-relaxed Si 0.8 Ge 0.2  buffer layer  16  and the lattice-relaxed Si 1-x Ge x  upper layer  13   b  is 200 nm or less, more preferably 10 nm or less. 
     The layer thickness of the strained Si layer  17  is preferably 30 nm or less, more preferably 5 nm to 10 nm. 
     Thus, the lattice-relaxed thin Si 1-x Ge x  layer  13   b  and  16  can be formed on the buried oxide layer  15  and the satisfactory strained Si layer  17  can be formed on the lattice-relaxed SiGe layer. The electron mobility in the strained Si layer thus formed is about 1.76 times that in a strain-free Si layer. In the case of forming a device, it is possible that the device is formed over the buried oxide layer  15 . On the other hands, the buffer layer  12  is located under the buried oxide layer  15 , so the buffer layer  12  does not need to be separated from each other. The forming process of the device is described in the embodiment 4. 
     FIG. 5 shows the relationship between the lowest concentration of an HF solution required in the hydrogen terminating treatment and the Ge content X of the lattice-relaxed Si 1-x Ge x  upper layer  13   b  to be treated discussed in this embodiment. In this experiment, the substrates differing from each other in the Ge content X of the lattice-relaxed Si 1-x Gex upper layer  13   b , which was 0%, 10%, 20% or 30%, were provided. The hydrogen terminates on the surface of the lattice-relaxed Si 1-x Ge x  upper layer  13   b  at varying HF concentration in the solution of hydrofluoric acid. 
     The HF concentration indicated here is a desirable lowest value. If HF solution less than HF concentration in the FIG. 5 is used, it can&#39;t remove oxygen impurities on the surface of the lattice-relaxed Si 1-x Ge x  upper layer  13   b  sufficiently since it can&#39;t hydrogen-terminate on the surface sufficiently. So it is possible that the impurities are remained in the interface between re-growing layer ant the lattice-relaxed layer or the crystal quality of the re-grown layer is deteriorated after re-growing process. 
     Thus it is preferable to employ a high HF concentration in the hydrogen terminating treatment, such as, for example, 1.5% or higher when the Ge content X of the lattice-relaxed Si 1-x Ge x  upper layer  13   b  is 20%. 
     The re-growing temperature can readily be adjusted since the hydrogen-terminated surface of the lattice-relaxed Si 1-x Ge x  upper layer  13   b  begins to release hydrogen at 400° C. to 500° C. 
     In order to ensure to remove oxygen or carbon impurities remained in trace amounts on the surface, a heat treatment at 850° C. to 900° C., in addition to the hydrogen release at 400° C. to 500° C., is further desirable. However, the surface of the lattice-relaxed Si 1-x Ge x  upper layer  13   b  may deteriorate such as causing a rough surface at a high temperature for a long period, since SiGe is susceptible to a heat treatment at a high temperature. Accordingly, the heat treatment for removing oxygen or carbon impurities without causing any surface deterioration of the lattice-relaxed Si 1-x Ge x  upper layer  13   b  whose Ge content is 20% is conducted preferably at 850° C. for 20 minutes or shorter, or at 900° C. for 5 minutes or shorter. 
     (Embodiment 2) 
     In this embodiment, an oxide layer is formed as a protecting layer on the surface of the lattice-relaxed Si 1-x Ge x  upper layer  13   b  instead of the hydrogen termination for layer protection. 
     Accordingly, the processes from FIG. 1 to FIG. 3 are similar to those in Embodiment 1 and thus are not described below. 
     After a part of the surface of the lattice-relaxed Si 1-x Ge x  upper layer  13   b  is etched off as described in Embodiment 1, the surface of the lattice-relaxed Si 1-x Ge x  layer  13   b  is oxidized to form an oxide layer (protecting layer). The thickness of such oxide layer is preferably 3 nm or less, more preferably 1.5 nm or less. This oxidation is conducted effectively by an acid reagent treatment using a mixture of hydrochloric acid and hydrogen peroxide. For example, a mixture of hydrochloric acid:hydrogen peroxide:water (about 1:1:6) heated at 90° C. or higher provides a satisfactory oxide layer. 
     The substrate is then placed in the ultra high vacuum CVD apparatus, and the oxide layer as a protecting layer is removed by heating under vacuum. 
     The heat treatment for removing the oxide layer is preferably 850° C. to 900° C. While the heat treatment there for removing the oxide layer should be a greater heat load than in the case of the hydrogen termination, a heat treatment at 850° C. for 30 minutes or shorter is preferable typically when handling a lattice-relaxed Si 1-x Ge x  layer whose Ge content is 20%. 
     As shown in FIG. 4, a lattice-relaxed Si 0.8 Ge 0.2  layer  16  is then grown to the layer thickness of 100 nm on the lattice-relaxed Si 1-x Gex upper layer  13  whose oxide layer has been removed using the ultra high vacuum CVD apparatus. The source gases for the lattice-relaxed Si 0.8 Ge 0.2  layer  16  are Si 2 H 6  and GeH 4 . The substrate temperature is at 650° C. with the Si 2 H 6  source gas partial pressure of 30 mPa and the GeH 4  source gas partial pressure of 60 mPa. 
     A strained Si layer  17  is then formed to the layer thickness of 20 nm on the lattice-relaxed Si 0.8 Ge 0.2  layer  16  using the ultra high vacuum CVD apparatus. The source gas for the strained Si layer  17  is Si 2 H 6 . The substrate temperature is at 650° C. with the Si 2 H 6  source gas partial pressure of 30 mPa. 
     In this process, a strained Si layer  17  having a further satisfactory crystal structure can be formed on an Si 0.8 Ge 0.2  buffer layer  16  instead of forming a strained Si layer directly on the lattice-relaxed Si 1-x Ge x  upper layer  13   b . It is a matter of course that a strained Si layer  17  may be formed directly on the lattice-relaxed Si 1-x Ge x  upper layer  13   b.    
     It is desirable that the total layer thickness of the lattice-relaxed Si 0.8 Ge 0.2  buffer layer  16  and the lattice-relaxed Si 1-x Ge x  upper layer  13   b  is 200 nm or less, more preferably 10 nm or less. 
     The layer thickness of the strained Si layer  17  is preferably 30 nm or less, more preferably 5 nm to 10 nm. 
     Thus, the lattice-relaxed thin Si 1-x Ge x  layer  13   b  and SiGe buffer layer  16  can be formed on the buried oxide layer  15 , and the strained Si layer  17  can be formed on the lattice-relaxed thin SiGe layer. 
     (Embodiment 3) 
     FIG. 7 shows the process for producing a semiconductor device described in Embodiment 3 of the invention. This embodiment 3 is the second aspect of the invention. 
     This embodiment employs an SOI (silicon-on-insulator) substrate, and a strained SiGe layer is grown by an epitaxial growth on an SOI layer to form a lattice-relaxed SiGe layer. 
     First of all, an SOI substrate in which a silicon substrate  41  is mounted with a silicon oxide layer  42  whose thickness is 100 nm and a silicon single crystal layer  43  whose thickness is 20 nm in this order is provided as shown in FIG.  7 ( a ). 
     Such SOI substrate is manufactured industrially and readily available, but a commercial low price SOI substrate frequently has a too thick silicon single crystal layer  43  whose thickness is 100 nm or more. Such silicon single crystal layer  43  may be oxidized in an ordinary heat oxidation furnace to make the SOI layer  43  (Si layer on a buried oxide layer  42 ) thinner. When the surface of an SOI layer  43 , for example, whose initial thickness is 100 nm is oxidized under a condition that an oxide layer about 160 nm can be formed, an SOI layer about 20 nm remains. In this case, the surface oxide film is removed by etching. 
     The following description concerns to a growth of Si 0.85 Ge 0.15  layer  44  (Ge content: 15%) whose layer thickness is 100 nm on the SOI substrate described above at a low temperature around 500° C. shown in FIG.  7 ( b ). To accomplish such growth at a low temperature, an MBE (molecular beam epitaxy) method employing a solid material is also useful similarly to the ultra high vacuum CVD method described in Embodiments 1 and 2. In this embodiment, an MBE method employing a solid material is discussed. 
     In the MBE method employing a solid material, an electron beam irradiates to a Si source to heat it and a Si vapor is supplied onto a substrate heated by a separate heat source (substrate heater). At the same time, a Ge vapor taken out of a Ge source heated by a furnace is supplied onto the substrate simultaneously, whereby forming a SiGe crystal layer. In this process, the temperatures of the Si source and the Ge source may be controlled to adjust the both vapor pressures, whereby devising a predetermined Ge composition. 
     The MBE method is employed to grow a Si 0.85 Ge 0.15  layer  44  (Ge content: 15%) to the thickness of 100 nm on the SOI layer  43  at a low temperature around 500° C. 
     After growing this Si 0.85 Ge 0.15  layer  44 , the Si 0.85 Ge 0.15  layer  44  has a tensile strain due to a Si crystal layer  43 . 
     As shown in FIG.  7 ( c ), this substrate is taken out into an atmosphere and then placed in a heat treatment furnace, where it is annealed at a temperature as high as 1100° C. for 1 hours. By taking out into an atmosphere, a very thin oxide layer  45  is formed on the surface of the Si 0.85 Ge 0.15  layer  44  and serves to suppress the precipitation or aggregation of Ge atom in a heat treatment. This heat treatment slides dislocation between the buried oxide layer  42  and the underlying SOI layer  43 , and then the Si 0.85 Ge 0.15  layer  44  is lattice-relaxed. 
     Since an Si oxide layer  45  is formed on the surface of the lattice-relaxed SiGe layer  44  after each process, this surface oxide layer is removed by an HF treatment simultaneously with a hydrogen termination of the surface of the lattice-relaxed SiGe layer  44  by the HF treatment. The condition of the HF treatment is similar to that in Embodiment 1. 
     As shown in FIG.  7 ( d ), this substrate is placed in the thin film growing apparatus, where the lattice-relaxed SiGe layer  46  is re-grown to ensure the crystal quality and a strained Si layer  47  is formed on the top. As a result, a multilayer structure of strained Si layer  47 /Si 1-x Ge x  layer  46 ,  44 /Si layer  43 /Si oxide layer  42  is obtained. The structure thus obtained diffuses Ge atoms into the initial SOI layer from the subsequently formed SiGe layer  46  when the heat treatment temperature is high, so it results in a reduced Ge concentration on average, which is 12.5% in the case discussed above. 
     While in this embodiment it anneals the substrate, next it hydrogen-terminates the surface of the SiGe layer  44 , and then it grows the strained Si layer  47 , it can be that it anneals the substrate, next it etches a part of the surface of the lattice-relaxed SiGe layer  44 , and then it hydrogen-terminates the etched surface of the SiGe layer  44 , next it grows the strained Si layer  47 . So an extremely thin lattice-relaxed SiGe layer  44  whose thickness was 120 nm and whose Ge content is 12.5% formed after the heat treatment as described above may be etched off by 90 nm from the surface to reduce the layer thickness to 30 nm, and the strained Si layer  47  whose layer thickness is 15 nm may be re-grown. 
     Also in this manner, a high temperature heat treatment is required to obtain a satisfactory lattice-relaxed SiGe layer  44 , resulting in the oxidation of the surface layer. Even if a Si cap layer  14  protects the surface, a large amount of Ge is migrated from the SiGe layer and prevents the Si layer from being preserved. Thus, in order to obtain a strained Si layer finally as a top layer, the re-growing process after the high temperature heat treatment is essential. 
     (Embodiment 4) 
     An example of an MOSFET produced using the multilayer structure obtained as described above is described below. 
     As shown in FIG. 6, a Si substrate  31  is mounted with a buried oxide layer  32 . A lattice-relaxed SiGe layer  35 , a strained Si layer  34 , a gate oxide layer  35  and a gate electrode  36  are formed on the buried oxide layer  32 . A source-drain  37  is formed in the strained Si layer  34  at the both sides of the gate electrode  36 . 
     The Ge content and the thickness of the lattice-relaxed Si 0.7 Ge 0.3  layer  35  employed here are 30% and 7 nm, respectively, and the initial thickness of the strained Si layer  34  is 6 nm. A 3 nm oxide layer and a 4.5 nm strained Si layer  34  are laminated on the relaxed SiGe/insulating layer (buried oxide layer) after completing the MOSFET, because the surface of the strained Si layer  34  is heat-oxidized as the gate oxide layer. 
     A method of producing this MOSFET is described below with referring to FIG.  8  and FIG.  9 . 
     As shown in FIG.  8 ( a ), it grows a SiGe gradient composition layer  82  (2.5 μm in thickness) having increasing Ge content on a Si substrate. It grows Si 0.7 Ge 0.3  layer  83  whose thickness is 2 μm on the SiGe gradient composition layer  82 . It grows a Si cap layer  84  whose thickness is 20 nm on the Si 0.7 Ge 0.3  layer  83 . These layers are formed by the ultra high vacuum CVD method starting from Si 2 H 6  and GeH 4 . 
     Then as shown in FIG.  8 ( b ) the multilayer substrate is subjected to an oxygen ion implantation. The accelerating energy here is 180 keV with the injection dose of 4×10 17  cm −2 . 
     The substrate is annealed at 1350° C. for 4 hours. As a result of this heat treatment, a buried oxide layer  85  whose thickness is 100 nm is formed mainly at the depth of 400 nm from the surface. This buried oxide layer  85  is located between the Si 0.7 Ge 0.3  layer  83  and the SiGe gradient composition layer  82 . Also as a result of this heat treatment, the Si 0.7 Ge 0.3  layer  83  is lattice-relaxed. 
     Then as shown in FIG.  8 ( c ) the surface of the Si 0.7 Ge 0.3  layer  83  whose thickness is 400 nm is etched to 7 nm with an HF:nitric acid solution mixture. In this process, the Si cap layer  84  is also etched. Other etching methods may also be employed here. 
     Then as shown in FIG.  8 ( d ) it forms a strained Si layer  86  whose thickness is 6 nm on the Si 0.7 Ge 0.3  layer  83  by the CVD apparatus. 
     Then as shown in FIG.  8 ( e ) the surface of the strained Si layer  86  is heat-oxidized. The layer thickness of a heat oxide layer  87  thus formed is 3 nm, resulting in a 3 nm oxide layer  87  and a 4.5 nm strained Si layer  86 . 
     Then as shown in FIG.  8 ( f ) a polycrystalline Si layer  88  whose thickness is 50 nm is formed on the oxide layer  87 . 
     Then as shown in FIG.  9 ( a ) the polycrystalline Si layer  88  is etched off except for the gate region and form a gate electrode  88 . It forms an insulating layer on the substrate and the insulating layer is etched off except for the side of the gate electrode  88  by RIE to form gate sidewall  89 . 
     Then as shown in FIG.  9 ( b ) the polycrystal Si gate electrode  88  and the source-drain region  90  at the both ends of the gate  88  are imparted with low resistances by an impurity ion implantation and a rapid thermal anneal. The rapid thermal anneal after the ion implantation is conducted preferably at a temperature not higher than 850° C. A higher temperature may relax the strained Si layer  86 . Also a higher temperature may deteriorate the Si/SiGe interface for diffusing Ge. 
     Finally, the source-drain  90  and the gate  88  are provided with aluminum electrodes to obtain a complete device. The buried oxide layer  85  is accordance with the buried oxide layer  32  in the FIG.  6 . The substrate  81  and the gradient composition layer  82  is accordance with the substrate  31  in the FIG.  6 . 
     Since an MOSFET produced as described above employs as a channel a strained Si layer, it serves as a high performance device. 
     Since the invention provides a thin lattice-relaxed SiGe layer regardless of the critical film thickness for the lattice relaxation when an SiGe layer is formed on an Si crystal, an extremely thin relaxed SiGe, whose thickness is equal to or less than the critical layer thickness of the SiGe layer on the Si crystal, can be obtained in a multilayer structure of a strained Si/relaxed SiGe/insulating layers. 
     Also lattice-relaxation SiGe layer on which the strain Si layer formed is so thin that it is easy to separate devices each other. 
     Also since it hydrogen-terminates the lattice-relaxed SiGe layer or it forms the oxide layer on the surface of the lattice-relaxed SiGe layer, and it etches the surface, and then it re-grows the strained Si layer on the surface, the interface characteristics between the strained Si layer and the lattice-relaxed SiGe layer becomes more satisfactory and the device performance can be improved.