Patent Publication Number: US-6709903-B2

Title: Relaxed SiGe layers on Si or silicon-on-insulator substrates by ion implantation and thermal annealing

Description:
RELATED APPLICATIONS 
     This application is a divisional of U.S. application Ser. No. 10/115,160, filed Apr. 3, 2002, now U.S. Pat. No. 6,593,625. 
     This application claims benefit of U.S. Provisional Application No. 60/297,496, filed Jun. 12, 2001, and is related to U.S. application Ser. No. 10/037,611, filed Jan. 4, 2002, the entire content of which is incorporated herein by reference. 
    
    
     FIELD OF THE INVENTION 
     The present invention relates to a process of fabricating a so-called “virtual substrate” as well as the virtual substrate and the use thereof in semiconductor devices such as modulation-doped field effect transistors (MODFETs), metal oxide field effect transistors (MOSFETs), strained silicon-based complementary metal oxide semiconductor (CMOS) devices and other devices that require fully-relaxed SiGe layers. The virtual substrate of the present invention contains Si and Ge in a crystalline layer that assumes the bulk lattice constant of a Si 1−x Ge x  alloy on either a lattice mismatched Si wafer or silicon-on-insulator (SOI) wafer. 
     BACKGROUND OF THE INVENTION 
     In the semiconductor industry, the Si/S 1−x Ge x  heteroepitaxial materials system is of strong interest for future microelectronic applications because the electronic properties of lattice mismatched heterostructures can be tailored for a variety of applications by exploiting band offsets at the interfaces. The most popular application of the Si/Si 1−x Ge x  system is heterojunction bipolar transistors (HBTs) that require deposition of a pseudomorphic, i.e., compressively strained so that the in-plane lattice parameter of the layer matches that of the Si substrate, compositionally graded Si 1−x Ge x  layer onto the Si substrate. Metal oxide semiconductor field effect transistors (MOSFETs) and modulation-doped field effect transistors (MODFETs) require Si layers under tensile strain to obtain proper conduction band offsets at the interface that enable the formation of a 2D electron gas in the Si quantum well which results in extremely high-electron mobility (on the order of about five-ten times larger than in unstrained Si at room temperature). Si layers under tensile strain are obtained by epitaxial growth on a strain-relaxed Si 1−x Ge x  buffer layer (x=0.15-0.35). As mentioned in P. Mooney, Mater. Sci. Eng. R17, 105(1996) and F. Schaeffler, Semiconductor Sci. Tech. 12, 1515 (1997), the strain-relaxed Si 1−x Ge x  buffer layer in conjunction with the Si or SOI substrate constitute the so-called “virtual substrate”. It is noted that the term “SiGe” is used sometimes herein to refer to the Si 1−x Ge x  layer. 
     The growth of the strain-relaxed Si 1−x Ge x  buffer layer itself is a challenging task since strain relaxation involves controlled nucleation, propagation and interaction of misfit dislocations that terminate with threading arms that extend to the wafer surface and are replicated in any subsequently grown epitaxial layers. These defects are known to have deleterious effects on the properties of electronic and optoelectronic devices. The crystalline quality of the relaxed SiGe layer can be improved by growing compositionally graded buffer layers with thicknesses of up to several micrometers. By using such a technique, the threading dislocation (TD) density in an epitaxial layer grown on top of a buffer layer was reduced from 10 10 -10 11  cm −2  for a single uniform composition layer to 10 6 -5×10 7  cm −2  for a graded composition buffer layer. The major drawback of thick SiGe buffer layers (usually a 1-3 micrometer thickness is necessary to obtain &gt;95% strain relaxation) is the high-TD density and the inhomogeneous distribution of TDs over the whole wafer surface. Some regions have relatively low TD densities and primarily individual TDs; but other areas contain bundles of TDs as a result of dislocation multiplication which creates dislocation pileups (see, for example, F. K. Legoues, et al., J. Appl. Phys. 71, 4230 (1992) and E. A. Fitzgerald, et al., J. Vac. Sci. and Techn., BIO 1807 (1992)). Moreover, blocking or dipole formation may occur, in some instances, due to dislocation interactions (see E. A. Stach, Phys. Rev. Lett. 84, 947 (2000)). 
     Surface pits that tend to line up in rows are typically found in the latter areas, thus making these regions of the wafer unusable for many electronic devices. Electronic devices on thick graded Si 1−x Ge x  buffer layers also exhibit self-heating effects since SiGe alloys typically have a much lower thermal conductivity than Si. Therefore, devices fabricated on thick SiGe buffer layers are unsuitable for some applications. In addition, the thick graded Si 1−x Ge x  buffer layers derived from dislocation pileups have a surface roughness of 10 nm on average, which typically makes such buffer layers unsuitable for device fabrication. For example, it is impossible to use these layers directly for wafer bonding. For that purpose an additional chemical-mechanical polishing (CMP) step is required. 
     Various strategies have been developed to further reduce the TD density as well as the surface roughness including: 
     1) The use of an initial low-temperature (LT) buffer layer grown at 450° C. and subsequent layer growth at temperatures between 750° and 850° C. This prior art method makes use of the agglomeration of point defects in the LT-buffer layers that occur at the higher growth temperatures. The agglomerates serve as internal interfaces where dislocations can nucleate and terminate. As a result, the misfit dislocation density that is responsible for the relaxation is maintained, while the TD density is reduced. LT buffer layers can only be grown by molecular beam epitaxy (MBE); this prior art approach cannot be implemented using UHV-CVD. 
     2) The use of substrate patterning, e.g., etched trenches, to create small mesas, approximately 10-30 micrometers on a side. The trenches serve as sources/sinks for dislocations to nucleate/terminate. When a dislocation terminates at a trench, no TD is formed; however, the misfit segment present at the Si/SiGe interface contributes to strain relaxation. The major drawback with this prior art method is loss of flexibility in device positioning and the loss of usable area. Moreover, it is difficult to obtain high degrees of relaxation (&gt;80%). 
     Neither the conventional graded buffer layer methods to achieve strain-relaxed Si 1−x Ge x  buffer layers for virtual substrates, nor the alternative approaches to reduce the density of TDs described above provide a solution that fully satisfies the material demands for device applications, i.e., a sufficiently low-TD density, control over the distribution of the TDs and an acceptable surface smoothness. 
     In some cases, He ion implantation has been employed in forming relaxed SiGe layers. Ion implantation of He into semiconductors is well-known to form bubbles that can be degassed and enlarged (Ostwald ripening) during subsequent annealing (see, for example, H. Trinkaus, et al., Appl. Phys. Lett. 76, 3552 (2000), and D. M. Follstaedt, et al., Appl. Phys. Lett. 69, 2059 (1996)). The bubbles have been evaluated for uses such as gettering metallic impurities or altering electronic properties of semiconductors. Moreover, the bubbles have also been evaluated as sources for heterogeneous dislocation nucleation. 
     It has also been shown that the binding energy between bubbles and dislocations is quite large (about 600 eV for a 10 ni radius of the bubble) and that the interaction of He bubbles with dislocations significantly alters the misfit dislocation pattern. It consists of very short (&lt;50 nm) misfit dislocation segments rather than the longer (&gt;1 μm) ones that occur in graded buffer layer growth. The interaction of He bubbles with dislocations also significantly changes the relaxation behavior of strained Si 1−x Ge x  layers. Moreover, the degree of relaxation is greater compared to an unimplanted control sample when the same heat treatment is applied to both samples. To achieve significant strain relaxation, a dose of 2×10 16  cm −2  He implanted about 80 nm below the Si/SiGe interface is required (M. Luysberg, D. Kirch, H. Trinkaus, B. Hollaender, S. Lenk, S. Mantl, H. J. Herzog, T. Hackbarth, P. F. Fichtner, Microscopy on Serniconducting Materials, IOP publishing, Oxford 2001, to be published). Although the strain relaxation mechanism is very different from that which occurs in graded buffer layers, the TD density remains unsatisfactorily large (&gt;10 7  cm −2  at best for Si 0.80 Ge 0.20 ). Lower TD densities are obtained only when little strain relaxation occurs. 
     In view of the drawbacks mentioned-above with prior art approaches for fabricating strain-relaxed Si 1−x Ge x  buffer layers on Si substrates as well as on silicon-on-insulator substrates (SOI), there exists a need to develop a new and improved process which is capable of fabricating strain-relaxed Si 1−x Ge x  buffer layers on Si or silicon-on-insulator (SOI) substrates having a reduced TD density, a homogeneous distribution of misfit dislocations and a remarkably low surface smoothness. 
     SUMMARY OF THE INVENTION 
     One aspect of the present invention relates to a process of fabricating a relaxed Si 1−x Ge x  buffer layer having a low-density of TDs on a single crystalline surface. Broadly, the inventive process, which forms a so-called ‘virtual substrate’ comprises the steps of: depositing a strictly pseudomorphic epitaxial layer of Si 1−x Ge x  (i.e., a layer that is completely free of dislocations) on a single crystalline surface of a substrate; ion implanting atoms of a light element such as He into the substrate; and annealing the substrate at a temperature above 650° C. 
     Even though He implantation is known, applicants have determined optimum processing conditions for implanting He ions below the Si/Si 1−x Ge x  interface and subsequent thermal annealing that yield a quite different relaxation mechanism resulting in a reduced threading dislocation density (e.g., 10 4 -10 6  cm −2  for Si 0.15 Ge 0.85 ) of a thin (&lt;300 nm) SiGe layer. 
     It is of key importance for successful device performance that the strain-relaxed single crystal Si 1−x Ge x  layer contains as few defects, which are primarily threading dislocations (TDs), as possible; the upper limit that can be tolerated for TDs mentioned in recent publications is 10 6  cm −2 . Using the inventive process, it is possible to obtain relaxed Si 1−x Ge x  layers having TD densities below this limit, in contrast to the commonly used state-of-the-art linearly- or step-graded buffer layers that typically have TDs in the range between 1×10 6  to 5×10 7  cm −2  on 8″ wafers at alloy compositions as high as Si 0.8 Ge 0.2 . 
     Another aspect of the present invention relates to a virtual substrate that is formed using the inventive process. Specifically, the inventive virtual substrate comprises 
     a substrate; and 
     a partially relaxed single crystalline Si 1−x Ge x  layer atop the substrate, wherein the partially relaxed single crystalline Si 1−x Ge x  layer has a thickness of less than about 300 nm, a threading dislocation density of less than 10 6  cm −2 , and significant relaxation of greater than 30%. 
     In some embodiments of the present invention, the epitaxial Si 1−x Ge x  layer includes C having a concentration of from about 1×10 19  to about 2×10 21  cm −3  therein. 
     A still further aspect of the present invention relates to semiconductor structures that are formed using the processing steps of the present invention. Broadly, the inventive semiconductor structure comprises: 
     a substrate; 
     a first single crystalline layer atop said substrate; 
     a second highly defective single crystalline layer atop said first single crystalline layer, said second highly defective single crystalline layer comprising planar defects which serve as sources and sinks of dislocation loops; 
     a third single crystalline layer of essentially the same composition as the first single crystalline layer, said third single crystalline layer comprising threading dislocations terminating at the interface formed between the third and fourth layers; and 
     a fourth relaxed single crystalline layer having a lattice parameter different from said third layer formed atop said third layer. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIGS. 1A-C are pictorial representations (through cross-sectional views) showing the basic processing step employed in the present invention in forming a thin, fully-relaxed SiGe buffer layer on a Si substrate or SOI wafer, i.e., virtual-substrate. 
     FIG. 2A illustrates the SIMS measurements of the Ge mole fraction vs. distance from the wafer surface for a relaxed ion-implanted nominally Si 0.85 Ge 0.15  buffer layer grown on a bulk Si substrate. 
     FIG. 2B is an atomic force micrograph (10 μm×10 μm) showing a faint cross hatch pattern on the surface of a relaxed ion-implanted Si 0.85 Ge 0.15  buffer layer on a bulk Si substrate. The Z-range for the whole image is about 3 nm. The RMS roughness is about 0.28 rm. Layer thickness is about 100 nm; He implant dose 8E15 cm −2 ; and annealed at 850° C. for 1 hr. 
     FIG. 3A (Prior Art) shows the SIMS measurements of the Ge mole fraction vs. distance from the wafer surface for a step-graded relaxed Si 0.85 Ge 0.15  layer grown on a bulk Si substrate. 
     FIG. 3B (Prior Art) is an atomic force micrograph (20 μm×20 μm) showing a pronounced cross hatch pattern on the step-graded relaxed Si 0.85 Ge 0.15  layer. The Z-range for the whole image is about 40 mm. The RMS roughness is about 6 nm. 
     FIG. 4A is a planar view TEM micrograph (weak beam (g 400 ), two beam conditions) of a relaxed ion-implanted buffer layer. White round structures stem from the platelets that reside below the Si/Si 1−x Ge x  interface. Orthogonal white lines along &lt;110&gt;-directions indicate 60 0  misfit dislocations that reside at, or close to the Si/Si 1−x Ge x  interface. He-implant; layer thickness is about 100 nm; implant dose 10E15 cm −2 ; and anneal 850° C., 1 hr. 
     FIG. 4B is a cross-sectional TEM micrograph (weak beam, two beam conditions) of an ion-implanted buffer layer. Under dark field conditions dislocations and He-induced platelets (or a width of about 100-150 nm and a spacing of that order) appear bright. 
     FIG. 5A (Prior Art) is a planar view TEM micrograph (weak beam, two beam conditions) of an ion-implanted buffer layer fabricated with a very high implant dose (2E16 cm −2 ). Under dark field conditions dislocations and He-induced bubbles (with a diameter of about 20-30 nm) appear bright. 
     FIG. 5B (Prior Art) is a cross-sectional TEM micrograph (weak beam, two beam conditions) of an ion-implanted buffer layer with a very high implantation dose. Under dark field conditions dislocations and He-induced bubbles appear bright. 
     FIG. 6 is a cross section of an inventive structure containing the relaxed buffer layer fabricated by the process of the present invention. 
     FIG. 7 is a schematic view of the cross section of the structure of FIG. 6 containing an optional graded composition SiGe layer  41  instead of the original uniform composition layer  40  of FIG.  6 . 
     FIG. 8 shows a cross section of an inventive structure containing the relaxed buffer layer fabricated by performing the inventive three-step procedure twice. 
     FIG. 9 is a schematic view of the cross section of the structure of FIG. 8 except that SiGe layers  43 ,  27  and  37  (original layer  41  of FIG. 7) and layer  46  have a graded alloy composition. 
     FIG. 10 is a schematic of the cross section of FIG. 6 where an additional single crystalline uniform composition SiGe layer  44  having a greater atomic % Ge is grown epitaxially on layer  40 . 
     FIG. 11 is a schematic of the cross section of FIG. 7 where an additional single crystalline graded composition SiGe layer  47  having a greater atomic % Ge is grown epitaxially on top of layer  41 . 
     FIG. 12 is a schematic of the cross section of FIG. 6 where an additional single crystalline uniform composition SiGe layer  400  of identical composition to layer  40  is deposited homo-epitaxially on layer  40  and a strained Si layer is deposited on top of layer  400 . 
     FIG. 13 is a schematic of the cross section of FIG. 7 where an additional single crystalline uniform composition SiGe layer  410  of identical composition as the top of layer  41  is deposited homo-epitaxially on layer  41 . A strained Si cap layer is deposited on layer  410 . 
     FIG. 14 is a schematic of the cross section of FIG. 8 where an additional single crystalline uniform composition SiGe layer  450  of identical composition to layer  45  is deposited homo-epitaxially on layer  45 . Additional strain relaxation may occur during the growth of this layer. A strained Si cap layer is deposited on layer  450 . 
     FIG. 15 is a schematic of the cross section of FIG. 9 where an additional single crystalline uniform composition SiGe layer  460  of identical composition as the top region of layer  46  is deposited homo-epitaxially on layer  46 . A strained Si cap layer  50  is deposited on top of layer  460 . 
     FIG. 16 is a schematic of FIG. 10 where an additional single crystalline uniform composition layer  440  of similar composition as layer  44  is deposited homo-epitaxially on layer  44 . A strained Si cap layer  50  is deposited on top of layer  440 . 
     FIG. 17 is a schematic of the cross section of FIG. 11 where an additional single crystalline uniform composition SiGe layer  470  of identical composition as the top region of layer  47  is deposited homo-epitaxially on layer  47 . A strained Si cap layer  50  is deposited on top of layer  470 . 
     FIG. 18 is a schematic of the cross section of FIG. 12 where a field effect transistor (FET) is fabricated on the structure. The FET comprises source contact  100 , drain contact  101 , gate oxide layer  102 , gate contact  103  and gate sidewall insulation  104 . 
     FIG. 19 is a schematic of the cross section of a n-type modulation-doped FET (MODFET) layer structure deposited on the structure of FIG.  12 . 
     FIG. 20 is a schematic of the cross section of a p-type MODFET structure deposited on the structure of FIG.  12 . 
     FIG. 21 is a schematic of the cross section of a structure where a MODFET device is fabricated on the structures of FIG. 19 or  20 . 
     FIG. 22 is a schematic view of the cross section of a structure comprising a superlattice consisting of alternating layers  550  and  560  deposited on top of the structure of FIG. 12 without the strained Si cap layer  50 . 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     The present invention, which provides a process of fabricating virtual substrates as well as structures containing the same, will now be described in more detail by referring to the drawings that accompany the present application. 
     Reference is first made to FIGS. 1A-C, which illustrate the basic processing steps employed in fabricating the inventive virtual substrate. It is noted that the term “virtual substrate” is used herein to denote a structure which includes a substrate (bulk Si or SOI) that has a relaxed single crystalline Si 1−x Ge x  layer formed thereon, wherein the relaxed single crystalline Si 1−x Ge x  layer has a thickness of less than about 300 nm, a threading dislocation density of less than 10 6  cm −2 , and a degree of relaxation depending on the layer thickness, i.e. between 30% for about 100 nm thick layers and 80% for about 200 nm thick layers. 
     First, and as shown in FIG. 1A, a thin, strictly pseudomorphic Si 1−x Ge x  layer  6  is deposited on a single crystalline surface of substrate  5  using any epitaxial growing process which is capable of forming such a layer atop substrate  5 ; substrate  5  may be comprised of bulk Si or an SOI material. An SOI material includes a buried insulating region that electrically isolates a top Si-containing layer from a bottom Si-containing layer. In one embodiment of the present invention, thin, strictly pseudomorphic Si 1−x Ge x  layer  6  is formed using an ultra-high-vacuum chemical vapor deposition (UHV-CVD) process. The Si 1−x Ge x  layer thickness exceeds the critical thickness for misfit dislocation formation by glide of a preexisting threading dislocation first proposed by J. W. Matthews, et al. J. Cryst. Growth 27, 188 (1974). 
     Next, ions of He or other like light elements are implanted through pseudomorphic Si 1−x Ge x  layer  6  into substrate  5  below Si/Si 1−x Ge x  interface  7 . Although the implanted ion may be implanted to any depth into substrate  5 , a good value for the projected range of the implanted ions is from about 90 to about 300 nm, preferably about 110 to about 200 nm below interface  7 . As shown in FIG. 1B, the implanted ions form damaged region  9  within substrate  5 . It is noted that the implanted atoms are essentially concentrated in substrate  5 , far below the single crystalline surface so that a minimum amount of implanted atoms is contained in the epitaxial layer and at interface  7 . 
     Finally, and as shown in FIG. 1C, the implanted substrate is annealed at temperatures above 650° C. such that platelets  12  are formed at a depth of about 100 to about 200 nm below Si/Si 1−x Ge x  interface  7 . The high strain in the region of the platelets results in the nucleation of dislocation half loops ( 11 ) at the platelets. The half loops glide to the Si/Si 1−x Ge x  interface where long misfit dislocation segments that relieve the lattice mismatch strain in the SiGe layer are formed. The density of misfit dislocation segments is large enough that 30%-80% of the lattice mismatch strain is relieved for layers as thin as 50-300 nm, respectively. 
     The inventive process produces a thin (&lt;300 nm) partially relaxed, single crystalline SiGe buffer layer on bulk Si or an SOI substrate with a very low-TD density, e.g., 10 5  cm −2  for Si 0.85 Ge 0.15  and &lt;10 6  cm −2  for Si 0.80 Ge 0.20 , and a high degree of surface smoothness. The commonly used strain relaxed graded SiGe buffer layers of comparable alloy composition have 1-2 orders of magnitude higher TD densities (at least on larger wafers such as 5″ or 8″ diameter), a surface roughness larger by at least a factor of 10 and total layer thickness larger by at least a factor of 10 as well. FIGS. 2 and 3 show a direct comparison of the layer thickness and the surface roughness. 
     Specifically, FIG. 2A shows a secondary ion mass spectroscopy (SIMS) profile that indicates the Ge composition variation as a function of the distance from the wafer surface; FIG. 2B shows the surface roughness as measured by atomic force microscopy (AFM); FIGS. 3A-B show the same types of data for a step-graded Si 0.85 Ge 0.15  layer. 
     The important requirements to obtain the low-TD density and smooth surface in thin (&lt;300 nm) SiGe buffer layers are: 
     a) Growth of a thin (&lt;300 nm) pseudomorphic Si 1−x Ge x  layer under conditions such that no strain relaxation occurs during the growth. This requires a method, such as UHV-CVD for example, where the initial wafer surface is extremely clean and the growth temperature is low (&lt;550° C.). Other suitable growth methods that can be employed in the present invention include: molecular beam epitaxy (MBE), chemical beam epitaxy (CBE), chemical vapor deposition (CVD), plasma-enhanced chemical vapor deposition (PECVD) and ion-assisted deposition. The strained SiGe layer is metastable, i.e., the layer exceeds the critical thickness for strain relaxation but no defects are nucleated during the layer growth. 
     b) The formation of a highly defective layer, i.e., damaged region  9 , at a depth of greater than 100 nm below the Si/Si 1−x Ge x  interface by ion implantation of He or other like light element at a dose in the range from about 5×10 15  to about 15×10 15  cm −2 . Strain relaxation occurs during subsequent annealing (e.g., at about 850° C. for about 1 hr. or equivalent rapid thermal anneal). 
     Having an ideal pseudomorphic SiGe layer in step (a) is key to achieve a low TD density in the final structure. The high degree of interfacial cleanliness and low growth temperature are key to avoiding any strain relaxation by the usual dislocation nucleation mechanism at the Si/Si 1−x Ge x  interface and the related dislocation multiplication that gives rise to dislocation pileups during the layer growth. As long as no dislocation multiplication occurs, the relaxation is exclusively governed by individual dislocations nucleated at platelets. However, if dislocation pileups are formed either, during the growth of the SiGe layer, or during annealing, the TD density will be higher and the surface will be rough. 
     The thin pseudomorphic Si 1−x Ge x  layer combined with a relatively large He implant depth are important since they do not result in a strong accumulation of He within the pseudomorphic layer and, more importantly, at the layer substrate interface. This accumulation is observed for the implant doses and conditions reported previously using prior art ion implantation conditions. The accumulation of He gives rise to He bubbles close to the Si/Si 1−x Ge x  interface, each of which gives rise to at least one TD extending from the He-induced bubble to the wafer surface. In contrast, applicants have found ion implantation conditions different from those reported in the literature that result in strain relaxation by a mechanism that is completely different from both the bubble mechanism previously reported for He implanted wafers and also the strain relaxation mechanism operative for graded buffer layer growth. 
     The new very effective strain relaxation-mechanism occurring in the present invention is dislocation nucleation at He-induced platelets (not bubbles) that lie parallel to the Si (001) surface, as shown in FIG. 4A, in a planar view transmission electron micrograph (PVTEM), and in FIG. 4B, in a cross sectional transmission electron micrograph (XTEM). The platelets can be as wide as 150 nm and eject dislocation half loops in the eight possible &lt;110&gt;-directions. The dislocation half loops having the right orientation extend to the interface where they deposit a misfit segment and where this misfit segment extends and relieves strain in the SiGe layer. The length of a misfit segment can be as long as several 10s of a μm so that the actual platelet spacing can be comparatively large (c.f. FIGS. 4A-B) and nevertheless result in a high degree of relaxation. The tremendous reduction of the TD density is a result of the nature of the platelets that act as intentionally inserted sources for dislocation nucleation. In graded buffer layers there is no control over the density and distribution of sources for dislocation nucleation. Thus, an irregular array of dislocations result in very uneven strain distribution in the relaxed SiGe layer, a very rough surface and regions of high and low TD densities. In the case of a high implant dose or low implant depth, a bubble rather than a platelet regime is entered. These bubble regimes are undesirable since they result in higher TD densities. 
     Bubbles that are induced using higher implant doses are shown in FIGS. 5A-B (Prior Art). The bubbles form at the Si/Si 1−x Ge x  interface at higher implant doses when the projected range of the implanted species is too close to the Si/Si 1−x Ge x  interface. The bubbles that reside at or close to the interface foster dislocation half loop nucleation due to their strain fields. The half loops are pushed from the bubbles to the layer surface, attracted by image forces as explained previously in H. Trinkaus, et al., Appl. Phys. Lett. 76, 3552 (2000) and M. Luysberg, et al., Microscopy on Semiconducting Materials, IOP Publishing, Oxford 2001, in press, and thereby create a high TD density. Bubbles that are induced by shallower implant are also undesirable. They are much smaller (only up to several 10s of nm) than the platelets and form at a much higher density and thus there is a much smaller average spacing between them as shown in the TEM micrographs in FIGS. 5A-B. This high bubble density creates a high density of dislocation nucleation sources in the SiGe layer resulting again in a high TD density. Thus, the platelet regime is the one that has to be met to obtain the lowest TD density. 
     At higher values of the Ge mole fraction (x&gt;0.25) it is difficult to grow a strictly pseudomorphic Si 1−x Ge x  layer due to the higher lattice mismatch strain, which induces surface roughening or islanding. Therefore, to achieve relaxed buffer layers having a higher Ge mole fraction, it may be necessary to first fabricate a relaxed Si 1−x Ge x  layer with x&lt;0.25 by the method proposed above and subsequently grow a second pseudomorphic Si 1−x Ge x  with higher x, implant He below the upper Si 1−x Ge x  layer and then anneal again to relax the upper Si 1−x Ge x  layer. This process can be repeated several times, increasing the Ge mole fraction of each successive layer, to achieve a relaxed Ge layer. 
     Dislocation nucleation is expected to occur by a similar platelet mechanism when other light elements such as H (hydrogen), D (deuterium), B (boron), or N (nitrogen) are implanted, or when a combination of elements such as H+B and He+B are implanted. The same element can be implanted at different depths using different implant energies. Combinations of different elements can be implanted at the same or at different depths by selecting suitable energies. This method of fabricating a relaxed SiGe buffer layer can also be applied to patterned Si or SOI substrates or to selected regions on blanket substrates. 
     Surprisingly, it has been determined that &gt;70% strain relaxation of a thin (about 200 nm) pseudomorphic Si 1−x Ge x  layer occurs by a platelet mechanism after ion implantation with relatively low doses of He and subsequent thermal annealing. This mechanism occurs when the projected range of the implanted species is greater than 100 nm below the Si/Si 1−x Ge x  interface. The thin SiGe layers fabricated by the inventive process are of very high quality, with smooth surfaces (RMS roughness &lt;1 nm) and TD densities &lt;10 6  cm −2 . This unexpected and efficient strain relaxation mechanism is distinctly different from the bubble mechanism that occurs when the projected range of the implanted species is &lt;100 nm from the interface (conditions that have been reported in the literature, e.g. in H. Trinkaus, et al., Appl. Phys., Lett. 76, 3552 (2000) and M. Luysberg, et al., Microscopy on Semiconducting Materials, IOP Publishing, Oxford 2001, in press). The inventive method of achieving a strain relaxed SiGe buffer layer is also completely different from the graded buffer layers that are now commonly used as ‘virtual substrates’ for a variety of devices. 
     The strain relaxed Si 1−x Ge x  buffer layers fabricated by the inventive process can be used as ‘virtual substrates’ for a wide variety of silicon-based devices including field effect transistors (FETs) of various types including strained silicon CMOS devices and modulation-doped field effect transistors (MODFETs). These buffer layers can also be used as ‘virtual substrates’ for various types of superlattices for many different applications. 
     The present invention discloses several methods for fabricating a strain relaxed epitaxial layer on a single crystalline surface with a mismatched lattice parameter and semiconductor structures that can be built on such a relaxed layer. More specifically, the present invention discloses methods for fabricating a partially strain relaxed SiGe, i.e, Si 1−x Ge x  buffer layer for application as a ‘virtual substrate’ for a variety of semiconductor devices having a strained Si or SiGe layer as the active region of the device. 
     According to one embodiment of the present invention and referring to FIG. 6, a thin, strictly pseudomorphic Si 1−x Ge x  layer  40  is grown epitaxially on a substrate having a single crystalline surface. The pseudomorphic layer is grown in a clean environment using a method such as ultra-high-vacuum chemical vapor deposition (UHV-CVD), MBE, PECVD, ion-assisted deposition or chemical beam epitaxy. In some embodiments, the Si 1−x Ge x  layer may include C therein. 
     The substrate  5  in FIG. 6 can be, for example, bulk Si or SOI and the single crystalline surface is of a layer selected from the group comprising Si, Si 1−x Ge x , Ge, Si 1−y C y , Si 1−x−y Ge x C y  and it can be patterned or not. The Si 1−x Ge x  layer thickness exceeds the critical thickness for misfit dislocation formation and due to the clean environment and a low growth temperature no dislocation nucleation occurs during the growth of this Si 1−x Ge x  layer. Helium is then implanted through the pseudomorphic Si 1−x Ge x  layer into the substrate below the Si/Si 1−x Ge x  interface. The He ions are implanted at doses in the range of from about 4×10 15  to about 4×10 16  cm −2 , preferably in the range of from about 7×10 15  to about 12×10 15  cm −2 . The wafer surface can be masked prior to implantation so that the He is implanted only into certain regions of the wafer, not over the entire wafer area. The projected range of the implanted He is about 100 to about 300 nm below the interface. Alternatively, the implanted ions can be from the group comprising H, D, B, or N. 
     The implanted wafer is then annealed in a furnace at temperatures above 650° C. for at least 30 minutes. As a result of the annealing, platelet-like defects are formed in layer  20  of FIG. 6, which is part of original single crystalline surface layer  10 . The platelets in layer  20 , which has a thickness of from about 20 to about 300 nm, give rise to dislocation nucleation. Layer  30 , which is also part of original single crystalline surface layer  10 , contains dislocations that thread to the interface with layer  40  where they form misfit segments. Layer  40  is between 50 nm and 500 nm thick (depending on the alloy composition), preferably about 100 nm. Moreover, layer  40  contains between 5 and 35 atomic % Ge and has a smooth surface (RMS roughness &lt;1 nm) and a threading dislocation (TD) density of less than 10 6  cm −2 . 
     In a second embodiment of the present invention, the procedure is similar to the one described in the first embodiment, except that the Si 1−x Ge x  layer  40  in FIG. 6 is replaced in FIG. 7 by layer  41  which has a graded alloy composition with x=0 at the bottom and 0&lt;x&lt;1.0 at the top of the layer. The composition of the graded layer  41  can change linearly or stepwise. 
     In a third embodiment of the present invention, the procedure is the same as described in the first two embodiments except that two different atomic species are implanted at the same or different depths from the Si/Si 1−x Ge x  interface. 
     In a fourth embodiment of the present invention, the procedure is the same as described in the first two embodiments except that the same atomic species is implanted at two different depths from the Si/Si 1−x Ge x  interface. 
     In a fifth embodiment of the present invention, a thin (50-300 nm), strictly pseudomorphic Si 1−y C y  layer, where y is as large as 0.02, is grown epitaxially on a substrate having a single crystalline surface layer. The substrate can be, for example, bulk Si or SOI, having a single crystalline surface from the group comprising Si, Si 1−x Ge x , Ge, Si 1−x−y Ge x C y . A 50-300 nm-thick strictly pseudomorphic crystalline Si layer is then grown on top of the Si 1−y C y  layer followed by a strictly pseudomorphic Si 1−x Ge x  layer. All the pseudomorphic crystalline layers are grown in a clean environment using a method such as ultra-high-vacuum chemical vapor deposition (UHV-CVD), MBE, PECVD, ion assisted deposition or chemical beam epitaxy. The Si 1−x Ge x  layer thickness exceeds the critical thickness for misfit dislocation formation and due to the clean environment and a low growth temperature no dislocation nucleation occurs during the growth of this Si 1−x Ge x  layer. The wafer is then annealed in a furnace at temperatures above 750° C. for at least 30 min. During annealing, defects formed in the carbon containing layer act as nucleation sources for dislocations which thread to the Si/Si 1−x Ge x  interface and form misfit dislocations that relieve the strain in the Si 1−x Ge x  layer. 
     In a sixth embodiment of the present invention, the relaxed SiGe buffer layer is fabricated by performing the steps described in the first and second embodiments at least twice, implanting either one or more atomic species as described in the third and fourth embodiments. This procedure may be necessary in order to achieve relaxed Si 1−x Ge x  buffer layers with x&gt;0.25. The Si 1−x Ge x  layer may have a uniform alloy composition or a graded alloy composition. Referring to FIG. 8, layers  5 ,  10 ,  20  and  30  are the same as in FIG.  6 . Layers  42 ,  25  and  35  together comprise layer  40  of FIG. 6 (i.e., the first relaxed SiGe layer) and therefore all have the same Ge content, which is between 5 and 35 atomic % Ge, and has a smooth surface (RMS &lt;1 nm) and a threading dislocation (TD) density less than 10 6  cm −2 . Layer  25  contains the second implant damage region with a thickness of about 150 nm containing platelets that give rise to dislocation nucleation. Layer  35 , like layer  30 , contains dislocations that thread to the interface to layer  45  where they form misfit segments. Layer  45  is the second relaxed uniform composition SiGe layer which has a larger atomic percent of Ge than layers  42 ,  25  and  35  and is between 50 nm and 500 nm thick. 
     Referring to FIG. 9, layers  43 ,  27  and  37  correspond to the original layer  41  of FIG. 7 which has a graded alloy composition with x=0 at the bottom and 0&lt;x&lt;1.0 at the top of the layer. The bottom of layer  46  has a composition equal to that of the top of layer  37  and the top of layer  46  has a greater alloy composition (up to x=1.0). The composition of the graded layer  46  can change linearly or stepwise. 
     A seventh embodiment is another variation of the method for fabricating a relaxed SiGe buffer layer in which a second Si 1−x Ge x  layer of higher atomic % Ge is grown epitaxially on the relaxed buffer layer fabricated according to one of the procedures described in the first five embodiments and then subsequently annealed so that strain relaxation may occur. This is done in order to achieve relaxed SiGe layers that have an alloy composition &gt;0.25. Referring to FIG. 10, layer  44 , which is grown epitaxially on top of layer  40  of FIG. 6, is between 50 and 500 nm thick, preferable between 100-200 nm and has Ge atomic % greater than layer  40 , between 15 and 60%, preferably between 20 and 40%. In FIG. 11, layer  47 , grown on top of layer  41  of FIG. 7, is between 50 and 500 nm thick, preferably 100-200 nm, and has a graded composition with Ge atomic % at the bottom that is equal to that of the top of layer  41  and is higher (up to x=1.0) at the top of the layer. The composition of the graded layer  47  can change linearly or stepwise. 
     As mentioned before, the methods described for the preparation of strain relaxed SiGe buffer layers on a Si containing single crystalline surface can by applied in similar ways to fabricate strain relaxed epitaxial layers of different materials on single crystalline lattice mismatched surfaces. 
     The relaxed Si 1−x Ge x  buffer layers fabricated by the methods described above may be used to fabricate SiGe-on-insulator substrates for integrated circuits using wafer bonding and layer transfer methods. These relaxed SiGe buffer layers may also be used as ‘virtual substrates’ for a variety of integrated circuits having at least one semiconductor device. 
     The structures obtained by the methods described above can be further expanded to fabricate more complex device structures. The device layer structures shown in FIGS. 12-17 are accordingly fabricated by growing additional epitaxial layers on the structures of FIGS. 6-11. 
     In FIG. 12, layer  400  is a SiGe layer that has the same atomic % Ge as layer  40 , thickness between 100 nm and 1000 nm, preferably between 300 nm and 500 nm, and the TD density is not higher than that of layer  40 . Layer  50  is a strictly pseudomorphic strained Si layer with a thickness between 50 and 350 nm, preferably about 200 nm. 
     In FIG. 13, layer  410  is a SiGe layer that has the same atomic % Ge as the top of layer  41 . The thickness of layer  410  is between 100 nm and 1000 nm, preferably between 300 nm and 500 nm and the TD density is not higher than that of layer  41 . Layer  50  is a strictly pseudomorphic strained Si layer with a thickness between 50 and 350 nm, preferably about 200 nm. 
     In FIG. 14, layer  450  is a SiGe layer that has the same atomic % Ge as layer  45 . The thickness of layer  450  is between 100 nm and 1000 nm, preferably between 300 nm and 500 nm and the TD density is not higher than that of layer  45 . Layer  50  is a strictly pseudomorphic strained Si layer with a thickness between 50 and 350 nm, preferably about 200 nm. 
     In FIG. 15, layer  460  is a SiGe layer that has the same atomic % Ge as the top of layer  46 . The thickness of layer  460  is between 100 nm and 1000 nm, preferably between 300 nm and 500 nm and the TD density is not higher than that of layer  46 . Layer  50  is a strictly pseudomorphic strained Si layer with a thickness between 50 and 350 nm, preferably about 200 nm. 
     In FIG. 16, layer  440  is a SiGe layer that has the same atomic % Ge as the top of layer  44 . The thickness of layer  440  is between 100 nm and 1000 nm, preferably between 300 nm and 500 nm and the TD density is not higher than that of layer  44 . Layer  50  is a strictly pseudomorphic strained Si layer with a thickness between 50 and 350 nm, preferably about 200 nm. 
     In FIG. 17, layer  470  is a SiGe layer that has the same atomic % Ge as the top of layer  47 . The thickness of layer  470  is between 100 nm and 1000 nm, preferably between 300 nm and 500 nm and the TD density is not higher than that of layer  47 . Layer  50  is a strictly pseudomorphic strained Si layer with a thickness between 50 and 350 nm, preferably about 200 nm. 
     The structures described above and in FIGS. 12-17 can be used to fabricate semiconductor devices. One embodiment is an integrated circuit consisting of at least one semiconductor device such as the field effect transistor (FET) shown in FIG.  18 . The FET shown in FIG. 18 is fabricated by way of illustration on the layer structure of FIG.  12 . In FIG. 18, the source contact is  100 , the drain contact is  101 , the gate dielectric is  102 , the gate contact is  103  and the sidewalls are  104 . The device structure of FIG. 18 could also be built on the layer structures of FIGS. 13,  14 ,  15 ,  16  and  17 , where layer  400  would be replaced by layer  410 ,  450 ,  460 ,  440  or  470  respectively. 
     The modulation-doped field effect transistor (MODFET) layer structures shown in FIGS. 19 and 20 can also be grown epitaxially on the layer structures of FIGS. 12-17. The structure of FIG. 19 is fabricated by way of illustration on the structure of FIG.  12 . The structure comprises a SiGe layer  120  of the same composition as layer  40  and  400 , an n+ doped SiGe layer  121  of otherwise the similar composition as layer  120 , and a pseudomorphic strained Si cap layer  51 . The same layer structure could be grown on the structures of FIGS. 13,  14 ,  15 ,  16  and  17 , where layer  400  would be replaced by layer  410 ,  450 ,  460 ,  440  or  470  respectively. 
     Alternatively, the MODFET layer structure in FIG. 20 can be grown epitaxially on the structure of FIG. 12 without the strained Si layer  50 . This structure comprises a p+ doped SiGe layer  60  of otherwise the same composition as layer  40  and  400 , a SiGe layer  48  of the same composition as layer  40  and  400 , a pseudomorphic compressively strained SiGe layer  130  with a Ge content that is substantially higher than in layer  40  and  400 , a SiGe layer  135  of the same composition as layer  40 , and a pseudomorphic strained Si cap  51 . The same layer structure can also be built on the structure of FIGS. 13,  14 ,  15 ,  16  and  17 , also without the strained Si layer  50 , where layer  400  would be replaced by layer  410 ,  450 ,  460 ,  440  or  470  respectively. 
     Another embodiment of an integrated circuit consisting of at least one semiconductor device such as the MODFET is illustrated in FIG.  21 . The device shown in FIG. 21 is built on the layer structure of FIG.  19 . In FIG. 21, layer  540  comprises all the layers above layer  400  as described in FIG.  19 . The MODFET comprises source contact  142 , drain contact  144 , and T-gate  150 . Alternatively the MODFET can be fabricated on the layer structure of FIG.  20 . In this case, layer  540  in FIG. 21 comprises all the layers above  400  as described in FIG.  20 . 
     Strain relaxed SiGe buffer layers can also be used for a variety of other applications. Some potential applications, e.g., thermoelectric cooling devices, require a superlattice structure which can be grown epitaxially on the layer structure shown in FIG. 12, but without the strained Si layer  50 , as shown in FIG.  22 . Layer  400  is optional. The superlattice structure consists of a repetition of alternating layers  550  and  560 , both pseudomorphic strained epitaxial layers wherein the composition of layer  550  is different from the compositions of layers  560 . In a specific case, the alternating layers are Si 1−x−y Ge x C y  and Si 1−z−w Ge z C w , wherein x and y are different from z and w, respectively and x and y can be equal to zero. The described superlattice structure can optionally be built on the structures of FIG. 13,  14 ,  15 ,  16  or  17 , also without the strained Si cap layer  50 , where layer  400  would be replaced by layer  410 ,  450 ,  460 ,  440  or  470 , respectively. The described superlattice structure can optionally be built on the structures of FIG. 13,  14 ,  15 ,  16  or  17 , also without the strained Si cap layer  50  and without the layers  410 ,  450 ,  460 ,  440  or  470 , respectively. 
     The following examples are given to illustrate the inventive process used in fabricating, a ‘virtual substrate’, i.e., a thin relaxed epitaxial Si 1−x Ge x  layer formed atop a Si or SOI substrate as well as the use of that ‘virtual substrate’ as a component of an electronic structure. 
     EXAMPLE 1 
     In this example, a ‘virtual substrate’ was fabricated by depositing a 100 nm-thick pseudomorphic Si 0.85 Ge 0.15  layer on a bulk Si substrate. The resultant structure was then implanted with He +  at a dose of about 1×10 16  cm −2 , using an implant energy of about 21 keV. The structure was subsequently annealed at approximately 850° C. for about 1 hour. HRXRD measurements after annealing show that 41% of the lattice mismatch strain was relieved. The sample had an RMS surface roughness of about 0.29 nm and an etch pit (TD) density of about 1×10 5  cm −2 . 
     EXAMPLE 2 
     A second implementation of the inventive process was also done according to the structure of FIG. 6, where layers  5  and  10  are a bulk Si substrate and layer  40  is a 100 nm-thick pseudomorphic Si 0.85 Ge 0.15  layer as measured by HRXRD prior to ion implantation. He +  was implanted at a dose of about 1×10 16  cm −2 , using an implant energy of about 21 keV. The wafer was subsequently annealed at approximately 850° C. for about 30 min. The SiGe layer was about 38% relaxed. 
     EXAMPLE 3 
     A third implementation of the inventive process was also done according to the structure of FIG. 6, where layers  5  and  10  are a bulk Si substrate and layer  40  is an 188 nm-thick Si 0.79 Ge 0.21  pseudomorphic layer as measured by HRXRD prior to ion implantation. He was implanted at a dose of about 0.8×10 16  cm −2  and at an energy of about 31 keV. The wafer was subsequently annealed at approximately 850° C. for about 1 hour. The SiGe layer was 69% relaxed. The RMS surface roughness was about 0.47 nm, and the etch pit (TD) density was about 2.7×10 5  cm −2 . 
     EXAMPLE 4 
     A fourth implementation of the inventive process was also done according to the structure of FIG. 6, where layers  5  and  10  are a bulk Si substrate and layer  40  is an 188 nm-thick pseudomorphic Si 0.79 Ge 0.21  layer as measured by HRXRD prior to ion implantation. He +  was implanted at a dose of about 1.2×10 16  cm −2  and at an energy of about 31 keV. The wafer was subsequently annealed at approximately 850° C. for about 1 hour. The SiGe layer was 68% relaxed, the RMS surface roughness was about 0.48 nm and the etch pit (TD) density was about 0.9×10 5  cm −2 . 
     While the present invention has been particularly shown and described with respect to preferred embodiments thereof, it will be understood by those skilled in the art that the foregoing and other changes in forms and details may be made without departing from the spirit and scope of the present invention. It is therefore intended that the present invention not be limited to the exact forms and details described and illustrated, but fall within the scope of the appended claims.