AU2007247955A1 - Semiconductor device having a semiconductor-on-insulator configuration and a superlattice and associated methods - Google Patents

Description

WO 2007/131119 PCT/US2007/068159 SEMICONDUCTOR DEVICE HAVING A SEMICONDUCTOR-ON INSULATOR CONFIGURATION AND A SUPERLATTICE AND ASSOCIATED METHODS Field of the Invention [0001]The present invention relates to the field of semiconductors, and, more particularly, to semiconductors having enhanced properties based upon energy band engineering and associated methods. Background of the Invention [0002] Structures and techniques have been proposed to enhance the performance of semiconductor devices, such as by enhancing the mobility of the charge carriers. For example, U.S. Patent Application No. 2003/0057416 to Currie et al. discloses strained material layers of silicon, silicon-germanium, and relaxed silicon and also including impurity-free zones that would otherwise cause performance degradation. The resulting biaxial strain in the upper silicon layer alters the carrier mobilities enabling higher speed and/or lower power devices. Published U.S. Patent Application No. 2003/0034529 to Fitzgerald et al. discloses a CMOS inverter also based upon similar strained silicon technology. [0003] U.S. Patent No. 6,472,685 B2 to Takagi discloses a semiconductor device including a silicon and carbon layer sandwiched between silicon layers so that the conduction band and valence band of the second silicon layer receive a tensile strain. Electrons having a smaller effective mass, and which have been induced by an electric field applied to the gate electrode, are confined in the second silicon layer, thus, an n-channel MOSFET is asserted to have a higher mobility. [0004] U.S. Patent No. 4,937,204 to Ishibashi et al. discloses a superlattice in which a plurality of layers, less than eight monolayers, and containing a WO 2007/131119 PCT/US2007/068159 fraction or a binary compound semiconductor layers, are alternately and epitaxially grown. The direction of main current flow is perpendicular to the layers of the superlattice. [0005] U.S. Patent No. 5,357,119 to Wang et al. discloses a Si-Ge short period superlattice with higher mobility achieved by reducing alloy scattering in the superlattice. Along these lines, U.S. Patent No. 5,683,934 to Candelaria discloses an enhanced mobility MOSFET including a channel layer comprising an alloy of silicon and a second material substitutionally present in the silicon lattice at a percentage that places the channel layer under tensile stress. [0006] U.S. Patent No. 5,216,262 to Tsu discloses a quantum well structure comprising two barrier regions and a thin epitaxially grown semiconductor layer sandwiched between the barriers. Each barrier region consists of alternate layers of SiO 2 /Si with a thickness generally in a range of two to six monolayers. A much thicker section of silicon is sandwiched between the barriers. [0007]An article entitled "Phenomena in silicon nanostructure devices" also to Tsu and published online September 6, 2000 by Applied Physics and Materials Science & Processing, pp. 391-402 discloses a semiconductor atomic superlattice (SAS) of silicon and oxygen. The SilO superlattice is disclosed as useful in a silicon quantum and light-emitting devices. In particular, a green electroluminescence diode structure was constructed and tested. Current flow in the diode structure is vertical, that is, perpendicular to the layers of the SAS. The disclosed SAS may include semiconductor layers separated by adsorbed species such as oxygen atoms, and CO molecules. The silicon growth beyond the adsorbed monolayer of oxygen is described as epitaxial with a fairly low defect density. One SAS structure included a 1.1 nm thick silicon portion that is about eight atomic layers of silicon, and another structure had twice this thickness of silicon. An article to Luo et al. entitled "Chemical Design of Direct-Gap Light-Emitting Silicon" published in Physical Review Letters, Vol. 89, No. 7 (August 12, 2002) further discusses the light emitting SAS structures of Tsu. 2 WO 2007/131119 PCT/US2007/068159 [0008] Published International Application WO 02/103,767 Al to Wang, Tsu and Lofgren, discloses a barrier building block of thin silicon and oxygen, carbon, nitrogen, phosphorous, antimony, arsenic or hydrogen to thereby reduce current flowing vertically through the lattice more than four orders of magnitude. The insulating layer/barrier layer allows for low defect epitaxial silicon to be deposited next to the insulating layer. [0009] Published Great Britain Patent Application 2,347,520 to Mears et al. discloses that principles of Aperiodic Photonic Band-Gap (APBG) structures may be adapted for electronic bandgap engineering. In particular, the application discloses that material parameters, for example, the location of band minima, effective mass, etc, can be tailored to yield new aperiodic materials with desirable band-structure characteristics. Other parameters, such as electrical conductivity, thermal conductivity and dielectric permittivity or magnetic permeability are disclosed as also possible to be designed into the material. [0010] Despite considerable efforts at materials engineering to increase the mobility of charge carriers in semiconductor devices, there is still a need for greater improvements. Greater mobility may increase device speed and/or reduce device power consumption. With greater mobility, device performance can also be maintained despite the continued shift to smaller device features. Summary of the Invention [0011] In view of the foregoing background, it is therefore an object of the present invention to provide a semiconductor device, such as a silicon-on insulator (SOI) device, having relatively high charge carrier mobility and related methods. [0012]This and other objects, features, and advantages in accordance with the present invention are provided by a semiconductor device which may include a substrate, an insulating layer adjacent the substrate, and a superlattice adjacent a face of the insulating layer opposite the substrate. More particularly, the superlattice may include a plurality of stacked groups of layers, with each group of layers including a plurality of stacked base 3 WO 2007/131119 PCT/US2007/068159 semiconductor monolayers defining a base semiconductor portion and an energy band-modifying layer thereon. Furthermore, the energy band-modifying layer may include at least one non-semiconductor monolayer constrained within a crystal lattice of adjacent base semiconductor portions. [0013] More particularly, the semiconductor device may further include a semiconductor layer between the insulating layer and the superlattice. Moreover, source and drain regions may be on the semiconductor layer, and the superlattice may extend between the source and drain regions. A gate may also overly the superlattice, and a contact layer may be on at least one of the source and drain regions. By way of example, the substrate may comprise silicon, and the insulating layer may comprise silicon oxide. [0014]Additionally, the superlattice may have a common energy band structure therein, and it may also have a higher charge carrier mobility than would otherwise be present without the energy band-modifying layer. Each base semiconductor portion may comprise at least one of silicon and germanium, and each energy band-modifying layer may comprise oxygen. Further, each energy band-modifying layer may be a single monolayer thick, and each base semiconductor portion may be less than eight monolayers thick. [0015]The superlattice may further have a substantially direct energy bandgap, and it may also include a base semiconductor cap layer on an uppermost group of layers. In one embodiment, all of the base semiconductor portions may be a same number of monolayers thick. In accordance with an alternate embodiment, at least some of the base semiconductor portions may be a different number of monolayers thick. In addition, each energy band modifying layer may include a non-semiconductor selected from the group consisting of oxygen, nitrogen, fluorine, and carbon-oxygen, for example. [0016]Another aspect of the invention is dirfected to a method for making a semiconductor device which may include forming an insulating layer adjacent a substrate, and providing a superlattice adjacent a face of the insulating layer opposite the substrate. More particularly, the superlattice may include a 4 WO 2007/131119 PCT/US2007/068159 plurality of stacked groups of layers, with each group of layers including a plurality of stacked base semiconductor monolayers defining a base semiconductor portion and an energy band-modifying layer thereon. Furthermore, the energy band-modifying layer may include at least one non semiconductor monolayer constrained within a crystal lattice of adjacent base semiconductor portions. The method may further include providing a semiconductor layer between the insulating layer and the superlattice. The source and drain regions may be formed on the semiconductor layer, and the superlattice may extend between the source and drain regions. A gate may also be formed overlying the superlattice, and a contact layer may be formed on at least one of the source and drain regions. By way of example, the substrate may comprise silicon, and the insulating layer may comprise silicon oxide. The substrate may also comprise germanium, for example. Brief Description of the Drawings [0017] FIG. 1 is schematic cross-sectional view of a semiconductor device in accordance with the present invention. [0018] FIG. 2 is a greatly enlarged schematic cross-sectional view of the superlattice as shown in FIG. 1. [0019] FIG. 3 is a perspective schematic atomic diagram of a portion of the superlattice shown in FIG. 1. [0020] FIG. 4 is a greatly enlarged schematic cross-sectional view of another embodiment of a superlattice that may be used in the device of FIG. 1. [0021] FIG. 5A is a graph of the calculated band structure from the gamma point (G) for both bulk silicon as in the prior art, and for the 4/1 Si/O superlattice as shown in FIGS. 1-3. [0022] FIG. 5B is a graph of the calculated band structure from the Z point for both bulk silicon as in the prior art, and for the 4/1 SilO superlattice as shown in FIGS. 1-3. [0023] FIG. 5C is a graph of the calculated band structure from both the gamma and Z points for both bulk silicon as in the prior art, and for the 5/1/311 SilO superlattice as shown in FIG. 4. 5 WO 2007/131119 PCT/US2007/068159 [0024] FIGS. 6A-6C are a series of schematic cross-sectional diagrams illustrating a method for making the semiconductor device of FIG. 1. Detailed Description of the Preferred Embodiments [0025]The present invention will now be described more fully hereinafter with reference to the accompanying drawings, in which preferred embodiments of the invention are shown. This invention may, however, be embodied in many different forms and should not be construed as limited to the embodiments set forth herein. Rather, these embodiments are provided so that this disclosure will be thorough and complete, and will fully convey the scope of the invention to those skilled in the art. Like numbers refer to like elements throughout, and prime notation is used to indicate similar elements in alternate embodiments. Also, the size of regions or thicknesses of various layers may be exaggerated in certain views for clarity of illustration. [0026] The present invention relates to controlling the properties of semiconductor materials at the atomic or molecular level to achieve improved performance within semiconductor devices. Further, the invention relates to the identification, creation, and use of improved materials for use in the conduction paths of semiconductor devices. [0027] Applicants theorize, without wishing to be bound thereto, that certain superlattices as described herein reduce the effective mass of charge carriers and that this thereby leads to higher charge carrier mobility. Effective mass is described with various definitions in the literature. As a measure of the improvement in effective mass Applicants use a "conductivity reciprocal effective mass tensor", M 1 and M.' for electrons and holes respectively, defined as: I f" (VkE(kn)), (VkE(kn))J Of (E(kn),E,,T) d 'k M-1 (EF T) - E>Er BZ. OE Jf (E(k,n),E,,T)d'k E>EF B.Z. 6 WO 2007/131119 PCT/US2007/068159 for electrons and: S (VkE(k,n)), (VkE(k,n))j f (E(k,n),E.,T) d3k M2Jf(E T)= E<E .aE ' J(1-f(E(k,n),EF,T))d 3 k E<Ep B.Z. for holes, where fis the Fermi-Dirac distribution, EF is the Fermi energy, T is the temperature, E(k,n) is the energy of an electron in the state corresponding to wave vector k and the n th energy band, the indices i and j refer to cartesian coordinates x, y and z, the integrals are taken over the Brillouin zone (B.Z.), and the summations are taken over bands with energies above and below the Fermi energy for electrons and holes respectively. [0028] Applicants' definition of the conductivity reciprocal effective mass tensor is such that a tensorial component of the conductivity of the material is greater for greater values of the corresponding component of the conductivity reciprocal effective mass tensor. Again Applicants theorize without wishing to be bound thereto that the superlattices described herein set the values of the conductivity reciprocal effective mass tensor so as to enhance the conductive properties of the material, such as typically for a preferred direction of charge carrier transport. The inverse of the appropriate tensor element is referred to as the conductivity effective mass. In other words, to characterize semiconductor material structures, the conductivity effective mass for electrons/holes as described above and calculated in the direction of intended carrier transport is used to distinguish improved materials. [0029] Using the above-described measures, one can select materials having improved band structures for specific purposes. One such example would be a superlattice 25 material for a channel region in a semiconductor device. A silicon-on-insulator (SOI) MOSFET 20 including the superlattice 25 in accordance with the invention is now first described with reference to FIG. 1. One skilled in the art, however, will appreciate that the materials identified 7 WO 2007/131119 PCT/US2007/068159 herein could be used in many different types of semiconductor devices, such as discrete devices and/or integrated circuits. [0030] The illustrated SOI MOSFET 20 includes a silicon substrate 21, an insulating layer (i.e., silicon oxide) 37 on the substrate, and a semiconductor (i.e., silicon) layer 39 on a face of the insulating layer opposite the substrate. Lightly doped source/drain extension regions 22, 23 and more heavily doped source/drain regions 26, 27 are formed in the semiconductor layer 39, as shown, and a channel region extending between the lightly doped source/drain extension regions is provided by the superlattice 25. Source/drain silicide layers 30, 31 and source/drain contacts 32, 33 overlie the source/drain regions, as will be appreciated by those skilled in the art. [0031] A gate 35 illustratively includes a gate insulating layer 36 (e.g., silicon oxide) adjacent the channel provided by the superlattice 25, and a gate electrode layer 38 (e.g., silicon) on the gate insulating layer. Sidewall spacers 40, 41 are also provided in the illustrated SOI MOSFET 20, as well as a silicide layer 34 on the gate electrode layer 36. For clarity of illustration, the insulating layer 37 and the gate insulating layer 36 are shown with stippling in the drawings. Moreover, the superlattice 25 is shown with dashes in regions where dopant from the source/drain implantations is present. It should be noted that other source/drain and gate configurations may be also used. [0032] As will be appreciated by those skilled in the art, the insulating layer 37 of the above-described SOI device advantageously provides reduced capacitance adjacent the source and drain regions 26, 27, thereby reducing switching time and providing faster device operation, for example. It should be noted that other materials may be used for the insulating layer 37, such as glass or sapphire, for example. Moreover, the substrate 21 and semiconductor layer 39 may comprise other semiconductor materials, such as germanium, for example. [0033] Applicants have identified improved materials or structures for the channel region of the SOl MOSFET 20. More specifically, the Applicants have identified materials or structures having energy band structures for which 8 WO 2007/131119 PCT/US2007/068159 the appropriate conductivity effective masses for electrons and/or holes are substantially less than the corresponding values for silicon. [0034] Referring now additionally to FIGS. 2 and 3, the materials or structures are in the form of a superlattice 25 whose structure is controlled at the atomic or molecular level and may be formed using known techniques of atomic or molecular layer deposition. The superlattice 25 includes a plurality of layer groups 45a-45n arranged in stacked relation, as perhaps best understood with specific reference to the schematic cross-sectional view of FIG. 2. [0035] Each group of layers 45a-45n of the superlattice 25 illustratively includes a plurality of stacked base semiconductor monolayers 46 defining a respective base semiconductor portion 46a-46n and an energy band modifying layer 50 thereon. The energy band-modifying layers 50 are indicated by stippling in FIG. 2 for clarity of illustration. [0036] The energy-band modifying layer 50 illustratively includes one non-semiconductor monolayer constrained within a crystal lattice of adjacent base semiconductor portions. In other embodiments, more than one such monolayer may be possible. It should be noted that reference herein to a non semiconductor or semiconductor monolayer means that the material used for the monolayer would be a non-semiconductor or semiconductor if formed in bulk. That is, a single monolayer of a material, such as semiconductor, may not necessarily exhibit the same properties that it would if formed in bulk or in a relatively thick layer, as will be appreciated by those skilled in the art. [0037] Applicants theorize without wishing to be bound thereto that energy band-modifying layers 50 and adjacent base semiconductor portions 46a-46n cause the superlattice 25 to have a lower appropriate conductivity effective mass for the charge carriers in the parallel layer direction than would otherwise be present. Considered another way, this parallel direction is orthogonal to the stacking direction. The band modifying layers 50 may also cause the superlattice 25 to have a common energy band structure. 9 WO 2007/131119 PCT/US2007/068159 [0038] It is also theorized that the semiconductor device described above enjoys a higher charge carrier mobility based upon the lower conductivity effective mass than would otherwise be present. In some embodiments, and as a result of the band engineering achieved by the present invention, the superlattice 25 may further have a substantially direct energy bandgap that may be particularly advantageous for opto-electronic devices, for example, as described in further detail below. [0039] As will be appreciated by those skilled in the art, the source/drain regions 22, 23 and gate 35 of the MOSFET 20 may be considered as regions for causing the transport of charge carriers through the superlattice in a parallel direction relative to the layers of the stacked groups 45a-45n. Other such regions are also contemplated by the present invention. [0040] The superlattice 25 also illustratively includes a cap layer 52 on an upper layer group 45n. The cap layer 52 may comprise a plurality of base semiconductor monolayers 46. The cap layer 52 may have between 2 to 100 monolayers of the base semiconductor, and, more preferably between 10 to 50 monolayers. [0041] Each base semiconductor portion 46a-46n may comprise a base semiconductor selected from the group consisting of Group IV semiconductors, Group III-V semiconductors, and Group II-VI semiconductors. Of course, the term Group IV semiconductors also includes Group IV-IV semiconductors, as will be appreciated by those skilled in the art. More particularly, the base semiconductor may comprise at least one of silicon and germanium, for example. [0042] Each energy band-modifying layer 50 may comprise a non semiconductor selected from the group consisting of oxygen, nitrogen, fluorine, and carbon-oxygen, for example. The non-semiconductor is also desirably thermally stable through deposition of a next layer to thereby facilitate manufacturing. In other embodiments, the non-semiconductor may be another inorganic or organic element or compound that is compatible with the given semiconductor processing as will be appreciated by those skilled in the 10 WO 2007/131119 PCT/US2007/068159 art. More particularly, the base semiconductor may comprise at least one of silicon and germanium, for example. [0043] It should be noted that the term monolayer is meant to include a single atomic layer and also a single molecular layer. It is also noted that the energy band-modifying layer 50 provided by a single monolayer is also meant to include a monolayer wherein not all of the possible sites are occupied. For example, with particular reference to the atomic diagram of FIG. 3, a 4/1 repeating structure is illustrated for silicon as the base semiconductor material, and oxygen as the energy band-modifying material. Only half of the possible sites for oxygen are occupied. [0044] In other embodiments and/or with different materials this one half occupation would not necessarily be the case as will be appreciated by those skilled in the art. Indeed it can be seen even in this schematic diagram, that individual atoms of oxygen in a given monolayer are not precisely aligned along a flat plane as will also be appreciated by those of skill in the art of atomic deposition. By way of example, a preferred occupation range is from about one-eighth to one-half of the possible oxygen sites being full, although other numbers may be used in certain embodiments. [0045] Silicon and oxygen are currently widely used in conventional semiconductor processing, and, hence, manufacturers will be readily able to use these materials as described herein. Atomic or monolayer deposition is also now widely used. Accordingly, semiconductor devices incorporating the superlattice 25 in accordance with the invention may be readily adopted and implemented, as will be appreciated by those skilled in the art. [0046] It is theorized without Applicants wishing to be bound thereto, that for a superlattice, such as the SilO superlattice, for example, that the number of silicon monolayers should desirably be seven or less so that the energy band of the superlattice is common or relatively uniform throughout to achieve the desired advantages. The 4/1 repeating structure shown in FIGS. 2 and 3, for SilO has been modeled to indicate an enhanced mobility for electrons and holes in the X direction. For example, the calculated conductivity

II

WO 2007/131119 PCT/US2007/068159 effective mass for electrons (isotropic for bulk silicon) is 0.26 and for the 4/1 SiO superlattice in the X direction it is 0.12 resulting in a ratio of 0.46. Similarly, the calculation for holes yields values of 0.36 for bulk silicon and 0.16 for the 4/1 SilO superlattice resulting in a ratio of 0.44. [0047] While such a directionally preferential feature may be desired in certain semiconductor devices, other devices may benefit from a more uniform increase in mobility in any direction parallel to the groups of layers. It may also be beneficial to have an increased mobility for both electrons or holes, or just one of these types of charge carriers as will be appreciated by those skilled in the art. [0048] The lower conductivity effective mass for the 4/1 Si/O embodiment of the superlattice 25 may be less than two-thirds the conductivity effective mass than would otherwise occur, and this applies for both electrons and holes. Of course, the superlattice 25 may further comprise at least one type of conductivity dopant therein as will also be appreciated by those skilled in the art. [0049] Indeed, referring now additionally to FIG. 4, another embodiment of a superlattice 25' in accordance with the invention having different properties is now described. In this embodiment, a repeating pattern of 31/1/5/1 is illustrated. More particularly, the lowest base semiconductor portion 46a' has three monolayers, and the second lowest base semiconductor portion 46b' has five monolayers. This pattern repeats throughout the superlattice 25'. The energy band-modifying layers 50' may each include a single monolayer. For such a superlattice 25' including SilO, the enhancement of charge carrier mobility is independent of orientation in the plane of the layers. Those other elements of FIG. 4 not specifically mentioned are similar to those discussed above with reference to FIG. 2 and need no further discussion herein. [0050] In some device embodiments, all of the base semiconductor portions of a superlattice may be a same number of monolayers thick. In other embodiments, at least some of the base semiconductor portions may be a 12 WO 2007/131119 PCT/US2007/068159 different number of monolayers thick. In still other embodiments, all of the base semiconductor portions may be a different number of monolayers thick. [0051] In FIGS. 5A-5C band structures calculated using Density Functional Theory (DFT) are presented. It is well known in the art that DFT underestimates the absolute value of the bandgap. Hence all bands above the gap may be shifted by an appropriate "scissors correction." However the shape of the band is known to be much more reliable. The vertical energy axes should be interpreted in this light. [0052] FIG. 5A shows the calculated band structure from the gamma point (G) for both bulk silicon (represented by continuous lines) and for the 4/1 SilO superlattice 25 as shown in FIGS. 1-3 (represented by dotted lines). The directions refer to the unit cell of the 4/1 Si/O structure and not to the conventional unit cell of Si, although the (001) direction in the figure does correspond to the (001) direction of the conventional unit cell of Si, and, hence, shows the expected location of the Si conduction band minimum. The (100) and (010) directions in the figure correspond to the (110) and (-110) directions of the conventional Si unit cell. Those skilled in the art will appreciate that the bands of Si on the figure are folded to represent them on the appropriate reciprocal lattice directions for the 4/1 SilO structure. [0053] It can be seen that the conduction band minimum for the 4/1 SilO structure is located at the gamma point in contrast to bulk silicon (Si), whereas the valence band minimum occurs at the edge of the Brillouin zone in the (001) direction which we refer to as the Z point. One may also note the greater curvature of the conduction band minimum for the 4/1 SilO structure compared to the curvature of the conduction band minimum for Si owing to the band splitting due to the perturbation introduced by the additional oxygen layer. [0054] FIG. 5B shows the calculated band structure from the Z point for both bulk silicon (continuous lines) and for the 4/1 SilO superlattice 25 (dotted lines). This figure illustrates the enhanced curvature of the valence band in the (100) direction. 13 WO 2007/131119 PCT/US2007/068159 [0055] FIG. 5C shows the calculated band structure from both the gamma and Z point for both bulk silicon (continuous lines) and for the 511/3/1 Si/O structure of the superlattice 25' of FIG. 4 (dofftted lines). Due to the symmetry of the 5/1/3/1 SilO structure, the calculated band structures in the (100) and (010) directions are equivalent. Thus the conductivity effective mass and mobility are expected to be isotropic in the plane parallel to the layers, i.e. perpendicular to the (001) stacking direction. Note that in the 5/1/3/1 SilO example the conduction band minimum and the valence band maximum are both at or close to the Z point. [0056] Although increased curvature is an indication of reduced effective mass, the appropriate comparison and discrimination may be made via the conductivity reciprocal effective mass tensor calculation. This leads Applicants to further theorize that the 5/1/3/1 superlattice 25' should be substantially direct bandgap. As will be understood by those skilled in the art, the appropriate matrix element for optical transition is another indicator of the distinction between direct and indirect bandgap behavior. [0057] Referring now additionally to FIGS. 6A-6C, a method for making the SOI MOSFET 20 will now be described. The method begins with providing a first semiconductor (e.g., silicon) substrate 61. By way of example, the substrate 61 may be an eight-inch wafer 21 of lightly doped P-type or N-type single crystal silicon with <100> orientation, although other suitable substrates may also be used. In accordance with the present example, a base undoped silicon layer 152 is epitaxially formed across the upper surface of the substrate 61. In some embodiments, the layer 152 may include a plurality of base semiconductor monolayers 46 to define a cap layer for the superlattice 25, which is then formed thereon. It should be noted that the superlattice here is being constructed in the reverse order as described above, since it will be "flipped" when combined with the SOI substrate 21, as discussed further below. [0058] The superlattice 25 material is deposited using atomic layer deposition, as will be appreciated by those skilled in the art. It should be noted 14 WO 2007/131119 PCT/US2007/068159 that in some embodiments the superlattice 25 material may be selectively deposited in those regions where channels are to be formed, rather than across the entire substrate 61, as will also be appreciated by those skilled in the art. The epitaxial silicon layer 39 is formed on the superlattice 25, and this layer is preferably undoped or lightly doped. The thickness of the layer 39 should be appropriate for either a partially or fully depleted device, as the case may be. Planarization may be used as necessary after forming the above layers to achieve the desired thickness and surface characteristics. [0059] Following the formation of the layer 39, the substrate 61 is implanted with ions (e.g., hydrogen ions), as illustrated with the downward pointing arrows in FIG. 6A, to form a separation layer 60 in the first substrate. The separation layer 60 is generally parallel to the upper surface of the substrate 61 and is located at a position corresponding to the mean penetration depth of the ion implantation, as will be appreciated by those skilled in the art. The implantation energy is chosen to give the desired overall layer thickness and the appropriate cap layer thickness in the finished device 20. [0060] The insulating layer 37 is formed on a second substrate 21, which may be similar to the substrate 61 discussed above, using conventional semiconductor processing techniques. While shown side by side for reference in FIG. 6A, it should be noted that the two substrates 21, 61 need not be processed simultaneously. [0061] The substrate 61 is then flipped and the layer 39 is bonded to the insulating layer 37 as shown in FIG. 6B. Once bonded, a thermal cycle is performed to fracture and cleave the first substrate 61 at the separation layer 60, as will be appreciated by those skilled in the art. The separated substrate 61 may then be discarded, and the substrate 21 becomes the base substrate for the device 20. [0062] The SOI MOSFET 20 is shown in FIG. 6C after the gate oxide 36, the gate electrode 38, and spacers 40, 41 are formed. More particularly, a thin gate oxide 36 is deposited, and steps of poly deposition, patterning, and 15 WO 2007/131119 PCT/US2007/068159 etching are performed. Poly deposition refers to low pressure chemical vapor deposition (LPCVD) of silicon onto an oxide (hence it forms a polycrystalline material). The step includes doping with P+ or As- to make it conducting, and the layer may be around 250 nm thick, for example. [0063] In addition, the pattern step may include performing a spinning photoresist, baking, exposure to light (i.e., a photolithography step), and developing the resist. Usually, the pattern is then transferred to another layer (oxide or nitride) which acts as an etch mask during the etch step. The etch step typically is a plasma etch (anisotropic, dry etch) that is material selective (e.g., etches silicon ten times faster than oxide) and transfers the lithography pattern into the material of interest. The sidewall spacers 40, 41 may be formed after patterning of the gate stack, as will be appreciated by those skilled in the art. [0064] Once the gate 35 and sidewall spacers 40, 41 are formed, they may be used as an etch mask to remove the superlattice 25 material and portions of the substrate 21 in the regions where the source and drain are to be formed, as will be appreciated by those skilled in the art. As may be seen in FIG. 1, this step forms an underlying step portion of the silicon layer 39 beneath the superlattice 25. [0065] The superlattice 25 material may be etched in a similar fashion to that described above for the gate 35. However, it should be noted that with the non-semiconductor present in the superlattice 25, e.g., oxygen, the superlattice may be more easily etched using an etchant formulated for oxides rather than silicon. Of course, the appropriate etch for a given implementation will vary based upon the structure and materials used for the superlattice 25 and substrate 21, as will be appreciated by those of skill in the art. [0066] The lightly doped source and drain ("LDD") extensions 22, 23 are formed using n-type or p-type LDD implantation, annealing, and cleaning. An anneal step may be used after the LDD implantation, but depending on the specific process, it may be omitted. The clean step is a chemical etch to remove metals and organics prior to depositing an oxide layer. It should be 16 WO 2007/131119 PCT/US2007/068159 noted that when the source and drain regions are said to be formed "on" the semiconductor layer 39 herein, this is meant to include implantations in the semiconductor layer as well as raised source/drain regions which may be formed on top of the semiconductor layer, as will be appreciated by those skilled in the art. [0067] To form the source and drain 26, 27 implants, an Si0 2 mask is deposited and etched back. N-type or p-type ion implantation is used to form the source and drain regions 26, 27. The structure is then annealed and cleaned. Self-aligned silicide formation may then be performed to form the silicide layers 30, 31, and 34, and the source/drain contacts 32, 33, are formed to provide the final SOl MOSFET device 20 illustrated in FIG. 1. The silicide formation is also known as salicidation. The salicidation process includes metal deposition (e.g. Ti), nitrogen annealing, metal etching, and a second annealing. [0068] The foregoing is, of course, but one example of a process and device in which the present invention may be used, and those of skill in the art will understand its application and use in many other processes and devices. In other processes and devices the structures of the present invention may be formed on a portion of a wafer or across substantially all of a wafer. Additionally, the use of an atomic layer deposition tool may also not be needed for forming the superlattice 25 in some embodiments. For example, the monolayers may be formed using a CVD tool with process conditions compatible with control of monolayers, as will be appreciated by those skilled in the art. [0069] It should be noted that devices other than MOSFETs may be produced in accordance with the present invention. By way of example, one type of insulator-on-substrate device that may be produced using the above described techniques is a memory device. Other potential insulator-on substrate devices include optical devices. [0070] Many modifications and other embodiments of the invention will come to the mind of one skilled in the art having the benefit of the teachings 17 WO 2007/131119 PCT/US2007/068159 presented in the foregoing descriptions and the associated drawings. Therefore, it is understood that the invention is not to be limited to the specific embodiments disclosed, and that modifications and embodiments are intended to be included within the scope of the appended claims. 18

Claims (30)

1. A semiconductor device comprising: a substrate; an insulating layer adjacent said substrate; a semiconductor layer adjacent a face of said insulating layer opposite said substrate; source and drain regions on said semiconductor layer; a superlattice adjacent said semiconductor layer and extending between said source and drain regions to define a channel; and a gate overlying said superlattice; said superlattice comprising a plurality of stacked groups of layers with each group of layers comprising a plurality of stacked base semiconductor monolayers defining a base semiconductor portion and an energy band-modifying layer thereon; said energy band-modifying layer comprising at least one non semiconductor monolayer constrained within a crystal lattice of adjacent base semiconductor portions.

2. The semiconductor device of Claim 1 further comprising a contact layer on at least one of said source and drain regions.

3. The semiconductor device of Claim 1 wherein said substrate comprises silicon, and wherein said insulating layer comprises silicon oxide.

4. The semiconductor device of Claim 1 wherein said superlattice has a common energy band structure therein.

5. The semiconductor device of Claim 1 wherein said superlattice has a higher charge carrier mobility than would otherwise be present without said energy band-modifying layer.

6. The semiconductor device of Claim 1 wherein each base semiconductor portion comprises silicon.

7. The semiconductor device of Claim 1 wherein each base semiconductor portion comprises germanium. 19 WO 2007/131119 PCT/US2007/068159

8. The semiconductor device of Claim 1 wherein each energy band-modifying layer comprises oxygen.

9. The semiconductor device of Claim 1 wherein each energy band-modifying layer is.a single monolayer thick.

10. The semiconductor device of Claim 1 wherein each base semiconductor portion is less than eight monolayers thick.

11. The semiconductor device of Claim 1 wherein said superlattice further has a substantially direct energy bandgap.

12. The semiconductor device of Claim 1 wherein said superlattice further comprises a base semiconductor cap layer on an uppermost group of layers.

13. The semiconductor device of Claim 1 wherein all of said base semiconductor portions are a same number of monolayers thick.

14. The semiconductor device of Claim 1 wherein at least some of said base semiconductor portions are a different number of monolayers thick.

15. The semiconductor device of Claim 1 wherein each energy band-modifying layer comprises a non-semiconductor selected from the group consisting of oxygen, nitrogen, fluorine, and carbon-oxygen.

16.A method for making a semiconductor device comprising: forming an insulating layer adjacent a substrate; forming a superlattice adjacent a semiconductor layer, and positioning the semiconductor layer adjacent a face of the insulating layer opposite the substrate; forming a gate overlying the superlattice; and forming source and drain regions on the semiconductor layer so that the superlattice extends therebetween to define a channel; the superlattice comprising a plurality of stacked groups of layers with each group of layers comprising a plurality of stacked base semiconductor monolayers defining a base semiconductor portion and an energy band-modifying layer thereon; 20 WO 2007/131119 PCT/US2007/068159 the energy band-modifying layer comprising at least one non semiconductor monolayer constrained within a crystal lattice of adjacent base semiconductor portions.

17.The method of Claim 16 further comprising forming a contact layer on at least one of the source and drain regions.

18.The method of Claim 16 wherein the substrate comprises silicon, and wherein the insulating layer comprises silicon oxide.

19.The method of Claim 16 wherein the superlattice has a common energy band structure therein.

20. The method of Claim 16 wherein the superlattice has a higher charge carrier mobility than would otherwise be present without the energy band-modifying layer.

21. The method of Claim 16 wherein each base semiconductor portion comprises silicon.

22. The method of Claim 16 wherein each base semiconductor portion comprises germanium.

23.The method of Claim 16 wherein each energy band modifying layer comprises oxygen.

24.The method of Claim 16 wherein each energy band modifying layer is a single monolayer thick.

25.The method of Claim 16 wherein each base semiconductor portion is less than eight monolayers thick.

26. The method of Claim 16 wherein the superlattice further has a substantially direct energy bandgap.

27. The method of Claim 16 wherein the superlattice further comprises a base semiconductor cap layer on an uppermost group of layers.

28.The method of Claim 16 wherein all of the base semiconductor portions are a same number of monolayers thick.

29.The method of Claim 16 wherein at least some of the base semiconductor portions are a different number of monolayers thick. 21 WO 2007/131119 PCT/US2007/068159

30.The method of Claim 16 wherein each energy band modifying layer comprises a non-semiconductor selected from the group consisting of oxygen, nitrogen, fluorine, and carbon-oxygen. 22