Patent Publication Number: US-9431243-B2

Title: Lattice-mismatched semiconductor structures with reduced dislocation defect densities and related methods for device fabrication

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application is a continuation in part of U.S. patent application Ser. No. 14/635,793, filed Mar. 2, 2015, which is a continuation of U.S. patent application Ser. No. 14/313,699, filed Jun. 24, 2014, (now U.S. Pat. No. 8,987,028, Issued Mar. 24, 2015) which is a continuation of U.S. patent application Ser. No. 14/104,924, filed Dec. 12, 2013, (now U.S. Pat. No. 8,796,734 B2, Issued Aug. 5, 2014) which is a continuation of U.S. patent application Ser. No. 13/903,762, filed May 28, 2013, (now U.S. Pat. No. 8,629,477 B2, Issued Jan. 14, 2014) which is a continuation of U.S. patent application Ser. No. 13/681,214, filed Nov. 19, 2012, (now U.S. Pat. No. 8,519,436 B2, Issued Aug. 27, 2013) which is a divisional of U.S. patent application Ser. No. 12/845,593, filed Jul. 28, 2010, (now U.S. Pat. No. 8,324,660 B2, Issued Dec. 4, 2012) which is a continuation of U.S. patent application Ser. No. 12/180,254, filed Jul. 25, 2008, (now U.S. Pat. No. 9,153,645 B2, Issued Oct. 6, 2015) which is a continuation-in-part of U.S. patent application Ser. No. 11/436,198, filed May 17, 2006, which claims priority to and benefit of U.S. Provisional Application Ser. No. 60/681,940 filed May 17, 2005. The entire disclosures of these applications are incorporated herein by reference. 
     U.S. patent application Ser. No. 12/845,593, filed Jul. 28, 2010, (now U.S. Pat. No. 8,324,660 B2, Issued Dec. 4, 2012) is a continuation-in-part of U.S. patent application Ser. No. 11/436,062, filed May 17, 2006. The entire disclosures of these applications are incorporated herein by reference. 
    
    
     FIELD OF THE INVENTION 
     This invention relates generally to lattice-mismatched semiconductor heterostructures and, more specifically, to the selective channel material regrowth in connection with the integration of dissimilar semiconductor materials. 
     BACKGROUND OF THE INVENTION 
     The increasing operating speeds and computing power of microelectronic devices have recently given rise to the need for an increase in the complexity and functionality of the semiconductor structures from which that these devices are fabricated. Hetero-integration of dissimilar semiconductor materials, for example, III-V materials, such as gallium arsenide, gallium nitride, indium aluminum arsenide, and/or germanium with silicon or silicon-germanium substrate, is an attractive path to increasing the functionality and performance of the CMOS platform. In particular, heteroepitaxial growth can be used to fabricate many modern semiconductor devices where lattice-matched substrates are not commercially available or to potentially achieve monolithic integration with silicon microelectronics. Performance and, ultimately, the utility of devices fabricated using a combination of dissimilar semiconductor materials, however, depends on the quality of the resulting structure. Specifically, a low level of dislocation defects is important in a wide variety of semiconductor devices and processes, because dislocation defects partition an otherwise monolithic crystal structure and introduce unwanted and abrupt changes in electrical and optical properties, which, in turn, results in poor material quality and limited performance. In addition, the threading dislocation segments can degrade physical properties of the device material and can lead to premature device failure. 
     As mentioned above, dislocation defects typically arise in efforts to epitaxially grow one kind of crystalline material on a substrate of a different kind of material—often referred to as “heterostructure”—due to different crystalline lattice sizes of the two materials. This lattice mismatch between the starting substrate and subsequent layer(s) creates stress during material deposition that generates dislocation defects in the semiconductor structure. 
     Misfit dislocations form at the mismatched interface to relieve the misfit strain. Many misfit dislocations have vertical components, termed “threading segments,” which terminate at the surface. These threading segments continue through all semiconductor layers subsequently added to the heterostructure. In addition, dislocation defects can arise in the epitaxial growth of the same material as the underlying substrate where the substrate itself contains dislocations. Some of the dislocations replicate as threading dislocations in the epitaxially grown material. Other kinds of dislocation defects include stacking faults, twin boundaries, and anti-phase boundaries. Such dislocations in the active regions of semiconductor devices, such as diodes, lasers and transistors, may significantly degrade performance. 
     To minimize formation of dislocations and associated performance issues, many semiconductor heterostructure devices known in the art have been limited to semiconductor layers that have very closely—e.g. within 0.1%—lattice-matched crystal structures. In such devices a thin layer is epitaxially grown on a mildly lattice-mismatched substrate. As long as the thickness of the epitaxial layer is kept below a critical thickness for defect formation, the substrate acts as a template for growth of the epitaxial layer, which elastically conforms to the substrate template. While lattice matching and near matching eliminate dislocations in a number of structures, there are relatively few lattice-matched systems with large energy band offsets, limiting the design options for new devices. 
     Accordingly, there is considerable interest in heterostructure devices involving greater epitaxial layer thickness and greater lattice misfit than known approaches would allow. For example, it has long been recognized that gallium arsenide grown on silicon substrates would permit a variety of new optoelectronic devices marrying the electronic processing technology of silicon VLSI circuits with the optical component technology available in gallium arsenide. See, for example, Choi et al, “Monolithic Integration of Si MOSFET&#39;s and GaAs MESFET&#39;s”, IEEE Electron Device Letters, Vol. EDL-7, No. 4, April 1986. Highly advantageous results of such a combination include high-speed gallium arsenide circuits combined with complex silicon VLSI circuits, and gallium arsenide optoelectronic interface units to replace wire interconnects between silicon VLSI circuits. Progress has been made in integrating gallium arsenide and silicon devices. See, for example, Choi et al, “Monolithic Integration of GaAs/AlGaAs Double-Heterostructure LED&#39;s and Si MOSFETs” IEEE Electron Device Letters, Vol. EDL-7, No. 9, September 1986; Shichijo et al, “Co-Integration of GaAs MESFET and Si CMOS Circuits”, IEEE Electron Device Letters, Vol. 9, No. 9, September 1988. However, despite the widely recognized potential advantages of such combined structures and substantial efforts to develop them, their practical utility has been limited by high defect densities in gallium arsenide layers grown on silicon substrates. See, for example, Choi et al, “Monolithic Integration of GaAs/AlGaAs LED and Si Driver Circuit”, IEEE Electron Device Letters, Vol. 9, No. 10, October 1988 (p. 513). Thus, while basic techniques are known for integrating gallium arsenide and silicon devices, there exists a need for producing gallium arsenide layers having a low density of dislocation defects. 
     To control dislocation densities in highly-mismatched deposited layers, there are three known techniques: wafer bonding of dissimilar materials, substrate patterning, and composition grading. Bonding of two different semiconductors may yield satisfactory material quality. Due to the limited availability and high cost of large size Ge or III-V wafers, however, the approach may not be practical. 
     Techniques involving substrate patterning exploit the fact that the threading dislocations are constrained by geometry, i.e. that a dislocation cannot end in a crystal. If the free edge is brought closer to another free edge by patterning the substrate into smaller growth areas, then it is possible to reduce threading dislocation densities. In the past, a combination of substrate patterning and epitaxial lateral overgrowth (“ELO”) techniques was demonstrated to greatly reduce defect densities in gallium nitride device, leading to fabrication of laser diodes with extended lifetimes. This process substantially eliminates defects in ELO regions but highly defective seed windows remain, necessitating repetition of the lithography and epitaxial steps to eliminate all defects. In a similar approach, pendeo-epitaxy eliminates substantially all defects in the epitaxial region proximate to the substrate but requires one lithography and two epitaxial growth steps. Furthermore, both techniques require the increased lateral growth rate of gallium nitride, which has not been demonstrated in all heteroepitaxial systems. Thus, a general defect-reduction process utilizing a minimum of lithography/epitaxy steps that does not rely on increased lateral growth rates would be advantageous both to reduce process complexity and facilitate applicability to various materials systems. 
     Another known technique termed “epitaxial necking” was demonstrated in connection with fabricating a Ge-on-Si heterostructure by Langdo et al. in “High Quality Ge on Si by Epitaxial Necking,” Applied Physics Letters, Vol. 76, No. 25, April 2000. This approach offers process simplicity by utilizing a combination of selective epitaxial growth and defect crystallography to force defects to the sidewall of the opening in the patterning mask, without relying on increased lateral growth rates. Specifically, as shown in  FIGS. 1A and 1B , in the (111)&lt;110&gt; diamond cubic slip system, misfit dislocations lie along &lt;110&gt; directions in the (100) growth plane while the threading segments rise up on (111) planes in &lt;110&gt; directions. Threading segments in &lt;110&gt; directions on the (111) plane propagate at a 45° angle to the underlying Si (100) substrate surface. Thus, if the aspect ratio of the holes in the patterning mask is greater than 1, threading segments will be blocked by the mask sidewall, resulting in low-defect top Ge “nodules” formed directly on Si. One important limitation of epitaxial necking, however, is the size of the area to which it applies. In general, as discussed in more detail below, the lateral dimensions (designated as  1  in  FIG. 1A ) in both dimensions have to be relatively small in order for the dislocations to terminate at sidewalls. 
     Thus, there is a need in the art for versatile and efficient methods of fabricating semiconductor heterostructures that would constrain dislocation defects in a variety of lattice-mismatched materials systems. There is also a need in the art for semiconductor devices utilizing a combination of integrated lattice-mismatched materials with reduced levels of dislocation defects for improved functionality and performance. 
     SUMMARY OF THE INVENTION 
     Accordingly, embodiments of the present invention provide semiconductor heterostructures with significantly minimized interface defects, and methods for their fabrication, that overcome the limitations of known techniques. In contrast with the prior art approach of minimizing dislocation defects by limiting misfit epitaxial layers to less than their critical thicknesses for elastic conformation to the substrate, in its various embodiments, the present invention utilizes greater thicknesses and limited lateral areas of component semiconductor layers to produce limited-area regions having upper portions substantially exhausted of threading dislocations and other dislocation defects such as stacking faults, twin boundaries, or anti-phase boundaries. As a result, the invention contemplates fabrication of semiconductor devices based on monolithic lattice-mismatched heterostructures long sought in the art but heretofore impractical due to dislocation defects. 
     In particular applications, the invention features semiconductor structures of Ge or III-V devices integrated with a Si substrate, such as, for example, an optoelectronic device including a gallium arsenide layer disposed over a silicon wafer, as well as features methods of producing semiconductor structures that contemplate integrating Ge or III-V materials on selected areas on a Si substrate. 
     In general, in one aspect, the invention is directed to a method of forming a semiconductor heterostructure. The method includes providing a substrate that contains, or consists essentially of, a first semiconductor material, and then providing a dislocation-blocking mask over the substrate. The mask has an opening extending to the surface of the substrate and defined by at least one sidewall. At least a portion of the sidewall meets the surface of the substrate at an orientation angle to a selected crystallographic direction of the first semiconductor material. The method further includes depositing in the opening a regrowth layer that includes a second semiconductor material, such that the orientation angle causes threading dislocations in the regrowth layer to decrease in density with increasing distance from the surface of the substrate. The dislocation-blocking mask may include a dielectric material, such as, for example, silicon dioxide or silicon nitride. 
     Embodiments of this aspect of the invention include one or more of the following features. An overgrowth layer that includes the second semiconductor material can be deposited over the regrowth layer and over at least a portion of the dislocation-blocking mask. At least at least a portion of the overgrowth layer can be crystallized. The regrowth layer can be planarized, for example, such that, following the planarizing step, a planarized surface of regrowth layer is substantially co-planar with a top surface of the dislocation-blocking mask. The planarizing step may include chemical-mechanical polishing. 
     In addition, in various embodiments of the invention, the first semiconductor material is silicon or a silicon germanium alloy. The second semiconductor material can include, or consist essentially of, either a group II, a group III, a group IV, a group V, or a group VI element, or a combination thereof, for example, germanium, silicon germanium, gallium arsenide, aluminum antimonide, indium aluminum antimonide, indium antimonide, indium arsenide, indium phosphide, or gallium nitride. In some embodiments, the second semiconductor material is compositionally graded. 
     In many embodiments of the invention, the selected crystallographic direction of the first semiconductor material is aligned with at least one direction of propagation of threading dislocations in the regrowth layer. In certain versions of these embodiment, the orientation angle ranges from about 30 to about 60 degrees, for example, is about 45 degrees. 
     The surface of the substrate may have (100), (110), or (111) crystallographic orientation. In some embodiments, the selected crystallographic direction is substantially aligned with a &lt;110&gt; crystallographic direction of the first semiconductor material. In other embodiments, the portion of the sidewall meets the surface of the substrate in substantial alignment with a &lt;100&gt; crystallographic direction of the first semiconductor material. 
     In certain embodiments of this and other aspects of the invention, the first semiconductor material is non-polar, the second semiconductor material is polar, and the orientation angle causes anti-phase boundaries in the regrowth layer to decrease in density with increasing distance from the surface of the substrate. In some embodiments, the threading dislocations terminate at the sidewall of the opening in the dislocation-blocking mask at or below a predetermined distance H from the surface of the substrate. In some versions of these embodiments, the opening in the dislocation-blocking mask has a variable width. In other versions, the sidewall of the opening in the dislocation-blocking mask includes a first portion disposed proximal to the surface of the substrate, and a second portion disposed above the first portion. A height of the first portion can be at least equal to the predetermined distance H from the surface of the substrate. The first portion of the sidewall can be substantially parallel to the second portion. Also, in some versions, the second portion of the sidewall is flared outwardly. Further, in certain embodiments of this and other aspects of the invention, the orientation angle causes stacking faults and/or twin boundaries in the regrowth layer to decrease in density with increasing distance from the surface of the substrate. 
     Further yet, in certain embodiments of this and other aspects of the invention, the sidewall of the opening in the dislocation-blocking mask has a height at least equal to a predetermined distance H from the surface of the substrate. In these embodiments, the opening is substantially rectangular and has a predetermined width W that is smaller than a length L of the opening. For example, the width W of the opening can be less than about 500 nm, and the length L of the opening can exceed each of W and H. In some versions of these embodiments, the substrate consists essentially of silicon and has a (100) crystallographic orientation, the orientation angle is about 45 degrees to the direction of propagation of defects in the regrowth layer, and the predetermined distance H is at least W √2. In other versions, the substrate consists essentially of silicon and has a (110) crystallographic orientation, the orientation angle is about 45 degrees, and the predetermined distance H is at least W √6/3. In still other versions, the substrate consists essentially of silicon and has a (111) crystallographic orientation, the orientation angle is about 45 degrees, and the predetermined distance H is at least 2 W. 
     In other embodiments of this aspect of the invention, the method additionally includes depositing a lattice-mismatched layer over at least a portion of the substrate prior to providing the dislocation-blocking mask thereon. The lattice-mismatched layer preferably includes a third semiconductor material and is at least partially relaxed. The lattice-mismatched layer can be planarized prior to providing the dislocation-blocking mask. The second semiconductor material and the third semiconductor material can be or include the same semiconductor material. 
     In general, in another aspect, the invention features a method of forming a semiconductor heterostructure that begins with providing a substrate including a first semiconductor material. The method additionally includes providing a dislocation-blocking mask over the substrate. The mask has an opening extending to the surface of the substrate and defined by at least one sidewall. At least a portion of the sidewall meets the surface of the substrate at an orientation angle to a selected crystallographic direction of the first semiconductor material. The method further includes the steps of depositing in the opening a regrowth layer that includes a second semiconductor material and subjecting the regrowth layer to thermal cycling, thereby causing threading dislocations to terminate at the sidewall of the opening in the dislocation-blocking mask at or below a predetermined distance from the surface of the substrate. 
     In various embodiments of this and other aspects of the invention, threading dislocations (and/or other dislocation defects such as stacking faults, twin boundaries, or anti-phase boundaries) in the regrowth layer decrease in density with increasing distance from the surface of the substrate. The first semiconductor material may include, or consist essentially of, silicon or a silicon germanium alloy. The second semiconductor material may include, or consist essentially of, a group II, a group III, a group IV, a group V, and/or a group VI element, and/or combinations thereof, for example, selected from the group consisting of germanium, silicon germanium, gallium arsenide, and gallium nitride. In some embodiments, the second semiconductor material is compositionally graded. 
     Generally, in yet another aspect, the invention focuses on a semiconductor structure that includes a substrate and a dislocation-blocking mask disposed over the substrate. The substrate includes, or consists essentially of, a first semiconductor material, such as, for example, silicon or a silicon germanium alloy. The dislocation-blocking mask may include a dielectric material, such as, for example, silicon dioxide or silicon nitride. The mask has an opening extending to the surface of the substrate and defined by at least one sidewall at least a portion of which meeting the surface of the substrate at an orientation angle to a selected crystallographic direction of the first semiconductor material. A regrowth layer comprising a second semiconductor material is formed in the opening, such that the orientation angle causes threading dislocations and/or other dislocation defects such as stacking faults, twin boundaries, or anti-phase boundaries in the regrowth layer to decrease in density with increasing distance from the surface of the substrate. 
     In various embodiments of this aspect of the invention, the threading dislocations terminate at the sidewall of the opening in the dislocation-blocking mask at or below a predetermined distance H from the surface of the substrate. In some embodiments of this aspect of the invention, the selected crystallographic direction of the first semiconductor material is aligned with at least one propagation direction of threading dislocations in the regrowth layer. In certain versions of these embodiments, the orientation angle ranges from about 30 to about 60 degrees, for example, is about 45 degrees. 
     The surface of the substrate may have (100), (110), or (111) crystallographic orientation. In some embodiments, the selected crystallographic direction is substantially aligned with a &lt;110&gt; crystallographic direction of the first semiconductor material. In other embodiments, the portion of the sidewall meets the surface of the substrate in substantial alignment with a &lt;100&gt; crystallographic direction of the first semiconductor material. 
     Also, certain embodiments of this aspect of the invention include an overgrowth layer disposed over the regrowth layer and over at least a portion of the dislocation-blocking mask, as well as a lattice-mismatched layer disposed over at least a portion of the substrate underneath the dislocation-blocking mask. The overgrowth layer and/or the lattice-mismatched layer may include a second semiconductor material and may be at least partially relaxed. 
     Further, in still another aspect, the invention features a semiconductor device formed over a substrate that includes a source region, a drain region, and a channel region therebetween. The substrate includes, or consists essentially of, a first semiconductor material, for example, a silicon. Also, a dislocation-blocking mask is disposed over the substrate. The mask has an opening extending to the surface of the substrate and is defined by at least one sidewall. The device additionally includes a regrowth region formed in the opening. At least a portion of the sidewall meets the surface of the substrate at an orientation angle to a selected crystallographic direction of the first semiconductor material, for example, about 45 degrees to the direction of propagation of threading dislocations in the regrowth region. The regrowth region has a first portion disposed proximal to the surface of the substrate, where threading dislocations and/or other dislocation defects such as stacking faults, twin boundaries, or anti-phase boundaries in the regrowth region substantially terminate, and a second portion disposed above the first portion and having the channel region formed therein. The first portion of the regrowth region includes a second semiconductor material and the second portion includes a third semiconductor material. The second and third semiconductor materials may be, or include, the same material. 
     In one embodiment, the semiconductor substrate includes a silicon wafer, an insulating layer disposed thereon, and a strained semiconductor layer disposed on the insulating layer. The strained semiconductor layer may include silicon or germanium. As used herein, the term “strain” encompasses uniaxial and biaxial strain, as well as tensile and compressive strain. In another embodiment, the semiconductor substrate includes a silicon wafer, a compositionally uniform relaxed Si 1-x Ge x  layer (where 0&lt;x&lt;1) deposited thereon, a strained silicon layer deposited on the relaxed Si 1-x Ge x  layer. A compositionally graded Si 1-x Ge x  layer can be disposed between the compositionally uniform Si 1-x Ge x  relaxed layer and the silicon wafer. Also, an insulating layer can be disposed between the compositionally uniform relaxed Si 1-x Ge x  layer and the silicon wafer. In yet another embodiment, at least partially relaxed lattice-mismatched layer is disposed between at least a portion of the substrate and the dislocation-blocking mask. 
     The second semiconductor material and/or the third semiconductor material can include, or consist essentially of, a group II, a group III, a group IV, a group V, and/or a group VI element, and/or combinations thereof, for example, germanium, silicon germanium, gallium arsenide, gallium nitride, indium aluminum arsenide, indium gallium arsenide, indium gallium phosphide, aluminum antimonide, indium aluminum antimonide, indium antimonide, and/or indium phosphide. In some embodiments, the first portion of the regrowth region may include silicon germanium and the second portion of the regrowth region may include a layer of strained germanium or strained silicon germanium. In other embodiments, the first portion of the regrowth region includes indium phosphide and the second portion of the regrowth region includes a layer of indium gallium arsenide disposed over a layer of indium aluminum arsenide. In other embodiments, the first portion of the regrowth region may include indium aluminum antimonide and the second portion of the regrowth region may include a layer of indium antimonide. 
     In various embodiments of the invention, the selected crystallographic direction of the first semiconductor material is aligned with at least one propagation direction of threading dislocations in the regrowth region. Threading dislocations in the regrowth region may substantially terminate at the sidewall of the opening in the dislocation-blocking mask at or below a predetermined distance from the surface of the substrate. The dislocation-blocking mask may include a dielectric material, for example, silicon dioxide or silicon nitride. In a particular embodiment, the dislocation-blocking mask includes a silicon nitride layer disposed over a silicon dioxide layer. 
     In certain embodiments, the source region and the drain region of the device are epitaxially deposited over the dislocation-blocking mask; for example, they may represent a structure epitaxially deposited over the dislocation-blocking mask proximal to the regrowth region following formation thereof. In some versions of these embodiments, the structure includes a first material forming a Schottky junction at the interface with the regrowth region. The structure may further include a second material, which may be strained, unstrained, or amorphous. A gate insulator can be disposed over the regrowth region, and, in some embodiments, a silicon layer having thickness ranging from about 5 Å to about 15 Å is disposed between the gate insulator and the regrowth region. 
     In general, in still another aspect, the invention features an integrated circuit that includes a substrate and a dislocation-blocking mask disposed over the substrate. The mask has an opening extending to the surface of the substrate and defined by at least one sidewall. The substrate includes, or consists essentially of, a first semiconductor material, such as, for example, silicon. At least a portion of the sidewall meets the surface of the substrate at an orientation angle to a selected crystallographic direction of the first semiconductor material. The integrated circuit also includes a regrowth region formed in the opening. The regrowth region has a first portion disposed proximal to the surface of the substrate, and threading dislocations and/or other dislocation defects such as stacking faults, twin boundaries, or anti-phase boundaries in the regrowth region substantially terminate in the first portion. The regrwoth layer also has a second portion disposed above the first portion. The first and second portions include, or consist essentially of, either different or the same semiconductor material(s). Further, a p-transistor is formed over a first area of the semiconductor substrate and an n-transistor is formed over a second area of the semiconductor substrate, each transistor has a channel through the second portion of the regrowth region. The transistors are interconnected in a CMOS circuit. 
     In yet another aspect, the invention relates to a method of forming a non-planar FET. The method begins with providing a substrate that includes, or consists essentially of, a first semiconductor material, such as, for example, silicon. The method further includes the steps of providing a dislocation-blocking mask over the substrate and forming an opening in the mask extending to the surface of the substrate and defined by at least one sidewall. The mask has a first dielectric layer disposed over a second dielectric layer. At least a portion of the sidewall meets the surface of the substrate at an orientation angle to a selected crystallographic direction of the first semiconductor material. The method additionally includes selectively forming in the opening a regrowth region that contains a second semiconductor material. The orientation angle and/or the image force causes threading dislocations and/or other dislocation defects such as stacking faults, twin boundaries, or anti-phase boundaries in the regrowth region to decrease in density with increasing distance from the surface of the substrate. The method further includes selectively removing at least a portion of the first dielectric layer to expose at least a portion of the regrowth region, thereby forming a semiconductor fin structure. A gate dielectric region is provided over at least a portion of the fin structure. A gate contact is disposed over the gate dielectric region. A source region and a drain region can be formed in the fin structure. The regrowth region can be planarized, for example, by chemical-mechanical polishing, prior to selectively removing at least a portion of the first dielectric layer. 
     Also, in a further aspect, the invention contemplates a method of forming an optoelectronic device. The method begins with providing a substrate that includes, or consists essentially of, a first semiconductor material, such as, for example, silicon. The method further includes the steps of providing a dislocation-blocking mask over the substrate and forming an opening in the mask extending to the surface of the substrate. The opening is defined by at least one sidewall. At least a portion of the sidewall meets the surface of the substrate at an orientation angle to a selected crystallographic direction of the first semiconductor material. The method additionally includes selectively depositing in the opening a first portion of the regrowth region that contains, or consists essentially of, a second semiconductor material, while in situ doping the second semiconductor material until thickness of the first portion approximates or exceeds the predetermined distance. The orientation angle causes threading dislocations and/or other dislocation defects such as stacking faults, twin boundaries, or anti-phase boundaries in the first portion to substantially terminate at or below a predetermined distance from the surface of the substrate. The method continues with the step of selectively depositing a second portion of the regrowth region that contains, or consists essentially of, a third semiconductor material, in the opening to a thickness selected to achieve a predetermined level of absorption of incident light; and then forming a doped region in the second portion. In various embodiments, the method further includes, prior to providing a dislocation-blocking mask, the step of forming a p-type or n-type region in the substrate. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       In the drawings, like reference characters generally refer to the same parts throughout the different views. Also, the drawings are not necessarily to scale, emphasis instead generally being placed upon illustrating the principles of the invention. In the following description, various embodiments of the present invention are described with reference to the following drawings, in which: 
         FIG. 1A  depicts a schematic cross-sectional side view of a silicon substrate with a germanium layer formed thereon according to an “epitaxial necking” technique known in the art; 
         FIG. 1B  is an XTEM image illustrating the semiconductor heterostructure of  FIG. 1A ; 
         FIGS. 2A, 2B, and 2C  are schematic diagrams showing the three types of crystalline orientation for silicon; 
         FIGS. 3A, 3B, 4A, 4B, 4C, 4D, 4E, 5A, and 5B  depict schematic views of different lattice-mismatched semiconductor heterostructures and structures for blocking dislocations therein, according to various embodiments of the invention; 
         FIGS. 6A, 6B, 6C, 6D, 6E, 6F, 6G, 6H, 7A, 7B, and 7C  depict schematic cross-sectional side views of the lattice-mismatched semiconductor heterostructures having increased active area, according to various embodiments of the invention; 
         FIGS. 8, 9, 10A, and 10B  depict various applications of dislocation blocking techniques for semiconductor devices, according to various embodiments of the invention; 
         FIGS. 11 and 12  depict Ge or III-V photodetector integration into Si substrate according to particular embodiments of the invention; 
         FIGS. 13A, 13B, and 13C  depict semiconductor heterostructures employing dislocation-blocking techniques according to alternative embodiments of the invention; and 
         FIGS. 14A, 14B, 14C, 15A, 15B, 15C, 15D, 16A, 16B, 16C and 16D  depict a method of trapping defects by forming grooves in a semiconductor substrate in accordance with various embodiments of the invention. 
     
    
    
     DETAILED DESCRIPTION 
     In accordance with its various embodiments, the present invention contemplates fabrication of monolithic lattice-mismatched semiconductor heterostructures with limited area regions having upper surfaces substantially exhausted of threading dislocations and other dislocation defects, as well as fabrication of semiconductor devices based on such lattice-mismatched heterostructures. 
     Silicon (Si) is recognized as presently being the most ubiquitous semiconductor for the electronics industry. Most of silicon that is used to form silicon wafers is formed from single crystal silicon. The silicon wafers serve as the substrate on which CMOS devices are formed. The silicon wafers are also referred to as a semiconductor substrate or a semiconductor wafer. While described in connection with silicon substrates, however, the use of substrates that include, or consist essentially of, other semiconductor materials, is contemplated without departing from the spirit and scope of the present invention. 
     In crystalline silicon, the atoms which make up the solid are arranged in a periodic fashion. If the periodic arrangement exists throughout the entire solid, the substance is defined as being formed of a single crystal. If the solid is composed of a myriad of single crystal regions the solid is referred to as polycrystalline material. As readily understood by skilled artisans, periodic arrangement of atoms in a crystal is called the lattice. The crystal lattice also contains a volume which is representative of the entire lattice and is referred to as a unit cell that is regularly repeated throughout the crystal. For example, silicon has a diamond cubic lattice structure, which can be represented as two interpenetrating face-centered cubic lattices. Thus, the simplicity of analyzing and visualizing cubic lattices can be extended to characterization of silicon crystals. In the description herein, references to various planes in silicon crystals will be made, especially to the (100), (110), and (111) planes. These planes define the orientation of the plane of silicon atoms relative to the principle crystalline axes. The numbers {xyz} are referred to as Miller indices and are determined from the reciprocals of the points at which the crystal plane of silicon intersects the principle crystalline axes. Thus,  FIGS. 2A-2C  show three orientations of the crystal plane of silicon. In  FIG. 2A , the crystal plane of silicon intersects the x-axis at  1  and never intersects the y or z-axis. Therefore, the orientation of this type of crystalline silicon is (100). Similarly,  FIG. 2B  shows (110) crystalline silicon and  FIG. 2C  shows (111) silicon. The (111) and (100) orientations are the two primary wafer orientations in commercial use. Notably, for any given plane in a cubic crystal there are five other equivalent planes. Thus, the six sides of the cube comprising the basic unit cell of the crystal are all considered (100) planes. The notation {xyz} refers to all six of the equivalent (xyz) planes. Throughout the description, reference will also be made to the crystal directions, especially the &lt;100&gt;, &lt;110&gt; and &lt;111&gt; directions. These are defined as the normal direction to the respective plane. Thus, the &lt;100&gt; direction is the direction normal to the (100) plane. The notation &lt;xyz&gt; refers to all six equivalent directions. 
     As discussed above, there is a need in the art for versatile and efficient methods of fabricating semiconductor heterostructures that would constrain substrate interface defects in a variety of lattice-mismatched materials systems. One conventional technique mentioned above that addresses control of threading dislocation densities in highly-mismatched deposited layers, termed “epitaxial necking,” is applicable only to devices with relatively small lateral dimensions. Specifically, in the prior art, metal oxide semiconductor (“MOS”) transistors are typically fabricated on (100) silicon wafers with the gates oriented such that current flows parallel to the &lt;110&gt; directions. Thus, for a FET device built on a (100) Si wafer with device channel orientation aligning with the &lt;110&gt; direction, both the channel width and channel length should be small compared to the height of a epitaxial necking mask, in order for the dislocations in a lattice-mismatched semiconductor layer to terminate at a sidewall of the mask on both directions. However, in modern CMOS circuits, the MOSFET device width often substantially exceeds the channel length, which, as a result of CMOS scaling, is frequently very small. Accordingly, under the conventional necking approach, a number of dislocations will not be terminated at the sidewall of the mask in the direction of the channel width. 
     In contrast with the prior art approach of minimizing dislocation defects, in its various embodiments, the present invention addresses the limitations of known techniques, by utilizing greater thicknesses and limited lateral areas of component semiconductor layers to produce limited-area regions having upper portions substantially exhausted of dislocation defects. Referring to  FIGS. 3A-3B , a substrate  310  is provided that includes, or consists essentially of, a first semiconductor material, such as, for example, a group IV element, e.g., germanium or silicon. The first semiconductor material may be crystalline. The substrate  310  may be, for example, a bulk silicon wafer, a bulk germanium wafer, a semiconductor-on-insulator (SOI) substrate, or a strained semiconductor-on-insulator (SSOI) substrate. In one embodiment, the substrate  310  includes or consists essentially of (100) silicon. The substrate  310  may include a material having a first conductivity type, e.g., n- or p-type, such as n +  Si. 
     A dislocation-blocking mask  320  is disposed over the substrate. The mask has an opening  325  extending to the surface of the substrate and defined by at least one sidewall  330 . In various embodiments, the opening  325  is generally rectangular. The dislocation-blocking mask may include a dielectric material, such as, for example, silicon dioxide or silicon nitride. At least a portion of the sidewall meets the surface of the substrate at an orientation angle α to a selected crystallographic direction of the first semiconductor material. In addition, at least a portion of the sidewall is generally vertical, i.e. disposed at about 80 to 120 degrees to the surface of the substrate, and, in a particular embodiment, substantially perpendicular to the surface of the substrate. 
     A regrowth layer  340  that includes a second semiconductor material is deposited in the opening. In one embodiment, the selected crystallographic direction of the first semiconductor material is aligned with direction of propagation of threading dislocations in the regrowth layer. In certain embodiments, the orientation angle ranges from about 30 to about 60 degrees, for example, is about 45 degrees to such crystallographic direction. The surface of the substrate may have (100), (110), or (111) crystallographic orientation. In some embodiments, the selected crystallographic direction is substantially aligned with a &lt;110&gt; crystallographic direction of the first semiconductor material. 
     In various embodiments, the first semiconductor material may include, or consist essentially of, silicon or a silicon germanium alloy. The second semiconductor material may include, or consist essentially of, a group II, a group III, a group IV, a group V, and/or a group VI element, and/or combinations thereof, for example, selected from the group consisting of germanium, silicon germanium, gallium arsenide, aluminum antimonide, indium aluminum antimonide, indium antimonide, indium arsenide, indium phosphide, and gallium nitride. 
     The regrowth layer can be formed in the opening by selective epitaxial growth in any suitable epitaxial deposition system, including, but not limited to, atmospheric-pressure CVD (APCVD), low-(or reduced-) pressure CVD (LPCVD), ultra-high-vacuum CVD (UHVCVD), by molecular beam epitaxy (MBE), or by atomic layer deposition (ALD). In the CVD process, selective epitaxial growth typically includes introducing a source gas into the chamber. The source gas may include at least one precursor gas and a carrier gas, such as, for example hydrogen. The reactor chamber is heated, such as, for example, by RF-heating. The growth temperature in the chamber ranges from about 300° C. to about 900° C. depending on the composition of the regrowth layer. The growth system also may utilize low-energy plasma to enhance the layer growth kinetics. 
     The epitaxial growth system may be a single-wafer or multiple-wafer batch reactor. Suitable CVD systems commonly used for volume epitaxy in manufacturing applications include, for example, EPI CENTURA single-wafer multi-chamber systems available from Applied Materials of Santa Clara, Calif., or EPSILON single-wafer epitaxial reactors available from ASM International based in Bilthoven, The Netherlands. 
     In some embodiments, the regrowth layer is compositionally graded, for example, includes Si and Ge with a grading rate in the range of &gt;5% Ge/μm to 100% Ge/μm, preferably between 5% Ge/μm and 50% Ge/μm, to a final Ge content of between about 10% to about 100% While the overall grading rate of the graded layer is generally defined as the ratio of total change in Ge content to the total thickness of the layer, a “local grading rate” within a portion of the graded layer may be different from the overall grading rate. For example, a graded layer including a 1 μm region graded from 0% Ge to 10% Ge (a local grading rate of 10% Ge/μm) and a 1 μm region graded from 10% Ge to 30% Ge (a local grading rate of 20% Ge/μm) will have an overall grading rate of 15% Ge/μm. Thus, the regrowth layer may not necessarily have a linear profile, but may comprise smaller regions having different local grading rates. In various embodiments, the graded regrowth layer is grown, for example, at 600-1200° C. Higher growth temperatures, for example, exceeding 900° C. may be preferred to enable faster growth rates while minimizing the nucleation of threading dislocations. See, generally, U.S. Pat. No. 5,221,413, incorporated herein by reference in its entirety. 
     In a particular embodiment, the first semiconductor material is silicon and the second semiconductor material is germanium. In this embodiment, threading dislocations  350  in the regrowth layer propagate along a &lt;110&gt; direction, and lie at an angle of 45-degrees to the surface of the first semiconductor material. The dislocation mask having a generally rectangular opening is disposed over the substrate such that the sidewall of the opening is disposed at a 45-degree angle to a &lt;100&gt; direction and is substantially aligned with a &lt;110&gt; crystallographic direction. As a result of such orientation of the opening, dislocations will reach and terminate at the sidewalls of the opening in the dislocation-blocking mask at or below a predetermined distance H from the surface of the substrate, such that threading dislocations in the regrowth layer decrease in density with increasing distance from the surface of the substrate. Accordingly, the upper portion of the regrowth layer is substantially exhausted of threading dislocations, enabling formation of semiconductor devices having increased channel width. 
     In certain versions of this and other embodiments of the invention, the sidewall of the opening in the dislocation-blocking mask has a height at least equal to a predetermined distance H from the surface of the substrate. In these embodiments, the opening is substantially rectangular and has a predetermined width W that is smaller than a length L of the opening. For example, the width W of the opening can be less than about 500 nm, and the length L of the opening can exceed each of W and H. In some versions of these embodiments, the substrate consists essentially of silicon and has a (100) crystallographic orientation, the orientation angle is about 45 degrees to propagation of dislocations in the regrowth layer, and the predetermined distance H is at least W √2. In other versions, the substrate consists essentially of silicon and has a (110) crystallographic orientation, the orientation angle is about 45 degrees, and the predetermined distance H is at least W √6/3. In still other versions, the substrate consists essentially of silicon and has a (111) crystallographic orientation, the orientation angle is about 45 degrees, and the predetermined distance H is at least 2 W. 
     In various embodiments of the invention, blocking of the dislocations is promoted both by geometry and orientation of the mask discussed above as well as because of the ‘image force’ whereby dislocations are attracted to substantially vertical surfaces, as explained in more detail below. In many embodiments, the image force alone is sufficient to cause the upper portion of the regrowth layer to be substantially exhausted of threading dislocations and other dislocation defects. 
     As skilled artisans will readily recognize, a dislocation near a surface experiences forces generally not encountered in the bulk of a crystal, and, in particular, is attracted towards a free surface because the material is effectively more compliant there and the dislocation energy is lower. See Hull &amp; Bacon,  Introduction to Dislocations,  4 th  edition, Steel Times (2001). Image force is determined by material properties of the semiconductor being grown, as well as the distance between a given dislocation and the free surface. Thus, even when the dislocations have an orientation that does not favor trapping at sidewalls, the approach discussed above is still effective at certain dimensions because of the boundary forces that draw dislocations to free surfaces in order to reduce the elastic energy of the crystal. Mathematically, these forces arise because the boundary conditions of the expressions for strain require strain components normal to a surface to be zero at that surface. Thus, force per unit of dislocation length on an edge dislocation, toward a vertical sidewall can be represented by the formula: 
               F   I     =       Gb   2       4   ⁢   π   ⁢           ⁢     d   ⁡     (     1   -   v     )                 
where
 
F 1 =Image force
 
G=Shear modulus
 
d=distance from free surface
 
b=Burgers vector
 
ν=Poisson&#39;s ratio
 
     Referring to  FIGS. 4A-4B , as used herein, the term “60° dislocation” refers to a dislocation for which the angle between the Burgers vector and the dislocation line is 60°. These dislocations typically form in diamond-cubic or zincblende lattice-mismatched systems where the strain is relatively low (e.g. &lt;2%). In the absence of forces on threads (which can come from other dislocations nearby or from a free surface nearby) they rise from the substrate surface at a 45° angle in &lt;110&gt; directions. However, when viewed from above (normal to the surface) they appear to lie in &lt;100&gt; directions. 
     Experimentally, it has been shown that for the case of germanium on silicon (4% mismatch) dislocations within approximately 300 nm of a SiO 2  sidewall are trapped. This is understood to be due to the influence of the image force. The angle between these dislocations and the sidewall appears to range between approximately 45-55°. 
     The relevant material constants for Ge are: 
     G=4.1e11 dyne/cm 2    
     ν=0.26; and 
     b=3.99 Å 
     Based on the above formula and the experimental observation that for d≦300 nm dislocations in Ge on Si are bent toward an SiO 2  sidewall, the force necessary to bend a dislocation in a cubic semiconductor crystal toward a free surface is approximately 2.3 dyne/cm. Thus, distance from free surface d for other materials can be estimated with certain degree of accuracy based on their known values for G, ν, and b. For example, by these calculations: 
     For GaAs d=258 nm 
     For InP d=205 nm 
     For AlSb d=210 nm 
     For InSb d=164 nm 
     Referring to  FIG. 4C , for full trapping, the hole or trench lateral dimension w is preferably less than or equal to approximately 2*d, while the vertical dimension h is preferably at least approximately d, where d is calculated discussed above. These criteria are expected to be substantially independent of the orientation of the boundary of the sidewall and the substrate surface. Thus, in various embodiments of the invention, dislocations in the lower portion of the regrowth layer are trapped by employing a dislocation-blocking mask with an elongated opening, e.g. a trench, having a width calculated as discussed above and oriented without any regard for the direction of propagation of dislocations in the regrowth layer. 
     Further, as shown in  FIG. 4D  and used herein, the term “90° dislocation” refers to a dislocation for which the angle between the Burgers vector and the dislocation line is 90°. These dislocations primarily form in mismatched systems where the strain is relatively high (e.g. &gt;2%). In the absence of forces on threading dislocation (which can come from other dislocations nearby or from a free surface nearby) they rise from the substrate surface at a 90° angle in &lt;100&gt; directions. Thus, these dislocations can be trapped most optimally by using a dislocation-blocking mask with slanted, rather than vertical sidewalls, as shown in  FIG. 4E . 
     The following summarizes mechanisms for trapping dislocations in different kind of diamond-cubic or zincblende semiconductor heterostructures: 
     1. Low mismatch, low image force
         60° dislocations predominate   Threads lie in &lt;110&gt; directions, rising from surface at 45°   Best approach for trapping dislocations is to rely on appropriate orientation of sidewalls and appropriate dimensioning of openings, as described above in connection with  FIGS. 3A-3B ;       

     2. Low mismatch, high image force
         60° dislocations predominate   Threads bend toward free substantially vertical surfaces   Best approach for trapping dislocations is described above in connection with  FIGS. 4A-4C ;       

     3. High mismatch, high image force
         90° dislocations predominate   Threads bend toward free substantially vertical surfaces   Best approach for trapping dislocations is described above in connection with  FIGS. 4A-4C ; and       

     4. High mismatch, low image force
         90° dislocations predominate   Threads lie in &lt;100&gt; directions, rising from surface at 90°   Best approach for trapping dislocations is described above in connection with  FIGS. 4D-4E         

     Hexagonal semiconductors, such as the III-nitride (III-N) materials, are of great interest for high-power high-speed electronics and light-emitting applications. For epitaxy of hexagonal semiconductors such as III-nitrides on Si, the (111) surface of Si is commonly preferred over the (100). This is because the (111) surface of Si is hexagonal (even though Si is a cubic crystal). This makes a better template for hexagonal crystal growth than the cubic (100) face. However, as mentioned above, epitaxial necking approach discussed above is less effective in these applications, because the threading dislocations in the hexagonal semiconductors disposed over the lattice-mismatched Si (111) substrates may not be effectively confined by the vertical sidewalls because the threading dislocations in such materials typically have a different orientation relative to the substrate, compared to the more commonly used cubic semiconductors, such as Si, Ge, and GaAs. For example, as described above in connection with  FIG. 4E , for certain surface orientations of substrate and crystalline structure of lattice-mismatched regrowth region, the threading defects tend to propagate perpendicular to the substrate, which may not favor trapping by vertical sidewalls of the dislocation-blocking mask. This is the case when GaN is grown on the (100) surface of Si. In such a case, in some embodiments, the angle of the sidewalls of the opening can be slanted relative to the substrate, as shown in  FIG. 4E  such that vertically propagating defects intersect the angled sidewalls. 
     In other embodiments, the surface of the underlying substrate itself exposed in the opening is configured to enable confinement of the threading dislocations. Referring to  FIG. 5A , after the dislocation-blocking mask is disposed over the Si (100) substrate and an opening is defined therethrough, an etch that is selective to the (111) crystallographic plane of Si, for example, a KOH solution, is applied to the portion of the substrate exposed at the bottom of the seed window to expose (111) surfaces. A lattice-mismatched semiconductor material is then deposited in the opening over the substrate, and the epitaxial deposition continues such that a heteroepitaxial region is grown over the material disposed in the opening, laterally expanding over the mask. Because of the configuration of the underlying surface, orientation of the threading dislocations in the heteroepitaxial region is at approximately 45° to the surface of the substrate, facilitating trapping of the dislocation by substantially vertical sidewalls of the mask, as shown in  FIG. 5B . Then, if small areas of hexagonal semiconductor material are desired for device active areas, the heteroepitaxial overgrowth regions expanding from the individual openings can be planarized (e.g. via CMP), to be substantially co-planar with the adjacent insulator areas. Alternatively, if a large area is desired, growth can proceed until neighboring regions coalesce, followed optionally by planarization of the resulting structure. In the latter case, because lateral growth rates of hexagonal semiconductor can be dramatically increased over growth rate normal to the surface employing various known approaches, these semiconductor materials afford process flexibility not available with cubic semiconductors grown on (100) surfaces. Specifically, differential growth rates of these materials allows for widely-spaced seed trenches; for example, spacing may be five times trench width or even greater, offering a substantial advantage over closely-spaced seed trenches, if the defects which are known to form when epitaxial growth fronts coalesce cannot be substantially eliminated. 
       FIGS. 6A-6F  depicts schematic cross-sectional side views of the lattice-mismatched semiconductor heterostructures having increased surface area according to various embodiments of the invention. Specifically, as discussed in more detail below, the area of the upper portion of the lattice-mismatched heterostructure substantially exhausted of threading dislocations is increased, compared to the embodiments described above with reference to  FIGS. 3A-3B . For example, as described in more detail below, in some embodiments, the opening in the dislocation-blocking mask has a variable width. In other versions, the sidewall of the opening in the dislocation-blocking mask includes a first portion disposed proximal to the surface of the substrate, and a second portion disposed above the first portion. A height of the first portion can be at least equal to a predetermined distance H from the surface of the substrate, where the threading dislocations terminate at the sidewall of the opening in the dislocation-blocking mask at or below the distance H. In some versions of these embodiments, the first portion of the sidewall can be substantially parallel to the second portion. Also, in some versions, the second portion of the sidewall is flared outwardly. 
     In many of the embodiments described below, a substrate  510  includes, or consists essentially of, silicon. The regrowth layer includes, or consists essentially of, a semiconductor material that is one of a group II, a group III, a group IV, a group V, and/or a group VI elements, and/or combinations thereof, for example, selected from the group consisting of germanium, silicon germanium, gallium arsenide, aluminum antimonide, indium aluminum antimonide, indium antimonide, indium arsenide, indium phosphide and gallium nitride. A dislocation-blocking mask  520  having an opening therein is disposed over the substrate. The dislocation-blocking mask may include a dielectric material, such as, for example, silicon dioxide or silicon nitride. At least a portion of the sidewall meets the surface of the substrate at an orientation angle α to a selected crystallographic direction of the first semiconductor material. A regrowth layer  540  that includes a second semiconductor material is deposited in the opening. In various embodiments, the selected crystallographic direction of the first semiconductor material is aligned with direction of propagation of threading dislocations in the regrowth layer. In various embodiments, the orientation angle ranges from about 30 to about 60 degrees, for example, is about 45 degrees. As mentioned above, in many embodiments of the invention, blocking of the dislocations is promoted by geometry and orientation of the mask discussed above and/or the ‘image force.’ 
     Referring to  FIG. 6A , in one embodiment, the dislocation-blocking mask is formed by depositing a first low-temperature oxide layer  521  having thickness h 1  over the substrate. The thickness h 1  is selected to be at least equal to the distance from the surface of the substrate at which the threading dislocations (and/or other dislocation defects such as stacking faults, twin boundaries, or anti-phase boundaries) terminate at the sidewall of the opening in the dislocation-blocking mask, as discussed above. A first aperture having a diameter d 1  or a first trench having a width w 1 , both the width w 1  and diameter d 1  being smaller than the thickness h 1 , are formed in the layer  521  by a conventional masking/etching technique. After the mask is stripped, a second low-temperature oxide layer  522  having a thickness h 2  is deposited over the layer  521 . Then, a second aperture of diameter d 2  or a second trench having a width w 2  is formed in the layer  522  by a conventional masking/etching technique, such that w 1 &lt;w 2  (or d 1 &lt;d 2 ). After the mask is stripped, the regrowth layer of second semiconductor material is deposited in the first and second apertures or in first and second trenches by selective epitaxy, according to any of the techniques disclosed in U.S. Patent Application Publication No. 2004/0045499A by Langdo et al., incorporated herein by reference. As discussed above, following deposition, threading dislocations and/or other dislocation defects substantially terminate in the first aperture (or in the first trench) at or below thickness h 1 . As a result, the regrowth layer portion having thickness h 2  that is substantially exhausted of threading dislocations is obtained with an upper surface that is larger compared to the embodiments described above with reference to  FIGS. 3A-3B . 
     Referring to  FIG. 6B , in another embodiment, an overgrowth layer  555  that includes the second semiconductor material is deposited over the regrowth layer  540  and over a portion of the dislocation-blocking mask  520  adjacent to the regrowth layer. At least a portion of the overgrowth layer may be deposited as non-crystalline (i.e. amorphous) material and can be crystallized subsequently, for example by an anneal step at a temperature higher than the deposition temperature. Thus, in this embodiment, crystallization of the overlayer is used to create crystal material in the overlayer regions over the regrowth layer, which is amorphous upon deposition. The arrows in  FIG. 6B  indicate a crystallized region expanding outward from the opening in the dislocation blocking mask, as amorphous material which may form at least a portion of the overgrowth layer  555  is crystallized. 
     Referring to  FIG. 6C , in yet another embodiment, deposition of the regrowth layer in the opening of the dislocation-blocking mask is followed by a step of lateral epitaxial deposition to increase the useful surface area. It may also utilize the higher growth rates typical of (100) surfaces compared to (110) or (111) surface to increase lateral overgrowth in this embodiment. For example, the overgrowth regions can be used as source/drain areas which typically have less stringent material quality requirement than the channel material. 
     Referring to  FIG. 6D , in still another embodiment, the useful upper area of the regrowth layer  540  is increased by gradually increasing the size of the regrowth region. Similar to the embodiment described above with reference to  FIG. 6A , the dislocation-blocking mask includes two layers—a first layer having thickness h 1 , and a second layer having thickness h 2 . The thickness h 1  is selected to be at least equal to the distance from the surface of the substrate at which the threading dislocations and/or other dislocation defects terminate at the sidewall of the opening in the dislocation-blocking mask, as discussed above. That is, a first aperture having a diameter d 1  or a first trench having a width w 1 , both the width w 1  and diameter d 1  being smaller than the thickness h 1 , are formed in the layer  521  by a conventional masking/etching technique. After the mask is stripped, a second low temperature oxide layer  522  having a thickness h 2  is deposited over the layer  521 . Then, a second aperture of diameter d 2  or a second trench having a width w 2  is formed in the layer  522  by a conventional masking/etching technique, such that w 1 &lt;w 2  (or d 1 &lt;d 2 ). In contrast to the embodiment depicted in  FIG. 6A , however, the width w 2  of the second trench is gradually increased such that the sidewall of the trench, i.e. the opening in the layer  522 , gradually flares outwardly. This effect can be achieved, for example, by conventional masking/etching techniques wherein the etchant and masking material are chosen such that the masking material is eroded laterally during the etching process, gradually exposing more of the dislocation-blocking mask below, resulting in an opening in the dislocation-blocking mask that flares outward. For example, the masking material could be conventional photoresist and the etchant could be a mixture of the gases CF 4  and H 2 , used in a conventional RIE system. After the mask is stripped, the regrowth layer of second semiconductor material is deposited by selective epitaxy in the opening defined by the layers  521 ,  522 . As discussed above, following deposition, threading dislocations (and/or other dislocation defects such as stacking faults, twin boundaries, or anti-phase boundaries) substantially terminate in the first aperture (or in the first trench) at or below thickness h 1 . Thus, in this embodiment, the dislocations are terminated in the first portion of the regrowth region at or below thickness h 1 , and then the regrowth layer becomes larger and larger gradually, allowing for high-quality epitaxial growth with large surface area for large device fabrication. 
     Referring to  FIG. 6E , in an alternative version of the embodiment discussed above in connection with  FIG. 6D , a dislocation-blocking mask having an opening with outward slanted sidewalls, i.e. the structure that is substantially narrower at the bottom than the top, can be formed with only one lithography step, followed by spacer deposition and etch. This technique is generally more economical and may overcome lithographic alignment problems, or lithographic minimum feature limitations, inherent with the lithography-and-etch approach. The spacers can be formed from the same or different material than the insulator layer. For either case, selective epitaxial growth follows creation of the opening or trench. 
       FIGS. 6F-6H  show further techniques to increase the surface area. Referring to  FIG. 6F  (as well as, again, to  FIG. 6B ), in one embodiment, silicon nitride is utilized instead of silicon dioxide as a dielectric material for the dislocation-blocking mask  520  that defines two openings  535 . After the regrowth regions  540  are epitaxially grown in the openings, overgrowth regions  560  are deposited thereover. Using silicon nitride facilitates merging two overgrown regions on the surface of dislocation-blocking mask  520  layer with fewer defects, resulting in larger surface area. Referring to  FIG. 6G , in one particular version of the embodiment of  FIG. 6F , a layer of second semiconductor material  570  is deposited over the substrate  510  before forming the dislocation-blocking mask  520  thereon, such that the regrowth regions  540  merge at the top of the dislocation-blocking mask with pre-defined lattice spacing. This lattice spacing in the regrowth regions follows the lattice spacing of the layer  570  and thus it has less lattice misalignment when two epitaxy structures merge. Referring to  FIG. 6H , in another version of the embodiment of  FIG. 6F , the dislocation-blocking mask defines two or more closely spaced flared openings, such that a horizontal top surface of the mask is minimized or, in certain implementations, eliminated. In this version, the lateral overgrowth region, often prone to defects, is negligible or altogether absent, thereby improving the quality of the resulting merged overgrowth region. 
     Further, referring to  FIGS. 7A-7C , in some embodiments, the invention focuses on creating large active areas within the heteroepitaxial region by a combination of epitaxial necking and ELO techniques, employing a self-assembled dislocation-blocking mask. Specifically, a dielectric layer defining an array of openings therethrough can be formed using self-assembly techniques, thereby avoiding traditional time-consuming lithography and etch approaches. For an example of how a self-assembled array of vertical openings in an insulator layer could be created on a Si substrate, see an article by Wenchong Hu et al. entitled “Growth of well-aligned carbon nanotube arrays on silicon substrates using porous alumina film as a nanotemplate,” published in Applied Physics Letters, Vol. 79, No. 19 (2001) and incorporated herein by reference, describing how anodic oxidation of the aluminum can be used to create a self-assembled array of vertical openings similar to that shown in  FIG. 7A-7B , where the insulator is alumina (Al 2 O 3 ). The process described by Hu et al., however, leaves a small residual layer of alumina at the bottom of each hole. To remove this residual layer, an anisotropic dry etch (much higher etch rate normal to the wafer surface than parallel to the wafer surface) could be performed, exposing the silicon which is the ‘seed’ for subsequent epitaxial necking. Then, heteroepitaxial regions are selectively grown within and out of the openings, at least until resulting overgrowth regions coalesce. Depending on lateral dimensions of the aperture, degree of mismatch, and rigidity of sidewall oxide, either plastic or elastic relaxation of the heteroepitaxial “pillars” may dominate. The resulting heteroepitaxial layer is then planarized ( FIG. 7C ), e.g. via CMP, and the active-area, substantially exhausted of threading dislocations and/or other dislocation defects is used for device fabrication. 
       FIGS. 8-10  depict various applications of dislocation-blocking techniques according to various embodiments of the invention for fabrication of CMOS devices.  FIG. 8  shows various device structures disposed over regrowth or overgrown regions fabricated according to the invention, such as MOSFET devices including Ge, InGaAs, strained Ge/SiGe and other materials, or HEMT devices, e.g. including InGaAs. The starting substrate can be Si substrate or SOI/SSOI substrate. In one example, n-FET and p-FET digital devices are fabricated on a SSOI substrate, while RF/analog devices are fabricated over a Ge region grown over the Si substrate using the approaches discussed above. By integrating advanced materials into Si substrate, electron and hole mobility can be enhanced significantly. In order to avoid the deleterious effects of dislocations defects on such FET devices, the channel, source, and drain region should be confined to an upper region of regrowth or overgrown material which is substantially defect-free. As discussed in detail above, blocking of the threading dislocations and other defects is promoted by geometry and orientation of the mask and/or the image force. In many embodiments, the image force alone is sufficient to cause the upper region of the regrowth or overgrown material to be substantially exhausted of threading dislocations and other dislocation defects. 
     Furthermore, still referring to  FIG. 8 , a wide bandgap material which will suppress junction leakage (such as AlSb) can be used for initial growth, followed by a material with high electron mobility for the FET channel (such as InAs). In this embodiment, preferably, the two semiconductor materials have similar lattice constants, to reduce the possibility of dislocations forming at the interface between them. Also in this embodiment, the growth of the wide bandgap material may be followed by a planarization step so that its surface is substantially planar with the top of the dislocation blocking mask; subsequently a thin layer of the high-mobility material can be grown to accommodate the MOS channel. Preferably, the bottom of the FET junctions is disposed within the wide bandgap region to suppress junction leakage. 
       FIG. 9  depicts another application of the dislocation-blocking techniques according to various embodiments of the invention in CMOS. This method allows the Ge/III-V necking technique to be used in relatively large CMOS devices. When applying the dislocation-blocking technique in a CMOS device as in the embodiment of  FIG. 8 , the length of device active region L active  should be small enough to satisfy the aspect ratio requirement discussed above. L active , which includes source/drain lengths as well, is, however, much larger than the device channel length Lg. The embodiment shown in  FIG. 9  addresses a situation where Ge or GaAs growth is performed at a narrow channel region only; source/drain materials are then deposited separately. This approach allows for Ge or GaAs growth techniques to be applied to much larger devices, for example, 90 nm node CMOS devices instead of 22 nm node devices. This channel-only Ge/III-V dislocation-blocking approach may also be combined with other desirable source/drain engineering techniques, such as raised source/drain techniques, Schottky source/drain approaches, or the use of materials on the source/drain region different from the material in the channel region for source/drain dopant/conductivity optimization. The quasi source/drain “on-insulator” structure also reduces the junction capacitance. Proper deposition of source/drain materials may also introduce localized strain in the channel region for mobility enhancement purpose. The approach discussed above can be applied to pre-defined small channel regions only. The epitaxial deposition in the source/drain regions may be defective, but as long as the dislocations terminate on the sidewalls of the narrow channel region, the defect density in source/drain is acceptable. 
     Besides the conventional planar MOSFETs, the dislocation-blocking technique of the invention can also be used to fabricate non-planar FETs. As mentioned above, blocking of the threading dislocations and other defects is promoted by geometry and orientation of the mask and/or the image force. In many embodiments, the image force alone is sufficient to cause the upper region of the regrowth or overgrown material to be substantially exhausted of threading dislocations and other dislocation defects.  FIGS. 10A and 10B  show body-tied finFETs or tri-gate transistor structures which takes the advantage of the vertical shape of the lattice-mismatched material. One exemplary method includes depositing or growing an oxide layer, followed by depositing a nitride layer, masking and etching a trench of width w&lt;0.5 h; (the trench orientation may be in a &lt;110&gt; direction, so all the threading dislocations along &lt;110&gt; directions (which will lie at an angle of 45-degrees to the surface of the first semiconductor material) will intersect sidewalls within the height of h); selectively growing Ge or III-V in the trench; chemical-mechanical polishing to remove the portion of selective growth outside of the trench; selectively removing nitride, which results in fin structures; and then growing and/or depositing insulator material around the fin structures; followed by depositing, masking and etching gate electrodes and ion implantation to create source/drain regions. A second exemplary method includes depositing or growing an oxide layer, masking and etching a trench of width w&lt;0.5 h; selectively growing Ge or III-V in the trench; chemical-mechanical polishing to remove the portion of selective growth outside of the trench; selectively removing a portion of the oxide, which results in fin structures; and then growing and/or depositing insulator material around the fin structures; followed by depositing, masking and etching gate electrodes and ion implantation to create source/drain regions. 
     Besides FET devices, the dislocation-blocking techniques of the invention can also be used to fabricate other types of devices, such as optical devices. Referring to  FIGS. 11-12 , in some embodiments, Ge or III-V photodetectors are integrated into a Si substrate using such techniques. In an exemplary embodiment shown in  FIG. 11 , a lower contact is implanted on a Si substrate to form p+-type region; low-temperature oxide is deposited; apertures or trenches are etched through the low-temperature oxide layer to explore the Si substrate; and Ge or III-V materials are selectively grown on the apertures or trenches with in-situ doping until past the defect regions (p-type). Further, epitaxial growth continues until the thickness is sufficient to allow for desirable levels of absorption of incident light, and then the top layer is implanted to form an n-type region. In another configuration, the light comes from the side (e.g. from in-plane waveguide) instead of from the top, as shown in  FIG. 12 . This allows light detection to occur in-plane with the wafer surface and also to allow growth thickness to be independent of absorption depth. 
     In various embodiments described above, the dislocation-blocking is performed in a vertical direction.  FIG. 13A  shows an alternative embodiment where the dislocation-blocking may conduct in a lateral direction, for example from the source or drain region. Therefore, the device can be an SOI structure. In one embodiment, the gate oxide and gate stack can be formed first, before the dislocation-blocking growth under the gate, using a channel-replacement-type process. This approach addresses the self-alignment issue and any surface roughness issues. 
       FIG. 13B  shows another method which allows dislocations be terminated for a large size epitaxial area. The method includes two steps of epitaxial growth, which take different growth directions, so that the dislocations in one direction terminate at the sidewall during the first epitaxial growth, and the dislocations in another direction, which may have large device dimensions, can terminate on the sidewall when the epitaxial growth changes the direction. 
     Conventional Ge/III-V necking forms crystal material in the vertical direction. Therefore, when building planar MOS or finFET type devices on that crystal, the device is typically a bulk-type or body-tied, not an “on-insulator” structure. Bulk-type of Ge or GaAs FET may exhibit large junction leakage and poor short-channel effect control. One solution is to build the device vertically instead of parallel to horizontal surface.  FIG. 13C  shows one embodiment of such structure: a vertical-channel FET, which incorporates the benefits that a vertical FET has, for example, SCE control, better scalability, etc. Another approach is to epitaxially grow an oxide layer that is lattice-matched to the second semiconductor material during selective deposition of the second semiconductor material. As result, there is an oxide layer within the regrowth region underlying a portion thereof subsequently used for device fabrication, as discussed in more detail in co-pending U.S. patent application Ser. No. 11/000,566 by Currie, incorporated herein by reference. 
       FIGS. 14-16  illustrate a method, in accordance with embodiments of the current invention, for reducing defects in lattice-mismatched semiconductor heterostructures by forming a recess or groove in a substrate, and also illustrate devices formed in accordance with the method.  FIG. 14A  shows a semiconductor structure  1400  that includes a substrate  310  and a dielectric layer  1404  disposed thereover. As discussed previously, with reference to  FIGS. 3A-3B , the substrate  310  may include, or consist essentially of, a first semiconductor material, such as, for example, a group IV element, e.g., germanium or silicon. The first semiconductor material may be crystalline. The substrate  310  may be, for example, a bulk silicon wafer, a bulk germanium wafer, a semiconductor-on-insulator ( 50 I) substrate, or a strained semiconductor-on-insulator (SSOI) substrate. In one embodiment, the substrate  310  includes or consists essentially of (100) silicon. The substrate  310  may include a material having a first conductivity type, e.g., n- or p-type, such as n +  Si. 
     The dielectric layer  1404  may be formed over the substrate  310 . The dielectric layer  1404  may include or consist essentially of a dielectric material, such as silicon nitride or silicon dioxide (SiO 2 ). The dielectric layer  1404  may be formed by any suitable technique, e.g., thermal oxidation or plasma-enhanced chemical vapor deposition (PECVD). In some embodiments, the height h 1  of the dielectric layer  1404  may be in the range of, e.g., 25-1000 nm. In a preferred embodiment, the height h 1  is approximately 600 nm. 
     An opening or trench  1406  may be formed in the dielectric layer  1404 , exposing a portion  1408  of the surface of the substrate  310 . More than one opening  1406  may be formed, and each opening  1406  may have a height equal to the height of the dielectric layer, e.g., height h 1 , and a width w 1 . The opening(s)  1406  may be created by forming a mask, such as a photoresist mask, over the substrate  310  and the dielectric layer  1404 . The mask may be patterned to expose a portion of the dielectric layer  1404 . The exposed portion of the dielectric layer  1404  may be removed by, for example, reactive ion etching (RIE) to define the opening  1406 . The opening  1406  may be defined by at least one sidewall  1407 . In one embodiment, the opening  1406  is formed within the substrate  310 , and the dielectric sidewall  1407  is formed within the opening  1406 . 
     The opening  1406  may be substantially rectangular in terms of cross-sectional profile, a top view, or both. With respect to a top view, the width w 1  may be smaller than the length l 1  (not shown) of the opening. For example, the width w 1  of the opening  1406  may be less than about 500 nm, e.g., about 10-500 nm, and the length l 1  of the opening  1406  may exceed w 1 . The ratio of the height h 1  of the opening to the width w 1  of the opening  1407  may be ≧0.5, e.g., ≧1. The opening sidewall  1407  may not be strictly vertical. 
     Referring to  FIG. 14B  and also to  FIG. 5A , a portion  1408  of the surface of the substrate  310  may be removed by, e.g., etching, to form a recess or groove  1410 . The recess  1410  may be formed by wet etching the substrate  310  with, for example, KOH or NaOH, or by dry etching, such as plasma etching. In one embodiment, the surface  1412  in the recess  1410  features non-(100) surfaces, such as (111) semiconductor surfaces, e.g., (111) Si surfaces. The recess  1410  may have a maximum depth d corresponding to its deepest point farthest from the substrate surface and may have a v-shaped profile. The ratio of the height of the dielectric layer  1404  plus the maximum depth of the recess  1410  to the width of the opening may be greater than or equal to one, i.e., (h 1 +d 1 )/w 1 ≧1. 
     In one embodiment, and with reference also to  FIG. 6H , another portion of the dielectric layer  1404  may be partially removed with, for example, a hydrofluoric acid etch. A portion of the dielectric layer  1404  distal to the substrate  310  may be removed at a faster rate than a portion proximate the substrate  310 , creating a non-vertical sidewall  1414 . The remaining portion of the dielectric layer  1404  may resemble an inverted V when viewed in cross-section. In one embodiment, the sidewall  1414  is substantially parallel to the surface  1412  of the opening  1410 . In another embodiment, a horizontal portion  1416  of the dielectric layer  1404  remains after the etch, and the width w 2  of the horizontal portion  1416  is much less than the width w of the opening  1406 . 
       FIG. 14C  shows an alternative embodiment of the current invention in which the dielectric layer  1404  is removed from the substrate  310  after the formation of the recess(es)  1410 . The dielectric layer  1404  including SiO 2  may be removed with, for example, a hydrofluoric acid etch. This embodiment of the invention may be suitable for the growth of III-nitride semiconductor layers or layers of other semiconductor materials having hexagonal lattice structures. Any dislocation defects that form in a III-nitride material (or other suitable material) deposited on the surface of the substrate  310  may form perpendicularly to the opening surfaces  1412 , rather than vertically, as the defects may typically form. The non-vertical formation of the defects may, therefore, create defect-free regions in the deposited material. In an alternative embodiment, the recesses  1410  improve the formation of GaAs (or other semiconductor materials having cubic or zinc blende lattice structures) heteroepitaxial structures. 
       FIG. 15A  illustrates an embodiment of the current invention in which the dielectric layer  1404  is not removed from the substrate  310  after the formation of the recess(es)  1410 . A second crystalline semiconductor material  1500 , which is lattice-mismatched to the first crystalline semiconductor material in the substrate  310 , may be formed within the recess  1410 . In one embodiment, the second crystalline semiconductor material  1500  may be further formed within the opening  1406 . The second crystalline semiconductor may be formed by, for example, metal-organic chemical vapor deposition (MOCVD) or molecular-beam epitaxy (MBE). The second crystalline semiconductor material  1500  may be a III-V material, such as GaAs or InP, a type-IV material, such as Ge or SiGe, or an alloy or mixture including any of these materials, such as InGaP. In one embodiment, an etch-stop layer (not shown), including a wide band-gap material, may be formed on top of the second crystalline semiconductor material  1500 . Dislocation defects  1502  may form in the second crystalline semiconductor material  1500  near the interface  1504  between the substrate  310  and the second crystalline semiconductor material  1500 . The defects may form along the (111) direction, which may be parallel to the surface  1412  of the substrate  310 . The defects may terminate at a height H 1  above the deepest point of the recess  1410 . In one embodiment, the defects terminate within the recess  1410 , and H 1 ≦d 1 . In another embodiment (e.g., the embodiment illustrated in  FIG. 15A ), the defects terminate within the opening  1406 , and H 1 ≦h 1 +d 1 . In a third embodiment, the defects terminate at a height H 1  that is less than or equal to the width w 1 . 
     The recess  1410  may effectively increase the height h of the opening  1406 . The surfaces  1412  along the interface  1504  may define an angle  1501  with the horizontal of approximately 57 degrees. The depth d may thus be equal to)tan(57°×w 1 /2, and the effective height may be equal to h 1 +)tan(57°×w 1 /2. The height h 1  may be effectively increased regardless of the material to be grown in the opening  1406 . In one embodiment, the recess  1410  allows a reduction in the height h 1  because the effective increase of h 1  may cause any dislocation defects to terminate at a lower height above the substrate  310 . 
     In one embodiment, the second crystalline semiconductor material  1500  does not extend above the height h of the dielectric layer  1404 . In an alternative embodiment, the second crystalline semiconductor material  1500  extends above the height h of the dielectric layer  1404 , and may coalesce with the second crystalline semiconductor material grown in a neighboring opening  1406  to form a single layer of the second crystalline semiconductor material  1500  above the dielectric layer  1404 . 
     In one embodiment, a buffer layer  1503 , comprising a third crystalline semiconductor material, is formed between the second crystalline semiconductor material  1500  and the substrate  310 . The buffer layer may be formed on the surface  1412  of the substrate  310 , and extend approximately up to the dielectric layer  1404 . In another embodiment, the buffer layer  1503  is confined to the recess  1410 . The boundary between the second  1500  and third  1503  crystalline semiconductor materials may be proximate the boundary defined by the interface between the exposed portion of the substrate  310  and the dielectric sidewall  1407 . The buffer layer  1503  may be used to facilitate the formation of the second crystalline semiconductor material  1500  if there is a large difference between the lattice constants of the second crystalline semiconductor material  1500  and of the substrate  310 . For example, the substrate  310  may include Si and the second crystalline semiconductor material  1500  may include InP, so that the two materials differ in lattice constants by approximately eight percent. In this example, GaAs may be used as the buffer layer, because its lattice constant differs from that of both Si and InP by approximately four percent. In another embodiment, Ge or another material having a lattice mismatch to the first and/or second crystalline semiconductor materials of less than eight percent may be used as a buffer layer. 
     In one embodiment, the buffer layer  1503  may include a constant concentration of the third crystalline semiconductor material, or the concentration may vary such that the lattice constant of the buffer layer  1503  is closer to that of the substrate  310  at the bottom of the buffer layer and closer to that of the second crystalline semiconductor material  1500  near the top of the buffer layer. In another embodiment, multiple buffer layers may be used. The use of one or more buffer layers may allow the formation of one or more heteroepitaxial material layers with large lattice-constant mismatches, while reducing the height h of the dielectric layer  1404  and/or depth d of the recess  1410 . The heteroepitaxial material layers may be formed inside the openings  1406  or above the dielectric layer  1404 . 
       FIG. 15B  illustrates that, in one embodiment, the second crystalline semiconductor material  1500  may be planarized by, for example, CMP. A substantially smooth surface  1506  may be formed as a result of the planarization. The upper portion  1508  of the second crystalline semiconductor material  1500  may be substantially free of defects. 
       FIG. 15C  shows, in one embodiment of the current invention, a device structure  1510  formed on the surface  1506  of the second crystalline semiconductor material  1500 . In an alternative embodiment, the device structure  1510  is wholly or partially formed within the opening  1406 . The device structure  1510  may include a diode; a transistor; a photonic device such as a photovoltaic device, LED, or laser diode; or any other passive or active semiconductor device. The device structure  1510  may include a single layer of a semiconductor material, or may include more than one layer. Each layer may include or consist essentially of a type-IV semiconductor material or a III-V semiconductor material. In one embodiment, semiconductor devices may be formed in regions of the substrate  310  proximate the device structure  1510 . 
       FIG. 15D  depicts, in one illustrative embodiment, a flip-chip wafer bonding process. A thin metal layer  1512  may first be formed on top of the device structure  1510 . The thin metal layer  1512  may serve as both an electrical conductor and, in the case where the device structure  1510  is a photonic device, as an optical reflector. In various embodiments, the thin metal layer is approximately 100-200 nm thick and includes or consists essentially of aluminum (for photonic devices emitting visible light) or gold or silver (for infrared light). The semiconductor structure  1400  may then be bonded to a handle wafer  1514  to define a bonded structure  1515 . The handle wafer  1514  may include or consist essentially of a semiconductor material such as Si. Finally, a top contact layer  1516  may be formed on a top surface of the handle wafer  1514  and a bottom contact layer  1518  may be formed on a bottom surface of the substrate  310 . 
       FIG. 16A  illustrates a flipped structure  1600 , which may be the bonded structure  1520  rotated by 180 degrees. The substrate  310  may be removed with, for example, an etching process, exposing surfaces  1602  of the second crystalline semiconductor material  1500 . The defects  1502  may now be near the exposed surfaces  1602  of the second crystalline semiconductor material  1500 . The exposed portion of the second crystalline semiconductor material  1500  may include a non-planar surface. 
     The wafer bonding and flipping process may present advantages during the formation of the second crystalline semiconductor material  1500 , because any layer or layers that include the second crystalline semiconductor material  1500  may ultimately be rotated 180 degrees. For example, with reference to  FIG. 15A , a layer with a high bandgap may first be deposited on the substrate  310  first, then a layer with a medium bandgap, and finally a layer with a low bandgap. Because the first layer with a high bandgap may require a higher growth temperature than the materials with lower bandgaps, depositing the layers in this order may spare the low bandgap layer from being subjected to the high temperature required for the high bandgap layer. The low bandgap layer may require a lower growth temperature, which the high bandgap material may be better able to withstand. When the device structure  1400 , including the illustrative high-, middle-, and low-bandgap layers of the second crystalline semiconductor material  1500 , is rotated by the flipping process, the layers may be in an optimal configuration to form a photonic device. 
     Similarly, doping types in the layer or layers comprising the second crystalline semiconductor material  1500  may be chosen to take advantage of the bonding and flipping process. For example, later processing steps may raise the temperature of the device structure  1400  sufficiently to cause the material of substrate  310 , e.g., Si, to diffuse into a first deposited layer or region in the second crystalline semiconductor material  1500 . Because the material of substrate  310  may be a n-type dopant in III-V materials such as GaAs and InP, atoms of that material that diffuse into a first deposited p-type doped III-V layer may deleteriously compensate the p-type dopants in that layer. Depositing n-type-doped III-V material on the substrate  310  first, however, may insulate other p-type doped III-V layers against diffusion from the substrate  310 . 
       FIG. 16B  depicts, in one embodiment, the removal of a portion of the second crystalline semiconductor material  1500  by, for example, an etching process. The dielectric layer  1404  may be relatively unaffected by the etching process. In an alternative embodiment, a portion of the second crystalline semiconductor material  1500  and a portion of the dielectric layer  1404  may be removed simultaneously by, for example, CMP. The portion of the second crystalline semiconductor material  1500  that is removed may contain at least a majority of the defects  1502 , leaving the remaining portion of the second crystalline semiconductor material  1500  substantially defect-free. The exposed surfaces  1604  of the second crystalline semiconductor material  1500  may comprise (100) surfaces. 
       FIG. 16C  illustrates the structure  1600 , in one embodiment, after the dielectric layer  1404  has been removed. A portion of the dielectric layer  1404  may have previously been removed by CMP, as illustrated in  FIG. 16B , in which case the remaining portion of the dielectric layer  1404  may be removed. The exposed second crystalline semiconductor material  1500  may comprise one or more ridges  1606 . Each ridge  1606  may have a width w, corresponding to the width w of the openings or trenches  1406 . Adjacent ridges  1606  may be separated by a spacing s. The width w and the spacing s may each be less than or equal to a visible light wavelength. In one embodiment, the exposed second crystalline semiconductor material  1500  comprises a two-dimensional array of raised features. The width w and the spacing s of the ridges  1606  in a one- or two-dimensional array may be selected such that the ridges  1606  enhance the light absorption or extraction efficiency of the structure  1600  and/or improve the light beam quality. In one embodiment, the spacing s is approximately equal to the wavelength of the absorbed or emitted light. 
       FIG. 16D  illustrates another embodiment of the structure  1600 . A metal contact  1608 , designed in accordance with standard methods, may be formed on the surface of the second crystalline semiconductor material  1500 . In one embodiment, the metal contact  1608  conforms to at least one ridge  1606 . In an alternative embodiment, the oxide layer  1404  is not removed before the metal contact  1608  is formed. In this embodiment, the top surface of the structure  1600  may be planarized before the metal contact  1608  is formed. 
     A photonic device  1510 , such as a light-emitting diode or a photovoltaic device, formed in accordance with the method described in  FIGS. 14-16 , may exhibit several advantages. For example, an optical reflection layer may be formed, prior to bonding, on the handle wafer  1514  or on the semiconductor structure  1400 . The optical reflection layer may therefore be disposed beneath the photonic device  1510 , and may thereby enhance backlight reflection and photon recycling. An LED photonic device  1510  may exhibit increased light-extraction efficiency as a result of the placement, width, and spacing of the ridges  1606  formed on the surface of the crystalline semiconductor material  1500 . In addition, the ridges  1606  may enhance the thermal cooling of the structure  1600 , thereby improving the thermal depletion of the structure. 
     Other embodiments incorporating the concepts disclosed herein may be used without departing from the spirit of the essential characteristics of the invention or the scope thereof. The foregoing embodiments are therefore to be considered in all respects as only illustrative rather than restrictive of the invention described herein. Therefore, it is intended that the scope of the invention be only limited by the following claims.