Abstract:
Buried layers are formed within a semiconductor. Metallic or insulating buried layers are produced several microns within a semiconductor substrate. The buried layer can confine current to the buried layer itself by using a conductive material to create the buried layer. The buried layer can also confine current to a specified area of the semiconductor, by using an insulating material inside of the buried layer or by leaving a created void within the material. The buried layer is useful in the construction of a semiconductor Vertical Cavity Laser (VCL). A buried isolation layer confines the current to a narrow active region increasing efficiency of the VCL. The buried layer is also useful in fabricating discrete devices, such as diodes, transistors, and photodetectors, as well as fabricating integrated circuits.

Description:
“This application is a Continuation of application Ser. No. 08/612,687, filed on Mar. 8, 1996, U.S. Pat. No. 5,977,604 entitled “BURIED LAYER IN A SEMICONDUCTOR FORMED BY BONDING”, which application is incorporated herein by reference.” 
    
    
     STATEMENT AS TO RIGHTS TO INVENTIONS MADE UNDER FEDERALLY SPONSORED RESEARCH AND DEVELOPMENT 
     This invention was made with Government support under Grant No. F19628-95C-0054, awarded by the Air Force Office of Scientific Research. The Government has certain rights in this invention. 
    
    
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention. 
     A semiconductor with a buried blocking or conducting layer is a valuable electronic or optoelectronic element. 
     This invention relates in general to creating buried layers within a semiconductor. More particularly, the invention is directed to creating metallic or insulating buried layers deep within a semiconductor substrate. The invention is further concerned with lasers formed using such a substrate. 
     2. Description of Related Art. 
     The optical fiber and the semiconductor laser have revolutionized the field of information transmission. The development of fiber optic communication systems has been fueled by the growing need for a high bit-rate and high volume communication medium that is more efficient than the coaxial cable. Optical fibers can guide information encoded in light signals uninterrupted over hundreds of kilometers, while the semiconductor laser provides an inexpensive source for such optical transmission. 
     Presently, optical fibers are being installed around the world: across the land and under the oceans. Both commercial telephone and business communications are being conducted over these links. Such links use a large number of semiconductor lasers to transmit and amplify the signals. Communication applications extend to numerous short distance applications, such as, local area networks, and chip-to-chip communication. Compact disc players and optical storage devices, as well as, laser printing were brought to mass production owing to the inexpensive semiconductor laser. Semiconductor lasers are pushing their way into many other fields where they are replacing large solid state or gas lasers with small devices barely visible to the naked eye. 
     In recent years, the vertical-cavity laser (VCL) has emerged as a new coherent light source alongside the conventional in-plane laser. This is due to its compactness, inherent single-longitudinal mode operation, circular beam profile and straightforward integration with other electronic circuitry. Vertical-cavity lasers hold promise of superior performance in many optoelectronic applications and lower manufacturing cost than in-plane lasers. 
     State of the art GaAs-based vertical-cavity lasers operate continuously at room-temperature with sub-100 μA threshold currents. The outstanding performance of these laser greatly relies on their monolithic fabrication process and the quality of Al(Ga)As/GaAs quarter-wave mirrors, which are presently the highest quality epitaxial mirrors that can be routinely fabricated. 
     The development of long-wavelength 1.3 μm and 1.55 μm vertical-cavity surface-emitting lasers has been driven by the need for low cost, high speed sources for optical communications and interconnects. However, the practical realization of these lasers has been a difficult process over the last decade due to numerous technological difficulties. A significant problem has been the fabrication of mirrors with sufficiently high reflectivity and adequate electrical and thermal properties. It has been recently demonstrated that, using the process of wafer fusion, InGaAsP active layers operating at 1.3 μm and 1.55 μm can be bonded to Al(Ga)As/GaAs mirrors, thereby enabling the fabrication of long-wavelength vertical-cavity lasers with mirrors grown on GaAs. 
     The methods for fabricating a VCL that have high reflectivity mirrors, proper current confinement to the active region of the VCL, and proper VCL structure have, to this point, been expensive and lengthy. The inability to grow GaAs, which is a preferred mirror material, on InP because of improper lattice matching creates problems for InP active layer VCLs. The low yields created by deeply etched well VCLs and the low reflectivity create efficiency and output problems. The use of amorphous silicon reduces the reflectivity of one of the mirrors, also reducing the efficiency and yields for these methods of constructing VCLs. 
     The creation of buried layers that can block current flow, or, conversely, channel current flow without appreciably affecting the semiconductor currents can be used in numerous semiconductor devices besides VCLs. VLSI devices also experience problems because of interference between layers of the device; buried layers formed via ion implantation are not deep enough into the semiconductor bulk to eliminate the effect on the conductive paths in a VLSI device. VLSI devices are combinations of transistors, diodes, resistors, and capacitors, and therefore, a buried blocking layer can be effective in enhancing the output characteristics of all types of semiconductor devices. Further, the use of buried insulating or conducting layers created via fusion can be used in place of or in conjunction with ion implantation or diffusion techniques to fabricate diodes, transistors, photodetectors, and other discrete and integrated semiconductor devices. 
     It can be seen then that there is a need for a method of creating a buried layer deep within a semiconductor device. It can also be seen that there is a need for a method of creating buried layers that are insulating in nature, thereby channeling current through certain regions of the semiconductor. There is also a need for a method of creating buried layers that carry current efficiently, thereby leaving the surrounding regions of the semiconductor unaffected. 
     SUMMARY OF THE INVENTION 
     To minimize the limitations described above, and other limitations that will become apparent upon reading and understanding the present specification, the present invention discloses a powerful device and method for creating a buried layer within a semiconductor device. The present invention is integrable with current fabrication techniques with both crystalline and amorphous substrates. 
     The present invention minimizes the above-described problems by providing a method for creating a doped region on the surface of a semiconductor wafer, and then by fusing or bonding that surface to another wafer, making that doped region a buried layer. 
     Further, by etching into the first wafer prior to fusing or bonding, a void may be created within a semiconductor device. The void will be completely surrounded with crystalline semiconductor material. The void may be partially or completely filled with material, either insulating or conductive, depending on the application desired. 
     A system in accordance with the principles of the present invention comprises creating a doped region within a wafer, where the doped region is exposed to one of the working planes of the wafer, and then fusing or bonding that surface to a second wafer, thus burying the doped region. 
     Similarly, if an etched area was created in the first wafer, and then the surface with the etched channel was fused to a second wafer, the void created by the etch would be sealed within the semiconductor bulk, having been covered by the second wafer. 
     One object of the present invention is to channel currents within a semiconductor device. Another object of the present invention is to channel currents within a conductive channel buried in a semiconductor device. Another object is to channel light within a semiconductor device. 
     These and various other advantages and features of the invention are pointed out with particularity in the claims and form a part hereof. A further understanding of the invention, its advantages, and the objects obtained by its use, is obtained from the following description and the drawings. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     Referring now to the drawings in which like reference numbers represent corresponding parts throughout: 
     FIGS. 1A-1F are cross sectional block diagrams of a planar method of performing the invention; 
     FIGS. 2A-2F are cross sectional block diagrams of a mesa and valley method of performing the invention; 
     FIGS. 3A-3C are cross sectional diagrams showing methods of constructing a Vertical Cavity Laser (VCL); 
     FIG. 4 is a cross sectional view of a completed VCL; 
     FIGS. 5A-5K are cross sectional views of the use of patterned bonding as a means for current confinement; 
     FIGS. 6A-6F are cross sectional views of a second method for constructing a VCL; 
     FIGS. 7A-7D are cross sectional views of a method for constructing a bipolar junction transistor; 
     FIGS. 8A-8E are cross sectional views of a method for constructing a heterojunction bipolar transistor; 
     FIGS. 9A-9F are cross sectional views of a method for constructing a photodetector; 
     FIG. 10 shows a flowchart of the steps of the method of the invention; and 
     FIG. 11 shows a flowchart of the steps of an alternative embodiment of the steps of the method of the invention. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     The present invention provides a method of constructing a buried layer within a semiconductor device. 
     FIG. 1A-1F are cross sectional block diagrams of a planar method of performing the invention. In FIG. 1A, a semiconductor wafer  10  is shown. The wafer  10  can be of a single crystalline element, such as silicon or germanium, or can be a compound, such as Gallium Arsenide (GaAs) or Indium Phosphide (InP). The wafer  10  is normally planar, but can also be piecewise linear. Piecewise linear surfaces are locally linear, but globally non-linear. 
     In FIG. 1B, wafer  10  initially has a dopant  12  deposited on the surface in a desired pattern to create a junction. The dopant can be of a conductive type, such as boron, phosphorous, or arsenic as used in silicon processing, or an insulating type, such as iron and oxygen as used in GaAs and InP processing, depending on the application desired for the device. 
     In FIG. 1C, the dopant  12  is then driven into the material to create a doped well  14  (also described as a region) within the wafer  10 . The atoms on the surface of the wafer  10  replace the atoms within the crystalline structure upon application of heat to the wafer  10 . Alternatively, FIGS. 1B and 1C can be performed by ion implantation of the dopant  12  into the wafer  10 . 
     FIG. 1D shows the wafer  10  physically in contact with cover wafer  16 . The cover wafer  16  can be a wafer of the same material as wafer  10 , or can be of a different material than wafer  10 . Once this physical connection is made, and the wafer  10  and the cover wafer  16  are properly aligned, heat is applied to the combination to fuse the wafer  10  and the cover wafer  16 . The fusion process normally requires placing the wafers in an autoclave and raising the temperature to approximately 600 degrees Centigrade for InP and GaAs, but may be higher or lower for other materials. A typical fusion process is described in U.S. Pat. No. 5,407,856, which is incorporated by reference. The junction created by the physical connection between the wafer  10  and the cover wafer  16  is the fusion junction  18 . The wells  14  now become a buried layer within the structure created by the wafer  10  and the cover wafer  16 . 
     FIG. 1E is a different construction technique than FIG.  1 D. Prior to the fusion step, dopant  12  may be introduced onto the cover wafer  16  to create cover wells  20 . FIG. 1E shows that the cover wafer  16  may have dopant wells that may overlap the wells  14  in the wafer  10 . Similarly, the cover wells  20  do not have to overlap the wells  14 ; the overlap is dependent on the design of the device and the requirements of the device. These cover wells  20  can be of the same dopant material as the wells  14 , or can be of a different dopant material, thus allowing for insulating wells in the cover wafer  16  and conducting wells in the wafer  10 , or wells of different types of conductor, or any combination desired by the designer or required by the device. 
     FIG. 1F is a further refinement of FIG.  1 E. By properly choosing the dopant material, the substrate material, and the alignment of the wells  14  and the cover wells  20 , diffusion can be designed into the fusion process across the fusion junction  18 . Typical fabrication processes shown in FIGS. 1B and 1C are performed by heat. The fusion process also requires heat to fuse the wafer  10  and the cover wafer  16 , and can be used for additional diffusion of the wells  14  and the cover wells  20 . The well  14  and the cover well  20  can be designed to not diffuse across the fusion junction  18  in certain sections as shown at junction  22 . The well  14  can also be designed, by using proper dopants and wafer  10  and cover wafer  16  materials, to diffuse across the fusion junction  18  during fusion. This process is called “fusion-diffusion,” or “cross diffusion.” The cross diffusion process creates a cross diffusion well  24  underneath the cover well  20  in the wafer  10 . By designing the cover well  20  and well  14  depths properly, the depth of the cross diffusion well  24  can be determined prior to the fusion process to prevent the cross diffusion well  24  from diffusing completely through the well  14 . 
     Similarly, a cover cross diffusion well  26  can be created in the cover wafer  16 . The use of geometry and different dopant densities can create cover cross diffusion wells  26  that do not intrude into the cover well  20  as shown, or, if desired, do intrude into the cover well with the dopant used in the well  14 . 
     The use of different dopant and wafer materials, geometries, and dopant depths creates various combinations of different charge carriers, insulating layers, and cross diffusion characteristics. These combinations are controlled by the designer. 
     FIGS. 2A-2F are cross sectional block diagrams of a mesa and valley method of performing the invention. FIG. 2A shows the wafer  10  prior to processing. FIG. 2B shows the wafer  10  patterned by a pattern material  28 . Typically, the pattern material  28  is photoresist, but the pattern material  28  can be anything which alternatively exposes and covers the top surface  30  of the wafer  10 . The wafer  10  is then etched with an etchant. Alternatively, the wafer  10  can have material grown on the top surface  30  of the wafer  10 , where material is grown to match the pattern created by the pattern material  28 . The material grown may be amorphous or crystalline. Typically, the wafer  10  is etched in a wet chemical etchant, but the wafer  10  can be etched or processed by any other means. 
     FIG. 2C shows the wafer  10  etched and the pattern material  28  removed. The etching will leave mesas  32  and valleys  34 . The valleys  34  are exposed areas of the wafer  10  which are lower in height than the original top surface  30  of the wafer  10 . The mesas  32  are areas which still extend to the original top surface  30  of the wafer  10 . 
     FIG. 2D shows the wafer  10  physically in contact with cover wafer  16 . The cover wafer  16  can be a wafer of the same material as wafer  10 , or can be of a different material than wafer  10 . Once this physical connection is made, and the wafer  10  and the cover wafer  16  are properly aligned, heat is applied to the combination to fuse the wafer  10  and the cover wafer  16 . The junction created by the physical connection between the wafer  10  and the cover wafer  16  is the fusion junction  18 . The fusion process typically does not appreciably change the shape of the valley  34  by more than a few percent, depending on the valley  34  dimensions, wafer materials, and fusion times. The valleys  34  now become a buried void within the structure created by the wafer  10  and the cover wafer  16 . This void can be buried several microns deep in a semiconductor structure. Since there are no charge carriers in the void, any current travelling between wafer  10  and cover wafer  16  must travel through the mesas  32 . The use of the valleys  34  to create cavities increases the current density in the mesas  32 . 
     FIG. 2E is a further refinement of FIG.  2 D. Prior to FIG. 2D, cover wafer  16  is also patterned and etched as illustrated in FIGS. 2B and 2C. This now creates cover mesas  36  and cover valleys  38  on cover wafer  16 . When the wafer  10  and the cover wafer  16  are properly aligned, this creates more geometric patterns than are possible if only the wafer  10  is etched. Larger voids created by overlapping the cover valley  38  and the valley  34  would be possible depending on the alignment and design of the device. Additionally, mesa  32  may be completely surrounded or enclosed by cover valley  38 , or cover mesa  36  may be completely surrounded or enclosed by valley  34 . This would allow the top of cover mesa  36  to extend into valley  34 , and possibly touch the bottom surface of valley  34 . 
     FIG. 2F is a further refinement of FIGS. 2D and 2E. At times, a design may require even more current confinement than an empty valley  34  and cover valley  38  can supply. Alternatively, a design may require current to be carried in the valley  34 , without affecting the current in the mesa  32 , or current to be carried in the cover valley  38  without affecting the current in the valley  34 . 
     By depositing valley material  40 , additional current confinement is possible if insulating material is used for the valley material  40 . The valley material  40  may be solid, liquid, or gaseous, or the valley  34  can be partially or fully evacuated by performing the fusing or bonding step in a vacuum. Alternatively, current flow in the valley  34  is possible if conductive material is deposited in the valley  34 . This is possible only when the height of the valley material  40  is less than or equal to the height of the mesa  32 . The valley material  40  may be less than or equal to the height of the mesa  32 , less than or equal to the width of the valley  34 , or any combination thereof. The deposition of the valley material  40  may also be offset in the valley  34 , such that the valley material  40  is closer to the wall of the mesa  32  at one end of the valley  34  than the valley material  40  is to the wall of the mesa  32  at the other end of the valley  34 . Further, the valley material  40  may, for some applications, be deposited at a level higher than the mesa  32 , or on top of the mesa  32 . Similarly, depositing cover valley material  42  in the cover valley  38  allows for additional current confinement or additional current carrying capabilities in the cover valley  38 , depending on the type of material used for the cover valley material  42 . Valley material  40  and cover valley material  42  may be metals, such as nickel or tin, and may be deposited on an insulating layer as well as directly on the semiconductor surface. 
     FIGS. 3A-3C are cross sectional diagrams showing methods of constructing a Vertical Cavity Laser (VCL). In FIG. 3A, illustrative of the prior art, the wafer  10  is first grown with an active region  44 , non-active regions  46 , and cladding layer  48 . The active region  44  may be an active, absorbing, or modulating region, depending on the desired device. The wafer  10  is then thinned in thinning region  50 , and a well is formed. If necessary, additional current confinement regions  52  can be added to the non-active regions  46  to force the current to pass through the active region  44  by etching, implantation, or regrowth. Contacts  54  are then added to the wafer  10 . 
     Finally, a top Distributed Bragg Reflector (DBR)  56  and a bottom DBR  58  are added. The top DBR  56  and the bottom DBR  58  are both amorphous dielectric mirrors to allow the energy to reflect through the active region  44 . Amorphous mirrors are deposited by low-temperature deposition techniques, such as evaporation, sputtering, or plasma enhanced chemical vapor deposition (PECVD), and are generally insulating. This method produces low-efficiency VCLs because the amorphous nature of the top DBR  56  and the bottom DBR  58  make them poor reflectors, thus giving the lasers typically higher thermal resistance and increasing the current density in the active region  44 . 
     FIG. 3B shows a VCL which has one conductive DBR grown epitaxially, and one amorphous DBR. The wafer  10  first has an epitaxial DBR  60  grown on the surface of the wafer  10 . Epitaxial DBRs  60  have reflectivities up to 99.9% versus≦99% for amorphous DBRs, and therefore allow for more energy to be reflected back to the active region  44  because the laser has a better resonator. An active region  44  and non-active regions  46  are epitaxially grown on a second wafer  62 , and the second wafer  62  is fused to the wafer  10  on top of the epitaxial DBR  60 . The entire second wafer  62  is then thinned to thinning junction  64 . Thinning junction  64  can be defined by an etchstop layer in the second wafer  62 , or other means. Once the second wafer  62  is thinned to the thinning junction  64 , current confinement regions  52  can be added to increase the current density in the active region  44 . Contacts  54  are added to the second wafer  62  on top of the thinning junction  64 , and to the bottom of wafer  10 . 
     Finally, a top Distributed Bragg Reflector (DBR)  56  is added. This method of construction allows for better reflection by one of the mirrors, but is still limited by the lower reflectivity of the top DBR  56 . 
     FIG. 3C shows a structure with two epitaxial DBRs. This structure has the advantage of having high reflectivity on both sides of the active region  44 , allowing for current gain through the active region  44 . The wafer  10  first has an epitaxial DBR  60  grown on the surface of the wafer  10 . Epitaxial DBRs  60  have higher electrical and thermal reflectivities and therefore allow for more energy to pass through the active region  44 . An active region  44  and non-active regions  46  are epitaxially grown on a second wafer  62 , and the second wafer  62  is fused to the wafer  10  on top of the epitaxial DBR  60 . The entire second wafer  62  is then thinned to thinning junction  64 . Thinning junction  64  can be defined by an etchstop layer in the second wafer  62 , or other means. 
     A top epitaxial DBR  66  is then grown on a third wafer  68 . If necessary, current confinement regions  52  can be implanted or diffused into the top epitaxial DBR  66  to provide current confinement. The top epitaxial DBR  66  is then fused to the active region  44  and non-active regions  46  at the thinning junction  64 . The third wafer  68  is then thinned to a second thinning junction  70 . Contacts  54  are then added to the third wafer  68  at the second thinning junction  70 , and to the wafer  10 . The opening  72  allows the laser light to escape from the VCL. Contacts  54  can be added at different locations, and the laser light can escape from the top of the VCL. Alternatively, light can be emitted through the wafer  10  substrate. 
     FIG. 4 is a cross sectional view of a completed VCL. The structure of the top epitaxial DBR  66  is smaller in width than the epitaxial DBR  60 , allowing for natural current confinement within the area below the top epitaxial DBR  66 . The laser light is emitted from the VCL through opening  72 . 
     FIGS. 5A-5K are cross sectional views of the use of patterned bonding as means for current confinement. In FIG. 5A, wafer  10  has an epitaxial DBR  60  grown on top of the wafer  10  surface. The wafer  10  is preferably Gallium Arsenide (GaAs), but can be other materials. The epitaxial DBR  60  is usually a p-type DBR, but if other materials are used for the wafer  10 , the epitaxial DBR can be an n-type DBR. If wafer  10  is GaAs, the epitaxial DBR  60  is preferably AlAs/GaAs layers or AlGaAs/GaAs layers. A cladding layer  74  is then epitaxially grown on top of the epitaxial DBR  60 , which ended with a GaAs layer. This cladding layer  74  is preferably InP, but can be other materials. 
     Similarly, in FIG. 5B, the cover wafer  16  has a top epitaxial DBR  66  grown on top of the cover wafer  16 . The cover wafer is preferably Indium Phosphide (InP), but can be other materials. The top epitaxial DBR  66  is preferably of the opposite carrier type than that of the epitaxial DBR  60 . If so, the layers of the top epitaxial DBR  66  are InGaAsP/InP. An active layer  44  is then grown on top of the top epitaxial DBR  66 . The active layer  44  can be either a quantum well or bulk layer. Finally, a second cladding layer  76  is epitaxially grown on top of the active layer  44 . The second cladding layer  76  is typically InP, but if the second wafer  16  or if the wafer  10  is a different material than preferred, the second cladding layer  76  can be a different material as well. 
     In FIG. 5C, one of the wafers is patterned. Either wafer can be patterned via etching; for illustration, the wafer  10  is shown as patterned. First, pattern material  28  is placed on top of the cladding layer  74 . The pattern material  28  is usually photoresist that has been masked and exposed to ultraviolet light. 
     In FIG. 5D, once the pattern material  28  is exposed, the cladding layer  74  is etched via either reactive ion etching (RIE) or wet chemical etching or other means to purposely undercut the pattern material  28  as shown by undercut area  78 . 
     In FIG. 5E, an isolating layer  80  is evaporated onto the cladding layer  74 . The undercut area  78  is not covered by the isolating layer  80 . The isolating layer is preferably SiO 2 , but can be any insulating material that does not allow current to pass through it easily. The thickness of the isolating layer  80  must be less than the height of the etch mesa  82  in the cladding layer  74 . This is the critical step in the construction process, as will be explained later. FIG. 5F shows the resulting structure after the pattern material  28  has been removed. 
     FIG. 5G shows wafer  10  and cover wafer  16  fused together. The wafers are fused with the second cladding layer  76  in contact with the cladding layer  74  and the isolating layer  80 . The cover wafer  16  may bow slightly during the fusion process because the height of the etch mesa  82  is higher than the isolating layer  80 . The criticality of the height of the isolating layer  80  with respect to the height of the etch mesa  82  comes into play because if the epitaxially grown layers of the cover wafer  16  have to bow too much to fuse to the etch mesa  82  and the isolating layer  80 , there will be overstress in the second cladding layer  76 , reducing device yield and device efficiency. In some cases, a void above isolating layer  80  may remain. 
     FIG. 5H shows wafer  10  and the remaining fused portions of the structure, after cover wafer  16  has been removed via etching. The relatively thin top epitaxial DBR  66 , active layer  44 , and second cladding layer  76  will reveal the location of the etch mesas  82  on the wafer  10 . If the etch mesas are still not visible, an alternative method would be to use an infrared aligner. Another alternative method would be to make either wafer  10  larger than cover wafer  16 , or cover wafer  16  larger than wafer  10 . The location of the etch mesas  82  would be known because some of the etch mesas  82  would not be covered with the cover wafer  16 . If the cover wafer  16  is to be made larger than wafer  10 , then the patterns and etch mesas  82  should be placed in the cover wafer  16 , and not in the wafer  10  as described above. The steps and processes for this are the same as described herein. 
     FIG. 5I shows contacts  84  deposited on top of the top epitaxial DBR  66  and wafer contact  86  deposited on the wafer  10 . The current flow between contacts  84  and wafer contact  86  is shown by arrows  88 . The current is confined to flow through the etch mesa  82  and is constricted by the isolating layer  80  and the undercut area  78 . The opening  72  can be used as the laser output of the device. 
     FIG. 5J shows a top view of the structure, showing that the etch mesa  82  diameter is smaller than the opening  72  defined by the contact  84 . 
     FIG. 5K shows the final processing step. A protective cover  90  may be added to the top of the isolating layer  80  and the top epitaxial DBR  66 . This protective cover  90  can be an additional DBR, depending on the efficiency of the epitaxial DBR  60  and the top epitaxial DBR  66 , or it can be a protective cover for the opening  72 . If the protective cover  90  is a DBR, it is preferably made of Si/SiO 2  or TiO 2 /SiO 2 , to further enhance the reflectivity of the top epitaxial DBR  66 , but can be made of other materials. Regardless of whether the opening is covered with a DBR or not, the protective cover  90  must be a material which will be able to withstand RIE or other etching means. RIE  92  is then performed on the wafer, and the protective cover  90  protects the contact  80 , top epitaxial DBR  66 , active layer  44 , and second cladding layer  76  from the RIE. The etching process, in this case RIE  92 , is stopped at the isolating layer  80 . This isolates the device from the remainder of the devices on the wafer  10 . Depending on whether the protective cover  90  is a DBR or just a protection material, it can be removed for final device construction. 
     FIGS. 6A-6F are cross sectional views of a second method for constructing a VCL. In FIG. 6A, an epitaxial DBR  60  is grown on wafer  10 . As before, wafer  10  is usually GaAs and the epitaxial DBR  60  is usually alternating layers of AlAs/GaAs. A cladding layer  74  is epitaxially grown on the epitaxial DBR  60 . Cladding layer  74  is the last GaAs layer of the epitaxial DBR  60 , and the cladding layer  74  may be thicker than a quarter wave layer of the epitaxial DBR  60 . 
     In FIG. 6B, a pattern material  28  is deposited on the top surface of the cladding layer  74 . The pattern material  28  is typically photoresist. The pattern material is then alternatively hardened and removed to leave the desired pattern in the pattern material  28 . This step alternatively exposes and covers the cladding layer  74 . The surface with the pattern material  28  is then exposed to an etchant, either wet chemical etching or RIE or other dry etching means. This will etch valleys  34  into the cladding layer  74 , and where the pattern material  28  covers the cladding layer  74 , the cladding layer  74  will have etch mesas  82 . There is typically some undercutting of the pattern material  28 , leaving undercut areas  78 . 
     FIG. 6C shows using the pattern material  28  as a self-aligning mask and evaporating metal contacts  94  into the valleys  34 . The pattern material  28  prevents evaporation of the metal contacts in the undercut areas  78 . The height of the metal contacts  94  must be less than the height of the etch mesa  82 . 
     FIG. 6D shows the pattern material  28  removed, leaving the top surface of cladding layer  74  ready for fusion to another wafer. The metal contacts  94  will become buried layers after the fusion process. For low temperature fusing, the metal contacts  94  can be Nickel, Titanium, or Gold; for higher temperature fusion processes, refractory metals, such as tungsten, tantalum, or molybdenum may have to be used. 
     FIG. 6E shows the structure of the VCL after fusion. Wafer  10 , epitaxial DBR  60 , and etched cladding layer  74  with metal contacts  94  have been fused at fusion junction  18  to thinned cladding layer  96 , active layer  44 , second cladding layer  76 , top epitaxial DBR  66 , and second wafer  62 . Protection layer  90  is selectively added to the areas on second wafer  62  to protect the areas above the etch mesas  82  of cladding layer  74 . An etch process, which can be RIE  92  or other etch means, is then performed on the reverse side of second wafer  62  to etch the thinned cladding layer  96 , active layer  44 , second cladding layer  76 , top epitaxial DBR  66 , and second wafer  62 . The RIE  92  process will be stopped at fusion junction  18 , and the protection layer  90  will be removed. 
     FIG. 6F shows the final structure of the VCL. After the areas of the thinned cladding layer  96 , active layer  44 , second cladding layer  76 , top epitaxial DBR  66 , and second wafer  62  are removed by RIE  92 , portions of the metal contacts  94  are exposed. Additional entry contacts  98  are evaporated onto the surface of the cladding layer  74 , and the entry contacts  98  overlap the metal contacts  94 , allowing for greater access to the metal contacts  94  from other parts of the substrate wafer  10 . The evaporation that deposits entry contacts  98  also deposits top contact  100 , which provides for current flow  88  through the active region of the VCL. Since top contact  100  is normally opaque, the VCL output  102  will be through the bottom of wafer  10 ; however, the output  102  may pass through the top of the VCL. 
     FIGS. 7A-D show a cross sectional view of a bipolar junction transistor (BJT) fabricated using the method of the invention. FIG. 7A shows the wafer  10  prior to processing. 
     FIG. 7B shows the wafer  10  after an etching process. The etching will leave mesas  32  and valleys  34 . The valleys  34  are exposed areas of the wafer  10  which are lower in height than the original top surface  30  of the wafer  10 . The mesas  32  are areas which still extend to the original top surface  30  of the wafer  10 . 
     FIG. 7C shows the wafer  10  physically in contact with cover wafer  16 . The cover wafer  16  can be a wafer of the same material as wafer  10 , or can be of a different material than wafer  10 . Cover wafer  16  has also been etched, leaving a cover mesa  36  and a cover valley  38 . Once this physical connection is made, and the wafer  10  and the cover wafer  16  are properly aligned, heat is applied to the combination to fuse the wafer  10  and the cover wafer  16 . The junction created by the physical connection between the wafer  10  and the cover wafer  16  is the fusion junction  18 . The fusion process does not appreciably change the shape of the valley  34  or the cover valley  38  by more than a few percent. The valleys  34  now become a buried void within the structure created by the wafer  10  and the cover wafer  16 . This void can be buried several microns deep in a semiconductor structure. Since there are no charge carriers in the void, any current travelling between wafer  10  and cover wafer  16  must travel through the mesas  32 . The use of the valleys  34  to create cavities increases the current density in the mesas  32 . 
     FIG. 7D shows the cover wafer  16  physically in contact with third wafer  68 . The third wafer  68  can be a wafer of the same material as wafer  10  and cover wafer  16 , or can be of a different material than wafer  10  or cover wafer  16 . Once this physical connection is made, and the cover wafer  16  and the third wafer  68  are properly aligned, heat is applied to the combination to fuse the cover wafer  16  and the third wafer  68 . The junction created by the physical connection between the wafer  10  and the cover wafer  16  is the second fusion junction  104 . The fusion process does not appreciably change the shape of the valley  34  or the cover valley  38  by more than a few percent. The valleys  34  and cover valleys  38  now become buried cavities within the structure created by the wafer  10 , the cover wafer  16 , and the third wafer  68 . These cavities can be buried several microns deep in a semiconductor structure. Since there are no charge carriers in the void, any current travelling between third wafer  68  and wafer  10  must travel through the mesas  32  and the cover mesas  36 . The use of the valleys  34  and cover valleys  38  to create cavities increases the current density in the mesas  32  and cover mesas  36  and reduces the capacitance. 
     The structure created by wafer  10 , cover wafer  16 , and third wafer  68  as shown in FIG. 7D is the structure of a Bipolar Junction Transistor (BJT). For an npn BJT, wafer  10  is n-type, cover wafer  16  is p-type, and third wafer  68  is n-type. If wafer  10  is the collector, cover wafer  16  is the base, and third wafer  68  is the emitter, the device created is a BJT. Wafer  10  or third wafer  68  may be formed of a material with a different bandgap than cover wafer  34 , which will create a heterojunction bipolar transistor (HBT). Current density and current flow can be altered by changing the shape of the mesas  32 , cover mesas  34 , valleys  34 , and cover valleys  38 . Further, the thickness of the wafer  10 , cover wafer  16 , and third wafer  68  can be altered to change the output characteristics of the BJT. Alternatively, the cover wafer  16  or third wafer  68  can be epitaxial layers grown on the wafer  10 . The use of the method of the invention creates a channel in the BJT that is defined by the mesa  32  and the cover mesa  36 . The geometric shaping of the channel of the BJT will create better switching characteristics in the finished device. 
     FIGS. 8A-8E show a cross sectional view of a heterojunction bipolar transistor (HBT) fabricated using the method of the invention. FIG. 8A shows the wafer  10  prior to processing. 
     FIG. 8B shows the wafer  10  after an etching process. The etching will leave mesas  32  and valleys  34 . The valleys  34  are exposed areas of the wafer  10  which are lower in height than the original top surface  30  of the wafer  10 . The mesas  32  are areas which still extend to the original top surface  30  of the wafer  10 . 
     FIG. 8C shows a cover wafer  16 . Cover wafer  16  is doped with dopant  12  on one surface of the cover wafer  16 . The use of cover wafer  16  allows doping an n-type cover wafer  16  with a p-type dopant. It is easier to dope a n-type cover wafer  16  with a p-type dopant, and thus desirable to do so, but it is not required to do so to utilize the benefits of the invention. If cover wafer  16  is not doped with dopant  12 , a diode will be created by the structure. Cover wafer  16  can be a wafer of the same material as wafer  10 , or can be of a different material than wafer  10 . 
     FIG. 8D shows cover wafer  16  in physical contact with wafer  10 . Once this physical connection is made, and the wafer  10  and the cover wafer  16  are properly aligned, heat is applied to the combination to fuse the wafer  10  and the cover wafer  16 . The junction created by the physical connection between the wafer  10  and the cover wafer  16  is the fusion junction  18 . The fusion process does not appreciably change the shape of the valley  34  by more than a few percent. The valleys  34  now become a buried void within the structure created by the wafer  10  and the cover wafer  16 . If the valleys  34  are filled with valley material  40 , the valley material  40  will lower the capacitance between the dopant  12  and the wafer  10 . This void can be buried several microns deep in a semiconductor structure. Since there are no charge carriers in the void, any current travelling between wafer  10  and cover wafer  16  must travel through the mesas  32 . The use of the valleys  34  to create cavities increases the current density in the mesas  32 . 
     FIG. 8E shows the structure of a transistor. An etching process has been applied to cover wafer  16  to thin the cover wafer  16 . A further etching process has been applied to cover wafer  16  and dopant  12  to remove the dopant from the region above at least part of the valley  34  and from the region above the ramainder of wafer  10 . The etching process can be either a wet or dry etching process. Contacts  54  are attached to wafer  10 , dopant  12 , and cover wafer  16 . 
     The structure of FIG. 8E is a transistor. The cover wafer  16 , after being thinned, is the emitter, the dopant  12  portion of the cover wafer  16  is the base, and the wafer  10  forms the collector. The electron flow from the cover wafer  16  (emitter) to the wafer  10  (collector) must pass through the mesa  32 , thereby increasing the current density in the mesa  32 . Current density and current flow can be altered by changing the shape of the mesas  32  and the wells  14 , as well as adding valley material  40  to the wells if desired. Further, the thickness of the wafer  10  and depth of the wells  14  can be altered to change the output characteristics of the HBT. 
     The importance of creating a valley  34  within the HBT is to create a lower capacitance region within the semiconductor. This lower capacitance region is critical for high speed operation, and eliminates back-biasing effects. Further, the use of valley material  40  which is conductive, such as a metal, solves resistance and thermal heating limitations which are caused by doped regions within semiconductor devices. 
     This invention can also be used to realize Metal Base Transistors. Present methods to bury metal layers deep within a semiconductor have not realized efficient devices because of the difficulty of burying metal layers within the semiconductor wafer. 
     FIGS. 9A-9F are cross sectional views of a method for constructing a photodetector. FIG. 9A shows the wafer  10  prior to processing. FIG. 9B shows the wafer  10  covered by a pattern material  28 . Typically, the pattern material  28  is photoresist, but the pattern material can be anything which will alternatively expose and cover the top surface  30  of the wafer  10 . The pattern material  28  is usually photoresist that has been masked and exposed to ultraviolet light. 
     In FIG. 9C, the pattern material  28  has been exposed to ultraviolet light, and the non-exposed areas of the pattern material  28  have been removed if the pattern material  28  is positive-reacting photoresist. Alternatively, if the pattern material  28  is negative-reacting photoresist, the exposed photoresist would be removed. the wafer  10  is then RIE etched  92 . The RIE  92  creates valleys  34 , mesas  32 , and undercut areas  78 . The undercut area  78  is important to create a gap underneath the pattern material  28 . 
     In FIG. 9D, valley material  40  is deposited in the valleys  34 . The pattern material  28  is left on the top surface  30  of the wafer  10  to shield the undercut area  78  and keep the undercut area  78  free of valley material  78 . The device will still operate without the undercut area  78 , but will not be as accurate as a device with an undercut area  78 . 
     In FIG. 9E, the pattern material  28  has been removed, exposing top surface  30  of wafer  10 . The height of valley material  40  must be as high as mesa  32 . 
     In FIG. 9F, top surface  30  of wafer  10  is physically in contact with cover wafer  16 . The cover wafer  16  can be a wafer of the same material as wafer  10 , or can be of a different material than wafer  10 . Once this physical connection is made, and the wafer  10  and the cover wafer  16  are properly aligned, heat is applied to the combination to fuse the wafer  10  and the cover wafer  16 . The fusion process normally requires placing the wafers in an autoclave and raising the temperature to approximately 600 degrees Centigrade. The junction created by the physical connection between the wafer  10  and the cover wafer  16  is the fusion junction  18 . The valleys  34 , along with the valley material  40 , now become a buried layer within the structure created by the wafer  10  and the cover wafer  16 . Contacts  84  are then added to the top surface  106  of cover wafer  16 . 
     The structure shown in FIG. 9F is then split into two structures along dicing line  108 . The two resulting structures are photodetectors, with photons entering the structure from the side of the structure. Alternatively, the structure would not be split along dicing line  108 , and contact  84  or valley material  40  would be deposited in a closed structure, such as a ring, square, or hexagonal structure, and the photons could enter the photodetector from the top or bottom of the structure. Current passes through contact  84  along current flow  88  to valley material  40 . When photons strike the region  110  between contact  84  and valley material  40 , the resistance of the material changes, allowing a different amount of current flow  88  to pass between contact  84  and valley material  40 . 
     The discrete devices described in FIGS. 7-9 are examples of what devices can be created using the present invention. The techniques described for individual devices can also be used to fabricate integrated circuits and for large scale integration (LSI) and very large scale integration (VLSI) devices. 
     The techniques of the present invention may also be used in fabricating metal-oxide semiconductor field-effect transistors (MOSFETs), metal-semiconductor field-effect transistors (MESFETs), and other transistor fabrication technologies. 
     FIG. 10 shows a flowchart of the steps of the method of the invention. In step  112  a doped region is created in a first wafer. Once the doped region is created, the surface with the doped region is fused to a second wafer in step  114 . 
     FIG. 11 shows a flowchart of the steps of an alternative embodiment of the steps of the method of the invention. In step  116 , a first surface of a first wafer is patterned using resist and masking techniques. Once the pattern is created, the first surface is etched in step  118 . Finally, the etched surface is fused to a second wafer in step  120 . 
     The description of the preferred embodiments of the invention have been presented for the purposes of illustration and description. It is not intended to be exhaustive or to limit the invention to the precise form disclosed. Many modifications and variations are possible in light of the above teaching. It is intended that the scope of the invention be limited not with this detailed description, but rather by the following claims.