Abstract:
A vertical cavity emitting laser (VCSEL) having a tunnel junction. The junction may be isolated with an implant into a top mirror and past the junction and p-layer. A trench around the VCSEL may result in reduced capacitance and more D.C. isolation of the junction. The implant may occur after the trench is made. Some implant may pass the trench to a bottom mirror. Additional isolation and current confinement may be provided with lateral oxidation of a layer below the junction. Internal trenches may be made from the top of the VCSEL vertically to an oxidizable layer below the junction. For further isolation, an open trench may be placed around a bonding pad and its bridge to the VCSEL and internal vertical trenches may be placed on the pad and its bridge down to the oxidizable layer.

Description:
BACKGROUND 
     The present invention pertains to vertical cavity surface emitting lasers (VCSELs) and particularly to tunnel junction VCSELs having long wavelengths, namely, 1200 to 1800 nanometer wavelengths. 
     A long wavelength VCSEL having a tunnel junction and a thick mirror layer is difficult to isolate because of the high doping used in the tunnel junction. Island isolation or trenches can provide direct current (D.C.) isolation for such VCSELs. To reduce capacitance of the VCSEL an implant needs to be used; however, it will not adequately compensate the tunnel junction. Because the implant can go deeper than the tunnel junction into the p region and down to the active region it can form an insulating region under the tunnel junction which reduces the capacitance. Some VCSEL isolation is shown in U.S. Pat. No. 5,903,588. U.S. Pat. No. 5,903,588 issued May 11, 1999, and entitled “Laser with a Selectively Changed Current Confining Layer,” is expressly incorporated herein by reference into this description. 
     SUMMARY 
     An isolation implant used for isolation of a VCSEL should go through and past the tunnel junction of the VCSEL into and optimally through the p layer. This implant results in a semi-insulating region that reduces capacitance under the tunnel junction. Internal trenches about the VCSEL aperture can be utilized to reduce a required oxidation distance and thus reduce variability of the aperture diameter. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 shows a cross-section of a VCSEL having tunnel junction, with implant and island or trench isolation; 
     FIG. 2 shows a cross-section of another configuration of a tunnel junction VCSEL; 
     FIG. 3 a  reveals a layout having a trench around a VCSEL including a bond pad and its connecting bridge to the VCSEL; 
     FIG. 3 b  shows a bond pad having several bridges connecting it to the VCSEL. 
     FIG. 4 a  reveals interior trenches on the VCSEL device for reducing a required oxidation distance; 
     FIG. 4 b  shows a cross-section of several interior trenches on the VCSEL device; and 
     FIG. 5 shows a bonding pad and the connecting bridge to a VCSEL also having interior oxidation trenches. 
    
    
     DESCRIPTION 
     The Figures are for illustrative purposes and not necessarily drawn to scale. FIG. 1 shows a manufacturable structure of a VCSEL having deep implant isolation. A VCSEL that emits light having a 1200 to 1800 nanometer (nm) wavelength is described, though the same structures and techniques adjusted for the wavelength are advantageous for other wavelength VCSELs such as 850 nm, 980 nm or 660 nm VCSELs. On a substrate  36  a distributed Bragg reflector N-mirror  11  is situated. Mirror  11  may be composed of 30 to 50 pairs of layers. Each layer of the pair is about one-fourth of the optical design wavelength (λ) of light to be emitted by the VCSEL. Each pair of layers may be InGaAsP and InP, AlGaAsSb and InP, or AlGaPSb and InP, respectively, for a 1550 nm InP VCSEL. These layers are lattice matched to InP and may or may not be fully N-doped. They may be partially doped for the intra-cavity type of device. An active region  12  having InAlGaAs strained quantum wells and InAlAs barriers, also of a strained composition, is on mirror  11 . Active region  12  is not doped or is unintentionally doped. An oxidizable layer(s) or region  13  is on active region  12 . Layer(s)  13  has InAlAs material. The composition of layer  13  may be high in Al content and thus easily oxidizable. Another kind of oxidizable material may be present in layer  13 . Lateral oxidation regions  23  are made and extend to the periphery of the inside aperture of current confinement. The material of region  13  may or may not be lattice matched. Region  13  is P-doped. A tunnel junction  14  may be on region  13 . This junction  14  has a highly doped P material adjacent (˜1e20/cm3) to a highly doped N material (˜3e19/cm3) that results in the junction. The materials may include InAlGaAs and InP. Another distributed Bragg reflector N-mirror  15  is on tunnel junction  14 . It may have about 35 pairs of layers of InGaAsP and InP, InAlGaAs and InP or InAlGaAs and InAlAs. These layers of mirror  15  may be lattice material to InP. 
     A 1310 nm VCSEL  10  may be a GaAs substrate based device. On an appropriate substrate  36  may be an N mirror  11  having from 25 to 40 pairs of layers of AlGaAs and GaAs or AlAs and GaAs, respectively. These materials are lattice matched and may or may not be fully N-doped. They may be partially doped for the intra-cavity type of device. On mirror  11  is an active region  12  having quantum wells and barriers. There may be one to five quantum wells. There may be included in active region  12  a spacer layer above or below the quantum wells to extend the cavity multiples of half wavelengths. The spacer may have periodic doping peaked at the nulls of the optical field. The material of active region  12  may include quantum wells of InGaAsN or InGaAsNSb, barrier layers of GaAs or GaAsN, GaAsSbN or a combination thereof, and confining layers of GaAs, AlGaAs, GaAsP or some combination thereof. The quantum wells of region  12  are not doped or may be unintentionally doped. On region  12  may be a region  13  having a partially oxidized layer that extends inward up to the periphery of where the current of an operating VCSEL  10  may be confined is centered on the null of the electric field. The material may include AlGaAs which has a high proportion of Al for lateral oxidation of the region. The material may be lattice matched and P-doped. On region  13  is a tunnel junction  14  having highly doped P and N materials adjacent to each other to form a junction. The materials may be GaAs, AlGaAs or InGaAs. They may be or may not be lattice matched. On tunnel junction  14  is an N mirror  15 . Mirror  15  may have 16 to 25 pairs of layers of AlGaAs and GaAs and may contain a spacer which is a multiple of half wavelengths thick which may be periodically doped. 
     A 1550 nm VCSEL  10  may be a GaAs substrate based device. The material structure of this VCSEL may be the same as that of the 1310 nm GaAs VCSEL. The content distribution of the various materials may vary from one illustrative embodiment to another. 
     VCSEL  10  of FIG. 1 may be structured as an island with volume  18  of material removed or with a trench  19  around it or at least partially around it. Both types of structures are illustrated on the right and left sides, respectively, of FIG.  1 . Island  20  is formed by the removal of material from volume  18 . Island  20  or trench  19  is around at least active region  12  of VCSEL  10  and may provide tunnel junction  14  with D.C. isolation. Island  20  or trench  19  may be used as an entry or source for lateral oxidation  23  to provide added isolation. Oxidation trenches  23  provide current confinement and thus may reduce variability in the diameter of the aperture. However, to obtain low capacitance, an isolation implant of volumes  21  and  22  may be utilized. Implants  21  and  22  are of sufficient depth to create a semi-insulating isolation to reduce the capacitance between tunnel junction  14  and the substrate. Buried implant  21  may go down through tunnel junction  14  and through a region including tunnel junction  14  and the quantum wells of active region  12 . The implant may be through more layers or regions, including down past active region  12 . Implant species may include H +  ions, D +  ions or He ++  ions. The energy range of an implant may be between 35 KeV and 2000 KeV. The energy dose of the implanting may be between 1E14 and 5E16 atoms/cm 2  and is optimally about 7E14 atoms/cm3. 
     FIG. 2 shows a VCSEL  30  having an upper mirror structure different than that of VCSEL  10 . However, VCSEL  30  may have a spacer in the active region and/or one of the mirrors as in VCSEL  10  noted above. Like VCSEL  10 , VCSEL  30  has an N mirror  11  on a substrate  36  and an active region  12  on mirror  11 . On active region  12  is an oxidation region  13  with lateral oxidized trenches  23 . Oxidation region  13  is p-doped. On region  13  is tunnel junction  14 . Up to this point, the material and numbers of pairs of layers may be the same as those of VCSEL  10 . But upper N-mirror  15  may have a stack of 11 pairs of layers at a maximum. Yet, the materials of these layers may be the same as those of mirror  15  of VCSEL  10 . From the top of mirror  11  up to the top of mirror  15 , that structure may be an island  24 . On top of mirror  15  is a mirror structure  25  that is an extension of mirror  15 . It is an island  26  relative to the top of mirror  15 . Around island  26  may be a contact  27  on top of mirror  15 . It is regarded as a “recessed contact” in that the distance from the top of mirror  15  to tunnel junction  14  is less than the distance between mirror  15  and junction  14  of VCSEL  10 . For a reduced energy of implantation of ions in VCSEL  10 , implant  21  can reach down into active layer  12 . Implant  22  may be situated in a portion of mirror  11  about or outside the perimeter of island  24 . One may do the same depth implant into or past active region  12  in VCSEL  10  as in VCSEL  30  with more energy but availability of implanters with sufficient energy may be a problem. 
     A distinguishing feature between VCSEL  10  and VCSEL  30  is the shorter mirror  15  with a dielectric mirror  25  stack or island  26  on mirror  15  of island  24 . Mirror  25  may have 3 to 4 pairs of TiO 2  and SiO 2 , 2 to 3 pairs of Si and SiO 2 , 2 to 3 pairs of Si and Al 2 O 3 , or 4 to 5 pairs of TiO 2  and Al 2 O 3 , respectively. 
     On top of mirror  15  of VCSEL  10  is a contact  16  and the rest of the top surface of mirror  15  has a layer  17  of dielectric such as, for example, SiO 2 . On the top of mirror  15  of VCSEL  30  is contact  27  and mirror  25 , as noted above. A layer  28  of dielectric on stack  25  may be, for example, SiO 2 . Another contact may be at the bottom of substrate  36  but may be brought up to be connectable from the top of the respective VCSEL  10  or  30 . In both VCSELs, the dielectric may cover light aperture  37  and not block the emitted light. 
     FIG. 3 a  shows VCSEL  10  having a bonding pad  29  and bridging connection  31 . Even though each of FIGS. 3 a ,  3   b  and  5  shows a VCSEL device with a bridge, device  10  or  30  may be made without bridging between the bond pad and the device by taking advantage of the buried implant that goes at least through and under the tunnel junction, but which may be also elsewhere in the chip. Trench  19  in FIG. 3 a  goes around VCSEL  10  and under bridge  31  at an area  38  close to its connection with contact  16 . Bridge  31 , at area  38  over trench  19 , may be an air bridge which might be made with the removal of sacrificial material beneath it, or it may be on a planarization layer, or have a trench filling under it or it may be electroplated with a patterned electroplating technique. Around and under bonding pad  29  and its connecting bridge  31  to contact  16  may be a dielectric  32 . Dielectric  32  may be contiguous with dielectric  17  of VCSEL  10 . Dielectric  17  is situated over VCSEL  10  light aperture  37 . Dielectric  32  might not be used in this pad  29  configuration. Trench  19  goes around pad  29  and along the sides of bridge  31 , but on the periphery of dielectric  32  if pad  29  utilizes a dielectric. The dielectric may provide both reduced capacitance and D.C. isolation of the VCSEL. VCSEL  10  of FIG. 3 a  may have interior trenches like those of VCSEL  10  in FIG. 4 a.    
     FIG. 3 b  shows VCSEL  10  having several bridges  39  connecting bonding pad  29  to contact  16 . VCSEL  10  of this Figure may have interior trenches like trenches  33  of VCSEL  10  in FIG. 4 a  as described below. VCSEL  10  of FIG. 3 b  may also have a trench at least around a portion of its perimeter. 
     FIG. 4 a  reveals VCSEL  10  having interior oxidation trenches  33 . VCSEL  30  of FIG. 2 may also have trenches  33 . FIG. 4 b  is a cross-section of trenches  33  which extend through top mirror  15 , tunnel junction  14  and oxidizable layer  13  having, for example, a high content of aluminum. Trenches  33  may extend into or past active area  12 . Trenches  33  enable oxidation in various layers, particularly layer  13 , having a high proportion of aluminum or other easily oxidizable material, to better provide isolation for tunnel junction  14  and current confinement. Trenches  33  may define aperture  37 . 
     FIG. 5 shows not only interior trenches  33  of VCSEL  10  but also trenches  34  along connecting bridge  31  and trenches  35  in pad  29 . The layers from mirror  15  of VCSEL  10  down to substrate  36  may be present through that portion of pad  29  and bridge  31 . Thus, trenches  34  and  35  may go down to oxidizable layer  13 . Trenches  34  and  35  may enable oxidation in layer  13  under pad  29  and bridge  31  for some isolation of the pad and connecting bridge. In an illustrative example, the trenches may be about 4 by 4 microns wide and be from 10 to 20 microns apart from one another. The bridge may or may not be present. 
     Although the invention has been described with respect to at least one illustrative embodiment, many variations and modifications will become apparent to those skilled in the art upon reading the present specification. It is therefore the intention that the appended claims be interpreted as broadly as possible in view of the prior art to include all such variations and modifications.