Abstract:
Various asymmetric InGaAsN VCSEL structures that are made using an MOCVD process are presented. Use of the asymmetric structure effectively eliminates aluminum contamination of the quantum well active region.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
     This application relates to the co-pending application Ser. No. 10/106,472 filed on the same day, entitled “Method for Obtaining High Quality InGaAsN Semiconductor Devices” by Takeuchi, Chang, Luan, Bour, Leary and Tan, owned by the assignee of this application and incorporated herein by reference. 
    
    
     BACKGROUND 
     Long wavelength vertical cavity surface emitting lasers (VCSELs) are attractive for long reach optical communication applications. Fabrication of long wavelength VCSELs incorporating well-developed AlAs/GaAs distributed Bragg reflectors (DBRs) is achieved by growing highly strained InGaAsN active regions on GaAs. Long wavelength VCSEL structures include structures where InGaAsN active regions are sandwiched between a first and second cladding region of AlGaAs/GaAs or InGaP/GaAs and top and bottom AlGaAs/GaAs DBR mirror layers. 
     Research by Kawaguchi et al. in Electronics Letters, 36, 2000, 1776 indicates that the material quality of metal-organic chemical vapor deposition (MOCVD) InGaAsN is severely degraded if the InGaAsN quantum well active layer is grown directly on the AlGaAs/GaAs DBR and lower cladding layers. To achieve acceptable material quality for the quantum well active layer, two separate reactors are used to grow the wafers for 1.3 μm wavelength VCSELs with InGaAsN quantum well active layers. A first reactor is used to grow the AlGaAs/GaAs DBR and lower cladding layers. Subsequently, the wafer is transferred to a second reactor for the growth of InGaAsN quantum well active layers, the top cladding layer and the top DBR mirror layers. These long wavelength InGaAsN VCSELs have “symmetric” structures where both the top and bottom cladding layers have the same composition. Sato et al. in Electronics Letters, 36, 2000, 2018 disclose an “asymmetric” VCSEL structure grown in a two reactor MOCVD process where a GaInP layer functions as an etch stop. 
     SUMMARY OF INVENTION 
     The use of an InGaAsN quantum well active layer allows VCSEL operation in the important 1300 nm or longer wavelength regime which is of interest for telecommunications and Internet infrastructure applications. In accordance with the invention, an asymmetric InGaAsN VCSEL structure may be made which allows all growth steps to be performed in the same metal-organic chemical vapor deposition (MOCVD) reactor. 
     In the asymmetric VCSEL structure, the first AlGaAs/GaAs DBR mirror layer is followed by growth of a sufficiently thick nitrogen or nitrogen and phosphorus containing layer such as GaAsN, InGaAsPN, GaAsPN, GaAsN, AlGaAsN, InGaPN, or similar compositions to improve growth of the InGaAsN quantum well active layer by serving to getter Al while not interrupting the MOCVD growth process. The top cladding layer may be AlGaAs to provide for higher band offset resulting in better electron confinement than is provided by a nitrogen or phosphorus containing cladding layer. However, AlGaAs requires a more complicated growth structure and typically GaAs is used for the top cladding layer. Instead of using a C-doped GaAs contact layer, a reverse-biased tunnel junction can be used to form the p-contact to reduce resistance and optical losses. 
     Using an asymmetric InGaAsN VCSEL structure results in the InGaAsN quantum well active layer having a quality that is comparable to that achieved by the conventional two reactor MOCVD process while providing good laser performance along with lower production costs by using a single reactor MOCVD process. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 a  shows a side view of a typical MOCVD reactor in accordance with the invention. 
     FIG. 1 b  shows a top view of a typical MOCVD reactor in accordance with the invention. 
     FIG. 2 shows arrangement of the chemical delivery system for the typical MOCVD reactor in FIG. 1 a  and FIG. 1 b    
     FIG. 3 shows an embodiment in accordance with the invention. 
     FIG. 4 shows an embodiment in accordance with the invention. 
    
    
     DETAILED DESCRIPTION OF INVENTION 
     In accordance with the invention, FIG. 1 a  shows MOCVD reactor  120  in side view with exhaust line  180 . MOCVD reactor  120  is a cold wall, quartz reactor. Group III source injection occurs at inlet  125  and Group V source injection occurs at inlet  130 . Group III and Group V gases begin mixing after passing from outlets  187  and  188 . Outlets  187  and  188  are both approximately 13 cm×2 cm rectangles. Vertical height  143  of MOCVD reactor  120  is approximately 4.5 cm while dimension  144  is approximately 3 cm. Dimension  148  is approximately 7 cm and is the distance from where the Group III and Group V gases begin to mix to where the vertical constriction of MOCVD reactor  120  starts. Dimension  147  is approximately 7 cm and Si-coated graphite susceptor  170  has diameter  146  of approximately 11 cm with thickness  145  of approximately 1.5 cm. Substrate  175  is positioned on susceptor  170  as shown in FIGS. 1 a  and  1   b.  With reference to FIG. 1 b , lateral dimension  185  of MOCVD reactor  120  is approximately 13 cm while dimension  149  is approximately 1.5 cm. 
     In accordance with the invention, FIG. 2 shows an arrangement of the chemicals and lines feeding into MOCVD reactor  120 . MOCVD reactor  120  is typically a cold wall, quartz reactor. Valves  250 ,  251 ,  252  and  253  control and direct flow from tanks  231 ,  232 ,  233 , and  234 , respectively. Valves  254 ,  255 ,  256 ,  257 ,  258 ,  259  and  260  control and direct flow from bubblers  235 ,  236 ,  237 ,  238 ,  239 ,  240  and  241 , respectively. Inlet  210  serves to introduce H 2  carrier gas into MOCVD reactor  120  via line  212  to inlet  130  of MOCVD reactor  120 . Line  212  serves as well for typically introducing Tertiarybutylarsine (TBAs) from bubbler  235  and Dimethylhydrazine (DMHy) from bubbler  236  into MOCVD reactor  120  via inlet  130 . Valves  254  and  255  direct flow from bubblers  235  and  236 , respectively, into either line  212  or vent line  220 . Vent lines  220  connect to exhaust line  180 . Line  221  serves to introduce H 2  into bubblers  235  and  236  while line  222  serves to introduce H 2  into bubblers  237 ,  238 ,  239 ,  240  and  241 . Line  211  serves to introduce NH 3  from tank  231 , AsH 3  from tank  232 , PH 3  from tank  233  and Si 2 H 6  from tank  234  into MOVCD reactor  120  via inlet  130 . 
     Valves  250 ,  251 ,  252  and  253  direct flow from tanks  231 ,  232 ,  233  and  234 , respectively, into either line  211  or vent line  220 . Line  213  serves to typically introduce Trimethylgalium (TMGa) from bubbler  237 , Triethylgallium (TEGa) from bubbler  238 , Trimethyaluminum (TMAl) from bubbler  239 , Trimethylindium (TMIn) from bubbler  240  and CBr 4  from bubbler  241  into MOCVD reactor  120  via inlet  125 . Valves  256 ,  257 ,  258 ,  259  and  260  direct flow from bubblers  237 ,  238 ,  239 ,  240  and  241 , respectively, into either line  213  or vent line  220 . Note that there is no back flow in any of the lines since a mechanical pump (not shown) maintains the pressure inside reactor  120  at about 100 mbar. 
     In accordance with an embodiment of the invention, asymmetric VCSEL structure  305  shown in FIG. 3 is grown by using an MOCVD reactor such as MOCVD reactor  120  shown in FIGS. 1 a  and  1   b.  With reference to MOCVD reactor  120 , Si-doped GaAs buffer layer  325  with a doping level typically in the range of 1.0×10 17 -5.0×10 18  cm −3  is grown on GaAs substrate  320  to a thickness typically in the range of about 1000-5000 Å at a typical temperature of about 600-800° C. Following growth of Si-doped GaAs buffer layer  325 , bottom n-type DBR mirror structure  330  is grown. N-type DBR mirror structure  330  is typically made up of about 35-45 pairs of alternating layers of which Si-doped Al 0.9 Ga 0.1 As layer  331  and Si-doped GaAs layer  332  are representative with Si-doping typically in the range of 5.0×10 17 -5.0×10 18  cm −3 . Si-doped Al x Ga 1−x As layer  331  where x is between about 0.8 and 1.0 is typically grown to a thickness corresponding to one quarter wavelength (of the emission wavelength) and Si-doped GaAs layer  332  is also typically grown to a thickness corresponding to one quarter wavelength with about 100-300 Å of grading at each interface. The grading profile is typically linear with distance from the interface. The grading serves to lower the hetero barrier between AlGaAs and GaAs layers resulting in lower operating voltages for the VCSEL device. The total amount of TMAl typically supplied to MOCVD reactor  120  for growth of all Si-doped Al 0.9 Ga 0.1 As type layers  331  layers in n-type DBR mirror structure  330  is about 7×10 −3  mol. 
     After completion of the growth for n-type DBR mirror structure  330 , GaAs layer  335  is grown to a thickness in the range of about 50-300 Å at a temperature typically in the range of 600-800° C. Following growth of GaAs layer  335 , GaAs 1−x N x  layer  336  is grown to a typical thickness of about 600 Å where x is between 0 and 0.1. TMGa, 100 sccm, AsH 3 , and 500 sccm of NH 3  are supplied for about 4 minutes while the growth temperature is decreased to about 500-550° C. from 600-800° C. in growing GaAs 1−x N x  layer  336 . The total amount of NH 3  introduced is typically about 8×10 −2  mol which is approximately ten times larger than the amount of TMAl that is typically supplied for the growth of all Si-doped Al 0.9 Ga 0.1 As type layers  331  that make up DBR mirror structure  330  when using MOCVD reactor  120 . NH 3  serves to getter the Al which would interfere with growth of InGaAsN quantum well active layers  350 ,  360  and  370  and can be replaced, for example, by Monomethylamine, Dimethylamine, Diethylamine, Tertiarybutylamine, hydrazine, Monomethylhydrazine, Dimethylhydrazine, Tertiarybutylhydrazine, Phenylhydrazine, phosphine or Tertiarybutylphosphine. In accordance with the invention, GaAs 1−x N x  in layer  336  may be replaced, for example, by GaAsNP, InGaAsPN, InGaAsN, or similar compositions. GaAs cladding layer  337  is grown over GaAs 1−x N x  layer  336  to a typical thickness in the range of about 700-900 Å but greater than about 200 Å. Alternatively, Si-doped GaAs 1−x N x  layer  336  may be grown embedded in one or more of Si-doped GaAs layers  332  of DBR mirror structure  330  having a typical thickness of about 600 Å. In addition, an Si-doped AlGa As 1−x N x  layer may be grown embedded in one or more of Si-doped Al 0.9 Ga 0.1 As layers  331  having a typical thickness of about 600 Å. 
     Then InGaAsN quantum well active layer  350  is grown to a thickness in the range of about 60-100 Å using TEGa, TMIn, TBAs and DMHy. The ratio of DMHy/(DMHy+TBAs) in the range of about 0.95-0.99 is typically used for growth of InGaAsN quantum well active layer  350 . GaAs barrier layer  351  is grown over InGaAsN quantum well active layer  350  to a thickness in the range of 50-300 Å. Then quantum well active layer  360  is grown to a typical thickness in the range of about 60-100 Å using TEGa, TMIn, TBAs and DMHy. The ratio of DMHy/(DMHy×TBAs) in the range of about 0.95-0.99 is typically used for growth of InGaAsN quantum well active layer  360 . GaAs barrier layer  361  is grown over InGaAsN quantum well active layer  360  to a thickness in the range of 50-300 Å. Then InGaAsN quantum well active layer  370  is grown to a thickness in the range of about 60-100 Å using TEGa, TMIn, TBAs and DMHy. The ratio of DMHy/(DMHy+TBAs) in the range of about 0.95-0.99 is typically used for growth of InGaAsN quantum well active layer  370 . The total number of quantum wells as well as the thickness of quantum well active layers  350 ,  360 ,  370  and barrier layers  351 ,  361  may be adjusted to obtain the best results. The distance from the first quantum well active layer, for example, quantum well active layer  350  to the last quantum well active layer, for example, quantum well active layer  370  is fixed to be no greater than 600 Å. The thickness of GaAs cladding layer  337  and GaAs layer  380  is typically adjusted appropriately in order to put the layers extending from the first quantum well layer to the last quantum well layer, for example, quantum well active layer  350  to quantum well active layer  370 , at a maximum of the standing wave cavity. 
     After growth of InGaAsN quantum well active layer  370 , GaAs layer  380  is grown to thickness in the range of about 1500-1700 Å while the temperature is typically increased to about 600-800° C. Then p-type DBR mirror structure  390  is grown. P-type DBR mirror structure  390  is made up of about 20-35 pairs of alternating layers of which C-doped Al x Ga 1−x As layer  391  and C-doped GaAs layer  392  are representative layer pairs with x typically in the range of between about 0.8 and 1 and with C-doping typically in the range of 5.0×10 17 -5.0×10 18  cm −3 . C-doped A x Ga 1−x As layer  391  is grown to a typical thickness corresponding to one quarter of the emission wavelength. C-doped Al y Ga 1−y As layer  392  where y is typically in the range of 0 to less than 0.2 is grown to a typical thickness corresponding to one quarter of the emission wavelength with about 100-300 Å of linear grading at each interface. Finally, the growth is completed by growing heavily C-doped GaAs contact layer  395  to a thickness in the range of about 500-1000 Å. C-doped GaAs contact layer  395  is typically doped in the range of 5.0×10 18 -1.0×10 20  cm −3 . 
     Typically, C-doped Al x Ga 1−x As layer  385  is used to make the laterally oxidized layer for the purposes of optical confinement and current confinement, if desired. The value of x for C-doped Al x Ga 1−x As layer  385  is selected to be higher than the value of x selected for typical C-doped Al x Ga 1−x As layer  391  since the rate of oxidation is strongly dependent on the Al content of C-doped Al x Ga 1−x As layer  385 . See, for example, U. S. Pat. No. 5,896,408, incorporated by reference in its entirety, for details. Ion implantation is used to realize current confinement either alone or in conjunction with laterally oxidized layer  385 . 
     In accordance with an embodiment of the invention, a reverse-biased tunnel junction can be utilized as a p-contact instead of C-doped GaAs contact layer  395 . This allows high current flow at a low bias voltage as well as low absorption of emission light in n-type DBR  490 . FIG. 4 shows asymmetric VCSEL structure  405  utilizing a reverse-biased tunnel junction. Sidoped GaAs buffer layer  325  with a doping level typically in the range of 1.0×10 17 -5.0×10 18  cm −3  is grown on GaAs substrate  320  to a thickness in the range of 1000-5000 Å at a temperature of about 600-800° C. Following growth of Si-doped GaAs buffer layer  325 , bottom n-type DBR mirror structure  330  is grown. N-type DBR mirror structure  330  is typically made up of about 35-45 pairs of alternating layers of which Sidoped Al x Ga 1−x As layer  331  and Si-doped GaAs layer  332  are typical with Si-doping typically in the range of 5.0×10 17 -5.0×10 18  cm −3 . Si-doped Al x Ga 1−x As layer  331  where x is between about 0.8 and 1.0 is grown to a thickness corresponding to one quarter wavelength (of emission wavelength) and Si-doped GaAs layer  332  is also grown to a thickness corresponding to one quarter wavelength length with about 100-300 Å of grading at each interface. The grading profile is typically linear with distance from the interface. The grading serves to lower hetero barrier between AlGaAs and GaAs layers resulting in lower operating voltages for the VCSEL device. The total amount of TMAl typically supplied to MOCVD reactor  120  for growth of all Si-doped Al 0.9 Ga 0.1 As type layers  331  in n-type DBR mirror structure  330  is about 7×10 −3  mol. 
     After completion of the growth for n-type DBR mirror structure  330 , GaAs layer  335  is grown to a thickness in the range of about 50-300 Å. Following growth of GaAs layer  335 , GaAs 1−x N x  non-active layer  336  is grown to a typical thickness of about 600 Å where x is between 0 and 0.1. TMGa, 100 sccm, AsH 3 , and 500 sccm of NH 3  are supplied for about 4 minutes while the growth temperature is decreased to about 500-550° C. from 600-800° C. in growing GaAs 1−x N x  layer  336 . The totalamount of NH 3  introduced is typically about 8×10 −3  mol which is approximately ten times larger than the amount of TMAl that is typically supplied for the growth of all Si-doped Al 0.9 Ga 0.1 As type layers  331  that make up DBR mirror structure  330  when using MOCVD reactor  120 . NH 3  serves to getter the Al which would interfere with growth of InGaAsN quantum well active layers  350 ,  360  and  370  and can be replaced, for example, by Monomethylamine, Dimethylamine, Diethylamine, Tertiarybutylamine, hydrazine, Monomethylhydrazine, Dimethylhydrazine, Tertiarybutylhydrazine or Phenylhydrazine. In accordance with the invention, GaAs 1−x N x  in layer  336  may be replaced, for example, by GaAsNP, InGaAsPN, InGaAsN, or similar compositions. GaAs cladding layer  337  is grown over GaAs 1−x N x  non-active layer  336  to a typical thickness in the range of about 700-900 Å but greater than about 200 Å so that the combined thickness of GaAs 1−x N x  non-active layer  336  and GaAs cladding layer  337  is in the range of from about 1000-2000 Å. 
     Then active layer  350  is grown to a thickness in the range of about 60-100 Å using TEGa, TMIn, TBAs and DMHy. The ratio of DMHy/(DMHy+TBAs) in the range of about 0.95-0.99 is typically used for growth of InGaAsN active layer  350 . GaAs barrier layer  351  is grown over InGaAsN active layer  650  to a thickness in the range of 100-300 Å. Then active layer  360  is grown to a thickness in the range of about 60-100 Å using TEGa, TMIn, TBAs and DMHy. The ratio of DMHy/(DMHy+TBAs) in the range of about 0.95˜0.99 is typically used for growth of InGaksN active layer  360 . GaAs barrier layer  361  is grown over InGaAsN quantum well active layer  360  to a thickness in the range of 100-300 Å. Then quantum well active layer  370  is grown to a thickness in the range of about 60-100 Å using TEGa, TMIn, TBAs and DMHy. The ratio of DMHy/(DMHy+TBAs) in the range of about 0.95-0.99 is typically used for growth of InGaAsN active layer  370 . The total number of quantum wells as well as quantum well active layer and barrier layer thickness may be adjusted to obtain the best results. The distance from the first quantum active layer to the last quantum well active layer is fixed to be no more than about 600 Å and the thickness of GaAs cladding layer  337  and GaAs layer  380  is adjusted appropriately in order to put the active region at a maximum of the standing wave cavity. 
     After growth of InGaAsN active layer  370 , GaAs layer  380  is grown to thickness in the range of about 1000˜2000 Å while the temperature is typically increased to about 600-800° C. Then C-doped Al 0.9 Ga 0.1 As layer  481  is grown to a typical thickness of about 260 Å, C-doped Al x Ga 1−x As layer  482  is grown to a typical thickness of about 100 Å and C-doped Al 0.9 Ga 0.1 As layer  483  is grown to a typical thickness of about 260 Å to make the laterally oxidized layer structure for the purpose of providing optical confinement and current confinement. Two graded interfaces, with a thickness of about 100-300 Å, are grown between GaAs layer  380  and C-doped Al 0.9 Ga 0.1 As layer  481  and between C-doped Al 0.9 Ga 0.1 As layer  483  and C-doped GaAs layer  484 . C-doped GaAs layer  484  is grown to a typical thickness in the range of 50˜100 A. 
     The value of x for C-doped Al x Ga 1−x As layer  482  is selected to be higher than the value of x selected for any other C-doped Al x Ga 1−x As layers in the structure since the rate of oxidation is strongly dependent on the Al content of C-doped Al x Ga 1−x As layer  482 . See, for example, U.S. Pat. No. 5,896,408, incorporated by reference in its entirety, for details. Ion implantation may also be used to realize current confinement either alone or in conjunction with laterally oxidized layer structure. 
     The tunnel junction which consists of heavily C-doped GaAs layer  485  grown to a typical thickness of about 200 A with C-doping typically in the range of about 2.0×10 19  to 2.0×10 20  cm −3  and heavily Si-doped In x Ga 1−x As layer  486 , where x is in the range of about 0 to 0.2, is grown to a typical thickness of about 100-200 Å with Si doping typically in the range of about 1.0×10 18  to 1.0×10 20  cm −3 . The use of the tunnel junction allows better lateral current spreading at the n-layers on the top of the tunnel junction as well as much lower absorption loss of emission light at top n-type DBR mirror structure  490  compared to a p-type DBR mirror structure. Then, Si-doped GaAs layer  487  with a thickness in the range of about 500˜600 A is grown on In x  Ga 1−x As layer  486  with Si doping typically in the range of about 1.0×10 17  to 1.0×10 18  cm −3 . The tunnel junction is located at a minimum of the standing wave in the laser cavity to minimize the absorption loss at the tunnel junction by adjusting the thickness of the GaAs layer  380  and Si-doped GaAs layer  487 . 
     Then n-type DBR mirror structure  490  is grown. N-type DBR mirror structure  490  is made up of about 20-35 pairs of alternating layers of which Si-doped Al x Ga 1−x As layer  491  and Si-doped GaAs layer  492  are typical layer pairs with x between 0.8 and 1 and with Si-doping typically in the range of 5.0×10 17 -5.0×10 18  cm −3 . Si-doped Al x Ga 1−x As layer  491  is grown to a thickness corresponding to one quarter of the emission wavelength. Si-doped GaAs layer  492  is also grown to a thickness corresponding to one quarter of the emission wavelength with about 100-300 Å of linear grading at each interface. Finally, the growth is completed by growing heavily Si-doped GaAs contact layer  406  to a thickness in the range of about 500-1000 Å. Si-doped GaAs contact layer  406  is typically doped in the range of 5.0×10 18 -1.0×10 20  cm −3 . 
     While the invention has been described in conjunction with specific embodiments, it is evident to those skilled in the art that many alternatives, modifications, and variations will be apparent in light of the foregoing description. Accordingly, the invention is intended to embrace all other such alternatives, modifications, and variations that fall within the spirit and scope of the appended claims.