Abstract:
Vertical cavity surface emitting lasers (VCSELs) and methods of making the same are described. The VCSELs include reliability-enhancing layers that perform specific functions at one or more critical locations within a VCSEL structure to reduce or prevent defect formation and migration that otherwise might degrade VCSEL performance, for example, by increasing optical absorption in the mirror stacks or by degrading the electro-optic properties of the active region. In particular, the reliability-enhancing layers are configured to perform one or more of the following functions within the VCSEL structure: gettering (i.e., removing defects or impurities from critical regions), strain balancing (i.e., compensating the lattice mismatch in the structure to minimize strain), and defect suppression (i.e., creating alloys that reduce the formation of defects during growth or post-growth activities). By strategically positioning one or more appropriately configured reliability-enhancing layers with respect to an identified defect source, the invention enables VCSEL structures to be modified in a way that enhances the reliability and performance of VCSEL devices.

Description:
TECHNICAL FIELD 
     This invention relates to reliability-enhancing layers for vertical cavity surface emitting lasers, and methods of making the same. 
     BACKGROUND 
     A vertical cavity surface emitting laser (VCSEL) is a laser device formed from an optically active semiconductor layer (e.g., AlInGaAs or InGaAsP) that is sandwiched between a pair of highly reflective mirror stacks, which may be formed from layers of metallic material, dielectric material or epitaxially-grown semiconductor material. Typically, one of the mirror stacks is made less reflective than the other so that a portion of the coherent light that builds in a resonating cavity formed between the mirror stacks may be emitted from the device. Typically, a VCSEL emits laser light from the top or bottom surface of the resonating cavity with a relatively small beam divergence. VCSELs may be arranged in singlets, one-dimensional or two-dimensional arrays, tested on wafer, and incorporated easily into an optical transceiver module and coupled to a fiber optic cable. 
     In general, a VCSEL may be characterized as a gain-guided VCSEL or an index-guided VCSEL. An implant VCSEL is the most common commercially available gain-guided VCSEL. An implant VCSEL includes one or more high resistance implant regions for current confinement and parasitic reduction. An oxide VCSEL, on the other hand, is the most common index-guided (laterally and vertically) VCSEL. An oxide VCSEL includes oxide layers (and possibly implant regions) for both current and optical confinement. 
     VCSELs and VCSEL arrays have been successfully developed for single-mode operation and multi-mode operation at a variety of different wavelengths (e.g., 650 nm, 850 nm, 980 nm, 1300 nm and 1550 nm). The commercial success of VCSEL technology, however, will depend in large part upon development of VCSEL structures that are characterized by high performance and high reliability. 
     Techniques have been proposed for improving the performance and reliability of a wide variety of different semiconductor laser devices, including VCSELs and edge-emitting lasers. 
     For example, U.S. Pat. No. 5,838,705 discloses VCSEL devices (i.e., a non-planar ridge VCSEL and a planar implant VCSEL) that include one or more defect inhibition layers that are positioned in a respective one of two cladding regions that are formed on opposite sides of an active area. According to the &#39;705 patent, the defect inhibition layers may be disposed anywhere outside of the active area. However, the only preferred locations for the defect inhibition layers are in close proximity and on either side of the active area to provide a barrier that does not allow defects formed outside of the active area to pass through and into the active area. The defect inhibition layers are formed from an indium-containing material that induces strain in the VCSEL device. The strain is believed to either prohibit movement of defects to the active area or attract and, subsequently, trap defects in the defect inhibition layers. 
     U.S. Pat. No. 4,984,242 discloses a GaAs/AlGaAs edge-emitting laser that includes at least one cladding layer that includes indium. According to the &#39;242 patent, the indium creates a local strain field that is sufficient to reduce and effectively stop defect migration through the cladding layer. The indium-containing strain layer may be spaced apart from the active region or may be positioned adjacent to the active region. Indium-containing layers may be added to the active region barrier layers to improve the performance of the edge-emitting laser. In one embodiment, a uniform doping of indium is provided throughout the edge-emitting laser heterostructure to impede the growth and migration of defects in the crystal lattice. Another embodiment includes indium in a cap layer to reduce the surface work function and, thereby, reduce the contact resistance of an overlying metallization layer. The &#39;242 patent does not teach or suggest the use of indium in a VCSEL, nor does it teach or suggest how indium might be translated to a VCSEL structure. 
     SUMMARY 
     The invention features reliability-enhancing layers that perform specific functions at one or more critical locations within a VCSEL structure to reduce or prevent defect formation and migration that otherwise might degrade VCSEL performance, for example, by increasing optical absorption in the mirror stacks or by degrading the electro-optic properties of the active region. In particular, the reliability-enhancing layers are configured to perform one or more of the following functions within the VCSEL structure: gettering (i.e., removing defects or impurities from critical regions), strain balancing (i.e., compensating the lattice mismatch in the structure to minimize strain), and defect suppression (i.e., block/reduce defects formation/migration during growth, processing or device operations). By strategically positioning one or more appropriately configured reliability-enhancing layers with respect to an identified defect source, the invention enables VCSEL structures to be modified in a way that enhances the reliability and performance of VCSEL devices. 
     In one aspect, the invention features a vertical cavity surface emitting laser (VCSEL) that includes a first mirror stack, a second mirror stack, and a cavity region that is disposed between the first mirror stack and the second mirror stack and includes an active region. The VCSEL also includes a defect source and a reliability-enhancing layer positioned with respect to the defect source to reduce defect-induced degradation of one or more VCSEL regions. 
     Embodiments in accordance with this aspect of the invention may include one or more of the following features. 
     The reliability-enhancing layer may be positioned between the defect source and the cavity region, within the defect source, or in close proximity to the defect source (above or below, or both). 
     As used herein, the term “cavity region” refers to the VCSEL structure that includes the active region and the spacer layers. 
     The reliability-enhancing layer may include one or more of the following elements: indium, boron, phosphorus, antimony, and nitrogen. The reliability-enhancing layer may be lattice-matched to surrounding layers. Alternatively, the reliability-enhancing layer may include one or more strained layers. The reliability-enhancing layer may include a superlattice, which may be tensile strained, compressive strained or strain compensated. The reliability-enhancing layers may be separated by non-reliability-enhancing layers. 
     The defect source may include one or more of the following: an oxidized portion of the VCSEL, an implant region of the VCSEL, an exposed region of the VCSEL, one or more dielectric layers, a doped region of the VCSEL, and the substrate. 
     The reliability-enhancing layer may be configured to balance strain created by the defect source. For example, the defect source may include an oxide region that induces a compressive strain field, and the reliability-enhancing layer may be positioned within the compressive strain field and may be characterized by tensile strain that substantially balances the compressive strain field. 
     In some embodiments, the defect source creates a concentration gradient that induces defect migration. In these embodiments, the reliability-enhancing layer may be configured to reduce the induced defect migration. For example, the defect source may be characterized by a relatively high group V vacancy concentration, in which case, the reliability-enhancing layer preferably is characterized by a lower diffusion rate of vacancy defects. 
     In another aspect, the invention features a method of manufacturing a VCSEL. In accordance with this method, a first mirror stack is formed, a second mirror stack is formed, and a cavity region having an active region is formed therebetween. A defect source is formed, and a reliability-enhancing layer is positioned with respect to the defect source to reduce defect-induced degradation of one or more VCSEL regions. 
     Other features and advantages of the invention will become apparent from the following description, including the drawings and the claims. 
    
    
     
       DESCRIPTION OF DRAWINGS 
         FIG. 1  is a diagrammatic cross-sectional side view of a portion of a VCSEL structure. 
         FIG. 2A  diagrammatic top view of a planar oxide VCSEL with a reliability-enhancing layer positioned between an oxidized portion of a first mirror stack and a cavity region. 
         FIG. 2B  is a diagrammatic cross-sectional side view of the planar oxide VCSEL of  FIG. 2A  taken along the line  2 B- 2 B. 
         FIG. 2C  is a diagrammatic cross-sectional side view of the planar oxide VCSEL of  FIG. 2A  taken along the line  2 C- 2 C. 
         FIG. 3  is a diagrammatic cross-sectional side view of an index-guided oxide VCSEL with a reliability-enhancing layer positioned between an oxidized portion of a first mirror stack and an active region. 
         FIG. 4  is a diagrammatic cross-sectional side view of a gain-guided implant VCSEL with a reliability-enhancing layer positioned between an implant region of a first mirror stack and an active region. 
     
    
    
     DETAILED DESCRIPTION 
     In the following description, like reference numbers are used to identify like elements. Furthermore, the drawings are intended to illustrate major features of exemplary embodiments in a diagrammatic manner. The drawings are not intended to depict every feature of actual embodiments nor relative dimensions of the depicted elements, and are not drawn to scale. 
     Referring to  FIG. 1 , in one generalized representation, a VCSEL  10  includes a cavity region  12  that is sandwiched between a first mirror stack  14  and a second mirror stack  16 , which is formed on a substrate  18 . Cavity region  12  includes one or more active layers  20  (e.g., a quantum well or one or more quantum dots) that are sandwiched between a pair of spacer layers  22 ,  24 . In other embodiments, active layer  20  may be located above or below a single spacer layer. A pair of electrical contacts  28 ,  30  enable VCSEL  10  to be driven by a suitable driving circuit. In operation, an operating voltage is applied across electrical contacts  28 ,  30  to produce a current flow in VCSEL  10 . In general, current flows through a central region of the VCSEL structure and lasing occurs in a central portion of cavity region  12  (hereinafter the “active region”). As shown in the embodiments described below, a confinement region (e.g., an oxide region or an implant region, or both) may provide lateral confinement of carriers and photons. Carrier confinement results from the relatively high electrical resistivity of the confinement region, which causes electrical current preferentially to flow through a centrally located region of VCSEL  10 . In an oxide VCSEL, optical confinement results from a substantial reduction of the refractive index of the confinement region that creates a lateral refractive index profile that guides the photons that are generated in cavity region  12 , whereas in an implant VCSEL optical confinement results primarily from thermal lensing and the injected carrier distribution. The carrier and optical lateral confinement increases the density of carriers and photons within the active region and, consequently, increases the efficiency with which light is generated within the active region. In some embodiments, the confinement region circumscribes a central region of VCSEL  10 , which defines an aperture through which VCSEL current preferably flows. In other embodiments, oxide layers may be used as part of the distributed Bragg reflectors in the VCSEL structure. 
     Active layer  20  may be formed from AlInGaAs (i.e., AlInGaAs, GaAs, AlGaAs and InGaAs), InGaAsP (i.e., InGaAsP, GaAs, InGaAs, GaAsP, and GaP), GaAsSb (i.e., GaAsSb, GaAs, and GaSb), InGaAsN (i.e., InGaAsN, GaAs, InGaAs, GaAsN, and GaN), or AlInGaAsP (i.e., AlInGaAsP, AlInGaAs, AlGaAs, InGaAs, InGaAsP, GaAs, InGaAs, GaAsP, and GaP). Other quantum well layer compositions also may be used. First and second spacer layers  22 ,  24  may be formed from materials chosen based upon the material composition of the active layers. First and second mirror stacks  14 ,  16  each includes a system of alternating layers of different refractive index materials that forms a distributed Bragg reflector (DBR) designed for a desired operating laser wavelength (e.g., a wavelength in the range of 650 nm to 1650 nm). For example, first and second mirror stacks  14 ,  16  may be formed of alternating layers of high aluminum content AlGaAs and low aluminum content AlGaAs. The layers of first and second mirror stacks  14 ,  16  preferably have an effective optical thickness (i.e., the layer thickness multiplied by the refractive index of the layer) that is about one-quarter of the operating laser wavelength. Substrate  18  may be formed from GaAs, InP, sapphire (Al 2 O 3 ), or InGaAs and may be undoped, doped n-type (e.g., with Si) or doped p-type (e.g., with Zn). A buffer layer  32  may be grown on substrate  18  before VCSEL  10  is formed. In the illustrative representation of  FIG. 1 , first and second mirror stacks  14 ,  16  are designed so that laser light is emitted from the top surface of VCSEL  10 ; in other embodiments, the mirror stacks may be designed so that laser light is emitted from the bottom surface of substrate  18 . 
     VCSEL  10  may be formed by conventional epitaxial growth processes, such as metal-organic chemical vapor deposition (MOCVD) and molecular beam epitaxy (MBE). VCSEL  10  may include a mesa structure that is formed by etching down at least to the confinement region of VCSEL  10 . The mesa structure may be etched by conventional wet or dry etching processes, including reactive ion etching (RIE) and reactive ion beam etching (RIBE). 
     As explained above, it has been observed that certain defect sources (e.g., an oxide region, an implant region, etched holes, etched trenches, etched mesas, substrate  18 , dielectric layers, and highly doped regions) tend to introduce strain, stress or defects (e.g., interfacial voids or point defects, or both) that may degrade the performance of VCSEL  10 . The following VCSEL embodiments feature reliability-enhancing layers that may be positioned at one or more critical locations within the VCSEL structure to reduce defect-induced degradation of the active region or other VCSEL regions (e.g., mirror stacks  14 ,  16 ), or both. In particular, reliability-enhancing layers may be positioned between the active region and the defect source, within the defect source, or in close proximity to the defect source (above or below, or both). The reliability-enhancing layers may include one or more of the following elements: In, P, B, N, and Sb. These elements may be incorporated into thick layers that are lattice matched to surrounding layers, thin strained layers, and superlattice structures that may be tensile strained, compressive strained or unstrained. Exemplary reliability-enhancing layers include GaAsP, InGaAsP, AlInGaAsP, AlGaAsSb, InGaAs, InAlGaAs, InGaAsP, and Ga x B 1−x N. The reliability-enhancing layers may be incorporated into one or more of the constituent layers of a VCSEL, including active layers, cavity layers, DBR mirror stacks, oxide layers, cap layers, and buffer layers. The reliability-enhancing layers may be separated by non-reliability-enhancing layers. A reliability-enhancing layer may be used to balance the strain fields induced by a defect source. For example, compressive strain that is induced by an oxide layer may be balanced by positioning a tensile strained reliability-enhancing layer (e.g., a GaAsP layer) in a region encompassed by the compressive strain fields of the oxide layer. A reliability-enhancing layer also may be positioned adjacent to or within a defect source to getter (or trap) defects. In addition, a reliability-enhancing layer may be used to block or reduce migration of defects or dopants. In particular, with respect to a defect source that creates a concentration gradient that induces defect migration, a reliability-enhancing layer may be configured to reduce the induced defect migration. For example, when a defect source creates a group V vacancy gradient, group VI dopant diffusion may be enhanced. In this case, a reliability-enhancing layer that is characterized by a low group V vacancy concentration may be positioned within the VCSEL structure to reduce or block the group VI dopant (or other point defect) migration. 
     Referring to  FIGS. 2A-2C , in one embodiment, a planar index-guided oxide VCSEL  40  includes a cavity region  12  sandwiched between a first mirror stack  14  and a second mirror stack  16 , which is formed on a substrate  18 . Cavity region  12  includes one or more active layers  20  (e.g., a quantum well or one or more quantum dots) that are sandwiched between a pair of spacer layers  22 ,  24 . As shown, in this embodiment, VCSEL  40  has a planar structure that includes a number of holes  42  that expose a number of respective side regions of first mirror stack  14  to be oxidized. At least a portion  26  of first mirror stack  14  is oxidized from the exposed side regions inwardly toward a centrally located aperture region  46 . In this embodiment, four holes  42  are opened that are equidistant from the center of a first electrode  44 . Holes  42  extend from the first surface of VCSEL  40  down at least to the layer (or layers) corresponding to oxidized portion  26 . When the VCSEL structure is exposed to heated water vapor, the heated water vapor enters holes  26  and oxidizes portion  26  in a radial direction away from holes  42 . The oxidation process continues until the oxidation front from each hole  42  merges to form an un-oxidized aperture  46 . Other oxide VCSEL embodiments may include more or fewer exposure holes  42  or exposed regions with other shapes, such as trenches or arches. 
     It has been observed that lateral oxidation of VCSEL  40  generates strain or stress in the VCSEL structure that may introduce defects (e.g., interfacial voids or point defects, or both) that tend to migrate into the active region or other VCSEL regions over time (e.g., during subsequent processing steps and during VCSEL operation) and, consequently, may compromise the reliability of VCSEL  40 . In the embodiment of  FIGS. 2A-2C , a reliability-enhancing layer  48  is disposed between oxidized portion  26  of first mirror stack  14  and cavity region  12 . As explained below, in other embodiments, reliability-enhancing layers may be positioned within oxidized portion  26  or in close proximity to oxidized portion  26  (above or below, or both). Reliability-enhancing layer  48  preferably is selected to produce between oxidized portion  26  of first mirror stack  14  and cavity region  12  a localized strain field that substantially reduces defect migration through reliability-enhancing layer  48  and, thereby, protects cavity region  12  from defect-induced degradation. In the embodiment of  FIGS. 2A-2C , reliability-enhancing layer  48  may be formed from a semiconductor alloy formed from Al, Ga, As and one or more of the following elements: In, B, P, Sb and N. For example, reliability-enhancing layer  48  may be formed from InGaAs, InAlGaAs, InGaAsP, or GaAsP. Reliability-enhancing layer  48  may be formed by adding one or more of, e.g., In, B, P, Sb and N during the epitaxial growth of one or more of the constituent layers of first mirror stack  14  that are located between oxidized portion  26  and cavity region  12 . 
     In some embodiments, at least a portion of second mirror stack  16  may be oxidized from the exposure holes  42  inwardly toward a centrally located region to achieve additional lateral confinement of carriers and photons. In these embodiments, a reliability-enhancing layer may be disposed between the oxidized portion of second mirror stack  16  and cavity region  12 . This reliability-enhancing layer preferably is selected to produce between the oxidized portion of second mirror stack  16  and cavity region  12  a localized strain field that substantially reduces defect migration through the reliability-enhancing layer and, thereby, protects cavity region  12  from defect-induced degradation. In these embodiments, the reliability-enhancing layer may have the same structure as reliability-enhancing layer  48 . As explained below, in other embodiments, reliability-enhancing layers may be positioned within the oxidized portion of second mirror stack  16  or in close proximity to the oxidized portion of second mirror stack  16  (above or below, or both). 
     For example, other embodiments may include reliability-enhancing layers that are designed to balance the strain that is introduced by the lateral oxidation of first mirror stack  14  or second mirror stack  16 , or both. In one embodiment, a reliability-enhancing layer may be positioned adjacent to (e.g., above or below, or both) the oxidized portions of first mirror stack  14  or second mirror stack  16 , or both. In another embodiment, a reliability-enhancing layer may be positioned within the oxidized portions of first mirror stack  14  or second mirror stack  16 , or both. In another embodiment, the oxidized portions of first mirror stack  14  or second mirror stack  16 , or both, may be sandwiched between respective pairs of reliability-enhancing layers. In these embodiments, the compressive strain created by the oxidation of portions of first mirror stack  14  or second mirror stack  16 , or both, may be balanced substantially by a reliability-enhancing layer that introduces a compensating tensile strain into the mirror stack. For example, in an embodiment that includes AlGaAs mirror stacks, the reliability-enhancing layers may be formed from In 1 Ga 1−x P, where x&lt;0.5 tensile. In these embodiments, the reliability-enhancing layers may suppress the formation of defects as well as block the migration of defects into cavity region  12  and other regions of the VCSEL structure. 
     In some embodiments, first mirror stack  14  may be implanted up to or through oxide layer  26  to further improve the performance of VCSEL  10 . In these embodiments, one or more reliability-enhancing layers may be formed above or within the implantation region to getter defects, or between the implantation region and cavity region  12  to suppress the propagation of defects into cavity region  12 . 
     Referring to  FIG. 3 , in another embodiment, a non-planar index-guided oxide VCSEL  50  is formed into a mesa (or pillar) structure with exposed sidewalls. At least a portion  52  of first mirror stack  14  is oxidized from the exposed mesa sidewalls inwardly toward a centrally located aperture region. As explained above, lateral oxidation of VCSEL  50  induces strain or stress in the VCSEL structure that may introduce defects (e.g., interfacial voids or point defects, or both) that tend to migrate into the active region over time (e.g., during subsequent processing steps and during VCSEL operation) and, consequently, may compromise the reliability of VCSEL  50 . In this embodiment, a reliability-enhancing layer  54  is disposed between oxidized portion  52  of first mirror stack  14  and cavity region  12 . In some embodiments, at least a portion of second mirror stack  16  may be oxidized from the exposed sidewalls inwardly toward a centrally located region to achieve additional lateral confinement of carriers and photons. In these embodiments, a reliability-enhancing layer may be disposed between the oxidized portion of second mirror stack  16  and cavity region  12 . Other embodiments may include reliability-enhancing layers that are designed to balance the strain that is introduced by the lateral oxidation of first mirror stack  14  or second mirror stack  16 , or both, as described above in connection with the embodiment of  FIGS. 2A-2C . In these embodiments, reliability-enhancing layers may be positioned within the oxidized portion of the first mirror stack or the oxidized portion of the second mirror stack, or both, or in close proximity to (above or below, or both) the oxidized portion of the first mirror stack or the oxidized portion of the second mirror stack, or both. 
     Referring to  FIG. 4 , in one embodiment, a gain-guided implant VCSEL  60  includes a cavity region  12  sandwiched between a first mirror stack  14  and a second mirror stack  16 , which is formed on a substrate  18 . Cavity region  12  includes one or more active layers  20  (e.g., a quantum well or one or more quantum dots) that are sandwiched between a pair of spacer layers  22 ,  24 . As shown in this embodiment, VCSEL  60  includes an implant region  62  that defines an aperture region  64 . It has been observed that implant region  62  of VCSEL  60 , especially at the implantation front, generates point defects that tend to migrate into the cavity region over time (e.g., during subsequent processing steps and during VCSEL operation) and, consequently, may compromise the reliability of VCSEL  60 . 
     In the embodiment of  FIG. 4 , a reliability-enhancing layer  66  is disposed between implant region  62  and cavity region  12 . In other embodiments, reliability-enhancing layers may be positioned within implant region  62  or in close proximity to implant region  62  (above or below, or both). Reliability-enhancing layer  66  is selected to produce a localized strain field and/or change the defect equilibria between implant region  62  and cavity region  12  that substantially reduces defect migration through reliability-enhancing layer  66  and, thereby, protects cavity region  12  from defect-induced degradation. In the embodiment of  FIG. 4 , reliability-enhancing layer  66  may be formed from a semiconductor alloy formed from Al, Ga, As and one or more of the following elements: In, B, P, Sb and N. For example, reliability-enhancing layer  66  may be formed from InGaAs, InAlGaAs, InGaAsP, or GaAsP. Reliability-enhancing layer  66  may be formed by adding one or more of, e.g., In, B, P, Sb and N during the epitaxial growth of one or more of the constituent layers of first mirror stack  14  located between implant region  62  and cavity region  12 . 
     In some embodiments, at least a portion of second mirror stack  16  may be implanted to achieve additional lateral confinement of carriers and photons. In these embodiments, a reliability-enhancing layer may be disposed between the implant region of second mirror stack  16  and cavity region  12 . This reliability-enhancing layer preferably is selected to produce between the implant region of second mirror stack  16  and cavity region  12  a localized strain field that substantially reduces defect migration through the reliability-enhancing layer and, thereby, protects cavity region  12  from defect-induced degradation. In these embodiments, the reliability-enhancing layer may have the same structure as reliability-enhancing layer  66 . In other embodiments, reliability-enhancing layers may be positioned within the implant region of second mirror stack  16  or in close proximity to the implant region of second mirror stack  16  (above or below, or both). 
     Other implant VCSEL embodiments may include reliability-enhancing layers that are designed to prevent degradation of active layer  20  and other VCSEL regions by defects that might be created during the implantation of first mirror stack  14  or second mirror stack  16 , or both. For example, in one embodiment, a reliability-enhancing layer may be positioned adjacent to (e.g., above or below, or both) the implant regions of first mirror stack  14  or second mirror stack  16 , or both. In another embodiment, a reliability-enhancing layer may be positioned within the implant regions of first mirror stack  14  or second mirror stack  16 , or both. In another embodiment, the implant regions of first mirror stack  14  or second mirror stack  16 , or both, may be sandwiched between respective pairs of reliability-enhancing layers. In such embodiments, the reliability-enhancing layers may modify strain fields, getter defects (e.g., point defects), suppress the formation of defects, and block the migration of defects into cavity region  12  and other regions of the VCSEL structure. 
     In each of the above-described embodiments, first mirror stack  14  preferably is doped with a dopant that has the opposite polarity as the substrate  18  (e.g., p-type dopants C or Mg for an n-type substrate  18 ). Second mirror stack  16  preferably is doped with a dopant that has the same polarity as the substrate  18  (e.g., n-type dopant Si for an n-type substrate  18 ). It has been observed that the process of doping the first and second mirror stacks  14 ,  16  also may introduce into the VCSEL structure defects that tend to migrate into critical VCSEL regions (e.g., the cavity region, which includes the active layers and the spacer layers) over time (e.g., during subsequent processing steps and during VCSEL operation). Accordingly, some embodiments may include reliability-enhancing layers that are disposed between the doped regions and the cavity region, within the doped regions, or in close proximity to the doped regions (above or below, or both). The reliability-enhancing layers preferably are selected to produce localized strain fields that substantially reduce defect migration therethrough and, thereby, protect cavity region  12  from defect-induced degradation. Each of the reliability-enhancing layers may be formed from a semiconductor alloy formed from Al, Ga, As and one or more of the following elements: In, B, P, Sb and N. For example, the reliability-enhancing layers may be formed from InGaAs, InAlGaAs, InGaAsP, or GaAsP. The reliability-enhancing layers may be formed by adding one or more of, e.g., In, B, P, Sb and N during the epitaxial growth of one or more of the constituent layers of first and second mirror stacks  14 ,  16  that are located between the doping regions and cavity region  12 . 
     Semiconductor substrates—especially semi-insulating semiconductor substrates—are potential sources of defects that might migrate into critical VCSEL regions (e.g., the cavity region, which includes the active layers and the spacer layers) over time. For this reason, some embodiments may include a defect-migration-suppressing reliability-enhancing layer that is positioned between the active region and the substrate or in close proximity to the substrate. In some embodiments, the reliability-enhancing layer may be positioned within second mirror stack  16 . The reliability-enhancing layer may be formed from semiconductor alloys formed from Al, Ga, As and one or more of the following elements: In, B, P, Sb and N. For example, the reliability-enhancing layer may be formed from InGaAs, InAlGaAs, InGaAsP, or GaAsP. The reliability-enhancing layer may be formed by adding one or more of, e.g., In, B, P, Sb and N during the epitaxial growth of one or more of the constituent layers of second mirror stack  16 . 
     In some embodiments, one or more reliability-enhancing layers may be positioned in buffer layer  32  to getter defects or buffer defect formation, or both. 
     Other embodiments are within the scope of the claims. 
     For example, although the above embodiments are described in connection with AlGaAs mirror stack systems, other semiconductor alloy compositions or dielectric layers may be used to form the DBR mirror structures. In addition, the reliability-enhancing layers may be formed from single layers, as described above, or from superlattice structures (e.g., strained layer superlattices or strain-compensated superlattices). 
     Still other embodiments are within the scope of the claims.