Abstract:
Semiconductor devices are described that include a vertical cavity surface emitting laser (VCSEL) and a structure formed on or near the surface of the VCSEL that acts as a filter that benefits high-frequency VCSEL modulation performance.

Description:
BACKGROUND 
     A vertical cavity surface emitting laser (VCSEL) is a type of semiconductor laser diode in which the optical beam is emitted in a direction normal to the top surface of a generally planar semiconductor structure. As illustrated in  FIGS. 1-2 , a conventional VCSEL  10  has a structure comprising a stack or number of layers that can be built up using photolithographic techniques. The structure can extend downwardly into a generally planar substrate stack  12 , with a portion of the structure extending above the surface of substrate stack  12  including a P-metal layer  14  that is deposited on a raised or mesa region  16  of substrate stack  12 . P-metal layer  14  represents the positive (P) electrical contact of the circuit that supplies current to VCSEL  10 . P-metal layer  14  has a substantially ring-like or annular shape. In operation, VCSEL  10  emits light through the opening in the center of P-metal layer  14  substantially in the direction of the arrow  18 . Note that arrow  18  is aligned along an optical axis  19  of VCSEL  10  that is normal to the planar semiconductor structure. 
     The remaining layers of the structure have similarly annular or circular shapes that are similarly symmetrically arranged with respect to optical axis  19 , though this aspect is not shown in the enlarged cross-sectional view of  FIG. 2 . The layers are shown in generalized or schematic form in  FIG. 2  for purposes of clarity. Also note that  FIGS. 1-2  are not to scale. At the bottom of the structure, an N-metal layer  20  is deposited over a semiconductor (e.g., GaAs) substrate layer  22 . N-metal layer  20  represents the negative (N) electrical contact of the circuit that supplies current to VCSEL  10 . Above substrate layer  22  is an N-type lower distributed Bragg reflector (N-DBR) layer  24 . Above N-DBR layer  24  is an active region  26  that can comprise one or more quantum wells. Above active region  26  is an oxide layer  28  having an annular shape that defines an oxide aperture  30 . A P-type upper distributed Bragg reflector (P-DBR) layer  32  is disposed above oxide layer  28  and extends into oxide aperture  30 . Oxide layer  28  helps direct electrical charge into active region  26 . N-DBR layer  24  is sometimes referred to as the lower DBR of the VCSEL, and P-DBR layer  32  is sometimes referred to as the upper DBR of the VCSEL. An isolation implant layer  34  surrounds the periphery of P-DBR layer  32 . Isolation implant layer  34  can be formed of P-DBR material in which ions are implanted to make the layer dielectric, so as to electrically insulate P-metal layer  14  from active region  26 . A dielectric layer  36  between isolation implant layer  34  and P-metal layer  14  provides further electrical insulation. When a voltage is applied between P-metal layer  14  and N-metal layer  20 , a current flows downwardly from P-metal layer  14  toward active region  26 , causing photons to be emitted in the area of active region  26  within oxide aperture  30 . The voltage is applied by coupling a source of high frequency electrical energy between a bondpad  35  and N-metal layer  20 . A metal neck region  37  electrically connects bondpad  35  to P-metal layer  14 . 
     The VCSEL described above is only one of several types known in the art. For example, in another common VCSEL configuration (not shown) the N-metal layer is on the top surface. A well can be etched beyond the active region, exposing the N-DBR region or N-type substrate, and the N-metal layer can be deposited over and in the well. 
     A VCSEL of the type described above can be modulated at high speeds (i.e., radio frequencies or RF) and used in a high-bandwidth optical data communication link. However, the modulation bandwidth is limited by several effects, including intrinsic properties of the optical-electrical conversion process, thermal effects, and electrical parasitic effects. The first of these effects relates to the VCSEL response rolling off as the VCSEL is driven above its resonant frequency. The second of these effects relates to the VCSEL response rolling off with an increase in temperature. The third effect relates to parasitic capacitances and inductances in the VCSEL that can cause frequency-dependent power transfer rolloff between the RF source and the VCSEL junction. 
     SUMMARY 
     Embodiments of the present invention relate to semiconductor devices that include a vertical cavity surface emitting laser (VCSEL) and a structure formed on or near the surface of the VCSEL that acts as a filter to promote good high-frequency VCSEL modulation performance. 
     In one aspect, a filter structure can comprise a pattern formed in a substantially annular region of one of the layers of materials forming the VCSEL. In another aspect, a filter structure can comprise a Schottky region in contact with the VCSEL electrical contact layer. In still another aspect, a filter structure can be formed on or near the surface of the semiconductor substrate on which the VCSEL is formed and be coupled between the VCSEL electrical contact layer and a bondpad. 
     Other systems, methods, features, and advantages will be or become apparent to one with skill in the art upon examination of the following figures and detailed description. It is intended that all such additional systems, methods, features, and advantages be included within this description, be within the scope of the specification, and be protected by the accompanying claims. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The invention can be better understood with reference to the following drawings. The components in the drawings are not necessarily to scale, emphasis instead being placed upon clearly illustrating the principles of the present invention. 
         FIG. 1  is a perspective view of a semiconductor device in generalized form, in accordance with the prior art. 
         FIG. 2  is a cross-sectional view taken on line  2 - 2  of  FIG. 1 , showing the layered structure of the semiconductor device in generalized form. 
         FIG. 3  is a cross-sectional view similar to  FIG. 2  but of a semiconductor device in accordance with an exemplary embodiment of the invention, in which the filter structure comprises a pattern formed in a VCSEL layer. 
         FIG. 4  is a top plan view of the semiconductor device of  FIG. 3 , with the top metal contact layer and dielectric layer removed to show the patterned filter structure. 
         FIG. 5  is a circuit diagram illustrating a circuit model of the semiconductor device of  FIGS. 3-4 . 
         FIG. 6  is a cross-sectional view similar to  FIGS. 2 and 3  but of a semiconductor device in accordance with another exemplary embodiment of the invention, in which the filter structure comprises a metallic Schottky layer. 
         FIG. 7  is a cross-sectional view similar to  FIGS. 2 ,  3  and  6  but of a semiconductor device in accordance with still another exemplary embodiment of the invention, in which the filter structure comprises a semiconductor Schottky layer. 
         FIG. 8  is a circuit diagram illustrating a circuit model of the semiconductor device of  FIGS. 6-7 . 
         FIG. 9  is a cross-sectional view similar to  FIGS. 2 ,  3 ,  6  and  7  but of a semiconductor device in accordance with yet another exemplary embodiment of the invention, in which the filter structure comprises a semiconductor Schottky layer. 
         FIG. 10  is a cross-sectional view similar to  FIGS. 2 ,  3 ,  6 ,  7  and  9  but of a semiconductor device in accordance with yet another exemplary embodiment of the invention, in which the filter structure comprises a semiconductor Schottky layer. 
         FIG. 11  is a top plan view of a semiconductor device in accordance with yet another exemplary embodiment of the invention, in which filter structures are formed on the surface of the VCSEL substrate. 
         FIG. 12  is a side elevational view showing a portion of  FIG. 11 . 
         FIG. 13  is a side elevational view showing another portion of  FIG. 11 . 
         FIG. 14  is a side elevational view showing still another portion of  FIG. 11 . 
     
    
    
     DETAILED DESCRIPTION 
     As illustrated in  FIGS. 3-4 , in an illustrative or exemplary embodiment of the invention, a VCSEL  40  has a structure comprising a number of layers, including a semiconductor substrate  42 . The remaining layers are formed around semiconductor substrate  42  and include an N-metal layer  44  below substrate  42 , an N-type distributed Bragg reflector (N-DBR) layer  46  above substrate  42 , an active region  48  above N-DBR layer  46 , an oxide layer  50  above active region  48 , a P-type distributed Bragg reflector (P-DBR) layer  52  above oxide layer  50 , an isolation implant layer  54  around the periphery of P-DBR layer  52 , a dielectric layer  56  above isolation implant layer  54 , and a P-metal layer  58  above dielectric layer  56 . Additional layers can be included in other embodiments, such as a narrow layer of P-DBR material between active region  48  and oxide layer  50 . The above-referenced layered structure can be formed by conventional photolithographic techniques, as well understood by persons skilled in the art to which the invention relates. But for substrate  42 , each of the layers can have a substantially circular or annular shape, symmetrically arranged with respect to an optical axis  60 . For example, the annular shape of oxide layer  50  defines an oxide aperture  62 . The references herein to “above” or “below” are with respect to optical axis  60  and the direction in which VCSEL  40  emits light in operation. N-metal layer  44  defines the bottom surface of VCSEL  40 . P-metal layer  58  defines the upper surface of VCSEL  40 . In operation, VCSEL  40  generates light within active region  48  in response to an electrical charge applied through P-metal layer  58  and N-metal layer  44 . The light is emitted through an opening in the center of P-metal layer  58  in a direction above the upper surface of VCSEL  40 , as indicated by the arrow  64 . Note that the above-described layers are shown in generalized or schematic form in  FIG. 2  for purposes of clarity. Also note that neither  FIGS. 3-4  nor any other drawing figures herein are to scale. 
     In the embodiment illustrated in  FIGS. 3-4 , a pattern  66  between P-metal layer  58  and P-DBR layer  52  acts as a filter to promote good high-frequency modulation performance of VCSEL  40  in operation. The term “pattern” as used herein refers to a repeating (e.g., geometric) shape or similar feature that is repeated at regular or substantially regular intervals. For example, pattern  66  can comprise a multiplicity of dielectric formations  68 , such as cuboid masses ( FIG. 4 ), distributed substantially evenly in a ring or annular region around optical axis  60 . Although as illustrated in  FIG. 4  each formation  68  can have, for example, a substantially square shape, in other embodiments such masses or other formations can have any other suitable shape, such as a drop-like shape, a dot-like shape, etc. Each formation  68  can have, for example, a square shape measuring between about one to two micrometers on each side and a thickness or depth in the range of, for example, a few hundred angstroms to about 1,000 angstroms. The region in which dielectric formations  68  are distributed extends substantially continuously around optical axis  60  but for the interstitial spaces  70  between adjacent dielectric formations  68 . In this embodiment, portions of P-metal layer  58  extend (e.g., a few hundred angstroms) into interstitial spaces  70  and contact P-DBR layer  52 . Thus, pattern  66  is defined or characterized by alternating regions of dielectric material (i.e., dielectric formations  68 ) and conductive material (i.e., portions of P-metal layer  58 ). Pattern  66  can also be characterized as a pattern or texture in the bottom surface of P-metal layer  58 , since dielectric formations  68  extend or protrude (e.g., a few hundred angstroms) into P-metal layer  58  in this embodiment. Interstitial spaces  70  can be on the same order of size as dielectric formations  68 , such as one to two micrometers. Pattern  66  can be formed through photolithographic techniques, such as, for example, depositing bump-like masses, drops, dots, etc., of a dielectric material on the surface of P-DBR layer  52  and then forming P-metal layer  58  over these dielectric masses. 
     Although in the exemplary embodiment, pattern  66  comprises a multiplicity of bump-like or cube-like dielectric formations  68  that are distributed substantially evenly in a ring or annular region around optical axis  60 , in other embodiments such a pattern can instead comprise, for example, a grid-like or cross-hatch pattern of dielectric material, or two or more concentric rings of dielectric material. 
     In operation, as pattern  66  has an annular shape, light emitted from active region  48  is not impeded by pattern  66  from being emitted from the surface of VCSEL  40  through the opening in P-metal layer  58 . Note that  FIG. 4  illustrates VCSEL  40  with P-metal layer  58  and dielectric layer  56  omitted so that pattern  66  can be seen. 
     As illustrated in  FIG. 5 , which electrically models the semiconductor structure of  FIGS. 3-4 , pattern  66  acts as a filter between VCSEL  40  and a source of RF electrical energy  72  that is coupled to a bondpad  74 . Although not shown in  FIGS. 3-4  for purposes of clarity, bondpad  74  is similar to bondpad  35  shown in  FIG. 1  and is similarly electrically connected to P-metal layer  58  by a metal neck region. Bondpad  74  exhibits a parasitic capacitance  78 . VCSEL  40  exhibits a parasitic junction resistance  80 , a parasitic junction capacitance  82  in parallel with junction resistance  80 , and a parasitic series resistance  84  in series with the parallel combination of junction resistance  80  and junction capacitance  82 . Pattern  66 , as it exists as part of the structure shown in  FIGS. 3-4 , exhibits a filter capacitance  86  in parallel with a filter resistance  88 , thereby defining a high-pass filter. Pattern  66 , acting as a high-pass filter, can selectively promote good high frequency modulation response of VCSEL  40  at the junction voltage of VCSEL  40  that is present at the VCSEL junction  90  in operation. Note that VCSEL junction  90  in  FIG. 5  electrically represents the diode junction that is formed at active region  48  between P-DBR layer  52  and N-DBR layer  46 . 
     As illustrated in  FIG. 6 , in another exemplary embodiment of the invention, a VCSEL  92  has a structure similar to that described above but includes a Schottky layer  94  instead of pattern  66 . Thus, VCSEL  92  includes a semiconductor substrate  96 , an N-metal layer  98  below substrate  96 , an N-DBR layer  100  above substrate  96 , an active region  102  above N-DBR layer  100 , an oxide layer  104  above active region  102 , a P-DBR layer  106  above oxide layer  104 , an isolation implant layer  108  around the periphery of P-DBR layer  106 , a dielectric layer  110  above isolation implant layer  108 , and a P-metal layer  112  above dielectric layer  110 . As in the embodiment described above with regard to  FIGS. 3-4 , each of the layers can have a substantially circular or annular shape, symmetrically arranged with respect to an optical axis  114 . For example, the annular shape of oxide layer  104  defines an oxide aperture  116 . In operation, VCSEL  92  generates light within active region  102  in response to an electrical charge applied through P-metal layer  112  and N-metal layer  98 . The light is emitted through an opening in the center of P-metal layer  112  in a direction extending above the upper surface of VCSEL  92 , as indicated by the arrow  118 . 
     In the embodiment illustrated in  FIG. 6 , Schottky layer  94  between P-metal layer  112  and P-DBR layer  106  acts as a filter to promote good high-frequency modulation performance of VCSEL  92  in operation. Although Schottky layer  94  can be made of any suitable material, in the embodiment shown in  FIG. 6  Schottky layer  94  is made of metal. For example, Schottky layer  94  can be similar to P-metal layer  112  but metallurgically altered to provide a Schottky contact rather than an ohmic contact with P-DBR layer  106 . As well understood in the art, in a general sense the term “Schottky” or “Schottky diode” refers to a contact region between two materials that acts like a diode junction with a forward voltage drop that is very low but greater than zero, such as, for example, 0.15 to 0.45 volts. A Schottky contact can be contrasted with an ohmic contact, which exhibits no diode-like behavior and effectively zero voltage drop. For example, the electrical contact between P-metal layer  14  and P-DBR layer  32  in VCSEL  10  described above with regard to  FIG. 2  is an ohmic contact. In the conventional VCSEL  10  shown in  FIG. 2 , ohmic contact between P-metal layer  14  and P-DBR layer  32  is desirable because it maximizes the transfer of electrical energy. However, in accordance with the present invention it has been found that Schottky contact between P-metal layer  112  and P-DBR layer  106  can provide a useful filter effect. Like P-metal layer  112 , Schottky layer  94  has an annular shape and thus does not impede the emission of light during operation. 
     As illustrated in  FIG. 7 , in yet another exemplary embodiment of the invention, a VCSEL  120  has a structure similar to that described above with regard to  FIG. 6  but includes another type of Schottky layer  122 . Thus, VCSEL  120  includes a semiconductor substrate  124 , an N-metal layer  126  below substrate  124 , an N-DBR layer  128  above substrate  124 , an active region  130  above N-DBR layer  128 , an oxide layer  132  above active region  130 , a P-DBR layer  134  above oxide layer  132 , an isolation implant layer  136  around the periphery of P-DBR layer  134 , a dielectric layer  138  above isolation implant layer  136 , and a P-metal layer  140  above dielectric layer  138 . As in the embodiment described above with regard to  FIG. 6 , each of the layers can have a substantially circular or annular shape, symmetrically arranged with respect to an optical axis  142 . For example, the annular shape of oxide layer  132  defines an oxide aperture  144 . In operation, VCSEL  120  generates light within active region  130  in response to an electrical charge applied through P-metal layer  140  and N-metal layer  126 . The light is emitted through an opening in the center of P-metal layer  140  in a direction above the upper surface of VCSEL  120 , as indicated by the arrow  146 . Like P-metal layer  112 , Schottky layer  94  has an annular shape and thus does not impede the emission of light during operation. 
     In the embodiment illustrated in  FIG. 7 , Schottky layer  122  between P-metal layer  140  and P-DBR layer  134 , acts as a filter to promote good high-frequency modulation performance of VCSEL  120  in operation. Schottky layer  122  can be made of a doped semiconductor material. For example, Schottky layer  122  can be similar to P-DBR layer  134  but doped more lightly to provide a Schottky contact rather than an ohmic contact with P-metal layer  140 . As well understood in the art, high semiconductor surface doping (e.g., greater than, for example, 5e18 atoms per cm 3 ) is used to provide an ohmic contact or interface between a P-DBR layer and P-metal layer in the conventional VCSEL  10  shown in  FIG. 2 . In contrast, to provide a Schottky contact the doping level should be less than about 5e17 atoms per cm 3 . Nevertheless, a contact can be sufficiently Schottky to provide an effect in accordance with an embodiment of the present invention even if the doping level is somewhat greater than 5e17 atoms per cm 3 . 
     As illustrated in  FIG. 8 , Schottky layer  94  in the embodiment shown in  FIG. 6  (or Schottky layer  122  in the embodiment shown in  FIG. 7 ) acts as a filter between VCSEL  92  (or VCSEL  120  in the embodiment shown in  FIG. 7 ) and a source of RF electrical energy  148  that is coupled to a bondpad  150 . Although not shown in  FIG. 8  or  9  for purposes of clarity, bondpad  150  is similar to bondpad  35  shown in  FIG. 1  and is similarly electrically connected to P-metal layer  112  (or P-metal layer  140  in the embodiment shown in  FIG. 7 ) by a metal neck region. Bondpad  150  exhibits a parasitic capacitance  152 . VCSEL  92  (or VCSEL  120  in the embodiment shown in  FIG. 7 ) exhibits a parasitic junction resistance  154 , a parasitic junction capacitance  156  in parallel with junction resistance  154 , and a parasitic series resistance  158  in series with the parallel combination of junction resistance  154  and junction capacitance  156 . Schottky layer  94  (or Schottky layer  122  in the embodiment shown in  FIG. 7 ), as it exists as part of the structure shown in  FIG. 6  (or  FIG. 7 ), exhibits a filter capacitance  160 , a filter resistance  162 , and a Schottky diode characteristic  164  in parallel with each other over a relatively low frequency range. Above this frequency range, in the high frequency range of the signal provided by RF source  148 , the Schottky diode characteristic  164  drops out or becomes of negligible effect, and Schottky layer  94  (or Schottky layer  122  in the embodiment shown in  FIG. 7 ) effectively becomes a high-pass filter like that described above with regard to pattern  66  in the embodiment shown in  FIG. 5 . Schottky layer  94  (or Schottky layer  122  in the embodiment shown in  FIG. 7 ), acting as a high-pass filter, can selectively promote good high frequency modulation response of VCSEL  92  (or VCSEL  120  in the embodiment shown in  FIG. 7 ) at the junction voltage of VCSEL  92  (or VCSEL  120 ) that is present at the VCSEL junction  166  in operation. 
     Although in the embodiments illustrated in  FIGS. 6-7 , VCSELs  92  and  120  have Schottky layers  94  and  122 , respectively, instead of a pattern, it should be understood that in still further embodiments that are similar to that illustrated in  FIGS. 3-4  the pattern itself can provide Schottky contact between the P-metal layer and P-DBR layer. That is, the Schottky layer can have a pattern. 
     For example, as illustrated in  FIG. 9 , a VCSEL  200  has a structure similar to that described above with regard to  FIGS. 6-7  but includes still another type of Schottky layer  202 . Accordingly, VCSEL  200  includes a semiconductor substrate  204 , an N-metal layer  206  below substrate  204 , an N-DBR layer  208  above substrate  204 , an active region  210  above N-DBR layer  208 , an oxide layer  212  above active region  210 , a P-DBR layer  214  above oxide layer  212 , an isolation implant layer  216  around the periphery of P-DBR layer  214 , a dielectric layer  218  above isolation implant layer  216 , and a P-metal layer  220  above dielectric layer  218 . As in the embodiments described above with regard to  FIGS. 6-7 , each of the layers can have a substantially circular or annular shape, symmetrically arranged with respect to an optical axis  222 . For example, the annular shape of oxide layer  212  defines an oxide aperture  224 . In operation, VCSEL  200  generates light within active region  210  in response to an electrical charge applied through P-metal layer  220  and N-metal layer  206 . The light is emitted through an opening in the center of P-metal layer  220  in a direction above the upper surface of VCSEL  200 , as indicated by the arrow  226 . Schottky layer  202  can be made of a material similar to P-DBR layer  214  but having a different doping level. Schottky layer  202  can be formed either by surface-etching a pattern in the material or by evaporation of a portion of P-metal layer  220 , resulting in a pattern of alternating regions of Schottky contact and resistive contact between P-metal layer  220  and P-DBR layer  214  similar to the pattern shown in  FIG. 4 . As the material of Schottky layer  202  is transparent to the emitted light, it does not impede the emission of light during operation. 
     As illustrated in  FIG. 10 , in an exemplary embodiment similar to that described above with regard to  FIG. 9 , a VCSEL  230  includes another type of Schottky layer  232 , as well as a semiconductor substrate  234 , an N-metal layer  236  below substrate  234 , an N-DBR layer  238  above substrate  234 , an active region  240  above N-DBR layer  238 , an oxide layer  242  above active region  240 , a P-DBR layer  244  above oxide layer  242 , an isolation implant layer  246  around the periphery of P-DBR layer  244 , a dielectric layer  248  above isolation implant layer  246 , and a P-metal layer  250  above dielectric layer  248 . As in the embodiments described above with regard to  FIGS. 6-7  and  9 , each of the layers can have a substantially circular or annular shape, symmetrically arranged with respect to an optical axis  252 . For example, the annular shape of oxide layer  242  defines an oxide aperture  254 . In operation, VCSEL  230  generates light within active region  240  in response to an electrical charge applied through P-metal layer  250  and N-metal layer  236 . The light is emitted through an opening in the center of P-metal layer  250  in a direction above the upper surface of VCSEL  230 , as indicated by the arrow  256 . Schottky layer  232  can be formed of ion-implanted regions of the surface (e.g., a few hundred to about 1,000 angstroms in depth) of P-DBR layer  244  that alternate with non-ion-implanted regions of P-DBR layer  244  in a pattern similar to that shown in  FIG. 4 . 
     As illustrated in  FIGS. 11-14 , in still other exemplary embodiments of the invention, a VCSEL  168  having a structure similar to those described above can be part of a semiconductor device that includes a metal bondpad  170  and one or more of the following filter elements or a similar filter element formed on the surface of the semiconductor substrate stack  172 : a metal airbridge  174 , an etched semiconductor bridge  176 , a surface capacitor  178 , and a metal coil  180 . Each of these filter elements is a two-terminal device having a first terminal or node coupled to bondpad  170  (via zero or more other such filter elements or via portions of a neck region  182  or  186 ) and a second terminal or node coupled to the P-metal layer  184  of VCSEL  168  (via zero or more other such filter elements or via portions of neck regions  182  and  186 ). 
     Metal airbridge  174 , which is further illustrated in a side view in  FIG. 12 , comprises a metal bridge portion  188  that extends over a gap  189 , i.e., through the air, between a raised or mesa region  190  of the upper surface of substrate stack  172  around VCSEL  168  and a similar mesa region  192  on another area on the surface of substrate stack  172 . Mesa region  192  is raised above the remainder of substrate stack  172  in a manner similar to that illustrated in  FIG. 1 . Such metal airbridges are, in and of themselves, known in the art, and are conventionally used to provide inductance. As persons skilled in the art understand how to form such airbridges, the formation and structure of metal airbridge  174  are not described herein in further detail. In accordance with the present invention, metal airbridge  174  can serve as an inductor between bondpad  170  and P-metal layer  184 . In a manner similar to the various filters described above with regard to the embodiments shown in  FIGS. 3-8 , metal airbridge  174  can serve as a high-pass filter that selectively promotes good high frequency modulation response of VCSEL  168  at the junction voltage in operation. As noted above, metal airbridge  174  can be included in combination with one or more of the other filter elements shown in  FIGS. 11-14  or as the sole filter element. 
     Etched semiconductor bridge  176 , which is further illustrated in a side view in  FIG. 13 , exhibits a resistance and can be used in combination with others of the filter elements shown in  FIGS. 11-14  to define additional filter elements. For example, the combination of etched semiconductor bridge  176  in parallel with surface capacitor  178  can exhibit a filter capacitance in parallel with a filter resistance, similar to filter capacitance  86  and filter resistance  88  shown in  FIG. 5 . 
     As illustrated in  FIG. 14 , surface capacitor  178  can comprise a top metal planar region  194  coupled to a portion of neck region  186 , a bottom metal planar region  196  coupled to a portion of neck region  182 , and a dielectric layer  198  between top and bottom metal planar regions  194  and  196 . Such surface capacitors are, in and of themselves, well known in the art. As persons skilled in the art understand how to form such surface capacitors, the formation and structure of surface capacitor  178  are not described herein in further detail. Surface capacitor  178  can serve as a capacitance between bondpad  170  and P-metal layer  184 . In a manner similar to the various filters described above, surface capacitor  178 , either alone or in combination with etched semiconductor bridge  176  or others of the filter elements described herein, can serve as a high-pass filter that selectively promotes good high frequency modulation response of VCSEL  168  at the junction voltage in operation. 
     Metal coil  180 , which exhibits an inductance, can similarly be used alone or in combination with others of the filter elements described herein. Such metal coils formed on the surface of a semiconductor substrate are, in and of themselves, well known and used to provide an inductance. As persons skilled in the art understand how to form such metal coils, the formation and structure of metal coil  180  are not described herein in further detail. In accordance with the present invention, metal coil  180  can serve as an inductor between bondpad  170  and P-metal layer  184 . In a manner similar to the various filters described above, metal coil  180  can serve as a high-pass filter that selectively promotes good high frequency modulation response of VCSEL  168  at the junction voltage in operation. As noted above, metal coil  180  can be included in combination with one or more of the other filter elements shown in  FIGS. 11-14  or as the sole filter element. 
     Metal airbridge  174 , etched semiconductor bridge  176 , surface capacitor  178 , metal coil  180 , and combinations thereof, are intended only as examples of filter elements that can be formed on the surface of a semiconductor substrate stack and coupled between a bondpad and a VCSEL P-metal contact. In view of these examples and other teachings herein, still other suitable types of filter elements that can be formed on the surface of a semiconductor substrate stack and coupled between a bondpad and a VCSEL P-metal contact will occur readily to persons skilled in the art and are therefore within the scope of the invention. 
     More generally, one or more illustrative embodiments of the invention have been described above. However, it is to be understood that the invention is defined by the appended claims and is not limited to the specific embodiments described.