Abstract:
In one exemplary embodiment of the invention, a method is employed that is directed to forming a resonant reflector on an optoelectronic device, such as a semiconductor laser for example. The exemplary method involves depositing a first material layer on the top layer of the optoelectronic device, where the first material layer having a refractive index and a thickness of about an odd multiple of a quarter of a wavelength to which the optoelectronic device is tuned. A patterned region is then created that extends at least partially into the first material layer. Selected patterned regions are at least partially filled with a second material that has a refractive index that is greater than the refractive index of the first material layer. Finally, a third layer, having a refractive index greater than the refractive index of the first material layer, is deposited immediately adjacent the first material layer.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application is a division, and claims the benefit, of U.S. patent application Ser. No. 09/751,423, now U.S. Pat. No. 6,727,520 entitled SPATIALLY MODULATED REFLECTOR FOR AN OPTOELECTRONIC DEVICE, filed Dec. 29, 2000, and incorporated herein in its entirety by this reference. 
    
    
     BACKGROUND OF THE INVENTION 
     Field of the Invention 
     This invention generally relates to methods for producing optoelectronic components such as semiconductor lasers. More particularly, embodiments of the invention are concerned with methods for producing a resonant reflector for use in facilitating mode control in optoelectronic components. 
     BRIEF SUMMARY OF AN EXEMPLARY EMBODIMENT OF THE INVENTION 
     In one exemplary embodiment of the invention, a method is employed that is directed to forming a resonant reflector on an optoelectronic device, such as a semiconductor laser for example. The exemplary method involves depositing a first material layer on the top layer of the optoelectronic device, where the first material layer having a refractive index and a thickness of about an odd multiple of a quarter of a wavelength to which the optoelectronic device is tuned. Next, a patterned region is created that extends at least partially into the first material layer. Selected patterned regions are then at least partially filled with a second material that has a refractive index that is greater than the refractive index of the first material layer. Finally, a third layer, having a refractive index greater than the refractive index of the first material layer, is deposited immediately adjacent the first material layer. In this way, a resonant reflector for facilitating mode control is formed on a device such as a semiconductor laser without necessitating significant additional processing and manufacturing steps. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       Other objects of the present invention and many of the attendant advantages of the present invention will be readily appreciated as the same becomes better understood by reference to the following detailed description when considered in connection with the accompanying drawings, in which like reference numerals designate like parts throughout the figures thereof and wherein: 
         FIG. 1  is a schematic diagram of a planar, current-guided, GaAs/AlGaAs top surface emitting vertical cavity laser in accordance with the prior art; 
         FIG. 2  is a schematic cross-sectional side view of a planar, current-guided, GaAs/AlGaAs top surface emitting vertical cavity laser with a first illustrative resonant reflector for increased mode control in accordance with the present invention; 
         FIGS. 3A-3D  are schematic cross-sectional side views showing an illustrative method for making the resonant reflector of  FIG. 2 ; 
         FIG. 4  is a schematic cross-sectional side view of a planar, current-guided, GaAs/AlGaAs top surface emitting vertical cavity laser with a second illustrative resonant reflector for increased mode control in accordance with the present invention; 
         FIGS. 5A-5D  are schematic cross-sectional side views showing an illustrative method for making the resonant reflector of  FIG. 4 ; 
         FIG. 6  is a schematic cross-sectional side view of a planar, current-guided, GaAs/AlGaAs top surface emitting vertical cavity laser with yet another illustrative resonant reflector for increased mode control in accordance with the present invention; 
         FIGS. 7A-7D  are schematic cross-sectional side views showing a first illustrative method for making the resonant reflector of  FIG. 6 ; 
         FIGS. 8A-8E  are schematic cross-sectional side views showing another illustrative method for making the resonant reflector of  FIG. 6 ; 
         FIGS. 9A-9E  are schematic cross-sectional side views showing yet another illustrative method for making the resonant reflector of  FIG. 6 ; 
         FIG. 10  is a schematic cross-sectional side view showing another illustrative embodiment of the present invention similar to that shown in  FIGS. 5A-5D ; 
         FIG. 11  is a schematic cross-sectional side view showing another illustrative embodiment of the present invention similar to that shown in  FIGS. 5A-5D ; and 
         FIG. 12  is a schematic cross-sectional side view showing another illustrative embodiment of the present invention similar to that shown in  FIGS. 8A-8E . 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
       FIG. 1  is a schematic illustration of a planar, current-guided, GaAs/AlGaAs top surface emitting vertical cavity laser  10  in accordance with the prior art. Formed on an n-doped gallium arsenide (GaAs) substrate  14  is an n-contact  12 . Substrate  14  is doped with impurities of a first type (i.e., n type). An n-type mirror stack  16  is formed on substrate  14 . Formed on stack  16  is a spacer  18 . Spacer  18  has a bottom confinement layer  20  and a top confinement layer  24  surrounding active region  22 . A p-type mirror stack  26  is formed on top confinement layer  24 . A p-metal layer  28  is formed on stack  26 . The emission region may have a passivation layer  30 . 
     Isolation region  29  restricts the area of the current flow  27  through the active region. Region  29  may be formed by deep H+ ion implantation. The diameter “g” may be set to provide the desired active area, and thus the gain aperture of the VCSEL  10 . Further, the diameter “g” may be set by the desired resistance of the p-type mirror stack  26 , particularly through the non-conductive region  29 . Thus, non-conductive region  29  performs the gain guiding function. The diameter “g” is typically limited by fabrication limitations, such as lateral straggle during the implantation step. 
     Spacer  18  may contain a bulk or quantum-well active region disposed between mirror stacks  16  and  26 . Quantum-well active region  22  may have alternating layers of aluminum gallium arsenide (AlGaAs) barrier layers and GaAs quantum-well layers. InGaAs quantum wells may also be used in the active region, particularly where an emission wavelength (e.g. λ=980 nm) is desired where GaAs is transparent. Stacks  16  and  26  are distributed Bragg reflector (DBR) stacks, and may include periodic layers of doped AlGaAs and aluminum arsenide (AlAs). The AlGaAs of stack  16  is doped with the same type of impurity as substrate  14  (e.g., n type), and the AlGaAs of stack  26  is doped with the other kind of impurity (e.g., p type). 
     Metal contact layers  12  and  28  are ohmic contacts that allow appropriate electrical biasing of laser diode  10 . When laser diode  10  is forward biased with a more positive voltage on contact  28  than on contact  12 , active region  22  emits light  31  which passes through stack  26 . 
     Most VCSELs of practical dimensions are inherently multi (transverse) mode. Single lowest-order mode VCSELs are favored for coupling into single-mode fibers, and are advantageous for free-space and/or wavelength sensitive systems, and may even be beneficial for use in extending the bandwidth-length product of standard 50 μm and 62.5 μm GRIN multi-mode fiber. However, it has long been known that, although the short optical cavity (2 λ) of the VCSEL favors single longitudinal mode emission, the multi-wavelength (10 λ) lateral dimensions facilitate multi-transverse mode operation. 
     As indicated above, higher order modes typically have a greater lateral concentration of energy away from the center of the optical or lasing cavity. Thus, the most obvious way to force the laser to oscillate in only a lowest order circularly symmetric mode is to make the lateral dimension “g” of the active area small enough to prevent higher-order modes from reaching threshold. However, this necessitates lateral dimensions of less than about 5 μm for typical VCSELs. Such small areas may result in excessive resistance, and push the limits obtainable from conventional fabrication methodologies. This is particularly true for implantation depths of greater than about 1 μm, where lateral straggle may become a limiting factor. Thus, control of transverse modes remains difficult for VCSEL&#39;s of practical dimensions. 
     One illustrative approach for controlling transverse modes of an optoelectronic device is shown in  FIG. 2 .  FIG. 2  is a schematic cross-sectional side view of a planar, current-guided, GaAs/AlGaAs top surface emitting vertical cavity laser, as in  FIG. 1 , with a top mounted mode control resonant reflector  50 . The resonant reflector  50  includes a patterned first material layer  56  that is positioned on top of the top mirror layer  52  of VCSEL  54 . A second material layer  58  is provided over the patterned first material layer  56 , as shown. 
     The first material layer  56  preferably has a refractive index that is greater than the refractive index of the second material layer  58 , and the first and second material layers  56  and  58  preferably have a refractive index that is less than the refractive index of the top mirror layer  52  of the optoelectronic device  54 . In one example, the first material layer  56  is SiO 2 , the second material layer  58  is Si 3 N 4  or TiO 2 , and the top mirror layer  52  is AlGaAs, although other suitable material systems are contemplated. Each layer is preferably an even multiple of one-quarter wavelength (λ/4) thick. This causes a reduction in reflectivity of the resonant reflector  50  in those regions that correspond to the etched regions  60  (see  FIG. 3B ) in the first material layer  56 , that is, those regions that are filled with the second material layer  58 . By designing the etched regions to circumscribe the desired optical cavity, this difference in reflectivity can be used to help provide mode control for VCSEL  54 . 
     In forming the resonant reflector  50 , and referring now to  FIG. 3A , the first material layer  56  is provided over the top mirror layer  52 . As shown in  FIG. 3B , the first material layer  56  is patterned, preferably by etching away the first material layer  56  in the region or regions circumscribing the desired optical cavity of the VCSEL  54 . As shown in  FIG. 3C , a second material layer  58  is provided over the first material layer  56 . The second material layer  58  is preferably provided over both the etched  60  and non-etched regions of the first material layer  56 , but may be confined to the non-etched regions if desired. Selected regions, such as regions  62   a  and  62   b  of the second material layer  58  may then be removed to provide access to the top mirror layer  52 . Then, and as shown in  FIG. 3D , a contact layer  64  may be provided on the exposed regions of the top mirror layer  52 . The contact layer  64  may provide electrical contact to the top mirror layer  52 . 
     In a related embodiment, a top mirror layer of the optoelectronic device may function as the first material layer  56  discussed above. Thus, the top mirror layer may be patterned, preferably by etching at least partially into the top mirror layer in the region or regions circumscribing the desired optical cavity of the optoelectronic device. In one embodiment, the layer  52  below the top mirror layer may function as an etch stop layer. Then, a second material layer  58  is provided over the top mirror layer. The second material layer is preferably provided over both the etched and non-etched regions of the top mirror layer, but may only be provided over the non-etched regions, if desired. In this embodiment, the regions labeled  56  in  FIGS. 2-3  should have the same cross-hatch pattern as layer  53 , and the refractive index of these regions should be less than the refractive index of layer  52 . 
     Another illustrative approach for controlling transverse modes of an optoelectronic device is shown in  FIG. 4 .  FIG. 4  is a schematic cross-sectional side view of a planar, current-guided, GaAs/AlGaAs top surface emitting vertical cavity laser, as in  FIG. 1 , with another illustrative top mounted mode control resonant reflector  70 . In this embodiment, the resonant reflector  70  is formed by etching down into but not all the way through one or more of the top mirror layers  72  of the optoelectronic device. The etched region, generally shown at  74 , preferably circumscribes the desired optical cavity of the optoelectronic device, and has a depth that causes a phase shift that reduces the reflectivity of the resonant reflector  70  at the desired operating wavelength, such as a depth that corresponds to an odd multiple of λ4. To provide further differentiation, a cap mirror  76  having one or more additional layers may be provided on selected non-patterned regions  78  of the top mirror layer  72 , such as over the desired optical cavity of the optoelectronic device. The cap mirror  70  may include one or more periods of a conventional semiconductor DBR mirror, or more preferably, a narrow band dielectric reflection filter. A metal layer may be provided on selected regions of the top mirror layer  72 . The metal layer may function as a top contact layer. 
     In forming the resonant reflector  70 , and referring now to  FIGS. 5A-5B , a top mirror layer  72  (or some other top layer) is patterned and etched to form one or more etched regions  74 . The etched regions  74  are preferably formed to circumscribe the desired optical cavity of the optoelectronic device. Also, the etched regions  74  are preferably etched down to a depth that causes a phase shift in the incident light, thereby reducing the reflectivity of the resonant reflector  70  in those regions. 
     Next, and as shown in  FIG. 5C , a cap mirror  76  is formed on the patterned top mirror layer  72 . As indicated above, the top mirror layer  72  may include one or more semiconductor DBR mirror periods, and/or a narrow band dielectric reflection filter. In either case, and to provide further differentiation in reflectivity, the cap mirror  76  may be etched away except on those areas that correspond to the desired optical cavity of the optoelectronic device. This is shown in  FIG. 5D . Alternatively, the patterned cap mirror  76  may be formed using well known lift-off techniques. Thereafter, a contact layer  80  may be provided on the selected regions of the top mirror layer  72 . The contact layer  80  may provide electrical contact to the top mirror layer  72 . 
     Another illustrative approach for controlling transverse modes of an optoelectronic device is shown in  FIG. 6 .  FIG. 6  is a schematic cross-sectional side view of a planar, current-guided, GaAs/AlGaAs top surface emitting vertical cavity laser, as in  FIG. 1 , with yet another illustrative top mounted mode control resonant reflector  90 . In this illustrative embodiment, the resonant reflector  90  has a refractive index that does not abruptly change across the optical cavity of the optoelectronic device. 
     In a preferred embodiment, the resonant reflector  90  has at least one resonant reflector layer  92  that has a refractive index. The refractive index may include, for example, contributions from both a first material  94  having a first refractive index and a second material  96  having a second refractive index. In a preferred embodiment, the first material  94  is confined to a first region and the second material is confined to a second region, wherein the first region and the second region co-extend along an interface  98 . By making the interface  98  non-parallel with the optical axis  100  of the optoelectronic device, the refractive index of the resonant reflector layer, at least when viewed laterally along the optical cavity of the optoelectronic device, does not change abruptly across the optical cavity. Rather, there is a smooth transition from one refractive index to another. This reduces the diffraction effects caused by abrupt changes in the refraction index. It is contemplated that one or more periods of a conventional semiconductor DBR mirror, or more preferably, a narrow band dielectric reflection filter  106 , may be provided on top of the resonant reflector  90 , as shown. Finally, a contact layer  102  may be provided around the periphery of the optical cavity. In the embodiment shown, the contact layer  102  is in direct contact with the top mirror layer  104  and provides electrical contact to the top mirror layer  104 . 
     The smooth transition from one refractive index to another is further illustrated in chart  110 . The X axis of chart  110  represents the lateral position along the optical cavity of the device shown above the chart. The Y axis of chart  110  corresponds to the reflectivity “R” of the top mirror, including the resonant reflector  90  and conventional semiconductor DBR mirror or dielectric reflection filter  106 . The reflectivity “R” of the top mirror, at least in the region of the resonant reflector  90 , is dependent on the refractive index of the resonant reflector layer  90 . 
     Traveling from left to right laterally along the optical cavity of the optoelectronic device, the reflectivity starts at a first value  112 . The first value  112  is shown relatively low because the resonant reflector  90  and the conventional semiconductor DBR mirror or dielectric reflection filter  106  do not extend out into this region. The contact layer  102  may also decrease the reflectivity in this region. 
     At the edge of the resonant reflector  90 , the reflectivity increases to a value  114 , which includes contributions from the second material  96  of the resonant reflector layer  90  and the conventional semiconductor DBR mirror or dielectric reflection filter  106 . Moving further to the right, the refractive index of the resonant reflector  90  begins to change due to the relative contributions of the first material  94  and the second material  96 . This causes the reflectivity of the resonant reflector  90  to smoothly increase toward the center of the desired optical cavity, as shown. Preferably, the reflectivity of the resonant reflector  90  reaches a maximum  116  in or around the center of the desired optical cavity. The reflectivity of the resonant reflector  90  then decreases to the right of the center of the desired optical cavity, in an opposite manner to that described above. As can be seen, the refractive index, and thus the reflectivity, of the resonant reflector  90  does not change abruptly across the optical cavity. Rather, there is a smooth transition from one refractive index to another. This reduces the diffraction effects often caused by abrupt changes in the refraction index of a resonant reflector. 
       FIGS. 7A-7D  are schematic cross-sectional side views showing a first illustrative method for making the resonant reflector of  FIG. 6 . In this illustrative embodiment, a first substantially planar layer of material  94  is provided on, for example, a top mirror layer  104  of a conventional DBR mirror. The top mirror layer  104  preferably has a refractive index that is higher than the refractive index of the first layer of material  94 . The top mirror layer  104  may be, for example, AlGaAs, and the first layer of material  94  may be, for example, TiO2, Si3N4, or a polymer such as polyamide or Benzocyclobuthene (BCB). 
     The first layer of material is then patterned, as shown in  FIG. 7A . This is typically done using a conventional etch process. As shown in  FIG. 7B , the patterned first layer of material  104  is then heated, which causes it to reflow. This results in a non-planar top surface  98 . Then, and as shown in  FIG. 7C , a second layer of material  96  is provided over the first layer of material  94 . The top surface  105  of the second layer of material  96  is preferably substantially planar, but it may be non-planar if desired. The second layer of material  96  preferably has a refractive index that is lower than the refractive index of the first layer of material  94 . The second layer of material  96  may be, for example, SiO2, a polymer, or any other suitable material. When desired, the top surface  105  of the second layer of material  96  may be planarized using any suitable method including, for example, reflowing the second layer of material  96 , mechanical, chemical or chemical-mechanical polishing (CMP) the second layer of material  96 , etc. In some embodiments, the top surface  105  is left non-planar. 
     The second layer of material  96  is preferably provided over the entire top surface of the resonant reflector, and etched away in those regions where a top contact  102  is desired. Once the second layer of material  96  is etched, a contact layer  102  is provided on the exposed regions of the top mirror layer  104 . The contact layer  102  provides electrical contact to the top mirror layer  104 . As shown in  FIG. 7D , a cap mirror  106  may then be provided above the second layer of material  96 . The cap mirror  106  may include one or more periods of a conventional semiconductor DBR mirror, or more preferably, a narrow band dielectric reflection filter. 
       FIGS. 8A-8E  are schematic cross-sectional side views showing another illustrative method for making the resonant reflector of  FIG. 6 . In this illustrative embodiment, and as shown in  FIG. 8A , a first substantially planar layer of material  94  is provided on, for example, a top mirror layer  104  of a conventional DBR mirror. The top mirror layer  104  preferably has a refractive index that is higher than the refractive index of the first layer of material  94 . The top mirror layer  104  may be, for example, AlGaAs, and the first layer of material  94  may be, for example, TiO2, Si3N4, or any other suitable material. Next, a photoresist layer  110  is provided and patterned on the first layer of material  94 , preferably forming an island of photoresist above the desired optical cavity of the optoelectronic device. 
     As shown in  FIG. 8B , the photoresist layer  110  is then heated, causing it to reflow. This results in a non-planar top surface on the photoresist layer  110 . That is, the top surface of the photoresist layer  110  may have portions that taper down toward the first layer of material  94 . Next, and as shown in  FIG. 8C , the photoresist layer  110  and the first layer of material  94  are etched for a specified period of time. The etchant preferably selectively etches both the photoresist layer  110  and the first layer of material  94 . This transfers the shape of the non-planar top surface of the photoresist layer  110  to the first layer of material  94 . 
     As shown in  FIG. 8D , a second layer of material  96  is then provided over the first layer of material  94 . The second layer of material  96  preferably has a refractive index that is less than the refractive index of the first layer of material  94 . The second layer of material  96  is preferably provided over the entire top surface of the resonant reflector, and etched away in those regions where a top contact  102  is desired. Once the second layer of material  96  is etched, a contact layer  102  is provided on the exposed regions of the top mirror layer  104 . The contact layer  102  provides electrical contact to the top mirror layer  104 . Preferably, the top surface of the second layer of material  96  is substantially planar. As shown in  FIG. 8E , a cap mirror  106  may be provided above the second layer of material  96 , if desired. The cap mirror  106  may include one or more periods of a conventional semiconductor DBR mirror, or more preferably, a narrow band dielectric reflection filter. 
       FIGS. 9A-9E  are schematic cross-sectional side views showing yet another illustrative method for making the resonant reflector of  FIG. 6 . In this illustrative embodiment, and as shown in  FIG. 9A , a first substantially planar layer of material  94  is provided on, for example, a top mirror layer  104  of a conventional DBR mirror. Like above, the top mirror layer  104  preferably has a refractive index that is higher than the refractive index of the first layer of material  94 . 
     Next, and as shown in  FIG. 9B , the first layer of material  94  is patterned, preferably forming an island above the desired optical cavity of the optoelectronic device. This results in the first layer of material  94  having lateral surfaces  118  that extend up to a top surface  116  that is defined by top peripheral edges  120 . A photoresist layer  114  is then provided over the patterned first layer of material  94 , including over the lateral surfaces  118 , the top peripheral edges  120  and the top surface  116 . Because of the step between the top surface  116  and the lateral surfaces  118 , the photoresist layer  114  is thinner near the top peripheral edges  120  than along either the lateral surfaces  118  or top surface  116  of the patterned first layer of material  94 , as shown. 
     As shown in  FIG. 9C , the photoresist layer  114  and the first layer of material  94  are then etched for a specified period of time. During this etch step, those regions of the first layer of material  94  that are adjacent the thinner regions of the photoresist layer  114  are subject to the etchant for a longer period of time than those regions that are adjacent thicker regions of the photoresist layer  114 . Thus, and as shown in  FIG. 9C , the top peripheral edges  120  of the first layer of material  94  are etched more than those regions away from the top peripheral edges  120 , resulting in tapered edges  122 . 
     After the etching step, and as shown in  FIG. 9D , a second layer of material  96  may be provided over the first layer of material  94 . Like above, the second layer of material  96  preferably has a refractive index that is less than the refractive index of the first layer of material  94 . The second layer of material  96  is preferably provided over the entire top surface of the resonant reflector, and etched away in those regions where a top contact  102  is desired. Once the second layer of material  96  is etched, a contact layer  102  is provided on the exposed regions of the top mirror layer  104 . The contact layer  102  provides electrical contact to the top mirror layer  104 . Preferably, the top surface of the second layer of material  96  is substantially planar. 
     As shown in  FIG. 9E , a cap mirror  106  may be provided above the second layer of material  96 , if desired. The cap mirror  106  may include one or more periods of a conventional semiconductor DBR mirror, or more preferably, a narrow band dielectric reflection filter. 
       FIG. 10  is a schematic cross-sectional side view showing another illustrative embodiment of the present invention similar to that shown in  FIGS. 5A-5D . In this embodiment, a top layer  110 , which may be the top mirror layer of the top DBR mirror stack of the optoelectronic device or an additional layer on top of the top mirror layer, may be etched down—but not all the way through. The etched region preferably circumscribes the desired optical cavity of the optoelectronic device, and has a depth that causes a phase shift that reduces the reflectivity of the resonant reflector at the desired operating wavelength, such as a depth that corresponds to an odd multiple of λ4. Like in  FIGS. 5A-5D , a cap mirror  112  having one or more additional layers may be provided on selected non-patterned regions of layer  110 , such as over the desired optical cavity of the optoelectronic device, to provide further differentiation in reflectivity. A metal layer  114  may then be provided on the etched region of layer  110 . The metal layer may function as the top contact. By extending the metal layer  114  all the way or near the cap mirror  112 , better current spreading can be achieved for the optoelectronic device. 
       FIG. 11  is a schematic cross-sectional side view showing another illustrative embodiment of the present invention similar to that shown in  FIGS. 5A-5D . This embodiment is similar to that of  FIG. 10 , but the metal layer, now labeled  116 , extends over the etched region of layer  110  and over the cap mirror  112 . For back illumination devices, this may provide even better current spreading for the optoelectronic device. 
       FIG. 12  is a schematic cross-sectional side view showing another illustrative embodiment of the present invention similar to that shown in  FIGS. 8A-8E . A resonant reflector is provided that has a refractive index that does not change abruptly across the optical cavity of the optoelectronic device. The illustrative resonant reflector includes at least one resonant reflector layer that has a refractive index that includes contributions from, for example, both a first material  120  having a first refractive index and a second material  122  having a second refractive index. In the embodiment shown, the first material  120  is confined to a first region and the second material  122  is confined to a second region, wherein the first region and the second region co-extend along an interface. A metal layer  124  is then provided over the entire structure. For back illumination devices, the metal layer  124  may provide enhanced current spreading when compared to the device shown in  FIGS. 8A-8E . 
     Having thus described the preferred embodiments of the present invention, those of skill in the art will readily appreciate that the teachings found herein may be applied to yet other embodiments within the scope of the claims hereto attached.