Abstract:
A top surface emitting vertical cavity laser with enhanced power handling capacity. The laser provides a patterned top electrode for uniformly injecting applied current to the active area. The diameter of the electrode relative to the vertical distance between it and the active layer is reduced by patterning. The result is an improved laser with significant power output.

Description:
FIELD OF THE INVENTION 
     The present invention relates to a vertical cavity surface emitting laser and more particularly, the present invention relates to a top emitting laser with top electrode current injection having an improved optical mode profile and power handling. 
     BACKGROUND OF THE INVENTION 
     Generally speaking, the known laser designs used so far have injected the current by means of a ring contact. However, the diameter of the device is normally larger than the vertical distance from the electrode to the active, light-emitting area. This limits the current spreading and since lasing occurs preferentially around the circumference of the device, several limitations are realized including: 
     i) design complications in high power lasers by increasing the area, since the maximum output power is proportional to the circumference and not to the area; 
     ii) coupling difficulties with the output of the light into the optical fibre since the fibre is emitted from a ring pattern along the circumference; 
     III) low efficiency since lasing takes place close to the current blocking region which surrounds the active volume. This area, whether oxidized, implanted, or designed in other ways, has a higher degree of defects; this leads to low quantum efficiency; 
     iv) high resistance of the device; 
     v) a mode pattern which is uncontrolled and varies with the forward current and between individual devices; 
     vi) high variation of output power between different devices; and 
     vii) a process which requires high precision when defining the electrode pattern. 
     In the art, current designs position the ring contact outside the emitting area and include a current blocking layer to force the current into the active volume. The current blocking layer may be ion implantation as taught in, for example, U.S. Pat. No. 5,115,442, issued May 19, 1992 to Lee et al. or by oxidation as set forth in U.S. Pat. No. 5,262,360, issued Nov. 16, 1993, to Holonyak, Jr. et al. More frequently employed is a ring electrode partly covering the active, light-emitting area. These designs may be combined with a layer(s) of graded resistivity with varying high and low resistivity to improve the current spreading. Current spreading is described in U.S. Pat. No. 5,343,487, issued Aug. 30, 1994, to Scott et al. 
     In view of the limitations of existing methodology, there exists a need for greater efficiency in top emitting laser power output. The present invention covers this need and provides a solution as well as a way to control the optical mode profile. 
     SUMMARY OF THE INVENTION 
     One object of the present invention is to provide an improved VCSEL with enhanced current spreading. 
     In accordance with a further object of the present invention, there is provided a light-emitting device, comprising: 
     a substrate; 
     a first layer of dielectric mirrors; 
     an active layer; 
     a passivated region; 
     a second layer of dielectric mirrors; 
     a bonding pad; and 
     an electrode connected to the pad and in electrical contact with the second layer of dielectric mirrors. 
     The top electrode of a top emitting VCSEL may take any shape which facilitates current injection in the active region. In the example, a circular electrode with fingers which overlie the conducting, active area is provided, however, it will be appreciated by those skilled that variations achieving the same result, may be used. In terms of variations, the electrode may comprise an arrangement with any number or shape of the fingers toward the center of the device to improve current spreading. The electrode may take a pattern which improves the mode pattern in the device. Further, the electrode may be unsymmetrically, for example, striped to improve the polarization of the device. 
     Other variations in the design of the device include positioning an inner ring coaxially with the electrode connected to the bond pad. It has also been found that the inventive concept set forth herein may be applied to non-circular forms for the emitting area. 
     The design of the top contact can be made to enhance polarization to get a higher degree of polarization on the emitted light. The design can be used for any electro optical device, i.e. micro cavity light-emitting diodes (MCLED&#39;s or RECLED&#39;s). Regarding fabrication, the contacts are normally defined by photolithography, on the top surface; designs using other production methods like electron beam lithography as well as the other suitable techniques may be employed. 
     A further object of the present invention is to provide a method of enhancing the output power of a light-emitting semiconductor, comprising: 
     providing a top emitting semiconductor laser having Bragg mirrors, an active region, a passivated region, substrate and bond pad connected to an electrode whereby applied current is concentrated in the active region for increased output power of the semiconductor. 
     Having thus generally described the invention, reference will now be made to the accompanying drawings illustrating preferred embodiments. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a schematic illustration of a VCSEL chip according to one embodiment of the present invention; 
     FIG. 2 is a top plan view of FIG. 1; 
     FIGS. 3 through 5 are top plan views of further embodiments of the invention; and 
     FIG. 6 is a graphical comparison of the output power for a ring electrode and patterned electrode. 
    
    
     Similar numerals employed in the text denote similar elements. 
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Referring now to the drawings, FIG. 1 illustrates a perspective view of a vertical cavity surface emitting laser (VCSEL), generally denoted by numeral  10 . The substrate of the device is denoted by numeral  12 . The arrangement provides a series of Bragg mirrors  14  adjacent quantum well stack  16 . A further series of Bragg mirrors  18  overlie wells  16 . An active region  20  is surrounded by a passivated area  22 . Generally, these elements are known in the art. 
     A metal bond pad  24  is positioned on the top of the device  10 . Mounted to the pad  24  is a patterned electrode  26 , shown in the example to include a semicircular member with projections or fingers  28 . The fingers are in equidistant spaced relation and are directed inwardly toward the active area  20 . The patterned electrode  26  has been found to be particularly effective for spreading current in comparison to VCSEL arrangements known in the art. 
     FIG. 2 illustrates a top view of the VCSEL chip  10 . A passivated area  22  surrounds the circular active area  20 . The active area is electrically connected to the bond pad  24  by means of the electrode  26  and the fingers  28 . 
     FIGS. 3 through 5 illustrate further variations in the formation of the electrode. 
     In the embodiment shown in FIG. 3, a quadruplet of electrodes  28  are shown in equidistant and orthogonal relation. Further, each electrode  28  provides a plurality of individual electrodes of different sizes in imbricated disposition and coaxially disposed relative to the active area  28 . 
     With respect to FIG. 4, the projecting electrodes  28  surround the electrode in an angular relationship. 
     FIG. 5 illustrates a further variation. 
     In each instance, significant power enhancement was realized when the patterned electrode was employed in comparison to the ring or annular arrangements known in the art. The results generated are illustrated in FIG.  6 . It is clear that the patterning has a definite effect on the power handling. 
     In operation of the devices, current is injected in a desired way by means of the design of the top electrode. By selecting a design for uniform injection it is possible to make a laser which preferably lases in the central part of the active area. Significant advantages flow from this arrangement, which include: 
     i) an increase in the maximum power with up to 20% for a device with a circular active area with a diameter of 56 um; 
     ii) an increased fibre coupling efficiency since lasing can be made to occur from the central part of the active area; 
     iii) the lasing takes preferably part in the central part of the device far away from the current blocking layers with a high density of defects; 
     iv) low resistance of the device because of an effective current spreading; 
     v) the mode pattern can be controlled, by means of the electrode pattern, either into a stable mode profile or to the central area of the device, or both; 
     vi) low variation in output power between devices; 
     vii) the design can be produced in photolithography without high precision aligning of the electrode mask with the mask that (either directly, or indirectly) causes the current blocking. Aligning two circular masks with almost the same radii (r 1  and r 2 ) is critical since the alignment error should be compared to the difference in radius for the circles (r 2 −r 1 ), any error comparable to r 2 −r 1  (normally around 2-3 microns in top emitting VCSELs) will case a very severe malfunction. Smaller errors also affects also the device properties. For an alignment of the masks of this invention (for example a cross instead of the smaller circle), the alignment error is to be compared with the radius of the circle, r 2 , which, for the devices on the market today, gives an alignment tolerance which is several times larger. 
     Although embodiments of the invention have been described above, it is not limited thereto and it will be apparent to those skilled in the art that numerous modifications form part of the present invention insofar as they do not depart from the spirit, nature and scope of the claimed and described invention.