Light-emitting device

Abstract

A light-emitting device comprising VCSELs formed in a die. The VCSEL distribution is characterized by an essentially linear decrease in VCSEL density over the die from a highest VCSEL density in a first die region to a lowest VCSEL density in another die region. The VCSELs share a common anode and a common cathode for collective switching of the plurality of VCSELs. A method of manufacturing such a VCSEL die is also described.

Claims

1. A light-emitting device comprising: a die; an anode on the die; a cathode on the die; and a plurality of vertical cavity surface-emitting lasers (VCSELs) in the die in a VCSEL distribution with a VCSEL density of the plurality of VCSELs over the die essentially linearly decreasing from a highest VCSEL density in a first die region to a lowest VCSEL density in a second die region and a diameter of the plurality of VCSELs essentially linearly increases from the first die region having the highest VCSEL density to the second die region having the lowest VCSEL density, each of the plurality of VCSELs are electrically coupled to the anode and the cathode.

2. The light-emitting device according to claim 1, wherein the first die region and the second die region are at opposite ends of the die.

3. The light-emitting device according to claim 1, wherein the first die region and the second die region are essentially annular in shape.

4. The light-emitting device according to claim 3, wherein the first die region is along edges of the die and the second die region is in a center of the die.

5. The light-emitting device according to claim 3, wherein the first die region is in a center of the die and the second die region is along edges of the die.

6. The light-emitting device according to claim 1, wherein the VCSEL density in the first die region is at least 600 VCSELs per mm.sup.2.

7. The light-emitting device according to claim 1, wherein the VCSEL density in the first die region is at least 1000 VCSELs per mm.sup.2.

8. The light-emitting device according to claim 1, wherein the VCSEL density in the first die region is at least 1,500 VCSELs per mm.sup.2.

9. The light-emitting device according to claim 1, wherein the VCSEL density in the second die region is at most 400 VCSELs per mm.sup.2.

10. The light-emitting device according to claim 1, wherein the VCSEL density in the second die region is at most 200 VCSELs per mm.sup.2.

11. The light-emitting device according to claim 1, wherein the VCSEL density in the second die region is at most 100 VCSELs per mm.sup.2.

12. The light-emitting device according to claim 1, wherein the diameter of VCSELs of the plurality of VCSELs in the first die region is in a range of 6 μm-8 μm.

13. The light-emitting device according to claim 1, wherein the diameter of VCSELs of the plurality of VCSELs in the second die region is in a range of 10 μm-15 μm.

14. The light-emitting device according to claim 1, wherein the plurality of VCSELs are arranged in the die in an essentially regular arrangement.

15. The light-emitting device according to claim 14, wherein the essentially regular arrangement is an essentially hexagonal array.

16. The light-emitting device according to claim 1, wherein the die comprises: a first distributed Bragg reflector mirror, a second distributed Bragg reflector mirror, and an active region between the first distributed Bragg reflector mirror and the second distributed Bragg reflector mirror.

17. A method of manufacturing a vertical cavity surface-emitting laser (VCSEL) die, the method comprising: allocating a first region of the VCSEL die to a highest VCSEL density; allocating a second region of the VCSEL die to a lowest VCSEL density; forming a plurality of VCSELs over the VCSEL die with an essentially linear decrease in VCSEL density from the first region to the second region and an essentially linear increase in diameter of the plurality of VCSELs from the first region having the highest VCSEL density to the second region having the lowest VCSEL density; and electrically coupling the plurality of VCSELs to a common cathode and a common anode.

18. The method according to claim 17, wherein the forming the plurality of VCSELs comprises depositing a plurality of metal contacts shaped to define an aperture on an emission face of the VCSEL die.

19. The method according to claim 17, wherein the forming the plurality of VCSELs further comprises forming respective VCSELs of the plurality of VCSELs in the first region with a density of at most 40 μm.

20. The method according to claim 17, wherein the forming the plurality of VCSELs further comprises forming respective VCSELs of the plurality of VCSELs in the second region with a density of at least 50 μm.

Description

BRIEF DESCRIPTION OF THE DRAWING(S)

(1) FIG. 1 shows an embodiment of the inventive VCSEL device;

(2) FIG. 2 shows the VCSEL density distribution in the embodiment of FIG. 1;

(3) FIG. 3 shows alternative VCSEL density distributions;

(4) FIG. 4 shows junction temperature in the embodiment of FIG. 1;

(5) FIG. 5 shows peak wavelength shift in the embodiment of FIG. 1;

(6) FIG. 6 shows a further embodiment of the inventive VCSEL device;

(7) FIG. 7 shows a cross-section through an embodiment of a VCSEL device;

(8) FIG. 8 shows a prior art VCSEL device.

(9) In the drawings, like numbers refer to like objects throughout. Objects in the diagrams are not necessarily drawn to scale.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT(S)

(10) FIG. 1 shows an embodiment of the inventive VCSEL device 1. The VCSEL device 1 (or “VCSEL array”) is realised as a die 100 with multiple VCSELs 10 formed at specific positions, and a wire bond bad 108 to one side. Each VCSEL 10 is identified by the corresponding aperture A.sub.10 in the electrode 101 deposited on the emission face of the die 100. The electrode 101 in this embodiment is a common anode 101, and a common cathode may be formed on the underside of the die 1. The dimensions of this exemplary die 100 can be 1.0×1.0 mm, and each aperture A.sub.10 can have a diameter as small as 10 μm. The diagram can only show a relatively small number of VCSELs 10, and it shall be understood that the array may comprise hundreds of VCSELs, possibly also in excess of 1,000 VCSELs.

(11) Instead of a regular distribution of VCSELs over the surface of the die 100, the approach taken by the invention is to form the VCSELs 10 in regions of different densities. This exemplary embodiment illustrates a high-density region R.sub.hi on the left, and a low-density region R.sub.lo on the right. In the high-density region R.sub.hi, the VCSELs 10 are arranged in a very dense configuration, indicated by the dense arrangement of apertures A.sub.10. In the low-density region R.sub.lo, the VCSELs 10 are distributed in a sparse configuration.

(12) The density of the VCSELs 10 gradually reduces with increasing distance from high-density region towards the low-density region. For example, VCSELs 10 can be distributed with a density D.sub.hi of 1,500 VCSELs per mm.sup.2 in the high-density region R.sub.hi, decreasing gradually towards a density D.sub.lo of only 300 VCSELs per mm.sup.2 in the low-density region R.sub.lo. Therefore, the smallest pitch P.sub.hi (i.e. distance between adjacent VCSELs) is found in the high-density region R.sub.hi, and the largest pitch P.sub.lo is found in the low-density region R.sub.lo. It shall be noted that the decrease in density applies to both the X-axis and the Y-axis, so that the maximum and minimum pitch P.sub.hi, P.sub.lo apply in both vertical and horizontal directions in this diagram.

(13) FIG. 2 shows a graph D1 of density D against distance along the die 100 for the embodiment of FIG. 1, showing that the rate of reduction in VCSEL density is linear.

(14) FIG. 3 shows a graph D2 of density D against distance along the die for a further exemplary embodiment in which the VCSELs are formed in the die such that the rate of reduction in VCSEL density is not clearly linear.

(15) FIG. 4 shows a corresponding graph of junction temperature T.sub.junction against distance along the die. This can be inferred from knowledge of the development of junction temperature in VCSELs of the type used in the array 1. In the closely-packed region R.sub.hi, there is limited space for heat to dissipate from the individual VCSELs 10, each one also being heated by its close neighbours. With decreasing density towards the sparsely-packed-packed region R.sub.lo, heat is better able to dissipate from the individual VCSELs 10, so that they have less effect on the junction temperatures of their neighbours. While the various junction temperatures cannot be observed directly, the large difference in junction temperatures in the closely-packed region and the sparsely-packed region may be observable as a slight temperature gradient on the bottom side of the die (a VCSEL array die with a uniform distribution of VCSELs would have an essentially even temperature spread across its underside).

(16) At the side of the die with highest VCSEL density R.sub.hi, the junction temperature T.sub.junction will also be highest during operation of the device. At the side of the die with lowest VCSEL density R.sub.lo, the junction temperature T.sub.junction will also be lowest. As explained above, the peak wavelength of the light emitted by a VCSEL will increase (i.e. shift towards longer wavelengths) with increasing junction temperature. FIG. 5 shows a graph of peak wavelength λ.sub.peak against distance along the die. The diagram shows that the VCSELs that are most closely packed in the high-density region R.sub.hi will also emit light at longer wavelengths than the less densely-packed VCSELs in the low-density region R.sub.lo. The peak wavelength λ.sub.max of the light emitted by the more closely-packed VCSELs in the high-density region R.sub.hi can be shifted by up to 5 nm relative to the peak wavelength λ.sub.min of the light emitted by the loosely-packed VCSELs in the low-density region R.sub.lo. The densities of the VCSELs in the die regions R.sub.hi, R.sub.lo and the relative sizes of the die regions can be chosen to achieve a desired peak wavelength shift Δλ.

(17) FIG. 6 shows an alternative embodiment of the inventive VCSEL device 1, in which the VCSEL density increases towards the centre of the die 100. Of course, the opposite arrangement is also possible, in which the VCSEL density decreases towards the centre of the die 100.

(18) FIG. 7 shows a cross-section through an embodiment of a VCSEL device 1 and illustrates the structure of a VCSEL 10 in one region of the die 100. The VCSEL device 1 essentially comprises an active layer 103 sandwiched between two DBR layers 102, 104. These layers 102, 103, 104 are formed on a substrate 105, and an electrode 106 is applied onto the base of the substrate 105. The upper DBR 102 includes an implanted region 107 shaped to leave a channel that defines a path for electric current, as indicated by the arrows. An electrode 101 is applied to the top of the upper DBR 102 and shaped to leave an aperture A.sub.10 above the channel in the implanted region 107. During operation of the device 1, light L from this VCSEL 10 is emitted through the aperture A.sub.10.

(19) In this embodiment, the upper electrode 101 is the anode, and is part of the common anode of the VCSEL array. The lower electrode 106 is the cathode, which is part of the common cathode of the VCSEL array. The order of the layers is as follows: p-DBR 102, active layer 103, n-DBR 104, n-substrate 105.

(20) As described above, a prior art VCSEL device 8 as shown in FIG. 8 generally has a uniform array of VCSELs 80 over a die 800, for example a regular hexagonal arrangement of VCSELs 80 with a single pitch P.sub.80 defining the distance between adjacent VCSELs 80. Because of the regular arrangement of closely-packed VCSELs, the junction temperature is essentially the same for all VCSELs 80. To counteract the speckle pattern that will result from this device 8, additional measures must be taken, for example to arrange a diffuser above the emission face of the VCSEL array.

(21) Although the present invention has been disclosed in the form of preferred embodiments and variations thereon, it will be understood that numerous additional modifications and variations could be made thereto without departing from the scope of the invention.

(22) For the sake of clarity, it is to be understood that the use of “a” or “an” throughout this application does not exclude a plurality, and “comprising” does not exclude other steps or elements.

REFERENCE SIGNS

(23) TABLE-US-00001 VCSEL device 1 VCSELs 10 die 100 anode 101 upper DBR 102 active layer 103 lower DBR 104 substrate 105 cathode 106 implanted region 107 wire bond pad 108 aperture A light L high-density region R.sub.hi low-density region R.sub.lo smallest pitch P.sub.hi largest pitch P.sub.lo density D highest density D.sub.hi lowest density D.sub.lo junction temperature T.sub.junction highest temperature T.sub.hi lowest temperature T.sub.lo peak wavelength λ.sub.peak max peak wavelength λ.sub.max min peak wavelength λ.sub.min desired peak wavelength shift Δλ prior art device 8 VCSEL 80 uniform pitch P.sub.80 die 800