EDGE EMITTING LASER DIODE AND METHOD FOR PRODUCING SAME

Abstract

The invention relates to an edge emitting laser diode comprising a semiconductor layer stack whose growth direction defines a vertical direction, and wherein the semiconductor layer stack comprises an active layer and a waveguide layer. A thermal stress element is arranged in at least indirect contact with the semiconductor layer stack, the thermal stress element being configured to generate a thermally induced mechanical stress in the waveguide layer that counteracts the formation of a thermal lens.

Claims

1. An edge emitting laser diode comprising: a semiconductor layer stack whose growth direction defines a vertical direction; and wherein the semiconductor layer stack comprises an active layer and a waveguide layer; characterized in that a thermal stress element is arranged in at least indirect contact with the semiconductor layer stack; wherein the thermal stress element is configured for generating a thermally induced mechanical stress in the waveguide layer which counteracts the formation of a thermal lens, and wherein the thermal stress element consists of a material with a thermal expansion coefficient α.sub.th which is smaller than 0.5*10.sup.−6 K.sup.−1 and preferably smaller than 0.25*10.sup.−6 K.sup.−1 in a temperature range from 20° C. to 300° C.

2. The edge emitting laser diode according to claim 1, wherein the thermal stress element is arranged in the vertical direction above the active layer and/or in a lateral direction perpendicular to the vertical direction next to the active layer in or adjacent to the semiconductor layer stack.

3. (canceled)

4. The edge emitting laser diode according to claim 1, wherein the thermal stress element consists of a material with a negative thermal expansion coefficient α.sub.th in the temperature range from 20° C. to 300° C.

5. The edge emitting laser diode according to claim 1, wherein the thermal stress element contains an oxide compound of zirconium and tungsten and preferably ZrW.sub.2O.sub.8.

6. The edge emitting laser diode according to claim 1, wherein the thermal stress element contains aluminum titanate and/or zirconium titanate.

7. The edge emitting laser diode according to claim 1, wherein the thermal stress element is part of a passivation layer of the semiconductor layer stack or is formed by a structured passivation layer.

8. The edge emitting laser diode according to claim 1, wherein the edge emitting laser diode is a broad area laser diode.

9. The edge emitting laser diode according to claim 8, wherein the broad area laser diode comprises an optical resonator having a mirror facet and an exit facet arranged spaced from each other in a longitudinal direction, wherein the longitudinal direction forms an orthogonal tripod with the vertical direction and the lateral direction; and wherein at least in a partial section of the optical resonator the extension of the thermal stress element in the vertical direction and/or in the lateral direction increases with decreasing distance from the exit facet.

10. The edge emitting laser diode according to claim 1, wherein the thermal stress element is arranged such that it adjoins a region of the waveguide for which, during operation of the laser diode, the greatest temperature gradient is present as a function of the lateral direction.

11. The edge emitting laser diode according to claim 1, wherein the edge emitting laser diode is gain-guided or index-guided.

12. A method for manufacturing an edge emitting laser diode, comprising: manufacturing a semiconductor layer stack, wherein the growth direction of the semiconductor layer stack defines a vertical direction; and wherein an active layer and a waveguide layer are provided in the semiconductor layer stack; characterized in that a thermal stress element is arranged in at least indirect contact with the semiconductor layer stack such that the thermal stress element generates a thermally induced mechanical stress in the waveguide layer during operation of the laser diode, which stress counteracts the formation of a thermal lens, and wherein the thermal stress element is made of a material having a coefficient of thermal expansion α.sub.th which is smaller than 0.5*10.sup.−6 K.sup.−1 and preferably smaller than 0.25*10.sup.−6 K.sup.−1 in a temperature range from 20° C. to 300° C.

13. The method for manufacturing a laser diode according to claim 12, wherein the thermal stress element is arranged in the vertical direction above the active layer and/or in a lateral direction perpendicular to the vertical direction next to the active layer in or adjacent to the semiconductor layer stack.

14. (canceled)

15. The method for manufacturing an edge emitting laser diode according to claim 12, wherein the thermal stress element is made of a material having a negative coefficient of thermal expansion α.sub.th in the temperature range from 20° C. to 300° C.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0028] In the following, exemplary embodiments of the invention are explained in connection with figure illustrations. These show, in each case schematically, the following:

[0029] FIG. 1 shows a cross-sectional view of an edge emitting laser diode according to the invention.

[0030] FIG. 2 shows a partial compensation of the refractive index curve during laser operation by a temperature-dependent mechanical stress in the waveguide.

[0031] FIG. 3 shows the sectional view A-A for the edge emitting laser diode of FIG. 1 according to the invention.

[0032] FIG. 4 shows a second embodiment of the edge emitting laser diode according to the invention in cross-sectional view.

[0033] FIG. 5 shows a third embodiment of the edge emitting laser diode according to the invention in cross-sectional view.

DETAILED DESCRIPTION

[0034] FIG. 1 shows a schematically simplified and not to scale cross-sectional view of an edge emitting diode with gain guidance according to the invention. A carrier substrate 1 is shown, on which an intermediate layer 2 is applied. An epitaxially grown semiconductor layer stack 3 is built up on the intermediate layer 2, the growth direction 4 of which defines the vertical direction 5. The semiconductor layer stack 3 comprises an active layer 8, in which the active region 9 is formed during laser operation. With respect to the vertical direction 5, an n-doped waveguide layer 10 is arranged below the active layer 8 and a p-doped waveguide layer 11 is arranged above it, over which a contact layer 12 is applied. No further functional layers, such as cladding layers, are shown. A metallic electrode layer 16 is used to provide the p-contact 15, and the n-contact 17 is arranged on the bottom surface of the carrier substrate 1.

[0035] According to the invention, a thermal stress element 13.1, 13.2, which is covered by a passivation layer 14, adjoins the contact layer 12 on both sides, i.e. in the lateral direction 6 perpendicular to the vertical direction 5. The temperature increase during laser operation leads to a temperature-dependent mechanical stress in the waveguide 10, 11 if the thermal expansion coefficient α.sub.th of the thermal stress element 13.1, 13.2 differs from that of the adjacent layers. Accordingly, for the embodiment shown, the thermal expansion coefficient α.sub.th of the contact layer 12, the waveguide layer 11, and the passivation layer 14 is considerable. For example, if Al.sub.xGa.sub.1-xAs/GaAs is used for the semiconductor layer stack 3, the thermal expansion coefficient of GaAs with α.sub.th.sup.GaAs=6*10.sup.−6 K.sup.−1 (at 20° C.) and the thermal expansion coefficient of Al.sub.xGa.sub.1-xAs depending on the aluminum content with α.sub.th.sup.AlxGa1-xAs=(1.76−6)*10.sup.−6 K.sup.−1 (at 20° C.) in the waveguide 10, 11 are to be used. Furthermore, the thermal expansion coefficient of the passivation layer 14, for example for SiN with α.sub.th.sup.SiN=4*10.sup.−6 K.sup.−1 (at 20° C.) or for SiO.sub.2 with α.sub.th.sup.SiO2=0.5*10.sup.−6 K.sup.−1 (at 20° C.) is relevant.

[0036] In addition to the selection of the material pairing, its dimensioning and arrangement in the semiconductor layer stack 3 must be taken into account for the function of the thermal stress element 13.1, 13.2. According to the invention, the thermal stress element 13.1, 13.2 is configured to generate a thermally induced mechanical stress in the waveguide layer 10, 11, which counteracts the formation of a thermal lens. For the embodiment shown in FIG. 1, the arrangement of the thermal stress elements 13.1, 13.2 is chosen above the active layer and for the lateral direction off-center but symmetrically, with a projection in the vertical direction 5 hitting the edge region of the active region 9. This ensures that the thermally induced mechanical stress in the waveguide 10 acts in that region (illustrated by arrows in FIG. 1) for which the greatest gradient of the inhomogeneous temperature profile occurs during laser operation. If the thermal expansion coefficient of the thermal stress elements 13.1, 13.2 is chosen sufficiently smaller than that of the surrounding material and the material thickness of the thermal stress elements 13.1, 13.2 is dimensioned sufficiently large, a lattice-widening temperature-dependent mechanical stress is generated in the waveguide 10. As shown in FIG. 2, this results in a refractive index curve during laser operation which changes the slope relevant for guiding the lateral modes.

[0037] FIG. 2 shows the real part of the refractive index n in laser operation as a function of the lateral position 1 for an arrangement without the thermal stress elements 13.1, 13.2. The formation of a thermal lens between the areas of the largest refractive index gradients is evident, which lie in the region of the not shown highest temperature gradients. The dashed curve in FIG. 2 shows the refractive index curve of the arrangement according to the invention with the thermal stress elements 13.1, 13.2, whereby the gain guidance can be maintained by smoothing the refractive index curve.

[0038] For the arrangement of the thermal stress element 13.1, 13.2 shown in FIG. 1, in particular for combination with a silicon oxide-based passivation layer 14, a material is used for the thermal stress element 13.1, 13.2 which has a coefficient of thermal expansion α.sub.th, which in a temperature range from 20° C. to 300° C. is less than 0.5*10.sup.−6 K.sup.−1 and particularly preferably less than 0.25*10.sup.−6 K.sup.−1. Particularly advantageous materials with a negative coefficient of thermal expansion α.sub.th are oxide compounds of zirconium and tungsten, in particular ZrW.sub.2O.sub.8, materials containing aluminum titanate and/or zirconium titanate, ZrMo.sub.2O.sub.8, NiSi, In.sub.2Mo.sub.3O.sub.12, (HfMg) (WO.sub.4).sub.3 or graphene oxide.

[0039] FIG. 3 shows the section A-A from FIG. 1. The contact layer 12 and the electrode layer 15 are visible. The passivation layer 14 contains the thermal stress elements 13.1, 13.2, which have a wedge-shaped structure. This shape adapts to the temperature curve in the longitudinal direction 7, whereby the temperature increases from the mirror facet 17 in the direction of the exit facet 18. Accordingly, the structure is designed in such a way that the lateral extension of the thermal stress elements 13.1, 13.2 increases towards the exit facet 18. Furthermore, it can be seen that the longitudinal extension of the thermal stress elements 13.1, 13.2 is selected to be smaller than the resonator length in order to keep the regions of the semiconductor layer stack 3 immediately adjacent to the mirror facet 17 and the exit facet 18 as free as possible from mechanical stresses.

[0040] FIGS. 4 and 5 show further embodiments of the invention, the same reference signs being used for the components corresponding to the first embodiment.

[0041] The embodiment sketched in FIG. 4 uses the entire passivation layer 14 as thermal stress element 13.3, 13.4. FIG. 5 concerns an embodiment with an index-guided edge emitting laser diode, wherein the waveguide 11 is laterally adjoined by index-guiding layers 20.1 and 20.2 which have a lower refractive index than the waveguide 11. The thermal stress elements 13.5, 13.6 provided according to the invention are incorporated in the index guiding layers 20.1 and 20.2.

[0042] Further embodiments of the invention result from the following claims.