LASER DIODE WITH INTEGRATED THERMAL SCREEN

Abstract

The present invention relates to a diode laser with an integrated thermal aperture. A laser diode (10) according to the invention comprises an active layer (14) formed between an n-doped semiconductor material (12) and a p-doped semiconductor material (16), wherein the active layer (14) forms an active zone (40) with a width w along a longitudinal axis for generating electromagnetic radiation; wherein in the p-doped semiconductor material (16) and/or in the n-doped semiconductor material (12) a thermal aperture (18) formed in a layer shape with a thermal conductivity coefficient k.sub.block smaller than a thermal conductivity coefficient k.sub.bulk of the corresponding semiconductor material (16, 12) is formed for a spatially selective heat transport from the active zone (40) to a side of the corresponding semiconductor material (16, 12) opposite to the active layer (14).

Claims

1. A laser diode, comprising: an active layer formed between an n-doped semiconductor material and a p-doped semiconductor material, wherein the active layer forms an active zone with a width w along a longitudinal axis for generating electromagnetic radiation; wherein in the p-doped or n-doped semiconductor material a thermal aperture formed in a layer shape with a thermal conductivity coefficient k.sub.block smaller than a thermal conductivity coefficient k.sub.bulk of the respective doped semiconductor material is formed for spatially selective heat transport from the active zone to a side of the respective doped semiconductor material opposite to the active layer (14).

2. The laser diode of claim 1, wherein the thermal aperture consists of the same semiconductor material as the respective doped semiconductor material.

3. The laser diode of claim 1, wherein the thermal aperture is formed of periodically alternating materials.

4. The laser diode of claim 1, wherein the thermal aperture forms a slit-shaped passage region, arranged parallel to the active layer, for a heat flow directed from the active zone towards an outer side of the laser diode.

5. The laser diode of claim 4, wherein the slit-shaped passage region is arranged medially with respect to the active zone.

6. The laser diode of claim 1, wherein the lateral distance dx between an outer edge of the active zone and a nearest inner edge of the thermal aperture is −w/6≤dx≤+w/6.

7. The laser diode of claim 1, wherein the vertical distance dy between the center of the active layer and the top of the thermal aperture is 0 μm≤dy≤1 μm.

8. The laser diode of claim 1, wherein the thermal aperture has an aperture thickness d.sub.block between 0.3 μm and 3 μm.

9. The laser diode of claim 1, wherein the thermal conductivity coefficient k.sub.block is at most 30% of the corresponding thermal conductivity coefficient k.sub.bulk.

10. The laser diode of claim 1, wherein a thermal aperture formed in a layer shape is formed in the n-doped semiconductor material and a thermal aperture formed in a layer shape is formed in the p-doped semiconductor material.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0031] The invention is explained below in embodiment examples with reference to the accompanying drawing, wherein:

[0032] FIG. 1 is a schematic illustration of an exemplary conventional laser diode without thermal aperture,

[0033] FIG. 2 is a schematic illustration of an exemplary first embodiment of a laser diode according to the invention with thermal aperture,

[0034] FIG. 3 is a simulation of the temperature as a function of the lateral position (x-axis) within the active zone,

[0035] FIG. 4 is a simulation of the normalized thermal lens curvature factor |B2| as a function of the thermal conductivity coefficient k.sub.KS of the p contact layer,

[0036] FIG. 5 is a simulation of the normalized thermal lens curvature factor |B2| as a function of the aperture thickness d.sub.block,

[0037] FIG. 6 is a simulation of the normalized thermal lens curvature factor |B2| as a function of the lateral distance dx,

[0038] FIG. 7 is simulation of the temperature difference ΔT between the temperature T as a function of lateral position (x-axis) and the peak temperature T.sub.peak at the position x=0 for structures with the KS material according to FIG. 4, and

[0039] FIG. 8 is a schematic illustration of an exemplary second embodiment of a laser diode according to the invention with two thermal apertures.

DETAILED DESCRIPTION

LIST OF REFERENCE NUMERALS

[0040] 10 Laser diode

[0041] 12 n-doped semiconductor material

[0042] 14 Active layer

[0043] 16 p-doped semiconductor material

[0044] 18 Thermal aperture

[0045] 20 Solder layer

[0046] 30 Submount

[0047] 30a First submount

[0048] 30b Second submount

[0049] 40 Active zone

[0050] 42 Heat flow

[0051] dx Lateral distance (slow axis)

[0052] dy Vertical distance (fast axis)

[0053] d.sub.block Aperture thickness

[0054] w Width

Description

[0055] FIG. 1 shows a schematic illustration of an exemplary conventional laser diode without thermal aperture. The diode laser shown comprises a laser diode 10 having an active layer 14 formed between an n-doped semiconductor material 12 and a p-doped semiconductor material 16, the active layer 14 forming along a longitudinal axis an active zone 40 having a width w for generation of electromagnetic radiation; and a submount 30, wherein the submount 30 is thermally conductively connected to the p-side underside of the laser diode 10 below the active zone 40. The thermally conductive connection may be formed by an intermediate solder layer 20, wherein the solder is intended to provide optimal heat transfer between the underside of the laser diode 10 and the submount 30.

[0056] In particular, the laser diode 10 may have a multilayer structure comprising an n-substrate, an n-cladding layer overlying the n-substrate, an n-waveguide layer overlying the n-cladding layer, an active layer 14 overlying the n-waveguide layer, a p-waveguide layer overlying the active layer 14, a p-cladding layer overlying the p-waveguide layer, a p-contact layer overlying the p-cladding layer, and a metallic p-contact overlying the p-contact layer.

[0057] The losses occurring as heat during operation of the laser diode in the active zone 40 must be dissipated from the active zone 40. For this purpose, a submount 30 is usually used as a corresponding heat sink. However, the heat flow directed from the active zone 40 to the submount 30 spreads out strongly in the lateral direction and leads to an inhomogeneous temperature distribution in the region below the active zone 40. The resulting temperature distribution can then have thermo-optical effects on the generated electromagnetic radiation and, by forming a thermal lens in this region, contribute to a deterioration of the beam quality during radiation emission.

[0058] FIG. 2 shows a schematic illustration of an exemplary first embodiment of a laser diode with thermal aperture according to the invention. The diode laser shown comprises a laser diode 10 with an active layer 14 formed between an n-doped semiconductor material 12 and a p-doped semiconductor material 16, the active layer 14 forming along a longitudinal axis (longitudinal direction, z-axis) an active zone 40 with a width w for generating electromagnetic radiation; and a submount 30, wherein the submount 30 below the active zone 40 is thermally conductively connected to the p-side underside of the laser diode 10. This corresponds as far as possible to the structure described for FIG. 1.

[0059] In the p-doped semiconductor material 16, however, a thermal aperture 18 formed in a layer shape with a thermal conductivity coefficient k.sub.block smaller than a thermal conductivity coefficient k.sub.bulk of the p-doped semiconductor material 16 (below the active zone 40) is formed for a spatially selective heat transport from the active zone 40 to the side of the p-doped semiconductor material 16 opposite to the active layer 14 (underside of the laser diode 10) and thus to the submount 30. As an approximation, an average thermal conductivity coefficient of the p-doped semiconductor material 16 can also be used for the thermal conductivity coefficient k.sub.bulk of the p-doped semiconductor material below the active zone 40. Alternatively, the thermal conductivity coefficient k.sub.bulk of the p-doped semiconductor material 16 can also be approximately equated with the thermal conductivity coefficient k.sub.KS of a p-contact layer of the p-doped semiconductor material 16.

[0060] Here, too, the thermally conductive connection can be formed by an intermediate solder layer 20, the solder being intended to enable optimum heat transfer between the underside of the laser diode 10 and the submount 30. The connection can also be made by bonding, for example by means of a thermally conductive adhesive.

[0061] The thermal aperture 18 forms a slit-shaped passage region arranged parallel to the active layer 14 for a heat flow 42 directed from the active zone 40 toward the underside of the laser diode 10. The slit-shaped passage region is arranged medially below the active zone 40 in the figure. Propagation of the heat flow 42 directed from the active zone 40 to the submount 30 in the lateral direction is suppressed by the thermal aperture 18 according to the invention, resulting in a largely parallel heat flow 42. The high thermal resistance of the thermal aperture 18 results in an increase in its local temperature (i.e., heating in the lateral regions) as more heat is generated by the active zone 40 with increasing output power. This results in a more uniform temperature distribution in the region below the active zone 40 between the central region (directly below the active zone) and the thermal aperture (the side regions). The formation of a thermal lens in this region is thus also suppressed, which can increase the beam quality during radiation emission.

[0062] The illustration further shows the horizontal distance dx between an outer edge of the active zone 40 and a nearest inner edge of the thermal aperture. Also shown is the vertical distance dy between the center of the active layer 14 and the thermal aperture 18. Also shown is the aperture thickness d.sub.block of the thermal aperture 18 and the total layer thickness d of the p-doped semiconductor material 16.

[0063] The description applies accordingly to a thermal aperture 18 formed in the n-doped semiconductor material 12. In this case, a corresponding submount 30 above the active zone 40 could be thermally conductively connected to the n-side top of the laser diode 10 to suppress a lateral widening of an upwardly directed heat flow 42.

[0064] FIG. 3 shows a simulation of the temperature as a function of the lateral position (x-axis) within the active zone. The simulation was performed at a vertical position (y-axis) of y=0, i.e., in the center of the active layer, for a GaAs-based broad-area diode laser (BAL) with a stripe width w=90 μm (see M. Elattar et al, High-brightness broad-area diode lasers with enhanced self-aligned lateral structure, Semicond. Sci. Technol. 35, 095011 (2020)), which operates at an optical power P.sub.opt=10 W. The simulated BAL corresponds to the typical structure consisting of an active zone (AZ) between an n-doped and a p-doped semiconductor material. The p-doped semiconductor material consists of a Al.sub.xGa.sub.1-xAs-waveguide layer (WL) grown on the AZ, followed by an Al.sub.xGa.sub.1-xAs cladding layer (MS), and finally a GaAs contact layer (KS) on which a contact metal is subsequently deposited. The simulation (matching corresponding experimental results) includes a thermal barrier at the KS metal interface. The term thermal lens curvature factor B2 is the quadratic term of a quadratic fit of the obtained thermal profile (Rieprich, J. et al., Chip-carrier thermal barrier and its impact on lateral thermal lens profile and beam parameter product in high power broad area lasers, J. Appl. Phys. 123, 125703 (2018)), where a quadratic fit was performed in the simulation for the region within the stripe width w=90 μm. The exemplary conventional diode laser in the simulation shows that a thermal profile with a curved profile between about 45° C. at the edges and about 51° C. in the center of the broad strip is obtained.

[0065] FIG. 4 shows a simulation of the normalized thermal lens curvature factor |B2| as a function of the thermal conductivity coefficient k.sub.KS of the p-contact layer. In the reference structure, the KS consists of GaAs (k.sub.KS≈44 W/(m.Math.K)). When GaAs is replaced by materials with lower thermal conductivity, such as InGaP (k.sub.block≈5 W/(m.Math.K)), InGaAsP (k.sub.block≈5 W/(m.Math.K)), an InGaP—InGaAsP superlattice (k.sub.block≈2.5 W/(m.Math.K)); see J. Piprek et al., Thermal conductivity reduction in GaAs—AlAs distributed Bragg reflectors, in IEEE Photon. Tech. Lett. 10, 81(1998)), or air (k.sub.air≈0.026 W/(m.Math.K)) is substituted, the normalized thermal lens curvature factor |B2| is reduced, corresponding to a weakened thermal lens. This results in a smaller far-field angle and thus improved beam quality. Specifically, simulation showed that a 5% reduction in normalized thermal lens curvature factor |B2| can be achieved with a reduced thermal conductivity coefficient k.sub.KS≈18 W/(m.Math.K). A 10% reduction can be achieved with a thermal conductivity coefficient k.sub.KS≈7 W/(m.Math.K). For a 15% reduction, the thermal conductivity coefficient should be k.sub.KS≈2.5 W/(m.Math.K).

[0066] FIG. 5 shows a simulation of the normalized thermal lens curvature factor |B2| as a function of the aperture thickness d.sub.block. When layers of GaAs (KS) or Al.sub.xGa.sub.1-xAs (MS, WL) are replaced by InGaP (low thermal conductivity coefficient k), d he normalized thermal lens curvature factor |B2| is reduced, corresponding to the formation of a weakened thermal lens. This results in a smaller far-field angle and thus improved beam quality. In particular, the simulation showed that a 5% reduction in the normalized thermal lens curvature factor |B2| can be achieved with an aperture thickness d.sub.block≈688 nm. A 10% reduction can be achieved with an aperture thickness d.sub.block≈1375 nm.

[0067] FIG. 6 shows a simulation of the normalized thermal lens curvature factor |B2| as a function of the lateral distance dx. The KS was assumed here to consist of InGaP. It can be observed that the thermal apertures can reduce the thermal lens curvature factor |B2| most effectively when dx=0, i.e., the thermally particularly conductive slit-shaped passage region below the active zone aligns perfectly medially with the laser stripe.

[0068] FIG. 7 shows a simulation of the temperature difference ΔT between the temperature T as a function of lateral position (x-axis) and the peak temperature T.sub.peak at position x=0 for structures with the KS material shown in FIG. 4. The curve shows the reduction of the curvature of the thermal lens when GaAs is replaced by materials with lower thermal conductivity.

[0069] FIG. 8 shows a schematic illustration of an exemplary second embodiment of a laser diode according to the invention with two thermal apertures. The laser diode 10 shown corresponds in principle to a first embodiment of a laser diode 10 according to the invention with a thermal aperture 18 shown in FIG. 2. The individual reference numerals and their respective assignment to the individual features therefore apply accordingly. In contrast to the illustration in FIG. 2, however, a structure with thermal apertures 18 according to the invention is shown here both in the p-doped semiconductor material 16 below the active layer 14 and in the n-doped semiconductor material 12 above the active layer 14. A first submount 30a is thermally conductively connected to an underside of the laser diode 10 below the active zone 40. Furthermore, a second submount 30b is thermally conductively connected above the active zone 40 to a top of the laser diode 10. Cooling can thus take place on both sides of the laser diode 10, whereby a lateral widening of the heat flow 42 both to the top and to the underside of the laser diode 10 can be effectively suppressed by thermal apertures 18. Such an embodiment is advantageous when the laser diode 10 is mounted for double-sided cooling, i.e., when heat extraction can occur to both sides of the laser diode 10. The laser diode shown is symmetrical with respect to the active layer 14.

[0070] From the foregoing, it will be appreciated that specific embodiments of the invention have been described herein for purposes of illustration, but that various modifications may be made without deviating from the scope of the invention. Accordingly, the invention is not limited except as by the appended claims.