Semiconductor laser

A semiconductor laser including an active zone and a waveguide, wherein the active zone includes an active layer configured to generate electromagnetic radiation during operation of the semiconductor laser, the waveguide is configured to guide the electromagnetic radiation generated during operation of the semiconductor laser within the semiconductor laser, the waveguide includes a subregion formed from a compound semiconductor material, wherein a proportion of a material of the compound semiconductor material gradually increases in the entire subregion along the vertical direction toward the active zone so that a refractive index of the subregion gradually decreases toward the active zone, and the proportion is an aluminum proportion or a phosphorus proportion.

TECHNICAL FIELD

This disclosure relates to a semiconductor laser.

BACKGROUND

Laser diodes require high efficiency and high beam quality. The laser diodes should have low electrical series resistances to achieve high efficiency even at high powers. Depending on the application, laser diodes should have certain radiation characteristics, e.g. predetermined aspect ratios with respect to beam divergences in vertical and lateral direction. In practice, low values of series resistance and vertical beam divergence or of beam parameter product are usually aimed at. In general, the series resistance and the vertical beam divergence are determined by the layer structure of the laser diode and cannot easily be adjusted independently from each other due to changes in the layer structure. To achieve high efficiency, leakage currents that occur increasingly in particular at high temperatures and/or high operating currents, should also be prevented as far as possible.

It could therefore be helpful to provide a semiconductor laser having improved efficiency as well as a semiconductor laser having a plurality of degrees of freedom to adjust the electrical series resistance, the vertical beam divergence and/or the lateral beam divergence.

SUMMARY

We provide a semiconductor laser including an active zone and a waveguide, wherein the active zone includes an active layer configured to generate electromagnetic radiation during operation of the semiconductor laser, the waveguide is configured to guide the electromagnetic radiation generated during operation of the semiconductor laser within the semiconductor laser, the waveguide includes a subregion formed from a compound semiconductor material, wherein a proportion of a material of the compound semiconductor material gradually increases in the entire subregion along the vertical direction toward the active zone so that a refractive index of the subregion gradually decreases toward the active zone, and the proportion is an aluminum proportion or a phosphorus proportion.

We also provide a semiconductor laser including an active zone and a waveguide, wherein the active zone includes an active layer configured to generate electromagnetic radiation during operation of the semiconductor laser, the waveguide is configured to guide the electromagnetic radiation generated during operation of the semiconductor laser within the semiconductor laser, the waveguide includes a subregion formed from a compound semiconductor material, wherein a proportion of a material of the compound semiconductor material gradually increases in the entire subregion along the vertical direction toward the active zone so that a refractive index of the subregion gradually decreases toward the active zone, the proportion is an aluminum proportion or a phosphorus proportion, the waveguide has a further subregion spatially spaced apart from the subregion, each of the subregions having an Al-proportion or phosphorus proportion gradually increasing towards the active zone, wherein among the subregions, an n-side subregion belongs to an n-side area of the waveguide and a p-side subregion belongs to a p-side area of the waveguide, a progression of the proportion of the one material of the n-side subregion and a progression of the proportion of the one material of the one material of the p-side subregion each take the form of a ramp gradually ascending toward said active zone, the p-side subregion and the n-side subregion have vertical extensions of different sizes, and the p-side subregion has a higher average proportion of the one material than the n-side subregion.

We further provide a semiconductor laser including an active zone and a waveguide, wherein the active zone includes an active layer configured to generate electromagnetic radiation during operation of the semiconductor laser, the waveguide is configured to guide the electromagnetic radiation generated during operation of the semiconductor laser within the semiconductor laser, the waveguide includes a subregion formed from a compound semiconductor material, wherein a proportion of a material of the compound semiconductor material gradually increases in the entire subregion along the vertical direction toward the active zone so that a refractive index of the subregion gradually decreases toward the active zone, and the proportion is a phosphorus proportion of the compound semiconductor material that gradually increases in the entire subregion along the vertical direction toward the active zone.

LIST OF REFERENCE NUMERALS

10Semiconductor laser1active zone11n-side layer of the active zone12p-side layer of the active zone13active layer of the active zone2waveguide21n-side area of the waveguide21a-dsubareas of the n-side area210n-side subregion of the waveguide22p-side area of the waveguide22a-csubareas of the p-side area220p-side subregion of the waveguide3semiconductor body31n-side cladding layer32p-side cladding layer41hole barrier42electron barrier5substrate6contact layer7ridgeRn, En n-side rampRp, Ep p-side ramp

DETAILED DESCRIPTION

The semiconductor laser has an active zone and a waveguide. The active zone has an active layer configured to generate electromagnetic radiation during operation of the semiconductor laser. The waveguide is configured to guide the electromagnetic radiation generated during operation of the semiconductor laser within the semiconductor laser. The waveguide comprises at least one subregion formed from a compound semiconductor material, wherein a proportion of the one material of the compound semiconductor material gradually increases in the entire subregion along the vertical direction toward the active zone, as a result of which a refractive index of the subregion gradually decreases toward the active zone. Gradually means for instance monotonously in small steps.

The active region may comprise an n-side layer and a p-side layer, wherein the active zone is disposed between the n-side layer and the p-side layer. For example, the semiconductor laser is a laser diode, especially an edge-side-emitting laser diode. The n-side layer of the active zone is arranged vertically between the active layer and an n-side area of the waveguide. The n-side and p-side layers can form secondary wells of the active zone of the semiconductor laser. Alternatively, it is possible that the active layer is embedded directly in the waveguide so that the semiconductor laser can be free of secondary wells.

The subregion of the waveguide is preferably formed from an aluminum-containing and/or phosphorus-containing compound semiconductor material, wherein an aluminum proportion (Al-proportion) and/or a phosphorus proportion (P-proportion) gradually increase/increases in the entire subregion along the vertical direction towards the active zone. Within the subregion, the Al-proportion or P-proportion decreases gradually with increasing vertical distance from the active zone, i.e. for instance monotonously in small steps.

Such a design, preferably with a gradual change of the Al-proportion in the waveguide, allows the quality with respect to wave guidance to be maintained while the overall series resistance of the semiconductor laser is reduced. This is due to the fact that the charge carrier mobility increases overproportionately with the reduction of the Al-proportion, while the refractive index increases only approximately proportionally. Due to the subregion having gradually decreasing Al-proportion away from the active zone, the charge carrier mobility increases overproportionately with increasing distance from the active zone, as a result of which the series resistance undergoes a significant change. At the same time, the properties of the waveguide with respect to wave-guiding are only slightly affected due to the comparatively smaller change in the refractive index. The Al-proportion is therefore preferably reduced in the subregion of the waveguide, where the influence on the resistance is greater than on the wave guidance and thus on the near and far-field divergence.

In this respect, the key characteristics of the semiconductor laser such as series resistance and beam divergence can be set somewhat more independently from each other than, for example, in a standard semiconductor laser, in which only a constantly high proportion of the one material is used for the entire n-side or p-side waveguide area, or in which even a proportion increasing away from the active zone is used. Starting from a reference semiconductor laser, without changing the total thickness, the vertical position or other properties of the active zone of the reference semiconductor laser, by varying the proportion of the one material in the subregion of the waveguide, the vertical near-field width and far-field width of a laser that are directly related to the vertical beam divergence, can be changed within certain limits in a targeted manner. This is particularly advantageous in the further development or in the adjustment of series resistance and/or beam divergence in existing products requiring established high-precision manufacturing processes or assembly processes, where very high accuracies are required for the vertical position of the light emission of the semiconductor laser.

A vertical direction is understood to mean a direction directed transversely, in particular perpendicularly, to a main extension surface of the active zone. For example, the vertical direction is parallel to a growth direction of the active zone. A lateral direction is generally understood to mean a direction running along, in particular parallel to the main extension surface of the active zone. In particular, the lateral direction is perpendicular to a longitudinal direction indicating the resonator direction of the semiconductor laser. In an edge-side-emitting semiconductor laser, the radiation generated during operation of the semiconductor laser is radiated in particular in a longitudinal direction, i.e. in a direction transverse or essentially perpendicular to the vertical and lateral directions.

The semiconductor laser may be based on a III-V or II-VI compound semiconductor material. The semiconductor laser can have a semiconductor body that, for example, is epitaxially applied to or arranged on a substrate of the semiconductor laser. The semiconductor body can only have semiconductor layers. In particular, the semiconductor body comprises the active zone, the waveguide and possible cladding layers of the semiconductor laser. The compound semiconductor material may contain aluminum or phosphorus. In particular, the waveguide is based on a compound semiconductor material containing aluminum and/or phosphorus.

A change, in particular an absolute change in the proportion of the one material within the subregion, may be at least 0.5%, preferably 0.5% to 20%, for example, 1% to 10%, or 1% to 5%, for instance 1% to 3%. For example, this may be a change in the Al-proportion and/or P-proportion of the compound semiconductor material.

A proportion and/or a change in the proportion of the one material in a semiconductor layer or in a compound semiconductor material may be expressed by the proportion x or y in a ratio formula such as AlxGa1-x-yInywith 0≤x≤1, 0≤y≤1 and x+y≤1 or in a ratio formula such as AlxGa1-x-yInywith 0≤x≤1, 0≤y≤1. For example, if the Al-proportion x increases from 0.1 to 0.2 in the entire subregion along the vertical direction to the active zone, the change is 10%. The semiconductor laser may be based on AlGaAs, AlGaAsP, InGaAlP, AlGaN, AlGaInN, GaN and other III-V or II-VI compound semiconductor materials depending on the desired emission wavelength.

The waveguide may have a p-side area. The active zone may be arranged vertically between the n-side area and the p-side area. The subregion having the gradually increasing proportion of the one material towards the active zone can be assigned to the n-side area or to the p-side area of the waveguide. In particular, the p-side area and the n-side area of the waveguide may each have a subregion whose Al-proportion or P-proportion gradually increases in the entire respective subregion along the vertical direction towards the active zone. In other words, the waveguide may have an n-side subregion and a p-side subregion whose Al-proportion or P-proportion is reduced as the distance from the active zone increases. The subregions can be asymmetrical with regard to their layer thickness, material composition, proportion of the one material and/or distance to the active zone. This additional degree of freedom allows a precise adjustment of a desired vertical beam divergence or of a desired vertical far-field width without significantly affecting other key characteristics of the semiconductor laser.

The waveguide may have two spatially spaced subregions, each having a gradually increasing proportion of the one material such as the Al-proportion or the P-proportion, towards the active zone. For example, the waveguide contains an n-side subregion associated with the n-side area and a p-side subregion associated with the p-side area. One subregion may have a vertical layer thickness of at most 100%, for instance 5% to 95%, 5% to 75%, or 10% to 50%, or 25% to 50% of a vertical layer thickness of the other subregion. One subregion may be the p-side subregion and the other subregion may be the n-side subregion, or vice versa.

The waveguide may be formed such that a progression of the proportion of the one material such as the Al-proportion of the n-side subregion and a progression of the proportion of the one material such as the Al-proportion of the p-side subregion each take the form of a ramp gradually increasing toward the active zone. Preferably, the p-side subregion has a smaller vertical extent and a smaller vertical distance to the active zone than the n-side subregion. The p-side region may have a higher average Al-proportion than the n-side subregion. This can form an energy barrier in the conduction band, in particular an electron barrier that hinders or prevents the electrons from leaving the active zone and thus suppresses leakage currents. Alternatively, it is also possible for the p-side subregion to have a lower average Al-proportion than the n-side subregion so that it is also possible that a hole barrier is formed in the waveguide to suppress leakage currents. Preferably, the electron and/or hole barrier/s are/is formed mainly or exclusively by different Al-proportions in the subareas of the waveguide adjacent to the active zone.

The subregion of the waveguide may directly adjoin the active zone. It is possible that the subregion of the waveguide is formed by the entire n-side area or the entire p-side area of the waveguide. Alternatively, it is also possible that the subregion having the gradually increasing proportion of the one material towards the active zone such the Al-proportion is formed exclusively by a subarea of the n-side or p-side area of the waveguide.

The waveguide may have an inner edge-side subarea arranged in the vertical direction between the subregion and the active zone. In this example, the subregion is spaced apart from the active zone at least by the inner edge-side subarea. The subregion and the edge-side subarea of the waveguide can be on the n-side or p-side. In particular, the edge-side subarea adjoins both the subregion and the active zone. Preferably, the subregion has a lower average proportion of the one material such as a lower average proportion of Al, than the inner edge-side subarea. Due to the different Al-proportions, the subregion may have a higher refractive index than the inner edge-side subarea as a result of which a desired wave guidance in the waveguide can be achieved.

The inner edge-side subarea of the waveguide is preferably formed such that its proportion of the one material, preferably its Al-proportion, is substantially constant or remains constant or even decreases along the vertical direction towards the active zone. In particular, an average Al-proportion of the inner edge-side subarea is higher than an average Al-proportion of the subregion. As the distance from the active zone increases, beginning at a certain distance from the active zone for instance given by the vertical layer thickness of the inner edge-side subarea, the Al-proportion is reduced. The active zone and its properties can remain virtually unaffected compared to an active zone of a reference structure, for example, in existing products requiring established high-precise manufacturing processes, while the series resistance is reduced overall. Essentially, the beam quality or the beam divergence can also be maintained. This is due to the fact that the gradual change in the proportion of the one material such as the Al-proportion takes place in a subregion or in subregions of the waveguide vertically spaced from the active zone. In these subregions, the variation of the proportion of the one material, preferably of Al, has a greater positive influence on the series resistance than on the wave guidance and thus on the beam divergence. The variation of the Al-proportion in the subregions of the waveguide thus contributes to increasing the efficiency of the semiconductor laser without significantly changing other key characteristics of the semiconductor laser.

The waveguide may have a further, in particular an outer edge-side subarea, wherein the subregion is arranged between the further edge-side subarea and the active zone. A proportion of the one material such as an Al-proportion or P-proportion, of the outer edge-side subarea may remain constant or in particular gradually decrease along the vertical direction toward the active zone. The outer edge-side subarea may have a higher refractive index than the subregion due to its material composition or Al or P-proportions. In particular, the semiconductor laser has a cladding layer adjacent to the outer edge-side subarea. To achieve a desired wave guidance within the waveguide, the outer edge-side subarea of the waveguide preferably has a higher refractive index than the cladding layer. The waveguide can have both an n-side and a p-side outer edge-side subarea.

The subregion having the gradually increasing proportion of the one material such as Al or P-proportion towards the active zone may be formed by an n-side subarea, i.e. by a subarea of the n-side area of the waveguide. The n-side subarea has a vertical layer thickness of at least 0.1 μm, 0.25 μm, 0.5 μm, for instance 0.8 μm or 1 μm or at least 1.5 μm. For example, the n-side subregion has a vertical layer thickness of 0.1 μm to 4 μm, for instance 0.1 μm to 3 μm, or 0.1 μm to 2 μm inclusive, or 0.1 μm to 1 μm.

The subregion having the gradually increasing proportion of the one material, for instance Al or P-proportion, towards the active zone may be formed by a p-side subarea, i.e. by a subarea of a p-side area of the waveguide. The p-side subregion may have a vertical layer thickness of at least 0.1 μm or at least 0.25 μm or at least 0.5 μm. The p-side subarea of the waveguide forming the subregion may have a vertical layer thickness of 0.1 μm to 4 μm, for instance 0.1 μm to 2 μm, or 0.1 μm to 1 μm.

The semiconductor laser may have an electron barrier and/or a hole barrier formed in an edge-side area of the waveguide, a border area or border areas between the waveguide and the active zone. The electron barrier or the hole barrier can be formed within the waveguide, e.g. in an edge-side area of the waveguide direct to the active zone.

To minimize leakage currents, a sufficiently high energy barrier can usually be formed by a strong contrast with respect to the material composition and/or in an area of increased doping at or near the border area between the waveguide and the active zone. However, an abrupt modification of the material composition and/or the doping in the vicinity of the active zone leads to undesirable disadvantages such as reduced mode stability, modified near-field and far-field and/or increased losses for the light circulating around the active zone. Preferably, the electron barrier or the hole barrier is formed, inter alia, when the waveguide has a first Al-proportion at its n-side border area to the active zone and a second Al-proportion different from the first Al-proportion at a p-side border area to the active zone. For example, exclusively by setting different Al-proportions on the n-side and on the p-side of the waveguide or of the active zone and/or by appropriate doping of the layers of the waveguide, energy barriers can be generated in the conduction band or in the valence band. The electron barrier and hole barrier formed exclusively by setting different Al-proportions can reduce the leakage current without causing the undesirable disadvantages mentioned above and thus increase the efficiency of the semiconductor laser. The energy barrier can be formed on the n-side to reduce hole leakage currents or on the p-side to reduce electron leakage currents or on both sides.

The semiconductor laser may have a p-side cladding layer. The subregion having the gradually increasing proportion of the one material towards the active zone, for instance the Al or P-proportion, can be formed by a subarea of the n-side area of the waveguide. In particular, the active zone, for instance the p-side layer of the active zone directly adjoins the p-side cladding layer. Alternatively, it is also possible that the active zone or the p-side layer of the active zone is spaced from the p-side cladding layer exclusively by a p-side area of the waveguide, wherein the p-side area can have a maximum layer thickness of 0.01 μm, 0.1 μm or 0.5 μm or of 2 μm.

In such a design of the semiconductor laser, it is free or essentially free of a p-side area of the waveguide. In other words, the semiconductor laser only has a very thin p-side waveguide or no real p-side waveguide. Such semiconductor lasers often greatly suffer from efficiency losses due to coupling to higher vertical modes. Due to the subregion of the waveguide comprising the proportion of the one material which decreases with increasing distance from the active zone, the coupling to higher vertical modes can be suppressed since the refractive index of the subregion increases with increasing distance from the active zone and thus promotes the wave guidance.

The semiconductor laser may be formed as a ridge laser. In this example, the semiconductor laser has a vertically protruding ridge formed as a p-side waveguide in the lateral direction. In a plan view, the ridge has in particular a smaller lateral width and/or length than the active zone. The ridge can have a contact layer, a p-side cladding layer and at least a subarea of the p-side waveguide. In particular, the p-side subregion is partially or completely included within the area of the ridge. The p-side waveguide may exhibit a local variation in the proportion of the one material, preferably in the Al- or P-proportion, and thus a related local modification of the refractive index formed preferably to adjust the lateral wave guidance within and below the ridge. Depending on the application, the near and far-field of the semiconductor laser can be adjusted by the design of the ridge accordingly. Alternatively, the p-side subregion may also be located outside the ridge.

In general, the series resistance is determined in particular by the Al-proportion in the entire waveguide. The vertical beam divergence and the vertical far-field width, on the other hand, are predominantly determined by the Al-proportions of the subareas and inner edge regions of the waveguide facing the active zone. In addition, leakage currents can be suppressed by local energy barriers. The energy barrier is formed in particular by different proportions of Al only in the inner edge regions of the waveguide. Other parameters of the semiconductor laser such as the near and far-field are determined, inter alia, by extension of the waveguide mode and thus by the waveguide properties of a larger area of the waveguide. Thus, different characteristics or parameters of the semiconductor laser can be adjusted at least partially independently by a gradual variation of the Al-proportion in one or more subregions of the waveguide. Such parameters are, for example, series resistance, near-field width, far-field width or mode stability of the semiconductor laser.

Further advantages, preferred configurations and further developments of the semiconductor layer will become apparent from the examples explained below in conjunction withFIGS. 1 to 10.

Identical, equivalent or equivalently acting elements are indicated with the same reference numerals in the figures. The figures are schematic illustrations and thus not necessarily true to scale. Comparatively small elements and particularly layer thicknesses can rather be illustrated exaggeratedly large for the purpose of better clarification.

An example of a semiconductor laser is shown schematically in sectional view in the xz plane inFIG. 1A, where x denotes a lateral direction and z a vertical direction. Another lateral direction y such as the longitudinal direction, which for instance indicates the resonator direction, is perpendicular to the x-direction. Along the dotted ZZ′ line, further parameters of the semiconductor laser, e.g. in connection with the band structure progression or with the refractive index progression, are schematically shown in followingFIGS. 3A to 9B.

The semiconductor laser10has a semiconductor body3arranged in the vertical direction between a substrate5and a contact layer6. In particular, substrate5is a growth substrate on which the semiconductor body3is epitaxially deposited, for example. The semiconductor body3has an active zone1and a waveguide2. The active zone1has an n-side layer11, a p-side layer12and an active layer13arranged between the n-side layer11and the p-side layer12. When the semiconductor laser10is in operation, the active layer13is configured to generate electromagnetic radiation. In particular, the semiconductor laser10is an edge-side-emitting semiconductor laser, wherein the radiation emitted during operation of the semiconductor laser is coupled out at a vertical side surface of the semiconductor laser in a direction transverse, for instance perpendicular, to this surface.

The waveguide2is configured to guide the electromagnetic radiation generated during operation of the semiconductor laser within the semiconductor laser10. Preferably, the waveguide2is formed with respect to its material composition and layer thicknesses such that at least 50%, preferably at least 70%, 80% or at least 90% of the total intensity of the basic mode of the laser is concentrated within the waveguide2. The waveguide2has an n-side area21and a p-side area22. The n-side area21may have one subarea or a plurality of subareas21a,21bor21c. Analogously, the p-side area22may have a subarea or a plurality of subareas22a,22bor22c.

It is possible that the n-side layer11and the p-side layer12partly contribute to guiding the basic mode. In this sense, the n-side layer11and the p-side layer12of the active zone1can form a so-called secondary well of the waveguide2. Preferably, the n-side layer11and the p-side layer12each have a higher refractive index than a subarea21aor22aadjacent to the n-side layer11or to the p-side layer12. These subareas21aand22aare referred to as inner edge-side subareas of the waveguide2.

The semiconductor laser10has an n-side cladding layer31and a p-side cladding layer32, wherein the waveguide2is arranged in the vertical direction between the n-side cladding layer31and the p-side cladding layer32. The cladding layers31and32generally have a lower refractive index than a subarea21cor22cof the waveguide2to achieve a desired wave guidance. The subareas21cand22cthat each adjoin a cladding layer31or32are referred to as outer edge-side subareas of the waveguide2. The inner and outer edge-side subareas of the n-side area21or of the p-side area22of the waveguide2may differ in their material composition, doping and/or layer thicknesses.

The semiconductor body3is based in particular on a III-V or II-VI compound semiconductor material. In particular, waveguide2is based on an aluminum-containing compound semiconductor material. The waveguide2preferably has a subregion210or220formed from an aluminum-containing and/or phosphorus-containing compound semiconductor material.

A modification in the Al- and/or P-proportion has direct consequences for the band structure profile and for the refractive index profile of the corresponding subregions. For example, while the band gap increases with increasing Al-proportion, the refractive index decreases with increasing Al-proportion. Also the charge carrier mobility, especially the hole mobility, directly depends on the Al-proportion in the corresponding aluminum-containing layer. We found that the charge carrier mobility increases overproportionately with the reduction of the Al-proportion, while the refractive index increases for instance only proportionally. On the basis of this knowledge, the Al-proportion in the subregion210and/or220can be varied such that the series resistance is kept low at least within these subregions, while the waveguide characteristic of the waveguide2remains essentially unchanged. Thus, the series resistance and the beam divergence or the far-field width of the semiconductor laser10can be adjusted somewhat more independently of each other.

This can be achieved by gradually increasing an Al-proportion in the entire subregion210or220along the vertical direction towards the active zone1. In other words, the Al-proportion of the waveguide2in the subregion210or220is gradually reduced as the distance from the active zone1increases. The subregion210or220may be formed by a subarea of the waveguide2, for instance by the subarea21bor22bas shown inFIG. 1A. Deviating fromFIG. 1A, it is also possible that the subregion210or220is formed by a subarea other than21bor22bof the waveguide2. InFIG. 1A, the waveguide2has two spatially spaced subregions210and220, each comprising an Al-proportion which preferably increases, especially gradually increases along the vertical direction towards the active zone1. The n-side subregion210is assigned to the n-side area21of the waveguide2and the p-side subregion220is assigned to the p-side area22of the waveguide2.

With respect to the active zone1, the areas21and22shown inFIG. 1Ahaving the associated subareas or subregions may be formed to be asymmetrical regarding their material composition, layer thicknesses, Al proportions and/or dopants. Deviating fromFIG. 1A, it is also possible that the semiconductor10is free from at least one or more edge-side subareas21a,21c,22aand/or22c. It is also possible that the semiconductor laser10is free or substantially free from the p-side area22. In this example, the semiconductor laser10has no real p-side waveguide layers or only a very thin p-side area22.

The semiconductor laser10according toFIG. 1Acan be formed as a wide-strip laser. In a wide-strip laser, generated radiation is decoupled especially over the entire lateral width of the active layer13. The radius of the beam waist is thus comparable to the lateral width of the semiconductor laser10. According toFIG. 1B, the semiconductor laser10is formed as a ridge laser which, in contrast to the example shown inFIG. 1A, has a ridge7. In a plan view of the semiconductor laser10, the ridge7has smaller lateral expansions than the active zone1, the active layer13and the substrate5. In a ridge laser, charge carriers are preferably impressed into the active zone1in the region of the ridge7so that electromagnetic radiation is generated more frequently in regions of the active layer13which, in a top view, are covered by the ridge7. Due to its geometry, the contact layer6is formed such that it electrically contacts the active zone1only in the region of the ridge7. By designing the ridge7, the radius of the beam waist of the semiconductor laser10can be adjusted according to the intended application.

The ridge7comprises in particular the p-side cladding layer32and at least a subarea22bof the n-side area22of the waveguide2. The ridge7projects vertically from its surrounding surfaces of the semiconductor laser10. To form the ridge7, the semiconductor laser10can be etched in places along the vertical direction. The ridge7can be configured as the p-side waveguide to laterally guide the wave, wherein the p-side waveguide has a local variation of the Al-proportion and a related local modification of the refractive index configured to adapt the lateral wave guidance within and below the ridge7. In particular, the ridge7comprises the p-side subregion220. Deviating fromFIG. 1B, it is possible that subarea22b, which in particular forms subregion220, is at least partially or completely outside the ridge7.

FIG. 2Ashows a normalized charge carrier mobility B, in particular the hole mobility, as a function of the Al-proportion x in AlxGa1-xAs with 0≤x≤1. The charge carrier mobility, in this case the hole mobility B, decreases with increasing Al-proportion x to approximately 0.6 and increases again from approximately 0.6 to 1.FIG. 2Bshows the refractive index of AlxGa1-xAs with 0≤x≤1 as a function of the Al-proportion x. The refractive index n decreases monotonously with increasing Al-proportion x. The refractive index n is determined at a wavelength of about 970 nm. For the sake of simplicity, the examples described here and below can refer to the AlGaAs-based layer structure of a semiconductor laser. However, the main statements can be transferred to other material systems such as to other III-V and II-VI material systems, in particular also to phosphorus-containing material systems and variation of the phosphorus proportion.

FIG. 3Aschematically shows the profile of the band gap E and the refractive index n of a conventional epitaxial structure for a semiconductor laser along a vertical direction. The n-side area21and the p-side area22of the waveguide2each have a constant Al-proportion along the vertical direction. Accordingly, the band gap and the refractive index n of the waveguide2remain substantially constant along the vertical direction.

Conventional methods used to reduce the series resistance of such a structure include reducing the layer thicknesses of the waveguide2, increasing the dopant concentration introduced into the waveguide2or keeping the Al-proportion low. However, all these methods have unwanted side effects. For example, due to the physical relationship between near-field and far-field, a reduced near-field normally results in an undesired increased vertical beam divergence due to reduced layer thicknesses. An aspect ratio of the vertical to the lateral far-field width is thus inevitably changed. Among other things, higher doping leads to higher optical losses and thus to reduced efficiency of the semiconductor laser. A low Al-proportion in the entire waveguide2usually leads to a reduced suppression of higher vertical modes so that the behavior of the laser can become unstable during operation. Such instabilities are expressed in abrupt jumps in output power, so-called kinks, in abrupt jumps in efficiency, in higher or multimode vertical near-fields and in widened and/or squinting far-fields, where outside the laser, the main direction of propagation differs from the resonator direction.

FIG. 3Bshows the band gap profile of a comparison example for a semiconductor laser. Corresponding toFIG. 3B,FIG. 3Cshows a refractive index profile of a comparison example for a semiconductor laser. Structurally, the layer structure shown inFIGS. 3B and 3Ccorresponds to the layer structure of a semiconductor laser shown inFIG. 3A.

In contrast toFIG. 3A,FIGS. 3B and 3Cschematically show the substrate5and the contact layer6. The layer structure shown inFIGS. 3B and 3Cserves in the following as a reference structure to explain further layer structures of different semiconductor lasers with varying Al-proportion in the waveguide2and the associated technical effects.

The layer structure for a semiconductor laser10shown inFIG. 4Aessentially corresponds to the reference structure shown inFIG. 3B. In contrast, the waveguide2has an n-side subregion210and a p-side subregion220, wherein an Al-proportion and thus the band gap E gradually increases in the entire n-side subregion210or in the entire p-side subregion220along a vertical direction towards the active zone1. In other words, the Al-proportion in the respective subregion210or220gradually decreases with increasing vertical distance from the active zone1. This is expressed by the corresponding band gap progression inFIG. 4Aor by the refractive index progression inFIG. 4B.

The n-side subregion210is formed by a subarea21bof the n-side area21of the waveguide2. The n-side subregion210can have a vertical layer thickness of 0.1 μm to 4 μm. The n-side area21has a further subarea21aadjacent to both the subregion210and the active zone1. In the vertical direction, the n-side subregion210is spaced apart from the active zone1by this inner edge-side subarea21a. The subarea21acan have a vertical layer thickness of 0.1 μm to 4 μm. In particular, the further subarea21ashows a constant Al-proportion is in particular higher than an average Al-proportion of the n-side subregion210.

Analogous to the n-side area21, the p-side area22of the waveguide2has a p-side subregion220which is formed by a subarea22bof the p-side area22. The p-side subregion220can have a vertical layer thickness of 0.1 μm to 4 μm. Throughout the entire p-side220subregion, an Al-proportion gradually increases along the vertical direction towards the active zone. In other words, the Al-proportion gradually decreases with increasing distance from the active zone1. Compared to the reference structure shown inFIG. 3B or 3C, this leads to a reduction of the aluminum content in the respective subregions210and220. This results in particular in an increased refractive index in the subregions210and220so that the basic mode is partly enhanced to be guided outside the usually highly doped and inefficient edge-side subareas21aand22a.

InFIG. 4A, the p-side area22has a further subarea22aarranged in the vertical direction between the p-side subregion220and the active zone1. InFIGS. 4A and 4B, the subarea22amay have a vertical layer thickness of 0.1 μm to 4 μm. The further subarea22ain particular adjoins the active zone1and is thus an inner edge-side subarea of the p-side area22. The further subarea22ain particular has a constant Al-proportion which is in particular higher than an average Al-proportion of the p-side subregion220. InFIGS. 4A and 4B, the n-side subregion210and the p-side subregion220each adjoin a cladding layer31and32, respectively.

Due to the gradual change, a profile of the Al-proportion in the n-side subregion210or in the p-side subregion220can take the form of a gradually ascending ramp towards the active zone1. Such ramps are indicated schematically inFIGS. 4A and 4Bby En and Ep in the band gap profile or by Rn and Rp in the refractive index profile. In particular, the band gap E has a ramp En or Ep in the respective subregions210and220which is analogous to the ramp with respect to the modification of the Al-proportion. The refractive index profile n has a ramp Rn or Rp in the respective subregions210and220which runs in the opposite direction to the ramp with respect to the modification of the Al-proportion.

In general, the vertical optical near-field and thus the vertical optical far-field are mainly determined by the vertical guidance in the areas21and22of the waveguide2, including layers11and12of active zone1if necessary. Along the vertical direction, the optical intensity is concentrated in the area of the active zone1and decreases towards the outer edge-side subareas of the waveguide2. Hence, the shape of the near-field and far-field is mainly determined by the surroundings of the active zone1and to a lesser extent by the edge-side subareas of the waveguide2. In contrast, in a first approximation, the series resistance of the semiconductor laser10depends on all vertical layers in equal parts.

The vertical far-field width is thus mainly determined by the Al-proportions of the edge-side areas of the waveguide2facing the active zone1. Compared to the reference structure shown inFIGS. 3B and 3C, the Al-proportion remains unchanged in the inner edge-side subareas21aand22a(seeFIGS. 4A and 4B) so that the vertical far-field width is only slightly or hardly influenced by the variation of the Al-proportions in subregions210and220. However, by reducing the Al-proportion in the subregions210and220, a lower series resistance of the semiconductor laser10can be achieved. The reduction of the series resistance is therefore mainly achieved by the variation of the Al-proportions in the subareas21band22bof the waveguide2remote from the active zone1and form the subregions210and220inFIGS. 4A and 4B, respectively.

The layer structure of a semiconductor laser10shown inFIGS. 3B to 4Bhas the same overall vertical height. By varying the Al-proportions in the subregions210and220each vertically spaced apart from the active zone1by a further subarea21aor22a, a reduction of the series resistance can be achieved while maintaining the overall height and an essentially unchanged far-field width of the semiconductor laser10.

The example of a layer structure of the semiconductor laser10shown inFIG. 5Aessentially corresponds to the examples of a layer structure shown inFIGS. 4A and 4B. In contrast, the subregions210and220directly adjoin the active zone1. The subregions210and220thus comprise the inner edge-side subareas21aand22a, respectively. As a further difference, the waveguide2has further outer edge-side subareas21cand22c, wherein the n-side subregion210is arranged between the n-side outer edge-side subarea21cand the active zone1, and the p-side subregion220is arranged between the p-side outer edge-side subarea22cand the active zone1. The further subareas21cand22cmay each have an Al-proportion that remains constant along the vertical direction to the active zone1.

The example of a layer structure shown inFIG. 5Bessentially corresponds to the example of a layer structure for a semiconductor laser shown inFIG. 5A. In contrast, the outer edge-side subarea21cof the waveguide2has an Al-proportion that decreases along the vertical direction towards the active zone1. This is expressed in the decreasing band gap E and in the increasing refractive index n in the direction of the active zone1. In addition, the waveguide2has a further subarea21dfor instance having a constant Al-proportion, wherein the further subarea21dis arranged in the vertical direction between the outer edge-side subarea21cand the n-side subregion210.

The example of a layer structure shown inFIG. 5Cessentially corresponds to the example of a layer structure shown inFIG. 5A. In contrast to this, the n-side area21of the waveguide2has an inner edge-side subarea21awhich is arranged in the vertical direction between the n-side subregion210and the active zone1.

The example of a layer structure shown inFIG. 5Dessentially corresponds to the layer structure shown inFIG. 5C. In contrast, the p-side area22of the waveguide2has an inner edge-side subarea22a, wherein the p-side subregion220is spaced apart from the active zone1by the subarea22a.

The example of a layer structure of a semiconductor laser10shown inFIGS. 6A and 6Bessentially corresponds to the example of a layer structure shown inFIGS. 4A and 4B. In contrast, the p-side subregion220has a significantly lower vertical layer thickness than the n-side subregion210. The vertical layer thickness of the subregion220inFIGS. 6A and 6Bcan be 0.1 μm to 4 μm. In addition, the semiconductor laser10has an electron barrier42formed in a border area of the waveguide2to the active zone1.

In particular, the electron barrier42is formed by different Al-proportions in the subareas21aand22aof the waveguide2adjacent to the active zone1. To form the electron barrier42, the p-side area22ahas a higher Al-proportion than the p-side area21a. In particular, the electron barrier42is formed exclusively by different Al-proportions in the inner edge-side areas21aand22a. Similarly, a hole barrier layer can be formed by adjusting the Al-proportion in the edge-side areas21aand22a. For example, the edge-side subarea21amay have a higher Al-proportion than the p-side edge-side subarea22a. By reducing the Al-proportion in the210and/or220subregions, the total Al-proportion in waveguide2can be compensated.

A reduction of leakage currents can be achieved by the electron barrier and/or hole barrier at the edge-side areas to the active zone1, wherein undesirable disadvantages such as reduced mode stability, modified near-field or far-field and/or increased losses can be avoided which occur for example in the case of an abrupt, in particular strong modification in the material composition and/or in the doping in the vicinity of the active zone1for the formation of the electron or hole barrier.

FIGS. 7A and 7Bshow a comparison example to the example shown inFIGS. 6A and 6B. A semiconductor laser10having the layer structure according toFIGS. 6A and 6Band a semiconductor laser10having the layer structure according toFIGS. 7A and 7Bhave a comparable vertical far-field divergence of about 16° (half width). Compared toFIGS. 6A and 6B, however, the layer structure shown inFIGS. 7A and 7Bdoes not show any subregion in which the Al-proportion is reduced or decreases with increasing distance from the active zone1. We found that the semiconductor laser10or the semiconductor body of the semiconductor laser10has an increased overall vertical height of several μm as shown inFIGS. 7A and 7Bcompared to the example shown inFIGS. 6A and 6B. Correspondingly, the semiconductor laser10having a layer structure as shown inFIGS. 7A and 7Bare formed to be thicker, with corresponding consequences for series resistance and efficiency.

The example of a layer structure of a semiconductor laser shown inFIG. 8Aessentially corresponds to the example of a layer structure shown inFIG. 5D. In contrast, the n-side subarea21ahas a higher Al-proportion and thus an increased band gap compared to the p-side subarea22a. Accordingly, a hole barrier41is formed at the edge Ev of the valence band, which is shown schematically for instance inFIG. 8B.

The example of a layer structure of a semiconductor laser shown inFIG. 8Cessentially corresponds to the example of a layer structure shown inFIG. 8A. In contrast toFIG. 8A, the p-edge-side subarea22ainFIG. 8Chas a higher Al-proportion than the n-edge-side subarea21a. Accordingly, an electron barrier42is formed at the edge of the conduction band Ec.

FIG. 9Ashows an example of a layer structure essentially corresponding to the examples shown inFIGS. 5C and 5D. In contrast to this, the p-side area22of the waveguide2is free from a subregion220having a varying Al-proportion. The p-side area22has only a subarea22awhich has a particularly low vertical layer thickness. In other words, the active zone1is separated from the p-side cladding layer32only by a particularly thin p-side area22aof the waveguide2. The subarea22ahas a constant Al-proportion along its vertical extent.

The example shown inFIG. 9Bessentially corresponds to the example shown inFIG. 9A. In contrast, the p-side cladding layer32directly adjoins the active zone1. According toFIG. 9B, the semiconductor laser10is essentially free or free from a p-side area of the waveguide. According toFIGS. 9A and 9B, the semiconductor laser10is formed with particularly thin or without real p-waveguide layers. In general, the suppression of higher vertical modes is particularly difficult in such a semiconductor laser. So far, to suppress higher modes, inter alia, several n-side waveguide layers having corresponding layer thicknesses and different refractive indices, coupling of the higher modes to substrate modes and radiation into the strongly absorptive substrate using a small refractive index jump between the n-side area2and the n-side cladding layer31have been used. However, such methods often lead to strong efficiency losses. By varying the Al-proportion in the n-side210subregion, the series resistance can be reduced, as a result of which a possible reduction in the efficiency of the semiconductor laser is partially compensated or even overcompensated.

The reduction of the Al-proportions in the subareas of the waveguide2facing away from active zone1also results in an enhancement of the refractive index, as a result of which the electromagnetic radiation in these subareas experiences a comparatively stronger wave guidance. This can lead to higher propagation losses for higher modes due to radiation and absorption in the often higher-doped outer areas of the waveguide2and thus to improved side mode suppression. In other words, higher modes are increasingly suppressed by the variation of the Al-proportion in the n-side subregion210and/or p-side subregion220. This applies to all the examples described here and in particular to semiconductor lasers with a particularly thin p-side area22of the waveguide2or without a real p-waveguide.

Overall, the efficiency of the semiconductor laser and the stability against suddenly occurring power jumps can be increased by improved suppression of undesired, e.g. higher vertical modes. For the desired mode, usually the basic mode, it is possible that the wave guidance in the layers11and12of the active zone1or in the subareas of the waveguide2adjacent to the active zone1is amplified for instance compared to the reference structure, for example, by increasing the corresponding refractive index.

FIG. 10shows a comparison of a vertical far-field divergence of a semiconductor laser without variation of the Al-proportion (curve D1) and a semiconductor laser with variation of the Al-proportion in the waveguide2(curve D2). The semiconductor laser associated with curve D2differs from the semiconductor laser associated with curve D1only in the Al-proportions in the corresponding layer structures. We found that the vertical far-field according to curve D1has a far-field divergence of approximately 19° (half width), while the vertical far-field according to curve D2has a far-field divergence of approximately 16° (half width). Thus, by varying the Al-proportions in the corresponding layers, especially in the n-side subregion210and/or in the p-side subregion220of the waveguide2, the vertical far-field divergence can be adjusted according to intended application and specific requirements.

A series resistance of a semiconductor laser can be reduced by gradually modifying the proportion of a material, preferably the Al or P-proportion, in at least one subregion of a waveguide, while maintaining the wave guidance quality at the same time. Also some characteristics of the semiconductor laser such as series resistance, vertical beam divergence and/or an aspect ratio of the beam divergences in vertical and lateral direction, can be influenced more independently from each other by targeted variation of the proportion of the one material in the waveguide.

This application claims priority of DE 10 2016 122 147.3, the subject matter of which is incorporated herein by reference.

Our semiconductor lasers are not restricted to the examples by the description made with reference to examples. This disclosure rather comprises any novel feature and combination of features, including in particular any combination of features in the appended claims, even if the feature or combination is not itself explicitly indicated in the claims or examples.