Vcsel

Abstract

A VCSEL has a semiconductor layer structure (12) which is stacked in a vertical direction and which comprises: a first mirror (14); an active region (16); and a second mirror (18), in order to form an optical cavity in which laser light is generated, the laser light being coupled out via a facet (26) of the VCSEL. The facet (26) has a first material (M1), which is a material of the semiconductor layer structure (12), and at least one layer (30) made of at least one second material (M2), said layer being arranged on parts of the first material (M1), the second material (M2) being different from the first material (M1). In a first region (I) of the facet (26) in which the second material (M2) is present, at least one first refractive index jump is formed, and in at least one second region (II) of the facet (26) which is different from the first region (I), at least one second refractive index jump is formed at least locally, the at least one second refractive index jump being different from the at least one first refractive index jump, and the facet (26) has a non-planar topography.

Description

VCSEL

[0001] The invention relates generally to the field of vertical cavity surface-emitting lasers, referred to as VCSELs for short.

[0002] Typically, a VCSEL has a semiconductor layer structure stacked in a vertical direction, which has a first mirror, an active region, and a second mirror to form an optical cavity in which laser light is generated. The laser light is output via a facet of the VCSEL. The facet of a VCSEL generally refers to the light-emitting surface of the VCSEL. The facet of a VCSEL can be modified, for example, to determine the polarization of the emitted laser light, to enable additional reflection or transmission of the facet, to select a defined mode through an angular dependence of the reflection, and / or to enable coherent coupling of different modes of the generated laser light. This can be achieved by structuring the output mirror of the VCSEL. The use of a periodic structure is common, e.g.a grating with a grating period smaller or larger than the emitted laser wavelength.

[0003] US 7,627,018 B1 discloses a VCSEL with a waveguide-grating coupler, wherein a resonant cavity is defined between a first and a second mirror. The waveguide-grating coupler comprises a waveguide having a first grating formed as part of the waveguide to couple light of a first polarization mode into the waveguide. A second grating is formed on the waveguide-grating coupler and runs orthogonally to the first grating. The second grating is intended to enhance the coupling of light into the waveguide by generating a loss difference between light of the first polarization mode and light of a second polarization mode, so that light of the first polarization mode propagates preferentially within the cavity.

[0004] EP 2 467 910 B1 discloses a VCSEL with a large-area facet, which has an output mirror whose reflectivity is selected such that the VCSEL does not lase without external feedback, wherein at least one optical feedback element is provided which provides angle-selective feedback for laser radiation emitted by the laser, wherein the angle-selective feedback is provided for at least part of the Laser radiation emitted at angles > 0 to the optical axis is higher than for laser radiation emitted along the optical axis, wherein the angle-selective feedback is higher for parts of the laser radiation that are point-symmetric about the optical axis than for laser radiation emitted on the optical axis, such that an intensity distribution of laser radiation in a working plane is point-symmetric about the optical axis, and wherein the optical feedback element is designed such that a product of the reflectivities of the output mirror and a surface section of the feedback element that provides the higher feedback is > 98%.

[0005] Existing technologies are limited when multiple VCSEL properties, such as the desired laser mode(s) and polarization stabilization, are to be combined. Furthermore, manufacturing tolerances are difficult to control with existing technologies. Furthermore, the performance of the VCSEL depends heavily on the chosen manufacturing process and the equipment used during production, such as the generation of the alignment of the grating structures of a local grating on the facet.

[0006] It is therefore an object of the present invention to provide a VCSEL that generates predetermined laser modes having defined properties, even if the manufacturing process is subject to tolerances.

[0007] According to the invention, the object is achieved by a VCSEL having a semiconductor layer structure stacked in a vertical direction, which has a first mirror, an active region, and a second mirror to form an optical cavity in which laser light is generated, wherein the laser light is coupled out via a facet of the VCSEL, wherein the facet has a first material, which is a material of the semiconductor layer structure, and at least one layer of at least one second material, which is arranged in regions on the first material, wherein the second material is different from the first material, such that in a first region of the facet, in which the second material is present, at least one first refractive index jump is formed, and in at least one second region of the facet, which is different from the first region, at least one second refractive index jump is formed,wherein the at least one second refractive index jump is from the at least one first, refractive index jump, and wherein the facet has a non-planar topography.

[0008] The inventive concept is based on the use of at least two refractive index jumps on a non-planar topography in localized regions of the facet, which enables targeted tuning of the laser threshold, for example, to separate or define laser modes and / or polarization states. This is achieved according to the invention by localizing the refractive index jumps at defined positions along the facet of the VCSEL, which are preferably radially symmetric. Therefore, parts of the optical mode interfere exclusively at these refractive index jumps. In the present invention, "position(s)" is / are also understood to include locally limited regions.

[0009] A refractive index jump occurs at an interface between a first material with a first refractive index and a second material with a second refractive index. In the VCSEL according to the invention, the first material is a material of the semiconductor layer structure, for example a material of one of the mirrors of the VCSEL. The second material is correspondingly different from the material of the semiconductor layer structure. The second material is only arranged in certain regions, i.e. in one or more selected regions on the first material, while it is not present in other regions of the facet. In the regions in which the second material is not present, the interface can be formed, for example, by the interface between the first material and air or, as provided in an optional embodiment, between the first material and a protective layer.In the areas where the second material is present, a refractive index jump occurs at the interface between the first material and the second material. For example, if the semiconductor layer structure is based on the gallium arsenide-aluminum gallium arsenide material system, the second material is a different material, such as silicon nitride. The refractive index jump from gallium arsenide or aluminum gallium arsenide to silicon nitride is smaller than the refractive index jump from gallium arsenide or aluminum gallium arsenide to air.

[0010] Furthermore, the facet of the VCSEL according to the invention has a non-planar topography. A non-planar topography of the facet can be achieved by only This can be achieved by arranging the second material on the first material in certain regions, and / or by the first and / or second regions themselves having a non-planar topography, for example, by means of depressions or elevations in these regions. This can provide further degrees of freedom in adjusting desired laser mode properties.

[0011] In the VCSEL according to the invention, several desired properties, such as the generation of one or more desired laser modes and polarization stabilization by localized tuning of the reflectivity at the facet, can be combined and controlled during the manufacture of the VCSEL without manufacturing tolerances having a detrimental effect.

[0012] Preferred embodiments of the VCSEL according to the invention are described below.

[0013] While the at least two different refractive index jumps can be at the same level when viewed in the vertical direction, it is preferred if the at least two different refractive index jumps are at different levels when viewed in the vertical direction.

[0014] For example, the refractive index jump in the second region may be lower or higher in the vertical direction than the refractive index jump in the first region of the facet in which the second material is present.

[0015] Furthermore, it is preferred if the first region is an outer region of the facet and the at least one second region is a region within the first region.

[0016] In particular, if the first refractive index jump in the first region is smaller than the second refractive index jump in the second region of the facet, the intensity of the laser mode in the first region, where the second material is present, is lower than in the center of the facet. By adjusting the thickness of the layer The reflectivity / transmissivity in the first area can be adjusted using the second material.

[0017] The first region can surround the second region in a ring, and the second material can be present throughout the first region, allowing the desired laser mode to be emitted by the VCSEL to be concentrated to the inner region of the facet. The width of the first region in the direction from an outer edge of the facet to the center of the facet is determined by the width of the layer of the second material.

[0018] Preferably, the at least one second refractive index jump is greater than the at least one first refractive index jump.

[0019] In particular in connection with the above-mentioned embodiment, according to which the first region is an outer region of the facet and the second region is an inner region of the facet, the laser threshold value can be set in a controllable manner in the two regions, wherein the laser threshold value in the second region with the higher refractive index jump is in particular lower than in the first region.

[0020] Further preferably, the at least one second region has at least one step-shaped depression or elevation.

[0021] The at least one stepped depression can be realized by a cutout in the uppermost layer or the upper layers of the semiconductor layer structure. In the case of an elevation in the at least one second region, this can also be formed from the first material by removing material of the semiconductor layer structure to the side of the elevation. In both cases, the optical cavity can be partially "opened" in the region of the cutout / elevation, i.e. the cutouts lie below the uppermost level of the semiconductor layer structure after its manufacture. However, an elevation can also be formed by a different material, in particular by the second material, on the surface of the semiconductor layer structure, without the optical cavity being opened. The at least one stepped depression or elevation results in a refractive index variation in the second range, with which the properties of the desired laser mode or laser modes can also be adjusted.

[0022] It is preferred if the at least one second region has a plurality of local step-shaped depressions or elevations, so that the refractive index varies along the second region in a plane transverse to and through the depressions or elevations.

[0023] The stepped depressions or elevations can serve to achieve specific mode properties or specific polarization states. A plurality of depressions / elevations can enhance this effect.

[0024] In this case, a depth or height and / or a width of the at least one stepped depression or elevation and / or in the case of a plurality of stepped depressions or elevations, a pitch of adjacent stepped depressions or elevations can be smaller than the wavelength of the laser wavelength propagating in the cavity.

[0025] With this choice of the above-mentioned parameters, the laser mode can be stabilized with regard to its polarization state due to the local refractive index variation.

[0026] If the above-mentioned parameters, as provided in a further embodiment, are equal to or greater than the wavelength of the laser wavelength propagating in the cavity, ie the depressions / elevations are "visible" to the laser mode, an additional tuning of the filtering (laser mode selection or suppression of unwanted modes and / or polarization directions) / reflectivity is possible.

[0027] The at least one second region may have at least one sub-region in which a plurality of parallel step-shaped depressions or elevations are present. Preferably, the at least one second region has at least two sub-regions, in each of which a plurality of parallel step-shaped depressions or elevations are present, wherein the depressions or elevations of one of the sub-regions run parallel or not parallel, in particular orthogonally, to the depressions or elevations of at least one other of the sub-regions.

[0028] This makes it possible to further utilize the non-planar refractive index variation in the second region to generate local differences in the laser modes in the subregions. The subregions can be generated on a single mesa of the VCSEL or on electrically separated mesae of the VCSEL.

[0029] The different sub-regions can have a predetermined distance from each other in order to achieve predetermined laser mode properties.

[0030] Furthermore, in a preferred embodiment, it is provided that a layer of a further material, which is at least different from the first material, is present on the at least one first region and the at least one second region.

[0031] The layer or coating made of the further material can be a protective coating or passivation layer. The further material can form a further refractive index jump with the second material in the first region, and in the second region, the further material can form a refractive index jump with the first material, the latter, however, being as high a refractive index jump as possible. The refractive index of the further material can be equal to or lower than the refractive index of the second material, and is in particular lower than the refractive index of the first material to ensure the high refractive index jump in the second region.

[0032] In a further preferred embodiment, the second material can be distributed on the facet asymmetrically with respect to an axis of symmetry of the facet, wherein the facet is in particular circular.

[0033] The asymmetric distribution of the second material can reduce noise between laser modes (intermode noise) and thus achieve natural mode separation in the VCSEL, even if the VCSEL has a circular mesa and thus usually a round current confinement aperture.

[0034] Further preferably, the second material can also be present locally in the second region. For example, the aforementioned elevations in the second region can be made of the second material instead of the first material if it is easier to produce such elevations from the second material than from the first material of the semiconductor layer structure. Thus, it can be easier to form a plurality of elevations from the second material rather than introducing them into the first material by etching.

[0035] Further advantages and features can be found in the following description and the attached drawing.

[0036] It is understood that the features mentioned above and those to be explained below can be used not only in the combination specified in each case, but also in other combinations or on their own, without departing from the scope of the present invention.

[0037] Embodiments of the invention are illustrated in the drawings and will be described in more detail below with reference to them. They show: Fig. 1 shows a schematic side view of the structure of a VCSEL; Fig. 2 schematically shows a VCSEL in the region of its facet in a section perpendicular to the semiconductor layer structure of the VCSEL in a first embodiment; Fig. 3 schematically shows a plan view of a facet of a VCSEL according to another embodiment; Fig. 4 schematically shows a plan view of a facet of a VCSEL according to another embodiment; Fig. 5 schematically shows a plan view of a facet of a VCSEL according to yet another embodiment; Fig. 6 schematically shows a VCSEL in the area of ​​its facet in a section corresponding to Fig. 2 of a further embodiment Figs. 7 to 9 are schematic plan views of a facet of a respective VCSEL according to three further embodiments; Fig. 10 schematically shows a VCSEL in the region of its facet in a section corresponding to Fig. 2 according to a further embodiment; Fig. 11 schematically shows a VCSEL in the region of its facet in a section corresponding to Fig. 2 according to a further embodiment; Fig. 12 schematically shows a VCSEL in the region of its facet in a section corresponding to Fig. 2 according to a further embodiment; and Figures 13A to 13N schematically show a process flow for manufacturing a VCSEL in the region of its facet.

[0038] Before describing embodiments of a VCSEL according to the invention, ie, a vertical cavity surface-emitting laser, the basic structure of a VCSEL 10 will first be described in general terms with reference to Fig. 1. This can be present in particular in the VCSELs 10 to be described below, without this being mentioned again later.

[0039] The VCSEL 10 has a semiconductor layer structure 12 comprising a first mirror 14, an active region 16, and a second mirror 18. The first mirror 14, the active region 16, and the second mirror 18 form an optical cavity in which laser light is generated. The first mirror 14 and the second mirror 18 can comprise a plurality of semiconductor layer pairs, the two layers of which alternately have a lower and a higher refractive index. The first mirror 14 and the second mirror 18 can each be designed, in particular, as a DBR (Distributed Bragg Reflector). The active region 16 can comprise one or more quantum wells. The semiconductor layer structure 12 can be based on a semiconductor material system, such as gallium arsenide / aluminum gallium arsenide, or other material systems typical for VCSELs.

[0040] The semiconductor layer structure 12 can be grown epitaxially on a substrate 20, ie the individual layers of the semiconductor layer structure are deposited epitaxially one after the other, starting on the substrate 20.

[0041] The VCSEL 10 further comprises an electrical contact arrangement (not shown) for electrically pumping the active region 16.

[0042] Furthermore, the VCSEL 10 can have a current confinement aperture 22, which can be formed as an opening in an oxide diaphragm 24. One of the two mirrors 14 or 18, here mirror 18, can serve as an output mirror to emit the laser light generated in the optical cavity, as indicated by an arrow 24. A surface 26 of the VCSEL 10 is also referred to as a facet of the VCSEL 10.

[0043] Embodiments of the facet 26 are described below with reference to the further Figures 2 to 12. For simplicity, the same reference numerals are used in Figures 2 to 12 for identical, similar, or comparable elements. In Figures 2 to 12, the respective VCSEL 10 is shown only in the region of its respective facet 26.

[0044] The embodiments to be described below have in common that they have two or more refractive index jumps or steps in a non-planar Topography of the facet 26, which enables localized or localized tuning of the laser welding value in order to separate or define laser modes and / or polarization states from one another. This is achieved by localizing the refractive index jumps at defined positions on the facet 26 of the VCSEL 10, which are preferably radially symmetrical. Parts of the optical mode or modes therefore interfere exclusively at these refractive index jumps. "Positions" of the facet 26 are not exclusively point locations, but also locally limited regions of the facet 26.

[0045] Fig. 2 shows a VCSEL 10, whose semiconductor layer structure 12 is shown only in a section in the area of ​​facet 26 of the VCSEL 10. The section of the semiconductor layer structure 12 shown in the area of ​​facet 26 can, for example, be the mirror 18 in Fig. 1. V denotes the end of the semiconductor layer structure 12 after its epitaxial production. V is also referred to as the epi level.

[0046] The facet 26 has a first material M1, which is a material of the semiconductor layer structure 12. The material M1 is, for example, gallium arsenide or aluminum gallium arsenide if the semiconductor layer structure 12 uses the gallium arsenide / aluminum gallium arsenide material system.

[0047] The facet 26 further comprises at least one layer 30 made of at least one second material M2. The layer 30 is arranged only partially on the first material M1. The second material M2 is also different from the first material M1 and, in particular, has a different refractive index than the material M1.

[0048] Since the second material M2 is only arranged in regions on the first material M1, a first region I results in which the second material 30 is present, so that in the region I, a first refractive index jump is formed at an interface 32 between the material M1 and the material M2. In the region II, which is different from the region I, at least one second refractive index jump is formed locally at an interface 34 with the material M4, wherein the material M4 can in particular be air. The second refractive index jump at the interface 34 is different from the refractive index jump at the interface 32.

[0049] The refractive index jump at the interface 32 between the material M1 and the material M2 is in particular smaller than the refractive index jump at the interface 34. The material M2 can be, for example, silicon nitride, whose refractive index is smaller than the refractive index of the material gallium arsenide or aluminum gallium arsenide. However, it is understood that the invention is not limited to the material systems mentioned, but can also be used with other material systems. The refractive index jump in the region II is formed locally at a non-planar structure or topography of the region II. The non-planar topography in the region II can, as shown in Fig. 2, be formed by one or more depressions 36 in the semiconductor layer structure 12 at the facet 26. In other words, the refractive index jump in the region II is formed at a non-planar structure formed by the depression(s) 36.The depression can be a single, in particular annular depression (see also Fig. 3 or 4), or it can comprise a plurality of individual depressions (see also Fig. 5). Even if the depressions 36 were missing in region II, a non-planar topography of the facet 26 is provided by an edge 37 of the layer 30; in particular, a refractive index jump and a second refractive index jump are present at a non-planar structure.

[0050] Fig. 2 shows a light mode 38 with an intensity distribution along the facet 26, wherein a higher density of the hatching illustrates a higher intensity and a lower density of the hatching illustrates a lower intensity of the light mode 38. At a position (or local region) P1, i.e. in region I, the light mode 38 sees the lower refractive index jump at the interface 32 between the material M1 and the material M2. The intensity of the light mode 38 in this region is correspondingly lower. By selecting the thickness of the layer 30 made of the material M2, the reflectivity / transmissivity of the facet 26 in the region I can be adjusted as required, i.e. by increasing the thickness of the layer 30, the reflectivity in the region I can be increased or the transmissivity can be reduced.

[0051] At a position (local region) P2, the light mode 38 sees the higher refractive index jump at the interface 34 between the material M1 and the material M4 (e.g., air). In this region, the intensity of the laser mode 38 is correspondingly high, particularly in the center of region II. Due to the recess or recesses 36, which extend into the semiconductor layer structure 12, e.g., the mirror 18 in Fig. 2, thereby partially opening the cavity, the reflectivity of the mirror 18 can be adjusted such that the desired laser mode 38 can be emitted with high intensity in region II, particularly in the region within the recess(es) 36.

[0052] A depth hi of the depression(s) 36 and / or a width D of the depression(s) 36 and, in the case where there is not a single annular depression 36 but rather a plurality of, for example, parallel depressions 36, a pitch P can be smaller or larger than the wavelength of the laser mode propagating in the optical cavity. If D and / or hi and / or P are selected to be smaller than the wavelength of the laser mode 38, the laser mode 38 can be stabilized with regard to its polarization state due to the local refractive index jump at the depression(s) 36. If D and / or hi and / or P are selected to be greater than or equal to the wavelength of the laser mode 38, so that the depression(s) 36 are visible to the laser mode 38, additional filtering (mode separation) or tuning of the reflectivity is possible.

[0053] Instead of depressions 36, the region II can also have local elevations made of the material M1 or M2, as is provided in embodiments to be described later, which have a corresponding effect on the laser mode separation and / or polarization stabilization.

[0054] In the embodiment in Fig. 2, the two different refractive index jumps M1 / M2 and M1 / M4 are located at different levels in the vertical direction of the VCSEL 10, where the vertical direction is defined as the direction from the first mirror 14 to the second mirror 18 in Fig. 1 (also referred to as the z-direction).

[0055] Region I in Fig. 2 is an outer region of facet 26, and region II is a region within region I. As shown in Figs. 3 to 5, region I can surround region II in a ring shape, wherein material M2 can be present throughout the entire region I, i.e., layer 30 is continuous in the circumferential direction. In Figs. 3 and 4, facet 26 is not circular, but oval or elliptical, although facet 26 can also be circular.

[0056] According to Figures 3 and 4, region II has two sub-regions lh and II2, wherein sub-region lh has, in addition to an annular depression 36i, a plurality of parallel stepped depressions 36; (here i = 1, 2, 3, 4) within it. Sub-region II2 has, in addition to an annular depression 36H, a plurality of parallel stepped depressions 36j, (here j = 1, 2, 3, 4). The parallel stepped depressions 36; in sub-region l do not run parallel to the stepped depressions 36j in sub-region II, but rather orthogonally here. As shown in Figures 3 and 4, it is possible to combine non-planar refractive index jumps at position P2 in Figure 2 with different effects in sub-regions lh and II2 respectively on a single mesa. The difference between the embodiment in Fig. 4 and the embodiment in Fig. 3 is that the sub-regions lh and II2 in Fig.4 are spaced apart from each other by a greater distance than in Fig. 3, whereby they partially merge into each other in Fig. 3. The selection of this distance also allows the properties of the VCSEL 10 with regard to laser threshold and polarization state to be specifically adjusted in combination.

[0057] In Fig. 5, the two subregions lh and II2 with the respective depressions 36; and 36j are distributed over two mesas 40, 42 of the VCSEL 10, which are electrically separated from each other.

[0058] Fig. 6 shows a further embodiment of a VCSEL 10 in the region of its facet 26. The facet 26 of the VCSEL 10 in Fig. 6 has a further layer 46 made of at least one further material M3 in both region I and region II, wherein the layer 46 can also be present in only one of the regions I and II. In region I, the layer 46 made of material M3 is arranged on the layer 30 made of material M2, and in region II, the layer 46 is arranged on the material M1. The material M4 is, for example, air again. In region I (position or local region P1), a refractive index jump occurs at the interface M1 / M2 that is smaller than the refractive index jump in region II (position or local region P2) at the interface M1 / M3. The refractive index of the material M3 can be less than or equal to the refractive index of the material M2. In region I, a first refractive index jump occurs at the interface M1 / M2 and, if applicable, a second refractive index jump occurs at the interface M2 / M3. Overall, the refractive index jumps or the thickness of the layers 30 and 46 in region I can be adapted such that the laser mode 38 has no intensity in region I, as shown in Fig. 6. In particular, the thicknesses of the layers 30 and 46 in region I can be adapted so that no or at most a very slight coupling-out of the laser mode 38 into region I occurs.In region II (position or local region P2), the laser mode 38 again sees a higher refractive index jump at the interface M1 / M3, whereby this refractive index jump can be adapted by appropriate choice of the material M3 to give the laser mode 38 defined properties.

[0059] In contrast to the embodiment in Fig. 2, region II in Fig. 6 has no depressions or elevations. The facet 26 of the VCSEL 10 in Fig. 6 has a non-planar topography due to the layer 30 being arranged only in certain regions on the material M1.

[0060] Figures 7 to 9 show schematic plan views of further embodiments of facets 26 of VCSELs 10 in which further aspects are realized.

[0061] The VCSELs 10 in Figures 7 to 9 have a circular facet 26. In the embodiment in Figure 7, the layer 30 made of material M2 is arranged asymmetrically on the material M1 with respect to the symmetry axis of the facet 26. This causes a natural laser mode separation in the VCSEL 10. This has the advantage of reducing intermode noise in the VCSEL 10, in particular between unwanted and desired laser modes. Due to the asymmetric coverage by the material M2, this advantage is achieved despite a circular facet 26 or a circular mesa and, accordingly, a circular oxide aperture 24, which is also schematically shown in Figures 7 to 9. Because at the position (or local Since the lower refractive index jump is present in the region P1, larger manufacturing tolerances can be tolerated before the effect of natural laser mode separation degrades. At position (or local region) P2, the reflectivity is high.

[0062] In the embodiment shown in Fig. 8, the coverage of layer 30 of material M2 on material M1 extends toward the center of facet 26 as far as the oxide aperture 24. Furthermore, in region II, depressions 36, which here are point-symmetrical to the center of facet 26, are introduced into the material M1 to provide an additional non-planar topography in region II, thereby locally opening the optical cavity and increasing the laser threshold there. In contrast, the threshold for the second-order laser mode with locally distributed intensity is reduced.

[0063] In the embodiment in Fig. 9, the coverage of material M1 with material M2 in region I is smaller than in the embodiment in Fig. 8; in particular, the coverage does not extend as far as the oxide aperture 24. Furthermore, material M2 is additionally applied locally to material M1 in region II in order to locally increase the reflectivity in region II by appropriately selecting the thickness of material M2 in region II. In these regions, the laser threshold is thus locally increased.

[0064] Fig. 10 shows an embodiment of a VCSEL 10 with a facet 26 with a non-planar topography. In region I, a layer 30 made of material M2 is arranged on material M1. In region II, material M2 is arranged locally with a plurality of elevations 50, also on material M1. The advantage here is that the structure can be produced more easily from material M2 than if it were produced from material M1 by etching. In region II, at position (local region) P1, depressions 36 are further present in material M1, whereby the optical cavity is locally opened in this region without this region being covered with material M2, in order to provide a reflectivity that is not too low there, but to confine the fundamental laser mode 38 in a plane perpendicular to the vertical direction, i.e., in the x, y direction.

[0065] At the position (local region) P2 where the material M2 is located, a smaller refractive index jump occurs at the interface between the material M1 and the material M2 than between the material M2 and the material M3, without the overall reflectivity of the VCSEL 10 being reduced. Larger manufacturing tolerances can be compensated for due to the smaller refractive index jump, i.e., by a smaller percentage change in the effective refractive index for tolerances in the parameters D, h, and / or P. Laser mode selection between TE and TM laser mode can also be achieved by adjusting the parameters D, h, and P accordingly.

[0066] If the parameters D, h, and P are chosen smaller than the laser wavelength, the polarization state of the laser mode can be stabilized due to the local refractive index jump. If the parameters D, h, and P are chosen equal to or larger than the laser wavelength, additional filter / reflectivity tuning is possible.

[0067] Fig. 11 shows a VCSEL 10 with a facet 26 which, in region II, has a plurality of locally limited elevations 56 made of material M1, i.e., made of the material of the semiconductor layer structure 12, the elevations being realized by removing material M1 to a depth h to the side of the elevations. In other words, depressions of depth h are located in the material M1 to the side of the elevations 56. The elevations 56 are present locally in the center of region II, i.e., not in the entire region II. At position P3, this results in a local periodic change in the refractive index perpendicular to the stepped elevations 56. If the parameters D and / or h and / or P are selected such that they are smaller than the laser wavelength of the laser mode 38, the mode can be stabilized with regard to its polarization state due to this local refractive index variation.If the parameters D and / or h and / or P are selected such that the structure is visible for laser mode 38, additional filter / reflectivity tuning is possible.

[0068] At position (or local area) P1, the refractive index jump is lowest, namely at the interfaces M1 / M2 and M2 / M3, further closing the optical cavity of the VCSEL 10. This allows the use of a larger oxide aperture 24 for reduced current density within the VCSEL 10, for lower Laser thresholds and increased ESD safety, i.e. reducing the risk of voltage breakdowns.

[0069] At the position (local region) P2, the optical cavity is locally opened, but without coverage by the material M2, in order to achieve a not too low reflectivity, but a confinement of the fundamental mode in the plane perpendicular to the vertical direction.

[0070] In the region I, a metallization 60 can further be provided, which prevents the re-entry of reflected laser light into the optical cavity.

[0071] Fig. 12 shows a VCSEL 10 with a facet 26 with a non-planar topography. In region I, in which the material M2 is arranged on the material M1, the semiconductor layer structure 12 is provided with a recess 66 in the material M1. This partially opens the optical cavity so that light 68, which is reflected back from a fiber end, for example, can penetrate into the optical cavity. However, the position (local region) P1 lies within the oxide aperture 24, so that losses of the VCSEL 10 due to unused pumped charge carriers in this region are reduced. In region II, the facet 26 has local elevations 56 made of the material M1, as in the embodiment in Fig. 11, so that a local periodic refractive index variation occurs at the position (local region) P3. The same applies to the parameters D, h, P as to the parameters D, h, P in Fig. 11.

[0072] At the position (local region) P2, the optical cavity is locally opened by a recess 36, where no material M2 is arranged on the material M1 in order to provide a reflectivity that is not too low, but to confine the fundamental mode in the x-, y-direction.

[0073] With reference to Figures 13A to 13N, a method for fabricating a VCSEL facet in accordance with the principles of the present invention is described below.

[0074] According to Fig. 13A, a layer 80 made of a material suitable, for example, for nanoimprint lithography is applied to the semiconductor layer structure 12, more precisely to the material M1. Nanoimprint lithography is typically used for the cost-effective production of nanostructures.

[0075] According to Fig. 13B and Fig. 13C, the layer 80 is structured by means of nanoimprint lithography, wherein a structure is shown as an example in Fig. 13C.

[0076] According to Fig. 13D, a photomask PH1 is arranged on layer 80. After exposure, the areas of layer 80 not covered by photomask PH1 are removed. According to Fig. 13F, photomask PH1 is removed again. In the next process step according to Fig. 13G, another photomask PH2 is arranged on layer 80. After exposure, areas 90 of material M1 are removed. According to Fig. 13I, photomask PH2 and the remaining areas of layer 80 are removed. According to Fig. 13J, a layer of material M2 is applied over the entire area of ​​material M1. Subsequently, according to Fig. 13K, another photomask PH3 is arranged on material M2. After exposure, the areas of material M2 not covered by photomask PH3 are removed, as shown in Fig. 13L. In the next step, photomask PH3 is removed.The resulting structure has a region-by-region arrangement of the material M2 on the material M1 of the semiconductor layer structure 12. Likewise, local structures in the form of depressions 36 are present in the material M1 as a result of the previous steps. According to a step 13N, a layer of the material M3 can be applied to the arrangement in Fig. 13M. Regions I and II are illustrated in Fig. 13N, wherein in region I the material M2 is arranged on the material M1 in order to realize a first refractive index jump in region I, while in region II a different refractive index jump is realized, and furthermore, local refractive index variations are formed due to the depressions 36.

Claims

Patent claims 1. A VCSEL comprising a semiconductor layer structure (12) stacked in a vertical direction, said semiconductor layer structure comprising a first mirror (14), an active region (16), and a second mirror (18) to form an optical cavity in which laser light is generated, wherein the laser light is coupled out via a facet (26) of the VCSEL, wherein the facet (26) comprises a first material (M1), which is a material of the semiconductor layer structure (12), and at least one layer (30) made of at least one second material (M2), which is arranged in regions on the first material (M1), wherein the second material (M2) is different from the first material (M1), wherein in a first region (I) of the facet (26), in which the second material (M2) is present, at least one first refractive index jump is formed, and in at least one second region (II) of the facet (26), which is different from the first region (I), at least one second refractive index jump is formed,wherein the at least one second refractive index jump is different from the at least one first refractive index jump, and wherein the facet (26) has a non-planar topography., 2. VCSEL according to claim 1, wherein the at least two different refractive index jumps are located at different levels when viewed in the vertical direction.

3. VCSEL according to claim 1 or 2, wherein the first region (I) is an outer region of the facet (26) and the at least one second region (II) is a region within the first region (I).

4. VCSEL according to claim 3, wherein the first region (I) surrounds the second region (II) in a ring shape and the second material (M2) is present in the entire first region (I).

5. VCSEL according to one of claims 1 to 4, wherein the at least one second refractive index jump is greater than the at least one first refractive index jump.

6. VCSEL according to one of claims 1 to 5, wherein the at least one second region has at least one stepped depression (36; 36j, 36j) or elevation (50; 56).

7. VCSEL according to claim 6, wherein the at least one second region (II) has a plurality of local step-shaped depressions (36j, 36j) or elevations (50; 56) such that the refractive index varies along the second region (II) in a plane transverse to the depressions (36j, 36j) or elevations (50; 56).

8. VCSEL according to claim 6 or 7, wherein a depth or height (h) and / or a width (D) of the at least one stepped depression (36) or elevation (50; 56) and / or in the case of a plurality of stepped depressions (36j, 36j) or elevations (50; 56) a pitch (P) of adjacent stepped depressions (36j, 36j) or elevations (50; 56) is smaller than the wavelength of the laser wavelength propagating in the cavity.

9. VCSEL according to claim 6 or 7, wherein a depth or height (h) and / or a width (D) of the at least one stepped depression (36) or elevation (50; 56) and / or in the case of a plurality of stepped depressions (36j, 36j) or elevations (50; 56) a pitch (P) of adjacent stepped depressions (36j, 36j) or elevations (50; 56) is equal to or greater than the wavelength of the laser wavelength propagating in the cavity.

10. VCSEL according to one of claims 1 to 9, wherein the at least one second region (II) has at least one sub-region (I h, II2) in which a plurality of step-shaped depressions (36j, 36j) or elevations are present.

11. VCSEL according to claim 10, wherein the at least one second region (II) has at least two subregions (lh, II2), in each of which a plurality of parallel step-shaped depressions (36j, 36j) or elevations are present, wherein the depressions (36j, 36j) or elevations of one of the subregions (lh, II2) run parallel or non-parallel, in particular orthogonally, to the depressions or elevations of at least one other of the subregions (lh, II2).

12. VCSEL according to one of claims 1 to 11, wherein on the at least one first region (I) and / or the at least one second region (II) there is a layer (46) made of a further material (M3) which is different at least from the first material (M1).

13. VCSEL according to one of claims 1 to 12, wherein the second material (M2) is distributed on the facet (26) asymmetrically with respect to an axis of symmetry of the facet (26), wherein the facet (26) is in particular circular.

14. VCSEL according to one of claims 1 to 13, wherein the second material (M2) is additionally present locally in the second region (II).

15. VCSEL according to claim 14, wherein the second material (M2) is arranged in the second region (II) in the form of individual elevations.

16. VCSEL according to one of claims 1 to 15, wherein the second refractive index jump is formed by an interface (34) of the first material (M1) to air.

17. VCSEL according to claim 12 or any one of claims 13 to 15 when dependent on claim 12, wherein the second refractive index jump is formed by an interface between the first material (M1) and the further material (M3).

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