Vcsel for emission of laser radiation and method of manufacturing a vcsel

Abstract

A VCSEL for emitting laser radiation is described, comprising a semiconductor layer structure having a plurality of semiconductor layers in a stacking direction, wherein the semiconductor layer structure comprises a resonator with a first mirror, a second mirror, and an active region between the first and second mirrors for generating laser radiation. The resonator further comprises an optical laser mode confinement structure decoupled from a current confinement structure that defines a laser mode guiding aperture having, in a plane perpendicular to the stacking direction, a greater extent in at least one dimension than a sum of extents of current apertures. An effective refractive index step from the laser mode guiding aperture to a region of the semiconductor layer structure that is laterally adjacent to the laser mode guiding aperture is greater than an effective refractive index step from the current apertures to regions of the semiconductor layer structure that are laterally adjacent to the current apertures.

Description

VCSEL for emission of laser radiation and method of manufacturing a VCSEL

[0001] The invention relates generally to a vertical cavity surface-emitting laser, referred to in abbreviated form as VCSEL (Vertical Cavity Surface Emitting Laser). More specifically, the invention relates to a VCSEL for emitting laser radiation, comprising a semiconductor layer structure having a plurality of semiconductor layers in a stacking direction, wherein the semiconductor layer structure comprises a resonator with a first mirror, a second mirror, and an active region between the first and the second mirror for generating laser radiation.

[0002] A VCSEL is a laser diode that emits laser radiation vertically from the surface of a manufactured wafer. VCSELs are advantageous due to their circular beam shape, which can even be Gaussian, their low beam divergence, and various possible light emission modes (multimode and single mode), making VCSELs suitable for a large number of applications.

[0003] VCSELs are known that emit a single fundamental mode. In addition, VCSELs are known that emit a single higher-order laser mode. A VCSEL capable of emitting a single higher-order laser mode is described in document EP 4 131 676 A1 and in document WO 2024 / 002906 A1. These known VCSELs have an electrically pumpable active region defined by a current confinement structure in the form of an oxide aperture. A homogeneous gain profile establishes itself in the electrically pumpable active region. To stabilize a desired higher-order laser mode, these known VCSELs have surface reliefs or localized reflective sub-wavelength gratings. The surface reliefs or sub-wavelength gratings result in a locally varying reflectivity of the outcoupling mirror such that the desired laser mode has the best overlap with these localized reliefs or gratings, whereby this laser mode has the lowest threshold at which gain occurs and can begin to lase, while all other laser modes suffer higher output coupling losses and should not lase or should at least be suppressed at the target operating current.

[0004] Laser mode stabilization in these known VCSELs thus takes place by means of measures at the surface of the VCSEL. Other higher-order laser modes than the desired laser mode can nevertheless be excited in the active medium of the active region of the laser cavity, as a result of which the efficiency of the VCSEL is not optimal. In other words, the gain profile in the active region is not optimally adapted to the desired laser mode profile. The stabilization of the desired higher-order laser mode by means of mode stabilization structures at the surface of the VCSEL also places high demands on the precision of the mode stabilization structure.

[0005] An object of the invention is to provide a VCSEL for emitting laser radiation in which the stabilization of a higher-order laser mode and the efficiency of the VCSEL are improved.

[0006] According to the invention, to achieve this object, a VCSEL for emitting laser radiation is provided, comprising a semiconductor layer structure having a plurality of semiconductor layers in a stacking direction, wherein the semiconductor layer structure comprises a resonator with a first mirror, a second mirror, and an active region between the first and the second mirror for generating laser radiation, wherein the resonator further comprises a current confinement structure between the first and second mirror that defines a plurality of current apertures that provide a locally varying current distribution in the active region during operation of the VCSEL, and wherein the resonator comprises an optical laser mode confinement structure decoupled from the current confinement structure that defines a laser mode guiding aperture having, in a plane perpendicular to the stacking direction, a greater extent in at least one dimension than the sum of the extents of the current apertures, wherein an effective refractive index step from the laser mode guiding aperture to a region of the semiconductor layer structure that is laterally adjacent to the laser mode guiding aperture is greater than an effective refractive index step from the current apertures to regions of the semiconductor layer structure that are laterally adjacent to the current apertures.

[0007] Whereas in the known VCSELs the oxide aperture defines both the gain profile in the active region and the waveguide effect, in the VCSEL according to the invention these two functions are decoupled from one another. The current confinementstructure of the VCSEL according to the invention has a plurality of current apertures that provide a locally varying current distribution in the active region during operation of the VCSEL. As a result, the active region has two or more electrically pumpable regions through which the drive current can flow, while no current flows through the remaining regions of the active region. This achieves the result that instead of a homogeneous gain profile as in the known VCSELs, a non-homogeneous, i.e. locally varying, gain profile is generated in the active region, with the advantage that, depending on the arrangement of the current apertures, only a specific laser mode whose intensity peaks have sufficient overlap with the current apertures can form in the active region, while other potential laser modes whose intensity peaks have no or only little overlap with the current apertures are already prevented from forming in the active region. The laser mode guiding aperture of the laser mode confinement structure, which is decoupled from the current confinement structure, acts as an optical waveguide that determines the shape and position of the laser mode. The laser mode guiding aperture is defined by an effective refractive index step to the region of the semiconductor layer structure that is laterally adjacent to the laser mode guiding aperture, which is sufficiently large to generate the waveguide effect. For a definition of the “effective refractive index step”, reference is made to the article by G. R. Hadley, “Effective-index model for vertical-cavity surface-emitting lasers”, Optics Letters 20, 1483 (1995). The current apertures, in contrast, have a smaller effective refractive index step to the regions of the semiconductor layer structure laterally adjacent to the current apertures, so that the current apertures exert substantially no waveguide effect on the laser mode and thus the laser mode guidance is well decoupled from the current flow or guidance, i.e. the current apertures do not, or do not substantially, alter the shape of the laser mode determined by the laser mode guiding aperture. The laser mode guiding aperture is, in a plane perpendicular to the stacking direction, larger in at least one dimension, preferably in two mutually perpendicular dimensions, than the total size of the current apertures in the corresponding dimension. By virtue of the fact that the current distribution in the active region varies locally due to the plurality of current apertures, only a specific higher-order laser mode is generated in the active region, whereby the stability of this laser mode and the efficiency of the VCSEL are improved.

[0008] Preferably, the current apertures are arranged at a node of the standing wave field forming in the resonator during operation of the VCSEL. This measure helps tokeep the effective refractive index step at the current apertures small, since the overlap of the standing wave field with the semiconductor layers, which have a different refractive index, contributes to the effective refractive index step. This contribution is smaller at a node of the standing wave field than outside a node of the standing wave field. In contrast, it is preferred if the laser mode guiding aperture is positioned outside a node of the standing wave field, whereby the effective refractive index step at the laser mode guiding aperture is increased.

[0009] Preferred embodiments of the VCSEL according to the invention are described below and / or are set out in the dependent claims.

[0010] Preferably, the laser mode guiding aperture is greater in the at least one dimension by at least 10%, preferably by at least 20%, further preferably by at least 30%, than the sum of the extents of the current apertures in that dimension.

[0011] In other words, it is preferred if the laser mode guiding aperture is, on the one hand, sufficiently spaced apart from the edges of the laterally outer current apertures in the length dimension (x-dimension) and / or in the width dimension (y-dimension) to stabilize the desired laser mode. On the other hand, the lateral extent of the laser mode guiding aperture should also not be too large relative to the current apertures, in order to avoid obtaining laser mode peaks outside the regions in which a current aperture exists.

[0012] Thus it is preferred if the extent of the laser mode guiding aperture is at most 10 pm, preferably 6 pm, further preferably 4 pm longer than the extent from an outer edge of a first laterally outer current aperture to an opposite outer edge of a last laterally outer current aperture.

[0013] Further preferably, the laser mode guiding aperture is spaced apart, in at least one dimension perpendicular to the stacking direction (13), from an outer edge of a laterally outer current aperture (26) facing the laser mode guiding aperture (34) by at least 0.25 pm, preferably at least 0.5 pm.

[0014] Preferably, the current apertures each have, laterally to the stacking direction of the semiconductor layer structure, a dimension in a range of 1 pm to 6 pm, preferably 1.5 pm to 5 pm, further preferably 2 pm to 4 pm.

[0015] Adjacent current apertures may be spaced apart from one another in the lateral direction or may partially overlap one another in the lateral direction. In the latter case, current apertures may, for example, partially overlap one another in a first dimension (x-dimension) perpendicular to the stacking direction, while the size of the current apertures varies locally in a second dimension (y-dimension) perpendicular to the first dimension.

[0016] A center-to-center spacing between directly adjacent current apertures may be in a range of 1.25 pm to 10 pm, preferably 2 pm to 8 pm, further preferably 2.75 pm to 4 pm. This preferably applies not only to current apertures that are spaced apart from one another, but also to current apertures that partially overlap one another.

[0017] The stated sizes of the current apertures and center-to-center spacings between directly adjacent current apertures can in particular be matched to the size and spacings of intensity peaks of a specific desired single higher-order laser mode.

[0018] Further preferably, a difference between the effective refractive index step at the laser mode guiding aperture and the effective refractive index step at the current apertures is greater than 0.002, preferably greater than 0.005, further preferably greater than 0.01.

[0019] The larger the differentiation of the effective refractive index step at the laser mode guiding aperture from the effective refractive index step at the current apertures, the more distinctly the waveguide effect is decoupled from the gain profile in the active region.

[0020] The effective refractive index step from the current apertures to the regions of the semiconductor layer structure that are laterally adjacent to the currentapertures is preferably as small as possible, preferably however less than 0.004, further preferably less than 0.002, further preferably less than 0.001, further preferably 0.

[0021] In contrast, the effective refractive index step from the laser mode guiding aperture to the region of the semiconductor layer structure that is laterally adjacent to the laser mode guiding aperture is preferably as large as possible, preferably greater than 0.002, further preferably greater than 0.004, still further preferably greater than 0.01.

[0022] The larger the effective refractive index step from the laser mode guiding aperture to the region of the semiconductor layer structure that is laterally adjacent to the laser mode guiding aperture, the more pronounced the waveguide effect and thus the better defined the shape and position of the laser mode.

[0023] As already described above, the current apertures are preferably arranged at or near a node of a standing wave field forming in the resonator during operation of the VCSEL, and the laser mode guiding aperture is preferably arranged outside a node of the standing wave field.

[0024] Preferably, positions of the current apertures are matched to positions of intensity peaks of a predetermined single higher-order laser mode to be emitted by the VCSEL.

[0025] If, for example, the predetermined single higher-order laser mode to be emitted has N intensity peaks, where N is a natural number, the current confinement structure in this example preferably has N current apertures arranged at positions along the active region that correspond to positions of the intensity peaks of the desired higher-order laser mode.

[0026] The current apertures are preferably arranged in the vicinity of the active region at positions at which the desired higher-order laser mode has an intensity greater than 5%, preferably greater than 10%, preferably greater than 15% of the maximum intensity of the laser mode.

[0027] The laser mode guiding aperture preferably collectively surrounds the plurality of current apertures. In this configuration, the laser mode confinement structure thus has a single laser mode guiding aperture that surrounds all current apertures.

[0028] Preferably, the current confinement structure comprises a plurality of tunnel diodes that define the current apertures.

[0029] The tunnel diodes may, as is customary for tunnel diodes in VCSELs, comprise a heavily n-doped layer (n++) and a heavily p-doped layer (p++). The tunnel diodes are poled in reverse with respect to the pn junction or junctions of the resonator, i.e. are connected in the reverse bias direction. The individual tunnel diodes may be realized by first arranging a tunnel diode layer with an n++and a p++layer in the semiconductor layer structure in the vicinity of the active region, while the tunnel diode layer is then removed, i.e. structured, in one or more regions in order to form the individual tunnel diodes. Alternatively, the tunnel diode layer may also be partially deactivated by ion implantation.

[0030] Preferably, the tunnel diodes are arranged at a node of a standing wave field forming in the resonator during operation of the VCSEL.

[0031] An advantage here is that optical absorption losses due to the high doping in the tunnel diodes are reduced, since the intensity of the standing wave field at the nodes is low.

[0032] It is also possible to form the current confinement structure — without providing a tunnel diode layer — by means of ion implantation, wherein the current apertures are defined in regions outside the ion implantation. In the region of the ion implantation, the conductivity of the semiconductor layers is reduced, so that current flow occurs only through the regions that have not been treated with ion implantation.

[0033] The current apertures may each be circular or elliptical. The size, shape, and / or mutual spacings of the current apertures may vary among the current apertures.

[0034] The laser mode guiding aperture may be elongated in the lateral direction, in particular rectangular, in particular rectangular with rounded corners, or ringshaped. The laser mode guiding aperture may also be square, preferably with rounded corners, whereby the current apertures can thus be distributed over a larger area within the perimeter of the laser mode guiding aperture, i.e. they are then not arranged in a single row. A ring-shaped configuration of the laser mode guiding aperture is also possible, in which case the current apertures form a ring-shaped arrangement.

[0035] In a further embodiment, the first or the second mirror may be an out-coupling mirror for coupling out laser radiation, wherein a reflective grating structure, in particular a polarization-selective grating structure, or a relief structure may be arranged above the outcoupling mirror and provides, on the emission side in combination with the reflectivity of the outcoupling mirror, an increased reflectivity of the resonator in the region of the respective current aperture.

[0036] This measure, which is implemented in the known VCSELs described above, may also be provided in the VCSEL according to the invention, however with the advantage that the precision requirements for the reflective grating structure or relief structure are reduced compared to the known VCSELs, since laser mode selection and stabilization already takes place in the active region by means of the non-homogeneous gain profile in the active region.

[0037] In a further preferred embodiment, the resonator may comprise, in at least one lateral region of the resonator outside the laser mode guiding aperture, at least one additional current aperture, preferably wherein the resonator comprises, on the emission side, a structure opaque to laser radiation generated in the resonator in the region of the at least one additional current aperture.

[0038] With this measure, at least one additional conductive region is created outside the actual current confinement structure, which does not contribute to the laser mode or the total output power of the VCSEL but serves to homogenize the temperature profile within the laser mode confinement structure. The structure opaque to laserradiation generated in the resonator, which is preferably provided in the region of the at least one additional current aperture, serves to prevent light from being emitted at these positions.

[0039] According to the invention, a method of manufacturing a VCSEL is furthermore provided, comprising the steps of:a) fabricating a semiconductor layer structure from a plurality of semiconductor layers arranged one above the other in a stacking direction, comprising a resonator with a first mirror, a second mirror, and an active region for generating laser radiation between the first mirror and the second mirror; wherein step a) comprises the steps of:b) fabricating a current confinement structure in the resonator that defines a plurality of current apertures that provide a locally varying current distribution in the active region during operation of the VCSEL;c) fabricating a laser mode confinement structure decoupled from the current confinement structure to define a laser mode guiding aperture in the resonator;wherein steps b) and c) are carried out such that the laser mode guiding aperture is larger in a plane perpendicular to the stacking direction in at least one dimension than the current apertures in total, and such that an effective refractive index step from the laser mode guiding aperture to a region of the semiconductor layer structure that is laterally adjacent to the laser mode guiding aperture is greater than an effective refractive index step from the current apertures to regions of the semiconductor layer structure that are laterally adjacent to the current apertures.

[0040] With the method according to the invention, a VCSEL according to the invention can be manufactured, and the method according to the invention has the same advantages as the VCSEL according to the invention itself.

[0041] The method may have the same preferred embodiments as the VCSEL according to the invention and / or as will be described further below.

[0042] In a preferred embodiment of the method, step b) comprises arranging a tunnel diode layer in the semiconductor layer structure.

[0043] Further preferably, the tunnel diode layer is removed in one or more subregions in order to define at least two current apertures, or the tunnel diode layer is treated with an ion implantation in one or more sub-regions in order to define at least two current apertures.

[0044] The region or regions in which the tunnel diode layer is removed may be overgrown with a material having approximately the same refractive index as the material of the tunnel diode layer. Furthermore, the tunnel diode layer may be very thin, so that only a very thin layer needs to be removed after application of the tunnel diode layer, which keeps the effective refractive index step small, in particular if the removed regions are overgrown with a material having approximately the same refractive index.

[0045] Step b) may also be realized by — without arranging a tunnel diode layer — carrying out an ion implantation in one or more sub-regions in order to define at least two current apertures.

[0046] Step c) may be realized by removing one or more laterally outer regions of one or more semiconductor layers. The removal of one or more laterally outer regions of one or more semiconductor layers for manufacturing the laser mode confinement structure may be carried out such that a sufficiently large effective refractive index step is thereby generated at the laser mode guiding aperture. After the removal of one or more laterally outer regions of one or more semiconductor layers for creating the laser mode confinement structure, the removed region or regions may be overgrown with material in order to planarize the VCSEL. Alternatively, the removed region or regions are not overgrown, whereby a phase mismatch of the outer regions of the subsequently grown mirror with respect to the central region of the mirror or of the resonator is generated. This phasemismatch results in a different reflectivity of the individual mirror sections, which can likewise be expressed as an effective refractive index step that confines the laser mode. As an alternative thereto and as described above, the laser mode confinement structure may be positioned in lateral layers that overlap with regions of higher intensity of the standing wave forming in the resonator, in order to increase the effective refractive index step.

[0047] Step a) may be realized by a conventional epitaxial method of growing semiconductor layers.

[0048] Further advantages and features will become apparent from the following description and the appended drawings.

[0049] It is understood that the features mentioned above and those yet to be explained below can be used not only in the respectively specified combination, but also in other combinations or in isolation, without departing from the scope of the present invention.

[0050] Exemplary embodiments of the invention are illustrated in the drawings and are described in greater detail below with reference thereto. In the drawings:FIG. 1 schematically shows a structure of a VCSEL according to an exemplary embodiment of the present invention;FIG. 2 shows the structure in FIG. 1 , additionally illustrating by way of example a locally varying gain profile in the active region of the resonator of the VCSEL;FIG. 3 shows an intensity profile of a specific desired laser mode in the lateral direction in the resonator and an intensity profile of another laser mode having a small overlap with the gain profile in the active region of the VCSEL in FIG. 2;FIGS. 4A to 4E show individual stages of a method of manufacturing a VCSEL with regard to the fabrication of a current confinement structure according to an exemplary embodiment of the present invention;FIGS. 5A to 5D show individual stages of a method of manufacturing a VCSEL with regard to the fabrication of a current confinement structure according to a further exemplary embodiment of the invention;FIGS. 6A to 6D show individual stages of a method of manufacturing a VCSEL with regard to the fabrication of a current confinement structure according to a further exemplary embodiment of the invention;FIGS. 7A to 7E show individual stages of a method of manufacturing a VCSEL with regard to the fabrication of a laser mode confinement structure according to an exemplary embodiment of the invention;FIGS. 8A to 8D show individual stages of a method of manufacturing a VCSEL with regard to the fabrication of a laser mode confinement structure according to a further exemplary embodiment of the invention;FIG. 9A to FIG. 9D show individual stages of a method of manufacturing a VCSEL with regard to the fabrication of a laser mode confinement structure according to yet a further exemplary embodiment of the invention;FIGS. 10A and 10B show individual stages of a method of manufacturing a VCSEL with regard to the fabrication of a laser mode confinement structure according to yet a further exemplary embodiment of the invention;FIG. 11 schematically shows a structure of a VCSEL according to a further exemplary embodiment of the invention;FIG. 12 schematically shows a structure of a VCSEL according to yet a further exemplary embodiment of the invention;FIG. 13 schematically shows the structure of a VCSEL according to yet a further exemplary embodiment of the invention; andFIGS. 14 to 30 show various exemplary embodiments of geometric shapes of a laser mode guiding aperture and of current apertures of a VCSEL according to the present invention.

[0051] FIGS. 1 and 2 schematically show a structure of a VCSEL for emitting laser radiation, designated by the general reference numeral 10. The VCSEL 10 comprises a semiconductor layer structure 12 having a plurality of semiconductor layers in a stacking direction 13 (also referred to as the z-dimension). The semiconductor layer structure 12 comprises a resonator 14 with a first mirror 16, a second mirror 18, and an active region 20, which has an active medium and serves for generating laser radiation, between the first and the second mirror 16, 18. The first mirror 16 and the second mirror 18 are preferably configured as distributed Bragg reflectors (DBRs). Accordingly, the mirrors 16 and 18 each comprise a plurality of mirror layer pairs, wherein each mirror layer pair comprises a layer having a high refractive index and a layer having a low refractive index. One of the two mirrors 16, 18 may have a reflectivity of > 99.9% and thus serve as an end mirror. The other of the mirrors 16, 18 may have a reflectivity of < 99.5% and thus serve as an outcoupling mirror for coupling out the laser radiation forming in the resonator 14. In the present description, the mirror 18 is considered as the outcoupling mirror.

[0052] The semiconductor layer structure 12 or the resonator 14 comprises a further region 22 having one or more semiconductor layers. The region 22 comprises a current confinement structure 24 that defines a plurality of current apertures 26 in a plane perpendicular to the stacking direction 13 of the semiconductor layer structure 12. In the example shown, the current confinement structure 24 has a total of four current apertures 26. The current apertures 26 are spaced apart from one another in the exemplary embodiment. In other embodiments, which will be described later, the current apertures maypartially overlap one another. During operation of the VCSEL 10, the drive current flows only through the current apertures 26, whereby the current flow in the active region varies locally in accordance with the current apertures 26. The active region 20 is thus electrically pumped during operation of the VCSEL 10 only in a plurality of locally limited regions 28. In the regions 28 of the active region 20, the threshold at which gain occurs in the active medium is lower than in regions 29 of the active region 20 outside the current apertures 26, so that a specific laser mode forms in the regions 28 and begins to lase. In other words, the current apertures 26 define a non-homogeneous gain profile in the active region 20. Regions 30 of the current confinement structure 24 outside or between the current apertures 26 are non-conducting. The current confinement structure 24 is preferably located as close as possible to the active region 20. Exemplary embodiments of the current confinement structure 24 and its fabrication are described further below.

[0053] Furthermore, the region 22 of the resonator 14 comprises a laser mode confinement structure 32 that is decoupled from the current confinement structure 24 and defines a laser mode guiding aperture 34 that is, in a plane perpendicular to the stacking direction 13, larger in at least one dimension — which is referred to as the x-dimension according to the coordinate system in FIG. 1 — preferably also in the dimension perpendicular thereto, which is referred to as the y-dimension, than the totality of the current apertures 26, and thus surrounds the totality of the current apertures 26 with a lateral spacing from the edges of the current apertures 26. The laser mode confinement structure 32 acts as a waveguide on the laser mode forming in the resonator 14. The waveguide effect is defined by an effective refractive index step from the laser mode guiding aperture 34 to an outer region 36 of the semiconductor layer structure 12 that is laterally adjacent to the laser mode guiding aperture 34, which effective refractive index step is sufficiently large. The effective refractive index step at the laser mode guiding aperture 34 to the region 36 is at least greater than an effective refractive index step from the current apertures 26 to the regions 30 of the semiconductor layer structure 12 that are laterally adjacent to the current apertures 26. With regard to a definition of the effective refractive index step, reference is made to the above-mentioned article by G. R. Hadley, “Effective-index model for vertical-cavity surface-emitting lasers”, Optics Letters 20, 1483 (1995).

[0054] Preferably, the laser mode guiding aperture 34 is larger not only in the x-dimension but also in a second dimension perpendicular thereto (y-dimension) than the current apertures 26, i.e. surrounds the current apertures 26 over their entire periphery with a spacing from the edges of the current apertures 26.

[0055] The laser mode guiding aperture 34 is decoupled from the current apertures 26, i.e. the current apertures 26 define (at least substantially) only the gain profile of the optical gain in the active region 20, and the laser mode guiding aperture 34 defines (at least substantially) only the optical waveguide effect and thus the shape and position of the laser mode in the resonator 14.

[0056] The laser mode guiding aperture 34 is preferably at least 10%, further preferably at least 20%, still further preferably at least 30% larger than the sum of the sizes of the current apertures 26, at least in one dimension, preferably in the x-dimension and the y-dimension.

[0057] In a practical exemplary embodiment, the laser mode guiding aperture 34 is spaced apart, in at least one dimension perpendicular to the stacking direction 13, from an outer edge of a laterally outer current aperture 26 facing the laser mode guiding aperture 34 (e.g. the current aperture 26 arranged furthest to the left or furthest to the right in FIG. 1) by at least 0.25 pm, preferably at least 0.5 pm.

[0058] The size of the individual current apertures 26 may be, in one dimension in the lateral direction, that is, in one dimension perpendicular to the stacking direction 13 of the semiconductor layer structure 12, in a range of 1 pm to 6 pm, preferably 1.5 pm to 5 pm, further preferably 2 pm to 4 pm. A center-to-center spacing between directly adjacent current apertures 26 may be, in one dimension in the lateral direction, in a range of 1.25 pm to 10 pm, preferably 2 pm to 8 pm, further preferably 2.75 pm to 4 pm.

[0059] A difference between the effective refractive index step at the laser mode guiding aperture 34 and the effective refractive index step at the current apertures26 is preferably greater than 0.002, preferably greater than 0.005, further preferably greater than 0.01.

[0060] As already mentioned, the effective refractive index step from the current apertures 26 to the regions 30 of the semiconductor layer structure 12 that are laterally adjacent to the current apertures should be small, preferably less than 0.004, preferably less than 0.002, further preferably less than 0.001, further preferably 0. The smallest possible effective refractive index step from the current apertures 26 to the laterally adjacent regions 30 can be achieved by making the thickness of the current confinement structure 24 in the stacking direction 13 as small as possible, and by the materials of the current apertures 26 and the laterally adjacent regions 30 having a refractive index that is as equal as possible. Furthermore, the effective refractive index step at the current apertures 26 can be reduced or kept small by arranging the current apertures 26 at or near a node of the standing wave field that forms in the resonator 14 during operation of the VCSEL 10.

[0061] In contrast, the effective refractive index step from the laser mode guiding aperture 34 to the laterally adjacent region 36 of the semiconductor layer structure 12 should be as large as possible, preferably greater than 0.002, preferably greater than 0.004, further preferably greater than 0.01. A high effective refractive index step can be realized by a correspondingly large extent of the refractive index step in the stacking direction 13 (z-direction), and / or by the semiconductor material or materials in the adjacent region 36 having a refractive index that is significantly smaller than the refractive index of the material within the laser mode guiding aperture 34. The laser mode guiding aperture 34 is preferably arranged outside a node of the standing wave field, i.e. in a region of high intensity of the standing wave field, which likewise contributes to increasing the effective refractive index step.

[0062] Due to the decoupling of the optical guidance of the laser mode from the current flow, the gain profile in the active region 20 can be individually optimized for the desired laser mode without changing the position and shape of the mode, which are defined by the laser mode guiding aperture 34, too greatly.

[0063] FIG. 3 shows a diagram with a curve IG illustrating the gain profile generated by the current apertures 26 in the active region 20. A curve IM shows the intensity profile of a desired laser mode whose intensity peaks optimally overlap with the gain profile IG, i.e. the intensity peaks of the desired laser mode form at positions corresponding to the positions of the current apertures 26. The current apertures 26 and the laser mode guiding aperture 34 are represented by arrows indicating the lateral extent of the current apertures 26 and of the laser mode guiding aperture 34. In the example shown, the desired laser mode has, in accordance with the gain profile IG, four intensity peaks at positions corresponding to the positions of the current apertures 26. It is understood that a desired laser mode that is to form in the resonator 14 may also have more or fewer than four intensity peaks, depending on how many current apertures are present. Furthermore, the diagram in FIG. 3 shows a curve I MU indicating the intensity profile of an undesired laser mode with only three intensity peaks. As illustrated in FIG. 3, the undesired laser mode has only a small overlap with the gain profile IG and is thus already prevented from forming in the active region 20. In other words, the selection and stabilization of a desired laser mode (here represented by IM) begins already in the gain medium, i.e. in the active region 20, so that no undesired laser mode can form in the active region 20 or the resonator 14 from the outset.

[0064] The laser mode guiding aperture 34 preferably collectively surrounds the current apertures 26, i.e. only one laser mode guiding aperture 34 is present that surrounds all current apertures 26 circumferentially with a spacing from the current apertures 26.

[0065] How the current confinement structure 24 and the laser mode confinement structure 32 can be configured and manufactured is described below with reference to exemplary embodiments of a method of manufacturing the VCSEL 10.

[0066] FIGS. 4A to 4E show various stages of a method of manufacturing the VCSEL 10 in FIG. 1, wherein the method stages in FIGS. 4A to 4D relate to the fabrication of the current confinement structure 24. The additional fabrication of the laser mode confinement structure 32 will be described later.

[0067] In FIGS. 4A to 4E, for simplicity, only the fabrication of a single current aperture 26 (FIG. 4E) is shown, it being understood that the described method can be used to fabricate a plurality of current apertures, such as the four current apertures 26 shown in FIGS. 1 and 2.

[0068] In the present exemplary embodiment, the current confinement structure 24 comprises a plurality of individual tunnel diodes 40 that are laterally spaced apart from one another in the exemplary embodiment. Each tunnel diode 40 defines a current aperture 26.

[0069] With reference to FIG. 4A, the mirror 16, the active region 20, and one or more semiconductor layers of the further region 22 are first grown epitaxially, standard methods being usable for this purpose. The active region 20 may comprise one or more quantum wells, or a plurality of groups of quantum wells with tunnel junctions between the quantum wells. As the last layer, a tunnel diode layer 42 is grown in the stage of FIG. 4A and growth is stopped. The tunnel diode layer 42 may, as is typically customary, comprise a heavily n-doped layer (n++) and a heavily p-doped layer (p++). If the mirror 16 is, for example, n-doped, the layer of the tunnel diode layer 42 facing the mirror is the p++layer.

[0070] According to FIG. 4B, the tunnel diode layer 42 is covered with a locally limited photoresist or a locally limited mask 44, and subsequently, according to FIG. 4G, the tunnel diode layer 42 is removed in the regions outside the mask 44, which regions outside the mask 44 may form a contiguous region, for example by etching. If a plurality of tunnel diodes are to be formed from the tunnel diode layer 42, a corresponding plurality of masks 44 are arranged along the tunnel diode layer 42 and subsequently all regions of the tunnel diode layer 42 that are not covered by masks are removed. This structures the tunnel diode layer 42 into a plurality of individual tunnel diodes 40, which form the current apertures 26 as shown, for example, in FIGS. 1 and 2. The current flow during operation of the VCSEL 10 takes place through the tunnel diodes 40.

[0071] Subsequently, according to FIG. 4D, the mask 44 is removed. According to FIG. 4E, the region or regions of the semiconductor layer structure from which thetunnel diode layer 42 was previously removed may be filled with a layer 46, wherein the layer 46 may have a small thickness and in particular is non-conducting. As already mentioned, the tunnel diode layer 42 is structured such that two or more tunnel diodes 40 are formed, as shown for example in FIG. 1. In particular, the structuring of the tunnel diode layer 42 may yield three, four, or even more tunnel diodes 40, each defining one of the current apertures 26 of the current confinement structure 24. The individual tunnel diodes 40 obtained by structuring the tunnel diode layer 42 thus represent a measure for providing a locally varying gain in the active medium of the active region 20 of the VCSEL.

[0072] Preferably, the tunnel diodes 40 are located near or at a node of the standing wave field forming in the resonator 14 of the VCSEL 10 during operation. The effective refractive index step from the individual tunnel diodes 40 to the respectively adjoining regions of the layer 46 is as small as possible, reference being made in this regard to the above description of FIG. 1.

[0073] FIGS. 5A to 5D show a method of manufacturing a VCSEL modified compared to FIGS. 4A to 4D with regard to the fabrication of the current confinement structure 24 such as that in FIG. 1 , for example. The method stages in FIGS. 5A and 5B are identical to the method stages in FIGS. 4A and 4B, so that reference is made to the above description.

[0074] Instead of removing the tunnel diode layer 42 region-wise as in FIG. 4C, the tunnel diode layer 42 is treated region-wise — here in regions 48, which may again form a contiguous region 48 — with an ion implantation, using a photoresist or a mask 44, according to FIG. 5C. The ion implantation in the regions 48 causes intermixing of the p++and n++dopings of the two layers of the tunnel diode layer 42, as a result of which the conductivity of the tunnel diode layer 42 in the regions 48 treated with the ion implantation is at least reduced. A local tunnel diode 40 then remains as a current aperture. Since the ion implantation is carried out at or near the surface of the semiconductor layer structure grown up to that point, the ion implantation can be carried out with small depth and low energy. According to FIG. 5D, the mask 44 is removed. It is understood that a plurality of tunnel diodes 40 can be formed by this method by treating the tunnel diode layer 42 withan ion implantation outside at least two regions in which the plurality of tunnel diodes 40 are to be present.

[0075] FIGS. 6A to 6D show a further exemplary embodiment of manufacturing a VCSEL with regard to the fabrication of the current confinement structure 24. In contrast to the methods described above, no tunnel diode layer is applied to the semiconductor layer structure grown up to that point; instead, a mask 44 is applied according to FIG. 6B, and an ion implantation is then carried out according to FIG. 60 as described with reference to FIG. 5C. In the regions 48 of the ion implantation, which may again form a contiguous region 48, the conductivity of the semiconductor layer structure 12 is reduced, so that the regions of the semiconductor layer structure 12 that have not been treated with ion implantation form the current apertures 26.

[0076] The following provides a more detailed description of the method of manufacturing a VCSEL additionally with regard to the fabrication of the laser mode confinement structure 32 in addition to the fabrication of the current confinement structure 24.

[0077] FIGS. 7A to 7E show an exemplary embodiment of a method of manufacturing a VCSEL with regard to the fabrication of the laser mode confinement structure 32.

[0078] FIG. 7A shows a method stage that corresponds to the method stage in FIG. 4D and continues therefrom. That is, the current confinement structure 24 with the current apertures 26 has already been fabricated. However, the method described below may equally continue from the method stage of FIG. 5D or FIG. 6D.

[0079] According to FIG. 7A, a mask 50 is applied to the semiconductor layer structure fabricated up to that point, including the current confinement structure 24. The mask 50 covers a region of the semiconductor layer structure fabricated up to that point that is larger than the current aperture 26 or the tunnel diode 40, as shown in FIG. 7A. In particular, the region covered by the mask 50 may be larger than the current aperture 26 in two mutually perpendicular dimensions, i.e. in the x- and in the y-dimension.

[0080] Subsequently, according to FIG. 7B, the region of the semiconductor layer structure laterally outside the mask 50 is removed, in particular by etching, to form the laser mode guiding aperture 34. It is essential that the laser mode guiding aperture 34 provides a sufficiently large effective refractive index step for the waveguide effect in a plane perpendicular to the stacking direction 13. According to FIG. 70, the mask 50 is removed.

[0081] The laser mode guiding aperture 34 is preferably not located at a node of the standing wave field forming in the resonator 14 during operation of the VCSEL. The removal of the region outside the mask 50 from the semiconductor layer structure may be carried out with a depth such that a step is created in the semiconductor layer structure that makes a significant contribution to the effective refractive index step that confines the laser mode. The depth of the removal (etching) of the region outside the mask 50 from the semiconductor layer structure can be adapted to set the desired refractive index step.

[0082] According to FIG. 7D, the resulting structure may then be overgrown or filled with a material 52. The material 52 may in particular have a refractive index that is smaller, in particular significantly smaller, than the refractive index of the material within the laser mode guiding aperture 34.

[0083] According to FIG. 7E, the remaining region of the further region 22 and the mirror 18 may subsequently be grown.

[0084] FIGS. 8A to 8D show a further exemplary embodiment of a method of manufacturing a VCSEL with regard to the laser mode confinement structure 32. The method stages in FIGS. 8A to 8C are identical to the method stages in FIGS. 7A to 7C, so that reference is made to the above description. While in the method stage of FIG. 7D the semiconductor layer structure is planarized after fabrication of the laser mode confinement structure 32, as shown in FIG. 7D, no such planarization takes place in the exemplary embodiment of FIG. 8D. Instead, the mirror 18 is grown according to FIG. 8C directly onto the stepped structure, optionally with one or more semiconductor layers 53, 55, 57 interposed that do not, however, planarize the structure. This brings about a phase mismatch of theouter regions 56, 58 of the mirror 18 compared to the central region 60 of the mirror 18. This phase mismatch results in a different reflectivity of the individual mirror portions 56, 58, 60, which can likewise be expressed as an effective refractive index step that optically confines the laser mode.

[0085] FIGS. 9A to 9D show a further exemplary embodiment of a method of manufacturing a VCSEL additionally with regard to the fabrication of the laser mode confinement structure 32. The method shown in FIGS. 9A to 9D continues from the method stage in FIG. 5D. According to FIG. 9A, a mask 50 is applied to the layer structure of the semiconductor layer structure 12 fabricated up to that point, including the current confinement structure 24. Subsequently, according to FIG. 9B, regions 62 outside the mask 50, in this exemplary embodiment the region or regions treated with ion implantation 48, are partially removed, in particular by etching, to form the laser mode confinement structure 32 with the laser mode guiding aperture 34. According to FIG. 9C, the mask 50 is removed. Instead of growing the mirror 18 onto the layer structure fabricated up to that point as in the previous exemplary embodiments, in this exemplary embodiment, according to FIG.9D, the mirror 18 is grown separately on a second wafer and then bonded to the remaining layer structure of the semiconductor layer structure 12, for example by wafer bonding.

[0086] FIGS. 10A and 10B show an alternative exemplary embodiment of a method of manufacturing a VCSEL with regard to the laser mode confinement structure 32. The current confinement structure 24 with the current apertures 26 may be fabricated as in the exemplary embodiments described above, wherein FIG. 10A shows by way of example the current confinement structure 24 fabricated according to FIGS. 6A to 6D. For fabricating the laser mode confinement structure 32, an aluminum-rich layer 68 is provided in the semiconductor layer structure 12, which, according to FIG. 10B, is laterally oxidized after etching a mesa 70 from the semiconductor layer structure. This likewise creates a sufficient effective refractive index step at the transition from the inner non-oxi-dized region 72 to the oxidized outer region 74. The non-oxidized region 72 has a lateral extent or width that is greater than the current aperture 26, as shown in FIG. 10B. It is understood that the oxide aperture thus formed, i.e. the non-oxidized region 72, does not form the current aperture or define the current path, but rather forms an optical guide,i.e. a laser mode guiding aperture 34. This optical guide can be adjusted by the position and thickness of such an oxide aperture.

[0087] FIGS. 11 and 12 show the VCSEL 10 in FIG. 1 with additional elements. According to FIG. 11, the VCSEL 10 in this exemplary embodiment comprises a polarization stabilization grating 90 fabricated on the mirror 18. FIG. 12 shows an exemplary embodiment in which a relief structure 92 with localized reliefs 94 is fabricated on the mirror 18, which, like the polarization stabilization grating 90, locally modifies the reflectivity of the mirror 18 such that the reflectivity is adapted to match the locally varying gain in the active medium of the active region 20. In the regions of the current apertures 26, i.e. at the maxima of the gain profile in the active region 20, the polarization stabilization grating 90 or the relief structure 92 increases the reflectivity, or conversely reduces the reflectivity in the regions of the minima of the gain profile, i.e. outside the current apertures 26. In the exemplary embodiment of FIG. 12, the reflectivity in the region of the reliefs 94, which are arranged between the respective current apertures 26, is reduced, so that a laser mode having intensity peaks in these regions is suppressed in addition to the absence of gain in the active medium of the active region 20.

[0088] FIG. 13 shows a further exemplary embodiment of a VCSEL 10. The VCSEL 10 of FIG. 13 differs from the VCSEL 10 of FIG. 1 in that the resonator 14 has one or more additional current apertures 26’ in lateral regions outside the laser mode guiding aperture 34. These additional current apertures 26’ provide for power dissipation but do not contribute to the laser mode to be emitted or to the total output power of the VCSEL. The additional current aperture or apertures 26’ serve to heat the VCSEL outside the laser mode guiding aperture 34 in order to homogenize the temperature profile within the laser mode guiding aperture 34. In order to prevent gain of a laser mode or even emission of laser radiation from the regions of the additional current apertures 26’, the resonator 14 comprises, on the emission side, a structure 98 opaque to the generated laser radiation in order to block the laser radiation. The structures 98 may, for example, be structures that absorb the laser radiation. Alternatively or cumulatively to such opaque structures 98, the resonator 14 may be disturbed in these sections in which the additional current apertures 26’ are located, for example by a reduced reflectivity of the mirror 18 in these regions, in order to prevent the lasing conditions from being reached in these regions.

[0089] FIGS. 14 to 30 show exemplary embodiments of possible shapes and sizes of the current apertures 26 and the laser mode guiding aperture 34 for different configurations. The current apertures may, as shown in FIG. 14, be circular, or elliptical, as shown in FIG. 15. Furthermore, the current apertures 26 may have different shapes and / or sizes and / or spacings relative to one another in one and the same VCSEL, as shown in FIGS. 16, 20, and 21. Furthermore, the current apertures 26 may overlap one another, as shown in FIGS. 17, 23, and 24, the size of the current apertures 26 varying locally in the shorter y-dimension.

[0090] The laser mode guiding apertures 34 according to the exemplary embodiments in FIGS. 14 to 17 are elongated rectangular, while in FIGS. 18 to 20 they are elongated with rounded corners. The spacings of the current apertures 26 may be the same within one and the same VCSEL, as shown in FIGS. 14, 15, 18, and 19, or they may vary, as shown in FIGS. 16, 20, and 22.

[0091] FIG. 21 shows a laser mode guiding aperture 34 having the shape of a bowtie. In this embodiment, the current apertures have different sizes along the laser mode guiding aperture 34. FIGS. 22 to 24 show laser mode guiding apertures 34 having an irregular contour, for example a wave-shaped contour.

[0092] Furthermore, the laser mode guiding aperture 34, as shown in FIGS. 25 to 28, may enclose a larger planar area, i.e. have a greater extent both in the x- and in the y-dimension. It is also possible, as shown in FIGS. 29 and 30, for the laser mode guiding aperture 34 to be ring-shaped, i.e. to form a ring-shaped waveguide, while the current apertures 26 in these examples stabilize a ring-shaped laser mode with eight intensity peaks in the waveguide.

Claims

1. Claims1. VCSEL for emitting laser radiation, comprising a semiconductor layer structure (12) having a plurality of semiconductor layers in a stacking direction (13), wherein the semiconductor layer structure (12) comprises a resonator (14) with a first mirror (16), a second mirror (18), and an active region (20) between the first and the second mirror (16, 18) for generating laser radiation, wherein the resonator (14) further comprises a current confinement structure (24) between the first and second mirror (16, 18) that defines a plurality of current apertures (26) that provide a locally varying current distribution in the active region (20) during operation of the VCSEL (10), and wherein the resonator (14) comprises an optical laser mode confinement structure (32) decoupled from the current confinement structure (24) that defines a laser mode guiding aperture (34) having, in a plane perpendicular to the stacking direction (13), a greater extent in at least one dimension than the sum of the extents of the current apertures (26), wherein an effective refractive index step from the laser mode guiding aperture (34) to a region (36) of the semiconductor layer structure (12) that is laterally adjacent to the laser mode guiding aperture (34) is greater than an effective refractive index step from the current apertures (26) to regions (30) of the semiconductor layer structure (12) that are laterally adjacent to the current apertures (26).

2. VCSEL according to claim 1, wherein the laser mode guiding aperture (34) is greater in the at least one dimension by at least 10%, preferably by at least 20%, further preferably by at least 30%, than the sum of the extents of the current apertures (26) in that dimension.

3. VCSEL according to claim 1 or 2, wherein the laser mode guiding aperture (34) is spaced apart, in at least one dimension perpendicular to the stacking direction (13), from an outer edge of a laterally outer current aperture (26) facing the laser mode guiding aperture (34) by at least 0.25 pm, preferably at least 0.5 pm.

4. VCSEL according to any one of claims 1 to 3, wherein the current apertures (26), in a direction lateral to the stacking direction (13) of the semiconductor layer structure (12), each have a dimension in a range of 1 pm to 6 pm, preferably 1.5 pm to 5 pm, further preferably 2 pm to 4 pm.

5. VCSEL according to any one of claims 1 to 4, wherein adjacent current apertures (26) are spaced apart from one another in the lateral direction or partially overlap one another in the lateral direction.

6. VCSEL according to claim 5, wherein a center-to-center spacing between directly adjacent current apertures (26) is in a range of 1.25 pm to 10 pm, preferably 2 pm to 8 pm, further preferably 2.75 pm to 4 pm.

7. VCSEL according to any one of claims 1 to 6, wherein a difference between the effective refractive index step at the laser mode guiding aperture (34) and the effective refractive index step at the current apertures (26) is greater than 0.002, preferably greater than 0.005, further preferably greater than 0.01.

8. VCSEL according to any one of claims 1 to 7, wherein the effective refractive index step from the current apertures (26) to the regions (30) of the semiconductor layer structure (12) that are laterally adjacent to the current apertures (26) is less than 0.004, preferably less than 0.002, further preferably less than 0.001, further preferably 0.

9. VCSEL according to any one of claims 1 to 8, wherein the effective refractive index step from the laser mode guiding aperture (34) to the region (36) of the semiconductor layer structure that is laterally adjacent to the laser mode guiding aperture (34) is greater than 0.002, preferably greater than 0.004, further preferably greater than 0.01.

10. VCSEL according to any one of claims 1 to 9, wherein positions of the current apertures (26) are matched to positions of intensity peaks of a predetermined single higher-order laser mode to be emitted by the VCSEL.

11. VCSEL according to any one of claims 1 to 10, wherein the laser mode guiding aperture (34) collectively surrounds the plurality of current apertures (26).

12. VCSEL according to any one of claims 1 to 11 , wherein the current confinement structure (24) comprises a plurality of tunnel diodes (40) distributed in the lateral direction, wherein the tunnel diodes (40) define the current apertures.

13. VCSEL according to claim 12, wherein the tunnel diodes (40) are arranged at or near a node of a standing wave field forming in the resonator (14) during operation of the VCSEL (10).

14. VCSEL according to any one of claims 1 to 13, wherein the current confinement structure (24) comprises one or more ion implantation regions, wherein the current apertures (26) are defined in regions outside the ion implantation.

15. VCSEL according to any one of claims 1 to 14, wherein the current apertures (26) are circular or elliptical.

16. VCSEL according to any one of claims 1 to 15, wherein the laser mode guiding aperture (34) is circular, elliptical, or rectangular.

17. VCSEL according to any one of claims 1 to 16, wherein the laser mode guiding aperture (34) is elongated in the lateral direction, in particular elongated rectangular, in particular elongated rectangular with rounded corners, or ringshaped.

18. VCSEL according to any one of claims 1 to 17, wherein the first or the second mirror (16, 18) is an outcoupling mirror for coupling out laser radiation,wherein a reflective grating structure (90), in particular a polarization-selective grating structure, or a relief structure (92) is arranged above the outcoupling mirror and provides, on the emission side in combination with the reflectivity of the outcoupling mirror, an increased reflectivity of the resonator (14) in the region of the respective current aperture (26).

19. VCSEL according to any one of claims 1 to 18, wherein the resonator (14) comprises, in at least one lateral region of the resonator (14) outside the laser mode guiding aperture (34), at least one additional current aperture (26’), preferably wherein the resonator (14) comprises, on the emission side, a structure (98) opaque to laser radiation generated in the resonator (14) in the region of the at least one additional current aperture (26’).

20. Method of manufacturing a VCSEL, comprising the steps of:a) fabricating a semiconductor layer structure (12) from a plurality of semiconductor layers arranged one above the other in a stacking direction (13), the semiconductor layer structure (12) comprising a resonator (14) with a first mirror (16), a second mirror (18), and an active region (20) for generating laser radiation between the first mirror (16) and the second mirror (18); wherein step a) further comprises:b) fabricating a current confinement structure (24) in the resonator (14) that defines a plurality of current apertures (26) that provide a locally varying current distribution in the active region (20) during operation of the VCSEL;c) fabricating a laser mode confinement structure (32) decoupled from the current confinement structure (24) to define a laser mode guiding aperture (34) in the resonator (14);wherein steps b) and c) are carried out such that the laser mode guiding aperture (34) is larger in a plane perpendicular to the stacking direction (13) in at least onedimension than the current apertures (26) in total, and such that an effective refractive index step from the laser mode guiding aperture (34) to a region (36) of the semiconductor layer structure (12) that is laterally adjacent to the laser mode guiding aperture (34) is greater than an effective refractive index step from the current apertures (26) to regions (30) of the semiconductor layer structure (12) that are laterally adjacent to the current apertures (26).

21. Method according to claim 20, wherein step b) comprises arranging a tunnel diode layer (42) in the semiconductor layer structure (12).

22. Method according to claim 21, wherein the tunnel diode layer (42) is removed in one or more sub-regions to define at least two current apertures (26).

23. Method according to claim 21, wherein the tunnel diode layer (42) is treated with an ion implantation in one or more sub-regions to define at least two current apertures (26).

24. Method according to claim 20, wherein step b) comprises an ion implantation in one or more sub-regions of the semiconductor layer structure to define at least two current apertures (26).

25. Method according to any one of claims 20 to 24, wherein step c) comprises removing one or more laterally outer regions of one or more semiconductor layers of the semiconductor layer structure (12).

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