RADIATION EMITTER

Description

Background of the invention

[0001] The energy required to transmit information as optical data bits within and between electronic and photonic integrated circuits, within and between computer servers, within and between data centers, and ultimately across the earth from one point to another spans from typically tens of picojoules-per-bit to well over tens of millijoules-per-bit for intercontinental distances. Internet use increases by 60% / year and larger and larger data centers (in size and energy consumption) with millions of optical interconnects are presently built. To keep up with the demand in communication capacity, data transmission across longer and longer distances using high power single mode emitters combined with dense wavelength multiplexing must be enabled.

[0002] Key enabling emitters for short distances up to 1-2 km are high power single mode vertical-cavity surface-emitting lasers (VCSELs).

[0003] VCSELs can be easily designed to emit at a variety of wavelengths across present multi- and single-mode optical cables for such short distances. Wavelength division multiplexing, WDM, increases enormously the bitrate per link. Optimized combination of the number of channels and the bit rate reduces the energy consumption and operating cost of the network as shown by Larisch et al. (G. Larisch, S. Tian, and D. Bimberg, "Optimization of VCSEL photon lifetime for minimum energy consumption at varying bit rates," Opt. Express, vol. 28, p. 6, 2020). The IEEE 802.3cd standard defines a 30 nm spacing between "channels", a number adapted to properties of typical multimode VCSELs. A further enormous increase of bandwidth per link, necessary to catch-up with demand, in line with additional reduction of energy consumption can be achieved by further reducing the spacing between the wavelengths. Narrow emission spectra are necessary to avoid cross talk between the channels. A further advantage of such spectra is the reduction of dispersion, supporting additionally increases of the transmission distances, as discussed in the publication by Larisch et al. (G. Larisch, A. A. Juarez, X. Chen, K. Li, D. Bimberg, and M.-J. Li, "910 nm Single-Mode VCSELs and its Application for Few-Mode Transmission over Graded-Index Single-Mode Fibers (Invited)," in 2020 22nd International Conference on Transparent Optical Networks (ICTON)).

[0004] Spectrally narrow emission is achieved today in particular by suppressing higher order modes of the laser. Higher order modes can be avoided by reduction of the size of the active area, leading to an increase of the D-factor: f R = D I − I th

[0005] With relaxation resonance frequency fR, current I and threshold current Ith. D = 1 2 π η i Γ ν g q ⋅ g ′ V a

[0006] With g' differential gain, Va active volume, ηi internal quantum efficiency, vg group velocity, Γ confinement-Factor.

[0007] The size of the active area of a VCSEL depends on its cavity length and its aperture diameter. Today's datacom VCSELs usually have a cavity lengths of lambda half. A further reduction is not possible. A reduction of aperture diameter leads to several drawbacks. The resistance and the differential resistance increase, and the output power drops. A low electrical resistance eases however impedance match of laser and driver, and reduces the total energy consumption of present driver designs.

[0008] European Patent Publication EP 1 501 162 A2 discloses a phase array of oxide confined VCSELs and a method for forming the phase array of oxide confined VCSELs. VCSELs in the array are designed to be simultaneously addressed such that the output of multiple VCSELs can be used to increase the light intensity at a point. In applications where beam coherence from the VCSEL array is desirable, high gain coupling regions break the continuity of the oxide wall surrounding each VCSEL aperture. The high gain coupling regions connect adjacent VCSELs in the VCSEL array thereby allowing mode coupling between adjacent lasers and the output of a coherent beam of light.

[0009] US Patent Publication US 2011 / 304684 A1 discloses a surface emitting laser device that includes a substrate and plural semiconductor layers laminated on the substrate, the plural semiconductor layers including a first semiconductor multi-layer film including aluminum (Al), an active layer, and a second semiconductor multi-layer film, a light emitting section having a mesa structure being formed on the first semiconductor multi-layer film. When viewed in a direction orthogonal to a surface of the substrate, an outer shape of the first semiconductor multi-layer film is a macroscopically smooth shape without an angular corner, and a side surface of the first semiconductor multi-layer film is coated with a passivation film and a protection film.Objective of the present invention

[0010] In view of the above, an objective of the present invention is to propose radiation emitters that may provide large output powers with relatively small aperture diameters.

[0011] A further objective of the present invention is to provide a method for fabricating radiation emitters that provide large output powers with relatively small aperture diameters.Brief summary of the invention

[0012] A first exemplary embodiment of the present invention relates to a method according to claim 1.

[0013] An advantage of this embodiment of the invention is that the method allows fabricating mesas which each have a plurality of relatively small apertures, in contrast to mesas of prior art VCSELS. In prior art VCSELs, apertures are formed by oxidizing oxidizable layers after the mesa is etched, i.e. by oxidizing via the sidewall of the mesa. Therefore, the prior art fabrication can provide a single aperture per mesa, only. In contrast thereto and according to the invention, the apertures are formed (either before the mesa is etched or thereafter) by etching blind holes and oxidizing oxidizable layer or layers from the inside of the holes via the holes' sidewalls. This procedure allows creating apertures which are independent from the formation of the mesa. Therefore, it is possible to fabricate a plurality of apertures within the same mesa. For example, the resulting radiation emitter may have a plurality of densely spaced apertures which each define a VCSEL sub-cell or VCSEL subunit within the same mesa. The VCSEL sub-cells may operate in parallel and allow the mesa to output larger amounts of energy than mesas with a single aperture are capable of.

[0014] A top contact layer is preferably fabricated on top of the second reflector.

[0015] The top contact layer is preferably provided with a first conducting material such that the conducting material partly covers the surface of the top contact layer, and such that sections of the top contact layer above the apertures are left uncovered in order to allow optical radiation to exit the mesa without additional attenuation.

[0016] Preferably, the steps of vertically etching the blind holes and oxidizing the oxidizable layer or layers via the sidewalls of the blind holes, are carried out before the mesa is etched.

[0017] The blind holes are preferably vertically etched inside the layer stack in an area which belongs to the at least one mesa after locally removing the layer stack.

[0018] A plurality of blind holes may be etched inside the layer stack in an area which will belong to the at least one mesa. The blind holes are preferably arranged in a lattice-like way forming a grid having a first grid spacing in a first direction and a second grid spacing in a second different direction.

[0019] The first grid spacing and the second grid spacing may be identical, for instance in order to generate symmetrical apertures in view of an emission of polarization independent radiation.

[0020] Alternatively, the first grid spacing may be between 10% and 30% larger than the second grid spacing, for instance in order to generate asymmetrical apertures in view of an emission of polarization dependent radiation.

[0021] According to a preferred embodiment, a 3x3 grid having nine blind holes or a 4x4 grid having sixteen blind holes is formed per radiation emitter.

[0022] The oxidation may be carried out using processing parameters causing circular oxidation fronts, for instance in order to generate symmetrical apertures in view of an emission of polarization independent radiation.

[0023] Alternatively, the oxidation may be carried out using processing parameters causing anisotropic (e. g. elliptical) oxidation fronts, for instance in order to generate asymmetrical apertures in view of an emission of polarization dependent radiation.

[0024] The oxidized material of the oxidizable layer or layers is preferably electrically non-conductive.

[0025] Said step of fabricating the layer stack may include forming two or more oxidizable layers inside the layer stack.

[0026] In case that two or more oxidizable layers are fabricated within the layer stack, at least two of said oxidizable layers are preferably each provided with at least two apertures.

[0027] If the layer stack comprises two or more oxidizable layers, at least two apertures may be formed within each of the oxidizable layers.

[0028] In case of two or more oxidizable layers, stacks of vertically aligned apertures may be fabricated. Each of the vertical stacks of apertures in combination with the adjacent section of the active region may be regarded as an individual VCSEL unit within the radiation emitter. In other words, in the latter embodiment, two or more apertures may be located in the same plane ("horizontal" plane) and two or more planes may be stacked vertically in order to form a group (for instance a grid) of vertically stacked apertures.

[0029] The at least one oxidizable layer or at least one of the oxidizable layers may be formed between the first reflector and the active layer.

[0030] The at least one oxidizable layer or at least one of the oxidizable layers may be formed inside the first reflector.

[0031] The at least one oxidizable layer or at least one of the oxidizable layers may be formed between the second reflector and the active layer.

[0032] The at least one oxidizable layer or at least one of the oxidizable layers may be formed inside the second reflector.

[0033] In a preferred embodiment, at least one of the oxidizable layers is formed inside the first reflector or between the first reflector and the active layer and at least one of the oxidizable layers is formed inside the second reflector or between the second reflector and the active layer.

[0034] In another preferred embodiment, at least two oxidizable layers are formed inside the first reflector or between the first reflector and the active layer and / or at least two oxidizable layers are formed inside the second reflector or between the second reflector and the active layer.

[0035] The method described above is preferably carried out in a wafer-scale fashion wherein a plurality of mesas (and therefore emitters), which each comprise a plurality of apertures, is fabricated simultaneously.

[0036] A second exemplary embodiment of the present invention relates to a radiation emitter according to independent claim 6.

[0037] The apertures preferably form VCSEL sub-cells that operate in parallel, inside said same mesa.

[0038] The apertures are preferably so narrowly spaced in said mesa that the resulting radiation emitter provides single mode emission. The radiation that the two apertures or VCSEL sub-cells generate, may differ with respect to the radiation wavelength. However, the difference is preferably smaller than 0,1 % of the average wavelength.

[0039] A top contact layer is preferably located on top of the second reflector.

[0040] A first conducting material is preferably located on top of the top contact layer, wherein the conducting material partly covers the surface of the top contact layer, but leaves sections of the top contact layer above the apertures uncovered in order to allow optical radiation to exit the mesa without additional attenuation.

[0041] The blind holes are preferably filled with a conducting material or at least the sidewalls of the holes are covered with the conducting material. The conducting material is preferably thermally conductive in order to dissipate heat that is generated during the operation of the radiation emitter.

[0042] The conducting material is isolated from the sidewalls of the blind holes by an intermediate isolating layer, to block a bypass of electrical current.

[0043] The apertures are subjected to electrical current flow as well as optical radiation when the radiation emitter operates.

[0044] Two or more apertures may be located inside the second reflector and / or between the active region and the second reflector.

[0045] At least the aperture that is the most adjacent to the active region, may be subjected to electrical current flow as well as optical radiation when the radiation emitter operates, wherein at least one of the remaining apertures may be bypassed.Brief description of the drawings

[0046] In order that the manner, in which the above-recited and other advantages of the invention are obtained, will be readily understood, a more particular description of the invention briefly described above will be rendered by reference to specific embodiments thereof, which are illustrated in the appended figures. Figures 1-12illustrate method steps for fabricating an exemplary embodiment of a radiation emitter in the form of a VCSEL, not falling under the scope of the invention, and Figures 13-18illustrate method steps for fabricating the inventive embodiment of a radiation emitter in the form of a VCSEL. Detailed description of the preferred embodiments

[0047] The preferred embodiments of the present invention will be best understood by reference to the drawings, wherein identical or comparable parts are designated by the same reference signs throughout.

[0048] It will be readily understood that the parameters of the embodiments of the present invention, as generally described herein, could vary in a wide range. Thus, the following more detailed description of exemplary embodiments of the present invention, is not intended to limit the scope of the invention but is merely representative of presently preferred embodiments of the invention.

[0049] Figures 1-12 show method steps for fabricating an exemplary embodiment, not being part of the invention, of a radiation emitter in the form of a VCSEL 10C (see Figure 7).

[0050] Figure 1 shows a cross-section of an exemplary layer stack 10 that comprises a first (lower) contact layer 11, a first reflector 12, an active region 13, a first (lower) oxidizable layer 21, a second (upper) oxidizable layer 22, a second reflector 14, and a second (top) contact layer 15.

[0051] The first contact layer 11 is preferably highly p-doped (doping level > 10 19< cm -3< ). The second contact layer 15 is preferably highly n-doped (doping level > 10 19< cm -3< ).

[0052] The first and second reflectors 12 and 14 may be distributed Bragg reflectors (DBRs) that each comprise a plurality of reflector layers with alternating reflective indices.

[0053] The layer stack 10 is preferably fabricated by depositing semiconductor material such as AlGaAs on a substrate 10a. In the exemplary embodiment of Figure 1, the two oxidizable layers 21-22 are located above the active region 13.

[0054] Figure 2 shows the layer stack 10 of Figure 1 after vertically etching blind holes 30 inside the layer stack 10. The blind holes 30 vertically protrude to the oxidizable layers 21-22 and expose the oxidizable layers 21-22.

[0055] Figure 3 shows the layer stack 10 of Figure 2 after oxidizing the oxidizable layers 21-22 via the sidewalls 31 of the blind holes 30 in lateral direction. From each blind hole 30, an oxidation front radially (see arrow R) moves outwards during the oxidation. The step of etching is terminated before the entire oxidizable layers 21-22 are oxidized in order to form unoxidized apertures 40. Each unoxidized aperture 40 is limited by at least three oxidation fronts of oxidized layer material 20. The oxidized layer material 20 is preferably electrically isolating.

[0056] Figure 4 shows the layer stack 10 of Figure 3 after depositing an intermediate isolating layer 50 on the bottom and the lower section of the blind holes 30. The intermediate isolating layer 50 covers the edges of the oxidized layer material 20 of the first oxidizable layer 21.

[0057] Figure 5 shows the layer stack 10 of Figure 4 after filling the blind holes 30 with a first conducting material 61. The first conducting material 61 is preferably electrically and thermally conductive. The conducting material 61 is preferably Gold, Platinum, Titanium, Nickel, Gold-Germanium alloy or a sequence of these materials, depending on the doping type of the semiconductor material that is to be contacted.

[0058] In the embodiment of Figure 5, the first conducting material 61 forms an ohmic contact with the uncovered sidewalls 31 of the blind holes 30.

[0059] Since the intermediate isolating layer 50 does not cover the edges of the oxidized layer material 20 of the second oxidizable layer 22, the apertures 40 formed in the second oxidizable layer 22 are electrically bypassed. This means that electrical current that is applied during the operation of the resulting radiation emitter 100 (see Figure 7) after completing the fabrication, may flow from the surface of the layer stack 10 through the conducting material 61 into the layer or layers that are located between the first and second oxidizable layers 21-22.

[0060] Since the intermediate isolating layer 50 covers the edges of the oxidized layer material 20 of the first oxidizable layer 21, the apertures 40 formed in the first oxidizable layer 21 are not electrically bypassed. This means that electrical current is forced to pass the apertures 40 that are located in the first oxidizable layer 21.

[0061] In addition to its electrical influence, the first conducting material 61 preferably forms a heat sink that dissipates heat during the operation of the resulting radiation emitter 100.

[0062] The conducting material 61 may partly cover the surface of the second contact layer 15. However, sections 15a of the second contact layer 15 above the apertures 40 are preferably left uncovered in order to allow optical radiation P to exit the radiation emitter 100 without additional attenuation.

[0063] Figure 6 shows the layer stack 10 of Figure 5 after etching a mesa M inside the layer stack 10. The mesa M may have one or more steps SS. The mesa M preferably extends to the first contact layer 11 in order to allow depositing a second conductive material 62 and thereby contacting the first contact layer 11.

[0064] Figure 7 shows the layer stack 10 of Figure 6 after depositing the second conductive material 62 and contacting the first contact layer 11. The layer stack 10 of Figure 7 provides a radiation emitter in the form of a VCSEL 100.

[0065] When applying an electrical voltage between the first and second conducting material 61 and 62, electrical current will flow through the apertures 40 of the first oxidizable layer 21 and the active region 13. The active region 13 generates optical radiation P that exits the radiation emitter 100 through the surface sections 15a of the second contact layer 15 that is uncovered by the first conductive material 61.

[0066] Each of the apertures 40 in combination with the adjacent section of the active region 13 may be regarded as an individual VCSEL unit within the radiation emitter 100 that comprises a plurality of these individual VCSEL units.

[0067] Figure 8 shows a top view of the layer stack 10 of Figure 2. after etching the blind holes 30 (for instance nine blind holes as depicted in Figure 8). The nine blind holes 30 are preferably arranged in a lattice-like way forming a grid having a first grid spacing d1 in a first direction D1 and a second grid spacing d2 in a second different direction D2.

[0068] In one preferably embodiment, the first grid spacing d1 and the second grid spacing d2 are identical. In another preferably embodiment, the first grid spacing d1 is between 10% and 30% larger than the second grid spacing d2.

[0069] The first and second direction D1, D2 can be perpendicular. Alternatively, the first and second direction D1, D2 may be angled, preferably with an angle between 60° and 85°.

[0070] Figure 9 shows a top view of the layer stack 10 of Figure 3 after oxidizing the oxidizable layers 21-22. From each blind hole 30, an oxidation front 32 radially moves outwards during the oxidation. Each unoxidized aperture 40 is limited by four oxidation fronts 32 of oxidized layer material 20 and therefore has a diamond-like shape. As depicted in Figures 9-11, the nine blind holes will lead to four apertures 40 in each of the two oxidizable layers 21 and 22. Each of the four vertical stacks of apertures 40 will be part of a future VCSEL subunit or subcell.

[0071] Figure 10 shows a top view of the layer stack 10 of Figure 5 after filling the blind holes 30 with the first conductive material 61. Sections 15a of the second contact layer 15 above the apertures 40 are left uncovered in order to allow the optical radiation P to exit the VCSEL subunits of the radiation emitter 100.

[0072] Figure 11 shows a top view of the layer stack 10 of Figure 7, i.e. the resulting radiation emitter 100 after contacting the first contact layer 11 by depositing the second conductive material 62.

[0073] In the exemplary embodiment of Figures 1-11, the layer stack 10 comprises two oxidizable layers 21 and 22. Therefore, four stacks, each of which comprises two vertically aligned apertures 40, are fabricated within each mesa M. Each of the four vertical stacks of apertures 40 in combination with the adjacent section of the active region 13 may be regarded as an individual VCSEL subunit within the radiation emitter 100.

[0074] Figures 12 - 18 show method steps for fabricating the inventive embodiment of a radiation emitter in the form of a VCSEL 100.

[0075] Figure 12 shows a cross-section of an exemplary layer stack 10 that comprises four oxidizable layers 21-24. Two oxidizable layers 21-22 are located below the active region 13, and two oxidizable layers 23-24 are located above the active region 13.

[0076] Figure 13 shows the layer stack 10 of Figure 1 after vertically etching blind holes 30 inside the layer stack 10. The blind holes 30 vertically extend to the lowest oxidizable layer 21 and expose all of the oxidizable layers 21-24.

[0077] Figure 14 shows the layer stack 10 of Figure 13 after oxidizing the oxidizable layers 21-24 via the sidewalls 31 of the blind holes 30 in lateral direction. From each hole, an oxidation front radially moves outwards during the oxidation. The step of etching is terminated before the entire oxidizable layer is oxidized in order to form unoxidized apertures 40. Each unoxidized aperture 40 is limited by at least three oxidation fronts of oxidized layer material.

[0078] Figure 15 shows the layer stack 10 of Figure 14 after depositing an intermediate isolating layer 50 on the bottom and the entire sidewalls 31 of the blind holes 30.

[0079] Figure 16 shows the layer stack 10 of Figure 15 after filling the blind holes 30 with a first conducting material 61. The conducting material 61 is preferably electrically and thermally conductive. The conducting material 61 is preferably Gold, Platinum, Titanium, Nickel, Gold-Germanium alloy or a sequence of these materials, depending on the doping type of the semiconductor material that is to be contacted.

[0080] Since the intermediate isolating layer 50 covers the sidewalls 31 of the blind holes 30, none of the apertures 40 is electrically bypassed. This means that electrical current that is applied during the operation of the resulting radiation emitter 100 after completing the fabrication, flows through all of the apertures 40.

[0081] Figure 17 shows the layer stack 10 of Figure 16 after etching a mesa M inside the layer stack 10. The mesa M may have one or more steps SS. The mesa M preferably extends to the first contact layer 11 in order to allow depositing a second conductive material 62 and contacting the first contact layer 11.

[0082] Figure 18 shows the layer stack 10 of Figure 17 after depositing the second conductive material 62 and contacting the first contact layer 11. The layer stack 10 of Figure 18 provides a radiation emitter in the form of a VCSEL 100.

[0083] In the embodiment of Figures 12-18, the layer stack 10 comprises four oxidizable layers 21-24. Therefore, four stacks, each of which comprises four vertically aligned apertures 40, are fabricated within each mesa M. Each of the four vertical stacks of apertures 40 in combination with the adjacent section of the active region 13 may be regarded as an individual VCSEL subunit within the radiation emitter 100.

[0084] In Figure 18, the vertical arrows indicate the radiation P that is emitted by the four individual VCSEL subunits that are each formed by a vertical stack of apertures 40 in combination with the adjacent section of the active region 13.

[0085] When applying an electrical voltage between the first and second conducting material 61,62, electrical current will flow through the apertures 40 of all oxidizable layers 21-22. The active region 13 generates optical radiation that exits the radiation emitter 100 through the surface sections 15a of the second conducting layer 15 that is uncovered by the first conductive material 61. Each of the apertures 40 (or each of the vertical stacks of apertures 40) in combination with the adjacent section of the active region 13 may be regarded as an individual VCSEL unit within the radiation emitter 100 that comprises a plurality of these individual VCSEL units.

[0086] For understanding the invention, it is summarized that the exemplary embodiments relate to a method for fabricating a vertical-cavity surface-emitting laser (VCSEL) as radiation emitter 100 with multiple apertures 40 narrowly spaced in a single mesa M that result in single mode emission together with large optical output power and small electrical resistance. The fabrication of the VCSEL 100 may be based on etching of narrow holes 30, e.g. 5 µm or less, in a regular array of a few µm distance between each hole into VCSEL wafers containing for instance AlGaAs (preferentially about 98% Al-contents) aperture layers. The arrangement of the holes 30 with respect to each other is variable and application dependent. The oxidation of the e.g. AlO apertures 40 is progressing from the inside of the holes 30. The orientation of the axes of the hole-arrays can be varied with respect to the crystal axes, thus leading to self-limiting orientation dependent oxidation processes. The novel VCSEL properties, including defined polarization, enable data transmission across large fiber distances ~ 1km and more, as well as dense wavelength division multiplexing (≤ 15 nm spacing). In addition, impedance matching of laser and driver circuits is eased due to the reduced impedance of the invention.

[0087] Preferred embodiments of the invention described above may have one or more of the following features and / or advantages: Preferred embodiments of the invention may consist of an active region sandwiched between two distributed Bragg reflectors (DBRs) and at least one oxid layer like a "normal" VCSEL structure. Any presently existing epi structure for high-speed oxide confined VCSELs can be used for the inventive approach. In contrast to presently employed VCSEL processing, the oxidation process may be based on etching holes of any shape into the wafer surface, exposing the oxide layer(s). These holes are serving as starting point for the lateral oxidation process. The speed of the wet oxidation of the oxidizable layer(s) 21-24 may depend on Al-contents, AlGaAs layer thickness, and most importantly on the crystallographic direction. In addition it is controlled by temperature, total pressure and water partial pressure. The arrangement of the etched holes in respect to each other and to the crystal axes, the distances between them, and finally the choice of the oxidation parameters impacts the shape of the resulting apertures and opens up new design roads. The oxidation fronts may be circular or elliptical. The resulting apertures may have a diamond shape. The final processing step may comprise mesa M etching, n-contact deposition, and planarization based on the processing steps developed for "normal" VCSELs. GaAs / AlAs heterostructures are enabling the growth of lattice matched DBRs and high Al-content layers suitable for wet oxidation and leading to current and optical field confinement. The apertures will not be oxidized from the outside after etching a mesa M, but from the inside of holes, being first of all etched in regular arrays. The arrangement of the blind holes with respect to each other and to the crystal axes presents a free design parameter. The resulting apertures however may have always the same size. A difference in distance of the holes in one direction and the other direction may lead to a difference in oxide diameter in both directions and may allow polarized emission if desired. The oxidation speed depends on the crystal axes. The impact on the shape of the oxidation front has an impact on the shape of the resulting apertures. The alignment between the series of holes defined by the mask and the crystal axes may enable a shape optimization of the oxide confined apertures and allows to control the polarization status of the emitted light. The distances between the holes (d1 in one direction and d2 in the other direction) and the size of the holes can be chosen to position a sufficient number of apertures so close to each other, that additionally the emitted light emission can be coupled into the 50 or 62,5 µm core of a multimode fiber (MMF) preferably without coupling optics. The optical power increases linearly with the number of apertures. If the size of the apertures is chosen in such a way that the laser emits single mode light, it intensity increases with the number of apertures, showing identical wavelength, polarization and transversal mode. The electrical resistance decreases similarly with the number of apertures. The total resistance Rt can be calculated by 1 R t = ∑ x = 1 n 1 R x With the resistance Rx of an individual aperture.

Claims

1. Method of fabricating a radiation emitter (100) comprising the steps of fabricating a layer stack (10) that comprises a first reflector (12), an active region (13), an oxidizable layer (21-24), and a second reflector (14); and locally removing the layer stack (10), and thereby forming a mesa (M) of the radiation emitter (100), wherein said mesa (M) comprises the first reflector (12), the active region (13), the oxidizable layer (21-24) and the second reflector (14), wherein before or after locally removing the layer stack (10) and forming said mesa (M) the following steps are carried out: vertically etching blind holes (30) inside the layer stack (10), wherein the blind holes (30) vertically extend at least to the oxidizable layer (21-24) and expose the oxidizable layer (21-24); and oxidizing the oxidizable layer (21-24) via the sidewalls (31) of the blind holes (30) in lateral direction, wherein from each hole an oxidation front (32) radially moves outwards and wherein the etching is terminated before the entire oxidizable layer (21-24) is oxidized, thereby forming at least four unoxidized apertures, (40) each of which is limited by at least three of said oxidation fronts (32), inside the mesa, wherein - at least nine blind holes (30) are vertically etched inside the layer stack (10), wherein the blind holes (30) vertically extend at least to the oxidizable layer (21-24) and expose the oxidizable layer (21-24); - wherein by oxidizing the oxidizable layer (21-24) via the sidewalls (31) of the blind holes (30) in lateral direction said at least four apertures are fabricated; - wherein an intermediate isolating layer (50) is deposited on the bottom and the entire sidewalls of the blind holes; - wherein the blind holes (30) are filled with the first conducting material (61) or at least the sidewalls (31) of the holes (30) are covered with the first conducting material (61); and - wherein the conducting material (61) is thermally and electrically conductive.

2. Method of claim 1 wherein said mesa is provided with at least four individual VCSEL units by fabricating said at least two apertures within said mesa and within the same oxidizable layer (21-24).

3. Method of any of the preceding claims wherein the at least four apertures form VCSEL sub-cells that operate in parallel.

4. Method of any of the preceding claims wherein the apertures (40) are so narrowly spaced in said mesa (M) that the resulting radiation emitter provides single mode emission.

5. Method of any of the preceding claims wherein - a top contact layer (15) is fabricated on top of the second reflector (14), and - the top contact layer (15) is provided with a first conducting material (61) such that the conducting material (61) partly covers the surface of the top contact layer (15) and sections (15a) of the top contact layer (15) above the apertures (40) are left uncovered in order to allow optical radiation (P) to exit the mesa (M) without additional attenuation.

6. Radiation emitter (100) comprising a layer stack (10) having a first reflector (12), an active region (13), at least four apertures (40) formed by unoxidized material (20) of an oxidizable layer (21-24) that is partly oxidized and partly unoxidized, and a second reflector (14); wherein a mesa (M) of the emitter (100) includes at least the first reflector (12), the active region (13), the oxidizable layer (21-24) and the at least four apertures (40), and the second reflector (14), wherein the mesa (M) further comprises at least nine blind holes (30) which vertically extend to oxidized sections of the oxidizable layer (21-24), wherein the at least four apertures (40) are each limited by oxidation fronts (32) of at least three of said oxidized sections, wherein each of the blind holes (30) forms a center point of one of the oxidation fronts (32), - wherein an intermediate isolating layer (50) is deposited on the bottom and the entire sidewalls of the blind holes, - wherein the blind holes (30) are filled with the first conducting material (61) or at least the sidewalls (31) of the holes (30) are covered with the first conducting material (61), and - wherein the conducting material (61) is thermally and electrically conductive.

7. Radiation emitter (100) of claim 6 wherein the at least two apertures form VCSEL sub-cells that operate in parallel.

8. Radiation emitter (100) of claim 6 or 7 wherein the apertures (40) are so narrowly spaced in said mesa (M) that the resulting radiation emitter provides single mode emission.

9. Radiation emitter (100) of any of the preceding claims 6-8, - wherein a top contact layer (15) is located on top of the second reflector (14), - wherein a first conducting material (61) is located on top of the top contact layer (15), - wherein the first conducting material (61) partly covers the surface of the first contact layer (15) and - wherein sections (15a) of the first contact layer (15) above the apertures (40) are uncovered in order to allow optical radiation (P) to exit the mesa (M) without additional attenuation.