Optoelectronic semiconductor chip and method for manufacturing an optoelectronic semiconductor chip

Abstract

Optoelectronic semiconductor chip with a semiconductor layer sequence comprising a plurality of superimposed layers. - an active layer (4) with an active area (11) suitable for emitting electromagnetic radiation along a direction of radiation (99) during operation, - a first grid layer (7) on the active layer (4) which has a plurality of strips (70) arranged as grid lines perpendicular to the direction of radiation (99) with spaces (79) arranged between them, and - a second grid layer (8) on the first grid layer (7) which covers the strips (70) of the first grid layer (7) and the spaces (79) and which has a transparent material applied by a non-epitaxial application method, wherein - the stripes (70) of the first grating layer (7) and the spaces (79) form a periodic variation of the effective refractive index in the direction of emission (99).

Description

[0001] An optoelectronic semiconductor chip and a method for manufacturing an optoelectronic semiconductor chip are described.

[0002] Document US 2004 / 0 135 166 A1 is directed to a light-emitting component and a method for its manufacture.

[0003] The printed document EP 18 792 72 A1 is directed at a photonic crystal laser.

[0004] For many applications of semiconductor lasers, it is necessary that the emission spectrum has the smallest possible spectral width, longitudinal single-mode capability, and a well-controlled temperature response of the wavelength.

[0005] An object of at least one embodiment is to specify an optoelectronic semiconductor chip with a semiconductor layer sequence comprising a plurality of superimposed layers. An object of at least one further embodiment is to specify a method for manufacturing such an optoelectronic semiconductor chip.

[0006] These problems are solved by the subject matter and method of the independent patent claims. Advantageous embodiments and further developments of the subject matter and method are characterized in the dependent claims and are further described in the following description and drawings.

[0007] An optoelectronic semiconductor chip has, in particular, a sequence of semiconductor layers with a plurality of layers arranged one above the other and includes, in particular, - an active layer with an active area suitable for emitting electromagnetic radiation along a direction of radiation during operation, - a first grid layer on the active layer, which has a plurality of strips perpendicular to the direction of radiation, formed as grid lines with spaces arranged between them, and - a second grid layer on top of the first grid layer, covering the stripes of the first grid layer and the spaces between them, and comprising a transparent material applied by a non-epitaxial application method, wherein - the stripes of the first grating layer and the spaces between them form a periodic variation of the effective refractive index in the direction of radiation.

[0008] A method for manufacturing an optoelectronic semiconductor chip with a semiconductor layer sequence having a plurality of superimposed layers includes in particular the following steps: A) Providing an active layer with an active area suitable for emitting electromagnetic radiation along a direction of radiation during operation, B) Applying a first grid layer to the active layer, which has a plurality of strips perpendicular to the direction of radiation, designed as grid lines with spaces between them, and C) Applying a second grid layer to the first grid layer, covering the stripes of the first grid layer and the spaces between them, and comprising a transparent material applied by a non-epitaxial application method, wherein - the stripes of the first grating layer and the spaces between them form a periodic variation of the effective refractive index in the direction of radiation.

[0009] The embodiments, features and combinations thereof described below refer equally to the optoelectronic semiconductor chip and to the method for manufacturing the optoelectronic semiconductor chip, unless explicitly stated otherwise.

[0010] Here and in the following, "light" or "electromagnetic radiation" can equally mean, in particular, electromagnetic radiation with at least one wavelength or a range of wavelengths from the infrared to ultraviolet spectrum. This light or electromagnetic radiation can encompass a visible, i.e., near-infrared to blue, wavelength range with one or more wavelengths between approximately 350 nm and approximately 1000 nm.

[0011] The fact that a layer or element is arranged or applied "on" or "over" another layer or element can mean, here and in the following, that the layer or element is in direct mechanical and / or electrical contact with the other layer or element. It can also mean that the layer or element is arranged indirectly on or over the other layer or element. In this case, further layers and / or elements may be arranged between the layer and the element.

[0012] The fact that a layer or element is arranged "between" two other layers or elements can mean, here and in the following, that the layer or element is in direct mechanical and / or electrical contact or in indirect contact with one of the other two layers or elements and in direct mechanical and / or electrical contact or indirect contact with the other of the two layers or elements. In the case of indirect contact, further layers and / or elements may be arranged between the layer and at least one of the other two layers, or between the layer and at least one of the other two elements.

[0013] The layers of the semiconductor sequence each have parallel orientations or planes of extension, with the stacked layers of the semiconductor sequence being arranged along a single orientation. This orientation is orthogonal to the orientations or planes of extension.

[0014] The first grid layer can, in particular, have grid lines that are designed as strip-like protrusions with intervening depressions forming the spaces. This can mean that the depressions or spaces do not extend completely through the first grid layer. Alternatively, the first grid layer can have separately arranged strips that are spatially separated from each other by the spaces. In this case, the second grid layer can project through the first grid layer between the strips of the first grid layer in the spaces between them, up to an underlying functional layer, as described in more detail below.

[0015] The transparent material of the second lattice layer can preferably be deposited non-epitaxially to exhibit a crystal lattice structure different from, and in particular non-matched to, that of the underlying layers, such as the active layer and / or other functional layers. Furthermore, the transparent material can differ from the material system of the active layer or from the material system of the active layer and other functional layers. Non-epitaxial deposition methods, including chemical and physical deposition processes, are suitable for not continuing the crystal lattice structure of the underlying layer(s), thus enabling the formation of a second lattice layer with a non-crystal lattice-matched structure.This means that the second lattice layer, according to the method and optoelectronic semiconductor chip described here, can be identified after the application of the epitaxially grown layer by its non-matched crystal lattice structure.

[0016] The semiconductor layer sequence or semiconductor chip described here is particularly preferably configured as a laser diode capable of generating coherent electromagnetic radiation, especially through stimulated emission, during operation. The emission direction is preferably directed along and parallel to the orientations or planes of the semiconductor layer sequence, and particularly along and parallel to the active layer. This means that the semiconductor chip is particularly preferably designed as an edge emitter, capable of emitting electromagnetic radiation from at least one face of the semiconductor layer sequence.

[0017] The semiconductor layer sequence can, for example, have a pn junction, a double heterostructure, a single quantum well (SQW) structure, or a multiple quantum well (MQW) structure as the active region in the active layer. Within the scope of this application, the term quantum well structure encompasses, in particular, any structure in which charge carriers can undergo quantization of their energy states by confinement. Specifically, the term quantum well structure does not specify the dimensionality of the quantization. It therefore includes, among other things, quantum wells, quantum wires, and quantum dots, and any combination of these structures.The semiconductor layer sequence can comprise, in addition to the active layer with its active region, further functional layers and functional regions, selected from p- and n-doped charge carrier transport layers (i.e., electron and hole transport layers), p-, n-, and undoped confinement, cladding, and waveguide layers, barrier layers, planarization layers, buffer layers, protective layers, and electrodes, as well as combinations of these layers. The electrodes can each have one or more metal layers containing Ag, Au, Sn, Ti, Pt, Pd, and / or Ni. Furthermore, additional layers, such as buffer layers, barrier layers, and / or protective layers, can also be arranged perpendicular to the orientation of the semiconductor layer sequence, for example, around the semiconductor layer sequence, such as on its side faces.

[0018] In a preferred embodiment, the semiconductor layer sequence comprises an active layer which includes a quantum well structure in the active region and is arranged between two waveguide layers configured as so-called secondary wells, thereby forming a so-called "separate confinement heterostructure" (SCH structure). The SCH structure enables the separate optimization of charge carrier confinement and vertical guidance of the generated electromagnetic radiation along the plane of extension of the active layer. The SCH structure can preferably be arranged between at least two further waveguide and / or at least two cladding layers. The aforementioned layers, as well as optionally further layers of the semiconductor layer sequence, can be arranged on a substrate.The provision of the active layer described in process step A above can also mean, for example, the provision of such a layer sequence or a modification thereof.

[0019] Furthermore, depending on the substrate design, an electrode can be positioned on the same substrate surface as the active layer, between the substrate and the active layer. Alternatively, the electrode can also be positioned on a surface facing away from the active layer. Another electrode is positioned above the active layer, viewed from the substrate.

[0020] The semiconductor layer sequence can be implemented as an epitaxial layer sequence, i.e., as an epitaxially grown semiconductor layer sequence. This semiconductor layer sequence can, for example, be based on AlGaAs. AlGaAs-based semiconductor chips and semiconductor layer sequences include, in particular, those in which the epitaxially grown semiconductor layer sequence typically comprises a sequence of different individual layers, containing at least one layer made of a material from the III-V compound semiconductor material system AlGaAs. x Ga 1-xThe material contains elements such as As with 0 ≤ x ≤ 1. In particular, an active layer comprising an AlGaAs-based material can be suitable for emitting electromagnetic radiation with one or more spectral components in a red to infrared wavelength range. Furthermore, such a material can additionally or alternatively contain In and / or P in addition to or as an alternative to the aforementioned elements.

[0021] Furthermore, the semiconductor layer sequence can alternatively or additionally be based on InGaAlN. InGaAlN-based semiconductor chips and semiconductor layer sequences include, in particular, those in which the epitaxially fabricated semiconductor layer sequence typically comprises a sequence of different individual layers, containing at least one layer made of a material from the III-V compound semiconductor material system InGaAlN. x Al y Ga 1-x-y N with 0 ≤ x ≤ 1, 0 ≤ y ≤ 1 and x + y ≤ 1.

[0022] Semiconductor layer sequences that have at least one active layer based on InGaAlN can, for example, preferentially emit electromagnetic radiation in an ultraviolet to green wavelength range.

[0023] Alternatively or additionally, the semiconductor layer sequence can also be based on InGaAlP, meaning that the semiconductor layer sequence can have different individual layers, at least one of which is a material from the III-V compound semiconductor material system In x Al y Ga 1-x-y P with 0 ≤ x ≤ 1, 0 ≤ y ≤ 1 and x + y ≤ 1. Semiconductor layer sequences or semiconductor chips that have at least one active layer based on InGaAlP can, for example, preferentially emit electromagnetic radiation with one or more spectral components in a green to red wavelength range.

[0024] Alternatively or additionally, the semiconductor layer sequence or semiconductor chip may also include II / VI compound semiconductor material systems alongside or instead of III-V compound semiconductor material systems. A II / VI compound semiconductor material may include at least one element from group 2, such as Be, Mg, Ca, Sr, and one element from group 6, such as O, S, Se. In particular, a II / VI compound semiconductor material comprises a binary, ternary, or quaternary compound containing at least one element from group 2 and at least one element from group 6. Such a binary, ternary, or quaternary compound may also include, for example, one or more dopants and additional components. Examples of II / VI compound semiconductor materials include: ZnO, ZnMgO, CdS, ZnCdS, and MgBeO.

[0025] The semiconductor layer sequence may further comprise a substrate on which the aforementioned III-V or II-VI compound semiconductor material systems are deposited. The substrate may include a semiconductor material, such as one of the aforementioned compound semiconductor material systems. In particular, the substrate may include or be made of GaP, GaN, SiC, Si, and / or Ge.

[0026] To enable operation of the semiconductor layer sequence or the semiconductor chip in a lateral fundamental mode, layers of the semiconductor layer sequence arranged on at least one side of the active region can, for example, be structured in a ridge and / or trapezoidal shape. Such configurations of the semiconductor layer sequence, known as ridge waveguides, finned waveguides, "ridge structure," "trapezoidal structure," or "tapered structure," are known to those skilled in the art and are not described further here.

[0027] To enable operation of the semiconductor layer sequence or the semiconductor chip in a selected longitudinal mode, the first and second grating layers can provide so-called distributed feedback of the electromagnetic radiation generated in the active region, thus forming a laser resonator or part thereof. The first and second grating layers can be arranged directly above the active region or longitudinally offset from it, but still within the resonator region. In the first case, this results in a so-called "distributed feetback structure" (DFB structure), and in the second case, a so-called "distributed Bragg reflector structure" (DBR structure). The basic functionality and structure of DFB and DBR structures are known to those skilled in the art and will not be described further here.

[0028] In particular, the second grating layer can have a lower refractive index than the first. This creates a spatial grating with a periodically varying refractive index along the radiation direction, enabling the distributed feedback or Bragg reflection typical of DFB and DBR structures. The wavelength of the electromagnetic radiation generated in the active region is determined by the grating, i.e., the first and second grating layers, through constructive interference of the wave fed back at the grating. The period of the grating, i.e., the distance of a strip of the first grating layer to an adjacent strip, can be chosen such that the wavelength oscillating in the resonator is preferably that which corresponds in the grating material to twice the period of the grating.The thickness of the additional layers arranged between the active layer and the first and second grid layers, such as waveguide and / or cladding layers, is chosen such that a sufficiently high overlap of the mode of the generated electromagnetic radiation guided in the waveguide region with the first grid layer is enabled.

[0029] In well-known DFB and DBR laser structures, a lattice structure is produced by epitaxial deposition of the lattice-forming layers. For this purpose, a material with a specific refractive index is produced in strip form and then epitaxially overgrown over a large area with a material having a different refractive index. This overgrowth process is technically feasible, for example, in the InGaAsP / InP material system, although complex and expensive. In aluminum-containing material systems, such as AlGaAs / GaAs systems, however, such epitaxial overgrowth is technically only possible with extremely high effort, or not at all, due to the very high oxidation potential of aluminum. Therefore, the technical approaches from the InGaAsP / InP material system, for example, have not been able to prevail, particularly in the AlGaAs / GaAs material system.Alternatively, the desired spectral properties can be achieved using surface gratings, but due to the high optical losses involved, such solutions are hardly commercially viable.

[0030] In the semiconductor chip and process described here, the second lattice layer is deposited in process step C using a non-epitaxial, and preferably a physical, deposition method, in particular by evaporation or sputtering. This enables the fabrication of a DFB- or DBR-equivalent structure over the active layer of the semiconductor layer sequence without having to perform the known and technically complex, or even practically impossible, epitaxial overgrowth step. For this purpose, after the formation of the stripes in the first lattice layer, i.e., after defining a periodic lattice, the second lattice layer is deposited over a large area and without structure onto the stripes of the first lattice layer and the spaces between them, instead of epitaxial overgrowth by evaporation or sputtering.A guided mode of the waveguide structure exhibits a slightly different propagation constant in regions with and without stripes in the first grating layer due to the difference in refractive indices between the first and second grating layers. This difference can also be described by an effective refractive index. By appropriately dimensioning the so-called duty cycle (i.e., the ratio of the stripe width to the grating period) and the layer thicknesses of the first and second grating layers, a sufficiently large difference in the effective refractive indices can be achieved through the different grating regions: the so-called "lines" (the stripes or grating lines of the first grating layer) and the so-called "spaces" (the spaces between the stripes of the first grating layer covered and filled by the second grating layer).This results in a distributed coupling of forward and backward propagating electromagnetic waves in the resonator of the semiconductor chips. The strength of this coupling is proportional to the difference in the effective refractive indices.

[0031] The transparent material of the second lattice layer can, in particular, also form an electrical contact with the layer below. For this purpose, the transparent material can especially preferably be an electrically conductive transparent material. Furthermore, the transparent material can have a lower refractive index than the layer below.

[0032] The second grating layer can have a refractive index less than or equal to 2.5. A refractive index of less than or equal to 2 is particularly preferred. The smaller the refractive index of the second grating layer compared to the first, the greater the difference in the effective refractive indices of the "lines" and "spaces"—that is, the areas with the lines of the first grating layer and the areas with the spaces between them. It may be possible to achieve a greater difference in refractive index compared to the first grating layer by using a suitable transparent material in the second grating layer than is possible with epitaxially overgrown gratings in known DFB or DBR lasers, since the epitaxial overgrowth layer typically has a refractive index relatively close to that of the overgrown grating layer.Furthermore, it is possible that the cladding and / or waveguide layers arranged between the active layer and the first or second grating layer can be thinner than in conventional DFB or DBR lasers with epitaxially overgrown gratings. Additionally, the electromagnetic radiation generated in the active region can have a very shallow penetration depth in the second grating layer due to the high refractive index difference, thus minimizing optical losses due to absorption in the second grating layer.

[0033] In particular, the transparent material can contain or consist of a described transparent conductive oxide (TCO). A TCO comprises a transparent, electrically conductive material containing a metal oxide such as zinc oxide, tin oxide, cadmium oxide, titanium oxide, indium oxide, or indium tin oxide (ITO). Besides binary metal-oxygen compounds, such as ZnO, SnO₂, or In₂O₃, ternary metal-oxygen compounds, such as Zn₂SnO₄, CdSnO₃, ZnSnO₃, MgIn₂O₄, GaInO₃, Zn₂In₂O₅, or In₄Sn₃O₄, are also included. 12or mixtures of different transparent conductive oxides belonging to the group of TCOs. Furthermore, it is possible that the TCOs do not necessarily have a stoichiometric composition and can also be p- or n-doped. Particularly preferred, due to its ease of application and good process compatibility, is zinc oxide and / or ITO in the second lattice layer. The refractive index of the second lattice layer with one or more TCOs as the transparent material can be less than 2 and, for example, approximately 1.6.

[0034] The first lattice layer can consist of a semiconductor material from the material system of the active layer and / or the subsequent layers, such as the waveguide and / or cladding layer. To ensure good electrical contact between the first and second lattice layers and the active layer, the first lattice layer can also be designed as a contact layer. For this purpose, the first lattice layer can, for example, have a sufficiently high doping concentration. In the AlGaAs material system, the first lattice layer can, for example, consist of GaAs.

[0035] To produce the first grid layer, the above-mentioned process step A or the above-mentioned process step B can include a further sub-step B1, and process step B can include further sub-steps B2 and B3 with B1) Large-area application of the first grid layer, B2) Application of a mask layer with a plurality of stripes running perpendicular to the direction of radiation of the active area and spaces between them and B3) Wet chemical etching of the first grid layer in the areas of the spaces between the mask layer.

[0036] Step B1 can, for example, be performed by epitaxial growth on the active layer provided in process step A, whereby, as described above, the active layer can be provided with additional waveguide and / or cladding layers in process step A. Step B1 is thus preferably carried out together with process step A in a single epitaxial step. In process step B2, the mask layer can be produced by applying a suitable material over a large area and subsequently structuring it in a lithographic step. In process step B3, the first lattice layer can be etched only partially in the spaces between the mask layer, so that depressions can be formed in the first lattice layer as the spaces between the layers.Alternatively, the first grid layer can be completely etched through in the spaces between the mask layer, so that the stripes of the first grid layer are spatially separated from each other by the etched spaces.

[0037] The height of the first grating layer can be greater than or equal to 20 nanometers, preferably greater than or equal to 50 nanometers, and particularly preferably greater than or equal to 80 nanometers. The greater the height of the first grating layer, and especially the greater the height of the stripes and the spaces between them, the greater the resulting effective refractive index difference between the grating regions and the greater the achievable coupling coefficient of the laser resonator produced in this way. In particular, the first and second grating layers described here make it possible to achieve a coupling coefficient of greater than or equal to 10 / cm, preferably greater than or equal to 20 / cm, and particularly preferably greater than or equal to 30 / cm.

[0038] In a further process step D, an electrode layer can be applied to the second grid layer, which has one of the materials mentioned above.

[0039] Furthermore, an unstructured, large-area contact layer can be arranged between the active layer and the first lattice layer. This contact layer can consist of an epitaxially grown semiconductor material. The additional contact layer can improve the electrical connection between the first and second lattice layers and the underlying layers, particularly the active layer. Specifically, the contact layer can enable a large-area, uniform electrical connection. The first lattice layer and the contact layer can consist of the same material.

[0040] Furthermore, an intermediate layer can be arranged between the first lattice layer and the contact layer. This intermediate layer, like the first lattice layer, has a strip-like structure. For example, the intermediate layer can be designed as an etch stop layer, which facilitates the fabrication of the first lattice layer on the contact layer. In the case of a first lattice layer made of GaAs, the intermediate layer can, for example, comprise or consist of a phosphorus-containing material, preferably InGaP and / or AlGaP. The thickness of the intermediate layer can have a size range mentioned above in connection with the first lattice layer.

[0041] The first lattice layer and the second lattice layer, made of a transparent material as described above, particularly a TCO, enable significant modification of the mode profile of the electromagnetic radiation generated in the active region, allowing for a coupling coefficient comparable to that of known laser structures. In particular, this makes it possible to develop a DFB or DBR laser semiconductor chip that does not exhibit an epitaxially overgrown lattice structure.

[0042] Further advantages and advantageous embodiments and developments of the invention will become apparent from the following in conjunction with the Fig. 1A to 5 described embodiments.

[0043] They show: Fig. 1A to 1E schematic representations of a method for manufacturing an optoelectronic semiconductor chip according to an exemplary embodiment, Fig. 2 and Fig. 3 schematic representations of optoelectronic semiconductor chips according to further embodiments, and Fig. 4 and Fig. 5 Graphene with properties of optoelectronic semiconductor chips according to further embodiments.

[0044] In the exemplary embodiments and figures, identical or similarly functioning components may be designated with the same reference numerals. The depicted elements and their relative sizes are generally not to be considered to scale; rather, individual elements, such as layers, components, building elements, and areas, may be exaggeratedly thick or large for clarity and / or better understanding.

[0045] In the Fig. 1A to 1E shows a method for manufacturing an optoelectronic semiconductor chip 100 according to an exemplary embodiment.

[0046] In a first procedural step A according to Fig. In 1A, an active layer 4 is provided. In particular, a semiconductor layer sequence with a plurality of layers is provided, which includes an active layer 4. The active layer 4 has an active region 11 which is suitable for emitting electromagnetic radiation along the emission direction marked 99 during the operation of the optoelectronic semiconductor chip 100. The arrangement direction of the layers of the semiconductor layer sequence is perpendicular to the emission direction 99 shown.

[0047] In the illustrated embodiment, the provided semiconductor layer sequence comprises a GaAs substrate 1 on which an n-type cladding layer 2 made of AlGaAs is arranged. Above this are an n-type waveguide layer 3 made of AlGaAs, the approximately 7 nanometer thick active layer 4 with a multi-quantum well structure made of InGaAs, and a p-type waveguide layer 5 made of AlGaAs. The active layer 4 and the waveguide layers 3 and 5 form a SCH structure as described above in the general section. The active layer 4 is suitable for generating electromagnetic radiation with a wavelength of approximately 920 nanometers. Above the waveguide layer 5 is a p-type cladding layer 6 made of AlGaAs. The cladding layers 2 and 6 have an Al content of approximately 20% or more and 30% or less, respectively, and a thickness of several hundred nanometers, approximately in the range of 400 to 1000 nanometers.Waveguide layers 3 and 5 have an Al content of approximately 5% or greater and approximately 15% or less and a thickness of several tens of nanometers, in the range of approximately 50 to 100 nanometers.

[0048] Substrate 1 can be a growth substrate on which functional layers 2 to 6 are epitaxially grown. Alternatively, the semiconductor layer sequence can be fabricated using thin-film technology. This means that functional layers 2 to 6 are grown on a growth substrate and then deposited onto a support substrate, which then forms the substrate 1 of the Fig. The semiconductor layer sequence shown in 1A is formed and transferred. Depending on the growth technique, the n-type or, alternatively, the p-type layers can face the substrate 1, as shown.

[0049] As an alternative to the layers and materials shown, semiconductor layer sequences with features and, in particular, an active layer as described in the general section can also be provided. The following description should therefore not be understood as being limited to the materials and combinations mentioned.

[0050] In a further process step B, a first grid layer 7 with stripes 70 running perpendicular to the radiation direction 99 and spaces 79 between them is produced. For this purpose, in a sub-step B1 according to Fig. In step 1B, the first lattice layer 7 is epitaxially grown over a large area and without structure on the cladding layer 6. Step B1 is preferably carried out in a single epitaxial step together with the epitaxial formation of the functional layers 2 to 6. In the illustrated embodiment, the first lattice layer 7 is made of p-doped GaAs and simultaneously serves as a contact layer. In further steps B2 and B3, a lattice structure with grid lines is transferred into the first lattice layer 7 in the form of stripes 70 running perpendicular to the radiation direction 99, which are separated from each other by the spaces 79. Fig. 1C). The strips 70 and spaces 79 form a periodic sequence along the emission direction 99 of regions containing the material of the first grating layer 7 and regions free of the material of the first grating layer 7. The dimensions of the strips 70 and spaces 79, as well as the length of the region containing the strips 70 and spaces 79, i.e., the length of the grating region, are selected according to considerations known to those skilled in the art with regard to DFB and DBR lasers and depend on the desired properties of the emitted electromagnetic radiation. In the illustrated embodiment, the first grating layer 7 is completely etched through, so that the spaces 79 spatially separate the strips 70 from one another. Alternatively, the spaces 79 can also form depressions in the first grating layer 7, which are created by not completely etching the first grating layer 7 (not shown).By completely separating the strips 70 from one another and by providing gaps 79 that extend completely through the first grid layer 7, a greater effective refractive index difference can be achieved. If the gaps 79 are formed as depressions, as also described below in connection with the embodiment of the... Fig. 2 explains how a more homogeneous current injection into the active layer 4 can be achieved.

[0051] In a further sub-step C according to Fig. In step 1D, a second lattice layer 8 is deposited onto the strips 70 and the spaces 79 of the first lattice layer 7 by vapor deposition or sputtering. Although the second lattice layer 8 differs from an epitaxially grown overgrowth layer in terms of its crystal structure due to the physical deposition method, similar electrical properties with respect to the contact resistance to the cladding layer 6 can be achieved. Furthermore, the oxidation problem that would arise during epitaxial overgrowth of the first lattice layer 7 due to the Al content in the layers of the semiconductor sequence can be avoided. After deposition, the second lattice layer 8 covers the first lattice layer 7 and thus has a height that is greater than the height of the first lattice layer 7.The second lattice layer 8 has a transparent material formed by a TCO and, in the embodiment shown, is made of zinc oxide or ITO with a refractive index of about 1.6.

[0052] In a further procedural step D according to Fig. An electrode layer 9 is applied to the second grid layer 8. A further electrode layer can be applied to the surface of the substrate 1 facing away from the active layer 4 for bilateral electrical connection of the active layer 4 to the active region 11 (not shown). Alternatively, the further electrode layer can also, for example, contact the cladding layer 2 directly and be arranged on the side of the substrate facing the active layer 4 (not shown).

[0053] The optoelectronic semiconductor chip 100 produced in this way according to Fig. 1E is constructed similarly to a known DFB laser, with the first and second grating layers 7, 8 arranged directly above the active region 11 in the orientation direction. The low refractive index of the second grating layer 8 causes the optical mode generated in the active region 11 of the active layer 4 and propagating in the semiconductor layer sequence to be guided predominantly in the (optically) low-loss semiconductor material of the first grating layer 7 and to exhibit only a very small overlap with the second grating layer.

[0054] This effect is shown in the graph of the Fig. Figure 4 illustrates this. The x-axis, which is shown in Fig. Figure 1E of section 98 corresponds to the distance x in micrometers from the surface of substrate 1 facing away from the active layer 4, along the arrangement direction of the semiconductor layer sequence. Curve 40, in conjunction with the left y-axis, shows the refractive index of the functional layers of the semiconductor layer sequence. The refractive index of the substrate is approximately 3.6, and that of the cladding layers is approximately 3.4. In the region of approximately 3 micrometers on the x-axis, the active layer with a refractive index of approximately 3.7 and the waveguide layers or SCH layers with a refractive index of approximately 3.5 can be seen. Adjacent to this, in the region labeled 41, the refractive index of the first grating layer 7 is shown as approximately 3.6. Following this, the second grating layer 8 can be seen with a refractive index of approximately 1.6.Curves 42 and 43, in conjunction with the right y-axis, show the power P of the optical mode in the semiconductor layer sequence in suitable units. Curve 42 corresponds to the power of the optical mode in the region of the intersection axis 98. Fig. 1E, i.e., in the region of a strip 70 of the first grating layer 7, while curve 43 shows the power of the optical mode in the region of an interval 79. It should be noted that, strictly speaking, curve 40 should be shown for curve 43, in which the refractive index shown in region 41 corresponds to that of the second grating layer, i.e., approximately 1.6. The comparison of curves 42 and 43 shows that the optical mode has only a small overlap with the second grating layer.

[0055] This effect, namely that the optical mode has hardly any overlap with the second grating layer, is crucial for the efficiency of the shown optoelectronic semiconductor chip 100, since the TCO of the second grating layer exhibits significantly higher optical losses than the semiconductor material of the first grating layer 7. However, this does not have a detrimental effect due to the small overlap. The periodic presence or absence of the material of the first grating layer 7, i.e., the periodic sequence of stripes 70 and spaces 79, in the longitudinal direction (radiation direction), thus creates a periodic variation in the effective refractive index. As with known DFB lasers, this leads to distributed feedback and the corresponding modification of the spectral properties of the electromagnetic radiation generated in the active region.

[0056] In this context, Fig. Figure 5 shows the refractive index difference Δn achievable as a function of the height d of the first grating layer 7 in nanometers on the left y-axis in conjunction with curve 51 and the resulting coupling coefficient k on the right y-axis in conjunction with curve 52. The first grating layer 7 is assumed to be a rectangular grating with a duty cycle, i.e., a width ratio of the stripes 70 to the spaces 79, of 1:1. Fig. Figure 5 shows that coupling coefficients can be achieved with a k of approximately 30 / cm, which is within the range of known DFB lasers. An additional way to vary the coupling coefficient k is by varying the thickness of the upper cladding layer as seen from the substrate.

[0057] In the Fig. 2 and Fig. 3 are modifications of the optoelectronic semiconductor chip 100 according to further embodiments. Fig. 1E shown.

[0058] The optoelectronic semiconductor chip 200 according to Fig. 2 has a contact layer 10 between the first grid layer 7 and the active layer 4 on the cladding layer 6. This contact layer 10 is applied to the cladding layer 6 in an unstructured and large-area manner. This allows for a homogeneous electrical connection between the first and second grid layers 7, 8, regardless of their materials and electrical properties. This ensures that the current injection into the active layer 4 is as homogeneous as possible, and in particular more homogeneous than in the previous embodiment. While in the previous embodiment the current injection mainly occurs via the strips 70 of the first grid layer 7 into the underlying functional layers, in this embodiment the Fig. 2. A full-surface current connection is made. This reduces the contact resistance between the grid layers 7, 8 and the underlying functional layers. The contact layer 10, like the first grid layer 7, has highly doped GaAs. An intermediate layer 71, made of AlGaP in the illustrated embodiment, is arranged between the contact layer 10 and the first grid layer 7 in the areas of the strips 70. The intermediate layer 71 serves as an etch stop layer in process step B described above for producing the strips 70 of the first grid layer 7.

[0059] In Fig. 3 is in contrast to the embodiments of the Fig. 1E and Fig.Figure 2 shows an optoelectronic semiconductor chip 300 in a DBR-like configuration. The first lattice layer 7 with the strips 70 and the spaces 79 containing the material of the second lattice layer 8 are arranged longitudinally offset from the active region 11 of the active layer 4. The active layer 4 can also be removed in the region of the first lattice layer 7, as indicated by the dashed line.

Claims

[1] Optoelectronic semiconductor chip with a semiconductor layer sequence comprising a plurality of layers arranged on top of each other, - an active layer (4) with an active area (11) suitable for emitting electromagnetic radiation along a direction of radiation (99) during operation, - a first grid layer (7) on the active layer (4) which has a plurality of strips (70) arranged as grid lines perpendicular to the direction of radiation (99) with spaces (79) arranged between them, and - a second grid layer (8) on the first grid layer (7) which covers the strips (70) of the first grid layer (7) and the spaces (79) and which has a transparent material applied by a non-epitaxial application method, wherein - the stripes (70) of the first grating layer (7) and the spaces (79) form a periodic variation of the effective refractive index in the direction of emission (99). [2] Semiconductor chip according to claim 1, wherein - the second lattice layer (8) has a smaller refractive index than the first lattice layer (7). [3] Semiconductor chip according to claim 1 or 2, wherein - the refractive index of the second lattice layer (8) is less than or equal to 2.5 and in particular less than or equal to 2. [4] Semiconductor chip according to any one of the preceding claims, wherein - the transparent material of the second lattice layer (8) comprises a transparent conductive oxide, in particular zinc oxide and / or indium tin oxide. [5] Semiconductor chip according to any one of the preceding claims, wherein - the second grid layer (8) is applied by vapor deposition or sputtering. [6] Semiconductor chip according to any one of the preceding claims, wherein - the first lattice layer (7) is formed as a contact layer. [7] Semiconductor chip according to any one of the preceding claims, wherein - an unstructured contact layer (10) is applied between the first grid layer (7) and the active layer (4). [8] Semiconductor chip according to claim 7, wherein - an intermediate layer (71) is arranged between the first grid layer (7) and the unstructured contact layer (10). [9] Semiconductor chip according to claim 8, wherein - the intermediate layer (71) is designed as an etch stop layer. [10] Semiconductor chip according to any one of claims 1 to 9, wherein - the first and second lattice layers (7, 8) are arranged along an arrangement direction of the semiconductor layer sequence above the active region (11). [11] Semiconductor chip according to any one of claims 1 to 9, wherein - the first and second grid layers (7, 8) are arranged longitudinally offset from the active area (11). [12] Semiconductor chip according to any one of the preceding claims, wherein - an electrode (9) is arranged on the second grid layer (8). [13] Semiconductor chip according to one of the preceding claims, which is a laser diode whose emission direction is parallel to the extension planes of the layers of the semiconductor layer sequence. [14] Semiconductor chip according to the preceding claim, wherein the first lattice layer and the second lattice layer form at least part of a resonator of the laser diode. [15] Method for manufacturing an optoelectronic semiconductor chip with a semiconductor layer sequence having a plurality of layers arranged one above the other, comprising the steps: A) Providing an active layer (4) with an active area (11) suitable for emitting electromagnetic radiation along a direction of radiation (99) during operation, B) Applying a first grid layer (7) to the active layer (4), which has a plurality of strips (70) arranged as grid lines perpendicular to the direction of radiation (99) with spaces (79) arranged between them, and C) Applying a second grid layer (8) to the first grid layer (7) which covers the strips (70) of the first grid layer (7) and the spaces (79) and which has a transparent material which is applied by a non-epitaxial application method wherein - the stripes (70) of the first grating layer (7) and the spaces (79) form a periodic variation of the effective refractive index in the direction of emission (99). [16] The method of claim 15, wherein - the transparent material comprises a transparent conductive oxide and is applied in process step C by vapor deposition or sputtering. [17] The method of claim 15 or 16, wherein process step A or process step B comprises a further sub-step B1 and process step B comprises further sub-steps B2 and B3, comprising B1) Large-area application of the first grid layer (7), B2) Application of a mask layer with a plurality of stripes running perpendicular to the direction of emission (99) of the active area (11) and spaces between them and B3) Wet chemical etching of the first grid layer (7) in the areas of the spaces between the mask layer.

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