SEMICONDUCTOR LASER AND METHOD FOR PRODUCING A SEMICONDUCTOR LASER

Description

Technical field of the invention:

[0001] The present invention relates to a method for producing a semiconductor laser, in particular a semiconductor laser diode, according to claim 1. Furthermore, the present invention relates to a semiconductor laser, in particular a semiconductor laser diode, according to claim 8, which relates to a single-mode semiconductor laser diode comprising a semiconductor substrate with a material layer sequence or multilayer structure arranged thereon, with a laser layer or active region arranged therein and at least one waveguide ridge arranged thereabove and with material removal regions arranged laterally to the waveguide ridge, or optionally the waveguide ridges, in which material removal regions a part of the multilayer structure or material layer sequence has been removed. State of the art:

[0002] Semiconductor laser diodes, or more generally semiconductor lasers, are a well-known source for generating coherent electromagnetic radiation according to the state of the art and are used for a wide variety of different applications. Typical applications for semiconductor laser diodes include data transmission in telecommunications or analysis and sensor technology. Many of these applications require a single-mode coherent radiation source with a wavelength determined by the application and also have other requirements specific to the respective application. These requirements include, for example, a certain tuning range of the wavelength of the light, e.g., to be able to measure the spectral properties of a material, a precisely specified output power, a specified far-field profile, or particularly high side-mode suppression.

[0003] In the following, the term semiconductor laser diode will be used predominantly, although this term should include all semiconductor lasers that, strictly speaking, do not form a diode or diode structure.

[0004] Semiconductor laser diodes are based on the principle of stimulated emission, according to which a light mode is exponentially amplified if it lies within the gain spectrum of the laser medium, while simultaneously minimizing its losses, especially mirror losses. Without further interference, the excited or generated light spectrum contains many so-called Fabry-Perot modes—light modes with minimal mirror losses—in the gain profile of the laser medium, so the laser diode emits a broad spectrum of modes.

[0005] To select from this multitude of modes, EP 0 984 535 B1, for example, discloses a semiconductor laser diode with a waveguide ridge and a periodic grating arranged parallel to the waveguide ridge. The grating period, i.e. the distance from the beginning of one grating ridge to the beginning of the next grating ridge, is set as half the wavelength of the desired laser mode in the waveguide medium. The grating material is selected such that it has a refractive index and absorption coefficient that differ from its surroundings, in order to establish a periodic modulation of the effective complex refractive index for modes in the waveguide. The periodic modulation of the refractive index leads to a strong wavelength dependence of the mirror losses, whereas the periodic modulation of the absorption coefficient leads to additional losses with a strong wavelength dependence.This mechanism allows monomode operation of the laser diode at the wavelength set by the grating.

[0006] However, known semiconductor laser diodes with periodic lateral gratings have the disadvantage that a grating consisting of periodically arranged ridges does not improve other properties of the semiconductor laser, or even worsens them. This particularly applies to other properties of the laser mode, the targeted influencing of the strain of the semiconductor material, and the optimized influencing of the thermal properties of the laser diode. Known laser diodes therefore have properties that cannot be influenced and / or reproduced during production, or can only be insufficiently influenced. This also means that a lot of waste is sometimes generated during production, since the lack of, or at least non-reproducibility influencing, optical, thermal, and / or mechanical properties means that otherwise functional semiconductor lasers do not meet the desired requirements or the required characteristics.

[0007] For example, the strain of the laser material generated by the grating of the known laser diodes has a strong influence on the gain spectrum, the detailed structure of the grating near the facet of the laser diode has an influence on its far field and the structuring further away from the waveguide has an influence on the lateral modes, whereby these influences could not be compensated for and in some cases could not be adjusted, so that the monomode nature of the known semiconductor lasers with grating structure was bought at a sometimes high price, which was achieved at the expense of the other properties of the semiconductor laser.

[0008] A semiconductor laser with a ridge waveguide and lateral gratings for mode selection is known, for example, from C.-A. Yang et al., Appl. Phys. Lett. 114 (2019) 021102.

[0009] Against this background, it is the object of the present invention to propose an improvement of the semiconductor laser diodes described above and of the manufacturing method used therein, with which, in addition to a known monomodal radiation, further optical, thermal and mechanical properties of the semiconductor laser can be specifically adjusted without having to accept a significant additional expenditure with regard to the process steps of the manufacturing process.

[0010] This object is achieved by a method for producing a semiconductor laser having the features of claim 1 or by a semiconductor laser having the features of claim 8. Advantageous embodiments are the subject of the subclaims.

[0011] The method according to the invention according to claim 1 provides in detail that a multilayer structure or material layer sequence is applied to a substrate, in particular a semiconductor substrate, in a manner known per se, wherein the layers of the multilayer structure run parallel to a layer propagation plane defined by a surface of the substrate, and wherein the application of the multilayer structure comprises at least the creation of an active region or laser layer. Furthermore, the method comprises the known method step of removing material from the multilayer structure in at least two separate material removal areas, wherein the material removal occurs substantially perpendicular to the layer propagation plane, thereby forming a waveguide ridge.However, it can also be provided that the waveguide ridge is trapezoidal in shape, with flanks that then run not at right angles but obliquely to the layer propagation plane.

[0012] In a further, known process step, an insulation layer is then produced on at least the material removal areas.

[0013] However, according to the invention, it is provided that in a further method step, which does not necessarily have to take place during production itself, but which can also be carried out in advance of the actual production of the semiconductor laser diode, the determination of a two-dimensional binary-valued, discrete lateral structure takes place in an optimization method taking into account at least the desired properties of the three-dimensional laser mode, in particular the wavelength, wherein, within the scope of the optimization method, the regions in which the lateral structure layer can be applied are divided into a regular grid, and a two-dimensional, preferably discrete and / or binary, start function relating to the grid cells is optimized to describe the lateral structure.This means that the area or surface of the semiconductor laser component under production, which is available for the arrangement of a lateral structure layer, is first divided into a plurality of grid cells without any actual influence or influencing of the component itself, for example, as part of a simulation or pre-calculation. Then, by defining the other properties of the semiconductor laser and the desired properties of the semiconductor laser, the grid cells are "filled" or "occupied" in the optimization process in order to test and optimize the resulting pattern of the filled grid cells and the resulting interaction of the simulated lateral structure layer with the semiconductor laser. The lateral structure has a two-dimensional discrete structure and assumes binary values. The start function can also already have a corresponding structuring and binary values.However, other starting functions can also be selected, which are then converted into two-dimensional, discrete and binary values at the beginning or end of the optimization.

[0014] After the optimization process is completed, the lateral structure previously determined or defined in the simulation and / or optimization is then generated and formed or deposited in the form of a lateral structure layer. In a further process step, a lateral structure layer is applied, whereby the material from which the lateral structure layer is formed has a different refractive index than the surrounding semiconductor material. The lateral structure layer is created at least in the area of the material removal regions, with the structures being formed in a masking and deposition process based on the optimized lateral structure. In terms of process steps, the application of the lateral structure layer therefore does not differ from that of a known grating structure layer.However, with regard to the structure created and its interactions with the semiconductor laser diode, the lateral structure layer differs significantly from the familiar regular gratings. In contrast to the familiar regular grating, the lateral structure layer is "truly" two-dimensional. This means that with increasing distance from the waveguide ridge, not only is the same pattern—preferably grating bars—repeated, but each row or column of the lateral structure layer running parallel to the waveguide ridge potentially reflects an individual arrangement of binary values and can therefore enable individual two-dimensional structures of the lateral structure layer.

[0015] The basic idea of the invention can therefore also be seen in the fact that, instead of arranging a very coarse geometry of the known grating structured only in one direction, along the waveguide ridge, a much finer and, moreover, two-dimensional structuring is carried out in order to achieve both an adjustability of optical properties and an adjustability of other properties of the semiconductor laser.

[0016] In other words, the method enables a semiconductor laser diode that, on the one hand, generates a monomode coherent radiation source and, at the same time, allows for a targeted desired adjustment of the strain of the laser material, as well as a targeted adjustment of the overlap of the lateral structure with the laser mode near the facet of the laser diode, and a targeted adjustment of the overlap of the lateral structure with lateral modes. This is achieved by a lateral structure layer adapted to the requirements, located laterally of the waveguide of the laser diode. This lateral structure layer consists of the targeted modification of the refractive index laterally of the waveguide according to a predetermined binary-valued function f(x,y), where, for example, x represents the coordinate parallel to the laser facet (measured from the left edge of the first waveguide ridge) and y represents the coordinate parallel to the waveguide ridges (measured from the preferred laser facet).This targeted modification of the refractive index can be achieved either by applying a material with a complex refractive index different from that of the semiconductor or by removing material at the points where the given function f(x,y) takes the value 1.

[0017] The refractive index contrast, which is achieved by selecting the material, and the specified function are optimized to optimally meet the application requirements of the laser diode.

[0018] The target wavelength is adjusted by structuring the first 500 nm adjacent to the waveguide ridge. Strain in the laser layer is adjusted by structuring the next region, which can be the next 1 µm. Undesired lateral modes are specifically suppressed by the structure located further away from the waveguide ridge. Furthermore, the adverse effect of the lateral structure on the far field can be minimized by deliberately reducing the structuring near the emission facet.

[0019] With regard to the optimization process, fundamentally different methods are possible, each of which is capable of capturing, comparing, and analyzing a large number of possible combinations. An example of an optimization process in the form of a genetic algorithm is described below: The lateral structure layer is optimized with the goal of optimally meeting the technical requirements of the laser. For example, it can be assumed that these technical specifications include the wavelength, the side mode suppression ratio, thermal and mechanical stability properties, and the shape of the laser's far field. For this purpose, an epitaxial or multilayer structure is selected that is suitable for a laser with these properties.

[0020] In practice, it can be assumed that the properties of this epitaxy that are important for the laser with a given geometry have been measured in preparatory experiments, so that a computer model for the epitaxy can be created with which the expected properties of the laser for a given lateral structure layer can be predicted.

[0021] This computer model can, for example, be used to generate an optimized lateral structure with the help of a genetic algorithm. To do this, a starting population of lateral structures is created, based on empirical values from already known lasers, with approximate properties and a targeted dispersion. This starting population can, for example, represent a number of certain variations of a starting function. The genetic makeup of the individuals or lateral structures is the discrete binary-valued functions f(x,y) that define the lateral structure. This means that the genetic algorithm is used to optimize this binary-valued function. To do this, a fitness function is defined that gives a higher score to laser properties predicted by the computer model if they better meet the specifications. An existing implementation of a genetic algorithm can then be used to optimize the binary-valued function.The promising lateral structures produced by this optimization procedure can then be experimentally realized and characterized to verify the theoretical predictions.

[0022] According to an advantageous embodiment of the invention, the starting function of the optimization method is provided as a row, preferably two rows, of parallel, evenly spaced ridges, which are arranged and aligned in particular such that, if the lateral structure layer were formed according to the non-optimized starting function of the lateral structure, the longitudinal extent of the parallel ridges would run substantially perpendicular to the longitudinal extent of the waveguide ridge of the semiconductor diode. This means that the optimization of the starting function can preferably have its starting point in a classic grating structure of a DFB (distributed feedback) grating. However, other starting functions are also possible.

[0023] In a further advantageous embodiment, it can also be provided that the optimization method optimizes the lateral structure at different distances from the waveguide ridge. This allows, for example, optimization of the optical properties in the region adjacent to the waveguide ridge and / or in the overlap with the waveguide ridge, optimization of the thermal properties in a more distant region, and optimization of mechanical stress in the region furthest from the waveguide ridge.

[0024] It can also be provided that the optimization method optimizes the lateral structure in the regions that are arranged adjacent to the facets that couple out the laser radiation during the formation of the lateral structure layer. This can particularly advantageously influence the far field.

[0025] It should be noted that the pre-simulated optimization of the entire lateral structure has several advantages over a conventional sequential experimental optimization: (1) the development time is significantly shortened because the parameter range is optimized as a whole; (2) the development costs are significantly reduced because the production of experimental samples is significantly more cost-intensive than performing computer simulations; (3) one can use optimization methods, such as the genetic algorithm used in the example, which allow the discovery of "unexpected" synergies that would otherwise be difficult to find. General description of the manufacturing process:

[0026] The process according to the invention begins with the selection of a suitable material system on which a sequence of material layers is applied.

[0027] The material layer sequence arranged on the semiconductor substrate is advantageously produced by means of a single and uninterrupted epitaxial process and arranged layer by layer on the semiconductor substrate, wherein the respective layers extend parallel to the layer propagation plane defined by the surface of the substrate or semiconductor substrate.

[0028] Epitaxy processes preferred within the scope of the invention are, for example, the so-called molecular beam epitaxy or the so-called gas phase epitaxy.

[0029] The material layer sequence produced by means of the epitaxial process advantageously comprises all layer structures required for generating the electromagnetic wave by means of stimulated emission, in particular an active zone or region and the layers surrounding it, such as an upper cladding layer, an upper waveguide layer, a lower waveguide layer and a lower cladding layer.

[0030] Furthermore, it is preferably provided that the active zone or region is formed as one or more quantum films. Likewise preferably, the active zone or region can also comprise one or more layers, each of which has quantum dots.

[0031] The material removal of the portion of the material layer sequence or multi-layer structure is preferably carried out by a known physical or chemical etching process.

[0032] The depth of material removal corresponds to the height of the waveguide ridge created by removing material from the remaining portion of the material layer sequence. It is possible and preferable to stop the etching process above the active zone, particularly in the upper waveguide layer, or even above the upper waveguide layer, or to continue the etching process until the active zone or region lateral to the waveguide ridge is completely removed, and the etching process only stops in a deeper layer, for example, the lower waveguide layer.

[0033] The etching depth can preferably be controlled via the etching time and the known reactivity or etching rate of the etching process used when applied to the particular material layer sequence. After the desired etching time has elapsed, a physical etching process, for example, can be interrupted by stopping the acceleration of further ions onto the material layer sequence. Likewise, a chemical etching process, for example, can be interrupted by rinsing the acid used from the material layer sequence using a rinsing agent.

[0034] Before performing the etching process, a masking process is advantageously carried out to apply a so-called etching mask to the multilayer structure or material layer sequence. The etching mask is preferably largely or, in particular, completely resistant to the etching process(es) used. This results in the advantage that the areas of the material layer sequence covered or masked by the etching mask are not etched. By applying an etching mask, for example, in the two-dimensional surface geometry of the waveguide ridge, the waveguide ridge can be precisely created. The etching mask can then be removed again, e.g., using a suitable solvent.

[0035] The etching mask can preferably be a metallic etching mask.

[0036] To prevent material removal, particularly the etching process, from penetrating undesirably deep into the material layer sequence, the semiconductor laser diode can comprise a so-called etch stop layer created during the epitaxial growth process of the material layer sequence. This etch stop layer is completely resistant to the etching process used, or at least has a comparatively high resistance. This etch stop layer preferably forms the base surface on which, for example, the waveguide ridge can be arranged, and the lateral structure can also be arranged laterally to the waveguide ridge.

[0037] Preferably, a comparatively thin barrier and / or insulation layer is first applied directly to the surface etched free by the etching process in the removed regions, for example the etch stop layer or any other layer of the material layer sequence at which the etching process is stopped. This barrier and insulation layer is located on both sides lateral to the waveguide ridge. It preferably consists of an oxide or nitride, for example SiO 2 or Si 3 N 4 , and is preferably 5 nm to 30 nm thick. In a trapezoidal design of the waveguide ridge, the barrier and / or insulation layer can also be arranged on lateral flanks of the waveguide ridge.

[0038] It is preferably provided that the lateral structure layer consists of chromium (Cr) or a chromium-containing alloy. Due to its refractive index and its comparatively high absorption coefficient, chromium enables effective complex coupling with the electromagnetic wave in the semiconductor laser diode, even though the lateral structure is arranged laterally next to the waveguide ridge and thus above or below the active zone. Since, during operation of the semiconductor laser diode, current injection usually occurs exclusively via the back of the waveguide ridge, the active zone is also only energized in the region below the waveguide ridge, so that stimulated emission essentially only occurs here. The lateral structure can therefore have not only a height offset from the region of the active zone in which the stimulated emission occurs, but also a lateral offset.It has been shown that despite this spacing of the lateral structure from the energized active zone, a sufficiently large portion of the laser mode or the other electromagnetic waves spatially overlaps with the lateral structure, so that due to the special material properties of chromium or the chromium-containing alloy, a corresponding interaction in the form of complex coupling takes place between the lateral structure and the laser mode or the other electromagnetic waves.

[0039] The strength of the coupling is generally determined by the coupling constant, which is proportional to the product of the filling factor of the lateral structure and the absorption of the lateral structure material.

[0040] Preferably, the lateral structure extends to the waveguide ridge, in accordance with the predeterminable function, and in particular touches it, especially if the edges of the waveguide ridge are not perpendicular. The closer the lateral structure extends to the waveguide ridge or is formed in the region of the waveguide ridge, the greater the overlap with the laser mode or the other electromagnetic waves and, accordingly, the stronger the coupling between the laser mode or the other electromagnetic waves and the lateral structure.

[0041] Advantageously, the lateral structure layer is applied to the semiconductor laser diode by evaporating the metallic material, in particular by thermal evaporation, evaporation by ion bombardment, or evaporation by electron bombardment. The application of the lateral structure is preferably preceded by a corresponding masking process and / or lithography process.

[0042] Alternatively, the lateral structure can also be applied to the semiconductor laser diode by means of a sputtering process.

[0043] According to a preferred embodiment of the invention, it is provided that the surface, in particular in the material removal regions, is divided into a grid with a predeterminable number of grid elements, wherein a grid element can be assigned to each grid element or grid cell in accordance with the binary function f(x,y). This means that the surface is divided into a finite number of discrete grid elements or grid cells and that the discrete binary function f(x,y) assigns only either the value 0 or the value 1 to each grid element. Preferably, the base area on which the lateral structure is formed as a lateral structure layer can have between 10 8< and 10 10< grid elements. With a base area of the surface to be structured, preferably in the material removal regions, of generally less than 0.25 mm 2<, very fine structuring is thus possible despite the use of discrete grid elements or grid cells.

[0044] In this context, it should be noted that the discrete properties of the lateral structure are not apparent or recognizable after their formation or creation in the form of the lateral structure layer, or are only negligibly so. The lateral structure layer, and in particular individual structures of the lateral structure layer, do not form a pixel structure, but rather coherent structures whose shape and size are determined by the raster elements. This means that two individual, adjacent, filled (value: 1) raster elements of the lateral structure layer are not recognized as such, but rather as a common or uniform structure that occupies or fills the area of two raster elements.This is due to the known methods for producing the lateral structure layer, in which connected grid elements or grid cells are coated or filled together with the material of the lateral structure layer, so that the finished lateral structure layer still shows the grid or grid property at certain points, for example an edge of a larger structure, at a suitable magnification, but a uniform homogeneous layer is present inside such structures.

[0045] The present invention assumes a discrete binary-valued function to define the lateral structure. In a series of experiments conducted by the applicant, it was found that the artifacts resulting from the discretization, which deviate from the optimal structure, do not adversely affect the monomode nature of the semiconductor laser diode. In particular, the emitted mode is determined by modes according to the so-called superlattice, which results from an inverse Fourier transformation of a coarsening of the Fourier transform of the discretized lateral structure.

[0046] According to the preferred embodiment of the invention, the function f(x,y) is additionally selected or optimized to optimize one or more properties of the semiconductor laser diode. As a result, factors for influencing the laser mode that have previously been ignored in the prior art can not only be considered, but can also be advantageously used and / or adjusted. This, in turn, enables a more targeted and effective influencing of desired or undesired properties of the laser mode, such as improvements in monomode, gain, temperature stability, or threshold current, significantly beyond the known framework. In other words, further possibilities for adapting the properties of the laser mode beyond the known framework are opened up.

[0047] According to a further preferred embodiment, the semiconductor laser diode is provided with a passivation layer at the edge of the waveguide ridge and above the lateral structure. The passivation layer preferably consists of an oxide layer or a nitride layer and preferably has a thickness of a few hundred nanometers. It is particularly preferred that the oxide layer consists of SiO 2 and the nitride layer consists of Si 3 N 4 . For this purpose, the waveguide ridge can form a trapezoidal basic structure, which can be achieved in a conventional manner using the etching methods already described above.

[0048] It is preferably provided that volumes of the material layer sequence or the multilayer structure removed by the material removal in the material removal regions lateral to the waveguide ridge are filled with benzocyclobutene up to a height of the waveguide ridge, so that a planar overall surface of the semiconductor laser diode exists.

[0049] According to a further preferred embodiment of the invention, it is provided that an entire surface of the semiconductor laser diode is planarized by means of benzocyclobutene up to a height of the waveguide ridge. The BCB is preferably applied by dripping a small amount of liquid BCB. Due to its liquid phase, the BCB forms a uniform film on the surface of the semiconductor laser diode, which initially completely covers both the waveguide ridge and the surfaces located laterally to the side of the waveguide ridge. By rapidly rotating the semiconductor laser diode with the liquid BCB applied to it, that portion of the film located above the back of the waveguide ridge can advantageously be spun off. Thus, even in the liquid phase of the BCB, an essentially planar overall surface of the semiconductor laser diode is produced in a very simple manner.After rotation and the resulting spin-off of the BCB, the BCB can then be baked. If necessary, material can be removed from the BCB, e.g., through a physical etching process, to remove excess BCB and further expose the first contact surface.

[0050] According to a further preferred embodiment of the invention, the semiconductor laser diode comprises a first metallic contact surface and a second metallic contact surface, wherein the first contact surface at least partially covers the waveguide ridge and wherein the second contact surface completely covers an underside of the semiconductor laser diode. The first and second metallic contact surfaces each serve to improve the electrical contacting of the semiconductor laser diode. Since the oxide layer, nitride layer, or BCB layer applied beneath the first contact surface represents a very good electrical insulator, the first contact surface can also be applied over a comparatively large area and, in particular, extending beyond the region of the waveguide ridges, without thereby unduly promoting the occurrence of leakage currents that would penetrate into the semiconductor substrate away from the waveguide ridges.Only the outer edge regions of the surface of the semiconductor laser diode are advantageously left free of the first contact surface in order to avoid complicating a subsequent mechanical cleaving or separation process due to the metallic properties of the first contact surface. The geometry of the first contact surface can be specified, for example, by a corresponding masking process.

[0051] The first contact area preferably represents the p-contact of the semiconductor laser diode.

[0052] Advantageously, the semiconductor substrate is thinned prior to applying the second contact surface, e.g., by mechanical grinding, to reduce the electrical resistance of the semiconductor substrate and simplify a subsequent cleaving process. In contrast to the first contact surface, the second contact surface is preferably applied over the entire surface, without requiring recourse to a specific contact geometry and a masking process.

[0053] The second contact surface preferably represents the n-contact of the semiconductor laser diode.

[0054] Preferably, the first contact surface and / or the second contact surface are made of aluminum (Au) or an aluminum-containing alloy or a layer sequence comprising at least one aluminum layer. Aluminum or aluminum-containing alloys or corresponding layer sequences are comparatively good electrical conductors and can also be applied relatively easily, e.g., by a sputtering process.

[0055] Preferably, the aluminum-containing alloy additionally contains platinum (Pt) and / or titanium (Ti) and / or the layer sequence additionally comprises at least one platinum layer or one titanium layer.

[0056] It is preferably provided that the first contact surface and / or the second contact surface is / is thickened by means of electroplating. The electroplating thus further thickens the first or second contact surface, so that the electrical conductivity as well as the mechanical stability of the first or second contact surface are improved. Electroplating leads to comparatively faster layer growth than a sputtering process, which is why the thickening of the first or second contact surface is preferably carried out by electroplating after a layer thickness sufficient for electroplating has been reached. The first contact surface is advantageously masked for this purpose with a mask which specifies the contact geometry to be thickened. The contact geometry to be electroplated has, in particular, at least a slight spacing from the respective outer edge of the first contact surface, ieThis means that the contact area thickened by the electroplating process is slightly smaller in every direction than the first contact area previously created by the sputtering process. The second contact area can also be thickened over the entire surface using the electroplating process.

[0057] Preferably, only the first contact surface is galvanized, but not the second contact surface.

[0058] After electroplating, the first or second contact surface is preferably 5 µm to 15 µm thick. This has proven particularly suitable. Without electroplating, however, the first or second contact surface is preferably 0.5 µm to 1 µm thick.

[0059] According to a further preferred embodiment of the invention, it is provided that a plurality of semiconductor laser diodes are manufactured on a common semiconductor substrate. The common semiconductor substrate preferably represents a so-called wafer or a portion of such a wafer. The wafer can be designed in different sizes and can carry several hundred to several thousand of the semiconductor laser diodes according to the invention. The plurality of semiconductor laser diodes is manufactured simultaneously by subjecting either the entire wafer or at least one or more portions of the wafer to the described manufacturing method.For example, the material layer sequence can be created using the epitaxial process for the entire wafer, while the material removal to create the waveguide ridge and the subsequent process steps are performed only on a single section of the wafer. Such a section can be, for example, 1 cm x 1 cm in size and can still contain several hundred of the semiconductor laser diodes according to the invention.

[0060] The individual semiconductor laser diodes can then preferably be separated by means of a mechanical cleavage process. The cleavage process advantageously involves cleaving, i.e., mechanically breaking, the semiconductor substrate with the material layer sequence arranged thereon along lattice lines of the crystal lattice structure of the semiconductor substrate. In this context, too, the inventive design of the lateral structure layer is particularly advantageous, since it also allows mechanical properties, such as material stresses, to be specified or adjusted in regions of the multilayer structure, so that predetermined breaking points for the mechanical cleavage process can be predefined or better defined. General description of the semiconductor laser diode:

[0061] The semiconductor laser diode preferably consists of a semiconductor substrate with a semiconductor laser diode having an epitaxial material layer sequence or multilayer structure thereon, which in particular comprises an active zone or an active region surrounded by an upper and / or lower cladding layer, which in turn is enclosed by upper and / or lower waveguide layers. The active zone or active region preferably has one or more so-called quantum films. Likewise preferably, the active zone comprises one or more layers comprising so-called quantum dots. The semiconductor substrate preferably comprises a III / V semiconductor material. Depending on the target wavelength range of the semiconductor laser diode, the semiconductor substrate is in particular GaN, GaAs, InP, or GaSb.

[0062] The semiconductor laser diode preferably has the geometry of a cuboid which is aligned such that the semiconductor substrate is arranged at the bottom and the epitaxial material layer sequence or multilayer structure is arranged at the top. The surface of the semiconductor substrate forms a layer propagation plane for the multilayer structure. This cuboid is preferably several hundred to a few thousand micrometers long and a few hundred micrometers wide. The cuboid is aligned such that the surfaces defined by the width and thickness form the front and rear sides of the semiconductor laser diode, i.e. these surfaces form the facets of the semiconductor laser diode that couple out the radiation. One or more waveguide ridges is / are formed on the top side of the cuboid, preferably by a material removal process.The waveguide ridge is preferably structured such that it is arranged perpendicularly on the front side and perpendicularly on the back side of the semiconductor laser diode, and such that its cross-section is dimensioned with a width of a few micrometers and a height of a few hundred nanometers to a few micrometers. In the case of an embodiment of the invention with multiple waveguide ridges, the spacing of the waveguide ridges is preferably dimensioned with a few micrometers.

[0063] According to the invention, a lateral structure layer optimized for the requirements of the semiconductor laser diode is arranged on the surface created by the material removal process to the side of the waveguide ridge. This lateral structure is preferably described by a binary-valued function f(x,y) of the position on the created surface, i.e. a function f that assigns the values 0 or 1 to the coordinates (x,y) on the created surface. In this case, for example, the coordinate system can be aligned such that the coordinate x indicates the distance from the left edge of the first waveguide ridge, i.e. the left one as seen from the front of the semiconductor laser diode, measured parallel to the facet of the semiconductor laser diode, and that the coordinate y indicates the distance from the front facet of the semiconductor laser diode measured perpendicular to this facet.

[0064] The lateral structure layer is preferably formed by applying a material, e.g., chromium, with a refractive index and absorption coefficient different from the surrounding semiconductor at the locations where the function f assumes the value 1, with no material being applied at the locations where the function f assumes the value 0. The thickness of the lateral structure layer is preferably a few tens of nanometers.

[0065] Also preferred is the generation of the lateral structure layer by removing material at the locations of the surface where the function f assumes the value 1, with no material being removed at the locations where the function f assumes the value 0. The depth of the lateral structure layer is preferably a few tens of nanometers.

[0066] The function f is preferably selected such that the lateral structure determined by it optimally influences the properties of the semiconductor laser diode. For this optimization, the material of the lateral structure layer or the production of the lateral structure layer by ablation of semiconductor material is also selected, as is the passivation material described below, so that the function f can optimally adapt the semiconductor laser diode to the application requirements.

[0067] The optimization process takes into account in particular (1) the influence of the refractive index curve on the light modes in the semiconductor laser diode, (2) the absorption of the lateral structure layer material for the light modes in the semiconductor laser diode, (3) the strain of the semiconductor material caused by the application of the lateral structure layer material, and (4) the change in the thermal properties of the semiconductor laser diode, in particular the modification of the tuning behavior by the application of the lateral structure layer material.

[0068] The optimization of the lateral structure, which forms the simulated precursor of the lateral structure layer produced or to be produced, is based on the fact that the structuring of the lateral structure layer influences the physical properties of the lateral structure layer and thus specifically changes the physical properties of the semiconductor laser diode, which in turn influences the operating properties of the semiconductor laser diode. The optimization process simulates the properties and behavior of the semiconductor laser diode, and the change in these properties and behavior is taken into account when adapting the also simulated lateral structure. Thus, after completion of the optimization process, a correspondingly optimized actual semiconductor laser diode can be produced, in which the properties are optimized by applying a lateral structure layer according to the previously optimized simulated lateral structure.

[0069] During optimization of the lateral structure, it turns out that the first 500 nanometers next to the waveguide ridge are crucial for the wavelength dependence of the absorption and the wavelength dependence of the effective refractive index of lateral fundamental modes, i.e., modes that are not excited in the lateral direction, whereas lateral structures further away from the waveguide ridge have no measurable influence on the effective refractive index and the absorption of the lateral fundamental modes. Therefore, the lateral structure layer is provided with a first region formed on the first 500 nanometers next to the waveguide ridge and which preferably has a periodic grating-like structure at least in sections. Particularly preferably, other sections are also provided or formed that deviate from a classic grating structure.

[0070] Furthermore, the micrometers following the first 500 nanometers have a comparable influence on the stresses of the semiconductor material as the first 500 nanometers. Consequently, it is envisaged that the lateral structure layer comprises a second region following the first 500 nanometers next to the waveguide ridge and preferably having a width of approximately 500 nanometers, wherein the second region has, for example, a comb-like structure.

[0071] Furthermore, the influence of the lateral structure layer on the thermal properties of the semiconductor laser diode is also possible through lateral structures located even further away from the waveguide ridge. Accordingly, the lateral structure layer preferably comprises a third region, which adjoins the first and / or second region and preferably also has a width of approximately 500 nanometers.

[0072] This allows the adjustment of the effective refractive index and absorption, as well as the strain of the semiconductor material and the thermal properties, to be optimized almost independently. The refractive index profile decisively influences the wavelength dependence of the mirror losses of the light modes in the semiconductor laser diode, and the spatially dependent modulation of the absorption coefficient decisively influences the internal losses of the light modes. These effects allow the light mode that is particularly strongly amplified by the semiconductor laser diode to be specifically adjusted, thus achieving a high side mode suppression ratio. The application of Bloch theory to the light mode equation in the semiconductor reveals that the so-called superlattice structure and so-called higher-order gratings, as well as the superposition of multiple superlattices, can be used to achieve high side mode suppression.

[0073] In addition to cleaving larger semiconductor material blocks (wafers), the strain of the semiconductor material has a decisive influence on the energy separations of the energy bands, particularly between the valence and conduction bands. Accordingly, the local strain of the semiconductor material in the active zone leads to a change in the local gain profile and, in the remaining layers, to a change in the absorption profile of the semiconductor laser diode. Thus, the targeted adjustment of the local strain influences which portion of the charge carriers contributes to the amplification of the light mode, since a light mode particularly efficiently stimulates the emission of photons from states with which its electromagnetic field has a particularly large effective cross section. The strain of the semiconductor material thus influences the efficiency and threshold current of the semiconductor laser diode.These two characteristics of the semiconductor laser diode can therefore be optimized by specifically adjusting the strain of the semiconductor material.

[0074] Another very important characteristic of a semiconductor laser diode is its tuning behavior, i.e., the fact that its light output and emitted wavelength can be tuned by adjusting the operating temperature and operating current of the semiconductor laser diode. In particular, it is desirable to achieve a particularly simple, linear relationship between the operating parameters of temperature and current and the output parameters of wavelength and power, so that the tuning behavior can be completely described by two proportionality constants, the so-called tuning parameters. The thermal properties of the semiconductor laser diode have a decisive influence on these tuning parameters. Therefore, by optimizing the thermal properties of the lateral structure, a positive effect on the tuning behavior of the semiconductor laser diode can be achieved.

[0075] An important aspect of semiconductor laser diodes with more than one waveguide ridge is the targeted control of the lateral mode spectrum, i.e. the spectrum of modes that have the same structure along the direction of the waveguides but have a different lateral structure. Different lateral modes can experience different mirror losses by specifically structuring the refractive index between the waveguide ridges, and different internal losses by specifically structuring the absorption coefficient between the waveguide ridges. This allows the lateral modes amplified by the semiconductor laser diode to be adjusted by specifically selecting the function f. Accordingly, in semiconductor laser diodes with more than one waveguide ridge, a further region of the lateral structure and / or the lateral structure layer can be provided, which enables the above-mentioned selection or optimization.

[0076] The far field of the semiconductor laser diode is determined by the electromagnetic field of the light mode at the front facet. This, in turn, is determined by the effective refractive index near the facet, which includes the influence of the lateral structure. To reduce the undesirable effect of the lateral structure on the far field, it is therefore possible to optimize the lateral structure near the facet. This can be done as part of the optimization of the first region. However, another separate region can also be provided for this purpose and optimized separately if necessary.

[0077] The advantage of the present invention over the prior art is that, through a targeted selection of the function f, more than one of the properties of the semiconductor laser diode mentioned here can be specifically adjusted. This leads to more efficient planning and thus to a reduction in development costs for semiconductor laser diodes with desired characteristics.

[0078] The semiconductor laser diode according to the invention preferably has a passivation layer at the edge of the waveguide ridge and above the lateral structure. This layer preferably consists of an oxide layer or a nitride layer with a thickness of a few hundred nanometers. Furthermore, the semiconductor laser diode according to the invention has a metallic contact on the underside and a metallic contact on the top side to enable better electrical contacting of the semiconductor laser diode.

[0079] The present invention also includes a method in which a plurality of semiconductor lasers are formed on a common semiconductor substrate according to the manufacturing method of the embodiments described above and a subsequent singulation of the semiconductor lasers, in particular a mechanical cleaving and / or sawing, takes place.

[0080] The invention is explained below by way of example with reference to embodiments shown in the figures.

[0081] They show: Fig. 1: a schematic view of an embodiment of a semiconductor laser diode according to the invention, in which the orientation of the lateral surfaces and the orientation of the coordinate system of an exemplary lateral structure layer are displayed, Fig. 2: a schematic plan view of a possible embodiment of a semiconductor laser diode according to the invention with the lateral structure layer and a waveguide ridge, Fig. 3: a schematic plan view of a further possible embodiment of a semiconductor laser diode according to the invention with several waveguide ridges and lateral structure layers and Fig. 4: a schematic flow diagram of a possible embodiment of the method according to the invention for producing a monomode semiconductor laser diode.

[0082] Identical objects, functional units, and comparable components are designated by the same reference symbols throughout the figures. These objects, functional units, and comparable components are identical in terms of their technical features, unless explicitly or implicitly stated otherwise in the description.

[0083] Fig. 1 shows, by way of example and schematically, the basic structure of an embodiment of the semiconductor laser 01 according to the invention or the semiconductor laser diode according to the invention. This consists of a semiconductor substrate 10, on which a multilayer structure 02 suitable for a laser diode, comprising the individual layers 11-16, is epitaxially applied. The multilayer structure 02 comprises an active zone or active region 13, which is surrounded by an upper and lower cladding layer 12, 14, which in turn has an upper and lower waveguide layer 03 comprising the individual layers 11, 15, 16.

[0084] A waveguide ridge 16 is created from the upper layers 15, 16 of the waveguide layer 03 by a material removal process in the material removal areas 04. The material removal process creates two lateral surfaces 20, 21 to the side of the waveguide ridge 16, as well as two flanks 30 on the waveguide ridge 16. The semiconductor laser diode or the semiconductor laser 01 is connected to electrical contacts via the upper contact surface 31 on the waveguide 16 and a lower, in the illustration of the Fig. 1 not shown, contact area on the underside of the substrate 10 is supplied with power.

[0085] The lateral surfaces 20, 21 are highlighted. The optimized lateral structure layer is patterned onto the highlighted surfaces according to an optimized binary function f(x,y) of the lateral structure. The axes of the coordinate system (x,y) are shown as dashed lines. The x=0 axis is defined by the intersection of the lateral surfaces 20, 21 with the front facet of the semiconductor laser diode. The y=0 axis is defined by the intersection of the left lateral surface 20 with the left flank of the waveguide ridge 16.

[0086] Fig. 2 shows, by way of example and schematically, a top view of a lateral structure layer 05, which was formed according to an optimized lateral structure for a semiconductor laser diode and is formed laterally adjacent to the waveguide ridge 16. The lateral surfaces 20, 21 to the side of the waveguide ridge 16 are equipped, by way of example, with optimized structural elements 41, 42, 43, 41 and 41', 42', 43', 44' of the lateral structure layer 05. In this example, the structural elements 41, 41' are provided and optimized in their shape and arrangement or placement to determine the waveguide mode through periodic absorption and modulation of the refractive index in the wavelength. The structural elements 42, 42' are optimized, for example, to largely homogenize the material strain in the y-direction, and the structural elements 44, 44' are optimized to ensure the thermal stability of the tuning behavior of the semiconductor laser diode.

[0087] Zones 43, 43' do not contain any structural elements to avoid negatively influencing the far field of the semiconductor laser diode. This means that the lateral structure f(x,y) in zones 43, 43' is assigned the value 0, and thus no deposition or application of structures of the aluminum-containing lateral structure layer 05 takes place. As in the previous exemplary embodiment, the binary-valued function f(x,y) defines the structural elements 41, 42, 43, 41, 41', 42', 43', and 44' by the points at which it assumes the value 1. The lateral structure layer 05 thus forms individual regions 06, 07, and 08, each of which essentially serves to influence different properties of the semiconductor laser 01.

[0088] Fig. 3 shows, by way of example and schematically, a top view of an optimized lateral structure for a semiconductor laser diode 01 with several waveguide ridges 16, 16'. The example shows two waveguide ridges 16, 16' and three zones of the lateral structure 21, 21, 21' with the optimized structural elements 41, 42, 43, 41, 41", 42", 43", 44" as well as 41' and 45 of the lateral structure layer 05. The coordinate system is analogous to Fig. 1 aligned with the left waveguide ridge 16 and the front facet 09. Structural elements 41, 41' are optimized within the framework of optimizing the lateral structure to determine the waveguide mode through periodic absorption and modulation of the refractive index in the wavelength. Structural elements 42, 42' are optimized, for example, to largely homogenize the material strain in the y-direction, and structural elements 44, 44' are optimized to ensure the thermal stability of the tuning behavior of the semiconductor laser diode. Zones 43, 43' do not contain any structural elements in order not to negatively influence the far field of the semiconductor laser diode. The semiconductor laser 01 according to Fig. 3 thus shows individual areas of the lateral structure or the lateral structure layer.

[0089] Fig. 4shows, by way of example, a possible embodiment of the method according to the invention for producing a single-mode semiconductor laser diode in the form of a flowchart or a sequence diagram. The individual method steps are advantageously provided, but at least in part are not mandatory or not necessarily carried out in the described order. In a first method step S10, a material layer sequence or multilayer structure is grown on a semiconductor substrate by means of molecular beam epitaxy. In the subsequent step S11, a masking process for masking a waveguide ridge 04 with an etching mask is carried out.

[0090] In step S12, material is removed from the region of the material layer sequence not masked by the etching mask, and the material removal region(s) are thus formed, so that the waveguide ridge is formed from the non-removed remainder of the material layer sequence.

[0091] In the following step S13, the discrete and binary function f(x,y) is generated as a lateral structure and thus as a simulated predecessor or precursor to the later formed or deposited lateral structure layer. This is done by an optimization process which is carried out in step S14, in which the function f(x,y) is optimized until the semiconductor laser diode, which is also simulated on the basis of data relating to the actual semiconductor laser diode and intended to interact with the simulated lateral structure, has achieved the desired properties in preliminary calculations or simulations.

[0092] In step S15, another masking process is then performed, this time to generate the lateral structure layer according to the lateral structure function f(x,y) specified by the optimization, whereby the lateral structure elements of the lateral structure layer are arranged wherever the binary-valued function f(x,y) assumes the value 1. In the following process step S16, the metallic material of the lateral structure layer is applied flatly to the material layer sequence of the semiconductor laser diode by means of vapor deposition. In step S17, the vapor deposition mask with the lateral structure material located on the vapor deposition mask is removed, so that only the lateral structure layer elements remain on the surface of the material layer sequence.

[0093] Thus, in an exemplary embodiment of the method, a semiconductor laser structure or semiconductor laser diode with a lateral structure layer according to the invention was produced. Fundamentally different method embodiments are also conceivable and encompassed by the present invention, as long as the method comprises the simulated optimization of a two-dimensional discrete binary-valued lateral structure and the subsequent formation of a lateral structure layer for optimizing the properties of the semiconductor laser according to the optimized lateral structure.

[0094] In step S18, a BCB layer is arranged on the lateral structure layer on the surface of the semiconductor laser diode by first applying liquid BCB to the surface of the semiconductor laser diode. Excess BCB is removed by spinning off the liquid BCB in step S19, and the BCB layer is baked in step S20. In the following step S21, a first metallic contact surface is created, which is then electroplated with additional material in step S22 to thicken it. In step S23, the semiconductor substrate is thinned on its underside, i.e., on the side facing away from the first contact surface, using a mechanical grinding process. The second metallic contact surface is then created in step S24 using a further sputtering process. This completes the production of an exemplary semiconductor laser diode.Steps S20 to S24 may also be varied, combined and / or omitted or adapted within the scope of the knowledge of the average person skilled in the art, as long as these combinations fall within the subject matter of the claims.

Claims

1. A method for producing a semiconductor laser (01), the method comprising the following steps: - applying a multilayer structure (02) on a semiconductor substrate (10), the layers of the multilayer structure (02) extending parallel to a layer expansion plane defined by a surface of the semiconductor substrate (10), and the application of the multilayer structure (02) including at least producing an active region (13). - removing material of the multilayer structure (02) in at least two separate material removal areas (04), the material removal taking place essentially perpendicular to the layer expansion plane, thus forming a waveguide ridge (16, 16'). - generating an insulation layer on at least the material removal areas (04). - determining a two-dimensional discrete binary lateral structure (f(x,y)) in an optimization method while taking into consideration at least the desired properties of the three-dimensional laser mode, in particular the wavelength, the areas in which a lateral structural layer (05) is applicable being divided into a uniform grid in the context of the optimization method and a two-dimensional start function pertaining to the grid cells being optimized for describing the lateral structure, the optimization taking place in a first area which is formed on the first 500 nm adjacent to the waveguide ridge (16, 16'), at least for setting the target wavelength, by structurization in a second area bordering the first area for setting a bracing in the multilayer structure and in a third area of structures further away from the waveguide ridge (16, 16') for the targeted suppression of undesired three-dimensional laser modes, zones (43, 43'') located on both sides of the waveguide ridge (16, 16') and bordering a front facet (09) being formed free of structural elements in the second and third areas in order to not negatively influence the far field of the semiconductor laser via a lateral structure on this front facet (09). - applying a lateral structural layer (05) made of a material which has a refractive index different to that of the surrounding semiconductor material at least in the area of the material removal areas (04), the structures (41 - 43, 41' - 43') of the lateral structural layer (05) being formed in a masking and deposition process based on the optimized lateral structure (f(x,y)).

2. The method according to claim 1 wherein the second area has a width of 500 nm.

3. The method according to claim 1 or 2, wherein the start function of the optimization method represents a row, preferably two or more rows, of parallel, evenly spaced ridges.

4. The method according to any one of claims 1 to 3, wherein the binary values of up to 108, preferably up to 1010, grid cells are optimized in the optimization method, starting from the binary values of the start function.

5. The method according to any one of the claims 1 to 4, wherein the optimization method optimizes the lateral structure (f(x,y)) also with regard to the thermal properties and / or the mechanical bracing of the semiconductor laser (01).

6. The method according to any one of the claims 1 to 5, wherein the optimization method optimizes the lateral structure (f(x,y)) in different distance ranges of the waveguide ridge.

7. The method according to any one of the claims 1 to 6, wherein a plurality of semiconductor lasers (01) are formed adjacent to each other or bordering each other on a shared semiconductor substrate (10) and a separation of the semiconductor lasers (01), in particular a mechanical splitting and / or sawing, takes place after the production.

8. A semiconductor laser (01), comprising a multilayer structure (02) comprising at least one waveguide ridge (16, 16') and material removal areas (04) laterally bordering the waveguide ridge (16, 16'), the multilayer structure (02) being disposed on a semiconductor substrate (10) and a layer expansion plane (26) being defined by a surface of the semiconductor substrate (10), the multilayer structure (02) having at least one active region (13) located perpendicular to the layer expansion plane between the semiconductor substrate and the waveguide ridge and / or the material removal area (04), the active region (13) having a layer structure and material structure for forming a laser layer based on the principle of the stimulated emission, the semiconductor laser (01) having a front facet (09) decoupling the emission mode, a lateral structural layer (05) being provided at least in the material removal areas (04), the lateral structural layer (05) having a lateral structure (f(x,y)) in the layer expansion plane, the lateral structure (f(x,y)) optimizing at least the properties of the three-dimensional laser mode and the material of the lateral structure layer (05), which has a refractive index different to that of the surrounding semiconductor material, being disposed in a part of the cells of a uniform two-dimensional grid, characterized in that the lateral structural layer (05) comprises a first area which is formed on the first 500 nm adjacent to the waveguide ridge (16, 16') and is configured for setting the target wavelength and a second area which borders the first area and is configured for setting a bracing in the multilayer structure and a third area which is disposed further away from the waveguide ridge and is configured for the targeted suppression of undesired three-dimensional laser modes, the second and third areas having zones (43, 43'') bordering the front facet (09) and disposed on both sides of the waveguide ridge (16, 16') and the zones (43, 43'') being formed free of structural elements.

9. The semiconductor laser according to claim 8, wherein the second area has a width of 500 nm.

10. The semiconductor laser according to claim 8 or 9, wherein the lateral structural layer (05) has a layer thickness of 25 nm to 70 nm perpendicular to the layer expansion plane.

11. The semiconductor laser according to any one of claims 8 to 10, wherein the lateral structural layer (05) is formed in the layer expansion plane by contiguous individual structures, which each have a base surface or a multiple of a base surface, the base surface being formed by a surface of less than 0.25 mm2 to be structured being divided into 108 to 1010 even grid cells.

12. The semiconductor laser according to any one of the claims 8 to 11, wherein the material of the lateral structural layer (05) comprises chrome or is made of chrome.

13. The semiconductor laser (01) according to any one of the claims 8 to 12, wherein the lateral structural layer (05) extends across an area of up to 1.5 µm perpendicular to the expansion or length of the waveguide ridge (16, 16').

14. The semiconductor laser according to any one of the claims 8 to 13, wherein the lateral structural layer (05) has several areas which each serve to optimize a property of the semiconductor laser.