Abstract
The invention relates to a surface-emitting photonic crystal laser (1). The laser has an active layer for generating electromagnetic radiation by combining charge carriers, wherein the active layer has a first main surface and a second main surface lying opposite the first main surface. The first main surface is equipped with a first waveguide layer, and the second main surface is equipped with a second waveguide layer, said waveguide layers having regions which are arranged periodically relative to one another and additional regions which have different refractive indices and which form a photonic crystal. The first waveguide layer is equipped with a first casing layer which has at least one p-connection region for injecting electrically positive charge carriers into the active layer and at least one n-connection region for injecting electrically negative charge carriers into the active layer. The invention additionally relates to a method for producing a surface-emitting photonic crystal laser and to an optoelectronic system.
Claims
1. A surface-emitting photonic-crystal laser (1), comprising: an active layer for generating electromagnetic radiation by charge carrier recombination, the active layer comprising a first main surface and a second main surface opposite the first main surface, a first waveguide layer arranged on the first main surface, a second waveguide layer arranged on the second main surface and comprising regions and further regions arranged periodically with respect to one another, wherein a refractive index of the regions differs from a refractive index of the further regions, and wherein the regions and the further regions form a photonic crystal, and a first cladding layer arranged on the first waveguide layer, the first cladding layer comprising at least one p-connection region for injecting electrically positive charge carriers into the active layer and at least one n-connection region for injecting electrically negative charge carriers into the active layer.
2. The surface-emitting photonic-crystal laser according to claim 1, wherein the second waveguide layer comprises a material which defines the regions of the second waveguide layer, and wherein the further regions of the second waveguide layer are defined by recesses of the second waveguide layer.
3. The surface-emitting photonic-crystal laser according to claim 2, wherein the recesses of the second waveguide layer are formed by trenches or holes in the second waveguide layer, the trenches or holes extending from a surface of the second waveguide layer facing away from the active layer into the second waveguide layer.
4. The surface-emitting photonic-crystal laser according to claim 1, wherein the first waveguide layer is undoped or intrinsically doped.
5. The surface-emitting photonic-crystal laser according to claim 1, wherein the second waveguide layer is undoped or intrinsically doped.
6. The surface-emitting photonic-crystal laser according to claim 1, wherein a thickness of the first waveguide layer is less than a thickness of the second waveguide layer.
7. The surface-emitting photonic-crystal laser according to claim 1, further comprising a second cladding layer arranged on the second waveguide layer.
8. The surface-emitting photonic-crystal laser according to claim 2, wherein the recesses of the second waveguide layer are formed by trenches in the second waveguide layer which extend from a surface of the second cladding layer facing away from the second waveguide layer into the second waveguide layer.
9. The surface-emitting photonic-crystal laser according to claim 7, wherein the second cladding layer is undoped or intrinsically doped.
10. The surface-emitting photonic-crystal laser according to claim 1, wherein the active layer forms at least one quantum well which is configured and formed to emit electromagnetic radiation of a predetermined wavelength when a driving current is applied.
11. The surface-emitting photonic-crystal laser according to claim 1, wherein a radiation direction of the laser is perpendicular to a main extension plane of the second waveguide layer and electromagnetic radiation is outcoupled via a surface of the second waveguide layer facing away from the active layer.
12. The surface-emitting photonic-crystal laser according to claim 1, further comprising a reflective layer arranged on or above the first cladding layer.
13. The surface-emitting photonic-crystal laser according to claim 1, wherein the first cladding layer comprises a plurality of individually and independently controllable p-connection regions and/or a plurality of individually and independently controllable n-connection regions.
14. The surface-emitting photonic-crystal laser according to claim 13, wherein the p-connection regions and the n-connection regions are arranged in a checkerboard pattern when viewed from a top view.
15. The surface-emitting photonic-crystal laser according to claim 1, wherein the at least one p-connection region and the at least one n-connection region are formed as concentric rings when viewed from a top view.
16. The surface-emitting photonic-crystal laser according to claim 1, further comprising at least one first electrical contact element and at least one second electrical contact element, wherein a respective first electrical contact element is arranged on and associated with each p-connection region, and wherein a respective second electrical contact element is arranged on and associated with each n-connection region.
17. An optoelectronic system comprising a surface-emitting photonic-crystal laser according to claim 1.
18. A method of manufacturing a surface-emitting photonic-crystal laser, comprising: forming a second waveguide layer, applying an active layer to the second waveguide layer, the active layer being formed to generate electromagnetic radiation by charge carrier recombination, applying a first waveguide layer to the active layer, applying a first cladding layer to the first waveguide layer, forming at least one p-connection region of the first cladding layer for injecting electrically positive charge carriers into the active layer and at least one n-connection region of the first cladding layer for injecting electrically negative charge carriers into the active layer, and forming regions and further regions of the second waveguide layer arranged periodically with respect to one another, wherein a refractive index of the regions differs from a refractive index of the further regions, and wherein the regions and the further regions form a photonic crystal.
19. The method according to claim 18, further comprising applying a first electrical contact element to each of the p-connection regions, and applying a second electrical contact element to each of the n-connection regions.
20. The method according to claim 18, wherein forming regions and further regions of the second waveguide layer arranged periodically with respect to one another comprises: forming trenches or holes in the second waveguide layer, which extend from a surface of the second waveguide layer facing away from the active layer into the second waveguide layer, wherein the regions are defined by the waveguide material, and the further regions are defined by recesses formed by the trenches or holes.
Description
BRIEF DESCRIPTION OF THE DRAWINGS
[0046] Further advantages, advantageous embodiments and further developments become apparent from the exemplary embodiments described below in conjunction with the figures.
[0047] In the exemplary embodiments and the figures, identical, similar or similarly effective elements may each be designated by the same reference numerals. The elements shown and their relative sizes are not to be regarded as true to scale; rather, individual elements, such as layers, components, structural elements, and regions, may be shown in exaggerated size for better illustration and/or understanding.
[0048] FIGS. 1 to 4 show a cross-section of a surface-emitting photonic-crystal laser according to embodiments.
[0049] FIGS. 5 to 7 show a top view of a surface-emitting photonic-crystal laser according to embodiments.
[0050] FIG. 8 shows a schematic representation of an optoelectronic system according to an exemplary embodiment.
[0051] FIGS. 9 to 13 show intermediates in the manufacture of a PCSEL according to an exemplary embodiment.
[0052] FIG. 14 shows an end product in the manufacture of a PCSEL according to an exemplary embodiment.
DETAILED DESCRIPTION
[0053] Referring to FIG. 1, a surface-emitting photonic-crystal laser 1, PCSEL, is shown. The PCSEL 1 comprises an active layer 5 which is configured to generate electromagnetic radiation 99 (illustrated by an arrow, following interaction with a photonic crystal) by charge carrier recombination. The active layer may comprise a semiconductor material. The active layer 5 comprises a first main surface 5 and a second main surface 5 opposite the first main surface 5. The active layer comprises a main extension plane extending in lateral directions x, y. In a transversal direction z, which is perpendicular to the main extension plane, the active layer has a thickness. The first main surface 5 and second main surface 5 are parallel to the lateral directions x, y. The active layer 5 may be formed by a plurality of layers (not shown). In particular, the active layer 5 may form at least one quantum well, which is intended and configured to emit electromagnetic radiation 99 of a predetermined wavelength when a driving current is applied. For example, the at least one quantum well is a 2D quantum well formed by a thin intermediate layer of a first material surrounded by barrier layers of a second material. The barrier layers have a larger band gap than the intermediate layer.
[0054] The PCSEL 1 further comprises a first waveguide layer 10 arranged on the first main surface 5 and a second waveguide layer 15 arranged on the second main surface 5. In other words, the active layer 5 is arranged between the first waveguide layer 10 and the second waveguide layer 15 and forms a respective interface therewith. The waveguide layers 10, 15 may comprise a semiconductor material. In the exemplary embodiment of FIG. 1, a thickness of the first waveguide layer 10 is less than a thickness of the second waveguide layer 15. Herein, the thicknesses of the waveguide layers 10, 15 refer to an expansion of the waveguide layers 10, 15 in the transversal direction z. The first waveguide layer 10 together with the second waveguide layer 15 forms a waveguide into which the active layer 5 is embedded. In the exemplary embodiment shown the different thicknesses of the waveguide layers 10, 15 result in an asymmetrical waveguide profile. The first waveguide layer 10 and the second waveguide layer 15 may be doped, undoped or intrinsically doped. However, the second waveguide layer 15 is preferably undoped or intrinsically doped.
[0055] The second waveguide layer 15 comprises regions 41 and further regions 42 arranged periodically with respect to one another. A refractive index of the regions 41 differs from a refractive index of the further regions 42. In this manner, the regions 41 and the further regions 42 form a photonic crystal 40. The electromagnetic radiation 99 is emitted in the transversal direction z due to interaction with the photonic crystal 40. The regions 41 and the further regions 42 may comprise different materials. For example, the regions 41 comprise a semiconductor material, while the further regions comprise another semiconductor material, air, gas, or an oxide. A difference of refractive indices between the regions 41 and the further regions 42 may be large. The further regions 42 may have been subsequently introduced into the waveguide layer 15. This may mean that the waveguide layer 15 is initially formed as a continuous layer comprising the material of the regions 41. The further regions 42 may be formed by changing the material, removing material or replacing material. The further regions 42 may be arranged in a matrix or may be located on intersections of a lattice, when viewed from a top view (see FIG. 7). The lattice may consist of a base comprising several elements. A lattice period may be chosen such that it substantially coincides with a wavelength of the radiation 99 generated by the active layer 5, or represents an integer or non-integer multiple thereof. In the exemplary embodiment shown in FIG. 1, the further regions 42 extend from a surface of the second waveguide layer 15 facing away from the active layer 5 into the second waveguide layer 15. In the transversal direction z, the further regions 42 are spaced apart from the active layer 5. It is also possible (but not shown) for the further regions 42 to be embedded in the second waveguide layer 15, i.e. to not extend to the surface of the second waveguide layer 15. Such an arrangement may, for example, be produced using a regrowth process. In this case, the further regions 42 may, for example, be formed by cavities or self-contained zones in the second waveguide layer 15.
[0056] The PCSEL 1 further comprises a first cladding layer 20 arranged on the first waveguide layer 10. The first cladding layer 20 forms an interface with the first waveguide layer 10, i.e. the first waveguide layer 10 is arranged between the first cladding layer 20 and the active layer 5. The first cladding layer 20 comprises a semiconductor material, for example. The first cladding layer is preferably doped at least in certain regions. The first cladding layer 20 comprises at least one p-connection region 21 for injecting electrically positive charge carriers into the active layer 5. The p-connection region may be formed, for example, by doping a region of the first cladding layer 20 with a p-type dopant. The first cladding layer 20 also comprises at least one n-connection region 22 for injecting electrically negative charge carriers into the active layer 5. The n-connection region may be formed, for example, by doping a further region of the first cladding layer 20 with an n-type dopant. The example shown shows a plurality of p-type and n-type connection regions, which may be electrically controlled independently and individually via contact elements 31, 32. The first cladding layer 20 forms a surface 20. The surface 20 is opposite the first waveguide layer 10, i.e. faces away from it. The surface 20 may also be referred to as the connection side 20.
[0057] The PCSEL 1 of FIG. 1 further comprises a second cladding layer 25, which is arranged on the second waveguide layer 15. The second cladding layer 25 forms an interface with the second waveguide layer 15. In other words, the second waveguide layer 15 is arranged between the second cladding layer 25 and the active layer 5. The second cladding layer 25 is optional. The second cladding layer 25 may comprise a semiconductor material. The second cladding layer 25 may preferably be undoped. The second cladding layer 25 may be thinner in the transversal direction z than the first cladding layer 20. The second cladding layer 25 forms a surface 25. The surface 25 is opposite the second waveguide layer 15, i.e. faces away from it. The surface 25 may also be referred to as the emission side 25. This may mean that the electromagnetic radiation 99 generated by the active layer 5 is emitted from the PCSEL 1 via the emission side 25 following manipulation by the photonic crystal 40.
[0058] The PCSEL 1 according to FIG. 1 further comprises a contact layer 34. The contact layer 34 is arranged on the first cladding layer 20, i.e. on the contact side 20, at least in certain regions. The contact layer 34 may be a highly doped semiconductor layer. The contact layer 34 may be part of a first or a second contact element 31, 32, by means of which the at least one n- or p-connection region 21, 22 is electrically contacted. In the example shown, the contact layer 34 is part of the second contact element 32, by means of which the n-connection region 22 is electrically contacted. The contact layer 34 is patterned such that only n-connection regions 22 of the first cladding layer 20 are covered by it and the other regions of the first cladding layer 20, which correspond to the p-connection regions 22, are exposed.
[0059] The PCSEL 1 according to FIG. 1 further comprises a further contact layer 33 which is arranged on the contact layer 34. The further contact layer 33 may comprise a metal, for example. The further contact layer 33 is patterned and covers regions of the contact layer 34. The contact layer 34 and the further contact layer 33 together form the second contact element 32. The PCSEL 1 comprises a plurality of second contact elements 32, corresponding to the number of n connection regions 22. The PCSEL 1 further comprises a first contact element 31 which is arranged on the p-connection region. The first contact element 31 may comprise a metal, for example. The first contact element 31 is patterned and covers the p-connection regions 21. The PCSEL 1 comprises a plurality of first contact elements 31, corresponding to the number of p-connection regions 21.
[0060] Referring to FIG. 2, a further exemplary embodiment of the PCSEL 1 is shown. The exemplary embodiment of FIG. 2 differs from the exemplary embodiment of FIG. 1 in that the further regions 42 of the second waveguide layer 15 are formed by recesses in the second waveguide layer 15. Here, the recesses of the second waveguide layer 15 are formed by trenches or holes extending from the surface 25 of the second cladding layer 25 into the second waveguide layer 15. The second cladding layer 25 is again optional. A diameter of the trenches may be in the nanometer range. The shape of the trenches may be cylindrical, i.e. when viewed from a top view, the trenches may have a circular or elliptical profile (see FIG. 7). However, it is also possible for the trenches to have a different profile, e.g. polygonal, in particular triangular or quadrangular. In the transversal direction z, the trenches may reach close to the active layer 5. However, a trench foot of the trenches remains at a distance from the active layer 5.
[0061] Referring to FIG. 3, a further exemplary embodiment of the PCSEL 1 is shown. The exemplary embodiment of FIG. 3 differs from the exemplary embodiment of FIG. 2 in that the active layer 5 uses a plurality of 0-dimensional quantum dots (structures limited in all three spatial directions) or 1-dimensional quantum wires (in this case, the structures only extend in the lateral y-direction, out of the image plane). The density of the quantum dots or quantum wires may vary along lateral directions x, y, for example. More radiation is then generated in regions of higher density than in regions of lower density. In this manner, the position of the radiation-emitting regions of the active layer 5 may be further adjusted. Furthermore, the PCSEL 1 of FIG. 3 does not comprise a contact layer 34. The first contact elements 31 are in direct contact with the p-connection regions 21. The second contact elements 32, consisting of the further contact layer 33, are in direct contact with the n-connection regions 22. The first cladding layer 20 is formed by a continuous layer in which the p-connection regions 21 and the n-connection regions 22 alternate laterally.
[0062] Referring to FIG. 4, a further exemplary embodiment of the PCSEL 1 is shown. The exemplary embodiment of FIG. 4 differs from the exemplary embodiment of FIG. 3 in that the p-connection regions 21 and the n-connection regions 22 are no longer part of a simply contiguous first cladding layer 20. Instead, the p-connection regions 21 and the n-connection regions 22 are separate and spaced-apart elements. For example, the first cladding layer 20 shown in the previous figures was patterned for this purpose. By configuring the connection regions 21, 22 as spaced-apart and separate elements, recombination of charge carriers outside the active layer 5 may be suppressed and pn junctions in the first cladding layer 20 may be avoided.
[0063] Referring to FIG. 5, a further exemplary embodiment of the PCSEL 1 is shown in a top view. The top view here refers to a view of the connection side 20 of the first cladding layer 20. As may be seen in FIG. 5, the first contact elements 31 and the second contact elements 32 are arranged in a checkerboard pattern. This means that a second contact element 32 is arranged between every two first contact elements 31 in lateral directions x, y, and vice versa. In this example, the PCSEL 1 comprises 8 first and 8 second contact elements 31, 32 which are arranged in a 44 array. These numbers are to be understood as arbitrary examples, and the PCSEL 1 may be expanded or reduced to larger or smaller areas comprising more or less than a total of 16 contact elements 31, 32. In addition, the lateral diameters of the contact elements 31, 32 may be adjusted as desired with respect to the area of the connection side 20. The number of first contact elements 31 (and thus the number of p-connection regions 21) may differ from the number of second contact elements 32 (and thus the number of n-connection regions 22).
[0064] Referring to FIG. 6, an alternative exemplary embodiment of a PCSEL 1 is shown in a top view of the connection side 20.
[0065] In this case, the first and second contact elements 31, 32 are formed as concentric rings or circles of different diameters. A second ring-shaped electrical contact element 32 surrounds a first innermost first contact element 31 in lateral directions x, y. A further ring-shaped first contact element 31 surrounds the second electrical contact element 32, and a further ring-shaped second contact element 32 surrounds the further first contact element 31. This scheme may be extended as desired and/or may start with a second contact element 32 as the innermost contact element.
[0066] Referring to FIG. 7, an exemplary embodiment of the PCSEL 1 is shown in a top view of the emission side 25. Only the second cladding layer 25 and the underlying second waveguide layer 15 are shown, further layers are not shown for reasons of clarity. As may be seen, the further regions 42 are formed as trenches that extend from the emission side 25 into the second waveguide layer 15. The areas of the second waveguide layer 15 located between the trenches form the regions 41 of the second waveguide layer 15. By way of example, the trenches are arranged in a hexagonal grid. The regions 41 and the further regions 42 form the photonic crystal 40.
[0067] As indicated in FIG. 8, the PCSEL 1 may be integrated into an optoelectronic system 100. For example, the optoelectronic system 100 is a LIDAR system. The optoelectronic system 100 may also include other systems in which VCSELs (vertical cavity surface-emitting lasers) or EELs (edge-emitting lasers) are commonly used. The electrical contact elements 31, 32 of the PCSEL 1 may be connected to a printed circuit board or PCB or to another semiconductor device (e.g. a driver IC) of the optoelectronic system 100 by means of wire bonds or flip-chip mounting. The optoelectronic system 100 may include further optical and/or electronic components, such as optical filters, lenses, photodetectors and/or integrated circuits.
[0068] FIGS. 9-14 show a possible manufacturing process for a PCSEL 1. FIG. 9 shows a layer stack that may be formed by epitaxial growth. The layer stack comprises a substrate 50, which may be a semiconductor substrate. The second cladding layer 25 is formed on the substrate 50. However, the second cladding layer 25 is optional, as explained above. As first layers on the substrate 50, for example, a buffer layer for improving growth and/or a release layer, which facilitates subsequent detachment of the substrate 50 or the buffer layer, may be deposited.
[0069] The second waveguide layer 15 is formed on the second cladding layer 25. The second waveguide layer 15 may also be formed directly on the substrate 50 (or the buffer layer or the release layer). The active layer 5, which is configured to generate electromagnetic radiation 99 by charge carrier recombination, is applied to the second waveguide layer 15. The first waveguide layer 10 is applied to the active layer 5. The first cladding layer 20 is applied to the first waveguide layer 10. In the example shown, the contact layer 34, which may in particular be a highly doped n-type semiconductor layer, is applied to the first cladding layer 20. The contact layer 34 is optional.
[0070] FIG. 10 shows the intermediate product of FIG. 9 after further process steps. According to FIG. 10, the contact layer 34 is patterned such that regions of the underlying first cladding layer 20 are exposed. Furthermore, p-connection regions 21 of the first cladding layer 20 are formed for injecting electrically positive charge carriers into the active layer. The p-connection regions 21 may be formed using ion implantation, for example. Alternatively, connection regions for different charge carriers may also be created by multiple growth processes. The details of such a process sequence may be deduced by a person skilled in the art. Furthermore, n-connection regions 22 of the first cladding layer 20 are formed for injecting electrically negative charge carriers into the active layer 5. The n-connection regions 22 may be defined, for example, by the regions of the first cladding layer 20 that are not formed as p-connection regions 21. In the example shown, the n-connection regions 22 are covered by the contact layer 34.
[0071] FIG. 11 shows the intermediate product of FIG. 10 after further process steps. According to FIG. 11, the further contact layer 33 is applied to the contact layer 34 or to its regions. The further contact layer 33 may comprise a metal and may be formed by means of a sputtering process. The contact layer 34 and the further contact layer 33 form the second electrical contact elements 32, which are consequently arranged on each of the n-connection regions 22. Furthermore, first contact elements 31 are applied to each of the p-connection regions. The first contact elements 31 may also preferably comprise a metal and be formed by means of a sputtering process.
[0072] FIG. 12 shows the intermediate product of FIG. 11 after further process steps. According to FIG. 12, the first and second contact elements 31, 32 are bonded to a further substrate 55. A bonding process may be used for this purpose. The further substrate 55 may have bonding pads 58 by means of which the respective contact elements 31, 32 are bonded to the further substrate 55. The further substrate 55 may be a carrier substrate which is removed again after the manufacturing process. Preferably, however, the further substrate 55 may comprise an application-specific integrated circuit, or ASIC for short. The circuit may comprise a plurality of switches, which may be transistors, in particular thin film transistors. Each switch may be electrically connected to at least one of the contact elements 31, 32. In this manner, the contact elements 31, 32, and therefore the connection regions 21, 22, may be controlled separately.
[0073] FIG. 13 shows the intermediate product of FIG. 12 after further process steps. According to FIG. 13, the intermediate product is rotated by 180 such that the additional substrate 55 acts as a carrier. Furthermore, the substrate 50 is removed, for example by grinding and/or etching. This exposes the second cladding layer 25. Alternatively, if no second cladding layer 25 is present, the second waveguide layer 15 is exposed. In this case, in an optional step, the second cladding layer 25 might now be formed by a deposition process. Alternatively, the second waveguide layer 15 may also be formed by a deposition process.
[0074] FIG. 14 shows the end product of the manufacturing process of FIGS. 9 to 13 after further process steps. By means of a patterning process (e.g. electron beam lithography and etching etc.), trenches are introduced into the optional second cladding layer 25 and the underlying second waveguide layer 15. As a result, regions 41 and further regions 42 of the second waveguide layer 15 are formed which are periodically arranged with respect to one another. The trenches form recesses in the second waveguide layer 15, which may be filled with air or gas. As a result, a refractive index of the regions 41 differs from a refractive index of the further regions 42, thereby forming a photonic crystal 40.
[0075] The features and embodiments described in conjunction with the figures may be combined with one another according to further embodiments, even if not all combinations have been explicitly described. Furthermore, the embodiments described in conjunction with the figures may alternatively or additionally comprise further features as described in the general part of the specification.
[0076] The invention is not limited by the description to the exemplary embodiments described. Rather, the invention includes any new feature as well as any combination of features, which includes in particular any combination of features in the patent claims, even if this feature or combination itself is not explicitly stated in the patent claims or embodiments.
