Surface-emitting semiconductor laser with vertical cavity and method for manufacturing such a laser

Abstract

A vertical cavity surface-emitting semiconductor laser (VCSEL) has a semiconductor multilayer structure comprising an optical resonator (12) composed of semiconductor layers, wherein the optical resonator (12) includes a first Bragg mirror (16), a second Bragg mirror (18), and an active region (20) for generating laser radiation between the first Bragg mirror (16) and the second Bragg mirror (18). The active region (20) has a plurality of active layers (22, 24, 40) with a first and at least one second active layer (22, 24), wherein the second active layer (24) is the last active layer before the second Bragg mirror (18).Between the first active layer (22) and the second active layer (24), a first oxide blend (26) for current confinement and a first tunnel diode (30) are arranged, with a second oxide blend (32) being arranged on the side of the second active layer (24) facing away from the first active layer (22). A second tunnel diode (34) is arranged on the side of the second active layer (24) facing away from the first active layer (22). Furthermore, a method for fabricating the VCSEL is described.

Description

[0001] The invention relates to a surface-emitting semiconductor laser with a vertical cavity, which has a semiconductor multilayer structure comprising an optical resonator made up of semiconductor layers, wherein the optical resonator has a first Bragg mirror, a second Bragg mirror and an active region between the first Bragg mirror and the second Bragg mirror for generating laser radiation.

[0002] The invention further relates to a method for manufacturing such a surface-emitting semiconductor laser with a vertical cavity.

[0003] Vertical cavity surface-emitting semiconductor lasers, or VCSELs, are used, for example, as radiation sources in sensor technology and telecommunications. VCSELs typically have a multilayer semiconductor structure in which semiconductor layers are epitaxially grown on a semiconductor substrate in a stacked arrangement. The multilayer structure typically includes a first Bragg mirror, an active region, and a second Bragg mirror, which together form an optical resonator. A Bragg mirror is also referred to as a DBR (distributed Bragg reflector). VCSELs typically have an oxidized region within the optical resonator, which consists of a semiconductor layer oxidized to a certain degree to form a current aperture in the resonator, also known as an oxide aperture.The semiconductor layer intended for oxidation is, for example, a layer with a high AlAs (aluminium arsenide) content, which can be selectively oxidized to Al2O3 up to a certain oxidation level.

[0004] VCSELs are known that feature an active region with multiple active layers, with a tunnel diode positioned between each adjacent active layer. For example, such a structure can have three active layers and two tunnel diodes. This arrangement significantly increases the power output of the VCSEL, even tripling it in pulsed operation. In other words, such an arrangement can generate 3 photons from a single charge carrier. With this type of optical resonator featuring multiple active layers, an oxide diaphragm is required near each active layer to channel or confine the current. Typically, a layer with a high aluminum arsenide (AlAs) content, for example, 90–100% AlAs, is used to fabricate the oxide diaphragm.It has been found that the integration of tunnel diodes between the active layers alters the crystal properties of the layers intended for oxidation, for example, by inducing intrinsic defects such as vacancies, and thereby changes the oxidation rate of the oxidizable layer(s), resulting in undesirably different current apertures. In other words, the oxidation rates of the oxidizable layers deviate significantly from the situation without tunnel diodes when tunnel diodes are integrated. This effect is particularly pronounced for oxide blends located near a tunnel diode, while an oxide blend not located near a tunnel diode oxidizes considerably faster.

[0005] Document US 2021 / 0104872 A1 describes various designs of VCSELs that incorporate one or more tunnel diodes within the semiconductor multilayer structure. An n-doped semiconductor layer of the tunnel diode is doped with at least one element such that the tunnel diode not only exhibits a high doping concentration but also maintains a relatively stable oxidation rate during the oxidation process. Alternatively, the n-doped semiconductor layer is doped with at least two elements. The tunnel diode is positioned between two active layers of the VCSEL. The solution described in this document for achieving a uniform oxidation rate during the oxidation of the oxidizable layer(s) relies on additional doping and layers, which, however, increase the manufacturing complexity of the VCSEL.

[0006] Against this background, it is an object of the present invention to provide a vertical cavity surface-emitting semiconductor laser in which the oxidation rate of all existing oxidizable layers for the production of oxide blends is uniform and thus a uniform oxidation range of all oxide blends is ensured, while keeping the manufacturing effort of the VCSEL low.

[0007] It is a further object of the present invention to provide a method for manufacturing a vertical cavity surface emitting semiconductor laser.

[0008] The first-mentioned object is achieved by a surface-emitting semiconductor laser with a vertical cavity according to patent claim 1.

[0009] The vertical cavity surface-emitting semiconductor laser according to the invention, hereinafter referred to as VCSEL, has an active region with a plurality of active layers, including a first and at least one second active layer. The second active layer is the last active layer before the second Bragg mirror. Between the first and second active layers, a first oxide aperture for current confinement and a first tunnel diode are arranged, with a second oxide aperture being located on the side of the second active layer facing away from the first active layer. This basic design is advantageous, as described above, because the VCSEL, due to the at least two active layers with a tunnel diode between these active layers, enables a higher luminous efficacy.

[0010] To solve the problem of differing oxidation rates during the production of the first and second oxide blends, the VCSEL according to the invention features a second tunnel diode on the side of the second active layer facing away from the first. This additional (second) tunnel diode on the side of the second active layer facing away from the first ensures that the environment of each oxide blend can be identical. Experiments have shown that the additional tunnel diode equalizes the oxidation rates during the production of the oxide blends, allowing the VCSEL to be processed without complex additional measures. Fine-tuning of the composition and precise control of the oxidation rates, both of which prove difficult, are not required in the production of the VCSEL according to the invention.The invention solves the problem of different oxidation rates by increasing the symmetry of the arrangement of active layers, oxide blends and tunnel diode by means of an additional tunnel diode.

[0011] A possible disadvantage of a slightly longer growth time and a slightly increased absorption due to the additional tunnel diode are more than compensated for by the significantly simplified and better controlled production of the VCSEL.

[0012] The first Bragg mirror can be the substrate-side Bragg mirror or the Bragg mirror facing away from the substrate, regardless of the fact that in the following description the first Bragg mirror is described as the substrate-side Bragg mirror.

[0013] Preferred embodiments of the VCSEL according to the invention are described below and are specified in the dependent claims.

[0014] In a preferred embodiment, the layer sequence of the arrangement consisting of a second active layer, a second oxide blend and a second tunnel diode is the same as the layer sequence of the arrangement consisting of a first active layer, a first oxide blend and a first tunnel diode.

[0015] In this configuration, a particularly high symmetry of the arrangement of active layers, oxide blends and tunnel diodes is created, resulting in a particularly good alignment of the oxidation rates in the first and second oxidizable layer for the production of the first and second oxide blends and thus uniform current apertures.

[0016] An advantageous sequence, starting from the first Bragg mirror of the first active layer, is: First active layer - first oxide diaphragm - first tunnel diode - second active layer - second oxide diaphragm - second tunnel diode. Another possible sequence is: First active layer - first tunnel diode - first oxide diaphragm - second active layer - second tunnel diode - second oxide diaphragm.

[0017] Preferably, the first and second tunnel diodes each have a highly doped n-layer and a highly doped p-layer with a doping concentration of at least 1E19 cm⁻¹. -3 on, wherein preferably the p-doped layer of the second tunnel diode of the second active layer and the n-doped layer of the second tunnel diode is facing the second Bragg mirror.

[0018] Due to their high doping, the first and second tunnel diodes advantageously exhibit very low resistance. As is typical for tunnel diodes, the first and second tunnel diodes are operated in reverse bias. The highly doped n-layer and the highly doped p-layer are very thin compared to the other layers of the semiconductor multilayer structure. Because the n-doped layer faces the second Bragg mirror, the second Bragg mirror becomes an n-Bragg mirror. In contrast to conventional VCSELs, where one Bragg mirror is an n-mirror and the other a p-Bragg mirror, in the VCSEL according to the invention, both Bragg mirrors are n-mirrors in this embodiment. An n-contact for supplying current to the VCSEL can be arranged on the second Bragg mirror.Although the n-type contacting of both Bragg mirrors is unusual, it has the advantage of reducing optical losses in both mirrors, not just one. n-type material absorbs approximately three times less laser light than p-type material at wavelengths of, for example, 940 nm.

[0019] In a further preferred embodiment, the active region has at least one third active layer arranged on the side of the first active layer facing away from the second active layer, and wherein a third oxide blend and a third tunnel diode are arranged between the third and the first active layer.

[0020] In this embodiment, the VCSEL has a total of three active layers, three oxide apertures and three tunnel diodes, with the aforementioned advantages of high luminous efficacy of the VCSEL with simultaneously uniform oxidation rates and thus uniform current apertures in the oxide apertures.

[0021] In the context of the aforementioned embodiment, the layer sequence of the arrangement consisting of the third active layer, third oxide blend and third tunnel diode is preferably the same as the layer sequence of the arrangement consisting of the first active layer, first oxide blend and first tunnel diode.

[0022] In this configuration, the symmetry is further increased even when the active area has three active layers, and the oxidation rates in the oxidizable layers are matched as much as possible.

[0023] It is understood that the active area may contain more than three active layers, more than three oxide blends and / or more than three tunnel diodes.

[0024] Preferably, the first and second and, if applicable, the third tunnel diode have GaAs (gallium arsenide) layers.

[0025] Furthermore, according to the invention, a method for producing a surface-emitting semiconductor laser with a vertical cavity is provided.

[0026] According to the method, a semiconductor multilayer structure is produced which has an optical resonator built from semiconductor layers, wherein the optical resonator has a first Bragg mirror, a second Bragg mirror and an active region for generating laser radiation between the first Bragg mirror and the second Bragg mirror, wherein the active region has a plurality of active layers with a first and at least one second active layer, wherein the second active layer is the last active layer before the second Bragg mirror.The semiconductor multilayer structure is produced according to the invention such that a first oxidizable layer and a first tunnel diode are arranged between the first active layer and the second active layer, and a second oxidizable layer is arranged on the side of the second active layer facing away from the first active layer, and a second tunnel diode is arranged on the side of the second active layer facing away from the first active layer. The first and the second oxidizable layers are oxidized to generate a first oxide blend and a second oxide blend for current confinement.

[0027] The method according to the invention has the same advantages as the VCSEL according to the invention. Likewise, the method according to the invention has preferred embodiments corresponding to the preferred embodiments of the VCSEL according to the invention.

[0028] Further features and advantages can be found in the following description and the attached drawing.

[0029] It is understood that the features mentioned above and those to be explained below can be used not only in the combinations specified, but also in other combinations or on their own, without leaving the scope of the present invention.

[0030] An embodiment of the invention is shown in the drawing and is described in more detail below with reference to it. The drawing shows: Fig. 1 a schematic longitudinal section through a VCSEL according to a first embodiment; and Fig. 2 a schematic longitudinal section through a VCSEL according to a second embodiment.

[0031] Fig. Figure 1 shows a vertical cavity surface-emitting semiconductor laser, designated VCSEL, with the general reference numeral 10. The VCSEL 10 has a semiconductor multilayer structure comprising an optical resonator 12 built from semiconductor layers. The semiconductor layers of the optical resonator 12 are arranged on a substrate 14, which is also made of a semiconductor material. The substrate 14 may also be removed from the finished VCSEL 10. The semiconductor layers of the optical resonator 12 are grown on the substrate 14 by epitaxy, as is known to those skilled in the art.

[0032] The optical resonator 12 has a first Bragg mirror 16 and a second Bragg mirror 18. The first Bragg mirror 16 and the second Bragg mirror 18 are also referred to as DBRs (Distributed Bragg Reflectors). Each of the two Bragg mirrors 16 and 18 is formed from a sequence of semiconductor layer pairs, with each pair comprising one layer with a higher and one layer with a lower refractive index.

[0033] Between the first Bragg mirror 16 and the second Bragg mirror 18, the optical resonator 12 has an active region 20. The active region 20 serves to generate laser radiation. The active region 20 has a first active layer 22 and a second active layer 24. The first active layer 22 faces the first Bragg mirror 16, and the second active layer 24 is located closer to the second Bragg mirror 18. The second active layer 24 forms the last active layer before the second Bragg mirror 18. The active layers 22 and 24 can each be configured as multiple quantum wells (MQWs).

[0034] A first oxide blend 26 is arranged between the first active layer 22 and the second active layer 24. The oxide blend 26 is formed from a layer that is easily oxidized. For example, a layer with a high aluminum arsenide (AlAs) content is a material that oxidizes readily and controllably. The layer is oxidized to a certain degree, leaving a central region 28 of the oxide blend 26 unoxidized. This central region 28 thus forms an aperture through which the current passes to drive the active region 20, while the oxidized outer region of the layer acts as an insulator. The oxide blend 26 therefore serves to limit the current.

[0035] Adjacent to, and in particular directly adjacent to, the first oxide diaphragm 26 is a first tunnel diode 30. The tunnel diode 30, located between the first active layer 22 and the second active layer 24, increases the light output from both active layers 22 and 24. Adjacent to, and in particular directly adjacent to, the second active layer 24 is a second oxide diaphragm 32, which is identical to the first oxide diaphragm 26 in terms of its material composition and function. The second oxide diaphragm 32 has a central region 29 that acts as a current aperture. A second tunnel diode 34, which has the same material composition as the first tunnel diode 30, follows the second oxide diaphragm 32.

[0036] It has been found that without the additional, or second, tunnel diode 34, the oxidation rate during the fabrication of the second oxide blend 32 is significantly higher than the oxidation rate during the fabrication of the first oxide blend 26. Without the second tunnel diode 34, the apertures in the respective central regions 28 and 29 would therefore be of different sizes. Experiments have shown that the oxidation rates during the fabrication of the first oxide blend 26 and the second oxide blend 32 are equalized when the second tunnel diode 34 is present in the semiconductor multilayer structure. By including the second tunnel diode 34, the symmetry of the layer structure in the region of oxide blends 26 and 32 is increased; in other words, the environment of the second oxide blend 32, due to the additional tunnel diode 34, looks the same as the environment of the first oxide blend 26.

[0037] The first tunnel diode 30 and the second tunnel diode 34 can each be composed of two thin layers, one of which is a highly doped p-layer and the other a highly doped n-layer. The respective p-doped layer faces the first Bragg mirror 16, and the n-doped layer faces the second Bragg mirror 18. Due to the additional second tunnel diode 34, the second Bragg mirror 18 becomes an n-Bragg mirror.

[0038] A top-side contact 36 for contacting the VCSEL 10 is accordingly an n-type contact. The highly doped n-type layers and the highly doped p-type layers of the first tunnel diode 30 and the second tunnel diode 34 can each have a doping of at least 1E19 cm -3 have.

[0039] In the VCSEL 10, the first Bragg mirror 16 and the second Bragg mirror 18 are n-doped mirrors.

[0040] The layer sequence of the arrangement consisting of the second active layer 24, the second oxide blend 32, and the second tunnel diode 34 is identical to the layer sequence of the arrangement consisting of the first active layer 22, the first oxide blend 26, and the first tunnel diode 30. In the exemplary embodiment in Fig. 1. The active region 20, viewed from the first Bragg mirror, has the layer sequence first active layer 22 - first oxide diaphragm 26 - first tunnel diode 30 - second active layer 24 - second oxide diaphragm 32 - second tunnel diode 34. The sequence first oxide diaphragm 26 - first tunnel diode 30 can also be reversed, in which case the sequence second oxide diaphragm 32 - second tunnel diode 34 is preferably also reversed.

[0041] The first tunnel diode 30 and the second tunnel diode 34, for example, are constructed from very thin gallium arsenide (GaAs) layers. For instance, they can have a thickness of 10–30 nm. Carbon, for example, can be used as a dopant for the highly doped p-layers of tunnel diodes 30 and 34, and tellurium can be used for the highly doped n-layers of tunnel diodes 30 and 34.

[0042] In the exemplary embodiment, the second Bragg mirror serves to couple out the generated laser radiation. For this purpose, contact 36 can be designed as a ring contact. The number of semiconductor layer pairs of the first Bragg mirror 16 is greater than the number of semiconductor layer pairs of the second Bragg mirror 18. For example, the first Bragg mirror can have forty mirror pairs, and the second Bragg mirror 18 can have fewer than twenty mirror pairs. The topmost layer of the second Bragg mirror or an additional layer on the second Bragg mirror 18 can be designed as an n-doped contact layer.

[0043] In a specific example, the semiconductor multilayer structure of the VCSEL 10 can be constructed as follows: The semiconductor multilayer structure can be based on the material system aluminum gallium arsenide-gallium arsenide (AlGaAs / GaAs). The substrate 14 can be a gallium arsenide substrate, including an n-doped growth layer. The first active layer 22 and the second active layer 24 can be formed as multiple quantum well (MQW) structures. The oxide diaphragms 26 and 32 can be made of semiconductor layers with a high aluminum content, in particular aluminum arsenide (AlAs). The first tunnel diode 30 and the second tunnel diode 34 can each be formed from two thin, highly doped gallium arsenide layers, wherein the first is p-doped and the second is n-doped as viewed from the substrate 14. The doping is preferably more than 1E19 cm -3 , for example 1E20 cm -3Dopants for the p-doped layers of the tunnel diodes 30 and 34 are, for example, carbon, and for the n-doped layers of the tunnel diodes 30 and 34, for example, tellurium. The second Bragg mirror 18 is constructed from n-doped semiconductor layers, with the second Bragg mirror having, for example, fewer than twenty mirror pairs for coupling out the laser radiation. An n-doped contact layer can be arranged on the second Bragg mirror 18. The contact 36 is designed as a metal contact.

[0044] Fig. Figure 2 shows another embodiment of a VCSEL 10', wherein elements of the VCSEL 10' are connected to elements of the VCSEL 10 in Fig. 1. are identical, similar or comparable, and are provided with the same reference numerals as in Fig. 1.

[0045] The following describes only the differences between VCSEL 10' and VCSEL 10.

[0046] The active region 20 of the VCSEL 10' additionally features a third active layer 40, which is arranged on the side of the first active layer 22 facing away from the second active layer 24. The VCSEL 10' thus has a total of three active layers, each of which can be configured as a multi-quantum trough structure. A third oxide blend 42 and a third tunnel diode 44 are arranged between the third active layer 40 and the first active layer 22. The oxide blend 42 has a central region 31 that acts as a current aperture.

[0047] The layer sequence of the arrangement consisting of the third active layer 40, the third oxide blend 42, and the third tunnel diode 44 is preferably identical to the layer sequence of the arrangement consisting of the first active layer 22, the first oxide blend 26, and the first tunnel diode 30. This results in a VCSEL with stacked pn junctions or a VCSEL with multiple active layers, in this case with a total of three active layers and high overall symmetry of the layer arrangement in the active region. Without the additional tunnel diode 34, different oxidation rates would occur during the fabrication of the oxide blends 26, 32, and 42, as demonstrated in experiments. It has been found that without the additional tunnel diode 34, the oxidation rates during the fabrication of the oxide blends 42 and 26 are lower than the oxidation rate during the fabrication of the oxide blend 32. This changes with the addition of the tunnel diode 34 and the resulting higher symmetry of the arrangement.With the additional tunnel diode 34, the same oxidation rates and thus the same current apertures in the respective central areas 28, 29, 31 of the oxide blends 26, 32, 42 are achieved during the production of all oxide blends 32, 26 and 42.

[0048] In a process for fabricating the VCSEL 10 or 10', the semiconductor layers of the first Bragg mirror, the semiconductor layers of the active region 20, and the semiconductor layers of the second Bragg mirror 18 are grown on the substrate 14 by epitaxy. The layers that serve to generate the oxide diaphragms 26 and 32 or 42 are oxidized in a subsequent process step to produce the oxide diaphragms 26 and 32 or 42. The contact 36 is then deposited on the semiconductor multilayer structure. QUOTES CONTAINED IN THE DESCRIPTION

[0000] This list of documents cited by the applicant was automatically generated and is included solely for the reader's convenience. The list is not part of the German patent or utility model application. The DPMA accepts no liability for any errors or omissions. Cited patent literature

[0000] US 2021 / 0104872 A1

[0005]

Claims

[1] A surface-emitting semiconductor laser with a vertical cavity, comprising a semiconductor multilayer structure having an optical resonator (12) constructed from semiconductor layers, wherein the optical resonator (12) has a first Bragg mirror (16), a second Bragg mirror (18), and an active region (20) for generating laser radiation between the first Bragg mirror (16) and the second Bragg mirror (18), wherein the active region (20) has a plurality of active layers (22, 24, 40) with a first and at least one second active layer (22, 24), wherein the second active layer (24) is the last active layer before the second Bragg mirror (18), and wherein a first oxide diaphragm (26) for current confinement and a first tunnel diode (30) are arranged between the first active layer (22) and the second active layer (24),wherein a second oxide diaphragm (32) is arranged on a side of the second active layer (24) facing away from the first active layer (22), wherein a second tunnel diode (34) is arranged on the side of the second active layer (24) facing away from the first active layer (22). [2] A vertical cavity surface emitting semiconductor laser according to claim 1, wherein the layer sequence of the arrangement of second active layer (24), second oxide diaphragm (32) and second tunnel diode (34) is the same as the layer sequence of the arrangement of first active layer (22), first oxide diaphragm (26) and first tunnel diode (30). [3] A vertical cavity surface emitting semiconductor laser according to claim 1 or 2, wherein, viewed from the first Bragg mirror (16), the layer sequence of the arrangement of first and second active layers (22, 24), first and second oxide diaphragms (26, 32), first and second tunnel diodes (30, 34) is as follows: first active layer (22) - first oxide diaphragm (26) - first tunnel diode (30) - second active layer (24) - second oxide diaphragm (32) - second tunnel diode (34). [4] A vertical cavity surface emitting semiconductor laser according to any one of claims 1 to 3, wherein the first and second tunnel diodes (30, 34) each comprise a highly doped n-layer and a highly doped p-layer with a doping of at least 1E19 cm -3 wherein the highly doped p-layer of the second tunnel diode (34) faces the second active layer (24) and the highly doped n-layer of the second tunnel diode (34) faces the second Bragg mirror (18). [5] A vertical cavity surface emitting semiconductor laser according to any one of claims 1 to 4, wherein the first Bragg mirror (16) and the second Bragg mirror (18) are each an n-Bragg mirror. [6] A vertical cavity surface emitting semiconductor laser according to any one of claims 1 to 5, wherein the active region (20) comprises at least a third active layer (40) arranged on the side of the first active layer (22) facing away from the second active layer (24), and wherein a third oxide diaphragm (42) and a third tunnel diode (44) are arranged between the third and the first active layer (40, 22). [7] A vertical cavity surface emitting semiconductor laser according to claim 6, wherein the layer sequence of the arrangement of the third active layer (40), the third oxide diaphragm (42) and the third tunnel diode (44) is the same as the layer sequence of the arrangement of the first active layer (22), the first oxide diaphragm (26) and the first tunnel diode (30). [8] A method for manufacturing a vertical cavity surface emitting semiconductor laser, comprising the steps of: Producing a semiconductor multilayer structure having an optical resonator (12) constructed from semiconductor layers, wherein the optical resonator (12) has a first Bragg mirror (16), a second Bragg mirror (18) and, between the first Bragg mirror (16) and the second Bragg mirror (18), an active region (20) for generating laser radiation, wherein the active region (20) has a plurality of active layers with a first and at least one second active layer (22, 24, 40), wherein the second active layer (24) is the last active layer before the second Bragg mirror (18), wherein the semiconductor multilayer structure is produced such that a first oxidizable layer and a first tunnel diode (30) are arranged between the first active layer (22) and the second active layer (24), and a second oxidizable layer is arranged on a side of the second active layer (24) facing away from the first active layer (22), and a second tunnel diode (34) is arranged on the side of the second active layer (24) facing away from the first active layer (22), and Oxidizing the first and second oxidizable layers to produce a first oxide aperture (26) and a second oxide aperture (32) for current confinement.

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