Vertical cavity surface emitting semiconductor laser and manufacturing method thereof

Abstract

A vertical cavity surface emitting semiconductor laser (VCSEL) has a semiconductor multilayer structure with an optical resonator (12) consisting of semiconductor layers, 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). The active region (20) has a plurality of active layers (22, 24, 40) having a first active layer and at least one second active layer (22, 24), the second active layer (24) being the last active layer located before the second Bragg reflector (18). A first oxide barrier (26) and a first tunnel diode (30) for limiting a current are arranged between the first active layer (22) and the second active layer (24), and a second oxide barrier (32) is arranged on a 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). The invention also relates to a method for manufacturing the VCSEL.

Description

TECHNICAL FIELD

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

[0002] The invention also relates to a method for producing such a vertical cavity surface emitting semiconductor laser. BACKGROUND

[0003] Vertical cavity surface emitting semiconductor lasers, in short VCSELs (English: Vertical Cavity Surface Emitting Laser), are used, for example, as radiation sources in sensor technology or in communication technology. A VCSEL usually has a semiconductor multilayer structure, wherein the semiconductor layers are epitaxially grown in a stacked arrangement on a semiconductor substrate. The semiconductor multilayer structure usually has 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 DBR (English: Distributed Bragg Reflector). A VCSEL usually has an oxidation region in the optical resonator, which has semiconductor layers that are oxidized to a certain oxidation range in order to form a current aperture in the resonator, which is also referred to as an oxide partition. The semiconductor layers provided for oxidation are, for example, layers with a high AlAs (aluminum arsenide) content, which can be oxidized to a specific oxidation range to Al2O3 in a targeted manner.

[0004] VCSELs are known which have an active region with a plurality of active layers, wherein between each adjacent active layer a tunnel diode is arranged. Such a structure can have, for example, three active layers and two tunnel diodes. By means of this arrangement the power of the VCSEL can be significantly increased, even for example by a factor of three in pulsed operation. In other words, by means of this arrangement 3 times as many photons can be generated from one carrier. In such a configuration of the optical resonator with a plurality of active layers, an oxide spacer is required in the vicinity of each active layer for channeling or limiting the current. In order to produce the oxide spacer, layers with a high content of aluminum arsenide (AlAs) are usually used, for example AlAs with a content of 90-100%. It has been found that the incorporation of a tunnel diode between the active layers changes the crystal properties of the layers provided for oxidation, for example induces intrinsic defects (for example vacancies), and thus changes the oxidation rate of the oxidizable layer(s), whereby differently sized current apertures undesirably occur. In other words, due to the incorporation of the tunnel diode, the oxidation rate of the oxidizable layer deviates greatly compared to the case without a tunnel diode. This effect is particularly pronounced at the oxide spacers arranged in the vicinity of the tunnel diode, while the oxide spacers not arranged in the vicinity of the tunnel diode oxidize significantly more quickly.

[0005] Different embodiments of a VCSEL are described in document US 2021 / 0104872 A1, which have one or more tunnel diodes in a semiconductor multilayer structure. The n-doped semiconductor layer of the tunnel diode is doped with at least one element, so that the tunnel diode not only has a high doping, but also a relatively stable oxidation rate during oxidation. Alternatively, the n-doped semiconductor layer is doped with at least two elements. The tunnel diode is arranged between two active layers of the VCSEL. The solution described in this document for achieving a uniform oxidation rate when oxidizing the oxidizable layer(s) is based on additional doping and layers, which however increase the manufacturing costs of the VCSEL. SUMMARY

[0006] Against this background, it is an object of the present application to provide a vertical-cavity surface-emitting semiconductor laser, in which the oxidation rate of all existing oxidizable layers for producing oxide spacers is uniform and in turn ensures a uniform oxidation range of all oxide spacers, wherein the manufacturing costs of the VCSEL remain low.

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

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

[0009] The vertical cavity surface emitting semiconductor laser according to the invention, in the following abbreviated as VCSEL, has an active region with a plurality of active layers, which have a first active layer and at least one second active layer. The second active layer is the last active layer before the second Bragg mirror. Between the first active layer and the second active layer a first oxidblende for current confinement and a first tunnel diode are arranged, wherein a second oxidblende is arranged on the side of the second active layer facing away from the first active layer. This basic structure is advantageous as described above, because the VCSEL can achieve a higher light efficiency due to the at least two active layers and the tunnel diode between these active layers.

[0010] In order to solve the problem of different oxidation rates when producing the first oxidblende and the second oxidblende, the VCSEL according to the invention has a second tunnel diode on the side of the second active layer facing away from the first active layer. By arranging an additional (second) tunnel diode on the side of the second active layer facing away from the first active layer, it is achieved that the environment of each oxidblende can be configured identically. It has been shown in experiments that the oxidation rate when producing the oxidblende is equalized to each other due to the additional tunnel diode and that the VCSEL can be processed without the need for expensive additional measures. When producing the VCSEL according to the invention, no fine adjustment of the composition and no special control of the oxidation rate is necessary, which has proven to be difficult. Rather, the invention solves the problem of different oxidation rates by the improved symmetry of the assembly consisting of active layer, oxidblende and tunnel diode due to the additional tunnel diode.

[0011] This possible disadvantage of a slightly longer growth time and a slightly increased absorption due to the additional tunnel diode is more than compensated by the significantly simplified and more controllable production of the VCSEL.

[0012] Irrespective of the description of the first Bragg mirror as Bragg mirror on the substrate side in the following description, the first Bragg mirror can be a Bragg mirror on the substrate side or a Bragg mirror facing away from the substrate.

[0013] Preferred configurations of the VCSEL according to the invention will be described in the following and given in the dependent claims.

[0014] In one preferred configuration, the layer sequence of the assembly consisting of the second active layer, the second oxidblende and the second tunnel diode is identical to the layer sequence of the assembly consisting of the first active layer, the first oxidblende and the first tunnel diode.

[0015] In this configuration, a particularly high symmetry of the assembly of active layer, oxide spacer and tunnel diode is achieved, whereby a particularly good use of the oxidation rates in the first and second oxidizable layers for manufacturing the first and second oxide spacers is achieved and thus a uniform current aperture is achieved.

[0016] From the view of the first Bragg mirror of the first active layer, a favorable sequence is: first active layer - first oxide spacer - first tunnel diode - second active layer - second oxide spacer - second tunnel diode. Another possible sequence is: first active layer - first tunnel diode - first oxide spacer - second active layer - second tunnel diode - second oxide spacer.

[0017] Preferably, the first and second tunnel diodes have a highly doped n-type layer and a highly doped p-type layer, respectively, with at least 1 E19 cm -3 Preferably, the p-type doped layer of the second tunnel diode faces the second active layer, and the n-type doped layer of the second tunnel diode faces the second Bragg mirror.

[0018] By the high doping, the first and second tunnel diodes advantageously have a very low electrical resistance. As is usual in tunnel diodes, the first and second tunnel diodes are operated in the blocking direction. The highly doped n-type layer and the highly doped p-type layer are very thin layers compared to the other layers of the semiconductor multilayer structure. Since the n-type doped layer faces the second Bragg mirror, the second Bragg mirror becomes an n-type Bragg mirror. In the VCSEL according to the application in the present embodiment form, both Bragg mirrors are n-type mirrors, in contrast to a usual VCSEL in which one Bragg mirror is an n-type mirror and the other is a p-type Bragg mirror. Accordingly, an n-type contact can be arranged on the second Bragg mirror for energizing the VCSEL. Although the n-type contact of both Bragg mirrors is unusual, this has the advantage that the optical losses are reduced not only in one but in both mirrors. At a wavelength of, for example, 940 nm, the n-type material absorbs about one third of the laser light compared to the p-type material.

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

[0020] In this embodiment form, the VCSEL has three active layers, three oxide spacers and three tunnel diodes in total, with the above-mentioned advantages of high light efficiency of the VCSEL while achieving a uniform oxidation rate and thus a uniform current aperture in the oxide spacers.

[0021] In the context of the aforementioned embodiments, preferably, the layer sequence of the assembly consisting of the third active layer, the third oxide spacer and the third tunnel diode is identical to the layer sequence of the assembly consisting of the first active layer, the first oxide spacer and the first tunnel diode.

[0022] In this configuration, the symmetry is further improved even in the configuration of the active region with three active layers, and in the oxidizable layers, the oxidation rate is as uniform as possible.

[0023] It is to be understood that the active region can have more than three active layers, more than three oxide spacers and / or more than three tunnel diodes.

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

[0025] Further in accordance with the present application, a method for manufacturing a vertical-cavity surface-emitting semiconductor laser is provided.

[0026] According to the method, a semiconductor multilayer structure is manufactured, which has an optical resonator composed of 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 active layer and at least one second active layer, wherein the second active layer is the last active layer before the second Bragg mirror. In accordance with the present application, the semiconductor multilayer structure is manufactured 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 second oxidizable layers are oxidized to produce a first oxide spacer and a second oxide spacer for limiting the current.

[0027] The method according to the present application has the same advantages as the VCSEL according to the present application. Likewise, the method according to the present application has preferred configurations corresponding to the preferred configurations of the VCSEL according to the present application.

[0028] Further features and advantages are derived from the following description and the drawings.

[0029] It is to be understood that the features mentioned above and still to be explained below can be used not only in the respectively specified combinations, but also in other combinations or on their own, without departing from the scope of the present application. BRIEF DESCRIPTION OF DRAWINGS

[0030] One embodiment of the present application is shown in the drawing and is described in more detail below with reference to the drawing. Shown is: Figure 1 A schematic longitudinal section of a VCSEL according to a first embodiment is shown; and Figure 2 A schematic longitudinal section of a VCSEL according to a second embodiment is shown. DETAILED DESCRIPTION

[0031] Figure 1 A vertical cavity surface emitting semiconductor laser, shortly VCSEL, is shown which is provided with the reference numeral 10. The VCSEL 10 has a semiconductor multilayer structure with an optical resonator 12 composed of semiconductor layers. The semiconductor layers of the optical resonator 12 are arranged on a substrate 14 which is likewise made of a semiconductor material. In the finished VCSEL 10, the substrate 14 can further also have been removed. The semiconductor layers of the optical resonator 12 are grown on the substrate 14 by means of epitaxy, which is familiar to the person skilled in the art.

[0032] The optical resonator 12 has a first Bragg mirror 16 and a second Bragg mirror 18. The first and second Bragg mirrors 16 and 18 are also referred to as DBR (English: Distributed Bragg Reflector). Each of the two Bragg mirrors 16 and 18 is composed of a sequence of pairs of semiconductor layers, wherein each pair has one layer with a higher refractive index 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 for generating 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 arranged closer to the second Bragg mirror 18. The second active layer 24 constitutes the last active layer before the second Bragg mirror 18. The active layers 22 and 24 can in particular each be configured as a multiple quantum well structure (MQW; English: Multiple Quantum Wells).

[0034] A first oxide partition 26 is arranged between the first active layer 22 and the second active layer 24. The oxide partition 26 is formed by a layer which is easily oxidizable. For example, a layer with a high arsenic content of aluminum arsenide (AlAs) is a good and controllably oxidizable material. The oxidation of this layer is carried out to a specific oxidation range, so that a central region 28 of the oxide partition 26 is not oxidized. The central region 28 thus forms an aperture or partition for the current driven through the active region 20, while the oxidized outer regions of the layer act as an insulator. The oxide partition 26 thus serves to limit the current.

[0035] The first tunnel diode 30 is adjacent, in particular directly adjacent, to the first oxide partition 26. The tunnel diode 30 between the first active layer 22 and the second active layer 24 leads to an increase in light efficiency from both active layers 22 and 24. The second oxide partition 32 is adjacent, in particular directly adjacent, to the second active layer 24 and is similar in its material composition and function to the first oxide partition 26. The second oxide partition 32 has a central region 29 which acts as a current aperture. Following the second oxide partition 32 is a second tunnel diode 34, which has the same material composition as the first tunnel diode 30.

[0036] It has been found that, in the absence of the additional or second tunnel diode 34, the oxidation rate in the production of the second oxide partition 32 is significantly higher than the oxidation rate in the production of the first oxide partition 26. Thus, in the absence of the second tunnel diode 34, the size of the aperture in the respective central regions 28 and 29 would be different. It has been shown in experiments that, when the second tunnel diode 34 is present in the semiconductor multilayer structure, the oxidation rates in the production of the first oxide partition 26 and the second oxide partition 32 match one another. By providing the second tunnel diode 34, the symmetry of the layer structure in the regions of the oxide partitions 26 and 32 is improved, in other words, the surroundings of the second oxide partition 32 appear to be identical to the surroundings of the first oxide partition 26 due to the additional tunnel diode 34.

[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-type layer and the other of which is a highly doped n-type layer. The respective p-type doped layer is here facing the first Bragg mirror 16, and the n-type doped layer is facing the second Bragg mirror 18. Due to the additional provision of the second tunnel diode 34, the second Bragg mirror 18 thus becomes an n-type Bragg mirror.

[0038] The upper-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 have a respective doping of at least 1 E19 cm -3 .

[0039] Therefore, 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 assembly consisting of the second active layer 24, the second oxide spacer 32 and the second tunnel diode 34 is identical to the layer sequence of the assembly consisting of the first active layer 22, the first oxide spacer 26 and the first tunnel diode 30. In the embodiment in Figure 1 In the embodiment in

[0040] , the active region 20 has the following layer sequence, seen from the first Bragg mirror: first active layer 22 - first oxide spacer 26 - first tunnel diode 30 - second active layer 24 - second oxide spacer 32 - second tunnel diode 34. The sequence of the first oxide spacer 26 - first tunnel diode 30 can also be reversed, wherein, preferably, the sequence of the second oxide spacer 32 - second tunnel diode 34 is then reversed as well.

[0041] The first tunnel diode 30 and the second tunnel diode 34 are, for example, composed of very thin gallium arsenide (GaAs) layers. They can have a thickness of 10-30 nm, for example. As a dopant of the highly doped p-type layer of the tunnel diodes 30 and 34, carbon can be used, for example, and as a dopant of the highly doped n-type layer of the tunnel diodes 30 and 34, tellurium can be used.

[0042] In this embodiment, the second Bragg mirror is used for coupling out the generated laser radiation. For this purpose, the contact 36 can be configured 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 has less than twenty mirror pairs. The uppermost layer of the second Bragg mirror or an additional layer on the second Bragg mirror 18 can be configured as an n-doped contact layer.

[0043] In one specific example, the semiconductor multilayer structure of the VCSEL 10 can be composed as follows: The semiconductor multilayer structure can be based on the aluminum gallium arsenide-gallium arsenide (AlGaAs / GaAs) material system. The substrate 14 can be a gallium arsenide substrate, comprising an n-doped growth layer. The first active layer 22 and the second active layer 24 can be configured as a multiple quantum well structure (MQW). The oxide spacers 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 by two thin, highly doped gallium arsenide layers, wherein, seen from the substrate 14, the first gallium arsenide layer is p-doped and the second gallium arsenide layer is n-doped. The doping concentration is preferably greater than 1 E19 cm -3for example 1 E20 cm -3 The dopant for the p-doped layer of the tunnel diodes 30 and 34 is, for example, carbon, and the dopant for the n-doped layer of the tunnel diodes 30 and 34 is, for example, tellurium. The second Bragg mirror 18 is composed of n-doped semiconductor layers, wherein the second Bragg mirror has, for example, less 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 configured as a metal contact.

[0044] Figure 2 A further embodiment of a VCSEL 10' is shown, wherein the active region 20 of the VCSEL 10' has, in addition to the first active layer 22 and the second active layer 24, a third active layer 40 arranged on the side of the first active layer 22 facing away from the second active layer 24. Figure 1 Elements of the VCSEL 10 which are identical, similar or comparable to elements of the VCSEL 10' are provided with the same reference signs as in the VCSEL 10. Figure 1 Elements of the VCSEL 10 which are identical, similar or comparable to elements of the VCSEL 10' are provided with the same reference signs as in the VCSEL 10.

[0045] In the following only the differences between the VCSEL 10' and the VCSEL 10 are described.

[0046] The active region 20 of the VCSEL 10' has, in addition to the first active layer 22 and the second active layer 24, a third active layer 40 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, which can each be configured as a multi-quantum well structure. Between the third active layer 40 and the first active layer 22 a third oxide spacer 42 and a third tunnel diode 44 are arranged. The oxide spacer 42 has a central region 31 which acts as a current aperture.

[0047] The layer sequence of the assembly consisting of the third active layer 40, the third oxide spacer 42 and the third tunnel diode 44 is preferably identical to the layer sequence of the assembly consisting of the first active layer 22, the first oxide spacer 26 and the first tunnel diode 30. Thus, in total a VCSEL with stacked p-n junctions or a VCSEL with multiple active layers is created, here with a total of three active layers, with a high overall symmetry of the layer arrangement in the active region. As could be shown in experiments, without the additional tunnel diode 34 different oxidation rates occur when manufacturing the oxide spacers 26, 32 and 42. It has been found that, without the additional tunnel diode 34, the oxidation rate when manufacturing the oxide spacers 42 and 26 is lower than the oxidation rate when manufacturing the oxide spacer 32. This is changed by the additional tunnel diode 34 and the thereby resulting higher symmetry of the arrangement. With the additional tunnel diode 34, the same oxidation rate occurs when manufacturing all oxide spacers 32, 26 and 42 and, in turn, the same size of the current aperture in the respective central regions 28, 29, 31 of the oxide spacers 26, 32, 42.

[0048] In the method for manufacturing 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 means of epitaxy. The layers for producing the oxide spacers 26 and 32 or 42 are oxidized in a later process step to manufacture the oxide spacers 26 and 32 or 42. Subsequently, the contacts 36 are applied on the semiconductor multilayer structure.

Claims

1. A vertical cavity surface emitting semiconductor laser having a semiconductor multilayer structure with an optical resonator (12) composed of 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 active layer 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 between the first active layer (22) and the second active layer (24) a first oxide spacer (26) and a first tunnel diode (30) for limiting the current are arranged, wherein on the side of the second active layer (24) facing away from the first active layer (22) a second oxide spacer (32) is arranged, wherein on the side of the second active layer (24) facing away from the first active layer (22) a second tunnel diode (34) is arranged.

2. The vertical cavity surface emitting semiconductor laser according to claim 1, wherein The layer sequence of the assembly consisting of the second active layer (24), the second oxide spacer (32) and the second tunnel diode (34) is identical to the layer sequence of the assembly consisting of the first active layer (22), the first oxide spacer (26) and the first tunnel diode (30).

3. The vertical cavity surface emitting semiconductor laser according to claim 1 or 2, wherein The layer sequence of the assembly consisting of the first and second active layers (22, 24), the first and second oxide spacers (26, 32), the first and second tunnel diodes (30, 34) is as follows, viewed from the first Bragg mirror (16): first active layer (22) - first oxide spacer (26) - first tunnel diode (30) - second active layer (24) - second oxide spacer (32) - second tunnel diode (34).

4. The vertical cavity surface emitting semiconductor laser according to any one of claims 1 to 3, wherein The first and the second tunnel diodes (30, 34) have at least 1E19 cm -3 doped highly doped n-type layers and highly doped p-type layers, respectively, wherein the highly doped p-type layer of the second tunnel diode (34) faces the second active layer (24) and the highly doped n-type layer of the second tunnel diode (34) faces the second Bragg mirror (18).

5. The 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-type Bragg mirror.

6. The vertical cavity surface emitting semiconductor laser according to any one of claims 1 to 5, wherein The active region (20) has at least one third active layer (40), which is arranged on the side of the first active layer (22) facing away from the second active layer (24), and wherein between the third and the first active layer (40, 22) a third oxide spacer (42) and a third tunnel diode (44) are arranged.

7. The vertical cavity surface emitting semiconductor laser according to claim 6, wherein The layer sequence of the assembly consisting of the third active layer (40), the third oxide spacer (42) and the third tunnel diode (44) is identical to the layer sequence of the assembly consisting of the first active layer (22), the first oxide spacer (26) and the first tunnel diode (30).

8. A method for manufacturing a vertical cavity surface emitting semiconductor laser, having the following steps: Manufacturing a semiconductor multilayer structure having an optical resonator (12) composed of a semiconductor layer, 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 with a first active layer 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 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), and the first and second oxidizable layers are oxidized to produce a first oxide spacer (26) and a second oxide spacer (32) for limiting the current.

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