Vertical cavity surface emitting semiconductor laser and method for producing same

Abstract

The invention relates to a vertical cavity surface emitting semiconductor laser (VCSEL) which has a multi-layered semiconductor structure having an optical resonator (12) composed of semiconductor layers, the optical resonator (12) comprising a first Bragg mirror (16), a second Bragg mirror (18), and an active region (20) between the first Bragg mirror (16) and the second Bragg mirror (18) for generating laser radiation. The active region (20) has a plurality of active layers (22, 24, 40) comprising a first and at least a second active layer (22, 24), the second active layer (24) being the last active layer in front of the second Bragg mirror (18). A first oxide aperture (26) for current constriction and a first tunnel diode (30) are located between the first active layer (22) and the second active layer (24), a second oxide aperture (32) being located on a side of the second active layer (24) which is remote from the first active layer (22). A second tunnel diode (34) is located on the side of the second active layer (24) which is remote from the first active layer (22). The invention also relates to a method for producing the VCSEL.

Description

Vertical cavity surface emitting semiconductor laser and method for producing such a laser

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

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

[0003] Vertical cavity surface-emitting semiconductor lasers, abbreviated as VCSELs (Vertical Cavity Surface Emitting Lasers), are used, for example, as radiation sources in sensor technology or communications engineering. VCSELs typically have a semiconductor multilayer structure in which semiconductor layers are grown epitaxially on a semiconductor substrate in a stacked arrangement. The semiconductor multilayer structure typically has a first Bragg mirror, an active region, and a second Bragg mirror, which together form an optical resonator. A Bragg mirror is also abbreviated to a DBR (Distributed Bragg Reflector). VCSELs typically have an oxidized region in the optical resonator, which has a semiconductor layer that is oxidized to a certain oxidation width to form a current aperture in the resonator, also called an oxide aperture.The semiconductor layer intended for oxidation is, for example, a layer with a high AlAs (aluminum arsenide) content, which can be specifically oxidized to AI2O3 up to a certain oxidation range.

[0004] VCSELs are known that have an active region with a plurality of active layers, with a tunnel diode arranged between each adjacent active layer. For example, such a structure can have three active layers and two tunnel diodes. Such an arrangement can significantly increase the power of the VCSEL, for example even threefold in pulsed operation. In other words, such an arrangement can generate 3x photons from one charge carrier. In such a design of the optical resonator with multiple active layers, an oxide diaphragm is required near each active layer to channel or confine the current. A layer with a high aluminum arsenide (AlAs) content, for example, a content of 90-100% AlAs, is typically used to produce the oxide diaphragm.It has been found that the incorporation of tunnel diodes between the active layers alters the crystal properties of the layers intended for oxidation, for example, inducing intrinsic defects such as vacancies, and thereby alters the oxidation rate of the oxidizable layer(s), resulting in undesirably different current aperture sizes. In other words, the oxidation rates of the oxidizable layers deviate significantly from the situation without a tunnel diode when tunnel diodes are incorporated. This effect is particularly pronounced for oxide apertures arranged near a tunnel diode. are, while an oxide aperture that is not located near a tunnel diode oxidizes significantly faster.

[0005] Document US 2021 / 0104872 A1 describes various designs of VCSELs that have one or more tunnel diodes in 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 has a high doping level, but the oxidation rate is relatively stable during the oxidation process. 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 approach described in this document for achieving a uniform oxidation rate during the oxidation of the oxidizable layer(s) is based on additional dopants 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 surface-emitting semiconductor laser with a vertical cavity in which the oxidation rate of all existing oxidizable layers for producing oxide apertures is uniform and thus a uniform oxidation width of all oxide apertures is ensured, wherein the manufacturing effort of the VCSEL remains 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 comprising 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 active layer and the second active layer, a first oxide diaphragm for current confinement and a first tunnel diode are arranged, wherein a second oxide diaphragm is arranged on a The second active layer 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 enables a higher light yield due to the at least two active layers with a tunnel diode between these active layers.

[0010] To solve the problem of different oxidation rates during the production of the first oxide diaphragm and the second oxide diaphragm, 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. The additional (second) tunnel diode on the side of the second active layer facing away from the first active layer ensures that the environment of each oxide diaphragm can be designed identically. Experiments have shown that the additional tunnel diode equalizes the oxidation rates during the production of the oxide diaphragms and that the VCSEL can be processed without complex additional measures. Fine adjustment of the composition and special control of the oxidation rates, both of which prove to be difficult, are not required in the production of the VCSEL according to the invention.Rather, the invention solves the problem of different oxidation rates by increasing the symmetry of the arrangement of active layers, oxide diaphragms 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 is more than compensated by the significantly simplified and better controllable production of the VCSEL.

[0012] The first Bragg mirror may 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 comprising the second active layer, the second oxide diaphragm and the second tunnel diode is the same as the layer sequence of the arrangement comprising the first active layer, the first oxide diaphragm and the first tunnel diode.

[0015] In this embodiment, a particularly high symmetry of the arrangement of active layers, oxide apertures and tunnel diodes is created, whereby a particularly good alignment of the oxidation rates in the first and second oxidizable layers for producing the first and second oxide apertures and thus uniform current apertures are created.

[0016] An advantageous sequence, viewed from the first Bragg mirror of the first active layer, is: first active layer – first oxide aperture – first tunnel diode – second active layer – second oxide aperture – second tunnel diode. Another possible sequence is: first active layer – first tunnel diode – first oxide aperture – second active layer – second tunnel diode – second oxide aperture.

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

[0018] Due to high doping, the first and second tunnel diodes advantageously have very low resistance. As is usual with tunnel diodes, the first and second tunnel diodes are operated in the reverse direction. The highly doped n-layer and the highly doped p-layer are very thin layers 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, in which one Bragg mirror is an n-mirror while the other is a p-Bragg mirror, in this embodiment of the VCSEL according to the invention both Bragg mirrors are n-mirrors. Accordingly, an n-contact can be arranged on the second Bragg mirror to supply current to the VCSEL. Although the n-type contact between both Bragg mirrors is unusual, it has the advantage of reducing optical losses not only in one but in both mirrors. 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 a third active layer which is arranged on the side of the first active layer facing away from the second active layer, and wherein a third oxide diaphragm 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 above-mentioned advantages of a high light yield of the VCSEL with simultaneous 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 comprising the third active layer, the third oxide diaphragm and the third tunnel diode is preferably the same as the layer sequence of the arrangement comprising the first active layer, the first oxide diaphragm and the first tunnel diode.

[0022] In this embodiment, the symmetry is further increased even when the active region is designed with three active layers, and the oxidation rates in the oxidizable layers are adjusted as much as possible.

[0023] It is understood that the active region may have more than three active layers, more than three oxide diaphragms and / or more than three tunnel diodes.

[0024] Preferably, the first and second and optionally the third tunnel diode comprise GaAs (gallium arsenide) layers.

[0025] Further 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 constructed from semiconductor layers, wherein the optical resonator has a first Bragg mirror, a second Bragg mirror and, between the first Bragg mirror and the second Bragg mirror, an active region for generating laser radiation, 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.According to the invention, 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, a second oxidizable layer is arranged on a 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 diaphragm and a second oxide diaphragm 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 combination specified in each case, but also in other combinations or on their own, without departing from the scope of the present invention.

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

[0031] Fig. 1 shows a vertical-cavity surface-emitting semiconductor laser, referred to as a VCSEL for short, designated by the general reference numeral 10. The VCSEL 10 has a semiconductor multilayer structure comprising an optical resonator 12 constructed 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 can 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 familiar 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 arranged 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 formed, in particular, as multiple quantum well (MQW) structures.

[0034] A first oxide aperture 26 is arranged between the first active layer 22 and the second active layer 24. The oxide aperture 26 is formed from a layer that can be easily oxidized. For example, a layer with a high aluminum arsenide (AlAs) content is a material that can be easily and controllably oxidized. The layer is oxidized up to a certain oxidation width, so that a central region 28 of the oxide aperture 26 is not oxidized. The central region 28 thus forms an aperture or diaphragm through which the current passes to drive the active region 20, while the oxidized outer region of the layer has an insulating effect. The oxide aperture 26 thus serves to confine the current.

[0035] Adjacent to the first oxide diaphragm 26, in particular directly adjacent, is a first tunnel diode 30. The tunnel diode 30 between the first active layer 22 and the second active layer 24 increases the light yield from the two active layers 22 and 24. Adjacent to the second active layer 24, in particular directly adjacent, is a second oxide diaphragm 32, which is similar 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. The second oxide diaphragm 32 is followed by a second tunnel diode 34, which has the same material composition as the first tunnel diode 30.

[0036] It has been found that without the additional or second tunnel diode 34, the oxidation rate during the production of the second oxide aperture 32 is significantly higher than the oxidation rate during the production of the first oxide aperture 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 production of the first oxide aperture 26 and the second oxide aperture 32 are aligned when the second tunnel diode 34 is present in the semiconductor multilayer structure. By providing the second tunnel diode 34, the symmetry of the layer structure in the region of the oxide apertures 26 and 32 is increased. In other words, the area surrounding the second oxide aperture 32, thanks to the additional tunnel diode 34, looks exactly like the area surrounding the first oxide aperture 26.

[0037] The first tunnel diode 30 and the second tunnel diode 34 can each be constructed from 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 layer faces the first Bragg mirror 16, and the n-type layer faces the second Bragg mirror 18. Due to the additionally provided second tunnel diode 34, the second Bragg mirror 18 thus becomes an n-type Bragg mirror.

[0038] A top-side contact 36 for contacting the VCSEL 10 is accordingly an n-contact. The highly doped n-layers and the highly doped p-layers of the first tunnel diode 30 and the second tunnel diode 34 can have a respective 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 comprising the second active layer 24, the second oxide diaphragm 32, and the second tunnel diode 34 is identical to the layer sequence of the arrangement comprising the first active layer 22, the first oxide diaphragm 26, and the first tunnel diode 30. In the exemplary embodiment in Fig. 1, the active region 20, as 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 of the first oxide diaphragm 26 - first tunnel diode 30 can also be reversed, in which case the sequence of the second oxide diaphragm 32 - second tunnel diode 34 is preferably also reversed.

[0041] The first tunnel diode 30 and the second tunnel diode 34 are constructed, for example, from very thin gallium arsenide (GaAs) layers. For example, they can have a thickness of 10-30 nm. Carbon, for example, can be used as dopants for the highly doped p-layers of the tunnel diodes 30 and 34, and tellurium can be used for the highly doped n-layers of the 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 1 E19 cm -3 , for example 1E20 cm -3Dopants for the p-doped layers of tunnel diodes 30 and 34 are, for example, carbon, and for the n-doped layers of 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. 2 shows a further embodiment of a VCSEL 10', wherein elements of the VCSEL 10' that are identical, similar, or comparable to elements of the VCSEL 10 in Fig. 1 are provided with the same reference numerals as in Fig. 1.

[0045] Only the differences between the VCSEL 10' and the VCSEL 10 are described below.

[0046] The active region 20 of the VCSEL 10' additionally has 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 formed as a multiple quantum well structure. A third oxide aperture 42 and a third tunnel diode 44 are arranged between the third active layer 40 and the first active layer 22. The oxide aperture 42 has a central region 31 that acts as a current aperture.

[0047] The layer sequence of the arrangement comprising the third active layer 40, the third oxide diaphragm 42, and the third tunnel diode 44 is preferably identical to the layer sequence of the arrangement comprising the first active layer 22, the first oxide diaphragm 26, and the first tunnel diode 30. Overall, this results in a VCSEL with stacked pn junctions or a VCSEL with multiple active layers, here with a total of three active layers with a high overall symmetry of the layer arrangement in the active region. Without the additional tunnel diode 34, as demonstrated in experiments, different oxidation rates would occur during the production of the oxide diaphragms 26, 32, and 42. It has been found that without the additional tunnel diode 34, the oxidation rates during the production of the oxide diaphragms 42 and 26 are lower than the oxidation rate during the production of the oxide diaphragm 32. This changes due to the additional 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 regions 28, 29, 31 of the oxide apertures 26, 32, 42 are achieved during the production of all oxide apertures 32, 26 and 42.

[0048] In one 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 epitaxy. The layers used to create the oxide apertures 26 and 32, or 42, are oxidized in a later process step to produce the oxide apertures 26 and 32, or 42. The contact 36 is then applied to the semiconductor multilayer structure.

Claims

Patent claims 1. A vertical-cavity surface-emitting semiconductor laser comprising a semiconductor multilayer structure having an optical resonator (12) constructed from semiconductor layers, wherein the optical resonator (12) comprises 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) comprises 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 comprising the second active layer (24), the second oxide diaphragm (32) and the second tunnel diode (34) is the same as the layer sequence of the arrangement comprising the first active layer (22), the first oxide diaphragm (26) and the 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 1 E19 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) has 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 comprising 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 comprising 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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