Vertical Cavity Surface Emitting Laser with Integrated Photodiode

Abstract

A vertical cavity surface emitting laser (VCSEL) is provided, comprising an optical resonator (18). The optical resonator (18) comprises a first reflector (12), an active region (14) for generating laser light, and a second reflector (16), wherein the active region (14) is arranged between the first reflector and the second reflector (12, 16). A photodiode (34) is integrated in the optical resonator (18). The photodiode (34) comprises an absorption region (20) having a plurality of absorption layers, wherein the plurality of absorption layers are configured to absorb the generated laser light, wherein the absorption layers are arranged to be spaced apart from each other by a distance d, wherein the distance d satisfies the condition: d=(2k‑1)λ / (4m), wherein λ is the wavelength of the laser light in the absorption region (20), and k and m are natural numbers greater than or equal to 1. An optical sensor and a method for producing a VCSEL are also described.

Description

Technical Field

[0001] The invention relates to a vertical cavity surface emitting laser (VCSEL) with an integrated photodiode. The invention also relates to an optical sensor comprising such a VCSEL. Furthermore, the invention relates to a method for producing such a VCSEL. Background Art

[0002] VCSELs with integrated photodiodes, often denoted ViPs, can be used as small sensors to measure, for example, distance, displacement, velocity, and even particle density. All of these measurements can be based on the principle of self-mixing interferometry (SMI). Optical sensors of this type can be simple enough to be integrated into mobile phones.

[0003] ViPs can be designed so that they operate at wavelengths in the range of about 850 nm to about 1200 nm. For such long wavelength operation of ViPs, strained semiconductor materials, such as InGaAs, must be used for the absorption region of the photodiode. However, when using strained semiconductor materials as the absorption layer of the photodiode, the thickness of such absorption layers is limited to only a few tens of nm due to the material strain of such materials and the ability to grow defect-free layers. In order to achieve high absorbance with such thin layers, these layers are usually placed at the position where the light intensity is maximum within the epitaxial stack of the VCSEL. However, small layer thickness deviations during the epitaxial growth of the epitaxial stack of the VCSEL will change the light intensity distribution within the VCSEL and thus lead to absorbance variations and corresponding performance variations between ViPs on the same wafer or between separate wafer production batches. In other words, the absorption capacity of the photodiode and the light detection capability of conventional ViPs are largely related to production tolerances. Summary of the invention

[0004] It is an object of the present invention to provide an improved vertical cavity surface emitting laser with an integrated photodiode, wherein the absorption of the photodiode is less dependent on production tolerances.

[0005] Another object is to provide an optical sensor having improved light detection properties, thereby enabling more accurate measurements to be made with the sensor.

[0006] Another object is to provide a method of producing a VCSEL with an integrated photodiode.

[0007] According to a first aspect, there is provided a vertical cavity surface emitting laser (VCSEL), comprising:

[0008] An optical resonator comprising a first reflector, an active region for generating laser light, and a second reflector, wherein the active region is arranged between the first reflector and the second reflector;

[0009] A photodiode is integrated in an optical resonator, the photodiode comprising an absorption region having a plurality of absorption layers, the plurality of absorption layers being configured to absorb generated laser light, wherein the absorption layers are arranged to be spaced apart from each other by a distance d, the distance d satisfying the condition: d=(2k-1)λ / (4m), wherein λ is the wavelength of the generated laser light in the absorption region, and k and m are natural numbers greater than or equal to 1.

[0010] The photodiode of the VCSEL according to the present invention comprises at least two absorption layers. The absorption layers are configured to absorb light in a wavelength range that includes at least the wavelength of the laser light generated by the active region. The at least two separate absorption layers are arranged at a distance d from each other, the distance d being an odd multiple of the wavelength of the laser light in the optical resonator divided by 4, so that the absorbance of the photodiode is less dependent on production tolerances. The distance d can be taken as the center-to-center distance from a single absorption layer to an adjacent absorption layer. The center of an absorption layer is to be understood as the middle of the absorption layer relative to its thickness. The space between the separate absorption layers can be filled with non-absorbing or at most low-absorbing semiconductor layers.

[0011] The above conditions for the distance d between the individual absorption layers are to be understood within typical production tolerances of 20%.

[0012] Due to the reflectors on both sides of the active area, the light intensity in the VCSEL forms a standing wave pattern. The maxima of this pattern are at a distance from each other that corresponds to half the laser wavelength in the optical resonator. The laser wavelength in the optical resonator, in particular in the absorption region of the photodiode, is understood to be the laser wavelength in the semiconductor material of the optical resonator or in particular in the absorption region of the photodiode, which is the laser wavelength in air divided by the refractive index of the semiconductor material used in the layer stack forming the absorption region of the optical resonator, in particular the photodiode. For example, when using the AlGaAs / GaAs material system to build a resonator, the refractive index is about 3.5 at wavelengths above 850 nm. InGaAs has a very similar refractive index.

[0013] If the photodiode has only one thin absorption layer, the absorption of the light generated in the active area is optimal when the absorption layer is arranged at the maximum intensity of the standing wave pattern. However, the epitaxial production process may have some extension deviations and thickness deviations from the theoretical design both across the wafer and between individual wafer production batches. These thickness deviations cause the standing wave pattern to shift relative to the photodiode, resulting in a reduction in the overlap between the maximum or highest light intensity and the absorption layer. The result is a significant reduction in absorption. Therefore, production tolerances lead to large variations in the photodiode signal across the wafer or between different wafer production batches.

[0014] As provided by the present invention, when the photodiode has at least two absorption layers spaced apart from each other by a distance d according to the above conditions, the reduction in absorbance caused by the relative displacement of the standing wave pattern with respect to the photodiode becomes smaller than that of a design having only one absorption layer, as will be described herein. Therefore, the VCSEL according to the present invention is much less sensitive to production tolerances.

[0015] The absorption layer of the photodiode may be monolithically integrated in one of the first reflector and the second reflector.The first reflector and / or the second reflector may be configured as a distributed Bragg reflector (DBR).

[0016] Preferably, the photodiode may comprise an even number of absorption layers. An even number of absorption layers is particularly suitable for reducing the sensitivity of the photodiode to tolerances in VCSEL production. The number of individual absorption layers may be 2, 4, 6 or more. 2, 4, 6 or more individual absorption layers may be arranged within half the wavelength of the laser light in the absorption region. 2, 4, 6 or more individual absorption layers may be present within the entire wavelength of the laser light in the absorption region.

[0017] The photodiode may include at least two absorption layers arranged at a distance d from each other, wherein k=1 and m=1. In this configuration, the at least two individual absorption layers are arranged to be spaced apart by a quarter of a wavelength within the optical resonator. In this case, the at least two absorption layers are arranged within half of the wavelength of the laser light within the absorption region.

[0018] In an alternative configuration, the photodiode may include at least two absorption layers arranged at a distance d from each other, where k = 2 and m = 1. In this configuration, the at least two absorption layers are spaced from each other by three quarters of the wavelength in the absorption region. Thus, the at least two absorption layers are arranged within the entire wavelength of the wavelength in the absorption region.

[0019] In another configuration, the photodiode may include at least four absorption layers spaced two by a distance d from each other, where k = 1 and m = 2. In this configuration, two adjacent layers of the at least four absorption layers are spaced one eighth of a wavelength within the optical resonator. In this configuration, the at least four absorption layers are arranged within half of a wavelength within the absorption region.

[0020] The absorption layer of the photodiode may comprise a strained semiconductor material. The invention is particularly advantageous when a strained semiconductor material, such as InGaAs, is used as the absorption layer. As mentioned above, such strained semiconductor materials only allow the production of thinner layers. Thus, the layer thickness of each absorption layer may be in the range of 25% to 200%, preferably in the range of 70% to 100%, of the critical layer thickness of the strained semiconductor layer. The critical layer thickness is generally defined as the limit of the layer thickness that can still elastically accommodate strain. For example, InGaAs with an indium content of 16.5% has a critical layer thickness of about 18 nm, as described in Fritz et al. in "Dependence of critical layer thickness on strain for InGaAs". x Ga 1-x As / GaAs strained-layer superlattices", Appl. Phys. Lett. Vol. 46 (10), pages 967-969, 1985".

[0021] In practical embodiments, the absorption layer may have a single layer thickness in the range from 5 nm to 50 nm, preferably in the range from 10 nm to 35 nm, for example in the range from 15 nm to 30 nm.

[0022] Preferably, the absorption layer may include In x Ga 1-x As, where x ranges from 0.05 to 0.3, depending on the wavelength output by the VCSEL. The active region may also include InGaAs, where the indium content in the single absorption layer is preferably higher than the indium content in one or more layers of the active region. The active region may include GaAs instead of InGaAs.

[0023] When one of the first reflector and the second reflector is configured as a DBR, the photodiode may be monolithically integrated in the layer stack of the DBR, thereby dividing the DBR into two or more parts. The DBR may include AlGaAs or GaAs as material system.

[0024] At least one layer comprising AlGaAs or GaAs or GaAsP may be arranged between adjacent absorption layers. For example, when the absorption layer is based on a strained semiconductor material such as InGaAs, GaAs y P 1-y Layers such as y>0.5, for example y>0.8, may be advantageous because they partially, completely or even excessively compensate for the strain introduced by the absorbing layer.

[0025] Preferably, at least one first layer including AlGaAs or GaAs, at least one second layer including AlGaAs or GaAs, and at least one third layer including GaAsP sandwiched between the first layer including AlGaAs or GaAs and the second layer may be arranged between adjacent absorption layers.

[0026] Further strain compensation layers may be arranged on one or both sides of the absorption region of the photodiode.

[0027] Preferably, the emission wavelength of the laser output by the VCSEL is in the range of 850 nm to 1200 nm.

[0028] The VCSEL may further comprise a contact arrangement arranged to provide a drive current to electrically pump the optical resonator.

[0029] According to a second aspect, there is provided an optical sensor comprising the vertical cavity surface emitting laser according to the first aspect.

[0030] The optical sensor may be included in the mobile communication device.

[0031] Optical sensors can be used for distance detection, speed detection, particle density detection, gesture control, and especially for all sensor applications based on self-mixing interferometry.

[0032] According to a third aspect, there is provided a method of producing a vertical cavity surface emitting laser (VCSEL), the method comprising:

[0033] Providing an optical resonator, the optical resonator comprising a first reflector, an active region for generating laser light, and a second reflector, wherein the active region is arranged between the first reflector and the second reflector;

[0034] A photodiode is integrated in an optical resonator, the photodiode comprising an absorption region having at least two absorption layers, wherein the absorption layers are arranged to be spaced apart from each other by a distance d, wherein the distance d satisfies the condition: d=(2k-1)λ / (4m), wherein λ is the wavelength of the laser in the absorption region, and k and m are natural numbers greater than or equal to 1.

[0035] The steps of the method may include depositing layers for forming the first and second reflectors, the active region and the photodiode by an epitaxial method such as MOCVD, MBE or the like.

[0036] The VCSEL may comprise a substrate on which the layer stack of the VCSEL with the integrated photodiode is grown. After production of the VCSEL, the substrate may be removed.

[0037] VCSELs can be top emitters or bottom emitters.

[0038] It should be understood that the VCSEL and the method of producing a VCSEL according to any embodiment described above and described below have similar and / or identical embodiments. Further advantageous embodiments are defined below. BRIEF DESCRIPTION OF THE DRAWINGS

[0039] These and other aspects of the invention will be apparent from and elucidated with reference to the embodiments described hereinafter. In the accompanying drawings:

[0040] Figure 1 shows a schematic diagram of a VCSEL with a monolithically integrated photodiode;

[0041] Figure 2 shows a standing wave pattern of light intensity within a VCSEL;

[0042] Figure 3 A single thin absorption layer of a photodiode is shown placed at the location of maximum light intensity within a VCSEL;

[0043] Figure 4 An embodiment of a photodiode of a VCSEL having two absorption layers is shown;

[0044] Figure 5 Shows Figure 4 two absorption layers of a photodiode in, wherein the standing wave pattern undergoes a first shift relative to the absorption layer;

[0045] Figure 6 Shows Figure 4 two absorbing layers in the embodiment of the present invention, wherein the standing wave pattern is secondly offset relative to the absorbing layers;

[0046] Figure 7 Graphs showing the effect of a shift in the standing wave pattern relative to a photodiode having one absorption layer and relative to a photodiode having two absorption layers;

[0047] Figure 8 An embodiment of a photodiode of a VCSEL having four absorption layers is shown;

[0048] Fig. 9Another embodiment of a photodiode of a VCSEL having two absorption layers is shown;

[0049] Fig.10 A schematic diagram showing an embodiment of an absorption region of a photodiode of a VCSEL;

[0050] Fig.11 shows a schematic diagram of an optical sensor including a VCSEL;

[0051] Fig.12 A schematic diagram showing a mobile communication device including an optical sensor; and

[0052] Fig.13 A schematic diagram showing the process flow of a method for producing a VCSEL. DETAILED DESCRIPTION

[0053] The present invention proposes a VCSEL with a monolithically integrated photodiode, wherein the photodiode comprises at least two absorption layers, which are arranged at a distance from each other that is an odd multiple of the laser wavelength in the absorption region of the photodiode of the VCSEL divided by 4, so that the absorption of the photodiode is less dependent on production tolerances. This will be explained in more detail below.

[0054] refer to Figure 1 , the overall design of a VCSEL with a monolithically integrated photodiode will be described. Figure 1 A VCSEL 10 is shown. The VCSEL 10 includes a first reflector 12, an active region 14 for laser emission, and a second reflector 16. The first reflector 12 and the second reflector 16 may be configured as a distributed Bragg reflector (DBR). The active region 14 is arranged between the first reflector 12 and the second reflector 16. The first reflector 12, the active region 14, and the second reflector 16 form an optical resonator 18. The VCSEL 10 also includes a photodiode 34, which includes a light absorbing region 20.

[0055] The layer stack of the first reflector 12, the active region 14, the second reflector 16 and the light absorbing region 20 of the photodiode may be epitaxially grown on the substrate 22. The layers of the first reflector 12 and the second reflector 16 may comprise a semiconductor material doped with AlGaAs.

[0056] The VCSEL 10 may be a top emitter, i.e., laser light is emitted by the VCSEL 10 as indicated by arrow 24. In the case of a top emitter, the reflectivity of the first reflector 12 is lower than the reflectivity of the second reflector 16. In other embodiments, the VCSEL 10 may be configured as a bottom emitter, i.e., laser light is emitted on the substrate side of the VCSEL 10. In the case of a bottom emitter, the reflectivity of the second reflector 16 is lower than the reflectivity of the first reflector 12. The substrate 22 may be at least partially removed.

[0057] exist Figure 1 In the design shown, the light absorbing region 20 of the photodiode 34 is monolithically integrated in the second reflector 16. In other embodiments, the light absorbing region 20 may be integrated in the first reflector 12.

[0058] When the first reflector 12 and the second reflector 16 are configured as DBRs, each of the first reflector 12 and the second reflector 16 may have one or more than two layer pairs, wherein two layers in the layer pair have different refractive indices. Figure 1 The number of layers shown is schematic and exemplary only. Figure 2 The thickness of the layers shown are not drawn to scale.

[0059] The optical resonator 18 may also include one or more than two oxide holes (not shown).

[0060] The light absorbing region 20 of the photodiode 34 is embedded in the layer stack of the second reflector 16 , thereby dividing the second reflector 16 into a first portion 24 and a second portion 26 .

[0061] The substrate 22 and the second portion 26 of the second reflector 16 may be n-doped. The first portion 24 of the second reflector 16 may have an n-doped first region 28, an n-doped second region 30, and a p-doped third region 32. The n-doped second region 30, the p-doped third region 32, the light absorbing region 20, which is preferably the entire intrinsic (undoped) region, and the n-doped second portion 26 of the second reflector 16 form a photodiode 34. Thus, the photodiode 34 may be an npin photodiode formed by the layers of the light absorbing region 20 and the regions 30, 32, 26 of the second reflector 16.

[0062] The VCSEL 10 includes an electrical contact arrangement that may include a p-contact 36 located on top of the first reflector 12, an n-contact 38 located on the bottom of the substrate 22, and an additional n-contact 40 located on top of the region 28 of the second reflector 16. The p-contact may be formed as an annular electrode. The p-contact 36 may be arranged on a cap layer (not shown) on top of the first reflector 12. The n-contact 38 may be formed as a metallization at the bottom of the substrate 22. In the case where the VCSEL 10 is designed as a bottom emitter, the n-contact 38 may be formed as an annular electrode.

[0063] The p-contact 36 may form an anode of the VCSEL 10 and the n-contact 40 may form a cathode of the VCSEL 10. At the same time, the n-contact 40 may form an anode of the photodiode 34 and the n-contact 38 may form a cathode of the photodiode 34.

[0064] The VCSEL 10 is preferably designed to be able to output laser light in a wavelength range from 850 nm to 1200 nm. In order to obtain emission with such a long wavelength, an InGaAs material system may be preferred for the active region 14 and for the absorption region 20. For example, the active region 14 may include one or more InGaAs layers. The higher the indium content, the higher the laser emission wavelength. For example, an InGaAs composition with a 10% indium content can provide laser emission of about 950 nm at a temperature of about 300 K. For a smaller emission wavelength, such as 850 nm, the active region 14 may include one or more GaAs layers. The indium content of the absorption layer in the absorption region 20 is preferably higher than the indium content in the active region 14.

[0065] InGaAs is a strained semiconductor material. The strain depends on the amount of indium in the composition and limits the layer thickness that can be grown without defects to only a few tens of nanometers. This is small compared to the period of the typical variation of the light intensity in the VCSEL 10, which follows a standing wave pattern due to the mirrors 12, 16 on both sides of the active area 14, as shown in FIG. Figure 2 shown. Figure 2 The normalized light intensity within the optical resonator 18 of the VCSEL 10 is shown as a function of position z along the layer stack from the first to the second reflector of the VCSEL. The light intensity is highest in the active region and decreases towards the DBR 12 and DBR 16.

[0066] Figure 3 1 shows a diagram in which a single absorption layer 42 made of InGaAs with an indium content of about 15% and a thickness of 20 nm is placed centrally at the location of maximum light intensity in the optical resonator of the VCSEL. Figure 3As shown, the thickness of the single absorber layer 42 is much smaller than the typical variation in light intensity 44. When the single absorber layer 42 is centered relative to the maximum light intensity, as shown in FIG. Figure 3 As shown, the photodiode signal is maximum. However, when producing VCSELs, the epitaxial production process will have some extension deviations and thickness deviations from the theoretical design across the wafer and between individual wafer production batches. These thickness deviations will cause the standing wave pattern to shift relative to the absorption region 20 of the photodiode 34, resulting in a reduction in the overlap between the maximum light intensity and the absorption layer. The result is that the photodiode signal varies across the wafer and between different production batches. When the absorption layer 42 is made of a strained semiconductor material, such as InGaAs with an indium content, such as an indium content exceeding, for example, 10%, the increase in the thickness of the absorption layer 42 can be strictly limited.

[0067] Therefore, according to the present disclosure, it is proposed to construct the absorption region 20 of the photodiode 34 using more than one absorption layer, wherein the absorption layers are arranged such that the absorption layers are spaced apart from each other by a distance d that satisfies the condition: d=(2k-1)λ / (4m), where λ is the wavelength of the laser light generated in the optical resonator 18, and k and m are natural numbers greater than or equal to 1. This will refer to Figures 4 to 9 Explain in more detail.

[0068] exist Figures 4 to 9 In the example, the light intensity 48 is given by the square sine function (sin 2 x). For example, the period of the squared sine function is chosen to be 134 nm, which is typical for a VCSEL with an output emission wavelength of 940 nm. Note that the light intensity varies with a period of half the wavelength of the laser light inside the VCSEL cavity. Therefore, a period of 134 nm light intensity variation corresponds to a laser wavelength of 268 nm inside the VCSEL cavity.

[0069] Figure 4 An embodiment of a photodiode is shown, wherein the absorption region of the photodiode is included in Figure 4 4 and 5. The two absorption layers 50, 52 are shown in dashed lines in FIG. As an example, in this example, the thickness of each individual absorption layer of the absorption layers 50 and 52 is selected to be 15 nm, for example. The thick line 54 shows the overlap of the light intensity curve 48 with the absorption profile provided by the absorption layers 50 and 52. In this embodiment, the absorption layers 50, 52 are arranged at a distance d from each other, which is half the period of the light intensity variation, corresponding to a quarter of the wavelength λ in the absorption region. Figure 4 As shown, the distance d can be taken as the distance from the center of the thickness of the absorbent layer 50 to the center of the thickness of the absorbent layer 52. The distance d can also be taken as the distance from the beginning of the absorbent layer 50 to the beginning of the absorbent layer 52, or from the end of the absorbent layer 50 to the end of the absorbent layer 52 (in Figure 4 (read from left to right in the figure).

[0070] exist Figure 4 , the relative spatial relationship between the light intensity and the absorbing layers 50 and 52 is such that layer 50 starts at a minimum light intensity, while the absorbing layer 52 starts at a maximum of the standing wave pattern.

[0071] Figure 5 The same two absorption layers 50 and 52 are shown, wherein the absorption layers 50 and 52 are offset by 10 nm relative to each other and the light intensity curve 48 without changing the distance d. This offset may be caused by production tolerances. For example, Figure 4 The absorbent layers 50, 52 and Figure 5 The absorber layers 50, 52 in FIG. 5 can be absorber layers of ViP along the same wafer or absorber layers of ViP of different production batches. Figure 5 As shown, while the light intensity overlap with the absorption of absorption layer 52 is reduced and thus the absorbance of layer 52 is reduced, the light intensity overlap with the absorption of absorption layer 50 is increased and thus the absorbance of layer 50 is increased. Therefore, the effect of the shift of the standing wave pattern relative to absorption layers 50 and 52 on the total absorbance is reduced to only a few percent of absorbance change.

[0072] Figure 6 A photodiode with two absorption layers 50 and 52 is shown again, where the Figure 4 Compared to the situation in FIG. 4 , the relative displacement between the light intensity curve 48 and the absorber layers 50, 52 is now 30 nm. Likewise, although the overlap of the light intensity with the absorber layer 52 is further reduced, the overlap of the light intensity with the absorber layer 50 is further increased, so that the effect of the shift of the standing wave pattern on the overall overlap of the light intensity profile and the absorbance profile is again reduced to only a few percent of absorbance change.

[0073] This is different from the case with only one absorbing layer ( Figure 3 ) is very different. In the case of only one absorption layer, the same shift of the standing wave pattern relative to the photodiode 34 results in a reduction of absorbance by more than 40%, as shown in FIG. Figure 7 shown. Figure 7 It shows that, relative to Figure 3 The photodiode with a single absorption layer shown and the photodiode with a single absorption layer as shown Figures 4 to 6 Comparison of the changes in the overall overlap of the standing wave patterns of the photodiode with two absorption layers, 50 and 52, when the offset is 10 nm and 30 nm.

[0074] Figure 8Another embodiment of a photodiode having an absorption region including four absorption layers 56, 58, 60, and 62 is shown. Adjacent ones of the absorption layers 56, 58, 60, 62 are separated by a distance d that is one eighth of the wavelength of the laser light in the absorption region 20.

[0075] like Figures 4 to 6 As shown, the two absorption layers 50 and 52 are arranged within half of the laser wavelength of the absorption region. Figure 8 As shown, all four absorption layers 56, 58, 60, 62 are arranged within half the laser wavelength. In other embodiments not shown, six or more absorption layers may be arranged within half the laser wavelength.

[0076] Fig. 9 An embodiment of a photodiode comprising an absorption region having two absorption layers 64, 66 arranged within the entire wavelength of the laser wavelength in the absorption region is shown. The absorption layers 64, 66 are arranged at a distance d from each other which is three quarters of the wavelength of the laser light in the absorption region.

[0077] Figures 4 to 6 , Figure 8 and Fig. 9 The embodiment in is an example of the overall condition of the distance d being related to the laser wavelength λ in the semiconductor material of the absorption region 20 by the following equation:

[0078] d = (2k-1)λ / (4 m) (1)

[0079] Wherein, k and m are natural numbers greater than or equal to 1.

[0080] The above condition (1) assumes that the individual absorbent layers should be understood within a typical production tolerance range of ±20%.

[0081] For Figure 4 , 5 In the embodiments of and 6, the natural numbers k and m in the above condition (1) are 1 (k=m=1) and d=67nm. Figure 8 In the above condition (1), k=1, m=2, and distance d=34nm. Fig. 9 In the embodiment, the numbers k and m in the above condition (1) are k=2, m=1, resulting in a distance d of 201 nm.

[0082] Reference Figure 4-6 , 8 and 9 described above embodiments of the absorption layer of the absorption region of the photodiode can be Figure 1 The photodiode 34 of the VCSEL 10 is implemented in the absorption region 20 .

[0083] Preferably, the absorption region 20 of the photodiode 34 includes an even number of absorption layers.

[0084] The disclosure herein is particularly applicable to VCSELs with monolithically integrated photodiodes (ViPs) that use strained semiconductor materials for the absorption layer of the photodiode. In particular, ViPs with output emission wavelengths between 850 nm and 1200 nm are contemplated.

[0085] As a practical example, a ViP with an output emission wavelength of 940 nm will be described. A suitable material for the photodiode absorption layer is In with x = 16.5%. x Ga 1-x As. According to the above-mentioned reference article by Fritz et al., “Dependence of critical layer thickness on strain for In x Ga 1-x As / GaAs strained-layersuperlattices", Appl.Phys.Lett.Vol.46(10),p.967-969,1985", the critical layer thickness of this material is about 18nm. The single layer thickness of the separate absorption layer should be greater than 25% of the critical layer thickness and less than 200% of the critical layer thickness. For example, a good choice is a thickness of 15nm (approximately 83% of the critical layer thickness). In this example, the refractive index of the semiconductor material in the absorption region is about 3.5 (typical for AlGaAs and InGaAs at a wavelength of about 940nm), and the laser wavelength in the semiconductor is λ=268nm.

[0086] Fig.10 Shows Figure 1 In an embodiment of an absorption region 20 in FIG. 1 , the absorption region comprises two absorption layers 70, 72 and three layers 74, 76, 78 located between the absorption layers 70 and 72. At least one of the layers 74, 76, 78, which are non-absorbing layers, comprises a strain compensating material that partially, completely or even excessively compensates for the strain introduced by the absorption layer, preferably an absorption layer comprising InGaAs. The material between the individual absorption layers 70, 72 may be AlGaAs or GaAs. The light absorbing region is preferably intrinsic (undoped) as a whole.

[0087] A suitable layer stack for the absorption region 20 may therefore include two InGaAs absorption layers 70, 72, a GaAsP strain compensation layer 76 embedded between two GaAs layers 74, 78. Fig.10 Also shown are regions 32 and 26, between which the absorption region 20 is arranged.

[0088] In the above example with an InGaAs absorber layer having an indium content of 16.5%, Figures 4 to 6 Combination of embodiments Fig.10 This can be achieved with a layer thickness of 15 nm for each of the absorption layers 70, 72, 3 nm for layer 74, 46 nm for layer 76 and 3 nm for layer 78. The absorption layers 70, 72 are thus spaced 67 nm apart, corresponding to a quarter of the laser wavelength in the absorption region.

[0089] It is conceivable Fig.10 For example, the GaAs layer may be omitted, or additional strain compensation layers may be arranged on both sides of the absorption region 20.

[0090] Fig.11 A schematic diagram of an optical sensor 100 according to an embodiment is shown. The optical sensor 100 is arranged to determine the presence, distance and movement of an object by means of self-mixing interferometry. The optical sensor 100 comprises a VCSEL 10 with a monolithically integrated photodiode as described above, a transmission window 102 and a drive circuit 104 for electrically driving the VCSEL 10. The drive circuit 104 is electrically connected to the VCSEL 10 via contacts 36 and 40 for supplying power to the VCSEL 10 in a defined manner. The drive circuit 104 may include a storage device for storing data and instructions for operating the drive circuit 104. The optical sensor 100 also includes an evaluator 106. The photodiode 34 included by the VCSEL 10 is arranged to determine changes in the standing wave pattern within the laser cavity, which are caused by interference of a return laser 110 based on an emitted laser 108 reflected by various objects with light waves within the optical resonator 18 of the VCSEL 10. The evaluator 106 may include at least one storage device, such as a memory chip, and at least one processing device, such as a microprocessor. The evaluator 106 is adapted to receive electrical signals from the VCSEL, and optionally from the drive circuit 104, to determine the distance or motion of one or more objects based on the interference (self-mixing interference) of the returning laser light 110 with the light waves within the optical resonator 18 of the VCSEL 10. The optical sensor 100 may be used for particle detection, distance / speed measurement, user interface, gesture control, etc.

[0091] Fig.12 A schematic diagram of a mobile communication device 120 comprising an optical sensor 100 is shown. The optical sensor 100 may be used, for example, in conjunction with a software application running on the mobile communication device 120. The software application may use the optical sensor 100 for a sensing application. Such a sensing application may be, for example, a self-mixing interferometry application, in particular a particle sensing application or an application for a user interface based on gesture recognition.

[0092] Fig.13 A schematic diagram showing a process flow of a method for producing a VCSEL according to the present disclosure is shown.

[0093] In step 150, a substrate 22 is provided. In step 152, a second portion 26 of a second reflector 16, which may be a DBR as described above, is provided on the substrate 22. In step 154, a light absorbing region 20 as described above is provided on the second portion 26 of the second reflector 16. Step 154 ​​includes separating the absorbing layers from each other by a distance d, the distance d satisfying condition (1) as described herein. Step 154 ​​may also include providing additional layers, in particular one or more strain compensation layers, such as a strain compensation layer, between the light absorbing layers of the absorbing region 20. Fig.10 Layer 76 in.

[0094] In step 156, the third region 32 and the second region 30 of the first portion 24 of the second reflector 16 as described above are disposed on the light absorbing region 20. In step 158, the first region 28 of the first portion 24 of the second reflector 16 is disposed on the second region 30. In step 160, the active region 14 as described above is disposed on the first region 28 of the first portion 24 of the second reflector 16. In step 162, the first reflector 12 is disposed on top of the active region 14. In step 164, the optical resonator 18 including the first and second reflectors 12, 16 and the active region 14 is electrically contacted as described above. In step 166, the photodiode 34 is electrically contacted as described above.

[0095] The electrical contact may include one or more of the following steps: Etching the layer structure of the VCSEL 10 down to the corresponding layers of the second region 30 of the first portion 24 of the second reflector 16 using a suitable etching technique. The process may also include an oxidation process to provide oxide holes in the VCSEL 10. The production process may also include a passivation or planarization process to provide a smooth surface for depositing the bonding pads. After depositing the semiconductor layers of the VCSEL structure, the substrate 22 may be removed. The n-contact 38 of the photodiode 34 may be provided on the thinned back side of the substrate 22 after thinning or grinding the substrate 22.

[0096] The above-mentioned layers of the layer stack of the VCSEL 10 may be deposited by an epitaxial method such as MOCVD or MBE.

[0097] Although the present invention has been illustrated and described in detail in the drawings and the foregoing description, such illustration and description should be considered illustrative or exemplary rather than restrictive; the present invention is not limited to the disclosed embodiments. Other variations to the disclosed embodiments can be understood and implemented by those skilled in the art by studying the drawings, the disclosure, the appended claims, and practicing the claimed invention.

[0098] In the claims, the word "comprising" does not exclude other elements or steps, and the singular form "a" or "an" does not exclude a plurality. A single element or other unit may fulfill the functions of several items recited in the claims. The fact that certain measures are recited in mutually different claims does not indicate that a combination of these measures cannot be used to advantage.

[0099] Any reference signs in the claims should not be construed as limiting the scope.

Claims

1. A vertical cavity surface emitting laser (VCSEL), the vertical cavity surface emitting laser (VCSEL) comprising: An optical resonator (18) comprising a first reflector (12), an active region (14) for generating laser light, and a second reflector (16), wherein the active region (14) is arranged between the first reflector and the second reflector (12, 16); A photodiode (34) is integrated in an optical resonator (18), the photodiode (34) comprising an absorption region (20) having a plurality of absorption layers (50, 52; 56, 58, 60, 62; 64, 66; 70, 72), the plurality of absorption layers being configured to absorb generated laser light, wherein the absorption layers (50, 52; 56, 58, 60, 62; 64, 66; 70, 72) are arranged to be spaced apart from each other by a center-to-center distance d, the distance d being sufficient to The condition is satisfied: d = (2k-1)λ / (4m), wherein λ is the wavelength of the laser in the absorption region (20), k and m are natural numbers greater than or equal to 1, wherein at least one first layer (74) comprising AlGaAs or GaAs, at least one second layer (78) comprising AlGaAs or GaAs, and at least one third layer (76) comprising GaAsP sandwiched between the first layer (74) and the second layer (78) are arranged between at least one pair of adjacent absorption layers.

2. The VCSEL according to claim 1, wherein: The photodiode (34) includes an even number of absorption layers (50, 52; 56, 58, 60, 62; 64, 66; 70, 72).

3. The VCSEL according to claim 1 or 2, wherein: The photodiode (34) comprises at least two absorption layers (50, 52; 70, 72) arranged at a distance d from each other, wherein k=1 and m=1.

4. The VCSEL according to claim 1 or 2, wherein: The photodiode (34) comprises at least two absorption layers (64, 66) arranged at a distance d from each other, wherein k=2 and m=1.

5. The VCSEL according to claim 1 or 2, wherein: The photodiode includes at least four absorption layers (56, 58, 60, 62) spaced apart from each other by a distance d, wherein k=1 and m=2.

6. The VCSEL according to claim 1 or 2, wherein: Absorbent layer (50, 52; 56, 58, 60, 62; 64, 66; 70, 72) include strained semiconductor material.

7. The VCSEL according to claim 6, wherein: Each absorbent layer (50, 52; 56, 58, 60, 62; 64, 66; 70, 72) has a layer thickness in the range of 25% to 200% of the critical layer thickness of the strained semiconductor layer.

8. The VCSEL according to claim 7, wherein: Each absorbent layer (50, 52; 56, 58, 60, 62; 64, 66; 70, 72) has a layer thickness in the range of 70% to 100% of the critical layer thickness of the strained semiconductor layer.

9. The VCSEL according to any one of claims 1, 2, 7 and 8, wherein: Each absorbent layer (50, 52; 56, 58, 60, 62; 64, 66; The layer thickness of 70, 72) is in the range of 5 nm to 50 nm.

10. The VCSEL according to claim 9, wherein: The layer thickness of each absorption layer (50, 52; 56, 58, 60, 62; 64, 66; 70, 72) is in the range of 10 nm to 35 nm.

11. The VCSEL according to any one of claims 1, 2, 7, 8, and 10, wherein: Absorbent layer (50, 52; 56, 58, 60, 62; 64, 66; 70, 72) include InGaAs.

12. The VCSEL according to any one of claims 1, 2, 7, 8, and 10, wherein: The emission wavelength of the laser output by the VCSEL (10) is in the range of 850nm to 1200nm.

13. An optical sensor comprising the VCSEL (10) according to any one of claims 1 to 12.

14. A method of producing a vertical cavity surface emitting laser (VCSEL), the method comprising: Providing an optical resonator (18), the optical resonator comprising a first reflector (12), an active region (14) for generating laser light, and a second reflector (16), wherein the active region (14) is arranged between the first reflector and the second reflector (12, 16); A photodiode (34) is integrated in an optical resonator (18), the photodiode (34) comprising an absorption region (20) having at least two absorption layers (50, 52; 56, 58, 60, 62; 64, 66; 70, 72), wherein the absorption layers (50, 52; 56, 58, 60, 62; 64, 66; 70, 72) are arranged to be spaced apart from each other by a center-to-center distance d, the distance d satisfying the condition: d=(2k-1)λ / (4m), wherein wherein λ is the wavelength of laser light in the absorption region, k and m are natural numbers greater than or equal to 1, and at least one first layer (74), at least one second layer (78), and at least one third layer (76) are arranged between at least one pair of adjacent absorption layers, wherein the first layer (74) includes AlGaAs or GaAs, the second layer (78) includes AlGaAs or GaAs, and the third layer (76) includes GaAsP and is sandwiched between the first layer (74) and the second layer (78).

Images