阅读说明：本技术 具有集成的光电二极管的垂直腔面发射激光器 (Vertical cavity surface emitting laser with integrated photodiode ) 是由 U·魏希曼 于 2021-06-02 设计创作，主要内容包括：提供一种垂直腔面发射激光器(VCSEL),其包括光学谐振器(18)。光学谐振器(18)包括第一反射镜(12)、用于产生激光的有源区域(14)和第二反射镜(16),其中,有源区域(14)布置在第一反射镜与第二反射镜(12、16)之间。光电二极管(34)集成在光学谐振器(18)中。光电二极管(34)包括具有多个吸收层的吸收区域(20),所述多个吸收层被构造为能够吸收产生的激光,其中,吸收层被布置为彼此间隔距离d,所述距离d满足条件：d＝(2k-1)λ/(4m),其中,λ是吸收区域(20)中的激光的波长,k和m是大于或等于1的自然数。还描述了一种光学传感器和一种生产VCSEL的方法。(A Vertical Cavity Surface Emitting Laser (VCSEL) is provided that includes an optical resonator (18). The optical resonator (18) comprises a first mirror (12), an active region (14) for generating laser light and a second mirror (16), wherein the active region (14) is arranged between the first and second mirrors (12, 16). The photodiode (34) is integrated in the optical resonator (18). The photodiode (34) comprises an absorption region (20) with a plurality of absorption layers configured to be able to absorb the generated laser light, wherein the absorption layers are arranged at a distance d from each other, which distance d fulfils the condition: and d is (2k-1) λ/(4m), where 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 of producing a VCSEL are also described.)

1. A Vertical Cavity Surface Emitting Laser (VCSEL) comprising:

an optical resonator (18) comprising a first mirror (12), an active area (14) for generating laser light and a second mirror (16), wherein the active area (14) is arranged between the first and second mirrors (12, 16);

a photodiode (34) integrated in the optical resonator (18), the photodiode (34) comprising an absorption region (20) with a plurality of absorption layers (50, 52; 56, 58, 60, 62; 64, 66; 70, 72) configured to be able to absorb the generated laser light, wherein the absorption layers (50, 52; 56, 58, 60, 62; 64, 66; 70, 72) are arranged at a distance d from each other, which distance d satisfies the condition: and d is (2k-1) λ/(4m), where 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.

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

3. The VCSEL of 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 of 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 of claim 1 or 2, wherein the photodiode comprises at least four absorption layers (56, 58, 60, 62) spaced two by a distance d from each other, wherein k-1 and m-2.

6. The VCSEL of any of claims 1 to 5, wherein the absorbing layer (50, 52; 56, 58, 60, 62; 64, 66; 70, 72) comprises a strained semiconductor material.

7. The VCSEL of claim 6, wherein a layer thickness of each absorption layer (50, 52; 56, 58, 60, 62; 64, 66; 70, 72) is in a range of 25% to 200%, preferably 70% to 100%, of a critical layer thickness of the strained semiconductor layer.

8. The VCSEL of any of claims 1 to 7, wherein a layer thickness of each absorption layer (50, 52; 56, 58, 60, 62; 64, 66; 70, 72) is in a range of 5nm to 50nm, preferably in a range of 10nm to 35 nm.

9. The VCSEL of any of claims 1 to 8, wherein the absorption layer (50, 52; 56, 58, 60, 62; 64, 66; 70, 72) comprises InGaAs.

10. The VCSEL of claim 9, wherein at least one layer (74, 76, 78) comprising AlGaAs or GaAs or GaAsP is arranged between adjacent absorption layers (70, 72).

11. The VCSEL of claim 9, 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 and second layers (74, 78) comprising AlGaAs or GaAs are arranged between adjacent absorbing layers (70, 72).

12. The VCSEL of any of claims 1 to 11, wherein the emission wavelength of the laser light output by the VCSEL (10) is in the range of 850nm to 1200 nm.

13. An optical sensor comprising a VCSEL (10) in accordance with any 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) comprising a first mirror (12), an active area (14) for generating laser light and a second mirror (16), wherein the active area (14) is arranged between the first and second mirrors (12, 16);

integrating a photodiode (34) in an optical resonator (18), the photodiode (34) comprising an absorption region (20) with 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 at a distance d from each other, which distance d satisfies the condition: d ═ 2k-1 λ/(4m), where λ is the wavelength of the laser light in the absorption region, and k and m are natural numbers greater than or equal to 1.

Technical Field

The present 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 of producing such a VCSEL.

Background

VCSELs with integrated photodiodes, commonly denoted ViP, can be used as small sensors for measuring e.g. distance, displacement, velocity, and even particle density. All these measurements can be based on the principle of self-mixing interference (SMI). This type of optical sensor may be simple enough to be integrated into a mobile phone.

The ViP may be designed such that it operates at a wavelength in the range of about 850nm to about 1200 nm. For such long wavelength operation of ViP, 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 a photodiode, the thickness of such absorption layer 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. To achieve high absorption with such thin layers, the layers are typically placed at the location where the optical intensity is greatest within the epitaxial stack of the VCSEL. However, small layer thickness deviations during epitaxial growth of the epitaxial stack of VCSELs will change the light intensity distribution within the VCSEL and thus lead to absorption variations and corresponding performance variations between vips on the same wafer or between individual wafer production lots. In other words, the absorption capability of the photodiode, as well as the light detection capability of conventional ViP, is largely related to production tolerances.

Disclosure of Invention

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.

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

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

According to a first aspect, there is provided a Vertical Cavity Surface Emitting Laser (VCSEL) comprising:

an optical resonator including a first mirror, an active area for generating laser light, and a second mirror, wherein the active area is disposed between the first mirror and the second mirror;

a photodiode integrated in the optical resonator, the photodiode comprising an absorption region having a plurality of absorption layers configured to be able to absorb the generated laser light, wherein the absorption layers are arranged at a distance d from each other, the distance d satisfying the condition: d ═ 2k-1 λ/(4m), where 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.

The photodiode of the VCSEL according to the present invention comprises at least two absorption layers. The absorption layer is configured to be capable of absorbing light in a wavelength range including at least a 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, which is an odd multiple of the laser wavelength in the optical resonator divided by a multiple of 4, so that the absorption of the photodiode is less dependent on production tolerances. The distance d may be taken as the center-to-center distance from a single absorbent layer to an adjacent absorbent layer. The center of the absorbent layer is understood to be the middle of the absorbent layer with respect to its thickness. The space between the individual absorption layers may be filled with a non-absorbing or at most a low-absorbing semiconductor layer.

The above conditions for the distance d between the individual absorption layers should be understood within a typical production tolerance of 20%.

Due to the mirrors on both sides of the active region, the light intensity within the VCSEL forms a standing wave pattern. The maxima of the pattern are at a distance from each other corresponding 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 of the photodiode. For example, when a resonator is built using an AlGaAs/GaAs material system, the refractive index is about 3.5 at wavelengths above 850 nm. NGaAs has very similar refractive indices.

If the photodiode has only one thin absorption layer, the absorption of the light generated by the active region is optimal when the absorption layer is arranged at the maximum intensity of the standing wave pattern. However, epitaxial production processes may have some spread and thickness deviations from the theoretical design, both across the entire wafer and between individual wafer production lots. These thickness deviations can cause a shift in the standing wave pattern relative to the photodiode resulting in a reduced overlap between the maximum or highest light intensity and the absorbing layer. The result is a significant decrease in absorbance. Thus, production tolerances result in large variations in photodiode signals across the entire wafer or between different wafer production lots.

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

The absorption layer of the photodiode may be monolithically integrated in one of the first mirror and the second mirror. The first mirror and/or the second mirror may be configured as a Distributed Bragg Reflector (DBR).

Preferably, the photodiode may include 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 the VCSEL production. The number of individual absorbent layers may be 2, 4, 6 or more. Within half the wavelength of the laser light within the absorption region, 2, 4, 6 or more individual absorption layers may be arranged. There may be 2, 4, 6 or more individual absorbing layers throughout the wavelength of the laser light within the absorbing region.

The photodiode may comprise 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 absorbing 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 the wavelength of the laser light within the absorption region.

In an alternative configuration, the photodiode may comprise at least two absorption layers arranged at a distance d from one another, where k is 2 and m is 1. In this configuration, the at least two absorbing layers are spaced from each other by three-quarters of the wavelength in the absorbing region. Thus, the at least two absorption layers are arranged within the entire wavelength of the wavelengths within the absorption region.

In another embodiment, the photodiode may comprise at least four absorption layers spaced apart from one another two by a distance d, k being 1 and m being 2. In this configuration, two adjacent layers of the at least four absorbing layers are spaced from each other by one eighth of a wavelength within the optical resonator. In this configuration, the at least four absorbing layers are arranged within half of the wavelength within the absorption region.

The absorption layer of the photodiode may include a strained semiconductor material. The invention is particularly advantageous when strained semiconductor materials, such as InGaAs, are used as the absorbing layer. As mentioned above, such strained semiconductor materials only allow for the production of thinner layers. Thus, the layer thickness of each absorption layer may be in the range of 25% to 200%, preferably 70% to 100% of the critical layer thickness of the strained semiconductor layer. Critical layer thickness is generally defined as the limit of 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 18nm, as In the Fritz et al, "Dependence of critical layer thickness on string for In_xGa_1-xAs/GaAs strained-layer superstrates ", appl. Phys. Lett. Vol.46(10), pages 967-.

In a practical embodiment, the absorption layer may have a monolayer thickness in the range from 5nm to 50nm, preferably in the range from 10nm to 35nm, for example in the range from 15nm to 30 nm.

Preferably, the absorption layer may include In_xGa_1-xAs, where x ranges from 0.05 to 0.3, depending on the wavelength output by the VCSEL. The active region may also comprise InGaAs, wherein 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 comprise GaAs instead of InGaAs.

When one of the first and second mirrors 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 the material system.

At least one layer comprising AlGaAs or GaAs or GaAsP may be disposed between adjacent absorption layers. For example, GaAs when the absorption layer is based on a strained semiconductor material, such as InGaAs_yP_1-yA layer, e.g. y > 0.5, e.g. y > 0.8, may be advantageous because it partially, completely or even excessively compensates for the strain introduced by the absorbing layer.

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 and second layers including AlGaAs or GaAs may be disposed between the adjacent absorption layers.

Additional strain compensation layers may be disposed on one or both sides of the absorption region of the photodiode.

Preferably, the emission wavelength of the laser light output from the VCSEL is in the range of 850nm to 1200 nm.

The VCSEL may further comprise a contact arrangement arranged to provide a driving current to electrically pump the optical resonator.

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

The optical sensor may be comprised in a mobile communication device.

The optical sensor may be used for distance detection, velocity detection, particle density detection, gesture control, in particular for all sensor applications based on self-mixing interferometry.

According to a third aspect, there is provided a method of producing a Vertical Cavity Surface Emitting Laser (VCSEL), the method comprising:

providing an optical resonator comprising a first mirror, an active area for generating laser light, and a second mirror, wherein the active area is disposed between the first mirror and the second mirror;

integrating a photodiode in an optical resonator, the photodiode comprising an absorption region having at least two absorption layers, wherein the absorption layers are arranged at a distance d from each other, the distance d satisfying the condition: d ═ 2k-1 λ/(4m), where λ is the wavelength of the laser light in the absorption region, and k and m are natural numbers greater than or equal to 1.

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

The VCSEL may include a substrate on which a layer stack of the VCSEL with an integrated photodiode is grown. After the VCSEL is produced, the substrate may be removed.

The VCSEL can be a top emitter or a bottom emitter.

It is to be understood that the VCSEL and the method of producing a VCSEL according to any of the embodiments described above and below have similar and/or identical embodiments, in particular embodiments as defined in the claims. Further advantageous embodiments are defined below.

Drawings

These and other aspects of the invention are apparent from and will be elucidated with reference to the embodiments described hereinafter. In the following drawings:

FIG. 1 shows a schematic diagram of a VCSEL with a monolithically integrated photodiode;

FIG. 2 shows a standing wave pattern of light intensity within a VCSEL;

FIG. 3 shows a single thinner absorbing layer of a photodiode placed at maximum light intensity within a VCSEL;

FIG. 4 shows an embodiment of a photodiode of a VCSEL with two absorption layers;

FIG. 5 shows two absorbing layers of the photodiode of FIG. 4, wherein the standing wave pattern is shifted first relative to the absorbing layers;

FIG. 6 shows the two absorbing layers of FIG. 4 with the standing wave pattern being a second offset from the absorbing layers;

FIG. 7 is a graph showing the effect of shifts in the standing wave pattern relative to a photodiode having one absorbing layer and relative to a photodiode having two absorbing layers;

FIG. 8 shows an embodiment of a photodiode of a VCSEL with four absorption layers;

FIG. 9 shows another embodiment of a VCSEL photodiode with two absorption layers;

FIG. 10 shows a schematic diagram of an embodiment of an absorption region of a photodiode of a VCSEL;

FIG. 11 shows a schematic diagram of an optical sensor comprising a VCSEL;

FIG. 12 shows a schematic diagram of a mobile communication device including an optical sensor; and

figure 13 shows a schematic diagram of a process flow of a method of producing a VCSEL.

Detailed Description

The 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 which is an odd multiple of the laser wavelength in the absorption region of the photodiode of the VCSEL divided by a multiple of 4, so that the absorption of the photodiode is less dependent on production tolerances. This will be explained in more detail below.

With reference to fig. 1, the general design of a VCSEL with monolithically integrated photodiodes will be described. Figure 1 shows a VCSEL 10. The VCSEL 10 includes a first mirror 12, an active area 14 for laser emission, and a second mirror 16. The first mirror 12 and the second mirror 16 may be configured as Distributed Bragg Reflectors (DBRs). The active area 14 is arranged between the first mirror 12 and the second mirror 16. The first mirror 12, the active area 14 and the second mirror 16 form an optical resonator 18. The VCSEL 10 further includes a photodiode 34, the photodiode 34 including the light absorbing region 20.

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

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

In the design shown in fig. 1, the absorption region 20 of the photodiode 34 is monolithically integrated in the second mirror 16. In other embodiments, the light absorbing region 20 may be integrated in the first mirror 12.

When the first mirror 12 and the second mirror 16 are configured as DBRs, each of the first mirror 12 and the second mirror 16 may have one or more layer pairs, wherein two layers in a layer pair have different refractive indices. The number of layers shown in fig. 1 is merely exemplary and illustrative. The thicknesses of the layers shown in fig. 2 are not drawn to scale.

The optical resonator 18 may also include one or more oxide apertures (not shown).

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

The substrate 22 and the second portion 26 of the second mirror 16 may be n-doped. The first portion 24 of the second mirror 16 may have a first region 28 that is n-doped, a second region 30 that is n-doped, and a third region 32 that is p-doped. 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 mirror 16 form a photodiode 34. Thus, the photodiode 34 may be an n-p-i-n photodiode formed by the light absorbing region 20 and the layers of the regions 30, 32, 26 of the second mirror 16.

The VCSEL 10 includes an electrical contact arrangement that may include a p-contact 36 on the top of the first mirror 12, an n-contact 38 on the bottom of the substrate 22, and an additional n-contact 40 on the top of the region 28 of the second mirror 16. The p-contact may be formed as a ring electrode. The p-contact 36 may be arranged on a cover layer (not shown) on top of the first mirror 12. The n-contact 38 may be formed as a metallization of 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 a ring electrode.

The p-contact 36 may form the anode of the VCSEL and the n-contact 40 may form the cathode of the VCSEL 10. Meanwhile, 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.

The VCSEL 10 is preferably designed to be able to output laser light in a wavelength range from 850nm to 1200 nm. To obtain emission with such long wavelengths, 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 lasing wavelength. For example, an InGaAs composition having an indium content of 10% can provide laser emission at about 950nm at a temperature of about 300K. For smaller emission wavelengths, such as 850nm, the active region 14 may include one or more than two GaAs layers. The indium content of the absorber layer in the absorber region 20 is preferably higher than the indium content in the active region 14.

InGaAs is a strained semiconductor material. The strain depends on the magnitude of the indium content in the composition and limits the layer thickness that can be grown defect-free to only a few tens of nm. This is small compared to the period of typical variations in light intensity in the VCSEL 10, which follow a standing wave pattern due to the mirrors 12, 16 on both sides of the active region 14, as shown in figure 2. Fig. 2 shows the normalized light intensity within the optical resonator 18 of the VCSEL 10 as a function of the position z along the layer stack from the first mirror to the second mirror of the VCSEL. The light intensity is highest in the active region and decreases towards the DBR 12 and the DBR 16.

Figure 3 shows a diagram in which a single absorption layer 42 made of InGaAs with an indium content of about 15% and a thickness of 20nm is centrally placed in the optical resonator of the VCSEL at the maximum light intensity. As shown in fig. 3, the thickness of the single absorbing layer 42 is much less than the typical variation in light intensity 44. The photodiode signal is at a maximum when the single absorbing layer 42 is centered with respect to the maximum light intensity, as shown in fig. 3. However, in producing VCSELs, the epitaxial production process will have some spread and thickness variation from the theoretical design across the entire wafer as well as between individual wafer production lots. These thickness deviations can cause a shift in the standing wave pattern relative to the absorption region 20 of the photodiode 34, resulting in a reduced overlap between the maximum light intensity and the absorption layer. The result is a variation in photodiode signal across the wafer and between production lots. When the absorption layer 42 is made of a strained semiconductor material, for example InGaAs with an indium content, for example with an indium content exceeding, for example, 10%, the increase of the thickness of the absorption layer 42 may be severely limited.

Therefore, according to the present disclosure, it is proposed to construct the absorption region 20 of the photodiode 34 with more than one absorption layer, wherein the absorption layers are arranged such that they are spaced apart from each other by a distance d which satisfies the condition: d is (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 be explained in more detail with reference to fig. 4 to 9.

In fig. 4-9, the light intensity 48 is represented by a squared sine function (sin)²x) approximation. For example, the period of the squared sine function is chosen to be 134nm, 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 within the VCSEL cavity. Thus, the period of the 134nm light intensity variation corresponds to a laser wavelength of 268nm within the VCSEL cavity.

Fig. 4 shows an embodiment of a photodiode, wherein the absorption region of the photodiode comprises two absorption layers 50, 52, which are shown in fig. 4 with dashed lines. As an example, in this example, the thickness of each individual one of the absorption layers 50 and 52 is selected to be, for example, 15 nm. 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 variation of the light intensity, corresponding to a quarter of the wavelength λ in the absorption region. As shown in fig. 4, the distance d may 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 may also be taken 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 (viewed from left to right in fig. 4).

In fig. 4, the relative spatial relationship between light intensity and the absorbing layers 50 and 52 is such that layer 50 starts with the minimum light intensity and absorbing layer 52 starts with the maximum of the standing wave pattern.

Fig. 5 shows the same two absorption layers 50 and 52, wherein the absorption layers 50 and 52 and the light intensity curve 48 are offset with respect to each other by 10nm without changing the distance d. This offset may be caused by manufacturing tolerances. For example, the absorber layers 50, 52 shown in fig. 4 and the absorber layers 50, 52 in fig. 5 may be absorber layers of ViP along the same wafer or absorber layers of ViP of different production lots. As shown in fig. 5, although the light intensity overlaps the absorption of the absorbing layer 52 and thus the degree of absorption of the layer 52 is reduced, the light intensity overlaps the absorption of the absorbing layer 50 and thus the degree of absorption of the layer 50 is increased. Thus, the effect of the shift of the standing wave pattern relative to the absorption layers 50 and 52 on the overall absorption is reduced to an absorption variation of only a few percent.

Fig. 6 again shows a photodiode with two absorption layers 50 and 52, wherein it is shown that the relative displacement between the light intensity curve 48 and the absorption layers 50, 52 is now 30nm compared to the situation in fig. 4. Likewise, although the overlap of the light intensity with the absorption layer 52 is further reduced, the overlap of the light intensity with the absorption layer 50 is further increased, so that the influence 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 an absorbance change of only a few percent.

This is quite different from the case with only one absorbing layer (fig. 3), where the same shift of the standing wave pattern relative to the photodiode 34 results in a reduction in absorbance by more than 40%, as shown in fig. 7. Fig. 7 shows a comparison of the variation when the overall overlap of the standing wave pattern is shifted by 10nm and 30nm relative to a photodiode with a single absorber layer as shown in fig. 3 and relative to a photodiode with two absorber layers as absorber layers 50 and 52 in fig. 4-6.

Fig. 8 shows another embodiment of a photodiode having an absorption region that includes four absorption layers 56, 58, 60, and 62. Adjacent ones of the absorbing layers 56, 58, 60, 62 are spaced apart by a distance d that is one eighth of the wavelength of the laser light in the absorbing region 20.

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

Fig. 9 shows an embodiment of a photodiode comprising an absorption region with two absorption layers 64, 66 arranged within the entire wavelength of the laser wavelength in the absorption region. 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.

The embodiments in fig. 4 to 6, 8 and 9 are examples of the overall condition of the distance d in relation to the laser wavelength λ in the semiconductor material of the absorption region 20 as follows:

d＝(2k-1)λ/(4m) (1)

wherein k and m are natural numbers greater than or equal to 1.

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

For the embodiments in fig. 4, 5 and 6, the natural numbers k and m in the above condition (1) are 1(k ═ m ═ 1) and d ═ 67 nm. In the above condition (1) shown in the example of fig. 8, k is 1, m is 2, and the distance d is 34 nm. In the embodiment of fig. 9, the numbers k and m in the above condition (1) are k-2 and m-1, resulting in a distance d of 201 nm.

The above-described embodiments of the absorption layer of the absorption region of the photodiode described with reference to fig. 4-6, 8 and 9 may be implemented in the absorption region 20 of the photodiode 34 of the VCSEL 10 of fig. 1.

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

The disclosure herein is particularly applicable to vcsels (vips) having monolithically integrated photodiodes, which use strained semiconductor materials for the absorption layer of the photodiode. In particular, a ViP with an output emission wavelength between 850nm and 1200nm may be considered.

As a practical example, a ViP having an output emission wavelength of 940nm will be described. A suitable material for the photodiode absorption layer is x 16.5% In_xGa_1-xAs. According to the Fritz et al reference "" Dependence of critical layer thickness on strain for In_xGa_1-xAs/GaAs strained-layer superstrates ", appl. Phys. Lett. Vol.46(10), p.967-969, 1985", critical layer thicknesses of this material are approximately 18 nm. The individual thickness of the absorber layer alone should be greater than 25% and less than 200% of the critical layer thickness. For example, a preferred choice is a thickness of 15nm (about 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 with a wavelength of about 940 nm), and the laser wavelength in the semiconductor is λ 268 nm.

Fig. 10 shows an embodiment of the absorbent region 20 of fig. 1, comprising two absorbent layers 70, 72 and three layers 74, 76, 78 between the absorbent layers 70 and 72. At least one of the layers 74, 76, 78, being a non-absorbing layer, comprises a strain compensating material that partially, completely or even excessively compensates the strain introduced by the absorbing layer, preferably an absorbing 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.

A suitable layer stack of the absorption region 20 may thus comprise two InGaAs absorption layers 70, 72, a GaAsP strain compensation layer 76 embedded between two GaAs layers 74, 78. Also shown in fig. 10 are regions 32 and 26 with absorbent region 20 disposed therebetween.

In the above example of an InGaAs absorber layer with an indium content of 16.5%, the embodiments of fig. 4-6 in combination with fig. 10 can be implemented as: the layer thickness of each of the absorption layers 70, 72 is 15nm, the layer thickness of layer 74 is 3nm, the layer thickness of layer 76 is 46nm, and the layer thickness of layer 78 is 3 nm. The absorbing layers 70, 72 are thus spaced 67nm apart, corresponding to a quarter of the wavelength of the laser in the absorption region.

Variations of the layer structure in fig. 10 can be envisaged. For example, the GaAs layer may be omitted, or additional strain compensation layers may be disposed on both sides of the absorption region 20.

Fig. 11 shows a schematic view of an optical sensor 100 according to an embodiment. 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 monolithically integrated photodiode as described above, a transmission window 102 and a driving circuit 104 for electrically driving the VCSEL 10. The driver circuit 104 is electrically connected to the VCSEL 10 via the contacts 36 and 40 for supplying power to the VCSEL 10 in a defined manner. The drive circuit 104 may include a memory device for storing data and instructions for operating the drive circuit 104. The optical sensor 100 further comprises an evaluator 106. The photodiode 34 included in the VCSEL 10 is arranged to determine variations in the standing wave pattern within the laser cavity caused by interference of the return laser light 110 based on the emitted laser light 108 reflected by various objects with the light wave within the optical resonator 18 of the VCSEL 10. The evaluator 106 may comprise at least one memory device, e.g. a memory chip, and at least one processing device, e.g. 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 return laser light 110 with the optical wave within the optical resonator 18 of the VCSEL 10. The optical sensor 100 may be used for particle detection, distance/velocity measurement, user interface, gesture control, and the like.

Fig. 12 shows a schematic view of a mobile communication device 120 comprising an optical sensor 100. 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 sensing applications. Such a sensing application may for example be a self-mixing interferometry application, in particular a particle sensing application or an application of a user interface based on gesture recognition.

Figure 13 shows a schematic diagram of a process flow of a method of producing a VCSEL according to the present disclosure.

In step 150, the substrate 22 is provided. In step 152, the second portion 26 of the second mirror 16, which may be a DBR as described above, is disposed on the substrate 22. In step 154, the light absorbing region 20 as described above is disposed on the second portion 26 of the second mirror 16. Step 154 includes separating the absorbent layers from each other by a distance d that satisfies condition (1) described herein. Step 154 may also include providing additional layers, particularly one or more strain compensating layers, such as layer 76 in fig. 10, between the light absorbing layers of the absorbing region 20.

In step 156, the third region 32 and the second region 30 of the first portion 24 of the second mirror 16 as described above are disposed on the light absorbing region 20. In step 158, the first area 28 of the first portion 24 of the second mirror 16 is positioned over the second area 30. In step 160, the active area 14 as described above is provided on the first area 28 of the first portion 24 of the second mirror 16. In step 162, a first mirror 12 is disposed on top of the active region 14. In step 164, the optical resonator 18 including the first and second mirrors 12, 16 and the active area 14 is electrically contacted as described above. In step 166, the photodiode 34 is electrically contacted as described above.

Electrical contact may include one or more of the following steps: the layer structure of the VCSEL 10 is etched down onto the corresponding layer of the second region 30 of the first portion 24 of the second mirror 16 using a suitable etching technique. The process may also include an oxidation process to provide an oxide aperture in the VCSEL 10. The production process may also include a passivation or planarization process to provide a smooth surface for depositing the bond pads. After deposition of 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.

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

While the invention has been illustrated and described in detail in the drawings and foregoing description, such illustration and description are to be considered illustrative or exemplary and not restrictive; the invention is not limited to the disclosed embodiments. Other variations to the disclosed embodiments can be understood and effected by those skilled in the art in practicing the claimed invention, from a study of the drawings, the disclosure, and the appended claims.

In the claims, the word "comprising" does not exclude other elements or steps, and the singular form of "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 mere fact that certain measures are recited in mutually different claims does not indicate that a combination of these measures cannot be used to advantage.

Any reference signs in the claims shall not be construed as limiting the scope.