DE102017108104A1 - Surface emitting semiconductor laser and method for its production - Google Patents

Abstract

A surface-emitting semiconductor laser (100) with a vertical resonator formed by reflectors (2, 6) is described, comprising a semiconductor layer sequence (10) having an active layer (4) arranged between the reflectors (2, 6), wherein in the semiconductor layer sequence (10) in the direction of the vertical resonator a plurality of blind holes (7) are formed, and wherein the semiconductor layer sequence (10) has at least one Stromeinengungsschicht (9), the oxidized regions (9a) extending from the blind holes (7) in the Stromeinengungsschicht (9) extend into it. Furthermore, a method suitable for producing the surface emitting semiconductor laser (100) will be described.

Description

A Vertical Cavity Surface Emitting Laser (VCSEL) surface emitting semiconductor laser and a method of making the same are described.

A surface emitting semiconductor laser with vertical resonator is for example from the document WO 02/063733 A2 known. The surface-emitting semiconductor laser described therein has a mesa structure, with current limiting diaphragms extending in the lateral direction into the semiconductor layer sequence for lateral current limitation from the side flanks of the mesa structure.

An object to be achieved is to specify an improved surface-emitting semiconductor laser, in which the semiconductor layer sequence is distinguished by a reduced defect density and a failure risk of the surface-emitting semiconductor laser is reduced. Furthermore, a method for producing the surface-emitting semiconductor laser is to be specified.

These objects are achieved by a surface emitting semiconductor laser and a method for the production thereof according to the independent claims. Advantageous embodiments and further developments are the subject of the dependent claims.

In accordance with at least one embodiment, the surface-emitting semiconductor laser has a vertical resonator formed by reflectors. In particular, the surface emitting semiconductor laser may be a VCSEL. The surface-emitting semiconductor laser comprises a semiconductor layer sequence which has an active layer arranged between the reflectors. The vertical resonator and thus the emission direction of the surface-emitting semiconductor laser are perpendicular to the layer planes of the semiconductor layer sequence. The reflectors are integrated, for example, in the semiconductor layer sequence and can be designed in particular as DBR (Distributed Bragg Reflection) mirror. DBR mirrors comprise a generally periodic arrangement of a plurality of alternating semiconductor layers which differ in their refractive indices and have maximum reflection at a wavelength dependent on the layer thickness.

The surface-emitting semiconductor laser can be provided in particular for the emission of laser radiation in the red or infrared spectral range. The semiconductor layer sequence is preferably based on an arsenide compound semiconductor. "Based on an arsenide compound semiconductor" in the present context means that the semiconductor layer sequence or at least one layer thereof comprises an arsenide compound semiconductor material, preferably Al _n Ga _m In _1-nm As, where 0 ≦ n ≦ 1, 0 ≦ m ≦ 1 and n + m ≤ 1. This material does not necessarily have to have a mathematically exact composition according to the above formula. Rather, it can have one or more dopants and additional constituents. For the sake of simplicity, however, the above formula contains only the essential components of the crystal lattice (Al, Ga, In, As), even though these may be partially replaced by small amounts of other substances.

According to at least one embodiment, a plurality of blind holes are formed in the semiconductor layer sequence in the surface-emitting semiconductor laser. The blind holes advantageously extend in the direction of the vertical resonator, ie perpendicular to the layer planes of the semiconductor layers, into the semiconductor layer sequence.

Furthermore, the semiconductor layer sequence has at least one current narrowing layer. The current constriction layer has oxidized regions extending from the blind holes, advantageously perpendicular to the vertical resonator, i. parallel to the layer planes of the semiconductor layer sequence, extend into the semiconductor layer sequence. The blind holes have, for example, a cylindrical shape, and the oxidized areas extend from the blind holes in the radial direction into the current constriction layer. The oxidized regions can be produced in particular by oxidizing the current constriction layer in the lateral direction starting from the blind holes.

The surface emitting semiconductor laser described herein makes use in particular of the idea of forming the oxidized regions of the current constricting layer starting from blind holes generated in the semiconductor layer sequence, in particular by a partial oxidation of the current constricting layer. This procedure has the advantage, in comparison to the mesa etching known from the prior art and subsequent oxidation of the mesa flank, that a mesa etching can be dispensed with.

It has been found that the generation of a mesa in the semiconductor layer sequence can lead to an increased defect density, which can continue into the active layer. In particular, in the case of mesa etching, impurities can form in the semiconductor layers, which can migrate into the active layer during operation of the surface-emitting semiconductor laser. It has been found that the preparation of the oxidized regions in the current constricting layer starting from in the Semiconductor layer sequence generated blind holes generated fewer impurities and thus advantageously reduces the risk of failure of the surface emitting semiconductor laser. In a preferred embodiment, the surface-emitting semiconductor laser is free of a mesa structure.

In accordance with at least one embodiment, the current constriction layer is a partially oxidized semiconductor layer. The at least one current constricting layer may in particular contain aluminum. In a preferred embodiment, the current constricting layer Al _x Ga _1-x As with 0.95 ≤ x ≤ 1, more preferably Al _x Ga _1-x As with 0.95 ≤ x ≤ 0.98. The aluminum contained in the current constriction layer can be oxidized in an oxidation process starting from the blind holes.

In accordance with at least one embodiment, the thickness of the current constricting layer is between 30 nm and 200 nm. A thickness in this range is advantageous for carrying out the oxidation and for achieving sufficient electrical insulation in the oxidized regions.

According to at least one embodiment, the number of blind holes in the semiconductor layer sequence is between 4 and 12. This number is advantageous in order to sufficiently narrow the current flow through the semiconductor layer sequence without significantly increasing the defect density.

The blind holes preferably each have a diameter between 2 .mu.m and 20 .mu.m, more preferably between 2 .mu.m and 10 .mu.m. In these areas of the diameter of the blind holes, effectively oxidized regions can be generated in the current constricting layer.

In a preferred embodiment, the blind holes are formed in a ring-shaped arrangement around a central region of the semiconductor layer sequence. The annular arrangement of the blind holes preferably has a diameter between 20 .mu.m and 60 .mu.m. The centers of the blind holes are preferably arranged on a circular line, particularly preferably at equal angular intervals. The arrangement of the blind holes can in particular be rotationally symmetrical.

In accordance with at least one embodiment, a central region of the current constriction layer is not oxidized. In the central region, a current flow through the semiconductor layer sequence and the active layer is made possible in this way. The central region preferably has a lateral extent of not more than 20 μm, particularly preferably not more than 10 μm. By narrowing the current flow to such a small central region, in particular a single-mode operation of the surface-emitting semiconductor laser can be achieved.

In a further preferred embodiment, the blind holes do not extend into the active layer. In this way, the formation of defects in the active layer is advantageously reduced. In particular, the oxidized regions are produced only outside the active layer. For example, the current constriction layer may be disposed between one of the reflectors and the active layer.

In a method for producing the surface-emitting semiconductor laser, according to at least one embodiment, firstly a semiconductor layer sequence is produced, which has an active layer arranged between reflectors, wherein the reflectors form a vertical resonator. In particular, the semiconductor layer sequence can be produced by an epitaxial method such as MOVPE.

In a subsequent step, blind holes are advantageously formed in the semiconductor layer sequence, which extend in the direction of the vertical resonator into the semiconductor layer sequence. The blind holes thus extend perpendicular to the layer planes of the semiconductor layer sequence into the semiconductor layer sequence.

In a further step, an oxidation process is advantageously carried out in the method in which, starting from the blind holes, at least one current constriction layer is oxidized in regions. The partial oxidation of the current constricting layer can be carried out, for example, in such a way that oxidized regions extend from the blind holes approximately 2 μm to 20 μm in the lateral direction into the current constriction layer.

In a preferred embodiment of the method, the blind holes are formed by means of an etching process. The etching depth is advantageously set in the etching process such that the blind holes do not penetrate the active layer. In this way, the risk is bypassed that form by the production of blind holes defects in the active layer.

Further advantageous embodiments of the method will become apparent from the previous description of the surface emitting semiconductor laser and vice versa.

The invention will be described below with reference to embodiments in connection with 1 and 2 explained in more detail.

• 1 eine schematische Darstellung eines Querschnitts durch einen oberflächenemittierenden Halbleiterlaser gemäß einem Ausführungsbeispiel, und
• 2A bis 2E eine schematische Darstellung eines Verfahrens zur Herstellung des oberflächenemittierenden Halbleiterlasers anhand von Zwischenschritten.

Show it:

• 1 a schematic representation of a cross section through a surface emitting semiconductor laser according to an embodiment, and
• 2A to 2E a schematic representation of a method for producing the surface emitting semiconductor laser based on intermediate steps.

Identical or equivalent elements are provided in the figures with the same reference numerals. The sizes of the individual elements and the size ratios of the elements with each other are not to be regarded as true to scale.

The in 1 illustrated surface emitting semiconductor laser 100 contains one on a substrate 1 applied semiconductor layer sequence 10 , which is an n-type semiconductor region 3 and a p-type semiconductor region 5 having. Between the n-type semiconductor region 3 and the p-type semiconductor region 5 is an active layer 4 arranged. The active layer 4 is preferably formed as a multiple quantum well structure.

The semiconductor layer sequence 10 is preferably based on an arsenide compound semiconductor, ie one or more in the semiconductor layer sequence 10 In particular, semiconductor layers contained include Al _x Ga _y In _1-xy As where 0 ≦ x ≦ 1, 0 ≦ y ≦ 1, and x + y ≦ 1. In particular, one or more layers of the n-type semiconductor region 3 , the active layer 4 and the p-type semiconductor region 5 formed from arsenide compound semiconductor materials. The substrate 1 In particular, it is a growth substrate suitable for growing an arsenide compound semiconductor material, for example a GaAs substrate.

In the surface emitting semiconductor laser 100 is the active layer 4 between a first reflector 2 and a second reflector 6 arranged, which form a vertical resonator. The resonator direction is vertical to the semiconductor layers of the semiconductor layer sequence 10 , The first reflector 2 is for example in the n-type semiconductor region 3 and the second reflector 6 in the p-type semiconductor region 5 arranged. The reflectors 2 . 6 are preferably each formed as a DBR mirror. For example, the reflectors 2 . 6 each have a periodic layer sequence of alternating first Al _x1 Ga _1-x1 As layers and second Al _x2 Ga _1-x2 As layers, wherein the aluminum components x1 and x2 are different from each other. For example, x1 is about 0.2 and x2 is about 0.8.

For electrical contacting are in the surface-emitting semiconductor laser 100 a p-contact layer 11 and an n-contact layer 12 are provided. In the embodiment shown here, the p-contact layer 11 is on a central region of the p-type semiconductor region 5 arranged, and the n-contact layer 12 is at a rear side of the substrate 1 arranged. In the embodiment shown here, the emission of laser radiation takes place in a main emission direction 8th through the substrate 1 therethrough. The radiation exit surface is at the back of the substrate 1 arranged. In the surface emitting semiconductor laser shown here 100 is therefore a so-called bottom emitter. Alternatively, however, other arrangements of the contact layers and / or orientations of the main radiation direction are possible.

The n-contact layer 12 is designed in the embodiment shown here as a ring contact, in which a central region is recessed as the radiation exit surface of the surface-emitting semiconductor laser. The recessed from the ring contact light exit surface is preferably with a reflection-reducing coating 13 Mistake.

The semiconductor layer sequence 10 contains a current narrowing layer 9 , The current narrowing layer 9 is advantageously a semiconductor layer, the oxidized areas 9a has, which are electrically insulating. A central area 9b the current constriction layer 9 is not oxidized and electrically conductive. By means of the current constriction layer 9 becomes the current flow through the surface emitting semiconductor laser 100 Therefore, to a central region of the semiconductor layer sequence 10 concentrated. The oxidized areas 9a the current constriction layer 9 thus form current limiting diaphragms in the semiconductor layer sequence 10 out.

The oxidized areas 9a the current constriction layer 9 extend starting from in the semiconductor layer sequence 10 trained blind holes 7 into the current constricting layer 9 into it. The blind holes 7 extend from a surface of the surface emitting semiconductor laser 100 in the direction of the vertical resonator in the semiconductor layer sequence 10 into it, with the blind holes 7 preferably not into the active layer 4 extend. The blind holes 7 have in the manufacture of the surface emitting semiconductor laser 100 the function, the partial oxidation of the Stromeinengungsschicht 9 to enable.

Because the oxidized areas 9a the current constriction layer 9 are electrically insulating, the current flow during operation of the surface emitting semiconductor laser is not concentrated on the oxidized central area 9b which is electrically conductive. The unoxidized central area 9b the current constriction layer 9 preferably has a lateral extension of not more than 20 microns, more preferably not more than 10 microns, on. In this way, it is possible, in particular, a single-mode operation of the surface-emitting semiconductor laser 100 to enable.

The preparation of the oxidized areas 9a the current constriction layer 9 by oxidation starting from the blind holes 7 has the advantage that in the semiconductor layer sequence 10 in comparison to a mesa etching with subsequent oxidation from the mesa flank, comparatively few defects in the semiconductor layer sequence 10 arise. This is due in particular to the fact that in the case of a mesa etching, the semiconductor material of the semiconductor layer sequence is completely removed around a central region, so that comparatively large areas are produced at the edges of the mesa structure, which are formed by etching flanks. At these comparatively large Ätzflanken defects may arise, which may propagate in the operation of the surface emitting semiconductor laser in the semiconductor material and in particular can reach the active layer. In this case, there is the risk that the function will be impaired by defects in the active layer or even failure of the surface-emitting semiconductor laser.

The surface-emitting semiconductor laser described here 100 On the other hand, it is advantageously free of a mesa structure. Although the blind holes are 7 typically by an etching process in the semiconductor layer sequence 10 produced, but much less semiconductor material is removed than in a Mesaätzung. Thus, the risk is the defect in the semiconductor layer sequence 10 arise and get into the active layer 4 can spread, advantageously reduced. The surface emitting semiconductor laser 100 is therefore characterized by a particularly low defect density and a low risk of failure.

Surface emitting semiconductor lasers 100 After their manufacture, the manufacturer often first undergoes a test operation in which they are operated at high power over a long period of time, for example several hours (so-called burn-in). Since components with high defect density do not survive such a load test, it is possible in this way to be able to sort out defective components before delivery. The low failure risk of the surface-emitting semiconductor laser described here 100 may make it possible to advantageously dispense with such a test operation.

In the following 2A to 2E an embodiment of a method is shown, with which a surface emitting semiconductor laser according to the 1 can be produced.

As in 2A can be seen in a cross section shown schematically, in a first intermediate step, the semiconductor layer sequence 10 on a growth substrate 1 grown, in particular with an epitaxial process such as metalorganic vapor phase epitaxy (MOVPE). The semiconductor layer sequence 10 includes the n-type semiconductor region 3 with the first reflector 2 , the active layer 4 and the p-type semiconductor region 5 with the second reflector 6 , where the reflectors 2 . 6 are each formed as a DBR mirror. The semiconductor layer sequence 10 For example, in the p-type semiconductor region 5 as a current narrowing layer 9 provided semiconductor layer. The current narrowing layer 9 preferably has Al _x Ga _1-x As with x ≥ 0.95, particularly preferably 0.95 ≤ x ≤ 0.98. Al _x Ga _1-x As with such a high aluminum content is easily oxidized by a heat treatment.

2 B schematically shows a cross section through the semiconductor layer sequence 10 after another intermediate step. In this intermediate step are in the semiconductor layer sequence 10 blind holes 7 been prepared. The blind holes 7 extend, for example, from a growth substrate 1 opposite surface in the semiconductor layer sequence 10 into it. The blind holes 7 can in particular in the p-type semiconductor region 5 be formed. The blind holes 7 are preferably produced with an etching process, in particular with a dry etching process. For this purpose, a mask layer is advantageous 14 used. The etching time is preferably set such that the blind holes 7 at least through the current constricting layer 9 but not into the active layer 4 into it. The current narrowing layer 9 For example, in the p-type semiconductor region 5 between the second reflector 6 and the active layer 4 be arranged.

2C schematically shows a plan view of the semiconductor body in the intermediate step of 2 B , where the 2 B the cross section along in 2C drawn line AB represents. As can be seen in the plan view, the blind holes 7 annularly arranged around a central region of the semiconductor layer sequence. The annular arrangement 15 preferably has a diameter of about 20 microns to about 60 microns. The blind holes 7 have a diameter of about 2 microns to 20 microns, preferably from 2 microns to 10 microns, on. The preferred number of blind holes 7 is between 4 and 12. In the embodiment shown here are eight blind holes 7 in the semiconductor layer sequence 10 educated.

In 2D is the semiconductor body in a further intermediate step schematically in a cross section and in 2E schematically shown in a plan view. In this step are in the current constriction layer 9 starting from the blind holes 7 oxidized areas 9a been generated. The oxidized areas 9a extend from the blind holes 7 in the radial direction in the current constricting layer 9 into it. The preparation of the oxidized areas 9a takes place by a temperature treatment, preferably in an oven at a temperature T> 100 ° C. The temperature treatment can be carried out in particular at elevated atmospheric humidity. The duration of the temperature treatment is chosen so that in the center of the Stromeinengungsschicht a central area 9b remains, which is not oxidized. The lateral extent of the unoxidized area 9b is preferably less than 20 microns, more preferably less than 10 microns.

After performing the temperature treatment, the surface emitting semiconductor laser 100 as he is for example in 1 is shown by the application of the electrical contact layers 11 . 12 and optionally the application of a reflection-reducing coating 13 be completed at the radiation exit surface. The features described herein in connection with the method can also be used for the surface-emitting semiconductor laser 100 of the 1 apply and vice versa.

The invention is not limited by the description with reference to the embodiments. Rather, the invention encompasses any novel feature as well as any combination of features, which in particular includes any combination of features in the claims, even if this feature or combination itself is not explicitly stated in the patent claims or exemplary embodiments.

LIST OF REFERENCE NUMBERS

1

substratum

2

first reflector

3

n-type semiconductor region

4

active layer

5

p-type semiconductor region

6

second reflector

7

blind

8th

main radiation

9

Current confinement layer

9a

oxidized area

9b

non-oxidized area

10

Semiconductor layer sequence

11

p-contact layer

12

n-contact layer

13

reflection-reducing coating

14

mask layer

100

surface-emitting semiconductor laser

QUOTES INCLUDE IN THE DESCRIPTION

This list of the documents listed by the applicant has been generated automatically and is included solely for the better information of the reader. The list is not part of the German patent or utility model application. The DPMA assumes no liability for any errors or omissions.

Cited patent literature

• WO 02/063733 A2 [0002]

Claims (15)

Surface-emitting semiconductor laser (100) having a vertical resonator formed by reflectors (2, 6), comprising a semiconductor layer sequence (10) having an active layer (4) arranged between the reflectors (2, 6), wherein in the semiconductor layer sequence (10) in the direction of the vertical resonator a plurality of blind holes (7) are formed, and wherein the semiconductor layer sequence (10) comprises at least one current constriction layer (9) having oxidized regions (9a) extending into the current constriction layer (9) from the blind holes (7). Surface emitting semiconductor laser according to Claim 1 , wherein the semiconductor layer sequence (10) is free of a mesa structure. A surface emitting semiconductor laser according to any one of the preceding claims, wherein said current constricting layer (9) comprises Al _x Ga _1-x As where x ≥ 0.95. A surface emitting semiconductor laser according to any one of the preceding claims, wherein said current constricting layer (9) is between 30 nm and 200 nm thick. A surface emitting semiconductor laser according to any one of the preceding claims, wherein a number of the blind holes (7) is between 4 and 12. Surface emitting semiconductor laser according to one of the preceding claims, wherein the blind holes (7) have a diameter of 2 microns to 20 microns. Surface emitting semiconductor laser according to one of the preceding claims, wherein the blind holes (7) have an annular arrangement. Surface emitting semiconductor laser according to Claim 7 , wherein the annular arrangement has a diameter between 20 microns and 60 microns. A surface emitting semiconductor laser according to any one of the preceding claims, wherein a central portion (9b) of the current constricting layer (9) is not oxidized, and wherein the central portion (9b) has a lateral extension of not more than 20 μm. Surface-emitting semiconductor laser according to one of the preceding claims, wherein the blind holes (7) do not extend into the active layer (4). A method of making a surface emitting semiconductor laser (100) comprising the steps of: Producing a semiconductor layer sequence (10) which has an active layer (4) arranged between reflectors (2, 6), wherein the reflectors (2, 6) form a vertical resonator, Forming a plurality of blind holes (7) in the semiconductor layer sequence (10) which extend in the direction of the vertical resonator into the semiconductor layer sequence (10), - Carrying out an oxidation process, in which, starting from the blind holes (7) a Stromeinengungsschicht (9) is partially oxidized. Method according to Claim 11 wherein the blind holes (7) are formed by means of an etching process, and wherein an etching depth is set such that the blind holes (7) do not penetrate the active layer (4). Method according to Claim 11 or 12 wherein the blind holes (7) are formed in an annular arrangement. Method according to Claim 13 , wherein the annular arrangement has a diameter between 20 microns and 60 microns. Method according to one of Claims 11 to 14 , wherein a number of the blind holes (7) is between 4 and 12.

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