WO2023247129A1

WO2023247129A1 - Surface emitting semiconductor laser diode

Info

Publication number: WO2023247129A1
Application number: PCT/EP2023/063901
Authority: WO
Inventors: Hubert Halbritter; Christoph Eichler; Jelena Ristic; Damir Borovac; Åsa HAGLUND; Joachim CIERS
Original assignee: Ams-Osram International Gmbh
Priority date: 2022-06-24
Filing date: 2023-05-24
Publication date: 2023-12-28

Abstract

A surface emitting semiconductor laser diode is specified herein, comprising: - a semiconductor layer sequence (1) with a p-doped side (2), an n-doped side (3) and an active layer (4) configured for emitting electromagnetic radiation arranged between the p- doped side (2) and the n-doped side (3), and - an optical resonator (5) with an optical axis (A) parallel to a growth direction (G) of the semiconductor layer sequence (1), wherein - the semiconductor layer sequence (1) is arranged inside the optical resonator (5) such that the electromagnetic radiation emitted during operation forms a standing wave (S) along the optical axis (A) of the optical resonator (5), - the p-doped side (2) comprises a contact layer (6) and an undoped spacer layer (7) arranged between the active layer (4) and the contact layer (6), and - a thickness (D) of the spacer layer (7) is configured such that the active layer (4) is arranged in a region of an anti- node (8) of the standing wave (S) and the contact layer (6) is arranged in a region of a node (9) of the standing wave (S).

Description

SURFACE EMITTING SEMICONDUCTOR LASER DIODE

A surface emitting semiconductor laser diode is specified herein .

At least one object of certain embodiments is to specify a surface emitting semiconductor laser diode with an increased efficiency.

According to at least one embodiment the surface emitting semiconductor laser diode comprises a semiconductor layer sequence with a p-doped side, an n-doped side and an active layer configured for emitting electromagnetic radiation arranged between the p-doped side and the n-doped side. For example, the semiconductor layer sequence comprises or consists of a III/V compound semiconductor material.

A III/V compound semiconductor material comprises at least one element from the third main group, such as B, Al, Ga, In, and one element from the fifth main group, for example N, P, As. In particular, the term "III/V compound semiconductor material" includes the group of binary, ternary or quaternary compounds containing at least one element from the third main group and at least one element from the fifth main group. Moreover, the III/V semiconductor material may comprise one or more dopants.

Preferably, the semiconductor layer sequence comprises or consists of a nitride compound semiconductor material. Nitride compound semiconductor materials are III/V compound semiconductor materials comprising nitrogen, such as the materials from the system In_xAl_yGai-_x-yN with 0 < x < 1 , 0 < y < 1 and x+y < 1 , denoted as indium aluminium gallium nitride in the following . For example , gallium nitride with x=y=0 or indium gallium nitride with y=0 are materials from this system .

In particular, the p-doped side comprises at least one semiconductor layer that is p-doped and the n-doped side comprises at least one semiconductor layer that is n-doped . Here and in the following "p-doped" refers to semiconductor materials comprising dopant atoms acting as electron acceptors , whereas "n-doped" refers to semiconductor materials comprising dopant atoms acting as electron donors .

The active layer may comprise a double heterostructure , a single quantum well structure or a multi quantum well structure . A multi quantum well structure comprises a plurality of quantum well layers separated by barrier layers . The barrier layers preferably exhibit a larger bandgap than the quantum well layers . The arrangement of quantum well layers and barrier layers leads to a confinement of electric charges in the quantum well layers , giving rise to discrete energy values for the confined electric charges . Preferably, the multi quantum well structure comprises at least 3 and at most 5 quantum well layers .

The active layer is configured to emit electromagnetic radiation in a spectral range between infrared light and ultraviolet light , for example . Preferably, the active layer is configured to emit electromagnetic radiation in a spectral range between red light and ultraviolet light . According to at least one embodiment , the surface emitting semiconductor laser diode comprises an optical resonator with an optical axis parallel to a growth direction of the semiconductor layer sequence . Here and in the following, the growth direction refers to a direction perpendicular to a main extension plane of layers in the semiconductor layer sequence .

The optical resonator preferably comprises two mirrors and/or reflectors , arranged such that the electromagnetic radiation is reflected multiple times between them . In particular, the optical axis of the optical resonator is parallel to a propagation direction of electromagnetic radiation between the two reflectors . For example , reflecting surfaces of the two reflectors are aligned parallel to each other and facing each other, such the optical axis is perpendicular to the reflecting surfaces of the two reflectors . Electromagnetic radiation generated by the active layer during operation is coupled out of the surface emitting semiconductor laser diode via one or both reflectors of the optical resonator . In other words , electromagnetic radiation is emitted from the surface emitting semiconductor laser diode in the direction of the optical axis of the optical resonator .

According to at least one embodiment of the surface emitting semiconductor laser diode , the semiconductor layer sequence is arranged inside the optical resonator such that the electromagnetic radiation emitted during operation forms a standing wave along the optical axis of the optical resonator . In particular, the semiconductor layer sequence is arranged between the two reflectors forming the optical resonator . Preferably, the optical resonator together with the active layer is configured to generate electromagnetic laser radiation during operation . Electromagnetic laser radiation is generated by stimulated emission . In comparison to electromagnetic radiation generated by spontaneous emission, electromagnetic laser radiation has a smaller bandwidth, a larger coherence length, and a larger degree of polari zation .

According to at least one embodiment of the surface emitting semiconductor laser diode , the p-doped side comprises a contact layer and an undoped spacer layer arranged between the active layer and the contact layer . Preferably, the contact layer is configured for an external electrical contacting of the semiconductor layer sequence . For example , a solder metal may be in direct contact with the contact layer .

Preferably, the undoped spacer layer comprises or consists of an unintentionally doped semiconductor material . In other words , no dopant atoms are intentionally introduced into the semiconductor material of the spacer layer . The spacer layer may comprise impurity atoms that are unintentionally introduced into the spacer layer during epitaxial growth of the spacer layer, for example . These impurity atoms may act as dopants in the semiconductor material of the spacer layer . Preferably, the concentration of impurity atoms is low, such that a free charge carrier concentration in the unintentionally doped semiconductor material does not exceed 10¹⁷ per cm³, for example .

According to at least one embodiment of the surface emitting semiconductor laser diode , a thickness of the spacer layer is configured such that the active layer is arranged in a region of an anti-node of the standing wave , and the contact layer is arranged in a region of a node of the standing wave . Here and the following the thickness refers to a spatial extension in growth direction . Moreover, here and in the following a "node" of the standing wave refers to a position along the optical axis of the optical resonator, where the electromagnetic intensity of the standing wave vanishes or has a local minimum . Likewise , here and in the following an "anti-node" of the standing wave refers to a position along the optical axis of the optical resonator, where the electromagnetic intensity of the standing wave has a local maximum .

Preferably the active layer is arranged at an anti-node of the standing wave , whereas the contact layer is arranged at a node of the standing wave . In particular, the active layer can be arranged at the anti-node of the standing wave , i f the anti-node is located within the active layer . Preferably, the anti-node is located at a center of the active layer . Likewise , the contact layer is arranged at the node of the standing wave , i f the node is located within the contact layer . Preferably, the node is located at a center of the contact layer .

I f the active layer is arranged in a region of the anti-node of the standing wave , the position of the anti-node along the optical axis may deviate from the active layer by at most a fi fth, preferably a seventh and most preferably a tenth of a wavelength of the standing wave . Likewise , i f the contact layer is arranged in a region of the node of the standing wave , the position of the node along the optical axis may deviate from the active layer by at most a fi fth, preferably a seventh and most preferably a tenth of the wavelength of the standing wave .

According to a preferred embodiment , the surface emitting semiconductor laser diode comprises :

- the semiconductor layer sequence with the p-doped side , the n-doped side and the active layer configured for emitting electromagnetic radiation arranged between the p-doped side and the n-doped side , and

- the optical resonator with the optical axis parallel to the growth direction of the semiconductor layer sequence , wherein

- the semiconductor layer sequence is arranged inside the optical resonator such that the electromagnetic radiation emitted during operation forms the standing wave along the optical axis of the optical resonator,

- the p-doped side comprises the contact layer and the undoped spacer layer arranged between the active layer and the contact layer, and

- the thickness of the spacer layer is configured such that the active layer is arranged in the region of an anti-node of the standing wave and the contact layer is arranged in the region of a node of the standing wave .

The surface emitting semiconductor laser diode described herein is based on the idea that the undoped spacer layer on the p-doped side increases an ef ficiency, in particular an external quantum ef ficiency, and thus a performance of the surface emitting semiconductor laser diode . For example , configuring the thickness of the spacer layer such the active layer is arranged in the region of the anti-node of the standing wave increases a coupling between the active layer and electromagnetic radiation inside the optical resonator . Similarly, arranging the contact layer in a region of the node of the standing wave decreases an absorption of the electromagnetic radiation by the contact layer .

Moreover, the undoped spacer layer absorbs less electromagnetic radiation emitted by the active layer during operation than a p-doped spacer layer, for example . Therefore , the undoped spacer layer gives rise to an improved ef ficiency and thus to an increased performance of the surface emitting semiconductor laser diode .

A thick undoped spacer layer directly arranged on the active layer also prevents di f fusion of p-type dopant atoms , such as magnesium in a nitride compound semiconductor material , into the active layer . Therefore , the active layer advantageously has a better material quality and the surface emitting semiconductor laser diode is improved with respect to aging and degradation . Moreover, the thick undoped spacer layer can improve the thermal management of the surface emitting semiconductor laser diode . For example , the thick undoped spacer layer may act as a heat-spreader, i f the optical resonator comprises a distributed Bragg reflector ( DBR) arranged on a main surface of the semiconductor layer sequence on the p-doped side . Here and in the following, the main surface is an outer surface of the semiconductor layer sequence that is parallel to the main extension plane of layers in the semiconductor layer sequence .

During operation of the surface emitting semiconductor laser diode , electric charge carriers , in particular holes , may be provided by a highly doped p-type layer on a side of the undoped spacer layer facing away from the active layer . The thick undoped spacer layer thus has little or no impact on the forward voltage of the surface emitting semiconductor laser diode .

According to at least one embodiment of the surface emitting semiconductor laser diode , the semiconductor layer sequence comprises a nitride compound semiconductor material and the spacer layer comprises unintentionally doped gallium nitride . Preferably, the semiconductor layer sequence comprises indium aluminium gallium nitride . Moreover, the spacer layer may comprise indium or aluminium . In other words , the spacer layer may comprise unintentionally doped indium aluminium gallium nitride .

According to at least one embodiment of the surface emitting semiconductor laser diode , the contact layer comprises a transparent conductive oxide and/or a tunnel j unction . Transparent conductive oxides are transparent and electrically conductive oxides , in particular metal oxides , such as zinc oxide , tin oxide , cadmium oxide , titanium oxide , indium oxide or indium tin oxide ( ITO) . Besides binary metal oxides , such as ZnO, Sn02 , or In₂03, also ternary metal oxides , such as Zn₂SnO4 , ZnSnO₂, GalnOa, or mixtures of di f ferent transparent conductive oxides are considered as transparent conductive oxides herein . Moreover, transparent conductive oxides may be p-doped or n-doped . The transparent conductive oxide preferably forms an outer layer of the semiconductor layer sequence .

The tunnel j unction is a pn-j unction formed by a highly p- doped semiconductor layer facing the active layer and a highly n-doped semiconductor layer facing away from the active layer . Here and in the following, "highly doped", "highly p-doped", or "highly n-doped" refers to a dopant concentration between 5*10¹⁸ per cm³ and 5*10²⁰ per cm³, preferably between 10¹⁹ per cm³ and 10²⁰ per cm³, inclusive. By contrast, "n-doped" or "p- doped" refers to a dopant concentration between 5*10¹⁷ per cm³ and 5*10¹⁹ per cm³, preferably between 10¹⁸ per cm³ and 2*10¹⁹ per cm³, inclusive.

Preferably, the thickness of the tunnel junction is small. For example, the tunnel junction has a thickness of at most 20 nanometers, preferably at most 10 nanometers, and particularly preferably at most 5 nanometers. If the contact layer comprises a tunnel junction, the contact layer preferably further comprises an n-doped semiconductor layer on a side of the tunnel junction facing away from the active layer .

According to at least one embodiment, the surface emitting semiconductor laser diode comprises an electron blocking layer arranged between the spacer layer and the contact layer. For example, the electron blocking layer is configured to decrease a leakage current of electrons from the active layer to the p-doped side.

According to at least one embodiment of the surface emitting semiconductor laser diode, the electron blocking layer comprises p-doped indium aluminium gallium nitride. In particular, the electron blocking layer comprises p-doped In_xAl_yGai-_x-yN with y > 0,1, preferably y > 0,15 and particularly preferably y > 0,2.

According to at least one embodiment of the surface emitting semiconductor laser diode, a highly doped p-layer is arranged between the electron blocking layer and the contact layer . The highly doped p-layer preferably has a dopant concentration of at least 10¹⁹ per cm³ , particularly preferably of at least 10²⁰ per cm³. The highly doped p-layer is advantageous for providing a good electrical contact to a transparent conductive oxide in the contact layer, for example .

According to at least one embodiment of the surface emitting semiconductor laser diode , the thickness of the undoped spacer layer is larger than a thickness of the highly doped p-layer and/or larger than a thickness of the electron blocking layer . For example , the spacer layer may have a larger thickness than all other semiconductor layers on the p-doped side .

For example , the thickness of the undoped spacer layer may be larger than a combined thickness of all p-doped semiconductor layers together . In particular, the thickness of the undoped spacer layer may be two , three , five or ten times larger than the combined thickness of all p-doped semiconductor layers together .

According to at least one embodiment , the surface emitting semiconductor laser diode comprises a first current spreading layer arranged in the region of a node of the standing wave on the n-doped side . In particular, the position of the node along the optical axis may deviate by at most a fi fth, preferably a seventh and most preferably a one tenth of the wavelength of the standing wave .

For example , the first current spreading layer is n-doped and has a higher concentration of dopants than other semiconductor layers on the n-doped side . In particular the first current spreading layer is configured for an improved lateral distribution of an electrical current flowing through the semiconductor layer sequence during operation of the surface emitting semiconductor laser diode . Here and in the following " lateral" refers to a direction parallel to the main extension plane of layers in the semiconductor layer sequence . In particular, a lateral direction is perpendicular to the growth direction .

Preferably, a thickness of the current spreading layer is smaller than a seventh or a tenth of the wavelength of the standing wave . Arranging the current spreading layer in a node advantageously decreases the absorption of electromagnetic radiation by the current spreading layer .

According to at least one embodiment of the surface emitting semiconductor laser diode , the first current spreading layer comprises highly n-doped gallium nitride .

According to at least one embodiment , the surface emitting semiconductor laser diode further comprises an n-contact configured for electrically contacting the semiconductor layer sequence on the n-doped side , and the n-contact is in direct contact with the first current spreading layer . For example , the n-contact comprises a metal or consists of a metal .

According to at least one embodiment of the surface emitting semiconductor laser diode , a second current spreading layer is arranged in the region of a node of the standing wave between the first current spreading layer and the active layer . Preferably, the second current spreading layer comprises highly n-doped indium aluminium gallium nitride and an n-doped indium aluminium gallium nitride layer is arranged between the first current spreading layer and the second current spreading layer . For example , the second current spreading layer is configured for an increased lateral distribution of the electrical current during operation of the surface emitting semiconductor laser diode . Preferably, the second current spreading layer has the same or a similar thickness as the first current spreading layer . The first current spreading layer and the second current spreading layer may have the same or a similar concentration of dopants .

According to at least one embodiment of the surface emitting semiconductor laser diode , the n-doped side comprises an n- doped indium gallium nitride layer adj acent to the active layer . Preferably, the active layer is directly disposed on the n-doped indium gallium nitride layer . In particular, the n-doped indium gallium nitride layer is configured to improve a crystal quality of the active layer during epitaxial growth of the semiconductor layer sequence . For example , by epitaxially growing the active layer on the n-doped indium gallium nitride layer, the active layer may have a smaller density of crystal defects .

According to at least one embodiment of the surface emitting semiconductor laser diode , the n-doped indium gallium nitride layer has a thickness of at least 10 nanometer . Preferably, the n-doped indium gallium nitride layer has a thickness of at least 30 nanometer, particularly preferably of at least 50 nanometer .

According to at least one embodiment of the surface emitting semiconductor laser diode , the n-doped indium gallium nitride layer comprises In_xG i-_xN with x > 0 , 01 and x < 0 , 08 , preferably with x > 0 , 03 and x < 0 , 05 .

According to at least one embodiment of the surface emitting semiconductor laser diode , the optical resonator comprises a distributed Bragg reflector . The distributed Bragg reflector is configured to reflect at least a part of the electromagnetic radiation emitted by the active layer during operation . For example , the distributed Bragg reflector reflects at least 90 % of incident electromagnetic radiation emitted by the active layer . Preferably, the reflectivity of the distributed Bragg reflector is at least 99 % for electromagnetic radiation emitted by the active layer during operation .

For example , the distributed Bragg reflector comprises a plurality of dielectric layers , with an alternate arrangement of layers with two di f ferent refractive indices . The dielectric layers may be disposed on the main surface of the semiconductor layer sequence . For example , the dielectric layers comprise a semiconductor material that is epitaxially grown on the main surface of the semiconductor layer sequence . Alternatively and/or in addition, the distributed Bragg reflector may be a hybrid reflector comprising dielectric and/or metallic layers disposed on main surfaces of the semiconductor layer sequence and/or on the main surface a growth substrate for the semiconductor layer sequence , for example .

According to at least one embodiment of the surface emitting semiconductor laser diode , two distributed Bragg reflectors are directly arranged on opposite main surfaces of the semiconductor layer sequence , and the semiconductor layer sequence is free of a growth substrate . In particular, a growth substrate configured for epitaxial growth of the semiconductor layer sequence is removed from the semiconductor layer sequence and the distributed Bragg reflectors are directly arranged on opposite main surfaces .

According to at least one embodiment of the surface emitting semiconductor laser diode , the semiconductor layer sequence comprises a step index waveguide on the n-doped side and/or on the p-doped side . The step index waveguide comprises a step in the main surface of the semiconductor layer sequence on the n-doped side and/or on the p-doped side , for example . In particular, the semiconductor layer sequence has a larger thickness in a region that defines an aperture of the surface emitting semiconductor laser diode . For example , a height of the step is at most 50 nm, preferably at most 10 nm . In other words , the thickness of the semiconductor layer sequence in the region of the aperture is by at most 50 nm, preferably by at most 10 nm, larger than outside the region of the aperture . Alternatively, the semiconductor layer sequence may have a thickness in the region of the aperture that is at most 50 nm, preferably at most 10 nm, smaller than a thickness of the semiconductor layer sequence outside the region of the aperture .

The step in the main surface of the semiconductor layer sequence may be filled and/or covered by a dielectric material . For example , the dielectric material is part of the distributed Bragg reflector . In particular, a shape of layers of the distributed Bragg reflector may follow the shape of the step index waveguide . In other words , the layers of the distributed Bragg reflector may have the same or a similar surface morphology as the main surface of the semiconductor layer sequence .

The step index waveguide is configured for a better lateral confinement of the standing wave within the optical resonator, for example . In particular, optical losses may be reduced due to the step index waveguide . Moreover, the step index waveguide may be configured for a better match of a wavefront of the standing wave to the active layer . For example , the step index waveguide is configured such that the standing wave has a plane wave- front in a region of the active layer .

Further advantageous embodiments and further embodiments of the surface emitting semiconductor laser diode will become apparent from the following exemplary embodiments described in connection with the figures .

Figure 1 shows a schematic cross-section of a surface emitting semiconductor laser diode according to an exemplary embodiment .

Figures 2 to 4 show schematic cross-sections of parts of a semiconductor layer sequence of a surface emitting semiconductor laser diode according to exemplary embodiments .

Figure 5 shows a schematic cross-section of a semiconductor layer sequence of a surface emitting semiconductor laser diode according to an exemplary embodiment .

Figures 6 to 12 show schematic cross-sections of surface emitting semiconductor laser diodes according to di f ferent exemplary embodiments . Elements that are identical , similar, or have the same ef fect , are denoted by the same reference signs in the figures . The figures and the proportions of the elements shown in the figures are not to be regarded as true-to-scale . Rather, individual elements , in particular layer thicknesses may be shown exaggeratedly large for better representability and/or better understanding .

The surface emitting semiconductor laser diode according to the exemplary embodiment in Figure 1 comprises a semiconductor layer sequence 1 arranged between two distributed Bragg reflectors 17 forming an optical resonator 5 . Figure 1 shows the distributed Bragg reflectors 17 arranged at a distance from the semiconductor layer sequence 1 . Preferably, the distributed Bragg reflectors 17 are in direct contact with the semiconductor layer sequence 1 . The optical axis A of the optical resonator 5 is arranged parallel to the growth direction G of the semiconductor layer sequence 1 . Electromagnetic radiation emitted by the active layer 4 during operation thus forms a standing wave S along the optical axis A of the optical resonator 5 .

The semiconductor layer sequence 1 comprises an n-doped side 3 , an active layer 4 and a p-doped side 2 arranged in sequence along the growth direction G . On the p-doped side 2 , an undoped spacer layer 7 is directly disposed on the active layer 4 . The undoped spacer layer 7 comprises unintentionally doped gallium nitride . In other words , the undoped spacer layer 7 is nominally undoped, but may contain a small concentration of impurity atoms acting as dopants . Moreover, the undoped spacer layer 7 may comprise indium and/or aluminium . Following the undoped spacer layer 7 in growth direction G is an electron blocking layer 10 . The electron blocking layer 10 comprises p-doped In_xAl_yGai-_x-yN with y > 0 , 1 . A highly p-doped layer 11 comprising gallium nitride , as well as a contact layer 6 , follow the electron blocking layer 10 in growth direction G . The highly p-doped layer 11 is in direct contact with the contact layer 6 and preferably has a dopant concentration of at least 10¹⁹ per cm³ to establish a good electrical contact with the contact layer 6 . The contact layer 6 either comprises a tunnel j unction, a transparent conductive oxide , or a highly p-doped gallium nitride layer . In particular, the contact layer 6 is configured for an external electrical contacting of the semiconductor layer sequence 1 and may be in direct contact with a p-contact 19 comprising a solder metal , for example .

The thickness D of the undoped spacer layer 7 is adj usted such that the active layer 4 is arranged in a region of an anti-node 8 of the standing wave S , while the contact layer 6 is arranged in a region of a node 9 of the standing wave S . This arrangement advantageously improves a coupling between the active layer 4 and a resonator mode corresponding to the standing wave S in the optical resonator 5 , while an absorption of the electromagnetic radiation by the contact layer 6 is advantageously reduced .

A schematic exemplary graph of a modulus of an electric field E of the standing wave S along the optical axis A of the optical resonator 5 is shown next to the surface emitting semiconductor laser diode in Figure 1 . In this example , a spatial distance between the active layer 4 and the contact layer 6 in growth direction G corresponds to approximately one quarter of the wavelength of the standing wave S . More generally, the distance between the active layer 4 and the contact layer 6 may correspond to an odd integer multiple of one quarter of the wavelength of the standing wave S .

The n-doped side 3 comprises an n-doped indium gallium nitride layer 14 as an underlayer for the active layer 4 , as well as a current spreading layer 12 and an n-doped gallium nitride layer 16 in between . In particular, the active layer 4 directly follows the n-doped indium gallium nitride layer 14 in growth direction G . The n-doped indium gallium nitride layer 14 is configured for improving the crystal quality of the active layer 4 during epitaxial growth of the semiconductor layer sequence 1 .

The current spreading layer 12 comprises highly n-doped gallium nitride and is configured for an improved lateral distribution of an electrical current flowing through the semiconductor layer sequence 1 during operation . In particular, the current spreading layer 12 may be in direct contact with an n-contact 15 comprising a solder metal for an electrical contacting of the semiconductor layer sequence 1 , as shown in Figures 9 to 12 , for example . By arranging the current spreading layer 12 in the region of a node 9 of the standing wave S , the absorption of electromagnetic radiation by the current spreading layer 12 is advantageously reduced .

The distributed Bragg reflectors 17 may be arranged directly on opposite main surfaces of the semiconductor layer sequence 1 . For example , the distributed Bragg reflectors 17 are epitaxially grown together with the semiconductor layer sequence 1 , and/or disposed on the main surface after epitaxial growth . In the former case , the distributed Bragg reflectors 17 comprise a plurality of alternating semiconductor layers with two di f ferent refractive indices . In the latter case , the alternating layers may comprise two di f ferent dielectric materials with di f ferent refractive indices , for example . The distributed Bragg reflectors 17 may also comprise dielectric and/or metallic layers .

Figure 2 shows a schematic cross-section of a part of the semiconductor layer sequence 1 , in particular the active layer 4 together with the p-doped side 2 while omitting the n-doped side 3 , according to an exemplary embodiment of the surface emitting semiconductor laser diode . Also shown are two exemplary schematic graphs of the electric field distribution of the standing wave S along the optical axis A.

The structure of the p-doped side 2 in Figure 2 is analogous to the p-doped side 2 described in connection with Figure 1 . In addition, a p-doped gallium nitride layer 20 is arranged between the electron blocking layer 10 and the highly p-doped layer 11 .

The active layer 4 has a thickness of approximately 30 nanometers and comprises three to five quantum well layers comprising indium aluminium gallium nitride separated by barrier layers comprising In_xGai-_xN with 0 , 02 < x < 0 , 05 , preferably x = 0 , 04 within a tolerance of 10% . A thickness of the contact layer 6 is approximately 20 nanometers i f it comprises a transparent conductive oxide , or approximately 20 nanometers i f it comprises a tunnel j unction .

The thicknesses of the remaining layers , in particular the thickness of the undoped spacer layer 7 , depends on the wavelength of the electromagnetic radiation emitted by the active layer 4 . For example , i f the surface emitting semiconductor laser diode is configured for emitting electromagnetic laser radiation at a wavelength of 450 nanometers and i f the contact layer 6 comprises a transparent conductive oxide , the total thickness of the undoped spacer layer 7 together with the electron blocking layer 10 , the p- doped gallium nitride layer 20 and the highly p-doped layer 11 is approximately 22 + n * 90 nanometers , where n is a nonnegative integer number . Here , 90 nanometers corresponds to one hal f of the wavelength of the standing wave S , i . e . one hal f of the wavelength of the electromagnetic radiation inside the semiconductor material . However, i f the contact layer 6 comprises a tunnel j unction instead of the transparent conductive oxide , the total thickness of the undoped spacer layer 7 together with the electron blocking layer 10 , the p-doped gallium nitride layer 20 and the highly p-doped layer 11 is approximately 30 + n * 90 nanometers , with n a non-negative integer number .

The two schematic graphs of the modulus of the electric field E of the standing wave S along the optical axis A shown in Figure 2 correspond to n=0 and n=l , respectively . In other words , the distance between the active layer 4 and the contact layer 6 is approximately equal to one quarter of the wavelength, or approximately equal to three quarters of the wavelength of the standing wave S . For example , for a surface emitting semiconductor laser diode emitting electromagnetic laser radiation at a wavelength of 450 nanometers and comprising gallium nitride with a refractive index of approximately 2 , 5 for light at 450 nanometers , one quarter of the wavelength of the standing wave S corresponds to approximately 45 nanometers . Figure 3 shows a schematic cross-section of a part of the semiconductor layer sequence 1 , in particular the active layer 4 together with the n-doped side 3 while omitting the p-doped side 2 , according to an exemplary embodiment of the surface emitting semiconductor laser diode . Also shown are two exemplary graphs of the electric field distribution of the standing wave S along the optical axis A.

The structure of the n-doped side 3 in Figure 3 is analogous to the n-doped side 3 described in connection with Figure 1 . In addition, an n-doped gallium nitride layer 16 and an unintentionally doped gallium nitride layer 21 are arranged on the side of the current spreading layer 12 facing away from the active layer 4 .

The n-doped indium gallium nitride layer 14 has a thickness between 10 nanometers and 50 nanometers , inclusive . The thickness of the current spreading layer 12 is at most a tenth of the wavelength of the standing wave S . For example , the thickness of the current spreading layer 12 is at most 20 nanometers for a surface emitting semiconductor laser diode comprising gallium nitride and emitting electromagnetic laser radiation at 450 nanometers .

While the active layer 4 is arranged in an anti-node 8 , the current spreading layer 12 is arranged in a node 9 of the standing wave S . The two schematic graphs of the modulus of the electric field E of the standing wave S along the optical axis A in shown Figure 3 show examples , where a distance between the active layer 4 and the current spreading layer 12 is approximately equal to one quarter of the wavelength, or approximately equal to three quarters of the wavelength of the standing wave S . Figure 4 shows a schematic cross-section of the active layer 4 together with the n-doped side 3 of the semiconductor layer sequence 1 according to a further exemplary embodiment of the surface emitting semiconductor laser diode . Also shown is a schematic exemplary graph of the electric field distribution of the standing wave S along the optical axis A.

Compared to Figure 3 , the n-doped side in Figure 4 comprises an additional , second current spreading layer 13 arranged between the first current spreading layer 12 and the n-doped indium gallium nitride layer 14 , separated from both by n- doped gallium nitride layers 16 . The second current spreading layer 13 is arranged in the region of a di f ferent node 9 of the standing wave S than the first current spreading layer 12 . For example , the distance between the active layer 4 and the second current spreading layer 13 is approximately one quarter of the wavelength, whereas the distance between the active layer 4 and the first current spreading layer 12 is approximately equal to three quarters of the wavelength of electromagnetic radiation inside the semiconductor material , as shown in the exemplary graph of the modulus of the electric field E along the optical axis A.

Figure 5 shows a schematic cross-section of a semiconductor layer sequence 1 of a surface emitting semiconductor laser diode according to an exemplary embodiment . In particular, the semiconductor layer sequence 1 comprises a p-doped side 2 according to the exemplary embodiment described in connection with Figure 2 , and an n-doped side 3 according to the exemplary embodiment described in connection with Figure 3 , with an active layer 4 in between . The surface emitting semiconductor laser diode according to the exemplary embodiment in Figure 6 comprises a semiconductor layer sequence 1 arranged between two distributed Bragg reflectors 17 forming an optical resonator 5 as in Figure 1 . Figure 6 shows the distributed Bragg reflectors 17 arranged at a distance from the semiconductor layer sequence 1 . Preferably, the distributed Bragg reflectors 17 are in direct contact with the semiconductor layer sequence 1 . The p-doped side 2 has a structure in accordance with the p-doped side 2 described in connection with the exemplary embodiment Figure 2 . However, the contact layer 6 neither comprises a transparent conductive oxide , nor a tunnel j unction . Instead, the contact layer is a highly p- doped gallium nitride layer . The n-doped side 3 of the semiconductor layer sequence 1 in Figure 6 has a structure according to one of the exemplary embodiments described in connection with Figure 3 or Figure 4 .

In contrast to Figure 6 , the p-doped side 2 of the semiconductor layer sequence 1 according to the exemplary embodiment of the surface emitting semiconductor laser diode in Figure 7 includes a contact layer 6 comprising indium tin oxide . The structure of the p-doped side 2 is in accordance with the exemplary embodiment described in connection with Figure 2 . Figure 7 shows the distributed Bragg reflectors 17 arranged at a distance from the semiconductor layer sequence 1 . Preferably, the distributed Bragg reflectors 17 are in direct contact with the semiconductor layer sequence 1 .

In contrast to Figure 7 , the contact layer 6 according to the exemplary embodiment of the surface emitting semiconductor laser diode in Figure 8 comprises a tunnel j unction instead of indium tin oxide . Moreover, an n-doped gallium nitride layer 16 is arranged on a side of the tunnel j unction facing away from the active layer 4 . The n-doped gallium nitride layer 16 is configured for an electrical contacting of the semiconductor layer sequence 1 . Figure 8 shows the distributed Bragg reflectors 17 arranged at a distance from the semiconductor layer sequence 1 . Preferably, the distributed Bragg reflectors 17 are in direct contact with the semiconductor layer sequence 1 .

Figure 9 shows an exemplary embodiment of a surface emitting semiconductor laser diode comprising a semiconductor layer sequence 1 according to the exemplary embodiment described in connection with Figure 1 . Moreover, Figure 9 shows an arrangement of an n-contact 15 and a p-contact 19 configured for electrically contacting the semiconductor layer sequence 1 . The n-contact 15 and the p-contact 19 comprise a solder metal , for example , whereas the contact layer 6 comprises indium tin oxide . The p-contact 19 is disposed directly on the contact layer 6 together with the distributed Bragg reflector 17 and laterally surrounds the latter, at least partially .

The semiconductor layer sequence 1 has a mesa etched structure such that the first current spreading layer 12 is partially exposed on a side facing the active layer 4 . The n- contact 15 is disposed such that it is in direct contact with the exposed part of the first current spreading layer 12 .

Furthermore , the surface emitting semiconductor laser diode has a step-index waveguide 18 on the p-doped side 2 . In particular, a main surface of the semiconductor layer sequence 1 upon which the contact layer 6 is disposed, is structured such that the semiconductor layer sequence 1 has a larger thickness by at most 10 nanometers in a region of an aperture 22 compared to a region outside the aperture 22 .

Electromagnetic laser radiation may be emitted from the surface emitting semiconductor laser diode through one or both of the two distributed Bragg reflectors 17 . Moreover, the semiconductor layer sequence 1 may be arranged on a substrate , such as a growth substrate or a carrier substrate . In this case , one of the two distributed Bragg reflectors 17 may be arranged on a main surface of the substrate facing away from the semiconductor layer sequence 1 .

Figure 10 shows a further exemplary embodiment of a surface emitting semiconductor laser diode . Compared to the surface emitting laser diode described in connection with Figure 9 , the semiconductor layer sequence 1 has a mesa etched structure such that the first current spreading layer 12 is partially exposed on a side facing away from the active layer 4 . The n-contact 15 is disposed such that it is in direct contact with the first current spreading layer 12 .

Compared to the exemplary embodiment described in connection with Figure 9 , the surface emitting semiconductor laser diode according to the exemplary embodiment shown in Figure 11 has an additional step-index waveguide 18 on the n-doped side 3 . In this case the surface emitting semiconductor laser diode is free of a growth substrate for the semiconductor layer sequence 1 . In particular, the main surface of the semiconductor layer sequence 1 on the n-doped side 3 is structured such that the thickness of the semiconductor layer sequence 1 is larger in the region of the aperture 22 . Moreover, the layers of the distributed Bragg reflector 17 disposed on the step-index waveguide 18 on the n-doped side 3 follow the shape of the step-index waveguide 18 . In other words , the layers of the distributed Bragg reflector 17 have a step structure in the region of the aperture 22 . Similarly, the layers of the distributed Bragg reflector 17 disposed on the step-index waveguide 18 on the p-doped side 2 follow the shape of the step-index waveguide 18 .

The aperture 22 on the n-doped side 3 is defined optically by the shape of the step-index waveguide 18 and/or by the shape of the distributed Bragg reflector 17 . On the p-doped side 2 , the aperture 22 is defined optically, as on the n-doped side 3 . Moreover, the aperture 22 on the p-doped side 2 is defined electrically, for example by a shape of the contact layer 6 and a corresponding lateral profile of an electrical current flowing through the semiconductor layer sequence 1 during operation of the surface emitting semiconductor layer sequence 1 .

Figure 12 shows a further exemplary embodiment of a surface emitting semiconductor laser diode . Compared to the exemplary embodiment described in connection with Figure 10 , the surface emitting semiconductor laser diode in Figure 12 has an additional step-index waveguide 18 on the n-doped side 3 , as described in connection with Figure 11 .

The invention is not restricted to the exemplary embodiments by the description on the basis of said exemplary embodiments . Rather, the invention encompasses any new feature and also any combination of features , which in particular comprises any combination of features in the patent claims and any combination of features in the exemplary embodiments , even i f this feature or this combination itself is not explicitly specified in the patent claims or exemplary embodiments.

This patent application claims the priority of German patent application DE 10 2022 115 807.1, the disclosure content of which is hereby incorporated by reference.

Reference signs

1 semiconductor layer sequence

2 p-doped side

3 n-doped side

4 active layer

5 optical resonator

6 contact layer

7 spacer layer

8 anti-node

9 node

10 electron blocking layer

11 highly p-doped layer

12 first current spreading layer

13 second current spreading layer

14 n-doped indium gallium nitride layer

15 n-contact

16 n-doped gallium nitride layer

17 distributed Bragg reflector

18 step-index waveguide

19 p-contact

20 p-doped gallium nitride layer

21 unintentionally doped gallium nitride layer

22 aperture

A optical axis

D thickness

E electric field

G growth direction

S standing wave

Claims

1. Surface emitting semiconductor laser diode comprising:

- a semiconductor layer sequence (1) with a p-doped side (2) , an n-doped side (3) and an active layer (4) configured for emitting electromagnetic radiation arranged between the p- doped side (2) and the n-doped side (3) , and

- an optical resonator (5) with an optical axis (A) parallel to a growth direction (G) of the semiconductor layer sequence ( 1 ) , wherein

- the semiconductor layer sequence (1) is arranged inside the optical resonator (5) such that the electromagnetic radiation emitted during operation forms a standing wave (S) along the optical axis (A) of the optical resonator (5) ,

- the p-doped side (2) comprises a contact layer (6) and an undoped spacer layer (7) arranged between the active layer (4) and the contact layer (6) , and

- a thickness (D) of the spacer layer (7) is configured such that the active layer (4) is arranged in a region of an antinode (8) and the contact layer (6) is arranged in a region of a node (9) of the standing wave (S) .

2. Surface emitting semiconductor laser diode according to the previous claim, wherein the semiconductor layer sequence (1) comprises a nitride compound semiconductor material and the spacer layer (7) comprises unintentionally doped gallium nitride.

3. Surface emitting semiconductor laser diode according to any of the previous claims, wherein the contact layer (6) comprises a transparent conductive oxide and/or a tunnel junction.

4. Surface emitting semiconductor laser diode according to any of the previous claims, wherein an electron blocking layer (10) is arranged between the spacer layer (7) and the contact layer (6) .

5. Surface emitting semiconductor laser diode according to the previous claim, wherein the electron blocking layer (10) comprises p-doped indium aluminium gallium nitride.

6. Surface emitting semiconductor laser diode according to any of claims 4 or 5, wherein a highly doped p-layer (11) is arranged between the electron blocking layer (10) and the contact layer (6) .

7. Surface emitting semiconductor laser diode according to the previous claim, wherein the thickness (D) of the undoped spacer layer (7) is larger than a thickness of the highly doped p-layer (11) and/or larger than a thickness of the electron blocking layer (10) .

8. Surface emitting semiconductor laser diode according to any of the previous claims, wherein a first current spreading (12) layer is arranged in the region of a node (9) of the standing wave (S) on the n- doped side (3) .

9. Surface emitting semiconductor laser diode according to the previous claim, wherein the first current spreading layer (12) comprises highly n-doped gallium nitride.

10. Surface emitting semiconductor laser diode according to any of claims 7 or 8, further comprising an n-contact (15) configured for electrically contacting the semiconductor layer sequence (1) on the n-doped side (3) , wherein the n-contact (15) is in direct contact with the first current spreading layer (12) .

11. Surface emitting semiconductor laser diode according to any of claims 8 to 10, wherein

- a second current spreading layer (13) is arranged in the region of a node (9) of the standing wave (S) between the first current spreading layer (12) and the active layer (4) ,

- the second current spreading layer (13) comprises highly n- doped indium aluminium gallium nitride, and

- an n-doped indium aluminium gallium nitride layer (16) is arranged between the first current spreading layer (12) and the second current spreading layer (13) .

12. Surface emitting semiconductor laser diode according to any of the previous claims, wherein the n-doped side (3) comprises an n-doped indium gallium nitride layer (14) adjacent to the active layer (4) .

13. Surface emitting semiconductor laser diode according to any of the previous claims, wherein the n-doped indium gallium nitride layer (14) has a thickness of at least 10 nanometer.

14. Surface emitting semiconductor laser diode according to any of the previous claims, wherein n-doped indium gallium nitride layer (14) comprises In_xGai-_xN with x > 0,01 and x < 0,08.

15. Surface emitting semiconductor laser diode according to any of the previous claims, wherein the optical resonator (5) comprises a distributed Bragg reflector (17) .

16. Surface emitting semiconductor laser diode according to the previous claim, wherein

- two distributed Bragg reflectors (17) are directly arranged on opposite main surfaces of the semiconductor layer sequence ( 1 ) , and

- the semiconductor layer sequence (1) is free of a growth substrate .

17. Surface emitting semiconductor laser diode according to any of the previous claims, wherein the semiconductor layer sequence (1) comprises a step-index waveguide (18) on the n-doped side (3) and/or on the p-doped side (2) .