VERTICAL LASER EMITTER AND MANUFACTURING METHOD THEREOF

Abstract

According to an aspect of the present inventive concept there is provided a light emitting unit, for emitting laser light at a laser wavelength, arranged on a planar surface of a substrate. The unit comprises a first reflective element to reflect light at the laser wavelength, a gain element to amplify the light, and a second reflective element to partially reflect the light, and to emit the laser light. The elements form a stack of layers integrated onto the planar surface. Each layer is parallel with the planar surface, and the gain element is arranged between the first and second reflective elements.

The unit comprises a beam shaping element integrated with the stack. The beam shaping element is configured to shape the emitted laser light. The beam shaping element comprises a plurality of structures spaced apart in a direction of an extension of a layer of the beam shaping element. A size of the structures and/or a distance between adjacent structures is smaller than the laser wavelength.

Claims

1. A light emitting unit for emitting laser light at a laser wavelength, the light emitting unit being arranged on a planar surface of a substrate, wherein the light emitting unit comprises: a first reflective element configured to reflect light at the laser wavelength; a gain element configured to amplify the light at the laser wavelength; a second reflective element configured to partially reflect the light at the laser wavelength, and to emit the laser light; wherein the first reflective element, the gain element, and the second reflective element form a stack of layers integrated onto the planar surface of the substrate, wherein each layer in the stack of layers is parallel with the planar surface, and wherein the gain element is arranged between the first reflective element and the second reflective element, wherein the light emitting unit further comprises a beam shaping element integrated with the stack of layers on the substrate, the beam shaping element being configured to shape the laser light being emitted, wherein at least a part of the beam shaping element is a separate element to the first reflective element, the gain element and the second reflective element or forms part of one or more of the first reflective element, the gain element and the second reflective element; and wherein the beam shaping element comprises a plurality of structures spaced apart in a direction of an extension of a layer of the beam shaping element and wherein a size of the structures of the plurality of structures and/or a distance between adjacent structures is smaller than the laser wavelength.

2. The light emitting unit according to claim 1, wherein the second reflective element comprises a plurality of structures spaced apart in a direction of an extension of a layer of the second reflective element and wherein a size of the structures of the plurality of structures and/or a distance between adjacent structures is smaller than the laser wavelength.

3. The light emitting unit according to claim 1, wherein the gain element comprises a plurality of structures spaced apart in a direction of an extension of a layer of the gain element and wherein a size of the structures of the plurality of structures and/or a distance between adjacent structures is smaller than the laser wavelength.

4. The light emitting unit according to claim 1, further comprising a first spacer element arranged between the gain element and the second reflective element, the first spacer element being configured to provide a desired distance between the first reflective element and the second reflective element.

5. The light emitting unit according to claim 1, wherein the beam shaping element is a separate element and wherein the beam shaping element and the gain element are arranged on opposite sides of the second reflective element.

6. The light emitting unit according to claim 5, further comprising a second spacer element arranged between the second reflective element and the beam shaping element, the second spacer element being configured to provide a desired distance therebetween.

7. The light emitting unit according to claim 1, further comprising a tunable element integrated with the stack of layers on the substrate, the tunable element being configured to provide tunability to the laser light.

8. The light emitting unit according to claim 1, wherein the substrate comprises Germanium.

9. The light emitting unit according to claim 1, further comprising a first electrode and a second electrode integrated with the stack of layers on the substrate, wherein the first electrode and the second electrode are arranged on opposite sides of the gain element in the stack of layers, and wherein the first electrode and the second electrode are configured to generate a population inversion in the gain element.

10. The light emitting unit according to claim 9, further comprising a first contact and a second contact, the first contact and the second contact extending through at least some of the layers in the stack, and being connected to the first electrode and the second electrode, respectively, wherein the first contact and the second contact enable electric control of the light emitting unit, and wherein at least one of the first contact and the second contact has a cross-sectional width of less than 1 μm.

11. The light emitting unit according to claim 1, wherein the first reflective element, the gain element, and the second reflective element respectively, are arranged along an optical axis such that a center of the first reflective element, the gain element, and the second reflective element respectively, does not deviate from the optical axis by more than 100 nm, and wherein the optical axis extends in a direction perpendicular to the planar surface of the substrate.

12. A light emitting device comprising: a substrate comprising a planar surface; an array of light emitting units according to claim 1, the array of light emitting units being arranged on the planar surface of the substrate.

13. The light emitting device according to claim 12, wherein the array of light emitting units has a pitch of light emitting units of less than 1000 μm, preferably less than 100 μm, and most preferably less than 60 μm.

14. A method for manufacturing a light emitting unit on a substrate comprising a planar surface, the light emitting unit being configured for emitting laser light at a laser wavelength, the method comprising: forming a light emitting unit arranged on the planar surface of the substrate, wherein the forming of the light emitting unit comprises: forming a first reflective element; forming a gain element by epitaxial deposition on the first reflective element; forming a second reflective element; wherein the first reflective element, the gain region, and the second reflective element form a stack of layers integrated onto the planar surface of the substrate, wherein each layer in the stack of layers is parallel with the planar surface, and wherein the gain element is formed to be arranged between the first reflective element and the second reflective element, wherein the forming of the light emitting unit further comprises: forming a beam shaping element integrated with the stack of layers on the substrate, the beam shaping element being configured to shape the laser light being emitted, wherein the beam shaping element is a separate element to the first reflective element, the gain element and the second element or forms part of one or more of the first reflective element, the gain element and the second element; wherein the forming of the beam shaping element involves lithography for forming a plurality of structures spaced apart in a direction of an extension of a layer of the beam shaping element such that a size of the structures of the plurality of structures and/or a distance between adjacent structures is smaller than the laser wavelength.

15. The method according to claim 14, wherein substrate is a wafer having a width of at least 200 mm.

Description

BRIEF DESCRIPTIONS OF THE DRAWINGS

[0104] The above, as well as additional objects, features and advantages of the present inventive concept, will be better understood through the following illustrative and non-limiting detailed description, with reference to the appended drawings. In the drawings like reference numerals will be used for like elements unless stated otherwise.

[0105] FIG. 1A illustrates a light emitting unit with the reflective elements being uniform mirrors.

[0106] FIG. 1B illustrates a light emitting unit with one of the reflective elements comprising a patterned structure.

[0107] FIG. 2A illustrates a light emitting unit comprising a first spacer element arranged between the gain element and the second reflective element.

[0108] FIG. 2B illustrates a light emitting unit comprising a second spacer element arranged between the second reflective element and the beam shaping element.

[0109] FIG. 2C illustrates a light emitting unit comprising a buffer spacer element arranged between the substrate and the first electrode.

[0110] FIG. 3 illustrates a light emitting unit comprising a tunable element integrated with the stack of layers on the substrate.

[0111] FIG. 4 illustrates a light emitting device comprising an array of light emitting units being arranged on the planar surface of the substrate.

[0112] FIG. 5 illustrates a schematic block diagram summarizing a method for manufacturing a light emitting unit on a substrate comprising a planar surface, the light emitting unit being configured for emitting laser light at a laser wavelength.

DETAILED DESCRIPTION

[0113] In cooperation with attached drawings, the technical contents and detailed description of the present inventive concept are described thereinafter according to a preferable embodiment, being not used to limit the claimed scope. This inventive concept may be embodied in many different forms and should not be construed as limited to the embodiments set forth herein; rather, these embodiments are provided for thoroughness and completeness, and fully convey the scope of the inventive concept to the skilled person.

[0114] FIG. 1A schematically illustrates a light emitting unit 100. The light emitting unit 100 comprises a stack of layers arranged on a planar surface 112 of a substrate 110. The substrate 110 comprises Germanium, however it should be understood that also other materials may be used for the substrate 110, as for example GaAs, Si, SiC, AlN, or Sapphire. The light emitting unit 100 is a semiconductor laser diode, and as such the light emitting unit 100 is configured for emitting laser light at a laser wavelength.

[0115] The laser beam emitted from the light emitting unit 100 is emitted in a vertically upward direction substantially perpendicular to the planar surface 112 of the substrate 110. It should be understood that even if the laser beam were not perfectly collimated, as for example if the laser beam were a divergent laser beam such that some rays of laser light may have a direction not being perpendicular to the planar surface 112 of the substrate 110, the average direction of all the rays of laser light emitted from the light emitting unit 100 may be perpendicular to the planar surface 112 of the substrate 110. In such a case the main direction of the emitted laser beam is still considered to be perpendicular to the planar surface 112 of the substrate 110, despite some individual rays having a different direction.

[0116] The light emitting unit 100 comprises a first reflective element 122 and a second reflective element 124. The first reflective element 122 and the second reflective element 124 form layers in the stack of layers which are arranged parallel to the planar surface 112 of the substrate 110, and spaced apart so as to form a resonator for the laser light. The first reflective element 122 is configured to reflect substantially all light at the laser wavelength incident onto the first reflective element 122, so that the first reflective element 122 functions as an end mirror of the resonator. The second reflective element 124 is configured to partially reflect the light at the laser wavelength. At least some of the light incident onto the second reflective element 124, and not being reflected by the second reflective element 124 may be emitted through the second reflective element 124 as laser light. The second reflective element 124 thus functions as the output coupler of the resonator.

[0117] Between the first and second reflective elements 122, 124 a gain element 130 is arranged. The gain element 130 is configured to amplify light at the laser wavelength by stimulated emission of radiation. Amplification by stimulated emission of radiation may occur if the gain element 130 is provided with population inversion. This may be provided in a number of different ways, however in the present embodiment of the light emitting unit 100, population inversion is provided by generation of an electric current in through the gain element 130. For this purpose, a first electrode 152 and a second electrode 156 are arranged on opposite sides of the gain element 130 in the stack of layers. In the present arrangement, the first electrode 152 is arranged outside the resonator, between the substrate 110 and the first reflective element 122. However, it should be realized that the first electrode 152 may alternatively be arranged inside the resonator, between the first reflective element 122 and the gain element 130. The second electrode 156 is arranged inside the resonator between the gain element 130 and the second reflective element 124. In order to enable electric control of the light emitting unit, the first electrode 152 is connected to a first contact 154, and the second electrode 156 is connected to a second contact 158. As illustrated in FIG. 1A, the first contact 154 and the second contact 158 extend in a direction substantially perpendicular to planar surface 112 of the substrate 110, through some of the layers in the stack, located above the respective electrodes 152, 154. In other words, the contacts 154, 158 extend in the vertical direction of the light emitting unit 100, whereas the planar surface 112 of the substrate 110 extends in the horizontal direction of the light emitting unit 100. The contacts provide access to apply an electric field between the electrodes 152, 154. The applied electric field may generate an electric current through the gain element 130, which may in turn provide population inversion in the gain element 130.

[0118] At least one of the first contact 154 and the second contact 158 has, and in the present arrangement both contacts 154, 158 have a cross-sectional width of less than 1 μm. By way of example, such small cross-sectional width may be achieved first providing all the layers in the stack of layers on the substrate, for example by epitaxial deposition, and subsequently applying advanced lithography in order to enable etching holes through the layers such that the holes reach down to the first electrode 152 and second electrode 156, respectively. After the holes have been etched to reach the electrodes, the holes may be filled with a metal to form the first contact 154 and the second contact 158, thereby providing electric connections to the electrodes 152, 156 from outside the light emitting unit 100.

[0119] The light emitting unit 100 further comprises a beam shaping element 140 integrated with the stack of layers on the substrate 110. In the present arrangement, the beam shaping element 140 is a separate element to the first reflective element 122, the gain element 130 and the second reflective element 124. The beam shaping element 140 is arranged external to the resonator such that the laser beam emitted through the second reflective element 124 subsequently propagates through the beam shaping element 140 prior to being emitted from the light emitting unit 100. In other words, the beam shaping element 140 and the gain element 130 are arranged on opposite sides of the second reflective element 124. However, it should be understood that the beam shaping element 140 may alternatively form part of one or more of the first reflective element 122, the gain element 130 and the second reflective element 124.

[0120] The beam shaping element 140 comprises a plurality of structures 142 spaced apart in a direction of an extension of a layer of the beam shaping element 140. In other words, the plurality of structures 142 are spaced apart and distributed across a plane parallel to the planar surface 112 of the substrate 110. The size of the structures 142 of the plurality of structures 142 and/or a distance between adjacent structures 142 is smaller than the laser wavelength. The beam shaping element 140 may be produced by successive epitaxial deposition and advanced lithography in order to provide the plurality of structures 142 with a size and/or distance being smaller than the laser wavelength. By epitaxial deposition a layer of a reflective material is provided and by use of advanced lithography structures 142 are formed by etching spaces in the layer, leaving the segments of the material forming the plurality of structures 142. By way of example, the reflective material may be, but is not limited to, GaAs or AlGaAs. Once the etching of the structures is completed the spaces may be filled with a dielectric material. Given as non-limiting examples, the dielectric material may be SiO.sub.2, with a refractive index of about 1.45, or other oxides such as TiO.sub.2. Other non-limiting examples of dielectric materials are SiN and polymers. The refractive index of the selected dielectric material may preferably be 1.5 or below. In this manner, a compact beam shaping element 140 integrated in the light emitting unit 100 may be provided. Given as non-limiting examples, the beam shaping element 140 may be in the form of a micro lens, a Fresnel phase plate, a photonic crystal, or any other suitable element for shaping the emitted laser beam.

[0121] The beam shaping element 140 is configured to shape the laser light being emitted by the light emitting unit 100. In the present embodiment, the beam shaping element 140 collimates the emitted divergent beam of laser light into a collimated laser beam being emitted from the light emitting unit 100. However, it should be understood that the beam shaping element 140 may alternatively be configured to re-shape the emitted laser beam in other manners, such as to focus the emitted beam of laser light into a focused laser beam, or to re-shape the cross-sectional shape of the beam of laser light into a desired shape.

[0122] FIG. 1B schematically illustrates an alternative variation of the light emitting unit 100. For the light emitting unit 100 in relation to FIG. 1A, the first and second reflective elements are illustrated as uniform mirrors. However, it should be understood that the first and/or second reflective element may alternatively comprise a patterned structure. FIG. 1B illustrates an alternative variation of the light emitting unit 100, in which the second reflective element 224 is illustrated as comprising a patterned structure. Thus, the second reflective element 224 comprises a plurality of structures 225 spaced apart in a direction of an extension of a layer of the second reflective element 224. In other words, the plurality of structures 225 are spaced apart and distributed across a plane parallel to the planar surface 112 of the substrate 110. The size of the structures 225 of the plurality of structures 225 and/or a distance between adjacent structures 225 is smaller than the laser wavelength.

[0123] Similarly to the beam shaping element 140, the second reflective element 224 may be produced by successive epitaxial deposition and subsequent etching using advanced lithography in order to provide the plurality of structures 225 with a size and/or distance being smaller than the laser wavelength. Once the etching is completed the spaces between the plurality of structures 225 may be filled with a dielectric material.

[0124] It serves to mention that by successive epitaxial deposition and subsequent etching using advanced lithography, the elements, and the structures of the elements, may be arranged with a very high precision and accuracy. For example, the elements may be aligned with high precision and accuracy with respect to an optical axis. The optical axis A of the light emitting unit 100 extends through the center of the resonator, in a direction perpendicular to the planar surface 112 of the substrate 110. The elements such as the first reflective element 122, the gain element 130, and the second reflective element 224 may respectively be arranged along the optical axis A such that the center of the respective elements does not deviate from the optical axis A by more than 100 nm. Preferably, the center of the respective elements does not deviate from the optical axis A by more than 30 nm.

[0125] In the present variation of the light emitting unit 100, the patterned structure of the beam shaping element 140 and the patterned structure of the second reflective element 224 jointly provide the effect of shaping the emitted laser beam, as for example collimating the emitted laser beam. In the present arrangement, it may be considered that at least part of the beam shaping element 140 forms part of the second reflective element 224.

[0126] Although not illustrated here, it is conceivable that additionally or alternatively the gain element and/or the first reflective element may be patterned to comprise a plurality of structures, so as to form part of the beam shaping element. It is further conceivable that at least part of the beam shaping element may be arranged between the second reflective element and the gain element.

[0127] FIGS. 2A-C schematically illustrates a light emitting unit 300 comprising one or more spacer elements. The light emitting unit 300 comprises a stack of layers comprising a first reflective element 122, a gain element 130, a second reflective element 224, and a beam shaping element 140, arranged onto a planar surface 112 of a substrate 110, and shares some of the features with light emitting unit 100 described in relation to FIG. 1, the details of which are not repeated here.

[0128] FIG. 2A schematically illustrates the light emitting unit 300 further comprising a first spacer element 362 arranged between the gain element 130 and the second reflective element 224. The first spacer element 362 is configured to provide a desired distance between the first reflective element 112 and the second reflective element 224.

[0129] The combination of the thickness and the refractive index of the first spacer element 362 provides an additional distance, or rather optical path length, between the first reflective element 122 and the second reflective element 224. In this manner, the optical path length for light of the laser wavelength may be selected so as to create a resonator for the laser wavelength, thereby controlling the laser wavelength of the emitted laser light.

[0130] It should be understood that in case at least part of the beam shaping 140 forms part of other elements in the light emitting unit 300, the first spacer element 362 may have the additional effect of contributing to the beam shaping properties of the light emitting unit 300 as a whole, since the optical path length of the resonator may affect properties of the propagating light beam.

[0131] FIG. 2B schematically illustrates the light emitting unit 300 further comprising a second spacer element 364 arranged between the second reflective element 224 and the beam shaping element 140. The second spacer element 364 is configured to provide a desired distance, or rather an optical path length, between the second reflective element 224 and the beam shaping element 140, in order to provide the intended beam shaping.

[0132] By varying the thickness and/or the refractive index of the second spacer element, the beam shaping effect may be varied. For example, a divergent laser beam being emitted through the second reflective element 224 will have a different beam diameter depending on the distance from the second reflective element 224. Thus, to provide a collimated laser beam with a predetermined diameter the beam shaping element 140 should be arranged at a predetermined distance, or optical path length, from the second reflective element 224. Hence, the effect of beam shaping, is a combination of the effect of the second spacer element 364 and the other elements in the stack.

[0133] In FIGS. 2A-C, the first spacer element 362 and the second spacer element 364 are illustrated as extending across the entire width of the light emitting unit 300. However, it should be understood that this is not necessary for practicing the invention. It is equally conceivable that the first spacer element 362 and the second spacer element 364 extend only to the outermost edge of the second contact 158, in the same manner as for example the first reflective element 122 and the gain element 130.

[0134] FIG. 2C schematically illustrates the light emitting unit 300 further comprising a buffer spacer element 366 arranged between the substrate 110 and the first electrode 152. The buffer spacer element 152 is made of a material providing a better fit between the lattice structure of the substrate 110 and the layers in the stack, such as the first reflective element 122, the gain element 130, and/or the first electrode 152. By way of example, the material out of which the buffer spacer is made may be, but is not limited to, SiO.sub.2.

[0135] It should be understood that, although the buffer spacer element 152 is here illustrated only in a light emitting unit 300 comprising also a first spacer element 362 and a second spacer element 364, it is conceivable that an alternative light emitting unit 300 may comprise a buffer spacer element 152 without any of the first spacer element 362 and the second spacer element 364 being included, or only one of the first spacer element 362 and the second spacer element 364 being included.

[0136] FIG. 3 schematically illustrates a light emitting unit 400 comprising a tunable element 470. The light emitting unit 400 comprises a stack of layers comprising a first reflective element 122, a gain element 130, a second reflective element 224, and a beam shaping element 140, arranged onto a planar surface 112 of a substrate 110, and shares some of the features with light emitting units 100 and 300, described in relation to FIGS. 1-2, the details of which are not repeated here.

[0137] As illustrated in FIG. 3, the tunable element 470 is integrated with the stack of layers on the substrate 110, and arranged as a separate element between the first reflective element 122 and the gain element 130. However, it should be understood that the tunable element 470 may alternatively be arranged within one of the first reflective element 122, the gain element 130, and the second reflective element 124.

[0138] In order to provide tunability to the laser light of the light emitting unit 400, a BaTiO.sub.3 based electro-optic layer is comprised in the tunable element 470. The electro-optic layer is transparent to the laser wavelength. The electro-optic properties of the electro-optic layer enables the refractive index of the electro-optic layer to be changed by varying an electric field applied across the electro-optic layer. By way of example, an electric field may be applied across the electro-optic layer of the tunable element 470 either by means of the first and second electrodes 152, 156 and the first and second contacts 154, 158, or by a set of separate electrodes and contacts (not shown here), or a combination thereof. By the present arrangement, tunability of wavelength, polarization, phase, intensity, and/or direction and thereby also collimation and illumination pattern of the laser light may be provided.

[0139] FIG. 4 schematically illustrates a light emitting device 1000 comprising an array of light emitting units 500 being arranged on a planar surface 1012 of a substrate 1010. It should be understood that the light emitting unit 500 may be any of the previously described light emitting units 100, 300, 400, or any other light emitting unit comprising a combination of the features described herein.

[0140] The substrate 1010 is a Germanium wafer. The substrate 1010 has a large width of 200 mm. The large width allows for a large number of light emitting units 500 to be arranged onto the substrate 1010. As previously described, by successive epitaxial deposition and subsequent etching by advanced lithography the structures of the different elements of the light emitting units 500 may be provided with a very high precision and accuracy. The structures may be provided to have sizes and/or distances being smaller than the laser wavelength.

[0141] Further, by successive epitaxial deposition and subsequent etching by advanced lithography the light emitting units 500 may be arranged close to each other. This is partially an effect of the high precision and accuracy of alignment of the elements. Put differently, the array of light emitting units 500 may be provided to have a pitch P of light emitting units 500 of less than 1000 μm, preferably less than 100 μm, and most preferably less than 60 μm. The pitch P of light emitting units 500 may be as low as 10 μm.

[0142] By conventional manufacturing of VCSEL diodes, the individual diodes may have a width of about 10 μm, however the pitch may be significantly larger, as for example 100 μm. This leaves gaps on the substrate between the individual diodes that are much larger than the diodes themselves. Consequently, the surface of the substrate is not efficiently used. However, by means of the present inventive concept, the light emitting units 500 may be arranged much more closely such that the gaps G between the individual light emitting units 500 may be smaller than the width W of the individual light emitting units 500. Put differently, the pitch P of light emitting units 500 may be close to the width W of individual light emitting units 500. Thus, in the present manner, the planar surface 1012 of the substrate 1010 may be much more efficiently used, and the production cost per unit may be decreased.

[0143] FIG. 5 illustrates a schematic block diagram shortly summarizing the method for manufacturing a light emitting unit on a substrate comprising a planar surface, the light emitting unit being configured for emitting laser light at a laser wavelength. It should be understood that the steps of the method, although listed in a specific order herein, may be performed in any order suitable.

[0144] The method may comprise forming S502 a first reflective element.

[0145] The method may comprise forming S504 a gain element by epitaxial deposition on the first reflective element.

[0146] The method may comprise forming S506 a second reflective element such that the gain element is arranged between the first reflective element and the second reflective element. The first reflective element, the gain region, and the second reflective element may form a stack of layers integrated onto the planar surface of the substrate, such that each layer in the stack of layers is parallel with the planar surface. The forming S506 the second reflective element may further involve lithography to form a plurality of structures, wherein a size of the structures of the plurality of structures and/or a distance between adjacent structures is smaller than the laser wavelength.

[0147] The method may comprise forming S508 a beam shaping element integrated with the stack of layers on the substrate. The forming S508 a beam shaping element may involve lithography for forming a plurality of structures spaced apart in a direction of an extension of a layer of the beam shaping element such that a size of the structures of the plurality of structures and/or a distance between adjacent structures is smaller than the laser wavelength. In the present manner, a beam shaping element being configured to shape the laser light being emitted by the light emitting unit may be provided. The forming S508 the beam shaping element may further comprise arranging the beam shaping element on an opposite side of the second reflective element with respect to the gain element.

[0148] The method may further comprises forming a first spacer element arranged between the gain element and the second reflective element, such that a desired distance is provided between the first reflective element and the second reflective element.

[0149] The method may further comprise arranging the first reflective element, the gain element, and the second reflective element respectively, along an optical axis such that a center of the first reflective element, the gain element, and the second reflective element respectively, does not deviate from the optical axis by more than 100 nm, and wherein the optical axis extends in a direction perpendicular to the planar surface of the substrate.

[0150] The method may comprise further comprise forming a light emitting unit on a substrate wherein the substrate is a wafer having a width of at least 200 mm.

[0151] In the above the inventive concept has mainly been described with reference to a limited number of examples. However, as is readily appreciated by a person skilled in the art, other examples than the ones disclosed above are equally possible within the scope of the inventive concept, as defined by the appended claims.