GaN-based VCSELS WITH MODE-MATCHING PARABOLIC META-MIRRORS

Abstract

A VCSEL device including: a stacked structure including: a hybrid mirror including a distributed Bragg reflector (DBR) and a mode-matched nanostructure disposed over the DBR; a semiconductor structure, the semiconductor structure including; a substrate layer; a first semiconductor layer disposed over the substrate layer; a second semiconductor layer disposed over the first semiconductor layer; and an active region disposed between the first semiconductor layer and the second semiconductor layer, wherein the substrate layer and the first semiconductor layer have a first conductivity and the second semiconductor layer has a second conductivity, and wherein in the substrate layer, the first semiconductor layer and the second semiconductor layer have a same semiconductor material; and a further DBR disposed over the semiconductor structure.

Description

GaN-based VCSELS WITH MODE-MATCHING PARABOLIC META-MIRRORSTECHNICAL FIELD

[0000] This disclosure generally relates to a Vertical-Cavity Surface-Emitting Laser (VCSEL).BACKGROUND

[0001] A VCSEL is a type of semiconductor laser diode that emits light perpendicular to the surface of the semiconductor chip. Unlike conventional edge-emitting lasers, VCSELs have their laser beam emission from the top surface or the bottom surface.BRIEF DESCRIPTION OF THE DRAWINGS

[0002] The accompanying drawings serve to provide an understanding of non-limiting embodiments of the present disclosure. The drawings illustrate non-limiting embodiments and, together with the description, serve for explanation thereof. Further non-limiting embodiments and many of the intended advantages will become apparent directly from the following detailed description. The elements and structures shown in the drawings are not necessarily shown to scale relative to each other. Similar reference numerals refer to similar or corresponding elements and structures.

[0003] FIG. 1 is a cross-sectional view of a VCSEL device in accordance with one or more embodiments of the present disclosure.

[0004] FIG. 2A is a cross-sectional view of a VCSEL device in accordance with one or more embodiments of the present disclosure.

[0005] FIG. 2B is a side cross-sectional view of the mode-matched photonic crystal structure of the VCSEL device of FIG. 2A in accordance with one or more embodiments of the present disclosure.

[0006] FIG. 2C is a top cross-sectional view of the mode-matched photonic crystal structure of the VCSEL device of FIG. 2A in accordance with one or more embodiments of the present disclosure.

[0007] FIG. 3A is a cross-sectional view of a VCSEL device in accordance with one or more embodiments of the present disclosure.

[0008] FIG. 3B is a side cross-sectional view of the mode-matched meta-surface structure of the VCSEL device of FIG. 3A in accordance with one or more embodiments of the present disclosure.

[0009] FIG. 3C is a top cross-sectional view of the mode-matched meta-surface structure of the VCSEL device of FIG. 3A in accordance with one or more embodiments of the present disclosure.

[0010] FIG. 4 is a cross-sectional view of a VCSEL device in accordance with one or more embodiments of the present disclosure.

[0011] FIG. 5 is a cross-sectional view of a VCSEL device in accordance with one or more embodiments of the present disclosure.

[0012] FIG. 6 is a top cross-sectional view of a VCSEL device in accordance with one or more embodiments of the present disclosure.

[0013] FIGS. 7A-7B are cross-sectional views of VCSEL devices in accordance with one or more embodiments of the present disclosure.

[0014] FIG. 8 is a cross-sectional view of a VCSEL device in accordance with one or more embodiments of the present disclosure.

[0015] FIG. 9 is a diagram illustrating diffraction loss vs cavity length of a VCSEL device with planar multilayer dielectric mirrors.

[0016] FIG. 10 is a flow diagram of a method for manufacturing a VCSEL device in accordance with one or more embodiments.

[0017] The above descriptions are for purposes of illustration and are not meant to be limiting. Numerous other examples, configurations, processes, algorithms, etc., may exist, some of which are described in greater detail below. Example embodiments will now be described with reference to the accompanying figures.DETAILED DESCRIPTION

[0018] The following description and the drawings sufficiently illustrate specific embodiments to enable those skilled in the art to practice them. Other embodiments may incorporate structural, logical, electrical, process, algorithm, and other changes. Portions and features of some embodiments may be included in, or substituted for, those of other embodiments. Embodiments set forth in the claims encompass all available equivalents of those claims.

[0019] In the following detailed description, reference is made to the accompanying drawings, which form a part of the disclosure and in which specific non-limiting embodiments are shown for purposes of illustration. In this context, directional terminology such as “top”, “bottom”, “front”, “back”, “over”, “on”, “in front”, “behind”, “leading”, “trailing”, etc. refers to the orientation of the figures just described and the relative position of the components of the figures just described.

[0020] The description of the non-limiting embodiments is not limiting. In particular, elements of the non-limiting embodiments described below may be combined with elements from others of the non-limiting embodiments described, unless the context indicates otherwise.

[0021] The terms “wafer” or “semiconductor substrate” used in the following description may include any semiconductor-based structure that has a semiconductor surface. Wafer and structure are to be understood to include doped and undoped semiconductors, epitaxial semiconductor layers, supported by a base, if applicable, and further semiconductor structures. For example, a layer of a first semiconductor material may be grown on a growth substrate made of a second semiconductor material or of an insulating material, for example sapphire. Further examples of materials for growth substrates include glass, silicon dioxide, quartz or a ceramic.

[0022] Depending on the intended use, the semiconductor may be based on a direct or an indirect semiconductor material. Examples of semiconductor materials particularly suitable for generating electromagnetic radiation include, without limitation, nitride semiconductor compounds, by means of which, for example, ultraviolet, blue or longer- wave light may be generated, such as GaN, InGaN, AIN, AIGaN, AIGalnN, AIGalnBN,phosphide semiconductor compounds by means of which, for example, green or longer- wave light may be generated, such as GaAsP, AIGalnP, GaP, AIGaP, and other semiconductor materials such as GaAs, AIGaAs, InGaAs, AllnGaAs, SiC, ZnSe, ZnO, Ga2O3, diamond, hexagonal BN and combinations of the materials mentioned. The stoichiometric ratio of the ternary compounds may vary. Other examples of semiconductor materials may include silicon, silicon germanium, and germanium. In the context of the present description, the term “semiconductor” also includes organic semiconductor materials.

[0023] The term “substrate” generally includes insulating, conductive or semiconductor substrates.

[0024] The terms “lateral” and “horizontal”, as used in the present description, are intended to describe an orientation or alignment which extends essentially parallel to a first surface of a semiconductor substrate or semiconductor body. This may be the surface of a wafer or a chip (die), for example.

[0025] The horizontal direction may, for example, be in a plane perpendicular to a direction of growth when layers are grown.

[0026] The term “vertical”, as used in this description, is intended to describe an orientation which is essentially perpendicular to the first surface of a substrate or semiconductor body. The vertical direction may correspond, for example, to a direction of growth when layers are grown.

[0027] To the extent used herein, the terms “have”, “include”, “comprise”, and the like are open-ended terms that indicate the presence of said elements or features, but do not exclude the presence of further elements or features. The indefinite articles and the definite articles include both the plural and the singular, unless the context clearly indicates otherwise.

[0028] In the context of the present disclosure, the term “photonic crystal structure” means a structure the structural elements of which are arranged at predetermined locations. The arrangement pattern of the structural elements is subject to a specific order. The functionality of the photonic structure results from the arrangement of the structural elements. The structural elements are, for example, arranged to be subwavelength to avoid any diffraction effect and act as a mode matched layer. Thestructural elements may be arranged in an array, for example, so that a photonic crystal is realized.

[0029] Any VCSEL with a long cavity (e.g., vertical length L>20 / zm) may suffer from diffraction loss that makes it more difficult to achieve lasing or that prevents lasing. That is, diffraction losses may increase the lasing threshold and may increase the lasing threshold sufficiently to eventually prevent lasing. FIG. 9 is a diagram illustrating diffraction loss vs cavity length of a VCSEL device with planar multilayer dielectric mirrors. Referring to FIG. 9, the diffraction loss per round trip increases by increasing the length of the cavity. In the gray portion on the right, lasing is prevented in a conventional VCSEL (i.e., planar DBRs). It is difficult to achieve a high-reflectivity (7?>0.995) for a long cavity VCSEL due to the diffraction loss with planar multilayer dielectric mirrors (e.g., DBRs).

[0030] For example, a GaN-based VCSEL with a long cavity exhibits significant diffraction loss which prohibits lasing. In particular, this problem arises in InGaN-based VCSELs for blue wavelengths. In a GaN blue VCSEL, the active material is made of ln_{x}Ga_{1-x}N for different indium content for barrier and quantum wells. For an InGaN based VCSEL at blue wavelength, the gain is low compared to other wavelengths, thus the diffraction loss needs to be further suppressed.

[0031] A general solution to minimize the diffraction loss in VCSELs with long cavities and achieve lasing is using a curved mirror. However, a curved mirror is difficult to manufacture in a consistent manner. The fabrication of a conventional long cavity InGaN- based VCSEL requires substrate removal as well as curved mirror fabrication. For example, substrate removal by etching may cause surface roughness. The present disclosure provides an alternative approach to suppress diffraction loss of a VCSEL with a long cavity, and in particular, to suppress diffraction loss in a long cavity InGaN-based VCSEL for blue wavelengths.

[0032] The present disclosure provides a hybrid parabolic meta-mirror (i.e., hybrid mirror) for VCSELs to suppress the diffraction loss. In some embodiments, an InGaN- based VCSEL with a hybrid parabolic meta-mirror is provided. A hybrid mirror may include a mode matched (or mode matching) structure (or layer) disposed over a distributed Bragg reflector (DBR). The mode matched structure of a hybrid mirror may include ameta-surface structure or photonic crystal structure. The mode matched structure may be provided instead of a curved mirror in a VCSEL, in particular, in an InGaN-based VCSEL.

[0033] A mode matched structure may be realized by using a photonic crystal structure or a meta-surface structure. The mode matched (mode matching) structure may be realized by an array of nanoelements and by controlling or sizing their dimensions to achieve mode matching with the phase of the fundamental mode of the cavity (e.g., Gaussian beam). The nanoelements may be nanopillars or nano-airholes. The nanoelements may have a circular planar cross-section or a planar cross-section of a different shape. For example, a shape of the nanoelement may be used to control polarization of the laser in addition to mode-matching. A planar cross-sectional shape of the nanoelement may be elliptical to provide polarization in addition to the matching provided by the dimension and grid arrangement of the nanoelements.

[0034] The fabrication of a hybrid mirror does not require substrate removal or curved mirror fabrication. A meta-surface or photonic crystal structure of the hybrid mirror may be fabricated by one of the state-of-the-art approaches including electron beam lithography (EBL), focused ion beam lithography (FIB) and deep ultraviolet lithography (DUV).

[0035] The meta-surface or photonic crystal structure may be configured such that the mode matching occurs only for the fundamental mode. Thus, the VCSEL cavity operates at single mode (fundamental mode). For the higher order modes, the cavity loss is very large because of diffraction loss.

[0036] A VCSEL with a long cavity (e.g., vertical length L>20 / zm) including a hybrid mirror has several advantages. For example:

[0037] A GaN-based VCSEL with a hybrid mirror has low diffraction loss and large reflectivity (e.g., 7?>0.995). In particular, a hybrid mirror including a mode matched structure (or layer) may lead to very small diffraction losses.

[0038] A GaN-based VCSEL with a hybrid mirror may include fewer distributed Bragg reflectors (DBRs). A small number of DBRs may still achieve a large reflection. For example, a large reflectivity (e.g., f?>0.995) may be realized by using a few bilayers of DBRs with the meta-surface or photonic crystal structure.

[0039] A GaN-based VCSEL with a hybrid mirror may have very small mode spacing and spectral filtering. A very small mode spacing makes it easy to match the cavity resonance and peak of the gain. Additionally, or alternatively, the mode-matched structure (meta-surface or photonic crystal layer) is narrow band, and thus it allows to spectrally filter the lasing mode. Accordingly, a more focused laser may be provided.

[0040] A GaN-based VCSEL with a hybrid mirror may be single mode. The parabolic meta-mirror may be configured to mode match only for the fundamental mode. This minimizes diffraction loss.

[0041] A GaN-based VCSEL with a hybrid mirror may have a reasonably thick cavity. This facilitates wafer fabrication without having any crack issues in the wafer.

[0042] A GaN-based VCSEL with a hybrid mirror may facilitate better heat management. A thick cavity allows management of heat similar to VCSELs with curved DBRs.

[0043] Various fabrication techniques may be used to fabricate meta-surfaces or photonic crystals: For example, electron beam lithography (EBL), focused ion beam lithography (FIB) or deep ultraviolet lithography (DUV) may be used.

[0044] A GaN-based VCSEL with a hybrid mirror may provide polarization control. A linearly or chirally polarized mode-matched structure (e.g., birefringent meta-surface or photonic crystal structure, e.g., by breaking in-plane symmetry) can provide linearly or chirally polarized laser emission.

[0045] As described in further detail herein, a mode-matched or mode-matching structure or layer may be implemented with photonic crystal or meta-surface with nanoelements.

[0046] Photonic Crystal

[0047] A photonic crystal structure may include an array of nano-air holes. The radius of each nano-air hole is designed such that it satisfies the phase of the fundamental Gaussian mode _(Gaussian beam) (x,y) of the long cavity. Reflection of a hybrid mirror including a photonic crystal structure with DBR is nearly one (reflectivity 7?>0.995).

[0048] Meta-Surface

[0049] A meta-surface structure may include an array of nanopillars made of high refractive index material. The radius of each nanopillar is designed such that it satisfiesthe phase of the fundamental Gaussian mode < / >_(Gaussian beam) (x,y) of the long cavity. Reflection of a hybrid mirror including a meta-surface structure with DBR is nearly one (reflectivity 7?>0.995).

[0050] FIG. 1 is a cross-sectional view of a VCSEL device 100 in accordance with one or more embodiments of the present disclosure.

[0051] A VCSEL device 100 may be a stacked structure including a first distributed Bragg reflector (DBR) 111 , a mode-matched nanostructure (layer) arranged over the first DBR, a semiconductor structure 130 arranged over the mode-matched nanostructure 131 and a second DBR 141 arranged over the semiconductor structure 130.

[0052] The VCSEL device 100 may include a hybrid mirror 105 including the mode- matched structure 121 (e.g., meta-mirror) disposed above the first DBR 111. The mode- matched structure 121 may be in direct contact with the first DBR 111.

[0053] The first DBR 111 may be a dielectric DBR. A dielectric DBR may be a multilayer periodic structure formed from alternating dielectric layers that can be used to achieve nearly total reflection within a range of frequencies, with minimal losses. The alternating dielectric layers may be alternating layers of dielectric material with different refractive index (or different effective refractive index). Each layer boundary causes a partial reflection and refraction of an optical wave. For example, the first DBR 111 may include alternating layers of niobium pentoxide (Nb2O5) and silicon dioxide (SiO2). The first DBR may include any combination of lossless dielectric material arranged as alternating layers of dielectric material of low refractive index and high refractive index material. For example, alternating layers of hafnium oxide (HfO2) and silicon dioxide (SiO2) or titanium dioxide (TiO2) and silicon dioxide (SiO2).

[0054] The mode-matched nanostructure 121 may be a layer including an array of nanostructures. The array of nanostructures may be arranged in a grid. In some embodiments, the distance between nanostructures may be periodic (i.e., the same). In some embodiments, the distance between nanostructures may be different when moving from the center of the mode matched layer to the edges of the mode matched layer. For example, the distance between two adjacent nanostructures at the center may be smaller than the distance between two adjacent nanostructures at the periphery. The nanostructures may have different sizes (e.g., different diameters). For example, the sizeof nanostructures at the center may be larger than the size of nanostructures at the periphery. The sizes of the nanostructures and the distance between the nanostructures may be determined to match a single mode (e.g., a fundamental mode of the semiconductor structure 130). The cross-sectional shape of the nanostructures may be circular or other shape. For example, the distance between two adjacent nanostructures at the center may be smaller than the distance between two adjacent nanostructures at the periphery. The mode-matched nanostructure 121 may have a layer thickness less than 1 micrometer (e.g., thickness LMML< 1 iim).

[0055] The semiconductor structure 130 may be a stacked structure including a substrate layer 131 , a first semiconductor layer 133 arranged over the substrate layer 131 , an active region 135 arranged over the first semiconductor layer 133 and a second semiconductor layer 137 arranged over the active region 135.

[0056] The substrate layer 131 may be a doped substrate layer including a semiconductor material having a first conductivity (e.g., n-type).

[0057] The first semiconductor layer 133 may include a semiconductor material having a first conductivity (e.g., n-type).

[0058] The second semiconductor layer 137 may include a semiconductor material having a second conductivity (e.g., p-type).

[0059] The semiconductor material of the substrate layer 131 , the first semiconductor layer 135 and the second substrate layer 137 may be the same semiconductor material. The first semiconductor layer 135 may have a higher doping than the substrate layer 131.

[0060] For example, the substrate layer 131 , first semiconductor layer 133 and second semiconductor layer 137 may contain a semiconductor material such as GaN or a GaN- containing compound. Other semiconductor material may also be used. Examples of semiconductor materials particularly suitable for generating electromagnetic radiation include, without limitation, nitride semiconductor compounds, by means of which, for example, ultraviolet, blue or longer-wave light may be generated, such as GaN, InGaN, AIN, AIGaN, AIGalnN, AIGalnBN, phosphide semiconductor compounds by means of which, for example, green or longer-wave light may be generated, such as GaAsP, AIGalnP, GaP, AIGaP, and other semiconductor materials such as GaAs, AIGaAs, InGaAs, AllnGaAs, SiC, ZnSe, ZnO, Ga2O3, diamond, hexagonal BN and combinationsof the materials mentioned. In particular, in some embodiments, the semiconductor material may be InGaN.

[0061] An active region 135 may be arranged between the first semiconductor layer 133 and the second semiconductor layer 137. The active region 135 may, for example, include a pn junction, a double heterostructure, a single quantum well structure (SQW, single quantum well) or a multiple quantum well structure (MQW, multi quantum well) for generating electromagnetic radiation. In particular, the active region 135 may be an InGaN-based active region.

[0062] The semiconductor structure 130 may be in direct contact with the hybrid mirror 105.

[0063] In some embodiments, the semiconductor structure 130 may be a VCSEL including a n-GaN substrate, an n-GaN layer disposed over the n-GaN substrate, MQWs disposed over the n-GaN layer, a p-GaN layer disposed over the MQWs.

[0064] The semiconductor structure 130 may emit electromagnetic radiation (e.g., light) when a lasing threshold is realized. For example, the semiconductor structure may be configured to emit blue light.

[0065] The semiconductor structure may be a vertical cavity structure having a long cavity. For example, the vertical cavity structure may have a cavity thickness of z,cav~ 15 - 500 gm.. In some embodiments, Lcavmay be 25 m to100 m thick.

[0066] The VCSEL device 100 may further include second dielectric DBR 141 arranged over the semiconductor structure 130. The second dielectric DBR 141 may include the same material as the first dielectric DBR 111. The number of layers of alternating dielectric material of the second DBR 141 may be greater than the number of layers of alternating dielectric material of the first DBR 111. For example, in a non-limiting embodiment, the first DBR 111 may include 5 pairs of alternating material layers and the second DBR 141 may include 10 pairs of alternating material layers. Said differently, the second DBR 141 may be thicker than the first DBR. For example, in a non-limiting embodiment, the second DBR 141 may have double the layers of the first DBR 111.

[0067] FIG. 2A is a cross-sectional view of a VCSEL device 200 in accordance with one or more embodiments of the present disclosure.

[0068] VCSEL device 200 may be similar to VCSEL device 100. Additional details of a mode-matched nanostructure is provided.

[0069] The mode-matched nanostructure 121 may be a mode-matched photonic crystal structure 221. The mode-matched photonic crystal structure 221 may include an array of nano-air holes 225. The mode-matched photonic crystal structure 221 may include a layer 223 patterned such that a plurality of nano-air holes 225 are formed in the layer 223.

[0070] FIG. 2B is a side cross-sectional view of the mode-matched photonic crystal structure of the VCSEL 200 device of FIG. 2A in accordance with one or more embodiments of the present disclosure. The side cross-sectional view is taken along a center line of the VCSEL device 200. The array of nano-air holes may be formed or provided at the bottom of the substrate layer 131 or a layer having the same material as the substrate layer 131. In some embodiments, the layer 223 may be a bottom-most portion of the substrate layer 131 . That is, the nano-air holes 225 may be formed in a lightly doped semiconductor material of the first conductivity. The layer 223 may include approximately the bottom 1 gm of the substrate layer 131. In some embodiments, the nano-air holes 225 may have a vertical length that is the same thickness as the layer 223, approximately 1 gm. The nano-air holes 225 may extend to a top surface of the first dielectric DBR 111. In some embodiments, the nano-air holes 225 may have a vertical length that is less than the thickness of the layer 223 and the nano-air holes 225 do not extend to a top surface of the first DBR 111.

[0071] FIG. 2C is a top cross-sectional view of the mode-matched photonic crystal structure of the VCSEL device 200 of FIG. 2A in accordance with one or more embodiments of the present disclosure. The mode-matched photonic crystal structure 221 may include an array or a grid of nano-air holes 225 of different sizes. The sizes of the nano-air holes may be larger towards the center and smaller towards the periphery. In some embodiments, the grid of nano-air holes 225 may be periodically arranged (i.e., the same). In some other embodiments, the distance between the nano-air holes 225 may be different when moving from the center of the mode-matched photonic crystal structure to the edges of the mode-matched photonic crystal. For example, the distance between two adjacent nano-air holes at the center may be smaller than the distancebetween two adjacent nano-air holes at the periphery. For example, a meta-mirror of the hybrid mirror as a mode-matching layer may have a similar structure to a meta-lens.

[0072] In some embodiments, the mode-matched photonic crystal structure 221 may include a grid or an array of circular nano-air holes 225 where the radii of the nano-air holes are different. To maximize reflectivity, the radius of each nano-airhole is determined such that it satisfies the position dependent phase of the fundamental Gaussian mode < / >_(Gaussian beam) (x,y) of the VCSEL device 200. For example, for an emitted light having a wavelength of 450 nm, the radius of a nano-air hole at the center may be about 200 nm and the radius of a nano-air hole at the periphery may be about 50 nm. In general, the dimensions of the nano-airholes should be scaled with the operating wavelength (and should be larger for larger wavelengths). The distance between the nano-air holes (i.e., periodicity) is subwavelength < / n_GaN, where n_GaN is the refractive index of the GaN. For example, the periodicity may be less than 450 nm. In some embodiments, the periodicity is less than 300 nm. The overall size of the mode matched photonic crystal structure should be at least twice the size of the fundamental mode size. For example, the overall size of the mode matched photonic crystal structure may be in the order of a few tens of microns.

[0073] The reflectivity of a hybrid mirror including a photonic crystal structure over a DBR may be nearly one (reflectivity ?>0.995). In some embodiments, the shape of the nano-air holes may be an elliptical shape, a square shape, rectangular shape, etc. to achieve linear or chiral polarization.

[0074] FIG. 3A is a cross-sectional view of a VCSEL device 300 in accordance with one or more embodiments of the present disclosure.

[0075] VCSEL device 300 may be similar to VCSEL device 100. Additional details of a mode-matched nanostructure is provided.

[0076] The mode-matched nanostructure 121 may be a mode-matched meta-surface structure 321. The mode-matched meta-surface structure 321 may include an array of nanopillars 325. The mode-matched matched meta-surface structure 321 may include a layer 323 patterned such that a plurality of nano-pillars 325 are formed in the layer 323.

[0077] FIG. 3B is a side cross-sectional view of the mode-matched meta-surface structure of the VCSEL 300 device of FIG. 3A in accordance with one or moreembodiments of the present disclosure. The side cross-sectional view is taken along a center line of the VCSEL device 300. The mode-matched meta-surface structure 321 may include a background layer 323 made of low-refractive index material (e.g., silicon dioxide (SiO2)). The background layer 323 may be patterned to include the array of nanopillars 325. The nanopillars may be made of a high-refractive index material (e.g., titanium oxide (TiO2)). The array of nanopillars may be formed in and surrounded by the background layer.

[0078] In some embodiments, the nano-pillars 325 may have a vertical length that is the same thickness as the background layer 323, approximately 1 fim. The nano-pillars 325 may extend to a top surface of the first dielectric DBR 111. In some embodiments, the nano-pillars 325 may have a vertical length that is less than the thickness of the background layer 323 and the nano-pillars 325 do not extend to a top surface of the first DBR 111.

[0079] FIG. 3C is a top cross-sectional view of the mode-matched meta-surface layer of the VCSEL device 300 of FIG. 3A in accordance with one or more embodiments of the present disclosure. The mode-matched meta-surface structure 321 may include a grid of nano-pillars 325 of different sizes. The sizes of the nano-pillars may be larger towards the center and smaller towards the periphery. In some embodiments, the grid of nano-pillars 325 may be periodically arranged (i.e., the same). In some other embodiments, the distance between the nano-pillars 325 may be different when moving from the center of the mode-matched meta-surface structure to the edges of the mode-matched metasurface structure. For example, the distance between two adjacent nano-pillars at the center may be smaller than the distance between two adjacent nano-pillars at the periphery.

[0080] In some embodiments, the mode-matched meta-surface structure 321 may include a grid of circular nano-pillars 325 where the radii of the nano-nanopillars are different. The radius of each nanopillar (e.g. made of high refractive index material such as TiO2) is determined such that it satisfies the position dependent phase of the fundamental Gaussian mode < / >_(Gaussian beam) (x,y) of the long cavity of VCSEL device 300. The mode-matched meta-surface structure mimics the phase of a curved mirror (parabolic phase) via the meta-surface. The distance between nanopillars (i.e.,periodicity) is subwavelength (<A / n_SiO2, where n_SiO2 is the refractive index of silicon dioxide (SiO2). For example, the periodicity may be less than 450 nm. In some embodiments, the periodicity is less than 300 nm. The overall size of the mode matched meta-surface structure is in the order of few tens of microns. For example, for an emitted light having a wavelength of 450 nm, the radius of a nano-pillar at the center may be about 200 nm and the radius of a nano-pillar at the periphery may be about 50 nm. In general, the dimensions of the nano-pillars should be scaled with the operating wavelength (and should be larger for larger wavelengths). For larger wavelength, the dimensions of the pillars should be scaled with the wavelength of operations (and will be larger for larger wavelength).

[0081] In some embodiments, the cross-sectional shape of the nano-pillars may be an elliptical shape, a square shape, rectangular shape, etc. The cross-section shape of the nano-pillars may be used to provide polarization.

[0082] FIG. 4 is a cross-sectional view of a VCSEL device 400 in accordance with one or more embodiments of the present disclosure.

[0083] The VCSEL device 300 of FIGS. 3A-3C may further include a dielectric spacer layer 450 disposed over the mode-matched meta-surface structure 321 and below the semiconductor structure 330. A dielectric spacer may be added to facilitate nanofabrication. In some circumstances, processing with electron beam lithography (EBL), focused ion beam lithography (FIB) or deep ultraviolet lithography (DUV) may be easier with the dielectric spacer instead of with only the semiconductor layers. The thickness of the dielectric spacer may be less than approximately one micron. The dielectric spacer 450 may have a refractive index close to the semiconductor material. For example, the dielectric spacer 450 may include niobium pentoxide with a gallium nitride semiconductor substrate (e.g. Nb2O5, n_Nb2O5»n_GaN). The addition of the dielectric spacer layer may extend the thickness of the vertical cavity to L_cav « 15-500 m.

[0084] FIG. 5 is a cross-sectional view of a VCSEL device 500 in accordance with one or more embodiments of the present disclosure.

[0085] The VCSEL device 500 may be similar to the VCSEL device 200 of FIGS. 2A- 2C and may further include a dielectric spacer layer 550 disposed over the mode-matchedphotonic crystal structure 521 and under the semiconductor structure 130. The dielectric spacer 550 may have a refractive index close to the material of the semiconductor layers. For example, the dielectric spacer may be niobium pentoxide which may have a refractive index close to gallium nitride (e.g. Nb2O5, n_Nb2O5«n_GaN). The addition of the dielectric spacer layer extends the thickness of the vertical cavity to L_cav » 15-500 f m. The mode-matched photonic crystal layer 521 may be formed at the bottom of the dielectric spacer 550 rather than at the bottom of the substrate layer 131 of the semiconductor structure. That is, an array of nano-air holes 525 may be formed in the bottom of the dielectric spacer 550 to provide the meta-mirror layer of the hybrid mirror.

[0086] FIG. 6 is a top cross-sectional view of VCSEL device 600 in accordance with one or more embodiments of the present disclosure.

[0087] VCSEL device may be similar to any of VCSEL devices 100-500. A linearly polarized mode-matched structure 621 is provided. A birefringent meta-surface or photonic crystal structure, e.g., by breaking in-plane symmetry can provide linearly polarized laser emission. For example, the shape of the FIG. 5 is a cross-sectional view of a VCSEL device 500 in accordance with one or more embodiments of the present disclosure.

[0088] FIGS. 7A-7B are cross-sectional views of VCSEL devices 700a and 700b in accordance with one or more embodiments of the present disclosure.

[0089] The VCSEL device 700a may be similar to any one of the VCSEL devices 100- 600 and may further include a metal layer 761 disposed under the hybrid mirror 105, 205, 305, 405, 505. The metal layer 761 provides additional reflectivity.

[0090] Alternatively, the VCSEL device 700b may be similar to any one of the VCSEL devices 100-600 and may further include a metal layer 763 disposed over the second DBR 141. The metal layer 763 provides additional reflectivity.

[0091] For example, the metal layer 761 , 763 may be made of gold or silver. Other material may be used. For example, the thickness of the metal layer may be 30 nm to 100 nm. The light will emit only on the side of the VCSEL away from the metal layer.

[0092] FIG. 8 is a cross-sectional view of a VCSEL device 800 in accordance with one or more embodiments of the present disclosure.

[0093] The VCSEL device 800 may be similar to any one of the VCSEL devices 100- 600 except the first DBR 111 is not included. In configurations where a VCSEL device is configured for emit infrared light or far infrared light, a mode matched layer may be sufficient to achieve lasing. In this case, the mode-matched nanostructure (e.g., metamirror) should have a very large reflectivity in addition to being mode-matched with the fundamental mode of the VCSEL.

[0094] FIG. 10 is a flow diagram of a method 1000 for manufacturing a VCSEL device in accordance with one or more embodiments of the present disclosure. A method 1000 for manufacturing an optoelectronic semiconductor device according to embodiments will be described below with reference to FIG. 10.

[0095] At 1001 , a first DBR 111 may be provided on a base layer. The first DBR 111 may be fabricated using any conventional method of manufacturing multilayer dielectric mirrors including pairs of alternating layers of high and low refractive index dielectric material. For example, such a process may include Sputter Deposition, Chemical Vapor Deposition (CVD), or Atomic Layer Deposition (ALD). With the meta-mirror, the first DBR 111 may have fewer pairs of layers than a planar DBR for a conventional VCSEL.

[0096] At 1003, a mode-matched nanostructure 120 may be provided on the first DBR 111.

[0097] For a mode-matched photonic crystal nanostructure, a layer of lightly doped n- type semiconductor material (e.g., n-GaN) may be provided on the first DBR 111. The layer of lightly doped n-type semiconductor material may be patterned using EBL, FIB, or DUV to form an array of nano-air holes.

[0098] For a mode-matched meta-surface nanostructure, a layer of dielectric material having a low refractive index (e.g., SiO2) may be provided on the first DBR 111. The dielectric layer may be patterned using EBL, FIB, or DUV to form an array of nano-air holes. The nano-air holes are filled with a dielectric material having a high refractive index (e.g. TiO2). The dielectric material may be deposited using electron beam (e-beam) evaporation, reactive magnetron co-sputtering, or ALD.

[0099] At 1005, a semiconductor structure 130 may be provided on the first DBR 111. The semiconductor structure may be a GaN-based VCSEL. The semiconductor structure 120 may be fabricated using any conventional method of manufacturing VCSELs. Forexample, such a process may include Metalorganic Chemical Vapor Deposition (MOCVD), Molecular Beam Epitaxy (MBE), or Hydride Vapor Phase Epitaxy (HVPE).

[0100] At 1007, a second DBR 141 may be provided on the semiconductor structure. The second DBR 141 may be fabricated using any conventional method of manufacturing multilayer dielectric mirrors including pairs of alternating layers of high and low refractive index dielectric material. For example, such a process may include Sputter Deposition, Chemical Vapor Deposition (CVD), or Atomic Layer Deposition (ALD). The second DBR 141 may have more pairs of layers than the first DBR 111. The second DBR 141 may be similar to a planar DBR for a conventional VCSEL.

[0101] The present disclosure provides a hybrid mirror including a mode-matched nanostructure and DBR. Diffraction loss is suppressed by a mode-matched layer. In combination with a DBR (e.g., 10 bilayers), high reflection is achieved. A large mode spacing allows easy matching of the cavity resonance and peak of the gain. A thick cavity minimizes crack issues into the wafer and eliminates heat management issues.

[0102] The following examples pertain to further embodiments.

[0103] Example 1 may include a VCSEL device including: a stacked structure including: a hybrid mirror including a distributed Bragg reflector (DBR) and a mode- matched nanostructure disposed over the DBR; a semiconductor structure, the semiconductor structure including; a substrate layer; a first semiconductor layer disposed over the substrate layer; a second semiconductor layer disposed over the first semiconductor layer; and an active region disposed between the first semiconductor layer and the second semiconductor layer, wherein the substrate layer and the first semiconductor layer have a first conductivity and the second semiconductor layer has a second conductivity, and wherein in the substrate layer, the first semiconductor layer and the second semiconductor layer have a same semiconductor material; and a further DBR disposed over the semiconductor structure.

[0104] Example 2 may include the VCSEL device of Example 1 , wherein the mode matched nanostructure includes a layer and a plurality of nanoelements arranged in a grid within the layer.

[0105] Example 3 may include the VCSEL device of Example 2 or 3, wherein the plurality of nanoelements have different sizes and wherein a distance between adjacent nanoelements are different.

[0106] Example 3.1 may include the VCSEL device of Example 3, wherein a size of a nanoelement at a center of the grid is larger than a size of a nanoelement at a periphery of the grid.

[0107] Example 4 may include the VCSEL device of any one of Examples 1-3.1 , wherein the semiconductor structure includes a vertical cavity, wherein the matchedmode structure matches a phase of the fundamental mode of the vertical cavity of the semiconductor structure.

[0108] Example 5 may include the VCSEL device of any one of Examples 1-4, wherein the cavity has a thickness L_cav ranging from 25 m to100 pm, and the mode-matched nanostructure has a thickness LMMLof less than 1 pm.

[0109] Example 6 may include the VCSEL device of any one of Examples 1-5, wherein a cross-sectional shape of the plurality of nanoelements is circular.

[0110] Example 7 may include the VCSEL device of any one of Examples 1-7, wherein the further DBR has more pairs of layers than the DBR of the hybrid mirror.

[0111] Example 8 may include the VCSEL device of any one of Examples 1-7, wherein the mode-matched nanostructure includes a mode-matched photonic crystal structure, wherein the nanoelements include nano-air holes.

[0112] Example 9 may include the VCSEL device of any one of Examples 1-8, wherein the mode-matched photonic crystal structure includes: the layer patterned with nano-air holes.

[0113] Example 10 may include the VCSEL device of any one of Examples 1-9, wherein the layer includes a bottom portion of the substrate layer, such that sides of the nano-air holes are surrounded by the material of the substrate layer.

[0114] Example 12 may include the VCSEL device of any one of claims 1-8, wherein the mode-matched nanostructure includes a mode-matched meta-surface structure, wherein the nanoelements include nano-pillars.

[0115] Example 13 may include the VCSEL device of any one of claims 1-8, the VCSEL, wherein the layer includes a low-refractive index material and the nanopillars includes a high-refractive index material.

[0116] Example 14 may include the VCSEL device of any one of claims 1-13, wherein a periodicity of the plurality of nanoelements is based on a material of the layer in which the nanoelements are provided.

[0117] Example 14 may include the VCSEL device of any one of claims 1-14, wherein the mode-matched nanostructure includes: a dielectric spacer layer disposed between the hybrid mirror and the semiconductor structure, wherein, preferably, a refractive index of the dielectric spacer layer is similar to a refractive index of the material of the substrate layer.

[0118] Example 15 may include the VCSEL device of any one of claims 1-5 and 7-14, wherein a cross-sectional shape of the plurality of nanoelements is asymmetrical, preferably, elliptical.

[0119] Example 16 may include the VCSEL device of any one of claims 1-15, further includes: a metal layer disposed under the hybrid mirror or over the further DBR.

[0120] The foregoing description of one or more implementations provides illustration and description, but is not intended to be exhaustive or to limit the scope of embodiments to the precise form disclosed. Modifications and variations are possible in light of the above teachings or may be acquired from practice of various embodiments.

[0121] Many modifications and other implementations of the disclosure set forth herein will be apparent having the benefit of the teachings presented in the foregoing descriptions and the associated drawings. Therefore, it is to be understood that the disclosure is not to be limited to the specific implementations disclosed and that modifications and other implementations are intended to be included within the scope of the appended claims. Although specific terms are employed herein, they are used in a generic and descriptive sense only and not for purposes of limitation.Reference Numeral List hybrid mirror first DBR mode-matched structure semiconductor structure substrate layer first semiconductor layer active region second semiconductor layer second DBR hybrid mirror mode-matched photonic crystal structure layer nano-air holes hybrid mirror mode-matched meta-surface structure background layer nano-pillars dielectric spacer linearly polarized mode-matched structure metal layer metal layer

Claims

Claims1. A VCSEL device (100) comprising: a stacked structure including: a hybrid mirror (105) including a distributed Bragg reflector (DBR)(111) and a mode-matched nanostructure (121) disposed over the DBR; a semiconductor structure (130), the semiconductor structure including; a substrate layer (131); a first semiconductor layer disposed over the substrate layer (133); a second semiconductor layer disposed over the first semiconductor layer (137); and an active region (135) disposed between the first semiconductor layer (133) and the second semiconductor layer (137), wherein the substrate layer (131) and the first semiconductor layer (133) have a first conductivity and the second semiconductor layer (137) has a second conductivity, and wherein in the substrate layer (131), the first semiconductor layer (133) and the second semiconductor layer (137) have a same semiconductor material; wherein the mode-matched nanostructure (121) is disposed directly on the DBR (111) and between the DBR (111) and the semiconductor structure (130); and a further DBR (141) disposed over the semiconductor structure (130).

2. The VCSEL device of claim 1 , wherein the mode matched nanostructure (121) comprises a layer and a plurality of nanoelements arranged in a grid within the layer.

3. The VCSEL device of claim 1 or 2, wherein the plurality of nanoelements have different sizes and wherein a distance between adjacent nanoelements are different, wherein, preferably, a size of a nanoelement at a center of the grid is larger than a size of a nanoelement at a periphery of the grid.

4. The VCSEL device of any one of claims 1 -3, wherein the semiconductor structure comprises a vertical cavity, wherein the matched-mode structure matches a phase of the fundamental mode of the vertical cavity of the semiconductor structure.

5. The VCSEL device of claim 4, wherein the cavity has a thickness L_cav ranging from 25 m to 100 gm, and the mode-matched nanostructure has a thickness LMMLof less than 1 gm.

6. The VCSEL device of any one of claims 1-5, wherein a cross-sectional shape of the plurality of nanoelements is circular.

7. The VCSEL device of any one of claims 1-6, wherein the further DBR (141) has more pairs of layers than the DBR (111) of the hybrid mirror (105).

8. The VCSEL device of any one of claims 1-7, wherein the mode-matched nanostructure (121) comprises a mode-matched photonic crystal structure (221), wherein the nanoelements comprise nano-air holes (225).

9. The VCSEL device of claim 8, wherein the mode-matched photonic crystal structure comprises: the layer (223) patterned with nano-air holes (225).

10. The VCSEL device of claim 9, wherein the layer (223) comprises a bottom portion of the substrate layer (131), such that sides of the nano-air holes are surrounded by the material of the substrate layer.

11. The VCSEL device of any one of claims 1-8, wherein the mode-matched nanostructure (121) comprises a mode-matched meta-surface structure (221), wherein the nanoelements comprise nano-pillars (225).

12. The VCSEL device of claim 11 , wherein the layer (223) comprises a low-refractive index material and the nanopillars (225) comprise a high-refractive index material.

13. The VCSEL device of any one of claims 1-12, wherein a periodicity of the plurality of nanoelements is based on a material of the layer in which the nanoelements are provided.

14. The VCSEL device of any one of claims 1-13, wherein the mode-matched nanostructure comprises: a dielectric spacer layer (450) disposed between the hybrid mirror (105) and the semiconductor structure (130), wherein, preferably, a refractive index of the dielectric spacer layer is similar to a refractive index of the material of the substrate layer.

15. The VCSEL device of any one of claims 1-5 and 7-14, wherein a cross-sectional shape of the plurality of nanoelements is asymmetrical, preferably, elliptical.

16. The VCSEL device of any one of claims 1-15, further comprises: a metal layer (761 , 763) disposed under the hybrid mirror (105) or over the further DBR (141).

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