Patent Publication Number: US-2023163262-A1

Title: Light emitting diode array containing metamaterial light collimating features and methods for forming the same

Description:
FIELD 
     The present invention relates to light emitting devices, and particularly to a light emitting diode array containing metamaterial collimating features and methods for forming the same. 
     BACKGROUND 
     Light emitting devices such as light emitting diodes (LEDs) are used in electronic displays, such as backlights in liquid crystal displays located in laptops or televisions, LED billboards, microdisplays, and LED televisions. A microLED refers to a light emitting diode having lateral dimensions that do not exceed 1 mm. A microLED has a typical lateral dimension in a range from 1 microns to 150 microns. An array of microLEDs can form an individual pixel element. A direct view display device can include an array of pixel elements, each of which includes several microLEDs which emit light having a different emission spectrum. 
     SUMMARY 
     According to an aspect of the present disclosure, a light emitting device includes a backplane, first, second and third light emitting diodes located on the backplane, a first patterned metamaterial lens containing first nanostructures located over the first light emitting diode, a second patterned metamaterial lens containing second nanostructures located over the second light emitting diode, and a third patterned metamaterial lens containing third nanostructures located over the light emitting diode. A configuration of the first nanostructures differs from a configuration of the second nanostructures, and a configuration of the third nanostructures differs from the configurations of the first and the second nanostructures. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG.  1    is a vertical cross-sectional view of a region of a first exemplary structure including a backplane according to an embodiment of the present disclosure. 
         FIG.  2    is a vertical cross-sectional view of a region of the first exemplary structure after attaching first light emitting diodes according to an embodiment of the present disclosure. 
         FIG.  3 A  is a vertical cross-sectional view of a region of the first exemplary structure after attaching second light emitting diodes and third light emitting diodes according to an embodiment of the present disclosure. 
         FIG.  3 B  is a top-down view of the first exemplary structure of  FIG.  3 A . 
         FIG.  4    is a vertical cross-sectional view of the first exemplary structure after formation of a dielectric matrix layer according to an embodiment of the present disclosure. 
         FIG.  5    is a vertical cross-sectional view of the first exemplary structure after forming a metamaterial layer on a front side of the array of light emitting diodes according to an embodiment of the present disclosure. 
         FIG.  6    is a vertical cross-sectional view of the first exemplary structure after patterning the metamaterial layer into an array of patterned metamaterial lenses according to an embodiment of the present disclosure. 
         FIGS.  7 A- 7 F  illustrate exemplary patterns that may be employed for a patterned metamaterial lens according to an embodiment of the present disclosure. 
         FIG.  8    is a vertical cross-sectional view of a region of a second exemplary structure after formation of a dielectric matrix layer over a backplane according to an embodiment of the present disclosure. 
         FIG.  9    is a vertical cross-sectional view of a region of the second exemplary structure after formation of an array of cavities through the dielectric matrix layer according to an embodiment of the present disclosure. 
         FIG.  10    is a vertical cross-sectional view of a region of the second exemplary structure after formation of an array of reflectors according to an embodiment of the present disclosure. 
         FIG.  11    is a vertical cross-sectional view of a region of the second exemplary structure after attaching an array of light emitting diodes to the backplane according an embodiment of the present disclosure. 
         FIG.  12    is a vertical cross-sectional view of a region of the second exemplary structure after formation of cavity-fill dielectric material portion according an embodiment of the present disclosure. 
         FIG.  13    is a vertical cross-sectional view of the second exemplary structure after forming a metamaterial layer on a front side of the array of light emitting diodes according to an embodiment of the present disclosure. 
         FIG.  14    is a vertical cross-sectional view of the second exemplary structure after patterning the metamaterial layer into an array of patterned metamaterial lenses according to an embodiment of the present disclosure. 
         FIG.  15    is a vertical cross-sectional view of a region of a third exemplary structure after attaching an array of light emitting diodes to the backplane according an embodiment of the present disclosure. 
         FIG.  16    is a vertical cross-sectional view of a region of the third exemplary structure after formation of cavity-fill dielectric material portion according an embodiment of the present disclosure. 
         FIG.  17    is a vertical cross-sectional view of a region of the third exemplary structure after formation of an array of color conversion medium portions according an embodiment of the present disclosure. 
         FIG.  18    is a vertical cross-sectional view of the third exemplary structure after forming a metamaterial layer on a front side of the array of light emitting diodes according to an embodiment of the present disclosure. 
         FIG.  19    is a vertical cross-sectional view of the third exemplary structure after patterning the metamaterial layer into an array of patterned metamaterial lenses according to an embodiment of the present disclosure. 
     
    
    
     DETAILED DESCRIPTION 
     Generally, photons emitted from a light emitting diode follows a Lambertian emission pattern, and are not collimated. Lack of collimation of emitted light from light emitting elements is an impediment in effective implementation of a high density light emitting device array, such as heads up display devices, virtual reality display devices or artificial reality display devices. In other words, a high degree of collimation of emitted light is advantageous for high-definition high-fidelity display devices. The embodiments of the present disclosure are directed to a light emitting diode array containing metamaterial light collimating features and methods for forming the same. The array may be used in heads up display devices, virtual reality display devices, artificial reality display devices, or other suitable devices. 
     The drawings are not drawn to scale. Multiple instances of an element may be duplicated where a single instance of the element is illustrated, unless absence of duplication of elements is expressly described or clearly indicated otherwise. Ordinals such as “first,” “second,” and “third” are employed merely to identify similar elements, and different ordinals may be employed across the specification and the claims of the instant disclosure. The same reference numerals refer to the same element or similar element. Unless otherwise indicated, elements having the same reference numerals are presumed to have the same composition. As used herein, a first element located “on” a second element can be located on the exterior side of a surface of the second element or on the interior side of the second element. As used herein, a first element is located “directly on” a second element if there exist a physical contact between a surface of the first element and a surface of the second element. As used herein, a “layer” refers to a continuous portion of at least one material including a region having a thickness. A layer may consist of a single material portion having a homogeneous composition, or may include multiple material portions having different compositions. 
     As used herein, a “conductive material” refers to a material having electrical conductivity greater than 1.0×10 5  S/cm. As used herein, an “insulator material” or a “dielectric material” refers to a material having electrical conductivity less than 1.0×10 −6  S/cm. As used herein, a “semiconducting material” refers to a material having electrical conductivity in the range from 1.0×10 −6  S/cm to 1.0×10 5  S/cm. As used herein, a “metallic material” refers to a conductive material including at least one metallic element therein. All measurements for electrical conductivities are made at the standard condition. 
     A display device, such as a heads up display device, virtual reality display device or artificial reality display device can be formed from an ordered array of pixels. Each pixel can include a set of subpixels that emit light at a respective emission spectrum. For example, a pixel can include a red subpixel, a green subpixel, and a blue subpixel. In one embodiment, each subpixel can include one or more light emitting diodes that emit light of one of several peak wavelengths (e.g., red light, green light and blue light). In another embodiment, all light emitting diodes emit radiation of the same peak wavelength (e.g., ultraviolet radiation or light in the blue color spectrum which includes blue and violet color light). Each pixel is driven by a backplane circuit such that any combination of colors within a color gamut may be shown on the display for each pixel. The display panel can be formed by a process in which LED subpixels are soldered to, or otherwise electrically attached to, a bond pad located on a backplane. The bond pad is electrically driven by the backplane circuit and other driving electronics. 
     Referring to  FIG.  1   , a first exemplary structure includes a backplane  400 . The backplane  400  includes a backplane substrate  410 , which can be an insulating substrate. A control circuitry for controlling operation of the light emitting diodes attached to the backplane  400  may be provided within the backplane. For example, switching devices  450  can be provided within the backplane  400 . In an illustrative example, the switching devices  450  can include field effect transistors, such as thin film transistors (TFTs). In this case, each field effect transistor  450  may include a gate electrode  420 , a gate dielectric  430 , a channel region  442 , a source region  446 , and a drain region  444 . While an inverted staggered TFT  450  is shown in  FIG.  1   , other types of TFTs, such as inverted coplanar, top gated staggered and top gated coplanar TFTs can be used instead. Other type of switching devices may also be used instead of or in addition to the TFTs  450 . Various electrical wirings can be provided to interconnect the various electrical nodes of the field effect transistors to electrical interfaces (not expressly shown) on the backplane  400 . A patterned passivation layer  454  may be optionally formed on the source regions  446  and the drain regions  444 . Additional interconnect wiring may be provided as needed. The switching devices  450  can be encapsulated by an encapsulation dielectric layer  465 . First-level metal interconnect structures  460  can be formed through the encapsulation dielectric layer  465  to a node of a respective switching device  450  such as a drain region  444 . An interconnect level dielectric layer  475  may be formed over the encapsulation dielectric layer  465 , and second-level metal interconnect structures  470  can be formed through the interconnect level dielectric layer  475  on the first-level metal interconnect structures  460 . The second-level metal interconnect structures  470  can include an array of bonding pads for attaching the array of light emitting diodes  10 . 
     In one embodiment illustrated in  FIGS.  2  to  6   , different color light emitting diodes are attached to the backplane in different subpixels. Referring to  FIG.  2   , first light emitting diodes  10 A can be attached to a first subset of physically exposed metal bonding pads, which are a subset of the metal interconnect structures ( 460 ,  470 ) located within the backplane  400 . In one embodiment, the first subset of the physically exposed metal bonding pads may be, for example, a first subset of the second-level metal interconnect structures  470 . While the present disclosure is described employing a backplane  400  including two levels of metal interconnect structures ( 460 ,  470 ), it is understood that the backplane  400  may include any number of metal interconnect levels. Generally, an array of bonding structures can be provided on the front side of the backplane  400 , and the array of light emitting diodes can be bonded to a respective bonding structure within the backplane  400  employing any bonding method known in the art. 
     Generally, an array of first light emitting diodes  10 A can be attached to a first subset of the metal interconnect structures ( 460 ,  470 ) (such as metal bonding pads) in the backplane  400 . The first light emitting diodes  10 A can be configured to emit light at a first peak wavelength, which may be, for example a red light having a wavelength range between 620 nm to 750 nm. Each first light emitting diode  10 A may comprise an optional buffer layer  24  and a first doped compound semiconductor layer  26  (such as an n-doped GaN layer). The first doped compound semiconductor layer  26  has a doping of a first conductivity type (such as n-type), and is epitaxially aligned to the single crystalline structure of the buffer layer  24  (if present). In an illustrative example, the buffer layer  24  may comprise a buffer III-V compound semiconductor material (e.g., GaN or AlGaN) having a doping of the first conductivity type, and may have a lattice constant that is substantially matched to the lattice constant of the single crystalline substrate  22 . In some embodiment, the buffer layer  24  may have a compositional gradient so that the top portion of the buffer layer  24  has a lattice constant of a first doped compound semiconductor material of the first doped compound semiconductor layer  26 . In an illustrative example, the first conductivity type may be n-type. The thickness of the buffer layer  24  may be in a range from 0.5 micron to 10 microns, such as from 1 micron to 3 microns, although lesser and greater thicknesses may also be employed. 
     In a non-limiting illustrative example, the first doped compound semiconductor layer  26  may comprise a single crystalline gallium nitride material in epitaxial alignment with the single crystalline structure of the buffer layer  24 . The single crystalline n-doped gallium nitride layer may be formed, for example, by an epitaxial deposition process such as metal-organic chemical vapor deposition (MOCVD) process. The single crystalline n-doped gallium nitride layer may be n-doped by introduction of silicon as n-type dopants during the epitaxial deposition process. 
     A first active layer  30 A is located on the first doped compound semiconductor layer  26 . The first active layer  30 A includes a set of doped compound semiconductor material layers that is configured to emit light at the first peak wavelength. In one embodiment, the first active layer  30 A may comprise a periodic repetition of first compound semiconductor layers  32 A and second compound semiconductor layers  34 A that forms a quantum well. The material compositions and the thicknesses of the first compound semiconductor layers  32 A and second compound semiconductor layers  34 A can be selected such that the first active layer  30 A emits light at the first peak wavelength. Additional material layers configured to increase the quantum efficiency of the light emission may be present within the first active layer  30 A. Alternatively, non-quantum-well structures may be employed for the first active layer  30 A. In a non-limiting illustrative example, the first active layer  30 A may comprise a planar light-emitting indium gallium nitride layer, a planar GaN barrier layer, and an optional planar p-doped III-nitride layer (such as a p-doped aluminum gallium nitride layer). Generally, the first active layer  30 A may comprise any set of doped compound semiconductor material layers that is configured to emit light at the first peak wavelength. 
     A second doped compound semiconductor portion  36  is located on the first active layer  30 A. The second doped compound semiconductor portion  36  includes a doped semiconductor material having a doping of a second conductivity type that is the opposite of the first conductivity type. In an illustrative example, the first doped compound semiconductor layer  26  may comprise an n-doped III-V compound semiconductor material (such as n-doped GaN), and the second doped compound semiconductor portions  36  may comprise a p-doped III-V compound semiconductor material (such as p-doped GaN or AlGaN). The second doped compound semiconductor portions  36  may have a thickness in a range from 100 nm to 1 micron, such as from 200 nm to 500 nm, although lesser and greater thicknesses may also be employed. 
     An optional transparent p-side electrode  38  may be formed on each second doped compound semiconductor region  36 . The p-side electrode  38  may comprise an optically transparent, electrically conductive material, such as a transparent conductive oxide. Examples of a transparent conductive oxide include indium tin oxide, aluminum zinc oxide, or fluorine doped tin oxide. 
     Each first light emitting diode  10 A may comprise an insulating spacer  60  that provides electrical isolation to the second doped compound semiconductor portion  36  and the first active layer  30 A. Further, each first light emitting diode  10 A may comprise a reflector  82  including an optically reflective material (such as aluminum, silver, gold, or copper), and configured to reflect light toward a distal surface of the first doped compound semiconductor layer  26  (such as an interface between the first doped compound semiconductor layer  26  and the buffer layer  24 ). In one embodiment, the reflector  82  may be electrically connected to the second doped compound semiconductor portion  36 , for example, by vertically extending through an opening in the insulating spacer  60  and directly contacting the second doped compound semiconductor portion  36 , or by contacting the p-side electrode  38  that is interposed between the vertically protruding portion of the reflector  82  and the second doped compound semiconductor portion  36 . The reflector  82  may have a thickness in a range from 10 nm to 300 nm, such as from 30 nm to 100 nm, although lesser and greater thicknesses may also be employed. 
     Each first light emitting diode  10 A may be attached to the backplane  400 , for example, through an array of solder contacts, such as an array of solder material portions  50 . For example, the array of solder material portions  50  can be formed in the recess regions of the reflectors  82  of the first light emitting diodes  10 A, and can be bonded to a respective metal bonding structure in the backplane  400 , which may be a metal interconnect structure such as a second-level metal interconnect structure  470 . Alternatively, the array of solder material portions  50  may be formed on metal bonding structures in the backplane  400  (such as the second-level metal interconnect structures  470 ), and can be bonded to a respective one of the recess regions of the reflectors  82 . 
     Referring to  FIGS.  3 A and  3 B , an array of second light emitting diodes  10 B can be attached to a second subset of the metal interconnect structures ( 460 ,  470 ) (such as metal bonding pads) in the backplane  400 . The second light emitting diodes  10 B can be configured to emit light at a second peak wavelength, which may be, for example a green light having a wavelength range between 530 nm to 570 nm. The second light emitting diodes  10 B may differ from the first light emitting diodes  10 A by the structure of an active layer therein, which is herein referred to as a second active layer  30 B. The second active layer  30 B includes a set of doped compound semiconductor material layers that is configured to emit light at the second peak wavelength. In one embodiment, the second active layer  30 B may comprise a periodic repetition of first compound semiconductor layers  32 B and second compound semiconductor layers  34 B that forms a quantum well. The material compositions and the thicknesses of the first compound semiconductor layers  32 B and second compound semiconductor layers  34 B can be selected such that the second active layer  30 B emits light at the second peak wavelength. For example, the first compound semiconductor layers  32 B in the second light emitting diodes  10 B may comprise indium gallium nitride having a higher indium concentration than the first compound semiconductor layers  32 A in the first light emitting diodes  10 A. Additional material layers configured to increase the quantum efficiency of the light emission may be present within the second active layer  30 B. Alternatively, non-quantum-well structures may be employed for the second active layer  30 B. Generally, the second active layer  30 B may comprise any set of doped compound semiconductor material layers that is configured to emit light at the second peak wavelength. 
     Each second light emitting diode  10 B may be attached to the backplane  400 , for example, through an array of solder contacts, such as an array of solder material portions  50 . For example, the array of solder material portions  50  can be formed in the recess regions of the reflectors  82  of the second light emitting diodes  10 B, and can be bonded to a respective metal bonding structure in the backplane  400 , which may be a metal interconnect structure such as a second-level metal interconnect structure  470 . Alternatively, the array of solder material portions  50  may be formed on metal bonding structures in the backplane  400  (such as the second-level metal interconnect structures  470 ), and can be bonded to a respective one of the recess regions of the reflectors  82 . 
     An array of third light emitting diodes  10 C can be attached to a third subset of the metal interconnect structures ( 460 ,  470 ) (such as metal bonding pads) in the backplane  400 . The third light emitting diodes  10 C can be configured to emit light at a third peak wavelength, which may be, for example a blue light having a wavelength range between 420 nm to 480 nm. The third light emitting diodes  10 C may differ from the first light emitting diodes  10 A by the structure of an active layer therein, which is herein referred to as a third active layer  30 C. The third active layer  30 C includes a set of doped compound semiconductor material layers that is configured to emit light at the third peak wavelength. In one embodiment, the third active layer  30 C may comprise a periodic repetition of first compound semiconductor layers  32 C and second compound semiconductor layers  34 C that forms a quantum well. The material compositions and the thicknesses of the first compound semiconductor layers  32 C and second compound semiconductor layers  34 C can be selected such that the third active layer  30 C emits light at the third peak wavelength. For example, the first compound semiconductor layers  32 C in the third light emitting diodes  10 C may comprise indium gallium nitride having a higher indium concentration than the first compound semiconductor layers  32 B in the second light emitting diodes  10 A. Additional material layers configured to increase the quantum efficiency of the light emission may be present within the third active layer  30 C. Alternatively, non-quantum-well structures may be employed for the third active layer  30 C. Generally, the third active layer  30 C may comprise any set of doped compound semiconductor material layers that is configured to emit light at the third peak wavelength. 
     Each third light emitting diode  10 C may be attached to the backplane  400 , for example, through an array of solder contacts, such as an array of solder material portions  50 . For example, the array of solder material portions  50  can be formed in the recess regions of the reflectors  82  of the third light emitting diodes  10 C, and can be bonded to a respective metal bonding structure in the backplane  400 , which may be a metal interconnect structure such as a second-level metal interconnect structure  470 . Alternatively, the array of solder material portions  50  may be formed on metal bonding structures in the backplane  400  (such as the second-level metal interconnect structures  470 ), and can be bonded to a respective one of the recess regions of the reflectors  82 . 
     Generally, an array of light emitting diodes ( 10 A,  10 B,  10 C) can be attached to the backplane  400 . In one embodiment, the array of light emitting diodes ( 10 A,  10 B,  10 C) comprises a first sub-array of first light emitting diodes  10 A configured to emit light at a first peak wavelength, a second sub-array of second light emitting diodes  10 B configured to emit light at a second peak wavelength, and a third sub-array of third light emitting diodes  10 C configured to emit light at a third peak wavelength. The first peak wavelength, the second peak wavelength, and the third peak wavelength are different from each other. As used herein, a sub-array refers to an array that is a subset of, and includes less elements than, another array of elements. 
     In one embodiment, each light emitting diode ( 10 A,  10 B,  10 C) within the array of light emitting diodes ( 10 A,  10 B,  10 C) comprises a vertical stack including a first doped compound semiconductor layer  26 , an active layer ( 30 A,  30 B, or  30 C) configured to emit light at a respective peak wavelength, and a second doped compound semiconductor portion  36 . In one embodiment, each light emitting diode ( 10 A,  10 B,  10 C) within the array of light emitting diodes ( 10 A,  10 B,  10 C) is attached to a respective metal interconnect structure (such as the second-level metal interconnect structures  470 ) within the backplane  400  via an array of solder material portions  50 . 
     As shown in  FIG.  3 B , each light emitting diode ( 10 A,  10 B,  10 C) may comprise a subpixel of a pixel “P” of a display device. For example, the pixel P may include at least one red, at least one green and at least one blue light emitting diode. Other color light emitting diodes may also be used. 
     Referring to  FIG.  4   , a dielectric fill material such as an underfill material, silicon oxide, or a polymer material can be deposited in gaps between neighboring pairs of light emitting diodes ( 10 A,  10 B,  10 C). The dielectric fill material may be deposited by injection, by a self-planarizing deposition method such as spin coating, or by a conformal deposition process such as chemical vapor deposition. Excess portions of the dielectric fill material can be removed from above the horizontal plane including distal horizontal surfaces of the light emitting diodes ( 10 A,  10 B,  10 C). In one embodiment, the distal horizontal surfaces of the light emitting diodes ( 10 A,  10 B,  10 C) may comprise backside surfaces of the buffer layers  24 , or may comprise backside surfaces of the first doped compound semiconductor layers  26  (in case buffer layers  24  are not employed or removed during the planarization process that removes excess portions of the dielectric material). 
     Generally, the dielectric fill material can be applied around the array of light emitting diodes ( 10 A,  10 B,  10 C) after attaching the array of light emitting diodes ( 10 A,  10 B,  10 C) to the backplane  400 , and portions of the dielectric fill material can be removed from above a horizontal plane including top surfaces of the array of light emitting diodes ( 10 A,  10 B,  10 C). The remaining portion of the dielectric fill material comprises the dielectric matrix layer  110 . An array of light emitting diodes ( 10 A,  10 B,  10 C) laterally surrounded by the dielectric matrix layer  110  can be formed over the backplane  400 . The array of light emitting diodes ( 10 A,  10 B,  10 C) is attached to a front side of the backplane  400 , and thus, is located above the backplane  400 . 
     In one embodiment, the dielectric matrix layer  110  contacts sidewalls of each of the light emitting diodes ( 10 A,  10 B,  10 C). In one embodiment, the dielectric matrix layer  110  may contact sidewalls of the buffer layer  24 , sidewalls of the first doped compound semiconductor layer  26 , sidewalls of the active layer ( 30 A,  30 B,  30 C), sidewalls of the second doped compound semiconductor layer  36 , sidewalls of the insulating spacer  60 , and surfaces of the reflector  82  within each light emitting diode ( 10 A,  10 B,  10 C). In one embodiment, the dielectric matrix layer  110  contacts, and laterally surrounds, each of the solder material portions  50 . 
     Referring to  FIG.  5   , a transparent conductive material layer  150  may be deposited on the front side of the array of light emitting diodes ( 10 A,  10 B,  10 C). The transparent conductive material layer  150  can be deposited directly on the buffer layers  24  or directly on the first doped compound semiconductor layers  26 , which are electrically connected to a first electrical node of a respective active layer ( 30 A,  30 B,  30 C), i.e., the first doped compound semiconductor layer  26 . In this case, the transparent conductive material layer  150  may function as a common ground node (e.g., common n-side electrodes) of the array of light emitting diodes ( 10 A,  10 B,  10 C), and the solder material portions  50  can be electrically connected to a p-side electrode  38  of a respective light emitting diode. A voltage bias across a solder material portion  50  and the common ground node of the transparent conductive material layer  150  can induce light emission from a respective active layer ( 30 A,  30 B, or  30 C). The transparent conductive material layer  150  may comprise a transparent conductive oxide material (such as indium tin oxide, fluorine-doped tin oxide, or a doped zinc oxide), a conductive polymer, a metal grid, or a random metallic or carbon nanotube network (e.g., network of metal nanowires or nanotubes above their percolation threshold). The thickness of the transparent conductive material layer  150  may be in a range from 3 nm to 300 nm, such as from 10 nm to 100 nm, although lesser and greater thicknesses may also be employed. 
     A metamaterial layer  120 L can be deposited on the front side of the array of light emitting diodes ( 10 A,  10 B,  10 C) directly on a distal horizontal surface of the transparent conductive material layer  150 . For example, the metamaterial layer  120 L may be deposited over the dielectric matrix layer  110  and over the array of light emitting diodes ( 10 A,  10 B,  10 C). In one embodiment, the metamaterial layer  120 L may comprise an optically transparent material having a wide band gap, i.e., a band gap having a magnitude greater than 2 eV. In one embodiment, the metamaterial layer  120 L may comprise, and/or consist essentially of, a semiconductor material, such as silicon, silicon germanium or a III-V compound semiconductor material (such as GaN) or a dielectric metal oxide material (such as titanium oxide, tantalum oxide, aluminum oxide, yttrium oxide, lanthanum oxide, etc.) or silicon oxide. In another embodiment, the metamaterial layer  120 L may comprise an optically opaque material, such as a metal, for example gold or silver. The metamaterial layer  120 L may be deposited by chemical vapor deposition or atomic layer deposition. The thickness of the metamaterial layer  120 L may be in a range from 30 nm to 2,000 nm, such as from 50 nm to 500 nm, although lesser and greater thicknesses may also be employed. 
     Referring to  FIGS.  6  and  7 A- 7 F , the metamaterial layer  120 L can be patterned into an array of patterned metamaterial lenses ( 120 A,  120 B,  120 C).  FIG.  6    is a vertical cross-sectional view of the first exemplary structure after patterning the metamaterial layer  120 L into the array of patterned metamaterial lenses ( 120 A,  120 B,  120 C). The metamaterial lenses may have at least one of length and/or width that is 800 nm or less, such as 10 nm to 800 nm, for example 50 nm to 200 nm, such as 50 nm to 150 nm. The metamaterial lenses ( 120 A,  120 B,  120 C) can bend and/or focus visible light that is incident on these lenses, similar to optical lenses. 
     The metamaterial lenses ( 120 A,  120 B,  120 C) are also known as optical metasurfaces, which are subwavelength (i.e., less than the wavelength (e.g., 400 to 800 nm) of visible light) dimension patterned layers or structures that interact strongly with light. Such lenses alter the light properties over a subwavelength thickness. In contrast to conventional optics, which rely on light refraction and propagation, the metamaterial lenses ( 120 A,  120 B,  120 C) manipulate light based on scattering from small nanostructures. Such nanostructures can resonantly capture the light and re-emit it with a defined phase, polarization, modality and spectrum, thus allowing the bending of light waves. 
     The metamaterial lenses ( 120 A,  120 B,  120 C) may comprise electrically conductive plasmonic nanostructures which bend light beams by phase manipulation, or dielectric (i.e., electrically insulating) nanostructures which exhibit strong Mie-type resonances of both electric and magnetic nature, where the resonant wavelength is proportional to the size of the nanostructure multiplied by the refractive index of the dielectric material. At resonance, the dielectric nanostructures have induced-electric or magnetic dipole (or higher-order) moments, the interference of which strongly affects the directionality of the light scattering. 
       FIGS.  7 A- 7 F  illustrate exemplary patterns that may be employed for a patterned metamaterial lens according to an embodiment of the present disclosure. The metamaterial lenses illustrated in  FIGS.  7 A- 7 F  are described in D. Neshev, et al., Light: Science &amp; Applications (2018) 7:58, incorporated herein by reference in its entirety.  FIG.  7 A  illustrates an optical metasurface composed of a gold antenna array. The unit cell of the plasmonic interface comprises eight gold V-shaped light antennas.  FIG.  7 B  illustrates a metamaterial lens operating at 660 nm and consisting of TiO 2  nanofins on a glass substrate. Scale bar is 300 nm.  FIG.  7 C  illustrates an Achromatic metamaterial lens with numerical aperture (NA) ˜0.1. Scale bar is 500 nm. The vertical boundary of nanopillars and Babinet structures is visible.  FIG.  7 D  illustrates a fabricated meta-hologram that produces 5 mm large images at a distance of 10 mm. The posts are silicon on SiO 2 ,  FIG.  7 E  is a SEM image of a dielectric metasurface lens based on Si nanobeams that results in a local Bessel spot focal length of 100 mm at λ=550 nm.  FIG.  7 F  illustrates a dielectric metasurface made from amorphous silicon pillars on SiO 2  that separates x-and y-polarized light and focuses them to two different points. 
     In one embodiment, a photoresist layer (not shown) can be applied over the metamaterial layer  120 L, and can be lithographically patterned to form an optical interference pattern that induces collimation of light that is emitted from a respective underlying light emitting diode ( 10 A,  10 B,  10 C). Since the light emitting diodes of different types emit light at different peak wavelengths, the optical interference pattern that is formed over each light emitting diode ( 10 A,  10 B,  10 C) may be optimized such that the optical interference pattern provides collimation for the wavelength of the light that is emitted from a respective underlying light emitting diode ( 10 A,  10 B,  10 C). The pattern in the photoresist layer can be transferred through the metamaterial layer  120 L by performing an etch process, which may comprise an anisotropic etch process (such as a reactive ion etch process). The metamaterial layer  120 L can be patterned into the array of patterned metamaterial lenses ( 120 A,  120 B,  120 C) by the etch process. The photoresist layer can be subsequently removed, for example, by ashing. 
     Each patterned metamaterial lens ( 120 A,  120 B,  120 C) can have a pattern that collimates light that is emitted from a respective underlying light emitting diode ( 10 A,  10 B,  10 C). First patterned metamaterial lenses  120 A can be formed over first light emitting diodes  10 A that emit light at the first peak wavelength, and can have a pattern that collimates light having the first peak wavelength with highest directionality. Second patterned metamaterial lenses  120 B can be formed over second light emitting diodes  10 B that emit light at the second peak wavelength, and can have a pattern that collimates light having the second peak wavelength with highest directionality. Third patterned metamaterial lenses  120 C can be formed over third light emitting diodes  10 C that emit light at the third peak wavelength, and can have a pattern that collimates light having the third peak wavelength with highest directionality. 
     Thus, the configuration, such as size (e.g., width and/or length) of the nanostructures, spacing between the nanostructures and/or direction (i.e., positional angle) of the nanostructures in the first patterned metamaterial lenses  120 A may differ from the configuration of the second patterned metamaterial lenses  120 B, and the configuration of the third patterned metamaterial lenses may different from the configurations of the first and the second patterned metamaterial lenses ( 120 A,  120 B). For example, if the first light emitting diodes  10 A emit red light, then the configuration of the first patterned metamaterial lenses  120 A may be configured to optimize collimation of red light. Likewise, if the second and third light emitting diodes ( 10 B,  10 C) emit green and blue light, respectively, then the configuration of the second patterned metamaterial lenses ( 120 B,  120 C) may be configured to optimize collimation of green and blue light, respectively. Generally, each patterned metamaterial lens ( 120 A,  120 B,  120 C) within the array of patterned metamaterial lenses ( 120 A,  120 B,  120 C) can be configured to collimate different color light emitted from a respective underlying light emitting diode ( 10 A,  10 B,  10 C) within the array of light emitting diodes ( 10 A,  10 B,  10 C) along a vertical direction that is perpendicular to a top surface of the backplane  400 . 
     According to an aspect of the present disclosure, the array of patterned metamaterial lenses ( 120 A,  120 B,  120 C) can be patterned from a single metamaterial layer  120 L that continuously extends over the entire array of light emitting diodes ( 10 A,  10 B,  10 C), and may be formed directly on a top surface of the transparent conductive material layer  150  that continuously extends over the entire array of light emitting diodes ( 10 A,  10 B,  10 C). In one embodiment, each patterned metamaterial lens ( 120 A,  120 B,  120 C) may comprise a set of non-periodic metamaterial portions (as illustrated in  FIGS.  7 A- 7 F ) that extend over an area of the respective underlying light emitting diode ( 10 A,  10 B,  10 C). In one embodiment, the set of non-periodic metamaterial portions has a same thickness. 
     Light collimation provided by the array of light emitting diodes ( 10 A,  10 B,  10 C) increases the directionality of the light from the light emitting device. Thus, the array of light emitting diodes ( 10 A,  10 B,  10 C) can make the light emitting device of the embodiments of the present disclosure are suitable for heads-up displays or for virtual reality applications. 
     Referring to  FIG.  8   , a second exemplary structure after according to a second embodiment of the present disclosure can be derived from the first exemplary structure illustrated in  FIG.  1    by forming a dielectric matrix layer  210  over the backplane  400 . The dielectric matrix layer  210  comprises a dielectric material such as silicon oxide, silicon nitride, a dielectric metal oxide (such as aluminum oxide or titanium oxide), or a polymer material (such as polyimide). The thickness of the dielectric matrix layer  210  may be in a range from 1 micron to 60 microns, such as from 2 microns to 30 microns, although lesser and greater thicknesses may also be employed. 
     Referring to  FIG.  9   , an array of cavities  219  can be formed through the dielectric matrix layer  210 . For example, a photoresist layer (not shown) may be applied over the dielectric matrix layer  210  and can be lithographically patterned to form an array of openings, and an anisotropic etch process may be performed to transfer the pattern of the openings in the photoresist layer into the dielectric matrix layer  210 . The photoresist layer can be subsequently removed, for example, by ashing. Alternatively, the dielectric matrix layer  210  may comprise a photosensitive material such as a photosensitive polyimide material. In this case, the material of the dielectric matrix layer  210  may be directly exposed and developed to form the array of cavities  219  therein. The cavities  219  may comprise optical cavities. Remaining portions of the dielectric matrix layer  210  comprise separators  212  which separate adjacent subpixels of a display device and which function of sidewalls of the optical cavities  219 . 
     Each cavity  219  may be located over a respective metallic bonding pad, which may be a respective metal interconnect structure ( 460 ,  470 ) such as a respective second-level metal interconnect structure  470 . In one embodiment, a second-level metal interconnect structure  470  may be physically exposed at a center region of the bottom of each cavity  219 , and another second-level metal interconnect structure  470  may be physically exposed at a peripheral portion of the bottom of each cavity  219 . In one embodiment, the cavities  219  may be formed with a taper angle in a range from 0.1 degree to 30 degrees, such as from 1 degree to 10 degrees so that the upper portion of each cavity  219  has a greater lateral dimension than the lower portion of each cavity  219 . 
     Referring to  FIG.  10   , a reflective material layer (such as a metal layer) can be conformally deposited in the cavities  219 . A reactive ion etch process can be performed to remove horizontally-extending portions of the reflective material layer. Remaining tubular portions of the reflective material layer comprise reflectors  282 . Alternatively, a photoresist layer (not shown) can be applied over the reflective material layer, and can be lithographically patterned to cover vertically-extending portions of the reflective material layer. An etch process, such as an anisotropic etch process, can be performed to remove unmasked portions of the reflective material layer. Remaining portions of the reflective material layer comprise reflectors  282 . 
     The reflectors  282  comprise a reflective material. For example, the reflectors  282  may comprise a metal, such as aluminum, silver, gold, or copper. The reflectors  282  may have a thickness in a range from 10 nm to 300 nm, such as from 30 nm to 100 nm, although lesser and greater thicknesses may also be employed. In one embodiment, each reflector  282  may have a configuration of a megaphone-shaped tube. In one embodiment, a bottom surface of each reflector  282  may contact a respective metal interconnect structure (such as a second-level metal interconnect structure  470 ), and a metal interconnect structure (such as another second-level metal interconnect structure  470 ) may be physically exposed at a center region of the bottom of each cavity  219 . Generally, an array of reflectors  282  can be formed in the array of cavities  219 . Each reflector  282  may be formed directly on sidewalls (i.e., the separator  212  sidewalls) of a respective cavity  219 . 
     Alternatively, the separators  212  (with or without the reflectors  282 ) may be formed separately and then attached to the backplane  400 . In another embodiment, the separators  212  may be formed of a reflective (e.g., metal) material. In that case, the sidewalls of the separators  212  function as the reflectors  282 . 
     Referring to  FIG.  11   , light emitting diodes ( 10 A,  10 B,  10 C) may be attached to a respective first metal interconnect structure (such as a respective second-level metal interconnect structure  470 ) through an array of solder material portions  50 . The light emitting diodes ( 10 A,  10 B,  10 C) employed in the second exemplary structure may be the same as the light emitting diodes ( 10 A,  10 B,  10 C) employed in the first exemplary structure, or may be different from the light emitting diodes ( 10 A,  10 B,  10 C) employed in the first exemplary structure by the absence of the reflectors  82 . In other words, the reflectors  82  may, or may not, be present in the light emitting diodes ( 10 A,  10 B,  10 C) in the second exemplary structure. 
     An array of first light emitting diodes  10 A can be attached to a first subset of the metal interconnect structures ( 460 ,  470 ) (such as metal bonding pads) in the backplane  400 . The first light emitting diodes  10 A can be configured to emit light at a first peak wavelength, which may be, for example a blue light having a wavelength range between 620 nm to 750 nm. 
     An array of second light emitting diodes  10 B can be attached to a second subset of the metal interconnect structures ( 460 ,  470 ) (such as metal bonding pads) in the backplane  400 . The second light emitting diodes  10 B can be configured to emit light at a second peak wavelength, which may be, for example a green light having a wavelength range between 530 nm to 570 nm. 
     An array of third light emitting diodes  10 C can be attached to a third subset of the metal interconnect structures ( 460 ,  470 ) (such as metal bonding pads) in the backplane  400 . The third light emitting diodes  10 C can be configured to emit light at a third peak wavelength, which may be, for example a blue light having a wavelength range between 420 nm to 480 nm. 
     Generally, an array of light emitting diodes ( 10 A,  10 B,  10 C) can be attached to the backplane  400 . In one embodiment, the array of light emitting diodes ( 10 A,  10 B,  10 C) comprises a first sub-array of first light emitting diodes  10 A configured to emit light at a first peak wavelength, a second sub-array of second light emitting diodes  10 B configured to emit light at a second peak wavelength, and a third sub-array of third light emitting diodes  10 C configured to emit light at a third peak wavelength. The first peak wavelength, the second peak wavelength, and the third peak wavelength are different from each other. 
     Referring to  FIG.  12   , n-side electrodes  250  may be formed within each cavity  219  to provide electrical connection between each first doped compound semiconductor layer  26  and the backplane  400 . The electrical connection may be located out of the plane of the drawing in  FIG.  12   . 
     A cavity-fill dielectric material can be deposited in remaining volumes of the cavities  219 . The cavity-fill dielectric material comprises an optically transparent dielectric material such as silicon oxide, silicon nitride, or an optically transparent polymer material. Excess portions of the cavity-fill dielectric material can be removed from above the horizontal plane including the top surface of the separators  212  by a planarization process, such as a chemical mechanical polishing process. An array of cavity-fill dielectric material portions  220  can be formed in the array of cavities  219  over the array of light emitting diodes ( 10 A,  10 B,  10 C). 
     Referring to  FIG.  13   , a metamaterial layer  120 L can be deposited over the cavity-fill dielectric material portions  220  and the front side of the array of light emitting diodes ( 10 A,  10 B,  10 C). In embodiment, the metamaterial layer  120 L can be deposited directly on distal horizontal surfaces of the separators  212 . For example, the metamaterial layer  120 L may be deposited over the separators  212 , the array of reflectors  282 , the cavity-fill dielectric material portions  220  and the array of light emitting diodes ( 10 A,  10 B,  10 C). 
     Referring to  FIG.  14   , the metamaterial layer  120 L can be patterned into an array of patterned metamaterial lenses ( 120 A,  120 B,  120 C). The same patterning process employed to pattern the metamaterial layer  120 L into the array of patterned metamaterial lenses ( 120 A,  120 B,  120 C) in the first exemplary structure may be employed to pattern the metamaterial layer  120 L into the array of patterned metamaterial lenses ( 120 A,  120 B,  120 C) in the second exemplary structure. 
     Each patterned metamaterial lens ( 120 A,  120 B,  120 C) can have a pattern that collimates light that is emitted from a respective underlying light emitting diode ( 10 A,  10 B,  10 C). First patterned metamaterial lenses  120 A can be formed over first light emitting diodes  10 A that emit light at the first peak wavelength, and can have a pattern that collimates light having the first peak wavelength with highest directionality. Second patterned metamaterial lenses  120 A can be formed over second light emitting diodes  10 B that emit light at the second peak wavelength, and can have a pattern that collimates light having the second peak wavelength with highest directionality. Third patterned metamaterial lenses  120 A can be formed over third light emitting diodes  10 C that emit light at the third peak wavelength, and can have a pattern that collimates light having the third peak wavelength with highest directionality. 
     Generally, each patterned metamaterial lens ( 120 A,  120 B,  120 C) within the array of patterned metamaterial lenses ( 120 A,  120 B,  120 C) can be configured to collimate light emitted from a respective underlying light emitting diode ( 10 A,  10 B,  10 C) within the array of light emitting diodes ( 10 A,  10 B,  10 C) along a vertical direction that is perpendicular to a top surface of the backplane  400 . As discussed above, the configuration, such as size (e.g., width and/or length), spacing and/or direction (i.e., positional angle), of the nanostructures in the first patterned metamaterial lenses  120 A may differ from the configuration of the second patterned metamaterial lenses  120 B, and the configuration of the third patterned metamaterial lenses may different from the configurations of the first and the second patterned metamaterial lenses ( 120 A,  120 B). 
     In one embodiment, the array of patterned metamaterial lenses ( 120 A,  120 B,  120 C) can be in contact with a top surface of the separators  212 , and can be vertically spaced from the array of light emitting diodes ( 10 A,  10 B,  10 C). In one embodiment, each of the patterned metamaterial lenses ( 120 A,  120 B,  120 C) can be located directly on a respective cavity-fill dielectric material portion  220  within the array of cavity-fill dielectric material portions  220 . 
     Referring to  FIG.  15   , a third exemplary structure according to a third embodiment of the present disclosure can be derived from the second exemplary structure of  FIG.  10    by attaching light emitting diodes  10  to the metallic bonding pads (which are a subset of the metal interconnect structures ( 460 ,  470 )) of the backplane  400 . 
     According to the third embodiment, each of the light emitting diodes  10  may have the same structure, and thus, may be configured to emit radiation at a same peak wavelength, which may be an ultraviolet wavelength or a blue wavelength (which includes blue and violet light). Each light emitting diode  10  may comprise a buffer layer  24 , a first doped compound semiconductor layer  26 , an active layer  30 , a second doped compound semiconductor portion  36 , and an insulating spacer  60 , and may optionally comprise a reflector (not shown). Each active layer  30  includes a set of doped compound semiconductor material layers that is configured to emit radiation at a peak wavelength. In one embodiment, the active layer  30  may comprise a periodic repetition of first compound semiconductor layers  32  and second compound semiconductor layers  34  that forms a quantum well. The material compositions and the thicknesses of the first compound semiconductor layers  32  and second compound semiconductor layers  34  can be selected such that the active layer  30  emits radiation at the peak wavelength, which may be an ultraviolet wavelength or a blue wavelength. Additional material layers configured to increase the quantum efficiency of the light emission may be present within the active layer  30 . Alternatively, non-quantum-well structures may be employed for the active layer  30 . Generally, the active layer  30  may comprise any set of doped compound semiconductor material layers that is configured to emit radiation at the peak wavelength. 
     The light emitting diodes  10  can be attached to the backplane  400  employing the same methods as the methods of attaching the light emitting diodes ( 10 A,  10 B,  10 C) of the second exemplary structure as described with reference to  FIG.  11   . 
     Referring to  FIG.  16   , n-side electrodes  250  and an array of cavity-fill dielectric material portions  220  can be formed by performing the processing steps described with reference to  FIG.  12   . 
     Referring to  FIG.  17   , arrays of color conversion medium portions ( 90 A,  90 B,  90 C) can be formed over, and optionally directly on, the separators  212 , the array of reflectors  282 , and the array of cavity-fill dielectric material portions  220 . Each color conversion medium portion ( 90 A,  90 B,  90 C) can be formed above, and can have an areal overlap in a plan view with, a respective underlying light emitting diode  10 . Each color conversion medium portion ( 90 A,  90 B,  90 C) comprises a material that converts incident radiation into an emission light having a different wavelength than the incident light. For example, the incident radiation as emitted by the active layers  30  of the light emitting diodes  10  may be a blue light or an ultraviolet radiation, and the emission light that is emitted from the color conversion medium portions ( 90 A,  90 B,  90 C) may be light having a longer wavelength than the incident radiation. For example, the color conversion medium portions ( 90 A,  90 B,  90 C) may comprise quantum dots, phosphors, dyes or any material that emits light upon excitation by the incident light. In an illustrative example, the emission light from the color conversion medium portions ( 90 A,  90 B,  90 C) may comprise a red light, a green light, and a blue light, respectively. 
     In one embodiment, the arrays of color conversion medium portions ( 90 A,  90 B,  90 C) may comprise first color conversion medium portions  90 A overlying a first subset of the light emitting diodes  10  within the array of the light emitting diodes  10  and configured to convert incident light into a first emission light having a first peak wavelength (such as red light), second color conversion medium portions  90 B overlying a second subset of the light emitting diodes  10  within the array of the light emitting diodes  10  and configured to convert incident light into a second emission light having a second peak wavelength (such as green light), and third color conversion medium portions  90 C overlying a third subset of the light emitting diodes  10  within the array of the light emitting diodes  10  and configured to convert incident light into a third emission light having a third peak wavelength (such as blue light). In one embodiment, the first peak wavelength, the second peak wavelength, and the third peak wavelength are different among one another. A conversion-level frame  92  may be optionally formed between neighboring pairs of color conversion medium portions ( 90 A,  90 B,  90 C) of the arrays of color conversion medium portions ( 90 A,  90 B,  90 C). The conversion-level frame  92 M may comprise a reflective material, such as aluminum, silver, gold, or copper. 
     Referring to  FIG.  18   , the above described metamaterial layer  120 L can be deposited on the distal horizontal surfaces of the arrays of color conversion medium portions ( 90 A,  90 B,  90 C). The metamaterial layer  120 L is deposited over and is vertically spaced from the separators  212 , the array of reflectors  282 , and the array of light emitting diodes ( 10 A,  10 B,  10 C) by the arrays of color conversion medium portions ( 90 A,  90 B,  90 C). 
     Referring to  FIG.  19   , the metamaterial layer  120 L can be patterned into an array of patterned metamaterial lenses ( 120 A,  120 B,  120 C). The same patterning process employed to pattern the metamaterial layer  120 L into the array of patterned metamaterial lenses ( 120 A,  120 B,  120 C) in the first exemplary structure may be employed to pattern the metamaterial layer  120 L into the array of patterned metamaterial lenses ( 120 A,  120 B,  120 C) in the third exemplary structure. 
     Each patterned metamaterial lens ( 120 A,  120 B,  120 C) can have a pattern that collimates radiation that is emitted from a respective underlying light emitting diode  10  and passes through a respective color conversion medium portion ( 90 A,  90 B,  90 C). Radiation emitted from a first subset of the light emitting diodes  10  and then down converted to longer wavelength light as it passes through the first color conversion medium portion  90 A may have a first peak wavelength. First patterned metamaterial lenses  120 A can be formed over the first color conversion medium portions  90 A that emit light at the first peak wavelength, and can have a pattern that collimates light having the first peak wavelength with highest directionality. Radiation emitted from a second subset of the light emitting diodes  10  and then down converted to longer wavelength light as it passes through passes through a second color conversion medium portion  90 B may have a second peak wavelength. Second patterned metamaterial lenses  120 B can be formed over the second color conversion medium portions  90 B that emit light at the second peak wavelength, and can have a pattern that collimates light having the second peak wavelength with highest directionality. Radiation emitted from a third subset of the light emitting diodes  10  and then down converted to longer wavelength light as it passes through passes through a third color conversion medium portion  90 C may have a third peak wavelength. Third patterned metamaterial lenses  120 C can be formed over third color conversion medium portions  90 C that emit light at the third peak wavelength, and can have a pattern that collimates light having the third peak wavelength with highest directionality. 
     Generally, each patterned metamaterial lens ( 120 A,  120 B,  120 C) within the array of patterned metamaterial lenses ( 120 A,  120 B,  120 C) can be configured to collimate light provided respective underlying color conversion medium portion ( 90 A,  90 B,  90 C) along a vertical direction that is perpendicular to a top surface of the backplane  400 . 
     The configuration, such as size (e.g., width and/or length), spacing and/or direction (i.e., positional angle), of the nanostructures in the first patterned metamaterial lenses  120 A may differ from the configuration of the second patterned metamaterial lenses  120 B, and the configuration of the third patterned metamaterial lenses may different from the configurations of the first and the second patterned metamaterial lenses ( 120 A,  120 B). For example, if the first color conversion medium portions  90 A emit red light, then the configuration of the first patterned metamaterial lenses  120 A may be configured to optimize collimation of red light. Likewise, if the second and third color conversion medium portions ( 90 B,  90 C) emit green and blue light, respectively, then the configuration of the second patterned metamaterial lenses ( 120 B,  120 C) may be configured to optimize collimation of green and blue light, respectively. 
     In one embodiment, the array of patterned metamaterial lenses ( 120 A,  120 B,  120 C) can be in contact with top surfaces of the color conversion medium portions ( 90 A,  90 B,  90 C), and can be vertically spaced from the array of light emitting diodes ( 10 A,  10 B,  10 C). In one embodiment, each of the patterned metamaterial lenses ( 120 A,  120 B,  120 C) can be located directly on a respective color conversion medium portion ( 90 A,  90 B,  90 C) within the array of color conversion medium portions ( 90 A,  90 B,  90 C). In one embodiment, each patterned metamaterial lens ( 120 A,  120 B,  120 C) within the array of patterned metamaterial lenses ( 120 A,  120 B,  120 C) is located directly on a respective color conversion medium portion ( 90 A,  90 B, or  90 C) selected from the first color conversion medium portions  90 A, the second color conversion medium portions  90 B, and the third color conversion medium portions  90 C. 
     Referring to all drawings and according to various embodiments of the present disclosure, a light emitting device includes a backplane  400 , first, second and third light emitting diodes { 10  or ( 10 A,  10 B,  10 C)} located on the backplane, a first patterned metamaterial lens  120 A containing first nanostructures located over the first light emitting diode, a second patterned metamaterial lens  120 B containing second nanostructures located over the second light emitting diode, and a third patterned metamaterial lens  120 C containing third nanostructures located over the light emitting diode. A configuration of the first nanostructures differs from a configuration of the second nanostructures, and a configuration of the third nanostructures differs from the configurations of the first and the second nanostructures. 
     In one embodiment, the configuration of the first nanostructures differs from the configuration of the second nanostructures by at least one of size, spacing or direction, and the configuration of the third nanostructures differs from the configurations of the first and the second nanostructures by at least one of size, spacing or direction. In one embodiment, the first nanostructures have a different size than the second nanostructures, and the third nanostructures have a different size than the second nanostructures. 
     In the first and second embodiments, the first light emitting diode  10 A is configured to emit red light, the second light emitting diode  10 B is configured to emit green light, and the third light emitting diode  10 C is configured to emit blue light. The first patterned metamaterial lens  120 A is configured to optimize collimation of red light, the second patterned metamaterial lens  120 B is configured to optimize collimation of green light, and the third patterned metamaterial lens  120 C is configured to optimize collimation of blue light. 
     In the first embodiment, a dielectric matrix layer  110  laterally surrounds the first, second and third light emitting diodes ( 10 A,  10 B,  10 C). 
     In the second embodiment, separators  212  surround optical cavities  219 . The first, second and third light emitting diodes ( 10 A,  10 B,  10 C) are located in respective optical cavities  219 . Reflectors  282  located are on sidewalls of the separators  212 . Cavity-fill dielectric material portions  220  are located within the optical cavities  219 , and overlying the first, second and third light emitting diodes ( 10 A,  10 B,  10 C). The first, second and third patterned metamaterial lenses ( 120 A,  120 B,  120 C) overly a distal surface of the cavity-fill dielectric material portions  220 . Each of the first, second and third light emitting diodes ( 10 A,  10 B,  10 C) comprises a vertical stack including a first doped compound semiconductor layer  26  of a first conductivity type, a second doped compound semiconductor layer  36  of a second conductivity type, and an active layer ( 30 A,  30 B,  30 C) configured to emit light at a respective peak wavelength located between the first and the second compound semiconductor layers ( 26 ,  36 ). 
     In the third embodiment, each of the first, second and third light emitting diodes  10  is configured to emit incident radiation having the same peak wavelength. The incident radiation may comprise ultraviolet radiation or blue light. 
     In the third embodiment, a first color conversion medium portion  90 A overlies the first light emitting diode, and configured to convert the incident radiation into a first emission light having a first peak wavelength, a second color conversion medium portion  90 B overlies the second light emitting diode, and configured to convert the incident radiation into a second emission light having a second peak wavelength, and a third color conversion medium portion  90 C overlies the third light emitting diode, and configured to convert the incident radiation into a third emission light having a third peak wavelength. The first peak wavelength, the second peak wavelength, and the third peak wavelength are different from each other. The first, second and third color conversion medium portions ( 90 A,  90 B,  90 C) may comprise quantum dots having a different size (e.g., diameter) from each other. 
     The first patterned metamaterial lens  120 A is located above the first light emitting diode (and over portion  90 A), the second patterned metamaterial lens  120 B is located above the second light emitting diode (and over portion  90 B); and the third patterned metamaterial lens  120 C is located above the third light emitting diode (and over portion  90 C). 
     In one aspect of the third embodiment, the first emission light comprises red light, the second emission light comprises green light, and the third emission light comprises blue light. The first patterned metamaterial lens  120 A is configured to optimize collimation of red light, the second patterned metamaterial lens  120 B is configured to optimize collimation of green light, and the third patterned metamaterial lens  120 C is configured to optimize collimation of blue light. 
     In one embodiment, the light emitting device comprises a display device. The first light emitting diode ( 10  or  10 A) and the first patterned metamaterial lens  120  are located in a first subpixel of a pixel “P” of the display device, the second light emitting diode ( 10  or  10 B) and the second patterned metamaterial lens  120 B are located in a second subpixel of the pixel of the display device, and the third light emitting diode ( 10  or  10 C) and the third patterned metamaterial lens  120 C are located in a third subpixel of the pixel of the display device. The display device may comprise a heads up display device, a virtual reality display device, or an artificial reality display device. 
     The various embodiments of the present disclosure can be employed to provide a light emitting device having enhanced light collimation along a forward direction for different colors of light. In one embodiment, a metamaterial layer  120 L can be deposited in a single deposition step, and can be patterned into an array of patterned metamaterial lenses ( 120 A,  120 B,  120 C) having different collimation characteristics (i.e., optimized for different wavelengths) employing a same set of lithographic patterning steps and a same set of pattern transfer steps. In this embodiment, the patterned metamaterial lenses ( 120 A,  120 B,  120 C) can be economically manufactured by employing a single material deposition step that forms the metamaterial layer  120 L, a single lithographic patterning step including application, exposure, and development of a photoresist layer, a single etch step that transfers the pattern in the photoresist layer through the metamaterial layer  120 L, and a single photoresist removal step. 
     The preceding description of the disclosed embodiments is provided to enable any person skilled in the art to make or use the present invention. Various modifications to these embodiments will be readily apparent to those skilled in the art, and the generic principles defined herein may be applied to other embodiments without departing from the spirit or scope of the invention. Thus, the present invention is not intended to be limited to the embodiments shown herein but is to be accorded the widest scope consistent with the following claims and the principles and novel features disclosed herein.