Patent Publication Number: US-2023155075-A1

Title: Light emitting devices including a quantum dot color conversion material and method of making thereof

Description:
FIELD 
     This disclosure relates to light emitting devices, and particularly to light emitting diodes formed in optical cavities with a color conversion material and methods of fabricating the same. 
     BACKGROUND 
     Light emitting devices are used in electronic displays, such as backlights in liquid crystal displays in laptops and televisions. Light emitting devices include light emitting diodes (LEDs) and various other types of electronic devices configured to emit light. 
     For light emitting devices, such as LEDs, the emission wavelength is determined by the band gap of the active region of the LED together with size dependent quantum confinement effects. Often the active region includes one or more bulk semiconductor layers or quantum wells (QW). For III-nitride based LED devices, such as GaN based devices, the active region (e.g., bulk semiconductor layer or QW well layer) material may be ternary, having a composition such as In x Ga 1-x N, where 0&lt;x&lt;1. 
     The band gap of such III-nitride materials is dependent on the amount of In incorporated in the active region. Higher indium incorporation yields a smaller band gap and thus longer wavelength of the emitted light. As used herein, the term “wavelength” refers to the peak emission wavelength of the LED. It should be understood that a typical emission spectra of a semiconductor LED is a narrow band of wavelength centered around the peak wavelength. 
     SUMMARY 
     An embodiment light emitting device includes a first optical cavity bounded by at least one first cavity wall, a first light emitting diode located in the first optical cavity and configured to emit blue or ultraviolet radiation first incident photons, a first color conversion material located over the first light emitting diode and configured to absorb the first incident photons emitted by the light emitting diode and to generate first converted photons having a longer peak wavelength than a peak wavelength of the first incident photons, and a first color selector located over the first color conversion material and configured to absorb or reflect the first incident photons and to transmit the first converted photons. 
     An embodiment method of forming an array of light emitting devices comprises forming a first via in a matrix material, depositing a first plurality of quantum dots in the first via to form a first portion of the color conversion material layer corresponding to a first color, forming a second via in the matrix material, depositing a second plurality of quantum dots in the second via to form a second portion of the color conversion material layer corresponding to a second color, forming a third via in the matrix material, and depositing a third plurality of quantum dots in the third via to form a third portion of the color conversion material layer corresponding to a third color. The first plurality of quantum dots are located over a first light emitting diode, the second plurality of quantum dots are located over a second light emitting diode, and the third plurality of quantum dots are located over a third light emitting diode. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG.  1 A  is a vertical cross-sectional view of an intermediate structure that may be used in the formation of an array of light emitting devices, according to various embodiments. 
         FIG.  1 B  is a vertical cross-sectional view of a further intermediate structure that may be used in the formation of an array of light emitting devices, according to various embodiments. 
         FIG.  1 C  is a vertical cross-sectional view of a further intermediate structure that may be used in the formation of an array of light emitting devices, according to various embodiments. 
         FIG.  1 D  is a vertical cross-sectional view of an array of light emitting devices, according to various embodiments. 
         FIG.  1 E  is a vertical cross-sectional view of a further array of light emitting devices, according to various embodiments. 
         FIG.  2 A  is a top perspective view of a first patterned matrix having a plurality of vias formed therein, according to various embodiments. 
         FIG.  2 B  is a top perspective view of a second patterned matrix having a plurality of vias formed therein, according to various embodiments. 
         FIG.  3 A  is a vertical cross-sectional view of an intermediate structure that may be used in the formation of an array of light emitting devices, according to various embodiments. 
         FIG.  3 B  is a vertical cross-sectional view of a further intermediate structure that may be used in the formation of an array of light emitting devices, according to various embodiments. 
         FIG.  3 C  is a vertical cross-sectional view of a further intermediate structure that may be used in the formation of an array of light emitting devices, according to various embodiments. 
         FIG.  3 D  is a vertical cross-sectional view of a further intermediate structure that may be used in the formation of an array of light emitting devices, according to various embodiments. 
         FIG.  3 E  is a vertical cross-sectional view of a further intermediate structure that may be used in the formation of an array of light emitting devices, according to various embodiments. 
         FIG.  3 F  is a vertical cross-sectional view of a further intermediate structure that may be used in the formation of an array of light emitting devices, according to various embodiments. 
         FIG.  3 G  is a vertical cross-sectional view of a further intermediate structure that may be used in the formation of an array of light emitting devices, according to various embodiments. 
         FIG.  3 H  is a vertical cross-sectional view of a further intermediate structure that may be used in the formation of an array of light emitting devices, according to various embodiments. 
         FIG.  3 I  is a vertical cross-sectional view of a further intermediate structure that may be used in the formation of an array of light emitting devices, according to various embodiments. 
         FIG.  3 J  is a vertical cross-sectional view of a further intermediate structure that may be used in the formation of an array of light emitting devices, according to various embodiments. 
         FIG.  3 K  is a vertical cross-sectional view of a further intermediate structure that may be used in the formation of an array of light emitting devices, according to various embodiments. 
         FIG.  3 L  is a vertical cross-sectional view of a further intermediate structure that may be used in the formation of an array of light emitting devices, according to various embodiments. 
         FIG.  4 A  is a vertical cross-sectional view of an intermediate structure that may be used in the formation of an array of light emitting devices, according to various embodiments. 
         FIG.  4 B  is a vertical cross-sectional view of a further intermediate structure that may be used in the formation of an array of light emitting devices, according to various embodiments. 
         FIG.  4 C  is a vertical cross-sectional view of a further intermediate structure that may be used in the formation of an array of light emitting devices, according to various embodiments. 
         FIG.  4 D  is a vertical cross-sectional view of a further intermediate structure that may be used in the formation of an array of light emitting devices, according to various embodiments. 
         FIG.  4 E  is a vertical cross-sectional view of a further intermediate structure that may be used in the formation of an array of light emitting devices, according to various embodiments. 
         FIG.  4 F  is a vertical cross-sectional view of a further intermediate structure that may be used in the formation of an array of light emitting devices, according to various embodiments. 
         FIG.  4 G  is a vertical cross-sectional view of a further intermediate structure that may be used in the formation of an array of light emitting devices, according to various embodiments. 
         FIG.  4 H  is a vertical cross-sectional view of a further intermediate structure that may be used in the formation of an array of light emitting devices, according to various embodiments. 
         FIG.  4 I  is a vertical cross-sectional view of a further intermediate structure that may be used in the formation of an array of light emitting devices, according to various embodiments. 
         FIG.  4 J  is a vertical cross-sectional view of a further intermediate structure that may be used in the formation of an array of light emitting devices, according to various embodiments. 
         FIG.  4 K  is a vertical cross-sectional view of a further intermediate structure that may be used in the formation of an array of light emitting devices, according to various embodiments. 
         FIG.  4 L  is a vertical cross-sectional view of a further intermediate structure that may be used in the formation of an array of light emitting devices, according to various embodiments. 
         FIG.  4 M  is a vertical cross-sectional view of a further intermediate structure that may be used in the formation of an array of light emitting devices, according to various embodiments. 
         FIG.  4 N  is a vertical cross-sectional view of a further intermediate structure that may be used in the formation of an array of light emitting devices, according to various embodiments. 
         FIG.  4 O  is a vertical cross-sectional view of a further intermediate structure that may be used in the formation of an array of light emitting devices, according to various embodiments. 
         FIG.  4 P  is a vertical cross-sectional view of a further intermediate structure that may be used in the formation of an array of light emitting devices, according to various embodiments. 
         FIG.  5 A  is a vertical cross-sectional view of an intermediate structure that may be used in the formation of an array of light emitting devices, according to various embodiments. 
         FIG.  5 B  is a vertical cross-sectional view of a further intermediate structure that may be used in the formation of an array of light emitting devices, according to various embodiments. 
         FIG.  5 C  is a vertical cross-sectional view of a further intermediate structure that may be used in the formation of an array of light emitting devices, according to various embodiments. 
         FIG.  5 D  is a vertical cross-sectional view of a further intermediate structure that may be used in the formation of an array of light emitting devices, according to various embodiments. 
         FIG.  5 E  is a vertical cross-sectional view of a further intermediate structure that may be used in the formation of an array of light emitting devices, according to various embodiments. 
         FIG.  5 F  is a vertical cross-sectional view of a further intermediate structure that may be used in the formation of an array of light emitting devices, according to various embodiments. 
         FIG.  5 G  is a vertical cross-sectional view of a further intermediate structure that may be used in the formation of an array of light emitting devices, according to various embodiments. 
     
    
    
     DETAILED DESCRIPTION 
     A display device, such as a direct view display may be formed from an ordered array of pixels. Each pixel may include a set of subpixels that emit light at a respective peak wavelength. For example, a pixel may include a red subpixel, a green subpixel, and a blue subpixel. Each subpixel may include one or more light emitting diodes that emit light of a particular wavelength. A traditional arrangement is to have red, green, and blue (RGB) subpixels within each pixel. Each pixel may be 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 may 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 may be electrically driven by the backplane circuit and other driving electronics. 
     Various embodiments provide a light emitting device configured to create high efficiency red, green, blue, and/or other color pixelated light from a shorter wavelength excitation source using photonically pumped quantum dots in a vertical cavity structure. Embodiment micron-scale light emitting diodes (micro-LED) which have a length and width less than 100 microns, such as 5 to 20 microns, may be used in display devices. This emerging technology offers ultimate black levels by using individual LEDs at each pixel location of a display device. Further, each pixel may be configured to generate a single color of light. A backplane upon which individual LEDs may be attached may include a substrate (e.g., plastic, glass, semiconductor, etc.) with thin-film transistor (TFT) structures, silicon CMOS, or other driver circuitry that may be configured to apply a voltage or current to each LED independently. For example, the backplane may include TFTs on a glass or plastic substrate, or bulk silicon transistors (e.g., transistors in a CMOS configuration) on a bulk silicon substrate or on a silicon-on-insulator (SOI) substrate. While micro-LEDs are described in the embodiments below, it should be noted that other types of LEDs (e.g., nanowire or other nanostructure LEDs) or macro-LEDs having a size (e.g., width and length) greater than 100 microns may also be used instead of or in addition to the micro-LEDs. 
     In some embodiments, a size of each micro-LED may be smaller than a pitch of the pixels used in a particular display device, such as a direct view display device or another display device. For example, a 300 ppi display may have pixels having a pitch of approximately 85 microns, while a typical micro-LED for such a display may have a width that is approximately 20 microns. Micro-LEDs that include an indium-doped GaN material (i.e., LEDs that emit a color that depends on indium doping of GaN) may suffer degradation of efficiency and uniformity with decreasing LED size (e.g., sizes less than 10 microns) due to difficulties associated with indium doping of GaN crystal structures. Thus, longer peak wavelength emitting III-nitride micro-LEDs (e.g., red LEDs) which utilize a higher indium content in their active regions may have insufficient efficiency and uniformity due to the degraded indium doping. 
     Some embodiments of the present disclosure include a photonic emitter based on a LED having an undoped GaN active region (e.g., a micro-LED having a GaN light emitting active layer) or a low indium doped InGaN active region (e.g., a micro-LED having a low indium content InGaN light emitting active layer) coupled with a photonically pumped color conversion material. Such LEDs may be ultraviolet (UV) radiation or blue light emitting micro-LEDs having a peak emission wavelength in the UV radiation or blue light spectral region (e.g., 370 to 460 nm, such as 390 to 420 nm, for example 400 to 410 nm). As used herein, the blue light spectral region includes blue and violet colors as perceived by the human observer. 
     In one embodiment, the color conversion material may include quantum dots. The quantum dots may be configured to absorb photons generated by the GaN emitter and to generate various colors of light depending on the properties of the quantum dots (e.g., quantum dot size and material composition). Such structures avoid problems associated with indium doping of small GaN structures. 
     In the size regime (i.e., sizes less than 10 microns) appropriate for augmented reality (AR) displays (e.g., smart glasses) and other applications, the use of a undoped GaN or low indium doped GaN LED active region and photonically pumped quantum dots to create various colors may provide display devices having better uniformity across an array of micro-LEDs. Such arrays may also exhibit higher efficiency than systems having colored LEDs based on relatively high indium doped GaN (e.g., red LEDs containing a higher amount of indium than blue LEDs). The increased efficiency and uniformity may be achieved because quantum dots may be manufactured with a high degree of uniformity of size and material composition. Such uniform quantum dots have corresponding uniform (i.e., narrow linewidth) emission properties. 
     Extraction of light emitted by micro-LEDs may be increasingly challenging with decreasing pixel pitch and micro-LED size. Disclosed embodiments provide improved optical extraction of photons (e.g., along a specific direction) generated by the quantum dots, while maintaining high efficiency by avoiding loss of photons to absorbing surfaces. Disclosed systems may also prevent or reduce pump photons from escaping the device, thereby ensuring purity of the color emitted by a given micro-LED. This may be accomplished by forming optical cavity walls that are reflective, including a light extracting material layer, and including other light extracting structures, such as micro lenses and/or a distributed Bragg reflector (DBR). 
       FIG.  1 A  is a vertical cross-sectional view of an intermediate structure  100   a  that may be used in the formation of an array of light emitting devices, according to various embodiments. The intermediate structure  100   a  may include a plurality of micro-LEDs  102  formed on a substrate  104 . As described above, the micro-LEDs  102  may include a micro-LEDs which have peak emission wavelength in the UV radiation or blue light spectral region (e.g., UV or blue emitting micro-LEDs, also referred to as UV or blue LEDs). Such LEDs may include undoped GaN active regions that are configured to emit ultraviolet (UV) photons and/or blue spectral range photons. 
     In one embodiment, the micro-LEDs  102  may have at least one electrode  103  located on the top of the LED and facing away from the substrate  104 . The electrode  103  may comprise an anode or a cathode electrode. In one embodiment, the micro-LEDs  102  may comprise vertical LEDs in which the second electrode (not shown for clarity) is located between the substrate  104  and the bottom of the micro-LED  102 . In another embodiment, the micro-LEDs may comprise lateral LEDs in which both electrodes are located on the same side of the LED (e.g., on top or on bottom sides of the LED). 
     The substrate  104  may be a backplane having electrical circuitry (e.g., TFT and/or CMOS circuits) configured to supply voltages and currents to the micro-LEDs  102  via the electrodes (including the electrodes  103 ) to thereby control light emission by the micro-LEDs  102 . A backplane may be an active or passive matrix backplane substrate for driving LEDs. As used herein, a “backplane substrate” refers to any substrate configured to affix multiple devices thereupon. In one embodiment, the backplane may include a substrate including silicon, glass, plastic, and/or at least other material that may provide structural support to devices attached thereto. In one embodiment, the backplane substrate may be a passive backplane substrate, in which metal interconnect structures (not shown) including metallization lines are present, for example, in a crisscross grid and dedicated active devices (e.g., TFTs) for each LED are not present. In another embodiment, the backplane substrate may be an active backplane substrate, which includes metal interconnect structures as a crisscross grid of conductive lines and further includes dedicated active devices (e.g., CMOS transistors or TFTs) for each LED at one or more intersections of the crisscross grid of conductive lines. 
       FIG.  1 B  is a vertical cross-sectional view of a further intermediate structure  100   b  that may be used in the formation of an array of light emitting devices, according to various embodiments. Intermediate structure  100   b  includes a plurality of optical cavities  106  formed over the micro-LEDs  102 . Each optical cavity may be bounded by cavity walls  108 . The optical cavities  106  may be constructed using a reflective material which has suitable mechanical properties to form high aspect ratio cavities (e.g., 5 microns or less, such as 1-2 microns, in diameter, and 10 microns or more, such as 20-30 microns, in height) with relatively thin side walls  108 . The cavity walls  108  may have a thickness of less than 10 microns, such as 0.5-5 microns, including 1-2 microns. The cavity walls  108  form an insulating matrix. 
     The matrix material may be chosen to be compatible with both thermal evaporative processing steps and solvent based fluidic depositions and evaporation. One such matrix material is alumina, although silica, titania, or other insulating metal oxide materials may be used. Various materials that are typically used to fabricate micro-electromechanical (MEMS) devices may be used to form the optical cavities  106  bounded by cavity walls  108  made of an electrically insulating material (e.g., alumina). Such materials have a relatively high index of refraction and are suitable for forming structures having high aspect ratios. A layer of such matrix material (not shown in  FIG.  1 B ) may be grown or deposited on the micro-LED  102  array located on the substrate  104  and techniques such as etching and other micro machining approaches may be used to generate optical cavities  106  in the material.  FIG.  2 A  is a top perspective view of a matrix  200   a  having a plurality of cylindrical optical cavities  106  bounded by cavity walls  108 .  FIG.  2 B  is a top perspective view of a matrix  200   b  having a plurality of hexagonal optical cavities  106  bounded by cavity walls  108 . 
     In one embodiment, a voltage may be applied to an anode or cathode electrode  103  of the micro-LEDs  102  to thereby form one side of an etch bias. For example, if the matrix  200   a  or  200   b  (i.e., the cavity walls  108 ) comprise alumina, then the porous alumina may be formed by anodic oxidation. In this embodiment, an aluminum metal layer may be deposited over the micro-LEDs  102 , and then electrochemically anodized to form a porous anodic alumina matrix with optical cavities (i.e., pores)  106  bounded by anodic alumina walls  108 . The substrate  104  containing the aluminum layer is placed in an acid electrolyte (e.g., oxalic acid, chromic acid, sulfuric acid and/or phosphoric acid), and a voltage is applied to the electrodes  103  of the micro-LEDs  102  and/or to an external electrode to form the porous anodic alumina matrix containing the optical cavities (i.e., pores)  106  bounded by the alumina cavity walls  108 . The optical cavities  106  may be arranged in a hexagonal array in an anodic alumina matrix. 
       FIG.  1 C  is a vertical cross-sectional view of a further intermediate structure  100   c  that may be used in the formation of an array of light emitting devices, according to various embodiments. Intermediate structure  100   c  may include a light extracting material layer  110  and a color conversion material ( 112   a ,  112   b ,  112   c ,  112   d ) formed in the optical cavities  106  over the array of micro-LEDs  102 . The light extracting material layer  110  may have an index of refraction that is lower than an index of refraction of the material forming the cavity walls  108 . For example, the light extracting material layer  110  may have an index of refraction of less than 1.7, such as 1.3 to 1.5 for alumina cavity walls  108 . The lower refractive index of the light extracting material layer  110  may cause pump photons (i.e., photons generated by the micro-LEDs  102 ) to be reflected from cavity walls  108  rather than being absorbed by or transmitted through that cavity walls  108 . Such reflection prevents loss of photons and thereby acts to increase the quantum efficiency of the device. 
     Various polymer materials may be used as a light extracting material layer  110 . One such polymer is Jet-144 (i.e., an inkjet compatible polymer), which has an index of refraction of 1.44 and which may be deposited into the optical cavities  106  using an inkjet system. A thickness of the cavity walls  108  may be configured to be as thick as possible to increase a probability that photons that do not reflect from the cavity walls  108  are absorbed (i.e., extinguished) so that they do no penetrate into an adjacent cavity. 
     The light extracting material layer  110  may be deposited using various techniques including ink jet, vacuum, pressure, and/or gravitational deposition. After deposition, the polymer may be cross-linked, for example, by exposure to ultra-violet (UV) radiation. In other embodiments, a solvent in which the polymer is dissolved may be drawn out by evaporation leaving a residual cross-linked polymer as the light extracting material layer  110  in each cavity. In various embodiments, the light extracting material layer  110  may be formed with various thicknesses and may or may not contain additional light scattering materials, such as TiO 2  or SiO 2  nano or micro beads. The light extracting material layer  110  partially fills the optical cavities  106  such that empty cavity space remains over the top of the light extracting material layer  110  in each cavity. 
     The color conversion material ( 112   a ,  112   b ,  112   c ,  112   d ) may then be formed in the optical cavities  106  (e.g., see  FIG.  1 B ) over the light extracting material layer  110  (e.g., see  FIG.  1 C ). The color conversion material ( 112   a ,  112   b ,  112   c ,  112   d ) may include quantum dots corresponding to various different colors. In this example, the color conversion material ( 112   a ,  112   b ,  112   c ,  112   d ) the plurality of first quantum dots  112   a , a plurality of second quantum dots  112   b , and plurality of third quantum dots  112   c , and a plurality of fourth quantum dots  112   d , which are configured to convert UV pump photons into photons having first, second, third, and fourth colors, respectively. The second and third colors may comprise different peak wavelengths in the green color spectrum range. Alternatively, only three quantum dot colors may be used. The quantum dots may comprise 1 to 10 nm, such as 2 to 8 nm nanocrystals of a compound semiconductor material, such as a Group III-V semiconductor material (e.g., indium phosphide, as described in U.S. Pat. No. 9,884,763 B 1, incorporated herein by reference in its entirety), a Group II-VI semiconductor material (e.g., ZnSe, ZnS, ZnTe, CdS, CdSe, etc., core-shell quantum dots, as described in U.S Patent Application Publication US 2017/0250322 A1, incorporated herein by reference in its entirety), and/or Group I-III-VI semiconductor material (e.g., AgInGaS/AgGaS core-shell quantum dots, as described in U.S. Pat. No. 10,927,294 B2, incorporated herein by reference in its entirety). The quantum dots may emit different color light (e.g., red, green or blue) depending on their diameter. The larger dots emit longer wavelength light while the smaller dots emit shorter wavelength light. The quantum dots may be suspended in a material (e.g., a polymer such as polyimide) having a different (e.g., higher) index of refraction from that of the light extracting material  110 . For example, the polyimide material may be a refractive index of 1.6 to 1.75, such as about 1.7. 
     As described in greater detail below (e.g., with reference to  FIGS.  3 A to  4 P ), quantum dots corresponding to various colors may be selectively deposited in respective cavities. For example, as described with reference to  FIGS.  3 A to  3 L , below, first cavities may be formed by etching first vias in a matrix material. First quantum dots corresponding to first color may then be introduced into the first cavities and a layer of protective material may then be formed over the first quantum dots. The process may then be repeated to form second cavities, third cavities, etc., and to respectively introduce second quantum dots, third quantum dots, etc. into the respective cavities. 
     In other embodiments (e.g., see  FIGS.  4 A to  4 P ), a photoresist may be deposited over all cavities except a plurality of first cavities. A first layer of quantum dots configured to generate a first color (e.g., red) may then be deposed into the plurality of first cavities corresponding to subpixels having the first color. A polymer in which the first quantum dots are suspended may then be cross linked by evaporation or by exposure to UV light. The process may then be repeated for the other optical cavities to respectively deposit quantum dots configured to generate other color light (e.g., green and blue). 
     An optional organic planarization layer may be formed over the color conversion material. The color conversion material and the optional organic planarization layer may partially fill the optical cavities  106 . 
       FIG.  1 D  is a vertical cross-sectional view of an array  100   d  of light emitting devices, according to various embodiments. As shown, the array  100   d  may include color selector  114  formed in and/or over the optical cavities  106 . The color selector  114  may comprise a color filter array and/or a distributed Bragg reflector. In one embodiment, the color selector  114  may be formed in the optical cavities and may extend to the top of the cavity walls  108  such that the optical cavities  106  are completely filled with the above materials. 
     The color conversion material ( 112   a ,  112   b ,  112   c ,  112   d ) may be configured to absorb the pump photons  118  and to convert them to emitted converted photons (e.g., visible light, such as red, green or blue light)  120 . In some embodiments, the color conversion material ( 112   a ,  112   b ,  112   c ,  112   d ) may not be sufficiently thick and/or dense to fully convert all pump photons  118  into converted photons  120 . Thus, the color selector  114  formed over the color conversion material ( 112   a ,  112   b ,  112   c ,  112   d ) absorbs and/or reflects all or a portion of the pump photons  118  that are not converted by the color conversion material ( 112   a ,  112   b ,  112   c ,  112   d ), without absorbing and/or reflecting the converted photon  120  emitted by the color conversion material. 
     Each of the micro-LEDs  102  may be configured to emit pump photons  118  having a common wavelength or within a range of the target wavelengths. For example, GaN-based micro-LEDs  102  may emit pump photons  118  having a wavelength that is 400 to 410 nm, such as approximately 405 nm (i.e., in the blue or near-UV part of the electromagnetic spectrum). The micro-LEDs  102  may exhibit a high degree of uniformity and may exhibit high efficiency. However, slight variations in the wavelength of such micro-LEDs  102  may not be easily visible to the eye. Further, any leakage of pump photons  118  through the color conversion material ( 112   a ,  112   b ,  112   c ,  112   d ) may cause minimal degradation of the color purity of converted photons  120 . 
     In one embodiment, the color selector  114  includes a color filter array comprising an organic dye embedded in an organic polymer. The dye may be configured to absorb UV radiation of the pump photons  118  but to not absorb blue, green, or red light of the converted photons. Optionally, a different dye may be applied over each of the colored subpixels (e.g., red, green, and blue subpixels). For example, a first dye filter material configured to primarily transmit red light may be applied to red subpixels, a second dye filter material configured to primarily transmit green light may be applied to green subpixels, and third dye filter material configured to primarily transmit blue light may be applied to blue subpixels. The color filters may by formed using a further photolithographic process. In various embodiments, a thin film encapsulation (TFE) layer or layer stack may then be applied over the color filter materials to provide protection against air or moisture ingress into the quantum dot layers of the color conversion material. In one embodiment, the TFE may comprise a tri-layer stack of two silicon nitride layers separated by a polymer layer. 
     In an alternative embodiment, the color selector  114  comprises a distributed Bragg reflector (DBR) formed over the color conversion material ( 112   a ,  112   b ,  112   c ,  112   d ). The DBR may be configured to reflect pump photons  118  which are transmitted through the color conversion material back into the cavity  106  as reflected photons  122  (e.g., UV or deep blue photons) and to allow the converted photons  120  to be transmitted out of the cavity  106 . The DBR may be formed as an alternating multi-layer stack of materials (not shown) having different indices of refraction. For example, the DBR may be formed as a stack of N layers alternating between TiO 2  (n=2.5) and SiO 2  (n=1.5) with N being 2 or more. In other embodiments, various other materials having respective indices of refraction may be used in constructing the DBR. 
     Embodiments in which the DBR includes TiO 2  and SiO 2  with N=2 may have a bandwidth of 164 nm at a center wavelength of 405 nm and a maximum reflectivity R of 84%. Embodiments in which the DBR stack includes a larger number of layers (i.e., N&gt;2) may have increased reflectivity. As such, the probability of a UV pump photon  118  passing through the DBR may be decreased. The UV reflected photons  122  reflected from the DBR back into the cavity  106  may circulate through the color conversion material ( 112   a ,  112   b ,  112   c ,  112   d ) and may thereby have an increased probability of also being converted to converted photons  120  having the target wavelength (e.g., green, blue, or red). In this way, any UV reflected photons  122  that are not initially absorbed by the color conversion material ( 112   a ,  112   b ,  112   c ,  112   d ) may eventually be absorbed and converted to converted photons  120  having the target emission wavelength. This process, which is sometimes called “photon recycling” may increase the quantum efficiency of the device. 
     If the micro-LEDs  102  comprise shorter wavelength blue light emitting LEDs, then the DRB  114  may block the shorter wavelength blue light (i.e., pump photons  118 ) of the micro-LEDs  102  but transmit the longer wavelength converted photons  120  emitted from blue quantum dots of the color conversion material. Alternatively, the DBR  114  may be omitted over the blue light emitting subpixels. 
     The DBR may be formed by a deposition (e.g., by evaporation) of a multi-layer stack (not shown) over all of the subpixels. As such, the DBR may provide additional protection against moisture and oxygen ingress into the quantum dot layer. A higher value of N may further increase both the DBR reflectivity and the protection from moisture and oxygen, leading to improved overall system performance and durability. 
     In various additional embodiments, other materials may be used for the various components of the device. For example, the DBR may include a wide range of materials each having respective refractive indices, for example, nitrides (TiN, AlN, TiN, etc.), polysilicon, etc. Some embodiments may include multiple layers of quantum dots, multiple DBR structures, etc. The light extracting material layer  110 , described above, may be omitted in some embodiments or multiple light extraction material layers  110  may be used. By using a more effective DBR  114 , the layer thickness and density of the color conversion material ( 112   a ,  112   b ,  112   c ,  112   d ) may be reduced. In further embodiments, the optical cavities  106  may be formed in various ways. For example, the optical cavities  106  may be formed in a separate matrix layer which may then be attached to the array of micro-LEDs  102  after the optical cavities  106  are formed, as described in greater detail below. Further embodiments may also include light-collimating elements to mitigate performance degradation that may otherwise occur due to lateral photon propagation. 
       FIG.  1 E  is a vertical cross-sectional view of a further array  100   e  of light emitting devices, according to various embodiments. As shown, the array  100   e  of light emitting devices includes micro lenses  124  formed over optical cavity  106 . Each micro lens  124  may help to improve light extraction from each micro-LED structure and may thereby improve efficiency of the array  100   e . In general, extraction of light emitted by micro-LEDs may be increasingly challenging with decreasing pixel pitch and micro-LED size. In this regard, the color conversion material ( 112   a ,  112   b ,  112   c ,  112   d ) may be chosen to be sufficiently thick to convert all of the pump photons  118  into converted photons  120 , each having a specific color. The thickness of the color conversion material ( 112   a ,  112   b ,  112   c ,  112   d ) may be very large compared to a lateral dimension of the subpixel. In such a structure, photons may move diffusively rather than ballistically out of the micro-LED subpixel. Such diffusively moving photons may spread to adjacent subpixels, potentially causing optical cross talk. 
     Disclosed embodiments provide improved optical extraction of photons (e.g., along a specific direction) generated by the quantum dots, while maintaining high efficiency by avoiding loss of photons to absorbing surfaces. As described above, this may be accomplished by forming a matrix structure that include cavity walls  108  that are reflective, including a light extracting material layer  110 , and/or including a color selector  114 , such as a DBR. 
     The use of quantum dots as a color conversion material ( 112   a ,  112   b ,  112   c ,  112   d ) for micro-LED displays may include deposition and patterning of dense quantum dot layers at very small feature sizes. To achieve sufficient absorption of pump photons  118  (e.g., see  FIGS.  1 D and  1 E ) in the quantum dot layer, subpixels with aspect ratios of greater than 1:1 may be used. Such subpixels may also be separated by cavity walls  108  formed of an opaque matrix material to prevent color crosstalk (i.e., photons from one micro-LED propagating into neighboring subpixels) in the display. 
     A high concentration of quantum dots used as a color conversion material ( 112   a ,  112   b ,  112   c ,  112   d ) may present additional challenges for fabrication of high-resolution structures. Since quantum dots strongly absorb UV light, the activity of photoinitiators or photo-acid generators, that are generally used in photoresists, may be diminished. Thus, the presence of quantum dots may require conventional fabrication materials and methods to be modified. As such, the patterning of tall and thin structures may be more difficult when using high loadings of quantum dots. Disclosed embodiments solve this problem by forming cavities as vias etched in a matrix material, as described in greater detail with reference to  FIGS.  3 A to  4 P , below. 
     Various embodiments include a matrix, such as a matrix  200   a  or  200   b , which may allow better light extraction from each subpixel and may mitigate photonic color crosstalk. Using the matrix as a template and sequentially opening vias corresponding to different color subpixels allows the deposition and curing of quantum dot inks without relying on a high-resolution photo-patternable resin formulation. Various embodiments, described below, include opening of vias corresponding to one color in a matrix layer, filling with quantum dot ink, curing and encapsulation, then repeating the same process with the second color, the third color, etc. 
       FIGS.  3 A to  3 L  are vertical cross-sectional views of intermediate structures that may be used in the formation of an array of light emitting devices, according to various embodiments. As shown in  FIG.  3 A , a continuous matrix layer  304 L may be deposited on a support  302 . In one embodiment, the continuous matrix layer  304 L may have a thickness of approximately 10 to 30 microns. The matrix layer  304 L may comprise an insulating material, such as silica, alumina, titania, etc. to form the optical cavity walls  108  described above with respect to  FIG.  1 B . Alternatively, the matrix layer  304 L may comprise a metal, such as aluminum which is then anodized to form anodic alumina. In another alternative embodiment, the matrix layer  304 L may be a reflective metal, such as aluminum, which is not converted to a metal oxide. In this alternative embodiment, the matrix layer  304 L is formed over the micro-LEDs  102  in such a manner as to avoid electrically shorting corresponding electrodes of adjacent micro-LED  102  to each other. 
     The support  302  may comprise the backplane  104  supporting the micro-LEDs  102  as described above with respect to  FIG.  1 A . In an alternative embodiment, the support  302  may comprise a separate substrate, such as a transparent glass or polymer substrate which is subsequently attached over the backplane  104  supporting the micro-LEDs  102 . 
     As shown in  FIG.  3 B , a patterned mask material  306  may be formed over the continuous matrix layer  304 L. In one embodiment, the patterned mask material  306  may be a photoresist and may be patterned using photolithography techniques. 
     The continuous matrix layer  304 L (e.g., see  FIG.  3 A ) may be etched to form an etched matrix layer  304  that includes first vias  308   a . In an exemplary embodiment, a continuous aluminum matrix layer  304 L may be etched using a BCl 3  dry etch process. The first vias  308   a  may correspond to optical cavities  106  for a first plurality of subpixels. For example, the first plurality of subpixels may correspond to a first color (e.g., red, green or blue color). After etching, the patterned mask material  306  may be removed. 
     As shown in  FIG.  3 C , the patterned mask material  306  (e.g., see  FIG.  3 B ) may be removed and replaced by a ultrahydrophobic (i.e., nonstick) coating  310 . The coating  310  may comprise a fluorinated silane coating, such as inorganic nanoparticles (e.g., silica nanoparticles) functionalized with a fluoroalkylsilane groups. A quantum dot ink, having a plurality of first quantum dots  112   a , may then be deposited by spin-coating, doctor-blading, inkjet-printing, or other method to fill the first vias  308   a , as shown in  FIG.  3 D . The fluorinated coating  310  may ensure that the majority of quantum dots do not stick to a top surface of the structure. The quantum dot ink may then be cured either by UV irradiation or by heating. The fluorinated coating  310  and excess quantum dots may then be washed off, as shown in  FIG.  3 E . 
     A protective layer  314  may then be formed over the first quantum dots  112   a , as shown in  FIG.  3 F . For example, the protective layer  314  may be a layer of alumina that may be deposited by atomic layer deposition (ALD). In an exemplary embodiment, the protective layer  314  may have a thickness of 3 to 10 nm, such as approximately 5 nm. Other embodiments may include other thicknesses, other materials, and other deposition methods for the protective layer  314 . 
     The above-described process (e.g., see  FIGS.  3 A to  3 F ) may then be repeated to deposit and cure quantum dot inks for other colors. For example, a patterned mask material  306  may be formed over the intermediate structure of  FIG.  3 F  and an etch process may be performed to form second vias  308   b  through the protective layer  314 , as shown in  FIG.  3 G . The patterned mask material  306  may be removed and replaced by the above described ultrahydrophobic (i.e., nonstick) fluorinated coating  310 , as shown in  FIG.  3 H . A quantum dot ink having a plurality of second quantum dots  112   b  (i.e., different color dots from the dots  112   a ) may then be deposited by spin-coating, doctor-blading, inkjet-printing, or other method to fill the second vias  308   b , as shown in  FIG.  3 I . The fluorinated coating  310  and excess quantum dots may then be washed off, as shown in  FIG.  3 J . A second protective layer having a first portion  314   a  and a second portion  314   b  may then be formed over the second quantum dots  112   b , as shown in  FIG.  3 K . The first portion  314   a  may be formed over the existing first protective layer  314 , while the second portion  314   b  may be formed over the second vias  308   b  filled with the second quantum dots  112   b.    
     Similarly, the process may be continued to form third vias  308   c , as shown in  FIG.  3 L . The third vias  308   c  may be filled with a third quantum dot ink including third quantum dots  112   c  (not shown in this figure). In various embodiments, the process may be continued to form additional vias that may be filled with quantum dots corresponding to additional respective colors. 
     If the support  302  comprises a transparent substrate, then the support  302  supporting the completed matrix containing the quantum dots may then be attached over the backplane  104  supporting the micro-LEDs  102 . If the support  302  comprises the backplane  104  supporting the micro-LEDs  102 , then the etched matrix layer  304  includes the cavity walls  108  surrounding the optical cavities  106  filled with the quantum dots ( 112   a ,  112   b , etc.). 
       FIGS.  4 A to  4 P  are vertical cross-sectional views of further intermediate structures that may be used in the formation of an array of light emitting devices, according to various embodiments. The processes of  FIGS.  4 A to  4 P  include providing matrix layer on support, deposition of a planar positive photoresist layer, and selective exposure and removal of this photoresist for sequentially opening vias. The processes of  FIGS.  4 A to  4 P  rely on removal of a photoresist to form the optical cavities. As such, in certain embodiments, the second process flow (i.e., described below with reference to  FIGS.  4 A to  4 P ) may be more versatile, cheaper, and safer. 
     As shown in  FIG.  4 A , a first intermediate structure may include a continuous matrix layer  108 L formed over the above described support  302 . The matrix layer  108 L may comprise an insulating layer, such as alumina, silica, titania, etc., or a conductive layer, such as a metal layer, for example aluminum. A patterned photoresist  406  may be formed over continuous matrix layer  108 L. In this regard, a blanket layer of photoresist (not shown) may be formed over the continuous matrix layer  108 L and may be patterned using photolithography techniques to form the patterned photoresist  406 . 
     As shown in  FIG.  4 B , using the patterned photoresist  406  as a mask layer, the continuous matrix layer  108 L may be etched to form vias or cavities (e.g., optical cavities)  106  that are bounded by cavity walls  108 , as shown in  FIG.  4 B . The patterned photoresist  406  may then be removed by ashing or by dissolution with a solvent. 
     In an alternative embodiment, rather than etching the continuous matrix layer  108 L, the cavity walls  108  may be formed by anodic oxidation. As described above, if the continuous matrix layer  108 L comprises aluminum, then it may be anodized in acid as described above to form the porous anodic alumina layer containing cavity walls  108  surrounding the optical cavities (i.e., pores)  106 . 
     As shown in  FIG.  4 C , a positive photoresist, having a first photoresist portion  408   a , a second photoresist portion  408   b , and a third photoresist portion  408   c , may then be deposited over the intermediate structure of  FIG.  4 B  into the optical cavities  106 . Each photoresist portion fills the respective optical cavity  106 . 
     As shown in  FIG.  4 D , an optional patterned mask  410  may be used with a UV radiation source (e.g., UV emitting lamp)  412  to selectively expose the first photoresist portion  408   a  of the positive photoresist through the mask  410  to UV radiation  414 . Exposure of the first photoresist portion  408   a  of the positive photoresist makes the first photoresist portion  408   a  soluble in a photoresist developer, which may be used to remove the first photoresist portion  408   a  of the positive photoresist. The second and third photoresist portions are not exposed to UV radiation. 
     Alternatively, if the support  302  comprises the backplane  104  supporting UV radiation emitting micro-LEDs  102 , then the micro-LED  102  located under the first photoresist portion  408   a  may be activated to irradiate the first photoresist portion  408   a  with UV radiation from the bottom to render the portion  408   a  soluble in the developer. In this alternative embodiment, the mask  410  and the radiation source  412  may be omitted. The micro-LED  102  located under the second and third photoresist portions  408   b ,  408   c  are not activated. 
     As shown in  FIG.  4 E , first vias  416   a  may be generated by removing the first photoresist portion  408   a  without removing the other photoresist portions  408   b ,  408   c  by immersing the structure in photoresist developer bath or spraying the positive photoresist with the developer solution. 
     As shown in  FIG.  4 F , a first quantum dot ink  418   a  may then be introduced into the first vias  416   a . The first vias  416   a  may thereby be filled with a uniform layer of first quantum dots  112   a . A polymer in which the first quantum dots  112   a  are suspended may then be cured thermally or by exposure to UV radiation. For example,  FIG.  4 G  illustrates selective exposure of the first quantum dots  112   a  to UV radiation using the patterned mask  410  and the source  412  of UV radiation. Alternatively, the UV emitting micro-LEDs  102  underlying the first quantum dots  112   a  may be activated to irradiate the first quantum dots  112   a  with UV radiation. 
     The above-described process shown in  FIGS.  4 C to  4 G  may then be repeated to form second quantum dots  112   b  in second optical cavities  106 . In this regard, the second photoresist portion  408   b  of the positive photoresist shown in  FIG.  4 C  may be exposed to UV radiation from the UV radiation source  412  or from the micro-LEDs  102 , as shown in  FIG.  4 H . The second photoresist portion  408   b  of the positive photoresist may then be removed with a photoresist developer to thereby generate second vias  416   b , as shown in  FIG.  4 I . The second quantum dot ink  418   b  may then be introduced into the second vias  416   b  to thereby form the uniform layer of second quantum dots  112   b , as shown in  FIG.  4 J . The uniform layer of second quantum dots  112   b  may then be cured by exposure to UV radiation from the UV radiation source  412  or from the micro-LEDs  102 , as shown in  FIG.  4 K . 
     The above-described process shown in  FIGS.  4 C to  4 G  may then be repeated to form third quantum dots  112   c  in third optical cavities  106 . In this regard, the third photoresist portion  408   c  of the positive photoresist may be exposed to UV radiation from the UV radiation source  412  or from the micro-LEDs  102 , as shown in  FIG.  4 L . The third photoresist portion  408   c  of the positive photoresist may then be removed with a photoresist developer to thereby generate third vias  416   c , as shown in  FIG.  4 M . The third quantum dot ink  418   c  may then be introduced into the third vias  416   c  to thereby form the uniform layer of third quantum dots  112   c , as shown in  FIG.  4 N . The uniform layer of third quantum dots  112   c  may then be cured thermally or by exposure to UV radiation from the UV radiation source  412  or from the micro-LEDs  102 , as shown in  FIG.  4 O . 
     Lastly, a protective layer  314  may then be formed over the uniform layer of first quantum dots  112   a , the uniform layer of second quantum dots  112   b , the uniform layer of third quantum dots  112   c , and the cavity walls  108 , as shown in  FIG.  4 P . As described above, the protective layer  314  may be an alumina layer that is deposited by ALD. Other materials and deposition processes may be used to deposit the protective layer  314  and/or the color selector  114  (e.g., DBR) as described above. 
     In the above-described embodiments, the shape of subpixels in an array of light emitting devices may be defined by the geometry of the cavities/vias. As such, the patternability requirements for the quantum dot inks ( 418   a ,  418   b ,  418   c ) may be significantly less stringent than requirements for embodiments that do not rely on a matrix template. In some embodiments, a UV curable quantum dot ink may be used for confining the quantum dots to the targeted subpixels. In other embodiments, thermally curable inks may also be used. The use of UV curable or thermally curable quantum dot inks enhances the choice of chemistries that may be used in forming the (quantum dot based) color conversion material ( 112   a ,  112   b ,  112   c ,  112   d ). 
     Various embodiments may include solvent-based or solvent-free quantum dot inks. The use of thermal curing for the quantum dot inks in each subpixel allows omission of photocurable acrylates/epoxies for the ink formulation. In further embodiments, quantum dot inks may be formed using inorganic ligands and matrix materials (for example, metal chalcogenides and metal oxides), which may offer alternative benefits such as high temperature stability. 
       FIGS.  5 A to  5 G  are vertical cross-sectional views of further intermediate structures that may be used in the formation of an array of light emitting devices, according to various embodiments. As shown in  FIG.  5 A , a plurality of micro-LEDs  102  may be formed on a substrate  104 . The substrate  104  may be a backplane having electrical circuitry (e.g., CMOS or TFT circuits) configured to supply voltages to the micro-LEDs  102  to thereby control light emission by the micro-LEDs  102 . As described above, the micro-LEDs  102  may comprise blue or UV emitting LEDs. The intermediate structure of  FIG.  5 A  may include a common cathode  502  for plural micro-LEDs  102  formed of a transparent conducting oxide (e.g., indium tin oxide) and separate anodes  503  for each micro-LED  102  which electrically connected to respective backplane circuitry (not shown for clarity). Thus, plural micro-LEDs  102  are shorted on their cathode (e.g., n-type) side, but are separately activated by the backplane circuitry on their anode (e.g., p-type) side. The common cathode  502  is also connected to the backplane circuitry outside of the micro-LED  102  area. 
     In one embodiment, the micro-LEDs  102  may comprise vertical LEDs with cathode and anode electrodes ( 502 ,  503 ) located on opposite sides of the LED. In one embodiment, the micro-LEDs  102  may have a reverse taper. In other words, the micro-LEDs  102  may be wider on the bottom side facing the anode  503  and the backplane  104 , than on the top side facing the common cathode  502 . 
     As shown in  FIG.  5 B , a first color conversion material (e.g., first color quantum dots)  504   a  may be formed over a first plurality of the micro-LEDs  102 . The first color conversion material  504   a  may be formed by an ink jet process that may be used to print a first quantum dot ink only directly over first portions of the common cathode  502  over respective micro-LEDs  102  in the first color subpixels. Alternatively a continuous quantum dot layer may be deposited directly on the common cathode, followed by photolithography and patterning to leave the first color quantum dots  504   a  only over the respective micro-LEDs  102  in the first color subpixels. 
     As shown in  FIG.  5 C , a second color conversion material  504   b  may be formed over a second plurality of the micro-LEDs  102 . The second color conversion material  504   b  may be formed by an ink jet process that may be used to print a second quantum dot ink directly over second portions of the common cathode  502  over respective micro-LEDs  102  or by depositing a continuous quantum dot layer followed by photolithographic patterning. The color conversion material may be omitted over blue emitting micro-LEDs  102 . Alternatively, a blue color conversion material may be formed over UV emitting micro-LEDs  102 . Lastly, a respective color selector  114  may be formed over the first color conversion material  504   a  and the second color conversion material  504   b . For example, the color selector  114  may be a DBR, as described above. If desired, an encapsulating layer, such as an alumina layer, may be formed over the color selector  114 . 
     Alternatively, the intermediate structures of  FIGS.  5 B and  5 C  may be formed using processes similar to those described above with reference to  FIGS.  4 A to  4 P . In this regard, a patterned photoresist (not shown) may be formed over the common cathode  502  and may be used as a mask material for deposition of the first color conversion material  504   a . In this regard, the mask material may include openings corresponding to places over which the first color conversion material  504   a  is to be deposited. After the first color conversion material  504   a  has been deposited and cured, the photoresist may be patterned to form openings corresponding to places over which the second color conversion material  504   b  is to be deposited, etc. 
     In further embodiments, the intermediate structures of  FIGS.  5 D and  5 E  may be formed by forming an etch stop layer  508  over the common cathode  502  of the structure of  FIG.  5 A . The first color conversion material  504   a  and the second color conversion material  504   b  may then be deposited, as shown in  FIGS.  5 D and  5 E . The etch stop layer  508  may comprise silicon oxide or other similar etch stop materials. The presence of the etch stop layer  508  may protect the transparent conductive oxide that forms the common cathode  502  during processes in which the photoresist is etched. 
     The processes of forming the additional alternative intermediate structures of  FIGS.  5 F and  5 G  may be similar to the processes used to form the intermediate structures of  FIGS.  5 D and  5 E  from the intermediate structure of  FIG.  5 A . In this regard, each of the intermediate structures of  FIGS.  5 F and  5 G  may include the etch stop layer  508  of  FIGS.  5 D and  5 E  formed over the common cathode  502  of  FIG.  5 A . The intermediate structures of  FIGS.  5 F and  5 G  may further include optical cavities  106  bounded by cavity walls  108 . The optical cavity  106  over the blue subpixel may remain unfilled if the micro-LEDs  102  comprise blue LEDs. As such, the intermediate structures of  FIGS.  5 F and  5 G  may be similar to the embodiments of  FIGS.  1 B to  1 E,  3 L, and  4 B to  4 P . 
     The preceding description of the disclosed embodiments is provided to enable persons of ordinary skill in the art to make or use the disclosed embodiments. Various modifications to these embodiments will be readily apparent to those of ordinary skill in the art, and the generic principles defined herein may be applied to other embodiments without departing from the spirit or scope of the disclosure. Thus, the disclosure 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.