Abstract
There is herein described light generating electronic components with improved light extraction and a method of manufacturing said electronic components. More particularly, there is described LEDs having improved light extraction and a method of manufacturing said LEDs.
Claims
1-48. (canceled)
49. A light emitting structure comprising: a light emitter configured to emit light; an transparent electrically conductive layer above the light emitter, the transparent electrically conductive layer configured to pass through the emitted light in first directions; a light emitting region below the transparent electrically conductive layer and the light emitter; and a reflective surface above the transparent electrically conductive layer, wherein: the reflective surface is curved to reflect the light passed through the transparent electrically conductive layer in the first directions back through the transparent electrically conductive layer in second directions toward the light emitting region; and the light reflected in the second directions include a reduced divergence relative to the light passed through the transparent electrically conductive layer in the first directions.
50. The light emitting structure according to claim 49, wherein the light emitting structure is a light emitting diode (LED) or a micro-LED.
51. The light emitting structure according to claim 49, wherein the light emitter includes a quantum well region configured to emit the light; and wherein the quantum well region is between 0.05 and 0.2 microns thick.
52. The light emitting structure according to claim 51, wherein the quantum well region is made from InGaN/GaN; and wherein the light emitted from the light emitter has a wavelength of about 300-700 nm.
53. The light emitting structure according to claim 49, wherein the reflective surface is disposed on a dome shaped portion of the electrically conductive layer.
54. The light emitting structure according to claim 49, wherein the transparent electrically conductive layer forms an ohmic contact to the light emitter.
55. The light emitting structure according to claim 49, wherein the transparent electrically conductive layer reduces Fresnel reflections at an interface of the transparent electrically conductive layer and the light emitter.
56. The light emitting structure according to claim 49, wherein the transparent electrically conductive layer includes one of: a conical cross-section; a domed cross-section; a spherical cross-section; a parabolic cross-section; an elliptical in cross-section; or a micro Fresnel reflector shape.
57. The light emitting structure according to claim 49, wherein the reflective surface is an electrical contact.
58. The light emitting structure according to claim 49, wherein the light emitting structure disposed on a substrate with other light emitting structures.
59. The light emitting structure according to claim 58, wherein the light emitting structures form a grid array or hexagonal array of light emitting structures on the substrate.
60. The light emitting structure according to claim 49, further comprising a substrate located below the transparent electrically conductive layer, a bottom surface of the substrate defining the light emitting region.
61. The light emitting structure according to claim 60, wherein the light emitter includes a quantum well region having a width that is smaller than a width of the substrate to facilitate collimation of the light transmitted through the transparent electrically conductive layer and toward the light emitting region.
62. The light emitting structure according to claim 49, further comprising side-walls including an electrically insulating layer.
63. The light emitting structure according to claim 62, wherein the light emitter includes a quantum well region having a width that is smaller than a width defined by the side-walls.
64. The light emitting structure according to claim 49, wherein the reflective surface is disposed on a series of dome shaped portions of the electrically conductive layer.
65. A light emitting diode (LED), comprising: a light emitter configured to emit light; an transparent electrically conductive layer above the light emitter, the transparent electrically conductive layer configured to pass through the emitted light in first directions; a reflective surface above the transparent electrically conductive layer, wherein: the reflective surface is curved to reflect the light passed through the transparent electrically conductive layer in the first directions back through the transparent electrically conductive layer in second directions; and the light reflected in the second directions include a reduced divergence relative to the light passed through the transparent electrically conductive layer in the first directions.
66. The LED according to claim 65, wherein the reflective surface is disposed on a dome shaped portion of the electrically conductive layer.
67. The LED according to claim 65, further comprising a substrate located below the transparent electrically conductive layer, a bottom surface of the substrate defining a light emitting region to output the light reflected in the second directions.
68. The LED according to claim 67, wherein the light emitter includes a quantum well region having a width that is smaller than a width of the substrate to facilitate collimation of the light transmitted through the transparent electrically conductive layer to the light emitting region.
Description
BRIEF DESCRIPTION OF THE DRAWINGS
[0054] Embodiments of the present invention will now be described, by way of example only, with reference to the accompanying drawings in which:
[0055] FIG. 1 is a representation of a required escape cone which light must fall within to be able to exit a light emitting device structure;
[0056] FIG. 2 is a representation of a light emitting structure comprising an integrated dome shaped transparent cap according to an embodiment of the present invention;
[0057] FIG. 3a is a representation of a light emitting structure where emitted light rays are reflected back against a metal contact/mirror according to the current state of the art;
[0058] FIG. 3b is a representation of a light emitting structure comprising a series of integrated dome shaped transparent cap according to an embodiment of the present invention;
[0059] FIGS. 4a-4h represent a method of manufacturing a light emitting structure according to an embodiment of the present invention;
[0060] FIG. 5a is a representation of a light emitting structure according to an embodiment of the present invention;
[0061] FIG. 5b is a representation of the embodiment shown in FIG. 5a deployed in a linear structure according to an embodiment of the present invention;
[0062] FIG. 6 is a representation of a light emitting structure where the quantum well area has been reduced according to an embodiment of the present invention;
[0063] FIG. 7 is a grid array of light emitting structures according to an embodiment of the present invention;
[0064] FIG. 8 is a hexagonal array of light emitting structures according to a further embodiment of the present invention;
[0065] FIG. 9 is a representation of a plurality of light emitting structures of the present invention joined together according to a further embodiment of the present invention; and
[0066] FIG. 10 is a representation of a cap layer with an array of domed structures (e.g. spherical) and adjacent conductive layers which provide electrical contact and reduce Fresnel reflections at this interface by design wherein a reflective structure is formed on the substrate so that light is output from the top surface according to a further embodiment of the present invention.
BRIEF DESCRIPTION
[0067] Generally speaking, the present invention resides in the provision of using a transparent (e.g. visible light transparent) conductive cap structure on top of a light emitting diode to improve light extraction and the amount of light emitted.
[0068] FIG. 1 is representation of light 110 being generated in an LED structure 100 according to the prior art. The light is emitted from a point source 112 in a quantum well. FIG. 1 shows that the light 110 is emitted in all directions and is shown to have an escape cone with an angle .sub.c. When the light reaches the boundary of, for example, the GaN or sapphire surface there is a change in refractive index of the material. If the light ray 110 reaching the interface has an angle within the escape cone it will be emitted from the LED structure 100. If the angle of incidence is greater than the escape cone the light 110 will be reflected back into the LED structure 100 and may be absorbed as heat. For the GaN-air interface the critical angle is only 21, 24 and 25 at the wavelengths of 365, 450 and 520 nm, respectively. It is beneficial to have light below this critical angle. This can be achieved by quasi collimating or directing the light in order to reduce the probability of internal reflection and so improve the efficiency of the device.
[0069] FIG. 2 is representation of an integrated light emitting structure 200 of the present invention. The light emitting structure may be an LED e.g. a micro-LED. As shown in FIG. 2 the light emitting structure 200 has a quantum well region 210 from which light 214 is emitted from a point source 212. The quantum well region 210 may be about 100 nm thick and may be made from InGaN. Emitted light rays 214 are shown passing through a transparent dome cross-sectioned cap 216. The dome cross-section cap 218 is integrally formed with the rest of the LED structure during fabrication. This is discussed in more detail below. The LED structure may consist of a single isolated p-n diode or may consist of multiple structures with a common p and/or n connections. The light 214 is shown being reflected from a metal contact/curved mirror 218 back into the main body of the light emitting structure 200. The light emitting structure 200 is also shown to have oxide covered side walls 220. The transparent cap 216 may in some embodiments be contoured so that light 214 emitted towards the transparent cap 216 is reflected back towards the emitting surface in a manner that reduces the divergence of light. In these types of embodiments the shaped transparent structure 216 is conductive and may form an ohmic contact to a Gallium Nitride layer. The cap 218 and the sidewall 220 of the Gallium Nitride emitter may both be contoured by etching to leave a suitable one or two dimensional cross-sectional shape which may be sloped, conical, domed parabolic, or alternatively an aspheric structure. The reflecting surface 218 may also be shaped to act as a Fresnel mirror. The reflective second surface mirror surface may extend to include the side wall below the quantum well. In an ideal device the distance from the top of the structure to the bottom of the sidewall will typically be between 1 and 10 microns. The diameter of the individual emitter should be less than 2 the height e.g. a device of 2 microns height will have emitters of 4 microns diameter or less The material chosen for the cap 216 should ideally have a high refractive index, normally greater than 1.5 and preferably more than 2.0 in order to reduce losses at the interface with the GaN layer. In an ideal device it should have a refractive index of 2.4 to match GaN. The sidewall angle of the structure should be chosen according to the type of cap shape chosen and the refractive index of the material used but should normally be greater than 5 from vertical, and preferably 4530 from vertical.
[0070] FIG. 3a is a representation of a light emitting structure 300 where emitted light rays 314 are reflected back against a metal contact/mirror 318. There is also a quantum well region 316.
[0071] FIG. 3b is a representation of a light emitting structure 350 comprising a series of integrated dome shaped transparent caps 354. There are also sidewalls 352 and a quantum well region 356. Light rays 358 are show exiting the light emitting structure 350.
[0072] FIGS. 4a-4h represent a method of manufacturing a light emitting structure according to an embodiment of the present invention. In FIG. 4a there is shown a transparent conductive oxide (TCO) layer 410, a GaN layer 412 and a sapphire layer 414. Sapphire substrates are therefore used with a proprietary GaN layer structure which is chosen to be suitable for the process used and the wavelength of emission required. The GaN layer 412 is coated with a TCO layer 410 with, for example, a refractive index greater than 1.8 and preferably greater than 2.0.
[0073] In FIG. 4b there is shown a photoresist pattern 416 being formed in the areas to be defined as emitters by photolithography.
[0074] In FIG. 4c the photoresist pattern 416 is baked to allow it to reflow into a rounded form.
[0075] In FIG. 4d the photoresist pattern 416 is transferred by a dry etch technique such as inductively coupled plasma etch (ICP) or Reactive Ion Etching (RIE) into the TCO layer 410 and then into the GaN layer 412. The photoresist pattern layer 416 will be sacrificial and erodes during the etch process.
[0076] In FIG. 4e the emitter structure is covered with a thin insulating layer 418 such as silicon dioxide or silicon nitride.
[0077] In FIG. 4f the insulating layer 418 on top of the TCO cap is uncovered to enable the p-contact to be formed. An interconnect metal layer 420 is then deposited.
[0078] The interconnect metal layer 420 may also act as a mirror to reflect light downwards in a back emitting device.
[0079] In FIG. 4g the sapphire carrier has been removed, preferably using a laser lift off technique.
[0080] In FIG. 4h the emitting face of the GaN layer is shown to be structured by etching where it may be patterned to form refractive, diffractive, Fresnel or 2D photonic crystal structures.
[0081] FIG. 5a is a light emitting structure 500 according to the present invention. As shown in FIG. 5 the light emitting structure 500 comprises a light generating quantum well region 510 which, for example, is typically less than 100 nm thick and is made from InGaN. Light 514 is emitted from the light generating quantum well region 510 and passes through a transparent conductive layer 516 which may be made from a visibly transparent conductive oxide. As shown there is also a transparent conductive layer 522. The light rays 514 are then reflected from a metal/contact curved mirror 518. The light emitting structure 500 also has a side wall oxide 520.
[0082] FIG. 5b shows a variant where a light emitting structure is in the form of a linear device where there is an extended metal/contact curved mirror 518. As before there is a quantum well region 510, sidewalls 520 and a transparent conductive layer 522.
[0083] In comparison, in FIG. 6 there is shown an embodiment where there is shown a light emitting structure 600 with a reduced quantum well area. The light emitting structure 600 has a light generating quantum well region 610 which, for example, is typically less than 100 nm thick and is made from InGaN. There is also a transparent dome cross-sectional cap 616 through which light 614 passes. As shown there is also a transparent conductive layer 622. The light rays 614 are then reflected from a metal/contact curved mirror 618. The light emitting structure 600 also has a side wall oxide 620. The point to note in the light emitting structure 600 is that the quantum well area 610 has been reduced to improve confinement of the light emission area to the centre of the pixel allowing more effective beam extraction from a shaped metal/contact curved mirror 618. For a 2 dimensional parabolic shape reflected light 614 will be near collimated. In this form, the p-GaN region and quantum well 610 are selectively exposed to a plasma treatment. This has the effect of stopping light emission from the treated area i.e. reducing the size of the active layer such that the light generated is close to the middle area of the active area. For the parabolic shaped reflector 618 this would coincide with the focal point. The application of the reduced quantum well area can be utilised with a conventional flat topped structure as long as the sidewalls are sloped, but preferably should be used with a cap structure as described.
[0084] FIG. 7 is a grid array 700 of light emitting structures 710 of the present invention. FIG. 8 is a hexagonal array of 800 of light emitting structures 810 of the present invention. As shown in FIG. 9 multiple shaped emitter regions may be placed in an array 900 to form a common emitter area. In this format p and/or n contact areas may be addressed either by a common connection or on an individual basis. Emitter sites may be formed in a grid structure or a close packed hexagonal array. FIG. 9 shows the n-contact regions 910 and the p-contact regions 912. FIG. 9 also shows in this case that there are seven light emitting structures as shown in FIG. 6 but the number may be more.
[0085] FIG. 10 shows a light emitting structure 1000 applicable to a top emission device where the transparent conductive layer 1010 has been structured in order to enhance light emission. The transparent conductive layer 1010 between structures should be of a thickness chosen to aid light transmission at the chosen wavelength. There is also shown side wall oxides 1012 and light rays 1014, 1016. The light emitting structure 1000 as before also has a quantum well area 1018. The light generated in the quantum well area 1018 is typically less than 0.5 micron from the GaN surface. The layer thickness of the transparent conductive layer 1010 is determined by the wavelength with the thickness being chosen to reduce reflection. An optional mirror as shown at the bottom of the light emitting structure 1000 shown in FIG. 10 may also be used.
[0086] Whilst specific embodiments of the present invention have been described above, it will be appreciated that departures from the described embodiments may still fall within the scope of the present invention. For example, any suitable type of integrated transparent conductive layer and transparent conductive layer (e.g. transparent cap) may be used.
