Patent Application
20240145629
The present disclosure is directed to a structured epitaxy for a light emitting diode array, e.g., one usable in a micro light emitting diode display. In one embodiment, a light emitting diode array includes a growth mask with an array of closed shapes on a III-N layer. The III-N layer is epitaxially grown on a substrate. An array of group III-N inverted pyramids is epitaxially grown around the growth mask. The inverted pyramids have {10-11} or {11-22} facets and hexagonal bases. An array of III-N c-plane surfaces are parallel with the substrate joining the {10-11} or {11-22} facets of adjacent ones of the III-N inverted pyramids. One or more quantum well layers include a III-N compound formed on the {10-11} or {11-22} facets and the c-plane surfaces. The quantum well layer has a first thickness on the {10-11} or {11-22} facets forming first light emitting elements. The quantum well layer has a second thickness on the c-plane surfaces forming second light emitting elements. The second thickness is greater than the first thickness such that the second light emitting elements emit light at a longer wavelength than the first light emitting elements. A pattern of electrical contacts is disposed over the light emitting array and is operable to separately activate the first light emitting elements and the second light emitting elements.
In another embodiment, a method involves epitaxially forming a III-N base layer on a growth template or substrate and patterning an array of closed shapes of a mask material on a top plane of the III-N base layer. The method involves selective epitaxial growing of III-N structures around the array of closed shapes and extending above the closed shapes to form a first array of III-N inverted pyramids and a second array of III-N c-plane surfaces. The inverted pyramids include {10-11} or {11-22} facets and hexagonal bases. The III-N c-plane surfaces are parallel with the III-N base layer and join the {10-11} or {11-22} facets of adjacent ones of the III-N inverted pyramids. The method further involves epitaxially growing a quantum well layer comprising a III-N compound on the {10-11} or {11-22} facets and the c-plane surfaces. The quantum well layer has a first thickness on the {10-11} or {11-22} facets forming first light emitting elements. The quantum well layer has a second thickness on the c-plane surfaces forming second light emitting elements. The second thickness is greater than the first thickness such that the second light emitting elements emit light at a longer wavelength than the first light emitting elements. The method further involves depositing a pattern of electrical contacts over the first light emitting elements and the second light emitting elements. The electrical contacts are operable to separately activate the first light emitting elements and the second light emitting elements.
These and other features and aspects of various embodiments may be understood in view of the following detailed discussion and accompanying drawings.
The discussion below makes reference to the following figures, wherein the same reference number may be used to identify the similar/same component in multiple figures.
The present disclosure is generally related to manufacturing of electronic devices on crystalline wafers. For example, crystalline group III and nitrogen alloy (III-N) semiconductor material such as GaN and more generally AlGaInN can be used for creating a wide variety of solid state devices, such as transistors, diodes, light-emitting diodes (LEDs), lasers, etc. In some cases, the III-N material can be grown on a growth template or substrate such as sapphire, silicon, GaN, or AlN. Under the proper growth conditions, the III-N material can achieve epitaxial growth, in which the added material has a highly regular crystalline structure. When forming a device, subsequent layers are added with different material compositions (e.g., AlGaN, GaInN, AlInN, AlGaInN) to have the electrical and/or optical characteristics desired for the device.
This disclosure relates to the challenges of accomplishing multi-wavelength light-emitting diode (LED) emission from the same integrated chip and specifically for emissions at longer wavelength, e.g., in the red spectral regime (X, =620-750 nm) using the InGaN material system. Today, red LED (and laser) emission is often realized with the AlGaInP material systems that offers decent device performance. However, multiple applications can be enumerated where the AlGaInN material system would be much more preferred. For example, this includes applications for full color displays where all the sub-pixel emissions for RGB (red, green, blue) would all be based on the same materials platform (e.g., group III-nitride). This may be the case for applications where high display resolution and a small pixel form factor are desired, such as augmented and virtual reality (AR/VR) displays, mobile (phone) displays, etc., having all colors implemented on one platform.
Efficient emission from red InGaN quantum wells is considered a particularly hard undertaking. This is mainly due to challenges related to high quality semiconductor crystal growth for the high Indium-containing films. For example, a low growth temperature T is needed for high In-incorporation and the larger lattice mismatch between GaN and InGaN with Indium concentrations of >30% becomes problematic for high crystalline quality. In addition, intrinsic material properties in III-N related to internal polarization fields (e.g., piezo-electric polarization in quantum wells, quantum confined Stark effect) reduce the radiative efficiency of such structures. The proposed implementations address these issues via an epitaxial growth concept on a pre-structured surface exposing different III-N crystal facets with the prospects of achieving high material quality for high In-containing films.
In embodiments described herein, the active zone of the longer wavelength, e.g., red emitting LED pixels using the challenging InGaN quantum wells (QWs), would be grown onto a non-planar, particularly structured surface. This is different to conventional LED fabrication where the device heterostructure is typically grown on a smooth, un-structured wafer surface. The lateral definition of individual pixel elements in conventional micro- and mini-LED fabrication is typically done via etching after the semiconductor crystal growth is completed. As described below, the wafer topology for the proposed embodiments can be realized via selective area growth prior to the growth of the light emitting layers. Choosing the proper growth conditions and mask design for the selective area growth can result in the desired topology. Relevant growth parameters to influence the growth mode between lateral and vertical directions include growth temperature, reactor pressure, and the V-to-III ratio.
For the subsequent growth of the InGaN QWs, gas phase diffusion and incorporation efficiency of the indium species strongly depend on the exposed crystallographic surfaces. As seen in the scanning electron microscope images in
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Note that the parts of the QW layers 304 parallel to the top surface 302 are thicker with a potentially higher In-content than those parts parallel to the sidewall 300. When a ternary compound such as InGaN is epitaxially grown on different surfaces such as these, various mechanisms occur and interplay. Gas phase diffusion and different incorporation probabilities for In and Ga can be expected for specific locations on the surfaces. It was found that thicker QWs 304 form on the (0001) c-plane areas versus the tilted {10-11} side facets when both are present. In addition, a higher growth rate leads to a higher In incorporation (at constant T) and the increased piezo-electric fields on the (0001) surface lead to longer emission wavelength (quantum-confined Stark effect). All of these factors result in an emission at longer wavelength (e.g., into the red part of the spectrum).
The pre-structured surface of devices built this way can be regarded as means for effectively strain-managing the highly compressively strained InGaN film (on GaN) as the material can partly relax laterally in-plane without disturbing the overall crystal quality. Furthermore, individual pixels or subpixels do not have to be structured via etching that typically induces many non-radiative recombination centers. Etching can degrade small-sized pixel elements as used in AR/VR displays.
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A pattern of electrical contacts over the display component are operable to separately activate one of the first light emitting elements or the second light emitting elements. As seen in
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In some embodiments, the second pixel/subpixels 904 will emit red, and the first pixel/subpixels 902 will emit blue and/or green. In some embodiments, the second pixel/subpixels 904 will emit green, and the first pixel/subpixels 902 will emit blue. In some embodiments, the first pixel/subpixels 902 can naturally emit blue over the entire structure. For some portion of the first pixel/subpixels 902, phosphors or quantum dot materials can be overlaid onto this portion to down-convert the blue light to green or red.
In the above embodiments, there are numerous mask designs possible for the initial three-dimensional GaN growth, some of which have been described above. However, a regular and homogeneous mask design might be preferred for devices such as micro LED displays. Growth conditions can be varied (e.g., T, V-to-III-ratio, pressure) to alter growth rates in the different crystallographic directions and, thus, have additional crystal facets exposed other than the ones enumerated in the implementations described above. Further, spacings of the mask material as well as build height of the three-dimensional structures can increase the surface areas of the c-plane regions, thus increasing a relative amount of red light emitted relative to other wavelengths emitted by the inverted pyramid facets.
Red (electro-)luminescence can also be achieved with AlGaInP materials. However, in the case of display applications the integration of dissimilar materials becomes particularly challenging (e.g., AlGaInP for red, AlGaInN for blue and green). Longer wavelength (red) emission can also be achieved through optical down-conversion (e.g., phosphors or quantum dot materials are optically pumped by blue light and emit red). Theoretically, conventional InGaN QWs on a planar growth surface could lead to the desired emission. However, conventional approaches typically result in poor device performance.
Unless otherwise indicated, all numbers expressing feature sizes, amounts, and physical properties used in the specification and claims are to be understood as being modified in all instances by the term “about.” Accordingly, unless indicated to the contrary, the numerical parameters set forth in the foregoing specification and attached claims are approximations that can vary depending upon the desired properties sought to be obtained by those skilled in the art utilizing the teachings disclosed herein. The use of numerical ranges by endpoints includes all numbers within that range (e.g. 1 to 5 includes 1, 1.5, 2, 2.75, 3, 3.80, 4, and 5) and any range within that range.
The terms “coupled” or “connected” refer to elements being attached to each other either directly (in direct contact with each other) or indirectly (having one or more elements between and attaching the two elements). Either term may be modified by “operatively” and “operably,” which may be used interchangeably, to describe that the coupling or connection is configured to allow the components to interact to carry out at least some functionality.
Terms related to orientation, such as “top,” “bottom,” “side,” and “end,” are used to describe relative positions of components (e.g., as arranged in the figures) and are not meant to limit the orientation of the embodiments contemplated. For example, an embodiment described as having a “top” and “bottom” also encompasses embodiments thereof rotated in various directions unless the content clearly dictates otherwise.
Reference to “one embodiment,” “an embodiment,” “certain embodiments,” or “some embodiments,” etc., means that a particular feature, configuration, composition, or characteristic described in connection with the embodiment is included in at least one embodiment of the disclosure. Thus, the appearances of such phrases in various places throughout are not necessarily referring to the same embodiment of the disclosure. Furthermore, the particular features, configurations, compositions, or characteristics may be combined in any suitable manner in one or more embodiment.
References to a “combination” of different elements is also meant to include each element on its own unless otherwise indicated. For example, a combination of A, B, and C may include any one of A, B, or C alone, as well as A+B, A+C, A+B+C, etc. Further, where the elements of the combinations are actions (e.g., steps of a method), the listing of actions is not meant to imply a specific order that the actions may be taken in the combination unless otherwise indicated.
The foregoing description of the example embodiments has been presented for the purposes of illustration and description. It is not intended to be exhaustive or to limit the embodiments to the precise form disclosed. Many modifications and variations are possible in light of the above teaching. Any or all features of the disclosed embodiments can be applied individually or in any combination and are not meant to be limiting, but purely illustrative. It is intended that the scope of the invention be limited not with this detailed description, but rather determined by the claims appended hereto.
