Light-emitting device

Abstract

Device successively including a substrate including a metal layer capable of reflecting a radiation; a first layer of a III/N type alloy, p-type doped, and including a first surface, opposite the metal layer, the first surface being provided with cavities; a light-emitting layer made of a III/N-type alloy, capable of generating the radiation; a second layer of a III/N-type alloy, n-type doped, having the radiation coming out therethrough; wherein a non-metallic filling material transparent in the spectral range is arranged within the cavities.

Claims

1. A light-emitting device for emitting a radiation in a spectral range, successively including: a substrate including a metal layer capable of reflecting the radiation; a first layer of a III/N type alloy, p-type doped, and comprising a first surface, opposite the metal layer, the first surface being provided with cavities; a light-emitting layer of a III/N-type alloy, capable of generating the radiation; a second layer of a III/N-type alloy, n-type doped, having the radiation coming out therethrough; wherein a filling material, non-metallic and transparent in the spectral range, is arranged within the cavities.

2. The device according to claim 1, wherein the filling material forms a planar layer extending between the metal layer and the first surface of the first layer, and the filling material is electrically conductive.

3. The device according to claim 2, wherein the filling material is an oxide, preferably selected from the group comprising indium-tin oxide, aluminum-doped zinc oxide ZnO, indium-doped zinc oxide ZnO, gallium-doped zinc oxide ZnO.

4. The device according to claim 1, wherein the filling material is flush with the first surface of the first layer, and wherein the first surface of the first layer is in contact with the metal layer.

5. The device according to claim 4, wherein the filling material is selected from the group comprising titanium dioxide TiO.sub.2, silicon dioxide SiO.sub.2, zinc oxide ZnO, aluminum-doped zinc oxide ZnO, indium-doped zinc oxide ZnO, gallium-doped zinc oxide ZnO, silicon nitride SiN, indium tin oxide.

6. The device according to claim 1, wherein the filling material forms dielectric balls, and wherein the first surface of the first layer is in contact with the metal layer.

7. The device according to claim 6, wherein the filling material is selected from the group comprising titanium dioxide TiO.sub.2, silicon nitride SiN, silicon dioxide SiO.sub.2, zinc oxide ZnO.

8. The device according to claim 1, wherein the metal layer is based on silver or on aluminum.

9. The device according to claim 1, wherein the alloy of the first and second layers and of the light-emitting layer is a binary alloy based on GaN, or a ternary alloy based on InGaN or on AlGaN.

10. The device according to claim 1, wherein the cavities have a surface density greater than 10.sup.8 cm.sup.2 at the first surface of the first layer.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0041] The foregoing and other features and advantages will be discussed in detail in the following non-limiting description of different embodiments of the invention, in connection with the accompanying drawings, among which:

[0042] FIG. 1 is a simplified perspective view of a device according to the invention,

[0043] FIG. 2 is a partial simplified cross-section view of a device according to a first embodiment of the invention,

[0044] FIG. 3 is a partial simplified cross-section view of a device according to a second embodiment of the invention,

[0045] FIG. 4 is a partial simplified cross-section view of a device according to a third embodiment of the invention,

[0046] FIG. 5 (already discussed) is a graph showing the total reflectivity of the interface between the metal layer (Ag) and the first surface of the first layer (GaN) of a device of the state of the art (axis of ordinates, in %) according to the wavelength of the emitted radiation (axis of abscissas, in nm) for different densities d of cavities (A: d=2.10.sup.9 cm.sup.2, B: d=10.sup.9 cm.sup.2, C: d=2.10.sup.8 cm.sup.2, D: d=0),

[0047] FIG. 6 is a partial simplified cross-section view of a device according to the first embodiment illustrating the metal layer,

[0048] FIG. 7 is a partial simplified cross-section view of a device according to the second embodiment illustrating the metal layer,

[0049] FIG. 8 is a partial simplified cross-section view of a device according to the third embodiment illustrating the metal layer.

DETAILED DESCRIPTION OF THE EMBODIMENTS

[0050] For the different embodiments, the same references will be used for identical elements or elements performing the same function, to simplify the description. The technical characteristics described hereafter for different embodiments are to be considered separately or according to any technically possible combination.

[0051] The device illustrated in FIGS. 1 to 8 is a light-emitting device 1 for emitting a radiation in a spectral range, the device successively comprising: [0052] a substrate 2 comprising a metal layer 20 capable of reflecting the radiation; [0053] a first layer 3 of a III/N type alloy, p-type doped, and comprising a first surface 30, opposite metal layer 20, first surface 30 being provided with cavities 300; [0054] a light-emitting layer 4 of a III/N-type alloy, capable of generating the radiation; [0055] a second layer 5 of a III/N-type alloy, n-type doped, having the radiation coming out therethrough.

[0056] A non-metallic filling material 6 transparent in the spectral range, is arranged within cavities 300. Filling material 6 is different from the material of substrate 2.

[0057] A contact pad 7 is advantageously formed on second layer 5.

[0058] Device 1 is preferably a light-emitting diode, more preferably of VTF (Vertical Thin Film) technology or TFFC (Thin Film Flip Chip) technology. The spectral range is preferably the ultraviolet or visible range, between 200 nm and 780 nm.

[0059] Metal layer 20 is made of a metallic material, the metallic material being a pure metal or a metal alloy. The metallic material is advantageously based on silver or on aluminum. Metal layer 20 may comprise sub-layers of a metallic material. Metal layer 20 forms a reflective mirror capable of reflecting the radiation. Metal layer 20 advantageously has a reflection coefficient greater than 0.8, preferably greater than 0.9, to obtain a high optical extraction efficiency. Metal layer 20 is capable of conducting an electric current. Metal layer 20 enables to bias device 1 by carrier injection.

[0060] Metal layer 20 extends at least partially in contact with filling material 6. Metal layer 20 extends between substrate 2 and filling material 6. In other words, device 1 successively comprises substrate 2, metal layer 20, filling material 6, first layer 3, as illustrated in FIGS. 6 to 8.

[0061] The alloy of first and second layers 3, 5 and of light-emitting layer 4 is advantageously a binary alloy or a ternary alloy, the binary alloy being preferably based on GaN, the ternary alloy being preferably based on InGaN or on AlGaN. Light-emitting layer 4 may comprise light-emitting sub-layers. Second layer 5 has an external surface forming an interface with the exit medium. The outer surface of second layer 5 is advantageously textured to avoid for a major part of the generated radiation to be trapped within device 1 by internal total reflections. The texturing of the outer surface of second layer 5 is preferably obtained by a selective chemical etching based on KOH. First and second layers 3, 5, and light-emitting layer 4 are preferably formed on an epitaxial growth substrate. The growth substrate is preferably made of sapphire when the spectral range is the visible range; the growth substrate is preferably made of AlGaN when the spectral range is the ultraviolet range. After the forming of metal layer 20 and transferring the assembly onto substrate 2 (host substrate), the growth substrate is preferably suppressed by laser lift-off.

[0062] Cavities 300 form hollow patterns in the shape of an upside-down pyramid having a hexagonal base (V-pits, the cross-section being V-shaped). The tops of the pyramids point towards a dislocation or a group of dislocations. Cavities 300 generally have a surface density in the range from 10.sup.8 to 10.sup.10 cm.sup.2 at first surface 30 of first layer 3. The applicant has observed that the total reflection, that is, the specular and diffuse reflection, of the radiation at the interface between metal layer 20 and first surface 30 of first layer 3 significantly decreases from a surface density in the order of 10.sup.8 cm.sup.2. Cavities 300 have a depth (that is, the pyramid height) in the order of 150 nm, and a diameter (that is, the diameter of the substantially regular hexagon forming the base of the pyramid) in the order of 100 nm.

[0063] In an embodiment illustrated in FIG. 2, filling material 6 forms a planar layer 60 extending between metal layer 20 and first surface 30 of first layer 3, and filling material 6 is electrically conductive. Filling material 6 advantageously is an oxide, preferably selected from the group comprising indium-tin oxide, aluminum-doped zinc oxide ZnO, indium-doped zinc oxide ZnO, or gallium-doped zinc oxide ZnO. The doping level is adapted according to the desired conduction level. Filling material 6 is deposited on first surface 30 of first layer 3 before the forming of metal layer 20. Planar layer 60 is advantageously obtained by chemical-mechanical polishing. Planar layer 60 prevents any direct contact between first layer 3 and metal layer 20.

[0064] In an embodiment illustrated in FIG. 3, filling material 6 is flush with first surface 30 of first layer 3, and first surface 30 of first layer 3 is in contact with metal layer 20. Filling material 6 is deposited on first surface 30 of first layer 3 before the forming of metal layer 20. Then, the flush state is preferably obtained by a chemical-mechanical polishing of filling material 6 down to first surface 30 of first layer 3. Filling material 6 is advantageously selected from the group comprising titanium dioxide TiO.sub.2, silicon dioxide SiO.sub.2, zinc oxide ZnO, aluminum-doped zinc oxide ZnO, silicon nitride SiN, indium tin oxide. Metal layer 20 is in direct contact with first layer 3 and filling material 6.

[0065] In an embodiment illustrated in FIG. 4, filling material 6 forms dielectric balls 61, and first surface 30 of first layer 3 is in contact with metal layer 20. Such dielectric balls 61 are advantageously formed by evaporation of a colloidal suspension, an example of implementation thereof being given in T. Pinedo et al, Assisted convective-capillary force assembly of gold colloids in a microfluid cell: Plasmonic properties of deterministic nanostructures, J. Vac. Sci. Technol., B 26(6), 2008, p. 2513-2519. This embodiment is advantageous since such a filling material 6 has a good flatness. A plurality of dielectric balls 61 may also occupy a cavity 300. Dielectric balls 61 advantageously have a radius in the range from 2 nm to 20 nm, so that filling material 6 protrudes or is recessed from first surface 30 of first layer 3 by a maximum distance smaller than or equal to 10 nm. A roughness of filling material 6 greater than 10 nm significantly alters the reflection of the radiation at the interface between filling material 6 and metal layer 20 by increasing the absorption. Further, such a radius of dielectric balls 61, much smaller than the size of cavities 300, enables a plurality of dielectric balls 61 to occupy a cavity 300, which enables to adapt to different sizes of cavities 300. Further, as small as possible a size of dielectric balls 61 will be searched for. Filling material 6 is advantageously selected from the group comprising titanium dioxide TiO.sub.2, silicon nitride SiN, silicon dioxide SiO.sub.2, zinc oxide ZnO.