OPTOELECTRIC ELEMENT

Abstract: A LED (D) is covered with an electroconductive layer (EL), and a luminescent layer (LU) is deposited on the electroconductive layer (EL) by means of electrophoresis. The conductivity of the electroconductive layer (EL) is chosen to be such that the layer can be used as one of the electrodes during the electrophoresis, while the LED (D) is not short-circuited by said layer during normal operation.

Description: Optoelectric element The invention relates to an optoelectric element comprising a semiconductor body provided on a substrate formed from an electrically insulating material and covered with a luminescent layer for converting electromagnetic radiation of a first wavelength range generated by the semiconductor body into visible light of a different wavelength range. The invention also relates to a method of covering an optoelectric element with a luminescent layer. ' An optoelectric element as mentioned in the opening paragraph is disclosed in US 5,813,752. In the known optoelectric element, the semiconductor body and the substrate form part of a LED. The luminescent layer converts UV light or blue light generated by the LED into visible light of a different wavelength range. In this manner, such LEDs can be made suitable for use in different applications requiring visible light of different colors. The problem is, however, that it is often difficult to apply the luminescent layer onto the optoelectric element in such a manner that a closely contacting layer of a readily controlled and substantially uniform thickness is obtained. Therefore, it is an object ofthe invention to provide an optoelectric element comprising a closely contacting luminescent layer of uniform thickness, which can be provided in a comparatively simple manner. To achieve this, an optoelectric element as described in the opening paragraph is characterized in accordance with the invention in that an electroconductive layer is situated between the substrate and the luminescent layer, the electric conductivity X of which is chosen to be such that, during operation ofthe optoelectric element, the current through the electroconductive layer is at most 5% ofthe current through the semiconductor element. In practice, the conductive layer usually connects different electrodes ofthe optoelectric element to each other. The electric conductivity of this layer is chosen to be so low that the layer does not cause a short-circuit between the electrodes ofthe optoelectric element, and that the functioning ofthe optoelectric element is hardly adversely affected. Surprisingly, it has been found that, at the same time, the conductivity is high enough to deposit luminescent material from a suitably chosen slurry by means of electrophoresis, in which process the electroconductive layer serves as one ofthe electrodes. In this manner, a closely contacting luminescent layer forming part ofthe optoelectric element can be readily provided in a uniform thickness, the optoelectric properties ofthe optoelectric element not being adversely affected by the conductive layer. It has been found that an optoelectric element in accordance with the invention can be embodied such that, during operation ofthe optoelectric element, the current through the electroconductive layer is at most 1 0/00 ofthe current through the semiconductor element. An optoelectric element in accordance with the invention has a comparatively high efficiency if the electroconductive layer is transparent to electromagnetic radiation generated by the semiconductor element. Favorable results were achieved using embodiments of an optoelectric element in accordance with the invention, wherein the electroconductive layer comprises a transparent oxide, more particularly an oxide selected from the group formed by indium tin oxide, antimony tin oxide and tin oxide. Favorable results were also achieved using embodiments of an optoelectric element in accordance with the invention, wherein the semiconductor body forms part of an LED. A method, as mentioned in the second paragraph of this description, for covering an optoelectric element comprising a semiconductor body provided on an electrically insulating substrate, which method comprises, in accordance with the invention, a step wherein the substrate is covered with an electroconductive layer, after which at least the substrate is brought into contact with a suspension of a luminescent material, which luminescent material is deposited by means of electrophoresis on the surface ofthe electroconductive layer, said electroconductive layer serving as a first electrode, and a second electrode being present in the suspension, a potential difference between the electrodes being maintained, the electric conductivity X ofthe electroconductive layer being higher than that ofthe suspension and lower than that ofthe semiconductor element. It has been found that the method can very suitably be used to cover an optoelectric element comprising an LED. It has also been found that the method in accordance with the invention can be advantageously applied to an optoelectric element connected to a carrier plate on which also a number of optoelectric elements are provided, so that all these optoelectric elements are simultaneously covered with a luminescent layer. These and other aspects ofthe invention will be apparent from and elucidated with reference to the embodiment(s) described hereinafter. In the drawings: Fig. 1 shows an example of a step in a method in accordance with the invention, and Fig. 2 shows an example of an optoelectric element in accordance with the invention. In Fig. 1, LF denotes a carrier plate of an electroconductive material. A denotes a heat sink, which is provided in a hole in the carrier plate LF and which is made from a metal. On the heat sink A there is provided a submount B composed of a Si wafer covered with a layer of aluminum. Ends ofthe submount B are connected to respective bonding wires, which are each connected to the carrier plate LF. D denotes a LED, which comprises a substrate made from an electrically insulating ceramic material on which a number of epitaxial layers consisting of a semiconductor material are provided. These epitaxial layers jointly form a semiconductor body. C denotes solder balls forming electric contacts between LED D and submount B. ST denotes a droplet of a suspension S of a luminescent material. The droplet ST is in contact with a part ofthe outside surface of heat sink A, submount B, contacts C and LED D. The part ofthe outside surface of heat sink A, submount B, contacts C and LED D contacting the suspension is provided with an electroconductive layer EL, the electric conductivity of which is chosen to be higher than that ofthe suspension, yet lower than that ofthe semiconductor body. The suspension is displaced by means of a pump in a direction indicated by means ofthe arrows, so that the part ofthe suspension that is near to the surface ofthe LED D is renewed continuously. A cathode of a voltage source Vg is connected via the carrier plate LF and via the bonding wires BD to the electroconductive layer, so that this layer forms a cathode which is in contact with the suspension. An anode of voltage source Vg is connected to an electrode E placed in the suspension. Under the influence of a voltage V generated by the voltage source, a layer of a luminescent material is deposited by means of electrophoresis on the surface ofthe electroconductive layer. In a practical embodiment ofthe step ofthe method shown in Fig. 1, the electroconductive layer is made of antimony tin oxide and has a thickness of approximately 50 nm. This electroconductive layer is provided by moistening the surface ofthe optoelectric element with a solution of antimony tin oxide. The optoelectric element comprises a sapphire substrate on which a semiconductor body is provided consisting of epitaxial layers AlGalnN. If a current flows through this semiconductor body, then blue light is generated. The suspension used contains strontium phosphate. The electric conductivity of this suspension is approximately 300 pS/m. It has been found that, under the influence of a voltage of 200 volts, the surface ofthe LED D is electrophoretically covered with a luminescent layer of approximately 50 μm in 50 seconds. In Fig. 2, parts corresponding to parts shown in Fig. 1 bear the same reference numerals. The electroconductive layer EL covers a part ofthe outside surface of heat sink A, submount B, contacts C and LED D. The electric layer EL in turn is completely covered with a luminescent layer LU. As a result, not only the surface ofthe LED D facing away from the heat sink is covered with luminescent material but also the side surfaces extending perpendicularly thereto. To influence the direction in which the light generated by the luminescent layer is emitted, a body in the form of a hemisphere is provided on the LED D, which hemispherical body is provided with a wall E which is transparent to visible light, and with a filling F which is also transparent to visible light. If a voltage is applied between the ends ofthe bonding wires contacting the carrier plate LF, the LED D generates electromagnetic radiation of a first wavelength range, which is converted by the luminescent layer LU to visible light of a different wavelength range. By virtue ofthe presence ofthe wall E and the transparent filling F, the light issues substantially in the direction ofthe longitudinal axis ofthe optoelectric element.

Claims: CLAIMS: 1. An optoelectric element comprising a semiconductor body provided on a substrate formed from an electrically insulating material and covered with a luminescent layer for converting electromagnetic radiation of a first wavelength range generated by the semiconductor body into visible light of a different wavelength range, characterized in that an electroconductive layer is situated between the substrate and the luminescent layer, the electric conductivity X of which is chosen to be such that, during operation ofthe optoelectric element, the current through the electroconductive layer is at most 5% ofthe current through the semiconductor element. 2. An optoelectric element as claimed.in claim 1, wherein, during operation of the optoelectric element, the current through the electroconductive layer is at most 1 0/00 of the current through the semiconductor element. 3. An optoelectric element as claimed in claim 1 or 2, wherein the electroconductive layer is transparent to the electromagnetic radiation generated by the semiconductor element. 4. An optoelectric element as claimed in claim 1, 2 or 3, wherein the electroconductive layer comprises a transparent metal oxide. 5. An optoelectric element as claimed in claim 1, 2, 3 or 4, wherein the electroconductive layer comprises one or more oxides chosen from the group formed by indium tin oxide, antimony tin oxide and tin oxide. 6. An optoelectric element as claimed in any one ofthe preceding claims, wherein the semiconductor body forms part of an LED. 7. A method for covering an optoelectric element with a luminescent layer, which optoelectric element comprises a semiconductor body provided on a substrate of an electrically insulating material, in which method the substrate is covered with an electroconductive layer, after which at least the electroconductive layer is brought into contact with a suspension of a luminescent material, which luminescent material is deposited by means of electrophoresis on the surface ofthe electroconductive layer, said electroconductive layer serving as a first electrode, and a second electrode being present in the suspension, a potential difference between the electrodes being maintained, the electric conductivity X ofthe electroconductive layer being higher than that ofthe suspension and lower than that ofthe semiconductor element. 8. A method as claimed in claim 7, wherein the optoelectric element comprises a LED. 9. A method as claimed in claim 7 or 8, wherein the optoelectric element is connected to a carrier plate on which also a number of optoelectric elements are provided.

Inventors: VAN LETH NICOLAAS J M, VERHOECKX GODEFRIDUS J, VISSER CORNELIS G, VAN LETH, NICOLAAS, J., M., VERHOECKX, GODEFRIDUS, J., VISSER, CORNELIS, G.
Applicants: KONINKL PHILIPS ELECTRONICS NV, KONINKLIJKE PHILIPS ELECTRONICS N.V.
Application year: 2001
Main classification: H01L33
IPC codes: C09K11, H01L33