The patent's assignee for patent number 8723207 is
News editors obtained the following quote from the background information supplied by the inventors: "The present invention relates to a radiation-emitting optical component, in particular a radiation-emitting optical component having a radiation-emitting semiconductor component, which is encapsulated by a transparent material.
"Known radiation-emitting semiconductor components have a layer structure which contains an active layer which, in operation, emits radiation with a certain spectral distribution. Electrical contacts are used to apply a current to the layer structure.
"It is customary to embed radiation-emitting semiconductor components of this type in a casting material which is transparent at the relevant useful wavelengths. However, particularly in the case of blue and white light-emitting diodes, the problem arises that after a prolonged operating time, the light-emitting diodes display a considerable decrease in brightness.
"The light loss is caused by ageing of the casting resin, which is subject to brown discoloration after a certain operating time of the light-emitting diode. This effect considerable shortens the service life of blue and white light-emitting diodes.
"It is in principle possible to slow the ageing of the casting resin. For example, unpublished tests carried out by the applicant have established that adding a stabilizer to the epoxy resins used as casting material delays the brown discoloration. The service life of blue and white light-emitting diodes can as a result be approximately doubled by the addition of a stabilizer.
"An alternative approach consists in replacing the epoxy resins of the casting material with more stable polymers. In this context, silicons, whose polymer network is composed predominantly of the very stable Si--O bonds, are particularly recommended. However, silicons entail a range of other disadvantages. They have poor adhesion and a very high expansion coefficient, which lead to considerable stresses during heating and cooling.
"On account of the poor adhesion, foreign substances can easily penetrate at boundary surfaces, leading to reliability problems. Moreover, silicons have a soft consistency, which can lead to problems in what are known as pick-and-place processes. Finally, silicons have a lower refractive index than hydrocarbon-based polymers, which leads to a lower light yield when they are used in light-emitting diodes."
As a supplement to the background information on this patent, VerticalNews correspondents also obtained the inventors' summary information for this patent: "One object of the present invention is to provide a radiation-emitting optical component of the type described above which has a long service life.
"This and other objects are attained in accordance with one aspect of the present invention directed to a radiation-emitting semiconductor component, having a layer structure that includes an active layer which, in operation, emits radiation with a spectral distribution, electrical contacts for applying a current to the layer structure, and a coating layer which at least partially surrounds the active layer and holds back a short-wave component of the emitted radiation.
"Another aspect of the invention is directed to a radiation-emitting optical component having a above-described radiation-emitting semiconductor component, in which the semiconductor component is encapsulated by a material which is transparent to the longer-wave component of the emitted radiation, and the coating layer is arranged between the active layer of the semiconductor component and the transparent encapsulation material.
"In addition to a layer structure which includes an active layer which, in operation, emits radiation of a spectral distribution and electrical contacts for applying a current to the layer structure, a radiation-emitting semiconductor component according to the invention has a coating layer which at least partially surrounds the active layer and holds back a short-wave component of the emitted radiation.
"In the context of the present invention, a coating layer is understood as meaning an optically substantially transparent layer which absorbs and/or reflects in the short-wave spectral region, in particular in the wavelength range below 430 nm.
"The invention is therefore based on the concept that high-energy photons from the short-wave component of the radiation emitted by the active layer have a substantial influence on the ageing of the casting material. Accordingly, the invention provides a coating layer which holds back the short-wave component of the emitted radiation, so that only a relatively small amount or even none of this component reaches the casting material.
"The coating layer can completely surround the active layer. However, for many applications it is sufficient if the coating layer only partially surrounds the active layer. By way of example, the short-wave radiation may already be prevented from entering the coating layer in some directions by bond pads, an absorbent substrate, a mounting base, housing parts or the like.
"Furthermore, it may even be necessary for certain regions of the active layer not to be surrounded or at least not to be directly surrounded, for example in order to avoid an electrical short circuit of a p/n junction in the case of an electrically conductive coating layer which is applied direct to the layer structure.
"Without wishing to be tied down to a specific explanation, it is assumed that the necessary precondition for damage caused by the action of light is the absorption of high-energy photons which are able to break relevant bonds during the ageing process of the irradiated, material.
"Relevant bonds include on the one hand the bonds of the increments which make up the polymer network (e.g. C--C, C--H, C--O, C--N). The associated bond energies are 330-380 kJ/mol. This means that these bonds can only be broken by photons with a wavelength below approximately 350 nm. However, such high-energy photons are practically not present in the spectrum of standard blue light-emitting diode chips.
"It is therefore assumed that the crucial damaging influence is derived from photons with a wavelength of below approximately 430 nm, in particular below approximately 420 nm. Such photons can break open hydroperoxides occurring in the ageing sequence, whose bond energy is approximately 270-300 kJ/mol.
"Typical blue light-emitting diodes with a dominant wavelength (i.e. the peak wavelength resulting from a convolution of the emission characteristic with the eye response curve) of approximately 460 to 470 nm only emit a small proportion of their overall capacity, generally no more than a few percent, in the spectral range below 400 nm.
"Converter substances which absorb in the blue spectral region and emit in the yellow spectral region are used to generate white light with the aid of blue light-emitting diodes. The conventional converter substances in this case have an absorption maximum at approximately 450-470 nm, whereas radiation in the range from 350 to 430 nm is scarcely absorbed.
"Therefore, the holding back of the high-energy and therefore short-wave photons, in the case of blue or white light-emitting diodes of photons with a wavelength below 430 nm, in particular below 420 nm, has no significant adverse effects.
"On the other hand, it can be demonstrated that the service life of blue and white light-emitting diodes is significantly increased by the holding back of the high-energy photons.
"A preferred configuration of the radiation-emitting semiconductor component has a coating layer which absorbs a short-wave component of the emitted radiation. This does not depend upon complete absorption; in some applications, to sufficiently extend the service life, it is sufficient merely to reduce the amount of radiation allowed to pass through.
"Rather, it is generally important to find a balance between a high transmission through the coating layer in the longer-wave emission range of the active layer, which is used as useful radiation, and a high absorption in the short-wave radiation range.
"In this configuration, the coating layer acts as a cut-off mass filter, but because the absorption coefficient does not generally change erratically, as a filter with a relatively shallow profile at the absorption edge. For this purpose, the use of an absorbent coating layer offers the advantage that there is no need to accurately set the layer thickness for the filter function, making the application of such a layer easy and inexpensive. Furthermore, the transmitted radiation does not have any pronounced angle dependency or polarization.
"Another preferred configuration of the radiation-emitting semiconductor component has a coating layer which reflects a short-wave component of the emitted radiation. In particular, it is expedient to form the coating layer by means of a layered stack of dielectric layers. However, metal and semiconductor layered stacks are also suitable for the reflective coating layer.
"By suitably selecting the layer materials and layer thicknesses, it is possible for an interference filter of this type to be formed as a sharp-edged interference cut-off filter, in which case the position of the absorption edge can readily be defined in such a way that the transmission in the desired emission range of the component is virtually unimpaired by the filter element, but at the same time the undesired short-wave component is efficiently held back.
"The reflection of the short-wave radiation back into the active layer also has the advantage that the reflected photons contribute to the emission again via photon recycling, i.e. do not, as in the case of an absorbent coating layer, lead to the latter being heated.
"A further benefit of this solution consists in its ease of integration in conventional production sequences. For example, an interference cut-off filter system can be applied and patterned, together with a standard passivation layer, by means of tried-and-tested processes.
"In a preferred configuration of the radiation-emitting semiconductor component, the coating layer substantially completely covers a main surface of the layered stack.
"Covering a main surface, for example the outwardly facing top face of the active layer, is often sufficient to reduce the amount of short-wave radiation which reaches the casting resin to an acceptable level. This is true in particular for conventionally constructed radiation-emitting semiconductor components, which emit the majority of the radiation via the component upper side (main surface). Regions of the main surface which are provided with connection pads and the like can be left free of coating.
"If possible, it is also possible for the side faces of the active layer to be provided with the coating layer. In embodiments in which the radiation emitted through a carrier substrate is also utilized, it may be expedient for a coating layer also to be applied to the substrate. In this case, the active layer is substantially completely surrounded by the coating layer.
"In a particularly expedient configuration, the coating layer applied to the main surface is electrically conductive. It can then fulfill a dual role, and as well as holding back the short-wave radiation can at the same time function as part of a first of the contacts for applying a current to the layer structure, in particular as a current spreading layer. It is therefore overall possible to dispense with one layer, making the component easier and less expensive to produce.
"In a preferred configuration of the radiation-emitting semiconductor component, the active layer emits radiation in the blue and ultraviolet spectral regions, and the coating layer holds back radiation in the ultraviolet spectral region.
"For the reasons explained in detail above, it is particularly expedient if the coating layer holds back radiation with a wavelength below approximately 430 nm, preferably below approximately 420 nm.
"The invention also comprises a radiation-emitting optical component with a radiation-emitting semiconductor component as described above, in which the semiconductor component is encapsulated by a material which is transparent in the longer-wave component of the emitted radiation, and the coating layer is arranged between the active layer of the semiconductor component and the transparent encapsulation material.
"The transparent material is preferably selected from the group consisting of epoxy resin, epoxy molding compounds and acrylates.
"The invention is particularly suitable for blue or white light-emitting diodes, since the encapsulation materials which are usually used in these applications are often subject to brown discoloration on account of the short-wave radiation component.
"At the same time, the invention can of course be used not only in light-emitting diodes which radiate incoherent light but also in laser diodes.
"Suitable materials for the active layer in the blue spectral region are in particular nitride-based III-V semiconductor material systems, in particular GaN, InN and AlN and the ternary and quaternary mixed crystals which are based thereon, such as for example AlGaN, InGaN, AlInN or AlGaInN.
"The growth substrates used may be insulating substrates, such as sapphire, or conductive substrates, such as SiC, Si, ZnO, GaAs or GaP. GaN single crystal wafers are also suitable for this purpose.
"Further advantageous configurations, features and details of the invention will emerge from the dependent claims, the description of the exemplary embodiments and the drawings."
For additional information on this patent, see: Jacob, Ulrich; Krauter, Gertrud; Plossl, Andreas. Radiation-Emitting Optical Component. U.S. Patent Number 8723207, filed
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