Light-emitting-diode chip comprising a sequence of gan-based epitaxial layer which emit radiation, and a method for producing the same

[0001] The invention relates to a light-emitting diode chip comprising a GaN-based, radiation-emitting epitaxial layer sequence, to a method for fabricating the same, and to a light-emitting diode component comprising a light-emitting diode chip of this type.

[0002] The term GaN-based as used herein encompasses in particular all ternary and quaternary GaN-based mixed crystals, such as AlN, InN, AlGaN, InGaN, INAlN and AlInGaN and gallium nitride itself.

[0003] A fundamental problem in the fabrication of GaN-based light-emitting diode (LED) chips is that the maximum attainable electrical conductivity of p-doped layers, especially p-doped GaN or AlGaN layers, is not sufficient to achieve current spread over the entire lateral cross section of the chip with conventional front contact metallization, as known from LED chips made of other material systems (to maximize radiation decoupling, this type of metallization covers only a fraction of the front face).

[0004] Growing the p-type layer on an electrically conductive substrate, which would make it possible to impress a current over the entire lateral cross section of the p-type layer, does not yield an economically viable result. The reasons for this are as follows. First, the fabrication of electrically conductive, lattice-matched substrates (e.g. GaN substrates) for growing GaN-based layers is technically onerous; second, the growth of p-doped GaN-based layers on non-lattice-matched substrates suitable for undoped and n-doped GaN compounds does not yield adequate crystal quality for an LED.

[0005] In a known approach designed to combat the above problem, to effect current spread, either a contact layer permeable to the radiation or an additional layer of good electrical conductivity is deposited with substantially full areal coverage on the side of the p-type layer facing away from the substrate, and is provided with a bonding contact.

[0006] However, the first-cited proposal has the disadvantage that a substantial portion of the radiation is absorbed in the contact layer. The second proposal requires an additional process step that greatly increases production expenditure.

[0007] The object of the invention is, first, to develop an LED chip of the type cited at the beginning hereof that offers improved current spread and whose additional production expenditure is kept to a minimum. An LED component with improved heat dissipation from the active region is also to be provided.

[0008] The first-cited object is achieved by means of an LED component having the features of claim 1. Advantageous improvements are the subject matter of claims 2 to 14. The second-cited object is achieved by means of an LED component having the features of claim 15 or 16. Methods for fabricating LED chips according to the invention are the subject matter of claims 17 and 18.

[0009] In an LED according to the invention, the p-doped layer is provided on its main surface facing away from the active layer with a reflective contact metallization. A suitable reflective metal layer is, for example, an Ag-based metal layer. The term Ag-based includes all metals whose electrical and optical properties are determined substantially by Ag. They are in particular those comprising Ag as their major constituent.

[0010] On the one hand, the contact metallization advantageously produces good ohmic contact with very low electrical transition resistance to the epitaxial layer sequence. On the other hand, it advantageously exhibits high reflectivity and very low absorption within the stated spectral range. This results in high back-reflection of the incident electromagnetic radiation into the chip. This back-reflected radiation can then be coupled out of the chip through its bare sides.

[0011] In a preferred embodiment, the reflective contact metallization is composed, at least in part, of a PtAg and/or PdAg alloy.

[0012] The reflective contact metallization preferably covers more than 50%, especially preferably 100%, of the main surface of the p-doped layer facing away from the active layer. This results in current supply to the entire lateral cross section of the active region.

[0013] To promote the adhesion of the reflective contact metallization to the p-doped layer, preferably provided therebetween is a radioparent contact layer substantially comprising at least one metal from the group Pt, Pd, Cr.

[0014] As a result, the reflective contact metallization can easily be optimized with respect to both its electrical and its reflective properties.

[0015] The thickness of a contact layer of the above-cited type is advantageously 10 nm or less. The optical losses in this layer can thereby advantageously be kept especially low.

[0016] Especially preferably, the contact layer has a non-closed, particularly island-like and/or net-like structure. This advantageously enables the Ag-based reflective layer to be in direct contact, at least in part, with the p-doped layer, which arrangement has a positive effect on the electrical and optical properties.

[0017] In another advantageous embodiment, the contact layer is substantially composed of indium tin oxide (ITO) and/or ZnO and preferably has a thickness10 nm. Very good current spread accompanied by very low radiation absorption can be achieved with this type of contact layer.

[0018] It is further preferred that disposed on the reflective layer is a bondable layer, in particular substantially composed of a diffusion barrier of Ti/Pt or TiWN and of Au or Al, thus improving the bondability of the reflective contact metallization.

[0019] In a further LED chip according to the invention, the chip comprises solely epitaxial layers whose total cumulative thickness is 30 m or less. To this end, a growth substrate is removed following the epitaxial growth of the epitaxial layer sequence. The reflective contact metallization is deposited, with substantially full areal coverage, on the main surface of the p-doped epitaxial layer facing away from the n-doped epitaxial layer. The main surface of the n-doped epitaxial layer facing away from the p-doped epitaxial layer is provided with an n-contact metallization that covers only a portion of this main surface. The decoupling of light from the chip takes place through the bare region of the main surface of the n-type epitaxial layer and through the sides of the chip.

[0020] The growth substrate in this type of LED chip can be both electrically insulating and radiopaque, and therefore can advantageously be selected solely with a view toward ideal growth conditions. The particular advantage of a so-called thin-film LED chip of this kind is that there are no light losses from a substrate and radiation decoupling is improved.

[0021] A further advantage associated with the LED chip according to the invention is that the radiation-emitting active region, in which the majority of the electrical energy conducted into the chip is converted to heat energy during operation, can be disposed very close to a heat sink, and the epitaxial layer sequence can thus be thermally connected to a heat sink with practically no intermediary, only the p-doped epitaxial layer being located between them. The chip can thus be cooled very effectively, thereby increasing the stability of the wavelength of the emitted radiation. Flow voltage is advantageously reduced in the LED chip according to the invention, owing to the full-area contacting.

[0022] In the LED component according to the invention comprising an LED chip according to the invention, the chip is mounted so that its p-side, i.e., its reflective contact metallization, rests on a chip mounting surface of an LED package, particularly a leadframe or a track of an LED package.

[0023] Further advantageous embodiments of the invention will become apparent hereinbelow in connection with the exemplary embodiments described in FIGS. 1a to 2. In the drawing:

[0024] FIG. 1a is a schematic representation of a section through a first exemplary embodiment;

[0025] FIG. 1b is a schematic representation of a preferred reflective contact metallization;

[0026] FIG. 2 is a schematic representation of a section through a second exemplary embodiment.

[0027] Like or like-acting elements have been given the same reference numerals in the figures illustrating the different exemplary embodiments.

[0028] In the LED chip 1 of FIG. 1a, deposited on an SiC substrate 2 is a radiation-emitting epitaxial layer sequence 3. The latter is composed of an n-type doped GaN or AlGaN epitaxial layer 4 and a p-type doped GaN or AlGaN epitaxial layer 5. There can equally well be provided, for example, a GaN-based epitaxial layer sequence 3 having a double heterostructure, a single quantum well (SQW) structure or a multi-quantum well (MQW) structure comprising one or more undoped layer(s) 19, for example of InGaN or InGaAlN.

[0029] The SiC substrate 2 is electrically conductive and is transparent to the radiation emitted by an active region 19 of the epitaxial layer sequence 3.

[0030] Deposited with substantially full areal coverage on epitaxial layer sequence 3, on its p-side 9 facing away from SiC substrate 2, is a reflective, bondable, Ag-based contact metallization 6. This is, for example, composed substantially of Ag, a PtAg alloy and/or a PdAg alloy.

[0031] As shown schematically in FIG. 1b, however, the contact metallization 6 can also be composed of a radioparent first layer 15 (starting from epitaxial layer sequence 3) and a reflective second layer 16.

[0032] The first layer 15 is, for example, composed substantially of Pt, Pd and/or Cr and has a thickness of 10 nm or less to keep radiation absorption to a minimum. Alternatively, it can be made of indium tin oxide and/or ZnO. In this case its thickness is preferably 10 nm or more, since these materials exhibit very little radiation absorption. The greater thickness is advantageous for current spread.

[0033] The second layer 16 is, for example, composed substantially of Ag, a PtAg alloy and/or a PdAg alloy.

[0034] To improve bondability, an additional metal layer 20 is deposited on the Ag-based layer. This additional layer is composed of Au or Al, for example. A layer of Ti/Pt or TiWN can be provided as a diffusion barrier 24 between the second layer 16 and the additional metal layer 20.

[0035] The SiC substrate 2 is provided on its main surface 10 facing away from epitaxial layer sequence 3 with a contact metallization 7 that covers only a portion of this main surface 10 and is realized as a bond pad for wire bonding. The contact metallization 7 is, for example, composed of an Ni layer deposited on the SiC substrate 2, followed by an Au layer.

[0036] The chip 1 is mounted by die bonding with its p-side, i.e., with the reflective contact metallization 6, on a chip mounting surface 12 of a leadframe 11 of an LED package. The n-contact metallization 7 is connected via a bonding wire 17 to a connecting part 18 of the leadframe 11.

[0037] The decoupling of light from the chip 1 takes place through the bare region of the main surface 10 of the SiC substrate 2 and through the sides 14 of the chip.

[0038] The chip 1 optionally comprises an SiC substrate 2 that is thinned after the growth of the epitaxial layer sequence 3 in order to optimize the thickness of the substrate 2 with regard to the absorption and decoupling of radiation.

[0039] The exemplary embodiment shown in FIG. 2 differs from that of FIG. 1a, on the one hand, by the fact that the chip 1 comprises solely epitaxial layers, i.e., epitaxial layer sequence 3 and no substrate layer. The latter was removed, for example by etching and/or grinding, after the growth of the epitaxial layers. The chip height is about 25 m.

[0040] The advantages of a so-called thin-film LED chip of this type are recited in the general part of the description. On the other hand, the epitaxial layer sequence 3 has a double heterostructure, a single quantum well (SQW) structure or a multi-quantum well (MQW) structure comprising one or more undoped layer(s) 19, for example of InGaN or InGaAlN.

[0041] The chip 1 is mounted by die bonding with its p-side, i.e., with the reflective contact metallization 6, on a chip mounting surface 12 of a track 22 of an LED package 21. The n-contact metallization 7 is connected via a bonding wire 17 to a further track 23.

[0042] Naturally, the description of the invention with reference to the above exemplary embodiments is not to be construed as limiting it thereto. On the contrary, the invention can be used in connection with all LED chips in which the epitaxial layer, remote from a growth substrate, has insufficient electrical conductivity.