FI122622B - Light-emitting semiconductor device and method of manufacture - Google Patents

Description

FIELD OF THE INVENTION FIELD OF THE INVENTION 1. Light Emitting Semiconductor Device and Method of Manufacture

The present invention relates generally to light emit-5 desirable semiconductor structures made from nitrides of Group III metals. More particularly, the invention relates to reflective contacts used, for example, in light emitting diodes (LEDs) of vertical geometry, and to the manufacture thereof.

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BACKGROUND OF THE INVENTION

Light-emitting semiconductor devices such as LEDs are playing an ever-increasing role in various areas of everyday life. They have a wide variety of applications 15 in areas such as telecommunications, lighting and display technology.

With regard to materials, a rapidly growing area of modern LED technology is based on nitrides of Group III metals such as gallium nitride GaN, aluminum gallium nitride AlGaN, indium um gallium nitride and their alloys. These materials are considered particularly suitable alternatives for use in very bright LEDs, for example in lighting applications.

25 In the intensive development of increasingly efficient LED structures, one of the prevailing technological trends in recent years has been a shift towards vertical LED geometry instead of the previously horizontally distributed structure. The two electrodes, O) 30, which form electrical connections to the n and x £ p-type layers of the component, were previously formed on about the same side of the LED chip horizontally apart

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one another. This configuration imposed several restrictions on CT.

o the operation of the device. Vertical LED geometry, where the contact electrodes are on opposite sides of the chip, can provide significant advantages in both current 2 uniformity and light extraction efficiency as well as thermal control of the device.

The simplified general principle of vertical LED manufacturing can be described as Example-5 as follows. First, a layer structure comprising, for example, a n-type semiconductor layer, an active region, and a p-type semiconductor layer is formed on a growing substrate made of sapphire. Next, a reflective contact structure 10 is formed over the structure, which provides an electrical connection to the p-type semiconductor layer and acts as a mirror to reflect the incident light back in the active region. Then, the reflective contact surface, a relatively thick metal layer 15, which forms the chip to the p-side contact electrode. The thick metal layer can also serve as a support structure for the finished LED chip. Next, the growth support is removed, whereupon the surface of the n-type layer originally grown on the culture substrate is exposed. Finally, this is formed on the exposed surface of the second thick metal layer which forms the n-side contact electrode.

One important point in the vertical LED structure is the mentioned reflective contact structure. 25 of the desired high reflectivity and low electrical retro-reflective sistiivisyyden addition to self-kontaktiraken tea should also remain stable throughout the ^ component life and, secondly, to produce good adhesion to the p-side electrode forming the metal layer and the actual 30 of the functional layers of the device. Conventional solutions include, for example, various metal combinations, such as one or more layers of aluminum or gold, directly deposited on or spaced over a p-Is »S-type semiconductor layer, but in an adhesive layer first formed in cm of semiconductor. surface. In the first case, both the adhesive strength and the long-term durability of the structure are generally insufficient. An intermediate adhesive layer, for example made of nickel, can improve adhesion. On the other hand, it increases the optical loss as light is absorbed into the adhesive layer.

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PURPOSE OF THE INVENTION

The object of the present invention is to provide a novel light-emitting semiconductor structure comprising a stable, reflective contact structure having excellent mechanical, optical and electrical properties.

SUMMARY OF THE INVENTION

The light emitting semiconductor device of the present invention and the method for manufacturing a light emitting semiconductor device 15 are known from those set forth in claims 1 and 8, respectively.

The light-emitting half conductor device of the present invention is made of Group III metal nitrides. One suitable material is gallium nitride 20 GaN and various variants thereof, such as indium gallium nitride InGaN and aluminum gallium nitride AlGaN. "Made of" said materials means that at least one of said materials is functionally relevant to the device. Of course, different parts of the device may also contain materials not included in the definition. The core component of the device is a layer structure having an n-cm type semiconductor layer, a p-type semiconductor layer and an active region for producing light between an n-type i σ> 30 semiconductor layer and a p-type semiconductor layer χ. The details of this sandwich structure may vary within the prior art and are not essential to the invention. Floor structure

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en is either a n-type or a p-type semiconductor layer 35 defined by its contact surface, and a reflective contact structure attached thereto. The reflective contact structure 4 provides an electrical contact to the semiconductor layer defining the contact surface and also acts as a mirror to reflect light from the active region.

According to the present invention, the reflective contact structure comprises a first transparent conductive oxide (TCO) contact layer having a polycrystalline structure attached to the contact surface of the sandwich structure, a second transparent conductive oxide (TCO) 10 contact layer having an amorphous structure and a second A metal reflective layer attached to the TCO layer.

Said structure, in which the reflective metal contact utilizes a two-layer TCO intermediate structure between the 15 metal reflective layers and the contact surface, offers significant advantages over prior art solutions. Each of the two TCO layers has its own purpose. The polycrystalline material structure of the first TCO layer provides high optical transparency and low electrical resistivity. The second TCO layer having an amorphous structure, in turn, allows high adhesion to the metallic reflection layer.

In addition to the various crystal structures, the precise chemical compositions of these two layers can also be optimized individually according to their different purposes. To take full advantage of this opportunity, in a preferred embodiment of the present invention, the chemical composition of the first TCO-30 contact layer is selected such that it promotes high adhesion to the Koni tactile layer, good transparency and good electrical conductivity of the first TCO contact layer. The chemical composition of its TCO contact layer is selected to promote high adhesion of the metallic reflection layer to the second TCO contact layer. In other words, in this embodiment, the properties of the two TCO layers are optimized separately for their different purposes.

The second TCO contact layer may be in direct contact with the first TCO contact layer. However, it is possible that there is some intermediate layer (s) between the two TCO contact layers.

Preferably, the contact surface determining layer is p-type indium gallium nitride InGaN. The first TCO contact layer, in turn, is preferably indium tin oxide, which, thanks to indium, can provide excellent adhesion to Group III metals. In order to ensure efficient current distribution over the entire surface area of the reflective contact structure and a good resistivity of the contact which does not significantly affect the total series resistance through the LED chip, the thickness of the first TCO contact layer is preferably 30-500 nm, more preferably 100-150 nm. Too little thickness would result in insufficient electrical properties of the layer. On the other hand, too thickening of this layer would undesirably increase the undesired absorption of light generated in the active region.

In a preferred embodiment of the present invention, a strong adhesion between the second TCO-25 contact layer and the reflective metal layer is ensured such that the second TCO contact layer comprises aluminum zinc oxide (AZO) and the metal reflective layer comprises aluminum deposited on the second TCO contact layer. These are of course just 30 examples of possible materials. The material of the $ 2 metallic reflection layer is also well suited for example to silver.

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Because the amorphous TCO has a lower optimum.

gJ transparency and electrical conductivity as with polycyclic <35 TCO, pack of second TCO contact layer

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^ the mouth must be restricted. On the other hand, too little thickness could further weaken the electrical conductivity 6 and possibly also the adhesion to the metal reflective layer above. A preferred thickness range is from 0.2 to 20 nm, more preferably from 1 to 3 nm.

The reflective metal layer may have a thickness of, for example, 20 to 1000 nm, but preferably at least 200 nm, to ensure that light does not penetrate through the layer, thereby maximizing the reflectivity of the reflective contact structure.

To protect the reflective metal layer 10 from oxidation during other processing of the device, a metallic reflective anti-oxidation layer having a thickness of 1 to 20, preferably 5 to 10 nm, may be deposited on the metal reflective mirror surface.

In the process of this invention for preparing a light emitting semiconductor structure made of nitrides of Group III metals, a layer structure having an n-type semiconductor layer, a p-type semiconductor layer and an active region between an n-type semiconductor layer and a p-type semiconductor layer is prepared. sa is the contact surface defined by the n-type or p-type semiconductor layer. The sandwich structure can be manufactured, for example, by standard, commonly used gas phase epitaxial processes well known in the LED industry. Therefore, no detailed description of the manufacturing processes is required here. The method

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In addition, a reflective contact structure is formed on the contact surface.

cp 30 According to the present invention, the formation of a reflective contact structure comprises the steps of forming a first transparent conductive oxide.

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(TCO) with a polycrystalline ra-

The structure, on the contact surface of the sandwich structure, is formed

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g 35 a second transparent conductive oxide (TCO) contact layer having an amorphous structure and forming a metallic reflection layer on the second TCO layer.

The TCO contact layers can be deposited, for example, by sputtering. As a layered sel-5, TCO is amorphous. Therefore, the first TCO contact layer needs to be heat treated to change the phase, i.e. to crystallize the initially amorphous layer. A suitable temperature range for the heat treatment, for example, depending on the exact material composition, can be, for example, 150-300 ° C. The second TCO contact layer may be deposited directly on the first TCO contact layer or on one or more intermediate layers first coated on a first TCO contact layer.

In the process of the present invention, the chemical composition of the first TCO contact layer is preferably selected to promote high adhesion to the contact surface of the layer structure, good transparency and good electrical conductivity of the first TCO contact layer 20 and the second TCO contact layer to promote the reflective metal layer. high adhesion to the second TCO contact layer. In other words, in this embodiment, the properties of the two TCO layers are optimized separately according to their different functions. The manner in which said material composition choices are made in practice to achieve said properties depends, for example, on the materials of the semiconductor layer structure. However, this is also a routine technique for one skilled in the art.

The layer defined by the contact surface preferably comprises p-type indium gallium nitride InGaN.

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One preferred material for the first TCO's.

The contact layers comprise indium tin oxide.

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The first TCO contact layer is preferably manufactured to have a thickness of 30 to 500 nm, more preferably 100 to 150 nm.

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In a preferred embodiment, the second TCO contact layer comprises aluminum-zinc oxide, and in the step of forming a metallic reflection layer, aluminum is deposited on the second TCO-contact layer.

The second TCO contact layer is preferably fabricated to have a thickness of 0.2 to 20 nm, more preferably 1 to 3 nm. Preferably, the metallic reflection layer is made to have a thickness of 20 nm to 1000 nm, more preferably, however, at least 200 nm.

In addition to the manufacturing steps described above related to the core principles of this invention, the entire manufacturing process of structures that provide electrical contact to the semiconductor conductive layer defining a contact surface may also include layering a plurality of other layers. First, to protect the metallic reflection layer from oxidation during subsequent process steps, an anti-oxidation layer, e.g. of gold 20-20, preferably 5-10 nm, of gold, for example, can be deposited on the metal reflecting layer. Next, an adhesive layer may be formed on the anti-oxidation layer, for example, to improve adhesion of the subsequent layers to the reflective contact structure. A diffusion barrier layer may then be coated to protect the reflective metal layer from a potentially aggressive contact pad forming the final surface electrode of the component.

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^ from metal diffusion. Finally, a forced 30-layer of solderable metal may be deposited to form said bonding substrate, for example, by galvanic plating. Suitable solderable metals include, for example, Q-

Au, Au / In mixture and Cu. Of said coating steps

In addition to the process, of course, the process may

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g 35, such as lithography to achieve the desired device geometry.

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On the other hand, the entire manufacturing process of a light emitting device, such as a light emitting diode of vertical geometry, an LED, may also require many other steps. These include, for example, removal of the growth substrate, for example by chemical etching, and the formation of electrical contact structures on the opposite side of the semiconductor device so exposed.

The manufacturing process 10 of the present invention is suitable for the cost-effective mass production of light emitting devices which can process several tens of discs simultaneously.

BRIEF DESCRIPTION OF THE FIGURES

In the following, the present invention will be further described with reference to the accompanying drawings, in which Figure 1 is a schematic overview of a vertical LED according to the present invention, and Figures 2a to 2f illustrate a manufacturing method according to the present invention. In the drawings, the corresponding layers are denoted by the same reference numerals. The drawings are not to scale.

DETAILED DESCRIPTION OF THE INVENTION

The vertical LED chip 1 of Figure 1 is based on a he-25 terror structure comprising an electron emitter layer 2 made of n-doped GaN, an aperture emitter layer 3 made of p-doped InGaN and an active region 4 light iq. to form between the two layers. Opening o) Below the 30 testers is a reflective contact structure x 6. The next layer down is an antioxidant layer 7, which protects the reflective contact structure cm from oxidation during the final stage of the manufacturing process after the reflective contact structure has been laid 35. The antioxidant layer is made of Au and has a thickness of about 5 nm. Below the antioxidant layer 10 is a Ti adhesive layer 8 which forms a strong bond between the layers above and below it. In the chip of Figure 1, the bottom layer is a metallic base layer 9 separated from the rest of the device by, for example, a diffusion barrier layer 10 formed therefrom which protects the upper device layers from diffusion of the metal atoms of the metallic base layer. The metal base layer serves as a p-side electrode, delivers the second of two electronic connection 10, which is required of the LED chip to electrically connect to an external power source. It also provides an efficient route for transferring excess heat away from the chip. In addition, it also serves as a protective and mechanical support structure for the chip. Through the metallic base layer 15, the chip can be attached to an electrically and thermally conductive pad (pad) on a circuit board or the like, for example by soldering. The metallic base layer may be formed of, for example, Au, Au / In, Cu or any other solderable metal, and may have a thickness of from 2 to 200 µm.

The surface of the n-doped GaN layer 2 above the chip of Figure 1 is structured to have an uneven surface topology. The roughened surface 25 reduces the total internal reflection of light 11 from the active region 4 on the surface of the device, thereby improving light extraction from the chip. For this uneven C \ | There is a mesh metal layer 12 on the surface which forms the second contact electrode of the chip.

As shown in Figure 1, the reflective contact structure 6 of this example contains three sub-layers.

g Next to the hole emitter layer is the first TCO-

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a layer 13 formed of polycrystalline radium tin oxide having a thickness in the range of 100 to 150 tn g at 35 nm. Below this is a second TCO layer 14, formed of amorphous aluminum-zinc oxide and having a thickness in the range of 1-3 nm. The lowest sublayer 11 attached to the second TCO layer is a mirror layer 15 made of aluminum and having a thickness of at least 200 nm.

The reflective contact structure 6 has two main purposes in size 5. First, it provides an electrical connection from the metallic base layer 9 to the aperture emitter layer 3. Secondly, it acts as a mirror that reflects light 16 downward from the active region 4 in a reciprocal direction which increases the likelihood of it escaping from the chip. At a more detailed level, each sublayer has its own specific purpose as part of a reflective contact structure. The metallic mirror layer 15 is naturally responsible for the actual reflection function of the reflective contact structure 15. The thickness of the mirror layer is selected to be large enough to ensure that practically no light can penetrate through this layer to the subsequent layers having higher absorbance. The main purpose of the TCO-20 layers is to provide strong adhesion between the mirror layer 15 and the gap emitter layer 3. The first TCO layer 13 of indium tin oxide provides a reflective contact structure with a strong adhesion to the indium-containing aperture emitter 25 layer 3. Its polycrystalline structure provides good optical transparency while minimizing the effect of the layer on the optical performance of the device. The multi-crystalline structure of the layer also provides high electrical conductivity which, together with the relatively high layer thickness 30, ensures efficient current distribution throughout the reflective contact structure and good x specific contact resistance to the gap emitter layer 3. From amorphous alumina '' .

a second TCO layer made of nickel oxide 14

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Instead, S 35 provides strong adhesion between the first TCO layer 13 and the aluminum mirror layer 15. Because the amorphous material structure 12 has lower optical transparency and electrical conductivity, the layer thickness is limited to a value significantly less than that of the first TCO layer.

As shown in Fig. 2a, an exemplary manufacturing method begins with growing a semiconductor heterostructure having an active region 4 disposed between the electron emitter layer 2 and the aperture emitter layer 3 10 on the insulating substrate disc 17. Next, the mask metal 18 is deposited on the heterostructure and patterned by photolithography according to the desired chip size and geometry. The heterostructure is then etched by reactive ion etching through openings 15 in the metallic mask layer to form discrete mesa-like layer stacks 19, as shown in Figure 2b. The mask metal is removed after etching. The first TCO layer 13 is formed on the disk by, for example, sputtering and is photolithographically patterned to remove a layer outside the mesa regions, after which the disk is heat treated to render the TCO structure polycrystalline. Next, a second TCO layer 14 having an amorphous structure, a metallic reflection layer 25 15 and a metallic antioxidant layer is formed and patterned photolithographically to remove the deposited material outside the mesa regions.

The dielectric passivation layer 20 is formed and patterned photolithographically to protect the sidewalls of the mesa regions 30 from the deposited material during subsequent processing steps. In addition, the passport x-x layer reduces leakage currents through the side walls.

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The grooves between the mesa regions can be protected by a layer-1-gJ rack with a resist 21 and a hard-baking 35 rack therewith. The situation at this stage of the process is shown in Figure 2c. Next, the adhesive layer 13 8 and the anti-diffusion layer 10, both formed of metal, are coated and patterned over the mesa areas. Thereafter, a thick metal layer 9 is formed by galvanizing the wafer. As shown in Figure 2d, the metal is formed into a continuous layer 5 over the entire disc. This metal layer forms a support structure which allows the next step, namely the removal of the original growing substrate 17, after which the mesa-like layer structures are superimposed on a thick metal layer 9 as shown in Figure 2e.

After removing the growth substrate, the exposed surface of the electron emitter layer 2 is roughened. The fragments of the side electrodes is formed on the roughened metal elektroniemitteripinnalle network. Finally, the mesa regions are separated into individual LED chips 1 as shown in Figure 2f.

As an alternative to the above process heterostruktuurin etching to form a separate layer stack, such as the mesa 20 could just as well be performed as the last step, the n-side electrode in the formation.

In general, it is also important to note that the embodiments described above with reference to the drawings are only some advantageous, but in no way exclusive, examples of all possible ways of carrying out the invention. In particular, all materials, layer thicknesses and processes used in the various manufacturing steps may vary freely

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within the scope of the invention as defined in the claims 30.

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Claims (14)

A light emitting semiconductor device (1) made of nitrides of Group III metals, the device comprising a layer structure comprising an n-type semiconductor layer (2), a p-type semiconductor layer (3), an active region (4) between a type semiconductor layer and a p-type semiconductor layer having a contact surface (5) defined by an n-type or p-10 type semiconductor layer, the structure further comprising a reflective contact structure (6) connected to the contact surface, 6) comprising: 15. a first transparent conductive oxide (TCO) contact layer (13) having a polycrystalline structure attached to a contact layer (5) of a layer structure, - a second transparent conductive oxide (TCO) contact layer (14) having an amorphous structure, and - a metallic reflection layer (15) connected to the second TCO layer. Semiconductor device (1) according to Claim 1, characterized in that the chemical composition of the first TCO contact layer (13) is selected to promote high adhesion to the contact surface (5) of the structure, good transparency. and the good electrical O) conductivity of the first TCO contact layer, and the chemical composition of the second TCO contact layer (14) is selected to promote strong adhesion of the ____ metal reflective layer (15) to the second TCO contact layer. LO g 35 o Semiconductor device (1) according to Claim 1 or 2, characterized in that the layer (3) defining the contact surface comprises a p-type In-GaN. Semi-conductor device (1) according to one of Claims 1 to 3, characterized in that the first TCO contact layer (13) comprises indium tin oxide. The semiconductor device (1) according to any one of claims 1 to 4, characterized in that the first TCO contact layer (13) has a thickness of 30 to 500 nm, preferably 100 to 150 nm. Semiconductor device (1) according to one of Claims 1 to 5, characterized in that the second TCO contact layer (14) comprises aluminum zinc oxide and that the reflective metal layer (15) comprises aluminum coated on the second TCO contact layer. 20 Semiconductor device (1) according to one of Claims 1 to 6, characterized in that the second TCO contact layer (14) has a thickness of 0.2 to 20 nm, preferably 1 to 3 nm. 25 A process for the preparation of a light emitting semiconductor device (1) made of nitrides of Group III metals, comprising a layer structure comprising an n-type semiconductor layer 30 (2), a p-type semiconductor layer (3), a - 2? between the dense region (4) of the n-type semiconductor layer and the ir p-type semiconductor layer, the layer CL structure having either a n-type or a p-type face} conductive layer contact surface (5), and wherein the LO g 35 method further structure (6) for the contact surface, characterized in that forming the reflective contact structure (6) comprises the following steps: - forming a first transparent conductive oxide (TCO) contact layer 5 (13) having a polycrystalline structure on the contact surface (5) of the layer structure. forming a second transparent conductive oxide (TCO) contact layer (14) having an amorphous structure, and 10. forming a metallic reflection layer (15) on the second TCO layer. Method according to Claim 8, characterized in that the chemical composition of the first TCO-contact layer (13) is selected to promote high adhesion to the contact layer (5) of the layer structure, good transparency and good electrical conductivity of the first TCO-contact layer. The chemical composition of the TCO contact layer (14) is selected to promote high adhesion of the metal reflective layer (15) to the second TCO contact layer. A method according to claim 8 or 9, characterized in that the layer (3) defining the contact surface (5) comprises p-type InGaN. A method according to any one of claims 8 to 10, characterized in that the first TCO-30 contact layer (13) comprises indium tin oxide. i '05 The o of any one of claims 8-11. A method, characterized in that the first TCO-h · contact layer (13) is manufactured so that its S 35 has a thickness of 30 to 500 nm, preferably 100 to 150 nm. o o {M Method according to one of Claims 8 to 12, characterized in that the second TCO contact layer (14) comprises aluminum-zinc oxide and that, in the step of forming a reflective metal layer (15), aluminum is deposited on the second TCO contact layer. Method according to one of Claims 8 to 13, characterized in that the second TCO-10 contact layer (14) is made to have a thickness of 0.2 to 20 nm, preferably 1 to 3 nm. c \ j δ (M i δ i O) X en CL h- · (M CD m O) o o (M