WO2010139860A1

WO2010139860A1 - Light emitting semiconductor device and method for manufacturing

Info

Publication number: WO2010139860A1
Application number: PCT/FI2010/050454
Authority: WO
Inventors: Vladislav E. Bougrov; Maxim A. Odnoblyudov; Mikael Mulot
Original assignee: Optogan Oy
Priority date: 2009-06-05
Filing date: 2010-06-03
Publication date: 2010-12-09
Also published as: TW201110419A; FI122622B; FI20095627A0; FI20095627A; RU2011144445A; EP2438628A1; KR20120030430A; CN102460743A; JP2012529170A

Abstract

The light emitting semiconductor device (1) of the present invention is made of nitrides of group III metals and comprises a layer structure comprising an n-type semiconductor layer (2), a p-type semiconductor layer (3), an active region (4) between the n-type semiconductor layer and the p-type semiconductor layer. The layer structure has a contact surface (5) defined by one of the n-type and p-type semiconductor layers and comprises further a reflective contact structure (6) attached to the contact surface. According to the present invention, the reflective contact structure (6) comprises: a first transparent conductive oxide (TCO) contact layer (13), having a poly- crystalline structure, attached to the contact surface (5) of the layer structure; a second transparent conductive oxide (TCO) contact layer (14) having an amorphous structure; and a metallic reflective layer (15) attached to the second TCO layer.

Description

LIGHT EMITTING SEMICONDUCTOR DEVICE AND METHOD FOR MANUFACTURING

FIELD OF THE INVENTION The present invention relates generally to light emitting semiconductor structures made of nitrides of group III metals. More precisely, the invention relates to reflective contacts used e.g. in vertical geometry Light Emitting Diodes (LEDs) , and manu- facturing thereof.

BACKGROUND OF THE INVENTION

Light emitting semiconductor devices like LEDs have a continuously growing role in different fields of everyday life. They have a great variety of applications e.g. in telecommunications, lighting, and display technology.

From the material point of view, one dramatically expanding branch of today' s LED technology is based on nitrides of group III metals like Gallium Nitride GaN, Aluminum Gallium Nitride AlGaN, Indium Gallium Nitride and the alloys thereof. These materials are found particularly suitable candidates for high brightness LEDs for e.g. lighting applications. In the intensive development of more and more efficient LED structures, one of the dominant technological trends in the recent years has been going towards a vertical LED geometry instead of the former laterally distributed structure. In the past, the two electrodes forming the electrical connections to the n- and p-type layers of the component were formed on the same side of the LED chip laterally separated from each other. This configuration set up several limitations for the device operation. The vertical LED ge- ometry having the contact electrodes on the opposite sides of the chip can provide essential advantages as well in current uniformity and light extraction efficiency as in heat management of the device.

The simplified general principle of manufacturing a vertical LED can be described, for example, as follows. First, a layer structure comprising an retype semiconductor layer, an active region, and a p- type semiconductor layer is formed on a growth substrate of e.g. sapphire. Next, a reflective contact structure is formed on top of the structure to provide electrical connection to the p-type semiconductor layer and to act as a mirror reflecting the incident light generated in the active region backwards. A relatively thick metal layer is then deposited on the reflective contact structure to form the p-side contact electrode of the chip. The thick metal layer can also act as a support structure of the completed led chip. Next, the growth support is removed, exposing the surface of the n-type layer originally grown on the growth substrate. Finally, another thick metal layer is formed on this exposed surface to form the the reside contact electrode.

One important point in a vertical LED structure is said reflective contact structure. In addition to the desired high reflectance and low electrical re- sistivity, the reflective contact structure itself should also remain stable throughout the component life-cycle and, on the other hand, provide good adhesion between the metal layer forming the p-side electrode and the actual operational device layers. Con- ventional solutions include e.g. different metallic combinations like one or more layers of aluminum or gold deposited directly on the p-type semiconductor layer or on an intermediate adhesion layer first formed on the semiconductor surface. In the first case, both adhesion strength and long-term stability of the structure are usually insufficient. An intermediate adhesion layer comprising e.g. nickel can en- hance the adhesion. On the other hand, it increases optical losses through light absorption in the adhesion layer.

PURPOSE OF THE INVENTION

The purpose of the present invention is to provide a novel light emitting semiconductor structure comprising a stable reflective contact structure with excellent mechanical, optical, and electrical proper- ties.

SUMMARY OF THE INVENTION

The light emitting semiconductor device and the method for manufacturing a light emitting semicon- ductor device of the present invention are characterized by what is presented in claims 1 and 8, correspondingly.

The light emitting semiconductor device according to the present invention is made of nitrides of group III metals. One suitable material is gallium nitride GaN and its different variations like indium gallium nitride InGaN and aluminum gallium nitride Al- GaN. Being made of said materials means that at least the operationally essential portions of the device comprise at least some of said materials. Naturally, the device can comprise, in different portions thereof, also materials not falling within said definition. As a core portion, the device comprises a layer structure comprising an n-type semiconductor layer, a p-type semiconductor layer, and an active region for light generation between the n-type semiconductor layer and the p-type semiconductor layer. The details of this layer structure can vary within the known technology and are not essential for the inven- tion. The layer structure has a contact surface defined by one of the n-type and p-type semiconductor layers, and a reflective contact structure attached to this contact surface. The reflective contact structure provides an electrical contact to the semiconductor layer defining the contact surface, and also acts as a mirror reflecting the incident light from the active region .

According to the present invention, the reflective contact structure comprises a first transparent conductive oxide (TCO) contact layer with a poly- crystalline structure attached to the contact surface of the layer structure, a second transparent conductive oxide contact layer having an amorphous structure, and a metallic reflective layer attached to the second TCO layer. Said structure of the reflective metal contact utilizing a two-layer intermediate TCO structure between the metallic reflective layer and the contact surface provides great advantages over the prior art solutions. Each of the two TCO layers has its own pur- pose. The polycrystalline material structure of the first TCO layer provides high optical transparency and low electrical resistivity. The second TCO layer of amorphous structure, for its turn, enables a strong adhesion to the metallic reflective layer. In addition to the different crystal structures, also the accurate chemical compositions of the two layers can be optimized separately 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 contact layer is selected to promote strong adhesion to the contact surface of the layer structure, good transparency, and high electrical conductivity of the first TCO contact layer, and the chemi- cal composition of the second TCO contact layer is selected to promote strong adhesion of the metallic reflective layer 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 purposes.

The second TCO contact layer can lie in di- rect contact with the first TCO contact layer. However, it is also possible to have some intermediate layer (s) between the two TCO contact layers.

The layer defining the contact surface comprises preferably p-type indium gallium nitride InGaN. The first TCO contact layer, for its part, comprises preferably indium tin oxide which due to the presence of indium can provide an excellent adhesion to metals of group III metals. To ensure efficient current spreading over the entire area of the reflective con- tact structure and good specific contact resistivity not significantly influencing the overall series resistance through the LED chip, the first TCO contact layer has preferably a thickness of 30 - 500 nm, more preferably 100 - 150 nm. A too low thickness would re- suit in inadequate electrical characteristics of the layer. On the other hand, thickening this layer too much would increase disadvantageously the unwanted absorption of the light generated in the active region.

In one preferred embodiment of the present invention, strong adhesion between the second TCO contact layer and the metallic reflective layer is ensured by that the second TCO contact layer comprises aluminum zinc oxide (AZO) and the metallic reflective layer comprises aluminum deposited on the second TCO contact layer. Naturally, these are just examples of possible materials. For example, another good material for the metallic reflective layer is silver.

Due to the lower optical transparency and electrical conductivity of the amorphous TCO in com- parison to the polycrystalline TCO, the thickness of the second TCO contact layer has to be limited. On the other hand, too low thickness could further decrease the electrical conductivity and possibly also the adhesion to the above metallic reflective layer. A preferable thickness range is 0.2 - 20 nm, more preferably 1 - 3 nm. The metallic reflective layer can have a thickness of e.g. 20 - 1000 nm, however preferably at least 200 nm to ensure that no light is penetrated through the layer and thus to maximize the reflectivity of the reflective contact structure. To protect the metallic reflective layer from oxidation during the further device processing, there can be an anti-oxidation layer formed of e.g. gold and having a thickness of 1 - 20, preferably 5 - 10 nm, deposited on the metallic reflective mirror surface. The method of the present invention for manufacturing a light emitting semiconductor structure made of nitrides of group III metals comprises fabricating a layer structure comprising an n-type semiconductor layer, a p-type semiconductor layer, and an ac- tive region between the n-type semiconductor layer and the p-type semiconductor layer, the layer structure having a contact surface defined by one of the n-type and p-type semiconductor layers. The layer structure can be fabricated e.g. by normal vapor phase epitaxial processes commonly used and well known in the LED industry. Thus, no detailed description of the fabrication processes is needed here. The method comprises further forming a reflective contact structure on the contact surface. According to the present invention, forming the reflective contact structure comprises the steps of forming a first transparent conductive oxide (TCO) contact layer, having a polycrystalline structure, on the contact surface of the layer structure, forming a second transparent conductive oxide (TCO) contact layer having an amorphous structure, and forming a metallic reflective layer on the second TCO layer. TCO contact layers can be deposited e.g. by sputtering. As-deposited TCO is amorphous. Thus, the first TCO contact layer has to be annealed to change the phase, i.e. to crystallize the originally amor- phous layer. Suitable temperature range for the annealing depending, for example, on the accurate material composition, can be e.g. 150 - 300°C. The second TCO contact layer can be deposited directly on the first TCO contact layer or on some intermediate layer (s) first deposited on the first TCO contact layer .

Preferably, in the method of the present invention the chemical composition of the first TCO contact layer is selected to promote strong adhesion to the contact surface of the layer structure, good transparency, and high electrical conductivity of the first TCO contact layer, and the chemical composition of the second TCO contact layer is selected to promote strong adhesion of the metallic reflective layer 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. How said material composition selections are made in practice in order to achieve said proper- ties depends, for example, on the materials of the semiconductor layer structure. However, it is routine engineering for a person skilled in the art.

The layer defining the contact surface comprises preferably p-type indium gallium nitride InGaN. One preferable material for the first TCO contact layers comprises indium tin oxide.

The first TCO contact layer is preferably fabricated to have a thickness of 30 - 500 nm, more preferably of 100 - 150 nm. In a preferred embodiment, the second TCO contact layer comprises aluminum zinc oxide, and the step of forming the metallic reflective layer com- prises depositing aluminum on the second TCO contact layer .

The second TCO contact layer is preferably fabricated to have a thickness of 0.2 - 20 nm, more preferably of 1 - 3 nm. The metallic reflective layer is preferably fabricated to have a thickness of 20 -

1000 nm, however preferably at least 200 nm.

In addition to the above-described manufacturing steps relating to the core principles of the present invention, the entire manufacturing process of the structures providing the electrical contact to the semiconductor layer defining the contact surface can also include depositing many further layers. First, to protect the metallic reflective layer from oxidation during the following process steps, an anti-oxidation layer of e.g. gold and having a thickness of e.g. 1 - 20, preferably 5 - 10 nm, can be deposited on the surface of the metallic reflective layer. Next, an adhesion layer of e.g. titanium can be formed on the anti- oxidation layer to enhance adhesion of the following layers to the reflective contact structure. A diffusion barrier layer can then be deposited to protect the metallic reflective layer from diffusion of possibly aggressive metal of a bonding pad finally defining the surface electrode of the component. Finally, a thick layer of solderable metal to form said bonding pad can be deposited by e.g. galvanic deposition. Examples of suitable solderable metals include Au, Au/In alloy, and Cu. Naturally, in addition to the deposi- tion steps mentioned, the process can also include different patterning steps by e.g. lithography to achieve the desired device geometry.

On the other hand, the manufacturing process of the light emitting device as a whole, e.g. in the case of a vertical geometry Light Emitting Diode LED, can necessitate also many further steps. Examples of these are removing the growth substrate by e.g. chemi- cal etching, and forming electrical contact structures also to this way exposed opposite side of the semiconductor device.

The manufacturing process according to the present invention is suitable for cost-effective mass production of light emitting devices in which up to several tens of wafers can be processed simultaneously.

BRIEF DESCRIPTION OF THE FIGURES

In the following, the present invention is described in more detail by means of the drawings attached, wherein Figure 1 presents a schematic overview of a vertical LED in accordance of the present inven- tion, and Figures 2a - 2f illustrate a manufacturing method according to the present invention. In the drawings, the corresponding layers are denoted by the same reference numbers. The drawings are not in scale.

DETAILED DESCRIPTION OF THE INVENTION

The vertical LED chip 1 of figure 1 is based on a heterostructure comprising an electron emitter layer 2 made of n-doped GaN, a hole emitter layer 3 made of p-doped InGaN, and an active region 4 for light generation between these two layers. Below the hole emitter is a reflective contact structure 6. As the next layer downwards is an anti-oxidation layer 7 to protect the reflective contact structure from oxidation during the rest of the manufacturing process after depositing the reflective contact structure. The anti-oxidation layer is formed of Au and has a thickness of about 5 nm. An adhesion layer 8 formed of Ti lies below the anti-oxidation layer to form a strong adhesion between the layers above and below it. The undermost layer in the chip of Figure 1 is a bottom metal layer 9 separated from the rest of the device by a diffusion barrier layer 10 formed of e.g. Ni, protecting the upper device layers from diffusion of metal atoms of the bottom metal layer. The bottom metal layer serves as a p-side electrode providing one of the two electrical connections needed to electrically connect the LED chip to an external power supply. It also provides an efficient route to transfer the excess heat out of the chip. Besides, it also acts as a protective and mechanical support structure of the chip. Via the bottom metal layer, the chip can be attached to an electrically and heat conducting pad on a circuit board or the like e.g. by soldering. The bottom metal layer can be formed of e.g. Au, Au/In alloy, Cu or some other solderable metal and have a thickness in the range of 2 - 200 μm.

The surface of the n-doped GaN layer 2 on the upper side of the chip of Figure 1 is structured to have an uneven surface topology. The roughened surface decreases the total internal reflection of the light 11 from the active region 4 at the device surface, thus enhancing the light extraction from the chip. On top of this uneven surface is a net-shaped top metal layer 12 forming the other contact electrode of the chip . As shown in Figure 1, the reflective contact structure 6 of the present example includes three sublayers. Next to the hole emitter layer is a first transparent conductive oxide (TCO) layer 13 formed of polycrystalline indium tin oxide and having a thick- ness in a range of 100 - 150 nm. Below this is a second transparent conductive oxide layer 14 formed of amorphous aluminum zinc oxide and having a thickness in a range of 1 - 3 nm. As the lowermost sub-layer, attached to the second transparent conductive oxide layer is a mirror layer 15 formed of aluminum and having a thickness of at least 200 nm. The reflective contact structure 6 as a whole has two main purposes. First, it provides an electrical connection from the bottom metal layer 9 to the hole emitter layer 3. Secondly, it acts as a mirror reflecting the downwards-directed light 16 from the active region 4 backwards, thus re-directing it to a direction increasing its probability to escape from the chip. Considered at a more detailed level, each of the sub-layers has its own specific purpose as a por- tion of the reflective contact structure. Naturally, the metallic mirror layer 15 is responsible for the actual reflection performance of the reflective contact structure. The thickness of the mirror layer is selected high enough to ensure that substantially no light can be penetrated through the layer to the next layers with a higher absorbance. Main purpose of the TCO layers is to provide a strong adhesion between the mirror layer 15 and the hole emitter layer 3. The first transparent conductive oxide layer 13 formed of indium tin oxide provides a strong adhesion of the reflective contact structure to the indium-containing hole emitter layer 3. Polycrystalline structure thereof provides good optical transparency, minimizing the effect of the layer to the device optical perform- ance . Polycrystalline structure of the layer also provides high electrical conductivity which, together with the relatively high layer thickness, ensures efficient current spreading over the entire area of the reflective contact structure and a good specific con- tact resistivity to the hole emitter layer 3. The second transparent conductive oxide layer 14 of amorphous aluminum zinc oxide, instead, provides a strong adhesion between the first TCO layer 13 and the mirror layer 15 made of aluminum. Due to the lower optical transparency and electrical conductivity of the amorphous material structure, the layer thickness is lim- ited to a value substantially lower than that of the first TCO layer.

As shown in Figure 2a, the exemplary manufacturing method starts by growing, on an insulating sub- strate wafer 17, a semiconductor heterostructure comprising an active region 4 sandwiched between an electron emitter layer 2 and a hole emitter layer 3. Next, 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 the openings in the mask metal layer to form separate mesa-like layer stacks 19 as shown in Figure 2b. The mask metal is removed after etching. A first transparent conductive oxide layer 13 is deposited on the wafer e.g. by sputtering and patterned by photolithography to remove the layer outside the mesas, after which the wafer is annealed to make the structure of the TCO polycrystal- line. Next, another TCO layer 14 of amorphous struc- ture, a reflective metal layer 15, and a metallic anti-oxidation layer 7 are deposited on top of each other and patterned by photolithography to remove the deposited material outside the mesas.

A dielectric passivation layer 20 is depos- ited and patterned by photolithography to protect the mesa sidewalls from deposited material during the following process steps. In addition, the passivation layer also decreases leakage currents via the side- walls. The trenches between the mesas can be protected by depositing and hard-baking resist 21 therein. The situation at this phase of the process is shown in figure 2c. An adhesion layer 8 and a diffusion barrier layer 10, both formed of metal, are next deposited and patterned on top of the mesas. After this, a thick metal layer 9 is deposited by electroplating on top of the wafer. As illustrated in Figure 2d, the metal deposits as a continuous film over the entire wafer. This metal layer forms a support structure enabling the next step, namely removal of the original growth substrate 17, after which the mesa-like layer structures lie on the thick metal layer 9 as shown in Fig- ure 2e.

After removing the growth substrate, the exposed surface of the electron emitter layer 2 is roughened. The n-side electrodes of the chips are formed as a metallic net on the roughened electron emitter surface. Finally, the mesas are separated to single LED chips 1 as shown in Figure 2f.

As an alternative to the above process, etching the heterostructure to form the separate mesa-like layer stacks could be as well performed as a final step after the n-side electrode formation.

Also in general, it is important to note that the embodiments described above with reference to the drawings are just some preferred but no way exclusive examples of all possible ways to carry out the present invention. Particularly all the materials, layer thicknesses and the processes used in different manufacturing steps can vary freely within the scope of the invention as determined in the claims.

Claims

1. 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 the n-type semiconductor layer and the p-type semiconductor layer, the layer structure having a contact surface (5) defined by one of the n- type and p-type semiconductor layers, the structure further comprising a reflective contact structure (6) attached to the contact surface, characterized in that the reflective contact structure (6) comprises : - a first transparent conductive oxide (TCO) contact layer (13), having a polycrystal- line structure, attached to the contact surface (5) of the layer structure,

- a second transparent conductive oxide (TCO) contact layer (14) having an amorphous structure, and

- a metallic reflective layer (15) attached to the second TCO layer.

2. A semiconductor device (1) according to claim 1, characteri zed in that the chemical composition of the first TCO contact layer (13) is selected to promote strong adhesion to the contact surface (5) of the layer structure, good transparency, and high elec- trical 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 metallic reflective layer (15) to the second TCO contact layer.

3. A semiconductor device (1) according to claim 1 or 2, characteri zed in that the layer (3) defining the contact surface comprises p-type InGaN.

4. A semiconductor device (1) according to any of claims 1 to 3, characterized in that the first TCO contact layer (13) comprises indium tin oxide.

5. A semiconductor device (1) according to any of claims 1 to 4, characterized in that the first

TCO contact layer (13) has a thickness of 30 - 500 nm, preferably of 100 - 150 nm.

6. A semiconductor device (1) according to any of claims 1 to 5, characterized in that the second

TCO contact layer (14) comprises aluminum zinc oxide, and the metallic reflective layer (15) comprises aluminum deposited on the second TCO contact layer.

7. A semiconductor device (1) according to any of claims 1 to 6, characterized in that the second TCO contact layer (14) has a thickness of 0.2 - 20 nm, preferably of 1 - 3 nm.

8. A method for manufacturing a light emitting semiconductor device (1) made of nitrides of group III metals, the method comprising fabricating a layer structure comprising an n-type semiconductor layer (2), a p-type semiconductor layer (3), an active re- gion (4) between the n-type semiconductor layer and the p-type semiconductor layer, the layer structure having a contact surface (5) defined by one of the retype and p-type semiconductor layers, the method further comprising forming a reflective contact structure (6) on the contact surface, characteri zed in that forming the reflective contact structure (6) comprises the steps of: - forming a first transparent conductive oxide (TCO) contact layer (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

- forming a metallic reflective layer (15) on the second TCO layer.

9. A method according to claim 8, characteri zed in that the chemical composition of the first TCO contact layer (13) is selected to promote strong adhesion to the contact surface (5) of the layer structure, good transparency, and high electrical 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 metallic reflective layer (15) to the second TCO layer.

10. A method according to claim 8 or 9, characterized in that the layer (3) defining the contact surface (5) comprises p-type InGaN.

11. A method according to any of claims 8 to 10, characteri zed in that the first TCO contact layer (13) comprises indium tin oxide.

12. A method according to any of claims 8 to 11, characteri zed in that the first TCO contact layer (13) is fabricated to have a thickness of 30 - 500 nm, preferably of 100 - 150 nm.

13. A method according to any of claims 8 to 12, characteri zed in that the second TCO contact layer (14) comprises aluminum zinc oxide, and the step of forming the metallic reflective layer (15) com- prises depositing aluminum on the second TCO contact layer .

14. A method according to any of claims 8 to 13, characteri zed in that the second TCO contact layer (14) is fabricated to have a thickness of 0.2 - 20 nm, preferably of 1 - 3 nm.