DIODE HAVING HIGH BRIGHTNESS AND METHOD THEREOF

Abstract

A light emitting device can include a substrate including first and second surfaces, the substrate having a thickness of less than 350 micrometers; a reflective layer on the second surface of the substrate; a light emitting structure on the first surface of the substrate and including first and second semiconductor layers with an active layer therebetween, the second semiconductor layer includes an aluminum-gallium-nitride layer, and the active layer includes aluminum and indium and has a multiple quantum well layer; a transparent conductive layer disposed on the second semiconductor layer and including an indium-tin-oxide; a first electrode on the first semiconductor layer and including multiple layers; a second electrode on the transparent conductive layer and including multiple layers; first and second pads on the first and second electrodes, respectively, in which the second pad includes the same material as the first pad and has a thickness of more than 500 nanometers.

Claims

1. A light emitting device, comprising: a substrate including a first surface and a second surface opposing the first surface of the substrate, the substrate having a thickness of less than 350 micrometers; a reflective layer disposed on the second surface of the substrate; a light emitting structure disposed on the first surface of the substrate, the light emitting structure including a first semiconductor layer, a second semiconductor layer, and an active layer between the first semiconductor layer and the second semiconductor layer, wherein the second semiconductor layer includes an aluminum-gallium-nitride layer, and wherein the active layer includes aluminum and indium and has a multiple quantum well layer; a transparent conductive layer disposed on the second semiconductor layer, the transparent conductive layer including an indium-tin-oxide; a first electrode disposed on the first semiconductor layer, the first electrode including multiple layers; a second electrode disposed on the transparent conductive layer, the second electrode including multiple layers; a first pad disposed on the first electrode; and a second pad disposed on the second electrode, wherein the second pad includes the same material as the first pad and has a thickness of more than 500 nanometers.

2. The light emitting device according to claim 1, wherein the reflective layer includes aluminum.

3. The light emitting device according to claim 1, wherein the reflective layer substantially entirely covers the second surface of the substrate and has a thickness of less than 300 nanometers.

4. The light emitting device according to claim 1, wherein the first electrode comprises at least one of Ti, Al, Cr, or Au.

5. The light emitting device according to claim 1, wherein the second electrode comprises at least one of Ni, Au, Pd, or Pt.

6. The light emitting device according to claim 1, wherein each of the first pad and the second pad includes Au.

7. The light emitting device according to claim 1, wherein the substrate includes at least two materials selected from a group of Zn, O, Ga, N, Si, C, Al, and N.

8. The light emitting device according to claim 1, wherein the second surface of the substrate has a surface roughness of less than 15 nanometers.

9. The light emitting device according to claim 1, wherein the second surface of the substrate has a surface roughness of less than 5 nanometers.

10. The light emitting device according to claim 1, further comprising a gallium nitride layer disposed between the first surface of the substrate and the first semiconductor layer, and wherein at least one of the substrate, the reflective layer, or the gallium nitride layer includes a slanted side surface.

11. The light emitting device according to claim 1, wherein the substrate has a thickness of less than 120 micrometers.

12. A light emitting device package, comprising: a substrate including a first surface and a second surface opposing the first surface of the transparent substrate; a first reflective layer disposed on the second surface of the substrate; a light emitting structure disposed on the first surface of the substrate, the light emitting structure including a first semiconductor layer, a second semiconductor layer, and an active layer between the first semiconductor layer and the second semiconductor layer, wherein the second semiconductor layer includes an aluminum-gallium-nitride layer; a transparent conductive layer disposed on the second semiconductor layer, the transparent conductive layer including an indium-tin-oxide; a first electrode disposed on the first semiconductor layer; a second electrode disposed on the transparent conductive layer; and a first pad and a second pad disposed on the first electrode and second electrode, respectively, the second pad having a thickness of more than 500 nanometers, wherein the second surface of the substrate has a surface roughness of less than 15 nanometers.

13. The light emitting device package according to claim 12, wherein the substrate has a thickness of less than 350 micrometers and includes at least two materials selected from a group of Zn, 0, Ga, N, Si, C, Al, and N.

14. The light emitting device package according to claim 12, wherein the first electrode includes at least one of Ti, Al, Cr, and Au, and wherein the second electrode includes at least one of Ni, Pd, Pt, and Au.

15. The light emitting device package according to claim 12, wherein at least one of the first electrode and second electrode includes Au, wherein at least one of the first pad and the second pad includes Au, and wherein the first pad has a thickness of more than 500 nanometers.

16. The light emitting device package according to claim 12, further comprising: a lead frame including a third surface and a fourth surface opposing the third surface of the lead frame; and a second reflective layer disposed on the third surface of the lead frame, wherein the first reflective layer is disposed on the second reflective layer.

17. A light emitting device package, comprising: a lead frame including a first surface and a second surface opposing the first surface of the lead frame; a first reflective layer on the first surface of the lead frame; and a light emitting device disposed on the first reflective layer, wherein the light emitting device comprises: a substrate including a third surface and a fourth surface opposing the third surface of the substrate, the substrate having a thickness of less than 350 micrometers; a second reflective layer disposed on the fourth surface of the substrate; a light emitting structure disposed on the third surface of the substrate, the light emitting structure including a first semiconductor layer, a second semiconductor layer, and an active layer between the first semiconductor layer and the second semiconductor layer, wherein the second semiconductor layer includes an aluminum-gallium-nitride layer, and wherein the active layer has a multiple quantum well layer; a transparent conductive layer disposed on the second semiconductor layer, the transparent conductive layer including an indium-tin-oxide; a first electrode disposed on the first semiconductor layer, the first electrode including multiple layers; a second electrode disposed on the transparent conductive layer, the second electrode including multiple layers; and a first pad and a second pad disposed on the first electrode and second electrode, respectively, each of the first pad and the second pad having Au.

18. The light emitting device package according to claim 17, wherein each of the first pad and the second pad has a thickness of more than 500 nanometers.

19. The light emitting device package according to claim 17, wherein the fourth surface of the substrate has a surface roughness of less than 15 nanometers.

20. The light emitting device package according to claim 17, wherein the substrate has a thickness of less than 120 micrometers.

Description

BRIEF DESCRIPTION OF THE DRAWING

[0022] The accompanying drawings, which are included to provide a further understanding of the invention and are incorporated in and constitute a part of this specification, illustrate embodiments of the invention and together with the description serve to explain the principles of the invention.

[0023] In the drawings:

[0024] FIG. 1 generally shows a conventional light emitting diode;

[0025] FIGS. 2A and 2B show two different embodiments of a light emitting diode of the present invention;

[0026] FIG. 3A-3F shows the manufacturing steps for forming the light emitting diode of the present invention;

[0027] FIGS. 4A and 4B each show a wafer having the light emitting diodes with scribe lines;

[0028] FIG. 5 shows another embodiment of the diode of the present invention; and

[0029] FIG. 6 is a graph showing a relationship between light output and current injection for an LED having a reflective layer of the present invention and an LED without a reflective layer.

DETAILED DESCRIPTION OF THE INVENTION

[0030] Reference will now be made in detail to the present invention, examples of which are illustrated in the accompanying drawings.

[0031] In order to fabricate GaN-based light emitting diodes (LEDs), sapphire substrate has been generally used since sapphire is very stable and relatively cheaper. The epitaxial layer quality of the AlInGaN grown on sapphire substrate is superior to the other substrate material due to their thermal stability and the same crystal structure of the GaN. However, there are some disadvantages in using sapphire as a substrate material for AlInGAN-based LED device fabrication. Because the sapphire is insulator, forming an n-type bottom contact is not possible. In addition, it is very difficult to perform the post fabrication processes that include the grinding, the polishing, and the scribing since sapphire is almost as hard as diamond. However, transparent sapphire substrate is beneficial for the light extraction compare to the other non-transparent compound semiconductor material such as GaAs and InP.

[0032] Nevertheless, it has not been possible to take advantage of this important benefit. When sapphire is used for the substrate, p and n electrodes should be placed on the same top electrode position. As a result, as shown in FIG. 1, the downward photons emitted in the active region can suffer absorption by thick substrate and the lead frame. Hence, only photons directing top portion and edge emitting can contribute to the optical output power. On the other hand, if a reflecting surface is provided in the bottom sapphire substrate, in addition to the top emitting and edge emitting photons, the photons emitted to the downward direction can be reflected to the side-wall of the sapphire substrate or can be reflected back to the top surface. In addition to the backside reflective coating, the light output can be increased by making a mirror-like or highly smooth interface between the reflective metal layer and the transparent substrate. Depending on the reflective index of the substrate material and the surface conditions, including surface roughness, there is a certain angle called an escaping angle in which the photons from the active layer reflect off of the interface back to the substrate crystal. Therefore, at a fixed reflective index of the sapphire substrate, for example, the amount of reflected photons can be controlled by reducing the surface roughness of the substrate. In the present invention, a new surface polishing technique is employed in addition to the conventional mechanical polishing techniques. An atomically flat sapphire surface was obtained using an inductively coupled plasma reactive ion beam etching (ICPRIE). By using ICPRIE, the sapphire surface having a surface roughness as small as I run was obtained. Moreover, the transmitted or escaped photons can be reflected back off of the smooth surface to the substrate crystal. This results in a considerable enhancement of the total optical light output of the LED device.

[0033] FIG. 2A illustrates an LED structure of the present invention. The light emitting diode structure includes substrate 100, which is a transparent substrate, such as sapphire. The sapphire has undergone backside lapping and polishing on its back surface to maximize the light output. Prior to the reflective metal coating, ICPRIE polishing was performed on a mechanically polished sapphire substrate to further reduce the surface roughness. In one sample, the ICPRIE polishing process conditions were as follows: [0034] 1600 watt RF power; [0035] 350V substrate bias voltage; [0036] gas mixture of 18% Cl.sub.2/72% BCl.sub.3/20% Ar; [0037] 20 degree Celsius substrate temperature; [0038] 40 minutes etching time; and [0039] resulting etch rate was 350 nm/min, respectively.

[0040] Referring to FIG. 2A, a reflective layer 200 is on the sapphire substrate 100 and can be made of an aluminum mirror, for example, to reflect the photons heading toward the bottom. The reflected photons contribute to dramatically increasing the brightness of the LED. As will be discussed throughout the description, the material for the reflective layer is not limited to aluminum but may be any suitable material that will reflect the photons to increase the brightness of the LED. Moreover, the substrate of the LED may also be made of suitable materials other than sapphire.

[0041] FIG. 2B illustrates another LED structure of the present invention. In FIG. 2B, the reflective layer is omitted. Although the reflective layer is omitted, the sapphire substrate 100 is polished using ICPRIE, for example, to maximize the smoothness of the surface of the surface. Such smooth surface allows the photons from the active layer directed toward the sapphire substrate to reflect off from the smooth surface of the sapphire surface to enhance the light output.

[0042] FIGS. 3A-3F illustrate the steps of making a light emitting diode, as an example application of the present invention.

[0043] Referring to FIG. 3A, a buffer layer 120 is formed on a substrate 100. The substrate 100 is preferably made from a transparent material including for example, sapphire. In addition to sapphire, the substrate can be made of zinc oxide (ZnO), gallium nitride (GaN), silicon carbide (SiC) and aluminum nitride (AlN). The buffer layer 120 is made of, for example, GaN (Gallium Nitride) and, in this instance, the GaN was grown on the surface of the sapphire substrate 100. An n-type epitaxial layer such as n-GaN 140 is formed on the buffer layer 120. In this instance, the n-GaN layer 140 was doped with silicon (Si) with a doping concentration of about 10.sup.17 cm.sup.3 or greater. An active layer 160 such as an AlInGaN multiple quantum well layer is formed on the n-GaN layer 140. The active layer 160 may also be formed of a single quantum well layer or a double hetero structure. In this instance, the amount of indium (In) determines whether the diode becomes a green diode or a blue diode. For an LED having blue light, indium in the range of about 22% may be used. For an LED having green light, indium in the range of about 40% may be used. The amount of indium used may be varied depending on the desired wavelength of the blue or green color. Subsequently, a p-GaN layer 180 is formed on the active layer 160. In this instance, the p-GaN layer 180 was doped with magnesium (Mg) with a doping concentration of about 10.sup.17 cm.sup.3 or greater.

[0044] Referring to FIG. 3B, a transparent conductive layer 220 is formed on the p-GaN layer 180. The transparent conductive layer 220 may be made of any suitable material including, for example, Ni/Au or indium-tin-oxide (ITO). A p-type electrode 240 is then formed on one side of the transparent conductive layer 220. The p-type electrode 240 may be made of any suitable material including, for example, Ni/Au, Pd/Au, Pd/Ni and Pt. A pad 260 is formed on the p-type electrode 240. The pad 260 may be made of any suitable material including, for example, Au. The pad 260 may have a thickness of about 5000 or higher.

[0045] Referring to FIG. 3C, the transparent conductive layer 220, the p-GaN layer 180, the active layer 160 and the n-GaN layer 140 are all etched at one portion to form an n-electrode 250 and pad 270. As shown in FIG. 3C, the n-GaN layer 140 is etched partially so that the n-electrode 250 may be formed on the etched surface of the n-GaN layer 140. The n-electrode 250 may be made of any suitable material including, for example, Ti/Al, Cr/Au and Ti/Au. The pad 270 is a metal and may be made from the same material as the pad 260.

[0046] Referring to FIG. 3D, the thickness of the substrate 100, such as made from sapphire, is reduced to form a thinner substrate 100A. In this regard, backside lapping is performed on the sapphire substrate 100 to reduce the wafer thickness. After backside lapping, mechanical polishing is performed to obtain an optically flat surface. After mechanical polishing, the surface roughness (Ra) may be less than about 15 nm. Such polishing technique can reduce the surface roughness up to about 5 nm or slightly less. Such low surface roughness adds to the reflective property of the surface.

[0047] In the present invention, the thickness of the substrate 100 can be controlled to be in the range of, for example, 350-430 m. Moreover, the thickness can be reduced to less than 350 m and to less than 120 m. Here, mechanical polishing and dry etching techniques are used. For dry etching, inductively coupled plasma (ICP) reactive ion beam etching (RIE) may be used as an example.

[0048] Referring to FIG. 3E, the surface roughness is further reduced to obtain a surface roughness of less than 1 nm. The surface roughness can be reduced from 5 nm up to less than 1 nm by using dry etching. One such dry etching technique is inductively coupled plasma (ICP) reactive ion beam etching (RIE) to obtain an atomically flat surface. The maximum reduction of the surface roughness further enhances the reflectivity of the surface. It is noted that depending on the type of material used for the substrate 100, the surface roughness may be further reduced for maximum reflectivity of the surface.

[0049] Referring to FIG. 3F, on the polished thin substrate 100A, a reflective material 200 is formed. The reflective material 200 can be any suitable material that can reflect light. In the present example, an aluminum coating of about 300 nm thick was formed on the polished sapphire surface 100A using an electron beam evaporation technique. Of course, other suitable deposition techniques may be used and different thicknesses of the aluminum are contemplated in the present invention. Here, the aluminum may have a concentration of about 99.999% or higher, which allows the aluminum to have a mirror-like property with maximum light reflectivity. Moreover, the reflective layer 200 entirely covers the second side of the substrate 100A.

[0050] FIG. 5 shows an alternative embodiment in which a cladding layer 170 is formed between the p-GaN layer 180 and the active layer 160. The cladding layer 170 is preferably formed with p-AlGaN. The cladding layer 170 enhances the performance of the diode. For simplicity, FIG. 5 does not show the p-electrode, n-electrode and the pads.

[0051] As conceptually shown in FIGS. 2A and 2B, the photons generated in the active layer which head toward the polished sapphire surface and the aluminum mirror coating 200 are reflected. Such reflected photons add to the brightness of the diode (photon recovery). Adding the reflective layer and making atomically flat surface greatly increases the brightness of the diode. In addition to the reflective surface of the reflective layer 200, it is important to note that the low surface roughness of the substrate 100 also enhances the photon reflection.

[0052] FIG. 6 is a graph showing a relationship between the light output and the injection current of, for example, a light emitting diode (LED). One curve of the graph depicts an LED having a reflective layer (in this case, an aluminum) and the other curve depicts an LED without a reflective layer. In this graph, only mechanical polishing was performed on both LED's. When the reflective aluminum layer was added to the mechanically polished surface of the sapphire substrate, the light output increased about 200% as compared to the device without the reflective layer.

[0053] FIG. 4A shows a wafer having LEDs formed thereon. Scribe lines 300 are formed on the wafer through the buffer layer 120 from the side having the LEDs (front scribing) to separate the LED chips. The scribe lines 300 are formed using, for example, a dry etching technique or mechanical scribing. The dry etching technique such as inductively coupled plasma (ICP) reactive ion beam etching (RIE) can form very narrow scribe lines on the buffer layer 120 and the substrate 100A. Using such dry etching technique greatly increased the number of LED chips on the wafer because the space between the chips can be made very small. For example, the space between the diode chips can be as narrow as 10 m or lower. FIG. 48 is an alternative method of forming the scribe lines in which the back side of the diode used.

[0054] The scribe lines may also be formed by a diamond stylus, which requires a large spacing between the diode chips due to the size of the diamond stylus itself. Also, a dicing technique may be used to separate the chips.

[0055] Once the diode chips are separated, each diode may be packaged. Such package may also be coated with a reflective material to further enhance the light output.

[0056] The present invention applies a simple and inexpensive light extraction process to the existing device fabrication process. According to this invention, adding just one more step of metallization after backside lapping and polishing allows a significant light output increase. With finer polishing using dry etching, in some cases, the light output can be as much as a factor of four without a substantial increase in production cost.

[0057] The diode of the present invention improves light intensity of a diode such as an AlInGaN-based light emitting diode (LED) using a reflective coating. The reflective coating recovers those photons, which would otherwise be absorbed by the substrate or the lead frame in the LED package. This increases the total external quantum efficiency of the quantum well devices. This invention can be applied not only to the current commercially available blue, green, red and white LEDs but also to other LED devices. Using this technique, the light output was increased by as much as a factor of four as compared to conventional LED devices (without the reflective coating) without significantly sacrificing or changing other characteristics of the diode.

[0058] Although the present invention has been described in detail with reference to GaN technology diodes, the reflector and substrate polishing technique of the present invention can easily be applied to other types of diodes including red LEDs and laser diodes including VCSELs. Although red LEDs do not use GaN, the substrate of the red LEDs may just as easily be polished and a reflective layer can easily be attached to the polished surface of the substrate, as described above. Such technique also recovers the photons to increase the light output of the diode. Similar technique is also applicable for laser diodes.

[0059] It will be apparent to those skilled in the art that various modifications and variation can be made in the present invention without departing from the split or scope of the invention. Thus, it is intended that the present invention cover the modifications and variations of this invention provided they come within the scope of the appended claims and their equivalents.