Diode having vertical structure

Abstract

A light emitting diode includes a conductive layer, an n-GaN layer on the conductive layer, an active layer on the n-GaN layer, a p-GaN layer on the active layer, and a p-electrode on the p-GaN layer. The conductive layer is an n-electrode.

Claims

1. A vertical light emitting device, comprising: a first pad; a first metal layer on the first pad, the first metal layer including titanium and gold; a gallium-nitride based semiconductor structure on the first metal layer, the gallium-nitride based semiconductor structure including: a first type semiconductor layer on the first metal layer, an active layer on the first type semiconductor layer; and a second type semiconductor layer, wherein the active layer includes multiple quantum well layer having indium; a second metal layer on the second type semiconductor layer, the second metal layer including platinum; and a second pad on the second metal layer.

2. The light emitting device according to claim 1, wherein the first type semiconductor layer is doped with silicon with a doping concentration of 10.sup.17 cm.sup.3 or greater.

3. The light emitting device according to claim 2, wherein the second type semiconductor layer is doped with magnesium with a doping concentration of 10.sup.17 cm.sup.3 or greater.

4. The light emitting device according to claim 3, further comprising an undoped gallium-nitride layer on the first metal layer.

5. The light emitting device according to claim 4, wherein the undoped gallium-nitride layer is a buffer layer.

6. The light emitting device according to claim 4, wherein a thickness of the undoped gallium-nitride layer is 30 m to 40 m.

7. The light emitting device according to claim 3, wherein the active layer includes 22% to 40% of indium composition.

8. The light emitting device according to claim 3, wherein the active layer comprises aluminum-indium-gallium-nitride.

9. The light emitting device according to claim 3, further comprising a transparent conductive layer on the second type semiconductor layer.

10. The light emitting device according to claim 9, wherein the transparent conductive layer comprises oxide such as indium-tin-oxide.

11. The light emitting device according to claim 1, wherein the second pad on the second metal layer has a thickness of 0.5 m or higher.

12. The light emitting device according to claim 1, wherein the first type is n-type and the second type is p-type.

13. The light emitting device according to claim 1, wherein the first pad on which the first metal layer is located is at a bottom of the device.

14. The light emitting device according to claim 1, wherein a width of the first metal layer is greater than a width of the second pad.

15. The light emitting device according to claim 1, wherein the first metal layer is overlapped with the second pad in a first direction at a cross-sectional edge of the vertical light emitting device, the first direction being a thickness direction of the vertical light emitting device.

16. The light emitting device according to claim 1, further comprising a cladding layer between the active layer and the second type semiconductor layer.

17. The light emitting device according to claim 16, wherein the cladding layer comprises AIGaN.

Description

BRIEF DESCRIPTION OF THE DRAWING

(1) 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.

(2) In the drawings:

(3) FIG. 1 shows a conventional lateral structure LED;

(4) FIG. 2 shows a vertical structure LED according one embodiment of the present invention;

(5) FIGS. 3-8 show the manufacturing steps for forming the light emitting diode according to the present invention; and

(6) FIG. 9 shows another embodiment of the vertical structure LED of the present invention.

DETAILED DESCRIPTION OF THE INVENTION

(7) Reference will now be made in detail to the present invention, examples of which are illustrated in the accompanying drawings.

(8) FIG. 2 shows a vertical structure light emitting diode in accordance with one embodiment of the present invention. Referring to FIG. 2, the vertical LED includes an re-contact 500. A buffer layer made of GaN 105 is on the n-contact 500. An n-GaN layer 140 is on the buffer layer 105. An active layer 160 made of, for example, a multiple quantum well (MQW) layer including AlInGaN is on the n-GaN layer 140. A p-GaN layer 180 is on the active layer 160. A p-contact layer 220 is on the p-GaN layer 180. A p-electrode 240 and a pad 260 are formed on the p-contact layer 220.

(9) In the LED shown in FIG. 2, the n-contact 500 may serve two functions. First, the n-contact 500 serves an electrical function as a conductive material. Second, the re-contact 500 can also serve to reflect photons emitted from the active layer 160 to the re-contact 500. This increases the brightness of the LED since photons that would otherwise be absorbed or wasted in some other manner would reflect off of the n-contact 500 and emit light. A material having good reflective characteristics such as that of a mirror can be used as the n-contact 500. One such example is a polished aluminum layer. Such reflective characteristics are described in more detail in an application entitled DIODE HAVING HIGH BRIGHTNESS AND METHOD THEREOF by Myung Cheol Yoo, U.S. patent application Ser. No. 09/905,969, filed on Jul. 17, 2001 (now U.S. Pat. No. 7,067,849), by the same assignee as the present application, the entirety of which contents is hereby incorporated by reference in this application. The material for the re-contact 500 is discussed in detail below.

(10) A benefit of this vertical structure LED of the present invention is the significant reduction in the size of the LED chip as compared to the lateral structure of the conventional LED. Due to its small chip size, significantly more chips can be formed on the same size wafer, such as sapphire. Moreover, the number of process steps for forming the vertical structure LED of the present invention is reduced, as discussed in more detail below. FIGS. 3-8 schematically illustrate a process for manufacturing vertical structure GaN-based light emitting diodes (LEDs) according to the present invention. In order to fabricate GaN-based LEDs, sapphire substrate has been generally used since sapphire is very stable and relatively inexpensive. The epitaxial layer quality of the various GaN layers grown on sapphire substrate is superior to other substrate materials due to their thermal stability and the similar crystal structure of the GaN.

(11) Referring to FIG. 3, a buffer layer 120, is formed on a transparent substrate 100, beneficially a sapphire substrate. The buffer layer 120, which eventually replaces the function of the sapphire substrate 100, may be formed as one, two or three layers. For example, the buffer layer 120 may have only the n-GaN layer that is grown by VPE. For a two layer buffer layer, a first layer of GaN layer 110 is grown on the sapphire substrate such as by VPE and a second layer of an n-GaN layer 120 is grown on the GaN layer 110 such as by VPE. For a three layer buffer layer, a first layer of GaN layer 110 is grown on the sapphire substrate such as by VPE, a second layer of an undoped GaN layer 130 is grown on the first layer of GaN layer 110 such as by VPE, and a third layer of an n-GaN layer 120 is grown on the undoped GaN layer 130 such as by VPE.

(12) The GaN layer 110 may be formed to have a thickness in a range of about 40-50 nm. The undoped GaN layer 130 may be formed to have a thickness in a range of about 30-40 .mu.m. The n-GaN layer 120 may be formed to have a thickness of about 1-2 .mu.m. For n-GaN 120, silene gas (SiH.sub.4) may be used as the n-type dopant.

(13) Referring to FIG. 4, an n-type epitaxial layer such as n-GaN 140 is epitaxially grown on the buffer layer 120 by a metal organic chemical vapor deposition (MOCVD) method. Beneficially, a chemical cleaning step (not shown in the figure) of the buffer layer 120 grown by VPE method can be added prior to growing the n-GaN layer 140 by MOCVD method in order to obtain a good quality of the n-GaN epitaxial layer 140. 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.

(14) Referring to FIG. 5, an active layer 160 such as an AlInGaN multiple quantum well (MQW) layer is formed by MOCVD method on the n-GaN layer 140. The active layer 160 may be of any suitable structure including a single quantum well layer or a double hetero structure. In this instance, the amount of indium (In) determines whether the diode takes on a green color or a blue color. For an LED with blue light, about 22% of indium may be used. For an LED with green light, about 40% of indium 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 by MOCVD method using, for example, CP.sub.2Mg as a p-type dopant 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.17cm.sup.3 or greater.

(15) Referring to FIG. 6A, the sapphire substrate 100 is separated from other layers preferably by a laser lift-off method. Other suitable techniques may be used to separate the sapphire substrate 100 from the other layers. The other layers include the buffer layer 120, n-GaN layer 140, active layer 160, and the p-GaN layer 180. By removing the sapphire substrate 100, which is an electrical insulator, from the device, an n-metal contact can be formed under the n-type GaN buffer layer 120, which is an electrical conductor.

(16) Referring to FIG. 6B, after the substrate 100 is removed, the layers below the buffer layer 120 may be removed as well using, for example, dry etching. This step will expose the n-GaN buffer layer 120 that will be electrically attached to the n-contact 500, as shown in FIG. 8.

(17) Referring to FIG. 8, 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, indium-tin-oxide (ITO). A p-type electrode 240 is formed on the transparent conductive layer 220. An n-type electrode 500 is formed on the bottom of the buffer layer 120. The p-type electrode 240 may be made of any suitable material including, for example, Ni/Au, Pd/Au, Pd/Ni and Pt. The n-type electrode 500 may be made of any suitable material including, for example, Ti/Al, Cr/Au and Ti/Au. 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 0.5 .mu.m or higher. Unlike the p-type electrode 240, the n-type electrode 500 does not require a pad, although one can be used, if desired.

(18) FIG. 9 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 by MOCVD method using CP.sub.2Mg as a p-type dopant. The cladding layer 170 enhances the performance of the LED device.

(19) According to the present invention, there are many advantages compared with both conventional lateral and vertical GaN-based LEDs. Compared with the conventional lateral structure GaN-based LEDs, the manufacturing process according to the present invention increases the number of LED devices fabricated on a given wafer size, since there is no n-metal contact on top of the devices. The device dimension can be reduced, for example, from 250.times.250 .mu.m to about 160.times.160 .mu.m or smaller. By not having the n-metal contact above the substrate or on top of the device, according to the present invention, the manufacturing process is significantly simplified. This is because additional photolithography and etch processes are not required to form the n-metal contact and there is no plasma damage which are often sustained on the n-GaN layer in the conventional lateral structure GaN-based LEDs. Furthermore, the LED devices fabricated according to the present invention are much more immune to static electricity, which makes the LED more suitable for high voltage applications than conventional lateral structure LED devices:

(20) In general, the deposition method of VPE is much simpler and requires less time to grow epitaxial layers with certain thickness than the deposition method of MOCVD. Therefore, the fabrication process is more simplified and the process time is more reduced even compared with those of the conventional vertical GaN-based LEDs in that the manufacturing process according to the present invention does not require growing buffer and n-GaN layers by MOCVD method. In total, the number of manufacturing steps is reduced, for example, from 28 steps with the conventional method to 15 steps with the method of the present invention. In addition, the manufacturing cost is reduced considerably compared with the conventional vertical structure GaN-based LEDs, which use silicon carbide (SiC) as a substrate, which can be 10 times more expensive than that of a sapphire substrate. Moreover, the method according to the present invention provides better metal adhesion between bonding pads and both n and p contacts than the conventional vertical structure GaN-based LEDs.

(21) With the present invention, mass production of GaN-based LEDs at an inexpensive cost is made possible without sacrificing or changing the desired characteristics of the LEDs. Moreover, the vertical structure of the LED of the present invention, with an added feature of a reflective bottom n-contact, enhances the brightness of the LED. This invention can be applied not only to the current commercially available blue, green, red and white LEDs but also to other suitable devices.

(22) Although the present invention has been described in detail with reference to GaN technology diodes, the present invention can easily be applied to other types of diodes including red LEDs and laser diodes including Vertical Cavity Surface Emitting Lasers (VCSELs).

(23) 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.