JP4154142B2 - Light emitting diode and manufacturing method thereof - Google Patents

Description

[0001]
BACKGROUND OF THE INVENTION
The present invention relates to a light emitting diode and a method for manufacturing the same, and more particularly, for example, to a light emitting diode with improved light extraction efficiency and a method for manufacturing the same.
[0002]
[Prior art]
A conventional light emitting diode (LED) will be described with reference to FIGS.
[0003]
FIG. 5 shows a conventional LED, which is a basic form of LED. The structure of this LED is as follows. A light emitting layer 22 and a current diffusion layer 23 are formed on the semiconductor substrate 21. An upper electrode 25 is formed on the current diffusion layer 23, and a lower electrode 24 is formed on the lower portion of the substrate 21. The light emitting layer 23 has a PN junction, for example, the upper side is P-type and the lower side is N-type. In this case, the substrate 21 is N-type, and the current diffusion layer 23 is P-type.
[0004]
In the LED configured as described above, a current supplied from the upper electrode 25, that is, a hole spreads left and right inside the current diffusion layer 23 and is supplied to the entire light emitting layer 22. On the other hand, the current, that is, electrons supplied from the lower electrode 24 spreads left and right inside the substrate 21 and is supplied to the entire light emitting layer 22. Inside the light emitting layer 22, holes and electrons respectively supplied from above and below the light emitting layer 22 are recombined to emit light.
[0005]
Here, luminance is one of the characteristics of the LED, and it is important that this luminance is high. The luminance depends on the light emission efficiency, and is proportional to the extraction efficiency of the emitted light to the outside. In order to increase the luminous efficiency and the like, various improvements have been made to the structure of the light emitting layer 22 and the current diffusion layer 23. Hereinafter, this will be described using some examples.
[0006]
FIG. 6 shows a first LED which is an improvement of the basic LED. In this LED structure, an N-type GaAs light-emitting layer 27 and a P-type GaAs light-emitting layer 28 are formed on an N-type GaAs substrate 26. An upper electrode 30 is formed on the P-type GaAs light emitting layer 28, and a lower electrode 29 is formed below the N-type GaAs substrate 26. The P-type GaAs light emitting layer 28 also serves as a current diffusion layer.
[0007]
In this LED manufacturing method, the P-type GaAs light emitting layer 28 is formed by liquid phase epitaxial growth using Si as a dopant. IV group Si is an amphoteric dopant with respect to III-V group GaAs, and the N-type GaAs light-emitting layer 27 and the P-type GaAs light-emitting layer 28 are separated by a single liquid phase epitaxial growth performed from high temperature to low temperature. It is formed. In general, the thickness of the substrate 26 is 150 to 300 μm. The N-type GaAs light emitting layer 28 is 5 to 50 μm. The P-type GaAs light emitting layer 28 is about 10-100 μm thick.
[0008]
This LED can be manufactured inexpensively with a simple process. However, since the light emitting layers 27 and 28 are simple PN junctions, the light emission efficiency is not so high. In addition, since the P-type GaAs light-emitting layer 28 is thick, the emitted light is absorbed in the P-type GaAs light-emitting layer 28, and thus the light extraction efficiency is low.
[0009]
FIG. 7 shows a second LED which is an improvement of the basic LED. In this LED, a P-type AlGaAs current diffusion layer 34 is formed on a P-type GaAs light emitting layer 33 by liquid phase epitaxial growth. Since other parts have the same configuration as the first LED, description thereof is omitted. In this LED, since the current diffusion layer 34 has a wide band gap, the light emitted from the light emitting layers 32 and 33 is not absorbed. For this reason, the light extraction efficiency is better than that of the first LED, and the luminance is high.
[0010]
FIG. 8 shows a third LED which is an improvement of the basic LED. This LED uses a DH (Double Hetero) structure with high luminous efficiency as a light emitting layer. The structure of this LED will be briefly described as follows. On the N-type GaAs substrate 37, an InGaAlP-based epitaxial layer is formed by MOCVD. More specifically, a light emitting layer including an N-type cladding layer 38, an active layer 39, and a P-type cladding layer 40 is formed on the N-type GaAs substrate 37. A current diffusion layer 41 is further formed on the light emitting layer. An upper electrode 43 is formed on the current diffusion layer 41, and a lower electrode 42 is formed below the N-type GaAs substrate 37.
[0011]
In this LED manufacturing method, the InGaAlP compound can be grown epitaxially on the N-type GaAs substrate 37 because the lattice constant matches GaAs when the In composition is 0.5. The band gap can be adjusted by changing the composition ratio of Al and Ga while maintaining the lattice matching. The Al composition ratio of the active layer 39 is adjusted to a desired emission wavelength, and both the cladding layers 38 and 40 have a higher Al composition ratio than that of the active layer 39 to increase the band gap. The current diffusion layer 41 also has a high Al composition ratio and a wide band gap.
[0012]
Thus, by making the light emitting layers 38, 39, and 40 have a DH structure, holes and electrons are confined in the active layer 39, and the light emission efficiency is increased. In addition, since the emitted light is not absorbed by the wide band gap layers 38, 40, 41, the emitted light can be extracted efficiently.
[0013]
In order to further increase the light extraction efficiency of the third LED, there is an LED in which the substrate 37 is made of a transparent material, for example, GaP. In the LED manufacturing method in this case, since InGaAlP has a lattice constant different from that of GaP and cannot be epitaxially grown, the InGaAlP layer grown on GaAs is bonded to the GaP substrate by wafer bonding.
[0014]
In addition to the basic LED and the first to third LEDs described above, the active layer is an MQW structure LED in which a thin active layer and a barrier layer are repeated, or only one side of the PN junction is a layer with a wide band gap. An LED having an SH (Single Hetero) structure is also known.
[0015]
[Problem to be Solved by the Invention]
In the LED as described above, the lower surface of the LED is mounted on an envelope, and emitted light is extracted from the upper surface and side surfaces of the LED. This light is often extracted from the upper surface in particular because the substrate is thick. Here, since the electrode does not transmit light, the light emitted from the light emitting layer directly below the upper electrode has a high rate of being wasted because it cannot be extracted. In the above-described LED, although the current from the upper electrode is diffused left and right by the current diffusion layer, the current is supplied most to the light emitting layer directly below the upper electrode, so the light emitting layer immediately below the upper electrode. A lot of useless light emission occurs.
[0016]
Thus, there are LEDs that attempt to solve this problem. For example, there is an LED that supplies an electric current to the light emitting layer from the outside of the upper electrode by attaching an X-shaped leg to the upper electrode that is usually circular. In addition, as shown in FIG. 9, there is an LED (fourth LED) provided with a current block 47 that prevents current from flowing below the upper electrode 48. Here, reference numeral 44 denotes a substrate, reference numeral 45 denotes a light emitting layer, reference numeral 46 denotes a current diffusion layer, and reference numeral 49 denotes a lower electrode. The current block 47 is provided at various positions in the current diffusion layer 46, between the current diffusion layer 46 and the light emitting layer 45, between the light emitting layer 45 and the substrate 44, etc. other than just below the upper electrode 48. As the structure of the current block 47, there are various structures using an insulating layer, a PN junction, a heterojunction barrier, a cavity and the like.
[0017]
Thus, although the device for transferring the light emitting portion of the LED to a portion other than the light emitting layer directly below the upper electrode has been devised, the light emission directly below the upper electrode could not be effectively suppressed. In addition, there is a problem that light emitted from a light-emitting layer other than just below the upper electrode, that is, light emitted from the light-emitting layer directly below the upper electrode, enters directly below the upper electrode, and the incident light is wasted.
[0018]
Therefore, the present invention has been made in view of the above problems, and suppresses light emission in the light emitting layer directly below the upper electrode of the light emitting diode, and light emitted from the light emitting layer directly below the upper electrode is on the upper side. The object is to prevent the light from entering directly under the electrode and increase the light extraction efficiency.
[0020]
In the method of manufacturing a light emitting diode according to the present invention, a conductive layer and a light emitting layer are sequentially formed on a dummy substrate, and a tapered groove having a bottom narrower than an opening is formed from the conductive layer to the light emitting layer. The substrate on one electrode side is integrally bonded to the surface of the light emitting layer, the dummy substrate is removed, and the exposed conductive layer is used as the other electrode side.
[0021]
DETAILED DESCRIPTION OF THE INVENTION
FIG. 1B is a top plan view of a light emitting diode (LED) to which the present invention is applied. FIG. 1A is a partial cross-sectional view taken along the line AA ′ of the LED.
[0022]
First, this embodiment will be briefly described as follows.
[0023]
The LED used in the present embodiment is used in such a manner that light is emitted from the light emitting layer by flowing current from above and below the semiconductor substrate (substrate) on which the light emitting layer is formed, and the emitted light is extracted from above the substrate. For example, it is used for display.
[0024]
More specifically, in the LED, a light emitting layer and a current diffusion layer (conductive layer) are formed in this order on a semiconductor substrate. A patterned circular insulating layer is formed on the current spreading layer, and a circular upper electrode is provided on the insulating layer. Outside the upper electrode, a wiring layer (wiring electrode) electrically connected to the upper electrode is formed in a rectangular shape so as to surround the upper electrode. A ring-shaped groove is formed in the region of the current diffusion layer between the upper electrode and the wiring layer so as to surround the upper electrode. The groove penetrates the light emitting layer from the current diffusion layer and reaches the inside of the semiconductor substrate. This groove is filled with air. On the other hand, a lower electrode is provided below the semiconductor substrate.
[0025]
In the LED configured as described above, when a voltage is applied between the upper electrode and the lower electrode, holes carried from the upper electrode through the wiring layer and electrons carried from the lower electrode through the substrate However, they combine in the light emitting layer outside the groove and emit light. This light is extracted from above the LED outside the groove. The light directly below the upper electrode is reflected by the groove and extracted from above the LED. On the other hand, in the light emitting layer inside the groove, no light is emitted because holes from the upper electrode do not reach the insulating film light emitting layer and the groove. In this embodiment, light is efficiently extracted by suppressing light emission in the light emitting layer directly below the upper electrode having low extraction efficiency and emitting light in the light emitting layer outside the upper electrode having high extraction efficiency. This is one of the major characteristics. Hereinafter, this embodiment will be described in more detail.
[0026]
First, the structure of the LED in this embodiment will be described.
[0027]
First, as can be seen from FIG. 1A, a light emitting layer 2 and a current diffusion layer 3 are provided on a semiconductor substrate (substrate) 1.
[0028]
Next, as can be seen from FIGS. 1B and 1A, an insulating layer 5 is formed on the diffusion current layer 3. The insulating layer 5 includes an insulating layer 5a and an insulating layer 5b. The insulating layer 5a is patterned in a circular shape. The insulating layer 5b is patterned in a straight shape, and a pair is formed with the insulating layer 5a interposed therebetween. An upper electrode (pad) 6 patterned in a circular shape is formed on the insulating layer 5a.
[0029]
Next, as can be seen from FIGS. 1A and 1B, a ring-shaped groove 4 is dug outside the insulating layer 5a so as to surround the insulating layer 5a. However, as can be seen from FIG. 1B, the groove 4 is not provided in two portions overlapping the insulating layer 5b. The groove 4 does not have to be continuous in this way. That is, even if the groove 4 is partially cut, the groove 4 may be only a part. Further, the groove 4 may have a shape other than the ring shape, for example, a square shape. In general, a square groove formed by straight lines is easier to make a mask. As can be seen from FIG. 1A, the groove 4 penetrates the light emitting layer 2 and reaches the inside of the substrate 1, and a substance having a refractive index lower than that of the constituent material of the LED is inside the groove 4. Filled. In the present embodiment, air having a refractive index of 1 is filled.
[0030]
As can be seen from FIGS. 1B and 1A, a wiring layer 7 is formed on the current diffusion layer 3 and the insulating layer 5. The wiring layer 7 includes a wiring layer 7a and a wiring layer 7b. The wiring layer 7 a is formed in a square shape so as to surround the groove 4. The wiring layer 7b is electrically connected to the upper electrode 6 as can be seen from FIG.
[0031]
In addition, although not shown, a lower electrode is formed on the lower side of the semiconductor substrate 1.
[0032]
Next, the operation of the LED having such a structure will be described with reference to FIG.
[0033]
When a voltage is applied between the upper electrode 6 and the lower electrode (not shown), a current flows through the upper electrode 6. This current is transmitted to the wiring 7 a via the wiring layer 7 b and flows to the current diffusion layer 3 a outside the groove 4. Then, the current to the current diffusion layer 3a is further diffused inside the current diffusion layer 3a and further supplied to the light emitting layer 2a to emit light in the light emitting layer 2a. Of the emitted light, the upward light is extracted from above the current diffusion layer 3a as it is. Of the emitted light, the light in the direction of the upper electrode 6 is reflected by the groove 4 as shown in FIG. 2, and is taken out from above the current diffusion layer 3a.
[0034]
As described above, according to the present embodiment, since the insulating layer 5a is provided below the upper electrode 6 and the groove 4 is provided between the upper electrode 6 and the wiring layer 7, the current from the upper electrode 6 is supplied to the wiring layer. The current flows to the current diffusion layer 3a outside the groove 4 through 7a and 7b, and is supplied to the light emitting layer 2a. Since the current from the upper electrode 6 to the current diffusion layer 3b and the light emitting layer 2b is blocked by the insulating layer 5a and the groove 4a, it does not flow. Thereby, unnecessary light emission in the light emitting layer 2b directly below the upper electrode 6 can be suppressed, and light can be emitted in the light emitting layer 2a outside the upper electrode 6 with good extraction efficiency. Further, among the light emitted from the light emitting layer 2 a outside the upper electrode 6, the light directly below the upper electrode 6 is reflected back at the groove 4, so that the light directly below the upper electrode 6 is also The light can be extracted from above the current diffusion layer 3a, and the light extraction efficiency can be increased.
[0035]
Here, the brightness of the LED chip in this embodiment was compared with that of the conventional LED chip shown in FIG. 7 having an epi wafer having the same structure as the epi wafer (layers 1 to 3 in FIG. 1) in this embodiment. State the results. Both LED chip shapes were 300 μm, and the upper electrode diameters were both 100 μm. As can be seen from FIG. 7, the LED chip in FIG. 7 is not provided with components that are one of the major features of the present embodiment, that is, an insulating layer, a groove, and a wiring layer.
[0036]
As a result of comparing the luminance of these LED chips under the condition of 20 mA energization, the LED chip in this embodiment showed 1.3 times the brightness of the LED chip in FIG. As can be seen from this comparison result, in the LED of this embodiment, the insulating layer 5a is provided directly below the upper electrode 6, the groove 4 is provided outside the upper electrode 6, and the upper electrode is provided around the groove 4. Since the wiring layers 7a and 7b electrically connected to the upper electrode 6 are provided, the light emission in the light emitting layer 2b directly below the upper electrode 6 can be suppressed, and the upper electrode 6 out of the light emitted from the light emitting layer 2a. It is possible to reflect the light heading in the direction directly below the groove 4 and extract the light to the outside, thereby improving the light extraction efficiency of the LED.
[0037]
Next, the manufacturing method of LED in this embodiment is demonstrated, referring FIG.
[0038]
As the substrate 1, an N-type GaAs substrate having a diameter of 3 inches and a thickness of 300 μm is used. Next, liquid phase epitaxial growth is performed on the substrate 1 using a source in which poly-GaAs and dopant Si are dissolved in Ga. An N-type GaAs light-emitting layer and a P-type GaAs light-emitting layer are formed on the substrate 1. As a result, a light emitting layer 2 composed of an N-type GaAs light-emitting layer and a P-type GaAs light-emitting layer is formed on the substrate 1. The thickness of the N-type GaAs light emitting layer is 40 μm, and the thickness of the P-type GaAs light emitting layer is 3 μm. When the thickness of the P-type GaAs light-emitting layer reaches 3 μm, the source is discharged, and liquid phase epitaxial growth is performed on the substrate 1 using a new source in which poly-GaAs, Al, and dopant Zn are dissolved in Ga. . Thus, a P-type AlGaAs current diffusion layer 3 having a thickness of 10 μm is formed on the light emitting layer 2 to obtain an epi wafer.
[0039]
Next, silicon oxide films for the insulating layers 5a and 5b are formed on the upper surface of the epi-wafer. Subsequently, a gold electrode is formed on the upper portion of the silicon oxide film and the lower portion of the substrate 1 by vapor deposition. Subsequently, the silicon oxide film and the gold electrode on the silicon oxide film are processed by PEP to form insulating layers 5a and 5b and an upper electrode 6 as shown in FIG. Subsequently, a groove 4 is dug from the current diffusion layer 3 to the inside of the substrate 1 by RIE. However, the groove 4 is not formed in a portion overlapping with the insulating layer 5b. Subsequently, as shown in FIG. 1B, the wiring layer 7 is formed on the current diffusion layer 3 and the insulating layer 5 by a lift-off method or the like. Thereafter, the wafer is cut by dicing to obtain an LED chip.
[0040]
The shape of the LED chip is 300 μm square, and the diameter of the upper electrode 6 is 100 μm. Further, the diameter of the insulating layer 5a is 120 μm, and as can be seen from FIG. 1B, the insulating layer 5b is left up and down with a width of 10 μm. The shape of the groove 4 is a ring shape having an inner diameter of 140 μm and an outer diameter of 150 μm. The depth of the groove 4 is about 20 μm and penetrates the light emitting layer 2. However, as described above, the groove 4 is not provided in the portion corresponding to the lower portion of the insulating layer 5b, as can be seen from FIG. The width of the wiring layer 7 is 5 μm, and the wiring 7 a is a square having a side of 250 μm.
[0041]
FIG. 3 is a partially enlarged view showing a modification of the groove 4 of the LED of FIG. The difference from FIG. 1 is that the groove 8 has a reverse taper shape. Since other parts are the same as those in FIG. 1, the same reference numerals are given to the same parts, and some of the parts are not shown.
[0042]
The operation of this LED chip will be described as follows.
[0043]
By making the groove 8 have an inversely tapered shape, the light (see FIG. 2) in the direction of the upper electrode 6 out of the light emitted from the light emitting layer 2a outside the groove 8 is reflected on the wall surface (reflecting surface) outside the groove 8. reflect. Since the reflecting surface is inclined inward, the reflected light is directed upward. This light is then extracted vertically from the top surface of the LED.
[0044]
As described above, according to the LED shown in FIG. 3, the light reflected in the groove 8 can be extracted in the vertical direction with high extraction efficiency, so that the light extraction efficiency can be further increased as compared with the LED of FIG.
[0045]
Next, a current diffusion layer 10 and a light emitting layer 11 are grown in this order by liquid phase epitaxial growth on a first substrate (dummy substrate) 9, which will be described with reference to FIG. FIG. 4 (a)). Next, a tapered groove 12 having an opening wider than the bottom is formed (FIG. 4B). Next, this is reversed upside down (FIG. 4 (b) -2). Next, this is integrated with the second substrate 13 by wafer bonding such as direct bonding or thermocompression bonding (FIG. 4C). Next, the first substrate 9 is removed to obtain an epi-wafer having a reverse-tapered groove 12. Thereafter, the insulating layer, the upper electrode, the lower electrode, and the like are formed in the same manner as in manufacturing the LED of FIG.
[0046]
【The invention's effect】
According to the present invention, in the light emitting diode, light emission in the light emitting layer immediately below the upper electrode is suppressed and light is emitted in the light emitting layer directly under the upper electrode. Can be taken out well.
[Brief description of the drawings]
FIG. 1 is a partial cross-sectional view of a light emitting diode to which the present invention is applied.
FIG. 2 is a diagram for explaining a path of emitted light in a light emitting diode to which the present invention is applied;
FIG. 3 is a partial cross-sectional view showing a partial modification of the groove of the light emitting diode of FIG.
4 is a view for explaining a manufacturing method of the light emitting diode of FIG. 3; FIG.
FIG. 5 is a cross-sectional view showing a basic form of a conventional light emitting diode.
FIG. 6 is a first conventional light emitting diode which is an improvement of the light emitting diode of FIG. 5;
7 is a conventional second light emitting diode which is an improvement of the light emitting diode of FIG. 5;
8 is a conventional third light emitting diode which is an improvement of the light emitting diode of FIG.
FIG. 9 is a conventional fourth light emitting diode which is an improvement of the light emitting diode of FIG. 5;
[Explanation of symbols]
1 Semiconductor substrate (substrate)
2, 2a, 2b Light-emitting layer 3 3a, 3b Current diffusion layer (conductive layer)
4 Groove 5, 5a, 5b Insulating layer 6 Upper electrode (pad)
7, 7a, 7b Wiring layer (wiring electrode)
8 Groove 9 First substrate (dummy substrate)
10 Current diffusion layer 11 Light emitting layer 12 Groove 13 Second substrate

Claims (1)

A conductive layer and a light emitting layer are sequentially formed on a dummy substrate,
From the conductive layer to the light emitting layer, a tapered groove having a bottom narrower than the opening is formed,
A substrate to be one electrode side is integrally bonded to the surface of the light emitting layer,
The dummy substrate is removed, and the exposed conductive layer is the other electrode side.
Manufacturing method of light emitting diode.

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