WO2002097902A1

WO2002097902A1 - Semiconductor led device

Info

Publication number: WO2002097902A1
Application number: PCT/KR2002/001027
Authority: WO
Inventors: Tae Kyung Yoo; Chang Tae Kim
Original assignee: Epivalley Co., Ltd.
Priority date: 2001-05-31
Filing date: 2002-05-30
Publication date: 2002-12-05
Also published as: KR100422944B1; KR20010070709A

Abstract

The present invention relates to a semiconductor LED device, and in particular, to a semiconductor LED device comprising a pumping layer and an active layer. The pumping layer has excellent luminous efficiency due to hetero-joined multi-layers. The active layer is formed of homo-joined multi-layers that transforms all received light into light of desired wavelength due to a smaller band gap. Light is generated in the pumping layer consisting of AlGaInN containing less In, projected onto the active layer containing much In, and then emitted. As a result, the blue shift caused by current is reduced, thereby resulting in improving the efficiency. In addition, light of various wavelength can be obtained in a device because light of two or more wavelength can be emitted in an LED device. The device formed by a continuous growth process has the excellent reproducibility, thereby resulting in improving mass production. When white light is formed, fluorescent materials are not used, thereby resulting in improving the efficiency. A single colored LED of single wavelength wherein the pumping layer is transformed into the wavelength of the active layer can be embodied by regulating the thickness and the number of the active layer and improving absorptance in the active layer.

Description

SEMICONDUCTOR LED DEVICE

BACKGROUND OF THE INVENTION

1. Field of the Invention The present invention generally relates to a semiconductor light emitting diode (hereinafter, referred to as 'LED') device, and more particularly, to an AlGalnN-based LED device comprising a pumping layer having high luminous efficiency and an active layer, wherein the pumping layer pumps light into the active layer emitting light of desired wavelength to convert the wavelength, thereby implementing LED device of a single color having its wavelength converted in one device, and LED device of white light by mixing two wavelengths.

2. Description of the Prior Art Fig. 1 is a cross-sectional diagram illustrating an embodiment of the conventional LED device using an insulating substrate.

A conventional AlGalnN-based LED device 1 comprises an insulating substrate 10 consisting of sapphire or quartz. A buffer layer 11, an n-type AlGalnN layer 12, an AlGalnN active layer 13, a p-type AlGalnN layer 14 and a transparent electrode 15 are sequentially stacked on the substrate 10. A p-type metal electrode 18 is formed on a first portion of the transparent electrode 15. An n- type metal electrode 17 is formed on an exposed portion of the n-type AlGalnN layer 12 formed by removing a second portion of the transparent electrode 15, and portions of the p-type AlGalnN layer 14, the active layer 13 and the n-type AlGalnN layer 12 therebelow. Fig. 2 is a cross-sectional diagram illustrating another embodiment of the conventional LED device using a conductive substrate such as SiC or Si. A buffer layer 20, an n-type AlGalnN layer 21, an AlGalnN active layer 22 and a p- type AlGalnN layer 23 are sequentially epitaxially grown on a fist side of a conductive substrate 19 using an MOCVD (Metal Organic Chemical Vapor Deposition) method. A p-type metal electrode 24 is formed on the p-type AlGalnN layer 23. An n-type metal electrode 24 is formed on a second side of the conductive substrate 19.

As shown in the above-described conventional LED devices, a common compound semiconductor light device has a structure in which holes transmitted through a p-type metal electrode combine with electrons transmitted through an n- type metal electrode in a single or multi layered active layer to emit a light corresponding to a bandgap of an active layer composition. Most of the light emitted from the active layer is emitted through an upper and an lower surfaces of the active layer. This is possible because the AlGalnN-based LED devices comprises the transparent electrode on the top of the active layer and the substrate which is transparent to light on the lower portion of the device.

Generally, significant characteristics of the LED such as an output and a wavelength are determined by an active layer. Accordingly, crystalline structure composition of an active layer is very important.

In a semiconductor device having a conventional AlGalnN-based LED structure, a light with one wavelength is emitted from an active layer. As a result, an LED package 3 shown in Fig. 3 is required to obtain white light.

An LED device 1 of Fig. 1 is mounted on an upper portion of a metal lead frame 29. Metal electrodes of the LED device are connected to the metal lead frame using wires. The LED device 1 is covered with fluorescent material 26. A body molded in a transparent resin is formed.

In the white light LED package 3, a light with a first wavelength 27 is emitted from the LED device by a voltage current applied to the LED device. When this light is incident on the fluorescent materials, a light with a second wavelength 28 is generated by fluorescent materials, and a mixed light of the lights with the first and the second wavelength 27 and 28 is obtained. When the colors of these two lights are complementary to each other, white light can be obtained from mixing only two lights of two wavelengths. For example, when fluorescent material containing a YAG is excited using the deep blue LED light having a wavelength of 450nm to generate a yellow light having a wavelength of 590nm which is a complementary color of a blue light having wavelength of 450nm, an LED package of white light which is a mixture of the two colors is obtained. Here, the white light is obtained by adjusting components of a YAG fluorescent layer to tune a yellow wavelength and adjusting a thickness of a fluorescent layer to adjust a ratio of intensity of two lights.

The white light LED package can substitute a light bulb or a light source of a back light of a display. However, while the conventional white light LED package has a relatively simple structure and manufacturing process, the reliability of fluorescent materials is inferior to an LED device resulting in fading of color or deterioration of luminous efficiency when used for a long period.

In addition, it is difficult to adjust components and the thickness of fluorescent materials according to a chip-to-chip variation of a wavelength and a power of a first blue LED so that a white light tuning is difficult in a mass production. SUMMARY OF THE INVENTION

Accordingly, it is an object of the present invention to provide a semiconductor LED device having a pumping layer capable of high output wherein light generated from the pumping layer is absorbed in an active layer and the absorbed light is recombined in the active layer to obtain a light with a desired wavelength.

In order to achieve the above-described object of the present invention, there is provided a semiconductor LED device, comprising: an active layer consisting of at least one Al_xGa_yIn_zN/Al_xlGa_y,In_zlN layer having homo-junction; a pumping layer consisting of at least Al_aGa_bIn_cN/Al_alGa_blIn_clN layer having hetero- junction, wherein the active layer and the pumpin layer are vertically stacked and a bandgap of an Al_xGa_yIn_zN in a well portion of the active layer is smaller than that of an Al_aGa_bIn_cN in a well portion of the pumping layer. The semiconductor LED device of the present invention is characterized in that an electrode formed on the active layer remote from the pumping layer is a front electrode, the front electrode comprises at least one window formed by patterning to emit light bilaterally, width of the window is 0~300μm, and the substrate is a transparent or a conductive substrate. There is also provided a semiconductor LED device comprising: an

AlGalnN buffer layer, an n-type AlGalnN layer, an active layer comprising repeatedly stacked Al_xGa_yIn_zN/Al_xlG<a_ylIn_zlN layers having homo-junction, an n- type AlGalnN layer, a pumping layer comprising repeatedly stacked Al_aGa_bIn_cN/Al_alGa_blIn_clN layers having hetero-junction, a p-type AlGalnN layer, a p-type metal electrode sequentially stacked on the transparent substrate, and an n- type metal electrode formed on one portion of the n-type AlGalnN layer, wherein a bandgap Eg(Al_xGa_yIn_zN) of an Al_xGa_yIn_zN layer corresponding to the active layer is smaller than a bandgap Eg(Al_xlGa_yiIn_zlN) of an Al_xlGa_ylIn_zlN layer corresponding to the barrier layer, and Eg(Al_aGa_bIn_cN) is smaller than Eg(Al_alGa_blIn_clN), where x+y+z=l and xl+yl+zl=l .

The semiconductor LED device of the present invention is characterized in that the p-type AlGalnN layer and the active layer are interposed between the pumping layer and the p-type AlGalnN layer to emit light of single or multiple wavelengths, the p-type metal electrode comprises windows formed by patterning to emit light bilaterally, and the LED device comprises a conductive substrate instead of the transparent substrate, a transparent electrode instead of the p-type metal electrode.

BRIEF DESCRIPTION OF THE DRAWINGS The present invention will be explained in terms of exemplary embodiments described in detail with reference to the accompanying drawings, which are given only by way of illustration and thus are not limitative of the present invention, wherein:

Fig. 1 is a cross-sectional diagram illustrating an embodiment of the conventional LED device;

Fig. 2 is a cross-sectional diagram illustrating another embodiment of the conventional LED device;

Fig. 3 is a cross-sectional diagram illustrating a white light LED package having the LED device of Fig. 1 mounted therein; Fig. 4 is a cross-sectional diagram illustrating an LED device in accordance with a first embodiment of the present invention;

Fig. 5 is a band diagram of the LED device of Fig. 4;

Fig. 6 is a cross-sectional diagram illustrating a LED device in accordance with a second preferred embodiment of the present invention; and Fig. 7 is a cross-sectional diagram illustrating a LED device in accordance with a third embodiment of the present invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

A semiconductor LED device of the present invention will be described in detail with reference to the accompanying drawings.

The principle of the present invention will be described hereinafter. Generally, the efficiency of an AlGalnN-based light device is decreased as the amount of In in an active layer is increased. A light emitted from an InGaN active layer containing about 35% of In has a wavelength of about 470nm. The power of output in this case is about 3~5mW depending on the structure of a device. When active layers contains about 5% of In or about 22% of In, lights from those active layers have wavelengths of 380nm or 430nm, respectively. In this case, powers of outputs are more than about lOmW even with the same structure. This is due to deterioration in the amount of crystal as the quantity of In in an active layer increases, resulting in low efficiency. When the amount of In in an active layer is increased, a phenomenon of a light being converted into a light with a shorter wavelength during the process of injection of current into the active layer, i.e. a blue shift phenomenon is increased, thereby deteriorating the efficiency. A novel structure according to the present invention takes advantages of a pumping layer having low In composition and an active layer having high In composition, particularly the active layer absorbing light to convert into light without recombination of electrons and holes by a current. Thus, light with a desired wavelength can be obtained from the same conductive type such as n-n active layer rather than from the conventional p-n structure.

According to the principle described above, inefficient lights having wavelengths of 470nm(blue), 525nm(green) and 635nm(red) can be easily obtained by using a pumping layer emitting highly efficient light having short wavelength such as wavelengths of 380~430nm. In addition, by utilizing this principle, light having more than one wavelength can be obtained from one LED device. Lights with new colors can be obtained by properly combining lights having two or more wavelengths. For example, when a deep blue light having a wavelength of 450nm generated from a pumping layer is absorbed in an active layer to generate a yellow light having a wavelength of 590nm, which is a complementary color of blue, white light can be obtained by adjusting a thickness of the number of active layers to adjust the amount of blue light absorbed in and penetrating through the active layer.

As an alternate method, a light having two wavelengths can also be obtained by emitting a portion of light of the pumping layer to combine with light from an active layer.

Since fluorescent materials are not used in an LED of multi-wavelengths or white light, the reliability of a device is improved. In addition, since epitaxial wafer obtained from an epitaxial growth process is applicable to the conventional blue LED manufacturing process, the manufacturing process is simplified and a device with uniform characteristics can be obtained, thereby improving yield. Fig. 4 is a cross-sectional diagram illustrating a semiconductor LED device in accordance with a first embodiment of the present invention, wherein n-n type homo-junction active layers and pumping layers are vertically stacked.

Referring to Fig. 4, a buffer layer 31, an n-type AlGalnN layer 32, a multi- layered active layer 33 comprising repeatedly stacked Al_xGa_yIn_zN/Al_xlGa_ylIn_zlN layers, an n-type AlGalnN layer 34, a multi-layered pumping layer 35 comprising repeatedly stacked Al_aGa_bIn_cN/Al_alGa_blIn_cιN layers and a p-type AlGalnN layer 36 are sequentially stacked on a transparent substrate 30 consisting of alumina, sapphire or quartz, preferably using MOCVD method. Here, an upper portion of the n-type AlGalnN layer 32 formed on a first portion of the transparent substrate 30 and stacked layers formed thereon are removed. An n-type metal electrode 37 is formed on the exposed portion of the n-type AlGalnN layer 32. A p-type metal electrode 38 consisting of opaque material is formed on the p-type AlGalnN layer 36. Although each of the active layer 33 and the pumping layer 45 is a stacked structure comprising repeatedly stacked two layers having different bandgaps, they are shown as single layers in Fig. 4. The active layer 33 which is n-type or p-type comprises a stacked structure of Al_xGa_yIn_zN/Al_xlGa_ylIn_zlN layers repeatedly stacked with different compositions. The pumping layer 35 comprises a multi- layer structure of Al_aGa_bIn_cN/Al_a|Ga_bl.In_clN layers having p-n junction. Here, the active layer 33 satisfies the equations x+y+z=l and xl+yl+zl=l . A bandgap Eg(Al_xGa_yIn_zN) of an Al_xGa_yIn_zN layer corresponding to an active layer is smaller than a bandgap Eg(Al_xlGa_ylIn_zlN) of an Al_xlGa_ylIn_z,N layer corresponding to a barrier layer. The principle of operation of the semiconductor LED device according to the present invention is described referring to Fig. 5.

Fig. 5 is a band diagram of the LED device of Fig. 4, wherein barrier structures of the active layer 33 and the pumping layer 35 are simplified as an

AlGalnN layer, i.e. when xl=0, yl = l, zl=0, al=0, bl=l, cl=0. A bandgap 40 of the well portion, Eg(Al_aGa_bIn_cN), of the pumping layer must be larger than a bandgap 41 of the well portion, Eg(Al_xGa_yIn_zN), of the active layer 33.

By a current supplied from two electrodes, holes 43 passed through a p- type AlGalnN layer 36 are restrained in the pumping layer 35 and electrons 42 passed through an n-type AlGalnN layer 32 are restrained in the pumping layer 35. The electrons and the holes are combined in the pumping layer 35 to emit a light 45 corresponding to the bandgap Eg 40 of the pumping layer 35. The light 45 emitted into the lower portion of the pumping layer 35 is absorbed in the active layer 32. The light emitted into the upper portion of the pumping layer 35 is reflected on a p-type metal electrode 38 and reabsorbed in the active layer 32. The absorbed lights are transformed into electrodes 46 and holes 47. The electrons 46 and the holes 47 are restrained by a barrier layer of the active layer 33 and recombined in the active layer 33 to emit a light 48 corresponding to the bandgap Eg 41 of the active layer 33. This light has the lowest energy possible in an LED structure and is emitted entirely through a substrate without being absorbed in any layer. A bandgap increases as the amount of Al increases while a bandgap decreases as the amount of In increases. Bandgaps of the active layer 53 and the pumping layer 55 can be adjusted by utilizing these characteristics.

Fig. 6 is a cross-sectional diagram illustrating a semiconductor LED device in accordance with a second embodiment of the present invention. The semiconductor LED device has a similar structure to that of the LED of Figure 4. However, an active layer is stacked on the upper portion of a pumping layer.

A buffer layer 51, an n-type AlGalnN layer 52, a multi-layered active layer 53 comprising repeatedly stacked Al_xGa_yIn_zN/Al_xlGa_ylIn_zlN layers, an n-type AlGalnN layer 54, a multi-layered pumping layer 55 comprising repeatedly stacked Al_aGa_bIn_cN/Al_alGa_blIn_clN layers, a p-type AlGalnN layer 56, a multi- layered active layer 57, a p-type AlGalnN layer 58 and a p-type metal electrode 59 are sequentially formed on a transparent substrate 50. An n-type metal electrode 60 is formed on the n-type AlGalnN layer 52 at one side of the substrate 50.

Light with multiple wavelengths or white light can be obtained from one device by adjusting composition and thickness of the active layers 53 and 57 because the active layers 53 and 57 are provided on both sides of the pumping layer 55 to emit a light from the both sides.

Also light of different wavelengths can be emitted bilaterally by forming the p-type metal electrode 59 with a transparent electrode. Fig. 7 is a cross-sectional diagram illustrating a semiconductor LED device in accordance with a third embodiment of the present invention. The semiconductor LED device has a similar structure to that of the LED of Fig. 4. However, the semiconductor LED device of Fig. 7 comprises a window for emitting a light formed by patterning one side of a p-type metal electrode. A buffer layer 71, an n-type AlGalnN layer 72, a multi-layered active layer 73 comprising repeatedly stacked Al_xGa_yIn_zN/Al_xιGa_ylIn_zlN layers, an n-type AlGalnN layer 74, a multi-layered pumping layer 75 comprising repeatedly stacked Al_aGa_bIn_cN/Al_aιGa_blIn_clN layers, a p-type AlGalnN layer 76 and a p-type metal electrode 77 are sequentially formed on a transparent substrate 70. The p- type metal electrode 77 is partially removed to form a window 78 exposing the p- type AlGalnN layer 76 and emitting a light therethrough. An n-type metal electrode 80 is formed on one side of the n-type AlGalnN layer 52.

The semiconductor LED device of Fig. 7 can be configured to control the amount of light emitted through the window by adjusting the size of window. A width of the window 78 Wo is preferably 0~300μm. The wavelength of light emitted from each side of the substrate can be set to be different.

A conductive substrate can be used instead of a transparent substrate. In the case of using a conductive substrate a transparent electrode is used for a p-type electrode so that a light is emitted therethrough. The substrates in the above embodiment are all transparent substrates such as sapphire, alumina or quartz substrates, but conductive substrates such as SiC or Si substrates can also be used.

The semiconductor layers may be formed using the methods such as MOCVD (Metal-Organic Chemical Vapor Deposition), MBE (Molecular Beam Epitaxy) or VPE (Vapor Phase Epitaxy).

As discussed above, a semiconductor LED device in accordance with the present invention comprises a highly efficient multi-layered pumping layer having hetero-junctions and a multi-layered active layer having homo-junctions which has smaller bandgap than that of the pumping layer and converts a received light into light of a desired wavelength. Since light is generated in a pumping layer consisting of AlGalnN having low In composition, and then transmitted into the active layer having high In composition to emit a light of a desired wavelength, a blue shift by a current is reduced and luminous efficiency improved. Light of two or more wavelengths can be obtained from one LED device. Since the LED device is formed using one continuous growth process, reproducibility of a device is improved, thereby yield is improved. Since fluorescent materials with low efficiency for generating white light is not used, efficiency is improved.

In addition, a single color LED having one wavelength wherein a light from a pumping layer is converted into a light with a wavelength of an active layer can be embodied by adjusting the thickness and the number of the active layer to increase absorptivity in the active layer.

Claims

WHAT IS CLAIMED IS:

1. A semiconductor LED device, comprising: an active layer on a substrate, formed by stacking a homo-junctioned Al_xGa_yIn_zN/Al_xlG<a_ylIn_zlN layer once or more; a pumping layer, above the active layer, formed by stacking a hetero- junctioned Al_aGa_bIn_cN/Al_alGa_blIn_clN layer once or more, wherein a bandgap of an Al_xGa_yIn_zN in a well portion of the active layer is smaller than that of an Al_aGa_bIn_cN in a well portion of the pumping layer.

2. The semiconductor LED device according to claim 1, further comprising a front electrode formed on a top portion of the active layer.

3. The semiconductor LED device according to claim 2, wherein the front electrode comprises at least one window exposing at least a portion of the active layer.

4. The semiconductor LED device according to claim 3, wherein width of the window ranges from 0 to 300μm.

5. The semiconductor LED device according to claim 1, wherein the substrate is a transparent or a conductive substrate.

6. A semiconductor LED device comprising: an AlGalnN buffer layer, an n-type AlGalnN layer, a first active layer repeatedly stacked homo-junctioned Al_xGa_yIn_zN/Al_xlG.a_y,_ιIn_zlN layers, an n-type AlGalnN layer, a pumping layer repeatedly stacked hetero-junctioned Al_aGa_bIn_cN/Al_aιGa_b,In_clN layers, a first p- type AlGalnN layer and a p-type metal electrode, sequentially stacked on a transparent substrate wherein a part of the n-type AlGalnN layer is exposed with being etched by a predetermined depth, and an n-type metal electrode formed on the exposed portion of the n-type AlGalnN layer, wherein a bandgap Eg(Al_xGa_yIn_zN) of an Al_xGa_yIn_zN layer corresponding to the active layer is smaller than a bandgap Eg(Al_xlGa_ylIn_zlN) of an Al_xlGa_ylIn_zlN layer corresponding to the barrier layer, and Eg(Al_aGa_bIn_cN) is smaller than Eg(Al_alGa_blIn_clN), where x+y+z=l and xl+yl+zl=l .

7. The semiconductor LED device according to claim 6, further comprising a second active layer and a second p-type AlGalnN layer between the pumping layer and the p-type AlGalnN layer to emit light of single or multiple wavelengths.

8. The semiconductor LED device according to claim 6, wherein the p- type metal electrode comprises at least one window exposing the active layer.

9. The semiconductor LED device according to claim 6, wherein the LED device comprises a conductive substrate instead of the transparent substrate, a transparent electrode instead of the p-type metal electrode.