WO2005015648A1

WO2005015648A1 - Method of forming grating on substrate and iii-nitride semiconductor light emitting device using the substrate

Info

Publication number: WO2005015648A1
Application number: PCT/KR2004/002029
Authority: WO
Inventors: Chang Tae Kim
Original assignee: Epivalley Co., Ltd.
Priority date: 2003-08-12
Filing date: 2004-08-12
Publication date: 2005-02-17

Abstract

The present invention relates to a method of manufacturing a substrate for use in a III-nitride semiconductor light emitting device and a III-nitride semiconductor light emitting device using the substrate, provides for a III-nitride semiconductor light emitting device including a substrate on which a light emitting unit having an active layer that generates light through recombination of electrons and holes is epitaxially grown, wherein the substrate comprises at least one grating on the surface on which the light emitting unit is epitaxially grown, and the grating includes a sidewall, and the sidewall includes a step and a method of manufacturing the substrate, thereby can increase the external quantum efficiency of the light emitting device.

Description

METHOD OF FORMING GRATING ON SUBSTRATE AND III -NITRIDE SEMICONDUCTOR LIGHT EMITTING DEVICE USING THE SUBSTRATE

[Technical Field] The present invention relates to a method of manufacturing a substrate for use in a III -nitride semiconductor light emitting device and a III -nitride semiconductor light emitting device using the substrate. In the present invention, the III -nitride semiconductor refers to AI_xGa_ylnι-χ-yN (0<x<1 , 0<y<1 , x + y ≤1).

[Background Art] FIG. 1 is a cross-sectional view showing a conventional III -nitride semiconductor light emitting device (WO 03/010831 and WO 02/75821). A manufacturing process will be described briefly. The surface of a substrate 10 is etched to form a grating A. A buffer layer 11 , a lower contact layer 12 composed of an n-type III -nitride semiconductor, an active layer 13 composed of a III -nitride semiconductor and an upper contact layer 14 composed of a p-type III -nitride semiconductor are sequentially grown on the substrate 10. Next, a transparent electrode layer 15 is formed on the upper contact layer 14 that comes in ohmic contact with the transparent electrode layer 15. The upper contact layer 14 and the active layer 13 are mesa-etched to expose the lower contact layer 12. An n-type ohmic metal electrode layer 16 is formed on the lower contact layer 12. Thereafter, a bonding pad 17 is formed on the transparent electrode layer 15. A transparent protection film 18 that covers the resulting surface but exposes the bonding pad 17 and the n-type ohmic metal electrode layer 16, is then formed. In this case, the substrate 10 can employ a sapphire or SiC substrate. FIG. 2 shows an example of a grating A formed on the substrate 10. A hexagonal shape can protrude and a portion between the hexagonal shapes can protrude. In both cases, the similar effect can be obtained. FIGS. 3 and 4 are views for explaining how external quantum efficiency is improved when the grating A is formed on the substrate 10 and when the grating A is not formed on the substrate 10. Referring to FIG. 3, if it is desired that light outputted from the active layer 13 goes into air (refractive index=1.0), as indicated by an optical path (1), i.e., the light escapes upwardly, the light must have an incidence angle of below 23.6° if it is composed of, e.g., GaN (refractive index=2.5). Therefore, light having an incidence angle of over 23.6° is reflected into the device and does not escape from the device, as indicated by an optical path (2). The same phenomenon occurs between the lower contact layer 12 and the substrate 10. If the substrate 10 is composed of sapphire (refractive index=1.8), the critical angle is 46.1°, which is relatively high. Light having an incidence angle of over 46J "returns to the lower contact layer 12, as indicated by an optical path (3). Accordingly, only a very small amount of light can escape outwardly and the remaining light is confined within the device. As this process is repeatedly performed several times, light is abruptly extinguished within the device. However, in the case of FIG. 4 in which the grating A is formed on the substrate 10, light that does not escape outwardly has its optical path changed by an inclined surface S of the grating and can thus escape outwardly, as indicated by an optical path (2). The grating A can be simply made to have a size W1 of 1 to 5 zm and a

distance W2 of 1 to 5/im between the gratings by a photolithography process.

Further, if a dry etching process such as ICP (Induction Coupled Plasma) etching or RIE (Reactive Ion Etching) is employed, the grating A can be made to have a depth of approximately 0J to 1 -ra.

The higher the density of the gratings is, the more the inclined surfaces of the gratings are formed. Therefore, as a probability that light can be refracted or scattered increases, external quantum efficiency also increases. That is, an increase in external quantum efficiency is dependent upon how the inclined surfaces formed by the gratings can be formed on a given substrate over a wide area at high density. When viewed from the top, it is advantageous that the density of the gratings is high. However, there is a limitation in increasing the density of the gratings because resolution through a photolithography process is restricted. [Disclosure] [Technical Problem] It is an object of the present invention to provide a method of manufacturing a substrate for use in a III -nitride semiconductor light emitting device in which a high grating density can be obtained though a photolithography process and the amount of light refracted and scattered on a grating surface are thus increased to improve external quantum efficiency, and a III -nitride semiconductor light emitting device using the substrate. [Technical Solution] According to an aspect of the present invention, there is provided a method of forming a grating in a substrate on which a light emitting unit including an active layer that generates light through recombination of electrons and holes is epitaxially grown, the method including a first step of forming a first mask pattern on the substrate, a second step of forming a second mask pattern on the substrate on which the first mask pattern is formed, wherein the second mask pattern at least partially overlaps with the first mask pattern but does not cover the entire first mask pattern, and a third step of etching the substrate on which the first mask pattern and the second mask pattern are formed, thus forming the grating. In the above, the grating refers to a projection or a concave portion that is formed on the substrate. Further, according to the present invention, there is provided a III -nitride semiconductor light emitting device including a substrate on which a light emitting unit having an active layer that generates light through recombination of electrons and holes is epitaxially grown, wherein the substrate comprises at least one grating on the surface on which the light emitting unit is epitaxially grown, and the grating includes a sidewall, and the sidewall includes a step. Further, the present invention provides a III -nitride semiconductor light emitting device in which a top width of the grating is wider than a bottom width of the grating. Also, the present invention provides a III -nitride semiconductor light emitting device in which the sidewall of the grating gets inclined outwardly as it goes toward the top of the sidewall. [Advantageous Effects] According to the present invention, the density of the gratings on a substrate 10 can be increased and the inclined surface H of the grating can be maximally obtained, by means of stepped sidewall grating B. Accordingly, as the amount of light scattered at the boundary between the substrate 10 and a III -nitride semiconductor layer increases, external quantum efficiency can be increased significantly.

[Description of Drawings] Further objects and advantages of the invention can be more fully understood from the following detailed description taken in conjunction with the accompanying drawings in which: FIGS. 1 to 4 are views for explaining a prior art III -nitride semiconductor light emitting device; FIG. 5 is a cross-sectional view for explaining a III -nitride semiconductor light emitting device according to the present invention; FIGS. 6 and 7 are schematic cross-sectional views for explaining that external quantum efficiency of the present invention is further increased in comparison with a prior art; FIG. 8 is a graph showing the relationship between an optical output and the etching number; FIG. 9 is a view for explaining an exemplary etch pattern according to the present invention; FIG. 10 is a graph showing a current-optical output characteristic of the III -nitride semiconductor light emitting device according to the present invention; FIGS. 11 to 14 are views for explaining a method of forming the gratings B in FIG. 5 according to an embodiment of the present invention; FIGS. 15 to 17 are views for explaining a method of forming the gratings B in FIG. 5 according to another embodiment of the present invention; FIG. 18 is a view for explaining a method of forming the gratings B in FIG. 5 according to still another embodiment of the present invention; and FIG. 19 is a photography showing a sapphire substrate in which steppedsidewall gratings are formed according to the present invention. [Mode for Invention] The present invention will now be described in detail in connection with preferred embodiments with reference to the accompanying drawings. In the drawings, the same reference characters as those in FIG. 1 designate parts that perform the same function. Thus, description on them will be omitted for simplicity. While the present invention has been described with reference to the particular illustrative embodiments, it is not to be restricted by the embodiments but only by the appended claims. It is to be appreciated that those skilled in the art can change or modify the embodiments without departing from the scope and spirit of the present invention. FIG. 5 is a cross-section ai view for explaining a III -nitride semiconductor light emitting device according to the present invention. In FIG. 5, the sidewall of a grating B is stepped or tiered unlike FIG. 1. The area of an inclined surface H in the grating becomes maximized. Thus, external quantum efficiency can increase since a probability of scattering of light that proceeds toward the substrate 10 increases. An n+ type or p+ type III -nitride semiconductor layer, or a superlattice layer composed of an n-type or p-type III -nitride semiconductor material can be interposed between an upper contact layer 14 and a transparent electrode layer 15. The transparent electrode layer 15 can be formed using any one selected from the group consisting of nickel, gold, silver, platinum, chrome, titanium, aluminum; rhodium and palladium, or a combination of two or more thereof. For light transparency, the transparent electrode layer 15 is thinly formed to a thickness of approximately 0.0001 to 10^-m. Preferably, the upper

contact layer 14 is formed to a thickness of 0.01 to 2/tm. FIGS. 6 and 7 are schematic cross-sectional views for explaining that external quantum efficiency of the present invention is further increased in comparison with a prior art. FIG. 6 is concerned with a prior art. Light that arrives at the edge portion of the grating is scattered at various angles and external quantum efficiency can thus increase. Referring to FIG. 7 corresponding to the present invention, as the density of the gratings increases, the probability of scattering and the probability of refraction increase. Also, light scattered at upper edges may collide against lower edges, so that second scattering occurs. Accordingly, more light can exit outwardly compared to FIG. 6 in which stepped sidewalls are not formed. FIG. 8 is a graph showing the relationship between an optical output and the number of etching. From FIG. 8, it can be seen that more light is emitted in twice etching (when a step is formed on the sidewall of the grating) compared to once etching (only grating is formed and a step is not formed on the sidewall of the grating). It can be also seen that an increase in the optical output blunts if the number of etching is 3 or more. FIG. 10 is a graph showing a current-optical output characteristic of the III -nitride semiconductor light emitting device according to the present invention. An optical output in a prior art having a grating is approximately 15% higher than that of the case where the grating is not formed. According to the present invention, however, the optical output is approximately 20% higher than that of the prior art where the grating is formed and approximately 40% higher than that of a structure not having the grating. An increase in the optical output may depend upon the size, depth, period, etc. of the gratings B. FIGS. 11 to 14 are views for explaining a method of forming the gratings B in FIG. 5 according to an embodiment of the present invention. Referring to FIGS. 11 to 14 in conjunction with FIG. 9, a first mask pattern 51 is formed on a substrate 10 (FIG. 11). A first etch pattern B1 is formed on the substrate 10 by means of a dry etching process such as RIE (Reactive Ion Etching), ICP (Inductive Coupled Plasma) etching or the like. The first mask pattern 51 is removed (FIG. 12). Further, a second mask pattern 52 is formed on the substrate 10 (FIG. 13). A second etch pattern B2 is formed by means of a dry etching process (FIG. 14). It is preferred that each of the mask patterns 51 and 52 used in the dry etching process has an etching selectivity higher than that of the substrate 10. If the substrate 10 is composed of sapphire, it is preferred that the mask patterns 51 and 52 are formed using nickel, platinum, gold, silver, chrome, titanium, aluminum, silicon oxide, silicon nitride, aluminum nitride, aluminum oxide or titanium oxide. The first etch pattern B1 and the second etch pattern B2 may have the same shape and size, the same shape but different sizes, or different shapes. If the first etch pattern B1 and the second etch pattern B2 are overlapped so that they cross each other regardless of the above factors, tiered gratings B can be obtained. In this case, a depth of the first etch pattern B1 and a depth of the second etch pattern B2 must be different from each other. In the case where the first etch pattern B1 and the second etch pattern B2 have the same shape and size, the tiered gratings B can be obtained even though these etch patterns are formed at the same location by changing the etch depths. FIG. 9 shows that the shape of the etch patterns B1 and B2 is a lozengeor a diamond. It is, however, to be noted that the shape of the etch patterns B1 and B2 is not limited to the lozenge, but can be circular, elliptical, square, triangular, trapezoid, parallelogrammic, hexagonal or any other type. Also, the etch pattern may have a stripe shape. It is preferred that the size of the etch patterns B1 and B2 is 1 to 10/-m,

a distance between the patterns is 0J to 10 m and a depth of the pattern is

0.001 to 10 -IH.

Preferably, a stepped longitudinal wall H becomes inclined outwardly as it goes upwardly. A top width and bottom width of the grating B are preferably OJ m to 1mm.

The deeper the depth of the grating B is, the wider the area of the inclined surface H is and the higher the external quantum efficiency is. If the grating B is too deep, however, it is difficult to obtain a flat surface of the lower contact layer 12. The quality of an epitaxial layer constituting the lower contact layer 12 is also degraded. It is also preferred that the depth of the grating B is approximately 0.001 to 10/-m. If GaN is grown on a sapphire substrate using a hexagonal pattern in which each side of which has a same size, it is more preferred that the distance W2 between the patterns is 1 to 4μm, the size W1 of the patterns is 2 to δμm, the

depth of the gratings is 0.5 to 3/-m and the bottom width of the gratings is 0.1 to

3 m. The substrate 10 can be etched by applying an output power of 700W and a bias power of 200W at a chamber pressure of 1.5mtorr under the condition where Cl₂ = 20sccm, BCI₃ = 7sccm if ICP is used. FIGS. 15 to 17 are views for explaining a method of forming the gratings B in FIG. 5 according to another embodiment of the present invention. In this embodiment, in forming a dry etching mask pattern on the substrate 10, two dry etching masks are used. The tiered grating B is formed through one dry etching process. After large-sized mask pattern 61 is formed using a photo mask for forming pattern as shown in FIG. 15, small-sized mask pattern 62 is consecutively formed on the mask pattern 61 , as shown in FIG. 16. The process of forming the mask pattern 62 on the mask pattern 61 is the same as the process of forming the mask pattern 61 on the substrate 10 except that the photo masks of different sizes are used. FIG. 18 is a view for explaining a method of forming the gratings B in FIG. 5 according to still another embodiment of the present invention. In this method, mask pattern 63 and 64 are overlapped with each other using the same photo mask, unlike FIGS. 15 and 16. In this case, a process in which the mask pattern 64 and the mask pattern 63 are formed so that they partially overlap with each other, is the same as the process of forming the mask pattern 63 on the substrate 10 except that the photo mask is located so that the mask pattern 64 is overlapped with some of the mask pattern 63. Hereinafter, a method of etching the substrate 10 on which the mask patterns such as those shown in FIGS. 16 and 18 are formed will be described. In this case, the gratings B are formed through one dry etching process unlike FIGS. 11 to 14. In the case where mask patterns as shown in FIG. 16 are used, in order to form the stepped sidewall gratings B of the present invention through one etching process, the mask pattern 62 that is not covered with the mask pattern 61 undergo dry etching and the region of the substrate 10 in which the mask pattern 62 is formed but the mask patterns 61 is not formed, is subjected to dry etching. Preferably, the mask pattern 62 is substantially completely removed through the dry etching process. Thus, there is an advantage in that an additional process for removing the mask pattern 62 is not needed. In the case where mask patterns as shown in FIG. 18 are used, in order to form the stepped sidewall gratings B of the present invention through one etching process, the region where the mask pattern 63 and the mask pattern 64 are not overlapped undergoes dry etching. Also, the substrate 10 in which the region where the mask pattern 63 and the mask pattern 64 are not overlapped is formed, undergoes dry etching. Preferably, the region where the mask pattern 63 and the mask pattern 64 are overlapped is substantially completely removed through the dry etching process. Therefore, there is an advantage in that an additional process for removing the region where the mask pattern 63 and the mask pattern 64 are overlapped is not required. The etch rates of materials constituting the mask patterns 61 , 62, 63 and 64 and the substrate 10 are different from each other. The thicknesses of the mask patterns 61, 62, 63 and 64 must be designed in view of the etch rates.

For example, if the etch rate of the sapphire substrate 10 is

and the

etch rate of nickel being the mask pattern is OJ m/min, nickel having a

thickness of 0.1 m is needed in order to etch the sapphire substrate 10 to a

depth of 1 m. If the mask patterns 61 are formed to a thickness of 0.05/-m and

the mask patterns 62 are formed to a thickness of 0.05 m, the stepped sidewall

gratings B as shown in FIG. 17 can be formed on the substrate 10 after dry etching. The mask patterns 61 , 62, 63 and 64 for dry etching may employ silicon oxide, silicon nitride, aluminum oxide, aluminum nitride, a photo resistor, polymers, BCB, etc. and also may employ a single layered metal having a relatively high resistance against chlorine (Cl₂) made of one of Cr, Ni, Pt and Ir having a relatively high resistance against chlorine (CI2) or a multi layers of two or more kinds thereof. FIG. 19 is a photography showing a sapphire substrate in which a grating with stepped sidewalls is formed according to the present invention. In FIG. 19, a stepped sidewall grating shape is shown clearly.

Claims

1. A method of forming a grating in a substrate on which a light emitting unit including an active layer that generates light through recombination of electrons and holes is epitaxially grown, the method including: a first step of forming a first mask pattern on the substrate; a second step of forming a second mask pattern on the substrate on which the first mask pattern is formed, wherein the second mask pattern at least partially overlaps with the first mask pattern but does not cover the entire first mask pattern; and a third step of etching the substrate on which the first mask pattern and the second mask pattern are formed, thus forming the grating.

2. The method of forming a grating in a substrate of claim 1 , wherein in the third step, the region of the substrate on which the first mask pattern and the second mask pattern are overlapped is not etched.

3. The method of forming a grating in a substrate of claim 1, wherein in the third step, the region of the substrate on which the first mask pattern and the second mask pattern are formed but the first mask pattern and the second mask pattern are not overlapped is etched.

4. The method of forming a grating in a substrate of claim 3, wherein the region of the substrate on which the first mask pattern and the second mask pattern are overlapped is not etched.

5. The method of forming a grating in a substrate of claim 1, wherein in the third step, the etching is a dry etching.

6. The method of forming a grating in a substrate of claim 1, wherein in the second step, the second mask pattern entirely overlaps with the first mask pattern.

7. The method of forming a grating in a substrate of claim 1, wherein in the second step, the second mask pattern partially overlaps with the first mask pattern.

8. The method of forming a grating in a substrate of claim 1, wherein the substrate is made of sapphire.

9. The method of forming a grating in a substrate of claim 1 , wherein the active layer is made of a compound semiconductor comprising at least Ga (gallium) and N (nitrogen).

10. The method of forming a grating in a substrate of claim 1, wherein the thicknesses of the first and second mask patterns are determined by the etch rates of materials constituting the first and second mask patterns, the etch rate of material constituting the substrate, and the depth of the grating.

11. A Ill-nitride semiconductor light emitting device including a substrate on which a light emitting unit having an active layer that generates light through recombination of electrons and holes is epitaxially grown, wherein the substrate comprises at least one grating on the surface on which the light emitting unit is epitaxially grown, the grating includes a sidewall, and the sidewall includes a step.0

12. The III -nitride semiconductor light emitting device of claim 11 , wherein the top width of the grating is wider than the bottom width of the grating.

5 13. The III -nitride semiconductor light emitting device of claim 11 , wherein the sidewall of the grating gets inclined outwardly as it goes toward the top of the sidewall.

14. The III -nitride semiconductor light emitting device of claim 11 , o wherein the grating is formed by a dry etching.

15. The Ill-nitride semiconductor light emitting device of claim 11, wherein the grating is formed by one etching process.

16. The III -nitride semiconductor light emitting device of claim 11 , wherein the grating is formed by a first etching process in which a first etch pattern is formed and a second etching process in which a second etch pattern is formed.

17. The III -nitride semiconductor light emitting device of claim 16, wherein the respective sizes of the first and second etch patterns are 1 to 10μm

and the distance between the patterns is 0.1 to 10 m.

18. The III -nitride semiconductor light emitting device of claim 16, wherein the grating is formed by forming the first etch pattern on the substrate and forming the second etch pattern at the same position in which the first etch pattern is formed, and wherein the first and second etch patterns have the same shape, but have the different size and depth.

19. The III -nitride semiconductor light emitting device of claim 16, wherein the first etch pattern has a shape selected from a group consisting of circular, elliptical, square, triangular, trapezoid, parallelogrammic and hexagonal shapes.

20. The III -nitride semiconductor light emitting device of claim 16, wherein the second etch pattern has a shape selected from a group consisting of circular, elliptical, square, triangular, trapezoid, parallelogrammic and hexagonal shapes.

21. The III -nitride semiconductor light emitting device of claim 11 , wherein the top width of the grating is OJ m to 1mm.

22. The III -nitride semiconductor light emitting device of claim 11 , wherein the bottom width of the grating is OJ m to 1mm.

23. The III -nitride semiconductor light emitting device of claim 11 , wherein the substrate is made of sapphire.

24. The III -nitride semiconductor light emitting device of claim 11 , wherein the substrate is made of SiC.

25. The III -nitride semiconductor light emitting device of claim 11 , wherein the depth of the grating is 0.001 to 10 m.

26. The III -nitride semiconductor light emitting device of claim 11 , wherein the active layer is made of a compound semiconductor comprising at least Ga (gallium) and N (nitrogen).

27. The III -nitride semiconductor light emitting device of claim 11 , wherein the grating is formed by two etching process.