JP2016012648A - Substrate for gallium nitride-based light-emitting devices - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a sapphire substrate, by which a GaN-based light-emitting device having a higher external quantum efficiency than that achieved in the past can be manufactured.SOLUTION: A substrate for GaN-based light-emitting devices comprises: a sapphire substrate 10; and protrusions 20 disposed on a surface of the sapphire substrate 10 and spaced part from each other by 0.2-4 μm, and made of a substance having a refraction index of 1.70-2.25. The protrusion 20 has a bottom part of 1.0-4.0 μm in diameter, a height of 1.0-4.0 μm, and a shape in which the area of its cross section in parallel with the surface becomes smaller with the increase in the distance from the surface.

Description

The present invention relates to a sapphire substrate used to manufacture a GaN-based light emitting device represented by the general formula _{In x Ga y Al 1-xy} N.

  A GaN-based light emitting device is, for example, an n-type semiconductor layer made of n-GaN on an sapphire substrate, an active layer (light emitting layer) having an InGaN / GaN multiple quantum well structure (MQW), It is manufactured by sequentially stacking p-type semiconductor layers made of GaN, and holes supplied from the p-type semiconductor layer and electrons supplied from the n-type semiconductor layer are combined in the active layer to emit light.

  One of the indexes for evaluating the performance of a light-emitting element is external quantum efficiency (EQE). The external quantum efficiency is an index for evaluating the light emission efficiency of the light emitting element, and is represented by a product of an internal quantum efficiency (IQE) and a light extraction efficiency (LEE). Therefore, if either the internal quantum efficiency or the light extraction efficiency is increased, the external quantum efficiency is increased, and the performance of the light emitting element is improved.

  Conventionally, as a method for improving light extraction efficiency, a method of forming a periodic uneven structure on the surface of a sapphire substrate has been employed. The light extraction efficiency is represented by the ratio of the number of photons extracted outside the light emitting element to the number of photons emitted from the light emitting layer. In a GaN-based light emitting device with a flat sapphire substrate surface, light incident on the surface of the sapphire substrate or the surface of the p-type semiconductor layer from the light emitting layer at an angle larger than the critical angle repeats total reflection, and thus is extracted outside the device I can't. On the other hand, if a concavo-convex structure is formed on the surface of the sapphire substrate to make it uneven, the traveling direction of such light changes on the surface, so that it can be extracted outside the device, and the light extraction efficiency is improved.

  One method for forming a concavo-convex structure on the surface of a sapphire substrate is to etch the surface of the sapphire substrate. However, when an n-type GaN layer is grown on the surface of a substrate (PSS: Patterned Sapphire Substrate) obtained by this method, the crystal growth of the GaN layer starts simultaneously from a different position on the surface of the sapphire substrate. Crystal defects are likely to occur inside the type GaN layer. Since crystal defects do not cause light emission due to electron-hole pair coupling, when such crystal defects increase, the number of photons emitted from the light-emitting layer with respect to the number of electron-hole pairs injected into the light-emitting layer is increased. The internal quantum efficiency expressed as a ratio decreases.

  On the other hand, a method has been proposed in which the substrate surface is made non-planar in the same manner as described above by arranging periodic protrusions made of silicon oxide or the like on the surface of the sapphire substrate, where GaN crystal growth hardly occurs (for example, Patent Document 1-3). When such a sapphire substrate is used, crystal growth of the GaN layer starts only from the portion of the surface of the sapphire substrate where the convex portions are not formed, so that defects are less likely to occur inside the n-type GaN layer.

JP 2009-54898 A JP 2008-10809 A JP 2011-60917 A

  As described above, methods for improving the external quantum efficiency by increasing the light extraction efficiency and the internal quantum efficiency have been proposed, but the external quantum efficiency achieved by them is still insufficient, and the external quantum efficiency is further improved. There is a need for a substrate that can produce a light-emitting element having a high value.

  The problem to be solved by the present invention is to provide a sapphire substrate capable of manufacturing a GaN-based light emitting device having higher external quantum efficiency than conventional ones.

The substrate for a GaN-based light emitting device according to the present invention, which has been made to solve the above problems,
a) a sapphire substrate;
b) A plurality of convex portions made of a material having a refractive index of 1.70 to 2.25, spaced apart by 0.2 μm to 4.0 μm on the surface of the sapphire substrate, and having a bottom diameter of 1.0 μm to 4.0 μm, high A convex portion having a shape that is 1.0 μm to 4.0 μm, and the area of the cross section parallel to the surface decreases as the distance from the surface increases.
It is characterized by providing.

  The diameter of the bottom portion refers to the length of the longest line segment connecting two points on the peripheral portion of the bottom surface of the convex portion. For example, when the bottom surface is circular, the diameter corresponds to the diameter of the bottom, and when the bottom surface is rectangular, the diagonal corresponds to the diameter of the bottom.

  The inventor enters the surface of the sapphire substrate or the surface of the p-type semiconductor layer from the light emitting layer at an angle larger than the critical angle due to the difference in refractive index between the sapphire substrate and the convex portion, the shape and the arrangement interval of the convex portion. The present invention has been conceived by focusing on the fact that the magnitude of the action of changing the traveling direction of light is different.

  The inventor first examined the refractive index of the convex material that affects the critical angle on the surface of the sapphire substrate. As a result, it was confirmed that when the refractive index of the convex portion was 1.70-2.25, the external quantum efficiency was higher than that of PSS. In addition, it was confirmed that the external quantum efficiency is particularly increased when the convex portion is made of SiN (refractive index 2.06).

  Subsequently, the shape of the convex portion and the arrangement interval were examined. Under a plurality of conditions in which the shape and arrangement of the protrusions are different, the PSS was created with a substrate in which the protrusions made of SiN were arranged on the surface of the sapphire substrate, and the light extraction efficiency was compared. As a result, it was confirmed that the external quantum efficiency was higher than that of PSS by setting the arrangement interval of the convex portions, the diameter of the bottom portion, and the height within the above ranges.

The shape of the convex portion can be various shapes such as a conical shape and a pyramid shape. Moreover, it is good also as the shape etc. which cut and flattened the upper part of the cone and the pyramid. Furthermore, the shape and size of the plurality of convex portions may be uniform or different as long as the above requirements are satisfied.
The plurality of convex portions are typically periodically arranged on the sapphire surface, but may be randomly arranged as long as the above requirements are satisfied.
For the convex portion, SiOxNy (0 ≦ x ≦ 2, 0 <y ≦ 1) can be suitably used. Since the hardness of these materials is lower than that of sapphire, it can be easily manufactured as compared with the conventional substrate in which the surface of the sapphire substrate is etched.

  By using the substrate for a GaN-based light-emitting element according to the present invention, a GaN-based light-emitting element having higher external quantum efficiency than the conventional one can be manufactured.

Schematic of one Example of the board | substrate for GaN-type light emitting elements which concerns on this invention. BRIEF DESCRIPTION OF THE DRAWINGS Schematic of the GaN-type light emitting element manufactured using the substrate for GaN-type light emitting elements of a present Example. The material and characteristics of the protrusions in the GaN-based light emitting device substrate of this example, and the type of material gas used in the plasma CVD method. Results of external quantum efficiency compared to PSS for GaN-based light-emitting devices with different refractive indices of protrusions. Model structure used in the simulation in this example. The graph which shows the result of having simulated the relationship between the diameter of a convex part, and light extraction efficiency. The graph which shows the result of having simulated the relationship between the height of a convex part and light extraction efficiency. The graph which shows another result which simulated the relationship between the height of a convex part, and light extraction efficiency. The graph which shows another result which simulated the relationship between the height of a convex part, and light extraction efficiency. The graph which shows the result of having simulated the relationship between the space | interval of a convex part, and light extraction efficiency.

  Embodiments of a GaN-based light emitting device substrate according to the present invention will be described below with reference to the drawings. As shown in FIG. 1, the substrate for a GaN-based light emitting device of this example (hereinafter also simply referred to as “substrate”) has a general formula SiOxNy (0 ≦ x ≦ 2, 0 <y) on the surface of the sapphire substrate 10. A plurality of convex portions 20 made of a substance represented by ≦ 1) are arranged. And on the surface of this substrate, for example, u-GaN layer (GaN layer without added impurities), n-GaN layer, SLS layer (Strained Layer Superlattice), InGaN / GaN multiple quantum well A structural layer, a p-AlGaN layer, a p-GaN layer, and an ITO layer (Indium Tin Oxide) are stacked in this order to manufacture a GaN light emitting device (see FIG. 2).

First, five types of substances represented by the general formula SiOxNy are formed on a sapphire substrate using plasma CVD (PECVD: plasma-enhanced chemical vapor deposition), and etched to form a cone-shaped convex part. Five types of substrates were prepared in which (height 1.6 μm, bottom diameter 3.6 μm) were arranged at intervals of 6 μm. For comparison, a PSS having the same surface shape was also produced. The refractive indexes of the five kinds of materials are 1.46 (SiO ₂ ), 1.55, 1.70, 1.85 (SiON), and 2.06 (SiN), respectively. FIG. 3 shows the refractive index of each substance, the light transmittance at a wavelength of 450 nm, and the type of material gas used in plasma CVD film formation.

Using the above-mentioned six types of substrates (a total of five types of substrates and protrusions composed of SiO ₂ , three types of SiON, and SiN, and PSS) and a sapphire substrate having a flat surface, FIG. A total of seven types of GaN-based light-emitting elements having the laminated structure shown above were fabricated, and the external quantum efficiency of each was determined. The external quantum efficiency was determined by measuring the amount of light emitted from the light emitting device when a current of 20 mA was injected into the light emitting device disposed inside the integrating sphere.

  FIG. 4 shows the external quantum efficiencies of the seven types of light emitting elements. In the figure, “flat” indicates a light emitting element manufactured using a sapphire substrate having a flat surface. From the value of the external quantum efficiency of each light emitting element, it can be seen that the external quantum efficiency is higher than that of PSS when the refractive index of the plurality of convex portions arranged on the sapphire substrate is 2.25 or less. Note that even when the refractive index of the convex part is smaller than that of sapphire, an external quantum efficiency higher than that of PSS can be obtained, but this is because GaN does not grow from the surface of the convex part by using the SiOxNy film. This is probably because the growth of ELO (Epitaxially Lateral Overgrowth) reduced the transition density of the GaN layer and improved the internal quantum efficiency. However, because the light extraction efficiency did not improve sufficiently depending on the composition of the SiOxNy film, the refractive index of the convex portion was 1.70 in the substrate of this example (the refractive index of SiOxNy with the same external quantum efficiency as PSS). Rate).

  As described above, by setting the refractive index of the plurality of convex portions arranged on the sapphire substrate within the range of 1.70 to 2.25, a GaN-based light emitting device having higher external quantum efficiency than the conventional PSS can be manufactured. It can be seen that the external quantum efficiency is highest (1.24 times that of PSS) when a convex portion made of SiN is arranged.

  Subsequently, a substrate with the convex portion diameter (circular diameter of the bottom surface), height, and arrangement interval changed was created, and the light extraction efficiency was compared with the PSS having the surface structure of the same shape and interval. . The shape of the convex portion was conical, and the material of the convex portion was SiN. The convex shape is conical because when the GaN layer is grown on the substrate, the growth of GaN starts from the part of the surface of the sapphire substrate where the convex part is not arranged, and gradually fills the convex part. This is to make it difficult for crystal defects to occur inside the GaN layer, taking into account the fact that it grows rapidly.

  The comparison of the light extraction efficiency is as follows. On the substrate (or PSS) of this example having a thickness of 80 μm, an n-GaN layer having a thickness of 8.5 μm, a p-GaN layer having a thickness of 0.10 μm, and an ITO having a thickness of 0.15 μm. The simulation was performed using a structural model (see FIG. 5) in which layers were sequentially stacked. Further, the active layer is arranged as a point light source in n-GaN directly under p-GaN.

  7 types of substrates (refractive index of 2.00) with the diameter of the bottom of the convex part (circle diameter) 1.0μm, 1.5μm, 2.0μm, 2.5μm, 3.0μm, 4.0μm, 5.0μm and on the surface The light extraction efficiency was simulated for seven types of PSS (with a refractive index of 1.77) having the same concavo-convex structure. In any substrate, the height of the protrusions was 1.0 μm, and the spacing between the protrusions was 1.0 μm. From the simulation results shown in FIG. 6, it was confirmed that when the diameter of the bottom is 1.5 μm to 2.5 μm, light extraction efficiency higher than that of PSS can be obtained. It was also confirmed that the light extraction efficiency was the highest when the diameter of the bottom was 2.0 μm. The same simulation was performed on a sapphire substrate having a flat surface, and the light extraction efficiency was 33.7%. The external quantum efficiency can be obtained by multiplying the light extraction efficiency by the internal quantum efficiency.

  Next, the height of the convex part is 1.00μm, 1.25μm, 1.50μm, 1.75μm, 2.00μm, 3.00μm, 4.00μm, and the seven types with the uneven structure of the same shape on the surface The light extraction efficiency was simulated for the PSS. In any substrate, the diameter of the bottom of the protrusions was 2.0 μm, and the spacing between the protrusions was 0.2 μm. From the simulation results shown in FIG. 7, it was confirmed that light extraction efficiency higher than that of PSS can be obtained when the height of the convex portion is in the range of 1.00 μm to 4.00 μm. It was also confirmed that the light extraction efficiency was the highest when the height of the convex portion was 1.25 μm.

  In addition, among the simulation conditions, the result of changing the spacing between the convex portions of each substrate from 0.2 μm to 1.0 μm is shown in FIG. 8, and the spacing between the convex portions of each substrate is changed from 0.2 μm to 2.0 μm. The results are shown in FIG. Similarly, from these simulation results, it was confirmed that if the height of the convex portion is within the range of 1.00 μm to 4.00 μm, light extraction efficiency higher than that of PSS can be obtained. From the results of the three simulations shown in FIG. 7, FIG. 8, and FIG. 9, when the height of the convex portion is 1.25 μm to 2.00 μm, the spacing between the convex portions is any of 0.2 μm, 1.0 μm, and 2.0 μm. Even in this case, it can be seen that high light extraction efficiency can be obtained.

  Lastly, about 6 types of substrates with the convex spacing of 0.2μm, 0.5μm, 1.0μm, 2.0μm, 3.0μm, 4.0μm, and 6 types of PSS with concavo-convex structure of the same shape on the surface The light extraction efficiency was simulated. In any substrate, the diameter of the bottom of the convex portion was 2.0 μm, and the height of the convex portion was 1.0 μm. From the simulation results shown in FIG. 10, it was confirmed that light extraction efficiency higher than that of PSS can be obtained when the height of the convex portion is in the range of 0.2 μm to 4.0 μm. It was also confirmed that the light extraction efficiency was the highest when the spacing between the convex portions was 1.0 μm or less, particularly 0.2 μm.

  The above-described embodiment is an example, and can be appropriately changed in accordance with the gist of the present invention. For example, in the above embodiment, the shape of the convex portion is a conical shape, but this shape may be a pyramid shape, a shape obtained by cutting and flattening the upper portion of the cone or the pyramid, and the like. Moreover, although said simulation was performed by the model structure which arrange | positioned the convex part of a uniform shape periodically on a sapphire substrate, the shape of a convex part is made nonuniform or a convex part is arrange | positioned at random. You can also.

10 ... Sapphire substrate 20 ... Projection

Claims (5)

a) a sapphire substrate;
b) A plurality of convex portions made of a material having a refractive index of 1.70 to 2.25, spaced apart by 0.2 μm to 4.0 μm on the surface of the sapphire substrate, and having a bottom diameter of 1.0 μm to 4.0 μm, high A convex portion having a shape that is 1.0 μm to 4.0 μm, and the area of the cross section parallel to the surface decreases as the distance from the surface increases.
A substrate for a GaN-based light emitting device, comprising:   2. The substrate for a GaN-based light emitting device according to claim 1, wherein the plurality of convex portions are made of a material represented by a general formula SiOxNy (0 ≦ x ≦ 2, 0 <y ≦ 1).   The GaN-based light emitting device substrate according to claim 1 or 2, wherein the height of the convex portion is 1.25 µm to 2.00 µm.   The GaN-based light emitting device substrate according to any one of claims 1 to 3, wherein a spacing between the convex portions is 0.2 µm to 1.0 µm.   The GaN-based light emitting device substrate according to claim 1, wherein the convex portion is made of SiN.