Semiconductor substrate and light emitting device using the same

There are provided a semiconductor substrate configured to improve the light extraction efficiency of a light emitting device, and a light emitting device using the substrate. The light emitting device includes the substrate, a buffer layer, and a light emitting structure, and the buffer layer and the light emitting structure being sequentially stacked on the substrate. The substrate includes a plurality of lenses disposed on a top surface thereof, and the lenses have a horn shape and are configured such that the buffer layer grows both on the top surface of the substrate and lateral surfaces of the lenses.

CROSS-REFERENCE TO RELATED APPLICATION

This application claims priority to Korean Patent Application No. 2010-0012279 filed on Feb. 10, 2010 and all the benefits accruing therefrom under 35 U.S.C. §119, the contents of which are incorporated by reference in their entirety.

BACKGROUND

The present disclosure relates to a light emitting device, and more particularly, to a substrate configured to improve light extraction efficiency of a light emitting device, and a light emitting device using the substrate.

The market of light emitting diodes (LEDs) has grown based on low-power LEDs used for keypads of portable communication devices such as cellular phones or small home appliances, and back light units of liquid crystal displays (LCDs). High-power, high-efficiency optical sources are recently required in the fields of interior lighting, exterior lighting, automobile interior or exterior lamps, back light units of large LCDs, etc., and thus the market of LEDs is now also concentrated on high-power products.

LEDs have a low light emitting efficiency. Generally, light emitting efficiency is determined by light generating efficiency (internal quantum efficiency), efficiency of guiding light outwardly (light extraction efficiency), and light conversion efficiency of a fluorescent material. Increasing the internal quantum efficiency by improving the characteristics of an active layer is effective to increase the output power of an LED; however, increasing the light extraction efficiency is more effective to increase the output power of an LED.

The biggest obstacle in guiding light to the outside of an LED may be internal total reflection caused by different refractive indexes of layers of the LED. Generally, due to different refractive indexes of layers of an LED, only about 20% of generated light can exit the LED. The rest of generated light is confined in the LED and is converted into heat as it moves in the LED. This results in a low light emitting efficiency and reduces the lifespan of the LED due to generation of heat.

Examples of light extraction efficiency increasing methods include a method of increasing the surface roughness of p-GaN or n-GaN, and a method of forming a rough or corrugated surface on a substrate which is a base of a light emitting device.

FIG. 1is a sectional view illustrating a gallium nitride (GaN) LED10of the related art, andFIG. 2is a perspective view illustrating a sapphire substrate11. The GaN LED10includes the sapphire substrate11and a GaN light emitting structure15formed on the sapphire substrate11.

The GaN light emitting structure15includes an n-type GaN cladding layer15a, a multi-quantum well (MQW) active layer15b, and a p-type GaN cladding layer15cthat are formed on the sapphire substrate11. The GaN light emitting structure15may be grown by a process such as metal-organic chemical vapor deposition (MOCVD). Predetermined parts of the p-type GaN cladding layer15cand the active layer15bmay be dry-etched to expose a topside part of the n-type GaN cladding layer15a, and an n-type contact electrode19and a p-type contact electrode17may be formed on the exposed topside of the n-type GaN cladding layer15aand the topside of the p-type GaN cladding layer15c, respectively, so as to apply a voltage to the GaN LED10. Generally, a transparent electrode16is formed on the topside of the p-type GaN cladding layer15cbefore the p-type contact electrode17is formed, so as to increase a current injection area without reducing brightness.

The sapphire substrate11includes lenses12to improve light extraction efficiency. The lenses12used for the GaN LED10of the related art are generally hemisphere-shaped as shown inFIG. 2. Optimization of the shape and arrangement density of the lenses12is necessary to improve light extraction efficiency and characteristics of the GaN light emitting structure15.

SUMMARY

The present disclosure provides a semiconductor substrate including lenses arranged more densely to improve light extraction efficiency and configured such that a light emitting structure having good characteristics can be formed on the substrate, and a light emitting device using the semiconductor substrate.

According to an exemplary embodiment, there is provided a substrate for a light emitting device including the substrate, a buffer layer, and a light emitting structure, the buffer layer and the light emitting structure being sequentially stacked on the substrate, the substrate including a plurality of lenses disposed on a top surface thereof, wherein the lenses have a horn shape and are configured such that the buffer layer grows both on the top surface of the substrate and lateral surfaces of the lenses.

According to another exemplary embodiment, there is provided a light emitting device including: a substrate comprising a plurality of lenses; a buffer layer disposed on the substrate; and a light emitting structure disposed on the buffer layer, wherein the lenses have a horn shape and are configured such that the buffer layer grows both on a top surface of the substrate and lateral surfaces of the lenses.

A part of the buffer layer growing on the top surface of the substrate may have the same crystal orientation as that of parts of the buffer layer growing on the lateral surfaces of the lenses

An angle between lateral and bottom surfaces of the lenses may be greater than 30° but smaller than 57.6°, and the lenses may have a height equal to or greater than 1.6 μm and are arranged at intervals of 1 μm or smaller.

The buffer layer may be formed of a nitride semiconductor such as AlN (aluminum nitride), and the buffer layer may have a thickness of 100 nm or greater.

DETAILED DESCRIPTION OF EMBODIMENTS

Hereinafter, a semiconductor substrate and a light emitting device using the semiconductor substrate will be described with reference to the accompanying drawings according to exemplary embodiments. The present invention may, however, be embodied in different forms and should not be construed as limited to the embodiments set forth herein. Rather, these embodiments are provided so that this disclosure will be thorough and complete, and will fully convey the scope of the present invention to those skilled in the art.

FIGS. 3A and 3Bare a schematic perspective view and a schematic section view illustrating a light emitting device substrate according to an exemplary embodiment;

Referring toFIGS. 3A and 3B, the light emitting device substrate300of the current embodiment includes a base substrate310and a plurality of lenses320. A buffer layer and a light emitting structure may be sequentially stacked on the light emitting device substrate300to form a light emitting device.

The kind of the base substrate310is not limited. For example, a sapphire substrate may be used as the base substrate310. In the case where a light emitting device is fabricated by using a gallium-nitride (GaN) compound semiconductor, a sapphire substrate may be used as the base substrate310.

The lenses320are disposed on the top surface of the base substrate310. The lenses320are spaced from each other as individual lenses and have a horn shape. The lenses320and the base substrate310may be formed in one piece by etching the base substrate310, or the lenses320may be formed of a separate material.FIG. 4is a scanning electron microscope (SEM) image illustrating a light emitting device substrate including a sapphire substrate310and conic lenses320formed on the top surface of the sapphire substrate310by etching the sapphire substrate310. As shown inFIG. 4, by etching the sapphire substrate310, the conic lenses320can be formed in a manner such that the conic lenses320have uniform sizes and are uniformly arranged.

The height (h) of the lenses320may be 1.6 μm or greater.FIG. 5shows light extraction efficiency with respect to lens height for different shapes of lenses.

InFIG. 5, CONIC denotes a conic lens, and HEMISPHERIC denotes a hemispheric lens. In addition, CYLINDRICAL denotes a cylindrical lens having a flat circular topside and a flat circular bottom side smaller than the flat circular topside. In any of the lenses ofFIG. 5, a section taken in parallel with a bottom side is circular.

Referring toFIG. 5, in the case of the hemispheric lens (-●-), the light extraction efficiency increases in proportion to the height of the lens until the height of the lens reaches 1 μm. However, the light extraction efficiency does not vary largely after the height of the lens becomes equal to or greater than 1 μm. In the case of the cylindrical lens (-▪-), the light extraction efficiency does not vary largely according to the height of the lens. However, in the case of the conic lens (-▴-), the light extraction efficiency increases as the height of the lens increases. As shown inFIG. 5, when the lens height is equal to or greater than about 1.6 μm, the light extraction efficiency is greater in the case of the conic lens (-▴-) than in the cases of the other lenses (-●-, -▪-). Therefore, in the case of a conic lens, highest light extraction efficiency can be obtained by adjusting the height of the conic lens equal to or greater than about 1.6 μm. In this case, the light extraction efficiency can be equal to or greater than about 60%.

Thus, when the lenses320have a horn shape having an apex at its upper end, light extraction efficiency can be increased by increasing the height (h) of the lenses320. In addition, when processes of forming conic lenses are considered, it is easy to form the lenses320if the lenses320have a large height (h). Furthermore, if a nitride semiconductor layer is epitaxially grown on the light emitting device substrate300, the surface of the nitride semiconductor layer can be easily leveled.

In addition to the height of the lenses320, the density of the lenses320affects light extraction efficiency. As the lenses320are densely formed, light extraction efficiency can be increased. In the current embodiment, to increase light extraction efficiency, the lenses320may be formed at intervals (d) of 1 μm or smaller. However, if the lenses320are densely formed, it is difficult to grow a nitride semiconductor layer on the light emitting device substrate300. This is explained with reference toFIGS. 6 through 7B.

FIG. 6is a SEM image illustrating a nitride semiconductor layer330agrown on a substrate in a low lens density condition.FIGS. 7A and 7Bare SEM images illustrating a nitride semiconductor layer330bgrown on a substrate in a high lens density condition by a method of the related art,FIG. 7Aillustrating the surface of the nitride semiconductor layer330bat an early stage of growth,FIG. 7Billustrating the surface of the nitride semiconductor layer330bafter the growth of the nitride semiconductor layer330bis completed;FIG. 6illustrates the case where lenses320aare arranged at intervals of 1 μm or greater, andFIGS. 7A and 7Billustrate the case where lenses320bare arranged at intervals smaller than 1 μm.

Referring toFIG. 6, when the lenses320aare not densely arranged, the nitride semiconductor layer330ais well grown and leveled on the substrate. However, referring toFIGS. 7A and 7Billustrating the case where the lenses320bare densely arranged, the nitride semiconductor layer330bis not evenly grown on the substrate. Particularly as shown inFIG. 7B, the nitride semiconductor layer330bhas discontinuous parts350even after the growth of the nitride semiconductor layer330bis completed.

In the related art, if it is intended to grow a nitride semiconductor layer is grown on a substrate where lenses are formed, although the nitride semiconductor layer is grown on exposed parts of the top surface of the substrate, the nitride semiconductor layer is not grown on lateral surfaces of the lenses. Therefore, in the case where the exposed parts of the top surface of the substrate are wide because the lenses320aare not densely arranged, as shown inFIG. 6, the nitride semiconductor layer330ais grown like water filled between the lenses320a, and then the lenses320ais surrounded by the grown nitride semiconductor layer330a. However, in the case where the exposed parts of the top surface of the substrate are narrow because the lenses320bare densely arranged, growth of the nitride semiconductor layer330bis hindered by the lenses320b. That is, since the exposed parts of the top surface of the substrate are narrow, at an early stage of growth (refer toFIG. 7A), the nitride semiconductor layer330bis epitaxially grown individually in narrow regions, and the individually grown parts of the nitride semiconductor layer330bare not merged in a later stage due to the lenses320b. Thus, although the nitride semiconductor layer330bis further grown, the nitride semiconductor layer330bis not leveled, and the discontinuous parts350are formed at the nitride semiconductor layer330b.

That is, in the related art, if the density of lenses is increased to improve light extraction efficiency, the properties of a nitride semiconductor layer grown on a substrate are degraded, and thus the performance of a light emitting device is lowered. Thus, to improve light extraction efficiency while maintaining or improving the properties of a nitride semiconductor layer, it is necessary to grow a nitride semiconductor layer simultaneously on the top surface of a substrate and lateral surfaces of lenses. For this reason, in the current embodiment, the lenses320are formed in a manner such that the buffer layer can be grown on the top surface of the base substrate310and the lateral surfaces of the lenses320. That is, if the buffer layer can be grown both on the top surface of the base substrate310and the lateral surfaces of the lenses320, although the exposed parts of the top surface of the base substrate310are narrow because the lenses320are densely arranged, the buffer layer can be evenly formed. In addition, if a part of the buffer layer grown on the top surface of the base substrate310has the same crystal orientation as that of parts of the buffer layer grown on the lateral surfaces of the lenses320, the parts of the buffer layer can merge with each other easily, and thus the buffer layer can have an even surface at the end of growth. Therefore, the lenses320may be formed in a manner such that a part of a buffer layer formed on the top surface of the base substrate310can have the same crystal orientation as that of parts of the buffer layer grown on the lateral surfaces of the lenses320. For this end, the crystal orientation of the lateral surfaces of the lenses320may be considered. For example, the lenses320may be formed in a manner such that the angle (refer to θ inFIG. 3B) between lateral and bottom surfaces of the lenses320is greater than 30° but smaller than 57.6°. This will now be explained with reference toFIGS. 8 and 9. The explanation is given on an exemplary case where a gallium nitride buffer layer is formed on a sapphire substrate.

FIG. 8is a schematic view illustrating a sapphire unit cell, andFIG. 9is a view illustrating crystal orientations of gallium nitride (GaN) grown on an R-plane (1102) of sapphire.

As shown inFIG. 8, sapphire includes stable low index planes: a C-plane (0001) orthogonal to the C-axis; an R-plane (1102) inclined 57.6° from the C-plane; an M-plane (1100) orthogonal to the C-plane (0001); and an A-plane (1120) orthogonal to the C-plane. (0001) gallium nitride grows on the C-plane (0001) of a sapphire substrate610(refer toFIG. 9). However, gallium nitride having different crystal orientations is grown on the R-plane (1102), M-plane (1100), and A-plane (1120) of the sapphire substrate610.

As shown inFIG. 9, (1120) gallium nitride620grows on the R-plane (1102) of the sapphire substrate610. In addition, (1122) gallium nitride (not shown) grows on the M-plane (1100) and A-plane (1120) of the sapphire substrate610. As described above, when the base substrate310is a C-plane (0001) sapphire substrate, if the lateral surfaces of the lenses320are R-planes (1102), M-planes (1100), or A-planes (1120), gallium nitride grown on the top surface of the base substrate310has a crystal orientation different from those of gallium nitride grown on the lateral surfaces of the lenses320.

As described above, if gallium nitride grown on the top surface of the base substrate310has a crystal orientation different from those of gallium nitride grown on the lateral surfaces of the lenses320, the gallium nitrides may not merge with each other at a late stage of growth, and the growth of the gallium nitrides may stop. In other words, if the angle (refer to θ inFIG. 3B) between the lateral and bottom surfaces of the lenses320is about 57.6° or 90°, growth of a gallium nitride buffer layer is restricted. Therefore, to easily grow a buffer layer having an even surface, the angle (θ) between the lateral and bottom surfaces of the lenses320may not be about 57.6° and 90°. In addition, if the angle (θ) between the lateral and bottom surfaces of the lenses320is greater than 60° or smaller than 30°, light extraction efficiency is not high. In addition, in terms of process technology, it is difficult to make the angle (θ) between the lateral and bottom surfaces of the lenses320greater than 60°. Therefore, the lenses320may be formed in a manner such that the angle (θ) between the lateral and bottom surfaces of the lenses320is greater than 30° but smaller than 57.6°.FIGS. 10A through 10Cillustrate a nitride semiconductor layer330cformed on a substrate in a condition where the angle (θ) between the lateral and bottom surfaces of lenses320cis greater than 30° but smaller than 57.6° and the distance between the lenses320is 1 μm or less.

In detail,FIGS. 10A through 10Care SEM images illustrating a nitride semiconductor layer330cgrown on a light emitting device substrate in a high lens density condition according to an embodiment.FIG. 10Aillustrates the surface of the nitride semiconductor layer330cin an early stage of growth of the nitride semiconductor layer,FIG. 10Billustrates the surface of the nitride semiconductor layer330cin a middle stage of growth of the nitride semiconductor layer, andFIG. 10Cillustrates the surface of the nitride semiconductor layer330cafter the growth of the nitride semiconductor layer is completed.

Referring toFIGS. 10A through 10C, although the lenses320care densely arranged at intervals of 1 μm or less, the nitride semiconductor layer330cis evenly grown unlike the case shown inFIGS. 7A and 7B. This is possible since the nitride semiconductor layer330cgrows on the lateral surfaces of the lenses320cas well as on the surface of the light emitting device substrate in the early stage of growth (FIG. 10A). In addition, a part of the nitride semiconductor layer330cgrown on the surface of the substrate has a crystal orientation similar to those of parts of the nitride semiconductor layer330cgrown on the lateral surfaces of the lenses320c, the parts of the nitride semiconductor layer330ccan merge with each other easily, and thus the surface of the nitride semiconductor layer330ccan be even as shown inFIG. 10C.

Explanation has been given on the light emitting device substrate on which a nitride semiconductor layer can be evenly grown and by which light can be efficiently extracted from a light emitting device. Hereinafter, a light emitting device using the substrate will be described.

FIG. 11is a schematic view illustrating a light emitting device900according to an exemplary embodiment.

Referring toFIG. 11, the light emitting device900of the current embodiment includes a base substrate910, a plurality of lenses920, a buffer layer930, a light emitting structure940, a transparent electrode950, a p-type contact electrode960, and an n-type contact electrode970.

The kind of the base substrate910is not limited. For example, a sapphire substrate may be used as the base substrate910. Particularly when the light emitting device900is fabricated by using a gallium-nitride (GaN) compound semiconductor, a sapphire substrate may be used as the base substrate910.

The lenses920are disposed on the top surface of the base substrate910. The lenses920are spaced from each other as individual lenses and have a horn shape so that light extraction efficiency can be improved. The lenses920and the base substrate910may be formed in one piece by etching the base substrate910, or the lenses920may be formed of a separate material. As described above, to improve light extraction efficiency, the lenses920is formed in a manner such that the height of the lenses920is 1.6 μm or greater and the distance between the lenses920is 1 μm or smaller. In addition, the lenses920are shaped such that the buffer layer930can be evenly formed on the base substrate910although the lenses920are densely arranged. That is, the lenses920are shaped such that the buffer layer930can be grown simultaneously on the top surface of the base substrate910and the lateral surfaces of the lenses920. If a part of the buffer layer930grown on the top surface of the base substrate910has the same crystal orientation as that of parts of the buffer layer930grown on the lateral surfaces of the lenses920, the parts of the buffer layer930can be easily merged, and thus the buffer layer930can be flat after the buffer layer930is completely grown. Therefore, the lenses920may be shaped in a manner such that a part of the buffer layer930formed on the top surface of the base substrate910can have the same crystal direction as that of parts of the buffer layer930grown on the lateral surfaces of the lenses920. For this, the angle between the lateral and bottom surfaces of the lenses920may be set to greater than 30° but smaller than 57.6°.

The buffer layer930is formed on the base substrate910to cover the lenses920partially or entirely. The buffer layer930is formed to reduce lattice mismatch between the light emitting structure940and the base substrate910. The buffer layer930may be formed of a nitride semiconductor such as aluminum nitride (AlN). The buffer layer930may be formed to a thickness of 100 nm or greater.

The light emitting structure940is formed on the buffer layer930for converting an electric signal into an optical signal. The light emitting structure940may be formed of a compound semiconductor. In the current embodiment, the light emitting structure940includes an n-type compound semiconductor layer941, an active layer942, and a p-type compound semiconductor layer943.

The n-type compound semiconductor layer941may be formed on the buffer layer930by using a semiconductor material having a compositional formula of InxAlyGa1-x-yN (0≦x≦1, 0≦y≦1, 0≦x+y≦1). The n-type compound semiconductor layer941may be a GaN or GaN/AlGaN layer doped with an n-type dopant and having a thickness of several micrometers (μm). The n-type dopant may be a group IV element such as silicon (Si). The n-type compound semiconductor layer941forms a p-n junction together with the p-type compound semiconductor layer943. The n-type compound semiconductor layer941supplies electrons to the active layer942.

The active layer942is formed on the n-type compound semiconductor layer941to generate and emit light. The active layer942may be formed of a semiconductor material having a compositional formula of InxAlyGa1-x-yN (0≦x≦1, 0≦y≦1, 0≦x+y≦1). In the active layer942, electric energy is converted into optical energy as electrons injected from the n-type compound semiconductor layer941recombine with holes injected from the p-type compound semiconductor layer943. Thus, light is emitted from the active layer942. For this, the active layer942may be formed into a quantum well structure in which quantum well layers and barrier layers are alternately stacked. To improve charge confinement in the quantum well layers, the active layer942may have a multi quantum well (MQW) structure in which a plurality of quantum well layers and a plurality of barrier layers are alternately stacked. The quantum well layers may be formed of a material having a relatively lower energy band gap such as InGaN, and the barrier layers may be formed of a material having a higher energy band gap such as GaN. The wavelength of light emitted from the active layer942is determined by the amount of indium (In).

The p-type compound semiconductor layer943may be formed on the active layer942by using a semiconductor material having a compositional formula of InxAlyGa1-x-yN (0≦x≦1, 0≦y≦1, 0≦x+y≦1). The p-type compound semiconductor layer943may be a GaN or GaN/AlGaN layer doped with a p-type dopant and having a thickness of several angstrom (Å). The p-type dopant may be a group II element such as magnesium (Mg). The p-type compound semiconductor layer943forms a p-n junction together with the n-type compound semiconductor layer941. The p-type compound semiconductor layer943supplies holes to the active layer942.

The transparent electrode950and the p-type contact electrode960are sequentially formed on the light emitting structure940. The transparent electrode950is disposed between the light emitting structure940and the p-type contact electrode960to increase a current injection area without reducing brightness. The transparent electrode950may be formed of a transparent conductive oxide (TCO) such as indium-tin oxide (ITO). Since light emitted from the light emitting structure940can be absorbed in the transparent electrode950, light extraction efficiency may be reduced if the transparent electrode950is thick. Therefore, for ohmic contact between the transparent electrode950and the light emitting structure940, large current injection area, and good light extraction efficiency, the transparent electrode950may be formed to a thickness of 80 nm or less.

The n-type contact electrode970is formed on a part of the n-type compound semiconductor layer941. The p-type contact electrode960and the n-type contact electrode970may be formed of one of titanium (Ti), chromium (Cr), aluminum (Al), palladium (Pd), vanadium (V), tungsten (W), and combinations thereof.

In the above embodiment, the light emitting structure940of the light emitting device900is formed of a material having a compositional formula of InxAlyGa1-x-yN (0≦x≦1, 0≦y≦1, 0≦x+y≦1) (GaN-based light emitting device). However, the light emitting structure940may be formed of another group III-V compound semiconductor material. The light emitting structure940may be formed of a semiconductor material having a compositional formula of AlxGayIn1-x-yP (0≦x≦1, 0≦y≦1, 0≦x+y≦1). In another embodiment, the light emitting structure940may be formed of a semiconductor material having a compositional formula of AlxGa1-xAs (0≦x≦1). In addition, another stacked structure capable of generating light may be used instead of the light emitting structure940. In any embodiment, if a plurality of lenses920are aimed on the base substrate910, light extraction efficiency can be improved to 69% or higher, and a high-quality buffer layer can be formed. That is, a high-performance light emitting device can be provided.

According to the substrate of the embodiments, since a nitride semiconductor layer can be grown on the substrate although the lenses are arranged more densely, the light emitting device can have largely improved light extraction efficiency and characteristics.

Although the semiconductor substrate and the light emitting device including the semiconductor substrate have been described with reference to the specific embodiments, they are not limited thereto. Therefore, it will be readily understood by those skilled in the art that various modifications and changes can be made thereto without departing from the spirit and scope of the present invention defined by the appended claims.