JP2007250714A - Light emitting element - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a high performance gallium nitride based compound semiconductor light emitting element in which light take-out efficiency can be enhanced easily while sustaining crystallinity of a compound semiconductor layer. <P>SOLUTION: In the light emitting element having a semiconductor layer 6 including a laminate where a first conductivity type gallium nitride based compound semiconductor layer 6a, a light emitting layer 6b composed of a gallium nitride based compound semiconductor, and a second conductivity type gallium nitride based compound semiconductor layer 6c are laminated sequentially, a reflection conductive layer 7 for scattering light by protrusions and recesses formed on the upper surface of the second conductivity type gallium nitride based compound semiconductor layer 6c is formed. <P>COPYRIGHT: (C)2007,JPO&INPIT

Description

  The present invention relates to a light emitting element such as a light emitting diode (LED) using a nitride gallium compound semiconductor.

In recent years, Al _x Ga _y In _1-xy N (0 ≦ x ≦ 1, 0 ≦ y ≦ 1, 0 ≦ x + y ≦ 1) is used as a light emitting element that emits light from the ultraviolet region to blue light. A light emitting element using a gallium nitride compound semiconductor (nitride semiconductor) is attracting attention (see, for example, Patent Document 1).

  A light-emitting element using such a gallium nitride-based compound semiconductor can emit white light when combined with a phosphor, and has an energy saving and long life. As it is considered promising as a product, its practical application has begun. However, since the luminous efficiency of a light-emitting element using a gallium nitride-based compound semiconductor is lower than that of a fluorescent lamp, further improvement in efficiency has been demanded, and various studies have been conducted for that purpose.

  By the way, the external quantum efficiency, which is the light emission efficiency of the light emitting element, is the internal quantum efficiency indicating the rate at which electrical energy is converted into light energy in the light emitting layer, and the light extraction indicating the rate at which the converted light energy is emitted to the outside. Determined by product of efficiency.

  The internal quantum efficiency is greatly affected by the crystallinity of the gallium nitride compound semiconductor layer forming the light emitting element. As a measure for improving the internal quantum efficiency, by forming a buffer layer of an amorphous or polycrystalline AlN-based or AlGaN-based material on a substrate and growing a gallium nitride-based compound semiconductor layer on the buffer layer, A method of relaxing the lattice mismatch between the substrate and the gallium nitride compound semiconductor layer and improving the crystallinity of the gallium nitride compound semiconductor layer is known as a known technique (for example, see Patent Document 2). .

On the other hand, various techniques for improving the light extraction efficiency have been disclosed, and by forming a concavo-convex structure on the surface of the gallium nitride compound semiconductor layer, the difference in refractive index from the outside is alleviated to suppress internal total reflection. A method (for example, see Patent Document 3), a method for efficiently extracting light incident on the substrate by irregularly reflecting the light incident on the substrate by forming an uneven structure on the back surface of the substrate (for example, see Patent Document 4), or a substrate There is a method of producing a similar effect by providing a protrusion and a recess on the surface (for example, see Patent Document 5).
JP-A-2-42770 Japanese Patent Publication No. 4-15200 Japanese Patent Laid-Open No. 15-69075 JP-A-14-368261 JP 11-274568 A

  A cross-sectional view of a conventional light emitting device is shown in FIG. An n-type gallium nitride compound semiconductor layer 2a, a light emitting layer 2b made of a gallium nitride compound semiconductor layer, and a semiconductor layer 2 made of a p-type gallium nitride compound semiconductor layer 2c are formed on the substrate 1, and n-type nitride An n-type electrode 3 and a p-type electrode 4 are formed on the gallium compound semiconductor layer 2a and the p-type gallium nitride compound semiconductor layer 2c, respectively.

As the substrate 1 used for forming the gallium nitride compound semiconductor layer, a sapphire substrate is generally used, but its refractive index is about 1 when the wavelength of light emitted from the light emitting layer 2b is 400 nm. The refractive index of a gallium nitride compound semiconductor is as high as about 2.55, while .78. For this reason, of the light emitted from the light emitting layer 2b, light incident at an angle exceeding the critical angle θ _r of about 44 ° (θ _r = arcsin (1.78 / 2.55)) is incident on the sapphire substrate. The total reflection is repeated inside the semiconductor layer 2 formed by laminating each gallium nitride compound semiconductor layer. Therefore, most of the light is absorbed in the process of repeating total reflection at the semiconductor layer 2, and the remaining light is emitted from the end of the semiconductor layer 2 to the outside.

  In order to solve the above problem, when the light extraction efficiency is improved by using the method of Patent Document 3, the substrate is mechanically polished and removed from the semiconductor layer, and then the concavo-convex structure is formed by wet etching or dry etching. Since it is necessary, not only the manufacturing process becomes complicated, but also there is a concern about damage to the semiconductor layer due to polishing.

  Further, in the method of Patent Document 4, light extracted from the side surface of the substrate due to irregular reflection due to the unevenness formed on the back surface of the substrate is effective only for light that has entered the substrate, but is emitted from the light emitting layer. Most of the emitted light is repeatedly totally reflected at the interface between the substrate and the electrode as described above, and is emitted from the end face of the semiconductor layer to the outside or absorbed in the semiconductor layer, thereby improving the light extraction efficiency. Is not enough.

  Furthermore, in the method of Patent Document 5, it is necessary to grow a semiconductor layer from the substrate surface provided with the protrusions and the recesses. That is, since the gallium nitride compound semiconductor layer grows from a rough substrate surface that is not flat, there is a problem that it is difficult to obtain a flat semiconductor layer with good crystallinity due to dislocations and defects.

  Therefore, the present invention has been completed in view of the problems in the above-described conventional technology, and the object thereof is not to require a complicated manufacturing process, and while maintaining the crystallinity of the gallium nitride compound semiconductor layer, It is an object to provide a high-performance light-emitting element that can easily improve the light extraction efficiency.

  The light emitting device of the present invention has a semiconductor layer including a first conductive type gallium nitride compound semiconductor layer, a light emitting layer made of a gallium nitride compound semiconductor, and a stacked body in which a second conductive type gallium nitride compound semiconductor layer is sequentially laminated. The light-emitting element is characterized in that a light-scattering reflective conductive layer that scatters light by unevenness formed on the surface is formed on the upper surface of the second conductivity type gallium nitride compound semiconductor layer.

  The light-emitting element of the present invention is preferably characterized in that the light-scattering reflective conductive layer has the irregularities formed on the surface on the interface side in contact with the second conductivity type gallium nitride compound semiconductor layer.

  In the light emitting device of the present invention, preferably, the irregularities are regularly formed, and the average period T is λ the wavelength of light emitted from the light emitting layer, and the p-type gallium nitride compound with respect to λ. When the refractive index of the semiconductor layer is n, 1.5λ / n ≦ T ≦ 3.5λ / n.

  The light emitting device of the present invention has a semiconductor layer including a first conductive type gallium nitride compound semiconductor layer, a light emitting layer made of a gallium nitride compound semiconductor, and a stacked body in which a second conductive type gallium nitride compound semiconductor layer is sequentially laminated. A light-scattering reflective conductive layer that scatters light by unevenness formed on the surface is formed on the upper surface of the second conductivity type gallium nitride compound semiconductor layer, so that the light-emitting layer emits light. The light directed toward the upper surface of the second conductivity type gallium nitride compound semiconductor layer is reflected by the light-scattering reflective conductive layer toward the first conductivity type gallium nitride compound semiconductor layer which is the light extraction direction. Therefore, light can be efficiently collected and emitted to the light extraction side.

  Furthermore, conventionally, the incident light exceeds the critical angle at the interface between the semiconductor layer and the external surface, and is totally reflected to the semiconductor layer side. The light absorbed in this way is scattered at various angles by the light-scattering reflective conductive layer, increasing the proportion of light that falls within the critical angle at the interface between the semiconductor layer and the outside, greatly increasing the light extraction efficiency. It becomes possible to improve.

  Even if the incident angle does not fall within the critical angle at the other interface due to one reflection / scattering in the light-scattering reflective conductive layer, the reflection / scattering in the light-scattering reflective conductive layer is repeated several times. Since the incident angle falls within the critical angle at the interface, the amount of light that can be extracted to the outside increases dramatically.

  In the light-emitting element of the present invention, preferably, the light-scattering reflective conductive layer has irregularities formed on the surface on the interface side in contact with the second-conductivity-type gallium nitride compound semiconductor layer. Scattered at the interface between the reflective conductive layer and the second conductive gallium nitride semiconductor layer, reflected and scattered toward the second conductive type gallium nitride semiconductor layer, so that it enters the light scattering reflective conductive layer and is absorbed. The amount of light that is generated can be greatly reduced.

  The light emitting device of the present invention preferably has irregularities regularly formed, the average period T of which is the wavelength of the light emitted from the light emitting layer is λ, and the p-type gallium nitride compound semiconductor layer has a wavelength λ. When the refractive index is n, it is 1.5λ / n ≦ T ≦ 3.5λ / n, so that the angular distribution of the light scattered by the unevenness can be maximized, and the light that can be extracted outside Can be most effectively increased.

  Hereinafter, the light emitting device of the present invention will be described in detail with reference to the drawings.

  FIG. 2 is a schematic cross-sectional view showing an example of an embodiment of a light emitting device of the present invention. In FIG. 2, 5 is a substrate used for epitaxial growth of a gallium nitride compound semiconductor layer, 6 is a semiconductor layer (stacked body) formed by laminating a plurality of gallium nitride compound semiconductor layers, and 6a is a first conductivity type. (Eg, n-type) gallium nitride compound semiconductor layer, 6b is a light emitting layer made of a gallium nitride compound semiconductor layer, 6c is a second conductivity type (eg, p-type) gallium nitride compound semiconductor layer, and 7 is a light-scattering reflective conductor. The layer 8 is a second conductive side conductive layer for forming a second conductive side electrode or a second conductive side electrode, 9 is a first conductive (for example n) side electrode, or This is a first conductive side conductive layer for forming one conductive side electrode.

  In the example of FIG. 2, the first conductivity type is n-type and the second conductivity type is p-type.

  The light-emitting element of this example has a flip-chip structure capable of so-called flip mounting, in which the p-side conductive layer 8 and the n-side conductive layer 9 are electrically connected to a wiring conductor or the like of an external mounting board to mount the light-emitting element. Therefore, the light extraction direction is the substrate 5 side. Accordingly, the substrate 5 is preferably made of a material that is transparent to the light emitted from the light emitting layer 6b. When the substrate 5 is removed from the semiconductor layer 6 by etching or the like, the substrate 5 may be made of an opaque material.

  The light-scattering reflective conductive layer 7 of the present invention is made of aluminum or the like having high reflectivity with respect to the light emitted from the light-emitting layer 6b, and the continuity between the p-type gallium nitride compound semiconductor layer 6c and the p-side conductive layer 8. It has conductivity so that it can be removed, and it also has a scattering property for the light emitted from the light emitting layer 6b.

  The light-scattering reflective conductive layer 7 is formed by forming an n-type gallium nitride compound semiconductor layer 6a, a light emitting layer 6b, and a p-type gallium nitride compound semiconductor layer 6c on the substrate 5 in this order, and then p-type nitrided. A mask is formed on the gallium compound semiconductor layer 6c, and irregularities are formed on the upper surface of the p-type gallium nitride compound semiconductor layer 6c by dry etching using a reactive ion etching (RIE) method. By depositing the conductive layer made of aluminum or the like having a vapor deposition, it can be easily formed while maintaining the high crystallinity of the p-type gallium nitride compound semiconductor layer 6c.

  The average period T of the unevenness of the light-scattering reflective conductive layer 7 is as follows: λ is the wavelength of light emitted from the light emitting layer 6b, and n is the refractive index of the p-type gallium nitride compound semiconductor layer 6c with respect to the wavelength λ. It is preferable that 5λ / n ≦ T ≦ 3.5λ / n. Specifically, when the refractive index of the p-type gallium nitride compound semiconductor layer 6c when λ is 400 nm is 2.5, T is 1.5 × 400 ÷ 2.5 = 240 nm to 3.5 × 400 ÷. A range of 2.5 = 560 nm is preferable. In the range of less than 240 nm or greater than 560 nm, the angular distribution of the scattered light scattered by the unevenness becomes narrow, the incident angle at the interface between the semiconductor layer 6 and the outside falls within the critical angle, and the scattering that can be extracted outside. Since the light is reduced, the effect of improving the light extraction efficiency cannot be sufficiently obtained.

  The plurality of irregularities formed in the light-scattering reflective conductive layer 7 has 2 to 4 irregularities per unit length of 1 μm when T is 240 nm to 560 nm.

  Further, the height of the unevenness of the light-scattering reflective conductive layer 7 is not particularly limited because the number of tip portions per unit length, that is, the period of the unevenness is greatly involved in the scattering, but is the same as the average period T. If it is about.

  The method for forming irregularities of the light-scattering reflective conductive layer 7 in the present invention will be specifically described below. First, after a resist layer for a photomask is formed on the p-type gallium nitride compound semiconductor layer 6c by spin coating, patterning is performed by exposure using an electron beam drawing method, and development processing is performed. Next, Ni or Ti is vapor-deposited and lift-off is performed to complete the mask pattern. Finally, the p-type gallium nitride compound semiconductor layer 6c is dry-etched by RIE from the mask pattern, thereby forming irregularities.

  Moreover, the thickness (thickness including unevenness) of the light-scattering reflective conductive layer 7 is preferably about 340 nm to 1 μm, and if it is less than 340 nm, a high reflectance cannot be obtained because the thickness is thin, and if it exceeds 1 μm, the electrode resistance The increase in current injection efficiency decreases.

  The growth method of the semiconductor layer 6 including the n-type gallium nitride compound semiconductor layer 6a, the light emitting layer 6b, and the p-type gallium nitride compound semiconductor layer 6c of the present invention is a metal organic vapor phase epitaxy (MOVPE) method. Other examples include molecular beam epitaxy (MBE), hydride vapor phase epitaxy (HVPE), and pulsed laser deposition (PLD).

The semiconductor layer 6 of the present invention has a configuration in which the light emitting layer 6b is sandwiched between the p-type gallium nitride compound semiconductor layer 6c and the n-type gallium nitride compound semiconductor layer 6a. 6c is an Al _0.15 Ga _0.85 N layer as a first p-type cladding layer, an Al _0.2 Ga _0.8 N layer as a second p-type cladding layer, and GaN as a p-type contact layer It consists of a laminate of layers. The p-type gallium nitride compound semiconductor layer 6c has a thickness of about 200 nm to 300 nm.

Further, for example, the n-type gallium nitride compound semiconductor layer 6a includes a stacked body of a GaN layer as a first n-type cladding layer, an In _0.02 Ga _0.98 N layer as a second n-type cladding layer, and the like. Consists of. The n-type gallium nitride compound semiconductor layer 6a has a thickness of about 2 μm to 3 μm.

Further, for example, the light emitting layer 6b includes an In _0.01 Ga _0.99 N layer as a barrier layer having a wide forbidden band and an In _0.11 Ga _0.89 N layer as a well layer having a narrow forbidden band. Are composed of a multiple quantum well structure (MQW: Muliti Quantum Well) or the like that is alternately and regularly stacked three times. The thickness of the light emitting layer 6b is about 25 nm to 150 nm.

  The p-type gallium nitride compound semiconductor layer 6c and the n-type gallium nitride compound semiconductor layer 6a may have opposite conductivity types.

  The material of the light-scattering reflective conductive layer 7, the p-side conductive layer 8, and the n-side conductive layer 9 reflects the light generated by the light emitting layer 6b without loss, and the light-scattering reflective conductive layer 7 and the n-side conductive layer 9 Are preferably those that can form a good ohmic connection with the p-type gallium nitride compound semiconductor layer 6c and the n-type gallium nitride compound semiconductor layer 6a, respectively.

Examples of such materials include aluminum (Al), titanium (Ti), nickel (Ni), chromium (Cr), indium (In), tin (Sn), molybdenum (Mo), silver (Ag), Gold (Au), Niobium (Nb), Tantalum (Ta), Vanadium (V), Platinum (Pt), Lead (Pb), Beryllium (Be), Indium oxide (In ₂ O ₃ ), Gold-silicon alloy (Au -Si alloy), gold-germanium alloy (Au-Ge alloy), gold-zinc alloy (Au-Zn alloy), gold-beryllium alloy (Au-Be alloy), or the like may be used. Among these, aluminum (Al) or silver (Ag) is preferable because it has a high reflectance with respect to light of blue light (wavelength 450 nm) to ultraviolet light (wavelength 350 nm) emitted from the light emitting layer 6b. Aluminum (Al) is also particularly suitable in terms of ohmic junction with the n-type gallium nitride compound semiconductor layer 6a. Further, a plurality of layers selected from the above materials may be stacked.

  In the present invention, the semiconductor layer 6 is preferably formed on the transparent substrate 5 by an epitaxial growth method. In this case, for example, of the light emitted inside the semiconductor layer 6, the light reflected to the substrate 5 side by the light-scattering reflective conductive layer 7 and the n-side conductive layer 9 composed of the light reflective layer is transmitted from the transparent substrate 5 to the outside. It becomes possible to release. The transparent substrate 5 is preferably made of sapphire, silicon carbide (SiC) or the like.

Further, when the substrate 5 may be opaque, a substrate made of a monoboride single crystal such as ZrB ₂ may be used. In this case, since the substrate 5 has excellent lattice matching and thermal expansion coefficient matching with the gallium nitride compound semiconductor layer, a gallium nitride compound semiconductor layer with few crystal defects can be formed. In this case, the substrate 5 is preferably removed by etching or the like, and the light emitted from the light emitting layer 6b can be emitted to the outside from the n-type gallium nitride compound semiconductor layer 6a side.

  Further, on the p-side conductive layer 8 and the n-side conductive layer 9, a p-side electrode and an n-side electrode (both not shown) for connecting a conductive wire for electrical connection with the outside are provided. Yes. Both electrodes may be, for example, a titanium (Ti) layer or a layer in which a gold (Au) layer is stacked with a titanium (Ti) layer as a base layer.

  According to the configuration of the present invention, the light emitted from the light emitting layer 6 b constituting the semiconductor layer 6 is directed toward the upper surface of the p-type gallium nitride compound semiconductor layer 6 c by the light scattering reflective conductive layer 7. Since light is scattered at various angles when reflected to the back side of the substrate 5 in the extraction direction, the proportion of light whose incident angle falls within the critical angle at the interface between the semiconductor layer 6 and the outside increases. In addition, it is possible to dramatically improve the light extraction efficiency.

  Of course, even when the light traveling toward the lower surface side of the n-type gallium nitride compound semiconductor layer 6a is totally reflected beyond the critical angle at the interface between the semiconductor layer 6 and the outside, the light-scattering reflective conductive layer 7 again. , The light is scattered again at various angles toward the substrate 5 side, which is the light extraction direction, so that the ratio of light whose incident angle falls within the critical angle at the interface between the semiconductor layer 6 and the outside increases. It is possible to increase the amount of light that can be extracted to the outside.

  Examples of the light emitting device of the present invention will be described below. In order to confirm the effect of the light-emitting element of the present invention, a light scattering property and a light extraction efficiency were simulated using a finite difference time domain (FDTD) method and a ray tracing method.

  First, light scattering simulation by the FDTD method was performed using a model with only irregularities, and the scattering angle distribution of scattered light was obtained. Next, the distribution is applied as a boundary condition of the light-scattering reflective conductive layer in the ray tracing method, and the light reflected by the light-scattering reflective conductive layer is given a distribution, whereby the light-emitting element (LED element) of the present invention. The light extraction efficiency was simulated.

  The emission wavelength is 400 nm, the refractive index of the substrate (thickness 330 μm) made of sapphire is 1.75, and the semiconductor layer (thickness) is made of an n-type gallium nitride compound semiconductor layer, a light emitting layer, and a p-type gallium nitride compound semiconductor layer. The refractive index of 3.2 μm) is 2.5 (the n-type gallium nitride compound semiconductor layer, the light-emitting layer, and the p-type gallium nitride compound semiconductor layer have almost no change in the refractive index, and thus all have the same refractive index) The light scattering reflective conductive layer (thickness 0.5 μm) made of aluminum (Al) was calculated to have a refractive index of 0.49.

  FIG. 3 shows a simulation result showing the distribution of scattered light when the period of unevenness is changed. Note that the plurality of irregularities formed on the actual light-scattering reflective conductive layer is not composed of irregularities having a regular but strictly uniform period, and therefore the period of the plurality of irregularities is an average as a whole. Although expressed by a period (average period), in this simulation, for the convenience of calculation, the calculation was performed on the assumption that the plurality of irregularities consist of irregularities having a uniform period.

  FIG. 3 shows that as the value of the scattering distribution exists in a range where the scattering angle is large, the scattered light extends over a wider angular range. From the figure, it can be seen that the scattered light extends over a wide range of angles in the range of the unevenness period from 240 nm to 560 nm. That is, when the uneven period is in the range of 240 nm to 560 nm, the scattered light intensity (scattering distribution) is large in the wide angle range of the scattered light, particularly in the range of 160 to 640 nm in the range of the scattering angle of about 0 to 30 °. The scattered light intensity is larger than that. In addition, even when the scattering angle is 30 to 60 °, the scattered light intensity is equal to or higher than that in the case of 160 nm and 640 nm.

  Next, FIG. 4 shows a simulation result of the light extraction efficiency when the period of the unevenness is changed. From the figure, it can be seen that when the period is in the range of 240 nm to 560 nm, the effect of improving the light extraction efficiency appears remarkably, and the effectiveness of the present invention clearly appears.

It is sectional drawing which shows an example of the conventional light emitting element. It is sectional drawing which shows an example of embodiment about the light emitting element of this invention. It is a graph of the result of having calculated | required the relationship between the scattering angle of light, and scattering distribution about the light-scattering reflective conductive layer in the light emitting element of this invention by simulation. It is a graph of the result of having calculated | required the light extraction efficiency of the light emitting element of this invention by simulation.

Explanation of symbols

5: Substrate 6: Semiconductor layer 6a: First conductivity type (n-type) gallium nitride compound semiconductor layer 6b: Light emitting layer 6c: Second conductivity type (p-type) gallium nitride compound semiconductor layer 7: Light scattering reflective conduction Layer 8: First conductive (n) side conductive layer 9: Second conductive (p) side conductive layer

Claims (3)

  A light emitting device having a semiconductor layer including a first conductive type gallium nitride compound semiconductor layer, a light emitting layer made of a gallium nitride compound semiconductor, and a second conductive type gallium nitride compound semiconductor layer sequentially laminated, A light-emitting element, characterized in that a light-scattering reflective conductive layer that scatters light is formed on the upper surface of the second-conductivity-type gallium nitride compound semiconductor layer by unevenness formed on the surface.   2. The light emitting device according to claim 1, wherein the light-scattering reflective conductive layer has the irregularities formed on a surface on an interface side in contact with the second conductive type gallium nitride compound semiconductor layer. The irregularities are regularly formed, and the average period T is λ the wavelength of light emitted from the light emitting layer, and n is the refractive index of the p-type gallium nitride compound semiconductor layer with respect to λ, 3. The light emitting device according to claim 2, wherein 1.5λ / n ≦ T ≦ 3.5λ / n.

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