JPS6133276B2 - - Google Patents

Description

[Detailed description of the invention]

The present invention relates to a light emitting diode, and more particularly to a semiconductor light emitting diode using a semiconductor with a high refractive index. FIG. 1 shows the structure of a typical conventional light emitting diode. In the figure, 1 is a semiconductor portion of a light emitting diode, 2 is a resin, 3 and 7 are electrode metals, 4 is an n-type semiconductor, 5 is a p-type semiconductor, and 6 is a p-type crystal substrate. 8 is a pn junction. In the example shown in FIG. 1, light is extracted from the surface 9 of the semiconductor 1.
Because of its large refractive index, light that enters from inside the semiconductor at an angle of incidence θc or more is totally reflected at the surface and does not exit to the outside. The refractive index _n2 of commonly used resins such as epoxy resins is approximately 1.5, whereas -
Intergroup compound semiconductor light emitting diode viz.
Approximately n ₁ = 3.4 for GaAlAs, GaPAs, GaP, etc.
It has a refractive index of The critical angle θc is sinθc=n ₂ /
Since it is given by n ₁ , when resin coating is performed with n ₁ =1.5, θc≈25. That is, the solid angle Ω
=2π(1-cosθc)≒2π×0.09strad, which is 9 compared to the solid angle of 2π of light incident on one plane.
Only % of the light is extracted to the resin side, that is, to the outside of the semiconductor. The rest is reflected by the surface 9 and returns to the interior. In direct transition semiconductors such as GaAlAs and GaPAs, the absorption coefficient for the emission wavelength is large, so it cannot be extracted from the back surface or reflected multiple times to the outside, so almost all of the portion reflected inward cannot be effectively extracted as light. . In the case of light incident perpendicularly to the surface, an antireflection film is appropriate to increase the transmittance, but the antireflection film requires control of the film thickness and does not have the role of increasing the critical angle. As shown in Figure 2, the semiconductor surface 9' is formed into a dome shape, but forming each light emitting diode into a lens shape is very expensive, and it is not possible to mass produce it at a very low cost. Therefore, it cannot be used for general purposes. A method of forming many lens-like irregularities on the surface is also known (for example, Japanese Patent Application Laid-Open No.
-52792 Publication), forming holes on a substrate using a mask to grow crystals has the disadvantage that the process is complicated and light emitting diodes, which are characterized by low cost, become expensive. Another problem is that the surface after growth tends to have lens-like irregularities, and a special method is required to obtain a flat surface. The present invention provides a bright light emitting diode that can extract a large amount of light from one surface regardless of the critical angle at a relatively low cost. As shown in Figure 3, the unevenness of the surface is somewhat larger than the wavelength, and if it is a so-called diffusing surface, no matter which direction the surface is coming from, the light reaching the surface will roughly speaking be in the same direction as shown by the arrows in Figure 3. It is scattered like this. (The diagram omits electrodes, etc.) Therefore, approximately 50% of the light that reaches the surface from any direction is scattered forward, giving a maximum efficiency of 50%, compared to the previous 9%. It can be made extremely large by more than 5 times. However, if the semiconductor surface of the light emitting diode is made uneven and damaged by sandblasting or the like, many defects may occur and the light may not emit light, which is completely undesirable. In addition,
It is known that roughening the surface of a photodiode lowers the effective reflectance, but in the case of a photodiode, the refractive index of the crystal is higher than that of the external air or light that enters the crystal from the outside. Since the refractive index is greater than that of the coating material, total internal reflection, which is a problem with the above-mentioned light emitting diode, does not occur. Therefore, roughening the surface merely serves to reduce the reflectance by effectively making the refractive index near the surface smaller than the refractive index of the crystal. It is extremely undesirable to roughen the surface of a light emitting diode, which suffers from a much greater reduction in efficiency due to defects than a photodiode. Therefore, this problem can be solved by forming a light-diffusing layer of a transparent material whose refractive index is as close as possible to the refractive index n of the semiconductor on the surface of the semiconductor. In general, the light diffusing surface itself is well known in optics, and for example, a display device is known in which a diffuser glass lid is placed over a small light bulb or an array of light emitting diodes so that the light bulbs or light emitting diodes themselves cannot be seen. In contrast, the diffusing surface of the present invention is formed directly on the light extraction surface of the light emitting diode or via another thin film having a wavelength smaller than the wavelength of light. In a typical resin-coated semiconductor light emitting diode, the refractive index of the material that makes up the diffusion surface is
Choose a refractive index that is larger than the refractive index of the transparent resin that covers the light extraction surface of the light emitting diode. Figure 4 is an example of a GaAlAs light emitting diode with a polycrystalline structure on the output surface.
GaP10 is deposited several times the wavelength by CVD (chemical vapor deposition) or vacuum evaporation. That is, if the emission wavelength is 6700 A and the refractive index of the gap is about 3.4, the wavelength within the film is 2000 A, so a film of about 1 μm to 2 μm is precipitated. However, the deposited film must have a large particle size so that it becomes a milky red-white light diffusing film rather than a transparent film. If it is precipitated at a temperature lower than the temperature at which a CVD film is normally formed, an opaque light-diffusing film can be obtained because the grain size is generally coarse. The fact that the film is milky white indicates that the diameter of the particles is so coarse that light is scattered on the surface in all directions as shown in FIG. In the figure, 10 schematically shows that light diffusion occurs because the grain size of the film is coarse. Even with the vapor deposition method, it is possible to vaporize at a higher speed than usual and obtain a coarse particle size. Temperature, substrate conditions, and CVD for diffusing light diffusing surfaces
In the case of , the flow rate etc. differ depending on the substance, but it is easy to determine the conditions experimentally because it is easy to clearly determine whether the film is a transparent film or a diffused film by measuring the reflected light or visually observing it. In order to form a film, instead of CVD or vapor deposition, another method that can be used is to mix a film-forming substance, such as GaP fine powder, with an organic solvent and spray it onto the semiconductor surface. Of course, the CVD method has better adhesion, but the spray method often provides adhesion that is sufficient for practical use. Epoxy coating is applied on the light diffusing film thus produced using a conventional method. In the above example, the refractive index of the film-forming material and the semiconductor are almost the same, so the light that reaches the surface enters the diffusion film and is scattered in all directions, so about 50% of it can be extracted to the outside. In other words, approximately five times more light can be obtained than when no diffusion film is used, resulting in an extremely bright light emitting diode. As for the film, as shown in Figure 6, if a transparent film 11 is first formed at a relatively high temperature and the light diffusing film 10 is formed by lowering the extraction temperature in the middle, reflection loss at the boundary can be greatly reduced. desirable. In this case, instead of forming two layers 10 and 11 with completely different grain sizes, by continuously changing the film deposition conditions at the boundary, an interface with discontinuous refractive index will not be created, and reflection at the boundary will be reduced. It is more preferable because it prevents Note that the refractive index of a deposited film changes depending on the deposition conditions and particle size, and it is known that, for example, in the case of a SiO film, there is a difference of about 1.7 to 1.9 depending on the deposition conditions. However, since the change is not so large as in the above example, the light extraction rate can be roughly evaluated using the refractive index of the crystal itself. The light diffusing film deposited by the CVD method or the vapor deposition method is considered to be a collection of minute particles, as shown in FIG. 5, and the light diffusing effect is produced because the particle size is larger than the wavelength of light to some extent. If the packing of particles is rough, the refractive index decreases, so it can be said that the rate of decrease in refractive index is almost the same as the rate of decrease in density, but in CVD or vapor deposited films, the decrease in density is less than a few percent. . In addition to GaP, other film materials include materials that have a high refractive index in the visible light region and are transparent to visible light, as shown in Table 1.

[Table] If the film material is ZnSe, the refractive index is 2.9, so −
When the refractive index of an intergroup semiconductor light emitting diode is approximately 3.4, the surface of the semiconductor is optically flat, so not all light enters the film, and a critical angle θc exists. That is, since Sinθc=n ₃ /n ₁ (n ₃ is the refractive index of the film material), θc=59°, and the solid angle Ω=2π(1−cosθc)=2π×0.5
becomes. The film is a light diffusing surface, but the refractive index of the film can be approximately estimated using the refractive index of a single crystal, etc. Light within a critical angle not only enters the film but also reflects between the film and the semiconductor surface. However, the reflection loss is given by R=(n ₁ -n ₃ /n ₁ +n ₃ ) ² , and although the reflection loss is 0.6% in the above example, it can be ignored. In this way, the light that enters the film is diffused; 50% of it is scattered forward, and the remaining 50% enters the crystal again and is lost, so the efficiency of light extraction to the outside is 1/1/2 of 50%. 2, or approximately 2.5%. Since the space between the film and the resin is not optically flat, it can be considered that almost all the light scattered forward from the light diffusing film enters the resin. Without the light diffusion film, the light extraction rate is 9% as mentioned earlier, so using the light diffusion film increases the brightness by about 2.8 times. If we use GaN with n=2.4, which has the lowest refractive index in the table, a similar calculation for an intergroup semiconductor light emitting diode will yield θc≒45°, which means that about 15% of the light can be extracted to the outside, which is about 1.5 times as much as when there is no film. The brightness will be . Therefore, it is much brighter to use GaP than GaN, but GaN has better insulating properties than GaP.
GaN is better, so depending on the purpose, GaN is preferable. In this way, in the case of an intergroup semiconductor light emitting diode, by selecting an appropriate material with a refractive index of about 2.4 or more from the substances shown in Table 1, mixtures thereof, etc., and forming a light diffusion layer, light emission can be achieved. The light extraction rate of a diode can be increased by approximately 1.5 times or more. Since the conditions for forming the light-diffusing film are relatively low temperatures, it is also advantageous that the light-diffusing film can be deposited after forming the electrodes of the light-emitting diode. Of course, it is also possible to form a film, then use a photomask to make holes for the electrode portion, and then attach the electrode metal. Note that GaOxNy is used as a passivation film on the semiconductor surface.
Even if there is a thin film with a different refractive index, such as SiO ₂ or SiO 2, if the film thickness is sufficiently thinner than the wavelength,
If it is less than 2 to 1/3, a sufficient effect can be obtained by depositing the light diffusing film of the present invention thereon. The manufacturing method, structure, and characteristics of GaOxNy have been patented by the inventor (Japanese Patent Publication No. 53-38580, Japanese Patent Publication No. 53-86573, etc.)
It is described in. It is also effective to use GaOxNy itself as the light diffusion film of the present application, or to use it as a first layer coating film and then deposit other films listed in Table 1 on top of it to form a two-layer film. It is. In GaOxNy, when the ratio of oxygen O to N increases, the refractive index becomes smaller than that of GaN, and the light extraction rate decreases.
It is better to reduce the ratio of , or use it as a two-layer film, so that it has both the effect of increasing the light extraction rate and the effect of acting as an inert film. Also,
Taking GaAlAs light emitting diodes as an example, GaP can be used as the deposited film, but GaAlAs can also be coated by CVD.
It may be coated by law. In this case, the forbidden band is close to the emission wavelength, so in order to prevent absorption loss from increasing, it is necessary to carefully control the CVD conditions so as to obtain a uniform composition of the deposited film. In this case, it is also possible to add the same conductive impurity as layer 4 to the deposited film to form a conductive film and then form an electrode. An impurity doped region may be selectively formed in the film by ion implantation or the like. Although specific examples have been shown above regarding intergroup compound semiconductor light emitting diodes, it goes without saying that the present invention can also be applied to intergroup compound semiconductor light emitting diodes. For example, a ZnSe ni junction is used as a blue light emitting diode, but the refractive index of ZnSe is 2.9 in the blue region, and since it is a direct transition semiconductor, it is effective to form a light diffusing film. ZnSe itself may be used as the light diffusing film, but since some absorption effect will be added, it is more effective to use a GaN film. That is, GaN has the advantage of being transparent even to the blue light emitted by ZnSe. In addition, -
The refractive index of intergroup compounds is 3.2 to 3.4, but −
Since the refractive index of intergroup compounds is 2.4 to 2.9, −
Intergroup compounds have much smaller internal critical angles,
Therefore, the effect of forming a light diffusing film is also significant. -In the case of intergroup compounds, the refractive index of the semiconductor is
2.9, if the refractive index of the resin is 1.5, the critical angle θc is 31
°, solid angle Ω = 2π × 0.14strad, and when the refractive index is 2.4, the critical angle θc = 39°, Ω = 2π × 0.22strad
becomes. In other words, if a light diffusing film is not used, the refractive index
In a light-emitting diode with a refractive index of 2.9, 14% of the light directed from the inside toward the surface is extracted, and in the case of a refractive index of 2.4, 22% is extracted to the outside. Needless to say, if no resin is used, the ratio will be even smaller. When the refractive index of the semiconductor is 2.4, the light extraction rate of 22% is increased by approximately 1.5 times.
It can be seen that in order to achieve 30%, the critical angle needs to be approximately 66° or more using the same calculation method as in the second embodiment. That is, in this case, it is necessary to form the light diffusing film with a substance having a refractive index of approximately 2.2 or more. In other words, the brightness increases by 50%, so even a film with a refractive index of 2.2 can be said to be quite effective. To summarize the present invention, the light emitted to the outside from a semiconductor light emitting diode has an angle greater than the critical angle with respect to the surface of the semiconductor because the refractive index of the semiconductor is much higher than that of air or coating resin. The purpose is to provide a bright light emitting diode that eliminates the disadvantage that the light that reaches the semiconductor surface is totally reflected and ultimately reabsorbed and cannot be effectively extracted to the outside. This is a light emitting diode characterized by depositing a film including a light diffusing film made of a transparent material with a high refractive index. The light diffusion layer may be any layer that can diffuse light emitted from a light emitting diode, and is not limited to a deposited film of particulate matter. The light-emitting diode of the present invention is much easier to manufacture than a light-emitting diode having a lens-shaped or dome-shaped semiconductor surface, and can have a light extraction rate of about 50%.

[Brief explanation of the drawing]

Fig. 1 is a schematic cross-sectional view of a conventional semiconductor light-emitting diode, Fig. 2 is a schematic cross-sectional view of an improved conventional semiconductor light-emitting diode, and Fig. 3 shows light scattering of a semiconductor light-emitting diode with a roughened surface. Pattern diagram,
4 is a schematic cross-sectional view showing an example of a light emitting diode of the present invention, FIG. 5 is an enlarged film diagram of the light diffusion film 10 in FIG. 4, and FIG. 6 is a diagram showing the light diffusion film 10 in FIG. 4. It is a schematic diagram which shows another example. 1... Semiconductor part of light emitting diode, 10...
Light diffusion film, 11...transparent layer.

Claims (1)

[Scope of Claims] 1. In a semiconductor light emitting diode constructed using a semiconductor with a high refractive index, polycrystalline particles of a substance that has no absorption in the light emission wavelength region of the light emitting diode are formed on the surface of a single crystal semiconductor from which light is extracted. 1. A semiconductor light-emitting diode, characterized in that a layer is provided, and the average diameter of polycrystalline particles in the layer is larger than the emission wavelength to serve as a light diffusion layer that causes light scattering. 2. The semiconductor light emitting diode according to claim 1, wherein the material forming the layer has a refractive index of about 2.4 or more, and the semiconductor forming the light emitting diode is an intergroup compound semiconductor. . 3. The semiconductor light emitting diode according to claim 1, wherein the material forming the layer has a refractive index of about 2.2 or more, and the light emitting diode is formed of an intergroup compound semiconductor. 4. Any one of claims 1 to 3, wherein the layer includes a light diffusion layer that diffuses light and an auxiliary layer that transmits light and is provided between the semiconductor surface and the semiconductor surface. The semiconductor light emitting diode according to item 1. 5. The light diffusing layer that diffuses light and the auxiliary layer that transmits light are composed of an integral film whose grain size changes in the thickness direction. semiconductor light emitting diode. 6. The semiconductor light emitting diode according to any one of claims 1 to 5, wherein the main component of the layer is GaP.