JP3260358B2 - Semiconductor light emitting device - Google Patents

Description

The present invention relates to a semiconductor light emitting device, and more particularly to a semiconductor light emitting device using an InGaAlP-based semiconductor material.

(Prior Art) In recent years, light emitting diodes (LEDs) have been widely used as light sources for display devices and the like in optical communication and optical information processing equipment. In general, light emission intensity and response speed are important characteristics of an LED. In particular, a display LED is required to emit light with high luminance. The luminous efficiency of an LED is determined by the internal quantum efficiency and the extraction efficiency, of which the extraction efficiency is greatly affected by the element structure. Figure 19 shows an LED using InGaAlP as the active layer.
FIG. In the figure, 100 is an n-GaAs substrate, 101 is an n-InGaAlP cladding layer, 102 is an InGaAlP active layer, 103 is a p-InGaAlP cladding layer, 105 is an n-electrode, and 106 is a p-electrode. The emission wavelength of an LED made of this material system is in the range from green to red, but since the GaAs substrate gives absorption loss to this wavelength band, all the light emitted from the active layer to the substrate side is extracted to the outside by absorption. Can not do. That is, the extraction efficiency is maximum, η ₁ = 0.5. On the other hand, only a part of the light emitted to the side opposite to the substrate can be extracted outside due to reflection loss and total reflection at the emission surface. For example a 3.4 refractive index n _c of the p-InGaAlP cladding layer, the outside is the air (n ₀ = 1), the proportion of non-light reflected to the inside by the total reflection is η _{2 =} 1- {1- (n ^{_{0 / n c) 2} 1/2}} = 0.044 Among them, the proportion taken out into the air eta ₃ = 1 - a _{{(n c -n 0) /} (n c -n 0)} 2 = 0.7. After all, when all these are multiplied, η ₀ = η ₁ η ₂ η ₃ = 0.0154, that is, the extraction efficiency is 1.5% at the maximum.

Further, the extraction efficiency is also limited by the current spread width. In general, p-type InGaAlP has a large resistivity,
The current hardly spreads in the cladding layer, and the emission diameter in the active layer almost matches the electrode diameter. Therefore, since most of the light emitting region is directly below the electrode, only a small part of the light can be extracted to the outside.

(Problems to be Solved by the Invention) As described above, the conventional LED has a problem that the extraction efficiency is low and it is difficult to increase the luminance.

The present invention has been made in view of the above circumstances, and an object of the present invention is to provide a high-brightness semiconductor light emitting device capable of obtaining a large extraction efficiency in a surface emitting semiconductor light emitting device. It is in.

[Configuration of the invention]

(Means for Solving the Problems) The gist of the present invention is that a high-intensity semiconductor light emitting device is obtained by extracting invalid light that cannot be extracted to the outside in a conventional semiconductor light emitting device by changing the element structure. Is to provide.

That is, the present invention provides a substrate, an InGaAlP active layer formed on one surface of the substrate, a current diffusion layer formed on the opposite side of the active layer from the substrate, and the other surface of the substrate. And a second electrode formed on the current spreading layer so as to face the first electrode with the active layer interposed therebetween, and the second electrode side has a light extraction direction. In the surface emitting type semiconductor light emitting device,
A reflection film composed of alternating layers of GaAs and InGaAlP having a large absorption coefficient for the emission wavelength is formed only on the substrate side with respect to the AlP active layer.

Also, in the present invention, the second electrode is selectively formed,
Light is extracted from the non-formation portion of the second electrode, and the ends of the opposed second electrodes separated by the non-formation portion reach the InGaAlP active layer from the second electrode. In addition, the distance from each end of the second electrode to the non-forming portion side is substantially equal to the width of current diffusion.

Further, the optical thickness of each of the layers constituting the reflection film is configured to correspond to 1/4 of the emission center wavelength.

(Operation) According to the present invention, a high-luminance semiconductor light emitting device can be realized by extracting light that has been disabled without being extracted to the outside to the outside by changing the element structure.

Hereinafter, embodiments of the present invention will be described with reference to the drawings. 1
FIG. 1 is a diagram showing a schematic structure of a semiconductor light emitting device according to one embodiment of the present invention. In the figure, 10 is an n-GaAs substrate, 11 is n-GaAs
In _0.5 (Ga _1-x Al _x ) _0.5 P clad layer, 12 is In _0.5 (Ga _1-y
Al _y ) _0.5 P active layer, 13 is a p-In _0.5 (Ga _{1 -z} Al _z ) _0.5 P cladding layer, 14 is a p-GaAlAs current diffusion layer, 15 is an n electrode, 16
Indicates a p-electrode, respectively. FIG. 2 shows the shape of the P electrode in FIG. In the embodiment of FIG. 1, the compositions of the active layer and the cladding layer are selected so as to satisfy y <x, z (1). The current diffusion layer 14 is a layer provided to increase current spread in the active layer. That is, since the p-GaAlAs 14 has a lower resistivity than the p-InGaAlP cladding layer 13, the current spreading width can be significantly increased as compared with the case where the current diffusion layer 14 is not provided. In order to explain the effect of the present invention, first, the structure dependence of the current spreading width in the active layer will be described. For simplicity, consider the current spread in the example shown in FIG. 3 where the layer structure is the same as in the example of FIG. 1 and there is only one electrode. The thickness and resistivity of each h ₂ of the current diffusion layer 24 in Figure 3, when the [rho _2, results effective current spreading width W _eff and extraction efficiency η is calculated whether changes to how for these parameters Is shown in FIGS. 4 and 5. As shown in the figure, ρ
_It can be seen that _Weff increases as _{2 decreases and h2} increases. It is clear that the current diffusion layer improves the extraction efficiency, but the point of the present invention is that the electrode is divided into a plurality of parts as shown in FIG. Hereinafter, the effect of the split electrode will be described.

In the example of FIG. 3, the above-mentioned effective current spread width W _eff is approximately expressed by the following equation (Yonezu et al .: Jpn.J.Appl.Phy.
s., Vol. 12, No. 10 (1973) pp. 1585-1592).

W _eff = W ₀ + 2W ₁ (2) W ₁ = {2nkT / (qρ ₅ J)} ^1/2 (3) 1 / ρ ₅ = h ₁ / ρ ₁ + h ₂ / ρ ₂ (4) where W ₀ is the electrode width, n is a value representing the diode characteristics (usually ~ 2), T is temperature, J is current density, or h ₁ , h ₂
Represents the thickness of the p-InGaAlP cladding layer 23 and the current diffusion layer 24, and ρ ₁ and ρ ₂ represent the resistivity of the cladding layer 23 and the current diffusion layer 24, respectively. The relationship between W _eff and ρ ₂ , h ₂ described above is apparent from equations (2) to (4).

As can be seen from equation (3), another parameter affecting _Weff is the current density J. When the total current value I is constant, the current density J changes depending on the size and shape of the electrode. In the case of a circular electrode, the effective light emitting area S _eff is represented by the following equation.

S _eff = π (W _eff / 2) ² (5) The extraction efficiency limited by the fact that the light emitting portion is below the electrode is approximately η ₅ = (S _eff −S ₀ ) / S _eff (6) Given. Where _{S 0 = π (W 0/} 2) 2 (7) On the other hand, because the relationship between the total current value I and the current density J is expressed by _{I = S eff J (8)} , (2), (3 ), (5), and (8), the following relational expression is obtained.

That is, the ratio of W ₁ and W ₀ is determined only by the [rho ₅ and I, does not depend on the size W ₀ of the electrode. This means that when an LED is used at a constant current value, the efficiency η ₅ determined by the equation (6) is used.
Means that it does not depend on the size of the electrode. Therefore, in the case of a single electrode, the luminous efficiency does not change even if the size of the electrode is changed. In the case of circular electrodes, the value of W _eff determining the S _eff strictly instead of W ₁ in the above _{equation, W 1 '= {(W} 0 + 2W 1) 2/4 + W 1 2} 1/2 -W 0 / 2 (10), where the ratio of W ₁ ′ to W ₀ is ρ ₅
Similarly, it is determined only by I and I and does not depend on the size W _{0 of the} electrode. FIGS. 6 and 7 show the ratio between the current spread width and W ₀ and the extraction efficiency η in the case of a single circular electrode.
The dependence of the current value I (values taking into account the absorption of the substrate, reflection on the extraction surface, and total reflection) is shown. As can be seen, the smaller the current value I, the higher the extraction efficiency. This is because, as shown by the equation (3), when the current density is small, the effective current spreading width becomes large.

Next, how the extraction efficiency depends on the electrode shape when the electrode shape is not a circle but a stripe electrode will be described. When the stripe width of the stripe electrode is _{W 0, (2), (} 4) expression is satisfied as it is. Assuming that the length of the stripe is L = aW ₀ (11), the equations corresponding to the equations (5) and (7) are S _eff = aW _eff ² (12) S ₀ = aW ₀ ² (13) It is derived that there is the following relationship between the W ₁ and W ₀ in the future.

W ₁ / W ₀ = ab / I + ｛(ab / I) ² + ab / I｝ ^1/2 (14) where b = {2nkT / (qρ ₅ )} ^1/2 (15) In the case of an electrode, the extraction efficiency is determined by the total current value I and the ratio a between the stripe length and the stripe width. FIGS. 8 and 9 show the dependence of W _eff / W ₀ and η on the parameter a in the case of a stripe electrode. It can be seen from the figure that the larger a is, the higher the extraction efficiency is.

As can be seen from the above two examples, in the case of a single electrode, if the electrode shape is the same, the extraction efficiency does not depend on the size of the electrode, but is determined only by the total current value I. Therefore, for example, in the case of a circular electrode, the extraction efficiency cannot be improved even if the electrode diameter is changed. On the other hand, if the current value I is reduced, the extraction efficiency is improved, but in this case, the light output itself is reduced, so that high-luminance light emission cannot be realized. However, according to the configuration of the present invention, by dividing the electrodes, the total current value I is kept constant,
Since the current flowing through each electrode can be reduced, the current spreading width can be increased, and the extraction efficiency can be improved. That is, if the divided electrodes into m, since the current flowing through each electrode becomes 1 / m, it is possible to increase the value of W _eff / W ₀ as shown in FIG. 6, the total current as a result Η can be increased while the value I is kept constant. Fig. 10 shows a comparison of the current spread between the single electrode and the split electrode. As shown in FIG. 10 (b), in the case of divided electrodes, the distance D between the electrodes is the current spreading width of one electrode in the active layer (= the width of the light emitting region).
The difference may be equal to or more than the difference ΔW (= W _eff −W ₀ ) between the electrode and the electrode width. In order to further reduce the overall area as much as possible, the interval between the electrodes may be set to be substantially equal to ΔW.
Another embodiment of the configuration using a plurality of circular electrodes is shown in FIG. From the above discussion, it is clear that as the number of divisions increases, the extraction efficiency can be improved. However, there is a problem in the electrode formation process, and the number of electrodes may be determined accordingly. Figures 12 and 13 _show that W _eff / W for I = 20 mA.
_This shows the dependence of ₀ and η on the number of electrodes. As you can see,
For example, when h ₂ = 1 μm, four divisions is sufficient. Furthermore, even when h ₂ = 0, that is, even when there is no current diffusion layer, a sufficiently large extraction efficiency can be realized by setting the number of divisions to about 6.

The above description is for the case of a circular electrode. Next, an example for a stripe electrode will be described. As shown in FIG. 6, in the case of a stripe electrode, the larger the ratio a between the stripe length and the stripe width, the higher the extraction efficiency. Therefore, efficiency can be improved by using an elongated electrode, but in that case, the light emitting pattern becomes elongated, which is not preferable. FIG. 14 shows a third embodiment of the present invention.
This is an example in which a stripe electrode is divided into a plurality. Also in this case, the interval between the electrodes may be made substantially equal to the above-described ΔW. FIG. 15 shows a modification of this embodiment, in which the electrodes are formed in a two-dimensional mesh pattern. In this case, the mesh interval in each direction may be made substantially equal to ΔW.

FIG. 16 shows a fourth embodiment of the present invention.
60 is an n-GaAs substrate, 61 is an n-InGaAlP cladding layer, 62 is I
nGaAlP active layer, 63 is a p-InGaAlP cladding layer, 64 is p-G
aAlAs electrode diffusion layer; 65, an n-electrode; 66, a p-electrode; 67, a reflective film made of a semiconductor thin film. The reflection film 67 has a function of reflecting light emitted from the active layer to the GaAs substrate side. In the example of FIG. 19, 50% of the light output was lost due to absorption of GaAs, but by providing a reflective film, this light can be reflected and effectively extracted. This reflection film is manufactured by alternately stacking semiconductor films having different refractive indexes. The optical thickness of each of the semiconductor film may be set to lambda _0/4 with respect to the emission center wavelength lambda _0. In addition, in the example of FIG. 16, since the reflection film is formed between the n-type conductivity layers, the semiconductor film forming the reflection film may be n-type.

For example, In _0.5 (Ga _0.7 Al _0.3 ) _0.5 P and In _0.5 (Ga _0.3 Al
_0.7 ) _0.5 P for alternating layers and GaAs and In _0.5 (Ga _0.3 Al
_0.7) for the case of _0.5 P of alternating layers, shown in Figure 17 the relationship between the number of layers and the reflectance characteristics of the lambda _0/4 of the film. As can be seen from the figure, In _0.5 (Ga _0.7 Al _0.3 ) _0.5 P / In _0.5 (Ga _0.3 Al _0.7 ) _0.5 P
In the case of alternating layers, a reflectance of 90% or more can be obtained with about 80 layers. On the other hand, in the case of the GaAs / In _0.5 (Ga _0.3 Al _0.7 ) _0.5 P alternating layer, a reflectivity of about 70% can be obtained with the 30 layers even though GaAs has a large absorption coefficient. This is GaAs
This is because the difference between the real part of the refractive index of In _0.5 (Ga _0.3 Al _0.7 ) _0.5 P is large. FIG. 18 shows the wavelength characteristics of the reflection film using the GaAs / In _0.5 (Ga _0.3 Al _0.7 ) _0.5 P alternating layer. Although this reflective film is designed with the center wavelength λ ₀ = 600 nm, as can be seen from the figure, for example, when the number of layers is 30, a reflectivity of 50% or more is obtained in a wavelength range of about 70 nm. It can be seen that it can be sufficiently used for an LED having a wide spectral width. Note that the same reflection characteristics can also be realized by using a GaAs / GaAlAs alternate layer.

The embodiments of the present invention related to the light extraction efficiency improving method in the surface emitting semiconductor light emitting device have been described above. These are effective even if used alone,
It is of course possible to use them in combination, in which case the effect is further enhanced.

〔The invention's effect〕

As described in detail above, according to the present invention, in a surface-emitting type semiconductor device, light that has been conventionally disabled without being extracted to the outside can be extracted to the outside, and a high-luminance semiconductor light-emitting device can be realized.

[Brief description of the drawings]

FIG. 1 is a diagram showing a schematic structure of a semiconductor light emitting device according to one embodiment of the present invention.

FIG. 2 is a diagram showing an example of an electrode pattern of the semiconductor light emitting device of the present invention.

FIG. 3 is a diagram for explaining an effect of the present invention.

FIG. 4 is a diagram for explaining an effect of the present invention.

FIG. 5 is a diagram for explaining the effect of the present invention.

FIG. 6 is a diagram for explaining the effect of the present invention.

FIG. 7 is a diagram for explaining an effect of the present invention.

FIG. 8 is a diagram for explaining the effect of the present invention.

FIG. 9 is a diagram for explaining an effect of the present invention.

FIG. 10 is a diagram illustrating an effect of the present invention.

FIG. 11 is a view showing another example of the electrode pattern of the semiconductor light emitting device of the present invention.

FIG. 12 is a diagram for explaining the effect of the number of electrodes.

FIG. 13 is a diagram for explaining the effect of the number of electrodes.

FIG. 14 is a view showing another example of the electrode pattern of the semiconductor light emitting device of the present invention.

FIG. 15 is a diagram showing another example of the electrode pattern of the semiconductor light emitting device of the present invention.

FIG. 16 is a diagram showing another embodiment of the present invention.

FIG. 17 is a view showing an example of characteristics of a reflection film.

FIG. 18 is a view showing a characteristic example of a reflection film.

FIG. 19 is a diagram showing an example of a conventional semiconductor light emitting device.

[Explanation of symbols]

 10, 20, 50, 60, 100 ... n-GaAs substrate, 11, 21, 51, 61, 101 ... n-InGaAlP cladding layer 12, 22, 52, 62, 102 ... InGaAlP active layer 13, 23, 53, 63, 103 ... p -InGaAlP cladding layer 14,24,54,64 ... p-GaAlAs layer 15,25,55,65,105 ... n electrode 16,26,56,66,106 ... p electrode 67 ... Reflecting film composed of semiconductor multilayer thin film

 ──────────────────────────────────────────────────続 き Continuation of front page (72) Inventor Hideto Sugawara 1 Toshiba Research Institute, Komukai, Kawasaki-shi, Kanagawa Prefecture (56) References JP-A-63-182876 (JP, A) JP-A-2-98984 (JP, A) JP-A-58-34984 (JP, A) JP-A-58-137274 (JP, A) JP-A-51-11387 (JP, A) JP-A-1-200678 ( JP, A) JP-A-2-20076 (JP, A) JP-A-2-170486 (JP, A)

Claims (3)

(57) [Claims] A substrate; an InGaAlP active layer formed on one surface of the substrate; a current diffusion layer formed on the opposite side of the active layer from the substrate; A first electrode formed on the current diffusion layer; and a second electrode formed on the current diffusion layer so as to face the first electrode with the active layer interposed therebetween. In a surface-emitting type semiconductor light-emitting device, a reflection film composed of alternating layers of GaAs and InGaAlP having a large absorption coefficient with respect to an emission wavelength is formed only on the substrate side with respect to the InGaAlP active layer. Semiconductor light emitting device. 2. The method according to claim 1, wherein the second electrode is selectively formed, and light is extracted from a non-formed portion of the second electrode.
The second opposing portions separated by the non-formed portion;
The ends of the electrodes are spaced from each other at a distance substantially equal to the width of current diffusion from each of the ends of the second electrode to the non-formation part before reaching the InGaAlP active layer from the second electrode. The semiconductor light emitting device according to claim 1, wherein 3. The semiconductor light emitting device according to claim 1, wherein the optical thickness of each of the layers constituting the reflection film is equivalent to / 4 of the central wavelength of light emission.