JPH0690020A - Light emitting diode - Google Patents

Abstract

PURPOSE:To make the current from an upper electrode diffuse over the entire light emitting layer by providing both a high resistance layer whose electric resistance is high and a low resistance layer whose electric resistance is low on an epitaxial growth emitting layer, mounted on a chemical compound semiconductor substrate. CONSTITUTION:A light emitting layer 2 is formed on a chemical compound semiconductor substrate 1, and on the top of that, a high resistance layer 3 with high resistance is formed. Then, adjoining to the high resistance layer 3, a low resistance layer 4 with low resistance is formed. Then an upper electrode 5 and lower electrode 6 are provided. With this, the electric current at an electrode part of light emitting diode can be diffused for wider light emitting area, even for the epitaxial wafer with which a thicker growth film is difficult to be obtained, such as an organic metal vapor growth method. Since an upper electrode can be miniaturized, application range is enlarged even with lamps.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a light emitting diode using a compound semiconductor, and more particularly to a light emitting diode in which a current is diffused over the entire surface of a semiconductor to improve the luminous efficiency.

[0002]

2. Description of the Related Art A light emitting diode using an epitaxial growth layer of a compound semiconductor is widely used. When making a light emitting diode by epitaxial growth on a semiconductor substrate, electrodes are attached to the front and back of the light emitting diode chip to serve as current supply ports. Generally, the upper electrode (the side opposite to the substrate) is the surface area of the chip on the light emitting diode chip. A ohmic electrode having an appropriate shape such as a circle or a square having an area of about 1/5 to 1/4 of that of the lower electrode (substrate side) is formed on the entire bottom surface or the entire bottom surface of the chip in a grid pattern. An electrode is formed on the surface (Japanese Patent Laid-Open No. 56-42).
390).

In this case, the upper electrode is preferably large from the viewpoint of diffusing a current over the entire chip, and the electrode area is preferably small from the viewpoint of extracting emitted light to the outside. The upper electrode is designed to have an appropriate size in consideration of the above. It has also been proposed to increase the thickness of the upper layer of the light emitting layer in order to diffuse the current from the upper electrode throughout the light emitting layer in order to widen the light emitting region (see Japanese Patent Laid-Open No. 3-171679).

[0004]

Problems to be Solved by the Invention In on a GaAs substrate
AlInGaP / InGa using GaP mixed crystal active layer
A light emitting diode having a light emitting layer having a P / AlInGaP double hetero structure has been attracting attention because it can obtain high brightness. However, when the upper layer of the light emitting layer is thin, the current does not spread in the lateral direction and only the light emission just below the electrode is emitted. There is a drawback that the light emitting region is narrow because it cannot be obtained. Generally, a mixed crystal thin film is formed by liquid phase epitaxial growth (LPE growth method) or metal-organic vapor phase epitaxy method (MOC).
The MOCVD growth method, which has been recently developed rapidly in this field, allows more precise crystal growth than the LPE growth method, but it takes time to grow a thick film and productivity is increased. When considering, it can be said that it is practically difficult to use.

Therefore, in the light emitting diode using the epitaxially grown film excellent in crystallinity obtained by this MOCVD growth method, the thick current diffusion layer cannot be used and the current from the upper electrode does not diffuse to the entire light emitting layer of the light emitting diode. For,
There is a drawback that only the peripheral portion of the electrode emits light.

[0006]

According to the present invention, in a light emitting diode having an epitaxially grown light emitting layer stacked on a compound semiconductor substrate, a high resistance layer having high electric resistance and a low resistance layer having low electric resistance are provided on the light emitting layer. It is provided so that the current from the upper electrode diffuses throughout the light emitting layer.

A typical structure of the light emitting diode of the present invention is shown in FIG. In FIG. 1, 1 is a compound semiconductor substrate, 2 is a light emitting layer, 3 is a high resistance layer, 4 is a low resistance layer, 5 is an upper electrode, and 6
Is the lower electrode.

The compound semiconductor substrate 1 is made of GaP, GaAs,
InP or the like can be used. The light emitting layer 2 is a simple pn junction,
Anything such as a single hetero structure and a double hetero structure can be used. The crystal used should be Ga according to the desired emission wavelength.
P, InGaP, AlInGaP, GaAlAs, Ga
Various things such as AsP can be selected. In order to avoid the influence of crystal defects of the substrate between the substrate 1 and the light emitting layer 2, an epitaxial growth layer of the same crystal as the substrate may be used as the buffer layer. It goes without saying that the p-type and the n-type can be formed in reverse.

Reference numerals 3 and 4 in FIG. 1 are current diffusion layers having different resistivities, and it is necessary that the electrodes are easy to form and do not absorb the emitted light. When it is difficult to form an electrode, a contact layer may be provided between the low resistance layer 4 and the electrode 5. The higher the resistance of the high resistance layer 3 in the vertical direction, the better the current spreading effect is. Therefore, it is considered that the resistivity is increased and the film thickness is increased. However, increasing the film thickness is against the object of the present invention. Therefore, it is desirable to adopt an epitaxial growth layer having a large resistivity and make the film thickness as thin as possible. Further, increasing the resistance increases the forward voltage of the light emitting diode, which is disadvantageous.
It should be within a range that does not pose a problem in practice. Considering the above, the resistivity of the high resistance layer 3 is set to 1 to 100 Ωcm. In general, when an impurity such as Zn is controlled to be low in order to provide a carrier to a semiconductor to obtain a high resistivity, the resistivity which can be actually controlled by a vapor phase growth method such as MOCVD is up to about 100 Ωcm. . Considering stable production, the target resistivity is set to 10 Ωcm, and the electric resistance of a semiconductor chip formed by forming this high resistance layer in a size of 1 μm and 300 × 300 μm is about 1 Ω. This is considered to be a level at which the voltage drop in this layer is 0.02 V when the normal working current of the light emitting diode is 20 mA, which is not a problem in practical use. When a layer having a higher resistivity is formed, the film thickness can be reduced to form a high resistance layer.

Next, a low resistance layer 4 having a low resistivity is provided adjacent to the high resistance layer 3. The lower the resistance in the lateral direction, the better the current spreading effect of the low resistance layer 4, and therefore, the lower the resistivity and the larger the film thickness, the better. However, increasing the film thickness diminishes the characteristics of the present invention. . The low resistivity that is actually obtained by doping impurities by MOCVD is 10
^{It is -1} to 10 ^-3 Ωcm. As described above, the higher the resistivity of the high resistance layer 3 is, the lower the resistivity of the low resistance layer 4 is, that is, the greater the ratio of the resistivity of the high resistance layer 3 to the low resistance layer 4 is, the more the current diffusion effect is The film thickness can be made large and thin. Considering the above, it is desirable to set the resistivity ratio to 50 or more. This is because if it is less than this, the effect of diffusing the current is weakened, or the advantage that the film thickness can be reduced.

The film thickness of the high resistance layer 3 is 5 μm or less, preferably 2 μm or less, and the film thickness of the low resistance layer 4 is 6 μm or less, preferably 3 μm or less, and it is desirable to make the resistivity ratio as large as possible. By increasing the resistivity ratio, the film thickness of each layer can be reduced accordingly.

As the type of crystal used as the current diffusion layer, one that lattice-matches with the light emitting layer is appropriately selected, and one that does not absorb the emitted light is selected. For example, the light emitting layer is AlI
nGaP (cladding layer) / InGaP (active layer) / Al
In the case of the InGaP (cladding layer) double heterostructure, AlInGaP, AlGaAs, or the like can be used.

Although the resistivity of the current diffusion layer can be controlled to some extent by changing the composition of each layer, it can be easily controlled by changing the carrier concentration in the crystal. For example, A
In the case of a p-type crystal in which 1InGaP is doped with Zn, the carrier concentration is 10 ¹⁶ cm ^-3 , the resistivity is about 10 Ωcm, and the carrier concentration is 10 ¹⁹ cm ^-3 , the resistivity is about 10 ^-2 Ωcm. or,
In the case of an n-type crystal in which AlInGaP is doped with Se, the resistivity is about 10 ¹⁶ cm ^-3 , the resistivity is about 10 Ωcm, and the carrier concentration is 10 ¹⁸ cm ^-3 , the resistivity is about 10 ^-2 Ωcm.

[0014]

According to the present invention, by controlling the resistivity and thickness of the current diffusion layer, the current supplied from the upper electrode is diffused throughout the light emitting layer. 3 and 4 are for considering the principle of the present invention. Originally, in order to make a precise electrical examination of such a structure, it is necessary to consider the case where a minute electric circuit exists in the current diffusion layer and consider it as a distributed constant circuit as shown in FIG. To do this, consider a circuit as shown in FIG. Where A is the upper electrode, B is a lower electrode, R _H1 is (the same area as the upper electrode) high-resistance layer immediately below the upper electrode, R _H2 is the high-resistance layer having the same area as R _H1 outside of R _H1 The resistance of the ring part, R _L is R
Equivalent resistance of low resistance layer from _H1 to R _H2 , I ₁ and I _2
Are currents that vertically pass through R _H1 and R _H2 , respectively. Figure 3
Since R _H1 and R _H2 have the same resistance value, the current I ₂ becomes I ₂ if R _L is sufficiently smaller than R _H1 and R _H2.
The value is close to ₁ , and the current from the upper electrode diffuses throughout the light emitting layer.

Next, the results of numerical analysis of the present invention using the finite element method will be shown. In order to simplify the analysis model, the light emitting diode chip was made into a cylindrical axisymmetric model as shown in FIG. 5 (C), and the numerical analysis results were obtained by changing the resistivity and thickness of the low resistance layer and high resistance layer respectively. It is FIG. Here, the substrate portion below the high resistance layer was a uniform semiconductor layer with a resistivity of 10 ^-1 Ωcm and a thickness of 240 μm. All of the high resistance layers had a resistivity of 10 Ωcm and a thickness of 1 μm. When the high resistance layer was not provided, the layer having the same resistivity of 10 ^-1 Ωcm as the substrate was 1 μm. The radius of the cylindrical model is 150 μm, and the electrodes 5,
A voltage of 1 V was applied between the six. In FIG. 6, the horizontal axis is the distance from the central axis of the light emitting diode chip, the vertical axis is the current density, A is the current density on the lower surface of the high resistance layer, B is the current density on the lower surface of the substrate, and broken line C is the position of the upper electrode end. It is shown. From this analysis result, it can be seen in FIG.
In the case of a1 to a4, the curve A becomes convex at the electrode end,
It can be seen that the current is concentrated on the edge of the upper electrode. This means that the light emitting portion is limited to this portion, which is not preferable from the viewpoint of the life of the light emitting diode. On the other hand, in the case of inserting the high resistance layer in FIG. 6, b1 to b4, the curve A is flattened, and this current concentration is greatly alleviated. In FIG. 6 and b4, it can be seen that the current spreads uniformly over the entire surface of the chip.

[0016]

EXAMPLES The present invention will be specifically described with reference to examples.
An LED epitaxial wafer having a double heterostructure having InGaP as an active layer and AlInGaP as a clad layer and a current diffusion layer adjacent to the double heterostructure was prepared on a GaAs substrate by MOCVD. The structure of each layer is as shown in Table 1. The structure of this wafer is shown in FIG. 41
Is a low resistance p-AlInGaP layer, and 31 is a high resistance p-
The AlInGaP layer has a resistivity ratio of 200.

[0017]

[Table 1]

Using this epitaxial wafer,
AuZn (7.5%) has a thickness of 0.5 μm as the upper electrode.
m and Au are vapor-deposited into a circle of 130 μmφ with a thickness of 1 μm,
AuGe (7.5%) as the lower electrode is 0.5μ thick
After depositing m and Au to a thickness of 1 μm on the entire surface and then forming by alloying in an Ar atmosphere at 500 ° C. for 10 minutes, the contact layer other than the electrode lower part and the p-InGaP layer are removed by etching, and 300 × 300 μm The chip was diced into a light emitting diode.

Table 2 shows the measurement results of the light emitting characteristics of the light emitting diode thus obtained. The device used for the measurement used an integrating sphere deflector as a test fixture and detected the light with a photodiode. Luminous efficiency was shown by the ratio of the detection current of the photodiode whose relative luminous efficiency was corrected and the current supplied to the light emitting diode.

[0020]

[Table 2]

Comparative Example For comparison, a light emitting diode having the same structure as in Example 1 except that the high resistance layer 31 was replaced with the same material as the low resistance layer 41 in the above example, the light emitting characteristics were obtained. Table 2 also shows the results of the measurement.

According to the above results, the luminous efficiency of the comparative sample was 2.5 nA / mA when a current of 20 mA was applied.
On the other hand, it was 6.5 nA / mA in the example of the present invention. Further, it was confirmed that the light emission pattern of the comparative example is light emission only in the peripheral portion of the electrode, whereas the light emission pattern of the present invention emits light from the entire surface.

[0023]

According to the present invention, even in the case of an epitaxial wafer such as a MOCVD method in which a thick grown film is difficult to obtain, the current of the electrode portion of the light emitting diode can be diffused to make the light emitting region wide, and the upper electrode can be obtained. Since it can be made smaller, the range of applications is expanded even when used as a lamp.

[Brief description of drawings]

FIG. 1 is a diagram illustrating a structure of a current spreading layer according to the present invention.

FIG. 2 is a diagram showing a chip structure of an example.

FIG. 3 is an explanatory diagram of the principle of the present invention.

FIG. 4 is an explanatory diagram of the principle of the present invention.

FIG. 5 is a model diagram used for numerical analysis.

FIG. 6 is a diagram showing a result of numerical analysis.

[Explanation of symbols] 1 ... GaAs substrate 2 ... Light emitting layer 3 ... High resistance layer 4 ... Low resistance layer 5 ... Upper electrode 6 ... Lower electrode 71 ... GaAs buffer layer 21 ... n-AlInGaP clad layer 22 ... p-InGaP active layer 23 ... p-AlInGaP clad layer 31 ... p ^-- AlInGaP high resistance layer 41 ... p ⁺ -AlInGaP low resistance layer 51 ... p-InGaP layer 61 ... GaAs contact layer

Claims (4)

[Claims] 1. A light emitting diode in which an epitaxially grown light emitting layer is stacked on a compound semiconductor substrate, wherein an epitaxial growth layer having a high electric resistance and an epitaxial growth layer having a low electric resistance are provided on the light emitting layer opposite to the substrate. And a light emitting diode. 2. The light emitting diode according to claim 1, wherein the ratio of the resistivity of the high electric resistance layer to the low electric resistance layer is 50 or more. 3. A light emitting layer having a double hetero structure having GaInP as an active layer and AlGaInP as a cladding layer is provided on a GaAs substrate, and the resistivity of 1 to 10 is provided on the light emitting structure layer.
High electrical resistance of 0Ω AlGaInP layer and resistivity of 1 to 1
A light-emitting diode comprising a low electric resistance AlGaInP layer of × 10 ⁻³ Ωcm sequentially stacked. 4. The high electric resistance layer has a thickness of 0.1 to 5 μm,
The light emitting diode according to claim 1, wherein the low electric resistance layer has a thickness of 0.5 to 6 μm.