JPS6158991B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a semiconductor Teherojunction light emitting diode whose main components are Ga, Al and As.

An example of the structure and change in forbidden band width of a Ga, Al, As heterojunction light emitting diode is shown in Figure 1A.
It is made up of a layer 2 of Ga _1-x Al _x As, a layer 3 of n-type Ga _1-y Al _y As, and electrodes 4 and 5, and the substrate 1 is made of, for example, p-type Ga or As. Conventionally, such a heterojunction light emitting diode has been known in Japanese Patent Application Laid-open No. 157084/1984 "Semiconductor light emitting diode and manufacturing method".
Both structures that extract light from the surface and structures that extract light from the sides are described. Also, JP-A-53-6591
No. ``Light-emitting diode for pattern display and its manufacturing method'' describes a structure for extracting light from the surface. However, the drawback is that the luminance is low because no special consideration is given to the carrier density value of each layer. The present invention provides a heterojunction light emitting diode with extremely high luminance, and as will be described in detail later, extremely high luminance can be achieved by setting the carrier density of each layer within a certain range. In particular, of the two structures mentioned above, the structure that extracts light from the surface is
-Although issue No. 157084 does not mention the difference in characteristics from the latter, by taking carrier density into consideration as described below, it was not possible to achieve this at all with a structure that extracts light from the side of the p-n junction. A high-brightness light emitting diode can be obtained. That is, in the light emitting diode according to the present invention, the forbidden band width is increased on the side opposite to the substrate via the pn heterojunction 6, as shown in FIG.
It relates to a type of light emitting diode in which light is extracted from the surface of the part without electrodes through the side with a large forbidden band width.

First, we will explain the principle of how the carrier density of each layer should be selected.

Usually, light is emitted mainly in the p region 2 due to recombination of electrons injected into the p region 2 via the heterojunction 6,
The emission wavelength corresponds to the forbidden band of this p region 2. Generally, in a light emitting diode made of a - group compound semiconductor, the luminous efficiency in the p region is higher than that in the n region. However, there is also a report that in the gap, the luminous efficiency due to recombination of holes injected into the n region is higher than the efficiency in the p region.

In the case of semiconductor lasers, the carrier lifetime of stimulated emission is extremely short, so recombination occurs mainly through stimulated emission rather than through non-emissive centers.
The effect of the non-emissive center is not so large, but in the case of light-emitting diodes, the carrier lifetime of spontaneous luminescent recombination is long, so the minority carriers injected from the p-n junction are captured by the non-emissive center, and do not emit light there. The probability of recombination is extremely high. Therefore, in light emitting diodes, the efficiency decrease due to non-light emitting centers is decisively large.

Whether the p-region or the n-region serves as the main light-emitting region is related to the likelihood of defects occurring. In other words, a region in which fewer defects, which serve as non-light emitting centers, occur becomes the main light emitting region. GaAs and a mixed crystal containing GaAs
In GaAlAs, defects are likely to occur in the n region, so the p
The side is the main light emitting area. An important element that causes defects in the n-region is the formation of vacancies that electrically compensate for donor impurities such as Te, Se, and S doped in the n-region, depending on the doping amount. This is thought to be because it combines with impurities or becomes a very effective non-luminescent center on its own.

Therefore, in the GaAlAs system, 2 in Figure 1a is p.
Layer 3 is an n layer, and this p layer is the main light emitting region, and the light emitted in the p layer is transmitted through the n layer, which has a large forbidden band width, without being absorbed, and is extracted from the surface.

By the way, in the case of a light emitting diode having a pn homojunction with no change in forbidden band, especially when manufactured using the impurity diffusion method, it is usually made by diffusing Zn as a p-type impurity into an n-type crystal, so a p-type region is used. The carrier density of the n-type region must be larger than that of the n-type region. Since there is no suitable n-type impurity with a large diffusion coefficient comparable to Zn, it is necessary to diffuse n-type impurities into the p-type substrate to make the carrier density in the p region lower than that in the n region. It is not possible.

It is well known that light emitting diodes manufactured by liquid phase growth have fewer defects and exhibit much higher luminous efficiency than light emitting diodes manufactured by impurity diffusion. Of course, the purpose of the light emitting diode used in the present invention is to obtain high brightness, and since it is a heterojunction, it is manufactured by a liquid phase growth method.
On the other hand, in a pn homojunction made by a liquid phase growth method, unlike a pn homojunction made by a diffusion method, there is no restriction on carrier density, so the carrier density is selected to increase the injection efficiency into the light emitting region. For example, when the p region is the main light emitting region, the amount of electrons injected into the p region is made sufficiently larger than the amount of holes injected into the n region.

That is, impurity doping is performed so that the carrier density in the p region, which is the light emitting region, is smaller than that in the n region. Of course, if holes are to be mainly injected into the n-region, the opposite is true, and in any case, the main light-emitting region side is doped with impurities so as to have a low impurity density.

However, in the case of a light emitting diode using a pn heterojunction as shown in the present application, the injection efficiency is determined by the potential barrier due to the difference in forbidden band width between the p region and the n region, and the impurity density is no longer the main factor. Therefore, in order to increase the number of luminescent centers, it is better to increase the impurity density, and therefore the carrier density, in the main luminescent region as much as possible. However, when the doping impurity density is increased, various defects including point defects occur, forming non-emissive centers. In GaAs and GaAlAs, defects are more likely to occur in the n-type, so the carrier density is appropriate.As a result, the impurity density is higher in the p-type region, which is the main light emitting region, than in the n-type region, making it the optimal homop-n junction. The conditions will be different.

In the n-type region, when the carrier density exceeds 10 ¹⁸ /cm ³ , the luminous efficiency is significantly reduced due to the occurrence of defects.

However, if the carrier density is too low, the resistance will increase, which is undesirable, so it is desirable that the carrier density is 10 ¹⁷ /cm ³ or more. On the other hand, the carrier density is
It can exceed 10 ¹⁸ /cm ³ .

The second factor that determines the upper limit of impurity density is the diffusion of impurities during growth. P-type impurities include Zn, Ge, etc., but the one with the highest luminous efficiency is
It is a Zn impurity. Since the diffusion coefficient of Zn is large,
Diffuses into the n-region during growth. If the diffusion of Zn is strong, part of the n-type region may be inverted to p-type, and the position of the p-n junction may shift. However, due to diffusion, p
Since the surface concentration of the region adjacent to the n region also decreases, even if the Zn impurity density increases beyond the impurity in the n region, the pn boundary will not immediately go deep into the region with a large forbidden band. .

Taking the configuration of FIG. 1 as an example, the impurity density distribution near the pn junction 6 between the p-type region 2 and the n-type region 3 will be explained using FIG. 2. Figure 2 a shows pn junction 6
Nearby impurity density distribution, FIG. 2b shows the forbidden band distribution near the same pn junction 6. The horizontal axis represents the distance perpendicular to the joint surface, and corresponds to both figures a and b. In FIG. 2a, the vertical axis indicates the difference N D _{-N A} between the donor density N _D and the acceptor density N _A , or the donor density N _D in the positive direction and the acceptor density N _A in the negative direction. The acceptor density toped in p-type region 2 is N
_AO , and the donor density doped into the n-type region 3 is N _DO . The acceptor density N _A has the same distribution as the solid line 10 indicating N _D -N _A in the p-type region 2 other than the vicinity of the pn junction 6, but is distributed like the dotted line 12 in the vicinity of the pn junction 6. Similarly, the donor density N _D is the same as the distribution 10 of N _D -N _A in the n-type region 3 other than the vicinity of the pn junction 6, but is distributed as shown by the dotted line 13 in the vicinity of the pn junction 6. pn
In the vicinity of the junction 6, the distribution 13 of the donor density N _D and the distribution 12 of the acceptor density N _A compensate each other, and N _A
-ND becomes like the solid line 10. Doped Zn acceptor density N _AO from doped donor density N _DO
Even if Zn is large, the Zn density near the pn interface decreases as Zn diffuses, so conductivity type reversal is unlikely to occur, and region 3 compensated by Zn diffusion also
It remains in shape. If the doping Zn density is too large compared to the donor density, the pn boundary will enter a region with a large forbidden band width, but the limit to which such intrusion occurs can be determined by observing the emission spectrum. That is, if the Zn density becomes too high and the pn interface moves into a region with a large forbidden band, the emission wavelength will correspondingly become shorter. At the same time, in the case of GaAlAs, a composition with a large forbidden band width results in an indirect transition, or even if it is a direct transition, the electron density in the indirect transition band increases relatively, resulting in a decrease in efficiency.

Based on the above considerations, p-type
A suitable range of impurity density when epitaxially growing the GaAlAs layer 2 and the n-type GaAlAs layer 3 was determined. The luminous intensity of the fabricated light emitting diode is shown in FIGS. 3 and 4 as a function of impurity density.

As shown in Figures 3 and 4, the carrier density on the p side is 4.5×10 ¹⁷ /cm ³ ＜p＜2.5×10 ¹⁸ /cm ³ The carrier density on the n side is 2×10 ¹⁷ /cm ³ ＜ Luminous efficiency is extremely high when n<1.0×10 ¹⁸ /cm ³ .

When the carrier densities of the p layer and n layer are selected from the above range, it is possible not only for p>n but also for p<n, but since the emissive layer is on the p side, the luminescent center density on the n side Since it is more important to reduce the defect density than to increase p, it is desirable that p>n in the above range.

As is clear from Figures 3 and 4, by selecting each carrier density within this range,
High brightness of 100mcd or more can be obtained. Furthermore, the third
To explain the figure, in this experiment in which the carrier density of the p-layer was varied, the carrier density of the n-layer was 2×10 ¹⁷ cm ^-3 <n<8×10 ¹⁷ cm ^-3 , that is, as the range of the carrier density of the n-layer. These are the results obtained by selecting from the range listed above, and the same can be said for Figure 4.

Furthermore, as is clear from Figures 3 and 4, within the carrier density range that provides this high brightness, all p>n except for the special case of p = 6 × 10 ¹⁷ cm ^-3 in Figure 3, and The length of the arrow indicates that in the special case described above, a diode with low brightness can also be obtained. Therefore, in order to reliably obtain a high-brightness light emitting diode, it is furthermore desirable that p>n, as shown in Figures 3 and 4.
The pictures tell a story.

To manufacture this light emitting diode, a patent number is required.
It is preferable to use the temperature difference method liquid phase growth described in No. 85754. In other words, the conventional manufacturing of light-emitting diodes by liquid phase growth uses a slow cooling method, that is, a method of epitaxial growth by gradually lowering the temperature of the solution. The degree of deviation of the growth layer changes as the temperature decreases, and therefore it is impossible to avoid changes in the amount in the thickness direction of the grown layer. On the other hand, according to the temperature difference method, the material is transported by diffusion from a high temperature part kept at a constant temperature to a low temperature part due to the temperature difference in the solution, and epitaxially grows on the substrate in contact with the solution in the low temperature part. Therefore, the crystal growth temperature is constant over time and, in principle, does not fluctuate as described above. To grow multiple layers, the substrates placed on a slider are brought into contact with different melt baths one after another. Therefore, except for fluctuations in the impurity density and mixed crystal composition near the hetero boundary due to solid diffusion during growth, it is possible to obtain a flat distribution for each layer that is incomparable to the slow cooling method. For example, p-type Ga _1-x Al _x As layer 2 and n-type
The variations in x and y of the Ga _1-y Al _y As layer 3 are △
Keeping it at 0.01, △y0.01 means that the growth layer thickness is 10
Even if it is more than μm, it is easy, △x＜0.002, △y
<0.002, whereas in the slow cooling method
The Al composition decreases as the growth progresses, and the thickness of 10μ
A decrease of △x = 0.02 per m is normal.

In light-emitting diodes, in order to increase luminous efficiency, the lower the density of non-emissive centers, the longer the diffusion length of minority carriers becomes, so the growth layer, especially p, becomes the main emissive region.
The thickness of the layer must be relatively thick. In a high-quality GaAlAs light-emitting diode, the diffusion length is several μm or more, so the thickness of the light-emitting layer is 1 μm as in the past.
It is insufficient to have a thickness of 2 to 3 μm or less, and it is necessary to make it at least 5 μm or thicker.
It is desirable that the thickness be 10 μm or more. At the same time, the temperature difference method allows extremely good control of impurity density, and as a result of the slow cooling method, there is less variation in the actual doping level from the designed value, making it possible to control the impurity doping amount within the range mentioned above. Can be done accurately. Therefore, the yield of highly efficient diodes is very high, for example, a luminous intensity of more than 80mcd at 20mA (value with epoxy resin coating)
We were able to increase the yield of diodes exhibiting 70% or more.

An example of a pn heterojunction light emitting diode manufactured in this manner has the structure shown in _{FIG .} n-type epitaxially grown on layer 2 and layer 1
A Ga _1-y Al _y As second layer 3 is formed so that x<y allows light to be extracted efficiently. Light emitted from the p-layer passes through the n-layer, which has a larger forbidden band, and is extracted in a direction perpendicular to the junction surface. Such a junction has the characteristic that light emitted closer to the pn interface has less absorption loss. Therefore pn
The brightness is extremely sensitive to defects near the interface.

On the other hand, in a light emitting diode having a structure different from the structure to which the present invention relates, in which light is extracted in the side direction of the junction, the luminance cannot be increased because the self-absorption loss of the light emitting layer itself is large. In addition, in the case of such a structure, since the emission is primarily observed from the p-side part, which is far from the p-n junction boundary, the effect of self-absorption is far greater than the effect of defects occurring on the n-side. Therefore, even if a carrier density range within the range of the present invention is selected, the brightness cannot be significantly increased.

In the above example, we talked about a heterojunction made of Ga, Al, and As, but if you add a small amount of P to this as a component of a semiconductor, you can create a heterojunction with well-matched lattice constants without significantly changing other physical properties of the crystal. Since junctions can be fabricated, defects such as misfit dislocations at hetero boundaries can be reduced, and luminous efficiency can be further improved.

[Brief explanation of the drawing]

Figure 1a shows an example of the configuration of a GaAlAs heterojunction light emitting diode, Figure 1b shows an example of the change in the forbidden band in Figure 1a, Figure 2a shows the impurity distribution near the pn junction, Figure 2b Figure 3 shows the distribution of prohibited band width.
4 and 4 are examples of characteristics for explaining the present invention.

Claims (1)

[Scope of Claims] At least one pn heterojunction of a group compound semiconductor in which the group element is Ga or a combination of Ga and Al as the main component, and the group element is As, and the pn heterojunction is The forbidden band width of the p-layer to be formed is smaller than the forbidden band width of the n-layer forming the p-n heterojunction, and the light generated in the p-layer passes through the n-layer and is extracted from the surface of the non-coherent layer. In a light emitting semiconductor device, the generation of defects due to an increase in the carrier density of the n-layer is suppressed, and
In order to increase the carrier density of the p-layer as long as the actual boundary of the p-n junction does not change in the emission wavelength due to entering the n-layer, the carrier density p of the p-layer and the carrier density n of the n-layer The emission brightness can be determined by selecting from the range of 4.5×10 ¹⁷ cm ^-3 <p<2.5×10 ¹⁸ cm ^-3 and 2×10 ¹⁷ cm ^-3 <n<1×10 ¹⁸ cm ^-3 and p>n A light-emitting semiconductor device characterized by increased . 2. The light emitting semiconductor device according to claim 1, wherein the impurity that substantially determines the carrier density of the p-layer is zinc.