JP2513550Y2 - Luminescent diode - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION [Industrial field of application] The present invention relates to a light emitting diode, and more particularly to a light emitting diode having a high emission output and uniform characteristics in a wafer plane.

[Prior Art] A light emitting characteristic is one of the most important characteristics for a light emitting diode. In particular, it is important that the light emission output is high, there is little variation within the wafer surface, and there is little variation in characteristics between growth lots. Various efforts have been made in terms of.

However, the variation of light emitting diode characteristics is about ± 10% within the wafer surface, and ± 20% if the growth lot is included.
It also becomes. The reason why such variations occur while growing under the same raw material / growth conditions is that the light emission output of the light emitting diode depends on an ambiguous factor such as crystallinity that has not yet been elucidated technically. Poor crystallinity causes defects in the active layer that serves as the light emitting portion, and electrons and holes are non-radiatively recombined via the defects.

By the way, when the light emitting diode is used for optical communication or a sensor, it is required to have a small variation in light emission output. This is because if the light emitting diode has a large variation, when it is used as a light emitting device and combined with a light receiving device to form a module, adjustment is required for each module. Therefore, it is considered that the demand for a light emitting diode having a small variation in light emission output will increase in the future.

The luminous efficiency of a light emitting diode is greatly influenced by crystal defects. There are roughly two ways to improve efficiency. One is to increase the probability of radiative recombination (conversion to light). The other is to reduce the probability of non-radiative recombination (converted to heat).

Conventionally, the latter method has been adopted in which the surface of the substrate is made flat and the crystal defects of the epitaxial layer grown on the flat substrate are reduced as much as possible to reduce the probability of non-radiative recombination.

[Problems to be Solved by the Invention] However, the conventional method of improving the light emission output and achieving the uniformity of the light emission output by performing the epitaxial growth with high reproducibility has reached a technical limit.

Therefore, the former method of increasing the injection carrier concentration to increase the radiative recombination probability has begun to be studied. this is,
It is intended to improve the light emission output and achieve the uniformity of the light emission output by changing a relatively simple structure in which a raised portion is provided on the surface of the substrate.

The principle of providing the raised portion on the substrate surface is as follows. In the conventional DH-LED (Double Hetero Light Emitting Diode), the current is spread over the entire active layer because the substrate surface is flat. However, when a ridge is formed on the surface of the substrate, current concentrates on the ridge. The luminous efficiency of the light emitting diode increases as the concentration of carriers injected into the active layer increases. Therefore, if the same current is flown, the light emission output is higher when the current is flown in a narrow area.

For example, as shown in FIG. 8, an annular raised portion 87 having a diameter slightly larger than that of the circular surface electrode 81 is formed by mesa etching on the surface of the substrate 85, and a current is applied to the annular portion of the active layer 83 corresponding to the raised portion 83. Are localized to form the localized light emitting portion 88. The epitaxial layers 84, 83, 82 formed on the substrate 85 are grown on this raised portion 87. Then, a light emitting diode is formed by attaching a circular surface electrode 81 to the wafer surface position corresponding to the center of the annular ridge and a back surface electrode 86 to the entire back surface.

However, in the light emitting diode in which the ring-shaped ridge is formed on the substrate, the light emission output and its variation are not yet sufficiently satisfactory.

Further, when forming the circular electrode 81 on the surface, it was necessary to match the mask of the raised portion 87 and the circular electrode 81. As shown in FIG. 8, the center of the circular electrode 81 must coincide with the center of the ring-shaped ridge 87. This is because when the center of the circular electrode 81 and the center of the raised portion 87 deviate, the active layer 83 under the electrode corresponding to the raised portion 87 emits the most light as shown in FIG. This is because 91 is shielded by the electrode 81 and does not come out to the outside.

The object of the present invention is to solve the above-mentioned drawbacks of the prior art,
Eliminates the need for mask alignment and increases the light emission output,
An object of the present invention is to provide a light emitting diode capable of reducing the variation.

[Means for Solving the Problem] The light emitting diode of the present invention is a light emitting diode in which an epitaxial layer composed of a single layer or a plurality of layers is grown on a substrate, and electrodes are formed on the front side and the back side of the substrate on which the epitaxial layer is grown. In the diode, fine irregularities are formed on the surface of the substrate on which the epitaxial layer is grown, and the total surface area of the convex portions of the irregularities is made smaller than the total surface area of the concave portions.

According to the present invention, the area of the active layer which becomes the light emitting portion is 2 × 10 ⁴ μm ^2.
It can be applied to the above light emitting diodes. Area of active layer is 2
If it is smaller than × 10 ⁴ μm ² , the current may be excessively concentrated on the localized light emitting portion formed in the active layer, and the temperature of the active layer may be increased to shorten the life. Further, similarly, since it is necessary to wire-bond the surface electrode, the area of the electrode cannot be reduced so much, and it is considered that the chip size is limited to about 150 μm from the viewpoint of extracting light.

Further, as the light emitting diode, GaAs, GaAlAs, Ga
It can be applied to all light emitting diodes such as P, GaAsP, GaAlAs, SiC and GaN.

[Operation] The characteristic variation of the light emitting diode depends on the crystal defect variation in the plane of the epitaxial wafer. However, if the radiative recombination probability is high, the luminous efficiency depends on the injected carrier concentration and not on the crystal defects. Therefore, the variation of the light emission characteristics correlated with the light emission efficiency is reduced. Most of all, if the area of the convex portion formed on the surface of the substrate varies within the wafer surface, the current density varies, which causes variation in the light emission output. But in this regard,
Since the convex area is usually formed by photolithography,
The variation is hardly a problem.

If the total surface area of the convex portions of the concave and convex portions is smaller than the total surface area of the concave portions, the current concentrates in a narrower area of the active layer, so that the light emission output of the light emitting diode increases,
The in-plane variation and the lot-to-lot variation are reduced.

Therefore, variations in light emission output within the surface and between wafers are significantly reduced.

Hereinafter, an embodiment of the present invention will be described with reference to the drawings.

Here, the case of using a light emitting diode having a GaAlAs double heterostructure with an emission wavelength of 660 nm has been described, but the present invention is not limited to GaAlAs, and a single hetero structure may be used.

First, a light emitting diode structure for explaining the present embodiment will be described.
Shown in the figure. The light emitting diode has a carrier concentration of 2 × 10 ¹⁹ cm.
Zn-doped GaAl with a mixed crystal ratio of 0.65 on a p-type GaAs substrate 15 of ^-3
Zn-doped GaAlAs layer 13 with As layer 14 of 20 μm and mixed crystal ratio of 0.35
(Active layer 13) 1.0 μm, Te-doped GaAlA with a mixed crystal ratio of 0.65
An epitaxial wafer in which the s layer 12 is grown to 35 μm is used. A circular n-side electrode 11 is formed on the front surface of this epitaxial wafer, and a p-side electrode 12 is formed on the entire back surface.

On the entire surface of the GaAs substrate 15 used here, fine irregularities 17 as shown in the cross section of FIG. 1 are formed in a matrix as shown in the substrate plan view of FIG. This unevenness
17 can be formed by etching. Unevenness 17
The flat portion of the convex portion 18 (mesa portion 18) that constitutes the
The top surface has a diameter of 10 μm, and the intervals are 40 μm in both the vertical and horizontal directions. The height of the convex portion 18 is 10 μm.

Epitaxial growth on the uneven substrate 15 on which the unevenness 17 is formed was carried out by the liquid phase epitaxial method as in the conventional case, but there is no problem even if it is grown under the same conditions as when growing on the conventional flat substrate. There is no.

The light emitting diode shown in FIG. 1 was manufactured on the substrate 15 on which such irregularities 17 were formed, and the characteristics of the light emitting diode were measured and compared with the light emitting diode grown on a flat substrate. A current of 20 mA was applied in a bare chip state and the in-plane distribution of the light emission output was examined.

When grown on a flat substrate and a concavo-convex substrate in the same lot, an emission output of about 1.3 times that of the flat substrate was obtained on the concavo-convex substrate. Moreover, the standard deviation of the variation within the wafer surface could be reduced to 60% compared with the flat substrate.

Here, FIG. 3 shows the relationship between the light emission output and the area ratio of the convex portions when the area of the convex portions of the uneven substrate is increased.
The light emission output increases as the ratio of the area of the convex portion to the area of the substrate decreases, and its variation also decreases. When the convex area ratio is 50% or less, a clear improvement in characteristics is recognized. However, 0% is omitted because it is the same as 100%. Incidentally, in the case of a raised substrate having an annular raised portion on the substrate surface, it cannot be discussed in the same row because the uneven pattern is different, but similarly, if the convex area ratio is small, the light emission output tends to increase. Seem. However, as compared with the annular ridge, the uneven pattern as in the present embodiment enables much finer patterning. As a result, the characteristics can be improved more effectively in the case of this embodiment than in the case of the annular ridge.

By the way, as the etching for forming the uneven portion 17 described above, dry etching can be used. The distance a between the convex portions shown in FIG. 7 and the depth b (FIG. 7 (a)) cannot be independently determined, and a certain relationship is inevitably established between them. Due to the etching technique, if the depth b is sought to be deep, the distance a becomes long. In order to increase the current concentration in the active layer, it is considered effective to increase the depth b while keeping the distance a. From this point of view, it is preferable that the depth of FIG. 7 (c) is deeper than that of the shallow depth of FIG. 7 (b) because the current can be concentrated more easily. The method using dry etching is most effective for increasing the depth. However, since dry etching causes defects on the surface, it is necessary to perform wet etching after dry etching.

Alternatively, the deep recesses may be formed by digging the grooves in a grid pattern with a dicing saw and then etching.

In the above description, the area standard is defined as the area of the convex portion. However, if the convex portion is formed, naturally the concave portion is also formed. Therefore, setting the concave portion area to 50% or more has the same meaning.

Further, although the convex portion having a circular cross section is formed in the present embodiment, the convex portion may have another shape because the area of the convex portion which causes current concentration in the active layer is important. For example, as shown in FIG. 5, the protrusions 58 may have a lattice shape and the protrusions may be connected to each other.

As described above, according to the present embodiment, as shown in FIG. 4, since the fine irregularities 17 are formed on the surface of the substrate 15, the current concentrates on each upper portion of the convex portion 18, The carrier concentration locally injected into the active layer 13 is increased, and the light emission output is increased. In addition, since it is possible to grow an epitaxial layer having high reproducibility without variations, it is possible to reduce variations in emission characteristics within the wafer surface and between lots. This has been confirmed by experiments. Therefore, even when the light emitting diode according to the present embodiment is used as a light emitting device such as optical communication or a sensor and combined with a light receiving device to form a module, adjustment for each module is not necessary.

Further, as shown in FIG. 6, since the unevenness 17 much smaller than the n-side electrode area is formed on the entire surface of the substrate 15, n is a mask of the side electrode 11 and the projection pattern of the unevenness 17. Since no alignment is required, the electrode forming process becomes very easy.

[Advantages of the Invention] As described above, according to the present invention, only the step of etching the substrate to form fine irregularities on the surface of the substrate is added, mask alignment becomes unnecessary, and the epitaxial growth step is changed. Therefore, it is possible to manufacture a light emitting diode having a high light emission output and a small variation.

[Brief description of drawings]

FIG. 1 is a cross-sectional view showing a light emitting diode structure according to an embodiment of the present invention, FIG. 2 is a plan view showing a first embodiment of a concavo-convex arrangement formed on a substrate according to the present embodiment, and FIG. 3 is this embodiment. FIG. 4 is a characteristic view showing the relationship between the convex area ratio and the light emission output according to FIG. 4, FIG.
FIG. 6 is a plan view showing a second embodiment of a concavo-convex arrangement formed on a substrate according to this embodiment, and FIG. 6 is a plan view illustrating that the light emitting diode according to this embodiment does not require mask alignment,
FIG. 7 is a sectional view of an essential part for explaining the interval and the depth of the uneven portion according to this embodiment, FIG. 8 is an explanatory view showing the structure of a conventional light emitting diode, and FIG. 9 is a mask alignment of the conventional light emitting diode. It is an explanatory view showing that is required. 11 …… n side electrode, 12 …… n type GaAl layer, 13 …… p type GaAlAs
Layer, 14 ... p-type GaAlAs layer, 15 ... p-type GaAs substrate, 16 ...
P-side electrode, 17 ... unevenness, 18 ... convex part.

Claims (1)

(57) [Scope of utility model registration request] 1. A light-emitting diode in which an epitaxial layer consisting of a single layer or a plurality of layers is grown on a substrate, and electrodes are formed on the front and back surfaces of a wafer on which the epitaxial layer is grown, respectively. A light emitting diode characterized in that irregularities are formed and the total surface area of the convex portions of the irregularities is smaller than the total surface area of the concave portions.