JPH04313281A - Light emitting diode - Google Patents

Abstract

PURPOSE:To obtain a higher luminance and a high-speed response by a double hetero structure. CONSTITUTION:Fine recessed and projecting portions 17 for forming a projecting portion 18 which allows a high-concentration carrier to be implanted into an activation layer are provided on a surface of a p-type GaAs substrate 15 by etching. A p-type GaAlAs-clad layer 14 and an epitaxial layer in a double hetero structure of an n-type GaAlAs activation layer 13 a conductive type which is opposite to the p-type of the substrate and an n-type GaAlAs window layer 12 are allowed to grow on this recessed and projecting substrate 15. A light emitting diode is produced by forming electrodes 11 and 16 on a surface and a rear face of a wafer which is subjected to epitaxial growth, thus enabling a p-type carrier with a different conductive type from that of the n-type activation layer 13 to be implanted into the activation layer 13 in a high carrier concentration state from an upper portion of the projection portion of the substrate 18.

Description

[Detailed description of the invention]

0001

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a double heterostructure light emitting diode, particularly to a light emitting diode with high brightness and high speed response.

0002

2. Description of the Related Art Light-emitting diodes are required to have high brightness characteristics and high-speed response, and to meet these requirements, heterostructure light-emitting diodes are used.

[0003] As a general-purpose light emitting diode, a light emitting diode is used which has a single-hetero structure, in which a circular electrode or an electrode close to the circular electrode is formed at the center of the front surface, and a full or partial electrode is formed on the back surface. Since this light emitting diode has a simple manufacturing process and is inexpensive, it accounts for nearly 90% of all light emitting diodes in use. However, the brightness characteristics and responsiveness are not sufficient.

On the other hand, as light-emitting diodes for special purposes such as communications, special ones are manufactured that have a current confinement structure in which only the center of the light-emitting diode chip emits light. Although both properties are satisfactory, they are not widely used due to their high cost.

[0005] In order for it to be widely used, it is important that it be inexpensive. Therefore, a product has been commercialized in which a double heterostructure light emitting diode is formed on a substrate using the conventional circular electrode structure. This consists of a cladding layer on the substrate with the same conductivity type as the substrate and a high mixed crystal ratio, an active layer with the same conductivity type and a thin film thickness and a low mixed crystal ratio, and an active layer with the opposite conductivity type as the substrate. Wind layers having a high mixed crystal ratio and forming a pn junction between the two are successively formed. By confining carriers in the active layer with this structure, brightness characteristics and high-speed response characteristics were greatly improved. However, the demand for improved characteristics is even greater.

In order to meet this demand, a device has been proposed in which a ring-shaped raised portion is provided on the substrate surface to form current concentration in the active layer and thereby obtain high brightness. The reason why high brightness can be obtained by providing an annular raised portion on the substrate surface is as follows. In conventional double heterostructure light emitting diodes, the flat substrate surface spreads the current across the active layer. However, if a raised portion is formed on the substrate surface, current will concentrate on the raised portion. The luminous efficiency of a light emitting diode increases as the carrier concentration injected into the active layer increases. Therefore, if the same current is passed through a narrower area, the light emission output will be higher.

[0007]

[Problems to be Solved by the Invention] However, although the light emitting diode in which an annular raised portion is formed on the substrate has significantly improved light emitting output and responsiveness, it is still not fully satisfactory.

SUMMARY OF THE INVENTION An object of the present invention is to provide a light emitting diode that can overcome the above-mentioned drawbacks of the prior art and achieve higher light output and higher response speed.

[0009]

[Means for Solving the Problems] In the light emitting diode of the present invention, a double heterostructure epitaxial layer having an active layer is grown on a substrate of one conductivity type, and the epitaxial layer is grown on the front and back sides of a wafer on which the epitaxial layer is grown. Applied to light emitting diodes with electrodes formed.

The conductivity type of the active layer is set to be the opposite conductivity type to that of the substrate having one conductivity type, and a raised portion is provided to locally inject carriers from the substrate having one conductivity type into the active layer having the opposite conductivity type. It is formed on the surface of the substrate. In order to increase the carrier concentration, the smaller the diameter of the protrusion, the better.

[0011] Note that as the light emitting diode, GaAs
, GaAlAs, GaP, GaAsP, GaAlAs,
It can be applied to all light emitting diodes such as SiC and GaN.

0012

[Operation] When the active layer is made of the conductivity type opposite to that of the substrate and a raised portion is formed on the surface as in the present invention, compared to the case where the active layer is made of the same conduction type as that of the substrate, It is not clear why high brightness and fast response are achieved, but when carriers are injected into the active layer from the raised part of the substrate, carriers with different conductivity types are injected into the active layer while the carrier concentration is high, resulting in high luminescence. It is believed that high output and fast response speed can be obtained.

[0013]

Embodiments Hereinafter, embodiments of the present invention will be described with reference to the drawings. Here, a case is described in which an infrared light emitting diode having a GaAlAs double heterostructure with an emission wavelength of 850 nm is used, but the invention is not limited to GaAlAs.

FIG. 1 shows a light emitting diode structure for explaining this embodiment. The light emitting diode has a carrier concentration of 2
A Zn-doped p-type GaAlAs layer 14 with a mixed crystal ratio of 0.30 is formed on a p-type GaAs substrate 15 of x1019 cm-3.
Te-doped n-type GaAlA with 0 μm and mixed crystal ratio of 0.05
S layer 13 (active layer 13) is 0.5 μm, mixed crystal ratio is 0.3
An epitaxial wafer on which a Te-doped n-type GaAlAs layer 12 of 30 μm is grown is used. In particular, what is important here is that the conductivity type of the active layer is not p-type, which is the same as the GaAs substrate 15, but n-type, which is the opposite. A circular n-side electrode 11 is formed on the surface of this epitaxial wafer.
A p-side electrode 16 is formed on the entire back surface.

The entire surface of the p-type GaAs substrate 15 used here has fine irregularities 17 forming convex portions 18 (mesa portions 18) as shown in the cross section of FIG. 1, as shown in the plan view of the substrate in FIG. It is formed like a matrix. This unevenness 17 can be formed by etching. The flat portions of the convex portions 18 constituting the unevenness 17, that is, the top surfaces of the convex portions 18, have a diameter of 10 μm, and are arranged at intervals of 40 μm in both the vertical and horizontal directions. The height of the convex portion 18 is 10~
It is 15 μm.

[0016] The epitaxial growth on the uneven substrate 15 on which the unevenness 17 was formed was carried out by the liquid phase epitaxial method as before, but there was no problem at all even if the growth was performed under the same conditions as when growing on a flat substrate. There is no

A light emitting diode having the double heterostructure shown in FIG. 1 was fabricated on the substrate 15 having such unevenness 17 formed thereon, and the characteristics of the light emitting diode were measured and compared with those of a light emitting diode with a p-type active layer. did. A current of 50 mA was passed in the bare chip state, and the in-plane distribution of light emission output was examined. As a result, compared to the case where the active layer is of the p-type, the case where the active layer is of the n-type as in this example was able to obtain approximately 1.4 times the light emission output. Also, the cutoff frequency is 20MHz, 1
．． It was possible to increase the frequency five times to 30MHz.

Now, the reason why the output of the light emitting diode has become higher and faster as described above is considered to be due to the following effect. When a p-type active layer is grown on a flat substrate, the light emission output P↓1 (↓ is a vector representation of P) can be expressed by equation (1).

[0019] P↓1 =hν・B・d
・p0Δn
(1) Here, hv is the energy of light, B is the emission recombination constant, d is the active layer thickness, p0 is the carrier concentration of the p-type active layer, and Δn is the density of injected electrons. In addition, p0
>>Δn.

In the case of a light emitting diode in which a p-type active layer is grown on a p-type substrate on which a mesa portion is formed, the light emission output P↓2
is shown by equation (2).

[0021] P↓2 =hν・B・d
・(p0 +Δp)Δn (2)(
Comparing Equation 1) and Equation (2), it can be seen that only the injected hole density ΔP component due to the effect of the mesa portion is increased. Note that p0 〓Δp >>Δn.

On the other hand, in the structure of this embodiment, the light emission output P↓3
is shown by equation (3).

[0023] P↓3 =hν・B・d
・n0 ・Δp
(3) Here, n0 is the carrier concentration of the n-type active layer.

[0024] Comparing P↓2 and P↓3, we find that p
0 is approximately equal to n0. Therefore, (p0 +Δp) is n
At most, it is only twice that of 0. On the other hand, Δn and Δp
Comparing , Δp is narrowed from the mesa portion and is injected into the active layer, so Δp >> Δn. Therefore, P↓3 > P
↓2.

It is thought that carriers of different conductivity types are injected into the active layer in a state where the carrier concentration is high as described above, so that high light emission output and fast response speed can be obtained.

As described above, this embodiment has the following various advantages.

■ Etching the p-type substrate to form fine mesa parts, and increasing the concentration of carrier injection into the active layer by concentrating current on each upper part of the mesa parts;
Moreover, by making the active layer n-type, the conductivity types of carriers injected from the p-type substrate into the active layer are different, so that the light emission output can be increased and the response speed can be increased. .

(2) Since the basic structure remains the same as the conventional double hetero structure, light emitting diodes can be produced at relatively low cost.

(2) Formation of a fine mesa portion requires only etching the substrate, so manufacturing is simple and yield is high.

(2) Characteristic variations in light emitting diodes depend on crystal defect variations within the epitaxial wafer surface. However, if the radiative recombination probability is high, the luminous efficiency depends on the injected carrier concentration and becomes independent of crystal defects. Therefore, if the probability of radiative recombination increases within the wafer plane due to current concentration due to the protrusions, variations in the luminescent characteristics will be reduced. In this respect, the device of this embodiment is of a current injection type using a p-type GaAs substrate, and has a fine mesa portion, so that variations in characteristics within the plane are small.

By the way, dry etching can be used as the etching to form the above-described unevenness 17. In order to further increase current concentration in the active layer, it is considered effective to increase the depth. The most effective method for increasing the depth is dry etching. However, since dry etching causes defects on the surface, it is necessary to perform wet etching after dry etching. Alternatively, deep recesses may be formed by digging grooves in a grid pattern with a dicing saw and then etching. Further, in this example, the protrusions having a circular cross section are formed, but since the area of the protrusions that causes current concentration in the active layer is important, the protrusions may have other shapes. For example, the shape of the convex part becomes a grid,
It does not matter if the convex parts are connected to each other. Note that the raised portion is not limited to minute irregularities, and the present invention can of course be applied to a light emitting diode having a ring-shaped raised portion.

In this example, the light emitting diode had a structure in which an n-type active layer was formed on a p-type substrate. However, on the contrary, n
The same effect can be expected with a structure in which a p-type active layer is used for a type substrate. These can also be achieved with double heterostructure light emitting diodes other than GaAlAs.

[0033]

According to the present invention, a raised portion is formed on the surface of the substrate, and the conductivity type of the active layer is made opposite to that of the substrate, so that a high concentration of carriers having a different conductivity type is transferred from the substrate to the active layer. By using injection, it is possible to manufacture a high speed light emitting diode with high light emitting output.

[Brief explanation 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 example of a concavo-convex arrangement formed on a substrate according to the present example.

[Explanation of symbols]

11 N-side electrode 12 N-type GaAl layer 13 N-type GaAlAs layer 14 p-type GaAlAs layer 15 p-type GaAs substrate 16 p-side electrode 17 Unevenness 18 Convex part (mesa part)

Claims (1)

[Claims] Claim 1: A light emitting diode in which a double heterostructure epitaxial layer having an active layer is grown on a substrate of one conductivity type, and electrodes are formed on the front and back sides of a wafer on which this epitaxial layer is grown. The conductivity type is the opposite conductivity type to the substrate of the first conductivity type, and a protrusion is formed on the substrate surface to locally inject carriers from the substrate of the first conductivity type into the active layer of the opposite conductivity type. light emitting diode.