JP2765256B2 - Light emitting diode - Google Patents

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 having a double hetero structure, and more particularly to a light emitting diode having high brightness and high speed response.

[0002]

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

As a general-purpose light-emitting diode, a light-emitting diode having a single-hetero structure, a circular electrode or an electrode close to the circular electrode formed at the center of the front surface, and an entire or partial electrode formed on the back surface is used. These light-emitting diodes account for nearly 90% of the light-emitting diodes used because of their simple and inexpensive manufacturing process. However, luminance characteristics and responsiveness are not sufficient.

On the other hand, as a light emitting diode for special use such as communication, a special light emitting diode having a current confinement structure in which light is emitted only at the center of a light emitting diode chip is manufactured. Although both properties are satisfactory, they are not widely used due to their high cost.

[0005] In order to be widely used, it is important that it be inexpensive. Therefore, a conventional circular electrode structure in which a light emitting diode having a double hetero structure is formed on a substrate has been commercialized. This consists of a cladding layer with the same conductivity type as the substrate and a high mixed crystal ratio, an active layer with the same conductivity type and a small thickness and a low mixed crystal ratio, and an active layer with the opposite conductivity type to the substrate. A window layer having a high mixed crystal ratio and constituting a pn junction between them is sequentially formed. By confining carriers in the active layer with such a structure, the luminance characteristics and the high-speed response characteristics have been greatly improved. However, the demand for improved characteristics is even greater.

In order to respond to this, there has been proposed an arrangement in which an annular ridge is provided on the surface of the substrate to form a current concentration in the active layer to obtain high luminance.
The reason why high luminance is obtained by providing an annular raised portion on the substrate surface is as follows. In a conventional double heterostructure light emitting diode, the current spreads over the entire active layer because the substrate surface is flat. However, when a ridge is formed on the substrate surface, 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 flows, the light emission output increases when the current flows in a narrow area.

[0007]

However, the light emitting diode in which the annular ridge is formed on the substrate has not been sufficiently satisfactory, though the light emitting output and the responsiveness have been greatly improved.

It is an object of the present invention to provide a light emitting diode which can solve the above-mentioned disadvantages of the prior art and can realize a higher light emitting output and a higher response speed.

[0009]

According to the light emitting diode of the present invention, an epitaxial layer having a double hetero structure having an active layer is grown on a substrate of one conductivity type. Applied to light emitting diodes with electrodes.

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

The light emitting diode is GaAs.
s, GaAlAs, GaP, GaAsP, GaAlA
It can be applied to all light emitting diodes such as s, SiC and GaN.

[0012]

According to the present invention, when the active layer has the conductivity type opposite to the conductivity type of the substrate and a raised portion is formed on the surface, the active layer has the same conductivity type as that of the substrate. Although it is not clear why high brightness and high-speed response are obtained, when carriers are injected into the active layer from the ridge of the substrate, carriers of different conduction types are injected into the active layer with a high carrier concentration, resulting in high light emission. It seems that output and fast response speed can be obtained.

[0013]

Embodiments of the present invention will be described below with reference to the drawings. Here, a case where an infrared light emitting diode having a GaAlAs double hetero structure with an emission wavelength of 850 nm is used is described, but the present 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
On a p-type GaAs substrate 15 of × 10 ¹⁹ cm ^-3 , a Zn-doped p-type GaAlAs layer 14 having a mixed crystal ratio of 0.30 was formed to a thickness of 20 μm.
m, an epitaxial wafer in which a Te-doped n-type GaAlAs layer 13 (active layer 13) having a mixed crystal ratio of 0.05 was grown to 0.5 μm, and a Te-doped n-type GaAlAs layer 12 having a mixed crystal ratio of 0.30 was grown to 30 μm. Used. In particular, what is important here is that the conduction type of the active layer is not the same p-type as that of the GaAs substrate 15 but the opposite n-type.
A circular n-side electrode 11 is formed on the surface of the epitaxial wafer.
And a p-side electrode 16 is formed on the entire back surface.

On the entire surface of the p-type GaAs substrate 15 used here, fine unevenness 17 for forming a convex portion 18 (mesa portion 18) as shown in the cross section of FIG. 1 is shown in the plan view of the substrate of FIG. In the form of a matrix. The irregularities 17 can be formed by etching. The flat portion of the projection 18 constituting the unevenness 17, that is, the top surface of the projection 18 has a diameter of 10 μm, and the interval is 40 μm in both the vertical and horizontal directions. The height of the projection 18 is 10
15 μm.

Incidentally, the epitaxial growth on the uneven substrate 15 having the unevenness 17 formed thereon is conventionally performed by the liquid phase epitaxial method. There is no.

A light emitting diode having a double hetero structure as shown in FIG. 1 is fabricated on the substrate 15 having such irregularities 17 and the characteristics of the light emitting diode are measured. did. A current of 50 mA was passed in a bare chip state, and the in-plane distribution of light emission output was examined. As a result, it was possible to obtain about 1.4 times the emission output when the active layer was an n-type as in this example, as compared with the p-type active layer. The cutoff frequency is 20 MHz,
It was possible to increase the frequency up to 30 MHz, which is 1.5 times.

The reason why the output of the light emitting diode is high and high as described above is considered to be as follows. When a p-type active layer is grown on a flat substrate, the light emission output P ↓ ₁ (↓ is a vector notation of P) can be expressed by equation (1).

P ↓ ₁ = hν · B · d · p ₀ Δn (1) where hν is light energy, B is emission recombination constant, d
Is the active layer thickness, p ₀ is the carrier concentration of the p-type active layer, Δn
Is the density of the injected electrons. Note that p ₀ >> 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 is formed, a light emission output P ↓ ₂
Is represented by the equation (2).

P ↓ ₂ = hν · B · d · (p ₀ + Δp) Δn (2) Comparing Equations (1) and (2), only the injected hole density ΔP component due to the effect of the mesa portion is increased. You can see that. Note that p ₀ 〓Δp >> Δn.

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

P ↓ ₃ = hν · B · d · n ₀ · Δp (3) where n ₀ is the carrier concentration of the n-type active layer.

When P ↓ ₂ and P ↓ ₃ are compared, p ₀ is almost equal to n ₀ . Therefore, (p ₀ + Δp) is at most twice n ₀ . On the other hand, when comparing Δn and Δp, Δp >> Δn because Δp is narrowed from the mesa portion and injected into the active layer. Therefore, P ↓ ₃ > P ↓ ₂ .

It is considered that high emission output and high response speed can be obtained because carriers having different conductivity types are injected into the active layer in a state where the carrier concentration is high.

As described above, according to the present embodiment, there are the following various advantages.

The p-type substrate is etched to form a fine mesa portion, and current is concentrated on each upper portion of the mesa portion to increase the carrier injection concentration into the active layer.
Moreover, since the conduction type of carriers injected from the p-type substrate into the active layer is made different by making the active layer n-type, the light emission output can be increased and the response speed can be increased. .

Since the basic structure is the same as the conventional double hetero structure, a light emitting diode can be produced at relatively low cost.

Since the formation of the fine mesa portion only requires etching the substrate, the fabrication is simple and the yield is high.

The variation in characteristics of the light emitting diode depends on the variation in crystal defects in the plane of the epitaxial wafer. However, if the luminescence recombination probability is high, the luminous efficiency depends on the injected carrier concentration and not on the crystal defects. Therefore, if the light emission recombination probability is increased due to the current concentration due to the protrusions in the wafer surface, the variation in the light emission characteristics is reduced. In this regard, the present embodiment is of a current injection type in which the GaAs substrate is a p-type and has a small mesa portion, so that the in-plane characteristic variation is small.

By the way, as the etching for forming the above-mentioned unevenness 17, dry etching can be used. In order to further increase the current concentration in the active layer, it is considered effective to increase the depth. A 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. Further, a deep concave portion may be formed by digging a groove in a lattice shape with a dicing saw and etching the groove. In this embodiment, the convex portion having a circular cross section is formed. However, since the area of the convex portion that causes current concentration in the active layer is important, the convex portion may have another shape. For example, the shape of the protrusions may be a lattice, and the protrusions may be connected to each other. It should be noted that the raised portions are not limited to fine irregularities, and the present invention can of course be applied to a light emitting diode having a ring-shaped raised portion.

In this embodiment, the light emitting diode has a structure in which an n-type active layer is formed on a p-type substrate. However, conversely, n
The same effect can be expected with a structure called a p-type active layer with respect to a mold substrate. These can also be achieved similarly with a double heterostructure light emitting diode other than a GaAlAs-based light emitting diode.

[0033]

According to the present invention, a ridge is formed on the surface of a substrate, and the conduction type of the active layer is reversed from the conduction type of the substrate. Since the injection is performed, a high-speed light-emitting diode having a high light-emitting output can be manufactured.

[Brief description of the drawings]

FIG. 1 is a 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 an uneven arrangement formed on a substrate according to the present embodiment.

[Explanation of symbols]

 Reference Signs List 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 portion (mesa portion)

Claims (1)

(57) [Claims] In a light emitting diode, an epitaxial layer having a double hetero structure having an active layer is grown on a substrate of one conductivity type, and electrodes are formed on the front and back of the wafer on which the epitaxial layer is grown. The conductivity type is the opposite conductivity type to the one conductivity type substrate, and a ridge for locally injecting carriers from the one conductivity type substrate into the active layer of the opposite conductivity type is formed on the substrate surface. Light emitting diode.