JPH07202262A - Light emitting diode - Google Patents

Abstract

PURPOSE:To largely increase a light emitting efficiency by eliminating excess doping of Te for an n-type window layer. CONSTITUTION:A GaAlAs light emitting diode has an n-type window layer having an AlAs mixed crystal ratio of 0.2 or more. The window layer is doped with In of a range of 1-4.5mol% in order to reduce DX center concentration. Thus, since the doping amount of the Te can be reduced, segregation of the Te is eliminated, and generation of the DX center is suppressed, thereby improving a light emitting efficiency.

Description

Detailed Description of the Invention

[0001]

FIELD OF THE INVENTION The present invention relates to light emitting diodes, especially G
The present invention relates to an aAlAs light emitting diode with improved light emission efficiency.

[0002]

2. Description of the Related Art As a light emitting diode (LED) for display, a high brightness visible light emitting diode is required.
As a red LED, a GaP or GaAs-based LED has been conventionally used, but a GaAlAs-based LED has attracted particular attention in terms of high brightness.

In a GaAlAs-based LED, a Zn-doped p-type substrate serving as a light-emitting layer is formed on a Zn-doped p-type GaAs substrate.
A well-known type has a single hetero structure (SH structure) in which a GaGaAs layer and a Te-doped GaGaAs layer are grown. In order to manufacture an LED of higher brightness, as shown in FIG. 5, a Zn-doped p-type GaAlAs layer (cladding layer) is formed on a Zn-doped p-type GaAs substrate 1.
2 is grown, and Zn-doped G to be an active layer is formed thereon.
A double heterostructure (DH structure) is used in which the aAlAs layer 3 and the Te-doped n-type GaAlAs layer 4 are further grown as window layers. This GaAl
In the As-based double heterostructure LED, the brightness is higher than that of the single heterostructure by utilizing the carrier confinement effect.

[0004]

The LED structure has been improved from the SH structure to the DH structure aiming at higher brightness.
However, even if the structure itself becomes suitable for higher brightness, if the crystallinity is not good, high brightness cannot be sufficiently achieved. Therefore, how to improve the crystallinity is important. Regarding the improvement of the crystallinity, one of the major problems is not only the impurities in the crystal but also the n-type window layer.
This is the segregation of the dopant Te that is doped to form the mold. Those generated in the vicinity of the active layer become non-emissive centers and significantly reduce the light emission efficiency. In order to prevent the segregation of Te, (1) the degree of supersaturation during the growth of the window layer is reduced.

(2) Decrease the Te doping amount.

Two measures are usually taken:

However, if the value of (1) is too small, it causes unstable factors such as meltback and cannot be reduced so much. Therefore, it cannot be said to be an effective means.

Regarding (2), the Te doping amount cannot be reduced for the following reasons (1) to (3).

Since an electrode is attached to the surface, at least 1 × 10 ¹⁷ -1.
A carrier concentration of about 5 × 10 ¹⁷ cm ^-3 is required.

A carrier concentration higher than the concentration for compensating the p-type dopant Zn diffused from the active layer and the p-type cladding layer is required, and if it is lowered too much, the forward current-voltage characteristic is deteriorated, or a thyristor by a parasitic transistor is used. Cause the effect.

This is the reason for the physical properties peculiar to GaAlAs. Reference (Chand N., et al .; phys. Rev. 1330,4481 (198
According to 4)), the donor ionization energy changes as shown in FIG. 2 depending on the AlAs mixed crystal ratio x. When x exceeds 0.2, most of the donors follow the Γ valley to the X valley, and the activation energy increases accordingly, and 0.2 eV, which is about 10 times higher than usual, near x = 0.4. Reach

Such a deep level donor is called a DX center. The ratio of DX donors to DX donors varies depending on the manufacturing method and growth conditions, but ranges from several 10% to more than half. Usually, in the case of a red LED for display, in order to increase the external quantum efficiency, the window layer needs to have a bandgap larger than the emission wavelength, so the AlAs mixed crystal ratio x = 0.5.
Since it is up to 0.7, it is a DX center generation area. D
The X center is an electron trap that exhibits a large electron relaxation and cannot be excited at room temperature and therefore does not contribute to electrical characteristics. Therefore, in order to compensate for the decrease of the electrons, T
It is necessary to dope the window layer with e, which is Te
Was caused to cause non-emissive centers to significantly reduce the luminous efficiency.

An object of the present invention is to solve the above-mentioned drawbacks of the prior art and to provide a novel LED capable of significantly increasing the luminous efficiency.

[0014]

The LED of the present invention comprises a p-type GaAlAs layer doped with a p-type impurity and an n-type GaAlA doped with an n-type impurity having an AlAs mixed crystal ratio of 0.2 or more.
In a GaAlAs-based light emitting diode having an s layer,
A small amount of In is added to the n-type GaAlAs layer.

The p-type impurity and the n-type impurity are, for example, Zn and Te, respectively, and the addition amount of In is preferably in the range of 1 to 4.5 mol%.

As a result, the occurrence of DX center is suppressed and T
The doping amount of e can be reduced and the luminous efficiency can be significantly improved.

The reason why the AlAs mixed crystal ratio is set to 0.2 or more is that a mixed crystal ratio smaller than 0.2 does not result in a DX center generation region.

[0018]

In the present invention, the added amount of In is 1 to 4.5.
The range of mol% is preferable, and the reason is as follows.

First, setting the lower limit of the In concentration to 1 mol% will be described. Figure 3 shows (NHK Giken Kobayashi et al., 19
89 Spring, Japan Society of Applied Physics), but Al _0.3 Ga _0.7 As
2 shows the relationship between the In doping concentration and the DX center concentration in FIG. The density of the DX center is DLTS (deep level t
When the In concentration measured by ransient spectroscopy is 2 mol%, it is 5% higher than when the In concentration is 0 mol%.
Although it decreased by 0%, it stopped at 30% when it was 1 mol%, and the concentration of DX center increased with the decrease of In concentration. In addition, FIG. 4 shows a MOCV on a semi-insulating substrate.
The In doping amount and the carrier concentration when an Al _x Ga _1-x As layer was grown at an AlAs mixed crystal ratio x = 0.3 by D (metalorganic vapor phase epitaxy) are shown. The carrier concentration depends on the CV method and the Pau method, but since the CV method excites and measures all the donors up to a deep level by high frequency, the total carrier concentration is measured. In the Pau method, only the carrier concentration activated at room temperature is measured. Therefore, the difference between the two measurements is the DX center density. The carrier concentration by the CV method is In
Although it is constant regardless of the concentration, the value obtained by the Pau method was half that of the value obtained by the CV method when the In content was 0 mol%, but started to increase from the point where it exceeded 0.5 mol% and the pau content at 1 mol%. It is almost equal to the law. From these two data, it can be seen that the effect cannot be expected unless the In concentration is at least 1 mol% or more.

On the other hand, explaining that the upper limit of the In concentration is 4.5 mol%, the lattice constant is GaAs: 5.
InAs is as large as 6.6 angstroms while 64 angstroms and AlAs: 5.66 angstroms. Therefore, if the In concentration is higher than 4.5 mol%, a problem of lattice mismatch occurs, which is not preferable.

Further, as will be described in detail later, as shown in FIG. 1, by setting the addition amount of In within the range of 1 to 4.5 mol%, the result with the best external quantum efficiency is obtained.

[0022]

EXAMPLES Examples of the present invention will be described below. An epitaxial wafer for a DH structure red LED was grown using a slide boat method which is one of liquid phase epitaxial growth methods. FIG. 5 shows the structure of this epitaxial wafer. The p-type GaAs substrate 1 has a thickness of 350 μm, a Zn-doped carrier concentration of 1.5 × 10 ¹⁹ cm ⁻³ , and a dislocation density of 1
It is less than 20,000 cm ^-2 . P-type GaAlA on substrate 1
The s-clad layer 2 is similarly doped with Zn, has an AlAs mixed crystal ratio of 0.65, and has a film thickness of 20 μm. p-type clad layer 2
The p-type GaAlAs active layer 3 on the top is similarly doped with Zn and has an AlAs mixed crystal ratio of 0.35 and a film thickness of 1 μm. The n-type GaAlAs clad layer 4 on the active layer 3 is A
The mixed crystal ratio of 1As is 0.65, the film thickness is 40 μm, and Te and In are contained as dopants.

When the In concentration is changed from 0 to 6 mol% and the Te concentration is adjusted to be measured by the Pau method,
A sample having a Te concentration of 1.5 × 10 ¹⁷ cm ⁻³ at the interface with the active layer 3 was prepared. LEDs from these samples
FIG. 1 shows the result of calculating the external quantum effect by creating Assuming that the external quantum efficiency when the In concentration is 0 mol% is 1, the external quantum efficiency increases by about 50% when the In concentration increases from 0 mol% to 2 mol%, and then In
The concentration shows the same value up to 4.5 mol%, but then drops sharply. In the figure, the reason why the emission efficiency is low at a low concentration of In is that non-emission centers are generated by adding a large amount of Te, and when it exceeds 4.5 mol%, it rapidly deteriorates. This is probably because the crystallinity deteriorated due to the lattice mismatch.

As described above, according to this embodiment, in a GaAlAs-based light emitting diode having an n-type GaAlAs window layer having an AlAs mixed crystal ratio of 0.2 or more, In is 1 mol% or more and 4.5 mol% or less in the window layer. Since the amount of Te doped can be reduced by performing the following doping,
It is possible to effectively suppress the occurrence of the DX center. Therefore, the luminous efficiency can be increased as much as possible.

[0025]

According to the present invention, the n-type window layer has an I
By doping n by 1 mol% or more and 4.5 mol% or less,
Since the amount of Te doped is small, the generation of DX centers can be suppressed without impairing the crystallinity, so that the luminous efficiency can be significantly increased.

[Brief description of drawings]

FIG. 1 is a characteristic diagram showing the relationship between In concentration in a window layer and external quantum efficiency according to the present embodiment.

FIG. 2 is a characteristic diagram showing the relationship between the Al composition ratio x in Al _x Ga _1-x As and the ionization energy of the donor.

FIG. 3 is a characteristic diagram showing In concentration dependency of DX center concentration in Al _0.3 Ga _0.7 As.

FIG. 4 is a comparative characteristic diagram showing the relationship between the In doping amount in Al _0.3 Ga _0.7 As and the carrier concentration by the Pau method and the CV method.

FIG. 5 is a sectional view of an epitaxial wafer for a GaAlAs-based double hetero structure red light emitting diode showing an embodiment according to the present invention.

[Explanation of symbols]

 1 p-type GaAs substrate 2 p-type GaAlAs clad layer 3 p-type GaAlAs active layer 4 n-type GaAlAs window layer

 ─────────────────────────────────────────────────── ─── Continuation of the front page (72) Inventor Yukio Kikuchi 5-1-1 Hidaka-cho, Hitachi-shi, Ibaraki Hitachi Cable Co., Ltd. Hidaka factory (72) Inventor Hiroyuki Kamogawa 5 Hidaka-cho, Hitachi-shi, Ibaraki 1-1-1 Hitachi Cable Co., Ltd. Hidaka Factory

Claims (4)

[Claims] 1. A GaAlAs-based light-emitting diode having a p-type GaAlAs layer doped with p-type impurities and an n-type GaAlAs layer doped with n-type impurities having an AlAs mixed crystal ratio of 0.2 or more, wherein the n-type GaAlAs layer is provided. Traces of In
A light emitting diode characterized by being added with. 2. The light emitting diode according to claim 1, wherein the p-type impurity is Zn. 3. The light emitting diode according to claim 1, wherein the n-type impurity is Te. 4. The light emitting diode according to claim 1, wherein the added amount of In is 1 to 4.5 mol%.