JPH0661525A - Light emitting diode - Google Patents

Abstract

PURPOSE:To increase the band-gap energy(Eg) of a light emitting diode so as to shorten the light emitting wavelength of the diode by using a GaAsxP1-x substrate crystal Which is grown on a GaAs substrate by gradually increasing phosphor content of the substrate and growing an A GaInP crystal lattice- matched to the substrate crystal. CONSTITUTION:At the time of epitaxially growing an AlGaInP crystal, a layer having a constant composition of GaAs1-xPx grown on a GaAs or GAP substrate crystal with a graded layer 4, in which the (x) of a GaAs1-xPx (0<x<=1) crystal is gradually changed so as to relieve a difference in lattice constant, in between is used as a substrate 8. As a result, a composition GaxIn1-xP (X>0.52) lattice- matched to the substrate 8, namely, an active layer which is >1.92cV in Eg and (6.45nm in light emitting wavelength can be used. The wavelength of the light emitted from the AlGaInP grown on the GaAsxP1-x substrate crystal is shorter than that of the light emitted from AlGaInP grown on a GaAs substrate. Therefore, light having a shorter wavelength can be obtained without adding Al to the active layer of a DH structure in which the active layer is put between layers having large band-gap energy.

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 used for indoor / outdoor character / graphic display boards, automobile direction indicators, and the like.

[0002]

2. Description of the Related Art Conventional p-AlGaInP / u-GaI
A light emitting diode having an nP / n-AlGaInP DH structure is formed on a GaAs substrate. The composition of GaInP that lattice-matches the GaAs substrate crystal is Ga _0.52 In
_{It is 0.48} P and Eg = 1.92 eV. Therefore, in order to obtain a light emitting diode having an emission wavelength of 645 nm or less, as shown in FIG. 2, aluminum is added to the active layer to form (Al _X Ga _1-X ) _0.52 In _0.48 P, and a larger Eg activity is obtained. It was necessary to make devices with layers (e.g. RMFletcher et al. J. Elec. Mat 20 [12] 1125-1130.
(1991); H. Sugawara et al. The 22nd Conf.Sol. Stat. Mat.,
Ext. Abstracts 1175-1176 (1990)). However, there is a problem in obtaining a high-luminance light emitting diode because the emission intensity sharply decreases as the Al concentration increases.

[0003]

In order to obtain a high-luminance and high-efficiency light emitting diode having a wavelength shorter than 645 nm,
a _X In _1-X P (X> 0.52), that is, E
A crystal with g> 1.92 eV is used. When using a quaternary mixed crystal AlGaInP for further shortening the wavelength, if the same wavelength of light is emitted, it is desirable to use AlGaInP with the lowest Al concentration as possible to prevent a decrease in light emission efficiency.
Therefore, if a GaInP crystal lattice-matched with GaAs is grown on a substrate crystal having a lattice constant smaller than that of GaAs, the composition becomes Ga _X In _{1 -X} P (X> 0.52), and Eg becomes Eg Is large, that is, the emission wavelength is short.
In addition, in this crystal system, a long-range ordered structure is generated depending on the growth conditions, so that Eg is reduced and therefore the emission wavelength is lengthened. In order to shorten the emission wavelength, it is necessary to reduce the occurrence of long-range ordered structure. On the other hand, in order to obtain high-luminance light emission, it is necessary to attach a low resistance layer as a layer that makes ohmic contact with the electrode so that the current spreads over the entire surface of the chip, and this layer is large in order to take out light emission efficiently. It must have Eg and have low absorption at the emission wavelength. If this layer is made of AlGaInP crystal, Al must be added to the crystal to increase Eg. However, since it is difficult to increase the p-type carrier density in a crystal having a large Al composition, a crystal that satisfies both of these requirements is required. In order to increase the external quantum efficiency and increase the emission intensity, it is necessary to have an electrode from the active layer.
It is necessary to take out both of the light emitted in one plane direction with less absorption of the light emission by the crystal.

[0004]

Means for Solving the Problems When epitaxially growing an AlGaInP-based crystal, GaAs or GaP is used.
Is used as the substrate crystal, and in order to alleviate the difference in lattice constant, GaAs _1-X P _X (0 <X ≦ 1) is formed on the GaAs layer with a constant composition via a graded layer in which X is gradually changed.
_A substrate obtained by growing a _1-X P _X layer is used. An active layer having a composition X of Ga _x In _{1 -X} P that is lattice-matched to this substrate> 0.52, that is, Eg> 1.92 eV and an emission wavelength of <645 nm can be used. In order to further reduce the wavelength, the crystal plane orientation of the substrate is selected and used in order to reduce the long-range order.

The layer that makes ohmic contact with the electrode is
Al _Y Ga _{1 -Y} P (0 ≦ Y ≦ 0.3) is highly doped as a crystal capable of increasing the carrier density and sufficiently reducing the absorption of light emitted from the active layer in the DH structure. Just grow up. However, since these crystals do not lattice match with GaAs _X P _1-X , in order to grow while reducing the crystal defects, the crystals grow from the lattice matched crystal through the graded layer with the composition gradually changed and the strain buffering is performed. Ultra thin film layer of Al _Z In _{1 -Z} P (0 ≦ Z ≦ 1) or Al
Put a superlattice structure of _{Y Ga 1-Y P / Al} Z In 1-Z P. When light emitted from the active layer is taken out from the upper side of the growth direction of the crystal from the substrate (the AlGaP layer side), the substrate is removed, a DBR reflective film is attached to this face, and light is taken out from the lower face. For this, a crystal with Eg smaller than the substrate is used for the active layer. At this time, the crystal that becomes the active layer does not lattice match with the substrate. Attach a film.

[0006]

As shown in FIG. 3, if, for example, the GaAs _X P _1-X substrate crystal is grown on a GaAs substrate while gradually increasing phosphorus, an AlGaInP-based crystal lattice-matched to this crystal is grown. , Al and Ga have a higher concentration than that of the crystal lattice-matched with the GaAs substrate, and thus Eg is large, and the emission wavelength can be shortened.

In Ga _X In _1-X P, in principle, a crystal having a band structure changing from a direct transition to an indirect transition X = 0.73, that is, a crystal having Eg of less than Eg = 2.23 eV does not need to have Al added. But Al, which is lattice-matched to the GaAs substrate
As compared with the GaInP crystal, the wavelength can be shortened while maintaining high luminous efficiency. For example, for red emission from GaInP grown on the GaAs substrate, GaAs
_With Ga _0.7 In _0.3 P grown on _0.61 P _0.39 , a yellow-green to yellow light emitting diode can be obtained. In order to further reduce the wavelength, a quaternary crystal of AlGaInP added with Al may be adopted. In this case, light emission of the same wavelength can be obtained at a lower Al concentration than that of AlGaInP lattice-matched on a GaAs substrate. it can. Therefore, high-efficiency light emission is possible due to the low Al concentration.

The Eg of an AlGaInP-based crystal may take different values depending on the degree of the long-range ordered structure even if the crystal has the same composition. As shown in FIG. 4, by selecting the plane orientation of the substrate crystal, the degree of generation of the long-range ordered structure of the AlGaInP-based crystal grown thereon can be adjusted, and the values of Eg and emission wavelength can be selected. When an LED is made of a crystal having a composition, the emission wavelength can be shortened by changing the substrate surface orientation. For example, GaInP is grown at 700 ° C,
By tilting the substrate plane orientation from the (100) plane to the (511) A plane, generation of a long-range ordered structure can be suppressed, and Eg can be increased by several tens to hundreds of meV, and the wavelength can be shortened accordingly.

According to, by selecting _X of the GaAs _X P _1-X substrate crystal, Ga that is lattice-matched to this crystal is selected.
_{Y of Y} In _1-Y P (Y> 0.52) is determined, the plane orientation of the substrate crystal is selected to control the long-range ordered structure, and the Al concentration of the active layer in the DH structure is selected to increase Eg. Can be achieved. The emission wavelength of the light emitting diode according to the present invention can be controlled by these three degrees of freedom.

Al _X Ga _1-X P (0≤0) capable of high-concentration doping as a layer for making ohmic contact with the electrode
By growing X ≦ 0.3), the current can be spread over the entire surface of the device. Moreover, since this crystal is an indirect transition type, the absorption coefficient is small, and light from the active layer can be efficiently extracted from the surface opposite to the substrate among the surfaces in contact with the electrodes of the element. When extracting light from the surface on the substrate side, the absorption of the emission wavelength of the substrate is reduced by setting the active layer having Eg smaller than the Eg of the substrate, and by using the reflective film on the AlGaP side, both directions from the active layer can be obtained. The emitted light can be extracted and the external quantum efficiency can be increased.

The Al _x Ga layer is formed by gradually reducing the In composition from an AlGaInP-based crystal lattice-matched with a GaAs _Z P _1-Z (0 ≦ Z <1) substrate crystal.
_1- XP is grown, and Al _Y In is used as a strain buffer layer on this layer.
_1-Y P (0 ≦ Y ≦ 1) Single ultra thin film layer or Al _X Ga
By inserting a superlattice structure of _{1-X P / Al Y In} 1-Y P, it can reduce crystal defects such as dislocations in the Al _X Ga _1-X P layer can be improved device life, the light emission efficiency.

[0012]

EXAMPLE 1 Example 1 will be described with reference to the cross-sectional view of the element shown in FIG. n-GaAs _0.61 P _0.39 substrate (n = 7 × 10
Si-doped n- (Al _0.7 Ga _0.3 ) _0.7 In _0.3 P layer (1 × 10 ³ on ¹⁷ cm ³ ) by metalorganic vapor phase epitaxy.
¹⁸ cm ³ ) 1.0 μm, undoped Ga _0.7
In _0.3 P 0.5 μm, Zn-doped p- (Al
_0.7 Ga _0.3 ) _0.7 In _0.3 P layer (p = 1.2 × 10 ¹⁷ cm
³ ) is grown to 0.2 μm, and the same (p = 5 × 10 ¹⁷ cm ³ ) is grown to 0.8 μm. (Hereinafter, all p-types are Zn-doped.) On top of that, p- (Al
_0.7 Ga _0.3 ) _X In _1-X P graded layer (X: 0.7 →
1.0, p = 5 × 10 ¹⁷ cm ³ ) to 2.0 μm, and p
-Al _0.8 In _0.2 P (80Å) / p-GaP (80Å)
Of the p-GaP layer (p = 3 ×
10 ¹⁸ cm ³ ) is grown to 8 μm. Here, the growth temperature is 7
It is 00 ° C. In this case, the active layer of Ga _0.7 In
The emission wavelength from the _0.3 P layer is 583 nm, and yellow emission is obtained. By including the graded layer and the ultra-thin layer, crystal defects in the GaP layer are reduced and luminous efficiency is increased.

(Embodiment 2) In Embodiment 1, the emission wavelength is 5 by setting the plane orientation of the substrate crystal to the (511) A plane.
Yellow-green light emission of the order of 60 nm is obtained. Where GaA
The wavelength is shortened by about 300 meV as compared with the emission of GaInP on the same (511) A surface of the s substrate. Also, GaAs
When quaternary AlGaInP lattice-matched to the substrate is used as the active layer, the external quantum efficiency of light emission decreases with an increase in Al concentration and becomes 0.1% at an emission wavelength of 560 nm, but in the present embodiment, it is relatively about 2%. It will be a large value.

(Embodiment 3) In Embodiment 2, by making the active layer a quaternary mixed crystal of (Al _0.05 Ga _0.95 ) _0.7 In _0.3 P, green light emission having an emission wavelength of 550 nm can be obtained.

(Embodiment 4) In Embodiments 1 to 3, the substrate crystal is an n-GaP crystal substrate on which GaAs _{X is formed.}
A GaAs _X P _1-X layer of 5 μm having a constant composition X is grown through a graded layer of 10 μm in which _{X of} P _1-X (0 ≦ X <1) is gradually increased, and lattice matching is performed thereon. AlGa
InP-based crystal is grown.

(Embodiment 5) Substrate crystal is n-type GaAs _0.08
P _0.92, and n− (Al _0.4
Ga _0.6 ) _0.96 In _0.04 P (1 × 10 ¹⁸ cm ³ ) 1 μm
m, u-Ga _0.7 In _0.3 P to 40 nm, p- (Al _0.4
Ga _0.6 ) _0.96 In _0.04 P (p = 1.2 × 10 ¹⁷ cm ³ )
Of 0.2 μm, p = 5.0 × 10 ¹⁷ cm ³ of 0.8 μm, and p-GaP layer (p = 3 × 10 ¹⁸ cm ³ ) of 8 μm are grown, and a DBR reflective film is attached to this surface. A part is removed to form a p-electrode. With this structure, of the light emitted from the active layer, the light emitted to the n and p electrode sides can be extracted, so that the external quantum efficiency further increases to about 4%.

[0017]

EFFECTS OF THE INVENTION A grown on a GaAs _X P _1-X substrate crystal
Light emission from lGaInP was grown on a GaAs substrate A
The wavelength is shorter than that of light emitted from 1GaInP, and by changing the crystal plane of the substrate, the generation of a long-range ordered structure is reduced and the emission wavelength is shortened even if the structure and growth conditions of other elements are the same. It is possible to take out light emission in the 560 nm emission wavelength range without adding Al to Al.

Further, by growing an Al _Y Ga _{1 -Y} P layer having a high carrier concentration through a graded layer in which the composition is lattice-matched to the GaAsP substrate and gradually changed to In, the electrode has a low resistance and a sufficient resistance. It is possible to obtain an element having high light emission efficiency, which is made of a crystal having a large ohmic contact, a current spreading over a wide range of the element, little absorption of emission wavelength, and few crystal defects such as dislocations. As compared with the conventional device in which Al is added to the active layer, the improvement of the luminous efficiency becomes larger as the wavelength becomes shorter, and the external quantum efficiency becomes 10 times or more in the vicinity of 560 nm.

Further, the light-extracting surface is determined to be one of the two surfaces in contact with the electrode, and the light emission to the opposite surface is D
The external quantum efficiency is further increased by about 1.8 times by taking out using the BR reflective film. Further, when the light is extracted from the substrate side, the Eg of the active layer is reduced as described above, so that the light absorption in the substrate can be reduced.

[Brief description of drawings]

FIG. 1 is a sectional view of a light emitting diode according to the present invention.

FIG. 2 is a conventional light emitting diode on a GaAs substrate.

FIG. 3 is a composition dependence of a band gap energy of an AlGaInP-based crystal.

FIG. 4 shows the substrate surface orientation dependence of Eg of Ga _0.7 In _0.3 P.

[Explanation of symbols]

1, 9 ... Electrode, 3 ... Superlattice, 4 ... Graded layer, 8 ...
substrate.

 ─────────────────────────────────────────────────── ─── Continuation of the front page (72) Inventor Shinichiro Yano 1-280, Higashi Koigokubo, Kokubunji, Tokyo Inside the Central Research Laboratory, Hitachi, Ltd.

Claims (6)

[Claims] 1. GaAs on a GaAs or GaP substrate
_1-X P _X (0 <X ≦ 1) GaAs _{Y having a} constant composition grown through a graded layer in which the composition X of phosphorus in the crystal is gradually changed
On a P _{1 -Y} (0 ≦ Y <1) crystal substrate, an AlGaInP-based semiconductor crystal that is lattice-matched with this is epitaxially grown, and (Al _X Ga _1-X ) _Y In _1-Y P (0 ≦ X ≦ 0. 8,
0.52 <Y ≦ 0.80) it from the band gap energy Eg larger the _{(Al X Ga 1-X)} Y In 1-Y P (0
A light emitting diode obtained by making a DH structure sandwiched by <X ≦ 1, 0.52 <Y ≦ 1). 2. AlGaInP epitaxially grown by selecting the crystal plane orientation of the GaAsP substrate.
The light emitting diode according to claim 1, wherein the long-range order of the system semiconductor crystal is controlled and thus the emission wavelength thereof is controlled. 3. An n- or p-type Al _{x having a} larger Eg than that of the active layer as a layer for making ohmic contact with the electrode.
The light emitting diode according to claim 1 or 2, wherein a Ga _1-X P layer (0 ≦ X ≦ 0.3) is grown. 4. A surface on the upper side in the crystal growth direction (the above AlGaP
When the light is mainly extracted from the layer side), the substrate is removed, and it is necessary to have a DBR reflective film on this surface when the light is mainly extracted from the lower surface (substrate side) in the crystal growth direction. Has a smaller Eg than the substrate (Al _Y Ga _1-Y ) _X In _1-X P
(0.52 <X ≦ 0.8, 0 ≦ Y ≦ 0.8) is used as an active layer, and the thickness of this active layer is set to 50 nm or less and has a strain. Further, a DBR reflective film is formed on AlGaP. The light emitting diode according to claim 3, wherein the light emitting diode is provided. 5. An AlGaI lattice-matched to a GaAsP crystal.
Ga is formed between the Al _X Ga _1-X P (0 ≦ X ≦ 0.3) layer that is not lattice-matched with the nP crystal and the GaAsP grown on the nP crystal.
Starting with AlGaInP that is lattice-matched with the AsP substrate, the _{Z of} (Al _Y Ga _1-Y ) _Z In _1-Z P (0 ≦ Y ≦ 1) is 1.
The light emitting diode according to claim 3 or 4, which is obtained by growing a graded layer which is gradually changed to 0. 6. The thickness 80 of Al _Y In _{1 -Y} P (0 ≦ Y ≦ 1) which is a strain buffer layer in the Al _X Ga _1-X P layer (0 ≦ X ≦ 0.3).
~ 150Å single ultra thin film layer or Al _X Ga _1-X P (thickness 80 to 150 Å) / Al _Y In _1-Y P (thickness 80 to 150
5. The superlattice (within 20 cycles) of Å) is included.
The light emitting diode described.