Active solid-state devices (e.g. – transistors – solid-state diode – Incoherent light emitter structure – With reflector – opaque mask – or optical element integral...
Reexamination Certificate
2002-09-12
2004-09-21
Elms, Richard (Department: 2824)
Active solid-state devices (e.g., transistors, solid-state diode
Incoherent light emitter structure
With reflector, opaque mask, or optical element integral...
C257S079000, C257S086000, C257S088000, C257S094000, C257S103000
Reexamination Certificate
active
06794688
ABSTRACT:
BACKGROUND OF THE INVENTION
The present invention relates to a semiconductor light-emitting device such as light-emitting diodes to be used for display, optical communications and the like, and also to a manufacturing method therefor. The invention further relates to an LED lamp and an LED display, whichever is equipped with such a semiconductor light-emitting device.
In recent years, there have been developed high-intensity light-emitting diodes (LEDs) which emit light of infrared to blue wavelengths. This is based on the fact that the crystal growth technique for direct-transition group III-V compound semiconductor materials has been improved dramatically so that crystal growth has become implementable for almost any semiconductor that belongs to the group III-V compound semiconductors. LEDs using these direct-transition materials, by virtue of their capability of high-output, high-intensity emission, have come to be widely used as high-intensity LED lamps such as outdoor display boards, display-use light sources such as indicator lamps for portable equipment of low power consumption, and light sources for optical transmission and optical communications by plastic optical fibers.
As a new high-output, high-intensity LED of this type, there has been known an LED using AlGaInP-based material as shown in FIG.
11
. This LED is fabricated by the following process. That is,
On an n-type GaAs substrate
1
, are stacked one after another:
an n-type GaAs buffer layer
2
;
a distributed Bragg reflector layer (dopant concentration: 5×10
17
cm
−3
)
4
made of a multilayer film in which n-type (Al
x
Ga
1−x
)
0.51
In
0.49
P (x=0.45) and n-type Al
0.51
In
0.49
P are stacked alternately;
an n-type (Al
x
Ga
1−x
)
0.51
In
0.49
P lower cladding layer (0≦x≦1, e.g. x=1.0; thickness: 1.0 &mgr;m; dopant concentration: 5×10
17
cm
−3
)
5
;
a p-type (Al
x
Ga
1−x
)
0.51
In
0.49
P active layer (0≦x≦1, e.g. x=0.42; thickness: 0.6 &mgr;m; dopant concentration: 1×10
17
cm
−3
)
6
; and
a p-type (Al
x
Ga
1−x
)
0.5
In
0.49
P upper cladding layer (0≦x≦1, e.g. x=1.0; thickness: 1.0 &mgr;m; dopant concentration: 5×10
17
cm
−3
)
7
,
and further thereon are formed:
a p-type (Al
x
Ga
1−x
)
v
In
1−v
P intermediate layer (x=0.2; v=0.4; thickness: 0.15 &mgr;m; dopant concentration: 1×10
18
cm
−3
)
8
;
a p-type (Al
x
Ga
1−x
)
v
In
1−v
P current spreading layer (x=0.05; v=0.05; thickness: 1.5 &mgr;m; dopant concentration: 5×10
18
cm
−3
)
10
; and
an n-type GaP current blocking layer (thickness: 0.3 &mgr;m; dopant concentration: 1×10
18
cm
−3
)
9
.
Thereafter, the n-type GaP current blocking layer
9
is subjected to selective etching by normal photolithography process so that a 50 &mgr;m to 150 &mgr;m-dia. portion thereof shown in the figure is left while its surrounding portions are removed. A p-type (Al
x
Ga
1−x
)
v
In
1−v
P current spreading layer (x=0.05; v=0.95; thickness: 7 &mgr;m; dopant concentration 5×10
18
cm
−3
)
10
is regrown in such a manner as to cover the top of the p-type (Al
x
Ga
1−x
)
v
In
1−v
P current spreading layer, which has been exposed by removing the n-type GaP current blocking layer
9
, as well as the n-type GaP current blocking layer
9
.
Finally, on the p-type current spreading layer
10
is deposited, for example, a Au—Be film. This film is patterned into a circular form, for example, so as to be inverse to the light-emitting region, thereby forming a p-type electrode
12
. Meanwhile, on the lower surface of the GaAs substrate
1
is formed, for example, an n-type electrode
11
made of a Au—Zn film by deposition.
It is noted here that, for simplicity' sake, the ratio x of Al to Ga, the ratio v of totaled Al and Ga to the other group III elements, or the like will be omitted as appropriate in the following description.
With respect to the p-type AlGaInP current spreading layer
10
, the Al composition “x” and the In composition (1−v) are set low, as already described, so that the current spreading layer becomes transparent to the emission wavelength range 550 nm-670 nm of this AlGaInP-based LED, low in resistivity, and makes ohmic contact with the p-side electrode (i.e., x=0.05, v=0.95). In the AlGaInP-based LED, normally, Si is used as the n-type dopant, and Zn is used as the p-type dopant. Also, the conductive type of the active layer is normally the p type.
As the substrate for (Al
x
Ga
1−x
)
v
In
1−v
P-based LEDs, normally, a GaAs substrate is used so as to obtain lattice matching with materials of individual layers. However, the GaAs substrate has a band gap of 1.42 eV, lower than those of (Al
x
Ga
1−x
)
v
In
1−v
P-based semiconductors, so that the GaAs substrate would absorb light emission of 550 nm to 670 nm, which is a wavelength range of (Al
x
Ga
1−x
)
v
In
1−v
P-based semiconductors. Therefore, out of light emitted from the active layer, the light emitted toward the substrate side would be absorbed within the chip, and could not be extracted outside. Accordingly, for (Al
x
Ga
1−x
)
v
In
1−v
P-based LEDs, with a view to fabricating a high-efficiency, high-intensity LED, it is important to provide a DBR (distributed Bragg reflector) layer
4
in which low-refractive-index layer and high-refractive-index layer are combined one after another between the GaAs substrate
1
and the active layer
6
as shown in
FIG. 11
so as to obtain an enhanced reflectance through multiple reflection. In this example of
FIG. 11
, (Al
0.65
Ga
0.35
)
0.5
In
0.49
P (refractive index: 3.51) that does not absorb the emission wavelength 570 nm of the active layer is selected as the high-refractive-index material, and Al
0.51
In
0.49
P (refractive index: 3.35) is selected as the low-refractive-index material, while optical film thicknesses of the individual low-refractive-index layer and high-refractive-index layer are set to &lgr;/4 relative to an emission wavelength of &lgr;. These materials are stacked alternately to an extent of 10 pairs so as to be enhanced in reflectance, by which the total photoreflection-layer reflectance is set to about 50%. In a case where such an AlGaInP-based light-reflecting layer is provided, reflectance characteristics against the number of pairs are shown in FIG.
13
A. The expression “AlInP/Q(0.4)” in the figure indicates a characteristic with the use of a pair of (Al
0.65
Ga
0.35
)
0.51
In
0.49
P and Al
0.51
In
0.49
P. Similarly, the expression “AlInP/Q(0.5)” indicates a characteristic with the use of a pair of (Al
0.55
Ga
0.45
)
0.51
In
0.49
P and Al
0.51
In
0.49
P. With this light-reflecting layer adopted, the chip luminous intensity can be improved from 20 mcd to 35 mcd, compared with the case where no light-reflecting layer is provided.
As is well known, if the layer thickness of crystals is “d” and the refractive index is “n,” then the optical film thickness is given by “nd.”
As shown in
FIG. 12
, in (Al
x
Ga
1−x
)
v
In
1−v
P-based LEDs is used a light-reflecting layer
14
which is formed by stacking a pair of Al
x
Ga
1−x
As and AlAs and which has lattice matching with the GaAs substrate. In a case where such an AlGaAs-based light-reflecting layer
14
is provided, reflectance characteristics against the number of pairs are shown in FIG.
13
B. In the figure, a broken line expressed as “Al
0.60
” shows a characteristic with the provision of a light-reflecting layer which is formed by selecting Al
0.65
Ga
0.35
As (refractive index: 3.66), which does not absorb the emission wavelength 570 nm of the active layer, as the high-refractive-index material and selecting AlAs (refractive index: 3.10) as the low-refractive-index material, and then stacking alternately these materials as a pair. Similarly, a broken line expressed as “Al
0.70
” in the figure shows a characteristic with the use of a pair of Al
0.70
Ga
Kurahashi Takahisa
Murakami Tetsuroh
Nakatsu Hiroshi
Ooyama Shouici
Elms Richard
Menz Douglas
Morrison & Foerster / LLP
Sharp Kabushiki Kaisha
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