Led display device

Details Led display device Led display device

1997-12-18

2001-05-08

Bovernick, Rodney (Department: 2874)

Coherent light generators

Particular active media

Semiconductor

C257S094000, C257S097000

Reexamination Certificate

active

06229834

ABSTRACT:

BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a high-power semiconductor light emitting diode (referred to hereinafter as “LED”) display device, and particularly, to a LED display device having a LED matrix circuit for an outdoor/indoor display panel, a railway sign board, a traffic sign board, or a vehicle-mounted light.
2. Description of the Prior Art
Semiconductor light emitting devices such as LEDs and semiconductor lasers are manufactured according to a liquid-phase epitaxial growth (LPE) technique and a vapor-phase epitaxial (VPE) growth technique such as a metal organic chemical vapor deposition (MOCVD) technique. Any of the techniques forms a double hetero (DH) structure to confine carriers in an active layer serving as a light emitting layer and realize high brightness.
FIG. 1
is a sectional view showing an InGaAlP-based LED having a DH structure according to a prior art. Successively laminated on an n
+
-type GaAs substrate
1
are an n-type GaAs buffer layer
2
, an n-type In
0.5
Al
0.5
P/GaAs reflection layer (quarter-wave stack mirror)
3
, an n-type In
0.5
(Ga
1−x
Al
x
)
0.5
P clad layer
41
, an undoped In
0.5
(Ga
1−y
Al
y
)
0.5
P active layer
61
, and a p-type In
0.5
(Ga
1−x
Al
x
)
0.5
P clad layer
81
. Here, x≦1 and x>y.
On a part of the clad layer
81
, an n-type GaAs current block layer
91
is formed. On the current block layer
91
and clad layer
81
, a p-type Ga
0.3
Al
0.7
As current diffusion layer
10
is formed. On a part of the current diffusion layer
10
, a p
+
-type GaAs contact layer
11
is formed. On the contact layer
11
, a p-type electrode
13
is formed. On the bottom surface of the substrate
1
, an n-type electrode
12
is formed.
The structure of
FIG. 1
is epitaxially grown according to a low pressure MOCVD (LPMOCVD) technique that employs trimethylindium (TMI), trimethylgallium (TMG), and trimethylaluminum (TMA) as materials of group III, arsine (AsH
3
) and phosphine (PH
3
) as materials of group V, silane (SiH
4
) and dimethylzinc (DMZ) as doping materials, and hydrogen as a carrier gas. These materials epitaxially grow crystals under a low pressure. More precisely, a wafer having an n-type GaAs substrate
1
is placed in a CVD reactor and is kept at a given pressure and temperature. A mass flow controller supplies the group III, V, and doping materials into the reactor at set flow rates, to epitaxially grow layers one after another on the substrate
1
. Namely, an n-type GaAs buffer layer
2
, an n-type In
0.5
Al
0.5
P/GaAs reflection layer
3
, an n-type In
0.5
(Ga
1−x
Al
x
)
0.5
P clad layer
41
, an undoped In
0.5
(Ga
1−y
Al
y
)
0.5
P active layer
61
, a p-type In
0.5
(Ga
1−x
Al
x
)
0.5
P clad layer
81
, and an n-type GaAs current block layer
91
are successively formed on the substrate
1
. Here, x≦1 and x>y.
The wafer with the layers is taken out of the reactor, and the current block layer
91
is selectively etched according to a photolithography technique into the shape of FIG.
1
. The MOCVD method is again employed to form a p-type Ga
0.3
Al
0.7
As current diffusion layer
10
and a p--type GaAs contact layer
11
.
Au-based material is deposited on each surface of the wafer according to a vacuum evaporation technique. The Au-based layer is selectively etched according to the photolithography technique to form a p-type electrode
13
as shown in FIG.
1
. The contact layer
11
is selectively etched to partly expose the current diffusion layer
10
. An n-type electrode
12
is formed over the bottom surface of the substrate
1
. The wafer Is diced into chips that serve each as the semiconductor light emitting device of FIG.
1
. Each chip is mounted on a stem, bonded, sealed with resin, and fabricated into a &phgr; 5-mm lamp.
FIG. 2
is a graph showing the initial brightness I
o
and remnant brightness ratio &eegr; of the &phgr; 5-mm lamp. The remnant brightness ratio is the ratio of the brightness I
1000
of the lamp measured after 1000 hours of operation at 50 mA to the initial brightness I
o
. Namely, &eegr;=(I
1000
/I
o
)×100. Here, the mole fraction “y” of Al of the active layer
61
is 0.3, and the mole fraction “x” of Al of each of the clad layers
41
and
81
is changed among 1.0, 0.9, 0.8, and 0.7.
When the Al mole fraction “x” of In
0.5
(Ga
1−x
Al
x
)
0.5
P of the clad layers
41
and
81
is increased, the initial brightness I
o
increases but the remnant brightness ratio &eegr; decreases. When the Al mole fraction “x” is decreased, the initial brightness I
o
decreases but the remnant brightness ratio &eegr; increases. In this way, the initial brightness I
o
and remnant brightness ratio &eegr; are trade-offs. It is difficult for the conventional DH structure to provide high brightness as well as long service life.
FIG. 3
shows an InGaAlP-based LED having a DH structure according to another prior art. Sequentially laminated on an n
+
type GaAs substrate
1
are an n-type GaAs buffer layer
2
, an n-type In
0.5
Al
0.5
P/GaAs reflective multilayer
3
, an n-type In
0.5
Al
0.5
P clad layer
42
, an n-type In
0.5
(Ga
0.72
Al
0.28
)
0.5
P active layer
62
, a p-type In
0.5
Al
0.5
P clad layer
82
, a p-type In
0.5
Ga
0.5
P contact layer
127
, and a p-type GaAs protection layer
128
. On a part of the protection layer
128
, there are sequentially laminated an n-type In
0.5
(Ga
0.3
Al
0.7
)
0.5
P current block layer
92
, an n-type GaAs protection layer
93
, a p-type Ga
0.3
Al
0.7
As current diffusion layer
10
, a p-type In
0.5
(Ga
0.7
Al
0.3
)
0.5
P diffusion layer
132
, and a p
+
-type GaAs contact layer
11
. The contact layer
11
is formed on a part of the diffusion layer
132
. On the contact layer
11
, a p-type electrode
13
is formed. An n-type electrode
12
is formed on the bottom surface of the substrate
1
.
When designing the clad layers
42
and
82
that sandwich the active layer
62
serving as a light emitting layer, the following opposing factors must be considered:
(a) To confine a sufficient quantity of minority carriers in the active layer
62
, the band gap (Eg) of the clad layers
42
and
82
must be properly larger than that of the active layer
62
. Namely, the Al mole fraction “x” of the clad layers
42
and
82
must be large.
(b) Crystal defects that trap minority carriers must be minimized in each interface between the clad layers
42
and
82
and the active layer
62
. Such defects, however, easily occur in the interfaces where the Al mole fraction of the clad layers
42
and
82
greatly differs from that of the active layer
62
. To reduce the crystal defects, the Al mole fraction “x” of the clad layers
42
and
82
must be small.
When the Al mole fraction of the clad layers
42
and
82
greatly differs from that of the active layer
62
that emits yellow light, initial brightness may be high but there will be many crystal defects in each interface among the layers. As a result, normalized light intensity P/P
o
deteriorates in proportion to an operating time as shown in FIG.
4
.
The Al mole fraction of the clad layers of the conventional DH structure LED must be high to provide high brightness when the LED is used outdoors. This, however, causes many crystal defects to shorten the service life of the LED. If the Al mole fraction of the clad layers is low to extend the service life of the LED, the light output of the LED will be low, and therefore, the brightness thereof will be improper for outdoor use. In this way, the light output and service life of the conventional LED are trade-offs.
The conventional DH structure semiconductor light emitting devices are incapable of simultaneously providing high brightness and long service life.
SUMMARY OF THE INVENTION
An object of the present invention is to provide a semiconductor light emitting device having a DH structure that realizes high brightness as well as long service life.
In order to accomplish the object, the present invention provides a semiconductor light emitting device having any one

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