Active solid-state devices (e.g. – transistors – solid-state diode – Incoherent light emitter structure – With heterojunction
Reexamination Certificate
2001-01-18
2003-09-16
Whitehead, Jr., Carl (Department: 2813)
Active solid-state devices (e.g., transistors, solid-state diode
Incoherent light emitter structure
With heterojunction
C257S103000
Reexamination Certificate
active
06621106
ABSTRACT:
BACKGROUND OF THE INVENTION
The present invention relates to a light emitting diode (LED) having a double hetero-structure and more particularly to an LED that has a high optical power and can be used at a large current.
A highly efficient LED having a double hetero-structure as shown in
FIG. 21
is known.
FIG. 21
is a vertical sectional view showing an AlGaInP LED in which layers are lattice-matched with a GaAs substrate
1
. The structure of each layer in the LED is as follows:
Substrate
1
:
made of an n-type GaAs
Buffer layer
2
:
made of n-type GaAs
N-type cladding layer
3
made of n-type (Ga
0.3
Al
0.7
)
0.5
In
0.5
P
impurity: Si, impurity concentration: 1×10
18
cm
−3
, and
thickness: 1 &mgr;m
Light-emitting layer
4
:
made of p-type (Ga
0.7
Al
0.3
)
0.5
In
0.5
P
thickness: 0.5 &mgr;m
P-type cladding layer
5
:
made of p-type Al
0.5
In
0.5
P
impurity: Zn, impurity concentration: 5×10
17
cm
−3
, and
thickness: 1 &mgr;m
First current diffusion layer
7
:
made of p-type Ga
0.3
Al
0.7
As
impurity: Zn, impurity concentration: 1×10
18
cm
−3
, and
thickness: 1 &mgr;m
Second current diffusion layer
8
:
made of p-type Ga
0.3
Al
0.7
As
impurity: Zn, impurity concentration: 3×10
18
cm
−3
, and
thickness: 6 &mgr;m
Contact layer
9
:
made of p-type GaAs
An n-side electrode
10
is formed on the underside of the n-type GaAs substrate
1
. A p-side electrode
11
is formed on the p-type GaAs contact layer
9
.
The n-type GaAs buffer layer
2
is intended to eliminate defects of the n-type GaAs substrate
1
and influence of contaminants in the substrate and is not required if the n-type GaAs substrate
1
is surface-treated favorably. The p-type GaAs contact layer
9
has a GaAs structure not containing Al to facilitate an ohmic contact between the p-type GaAs contact layer
9
and the p-side electrode
11
. The GaAs composing the contact layer
9
does not transmit light generated from the p-type (Ga
0.7
Al
0.3
)
0.5
In
0.5
P light-emitting layer
4
, but no problem is raised because the contact layer is formed immediately below the p-side electrode
11
.
Sharp K. K. has recently proposed an LED, a vertical sectional view of which is shown in
FIG. 22
, to achieve higher reliability than the above LED (Japanese Patent Application No. 10-338656). The structure of each layer in the LED is as follows:
Substrate
21
:
made of n-type GaAs
Buffer layer
22
:
made of n-type GaAs
N-type cladding layer
23
:
made of n-type (Ga
0.3
Al
0.7
)
0.5
In
0.5
P
impurity: Si, impurity concentration: 1×10
18
cm
−3
, and
thickness: 1 &mgr;m
Light-emitting layer
24
:
made of p-type (Ga
0.7
Al
0.3
)
0.5
In
0.5
P
thickness: 0.5 &mgr;m
First p-type cladding layer
26
:
p-type (Ga
0.5
Al
0.5
)
0.5
In
0.5
P
impurity: Zn, impurity concentration: 1×10
17
cm
−3
, and
thickness: 0.2 &mgr;m
Second p-type cladding layer
27
:
made of p-type Al
0.5
In
0.5
P
impurity: Zn, impurity concentration: 5×10
17
cm
−3
, and
thickness: 1.0 &mgr;m
Current diffusion layer
28
:
made of p-type Ga
0.9
In
0.1
P
impurity: Zn, impurity concentration: 1×10
18
cm
−3
, and
thickness: 7 &mgr;m
Contact layer
29
:
made of p-type Gals
An n-side electrode
30
is formed on the underside of the n-type GaAs substrate
21
. A p-side electrode
31
is formed on the p-type GaAs contact layer
29
.
A p-type cladding layer
25
is formed as a two-layer structure consisting of the p-type (Ga
0.5
Al
0.5
),
0.5
In
0.5
P first cladding layer
26
and the p-type Al
0.5
In
0.5
P second cladding layer
27
. Accordingly, it is possible to prevent a p-type impurity from diffusing to the p-type (Ga
0.7
Al
0.3
)
0.5
In
0.5
P light-emitting layer
24
although the p-type impurity has a large impurity gradient and is liable to diffuse when electric current flows through the LED for a long time. Thus it is possible to prevent deterioration of the optical power of the LED.
The LED is used in the form of a chip. Conventionally, an LED wafer is divided into chips of a size of 200 &mgr;m-300 &mgr;m by 200 &mgr;m-300 &mgr;m. In the above LEDs, the p-type GaAs contact layers
9
,
29
and the p-side electrodes
11
,
31
are formed circular and disposed at the center of each chip.
FIG. 23
shows a planar configuration of the chip.
The above LEDs have the following problem: Electric current flows immediately below the p-side electrodes
11
,
31
. Both the p-side electrodes
11
,
31
and the p-type Gabs contact layers
9
,
29
disposed under the p-side electrodes
11
,
31
are opaque. Thus, the p-side electrodes
11
,
31
and the contact layers
9
,
29
intercept light coming from parts of the p-type (Ga
0.7
Al
0.3
)
0.5
In
0.5
P light-emitting layers coming from parts of the p-type (Ga
0.7
Al
0.3
)
0.5
In
0.5
P light-emitting layers
4
,
24
that are located immediately below the p-side electrodes
11
,
31
. Thus, the light coming from those parts cannot be taken out to the outside. Accordingly, the above LEDs have a low light-emitting efficiency.
The LED chips are conventionally used at electric current having an intensity of several milliamperes to 50 mA. If the LED chip is used for an electric current having intensity higher than that, the optical power of the LED chip will saturate and characteristics will deteriorate with the passage of a current.
SUMMARY OF THE INVENTION
Therefore, it is an object of the present invention to provide an LED in which light emission immediately below an electrode is restricted to improve light take-out efficiency so that the LED has an improved light-emitting characteristic when it is used at a large current of several milliamperes to 50 mA or more.
In order to accomplish the above object, there is provided, according to an aspect of the invention, a light emitting diode of a double hetero-junction type comprising:
a light-emitting layer composed of a GaAlInP material;
a p-type cladding layer and an n-type cladding layer sandwiching the light-emitting layer therebetween;
a p-side electrode formed on the p-type cladding layer side; and
an n-side electrode formed on the n-type cladding layer side;
the p-type cladding layer consisting of a first p-type cladding layer positioned closer to the light-emitting layer and having a lower aluminum content and a lower impurity concentration, and a second p-type cladding layer positioned farther from the light-emitting layer and having a higher aluminum content and a higher impurity concentration; and
a current blocking layer for locally blocking electric current flowing from the p-side electrode to the n-side electrode.
The current blocking layer may be provided immediately below the p-side electrode which is opaque. With this arrangement, electric current flowing to those parts of the light-emitting layer that are positioned immediately below the p-side electrode is restricted. By thus suppressing emission of unrequired light which would be intercepted by the opaque p-side electrode, it is possible to enhance the light take-out efficiency and thus improve the optical power. That is, the external light emission efficiency can be enhanced.
If the thickness of the first p-type cladding layer is within a range of 0.2 &mgr;m to 0.5 &mgr;m inclusive, an initial luminous intensity ratio of 100% can be obtained. Thus, it is possible to increase reliability of the LED.
In one embodiment, the p-side electrode has an electrode window consisting of an opening, and the current blocking layer has an opening at a position confronting the electrode window of the p-side electrode, and the opening of the current blocking layer serves as a current path for intensively passing electric current from the p-side electrode through the light emitting diode.
According to the structure, the current density is increased and thus the internal light-emitting efficiency is also increased. There is a fear that the increase of the current density will reduce the optical power if electric current is passed through the LED for a long time. But such reduction of the optical power can be suppressed because the p-t
Hosoba Hiroyuki
Kurahashi Takahisa
Murakami Tetsuroh
Nakatsu Hiroshi
Jr. Carl Whitehead
Morrison & Foerster / LLP
Sharp Kabushiki Kaisha
Vesperman William C.
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