Active solid-state devices (e.g. – transistors – solid-state diode – Incoherent light emitter structure – With particular semiconductor material
Reexamination Certificate
2000-06-09
2001-08-21
Meier, Stephen D. (Department: 2822)
Active solid-state devices (e.g., transistors, solid-state diode
Incoherent light emitter structure
With particular semiconductor material
C257S099000, C257S096000
Reexamination Certificate
active
06278139
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates to a semiconductor light emitting diode fabricated on an n-type GaAs substrate for emitting visible light and, more particularly, to a high-intensity semiconductor visible light emitting diode having a double hetero-junction structure made of AlGaInP based materials.
BACKGROUND
With remarkable developments of information processing technologies, semiconductor light emitting devices have increasingly large demands as light sources for optical communications, optical recording media, and image displays. In particular, visible light emitting diodes (hereinafter, a light emitting diode will be abbreviated to LED) of higher intensities are strongly required for development thereof as effective light sources for image displays.
AlGaInP based mixed-crystal semiconductor lattice matched with a GaAs substrate, as it is capable of forming excellent hetero-junctions and suited to obtain high luminance LEDs, is widely used in recent years as a material for forming high luminance LEDs which can display visible colors from red to green.
To obtain high luminance LEDs, it is necessary to generate light from a prescribed limited region thereof. Basically, conventional AlGaInP based LEDs, despite of their various systems for guiding the light from the prescribed limited region, were classified roughly into only two types based on the shapes of contact layers thereof.
One example is an LED having a contact layer of the shape as shown in FIG.
5
. FIG.
5
(
a
) is a schematic sectional view showing the layered structure of a conventional AlGaInP based LED, and FIG.
5
(
b
) is a diagram showing the emission pattern of the conventional LED shown in FIG.
5
(
a
).
The conventional LED
50
shown in
FIG. 5
includes, on an n-type GaAs substrate
52
, a double hetero-junction structure composed of an n-AlGaInP lower cladding layer
54
, an active layer
56
, and a p-AlGaInP upper cladding layer
58
which are consecutively layered. The LED
50
also has: a p-GaAs contact layer
60
formed on the double hetero-junction structure so as to have the same diameter as an upper electrode, and to be concentrically thereto; the upper electrode
64
formed on the contact layer
60
and having a circular central opening
62
as a light emitting region; and an n-side electrode
66
formed on the bottom surface of the GaAs substrate
52
.
The other example is an LED
70
having a contact layer of the shape as shown in FIG.
6
. FIG.
6
(
a
) is a schematic sectional view showing the layered structure of the another conventional AlGaInP based LED, and FIG.
6
(
b
) is a diagram showing the emission pattern of the conventional LED shown in FIG.
6
(
a
).
The conventional LED
70
shown in
FIG. 6
has a structure similar to that of the LED
50
shown in
FIG. 5
, except that a current diffusion layer
72
made of p-AlGaAs and a p-GaAs contact layer
74
having a shape similar to that of the upper electrode
64
are formed on the double heterojunction structure.
In the LED
50
of
FIG. 5
having the circular light emitting region
62
, the contact layer
60
made of GaAs is usually formed in order to provide lattice matching with the AlGaInP based double heterojunction structure being in lattice match with GaAs, followed by etching thereof to form a circular contact layer, and by providing thereon the ring-shaped upper electrode
64
having the circular opening as the light emitting region, to thereby obtain the circular emission pattern.
GaAs, however, has a high absorbance with respect to the emission wavelength; therefore, in the LED
50
, the contact layer has a large absorbance for radiated light, resulting in a large factor of decreasing the radiation from the LED.
Meanwhile, the LED
70
shown in FIG.
6
and having the contact layer of a ring shape similar to that of the upper electrode is a conventional example which avoids the aforementioned decrease in brightness resulting from the absorption by the GaAs contact layer. Here, the high-absorptive GaAs contact layer
74
is formed at the limited area directly below the ring-shaped upper electrode
64
; besides, the current diffusion layer
72
is arranged below the contact layer
74
for sufficient diffusion of the injected current.
This, while reducing the absorption of emitted light in the contact layer to obtain emission of higher intensities, leads to the drawbacks in that: the injected current diffuses beyond the upper electrode to result in a poor emission efficiency; and, as shown in FIG.
6
(
b
), a doughnut-shaped emission pattern occurs having a blank at the center, which is not acceptable as a circular emission pattern.
Thus, as an LED for preventing the absorption in the GaAs contact layer and having an emission pattern of approximately circular shape, there is a proposal for an LED
80
having the configuration as shown in
FIG. 7
in which the current diffusion layer outside the electrode is etched for removal.
FIG. 7
is a schematic sectional view showing the layered structure of yet another conventional LED. This LED
80
has a configuration similar to that of the LED
70
shown in
FIG. 6
, except that a p-type current diffusion layer
82
made of AlGaAs is formed as a circular layer which has the same outer diameter as those of the upper electrode
64
and the contact layer
74
and is concentric thereto.
While the LED
80
shown in
FIG. 7
, due to suppression of the light emission outside the upper electrode, is improved in emission efficiency as compared with the LED shown in
FIG. 6
, it has a drawback in that its emission pattern tends to take a doughnut shape and is difficult to form in an acceptable circular shape. Besides, a step is needed for deeply etching the current diffusion layer, which yielded the drawbacks of increasing the steps in fabricating the elements and of the complicated processes.
As described above, the conventional AlGaInP based LEDs have various problems, and it is difficult, by relatively simple processes, to fabricate an LED for emitting light in a desired emission pattern and at higher intensities.
SUMMARY OF THE INVENTION
An object of the present invention is to provide a semiconductor light emitting diode which emits light in a desired emission pattern and at higher intensities, and has a configuration capable of being fabricated by relatively simple processes.
The present inventors conceived that: since AlGaAs or AlGaAsP having an Al content of 0.5 or more has a larger bandgap as compared with GaAs, a contact layer formed from AlGaAs or AlGaAsP makes a contact layer approximately transparent to the light emitted by an AlGaInP based active layer to allow a large reduction of the light absorption in the contact layer; and the contact layer is doped with impurities of the same conductivity type at a higher concentration to elevate the carrier concentration so that the current diffusion in the contact layer is enhanced to eliminate the need for the current diffusion layer provided in the conventional art. The present inventors had repeated experiments along the concept for the accomplishment of the present invention.
In order to achieve the foregoing objects, on the basis of the knowledge thus obtained, the present invention provides a semiconductor light emitting diode formed on an n-type GaAs substrate and includes an AlGaInP based double heterojunction structure having an active layer sandwiched between cladding layers, and a p-side electrode having an opening and arranged on the double heterojunction structure with an intervention of a p-type contact layer, characterized in that:
the p-type contact layer is formed as a semiconductor layer made of AlGaAs or AlGaAsP having an Al content of 0.5 or more and doped with p-type impurities at a carrier concentration of 5×10
18
cm
−3
or more; and
light is emitted through the p-type contact layer and the opening in the p-side electrode.
According to the present invention, since the upper p-type contact layer of the semiconductor light emitting diode formed on the n-type GaAs substrate is formed as
Arakawa Satoshi
Ishikawa Takuya
Kasukawa Akihiko
Mukaihara Toshikazu
Helfgott & Karas P.C.
Meier Stephen D.
The Furukawa Electric Co. Ltd.
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