Active solid-state devices (e.g. – transistors – solid-state diode – Incoherent light emitter structure
Reexamination Certificate
2000-06-22
2004-10-12
Pham, Long (Department: 2814)
Active solid-state devices (e.g., transistors, solid-state diode
Incoherent light emitter structure
C257S082000, C257S085000, C257S086000, C257S087000, C257S095000, C257S098000, C257S103000
Reexamination Certificate
active
06803603
ABSTRACT:
BACKGROUND OF THE INVENTION
The present invention relates to a semiconductor light-emitting element.
In recent years, semiconductor light-emitting elements are widely used in an outdoor display, automobile indicator, and the like. The semiconductor light-emitting element is a device using emission recombination of electrons and holes injected in a p-n junction region. Emission ranging from infrared radiation to ultraviolet radiation can be realized by changing the semiconductor material of a light-emitting layer.
FIG. 30
shows the structure of a conventional semiconductor light-emitting element. An n-type GaAs buffer layer
3202
, an n-type DBR (Distributed Bragg Reflector) reflective layer
3203
made of InGaAlP and GaAs to reflect light using the Bragg reflection effect, an n-type InGaAlP cladding layer
3204
, an active layer
3205
, a p-type InGaAlP cladding layer
3206
, a p-type AlGaAs window layer
3207
, and a p-type GaAs contact layer
3208
are sequentially formed on the upper surface of an n-type GaAs substrate
3201
.
An n-type electrode
3209
is formed on the lower surface of the n-type GaAs substrate
3201
, and a p-type electrode
3210
is formed on the p-type GaAs contact layer
3208
. Power is supplied to the light-emitting element to emit light from the active layer
3205
. Light emitted downward in
FIG. 30
by the active layer
3205
is reflected by the reflective layer
3203
, and radiated to above the element via the window layer
3207
together with the light emitted upward.
The conventional semiconductor light-emitting element suffers the following problem.
Part of light that is emitted downward by the active layer
3205
and travels straight toward the reflective layer
3203
is reflected by the reflective layer
3203
without being absorbed by the substrate
3201
, and can be effectively extracted outside.
However, the reflective layer
3203
exhibits a very low reflectivity with respect to light traveling diagonally toward the reflective layer
3203
, so not all the light from the active layer
3205
can be extracted outside.
The semiconductor light-emitting element absorbs light by a substrate which provides a critical angle defined by the difference in refractive index between the semiconductor crystal and the atmosphere or enables crystal growth. For this reason, light which can be extracted outside is only several % of internally emitted light.
FIG. 26
shows the structure of another semiconductor light-emitting element relating to the present invention.
A multilayered reflective film
1001
, p-type contact layer
1002
, p-type cladding layer
1003
, active layer
1004
functioning as a light-emitting layer, n-type cladding layer
1005
, and n-type contact layer
1006
are formed on a p-type semiconductor substrate
1000
. An n-type electrode
1007
is formed on the contact layer
1002
, whereas a p-type electrode
1008
is formed on the contact layer
1006
.
Part of light emitted by the active layer
1004
that travels toward the n-type cladding layer
1005
is extracted outside via the cladding layer
1005
.
Light that travels toward the p-type cladding layer
1003
is reflected by the multilayered reflective film
1001
, and extracted outside via the n-type cladding layer
1005
.
In this structure, light emitted toward the substrate
1000
can be reflected by the reflective film
1001
, and extracted outside.
However, the reflectivity of light which is not vertically incident on the reflective film
1001
is low, the electrodes
1007
and
1008
which shield light exist on the light extraction surface, and the active layer
1004
is formed on the reflective film
1001
. This results in low crystallinity and short service life.
FIG. 27
shows still another semiconductor light-emitting element relating to the present invention. An n-type InGaP buffer layer
1102
, n-type InAlP cladding layer
1103
, InGaAlP active layer
1104
functioning as a light-emitting layer, p-type InAlP cladding layer
1105
, and p-type GaAs contact layer
1106
are formed on the upper surface of an n-type GaP substrate
1101
. A p-type electrode
1107
is formed on the p-type GaAs contact layer
1106
, while an n-type electrode
1100
is formed on the lower surface of the substrate
1101
.
Light emitted by the InGaAlP active layer
1104
is reflected by the n- and p-type electrodes
1100
and
1107
, and extracted outside from a region of the contact layer
1106
which is not shielded by the p-type electrode
1107
.
In this structure, however, light concentrated immediately below the electrode
1107
is shielded by the electrode
1107
, and cannot be extracted outside.
In the element shown in
FIG. 27
, only several % of light emitted by the active layer
1104
can be extracted outside owing to the difference in refractive index between the crystal and the air.
As the semiconductor light-emitting element, a compound semiconductor light-emitting element using a GaAs-based semiconductor material is adopted to emit light ranging from red to green, and a gallium nitride-based compound semiconductor light-emitting element using Al(x)Ga(y)In(1−x−y)N (0≦x, y≦1, x+y≦1) is adopted to emit light from the ultraviolet range to the blue/green range.
However, the refractive indices of these light-emitting elements are high (GaN=2.67, GaAs=3.62), their critical angles are small (GaN=21.9°, GaAs=16.0°), and thus their light extraction efficiencies are low.
The GaAs system exhibits large light absorption on the substrate. Emitted light is absorbed by the substrate to decrease the light extraction efficiency.
FIG. 29
shows still another semiconductor light-emitting element relating to the present invention.
An n-type GaAs buffer layer
1301
, n-type InGaAlP cladding layer
1302
, InGaAlP active layer
1303
, p-type InGaAlP cladding layer
1304
, and p-type AlGaAs current diffusion layer
1305
are sequentially grown on the upper surface of an n-type GaAs substrate
1300
. A p-side electrode pad
1307
is formed on the p-type AlGaAs current diffusion layer
1305
, whereas an n-side electrode
1306
is formed on the lower surface of the n-type GaAs substrate
1300
.
In this structure, a current flowing from the p-side electrode
1307
is widened by the p-type. AlGaAs current diffusion layer
1305
, and injected from the p-type InGaAlP cladding layer
1304
to the InGaAlP active layer
1303
. The light is extracted outside the element via the p-type AlGaAs current diffusion layer
1305
.
In the GaAs-based compound semiconductor light-emitting element having this structure, part of light emitted by the active layer
1303
that travels toward the substrate
1300
is absorbed by the substrate
1300
, and cannot be extracted outside the element. More specifically, 50% of the emitted light cannot be extracted, which is fatal to high luminance.
As described above, the elements relating to the present invention suffer low light extraction efficiency.
SUMMARY OF THE INVENTION
The present invention has been made in consideration of the above situation, and has as its object to provide a semiconductor light-emitting element capable of efficiently extracting light emitted by a light-emitting layer outside the element.
According to the present invention, there is provided a semiconductor light-emitting element comprising a substrate, a reflective layer which is formed on the substrate, contains a metal, and reflects light, a light-emitting layer formed on the reflective layer to emit light, and a transparent electrode formed on the light-emitting layer to transmit light.
The light-emitting layer desirably has a double-heterostructure in which an active layer is sandwiched between first and second cladding layers.
The semiconductor light-emitting element can further comprise an electrode of one conductivity type between a surface of the substrate and the reflective layer, a contact layer of the one conductivity type between the reflective layer and the light-emitting layer, and a contact layer of an opposite conductivity type between the light-em
Furukawa Chisato
Nitta Koichi
Okazaki Haruhiko
Watanabe Yukio
Gray Cary Ware & Freidenrich LLP
Louie Wai-Sing
Pham Long
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