Active solid-state devices (e.g. – transistors – solid-state diode – Thin active physical layer which is – Heterojunction
Reexamination Certificate
2001-09-10
2003-09-09
Baumeister, Bradley W. (Department: 2815)
Active solid-state devices (e.g., transistors, solid-state diode
Thin active physical layer which is
Heterojunction
C257S013000, C257S101000, C257S102000, C257S103000, C257S196000
Reexamination Certificate
active
06617606
ABSTRACT:
BACKGROUND OF THE INVENTION
The present invention relates to light-emitting semiconductor elements and, more specifically, to light-emitting semiconductor elements such as light-emitting diodes effectively usable for plastic fiber optic communications according to IEEE1394 and USB-2 standards.
Among group III-V compound semiconductor materials except for nitrides, industry attention has been focused on AlGaInP compound materials as of light-emitting semiconductor elements which possess the largest direct-transition band gap and a large emission output with wavelengths of 0.5 to 0.6 &mgr;m. Especially, p-n junction type light-emitting diodes (LED) each having a light-emitting portion (laminated structure including an active layer) formed by lattice matching crystal growth on a substrate GaAs are capable of emitting light having wavelengths corresponding to that of red and green at a higher response speed and greater output in comparison with conventional LEDs made of GaP and AlGaAs materials. Hence, they can be effectively used as light sources for plastic fiber optic transmission and communications systems.
FIG. 1
is a sectional view of a first conventional light-emitting diode having a light-emitting portion of AlGaInP material.
The structure of the first conventional LED of AlGaInP is described bellow.
As shown in
FIG. 1
, the first conventional LED comprises an n-type GaAs substrate with subsequently formed thereon layers, i.e., an n-type GaAs buffer layer
2
, an light-reflecting layer
3
(a DBR layer of carrier concentration of 5×10
17
cm
−3
) composed of an n-type (Al
x
Ga
1−x
) As (x=0.45) and an n-type AlAs, a lower clad layer
4
of n-type (Al
x
Ga
1−x
)
0.51
In
0.49
P (0≦x≦1) (e.g., x=1.0, width 1.0 &mgr;m and carrier concentration 5×10
17
cm
−3
), an active layer
5
of p-type (Al
x
Ga
1−x
)
0.51
In
0.49
P (0≦x≦1) (e.g.,x=0.0, width 0.1 &mgr;m and carrier concentration 5×10
17
cm
−3
), an upper clad layer
6
of p-type (Al
x
Ga
1−x
)
0.51
In
0.49
P)(0≦x≦1) (e.g., x=1.0, width 1.0 &mgr;m and carrier concentration 5×10
17
cm
−3
), an intermediate layer
7
of p-type (Al
x
Ga
1−x
)
y
In
1−y
P (x=0.2, y=0.4, thickness 0.15 &mgr;m and carrier concentration 1×10
8
cm
−3
), a current spreading layer
9
of p-type (Al
x
Ga
1−x
)
y
In
1−y
P (x=0.05, y=0.95, thickness 1.5 &mgr;m and carrier concentration 5×10
18
cm
−3
), and a current blocking layer
8
of n-type GaP (thickness 0.3 &mgr;m and carrier concentration 1×10
18
cm
−3
).
Then, the n-type GaP current blocking layer
8
is selectively etched by photolithography to form a current path of 50 to 150 &mgr;m&phgr; in diameter therein, and the current spreading layer
9
of p-type (Al
x
Ga
1−x
)
y
In
1−y
P (x=0.05, y=0.05, thickness 7 &mgr;m and carrier concentration 5×10
18
cm
−3
) is grown again to complete the element structure.
On the p-type current spreading layer
9
, a film of, e.g., Au—Be is deposited and then circularly patterned to form a p-type electrode
10
and a light-emission area.
On the other hand, an n-type electrode
11
of, e.g., Au—Ge film is formed by deposition on the bottom surface of the GaAs substrate
1
.
Thus, the first conventional light-emitting diode is completed.
The mixed crystal ratio “1−y” of indium (In) in the lower clad layer
4
of n-type Al
y
In
1−y
P, the active layer
5
of p-type Ga
y
In
1−y
P and the upper clad layer
6
of p-type Al
y
In
1−y
P is set to 0.5 so as to match the crystal lattice of the GaAs substrate
1
.
The p-type AlGaInP current spreading layer
9
in a (Al
x
Ga
1−x
)
y
In
1−y
P semiconductor is transparent to a light-emission range of wavelengths of 550 nm to 650 nm and a low resistance. To easily make an ohmic contact with the p-side electrode, the mixed crystal ratio of Al and the mixed crystal ratio of In are set to lower values, usually x=0.05 and y=0.95 respectively.
Generally, in LEDs of AlGaInP, Si is used as an n-type dopant and Zn is used as a p-type dopant.
The (Al
x
Ga
1−x
)
y
In
1−y
active layer
5
is usually of the p-type. Since the element is forced to emit light of 650 nm to 670 nm that is hardly absorbed in a plastic optical fiber, the mixed crystal ratio of Al in the active layer is selected within the range of 0.03 to 0.00. Since the thinner active layer may confine injected carriers to a smaller area thereof to increase the density of current therein and the higher density of carriers may shorten the lifetime of carrier recombination, the thickness of the active layer is set to 0.1 &mgr;m or less and the density of carriers therein is set to 7×10
17
cm
−3
or more. However, LEDs of AlGaInP involve such a problem that decreasing of the thickness of the active layer and/or increasing of doping concentration is accompanied in practice by decreasing of initial radiant output light power and intensive variation (increase or decrease) of output light power during the operation.
Recently, there has been proposed a LED having a quantum well structure of an active layer
5
for increasing radiant output light power and having an increased current density for improving the response characteristic.
FIG. 2
is a sectional view of a second conventional light-emitting diode.
Referring to
FIG. 2
, the quantum well structure of the second conventional light-emitting diode is described below.
The LED shown in
FIG. 2
differs from the first LED of
FIG. 1
by merely its active layer portion that, in replace of p-type (Al
x
Ga
1−x
)
0.51
In
0.49
P (0≦x≦1) active layer
5
, comprises an undoped (Al
x
Ga
1−x
)
0.51
In
0.49
P (0≦x≦1) lower barrier layer
12
(e.g., x=0.3, width 500 Å and carrier concentration 1×10
16
cm
−3
), an undoped (Al
x
Ga
1−x
)
0.51
In
0.49
P (0≦x≦1) upper barrier layer
14
(e.g., x=0.3, width 500 Å and carrier concentration 1×10
16
cm
−3
) and 4-10 undoped (Al
x
Ga
1−x
)
0.51
In
0.49
P (0≦x≦1) quantum-well layers
13
(e.g., x=0.0, width 80 Å and carrier concentration 1×10
16
cm
−3
) sandwiched between the two barrier layers.
The quantum well structure of the active layer is described below with reference to
FIG. 3
depicting energy bands therein.
4 quantum-well layers
13
each having 80 Å in thickness is sandwiched between the lower barrier layer
12
and the upper barrier layer
14
.
The LED having the quantum well type active layer may have a small overflow of injection carriers and attain a higher light output than that of the conventional LED of FIG.
1
. However, this LED cannot attain high-speed response characteristic because of a large number of wells, decreasing the density of injected carriers and elongating the lifetime of carrier recombination. In other words, the multiple quantum-well structure of the LED is advantageous to attain a high light output but disadvantageous to improve the response characteristic. In contrast to the first conventional LED, the second conventional LED cannot possess a high-speed response characteristic.
As described above, the LED having a usual bulk type active layer may have high-speed response characteristic by reducing the thickness of the active layer and increasing the carrier concentration but cannot attain a sufficient initial radiant output light power and a stable light emission during the operation.
On the contrary, the LED having the multi-quantum-well structure may attain a sufficient initial light output and a stable light emission during energizing period but may not attain a high-speed response characteristic.
SUMMARY OF THE INVENTION
An object of the present invention is to provide a light-emitting diode that has an excellent high-speed response characteristic as well as a large output light power and a very small variation
Kurahashi Takahisa
Murakami Tetsurou
Nakatsu Hiroshi
Ohyama Shouichi
Baumeister Bradley W.
Nixon & Vanderhye P.C.
Sharp Kabushiki Kaisha
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