Active solid-state devices (e.g. – transistors – solid-state diode – Incoherent light emitter structure – Active layer of indirect band gap semiconductor
Reexamination Certificate
1998-12-24
2001-04-17
Tran, Minh Loan (Department: 2826)
Active solid-state devices (e.g., transistors, solid-state diode
Incoherent light emitter structure
Active layer of indirect band gap semiconductor
C257S087000, C257S190000, C257S191000
Reexamination Certificate
active
06218681
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates to a gallium arsenide phosphide epitaxial wafer and a light emitting diode (hereinafter referred as “LED”) comprising the epitaxial wafer.
BACKGROUND OF THE INVENTION
LED devices made of semiconductor crystal are widely used as display elements. These LED devices are mostly made of II-V compound semiconductors. III-V compound Semiconductors have a band gap corresponding to the wavelength of visible light and infrared light and thus have been applied to light emitting elements. Among these III-V compound semiconductors, GaAsP has been in great demand as an LED. Since the most important characteristics of GaAsP LED devices are light emitting power and life, improvements in these characteristics have been desired.
GaAs
1-x
P
x
(0.45<x<1) is doped with nitrogen (N) as an isoelectronic trap to thereby enhance light emitting efficiency. This LED device exhibits a light output enhanced about 10 times. In general, this LED device can be stably obtained by a process which comprises vapor phase epitaxial growth of N-type layers using a quartz reactor, and then diffusion of zinc from the surface of a light emitting layer to form a P-type layer therein, thereby forming a PN-junction.
FIG. 4
illustrates a general structure of a GaAsP epitaxial wafer.
In
FIG. 4
, the GaAsP epitaxial wafer is made of a GaP single crystal substrate
1
, on which a homo-layer
2
having the same composition as that of the substrate
1
, a GaAs
1-x
P
x
graded composition layer
3
having a composition continuously changing from 1.0 to x
0
to relax the lattice mismatch between the substrate and the uppermost layer, a GaAs
1-x0
P
x0
constant composition layer
4
, and a GaAs
1-x0
P
x0
N-doped layer
5
doped with nitrogen are formed in this order. The uppermost layer of the epitaxial wafer is a light-emitting layer having a constant composition x
0
, which is arranged to obtain the desired emission wavelength of the LED. This light emitting layer has been doped with nitrogen and tellurium (Te) or sulfur (S) in such an amount that a predetermined carrier concentration is realized. Usually, the composition x
0
is about 0.65 for emission of red light (wavelength: 630 nm). Nitrogen acts as an isoelectronic trap which serves as a light emitting center. However, nitrogen is electrically inactive and thus does not contribute to the carrier concentration.
In order to put this LED material into practical use as an LED device, it is necessary that the electrical characteristics of this LED material be improved. JP-A-53-64488 (The term “JP-A” used herein means an “unexamined published Japanese patent application”) discloses a structure having a carrier concentration gradient in which the carrier concentration is from 1 to 20×10
16
only in the portion that serves as a light emitting layer, and but not less than 10×10
16
in the other epitaxial layers. The above-cited patent describes that the thickness of the GaAsP epitaxial layer in its attached drawings is 100 &mgr;m at one end of the wafer and 230 &mgr;m at the other.
It is also known that the carrier concentration should be from 3.5 to 8.8×10
15
cm
−3
to minimize the destruction of the completeness in crystallinity of the light emitting layer during the formation of the PN-junction by the diffusion of zinc from in the surface of the light emitting layer. This prolongs the life of carriers thus injected, resulting in an LED device having a high light output (JP-A-55-9467). It is further known that when the carrier concentration of the light emitting layer is further reduced to not more than 3×10
15
cm
−3
, the enhancement of light output and the prolongation of LED life can be realized at the same time as shown in
FIGS. 5 and 6
. It is further known that the layers other than the light emitting layer are droped with Te or S to a carrier concentration as high as from 0.5 to 5×10
17
cm
−3
, to thereby reduce the electrical resistance of the LED (JP-A-6-196756).
SUMMARY OF THE INVENTION
In recent years, as the applications of LEDs have diversified, LEDs having an extremely high reliability have come to be in great demand.
Past studies show that the optimization of the carrier concentration profile of the epitaxial layer makes it possible to prolong LED life. However, the further prolongation of LED life is limited if only the optimization of the carrier concentration of the epitaxial wafer is effected.
In other words, a structure having two ranges of carrier concentration, as described in JP-A-53-64488, is effective for the improvement of the electrical characteristics of an LED. However, the carrier concentration in the low carrier concentration layer which serves as a light-emitting layer, is from 1 to 20×10
16
cm
−3
. this structure cannot realize the prolongation of life and the enhancement of brightness to an extent which has been desired so far as can be seen in JP-A-6-196756 and JP-A-55-9467. Further, as described in JP-A-6-196756, a structure having two ranges of carrier concentration, wherein the carrier concentration in the higher carrier concentration layer is as high as from 0.5 to 5×10
17
cm
−3
, provides some improvement in life and light output. However, this improvement falls short of the desired level.
The inventors made extensive studies in the development of a solution to the foregoing problems. As a result, it was found that the optimization of the carrier concentration in specific layers and the restruction of the thickness of othe entire GaAs
1-x
P
x
(0<x<1) epitaxial layer to not less than 80 &mgr;m had a good effect on the internal stress applied to the PN-junction, making it possible to drastically prolong the life of the light emitting diode. Thus, the present invention has been developed.
An essence of the present invention lies in an epitaxial wafer having compound semiconductor epitaxial layers provided on a substrate a total thickness of the portion of the epitaxial layers that comprises Ga, As and P as constituent elements being not less than 80 &mgr;m and in the epitaxial layer a low carrier concentration region with a carrier concentration of from 0.5 to 9×10
15
cm
−3
doped with nitrogen being formed. Another essence of the present invention is a light emitting diode produced from such an epitaxial wafer.
In the epitaxial wafer according to the present invention, the compound semiconductor eptiaxial layer comprising Ga, As and P as constituent elements preferably has a thickness of not more than 200 &mgr;m.
The foregoing low carrier concentration region preferably has a thickness of not less than 2 &mgr;m.
The low carrier concentration region preferably has a thickness of not more than 80 &mgr;m.
The compound semiconductor epitaxial layer comprising Ga, As and P as constituent elements preferably has an in-(wafer surface-)plane thickness difference of not more than 120 &mgr;m.
The compound semiconductor epitaxial layer comprising Ga, As and P as constituent elements preferably has an average carrier concentration of from 0.5 to 5×10
17
cm
−3
in the region other than the low carrier concentration region.
The compound semiconductor epitaxial layer comprising Ga, As and P as constituent elements preferably is a GaAs
1-x
P
x
(0<x<1) epitaxial layer wherein the GaAs
1-x
P
x
epitaxial layer preferably has a composition x from greater than 0.45 to less than 1.
The compound semiconductor epitaxial layer comprising Ga, As and P as constituent elements preferably comprises a graded composition layer and a constant composition layer in which the low carrier concentration region lies, wherein the GaP composition x in the graded composition layer is preferably from greater than 0.45 to less than 1, and the GaP composition x
0
in the constant composition layer is preferably from greater than 0.45 to 0.95 or less.
The foregoing substrate is preferably a single crystal, more preferably GaAs single crystal or GaP single crystal.
A homo-layer, which is an epitaxial layer
Mitsubishi Chemical Corporation
Oblon & Spivak, McClelland, Maier & Neustadt P.C.
Tran Minh Loan
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