Light emitting device for achieving high luminous efficiency and high saturation level of light output

A light emitting device has an indium gallium arsenide phosphide luminescent layer between a first clad layer of n-type indium phosphide and a second clad layer of p-type indium phosphide, and a strained barrier layer of p-type indium aluminum arsenide is inserted between the luminescent layer and the second clad layer so as to increase the potential barrier therebetween, thereby improving the luminous efficiency and the saturation point of the light output.

FIELD OF THE INVENTION 
This invention relates to a semiconductor light emitting device and, more 
particularly, to a light emitting device fabricated from compound 
semiconductor films. 
DESCRIPTION OF THE RELATED ART 
A semiconductor light emitting devices is an indispensable component of an 
optical communication system, and has found a wide variety of application 
therein. In fact, the semiconductor light emitting device is used not only 
in the main network but also in subscriber's loops, a local area network 
and a data link network. User expects the optical communication system to 
be improved in performance, and research and development efforts have been 
made on a high-speed light emitting device. 
One of the high-speed light emitting devices applicable as a light source 
in a low-loss fiber optics communication system has a double 
hetero-junction structure fabricated from In.sub.x Ga.sub.1-x As.sub.y 
P.sub.1-y / InP films, and is operative in wavelength of the order of I 
micron. However, a problem is encountered in the p-type InP/In.sub.x 
Ga.sub.1-x As.sub.y P.sub.1-y / InP double hetero-junction light emitting 
device operative in the 1 micron wavelength in that the luminous 
efficiency is lower than a GaA1As/ GaAs light emitting device operative in 
the 0.8 micron wavelength as described in "Optical Communication Device 
Engineering". The author of the book further teaches that the p-type InP/ 
In.sub.x Ga.sub.1-x As.sub.y P.sub.1-y /InP double hetero-junction light 
emitting device is saturated while the light output is relatively low. 
The reason for the low luminous efficiency as well as the low saturation 
output is recombination between holes and electrons injected into the 
luminescent layer through a leak process and/ or the Auger process without 
participation in production of light. The band offset ratio is defined as 
dEc/dEv where dec is the discontinuity between the conduction bands and 
dEv is the discontinuity between the valence bands. In the In.sub.x 
Ga.sub.1-x As.sub.y P.sub.1-y system, the band offset ratio is 0.22/0.38, 
and such a small band offset ratio allows electrons injected into the 
InGaAs luminescent layer to exceed the hetero-junction and flow into the 
p-type InP clad layer. Especially, the InGaAs luminescent layer can 
produce high-kinetic energy electrons through the Auger recombination 
process. However, such a narrow potential barrier hardly confines the high 
kinetic energy electrons in the luminescent layer, and allows the high 
kinetic energy electrons to flow into the p-type InP clad layer without 
producing light. 
SUMMARY OF THE INVENTION 
It is therefore an important object of the present invention provide a 
light emitting device which is large in luminous efficiency and is hardly 
saturated in the light output. 
To accomplish the object, the present invention proposes to insert a 
strained barrier layer between a luminescent layer and a second clad layer 
for increasing potential barrier therebetween. 
In accordance with the present invention, there is provided a light 
emitting device fabricated on a substrate formed of indium phosphide of a 
first conductivity type, comprising: a) a first clad layer formed on the 
substrate and of indium phosphide of the first conductivity type; b) a 
luminescent layer formed on the first clad layer, and having a luminescent 
film formed of compound semiconductor selected from the group consisting 
of indium gallium arsenic phosphide and indium gallium arsenide; c) a 
strained barrier layer formed on the luminescent layer and of indium 
aluminum arsenide of a second conductivity type expressed by the molecular 
formula of In.sub.1-x Al.sub.x As where x ranges between 0.48 and 1.00, 
the second conductivity type being opposite to the first conductivity 
type; and d) a second clad layer formed on the strained barrier layer and 
of indium phosphide of the second conductivity type.

DESCRIPTION OF THE PREFERRED EMBODIMENT 
Referring first to FIG. 1 of the drawings, a light emitting device 
embodying the present invention is fabricated on a substrate I of n-type 
indium phosphide. On the substrate I are epitaxially grown through a vapor 
phase exitaxy or a molecular beam epitaxy an n-type indium phosphide film 
2, a p-type indium gallium arsenic phosphide film 3, a p-type indium 
aluminum arsenide film 4, a p-type indium phosphide film 5 and a p-type 
indium gallium arsenic phosphide film 6 which respectively serve as a 
first clad layer, a luminescent layer, a strained barrier layer, a second 
clad layer and a contact layer. In the following description, the first 
clad layer, the luminescent layer, the strained barrier layer, the second 
clad layer and the contact layer are also accompanied with the same 
references as the corresponding compound semiconductor films, 
respectively. The n-type indium phosphide film 2 is about 1 micron in 
thickness, and the carrier concentration thereof is about 
2.times.10.sup.17 cm.sup.-3. The p-type indium gallium arsenic phosphide 
film 3 is lattice matched with the n-type indium phosphide film 2, and the 
energy band gap created therein allows the light emitting device to 
produce light of 1.3 micron wavelength. The p-type indium gallium arsenic 
phosphide film 3 is of the order of 0.5 micron, and the carrier 
concentration thereof is about 2.times.10.sup.17 cm.sup.-3. The p-type 
indium aluminum arsenide film 4 is as thin as about 0.02 micron, and the 
carrier concentration thereof is 5.times.10.sup.17 cm.sup.-3. The p-type 
indium aluminum arsenide film 4 is expressed by the molecular formula of 
In.sub.1-x Al.sub.x As where x is 0.54. However, x can range from 0.48 to 
1.0. 
If x is 0.48, a wide potential discontinuity takes place between the 
conduction bands as shown in FIG. 2, and the band offset ratio dEc/dEv is 
0.50/0.22. In FIG. 2, dots in the conduction bands are indicative of 
electrons, and bubbles in the valence bands stand for holes. If x is 
increased from 0.48 to 1.00, the band offset ratio dEc/dEv is increased 
together with x, because the strain is enlarged. Thus, the strained 
barrier layer 4 increases the hetero-junction barrier between the 
conduction band of the luminescent layer 3 and the conduction band of the 
second clad layer 5, and electrons injected into the luminescent layer 3 
hardly flow into the second clad layer 5 over the potential discontinuity 
dec. The wide potential discontinuity dec is especially effective against 
high kinetic energy electrons produced through the Auger process, and the 
high kinetic energy electrons confined in the luminescent layer 3 are 
effectively recombined with holes for producing light without flowing-out 
into the second clad layer 5. Thus, the electrons confined in the 
luminescent layer 3 are effectively recombined with the holes, and the 
luminous efficiency is improved. Moreover, the light output is not 
saturated over wide operation range. 
Turning back to FIG. 1, the p-type indium phosphide film 5 is about 1 
micron in thickness, and the carrier concentration thereof is about 
5.times.10.sup.17 cm.sup.-3. The contact layer 6 is overlain by a 
dielectric film 7 which is patterned so as to form a window 7a. Cadmium or 
zinc is introduced through the window 7a into the contact layer 6, and 
heavily doped o-type contact region 6a is formed in the contact layer 6. A 
p-electrode 8 is held in contact with the heavily doped contact region 6a, 
and is formed of gold-zinc alloy. The back surface of the substrate 1 is 
grinded to about 100 microns, and an anti-reflecting film 9 and an 
n-electrode 10 are formed on the back surface of the substrate 1. 
As will be appreciated from the foregoing description, the light emitting 
device according to the present invention confines electrons in the 
luminescent layer 3 by virtue of the wide potential discontinuity dec, and 
the high kinetic energy electrons thus confined in the luminescent layer 3 
are effectively recombined with holes for producing light. This results in 
high luminescent efficiency as well as in high saturation output. 
In the description on the embodiment, the light emitting device is exactly 
defined with the thicknesses of the compound semiconductor films and the 
carrier concentrations thereof. However, these dimensions and the 
concentrations merely illustrate the present invention by example, and 
never set limit to the present invention. Similarly, there is no 
limitation to the substances of the electrodes 8 and 10 as well as the 
substance of the dielectric films 7 and 9. 
Although particular embodiments of the present invention have been shown 
and described, it will be obvious to those skilled in the art that various 
changes and modifications may be made without departing from the spirit 
and scope of the present invention. For example, a luminescent layer may 
be formed in a quantum well structure.