Light emitting diode

A light emitting diode composed of a compound semiconductor containing aluminum having a GaAlAs layer and the like and having a structure which is superior in humidity resistance and has a long life. For example, in a double hetero light emitting diode constructed by comprising a p-GaAlAs layer, an n-GaAlAs layer, and an active layer interposed between both the GaAlAs layers, an oxide layer containing the oxide of gallium is provided inside of the surface excluding a portion where an electrode is provided of its semiconductor device, so that the surface of the device is stabilized. On the other hand, in the structure of a light emitting diode in which stepped portions are provided in the peripheral part of its semiconductor device, an oxide layer is provided inside of the surface of the device and inside of the peripheral part, and an insulating layer is formed on the oxide layer on the surface of the device.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
The present invention relates generally to a light emitting diode composed 
of a compound semiconductor containing aluminum, and more particularly, to 
a light emitting diode having a structure which is superior in humidity 
resistance and has a long life. 
2. Description of the Prior Art 
In a light emitting diode composed of a compound semiconductor containing 
aluminum having a gallium aluminum arsenic (GaAlAs) layer and the like, an 
insulating layer (coating film) composed of silicon nitride has been 
conventionally formed on the surface of its device so as to increase 
humidity resistance and improve light extraction efficiency, as disclosed 
in, for example, Japanese Patent Laid-Open Gazette No. 226181/1989. Such a 
structure is formed as the measures of the problem peculiar to a compound 
semiconductor containing aluminum that the oxide or the hydroxide of 
aluminum is liable to be mainly formed on the upper surface of the device, 
thereby to form a light absorbing layer as well as to degrade the device 
and decrease the luminance under humidity conditions, so that the life of 
the device is significantly reduced. 
In such a structure, however, the above described insulating layer does not 
sufficiently adhere to the upper surface of the above described device or 
is damaged due to, for example, heat treatment for ohmic contact of 
electrodes and the impact in the subsequent process of assembling a lamp 
and the like. Consequently, such circumstances have often occur that the 
above described oxide or hydroxide of aluminum is expanded, irrespective 
of the presence of the insulating layer, on the surface of the device 
centered around a portion where the insulating layer does not sufficiently 
adhere or is damaged, so that the life of the device is reduced. 
Particularly in a structure in which stepped portions are formed in the 
peripheral part of the above described device by mesa etching, such a 
phenomenon is, in many cases, centered around edge parts of the stepped 
portions. 
SUMMARY OF THE INVENTION 
A primary object of the present invention is to provide a new light 
emitting diode composed of a compound semiconductor containing aluminum in 
which the above described problems are solved. 
Another object of the present invention is to provide a light emitting 
diode composed of a compound semiconductor containing aluminum which is 
superior in humidity resistance and has a long life by comprising an oxide 
layer containing the oxide of gallium inside of the upper surface of its 
semiconductor device. 
The present invention provides a light emitting diode composed of a 
compound semiconductor containing aluminum which is constructed, in the 
structure of no treating mesa etching type semiconductor device having no 
stepped portions in its peripheral part, by having a thermal oxide layer 
containing the oxide of gallium provided inside of at least the upper 
surface excluding a portion where an electrode is provided of the 
semiconductor device. 
Furthermore, the present invention provides a light emitting diode composed 
of a compound semiconductor containing aluminum which is constructed, in 
the structure of a treating mesa etching type semiconductor device having 
stepped portions in its peripheral part, by having a thermal oxide layer 
containing the oxide of gallium provided inside of at least the upper 
surface excluding a portion where an electrode is provided of the 
semiconductor device as well as laminating an insulating layer on the 
oxide layer on the upper surface of the above described device. 
The foregoing and other objects, features, aspects and advantages of the 
present invention will become more apparent from the following detailed 
description of the present invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
First Embodiment 
A light emitting diode shown in FIG. 1 is a double hetero gallium aluminum 
arsenic (GaAlAs) light emitting diode in which the effect of the present 
invention is most largely recognized. 
The light emitting diode comprises, for example, a p-GaAlAs layer 1 serving 
as a first GaAlAs layer, an n-GaAlAs layer 2 serving as a second GaAlAs 
layer, and a p-GaAlAs active layer 3 interposed between both the GaAlAs 
layers 1 and 2. The light emitting diode further comprises stepped 
portions 4 provided in the peripheral part of its semiconductor device, as 
well as an n electrode 5 and a p electrode 6 provided on the upper surface 
and lower surface of the semiconductor device. 
An oxide layer 7 containing the oxide of gallium is, provided centered 
around the exposed parts of the upper surface of the above described 
device and the above described stepped portions 4. In addition, the oxide 
layer 7 is provided on the lower surface of the device. However, even if 
it is not provided on the lower surface of the device, the effect as 
described later is obtained. An insulating layer (coating film) 8 composed 
of silicon nitride of the like is formed on the oxide layer 7 on the upper 
surface of the device. It is preferable that the insulating layer 8 is 
also provided in the stepped portions 4. However, even if it is not 
provided in the stepped portions 4, the effect as described later is 
obtained. Accordingly, when the workability in a case where the insulating 
layer 8 is provided is considered, the insulating layer 8 need not be 
necessarily provided. 
Such a light emitting diode is constructed in, for example, the following 
manner. 
First, a p-GaAlAs layer 1, a p-GaAlAs active layer 3, and an n-GaAlAs layer 
2 are sequentially formed on a GaAs substrate 9 by epitaxial growth, an 
insulating layer 80 composed of silicon nitride or the like is formed on 
the upper surface thereof, as shown in FIG. 2A and then, a part of the 
insulating layer 80 which corresponds to a portion 50 where an electrode 
is to be provided is removed. 
It is preferable for the reasons as described later that a thermal oxide 
layer (a layer equivalent to the oxide layer 7) has been formed on the 
upper surface of the above described n-GaAlAs layer 2 by previously 
subjecting the above described substrate to heat treatment at temperatures 
of 455.degree. to 480.degree. C. before the insulating layer 80 is formed. 
A part of the above described insulating layer 80 which corresponds to the 
above described portion 50 where an electrode is to be provided is removed 
by, for example, CF.sub.x dry etching. At this time, even if the above 
described thermal oxide layer has been formed on the upper surface part of 
the n-GaAlAs layer 2, this thermal oxide layer is removed by the described 
above dry etching, not to interfere with ohmic contact of electrodes. 
An n electrode 5 is formed on the n-GaAlAs layer 2, as shown in FIG. 2B. 
Thereafter, the above described p-GaAs substrate 9 is removed as required, 
so that a p electrode 6 is formed on the lower surface of the above 
described p-GaAlAs layer 1 which is exposed by this removal (see FIG. 2C). 
When the p-GaAs substrate 9 is not removed, the p electrode 6 is formed on 
the lower surface of the p-GaAs substrate 9 (which is not shown). 
The shapes, the numbers, and the arrangements of both the above described 
electrodes 5 and 6 are respectively determined in consideration of the 
current distribution, the direction of light extraction and the like, as 
the conventional example. 
Thereafter, mesa etching is made from the upper surface of the above 
described n-GaAlAs layer 2 to conform to the size of the device, so that 
recess portions corresponding to the above described stepped portions 4 
are formed (see FIG. 2D). Subsequently, an oxide layer 7 is formed on the 
exposed upper surface parts of the stepped portions 4 by annealing 
treatment also serving as ohmic contact of the electrodes, resulting in 
device isolation. In addition, the oxide layer 7 is also formed on the 
exposed part of the lower surface of the device. 
The above described annealing treatment must be carried out to a sufficient 
degree to form the oxide layer 7, for example, at temperatures of 
455.degree. to 490.degree. C. for 5 to 30 minutes. It is not preferable 
that the heating temperature at this time is excessively increased because 
the above described electrodes are adversely affected. 
In such an oxide layer 7, gallium and aluminum are oxidized at a rate of 
approximately 100 per cent, and arsenic is also oxidized at a high rate, 
for example, on the upper surface part of the above described device. In 
such oxide 7, for example, for gallium and aluminum are oxidized at almost 
100% oxidation rate in an upper surface of the above-mentioned device, and 
arsenic is also oxidized at a high oxidation rate. An oxidation rate of 
gallium means a numeric value obtained by dividing the number of molecules 
of gallium oxide by the sum of the number of molecules of gallium and the 
number of molecules of the gallium oxide, and then multiplying a quotient 
by 100. An oxidation rate of aluminum means a numeric value obtained by 
dividing the number of molecules of aluminum oxide by the sum of the 
number of molecules of aluminum and the number of molecules of the 
aluminum oxide, and then multiplying the quotient by 100. An oxidation 
rate of arsenic means a numeric value obtained by dividing the number of 
molecules of arsenic oxide by the sum of the number of molecules of 
arsenic and the number of molecules of the arsenic oxide, and then 
multiplying a quotient by 100. In this specification, an average oxidation 
rate means an arithmetic average value of rates of oxidation of the 
gallium, aluminum and arsenic respectively. 
In respect of this point, even when the device is left in a natural state, 
oxidation is carried out on the surface part of the device. In this case, 
however, aluminum is oxidized at a rate of 10 per cent and gallium is 
slightly oxidized, while arsenic is hardly oxidized. Moreover, such 
natural oxidation is carried out only in the outermost surface part of the 
device. The oxidation of aluminum is only recognized in an area where the 
surface part of the device is removed by 0.001 to 0.009 .mu.m. On the 
other hand, in the present invention, the oxidation rate of arsenic is 
only lowered, while the oxidation rates of aluminum and gallium are hardly 
lowered even in the above described area. 
Consequently, in the present invention, the oxide layer 7 containing the 
oxide of gallium is obtained. The oxide layer 7 can be easily confirmed if 
it is analyzed, for example, in the usual analysis of elements after 
previously cutting the upper surface of the device by sputtering. 
The average oxidation rate within a depth of 0.01 .mu.m from the upper 
surface of the above described device in the present invention is 
approximately 50 to 95% and more preferably, 80 to 95%. The average 
oxidation rate under the same condition in a case where the effect cannot 
be confirmed is not more than 40%. 
Although it becomes clear that the humidity resistance of the above 
described light emitting diode is improved and the degradation of the 
semiconductor device is reduced by the formation of such an oxide film 7, 
the reason for this is not clearly known at the present time. However, 
excessive arsenic in the outermost surface part of the device is scattered 
by the above described heat treatment, and the dispersion of arsenic 
inside of the device is restrained by the above described oxide of 
gallium. In addition, the movement and the deposition of aluminum are 
restrained due to the physical surface coating of the oxide of gallium and 
the stabilization of a pair of aluminum arsenide caused by the formation 
of the oxide of gallium. Consequently, it is presumed that the promotion 
of the oxidation of aluminum is restrained. 
Therefore, it is preferable that the thermal oxide layer is provided even 
in the early stage of the above described fabrication. The reason for this 
is that the surface of the device which is brought into contact with the 
insulating coating film is stabilized by the oxide layer containing the 
oxide of gallium, so that it can be expected that the cause of the 
degradation during the fabrication is removed. 
Second Embodiment 
FIG. 3 shows a double hetero junction and no treating mesa etching type 
light emitting diode. 
The light emitting diode comprises, for example, a p-GaAlAs layer 1a, an 
n-GaAlAs layer 2a, and a p-GaAlAs active layer 3a interposed between both 
the GaAlAs layers 1a and 2a. In addition, the light emitting diode 
comprises an electrode 5a and a p electrode 6a provided on the upper 
surface and lower surface of its semiconductor device. An oxide layer 7a 
containing the oxide of gallium is provided centered around the exposed 
parts of the surface of the above described device and side surfaces 10a 
of the device. In the present embodiment, the oxide layer 7a is also 
formed on the exposed part of the lower surface of the device. An 
insulating layer (coating film) 8a composed of silicon nitride or the like 
is formed on the oxide layer 7a on the upper surface of the device. 
In forming this light emitting diode, the p-GaAlAs layer 1a, the p-GaAlAs 
active layer 3a, and the n-GaAlAs layer 2a are formed on a p-GaAs 
substrate (not shown) by epitaxial growth. Thereafter, the insulating 
layer 8a as well as the n electrode 5a are formed on the upper surface of 
the n-GaAlAs layer 2a. 
The above described p-GaAs substrate is then removed as required, the p 
electrode 6a is formed on the lower surface of the device and then, device 
isolation is achieved. The above described side surfaces 10a of the device 
are subjected to etching of approximately 1 to 10 .mu.m, to remove 
cracking and crystal defects which occur by the above described device 
isolation. Subsequently, the oxide layer 7a is formed in the exposed parts 
of the device by annealing treatment also serving as ohmic contact of the 
electrodes. 
Meanwhile, in the light emitting diode having a p-GaAs substrate according 
to the present embodiment, light emitted from a PN junction surface is 
partly absorbed into this p-GaAs substrate, thereby to reduce light 
extraction efficiency to the exterior. 
Third Embodiment 
FIG. 4 shows a single hetero and treating mesa etching type light emitting 
diode. 
The light emitting diode comprises a p-GaAlAs layer 12 and an n-GaAlAs 
layer 13 on a p-GaAs substrate 11. In addition, the light emitting diode 
comprises an n electrode 15 and a p electrode 16 provided on the upper 
surface and lower surface of its semiconductor device. An oxide layer 18 
containing the oxide of gallium is provided centered around the exposed 
parts of the surface of the above described device and stepped portions 
17. An insulating layer (coating film) 14 composed of silicon nitride is 
formed on the oxide layer 18 on the upper surface of the device. 
In forming this light emitting diode, the p-GaAlAs layer 12 and the 
n-GaAlAs layer 13 are formed on the p-GaAs substrate 11 by epitaxial 
growth and then, the insulating layer 14 is formed on the upper surface of 
the n-GaAlAs layer 13. Then, the insulating layer 14 in a portion where 
the n electrode 15 is to be provided is removed and then, the n electrode 
15 is formed in this portion. 
The above described p electrode 16 is formed on the lower surface of the 
above described device and then, mesa-etching is made from the n-GaAlAs 
layer 13 to the p-GaAlAs layer 12, to form the above described stepped 
portions 17. Subsequently, the above described oxide layer 18 is formed in 
the exposed parts by annealing treatment also serving as ohmic contact of 
the electrodes and then, device isolation is achieved. 
Meanwhile, the single hetero light emitting diode according to the present 
embodiment is so constructed that the respective mixture ratios of 
aluminum in the p-GaAlAs layer 12 and the n-GaAlAs layer 13 differ from 
each other. 
Fourth Embodiment 
FIG. 5 shows a single hetero and no treating mesa etching type light 
emitting diode. 
The light emitting diode comprises, for example, a p-GaAlAs layer 12a and 
an n-GaAlAs layer 13a. In addition, the light emitting diode comprises an 
n electrode 15a and a p electrode 16a provided on the upper surface and 
lower surface of its semiconductor deice. An oxide layer 18a containing 
the oxide of gallium is provided centered around the exposed parts of the 
surface of the above described device and side surfaces 19a of the device. 
An insulating layer (coating film) 14a composed of silicon nitride or the 
like is formed on the oxide layer 18a on the upper surface of the device. 
In forming this light emitting diode, the p-GaAlAs layer 12a and the 
n-GaAlAs layer 13 are formed on a p-GaAs substrate 11a by epitaxial growth 
and then, the insulating layer 14a is formed on the upper surface of the 
n-GaAlAs layer 13a. Subsequently, the insulating layer 14a in a portion 
where the n electrode 15a is to be provided is removed and then, the n 
electrode 15a is formed in this portion. 
The p electrode 16a is formed on the lower surface of the above described 
device and then, device isolation is achieved. The above described side 
surfaces 19a of the device are then subjected to etching of approximately 
1 to 10 .mu.m, to remove cracking and crystal defects which occur by the 
above described device isolation. Subsequently, the oxide layer 18a is 
formed in the exposed parts by annealing treatment also serving as ohmic 
contact of the electrodes. 
Meanwhile, when the above described etching is made, the p-GaAlAs layer 12a 
and the n-GaAlAs layer 13a are etched easier than the p-GaAs substrate 
11a, so that the side surfaces 19a of the device are inclined as shown in 
FIG. 5. 
Fifth Embodiment 
FIG. 6 shows a homo PN junction and treating mesa etching type light 
emitting diode. 
The light emitting diode comprises an n-GaAlAs layer 21 and a p-GaAlAs 
layer 22 on an n-GaAs substrate 20. In addition, the light emitting diode 
comprises a p electrode 24 and an n electrode 25 provided on the upper 
surface and lower surface of its semiconductor device. An oxide layer 27 
containing the oxide of gallium is provided centered around the exposed 
parts of the upper surface of the above described device and stepped 
portions 26. In the present embodiment, the oxide layer 27 is also formed 
in the exposed part of the lower surface of the device. An insulating 
layer (coating film) 23 composed of silicon nitride or the like is formed 
on the oxide layer 27 on the upper surface of the device. 
In forming this light emitting diode, the n-GaAlAs layer 21 and the 
p-GaAlAs layer 22 are formed on the n-GaAs substrate 20 by epitaxial 
growth and then, the insulating layer 23 is formed on the upper surface of 
the p-GaAlAs layer 22. Then, the insulating layer 23 in a portion where 
the p electrode 24 is to be provided is removed and then, the p electrode 
24 is formed is this portion. 
After the n electrode 25 is formed on the lower surface of the above 
described device and then, mesa-etching is made from the p-GaAlAs layer 22 
to the n-GaAlAs layer 21, thereby to form the above described stepped 
portions 26. Subsequently, the above described oxide layer 27 is formed in 
the exposed parts by annealing treatment also serving as ohmic contact of 
the electrodes and then, device isolation is achieved. 
Meanwhile, the homo PN junction light emitting diode according to the 
present embodiment is so constructed that the respective mixture ratios of 
aluminum in the n-GaAlAs layer 21 and the p-GaAlAs layer 22 are 
approximately the same. 
Furthermore, the n electrode 25 is formed partly in a stripe shape, as 
shown in FIG. 6, thereby to make the area of the electrode smaller than 
those in the embodiments 3 and 4. Accordingly, the amount of absorption of 
light emitted from a PN junction surface into the n electrode 25 is 
decreased, thereby to increase light extraction efficiency to the 
exterior. 
Sixth Embodiment 
FIG. 7 shows a homo PN junction and no treating mesa etching type light 
emitting diode. 
The light emitting diode comprises, for example, an n-GaAlAs layer 21a and 
a p-GaAlAs layer 22a on an n-GaAs substrate 20a. In addition, the light 
emitting diode comprises a p electrode 24a and an n electrode 25a provided 
on the upper surface and lower surface of its semiconductor device. An 
oxide layer 27a containing the oxide of gallium is provided centered 
around the exposed parts of the surface of the above described device and 
side surfaces 28a of the device. In the present embodiment, the oxide 
layer 27a is also formed in the exposed part of the lower surface of the 
device. An insulating layer (coating film) 23a composed of silicon nitride 
or the like is formed on the oxide layer 27a on the upper surface of the 
device. 
In forming this light emitting diode, the n-GaAlAs layer 21a and the 
p-GaAlAs layer 22a are formed on the n-GaAs substrate 20a by epitaxial 
growth and then, the insulating layer 23a is formed on the upper surface 
of the p-GaAlAs layer 22a. Then, the insulating layer 23a in a portion 
where the electrode 24a is to be provided is removed and then, the 
electrode 24a is formed in this portion. 
The n electrode 25a is formed on the lower surface of the above described 
device and then, device isolation is achieved. Subsequently, the above 
described side surfaces 28a of the device are subjected to etching of 
approximately 1 to 10 .mu.m, to remove cracking and crystal detects which 
occur by the above described device isolation. Subsequently, the oxide 
layer 27a is formed in the exposed parts by annealing treatment also 
serving as ohmic contact of the electrodes. 
Although in the above described embodiments 3 to 6, the GaAs substrates 11, 
11a, 20 and 20a are formed, the GaAs substrates may be removed as 
required. In this case, the electrodes 16, 16a, 25 and 25a are formed on 
the lower surfaces of the exposed p or n-GaAlAs layers 12, 12a, 21 and 
21a. 
Description is now made of the function and the effect of each of the above 
described embodiments with reference to a characteristic diagram of FIG. 
8. 
The characteristic diagram of FIG. 8 shows the results of continuous 
energization of each of the light emitting diodes constructed a described 
above. It has been said that the light emitting diode containing aluminum 
is particularly inferior in humidity resistance. Accordingly, this 
characteristic diagram shows characteristics with time in a case where the 
luminous intensity in the early stage of a period during which a current 
of 30 mA is continuously caused to flow in a laboratory at a temperature 
of 80.degree. C. and at a humidity of 96% is set to 100. 
In this characteristic diagram, a characteristic A indicated by a solid 
line shows the average luminous intensity of lots of the light emitting 
diodes having the thermal oxide layers 7, 7a, 18, 18a, 27 and 27a and the 
insulating layers 8, 8a, 14, 14a, 23 and 23a, a characteristic B indicated 
by a broken line shows characteristics of light emitting diodes having the 
above described thermal oxide layers whose average oxidation rate is 75% 
and having no insulating layers, and a characterstic C indicated by a 
dotted line shows characteristics of prior light emitting diodes having no 
the above described thermal oxide layers. In the light emitting diodes 
having no stepped portions 4, 17 and 26, an intermediate characteristic 
between the above described characteristics A and B is obtained even if 
they have no insulating layers. 
In the above described respective embodiments, the oxide layers 7, 7a, 18, 
18a, 27 and 27a containing the oxide of gallium are respectively provided 
inside of the upper surface and lower surface excluding portions where the 
electrodes 5 and 6, 5a and 6a, 15, 15a, 24 and 25, and 24a and 25a are 
provided and the side surfaces. If the oxide layers 7, 7a, 18, 18a, 27 and 
27a are respectively provided inside of at least the upper surfaces 
excluding the portions where the electrodes 5, 5a, 15, 15a, 24 and 24a are 
provided, however, the humidity resistance of each of the light emitting 
diodes are improved and the degradation of the semiconductor device can be 
restrained. The reason for this is that the upper surface is protected by 
providing the oxide layer inside of the upper surface because the upper 
surface is liable to be inferior in humidity resistance to the lower 
surface and the side surfaces. 
The function and the effect of the present invention are summarized as 
follows. 
(1) The thermal oxide layer is provided between the insulating layer and 
the upper surface of the semiconductor device as in the present invention, 
so that the upper surface of the semiconductor device is stabilized. As a 
result, the adhesion of the insulating layer and the upper surface of the 
semiconductor device is improved, so that the insulating layer is not 
damaged. Consequently, the humidity resistance of the light emitting diode 
is improved and the degradation of the semiconductor device is delayed. In 
addition, the average oxidation rate of the above described thermal oxide 
layer and particularly, the oxidation rate of gallium under the upper 
surface of the semiconductor device is raised, thereby to lengthen the 
life of the semiconductor device. 
(2) Particularly, the humidity resistance of the above described treating 
mesa etching type light emitting diode having stepped portions is very 
good, as shown by the characteristic A. More specifically, in this 
treating mesa etching type light emitting diode, the above described 
insulating layer is provided on the upper surface of the semiconductor 
device also in the sense that the edge parts of the stepped portions are 
protected, so that the humidity resistance of the light emitting diode is 
further improved and the degradation of the semiconductor device is 
reduced. 
(3) Furthermore, the humidity resistance of the above described no treating 
mesa etching type light emitting diode having no stepped portions is 
intermediate between the characteristics A and B, that is, is slightly 
inferior. However, the thermal oxide layer is provided inside of the side 
surfaces of the semiconductor device, so that the humidity resistance of 
the light emitting diode is good. 
(4) Additionally, the above described treating mesa etching type light 
emitting diode is in a mesa shape. Accordingly, no cracking and crystal 
defects occur at the time of device isolation. 
Although the present invention has been described and illustrated in 
detail, it is clearly understood that the same is by way of illustration 
and example only and is not to be taken by way of limitation, the spirit 
and scope of the present invention being limited only by the terms of the 
appended claims.