Semiconductor device manufacturing: process – Making device or circuit emissive of nonelectrical signal
Reexamination Certificate
2002-06-03
2003-04-01
Nelms, David (Department: 2818)
Semiconductor device manufacturing: process
Making device or circuit emissive of nonelectrical signal
C438S933000
Reexamination Certificate
active
06541293
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The invention relates to a semiconductor light-emitting device and a method of manufacturing said semiconductor light-emitting device. The semiconductor light-emitting device can be utilized as, e.g., a light-emitting diode and a laser diode.
2. Description of the Related Art
Light-emitting devices using compound semiconductors are known as those covering visible to short wavelength regions. Among others, group III nitride semiconductors have attracted attention in recent years, not only because these semiconductors are of direct transition type, so that they exhibit high light-emitting-efficiency, but also because these semiconductors emit blue light, which is one of the three primary colors.
One example of such light-emitting device is formed by laminating an AlN buffer layer, a first clad layer, a light-emitting layer, and a second clad layer sequentially on a sapphire substrate. Here, the first and the second clad layers are made of Al
X
In
Y
Ga
1−X−Y
N (including X=0, X=Y, X=Y=0). The light-emitting layer has a superlattice structure formed by laminating a barrier layer made of In
Y1
Ga
1−Y1
N (Y
1
≧0) and a quantum well layer made of In
Y2
Ga
1−Y2
N (Y
2
≧Y
1
and Y
2
>0) repetitively.
These semiconductor layers are formed in accordance with an ordinary technique based on a metal organic vapor phase epitaxial growth method (hereinafter referred to as the “MOVPE method”).
The thus superlattice-structured light-emitting layer, requiring steepness in difference of composition between the barrier layers and the quantum well layers, is formed at relatively low growth temperatures. Further, the respective barrier layers are generally formed to have the same thickness, and similarly the respective quantum well layers are formed to have the same thickness. This is because there is a danger that the wavelengths of light emitted from the respective quantum well layers will be slightly varied by the quantum effect if thicknesses differ between layers.
On the other hand, the second. clad layer that is formed on the light-emitting layer is formed at higher temperatures than the light-emitting layer in order to meet thickness and composition requirements (the second clad layer is thicker than the barrier layers and the quantum well layers).
The inventors have found that the following problems have been addressed in manufacturing the semiconductor light-emitting device.
In the superlattice-structured light-emitting layer, if layers adjacent to the respective clad layers are quantum well layers, the following problems are encountered. When a clad layer is of the p-type and a quantum well layer is contiguous to such clad layer, the depth of the well of such quantum well layer differs from those of the other quantum well layers because the clad layer has a different energy level from a barrier layer. Therefore, there is a danger that the wavelengths of light will be shifted. Further, if a clad layer is of the n-type and a quantum well layer is contiguous to such clad layer, the well is hard to form in such quantum well layer because the energy level of the clad layer is lower than that of the quantum well layer. As a result, emission of light cannot be expected.
SUMMARY OF THE INVENTION
To overcome the aforementioned problems, a first aspect of the invention is applied to a semiconductor light-emitting device that includes:
a first semiconductor layer that is made of n-type GaN;
a light-emitting layer of superlattice structure that is formed on the first semiconductor layer by laminating a barrier layer being made of In
Y1
Ga
1−Y1
N (Y
1
≧0) and a quantum well layer being made of In
Y2
Ga
1−Y2
N (Y
2
>Y
1
and Y
2
>0); and
a second semiconductor layer that is made of p-type Al
X
Ga
1−X
N (0.05<X<0.2), and
in such a semiconductor light-emitting device, layers that are adjacent to clad layers are the barrier layers in the light-emitting layer. That is, the light-emitting layer is designed to have a barrier layer—a quantum well layer—a quantum well layer—a barrier layer.
It may be noted that the first semiconductor layer and the second semiconductor layer correspond to clad layers or optical guide layers in the following description. It may further be noted that impurities introduced due to the background at the time of growing the semiconductor layers such as the barrier layers and the quantum well layers are not intentional impurities.
However, when the inventors have examined again, the following problems have further been found out.
When the second clad layer is formed on the light-emitting layer of superlattice structure, the barrier layer that comes on top of the light-emitting layer (hereinafter referred to as the “uppermost barrier layer”) becomes thin. The reason therefor is assumed to be as follows. Since the second clad layer is formed at higher temperatures than the uppermost barrier layer, the material of which the uppermost barrier layer is formed is blown away from the upper surface of the second clad layer at the time of forming the second clad layer.
It is not desirable that the uppermost barrier layer become thin, because the wavelengths of light are shifted toward the short wavelength side by the quantum effect.
Further, if the barrier layer is designed to be thin (e.g., to a thickness of some nanometers), there is a danger that no uppermost barrier layer substantially exists.
To overcome this problem, a second aspect of the invention is applied to a method of manufacturing a semiconductor light-emitting device that includes the steps of:
forming a first semiconductor layer that is made of Al
X
In
Y
Ga
1−X−Y
N (including X=0, Y=0, X=Y=0);
forming a light-emitting layer of superlattice structure that is formed on the first semiconductor layer by laminating a barrier layer being made of In
Y1
Ga
1−Y1
N (Y
1
≧0) and a quantum well layer being made of In
Y2
Ga
1−Y2
N (Y
2
>Y
1
and Y
2
>0); and
forming a second semiconductor layer that is made of Al
A
In
B
Ga
1−A−B
N (including A=0, B=0, A=B=0) on the light-emitting layer, wherein
an uppermost barrier layer, which is an uppermost layer of the light-emitting layer, is formed thicker than the other barrier layers.
Further, a third aspect of the invention is applied to a method of manufacturing a semiconductor light-emitting device according to the second aspect of the invention, which is characterized in that at the time of forming the second semiconductor layer, an upper surface of the uppermost barrier layer is caused to disappear and the uppermost barrier layer is made to have substantially the same thickness as the other barrier layers.
Further, an object of the invention is to provide a semiconductor light-emitting device in which the peak wavelengths of emitted light do not vary even if an applied current is varied.
Another object of the invention is to provide a semiconductor light-emitting device having a narrow wavelength distribution, i.e., a semiconductor light-emitting device that emits light that is close to ideal monochromatic light.
Still another object of the invention is to provide a semiconductor light-emitting device that has high light-emitting efficiency and an active layer of superlattice structure exhibiting strong emission of light.
In the semiconductor light-emitting device according to the first aspect of the invention, layers that are adjacent to the first semiconductor layer and the second semiconductor layer are barrier layers in the light-emitting layer. Therefore, the shape of a quantum well, i.e., the potential wells in the quantum well layers closest to the respective semiconductor layers are substantially the same as those of the other quantum well layers. As a result, the wavelengths of light emitted from the respective quantum well layers become substantially the same.
Further, a crystal of the barrier layer made of In
Y1
Ga
1−Y1
N in the lig
Asami Shinya
Koide Norikatsu
Koike Masayoshi
Nagai Seiji
Umezaki Junichi
Dang Phuc T.
McGinn & Gibb PLLC
Nelms David
Toyoda Gosei Co,., Ltd.
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