Active solid-state devices (e.g. – transistors – solid-state diode – Thin active physical layer which is – Heterojunction
Reexamination Certificate
1999-08-24
2001-07-24
Tran, Minh Loan (Department: 2826)
Active solid-state devices (e.g., transistors, solid-state diode
Thin active physical layer which is
Heterojunction
C257S087000, C257S094000, C257S101000, C257S102000, C257S103000
Reexamination Certificate
active
06265726
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a light-emitting semiconductor device that emits blue light and uses a group III nitrogen compound.
2. Description of the Prior Art
It has been known that an aluminum gallium indium nitride (AlGaInN) compound semiconductor may be used to obtain a light-emitting diode (LED) which emits blue light. This semiconductor device is useful because of its high luminous efficiency resulting from direct electron transition and because of its ability to emit blue light, which is one of the three primary colors.
Irradiating an electron beam into an i-layer to which magnesium (Mg) is doped and heat treatment is carried out enables the i-layer to have a p-type layer of the AlGaInN semiconductor device. As a result, a LED with a double hetero p-n junction structure includes an aluminum gallium nitride (AlGaN) p-layer, a zinc (Zn) doped indium gallium nitride (InGaN) emission layer and an AlGaN n-layer, becomes useful instead of a conventional LED of metal insulator semiconductor (MIS) structure which includes an n-layer and a semi-insulating i-layer.
The conventional LED with a double hetero p-n junction structure is doped with Zn as an emission center. Luminous intensity of this type of LED has been improved fairly. Still, there exists a problem in luminous efficiency and further improvement is necessary.
The emission mechanism of a LED with an emission layer doped with only Zn, or only an acceptor impurity, as the emission center is electron transition between conduction band and acceptor energy levels. However, a large difference of their energy levels makes recombination of electrons through deep levels dominant which deep level recombination does not contribute to emission. This results in lower luminous intensity. Further, the wavelength of light from the conventional LED is about 380 to 440 nm, or shorter than that of pure blue light.
Further, the emission layer doped with Zn as the emission center exhibits semi-insulative characteristics. Its emission mechanism is explained by recombination of an electron through acceptor level injected from an n-layer and a hole injected from a p-layer. However, the diffusion length of the hole is shorter than that of the electron. It results in high ratio of holes disappearing in a non-emission process before recombination of the hole and electron occurs in the emission layer. This phenomenon impedes higher luminous intensity.
SUMMARY OF THE INVENTION
It is an object of the present invention to solve the above problem and improve the luminous intensity of the LED of AlGaInN semiconductor, or obtain enough spectrum to emit a purer blue light.
According to the first aspect of the invention, there is provided a light-emitting semiconductor device comprising:
an n-layer with n-type conduction of group III nitride compound semiconductor satisfying the formula Al
x3
Ga
y3
In
1−x3−y3
N, inclusive of x3=0, y3=0 and x3=y3=0,
a p-layer with p-type conduction of group III nitride compound semiconductor satisfying the formula Al
x1
Ga
y1
In
1−x1−y1
N, inclusive of x1=0, y1=0 and x1=y1=0,
an emission layer of group III nitride compound semiconductor satisfying the formula Al
x2
Ga
y2
In
1−x2−y2
N, inclusive of x2=0, y2=0 and x2=y2=0;
the junction layer of the n-layer, the p-layer, and the emission layer being any one of a homo-junction structure, a single hetero-junction structure, and a double hetero-junction structure; and
wherein the emission layer is formed between the n-layer and the p-layer, and doped with both a donor and an acceptor impurity.
It is preferable that the donor impurity is one of the group IV elements and that the acceptor impurity is one of the group II elements.
Preferable combinations of a donor and an acceptor impurity include silicon (Si) and cadmium (Cd), silicon (Si) and zinc (Zn), and silicon (Si) and magnesium (Mg), respectively.
The emission layer can be controlled to exhibit any one of n-type conduction, semi-insulative, and p-type conduction depending on the concentration ratio of a donor impurity and an acceptor impurity doped thereto.
Further, the donor impurity can be one of the group VI elements.
Further, it is desirable to design the composition ratio of Al, Ga, and In in the n-layer, p-layer, and emission layer to meet each of the lattice constants of the three layers to an n
+
-layer of high carrier concentration on which the three layers are formed.
Further, a double hetero-structure sandwiching of the emission layer of p-type conduction by the n-layer and p-layer improves luminous efficiency. Making the concentration of acceptor impurity larger than that of the donor impurity and processing by electron irradiation or heat treatment changes the emission layer to exhibit p-type conduction. Magnesium, an acceptor impurity, is especially efficient for obtaining p-type conduction.
Further, doping any combinations of the described acceptor and donor impurity to an emission layer of p-type conduction also improves luminous efficiency. The luminous mechanism doped with acceptor and donor impurities is due to recombination of an electron at donor level and a hole at the acceptor level. This recombination occurs within the emission layer, so that luminous intensity is improved.
Further, a double hetero-junction structure of a triple-layer sandwiching the emission layer having a narrower bad gap by the n-layer and p-layer having a wider band gap improves luminous intensity. Since the emission layer and the p-layer exhibit p-type conduction, valence bands of those layers are successive even without applying external voltage. Consequently, holes readily highly exist within the emission layer. In contrast, conduction bands of the n-layer and the emission layer are not successive without applying an external voltage. Applying a voltage enables the conduction bands to be successive and electrons to be injected deeper into the emission layer. Consequently, the number of injected electrons into the emission layer increases ensuring recombination with holes and a consequent improvement in luminous intensity.
Other objects, features, and characteristics of the present invention will become apparent upon consideration of the following description in the appended claims with reference to the accompanying drawings, all of which form a part of the specification, and wherein referenced numerals designate corresponding parts in the various figures.
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Nakamura et al., “P-GaN/N-InGaN/N-GaN Double-Heterostructure Blue-Light-Emitting Diodes”, Japanese J of Applied Physics, Jan. 1993, vol. 32, No. 1A/B, Part 2, pp. L8-L11.
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Asai Makoto
Kato Hisaki
Koike Masayoshi
Manabe Katsuhide
Sassa Michinari
Pillsbury & Winthrop
Toyoda Gosei Co,., Ltd.
Tran Minh Loan
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