Active solid-state devices (e.g. – transistors – solid-state diode – Incoherent light emitter structure – With particular dopant concentration or concentration profile
Reexamination Certificate
1999-08-04
2001-07-10
Tran, Minh Loan (Department: 2826)
Active solid-state devices (e.g., transistors, solid-state diode
Incoherent light emitter structure
With particular dopant concentration or concentration profile
C257S102000, C257S103000
Reexamination Certificate
active
06259122
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
This invention relates to a Group III nitride semiconductor light-emitting device, and more particularly to a method of forming single crystal films for use in the light-emitting device.
2. Description of the Related Art
In the manufacturing of a light-emitting device using semiconductors, such as a light-emitting diode (LED) and a laser diode (LD), semiconductor layers with varied forbidden band width (hereinafter simply referred to as “band gap” or “E
g
”) are formed one upon another to form a basic structure of the device. In a Group III nitride semiconductor light-emitting device according to the present invention, x and y values of (Al
x
Ga
1−x
)
1−y
In
y
N (0≦x1, 0≦y≦1) are varied to thereby change values of the band gap.
FIG. 1
shows an example of the basic structure of a laser diode device using Group III nitride semiconductors, in which a GaN or AIN layer
2
is formed on a single crystal sapphire substrate
1
at a low temperature, and then on the layer
2
, an n-type GaN layer
3
, an n-type Al
0.1
Ga
0.9
N layer
4
, an n-type GaN layer
5
, an active layer
6
having InGaN as a major constituent thereof, a p-type GaN layer
7
, a p-type Al
0.1
Ga
0.9
N layer
8
, and a p-type GaN layer
9
are formed one upon another in the mentioned order. An n-type electrode
101
and a p-type electrode
102
are formed on the n-type GaN layer
3
and the p-type GaN layer
9
, respectively.
In this laser diode device constructed as above, light is emitted by electron-hole recombination in the active layer
6
. The n-type GaN layer
5
and the p-type GaN layer
7
are guide layers, within which light generated in the active layer
6
is waveguided. Further, it is possible to confine electrons and holes within the active layer
6
effectively by setting the band gap of each of the guide layers to be larger than that of the active layer
6
. The n-type Al
0.1
Ga
0.9
N layer
4
and the p-type Al
0.1
Ga
0.9
N layer
8
are clad layers having a refractive index which is lower than that of the p-type GaN layer
7
, and the aforementioned waveguiding of the light is effected by the difference between the refractive index of the clad layers and that of the guide layers.
The n-type GaN layer
3
is an underlying layer which provides a current path because the sapphire substrate has no conductivity. Further, the low-temperature growth layer
2
, so-called a buffer layer, is formed for producing a smooth GaN film on the sapphire substrate which is a dissimilar material to GaN.
In the Group III nitride semiconductors (Al
x
Ga
1−x
)
1−y
In
y
N (0≦x≦1, 0≦y≦1), considering GaN is a basic binary compound, it is possible to replace part of the Ga atoms with Al atoms by doping the compound with Al to thereby increase the band gap of the same. Further, it is possible to replace part of the Ga atoms with In atoms by doping the compound with In to thereby decrease the band of the same. As the value of the band gap becomes larger, the refractive index is reduced.
In an Al
z
Ga
1−z
As/GaAs system used in a laser diode operative in the infrared region, a lattice constant thereof hardly changes irrespective of the z value, whereas in the Group III nitride semiconductors (Al
x
Ga
1−x
)
1−y
In
y
N (0≦x≦1, 0≦y≦1), if the x and y values are varied, a lattice constant thereof changes sharply. This difference results from the fact that in the case of Al
z
Ga
1−z
,As system, the lattice constant of GaAs and that of AlAs are approximately equal to each other, so that no lattice mismatch occurs.
In the manufacturing of the semiconductor device described above by using Group III nitride semiconductors (Al
x
Ga
1−x
)
1−y
In
y
N (0≦x≦1, 0≦y≦1), cracking (or crazing) occurs during formation of the n-type AlGaN layer
4
. The lattice constant of AlN is smaller than that of GaN by approximately 2.4% in a-axis direction, so that when the AlGaN-clad layer
4
is formed on the GaN underlying layer
3
, tensile stress is generated paralell to the interface between the two layers
3
and
4
. In general, a semiconductor crystal is resistant to compressive stress, but brittle to tensile stress. For this reason, cracking occurs very easily in the AlGaN-clad layer
4
.
This cracking in the AlGaN-clad layer
4
propagates through the underlying layer
3
as well as through the guide layer
5
formed on the AlGaN-clad layer
4
. A laser diode is a device which is operated by guiding optical waves (i.e. by waveguiding light) within a layered structure thereof. Therefore, the cracking can fatally affect the characteristics of the device.
SUMMARY OF THE INVENTION
An object of the present invention is to provide a semiconductor light-emitting device having a multilayered structure constructed by sequentially forming layers of Group III nitride semiconductors one upon another such that cracking at the interface region of said multilayered structure is prevented, thereby maintaining an excellent optical property of the device, and a method of manufacturing the device.
To attain the object, according to a first aspect of the a invention, there is provided a semiconductor light-emitting device having a multilayered structure formed by sequentially forming layers of Group III nitride semiconductors (Al
x
Ga
1−x
)
1−y
In
y
N (0≦x≦1, 0≦y≦1) which are different from each other in chemical composition ratio, one upon another, on a substrate.
This semiconductor light-emitting device is characterized in that two adjacent layers within the multilayered structure, which include a lower layer having a lattice constant larger than a lattice constant of an upper layer, have a portion close to an interface of the two adjacent layers doped such that an element different from the Group III nitride semiconductors is added to the portion in a higher concentration, i.e. in a higher distribution density than in other portions of the two adjacent layers.
To attain the object, according to a second aspect of the invention, there is provided a method of manufacturing a semiconductor light-emitting device having a multilayered structure formed by sequentially forming layers of Group III nitride semiconductors (Al
x
Ga
1−x
)
1−y
In
y
N (0≦x≦1, 0≦y≦1) which are different from each other in chemical composition ratio, one upon another, on a substrate by a metal-organic chemical vapor deposition method.
This method is characterized by comprising: a first film-forming step of forming a first crystal layer; a second film-forming step for forming a second crystal layer on the first crystal layer, the second crystal layer having a lattice constant which is smaller than a lattice constant of the first crystal layer; and an impurity-adsorbing step carried out before the second film-forming step, for causing an element different from the Group III nitride semiconductors to be adsorbed onto the surface of the first crystal layer.
To attain the object, according to a third aspect of the invention, there is provided a method of manufacturing a semiconductor light-emitting device having a multilayered structure formed by sequentially forming layers of Group III nitride semiconductors (Al
x
Ga
1−x
)
1−y
In
y
N (0≦x≦1, 0≦y≦1) which are different from each other in chemical composition ratio, one upon another, on a substrate by metal-organic chemical vapor deposition method.
This method is characterized by comprising: a first film-forming step of forming a first crystal layer; and a second film-forming step of forming a second crystal layer on the first crystal layer, the second crystal layer having a lattice constant which is smaller than a lattice constant of the first crystal layer, the second film-forming step including an impurity-doping step of adding a dopant gas to a source gas at an initial stage of the second film-forming step such that concentration of the dopant gas in the source gas becomes higher than in the first film-forming step,
Ota Hiroyuki
Tanaka Toshiyuki
Watanabe Atsushi
Perman & Green LLP
Pioneer Corporation
Tran Minh Loan
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