Coherent light generators – Particular active media – Semiconductor
Reexamination Certificate
1998-08-07
2001-07-03
Font, Frank G. (Department: 2877)
Coherent light generators
Particular active media
Semiconductor
C372S046012, C372S043010, C372S044010, C372S096000, C372S075000
Reexamination Certificate
active
06256331
ABSTRACT:
BACKGROUND OF THE INVENTION
The present invention relates to a semiconductor laser device suitably used as a light source for optical communication and to an optical communication system using the same. The present invention also relates to technology for crystal-growing a compound semiconductor as a material for an active layer included in the semiconductor laser device and the optical communication system using the same.
A conventional semiconductor laser device as a light source for optical communication employs an InP substrate and InGaAsP mixed crystals as a material for the active layer thereof. This is because the InGaAsP mixed crystals have band gap energy on the bands of 1.3 &mgr;m and 1.55 &mgr;m, which are low transmission loss bands of an optical fiber.
A conventional semiconductor laser device for optical communication is illustrated in FIG.
10
.
The semiconductor laser device shown in
FIG. 10
includes: an n-type InP substrate
101
; and a mesa-shaped multi-layer structure formed on the substrate
101
. The mesa-shaped multi-layer structure includes: an n-type InGaAsP light confinement layer
102
; an InGaAsP active layer
103
; and a p-type InP cladding layer
104
. A p-type InP current blocking layer
105
and an n-type InP current blocking layer
106
are buried in the regions interposing the mesa-shaped multi-layer structure therebetween. A p-type InP buried layer
107
and a p-type InGaAsP contact layer
108
are formed so as to cover these current blocking layers and the mesa-shaped multi-layer structure. An insulating film
109
having stripe-shaped openings is deposited over the p-type InGaAsP contact layer
108
. An An/Zn electrode
110
and a Ti/Au electrode
111
are formed thereon. An Au/Sn electrode
112
is formed on the reverse surface of the substrate
101
.
The InGaAsP/InP semiconductor laser device shown in
FIG. 10
has a problem that the threshold current and the light emission efficiency thereof are variable to a large degree with respect to the variation in temperatures. Thus, various measures to keep the temperature of the semiconductor laser device constant, e.g., using a Peltier device, have been taken. However, the price of a laser module is raised partly because of such measures.
A very small band offset &Dgr;c on the conduction band is presumably one of the reasons why the characteristics of an InGaAsP/InP semiconductor laser device are variable to a large extent with respect to the variation in temperatures. This phenomenon will be described with reference to
FIGS. 11A through 11C
.
FIGS. 11A and 11B
illustrate cases where an active layer has a quantum well structure including barrier layers and a well layer sandwiched therebetween. If &Dgr;Ec between the barrier layers and the well layer is as small as about 100 meV and if the temperature is low, then a sufficiently large number of electrons are confined within the well layer functioning as a light-emitting region as shown in FIG.
11
A. However, if the temperature rises, then the electrons are likely to overflow from the well layer owing to the thermal energy applied and cease to contribute to the emission of light. Thus, the threshold current thereof increases and the slope efficiency declines shown in FIG.
11
C.
As described above, &Dgr;Ec of the InGaAsP/InP semiconductor laser device is about 100 meV, which is much smaller than that of an AlGaAs/GaAs semiconductor laser device in the range from about 200 to about 300 meV.
In view of the above-described problems, the present invention was made in order to accomplish the objects of (1) providing a semiconductor laser device having low threshold current and exhibiting high slope efficiency over a wide temperature range and an optical communication system using the same, and (2) providing a method for producing an InN
x
As
y
P
1−x−y
(where 0<x<1 and 0≦y<1) mixed crystal having excellent crystallinity suitable for the active layer of the semiconductor laser device.
SUMMARY OF THE INVENTION
In order to accomplish the above-described object, according to the present invention, an InN
x
As
y
P
1−x−y
(where 0<x<1 and 0≦y<1) layer that is lattice-matched with a GaAs substrate, a GaP substrate or an Si substrate is used as the active layer, thereby realizing a semiconductor laser device oscillating on a wavelength band suitable for optical communication and having &Dgr;Ec of 200 meV or more.
A semiconductor laser device according to the present invention includes a GaAs substrate and a multi-layer structure formed on the GaAs substrate. The multi-layer structure includes an active layer for emitting light. The active layer includes an InN
x
As
y
P
1−x−y
(where 0<x<1 and 0≦y<1) layer that is lattice-matched with the GaAs substrate.
By employing such a structure, a semiconductor laser device for long-distance optical communications (band gap energy is on the band from 1.1 to 1.6 &mgr;m) is realized by using a GaAs substrate. This is because the band gap energy of the InN
x
As
y
P
1−x−y
(where 0<x<1 and 0≦y<1) layer that is lattice-matched with the GaAs substrate decreases owing to the bowing effect to be an optimum value for laser oscillation on the band from 1.1 to 1.6 &mgr;m. In addition, since the decrease in the conduction band level of the InN
x
As
y
P
1−x−y
(where 0<x<1 and 0≦y<1) layer owing to the bowing effect is more remarkable than the decrease in the valence band level thereof, &Dgr;Ec can be increased to 200 meV or more in the multi-layer structure. Thus, even when the energy of the carriers increases because of the rise in temperature and/or temperature of the semiconductor laser device itself, the increase in number of carriers overflowing from the active layer can be suppressed. Therefore, the semiconductor laser device can show excellent performance in terms of temperature characteristics.
In one embodiment, the active layer preferably has a quantum well structure including at least one well layer and at least two barrier layers, and the well layer is preferably an InN
x
As
y
P
1−x−y
(where 0<x<1 and 0≦y<1) layer.
In such an embodiment, the carriers in the well layer behave as quantum mechanical wave propagation. As a result, laser oscillation is realized by injecting a smaller amount of current.
In another embodiment, the barrier layers may be made of a material selected from the group consisting of AlGaInP, AlGaAs, GaAs, InGaAsP and InGaP.
In still another embodiment, the multi-layer structure may further include a first cladding layer, which has the same conductivity type as that of the substrate and is located below the active layer and a second cladding layer and a contact layer, which have a different conductivity type from that of the substrate and are located above the active layer. And an electrode may be disposed on the contact layer to be in contact with each other in a stripe region.
In such an embodiment, the injected current is confined to the stripe region. As a result, the carriers can be confined transversally.
In still another embodiment, a portion of the multi-layer structure, including the second cladding layer and the contact layer having the different conductivity type from that of the substrate, may be formed in a ridge shape.
In such an embodiment, the effective refractive index varies in a transverse direction in the ridge portion and the regions sandwiching the ridge portion. As a result, light can be confined transversally.
In still another embodiment, the multi-layer structure may further include: a first cladding layer, which has the same conductivity type as that of the substrate and is located below the active layer; and a second cladding layer, which has a different conductivity type from that of the substrate and is located above the active layer. The second cladding layer having the different conductivity type from that of the substrate may have a ridge-shaped portion. A current blocking layer having the same conductivity type as that of t
Ishino Masato
Kitoh Masahiro
Matsui Yasushi
Flores Ruiz Delma R.
Font Frank G.
Matsushita Electric - Industrial Co., Ltd.
McDermott & Will & Emery
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