Coherent light generators – Particular active media – Semiconductor
Reexamination Certificate
1998-06-03
2001-09-18
Leung, Quyen P. (Department: 2881)
Coherent light generators
Particular active media
Semiconductor
C372S046012
Reexamination Certificate
active
06292502
ABSTRACT:
BACKGROUND OF THE INVENTION
The present invention relates generally to a semiconductor laser device having an active region of a quantum well structure of an InGaP crystal, and a DVD system using the semiconductor laser.
Semiconductor laser devices have excellent characteristics, such as small size, steadiness, easiness of modulation, operability at a low voltage and a low current, and a long life in comparison with gas lasers and solid lasers. Therefore, semiconductor laser devices are not only substituted for gas lasers in fields which have been practically achieved, but there are often also application fields in which can be first practically achieved by semiconductor laser devices. As examples thereof, there are optical communication systems and optical information processing systems.
However, conventional semiconductor laser devices can not sufficiently clear specifications necessary for these application fields.
For example, a DVD system, such as a DVD-ROM (Digital Versatile Disc-Read Only Memory) and DVD-RAM(Random Access Memory), has been noticed as an optical information processing system capable of performing rapid access at a large capacity. It has been expected that a semiconductor laser device is used for a light source for an optical pickup head of the DVD systems.
However, according to the standard for light sources of the DVD systems, which has been standardized all over the world, it is required that the light sources have an oscillating wavelength of 650 nm, a very low threshold current, a high efficiency, stable temperature characteristics, a high maximum oscillating temperature (Tmax) and so forth.
On the other hand, a semiconductor laser device using an InGaAlP crystal, which lattice-matches with a GaAs substrate, practically has an oscillating waveband which is in the range of from 630 nm to 700 nm. In recent years, semiconductor laser devices using a quantum well structure, which uses an InGaP semiconductor layer having a thickness of a few nm as a well layer so that an active region has quantum effects, have been produced.
However, conventional semiconductor lasers having an oscillating wavelength of 650 nm have a high threshold current of about 60 mA and a low maximum oscillating temperature (Tmax) of about 80° C. For that reason, conventional semiconductor lasers do not meet the specifications of the DVD standard, so that it is required to much improve the characteristics of the semiconductor lasers.
SUMMARY OF THE INVENTION
It is therefore an object of the present invention to eliminate the aforementioned problems and to provide a semiconductor laser device having an MQW active region, which can greatly increase gain while maintaining a specific oscillating wavelength, such as 650 nm in the DVD standard, and which has a reduced threshold, an improved emission efficiency and improved temperature characteristics, such as a reduced current when operating at a high temperature. The semiconductor device of the present invention can be very suitably used as a light source for DVD systems or the like.
That is, according to the present invention, it is possible to much improve various characteristics, such as gain, while maintaining the required oscillating wavelength, by introducing a compressive strain into well layers while adjusting the thickness of the well layers.
In order to accomplish the aforementioned and other objects, according to one aspect of the present invention, a semiconductor laser device comprises: a GaAs substrate of a first conductive type; at least one In
0.5
(Ga
1−x
Al
x
)
0.5
P cladding layer of the first conductive type, which is provided on a first principal plate of the GaAs substrate; a plurality of well layers provided on the cladding layer of the first conductive type; a plurality of In
0.5
(Ga
1−y
Al
y
)
0.5
P barrier layers associated with the plurality of well layers for forming a multiple quantum well structure, each of the plurality of In
0.5
(Ga
1−y
Al
y
)
0.5
P barrier layers being provided between adjacent well layers of the plurality of well layers; and at least one In
0.5
(Ga
1−x
Al
x
)
0.5
P cladding layer of a second conductive type, which is provided on the multiple quantum well structure, wherein a compressive strain is introduced into the well layers.
With this construction, according to the present invention, it is possible to achieve a reduced threshold and improved temperature characteristics, such as a maximum oscillating temperature rise, by introducing a compressive strain into well layers of an active region having a MQW structure in a semiconductor laser which has an oscillating wavelength of 650±20 nm. In addition, the temperature characteristics can be improved by increasing the compressive strain in the well layers and introducing a tensile strain into guiding layers and/or barrier layers. Moreover, it is possible to achieve a more reduced threshold and a more improved temperature characteristics by providing two or more cladding layers.
The semiconductor laser device of the present invention can be very suitably used as a light source for DVD systems. According to the present invention, various kinds of DVD systems having high performance and high reliability can be actually put to practical use.
REFERENCES:
patent: 4429378 (1984-01-01), Sato
patent: 5299216 (1994-03-01), Van Der Poel et al.
patent: 5850411 (1998-12-01), Major, Jr. et al.
patent: 6-196805 (1994-07-01), None
patent: 8-181385 (1996-07-01), None
Shimada, N., H. Matsuura, M. Watanabe, H. Okuda, K. Nitta, M. Okajima, “40mW Operation of InGaAIP Visible-Light Laser Diodes,” 1993 ISOM/ODS'93 Conference Digest. pp. 170-171, (No Month Available).
Hogan & Hartson LLP
Kabushiki Kaisha Toshiba
Leung Quyen P.
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