Coherent light generators – Particular active media – Semiconductor
Reexamination Certificate
2000-02-29
2004-11-23
Wong, Don (Department: 2828)
Coherent light generators
Particular active media
Semiconductor
C372S046012
Reexamination Certificate
active
06822989
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a semiconductor laser used as a light source in an optical disc device and to a manufacturing method for such semiconductor laser.
2. Description of the Prior Art
Optical disc drives for digital video discs (DVDs) and other such media have been developed in recent years. Of the semiconductor lasers currently available, such devices mainly use AlGaInP-type semiconductor lasers that emit laser light of a short wavelength as their light sources.
In the field of AlGaInP-type semiconductor lasers, much attention has been placed on the favorable characteristics exhibited by RISA (Real refractive Index guided Self-Aligned structure) type lasers. These have been described by Osamu Imafuji et al. on page 1223 of “Electronics Letters” Volume 33 (1997), for example.
FIG. 7
shows a cross-section of a RISA-type laser. The expressions “above” and “below” in the following explanation refer to the structure when
FIG. 7
is in an upright position. The illustrated RISA-type laser has an n-type GaAs substrate
1
, on which an n-type GaAs buffer layer
2
, an n-type cladding layer
3
made of (Al
x
Ga
1-x
)
y
In
1-y
P (where x=0.7, y=0.5), an active layer
4
, a p-type cladding base layer
5
made of (Al
x
Ga
1-x
)
y
In
1-y
(where x=0.7, y=0.5), and a current-blocking layer
6
made of AlInP are successively formed in the stated order. Etching is performed next on a stripe-shaped part of the current-blocking layer
6
. Above this, a p-type buried cladding layer
7
, an ohmic contact layer
8
made of p-type Ga
0.5
In
0.5
P, a cap layer
9
made of p-type GaAs are formed in the stated order. A p-type electrode
10
is formed on the cap layer
9
, and an n-type electrode
11
is formed on the back of the n-type GaAs substrate
1
. Note that the materials cited here are mere examples, so that other combinations of materials may be used.
As shown in
FIG. 7
, the p-type buried cladding layer
7
that covers the current-blocking layer
6
is buried in a groove formed in the center of the construction. This produces a current flow concentrating effect whereby the flow of current between the top and bottom of the p-type buried cladding layer
7
is narrowed. Light is confined within the n-type cladding layer
3
, the p-type cladding base layer
5
, and the p-type buried cladding layer
7
. The current flow concentrating and light-confining effects of this construction ideally mean that laser light can be produced using a relatively low operating current.
FIG. 8
shows the manufacturing process for the above type of laser. Each layer from the n-type GaAs buffer layer
2
to the cap layer
9
is successively formed using metalorganic vapor phase epitaxy (MOVPE). In more detail, each layer up to the current-blocking layer
6
is successively formed on the n-type GaAs substrate
1
(process
1
). Once the current-blocking layer
6
has been provided, a stripe is formed by etching a central part of the current-blocking layer
6
. Before the p-type buried cladding layer
7
is formed, impurities (which are mainly composed of the etching solution that remains after the etching process) need to be removed from the surface of the multilayer structure formed of the n-type GaAs substrate
1
to the current-blocking layer
6
. These impurities are removed by a thermal cleaning process where the multilayer structure is heated at a high temperature (generally 700° C. or higher) that is near the crystal growth temperature of the layers (process
2
). To prevent phosphorous from being vaporized from the surface of the multilayer structure, a phosphorous compound such as phosphine (PH
3
) or the like is supplied during this process. In this way, the cleaning is performed in the presence of a phosphorous compound. The remaining layers are thereafter formed using MOVPE, and the manufacturing procedure ends with the formation of the electrodes (process
3
).
To improve the performance of optical disc devices in which the above laser is used, however, it is desirable to further improve the laser characteristics, such as by lowering the laser threshold current (see Optical Device Dictionary, page 8).
SUMMARY OF THE INVENTION
It is a first object of the present invention to provide a durable semiconductor laser with improved laser characteristics, such as a lowered threshold current.
It is a second object of the present invention to provide a manufacturing method for efficiently producing a durable semiconductor laser with improved laser characteristics, such as a lowered threshold current.
In order to achieve the stated objects, the inventors studied the manufacturing method of the semiconductor lasers described in the prior art, and tried to find points that could be improved. As a result, the inventors found that when thermal cleaning is performed as part of the manufacturing method of the above semiconductor laser, the etching proceeds in the horizontal direction at the joins between the current block layer and the p-type cladding base layer, creating concaves in the current-blocking layer. If the current-blocking layer is embedded and reconstructed in this state having concaves, crystal growth will not proceed in certain areas and, as shown in
FIG. 7
, hollows
12
will be left in the construction. While these hollows
12
may be small, they are formed along the interface between the p-type cladding base layer
5
and the current-blocking layer
6
and cause waveguide loss in the semiconductor laser. This worsens the semiconductor laser characteristics (such as by raising the threshold current) and so has made it impossible to realize an ideal semiconductor laser.
The inventors next focused on the horizontal progression of the etching that occurs at the interface between the p-type cladding base layer
5
and the current-blocking layer
6
during the thermal cleaning process in terms of the concentration of carriers in the current-blocking layer
6
. As a result, the inventors discovered that the extent to which etching progresses in the horizontal direction at the interface between the p-type cladding base layer
5
and the current-blocking layer
6
has a high correlation with the concentration of carriers in the current-blocking layer
6
. This discovery led to the conception of the present invention.
In order to achieve the stated first object, the present invention is a semiconductor laser, including: an n-type cladding layer that has n-type conductivity; an active layer formed on top of the n-type cladding layer; a p-type cladding base layer that is formed on top of the active layer and has p-type conductivity; a current-blocking layer that is formed on specified parts of an upper surface of the p-type cladding base layer and substantially has n-type conductivity; and a p-type buried cladding layer that has p-type conductivity and is formed so as to cover the current-blocking layer and contact remaining parts of the upper surface of the p-type cladding base layer, wherein the current-blocking layer has at least two regions having different concentrations (hereafter “N1” and “N2” where N1<N2) of n-type carriers, a region adjacent to an interface between the p-type cladding base layer and the p-type buried cladding layer having the N1 concentration of n-type carriers and a part or all of a remaining region of the current-blocking layer region having the N2 concentration.
Note that the term “substantially” used in the description of the current-blocking layer means that while one or more parts of the current-blocking layer
6
may not have a current blocking action, such action will be achieved by some other part of the current-blocking layer. This is also the case for the other independent claims.
In the semiconductor laser of the stated construction, the concentration of carriers in the current-blocking layer near the joins between the p-type cladding base layer and the p-type buried cladding layer is lower than the concentration of carriers in some or all of a remaining part of the current-bloc
Fukuhisa Toshiya
Manno Masaya
Yoshikawa Akio
Matsushita Electric - Industrial Co., Ltd.
Nguyen Tuan N.
Wong Don
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