Coherent light generators – Particular active media – Semiconductor
Reexamination Certificate
2000-02-29
2003-10-07
Ip, Paul (Department: 2828)
Coherent light generators
Particular active media
Semiconductor
C342S046000, C342S045000
Reexamination Certificate
active
06631148
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.
FIG. 7
shows a cross-section of a real index guided-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 real index guided-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
0.35
Ga
0.15
In
0.5
P, an active layer
4
, a first p-type cladding layer
5
made of Al
0.35
Ga
0.15
In
0.5
P, and an etch-stop layer
6
made of (Al
x
Ga
1−x
)
0.5
In
0.5
P (where 0≦x≦0.1), Al
z
Ga
1−z
As (where 0.4≦z≦1) or the like are successively formed in the stated order. A second p-type cladding layer
7
is then formed from Al
0.35
Ga
0.15
In
0.5
P as a ridge in the center of the upper surface of the etch-stop layer
6
. An ohmic contact layer
8
made of p-type Ga
0.5
In
0.5
P is then formed on top of this second p-type cladding layer
7
. A current-blocking layer
9
made of n-type Al
0.35
Ga
0.15
In
0.5
P is formed on both sides of the second p-type cladding layer
7
and the ohmic contact layer
8
, and a cap layer
10
made of p-type GaAs is then formed on top of the ohmic contact layer
8
and the current-blocking layer
9
. A p-type electrode
11
is formed on the cap layer
10
, and an n-type electrode
12
is formed on the back of the n-type GaAs substrate
1
. The second p-type cladding layer
7
and the current-blocking layer
9
form a light-confining construction, with light being confined within this and the n-type cladding layer
3
. Note that the materials cited here are mere examples, so that other combinations of materials may be used.
Each layer in the AlGaInP-type semiconductor laser described above is successively formed using metalorganic vapor phase epitaxy (MOVPE). The light-confining construction is formed as follows. A material layer used to produce the second p-type cladding layer
7
is first provided on top of the etch-stop layer
6
, an etching mask is applied to the part of the material layer that corresponds to the second p-type cladding layer
7
, and an etching solution including sulfuric acid is applied to the unmasked parts of the material layer. This results in the unmasked parts being etched as far as the etch-stop layer
6
, leaving a ridge of the material layer that forms the second p-type cladding layer
7
. The etching mask is then removed, and the current-blocking layer
9
made of Al
0.35
Ga
0.15
In
0.5
P is formed through crystal growth using MOVPE. Before this current-blocking layer
9
is formed, however, 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 ohmic contact layer
8
. These impurities are removed by a thermal cleaning process where the multilayer structure is heated to a high temperature (generally 700° C. or higher) that is near the crystal growth temperature of the current-blocking layer
9
that is formed next. To prevent phosphorous from being vaporized from the surface of the ohmic contact layer
8
, a gas such as phosphine (PH
3
) is supplied during the heating.
An AlGaInP semiconductor laser used in an optical disc device needs to have improved laser characteristics in keeping with the optical disc device, which is to say, to oscillate in a unified lateral mode and to have a low threshold current. As a result, the thickness and shape of the first p-type cladding layer
5
and second p-type cladding layer
7
need to be appropriately determined, while the crystallization of the current-blocking layer
9
needs to be improved.
SUMMARY OF THE INVENTION
It is a first object of the present invention to provide a semiconductor laser that has improved laser characteristics, such as a lower threshold current.
It is a second object of the present invention to provide a manufacturing method for a semiconductor laser that has improved laser characteristics, such as a lower threshold current.
In order to achieve the stated objects, the inventors first examined the following aspects of conventional semiconductor lasers.
Firstly, the inventors studied why irregularities (bumps and concaves) appear in the surface of the etch-stop layer. When the etch-stop layer is formed of (Al
x
Ga
1−x
)
0.5
In
0.5
P (where 0≦x≦0.1), Al
z
Ga
1−x
As (where 0.4≦z≦1) or the like, crystal growth has to be performed at a temperature of 700° C. or higher to obtain a second p-type cladding layer and a current blocking layer with a high degree of crystallization. Thermal cleaning also has to be performed at a temperature of 700° C. or higher. Due to these high temperatures, the (Al
x
Ga
1−x
)
0.5
In
0.5
P (where 0≦x≦0.1) or Al
z
Ga
1−z
As (where 0.4≦z≦1) sublimates at the surface of the etch-stop layer. In order words, the heating performed when the second p-type cladding layer is formed using MOVPE causes sublimation which results in irregularities being formed in the surface of this layer. The heating performed when the current blocking layer is formed using MOVPE and the heating performed as part of the thermal cleaning also cause sublimation in the surface of the etch-stop layer, which also causes irregularities. The part of the structure forming the current blocking layer is subjected to high temperature at least twice during the manufacturing process, so that the irregularities in its surface are more prominent than those in the surface of the second p-type cladding layer. As a result, the second p-type cladding layer and the current blocking layer cannot be formed with a high degree of crystallization, which makes it impossible to produce a semiconductor laser with the desired characteristics. Through experimentation, the inventors found that the sizes of the irregularities in the surface of the etch-stop layer depend on the proportion of aluminum to gallium in the (Al
x
Ga
1−x
)
y
In
1−y
P material forming the etch-stop layer.
A second phenomenon is the formation of a metamorphosed layer on the surface of the etch-stop layer. When the etch-stop layer is formed of (Al
x
Ga
1−x
)
0.5
In
0.5
P (where 0≦x≦0.1) or Al
z
Ga
1−z
As (where 0.4≦z≦1) and thermal cleaning is performed in the presence of a gas such as phosphine (PH
3
) at a temperature of 700° C. or higher, the (Al
x
Ga
1−x
)
0.5
In
0.5
P (where 0≦x≦0.1) or Al
z
Gal
1−z
As (where 0.4<z<1) surface of the etch-stop layer reacts with the phosphine or other gas, forming a metamorphosed layer. Also, crystal growth has to be performed at a temperature of 700° C. or higher to obtain a second p-type cladding layer and a current blocking layer with a high degree of crystallization. When such high temperature is used, however, the surface of the etch-stop layer will absorb more of the impurities such as silicon that remain inside the reactor (while the reaction takes place after first evacuating the reactor, it is practically impossible to remove all such impurities) than are absorbed when a lower temperature is used. These absorbed impurities are one cause in the formation of a region (metamorphosed layer) of crystal defects. The part of the structure forming the current blocking layer is subjected to high temperatures at least twice, during which the part comes into contact with the impurities and phosphine gas, making the formation of a metamorphosed layer more evident for the surface of the etch-stop layer than for the surfa
Fukuhisa Toshiya
Yoshikawa Akio
Ip Paul
Matsushita Electric - Industrial Co., Ltd.
Vy Hung
LandOfFree
Semiconductor laser and a manufacturing method for the same does not yet have a rating. At this time, there are no reviews or comments for this patent.
If you have personal experience with Semiconductor laser and a manufacturing method for the same, we encourage you to share that experience with our LandOfFree.com community. Your opinion is very important and Semiconductor laser and a manufacturing method for the same will most certainly appreciate the feedback.
Profile ID: LFUS-PAI-O-3146576