Coherent light generators – Particular active media – Semiconductor
Reexamination Certificate
2001-12-19
2004-01-27
Davie, James (Department: 2828)
Coherent light generators
Particular active media
Semiconductor
Reexamination Certificate
active
06683899
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a semiconductor laser to be used as a writing/reading light source for an optical disc device such as a DVD-ROM drive, a DVD-R drive, a DVD-RW drive, a DVD-RAM drive or the like, a writing light source for an LBP (laser beam printer) or the like, and a reading light source for a bar code scanner or the like. The present invention also relates to a method of producing the semiconductor laser above-mentioned.
2. Description of Related Arts
A writing/reading light source for an optical recording device indispensably requires a semiconductor laser capable of conducting oscillation in a short wavelength. A semiconductor laser having a composition of the AlGaInP type satisfies this requirement. To improve the recording speed, it is also required to drive a high-output laser light in a moment. A ridge-type semiconductor laser using a compound semiconductor satisfies this requirement.
FIG. 3
is a schematic section view illustrating the arrangement of a general ridge-type semiconductor laser of prior art. A lower clad layer
22
is formed on a substrate
21
. An active layer
23
is formed on the lower clad layer
22
. A first upper clad layer
24
is formed on the active layer
23
. An etching stop layer
25
is formed on the first upper clad layer
24
.
A second upper clad layer
26
and a contact layer
27
both in the form of a ridge, are laminated on the etching stop layer
25
in the form of a continuous film, thus forming a ridge portion
32
. On the etching stop layer
25
, block layers
30
are formed at both sides of the ridge portion
32
. That is, the etching stop layer
25
is present under the second upper clad layer
26
and also under the block layers
30
.
A cap layer
31
is formed on the contact layer
27
and the block layers
30
.
FIG.
4
(
a
) to FIG.
4
(
e
) are schematic section views illustrating the steps of producing the above-mentioned general ridge-type semiconductor laser of prior art. Successively formed on the substrate
21
are the lower clad layer
22
, the active layer
23
, the first upper clad layer
24
, the etching stop layer
25
, the second upper clad layer
26
and the contact layer
27
. A mask layer
28
is then formed, and a resist pattern
29
is formed on the mask layer
28
(FIG.
4
(
a
)).
The etching stop layer
25
has a composition of the GaInP type or the like, and the active layer
23
has a composition of the AlGaInP type, the GaInP type or the like. Each of the substrate
21
and the contact layer
27
has a composition of GaAs or the like. Other semiconductor portions have a composition of the AlGaInP type or the like. The mask layer
28
is generally composed of SiO
2
or SiN.
Then, the mask layer
28
is etched in the pattern of the resist
29
(FIG.
4
(
b
)).
After the resist
29
is removed, the contact layer
27
and the second upper clad layer
26
are etched in the form of a ridge (FIG.
4
(
c
)). Since the etching stop layer
25
is resistant to the etching medium, the layers on and lower than the etching stop layer
25
are not etched.
Then, the block layers
30
are selectively grown at both sides of the ridge portion
32
(FIG.
4
(
d
)). After the selective growth of the block layers
30
, the mask layer
28
is removed and the cap layer
31
is then grown (FIG.
4
(
e
)).
Each of the block layers
30
and the cap layer
31
has a composition of GaAs or the like.
In the semiconductor laser having the arrangement above-mentioned, the etching stop layer
25
is present between the block layers
30
and the first upper clad layer
24
. This disadvantageously relaxes the hetero-barrier between the block layers
30
and the first upper clad layer
24
, thereby to increase a leak electric current from the block layers
30
. This lowers the current confinement effect for the second upper clad layer
26
. The oscillation efficiency of the semiconductor laser is therefore lowered.
In view of the foregoing, the etching stop layer
25
is preferably not present, in a finished product, between the block layers
30
and the first upper clad layer
24
.
In this connection, it may be proposed to remove, using an etching solution such as a solution of sulfuric acid or the like, the etching stop layer
25
exposed after the second upper clad layer
26
has been etched. However, as soon as the etching stop layer
25
is removed, there is exposed the first upper clad layer
24
which is etched at a higher etching rate. Etching control in the depth direction is therefore difficult.
It is also proposed that the exposed etching stop layer
25
is removed by thermal decomposition (Japanese Laid-Open Patent Publication (KOKAI) 4-130692). In this case, however, the application of heat causes the elements (including dopants) forming the respective portions of the semiconductor laser, to mutually diffuse, thus making laser oscillation impossible.
SUMMARY OF THE INVENTION
It is a first object of the present invention to provide a semiconductor laser in which the leak electric current flowing between the block layers and the first upper clad layer is small, thus making the oscillation efficiency high, and also to provide a method of producing the semiconductor laser above-mentioned.
It is a second object of the present invention to provide a semiconductor laser having no etching stop layer between the block layers and the first upper clad layer, and also to provide a method of producing the semiconductor laser above-mentioned.
It is a third object of the present invention to provide a mass-productive semiconductor laser in which only the etching stop layer can be removed with high precision, and also to provide a method of producing the semiconductor laser above-mentioned.
A semiconductor laser according to the present invention comprises: a lower clad layer, an active layer, a first upper clad layer, and a dry-etching stop layer, all these layers being successively laminated in this order on a compound semiconductor substrate; a second upper clad layer in the form of a ridge disposed above the first upper clad layer; an etching stop layer present only between the dry-etching stop layer and the second upper clad layer; and block layers formed at sides of the second upper clad layer.
According to the present invention, the etching stop layer is not present between the block layers and the first upper clad layer. Accordingly, the current confinement effect for the second upper clad layer is well produced. Thus, the semiconductor laser having the arrangement above-mentioned has a high oscillation efficiency.
According to the arrangement above-mentioned, it is preferred that each of the dry-etching stop layer, the first upper clad layer and the second upper clad layer is a compound semiconductor of the AlGaInP type, that the etching stop layer is a compound semiconductor of the GaInP type, and that the dry-etching stop layer is higher in Al concentration than the first upper clad layer.
By making the dry-etching stop layer higher in Al concentration than the first upper clad layer, the band barrier between the dry-etching stop layer and the block layers becomes higher. This increases the current confinement effect for the ridge portion.
A semiconductor laser producing method according to the present invention comprises the steps of: successively forming, on a compound semiconductor substrate, a lower clad layer, an active layer, a first upper clad layer, an etching stop layer, and a second upper clad layer in this order; etching the second upper clad layer, into the form of a ridge, up to the etching stop layer; removing, by dry-etching, the etching stop layer exposed after the etching step above-mentioned; and selectively growing block layers at sides of the second upper clad layer in the form of a ridge.
According to the present invention, the semiconductor laser producing method may further comprise, after the first upper clad layer has been formed and before the etching stop layer is formed, the step of forming a dry-etching stop layer on the first upper cl
Davie James
Rabin & Berdo P.C.
Rohm & Co., Ltd.
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