Coherent light generators – Particular active media – Semiconductor
Reexamination Certificate
2002-07-16
2004-04-06
Wilson, Allan R. (Department: 2815)
Coherent light generators
Particular active media
Semiconductor
C257S014000, C257S023000, C372S043010
Reexamination Certificate
active
06717969
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a semiconductor laser device having a current stopping layer for confining current. The present invention also relates to a short-wavelength laser light source which converts a laser beam emitted from a semiconductor laser device having a current stopping layer for confining current, into a second harmonic laser beam.
2. Description of the Related Art
Generally, semiconductor laser devices used as a light source in information processing or printing equipment are required to efficiently operate with low-level current. In a conventional semiconductor laser device, which is disclosed, for example, in the registered Japanese patent No. 2746131, a current confinement region including a reverse bias pn junction is provided so that current is injected into only a very small region of an active layer. This semiconductor laser device basically includes the active layer formed over a substrate, and a current confinement structure realized by p-type and n-type layers being formed above the active layer and including a current stopping layer which has an opening for current injection into only a predetermined stripe region of the active layer.
FIG. 6
is a vertical cross-sectional view of a typical example of the above semiconductor laser device. In the semiconductor laser device of
FIG. 6
, an n-type InGaP lower cladding layer
11
′, semiconductor multiple layers
12
′, and a p-type InGaP first upper cladding layer
13
′ are formed on an n-type GaAs substrate
10
′, where the semiconductor multiple layers
12
′ include an i-type InGaAsP barrier layer, an i-type InGaAs quantum-well active layer, and an i-type InGaAsP barrier layer.
On the p-type InGaP first upper cladding layer
13
′, an n-type InGaP current stopping layer
31
′ and a p-type AlGaAs second upper cladding layer
23
′ are formed so that the n-type InGaP current stopping layer
31
′ exists on each side of the p-type AlGaAs second upper cladding layer
23
′, and a current confinement structure is realized by the n-type InGaP current stopping layer
31
′ and the p-type InGaP first upper cladding layer
13
′. That is, the n-type InGaP current stopping layer
31
′ has an opening filled with the p-type AlGaAs second upper cladding layer
23
′, and a reverse bias state is realized by pn junctions between the n-type InGaP current stopping layer
31
′ and the p-type InGaP first upper cladding layer
13
′.
In addition, a p-type AlGaAs third upper cladding layer
24
′, a p-type GaAs contact layer
14
′, an insulation film
15
′, and a p electrode
16
′ are formed in this order on the n-type InGaP current stopping layer
31
′ and the p-type AlGaAs second upper cladding layer
23
′. Further, an n electrode
17
′ is formed on the lower surface of the n-type GaAs substrate
10
′.
However, when the current confinement structure including the reverse pn junctions is provided, the pn junctions generate parasitic capacitance. Therefore, when the semiconductor laser device is modulated at high speed, the high-frequency components pass through the pn junctions, and thus high-frequency modulation is impossible.
In addition, when the semiconductor laser device having the above problem is used in a short-wavelength laser light source in combination with an optical wavelength conversion element which converts a laser beam emitted from the semiconductor laser device, into a second harmonic laser beam having a blue or green wavelength, it is difficult to use the short-wavelength laser light source for image recording or the like.
Further, when a semiconductor laser device used in reading data from an optical disk or the like is driven at high frequency for reducing noise, high-frequency components pass through the pn junctions, and the current is not efficiently injected into the active layer.
In
FIG. 6
, an equivalent circuit of the semiconductor laser device is also diagrammatically indicated. As illustrated in
FIG. 6
, it is considered that the semiconductor laser device of
FIG. 6
has as resistance components an ohmic resistance R
1
in the p electrode
16
′, a resistance R
2
in the active layer, and resistances R
3
and R
4
in a distributed constant circuit which represents influences of the spread of the active layer in the lateral directions. In addition, the semiconductor laser device of
FIG. 6
has as capacitance components a capacitance C
1
existing between the p electrode
16
′, the insulation film
15
′, and the p-type GaAs contact layer
14
′, capacitances C
2
and C
3
generated by the pn junctions at the upper and lower boundaries of the n-type InGaP current stopping layer
31
′, a capacitance C
4
generated by the junctions of the active layer, and a capacitance C
5
in the above distributed constant circuit.
The parasitic capacitances C
2
and C
3
generated by the pn junctions at the upper and lower boundaries of the n-type InGaP current stopping layer
31
′ become most dominant in operation with high-speed modulation, and are the major cause of the damage to the high-frequency characteristics. In particular, the areas of the pn junctions almost correspond to the area of the semiconductor laser device. In addition, viewed as an electric circuit, the pn junctions extend in parallel with the active layer. Therefore, high-frequency components can pass through the current stopping layer, and the current is not efficiently injected into the active layer.
In order to solve the above problem, Japanese Patent Publication No. 5(1993)-9951 discloses a technique for reducing parasitic capacitance existing in a current stopping layer in a buried heterostructure semiconductor laser device, which is widely used for oscillation at the wavelength of 1.3 micrometers or greater. As illustrated in
FIG. 7
, the semiconductor laser device has a structure in which an active layer
201
is formed above an n-type InP substrate
200
, and both sides of the active layer are etched off and filled with an n-type current stopping layer
205
. In addition, a pair of trenches
208
having such a depth as to reach the substrate
200
are formed on both sides of the active layer
201
so that parasitic capacitance existing in the current stopping layer
205
is reduced. Further, in
FIG. 7
, reference numeral
202
denotes a p electrode,
203
denotes an insulation film,
204
denotes a p-type InGaAs contact layer, and
207
denotes an n electrode.
The above technique is very useful for reducing parasitic capacitance in the current stopping layer
205
which extends through the entire area of the semiconductor laser device. However, the above structure can be formed mainly in semiconductor laser devices made of InP-based materials. In particular, from the viewpoint of the production process and reliability, the above structure cannot be formed in semiconductor laser devices made of materials which can realize oscillation at a short wavelength of 1 micrometer or smaller. The semiconductor laser devices which oscillate at a wavelength of 1.3 micrometers or greater are made of InP/InGaAsP materials, and the etching characteristics of the constituent materials of the structure of
FIG. 7
are similar. That is, the structure of
FIG. 7
can be realized because the formation of the trenches as illustrated in
FIG. 7
is easy. On the other hand, the semiconductor laser devices which oscillate at a short wavelength of 1 micrometer or smaller are made of various materials as GaAs/AlGaAs/InGaP/InGaAsP/AlGaInP, and the etching characteristics of these materials are different. Therefore, formation of the trenches as illustrated in
FIG. 7
is not easy in the semiconductor laser devices which oscillate at a short wavelength of 1 micrometer or smaller.
The registered Japanese patent No. 2746131 also discloses another technique for reducing parasitic capacitance. In the registered Japanese patent No. 2746131
Fuji Photo Film Co. , Ltd.
Wilson Allan R.
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