Semiconductor laser device with non-oxidized facet regions

Details Semiconductor laser device with non-oxidized facet regions Semiconductor laser device with non-oxidized facet regions

2000-06-27

2002-08-06

Ip, Paul (Department: 2828)

Coherent light generators

Particular active media

Semiconductor

C372S050121

Reexamination Certificate

active

06430203

ABSTRACT:

BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a semiconductor laser device, and, more particularly, to a semiconductor laser device which has the ratio of a defective laser-beam emitting face reduced at the time of production, thus reducing the production cost and making the deterioration of characteristics over a long operational period difficult to occur.
2. Discussion of the Background
A ridge stripe semiconductor laser device as shown in
FIG. 1
is known.
This semiconductor laser device has a stripe-shaped ridge structure B
0
formed on a substrate
1
and one facet formed as a low-reflection face and the other facet as a high-reflection face.
This semiconductor laser device has a strained quantum well active layer
2
of, for example, i-type (nondoped) GaInAsP stacked, by epitaxial growth, on the substrate
1
of, for example, n-type InP and an n-type semiconductor buffer layer (not shown) formed on the substrate. Formed in order on the layer
2
are a spacer layer
3
of p-type InP, a layer
4
of an Al-containing compound semiconductor such as p-type AlInAs, and a layer
5
of p-type InP, thereby forming an upper clad layer as a whole on which a contact layer
6
of, for example, p-type GaInAsP is stacked. This layer structure forms the ridge structure B
0
.
The surfaces of the ridge structure B
0
and the substrate
1
are coated with an insulation protection film
7
of, for example, SiNx (silicon nitride). The insulation protection film
7
is removed at the top portion of the ridge structure B
0
where an upper electrode
8
of, for example, Ti/Pt/Au is formed in contact with the contact layer
6
, and a lower electrode
9
of, for example, an AuGeNi alloy is formed at the bottom surface of the substrate.
The structural feature of this semiconductor laser device lies in that both side portions
4
a
,
4
a
of the aforementioned layer
4
are insulation areas essentially consisting of an Al oxide, so that when a current is supplied from the upper electrode
8
, both side portions
4
a
,
4
a
and a center portion
4
b
together serve as a current constriction layer.
This semiconductor laser device is, roughly, produced as follows. First, the buffer layer of, for example, n-type InP, the strained quantum well active layer
2
of i-type GaInAsP, the spacer layer
3
of p-type InP, the layer
4
of p-type AlInAs, the layer
5
of p-type InP and the contact layer
6
of p-type GaInAsP are stacked in order on a wafer of n-type InP, which has a predetermined planar dimension, by epitaxial growth, thereby yielding a semiconductor layer structure which is flat as a whole. Then, a thin film of SiNx (silicon nitride) is formed covering the entire surface of this layer structure.
Then, the SiNx (silicon nitride) thin film is made into a pattern of a plurality of stripes having the equal width in the lengthwise direction by using an ordinary photolithography technique. With this pattern as a mask, for example, reactive ion beam etching is performed to etch out at least that portion of the aforementioned layer structure which extends to the spacer layer
3
, forming a plurality of stripe-shaped ridge structures B
0
. Therefore, the contact layer and both side portions of the upper clad layer are exposed at the sides of each ridge structure.
Then, the entire structure is heated under a vapor atmosphere for a predetermined time. During this process, the Al component in the layer
4
of an Al-containing compound semiconductor in the upper clad layer is sequentially oxidized to become an Al oxide from both side portions toward the center portion. As a result, both side portions
4
a
,
4
a
of this layer
4
become an insulation area and the center portion
4
b
stays as a non-oxidized area. Because the other layers in the upper clad layer do not contain an Al component, no oxidation occurs at both side portions.
As the portion directly above the active layer
2
stays as a layer structure having a conductivity after the aforementioned oxidizing process and both side portions of its upper portion are insulation areas, and therefore, a current constriction structure with respect to the active layer is formed there.
Then, SiNx (silicon nitride) is deposited again covering the entire surface, thereby forming the insulation protection film
7
, after which the insulation protection film only at the top portion of the ridge structure B
0
is removed to expose the contact layer
6
.
Then, for example, Ti/Pt/Au is vapor-deposited on the entire surface by electron beam vapor deposition, thereby forming the upper electrode
8
in contact with the contact layer
6
, and after the bottom surface of the wafer is polished, an AuGeNi alloy, for example, is vapor-deposited there, thus forming the lower electrode
9
.
Next, this wafer is cleaved into a bar in a direction perpendicular to the lengthwise direction of each of the ridge structure formed in stripes. The cleaved face of the acquired bar becomes the laser-beam emitting face. Thereafter, the individual ridge structures are separated in the lengthwise direction, yielding intended semiconductor laser devices.
In this semiconductor laser device, the area R around that portion of the active layer
2
which is located directly below the center portion
4
b
of the layer
4
is a luminescence area.
This semiconductor laser device has such an advantage that its production, especially, the formation of the current constriction structure is easy. The semiconductor laser device however has the following shortcomings.
First, when both side portions
4
a
,
4
a
of the layer
4
comprised of the Al-containing compound semiconductor are oxidized to be insulation areas, the volumes of both side portions
4
a
,
4
a
shrink during this oxidation process so that a large strain might storage in the crystal in this portion. When the wafer is cleaved, cracks C may run upward or downward from the layer
4
at the cleaved face as shown in
FIG. 2
due to this strain so that the cleaved face does not become flat, or the cracks C may extend to the luminescence area R. When such a crack C occurs on the cleaved face, the threshold current of the laser beam increases and what is more, there occurs a problem of a time-dependent degradation of the optical output characteristics.
That is, the above-described semiconductor laser device has such a problem that the laser-beam emitting face or the cleaved face is likely to become defective at the time of cleaving the wafer in the production process, which results in a higher production cost and is likely to lead to the degradation of the characteristics over a long operational period.
SUMMARY OF THE INVENTION
It is an object of the present invention to provide a semiconductor laser device which has no cracks formed on the cleaved face (laser-beam emitting face) at the time of cleaving a wafer or can prevent the threshold current of the laser beam from rising due to cracks if produced, thus reducing defective laser-beam emitting faces, which makes it difficult to cause the degradation of the characteristics over a long operational period.
To achieve the above object, according to this invention, there is provided a semiconductor laser device (hereinafter called “device B
1
”) comprising:
a ridge structure including a layer comprised of an Al-containing compound semiconductor formed at an upper portion, both side portions of the layer being oxidized; and
a laser-beam emitting face of the ridge structure being a non-oxidized area.
According to this invention, there is also provided a semiconductor laser device (hereinafter called “device B
2
”) comprising:
a ridge structure including a layer comprised of an Al-containing compound semiconductor formed at an upper portion, both side portions of the layer being oxidized; and
the width of the ridge structure in the vicinity of a laser-beam emitting face being wider than the width of the ridge structure in the center portion.
To begin with, the device B
1
will be discussed with reference to the drawings.
FIG. 3
is a perspective view sh

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