Coherent light generators – Particular active media – Semiconductor
Reexamination Certificate
2001-09-13
2003-11-11
Ip, Paul (Department: 2828)
Coherent light generators
Particular active media
Semiconductor
C372S045013, C372S043010
Reexamination Certificate
active
06647045
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to an air ridge type semiconductor laser device and a manufacturing method of the semiconductor laser device. In particular, the present invention relates to a long-life air ridge type semiconductor laser device having an excellent characteristic in relation between injection current and light output and a manufacturing method thereof.
2. Description of the Related Art
There are a variety of structures for a waveguide structure of a semiconductor laser device. Among those known structures, an air ridge waveguide type semiconductor laser device, which is often compared with that of an embedded waveguide type semiconductor laser device, has been focused as a waveguide structure which can be easily fabricated. Such an air ridge waveguide type semiconductor laser device (hereinafter, simply referred to as an “air ridge type semiconductor laser device”) can be fabricated in a following manner. First, an upper portion of an upper cladding layer is etched to form a ridge stripe. Due to a thickness of a lower portion of the upper cladding layer, which is left to elongatedly extend outwardly from a lower end edge, a predetermined difference in refractive index in a lateral (horizontal) direction is provided. Thereby, a waveguide having an optical confinement structure in the lateral direction can be realized. The air ridge type semiconductor laser device has advantages in that it is easy to form an optical waveguide, and that it requires less operational current because it is based on real refractive index waveguide which brings small internal loss.
Now, with reference to
FIG. 6
, an AlGaInP-containing air ridge type semiconductor laser device is taken as an example to explain a structure of a conventional air ridge type semiconductor laser device.
FIG. 6
is a cross sectional view showing the structure of the conventional air ridge type semiconductor laser device. The conventional air ridge type semiconductor laser device
50
has a multi-layer structure including an n-GaAs substrate
52
, and epitaxially grown layers are formed thereon in the sequential order of a first cladding layer (lower cladding layer)
54
of n-AlGaInP, an active layer
56
of GaInP, a second cladding layer (upper cladding layer)
58
of p-AlGaInP, an intermediate layer
59
of p-GaInP and a contact (capping) layer
60
of p-GaAs.
In the multi-layer structure, the contact layer
60
, the intermediate layer
59
of p-GaInP and an upper portion of the second cladding layer
58
are etched to be a ridge stripe
62
.
In other words, the second cladding layer
58
comprises an upper layer
58
a
which, together with the contact layer
60
and the intermediated layer
59
, forms the ridge stripe
62
, and a thin lower layer
58
b
which is positioned below the upper layer
58
a
and elongatedly extends from both bottom ends of the upper layer
58
a
outwardly. A dielectric film, for example, an SiO
2
film is stacked as a protection layer
64
on an upper surface of the lower layer
58
b
and side planes of the ridge
62
. However, an upper surface of the ridge
62
to be a current injection region is not covered with the protection layer
64
. In addition, a p-electrode
66
is formed on the protection layer
64
and on the contact layer
60
which is exposed from the protection layer
64
, and n-electrode
68
is formed on a back surface (bottom surface in the figure) of the GaAs substrate
52
.
Such a conventional air ridge type semiconductor laser device has a fatal problem that the period during which the device can operate exhibiting a predetermined operational property, that is, the device has a short life. For example, there is a problem that when a fixed light output is required, the longer the operating period is, the higher the operational current becomes. In other words, if the operational current is constant, the light output diminishes as the operating period becomes longer. This problem has been noticeably found in air ridge type semiconductor laser devices made of materials containing AlGaAs or AlGaInP, in particular.
As a result, it is difficult to use the air ridge type semiconductor laser device in a field of a light source for an optical pickup used in an optical disk recording/reproducing apparatus, for example, which requires high reliability. Accordingly, under the present circumstances, the air ridge type semiconductor laser devices are used only in a field of a laser pointer, for example, which requires relatively low reliability.
Accordingly, there is a need for a high-reliable long-life air ridge type semiconductor laser device.
During the study of the conventional air ridge type semiconductor device, the Inventors have conceived that one of the factors of the short life of the air ridge type semiconductor laser device is brought by follows. That is, since an interface between the cladding layer exposed by etching and the dielectric film or an ohmic metal (a metal used for an ohmic electrode) at the formation of the ridge is chemically instable, at the time of laser operation, operational current injection accelerates the deterioration of crystals in the cladding layer. In consideration thereof, the Inventors have hit upon an idea to have an epitaxially grown layer having a lattice constant close to that of the second cladding layer on the second cladding layer, as a protection layer, so as to chemically stabilize the second cladding layer. After carrying out various experiments based on the idea, the Inventors have invented the semiconductor laser device and the manufacturing method thereof, which are claimed in the application.
SUMMARY OF THE INVENTION
According to a first aspect of the present invention, there is provided an air ridge type semiconductor laser device comprising a structure including an active layer sandwiched between a first cladding layer (lower cladding layer) and a second cladding layer (upper cladding layer) each having a conductivity type different from each other. The second cladding layer comprises an upper layer which forms a ridge stripe, and a lower layer positioned below the upper layer, which elongatedly extends from both lower ends of the upper layer outwardly. A protection layer is provided on an upper surface of the lower layer of the second cladding layer and side planes of the ridge stripe other than an upper surface thereof. In the air ridge type semiconductor laser device, the protection layer is a compound semiconductor layer epitaxially grown on the upper surface of the lower layer of the second cladding layer and the side planes of the ridge stripe.
According to the present invention, the epitaxially grown layer is provided as the protection layer so that crystals of the second cladding layer are prevented from deterioration. Accordingly, the conventional problem that, when a constant light output is required, a longer operating period raises the operational current, that is, the problem that, if the operational current is constant, the longer operating period lowers the light output does not occur in the present invention.
The present invention may be applied without any limitation to materials for the compound semiconductor multi-layer structure which forms the structure in which the active layer is sandwiched between the cladding layers having different conductivity types. In addition, there is no limitation to the composition of the second cladding layer.
It is preferable that the compound semiconductor layer constituting the protection layer is a compound semiconductor layer having a conductivity type different from that of the second cladding layer. According to the arrangement above, the protection layer functions as a current confinement region due to p-n junction separation, which leads to better injection current-light output characteristics.
In addition, since it is required to have the protection layer epitaxially grown on the lower layer of the second cladding layer and side planes of the ridge, in order to have the protection layer and the second clad
Iwamoto Koji
Nagasaki Hiroki
Ip Paul
Kananen, Esq. Ronald P.
Nguyen Dung
Rader & Fishman & Grauer, PLLC
Sony Corporation
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