Semiconductor laser device capable of suppressing leakage...

Details Semiconductor laser device capable of suppressing leakage... Semiconductor laser device capable of suppressing leakage...

2002-11-08

2003-11-11

Scott, Jr., Leon (Department: 2828)

Coherent light generators

Particular active media

Semiconductor

C372S087000

Reexamination Certificate

active

06647047

ABSTRACT:

BACKGROUND OF THE INVENTION
The present invention relates to a semiconductor laser device having a protective coating with a high-reliability formed on an end surface, and to a method for manufacturing the same.
As shown in
FIG. 5
, most semiconductor laser devices are composed of, for example, protective coatings
43
a
and
43
b
, each having an identical reflectance, formed on light emitting end surfaces
41
a
and
41
b
of a GaAs laser chip
4
. Reference numeral
42
denotes an active layer of the laser chip
4
. In the case where the protective coatings
43
a
and
43
b
are composed of Al
2
O
3
in
FIG. 5
, if a refractive index of the Al
2
O
3
film is set to be 1.60 while a refractive index of the laser chip
4
is set to be 3.50, a reflectance of the protective coatings
43
a
and
43
b
corresponding to a coating thickness d varies as shown in
FIG. 6
(provided that a laser emission wavelength &lgr;=7800 Å).
FIG. 6
indicates that regardless of the coating thickness d of the protective coatings
43
a
and
43
b
, the reflectance thereof is smaller than that of the case without the protective coatings
43
a
and
43
b
(i.e. the reflectance of the light emitting end surfaces
41
a
and
41
b
). The reflectance becomes smallest when an optical coating thickness (refractive index n×coating thickness d) is an odd multiple of &lgr;/4, while the reflectance becomes approximately equal to that in the case without the protective coatings
43
a
and
43
b
when the optical coating thickness is an integral multiple of &lgr;/2. This is because the refractive index (1.60) of the protective coatings
43
a
and
43
b
is smaller than the refractive index (3.50) of the GaAs laser chip
4
.
Contrary to this, in the case where the refractive index of the protective coatings
43
a
and
43
b
is larger than the refractive index of the GaAs laser chip
4
(for example, if such material as Si is used as the protective coatings
43
a
and
43
b
, the reflectance thereof is larger than that in the case without the protective coatings
43
a
and
43
b
, regardless of the coating thickness), the reflectance becomes largest when the optical coating thickness is an odd multiple of &lgr;/4, while the reflectance becomes approximately equal to that in the case without the protective coatings
43
a
and
43
b
when the optical coating thickness is an integral multiple of &lgr;/2.
In the case of high output semiconductor laser devices with optical output as high as 20 mW or more, as shown in
FIG. 7
, for increasing optical output Pf from the side of a main emitting end surface (front end surface), the reflectance of the protective coating
43
a
on the side of the main emitting end surface
41
a
is generally set lower than that in the case without the protective coating
43
a
, while the reflectance of the protective coating
43
b
on the side of a rear emitting end surface
41
b
is set higher than that in the case without the protective coating
43
b.
For example, the reflectance of the protective coating (Al
2
O
3
)
43
a
on the main emitting end surface
41
a
is set to be approx. 15% or less. This reflectance is obtained with the coating thickness of approx. 700 Å to 1600 Å.
The protective coating
43
b
on the rear emitting end surface
41
b
, if composed with use of a film having a refractive index larger than that of the laser chip
4
, is not capable of providing a sufficiently high reflectance as a single layer. Accordingly, an Al
2
O
3
film with a thickness of &lgr;/4 as a first layer
431
and a third layer
433
, and an amorphous Si with a thickness of &lgr;/4 as a second layer
432
and a fourth layer
434
, are laminated. Then finally, an Al
2
O
3
film with a thickness of &lgr;/2 as a fifth layer
435
is laminated. This makes it possible to form a protective coating
43
b
having a reflectance as high as approx. 85% or more. It is noted that reference numeral
43
denotes an active layer.
Description will now be given of a method for forming protective coatings
43
a
and
43
b
having the above-described reflectance on light emitting end surfaces
41
a
and
41
b
of a semiconductor laser chip
4
.
First, after one side of an n-GaAs substrate is polished, a p-electrode (comprising an ohmic electrode and a bonding electrode) is formed by evaporation or sputtering. A photomask is made thereon and, then, the p-electrode is patterned by etching.
Subsequently, after the other side of the n-GaAs substrate is polished, an n-electrode (comprising an ohmic electrode and a bonding electrode) is formed by evaporation or sputtering according to the above procedures. After that, the electrodes and the substrate are alloyed at appox. 400 to 500° C.
Next, as shown in
FIG. 8
, a cleavage line
49
is formed by scribing extensively disposed between an electrode
44
of an arbitrary element in a semiconductor laser wafer
5
and an electrode
44
′ of an adjoining element in direction orthogonal to an emitting section (channel)
42
. Then, as shown in
FIG. 9
, the semiconductor laser wafer
5
is cleaved and divided into a plurality of laser bars (bar-shaped laser chips)
51
.
Next, as shown in
FIG. 10
, a plurality of the laser bars
51
obtained by dividing are loaded in a laser bar fixing device
6
such that the electrodes
44
face towards the same side. All the laser bars
51
should be loaded so that all the emitting end surfaces
41
a
are positioned on the same side and, therefore, all the emitting end surfaces
41
b
are positioned on the same side. Next, on the emitting end surfaces
41
a
and
41
b
of laser bars
51
loaded in the laser bar fixing device
6
, a protective coating having a specified reflectance is formed generally by using a vacuum evaporator
7
schematically shown in FIG.
11
. The vacuum evaporator
7
is provided with a vapor source
72
, a holder
73
for holding a plurality of the laser bar fixing devices
6
, and a crystal oscillator
74
for monitoring the thickness of evaporated films, all in a chamber
71
.
Following description discusses procedures of forming the protective coating. First, in the case for evaporating a protective coating onto the emitting end surface
41
a
, the holder
73
is disposed such that the emitting end surface
41
a
of a laser bar
51
faces the vapor source
72
side as shown in FIG.
11
. Then, the chamber
71
is evacuated through a duct
75
. After a specified degree of vacuum is obtained, an evaporation material
76
put in the vapor source
72
is heated and evaporated by electron beams and the like so that a protective coating is evaporated onto the emitting end surface
41
a
of the laser. After evaporation is completed, the holder
73
is then rotated 180° for evaporating a protective coating onto the emitting end surface
41
b
according to the above procedures.
Here, a forming speed (evaporation rate) for forming a protective coating on the both light emitting end surfaces
41
a
and
41
b
is controlled so as to be approximately constant till completion of evaporation. The evaporation rate is in this case controlled with a heating temperature. In the case of electron beam evaporation, therefore, the evaporation rate may be controlled with the intensity of electron beams. It is well known that in the case of resistance heating, the evaporation rate is controlled with an amount of electric current passed through a resistor. The evaporation rate is generally set in a range between several Å/sec to 30 Å/sec in the case of the evaporation material of Al
2
O
3
Evaporation is conducted while coating thickness is monitored with use of the crystal oscillator
74
. Evaporation is terminated when a prescribed coating thickness is obtained.
Although not shown in
FIG. 7
, in the case of a high output type semiconductor laser device, a low reflecting protective coating
43
a
(having a reflectance of approx. 15% or less) is formed on the side of the main emitting end surface
41
a
, and then a multilayered high reflecting protective coating
43
b
is formed on the side

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