Coherent light generators – Particular active media – Semiconductor
Reexamination Certificate
2000-05-08
2002-05-07
Smith, Matthew (Department: 2825)
Coherent light generators
Particular active media
Semiconductor
C372S044010, C372S045013
Reexamination Certificate
active
06385225
ABSTRACT:
FIELD OF THE INVENTION
This invention relates to a window type semiconductor laser light emitting device and a process for fabricating the window type semiconductor laser light emitting device.
DESCRIPTION OF THE RELATED ART
The internet has rapidly spread far and wide, and the audiographics are drastically improved. The development is achieved on the basis of the large-capacity optical transmission technologies such as, for example, the optical fiber amplification/wavelength division multiplex communication technologies. A low-power consumption low-noise semiconductor laser light emitting device for 1 micron wavelength is used as a light source for optical pumping in the optical fiber amplification technology. When the semiconductor laser light emitting device is employed in the wavelength division multiplex communication system, the wavelength division multiplex communication system requires a high-power semiconductor laser light emitting device, the output power of which is proportional to the amplification ratio.
Crystal degradation called as “optical abrasion” is well known to skilled person. The optical abrasion is the phenomenon that the crystal at the end surface of the active layer is degraded due to the high-power laser light output. The optical abrasion proceeds as follows. The laser light is generated in the active layer, and is emitted from the end surface. While the laser light is being radiated from the end surface of the active layer, the surface state at the end surface absorbs part of the laser light, and the active layer generates heat. The heat makes the band gap around the end surface narrower. The narrow band gap absorbs the laser light more than before, and, accordingly, promotes the heat generation. The undesirable feedback loop results in the degradation of the crystal around the end surface, and malfunction takes place in the semiconductor laser emitting device. The optical abrasion is serious in the semiconductor laser light emitting device of 0.6-1 micron wavelength.
A window is effective against the optical abrasion. The window is implemented by using a buried layer wider in band gap than the active layer. The buried layer is transparent to the laser light by virtue of the wide band gap, and forms a window region at the end surface of the active region.
The prior art window type semiconductor laser light emitting device is fabricated as follows. A double heterojunction structure is formed on a semiconductor substrate, and forms a semiconductor laminated structure together with the semiconductor substrate. The semiconductor layers are partially removed from the semiconductor laminated structure for forming the window, and the semiconductor laminated structure is shaped into a striped mesa. The periphery of the striped mesa structure is covered with semiconductor material, which is transparent to the oscillating laser light and electrical insulating. Part of the transparent semiconductor layer serves as the window or the buried layer.
A problem is encountered in the prior art semiconductor laser light emitting device of the type having the striped mesa structure covered with the semiconductor transparent layer in serious coupling loss in the window region. The serious coupling loss is derived from the optical properties of the transparent semiconductor layer. The transparent semiconductor layer does not have any wave-guide in the window region in both of the vertical direction and the horizontal direction. The oscillating laser light is radiated from the internal active region of the resonator toward the boundary between the buried layer and the active layer, and is reflected on the end surface of the resonator. The reflection goes to and comes back in the window region. While the reflection is going to and coming back, the beam shape is deformed, and is coupled to the internal active region, again. When the laser light beam is coupled to the internal active region, energy loss takes place. The energy loss is referred to as “coupling loss”. Increase of the coupling loss results in decrease of the external differential quantum efficiency. Thus, the prior art window type semiconductor laser light emitting device has the problem in that the external differential quantum efficiency is low. Since the boundary is not perfectly perpendicular to the longitudinal direction of the resonator, the laser light beam is bend around the boundary of the buried layer. This results in that the output laser light beam inclines from the longitudinal direction of the resonator toward the vertical direction. Thus, the prior art window type semiconductor laser light emitting device has another problem in that the output laser light beam inclines. Research and development efforts are being made on a solution of the problems.
A solution is disclosed in Japanese Patent Publication of Unexamined Application (laid-open) No. 3-14281.
FIG. 1
illustrates the prior art window type self-aligned semiconductor laser light emitting device disclosed therein.
The prior art window type self-aligned semiconductor laser light emitting device is fabricated as follows. On an n-type GaAs substrate
31
are formed an n-type Al
y
Ga
1−y
As clad layer
32
, an active layer
33
, a p-type Al
y
Ga
1−y
As clad layer
34
and a p-type GaAs cap layer
35
which are successively grown by using a metal organic vapor phase epitaxial growing technique. Only an active region is covered with a photo-resist mask (not shown), and the p-type GaAs cap layer
35
, the p-type Al
y
Ga
1−y
As clad layer
34
and the active layer
33
are partially etched away through a wet etching technique. The photo-resist mask is stripped away. The n-type Al
y
Ga
1−y
As clad layer is exposed to the space where a window is formed.
Subsequently, a p-type Al
z
Ga
1−z
As optical guide layer
38
, an n-type Al
z
Ga
1−z
As current blocking layer
39
and an n-type GaAs current blocking layer
40
are successively grown on the entire surface of the resultant semiconductor structure. The space is buried with these layers
38
/
39
/
40
. The layers
38
/
39
/
40
in the space are referred to as a window region. The resultant semiconductor structure is partially covered with a photo-resist mask, and the part of the resultant semiconductor structure over the active region is exposed to wet etchant. The p-type Al
z
Ga
1−z
As optical guide layer
38
, the n-type Al
z
Ga
1−z
As current blocking layer
39
and the n-type GaAs current blocking layer
40
are partially etched away in the wet etchant until the p-type GaAs cap layer
35
is exposed, again. The part of the semiconductor structure over the active region is made coplanar with the remaining part of the semiconductor structure or the window region.
An SiO
2
stripe
51
is formed on the resultant semiconductor structure, and extends over the window region and the part of the layers
38
/
39
/
40
over the active region. Using the SiO
2
stripe
51
as an etching mask, the semiconductor structure is shaped into the striped mesa by using a wet etching, and the p-type Al
y
Ga
1−y
As clad layer
34
are decreased to 0.3-0.4 micron thick on both sides of the remaining semiconductor structure under the SiO
2
stripe
51
. An n-type GaAs current blocking layer
41
is selectively grown without removing the SiO
2
stripe
51
so as to bury the space on both sides of the remaining semiconductor structure under the SiO
2
stripe
51
.
Finally, the SiO
2
stripe is removed from the part of the semiconductor structure over the active region. A p-type electrode
36
is formed over the resultant semiconductor structure, and an n-type electrode
37
is formed on the reverse surface of the n-type GaAs substrate
31
. The resultant semiconductor structure is cleaved along the center of the window region, and the prior art window type semiconductor laser light emitting device is completed as shown in FIG.
1
. The p-type Al
z
Ga
1−z
As optical guide layer
38
vertically extends in the window region, and the n-type GaAs current blocking layer
Luu Chuong Anh
NEC Corporation
Smith Matthew
LandOfFree
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