Semiconductor optical waveguide

Details Semiconductor optical waveguide Semiconductor optical waveguide

1997-10-31

2001-08-28

Fourson, George (Department: 2823)

Active solid-state devices (e.g., transistors, solid-state diode

Incoherent light emitter structure

With reflector, opaque mask, or optical element integral...

C372S045013

Reexamination Certificate

active

06281523

ABSTRACT:

BACKGROUND OF THE INVENTION
(a) Field of the Invention
The present invention relates to a structure and a method for manufacturing a semiconductor optical waveguide and, more particularly, to fabrication of an improved optical waveguide for achieving a high coupling efficiency with an optical fiber by forming a circular and narrow optical beam in a semiconductor laser or a photodetector.
(b) Description of the Related Art
The mode field of an optical signal from a semiconductor laser device, for example, should be adjusted to the mode field of an optical fiber to be coupled for obtaining a high coupling efficiency as much as possible. A mode field converter (MFC) is generally installed for this purpose in the semiconductor laser device.
FIG. 1
is a cross-sectional view of a conventional product of a semiconductor laser device having a MFC, and
FIGS. 2A and 2B
are cross-sectional views thereof taken along lines I-I′ and II-II′ in FIG.
1
.
FIGS. 3
,
4
A,
4
B,
5
A and
5
B show the semiconductor laser device of
FIG. 1
in consecutive steps of fabrication process therefor, wherein
FIG. 3
showing a first step thereof corresponds to
FIG. 1
,
FIGS. 4A and 4B
showing a second step correspond to
FIGS. 2A and 2B
, respectively, and
FIGS. 5A and 5B
showing a third step correspond to
FIGS. 2A and 2B
, respectively. The structure and the fabrication process for the conventional semiconductor laser will be described with reference to these drawings.
In general, a semiconductor laser device having a MFC section is fabricated by an epitaxial growth process using a low-pressure MOCVD (metal-organic chemical vapor deposition) method from the viewpoint of process simplification. In the fabrication process, first, a SiO
2
film is deposited on a n-type InP substrate (n-InP substrate)
101
by using a plasma-enhanced CVD technique. The SiO
2
film is then selectively etched by a photolithography and a wet etching technique using a BHF (buffered hydrofluoric) solution to obtain a plurality of stripe SiO
2
patterns
120
shown in FIG.
3
. Each of the stripe patterns
120
has a rectangular shape which is 800-&mgr;m long (L) and 60-&mgr;m wide (W), and each two of the stripe patterns
120
form a stripe pair with the distance (d) therebetween being, for example, 10 &mgr;m. The stripe pairs are arranged in a matrix, with a gap (D
1
) of 300 &mgr;m in the column direction and a pitch (D
2
) of 250 &mgr;m in the row direction. Each of the stripe pairs and the area adjacent thereto in the column direction is formed as a semiconductor laser device, and accordingly,
FIG. 3
shows an area for a plurality of semiconductor laser devices formed in a single process.
After the stripe SiO
2
patterns
120
are formed on the substrate
101
, as shown in
FIG. 4A
, an n-InP cladding layer
102
, an InGaAsP/InGaAsP quantum well active layer
103
and a p-InP cladding layer
104
are consecutively grown on the exposed surface of the n-InP substrate
101
not covered by the stripe SiO
2
patterns
120
. In this epitaxial step, thick epitaxial layers
102
to
104
are formed in the belt area
124
(
FIG. 3
) disposed between each stripe pair, as shown in
FIG. 4A
, whereas thin epitaxial layers
102
to
104
re formed in the other area, as shown in FIG.
4
B.
After the stripe SiO
2
patterns
120
are removed by a BHF solution, a second SiO
2
film is deposited on the entire surface by a plasma-enhanced CVD technique. Thereafter, the second SiO
2
film is patterned using a photolithography and a wet etching technique to leave a belt SiO
2
film
121
on each 4.0-&mgr;m-wide belt zone defined by the belt areas
124
arranged in a column direction and the spaces between the adjacent belt areas
124
arranged in the column direction. A wet etching is then performed using the belt SiO
2
film
121
as a mask and bromomethanol as an etchant to selectively remove the n-InP cladding layer
102
, the quantum well active layer
103
and p-InP cladding layer
104
, as a result of which 1.5-&mgr;m-wide mesa stripe
123
is left below the 4.0-&mgr;m-wide belt SiO
2
film
121
, as shown in
FIGS. 5A and 5B
.
Subsequently, blocking layers including p-InP layer
105
and n-InP layer
106
are laminated on the side surface of the mesa stripe
123
, thereby embedding the mesa stripe
123
by using a MOCVD method as shown in
FIGS. 2A and 2B
. Thereafter, the belt SiO
2
film
121
is removed using a BHF solution, followed by a MOCVD process to form consecutively a p-InP cladding layer
107
and a p-InGaAs contact layer
108
. Next, the p-InGaAs contact layer
108
in the upper part of a MFC section II′ is selectively removed by a photolithography and a wet etching technique using a tartaric acid based etchant, the MFC section II′ being shown in FIG.
2
B.
Then, a third SiO
2
film
122
is deposited by a plasma-enhanced CVD process, and patterned to have an opening for an electric contact to be used for injection of carries in a laser section I′ as shown in FIG.
2
A. Thereafter, the n-InP substrate
101
is polished at the bottom surface thereof to reduce the thickness thereof down to about 100 &mgr;m, followed by formation of p-side electrode
109
and n-side electrode
110
on the top surface and the bottom surface, respectively, of the resultant wafer, to obtain the structure shown in
FIGS. 1
,
2
A and
2
B.
In the conventional semiconductor laser device as described above, the cladding layer
102
and the laser active layer
103
have smaller thicknesses in the laser section I′ than in the MFC section II′. By this configuration, a narrow and excellent optical beam can be obtained from the MFC section II′ because of the smaller optical confinement area of the MFC section II′. In this case, because the MFC section II′ is transparent for laser light, the optical loss is small in the optical transmission.
For the conventional semiconductor laser device having a MFC section as described above, a complicated process is required to form the optical waveguide therein. In addition, since the waveguide does not have a current confinement function in the direction of the resonator of the laser device, there arise a problem in that the carriers supplied to the laser section leak tat the MFC section to raise the threshold current for the lasing of the laser device.
In the case of the above described semiconductor laser device, the n-InP substrate used therein requests a p-InP epitaxial layer as the top layer for the layer structure, wherein the carrier leakage is effected by holes (not by electrons), which fact reduces the carrier leakage compared to the case wherein a p-type substrate is used and thereby the carrier leakage is effected by electrons. If a p-type substrate is used instead in the above laser device, the carrier leakage effected by electrons raises a larger problem.
SUMMARY OF THE INVENTION
It is therefore an object of the present invention to provide a method for manufacturing an optical waveguide which has a high coupling efficiency with an optical fiber, and which is capable simplifying the fabrication process of the semiconductor optical device.
It is another object of the present invention to provide a semiconductor optical device having a spot size converter instead of the conventional MFC converter and less susceptible to the carrier leakage problem.
The present invention provides a method for manufacturing a semiconductor optical waveguide comprising the steps of forming a first semiconductor layer overlying a semiconductor substrate, the first semiconductor layer having an aluminum concentration which increases from a central part, as viewed in the thickness direction of the first semiconductor layer, toward both surfaces of the first semiconductor layer, and selectively oxidizing the first semiconductor layer to obtain a non-oxidized region constituting an optical waveguide and an oxidized region surrounding the non-oxidized region.
The present invention also provides, in another aspect thereof, a semiconductor laser device comprising

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