Coherent light generators – Particular resonant cavity – Distributed feedback
Reexamination Certificate
1998-03-13
2001-02-27
Davie, James W. (Department: 2881)
Coherent light generators
Particular resonant cavity
Distributed feedback
C372S046012
Reexamination Certificate
active
06195380
ABSTRACT:
BACKGROUND TO THE INVENTION
1. Field of the Invention
The present invention relates to semiconductor lasers and methods of fabricating such lasers. In particular, the present invention relates to digital versatile disks (hereafter referred to as “DVD”) laser diode structure and fabrication methods.
2. Description of Prior Art
Semiconductor lasers are widely used in communication and data storage. For communication, an IR laser with a long wavelength is normally used. On the other hand, a laser with a relative short wavelength, such as an AlGaInP-based laser diode, is used for data storage.
AlGaAs-based laser diodes, which have a wavelength of 780 nm, are commonly used to record data on compact discs and CD-ROM discs. With the development of DVD, AlGaInP-based laser diodes are now in great demand since the wavelength of the laser light source used in the DVD specification is set to 630~650 nm.
The formula of AlGaInP alloy can be represented by (Al
x
Ga
1-x
)
0.5
In
0.5
P where x is a value between 0 and 1. The lattice of AlGaInP alloy is matched with that of GaAs. The AlGaInP alloy is a direct semiconductor when x is between 0 and 0.7. That is, the energy gap of Ga
0.5
In
0.5
P is 1.9 eV for x=0; the energy gap of (Al
0.7
Ga
0.3
)
0.5
In
0.5
P is 2.3 eV for x=0.7. Therefore, the light-emitting AlGaInP-based devices emit red light. The (Al
0.7
Ga
0.3
)
0.5
In
0.5
P alloy, having a larger energy gap, can serve as a barrier layer in a quantum well, a waveguide or a cladding layer in the light-emitting device.
In the past, many structures and fabricating methods of AlGaInP-based laser diode have been proposed. Reference may be made to FIG.
1
, which illustrates a cross section of a selectively buried ridge waveguide laser diode. Such a laser diode includes an n-type AlGaInP cladding layer
12
, a GaInP active layer
14
, a p-type AlGaInP cladding layer
16
, an n-type GaAs blocking layer
18
, a p-type GaInP layer
19
, a p-type GaAs contact layer
20
and a p-type electrode
22
formed in this order on an n-type GaAs substrate
10
. Then, an n-type electrode
21
is formed on the other side of the n-type GaAs substrate
10
.
The structure mentioned above is fabricated by the MOCVD (Metalorganic Chemical Vapor Deposition) method, which grows an n-type AlGaInP cladding layer
12
, a GaInP active layer
14
, a p-type AlGaInP cladding layer
16
, an n-type blocking layer
18
and a p-type GaInP layer
19
in order on an n-type GaAs substrate
10
. A layer of Si
3
N
4
or SiO
2
is then formed on the p-type GaInP layer
19
. By photolithography and etching processes, a pattern of a strip having a width of 3~5 &mgr;m is defined on the Si
3
N
4
or SiO
2
layer. The strip of Si
3
N
4
or SiO
2
layer is used to serve as a mask in the etching process. The deposited layers are then etched back to the p-type AlGaInP cladding layer
16
with the mask. By selectively applying MOCVD processes, a layer of n-type GaAs
18
is formed on the sidewall and the area except the strip area, i.e., the mask, to serve as a current blocking layer. A p-type GaAs contact layer
20
is then formed thereon after removing the strip mask.
Based on the above description, the fabrication process is very complex, because there are two regrowth steps in the fabrication process for a selectively buried ridge waveguide laser diode.
FIG. 2
shows another conventional semiconductor laser, a hetero-barrier blocking laser diode, which includes an n-type AlGaInP cladding layer
26
, a GaInP active layer with quantum wells
28
, a p-type AlGaInP cladding layer
30
, a p-type GaInP cap layer
32
, a p-type GaAs contact layer
34
and a p-type electrode
36
formed in this order on an n-type GaAs substrate
24
, and an n-type electrode
35
formed on the other side of the n-type GaAs substrate
24
.
The hetero-barrier blocking laser diode confines the current by the hetero-barrier blocking effect. That is, the current is confined by the difference between voltage drops of the p-type AlGaInP cladding layer
30
/the p-type GaAs contact layer
34
and the p-type AlGaInP cladding layer
30
/the p-type GaInP cap layer
32
/the p-type GaAs contact layer
34
.
Compared to the fabrication of a selectively buried ridge waveguide laser diode, a regrowth step can be reduced in the fabrication process of a hetero-barrier blocking laser diode. However, such structure has an inferior current confinement, and thus a low threshold current for the laser diode may not be obtained.
Referring to
FIG. 3
, the other conventional laser diode, a ridge waveguide laser diode, is provided. Such laser diode includes an n-type AlGaInP cladding layer
42
, an active layer
44
, a p-type AlGaInP cladding layer
46
, a dielectric layer
48
, a p-type GaInP layer
50
, a p-type GaAs layer
52
and a p-type electrode
54
formed in this order on an n-type GaAs substrate
40
, and an n-type electrode
53
formed on the other side of the n-type GaAs substrate
40
. A ridge waveguide is formed on the p-type AlGaInP cladding layer
46
, and both of the p-type GaInP layer
50
and the p-type GaAs layer
52
are formed on the ridge waveguide.
The above dielectric layer
48
can be nitride or oxide. Both of the n-type AlGaInP cladding layer
42
and the p-type AlGaInP cladding layer
46
can be represented by (Al
x
Ga
1-x
)
0.5
In
0.5
P, where x is 0.7. The active layer
44
has a quantum well structure, which consists of Ga
y
In
1-y
P and (Al
x
Ga
1-x
)
0.5
In
0.5
P formed in turn.
The ridge waveguide laser diode can be fabricated without the using of regrowth steps during the MOCVD process, but the etching of very small ridge width is difficult. Besides, the power output for single mode operation is limited.
As described above, the methods to fabricate AlGaInP laser diode of the prior art are too complicated and can be simplified by this invention.
SUMMARY OF THE INVENTION
Accordingly, one object of the present invention is to provide a laser diode structure for DVD and the fabrication method, which can significantly simplify the fabrication process.
A further object of the present invention is to provide a laser diode structure for DVD and the fabrication method, which allows for a relatively low threshold current for the laser diode.
One feature of the fabrication method of this invention is the forming of a layer of Al
x
Ga
1-x
As (x>0.8) between the top and/or the bottom AlGaInP cladding layer, then oxidizing the AlGaAs layer to form AlO
x
by an oxidization step, and forming apertures on the AlO
x
layer. The AlO
x
layer can serve as a current confinement layer. Hence, since there is no need for regrowth steps, the fabrication process can be significantly simplified. Furthermore, since the opening or the aperture on the AlO
x
layer can be easily controlled, small size openings can be easily obtained.
REFERENCES:
patent: 4949349 (1990-08-01), Ohba et al.
patent: 5065402 (1991-11-01), Kawano
patent: 5903588 (1999-05-01), Guenter et al.
patent: 6014400 (2000-01-01), Kobayashi
Hong Ming-Huang
Huang Man-Fung
Shih Kwang-Kuo
Davie James W.
Industrial Technology Research Institute
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