Semiconductor laser device

Details Semiconductor laser device Semiconductor laser device

2002-04-11

2004-03-23

Ip, Paul (Department: 2828)

Coherent light generators

Particular active media

Semiconductor

C372S045013

Reexamination Certificate

active

06711197

ABSTRACT:

RELATED APPLICATION DATA
The present application claims priority to Japanese Application(s) No(s). P2001-114268 filed Apr. 12, 2001, which application(s) is/are incorporated herein by reference to the extent permitted by law.
BACKGROUND OF THE INVENTION
The present invention relates to a semiconductor laser device of ridge waveguide type. More particularly, the present invention relates to a semiconductor laser device of ridge waveguide type which has a desirably controlled half-width value &thgr;
//
of a far field pattern (FFP) in a direction horizontal to a hetero interface, exhibits good laser characteristics during high-output operation and merely requires a low driving voltage.
Semiconductor laser devices of ridge waveguide type, including those which are based on GaAs or InP for long wavelengths and a nitride based III-V group compound for short wavelengths, find use in a various application areas because they are easy to manufacture.
The semiconductor laser device of ridge waveguide type belongs to the category of index guided device. It has an upper portion of an upper cladding layer and a contact layer, both resembling a striped-shaped ridge. The ridge is formed such that an insulating film covers both sides of the ridge and the upper cladding layer extending sideward from the base of the ridge. This insulating film functions as a layer to constrict electric current and provides an effective refractive index difference in the lateral direction for mode control.
An explanation is given below, with reference to
FIG. 11
, of the structure of a related-art nitride based III-V group compound semiconductor laser device of ridge waveguide type which emits light with a wavelength of about 410 nm. This laser device is referred to as “nitride based semiconductor laser device” hereinafter.
FIG. 11
shows a related-art nitride based semiconductor laser device of ridge waveguide type
10
has basically a stacked structure in which a plularity of layers are stacked on a sapphire substrate
12
. The plularity of layers stacked on the sapphire substrate
12
are a laterally grown GaN layer
14
, an n-GaN contact layer
16
, an n-AlGaN cladding layer
18
, an active layer
20
, a p-AlGaN cladding layer
22
, and a p-GaN contact layer
24
.
In the stacked structure, the upper portion of the p-AlGaN cladding layer
22
and the p-GaN contact layer
24
are formed as a striped-shaped ridge
26
. A mesa structure extending in the same direction as the ridge
26
is formed by the upper portion of the n-GaN contact layer
16
, the n-AlGaN cladding layer
18
, the active layer
20
, and the remaining portion
22
a
of the p-AlGaN cladding layer
22
.
The ridge
26
has a width (W) of about 1.7 &mgr;m. The remaining portion
22
a
of the p-AlGaN cladding layer
22
which extends sideward from the base of the ridge
26
has a thickness (T) of about 0.17 &mgr;m.
An insulating film
28
of SiO
2
(about 2000 Å thick) is formed on both sides of the ridge
26
, the side of the mesa structure above the p-AlGaN cladding layer
22
extending sideward from the base of the ridge
26
, and the n-AlGaN contact layer
16
.
On the insulating film
28
is formed a p-side electrode
30
, which is in contact with the p-GaN contact layer
24
through a window in the insulating film
28
. On the n-GaN contact layer
16
is formed an n-side electrode
32
.
The nitride based semiconductor laser device of ridge waveguide type
10
mentioned above is considered as a highly efficient one because the insulating film
28
covering both sides of the ridge
26
is transparent to the emitted laser beam with little waveguide loss and the threshold current is small.
In the meantime, as its application areas expand, the nitride based semiconductor laser device of ridge waveguide type is required to have a higher kink level so that it maintains good characteristic property for light output vs. injected current throughout the region up to the high-output level. It is also required to have a larger half-width value &thgr;
//
of a far field pattern (FFP) in a direction horizontal to the hetero interface.
For example, in the case where the nitride based semiconductor laser device is used as a light source of an optical pickup, it is required to have a larger half-width value &thgr;
//
.
The results of the present inventors' investigation revealed that the value of &thgr;
//
is related closely with the difference (&Dgr;n) of effective refractive index of the ridge waveguide, as shown in FIG.
12
. In order to obtain a larger value of &thgr;
//
, it is necessary to have a larger value of &Dgr;n. Incidentally, the difference (&Dgr;n) of effective refractive index of the ridge waveguide is defined as n
eff1
−n
eff2
or a difference between n
eff1
which is the effective refractive index of the ridge for the oscillation wavelength and n
eff2
which is the effective refractive index of the ridge's side, as shown in FIG.
11
. Closed and open circles in
FIG. 12
denote the values obtained by experiments.
Unfortunately, any attempt to increase the value of &Dgr;n ends up with a narrow cutoff ridge width of high-order horizontal lateral mode. The cutoff ridge width of high-order horizontal lateral mode is defined as a ridge width which gives rise to no high-order horizontal lateral mode. When the ridge width is larger than the cutoff ridge width, the horizontal lateral mode tends to shift from the fundamental mode to the primary high-order mode at the time of laser oscillation.
When a hybrid mode consisting of the fundamental horizontal lateral mode and the high-order horizontal lateral mode occurs, a kink occurs in the light output-injected current characteristics, as shown in FIG.
13
. The result is a deterioration in the laser characteristics at the time of high-output operation.
The foregoing holds true particular for the nitride based semiconductor laser device of ridge wave-guide type, which has a small value of &Dgr;n and a short oscillation wavelength and hence has a narrow cutoff ridge width of high-order horizontal lateral mode, as shown in FIG.
14
.
FIG. 14
is a graph showing the relation between the value of &Dgr;n and the cutoff ridge width in the case where the GaN layer has a refractive index of 2.504 and an oscillation wavelength (&lgr;) of 400 nm. &Dgr;n stands for the difference between the effective refractive index of the ridge and the effective refractive index of the ridge's side. For example, if the value of &Dgr;n is 0.005 to 0.01, the ridge width should be reduced to about 1 &mgr;m so that the ridge width is smaller than the cutoff ridge width.
As mentioned above, any attempt to increase the value of &Dgr;n, thereby increasing the value of &thgr;
//
, ends up with a decreased cutoff ridge width, which leads to a deterioration in laser characteristics at the time of high-output operation. In other words, there is a tradeoff for ridge width between the value of &thgr;
//
and the laser characteristics at the time of high-output operation.
Moreover, the nitride based semiconductor laser device of ridge waveguide type has found an increasing use in the area of portable machines. The one for this purpose is required to have a lower drive voltage. One way to reduce the drive voltage is to increase the ridge width so that the contact area between the contact layer and the p-side electrode is increased. However, this suffers the disadvantage that the ridge width exceeds the cutoff ridge width, resulting in a deterioration in the laser characteristics at the time of high-output operation. In other words, there is a trade-off for the ridge width between the reduced drive voltage and the improved laser characteristics at the time of high-output operation.
The foregoing indicates that reducing the ridge width, thereby improving the laser characteristics at the time of high-output operation, contradicts increasing the value of &thgr;
//
and decreasing the drive voltage.
As mentioned above, the related-art nitride based semiconductor laser device poses several problems. That is, it does not

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