Coherent light generators – Particular active media – Semiconductor
Reexamination Certificate
2000-07-07
2002-12-10
Leung, Quyen (Department: 2828)
Coherent light generators
Particular active media
Semiconductor
Reexamination Certificate
active
06493367
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a semiconductor laser device using a gallium nitride type semiconductor and an optical information reproduction apparatus using such a semiconductor laser device. More particularly, the present invention relates to a semiconductor laser device having a desirable FFP (Far Field Pattern).
2. Description of the Related Art
Prototype semiconductor laser devices have been produced in the art using a nitride type semiconductor material, such as GaN, InN, AlN, or a mixture thereof, which emit light whose wavelength ranges from a blue region to a UV region.
FIG. 16
illustrates a nitride semiconductor laser device
1600
oscillating at a wavelength of 405 nm, which was reported in Masaru KURAMOTO, et al., Jpn. J. Appl. Phys. vol. 38 (1999) pp. L184-L186. The semiconductor laser device
1600
includes an n-GaN layer
1601
(thickness: 100 &mgr;m). On the n-GaN layer
1601
, the semiconductor laser device
1600
further includes an n-Al
0.7
Ga
0.93
N lower cladding layer
1602
(thickness: 1 &mgr;m), an n-GaN lower guide layer
1603
(thickness: 0.1 &mgr;m), an In
0.2
Ga
0.8
N (thickness: 3 nm)/In
0.05
Ga
0.95
N (thickness: 5 nm)-triple quantum well active layer
1604
, a p-Al
0.19
Ga
0.81
N cap layer
1605
(thickness: 20 nm), a p-GaN upper guide layer
1606
(thickness: 0.1 &mgr;m), a p-Al
0.07
Ga
0.93
N upper cladding layer
1607
(thickness: 0.5 &mgr;m), and a p-GaN contact layer
1608
(thickness: 0.05 &mgr;m), which are deposited in this order. Electrodes
1609
and
1610
are provided on the lower side and the upper side of the device, respectively. The semiconductor laser device
1600
has a waveguide structure in which the active layer
1604
and the guide layers
1603
and
1606
are interposed between the cladding layers
1602
and
1607
, so that light generated in the active layer
1604
is confined in the waveguide structure so as to cause laser oscillation.
However, the conventional semiconductor laser device
1600
has the following problems. The present inventors have produced the semiconductor laser device
1600
with the above-described structure, and obtained an FFP as shown in FIG.
17
. In
FIG. 17
, the horizontal axis represents the angle of the beam along a plane which is perpendicular to the plane of the active layer
1604
and parallel to the longitudinal direction of the optical cavity. The vertical axis represents a relative beam intensity value. In the present specification, the term “FFP” refers to an FFP (i.e., an angular distribution of the light beam intensity measured at a position apart from the laser light opening of the laser device) along a direction perpendicular to the plane of the active layer. In the graph of
FIG. 17
, FFPs
1701
and
1702
are FFPs which have been obtained with the semiconductor laser device
1600
having the above-described structure. The FFPs
1701
and
1702
have a sub-peak in the vicinity of +20° and have many ripples. As shown in
FIG. 17
, the ripples are very suppressed for some individual devices, e.g., as shown by the FFP
1701
, and very significant for some other individual devices, e.g., as shown by the FFP
1702
. An FFP
1703
is an FFP obtained with the semiconductor laser device
1600
in which the thickness of the n-Al
0.07
Ga
0.93
N lower cladding layer
1602
is reduced from 1 &mgr;m to 0.7 &mgr;m. The FFP
1703
has a very large sub-peak in the vicinity of ±20°.
Although not shown in
FIG. 17
, research by the present inventors has demonstrated that ripples, including the sub-peak in the vicinity of 20° , are reduced by reducing the crystalline quality of the n-GaN layer
1601
, which is used as a substrate, or by increasing the amount of impurity. Conversely, the ripples in the vicinity of ±20° are increased when using a high quality crystal with little crystalline defect for the GaN layer
1601
and/or reducing the impurity concentration of the GaN layer
1601
in order to obtain a semiconductor laser device having a long operating life. It is believed that the differences between the ripples of FFP
1701
and those of FFP
1702
occurs due to the slight difference in terms of the conditions as described above. Moreover, it was also experimentally demonstrated that the ripples are generally more significant when the thickness of the GaN layer
1601
under the lower AlGaN cladding layer
1602
is greater. Since the thickness of the substrate is normally as great as 50 &mgr;m or more, it is very difficult to suppress these ripples when using GaN as a substrate as compared to when sapphire is used as a substrate.
Thus, in the prior art, ripples occur in the FFP, and in worst cases, it is not possible to obtain a single-peak FFP intensity pattern. This can be suppressed by taking one of the above-described measures: (1) increasing the thickness of the lower cladding layer
1602
: (2) reducing the crystalline quality of the GaN layer
1601
; and (3) increasing the amount of impurity in the GaN layer
1601
. However, if the AlGaN lower cladding layer
1602
is formed to be thick on the GaN layer
1601
, as shown in (1) above, a crack may occur. If the crystalline quality of the GaN layer
1601
is reduced, as shown in (2) above, or the amount of impurity in the GaN layer
1601
is increased, as shown in (3) above, the operating life of the obtained semiconductor laser device
1600
may be reduced. Thus, these measures (1) to (3) have limited effects, and it has been difficult to adequately control the production process with a good yield.
Ripples occurring in an FFP are undesirable because they may result in insufficient focusing or generation of stray light when the device is used in an optical pickup, or the like.
SUMMARY OF THE INVENTION
According to one aspect of this invention, there is provided a semiconductor laser device, including, in this order: a GaN layer; an Al
x1
Ga
1-x1
N (0.05≦x1≦0.2) lower cladding layer; an In
y1
Ga
1-y1
N (0<y1<1) lower guide layer (thickness: d1 [&mgr;m]); an active layer (thickness: Wa [&mgr;m]) having a multilayer structure comprising of alternating layers of a well layer and a barrier layer, the well layer comprising Al
a1
In
b1
Ga
1-a1-b1
N
1-e1-f1
P
e1
As
f1
(0≦a1, 0≦b1, a1+b1≦1, 0≦e1, 0≦f1, e1+f1<0.5), and the barrier layer comprising Al
a2
In
b2
Ga
1-a2-b2
N
1-e2-f2
P
2e
As
f2
(0≦a2, 0≦b2, a2+b2≦1, 0≦e2, 0≦f2, e2+f2<0.5); an In
y2
Ga
1-y2
N (0<y2<1) upper guide layer (thickness: d2 [&mgr;m]); and an Al
x2
Ga
1-x2
N (0.05≦x2≦0.2) upper cladding layer, wherein: the thicknesses and the compositions of the lower guide layer and the upper guide layer are set such that ripples in a far field pattern in a direction perpendicular to a stack plane are suppressed.
According to another aspect of this invention, there is provided a semiconductor laser device, including, in this order: a GaN layer; an Al
x1
Ga
1-x1
N (0.05≦x1≦0.2) lower cladding layer; an In
y1
Ga
1-y1
N (0<y1<1) lower guide layer (thickness: d1 [&mgr;m]); an active layer (thickness: Wa [&mgr;m]) having a multilayer structure comprising of alternating layers of a well layer and a barrier layer, the well layer comprising Al
a1
In
b1
Ga
1-a1-b1
N
1-a1-f1
P
a1
As
f1
(0≦a1, 0≦b1, a1+b1≦1, 0≦e1, 0≦f1, e1+f1<0.5), and the barrier layer comprising Al
a2
In
b2
Ga
1-a2-b2
N
1-e2-f2
P
e2
As
f2
(0≦a2, 0≦b2, a2+b2<1, 0≦e2, 0≦f2, e2+f2<0.5); an In
y2
Ga
1-y2
N(0<y2<1) upper guide layer (thickness: d2 [&mgr;m]); and an Al
x2
Ga
1-x2
N (0.05≦x2≦0.2) upper cladding layer, wherein: the thicknesses and the compositions of the lower guide layer and the upper guide layer are set such that an oscillating mode effective refractive index n
eq
of oscillation light from the semiconductor laser device and a refractive index n
GaN
of the GaN layer have a relationship of n
eq
≧n
GaN
.
According to still anot
Ito Shigetoshi
Taneya Mototaka
Yamasaki Yukio
Leung Quyen
Morrison & Foerster / LLP
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