Coherent light generators – Particular active media – Semiconductor
Reexamination Certificate
2000-09-11
2003-04-29
Davie, James W. (Department: 2828)
Coherent light generators
Particular active media
Semiconductor
Reexamination Certificate
active
06556603
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a semiconductor laser element using a gallium-nitride-contained semiconductor laser element as well as an optical information reproducing device using the same, and particularly relates to a semiconductor laser device providing a good FFP (Far Field Pattern).
2. Description of the Background Art
Such a prototype of a semiconductor laser element is already prepared that is made of a nitride-contained semiconductor material such as GaN, InN, AlN or mixed crystal thereof, and can emit light in a region from blue to ultraviolet light
FIG. 22
is a schematic view showing a nitride semiconductor laser element (or light-emitting diode) disclosed in Japanese Patent Laying-Open No. 9-55560 of the same inventors, and shows a cross section of a waveguide stripe portion of the semiconductor laser taken along a plane perpendicular to a resonator. This semiconductor laser element (or light-emitting diode) includes a low-temperature buffer layer
192
which is a thin layer of GaN, AlN or AlGaN, and is formed on a sapphire substrate
191
. The semiconductor laser element further includes an n-type GaN contact layer
193
, an n-type lower clad layer
202
, a non-doped or silicon-doped AlGaInN active layer
196
(or light-emitting layer), a p-type upper clad layer
197
and a p-type GaN contact layer
198
, which are layered on buffer layer
192
. A positive electrode
200
is formed on p-type contact layer
198
. n-type GaN layer
193
has a partially exposed portion, on which a negative electrode
201
is formed, For forming this exposed portion, the n-type lower clad layer, active layer, p-type upper clad layer and a p-type GaN cap layer
199
located above the n-type GaN layer are partially removed.
In this semiconductor laser element, the n-type lower clad layer is configured by repetitively growing the n-type Al
0.1
Ga
0.9
N clad layer
194
of about 0.15-0.3 &mgr;m in thickness and an n-type Al
a
Ga
1−a
N (0≦a≦1) buffer layer
195
and about 200 Å in thickness, and thereby lower clad layer
202
having a thickness of about 1 &mgr;m is prepared.
It is desired that the total thickness of the n-type lower clad layer is large from the viewpoint of preventing such a situation that the electric field distribution in the laser oscillation mode spreads to the n-type GaN contact layer, and thereby causes ripples in FFP. Due to difficulty in crystal growth, however, the thickness of the n-type Al
0.1
Ga
0.9
N clad layer, which can be grown on the n-type GaN layer with good yield, is restricted. Although this specification relates to the photoelectric field distribution and FFP in the semiconductor laser element structure, it particularly relates to the photoelectric field distribution and FFP in the direction perpendicular to the semiconductor multilayer film. The photoelectric field distribution and the FFP in the above direction are merely referred to as the electric field distribution and the FFP, respectively. In this semiconductor laser element, therefore, the n-type lower clad layer having a total thickness of about 1 &mgr;m is formed by repetitive growth of the n-type Al
0.1Ga
0.9
N clad layer of about 0.15-0.3 &mgr;m in thickness and the n-type Al
a
Ga
1−a
N (0≦a≦1) buffer layer of about 200 Å.
However, the foregoing semiconductor laser element in the prior art suffers from the following problems. The inventors and others actually prepared the semiconductor laser wafer of the foregoing structure, and observed its surface with an optical microscope of a magnification of about 200 times. As a result, it was found that some wafers have fine or good surfaces, but the others have hexagonal cracks at peripheral portions. More specifically, the hexagonal cracks were found in eight among ten wafers.
Elements each formed of a portion of the wafer of the laser element structure, in which the crack was not present, were prepared. According to these element, laser oscillation occurred with a threshold current density of 2-3 kA/cm
2
. Elements each made of a cracked wafer portions were also prepared. According to these elements, the laser oscillation occurred only 15 among 30 elements.
As described above, the structure which employs the buffer layers containing AlN, GaN or AlGaN cannot sufficiently achieve the effect of preventing cracks, and therefore suffers from a problem of serious lowering of the yield.
An object of the invention is to overcome the foregoing problem, to provide a nitride semiconductor laser element which is optimum for application to an optical pickup or the like, and to achieve an optical information reproducing device having good light focusing characteristics.
SUMMARY OF THE INVENTION
For achieving the above object, the invention provides a nitride-contained semiconductor laser element including a layer formed of an Al
x1
Ga
1−x1
N (0.08≦x1≦0.2) lower clad layer, an active layer formed of an alternate multilayer structure including an In
w
Ga
1−w
N well layer and an In
v
Ga
1−v
N barrier layer, and an Al
x2
Ga
1−x2
N (0.08≦x2≦0.2) upper clad layer layered in this order on a substrate, one or a plurality of In
z
Ga
1−z
N (0≦z≦0.2) buffer layer(s) of 200 nm or less in thickness being disposed in the lower clad layer and/or the upper clad layer
Preferably, in the above structure, the In
z
Ga
1−z
N (0≦z≦0.2) buffer layer(s) have a thickness and a composition determined to suppress ripples in a far field pattern in a direction perpendicular to a layer surface.
Preferably, in the above structure, either or each of the upper and lower clad layers had a total thickness in a range from 0.8 &mgr;m to 10 &mgr;m.
Preferably, in the above structure, at least one of the In
z
Ga
1−z
N (0≦z≦0.2) buffer layers disposed in the upper and/or lower clad layers is configured such that each of the In
z
Ga
1−z
N (0≦z≦0.2) buffer layers (having a dielectric constant ∈
1
) and the lower/upper clad layers (having a dielectric constant ∈
A
, neighboring to the opposite sides form a waveguide providing a waveguide mode (effective refractive index n
i
, electric field distribution f
i
(x) in a direction perpendicular to the semiconductor layer); a waveguide layer for oscillation light of the semiconductor laser element as well as the lower clad layer and the upper clad layer form a waveguide providing a waveguide mode (effective refractive index n
eq
, electric field distribution f
eq
(x) in the direction perpendicular to the semiconductor layer); these waveguide modes determine a parameter F; and the buffer layer has the thickness and the composition determined to satisfy a relationship of F<0.4, assuming that:
F
=1/{1+(&Dgr;/&kgr;)
2
}
&Dgr;=(&pgr;/&lgr;)(
n
eq
−n
i
)
&kgr;(&ohgr;·∈
0
/4)∫{
f
eq
(
x
)*·&dgr;∈(
x
)·
f
i
(
x
)}
dx
&dgr;∈(
x
)=∈
I
−∈
A
(
x:
within In
z
Ga
1−z
N buffer layer), 0 (
x:
other than it)
&ohgr;=2&pgr;
c
0
/&lgr;
&lgr;: wavelength of oscillation light in vacuum
c
0
: velocity of light in vacuum
∈
0
: dielectric constant in vacuum
Preferably, the above structure includes a GaN lower guide layer (thickness; 0.08-0.15 &mgr;m) located between the lower clad layer and the active layer, and a GaN upper guide layer (thickness: 0.08-0.15 &mgr;m) located between the Al
x2
Ga
1−x2
N (0.08≦x
2
≦0.2) upper clad layer and the active layer; a distance from the In
z
Ga
1−z
N (0≦z≦0.2) buffer layer disposed in the lower clad layer to the GaN upper guide layer, or a distance from the In
z
Ga
1−z
N (0≦z≦0.2) buffer layer disposed in the upper clad layer to the GaN upper guide layer is equal to d [&mgr;m]; an average aluminum composition of the lower clad layer disposed in a region from the In
z
Ga
1−z
N (0≦z≦0.2) buffer layer to the GaN lower guide layer, or an average aluminum composition of the upper clad layer dispos
Okumura Toshiyuki
Yamasaki Yukio
Davie James W.
Morrison & Foerster / LLP
Sharp Kabushiki Kaisha
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