Coherent light generators – Particular active media – Semiconductor
Reexamination Certificate
2001-12-20
2004-03-23
Ip, Paul (Department: 2828)
Coherent light generators
Particular active media
Semiconductor
C372S043010
Reexamination Certificate
active
06711193
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a semiconductor laser device formed by a gallium nitride-based semiconductor, and to an optical information reproducing apparatus using the same.
2. Description of the Background Art
In recent years, a semiconductor laser device that emits light in a range from a blue region to an ultraviolet region has been prototyped using a nitride-based semiconductor material represented by GaN, InN, AlN and a mixed crystal thereof.
FIG. 19
shows a nitride semiconductor laser device oscillating at a wavelength of 405 nm, which was reported by Masaru Kuramoto et al. in Japanese Journal of Applied Physics vol. 38 (1999), pp. L184-L186.
A semiconductor laser device
500
has hexagonal nitride-based semiconductor layers (hereinafter referred to as a “layered lump”)
12
formed on an n-GaN substrate
10
(film thickness of 100 &mgr;m). Layered lump
12
is constituted by an n-Al
0.07
Ga
0.93
N lower clad layer (film thickness of 1 &mgr;m), an n-GaN lower guide layer (film thickness of 0.1 &mgr;m), an In
0.2
Ga
0.8
N (film thickness of 3 nm)/In
0.05
Ga
0.95
N (film thickness of 5 nm)-3 quantum well active layer, a p-Al
0.19
Ga
0.81
N evaporation preventing layer (film thickness of 20 nm), a p-GaN upper guide layer (film thickens of 0.1 &mgr;m), a p-Al
0.07
Ga
0.93
N upper clad layer (film thickness of 0.5 &mgr;m), and a p-GaN contact layer (film thickness of 0.05 &mgr;m) that are layered in this order. Moreover, a positive electrode
14
and a negative electrode
15
are formed at the top and bottom sides of the layered lump, respectively. A mirror end face
400
is formed by a conventional cleaving method, which will be described later in detail. Furthermore, a stripe optical waveguide
13
is provided within layered lump
12
, and serves to guide laser light. Semiconductor laser device
500
has a waveguide structure in which an active layer and a guide layer are interposed between clad layers. Thus, light emitted at the active layer is trapped into the waveguide structure, and the mirror end face functions as a laser cavity mirror, generating laser oscillating operation.
Mirror end face
400
is manufactured by the conventional cleaving method, for example, as described below.
FIG. 20
illustrates the conventional cleaving method, showing an enlarged plan view of a substantial part of a wafer
121
in which hexagonal gallium nitride-based semiconductors are layered with a required layer structure on n-GaN substrate
10
.
First, wafer
121
is prepared in which hexagonal gallium nitride-based semiconductors are layered with a required layer structure on an n-GaN substrate
10
, and positive electrode
14
is formed. At a peripheral portion on the surface of wafer
121
(a surface opposite to n-GaN substrate
10
), a groove
122
having a length of approximately 0.1 to a few millimeters is formed in a direction parallel to a cleavage plane which is unique to the material of a hexagonal nitride-based semiconductor. Here, the groove is formed by dicing or scribing, and specifically, a (1-100) plane is selected as the cleavage plane of the semiconductor layer described above.
Next, an external force is applied to n-GaN substrate
10
to divide wafer
121
into pieces in the direction parallel to the cleavage plane unique to the hexagonal nitride-based semiconductor, to obtain a plane which is to be mirror end face
400
of semiconductor laser device
500
(see FIG.
19
). Here, a cleavage line
123
presents an issue. Details will be described later.
The semiconductor laser device according to the conventional technique described above has the problems as indicated below.
[Problem 1] Effects by Substrate-Side End face
Inventors of the present invention fabricated the semiconductor laser device according to the above-described conventional technique, to find one or two in every ten semiconductor laser devices that oscillate at two different wavelengths.
FIG. 21
shows an oscillation spectrum of a semiconductor laser device oscillated at two different wavelengths. While vertical multimode oscillation occurred in the vicinity of threshold current, an envelope shaped by each peak is bimodal. This means that, in addition to a peak group
131
of a plurality of vertical modes around a primary oscillation wavelength, another peak group
132
of a plurality of vertical modes has occurred around a point a little toward a longer wavelength side or a shorter wavelength side. In
FIG. 21
, location of peak group
132
, whether it is on the longer wavelength side or on the shorter wavelength side with respect to peak group
131
, is not constant for each device. Peak group
132
may be located on a longer wavelength side in one semiconductor laser device, whereas it may be located on the shorter wavelength side in a different semiconductor laser device fabricated in the same lot.
The inventors of the present invention examined and found that this was due to a leaking mode to the n-GaN substrate in some ways, and proved that the leakage mode was caused by oscillation light generated within the cavity formed by a pair of n-GaN substrate portions on the mirror end face (hereinafter referred to as “substrate-side end faces”).
Thus, even with semiconductor laser devices formed in the same lot, a ratio of the length of a cavity constituted by a pair of substrate-side end faces to the length of a cavity constituted by a pair of the layered lump portions on the mirror end face (hereinafter referred to as “layered-lump-side end faces”). Thus, relative positions of peak group
131
and peak group
132
in
FIG. 21
may be different in each device. In the present specification, laser light oscillated at a wavelength different from the primary wavelength is referred to as a “substrate leaking mode.”
Threshold current of the substrate leaking mode is somewhat higher than that of the primary laser oscillation mode, resulting in nonlinear I-L property of the semiconductor laser, which is undesirable in operation of the semiconductor laser device. Moreover, the substrate leaking mode emits light in the same direction as that of the primary oscillation light, which makes it impossible to separate the substrate leaking mode from the primary laser light in a spatial sense. Therefore, when such a laser device is mounted to an optical information reproducing apparatus such as an optical pickup, noise may be caused, resulting in lowering of an SIN ratio.
[Problem 2] Deterioration in Flatness of Mirror End Face
The inventors of the present invention fabricated the semiconductor laser device according to the conventional technique, and failed in some cases to obtain a good cleavage plane on the mirror end face. On mirror end face
400
of semiconductor laser device
500
shown in
FIG. 19
produced according to the conventional technique, a number of vertical streaks
16
were observed within layered lump
12
of hexagonal nitride-based semiconductors, including the portion of optical waveguide
13
. When observed in detail, it was found that vertical streaks
16
were concavities and convexities, i.e. surface roughness, generated across a region extending from the lower surface of n-GaN substrate
10
to the upper surface of layered lump
12
. The size of each streak is evaluated along a line perpendicular to the direction of layering (the left to right direction in FIG.
19
), and a RMS (Root Mean Square) value of approximately 1 to 6 nm along the length of 4 &mgr;m is obtained. Three to ten such semiconductor laser devices were observed in every ten devices. Though the cause thereof is unknown, it can be interpreted as follows.
The inventors of the present invention examined and evaluated the n-GaN substrate by XRD (X-Ray Diffraction), to find that a half band width of a peak indicating the <0001> direction was approximately four minutes, which bears comparison with the GaN film grown by a normal MOCVD Metal Organic Chemical Vapor Deposition) device, whereas a half band width of the peak indicating the <1-100>
Ip Paul
Nguyen Dung T
Sharp Kabushiki Kaisha
LandOfFree
Semiconductor laser device and method of manufacturing the same does not yet have a rating. At this time, there are no reviews or comments for this patent.
If you have personal experience with Semiconductor laser device and method of manufacturing the same, we encourage you to share that experience with our LandOfFree.com community. Your opinion is very important and Semiconductor laser device and method of manufacturing the same will most certainly appreciate the feedback.
Profile ID: LFUS-PAI-O-3281552