Semiconductor device manufacturing: process – Making device or circuit emissive of nonelectrical signal – Compound semiconductor
Reexamination Certificate
2002-08-13
2004-07-20
Whitehead, Jr., Carl (Department: 2813)
Semiconductor device manufacturing: process
Making device or circuit emissive of nonelectrical signal
Compound semiconductor
C438S047000, C438S604000, C438S740000
Reexamination Certificate
active
06764870
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a gallium nitride type semiconductor laser device for use in a light source of an optical disk system.
2. Description of the Related Art
A gallium nitride type semiconductor (e.g., GaInAlN) is used as a semiconductor material for a semiconductor laser device (LD) having an emission wavelength in a wavelength range from ultraviolet to green. For example, MRS Internet J. Nitride Semicond. Res., vol.2, no.5 (1997) describes a semiconductor laser device using such a gallium nitride type semiconductor, as illustrated in a cross-sectional view in FIG.
11
.
Referring to
FIG. 11
, the semiconductor laser device has, on a sapphire substrate
201
, a GaN buffer layer
202
, an n-GaN contact layer
203
, an n-In
0.05
Ga
0.95
N layer
204
, an n-Al
0.08
Ga
0.92
N cladding layer
205
, an n-GaN guide layer
206
, a multiquantum well structure active layer
207
including In
0.15
Ga
0.85
N quantum well layers and In
0.02
Ga
0.98
N barrier layers, a p-Al
0.2
Ga
0.8
N layer
208
, a p-GaN guide layer
209
, a p-Al
0.08
Ga
0.92
N cladding layer
210
, a p-GaN contact layer
211
, a p-side electrode
212
and an n-side electrode
213
. The multiquantum well structure active layer
207
includes seven layers in total, i.e., four In
0.15
Ga
0.85
N quantum well layers each having a thickness of about 3.5 nm and three In
0.02
Ga
0.98
N barrier layers each having a thickness of about 7 nm. In the multiquantum well structure active layer
207
, the quantum well layers and the barrier layers alternate with each other.
In this conventional example, the p-Al
0.08
Ga
0.92
N cladding layer
210
and the p-GaN contact layer
211
are formed in a ridge stripe pattern so as to constrict an injected current. The width of the stripe pattern is about 4 &mgr;m.
Japanese Laid-open Publication No. 9-232680 describes a semiconductor laser device similarly using a gallium nitride type semiconductor, which also includes a ridge stripe structure having a stripe width of about 5 &mgr;m to about 10 &mgr;m for constricting an injected current.
When employing a semiconductor laser device using a gallium nitride type semiconductor as a light source of an optical disk system, in order to prevent a read error from occurring due to noise during a data read operation, a self-pulsation type semiconductor laser is employed in which an optical output is modulated for a constant current injected. Such a semiconductor laser device is described in Japanese Laid-open Publication No. 9-191160, for example.
FIG. 12
is a cross-sectional view illustrating such a semiconductor laser device. Referring to
FIG. 12
, the semiconductor laser device includes an n-SiC substrate
221
, an n-AlN buffer layer
222
, an n-Al
0.15
Ga
0.85
N cladding layer
223
, an In
0.15
Ga
0.85
N active layer
224
having a thickness of about 50 nm, a p-Al
0.15
Ga
0.85
N first p-type cladding layer
225
, a p-In
0.2
Ga
0.8
N saturable absorbing layer
226
, an n-Al
0.25
Ga
0.75
N current blocking layer
227
, a p-Al
0.15
Ga
0.85
N second p-type cladding layer
228
, a p-GaN cap layer
229
, a p-GaN contact layer
230
, a p-side electrode
231
and an n-side electrode
232
.
In this conventional example, a portion of light generated by the active layer
224
is absorbed by the saturable absorbing layer
226
, thereby causing an absorption coefficient of the saturable absorbing layer
226
to change. Accordingly, an intensity of light emission by a laser oscillation from the active layer
224
is changed periodically. As a result, coherence of the emitted light from the laser is reduced. This conventional example also includes a ridge stripe structure having a stripe width of about 2 &mgr;m for constricting an injected current.
When employing such a semiconductor laser device with reduced coherence as a light source of an optical disk system, even if light reflected by the disk returns to the semiconductor laser, the emitted light from the laser does not interfere with the reflected return light, thereby suppressing generation of noise and thus preventing a data read error from occurring.
However, the conventional laser device using a gallium nitride type semiconductor has the following problems.
First, in the self-pulsation type semiconductor laser device having the saturable absorbing layer, light generated by the active layer is absorbed by the saturable absorbing layer, thereby increasing the loss of light within the laser cavity. As a result, the oscillation threshold current of the semiconductor laser device increases, and the emission efficiency is reduced. Moreover, in this conventional self-pulsation type semiconductor laser device, since the saturable absorbing layer is added only to one of the cladding layers interposing the active layer therebetween or only to one of the guide layers interposing the active layer therebetween, the far field pattern of the emitted light from the laser is asymmetric, whereby the focused spot size cannot be made sufficiently small when focusing the emitted light with a lens.
The conventional laser device using a gallium nitride type semiconductor to which the saturable absorbing layer is not added does not have such problems (e.g., the increased oscillation threshold current, the reduced emission efficiency, and incapability to have a small focused spot size) as those seen in the conventional self-pulsation type semiconductor laser device. However, when this semiconductor laser device is used as a light source of an optical disk system, noise occurs due to the return light from the disk, thereby causing a read error during a data read operation. Therefore, the conventional laser device using a gallium nitride type semiconductor to which the saturable absorbing layer is not added is not suitable for a light source of an optical disk system.
SUMMARY OF THE INVENTION
A gallium nitride type semiconductor laser device of the present invention includes: a substrate; and a layered structure formed on the substrate. The layered structure at least includes an active layer of a nitride type semiconductor material which is interposed between a pair of nitride type semiconductor layers each functioning as a cladding layer or a guide layer. A current is injected into a stripe region in the layered structure having a width smaller than a width of the active layer. The width of the stripe region is in a range between about 0.2 &mgr;m and about 1.8 &mgr;m.
Preferably, a portion of the active layer existing outside the stripe region has a width of at least about 3 &mgr;m.
The active layer may include a single quantum well layer.
Alternatively, the active layer may include a multiquantum well structure including a plurality of quantum well layers and at least one barrier layer each interposed between the adjacent two quantum well layers, the number of the quantum well layers being two, three or four.
A thickness of each quantum well layer in the active layer may be about 10 nm or less.
A thickness of each of the at least one barrier layer in the active layer may be about 10 nm or less.
In one embodiment, the layered structure at least includes a first cladding layer having a first conductivity type, the active layer, a second cladding layer having a second conductivity type, and a contact layer having the second conductivity type, which are deposited in this order. The second cladding layer and the contact layer are formed in a stripe having a width smaller than the width of the active layer. And the layered structure further includes a current blocking layer deposited outside the stripe.
In another embodiment, the layered structure at least includes a first cladding layer having a first conductivity type, the active layer, a guide layer or a second cladding layer having a second conductivity type, and a current blocking layer. A striped groove is provided in the current blocking layer so as to reach the guide layer or the second cladding layer having the second conductivity type, the groove having a width smaller than the width of the active layer. An
Jr. Carl Whitehead
Morrison & Foerster / LLP
Sharp Kabushiki Kaisha
Smoot Stephen W.
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