Active solid-state devices (e.g. – transistors – solid-state diode – Incoherent light emitter structure – With particular semiconductor material
Reexamination Certificate
2000-11-15
2004-04-13
Zarabian, Amir (Department: 2822)
Active solid-state devices (e.g., transistors, solid-state diode
Incoherent light emitter structure
With particular semiconductor material
C257S012000, C257S013000, C257S015000, C257S021000, C257S094000, C257S095000, C257S079000
Reexamination Certificate
active
06720586
ABSTRACT:
BACKGROUND OF THE INVENTION
The present invention relates to a method of fabricating a nitride semiconductor for use in a short-wavelength semiconductor laser diode and the like expected to be applied to the fields of optical information processing and the like, a semiconductor device and a semiconductor light emitting device using the nitride semiconductor and a method of fabricating the same.
Recently, a nitride semiconductor of a group III-V compound, that is, a group V element including nitride (N), is regarded as a promising material for a short-wavelength light emitting device due to its large energy gap. In particular, a gallium nitride-based compound semiconductor (Al
x
Ga
y
In
z
N, wherein 0≦x, y, z≦1 and x+y+z=1) has been earnestly studied and developed, resulting in realizing a practical blue or green light emitting diode (LED) device. Furthermore, in accordance with capacity increase of an optical disk unit, a semiconductor laser diode lasing at approximately 400 nm is earnestly desired, and a semiconductor laser diode using a gallium nitride-based semiconductor is to be practically used.
CONVENTIONAL EXAMPLE 1
Now, a gallium nitride-based semiconductor laser diode according to Conventional Example 1 will be described with reference to drawings.
FIG. 37
shows the sectional structure of the conventional gallium nitride-based semiconductor laser diode showing laser action. As is shown in
FIG. 37
, the conventional semiconductor laser diode includes a buffer layer
302
of gallium nitride (GaN), an n-type contact layer
303
of n-type-GaN, an n-type cladding layer
304
of n-type aluminum gallium nitride (AlGaN), an n-type light guiding layer
305
of n-type GaN, a multiple quantum well (MQW) active layer
306
including gallium indium nitride layers having different composition ratios of indium (Ga
1−x
In
x
N/Ga
1−y
In
y
N, wherein 0<y<×<1), a p-type light guiding layer
307
of p-type GaN, a p-type cladding layer
308
of p-type AlGaN and a p-type contact layer
309
of p-type GaN successively formed on a substrate
301
of sapphire by, for example, metal organic vapor phase epitaxial growth (MOVPE).
An upper portion of the p-type cladding layer
308
and the p-type contact layer
309
is formed into a ridge with a width of approximately 3 through 10 &mgr;m. A lamination body including the MQW active layer
306
is etched so as to expose part of the n-type contact layer
303
, and the upper face and the side faces of the etched lamination body are covered with an insulating film
310
. In a portion of the insulating film
310
above the p-type contact layer
309
, a stripe-shaped opening is formed, a p-side electrode
311
in ohmic contact with the p-type contact layer
309
through the opening is formed over a portion of the insulating film
310
above the ridge. Also, on a portion of the n-type contact layer
303
not covered with the insulating film
310
, an n-side electrode
312
in ohmic contact with the n-type contact layer
303
is formed.
In the semiconductor laser diode having the aforementioned structure, when a predetermined voltage is applied to the p-side electrode
311
with the n-side electrode
312
grounded, optical gain is generated within the MQW active layer
306
, so as to show laser action at a wavelength of approximately 400 nm.
The wavelength of laser action depends upon the composition ratios x and y or the thicknesses of the Ga
1−x
In
x
N and Ga
1−y
In
y
N layers included in the MQW active layer
306
. At present, the laser diode having this structure has been developed to show continuous laser action at room temperature or more.
Furthermore, laser action in the fundamental mode of the lateral mode along a horizontal direction (parallel to the substrate surface) can be shown by adjusting the width or height of the ridge. Specifically, the laser action of the fundamental lateral mode can be shown by providing a difference in the light confinement coefficient between the fundamental lateral mode and a primary or higher mode.
The substrate
301
is formed from, apart from sapphire, silicon carbide (SiC), neodymium gallate (NdGaO
3
) or the like, and any of these materials cannot attain lattice match with gallium nitride and is difficult to attain coherent growth. As a result, any of these materials includes a large number of mixed dislocations, namely, mixed presence of edge dislocations, screw dislocations and other dislocations. For example, when the substrate is made from sapphire, the substrate includes dislocations at a density of approximately 1×10
9
cm
−2
, which degrades the reliability of the semiconductor laser diode.
As a method for reducing the density of dislocations, epitaxial lateral overgrowth (ELOG) has been proposed. This is an effective method for reducing threading dislocations in a semiconductor crystal with large lattice mismatch.
CONVENTIONAL EXAMPLE 2
FIG. 38
schematically shows the distribution of crystal dislocations in a semiconductor layer of gallium nitride formed by the ELOG.
The outline of the ELOG will be described with reference to FIG.
38
. First, a seed layer
402
of GaN is grown on a substrate
401
of sapphire by the MOVPE or the like.
Next, a dielectric film of silicon oxide or the like is deposited by chemical vapor deposition (CVD) or the like, and the deposited dielectric film is formed into a mask film
403
having an opening pattern in the shape of stripes with a predetermined cycle by photolithography and etching.
Then, a semiconductor layer
404
of GaN is formed on the mask film
403
by selective growth with portions of the seed layer
402
exposed from the mask film
403
used as a seed crystal by the. MOVPE or halide vapor phase epitaxial growth.
At this point, although a dislocation high-density region
404
a
where the dislocation density is approximately 1×10
9
cm
2
is formed in a portion of the semiconductor layer
404
above the opening of the mask film
403
, a dislocation low-density region
404
b
where the dislocation density is approximately 1×10
7
cm
−2
can be formed in a portion of the semiconductor layer
404
laterally grown on the mask film
403
.
FIG. 39
shows the sectional structure of a semiconductor laser diode whose active area, namely, a ridge working as a current injecting region, is formed above the dislocation low-density region
404
b
. In
FIG. 39
, like reference numerals are used to refer to like elements shown in
FIGS. 37 and 38
.
When the current injecting region is formed above the dislocation low-density region
404
b
of the MQW active layer
306
in this manner, the reliability of the laser diode can be improved.
As a result of various examinations, the present inventors have found that semiconductor laser diodes according to Conventional Examples 1 and 2 have the following problems:
First, the problems of the growth method of a nitride semiconductor by the ELOG according to Conventional Example 2 will be described.
FIGS.
40
(
a
) through
40
(
d
) schematically show a state where polycrystals
405
of gallium nitride are deposited on the mask film
403
during the growth of the semiconductor layer
404
so as to degrade the crystallinity of the semiconductor layer
404
.
Specifically, the mask film
403
having the openings is first formed on the seed layer
402
as is shown in FIG.
40
(
a
), and plural semiconductor layers
404
are respectively grown by using, as the seed crystal, the portions of the seed layer
402
exposed in the openings of the mask film
403
as is shown in FIG.
40
(
b
). At this point, since the mask film
403
is formed from a dielectric, plural polycrystals
405
that cannot be crystallized on a dielectric may be deposited on the mask film
403
.
Next, as is shown in FIGS.
40
(
c
) and
40
(
d
), when the plural semiconductor layers
404
are grown to be integrated and to have a flat face with the polycrystals
405
deposited, a region
404
c
with poor crystallinity is formed on each polycrystal
405
.
The present inventors ha
Ban Yuzaburo
Hasegawa Yoshiaki
Ishibashi Akihiko
Kidoguchi Isao
Kume Masahiro
Matsushita Electric - Industrial Co., Ltd.
Soward Ida M.
Zarabian Amir
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