Semiconductor device manufacturing: process – Making device or circuit emissive of nonelectrical signal
Reexamination Certificate
2003-07-17
2004-10-19
Mulpuri, Savitri (Department: 2812)
Semiconductor device manufacturing: process
Making device or circuit emissive of nonelectrical signal
C438S033000, C438S438000, C438S479000, C438S464000, C438S977000
Reexamination Certificate
active
06806109
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Technical Field
The present invention relates to a method of fabricating a nitride based semiconductor substrate for use in a nitride based semiconductor laser or the like which is expected to be applied to fields such as optical information processing and radio communication, and a method of fabricating a nitride based semiconductor device.
2. Related Art
A nitride based semiconductor having nitride (N) as V-group element is considered to be promising as a material of a short wavelength light-emitting element and a high power semiconductor circuit because of its large bandgap. In particular, a gallium nitride based compound semiconductor (GaN based semiconductor: Al
x
Ga
y
In
z
N(0≦x,y,z≦1, x+y+z=1)) has been intensively studied, and a blue light-emitting diode (LED) and a green LED have been put into practical use. Meanwhile, for achieving a large-capacity optical disc system, a semiconductor laser having an oscillation wavelength in 400 nm band has come to draw attention. At present, the semiconductor laser is practically used.
FIG. 1
is a cross-sectional view schematically showing a structure of the conventional GaN based semiconductor laser. As shown in
FIG. 1
, on a sapphire base
1701
, a GaN buffer layer
1702
, a n-GaN layer
1703
, a n-AlGaN cladding layer
1704
, a n-GaN light guiding layer
1705
, a multiple quantum well (MQW) active layer
1706
comprised of Ga
1-x
In
x
N/Ga
1-y
In
y
N (0<y<x<1), a p-GaN light guiding layer
1707
, a p-AlGaN cladding layer
1708
, a p-GaN contact layer
1709
are deposited as crystals grown by a metalorganic Vapor Phase Epitaxy (MOVPE) process. On the p-GaN contact layer
1709
, a ridge strip having a width of approximately 3 &mgr;m is provided, and both sides thereof are covered by the insulating film such as SiO
2
1711
. On the ridge strip and the SiO
2
1711
, a p electrode
1710
comprised of, for example, Ni/Au, is provided, and an electrode
1712
comprised of, for example, Ti/Al is provided on a surface of part of the n-Ga N layer
1703
exposed by etching.
In the semiconductor laser so structured, upon the n electrode
1712
being grounded and a forward voltage being applied to the p electrode
1710
, positive holes migrate from the p electrode
1710
side toward the MQW active layer
1706
and electrons migrate from the n electrode
1712
side toward the MQW active layer
1706
. This results in optical gain inside the MQW active layer
1706
and laser oscillation having an oscillation wavelength of 400 nm band. The oscillation wavelength varies depending on a composition and thickness of Ga
1-x
In
x
N/Ga
1-y
In
y
N thin film as a material of the MQW active layer
1706
. At present, continuous oscillation at temperatures higher than a room temperature, is implemented. A high-power semiconductor circuit using these techniques is studied and is expected to be achieved in fields such as semiconductor devices for radio communications.
As a substrate on which GaN based crystal is grown, a sapphire base, a SiC (silicon carbide) substrate, or a Si (silicon) substrate is used. But, these substrates lattice-mismatch to GaN, and therefore crystal growth becomes difficult. This results in a number of dislocations (blade-shaped dislocation, spiral dislocation, mixed dislocation). For example, when using the sapphire base or the SiC substrate, dislocations of approximately 1×10
9
cm
−2
exist. As a result, a threshold current of a semiconductor laser is increased and reliability of the semiconductor laser is degraded.
A first article as a known literature “Journal of Material Research, Vol. 14 (1999) pp. 2716-2731” proposes an Epitaxial Lateral Over Growth (ELOG) as a method of reducing dislocation density. This method is effective in reducing through dislocations in a system having large lattice mismatching.
FIG. 2
is a cross-sectional view schematically showing a structure of GaN crystal formed by ELOG. On a sapphire base
1801
, GaN crystal
1802
is formed by the MOVPE process or the like. On the GaN crystal
1802
, SiO
2
1803
is formed by a CVD (Chemical Vapor Deposition) process or the like. The SiO
2
1803
is processed in stripes by photolithography and etching. A GaN based semiconductor layer
1804
is deposited by selectively growing an exposed portion of the GaN crystal
1802
as seed crystal. As a growing process, the MOVPE process or a hydride vapor phase epitaxy process (HVPE process) is used. Above the seed crystal, a region
1806
having a number of dislocations of approximately as high as 1×10
9
cm
−2
exists, but a region
1805
which is laterally grown has dislocation density as low as approximately 1×10
7
cm
−2
. An active region is provided above the region
1805
with fewer dislocations, thereby improving reliability. Since the other structure in
FIG. 2
is identical to a structure of the conventional semiconductor laser in
FIG. 1
, the same or corresponding parts are identified by the same reference numerals and will not be further described.
In recent years, fabrication of a GaN substrate has been intensively studied.
A second article as a known literature “Japanese Journal of Applied Physics, Vol. 37 (1998) pp. L 309-L312” illustrates a method in which a sapphire base is removed by polishing in a GaN based semiconductor layer grown on the sapphire base, thereby obtaining a GaN substrate. A third article as a known literature “Japanese Journal of Applied Physics, Vol. 38 (1999) pp. L217-L219” illustrates a method in which a GaN based semiconductor layer is separated (lifted off) from the vicinity of a sapphire base by irradiation of a laser beam using a third harmonic (wavelength of 355 nm) of Nd:YAG laser. It is considered that the GaN based semiconductor layer is thus separated by irradiation of the laser beam due to the fact that the GaN based semiconductor layer in the vicinity of the sapphire base has low quality and high carrier concentration.
Related Arts are disclosed in Japanese Laid-Open Patent Application Publication No. Hei. 11-191657 that discloses a method of growing nitride semiconductor and Japanese Laid-Open Patent Application Publication No. 2001-93837 that discloses a structure of a semiconductor thin film and a fabrication method thereof.
However, in the methods illustrated in the second and third articles, due to difference in thermal expansion coefficient between sapphire and GaN, a number of cracks occur in the GaN based semiconductor layer when separating the GaN based semiconductor layer from the sapphire base. For this reason, a GaN substrate having a large area equal to a two-inch wafer level is impossible to obtain. In addition, in these methods, it is not easy to control separation between the sapphire base and the GaN based semiconductor layer.
In a semiconductor device with the GaN based semiconductor layer disposed on the sapphire base, the GaN based semiconductor layer is subjected to a stress due to large difference in lattice constant between the sapphire base and the GaN based semiconductor layer grown thereon. This reduces reliability of yield and productivity as well as an electric property. Therefore, it is necessary to separate the GaN based semiconductor layer from the sapphire base and form constituents on the GaN based semiconductor substrate.
The present invention has been made under the circumstances, and an object of the present invention is to provide a method of fabricating a nitride based semiconductor substrate with high controllability in separation between the sapphire base and the GaN based semiconductor layer.
SUMMARY OF THE INVENTION
To achieve the above-described object, according to the present invention, there is provided a method of fabricating a nitride based semiconductor substrate, comprising the steps of depositing a first nitride based semiconductor layer on a base; processing the first nitride based semiconductor layer to have ridge portions and recess portions; coating side surfaces of the ridge portions and bottom surfaces of the recess
Furuya Hiroyuki
Hasegawa Yoshiaki
Ishibashi Akihiko
Yokogawa Toshiya
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