Single-crystal – oriented-crystal – and epitaxy growth processes; – Processes of growth from liquid or supercritical state – Having growth from a solution comprising a solvent which is...
Reexamination Certificate
2000-06-08
2003-07-15
Kunemund, Robert (Department: 1765)
Single-crystal, oriented-crystal, and epitaxy growth processes;
Processes of growth from liquid or supercritical state
Having growth from a solution comprising a solvent which is...
C117S013000, C117S073000, C117S074000, C117S075000, C117S077000, C117S078000, C117S223000, C117S224000, C117S952000
Reexamination Certificate
active
06592663
ABSTRACT:
BACKGROUND OF THE INVENTION
The present invention generally relates to semiconductor devices and more particularly to a semiconductor device having a GaN bulk crystal substrate.
GaN is a III-V compound semiconductor material having a large bandgap of blue to ultraviolet wavelength energy. Thus, intensive investigations are being made with regard to development of optical semiconductor devices having a GaN active layer for use particularly in optical information storage devices including a digital video data recorder (DVD). By using such a light emitting semiconductor device producing blue to ultraviolet wavelength optical radiation for the optical source, it is possible to increase the recording density of optical information storage devices.
Conventionally, a laser diode or light-emitting diode having a GaN active layer has been constructed on a sapphire substrate in view of the absence of technology of forming a GaN bulk crystal substrate.
FIG. 1
shows the construction of a conventional GaN laser diode according to Nakamura, S., et al., Jpn. J. Appl. Phys. vol.36 (1997) pp.L1568-L1571, Part 2, No.12A, Dec. 1, 1997, constructed on a sapphire substrate
1
.
Referring to
FIG. 1
, the sapphire substrate
1
has a (
0001
) principal surface covered by a low-temperature GaN buffer layer
2
, and includes a GaN buffer layer
3
of n-type grown further on the buffer layer
2
. The GaN buffer layer
3
includes a lower layer part
3
a
and an upper layer part
3
b
both of n-type, with an intervening SiO
2
mask pattern
4
provided such that the SiO
2
mask pattern
4
is embedded between the lower layer part
3
a
and the upper layer part
3
b
. More specifically, the SiO
2
mask pattern
4
is formed on the lower GaN buffer layer part
3
a
, followed by a patterning process thereof to form an opening
4
A in the SiO
2
mask pattern
4
.
After the formation of the SiO
2
mask pattern
4
, the upper GaN layer part
3
b
is formed by an epitaxial lateral overgrowth (ELO) process in which the layer
3
b
is grown laterally on the SiO
2
mask
4
. Thereby, desired epitaxy is achieved with regard to the lower GaN layer part
3
a
at the opening
4
A in the SiO
2
mask pattern
4
. By growing the GaN layer part
3
b
as such, it is possible to prevent the defects, which are formed in the GaN layer part
3
a
due to the large lattice misfit between GaN and sapphire, from penetrating into the upper GaN layer part
3
b.
On the upper GaN layer
3
b
, a strained super-lattice structure
5
having an n-type Al
0.14
Ga
0.86
N/GaN modulation doped structure is formed, with an intervening InGaN layer
5
A of the n-type having a composition In
0.1
Ga
0.9
N interposed between the upper GaN layer part
3
b
and the strained superlattice structure
5
. By providing the strained superlattice structure
5
as such, dislocations that are originated at the surface of the sapphire substrate
1
and extending in the upward direction are intercepted and trapped.
On the strained superlattice structure
5
, a lower cladding layer
6
of n-type GaN is formed, and an active layer
7
having an MQW structure of In
0.01
Ga
0.98
N/In
0.15
Ga
0.85
N is formed on the cladding layer
6
. Further, an upper cladding layer
8
of p-type GaN is formed on the active layer
7
, with an intervening electron blocking layer
7
A of p-type AlGaN having a composition of Al
0.2
Ga
0.8
N interposed between the active layer
7
and the upper cladding layer
8
.
On the upper cladding layer
8
, another strained superlattice structure
9
of a p-type Al
0.14
Ga
0.86
N/GaN modulation doped structure is formed such that the superlattice structure
9
is covered by a p-type GaN cap layer
10
. Further, a p-type electrode
11
is formed in contact with the cap layer
10
and an n-type electrode
12
is formed in contact with the n-type GaN buffer layer
3
b.
It is reported that the laser diode of
FIG. 1
oscillates successfully with a practical lifetime, indicating that the defect density in the active layer
7
is reduced successfully.
On the other hand, the laser diode of
FIG. 1
cannot eliminate the defects completely, and there remain substantial defects particularly in correspondence to the part on the SiO
2
mask
4
as represented in FIG.
2
. See Nakamura S. et al., op cit. It should be noted that such defects formed on the SiO
2
mask
4
easily penetrate through the strained superlattice structure
5
and the lower cladding layer
6
and reach the active layer
7
.
In view of the foregoing concentration of the defects in the central part of the SiO
2
mask pattern
4
, the laser diode of
FIG. 1
uses the part of the semiconductor epitaxial structure located on the opening
4
A of the SiO
2
mask
4
, by forming a mesa structure M in correspondence to the opening
4
A. However, the defect-free region formed on the opening
4
A has a lateral size of only several microns, and thus, it is difficult to construct a high-power laser diode based on the construction of FIG.
1
. When the laser diode of
FIG. 1
is driven at a high power, the area of optical emission in the active region extends inevitably across the defects, and the laser diode is damaged as a result of optical absorption caused by the defects. Further, the laser diode of
FIG. 1
having such a construction has other various drawbacks associated with the defects in the semiconductor epitaxial layers, such as large threshold current, limited lifetime, and the like. Further, the laser diode of
FIG. 1
has a drawback, in view of the fact that the sapphire substrate is an insulating substrate, in that it is not possible to provide an electrode on the substrate. As represented in
FIG. 1
, it is necessary to expose the top surface of the n-type GaN buffer layer
3
by an etching process in order to provide the n-type electrode
12
, while such an etching process complicates the fabrication process of the laser diode. In addition, the increased distance between the active layer
7
and the n-type electrode
12
causes the problem of increased resistance of the current path, while such an increased resistance of the current path deteriorates the high-speed response of the laser diode.
Further, the conventional laser diode of
FIG. 1
suffers from the problem of poor quality of mirror surfaces defining the optical cavity. Due to the fact that the sapphire single crystal constituting the substrate
1
belongs to hexagonal crystal system, formation of the optical cavity cannot be achieved by a simple cleaving process. It has been therefore necessary to form the mirror surfaces, when fabricating the laser diode of
FIG. 1
by conducting a dry etching process, while the mirror surface thus formed by a dry etching process has a poor quality.
Because of the foregoing reasons, as well as because of other various reasons not mentioned here, it is desired to form the substrate of the GaN laser diode by a bulk crystal GaN and form the laser diode directly on the GaN bulk crystal substrate.
With regard to the art of growing a bulk crystal GaN, there is a successful attempt reported by Porowski (Porowski, S., J. Crystal Growth 189/190 (1998) pp.153-158, in which a GaN bulk crystal is synthesized from a Ga melt under an elevated temperature of 1400-1700° C. and an elevated N
2
pressure of 12-20 kbar (1.2-2 GPa). This process, however, can only provide an extremely small crystal in the order of 1 cm in diameter at best. Further the process of Porowski requires a specially built pressure-resistant apparatus and a long time is needed for loading or unloading a source material, or increasing or decreasing the pressure and temperature. Thus, the process of this prior art would not be a realistic solution for mass-production of a GaN bulk crystal substrate. It should be noted that the reaction vessel of Porowski has to withstand the foregoing extremely high pressure, which is rarely encountered in industrial process, under the temperature exceeding 1400° C.
Further, there is a known process of growing a GaN bulk crystal without using an extremely high pressure environment for growing a
Iwata Hirokazu
Sarayama Seiji
Shimada Masahiko
Yamane Hisanori
Cooper and Dunham, LLP.
Kunemund Robert
Ricoh & Company, Ltd.
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