Active solid-state devices (e.g. – transistors – solid-state diode – Thin active physical layer which is – Heterojunction
Reexamination Certificate
2000-05-22
2002-10-08
Crane, Sara (Department: 2811)
Active solid-state devices (e.g., transistors, solid-state diode
Thin active physical layer which is
Heterojunction
C257S096000, C257S097000, C257S103000
Reexamination Certificate
active
06462354
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
This invention relates to a semiconductor device and a semiconductor light emitting device, and in particular, those using nitride III-V compound semiconductors.
2. Description of the Related Art
Nitride III-V compound semiconductors represented by gallium nitride (GaN) (hereinafter also called “GaN semiconductors”) are hopeful materials of light emitting devices capable of emitting light in the green to blue and ultraviolet ranges, high-frequency electronic devices and environment-enduring electronic devices. Especially since light emitting diodes (LED) using GaN semiconductors were brought into practice, GaN semiconductors have become a center of attraction. Realization of semiconductor lasers using GaN semiconductors was also reported, and their application to various purposes, starting from the light source of an optical disc device, is expected.
There is known a GaN semiconductor laser having an AlGaN/GaN/GaInN SCH (separate confinement heterostructure) structure which includes a cladding layer of AlGaN, optical guide layer of GaN and active layer of GaInN.
FIG. 1
shows a GaN semiconductor laser of this conventional type.
As shown in
FIG. 1
, in the conventional GaN semiconductor laser, sequentially stacked on a c-plane sapphire substrate
101
are, via an undoped GaN buffer layer
102
by low-temperature growth: an undoped GaN layer
103
, n-type GaN contact layer
104
, n-type Al
0.07
Ga
0.93
N cladding layer
105
, n-type GaN optical guide layer
106
, active layer
107
having quantum well layers of undoped Ga
0.9
In
0.1
N, p-type Al
0.2
Ga
0.8
N cap layer
108
, p-type GaN optical guide layer
109
, p-type Al
0.07
Ga
0.93
N cladding layer
110
and p-type GaN contact layer
111
.
Upper part of the n-type GaN contact layer
104
, n-type Al
0.07
Ga
0.93
N cladding layer
105
, n-type GaN optical guide layer
106
, active layer
107
, p-type Al
0.2
Ga
0.8
N cap layer
108
, p-type GaN optical guide layer
109
, p-type Al
0.07
Ga
0.93
N cladding layer
110
and p-type GaN contact layer
111
have the shape of a stripe extending in a direction with a predetermined width.
On the p-type GaN contact layer
111
, a stripe-shaped p-side electrode
112
such as Ni/Pt/Au electrode or Ni/Au electrode is provided, and on the n-type GaN contact layer
104
in the region adjacent to the stripe portion, an n-side electrode
113
such as Ti/Al/Pt/Au electrode is provided.
According to the knowledge of the Inventor, it has been confirmed that, from the viewpoint of realizing continuous oscillation of a GaN semiconductor, it is sufficient that the band gap between its cladding layer and active layer is not less than 500 meV. However, in conventional AlGaN/GaN/GaInN SCH-structured GaN semiconductor lasers, if the Al composition of the cladding layer is increased for the purpose of increasing the band gap between the cladding layer and the active layer, their growth becomes difficult. Additionally, since the p-type carrier concentration decreases in AlGaN with a high Al composition, resistance of the p-type cladding layer undesirably increases. These problems become more serious as the band gap of the active layer increases, namely, as the emission wavelength becomes shorter.
Furthermore, in conventional GaN semiconductor lasers, although the p-type Al
0.2
Ga
0.8
N cap layer
108
is interposed between the active layer
107
and the p-type GaN optical guide layer
109
, the p-type Al
0.2
Ga
0.8
N cap layer
108
with a high Al composition is difficult to grow and decrease in resistance as mentioned above. In due course, there were also problems such as adverse influences to the laser property from an increase of electric resistance by the p-type Al
0.2
Ga
0.8
N cap layer
108
.
Moreover, In conventional GaN semiconductor lasers, since there is a difference in lattice constant between the sapphire substrate
101
and the GaN semiconductor layers forming the laser structure, the GaN buffer layer
102
is grown on the sapphire substrate
101
at a low temperature, and it is crystallized when GaN semiconductor layers are grown thereon for the purpose of improving the quality of the GaN semiconductors grown on the GaN buffer layer
102
. However, even when the GaN buffer layer
102
by low-temperature growth is used, there is a limit in density of defects which can be decreased.
OBJECTS AND SUMMARY OF THE INVENTION
It is therefore an object of the invention to provide a high-quality, high-performance semiconductor light emitting device using nitride III-V compound semiconductors, which can be reduced in threshold current density and operation voltage and can be shortened in emission wavelength down to the ultraviolet region.
Another object of the invention is to provide a semiconductor device using nitride III-V compound semiconductors with a high band gap, having excellent electric property and optical property.
To solve the problems contained in the conventional techniques, the Inventor made studies with every efforts. Its outline is explained below.
B-based semiconductors such as BN containing boron (B) as a group III element are stable in the sense of energy, strong against high energy light such as ultraviolet light; for example, and are hopeful for the future use. In particular, with nitride III-V compound semiconductors containing B, an increase in band gap by addition of B can be expected. Further, according to the knowledge of the Inventor, it has been confirmed that II-VI compound semiconductors and other III-V compound semiconductors are more easily p-typed as the covalent bond diameter of positive ions decreases (Hiroyuki Okuyama, Akira Ishibashi, Applied Physics 65,687(1996), herein after called Document 1). Therefore, analogically inferring from Document 1, it is considered that relatively high p-type carrier concentrations can be obtained with nitride III-V compound semiconductors containing B, in which the covalent radius of positive ion elements is reduced by addition of B.
Here is taken B
p
Al
q
Ga
r
In
s
N (0<p≦1, 0≦q<1, 0<r<1, 0≦s<1, p+q+r+s=1) as a nitride III-V compound semiconductor containing B, and reviews are made on the use of this B
p
Al
q
Ga
r
In
s
N as a material of a semiconductor layer forming a light emitting structure.
In semiconductor light emitting devices, in general, it is considered desirable to make layers from the active layer to the cladding layer by using direct transition type semiconductors, taking influences of optical absorption into consideration. However, since BN is an indirect transition type semiconductor, for making mixed crystals with other direct transition type semiconductors, such as GaN and AlN, the range of composition of B must be fixed.
FIG. 2
shows relations between lattice constants and energy gaps of typical GaN semiconductors. Shown in
FIG. 2
are minimum energy gaps of r points and minimum energy gaps other than r points of GaN, AlN, InN and BN. If the Vegard law that a lattice constant linearly changes with composition ratio holds, the minimum of the wurtzite structure appears approximately at L point. Comparing them and calculating the intersection between direct transition and indirect transition from the interpolation, BAlN becomes an indirect transition type semiconductor with B composition not less than 10%, and BGaN becomes an indirect transition type semiconductor with B composition not less than 30%. In other words, in order to ensure that B
p
Al
q
Ga
r
In
s
N be of an indirect transition type, B composition not larger than 0.3, or more preferably not more than 0.1 will be acceptable.
Further, since B
p
Al
q
Ga
r
In
s
N enables an increase of the band gap by addition of B as explained above, it will be suitable as the material of the cladding layer which is desired to have a large band gap. Additionally, since B
p
Al
q
Ga
r
In
s
N can be more easily increased in p-type carrier concentration than AlGaN, it is advantageous for reducing the resistance of the p-type cladding layer as well. However, when growth of B
p
A
Crane Sara
Sonnenschein Nath & Rosenthal
Sony Corporation
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