Semiconductor device manufacturing: process – Making device or circuit emissive of nonelectrical signal
Reexamination Certificate
2001-12-18
2004-08-17
Nelms, David (Department: 2818)
Semiconductor device manufacturing: process
Making device or circuit emissive of nonelectrical signal
C438S022000, C438S031000, C438S033000, C438S046000, C438S479000
Reexamination Certificate
active
06777253
ABSTRACT:
BACKGROUND OF THE INVENTION
The present invention relates to a method for fabricating a semiconductor of group III-V nitrides constituting a semiconductor laser device of which application to the optical information processing field and the like is expected, a method for fabricating a semiconductor substrate, and semiconductor light emitting devices fabricated using such methods.
Group III-V nitride semiconductors using nitrogen (N) as a group V element have received attention as promising materials for short-wavelength light emitting devices because they have a comparatively large band gap. Among others, gallium nitride (GaN) based compound semiconductors (Al
x
Ga
y
In
z
N (0<x≦1, 0≦y≦1, 0≦z≦1, x+y+z=1)) have been studied vigorously, and blue light emitting diode (LED) devices and green LED devices made of GaN-based semiconductors have already been commercialized.
To increase the storage capacity of optical disc devices, semiconductor laser devices having an oscillating wavelength in the 400 nm band are eagerly demanded. In this relation, semiconductor laser devices made of GaN-based compound semiconductors have received attention, and are now about to reach a level of commercialization.
A GaN-based semiconductor laser device has a device structure generally formed by growing crystals on a substrate made of sapphire (Al
2
O
3
single crystal), silicon carbide (SiC), or the like by metal-organic vapor phase epitaxy (MOVPE).
A conventional GaN-based semiconductor laser device will be described with reference to FIG.
8
.
FIG. 8
shows a cross-sectional construction of a conventional GaN-based semiconductor laser device capable of providing laser oscillation.
As shown in
FIG. 8
, on a substrate
101
made of sapphire, formed sequentially by crystal growth are a low-temperature growth buffer layer
102
, a strain suppression layer
103
made of n-type Al
0.05
Ga
0.95
N, an n-type cladding layer
104
made of n-type Al
0.07
Ga
0.93
N, an n-type optical guide layer
105
made of n-type GaN, a multiple quantum well (MQW) active layer
106
made of GaInN, a block layer
107
made of p-type AlGaN, a ptype optical guide layer
108
made of p-type GaN, a P-type cladding layer
109
made of p-type Al
0.07
Ga
0.93
N, and a p-type contact layer
110
made of p-type GaN.
A feature of the above conventional semiconductor laser device is the strain suppression layer
103
formed on the low-temperature growth buffer layer
102
. The strain suppression layer
103
is made of Al
0.05
Ga
0.95
N. The mole fraction of Al of this composition, 0.05, is determined to be a value close to the Al mole fraction of the n-type cladding layer
104
made of Al
0.07
Ga
0.93
N, of which the lattice constant is smallest among the plurality of semiconductor layers constituting the laser structure. The strain suppression layer
103
having this composition serves to reduce strain as the underlying layer of the n-type cladding layer
104
. Thus, with the existence of the strain suppression layer
103
, it is possible to reduce occurrence of cracking in the cladding layer
104
or warping of the substrate
101
that may be caused by crystal strain during the formation of the laser structure.
The n-type and p-type cladding layers
104
and
109
have a thickness of about 0.5 &mgr;m, the largest among the layers of the laser structure, and also have the largest Al mole fraction among the layers because they must secure a large band gap and a small refractive index. Therefore, cracking generally tends to occur in the cladding layers.
To overcome the above problem, in the conventional semiconductor laser device, the Al mole fraction of the strain suppression layer
103
is simply determined so that the lattice constant of the strain suppression layer
103
is a value somewhere between the lattice constant of the substrate
101
made of sapphire and that of the cladding layers
104
and
109
made of AlGaN.
Another crystal growth method is also reported where a substrate made of gallium nitride formed by hydride vapor phase epitaxy (H-VPE) or the like is used as the substrate
101
in place of the sapphire substrate.
However, in the above conventional semiconductor growth method, the lattice constants of the strain suppression layer
103
and the cladding layers
104
and
109
, which are both made of AlGaN, are not determined by strict designing, but determined by simply setting the Al mole fraction of the strain suppression layer
103
at a value close to that of the cladding layers
104
and
109
. Therefore, when the temperature is lowered to room temperature after the crystal growth, the strain suppression layer
103
undergoes strain due to a difference in thermal expansion coefficient between the substrate
101
and the strain suppression layer
103
and therefore changes in lattice constant. As a result, the lattice constant of the strain suppression layer
103
differs from that of the cladding layers
104
and
109
, and this causes occurrence of cracking or warping.
In the case of the substrate
101
made of gallium nitride, also, in which the lattice constant is decisively different between the cladding layers and the substrate, cracking or warping occurs in the cladding layers.
SUMMARY OF THE INVENTION
An object of the present invention is to ensure that no cracking or the like occurs particularly in a semiconductor layer having a small lattice constant among a plurality of layered semiconductor layers made of group III-V nitrides.
To attain the above object, in a structure using a substrate made of a material different from a group III-V nitride, the lattice constant of a semiconductor layer having a comparatively small lattice constant among a plurality of semiconductor layers grown on the substrate, that is, a semiconductor layer containing aluminum, is made to substantially match with the lattice constant of a strain suppression layer at room temperature after thermal shrinkage or thermal expansion.
In a structure using a substrate made of a group III-V nitride, the lattice constant of a semiconductor layer having a comparatively small lattice constant among a plurality of semiconductor layers grown on the substrate, that is, a semiconductor layer containing aluminum, is made to substantially match with the lattice constant of the substrate.
The first method for fabricating a semiconductor of the present invention includes the steps of: (1) growing a first semiconductor layer made of Al
x
Ga
1−x
N (0≦x≦1) on a substrate at a temperature higher than room temperature; and (2) growing a second semiconductor layer made of Al
u
Ga
v
In
w
N (0<u≦1, 0≦v≦1, 0≦w≦1, u+v+w=1) over the first semiconductor layer, wherein in the step (1), the mole fraction x of Al of the first semiconductor layer is set so that the lattice constant of the first semiconductor layer at room temperature substantially matches with the lattice constant of the second semiconductor layer in the bulk state after thermal shrinkage or thermal expansion.
According to the first method for fabricating a semiconductor of the present invention, cracking and the like will not occur in the aluminum-containing second semiconductor layer having a comparatively small lattice constant even when the temperature is lowered to room temperature after the growth of the second semiconductor layer.
Preferably, the first method for fabricating a semiconductor further includes the step of growing a third semiconductor layer having an Al mole fraction smaller than the second semiconductor layer between the first semiconductor layer and the second semiconductor layer or over the second semiconductor layer. With this construction, the third semiconductor layer can function as an active layer including a quantum well layer. Thus, the second semiconductor layer having an Al mole fraction larger than the third semiconductor layer can function as a cladding layer.
In the first method for fabricating a semiconductor, the substrate preferably is composed of sapphire, silicon carbide, or si
Ishibashi Akihiko
Kawaguchi Yasutoshi
Ohnaka Kiyoshi
Otsuka Nobuyuki
Tsujimura Ayumu
Matsushita Electric - Industrial Co., Ltd.
McDermott & Will & Emery
Nelms David
Tran Mai-Huong
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