Active solid-state devices (e.g. – transistors – solid-state diode – Incoherent light emitter structure – With particular semiconductor material
Reexamination Certificate
2000-09-21
2003-03-18
Meier, Stephen D. (Department: 2822)
Active solid-state devices (e.g., transistors, solid-state diode
Incoherent light emitter structure
With particular semiconductor material
Reexamination Certificate
active
06534800
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a semiconductor device having a compound semiconductor layer composed of GaN (gallium nitride), AlN (aluminum nitride), InN (indium nitride), BN (boron nitride), or TlN (thallium nitride) or an III-V group nitride based semiconductor (hereinafter referred to as a nitride based semiconductor) which is their mixed crystal, and a method of fabricating the same.
2. Description of the Background Art
In recent years, GaN based semiconductor light emitting devices have been put to practical use as semiconductor light emitting devices such as light emitting diodes and semiconductor laser devices which emit light in blue or violet.
FIG. 8
is a cross-sectional view showing an example of a conventional GaN based semiconductor laser device.
The semiconductor laser device shown in
FIG. 8
is fabricated in the following manner.
In a crystal growth device such as an MOCVD (Metal Organic Chemical Vapor Deposition) device or an MBE (Molecular Beam Epitaxy) device, an AlGaN buffer layer
102
composed of undoped AlGaN, an undoped GaN layer
103
, an n-GaN contact layer
104
, an n-AlGaN cladding layer
105
, an n-GaN optical guide layer
106
, an InGaN quantum well active layer
107
, a p-AlGaN layer
108
, a p-GaN optical guide layer
109
, a p-AlGaN cladding layer
110
, and a p-AlGaN cap layer
111
are successively grown on a C(0001) plane of a sapphire substrate
101
.
Subsequently, a wafer is taken out of the crystal growth device, to etch predetermined regions of the p-AlGaN cap layer
111
and the p-AlGaN cladding layer
110
by RIBE (Reactive Ion Beam Etching) or the like. A ridge portion is thus formed.
After the ridge portion is formed, the wafer is returned to the crystal growth device again, to grow an n-AlGaN current blocking layer
112
on side surfaces and an upper surface of the ridge portion as well as on a flat portion of the p-AlGaN cladding layer
110
. Further, the wafer is taken out of the crystal growth device, to etch the n-AlGaN current blocking layer
112
on the upper surface of the ridge portion to form a striped opening. The upper surface of the ridge portion is thus exposed. Thereafter, the wafer is returned to the crystal growth device again, to grow a p-GaN contact layer
113
on the n-AlGaN current blocking layer
112
and on the upper surface of the ridge portion.
Subsequently, the wafer is taken out of the crystal growth device, to etch a partial region from the p-GaN contact layer
113
to the n-GaN contact layer
104
away. A predetermined region of the n-GaN contact layer
104
is thus exposed. Further, an n electrode
50
is formed on the exposed predetermined region of the n-GaN contact layer
104
. Further, a p electrode
51
is formed on a predetermined region of the p-GaN contact layer
113
. Finally, the sapphire substrate
101
is cleaved, to form an end surface of a cavity.
In the semiconductor laser device having a ridge wave-guided structure as shown in
FIG. 8
, the ridge portion is formed, thereby creating a refractive index distribution in the horizontal direction of the InGaN quantum well active layer
107
as well as narrowing down a current. Light is horizontally confined, that is, transverse mode control is carried out in the semiconductor laser device utilizing the refractive index distribution and the current narrowed down.
Generally when the nitride based semiconductor layer is grown such that it is thick, it is liable to be cracked. In the nitride based semiconductor layer, an AlGaN layer containing Al is liable to be particularly cracked. In fabricating the above-mentioned semiconductor laser device having a ridge wave-guided structure, it is necessary to take the wafer out of the crystal growth device when the ridge portion is formed and when the striped opening in the n-AlGaN current blocking layer
112
is formed to subject the wafer taken out to etching, and then return the wafer to the crystal growth device again to grow the n-AlGaN current blocking layer
112
and the p-GaN contact layer
113
.
Particularly in the n-AlGaN current blocking layer
112
, the refractive index must be made lower (the band-gap must be made larger), as compared with that in the cladding layer in order to carry out the transverse mode control. In the n-AlGaN current blocking layer
112
, therefore, the Al composition ratio is increased. The thickness of the n-AlGaN current blocking layer
112
is increased such that the current is sufficiently narrowed down by the n-AlGaN current blocking layer
112
. The n-AlGaN current blocking layer
112
having a high Al composition ratio and having a large thickness is liable to be particularly cracked.
Since the thickness of the p-GaN contact layer
113
is also large, the p-GaN contact layer
113
is liable to be cracked.
When the wafer is taken out of the crystal growth device as described above, a surface of the wafer is oxidized. At the time of regrowth, the nitride based semiconductor layer is grown on the oxidized surface. Accordingly, lattice defects occur in the regrown layer. That is, in fabricating the semiconductor laser device, the wafer is taken out of the crystal growth device at the time of forming the ridge portion. Consequently, the flat portion of the p-AlGaN cladding layer
110
and the ridge portion as well as the surface of the p-GaN cap layer
111
are oxidized. The n-AlGaN current blocking layer
112
is regrown on the flat portion of the p-AlGaN cladding layer
110
and the ridge portion as well as the surface of the p-GaN cap layer
111
, which have been oxidized. Lattice defects occur in the n-AlGaN current blocking layer
112
. When a striped opening is also formed in the n-AlGaN current blocking layer
112
, the wafer is taken out of the crystal growth device. Consequently, the surfaces of the p-GaN cap layer
111
and the n-AlGaN current blocking layer
112
are oxidized. The p-GaN contact layer
113
is regrown again on the oxidized surfaces of the p-GaN cap layer
111
and the n-AlGaN current blocking layer
112
. Accordingly, lattice defects also occur in the p-GaN contact layer
113
.
The occurrence of the crack and the degradation of crystallizability in the n-AlGaN current blocking layer
112
and the p-GaN contact layer
113
which have been regrown, as described above, degrade device characteristics and decrease reliability in the semiconductor laser device.
Particularly, the occurrence of the crack and the degradation of the crystallizability in the n-AlGaN current blocking layer
112
degrade the device characteristics and decrease the reliability. Therefore, a method of fabricating a semiconductor laser device with a transverse mode is difficult.
SUMMARY OF THE INVENTION
An object of the present invention is to provide a semiconductor device in which the occurrence of a crack and the degradation of crystallizability in a layer regrown after processing such as etching are prevented.
Another object of the present invention is to provide a method of fabricating a semiconductor device in which the occurrence of a crack and the degradation of crystallizability in a layer regrown after processing such as etching can be prevented.
A semiconductor device according to an aspect of the present invention comprises a first semiconductor layer composed of a nitride based semiconductor whose upper surface is patterned; a buffer layer composed of a nitride based semiconductor positioned on the first semiconductor layer; and a second semiconductor layer composed of a nitride based semiconductor positioned on the buffer layer.
The buffer layer is a layer which can be grown without being affected by lattice defects in the underlying nitride based semiconductor layer. The buffer layer makes it possible to reduce the number of lattice defects in the nitride based semiconductor layer positioned on the buffer layer. Further, the buffer layer is a layer capable of reducing the difference in the coefficient of thermal expansion between two types of nitride based semiconductor layers, which differ in compositio
Hayashi Nobuhiko
Ohbo Hiroki
Armstrong Westerman & Hattori, LLP
Sanyo Electric Co,. Ltd.
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