Method of manufacturing high performance semiconductor...

Details Method of manufacturing high performance semiconductor... Method of manufacturing high performance semiconductor...

2001-02-23

2003-05-20

Nelms, David (Department: 2818)

Semiconductor device manufacturing: process

Formation of electrically isolated lateral semiconductive...

Recessed oxide by localized oxidation

C438S042000

Reexamination Certificate

active

06566231

ABSTRACT:

BACKGROUND OF THE INVENTION
The present invention relates to a semiconductor device, in particular, a nitride semiconductor device functioning as a blue laser, a field effect transistor for high-speed operation, and the like, a method for fabricating such a semiconductor device, and a method for fabricating a semiconductor substrate used in a semiconductor device
There are conventionally known lasers and field effect transistors that use as the active region thereof a compound semiconductor layer made of a nitride semiconductor, in particular, a group III nitride typified by gallium nitride (GaN), aluminum nitride (AlN), and indium nitride (InN). In other words, many techniques utilizing features of nitride semiconductors already exist, including forming lasers (for example, blue lasers) that emit short-wavelength light utilizing the broad band gap of nitride semiconductors, and forming field effect transistors achieving high-speed operation utilizing the high mobility (traveling velocity) of carriers in nitride semiconductors.
FIG. 12
is a cross-sectional view of a conventional semiconductor device as a semiconductor laser using a nitride semiconductor. In
FIG. 12
, hatching of the cross section is omitted for clarification of the structure of defects in crystals. Referring to
FIG. 12
, on a substrate
101
made of n-type GaN, sequentially grown by epitaxy are an n-type GaN layer
111
, an n-type AlGaN cladding layer
112
, an n-type GaN optical guide layer
113
, an undoped GaN active layer
114
, a p-type GaN optical guide layer
115
, a first p-type AlGaN cladding layer
116
, a current narrowing layer
117
having an opening, a second p-type AlGaN cladding layer
118
, and a p-type GaN contact layer
119
in this order. An n-side electrode
120
is formed on the bottom surface of the substrate
101
, and a p-side electrode
121
is formed on the top surface of the p-type GaN contact layer
119
.
The semiconductor device with the above construction includes the undoped GaN active layer
114
made of a nitride semiconductor. Therefore, by applying a voltage through the n-side electrode
120
and the p-side electrode
121
, the semiconductor device can be used as a semiconductor laser device that oscillates blue light at an active region
114
a
of the undoped GaN active layer
114
located below the opening of the current narrowing layer
117
.
The above conventional semiconductor device has a problem as follows. The substrate
101
intrinsically includes streaky lattice defects D (in particular, dislocations) extending vertically. Note that the substrate
101
also includes lattice defects such as dislocations extending in parallel with or in directions declined from the substrate plane. These dislocations have little relation to the cause of the problem to be described hereinafter, and thus are not shown in the figures. With the sequential epitaxial growth of the n-type GaN contact layer
111
, the n-type AlGaN cladding layer
112
, . . . on the substrate
101
, the lattice defects extend upward, reaching the active region
114
a
of the undoped GaN active layer
114
located below the opening of the current narrowing layer
117
.
In the semiconductor laser device, for laser oscillation, a high current must be applied to the active region
114
a
to generate an inversion state in the active region
114
a
. However, when such a high current is applied to the active region
114
a
that includes a number of lattice defects, deterioration of the laser oscillation function may possibly develop from the positions of the lattice defects and, as a result, the life and reliability of the semiconductor laser may be significantly reduced.
The above problem due to the existence of defects may arise, not only in semiconductor laser devices, but also in other semiconductor devices such as high-speed field effect transistors and Schottky diodes. For example, if a number of lattice defects exist in the channel region below the gate of a field effect transistor, the mobility of carriers decreases. This may possibly deteriorate the performance of the transistor.
As described above, a semiconductor device may possibly be deteriorated in performance due to lattice defects existing in the active region (carrier traveling region) thereof, such as the active layer in the case of a semiconductor laser device and the channel region in the case of a transistor.
SUMMARY OF THE INVENTION
An object of the present invention is providing a semiconductor device with high reliability and high performance capable of reducing the number of lattice defects in the active region thereof, a method for fabricating such a semiconductor device, and a method for fabricating a semiconductor substrate used in a semiconductor device.
The semiconductor device of the present invention includes: a substrate having a first semiconductor layer; at least one convex portion formed in the first semiconductor layer, the convex portion having a top surface and a side face intersecting with the top surface; a coat layer formed to cover at least part of the top surface and leave open at least part of the side face of the convex portion of the first semiconductor layer, the coat layer having a function of suppressing epitaxial growth of a semiconductor on the first semiconductor layer; and a second semiconductor layer formed on the first semiconductor layer by epitaxial growth, wherein a region of the second semiconductor layer located above the convex portion operates as an active region.
With the above construction, the following effects are obtained. A semiconductor crystal epitaxially growing from the side face of the convex portion of the first semiconductor layer is deposited in a direction roughly normal to the side face. During this crystal growth, lattice defects exposed on the side face of the first semiconductor layer are incorporated in the crystal constituting the second semiconductor layer, extending in the second semiconductor layer in a direction roughly normal to the side face of the convex portion, that is, in a direction away from the convex portion. Therefore, the lattice defects in the first semiconductor layer deposited by side-direction growth of the crystal epitaxially grown from the side face of the convex portion, such as the portion located above the coat layer. Thus, the region of the second semiconductor layer located above the convex porion constitutes a low defect region, and a semiconductor device having its active region in this low defect region can exhibit good characteristics. For example, when the device is a semiconductor laser device, the light emitting characteristics are suppressed from deteriorating. When the device is a field effect transistor, the carrier traveling characteristics are improved.
The coat layer may cover a portion of the semiconductor layer other than the top surface of the convex portion. This reduces the possibility of propagation of the lattice defects in the first semiconductor layer into the second semiconductor layer, and thus it is possible to provide a semiconductor device in which the defect density of the second semiconductor layer is lower.
At least two convex portions may be formed, and the coat layer may also cover a bottom surface of a concave portion formed between the at least two convex portions. Lattice defects extending from the side faces of the two convex portions sandwiching the concave portion (side faces of the concave portion) in a direction roughly normal to the side faces in the second semiconductor layer concentrate near the center of the concave portion and are united into roughly one streak, which then extends upward. Contrarily, no lattice defects propagate from the bottom surface of the concave portion into the second semiconductor layer. This greatly reduces the defect region in the second semiconductor layer, and thus further reduces the defect density of the entire second semiconductor layer.
A plurality of convex portions may be formed, and the top surfaces of the convex portions may constitute a stripe pattern. Thus, a semiconducto

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