Active solid-state devices (e.g. – transistors – solid-state diode – With means to control surface effects – In compound semiconductor material
Reexamination Certificate
2002-02-28
2004-03-30
Le, Dung A. (Department: 2818)
Active solid-state devices (e.g., transistors, solid-state diode
With means to control surface effects
In compound semiconductor material
C257S622000
Reexamination Certificate
active
06713845
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a nitride-based semiconductor element and a method of forming a nitride-based semiconductor, and more specifically, it relates to a nitride-based semiconductor element containing a nitride-based semiconductor formed by epitaxial lateral overgrowth and a method of forming a nitride-based semiconductor.
2. Description of the Prior Art
A technique of growing a nitride-based semiconductor on an underlayer is known in general. For example, a crystal of GaN, which is one of nitride-based semiconductors lattice-matching with only a small number of types of substrates, is grown on a substrate such as a sapphire substrate. In relation to this, generally known is a technique of inserting a buffer layer grown at a low temperature between the substrate and a GaN layer for growing GaN in excellent crystallinity with a small number of crystal defects.
Even if the aforementioned low-temperature buffer layer is employed, however, the density of reducible defects is limited and it is difficult to reduce the number of dislocations. To this end, a technique of reducing the number of dislocations by epitaxial lateral overgrowth (ELOG) of GaN is proposed in general. This epitaxial lateral overgrowth is disclosed in Journal of Oyo Denshi Bussei Bunkakai, Vol. 4 (1998), pp. 53 to 58 and 210 to 215, for example.
FIG. 29
is a sectional view for illustrating a conventional method of forming a nitride-based semiconductor by epitaxial lateral overgrowth. Referring to
FIG. 29
, a low-temperature buffer layer
102
is first formed on a sapphire substrate
101
, and thereafter a GaN layer
103
for serving as an underlayer is grown on the low-temperature buffer layer
102
in the conventional method of forming a nitride-based semiconductor by epitaxial lateral overgrowth.
Then, striped (elongated) mask layers
104
of SiO
2
or the like are formed on prescribed regions of the GaN layer
103
. The mask layers
104
are employed as selective growth masks for epitaxially laterally overgrowing a GaN layer
105
from the GaN layer
103
serving as an underlayer, so that the GaN layer
105
is vertically (upwardly) grown from exposed portions of the GaN layer
103
and thereafter laterally grown on the mask layers
104
. Dislocations extending in the c-axis direction are laterally bent due to this lateral overgrowth, not to reach a portion around the upper surface of the GaN layer
105
. Thus, the number of dislocations reaching the flat upper surface of the finally formed GaN layer
105
is remarkably reduced as compared with the GaN layer
103
forming the underlayer.
In the conventional method of forming a nitride-based semiconductor by epitaxial lateral overgrowth shown in
FIG. 29
, however, the c-axis of the overgrowth region of GaN layer
105
is disadvantageously inclined from the normal direction of the substrate although the number of dislocations resulting from epitaxial lateral overgrowth can be reduced in the GaN layer
105
. In other words, the growth layer of the GaN layer
105
laterally grown on the mask layers
104
is strained due to stress applied thereto. Therefore, the c-axis perpendicular to the sapphire substrate
101
is disadvantageously inclined by about 2° at the maximum as shown by arrows in FIG.
29
. When the c-axis is displaced, crystallinity is deteriorated to result in inferior element characteristics.
In order to suppress such inclination of the c-axis, therefore, a method shown in
FIG. 30
is proposed in general. Referring to
FIG. 30
, a GaN layer
113
serving as an underlayer is formed on a sapphire substrate
111
through a low-temperature buffer layer
112
in this proposed method. Recess portions are formed on the surface of the GaN layer
113
for thereafter forming mask layers
114
having recess portions
114
a
of SiO
2
or the like in recess portions
113
a
of the surface. The mask layers
114
are employed as selective growth masks for epitaxially laterally overgrowing a GaN layer
115
on projection portions of the GaN layer
113
serving as an underlayer. In this case, voids
120
are defined between the mask layers
114
and the epitaxially laterally overgrown GaN layer
115
, thereby reducing the contact areas between the GaN layer
115
and the mask layers
114
when the GaN layer
115
is laterally grown on the mask layers
114
. Thus, stress is hardly applied to the GaN layer
115
laterally grown on the mask layers
114
, and hence strain of the GaN layer
115
is relaxed. Consequently, inclination of the c-axis can be relaxed as compared with the prior art shown in
FIG. 29
, as shown by arrows in FIG.
30
.
In the conventional proposed method shown in
FIG. 30
, however, it is necessary to etch the GaN layer
113
serving as an underlayer for forming the recess portions thereof. In this case, a long time is required for etching the GaN layer
113
, disadvantageously leading to a long process time.
In the conventional proposed method shown in
FIG. 30
, most dislocations
116
are bent in intermediate portions during lateral growth of the GaN layer
115
not to reach the surface, as shown in FIG.
31
. However, some of dislocations
116
are not bent but reach the surface as such. In the conventional proposed method, therefore, it is difficult to further reduce the number of dislocations.
SUMMARY OF THE INVENTION
An object of the present invention is to provide a nitride-based semiconductor element having excellent device characteristics by obtaining a nitride-based semiconductor layer having excellent crystallinity without performing a long-time etching process.
Another object of the present invention is to reduce the number of dislocations in an epitaxially laterally overgrown nitride-based semiconductor layer in the aforementioned nitride-based semiconductor element.
Still another object of the present invention is to provide a method of forming a nitride-based semiconductor capable of readily forming a nitride-based semiconductor layer having excellent crystallinity without performing a long-time etching process.
A further object of the present invention is to more effectively reduce the number of dislocations reaching the surface of the nitride-based semiconductor in the aforementioned method of forming a nitride-based semiconductor.
A nitride-based semiconductor element according to a first aspect of the present invention comprises a mask layer, having a recess portion on the upper surface of the mask layer, formed on a substantially flat upper surface of an underlayer to partially expose the upper surface of the underlayer, a nitride-based semiconductor layer formed on the exposed part of the underlayer and the mask layer while forming a void on the recess portions of the mask layer, and a nitride-based semiconductor element layer, formed on the nitride-based semiconductor layer, having an element region.
In the nitride-based semiconductor element according to the first aspect, the mask layer having a recess portion is formed on the substantially flat upper surface of the underlayer as hereinabove described, whereby the nitride-based semiconductor layer is formed with the void on the recess portion of the mask layer when laterally grown on the mask layer. This void relaxes strain of the laterally grown nitride-based semiconductor layer, whereby inclination of the c-axis (crystal axis) of the nitride-based semiconductor layer can be suppressed. Further, the underlayer is formed in a substantially flat shape, whereby no etching may be performed over a long time for forming recess portions on the surface of the underlayer consisting of a nitride-based semiconductor or the like, dissimilarly to the prior art. Therefore, inclination of the c-axis (crystal axis) of the nitride-based semiconductor layer can be suppressed without performing an etching process for a long time. Consequently, a nitride-based semiconductor layer having excellent crystallinity can be obtained without performing a long-time etching process. When the nitride-based semiconductor element layer
Hayashi Nobuhiko
Kunisato Tatsuya
Ohbo Hiroki
Yamaguchi Tsutomu
Le Dung A.
McDermott & Will & Emery
Sanyo Electric Co,. Ltd.
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