Stock material or miscellaneous articles – All metal or with adjacent metals – Composite; i.e. – plural – adjacent – spatially distinct metal...
Reexamination Certificate
2002-02-25
2004-07-06
Lam, Cathy (Department: 1775)
Stock material or miscellaneous articles
All metal or with adjacent metals
Composite; i.e., plural, adjacent, spatially distinct metal...
C428S698000, C257S085000, C257S103000, C257S631000, C438S604000
Reexamination Certificate
active
06759139
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 particularly, it relates to a nitride-based semiconductor element having a nitride-based semiconductor, consisting of a material different from that of an underlayer, formed on the underlayer by hetero growth and a method of forming a nitride-based semiconductor.
2. Description of the Prior Art
A technique of hetero-growth about a nitride-based semiconductor consisting of a material different from that of an underlayer on the 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 heterogeneous substrate such as a sapphire substrate to form hetero structure. 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 employing an underlayer reduced in 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.
FIGS. 20
to
25
are sectional views for illustrating an exemplary conventional method of forming a nitride-based semiconductor through epitaxial lateral overgrowth. This conventional method of forming a nitride-based semiconductor through epitaxial lateral overgrowth is now described with reference to
FIGS. 20
to
23
.
First, a low-temperature buffer layer
202
is formed on a sapphire substrate
201
, and thereafter a GaN layer
203
for serving as an underlayer is grown on the low-temperature buffer layer
202
, as shown in FIG.
20
.
Then, striped (elongated) mask layers
204
of SiO
2
or the like are formed on prescribed regions of the GaN layer
203
, as shown in FIG.
21
. The mask layers
204
are employed as selective growth masks for making re-growth from the GaN layer
203
serving as an underlayer, thereby forming GaN layers
205
having a facet structure with a triangular section on exposed parts of the GaN layer
203
.
Then, the facet GaN layers
205
are bonded to each other and dominated by lateral growth, as shown in FIG.
22
. Therefore, dislocations extending in the c-axis direction are bent on the bonded portions of the facets not to reach upper portions. However, the dislocations remain on the bonded portions of the facets.
Then, the facet GaN layers
205
coalesce into a continuous GaN layer
205
having a flat upper surface, as shown in FIG.
23
. The number of dislocations reaching the flattened surface of the GaN layer
205
is remarkably reduced as compared with the underlayer.
In the conventional method of forming a nitride-based semiconductor shown in
FIGS. 20
to
23
, dislocations concentrically remain on upper portions of the mask layers
204
where the facets are bonded to each other when the GaN layer
205
is formed by epitaxial lateral overgrowth. In order to reduce the number of dislocations, therefore, the mask layers
204
are preferably reduced in width. If the width of the mask layers
204
is reduced in order to reduce the number of dislocations, however, the exposed parts of the GaN layer
203
serving as an underlayer are increased in width, and hence the facets of GaN formed on the exposed parts of the GaN layer
203
are also increased in width (height). In order to bond the large facets and flatten the upper surface of the GaN layer
205
, therefore, the GaN layer
205
must be thickly formed. Thus, it is generally difficult to obtain the GaN layer
205
having a small number of dislocations with a small thickness.
A method of growing a GaN layer through epitaxial lateral overgrowth by directly forming mask layers on a substrate is also proposed in general.
FIG. 24
is a sectional view for illustrating the conventional proposed method of forming a nitride-based semiconductor. Referring to
FIG. 24
, mask layer
212
of SiO
2
are directly formed on a sapphire substrate
211
for forming low-temperature buffer layers
213
of GaN and a GaN layer
214
of high-temperature growth thereon, thereby forming a GaN layer
214
reduced in number of dislocations by single growth. According to this conventional proposed method, the mask layers
212
are directly formed on the sapphire substrate
211
, and hence the total thickness is reduced due to absence of an underlayer.
However, the conventional proposed method shown in
FIG. 24
has a problem similar to that in the prior art shown in
FIGS. 20
to
23
. Also when the mask layers
212
are directly formed on the sapphire substrate
211
for making epitaxial lateral overgrowth, the mask layers
212
must be reduced in width in order to reduce the number of dislocations. If the width of the mask layers
212
is reduced, however, exposed areas of the sapphire substrate
211
are increased to increase the size (height) of GaN facets formed on the low-temperature buffer layers
213
located on the exposed parts. In order to bond the large facets to each other for flattening the GaN layer
214
, therefore, the GaN layer
214
must be formed in a large thickness of at least about 5 &mgr;m. Consequently, it is difficult to obtain the GaN layer
214
having a small number of dislocations with a small thickness also in the conventional proposed method shown in FIG.
24
.
When a mixed crystal of AlGaN, InN, InGaN, BGaN, BAlGaN, BInGaN or AlInGaN is thickly grown, it is more difficult to obtain a lattice-matching substrate in general. For example, it is difficult to thickly grow an InGaN layer directly on a sapphire substrate, due to remarkable difference between lattice constants. In general, therefore, a GaN layer
223
is first grown on a sapphire substrate
221
through a buffer layer
222
, as shown in FIG.
25
. Mask layers
224
are formed on the GaN layer
223
and thereafter employed as growth masks for making epitaxial lateral overgrowth, thereby forming a low-dislocation GaN layer
225
. An InGaN layer
226
is grown on the low-dislocation GaN layer
225
. Thus, the InGaN layer
226
having low dislocation density can be somewhat thickly grown on the low-dislocation GaN layer
225
formed by epitaxial lateral overgrowth.
In the conventional method of forming a nitride-based semiconductor consisting of a mixed crystal shown in
FIG. 25
, however, the low-dislocation GaN layer
225
must be formed as an underlayer by epitaxial lateral overgrowth in order to obtain the InGaN layer
226
having a small number of dislocations, as hereinabove described. In the prior art shown in
FIG. 25
, therefore, the total thickness is so increased that it is consequently difficult to obtain the InGaN layer
226
having a small number of dislocations with a small thickness as a whole. In the conventional method of forming a nitride-based semiconductor consisting of a mixed crystal shown in
FIG. 25
, further, the InGaN layer
226
is grown on the GaN underlayer
225
formed by epitaxial lateral overgrowth, and hence the steps are disadvantageously complicated.
SUMMARY OF THE INVENTION
An object of the present invention is to provide a nitride-based semiconductor element having a structure enabling formation of a nitride-based semiconductor layer having a small number of dislocations with a small thickness when a nitride-based semiconductor consisting of a material different from that of an underlayer is formed on the underlayer by hetero growth.
Another object of the present invention is to provide a method of forming a nitride-based semiconductor capable of readily growing a nitride-based semiconductor layer having a small number of di
Hata Masayuki
Hayashi Nobuhiko
Kunisato Tatsuya
Ohbo Hiroki
Yamaguchi Tsutomu
Lam Cathy
McDermott & Will & Emery
Sanyo Electric Co,. Ltd.
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