Active solid-state devices (e.g. – transistors – solid-state diode – Incoherent light emitter structure – With particular semiconductor material
Reexamination Certificate
2000-11-15
2002-07-30
Meier, Stephen D. (Department: 2822)
Active solid-state devices (e.g., transistors, solid-state diode
Incoherent light emitter structure
With particular semiconductor material
C257S522000, C438S511000
Reexamination Certificate
active
06426519
ABSTRACT:
BACKGROUND OF THE INVENTION
(1) Field of the invention:
This invention relates to an epitaxial growth substrate comprising a base material composed of a sapphire substrate, a SiC substrate, a GaN substrate or the like and an Al
x
Ga
y
In
z
N film (x+y+z=1,x>0,y,z≧0) on the base material, and a method for producing the same.
(2) Related Art Statement:
An epitaxial growth substrate is employed for an electronic device such as a light emitting diode (LED), a laser diode (LD) or a field effect transistor (FET). In producing such an electronic device, semiconductor films such as Al
a
Ga
b
In
c
N film (a+b+c=1,a,b,c≧0) are formed on the epitaxial growth substrate by epitaxial growth. Attention is paid to an Al
a
Ga
b
In
c
N film (a+b+c=1,a,b,c≧0) because it has a large bandgap to generate and emit a short wavelength light in a light emitting element.
FIG. 1
is a cross sectional view showing a conventional light emitting diode to generate a blue light, which is composed of the above Al
a
Ga
b
In
c
N films.
For example, a GaN film
2
as a buffer layer is formed on a C-faced sapphire (Al
2
O
3
) substrate
1
at a low temperature by CVD, and an n-type Al
a
Ga
b
In
c
N film
3
is formed on the GaN film
2
by epitaxial growth through CVD. Then, a p-type Al
a
Ga
b
In
c
N film
4
is epitaxially grown on the n-type Al
a
Ga
b
In
c
N film
3
by epitaxial growth through CVD, and a low resistive p-type Al
a
Ga
b
In
c
N film
5
is epitaxially grown on the p-type Al
a
Ga
b
In
c
N film
4
.
Electrodes
6
and
7
are formed on the n-type Al
a
Ga
b
In
c
N film
3
and the p-type Al
a
Ga
b
In
c
N film
5
, respectively.
If the n-type Al
a
Ga
b
In
c
N film
3
is directly formed on the sapphire substrate
1
by CVD, it has a large amount of defect, its depleted crystallinity and its rough surface. Therefore, in this case, the light emitting diode having the directly formed n-type Al
a
Ga
b
In
c
N film can have a relatively small light emission efficiency.
Therefore, as shown in
FIG. 1
, the GaN film
2
, which is formed at a low temperature by CVD, intervenes as the buffer layer between the sapphire substrate
1
and the n-type Al
a
Ga
b
In
c
N film
3
. Since the GaN film
2
is epitaxially grown at a low temperature by CVD, the lattice constant difference of approximately 10% between the sapphire substrate
1
and the n-type Al
a
Ga
b
In
c
N film
3
is compensated, and the Al
a
Ga
b
In
c
N film can have its flat surface, which is very important in a heterojunction of the light emitting diode.
Moreover, it is proposed that an AlN film, which is epitaxially grown at a low temperature by CVD, is employed as the buffer layer instead of the GaN film
2
.
However, if the Al
a
Ga
b
In
c
N film incorporating Al as an indispensable component is epitaxially grown on the GaN film or the AlN film formed at a low temperature by CVD, it has a large amount of dislocation, for example a dislocation density of 10
10
/cm
2
.
Since the large amount of dislocation forms light absorption centers, a semiconductor device having the Al
a
Ga
b
In
c
N film with the large amount of dislocation is degraded. It is a large problem in an optical device such as a laser diode requiring a large light emitting efficiency. Moreover, the large amount of dislocation deteriorates the condition of the pn junction of the optical device or the like. Consequently, it is very important to reduce the dislocations.
In view of the reduction of the dislocations, a selective lateral epitaxial growth method is disclosed in “J. Appl. Phys, Vol. 68, No. 7, p.774-779”. In the document, the selective lateral epitaxial growth method is employed in epitaxially growing a GaN film on a sapphire substrate. Therefore, the inventors of the present invention make an attempt to epitaxially grow the Al
a
Ga
b
In
c
N film by taking advantage of the above selective lateral epitaxial growth method.
In this case, as shown in
FIG. 2
a
, a GaN film
12
is epitaxially grown in a thickness of 1-2 &mgr;m on one main surface of a sapphire substrate
11
by CVD, and thereafter, a strip-shaped mask
13
is formed of a SiN material on the GaN film
12
. Then, a Al
x
Ga
y
In
z
N films (x+y+z=1,x,y,z≧0)
14
are epitaxially grown on the GaN film
12
.
As shown in
FIG. 2
b
, the Al
x
,Ga
y
In
z
N films
14
are epitaxially grown on the GaN film
12
alongside the mask
13
, and then, are laterally grown on the mask
13
as they reach the upper surfaces of the mask
13
. Lastly, as shown in
FIG. 2
c
, the laterally grown Al
x
Ga
y
In
z
N films are coalesced on the mask
13
.
In this case, the dislocations at the interfaces between the GaN film
12
and the Al
x
Ga
y
In
z
N films
14
are transmitted longitudinally, not laterally on the mask
13
. Therefore, the Al
x
Ga
y
In
z
N films
14
have respective small amounts of dislocation, for example dislocation density of about 10
7
/cm
2
, within areas W on the mask
13
.
Therefore, if a light emitting element as shown in
FIG. 1
is formed on the low dislocation density area W, it can exhibit good performances.
In the above conventional method, however, the sapphire substrate
11
is set into a CVD chamber, and then, the GaN film
12
is formed on the sapphire substrate
11
. Thereafter, the sapphire substrate
11
is taken out of the CVD chamber, and the strip-shaped mask
13
is formed on the GaN film
12
by photolithography. Then, the sapphire substrate
11
is set into the CVD chamber again, and the Al
x
Ga
y
In
z
N films
14
are formed.
That is, since the conventional method requires the above complicated process in forming the Al
x
Ga
y
In
z
N films
14
, it can not provide an epitaxial growth substrate efficiently.
Moreover, in forming step of the mask
13
after forming the GaN film
12
as a buffer layer, impurities are incorporated into the GaN film, resulting in the degradation of the properties of the GaN film.
The above phenomenon can be observed in the case of forming the GaN film on a SiC substrate or a GaN substrate instead of the sapphire substrate. Consequently, the above conventional method can not provide an epitaxial growth substrate having the above Al
x
Ga
y
In
z
N film, particularly the Al
x
Ga
y
In
z
N film (x+y+z=1,x>0,y,z≧0) incorporating Al as an indispensable component with low dislocation density and good properties, efficiently.
SUMMARY OF THE INVENTION
It is an object of the present invention to provide an epitaxial growth substrate comprising a base material composed of a sapphire substrate, a SiC substrate, a GaN substrate or the like and an Al
x
Ga
y
In
z
N (x+y+z=1,x>0,y,z≧0) film with low dislocation density and good properties thereon, on which Al
a
Ga
b
In
c
N (a+b+c=1,a>0,b,c≧0) films having good properties can be formed by epitaxial growth.
It is another object of the present invention to provide a method for producing the above epitaxial growth substrate.
This invention relates to an epitaxial growth substrate comprising a base material having
a strip-shaped concave-convex structure thereon and an epitaxially grown Al
x
Ga
y
In
z
N (x+y+z=1,x>0,y,z≧0) film formed so as to embed the concave-convex structure by a selective lateral epitaxial growth method and having low dislocation density areas on at least one of the concave portions and the convex portions of the concave-convex structure.
In a preferred embodiment of the present invention, the above concave-convex structure may be composed of plural kinds of strip-shaped ditches formed on the surface of the base material.
In the epitaxial growth substrate of the present invention, the cross sectional shape, size, ditch direction and aspect ratio of the above concave-convex structure can be determined appropriately in accordance with the composition and forming conditions of the Al
x
Ga
y
In
z
N film and a kind of device to be produced.
Moreover, this invention relates to A method for producing an epitaxial growth substrate comprising the steps of:
prep
Asai Keiichiro
Nakamura Yukinori
Shibata Tomohiko
Burr & Brown
Meier Stephen D.
NGK Insulators Ltd.
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