Active solid-state devices (e.g. – transistors – solid-state diode – Heterojunction device – Light responsive structure
Reexamination Certificate
2002-07-08
2004-11-23
Wilczewski, Mary (Department: 2822)
Active solid-state devices (e.g., transistors, solid-state diode
Heterojunction device
Light responsive structure
C257S011000, C257S013000, C257S079000, C257S103000
Reexamination Certificate
active
06822272
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates to a light emitting element such as laser diode element (LD), a light emitting diode element (LED) and the like comprising a nitride semiconductor (In
x
Al
y
Ga
1−x−y
N, 0≦X, 0≦Y, X+Y≦1).
DESCRIPTION OF THE RELATED ART
Since a multilayered reflective membrane layer formed by alternately depositing two layers with different reflectivities has an extremely high reflectivity, the layer is used in a variety of purposes. Such a multilayered reflective membrane layer is generally formed by depositing pairs of a first layer with a film thickness of &lgr;/4n
a
(&lgr;: incident light wavelength, and n
a
: index of refraction) and a second layer with a film thickness of &lgr;/4n
b
[&lgr;: incident light wavelength, and n
b
: index of refraction (n
b
≠n
a
)] and in order to obtain a further high reflectivity, the reflectivity difference of the first layer and the second layer is required to be large.
For example, in the case of forming a multilayered reflective membrane using Al
a
Ga
1−a
N (0<a<1) for the above first layer and GaN for the second layer, in order to make the reflectivity difference between these layers wide, the Al mixed crystal ratio a of the Al
a
Ga
1−a
N is required to be high.
As a light emitting element using such a multilayered reflective membrane, applicant of the present invention has developed a short wavelength laser oscillating in a violet to green region as disclosed in Japanese Laid-open Patent Publication No. 2001-7444. A schematic cross-sectional view of the laser element is illustrated in FIG.
7
. The laser element
10
of
FIG. 7
is a surface emitting laser element and formed by depositing an n-type nitride semiconductor layer, an active layer
6
composed of In
x
Ga
1−x
N (0<x<1) and a p-type nitride semiconductor layer in this order on a sapphire substrate
1
through a buffer layer
2
. In the laser element
10
, the n-type nitride semiconductor layer is composed of an n-type contact layer
3
, a second n-type clad layer
4
, an n-type multilayered reflective membrane
44
, and a first n-type clad layer
5
formed on the buffer layer
2
. On the other hand, the p-type nitride semiconductor layer formed on the active layer
6
is composed of a second p-type clad layer
7
, a first p-type clad layer
8
, and a p-type contact layer
9
. Further, a negative electrode is formed on the n-type contact layer
3
and a positive electrode is formed on the p-type contact layer
9
.
In such a laser element
10
, the multilayered reflective membrane
44
is formed in the n-type nitride semiconductor layer nearer to the substrate
1
side than the active layer
6
. The multilayered reflective membrane
44
functions as a mirror (light reflecting) layer and reflects the emitted light from the active layer
6
and enclosed it in the active layer
6
. In the laser element
10
of
FIG. 7
, the multilayered reflective membrane
44
is formed by alternately depositing, for example, each 10 layers of Al
a
Ga
1−a
N (0<a<1) and GaN.
SUMMARY OF THE INVENTION
In a multilayered reflective membrane comprising Al
a
Ga
1−a
N and GaN, if the Al mixed crystal ratio a of the Al
a
Ga
1−a
N layer is increased in order to increase the reflectivity difference between these layers, as a is increased, the crystallinity of the Al
a
Ga
1−a
N layers is deteriorated. If multilayered reflective membrane with deteriorated crystallinity is formed in a laser element
10
, there occurs a problem that the light emitted from an active layer
6
is diffused in the multilayered reflective membrane
44
and the multilayered reflective membrane
44
cannot sufficiently exhibit the function as the reflective membrane to result in increase of the threshold electric current value and the threshold voltage for laser oscillation.
Further, in the laser element
10
, if the crystallinity of the multilayered reflective membrane
44
is low, there is a problem that the crystallinity of the respective nitride semiconductor layers to be grown on the multilayered reflective membrane
44
is deteriorated and morphological abnormality takes place and cracks are formed.
On the other hand, if the Al mixed crystal ratio a is lowered in order to suppress the crystallinity deterioration of the Al
a
Ga
1−a
N layer, the reflectivity difference between the Al
a
Ga
1−a
N layer and the GaN layer becomes small and the reflectivity of the multilayered reflective membrane is decreased. If the multilayered reflective membrane with a low reflectivity is formed in the laser element
10
, the light cannot effectively be enclosed in the active layer
6
to result in difficulty of laser oscillation.
The present invention is to solve the above-described problems and to provide a gallium nitride-based multilayered reflective membrane with an excellent crystallinity while keeping a high reflectivity and a gallium nitride-based light emitting element using such a multilayered reflective membrane.
A multilayered reflective membrane of the present invention includes an Al
a
Ga
1−a
N layer (0<a<1) having a thickness of (&agr;
1
·&lgr;)/(4n
1
) (&lgr;: incident light wavelength, n
1
: index of refraction) and a GaN layer having a thickness of (&agr;
2
·&lgr;)/(4n
2
) (n
2
: index of refraction) which are deposited alternately and satisfy the relationship of 0<&agr;
1
<1 and &agr;
1
+&agr;
2
=2.
Conventionally, in a multilayered reflective membrane depositing a plurality of pairs of an Al
a
Ga
1−a
N layer and a GaN layer, the film thickness of the Al
a
Ga
1−a
N layer and the film thickness of the GaN layer composing one pair are &lgr;/4n
1
(that is, &agr;
1
=1) and &lgr;/4n
2
(that is, &agr;
2
=1), respectively. Whereas, according to the present invention, while keeping &agr;
1
+&agr;
2
=2 as it is before, &agr;
1
is kept less than 1 to make the film thickness of the Al
&agr;
Ga
1−&agr;
N layer thinner than the conventional value &lgr;/4n
1
, so that a multilayered reflective membrane with an excellent crystallinity while keeping a high reflectivity can be obtained. Further, since the Al
a
Ga
1−a
N layer is made thinner than before, even if the Al mixed crystal ratio a is made relatively high, the crystallinity deterioration can be suppressed and a multilayered reflective membrane with a high reflectivity can be obtained.
In such a multilayered reflective membrane, the Al mixed crystal ratio a of the Al
a
Ga
1−a
N layer is preferable to satisfy 0.2≦a≦0.8. It is because if a exceeds 0.8, the crystallinity deterioration of the multilayered reflective membrane probably becomes significant, whereas if a is less than 0.2, the reflectivity difference between the Al
a
Ga
1−a
N layer and the GaN layer becomes small and it probably becomes difficult to obtain the multilayered reflective membrane with a sufficient reflectivity. The Al mixed crystal ratio a is more preferable to satisfy 0.3≦a≦0.7 and in such a case, it is made possible to obtain a remarkably high reflectivity difference and excellent crystallinity.
Further, in the above-described multilayered reflective membrane, &agr;
1
has preferably a value of not greater than 0.75. It is because if &agr;
1
exceeds 0.75, the film thickness of the Al
a
Ga
1−a
N layer becomes too thick and the crystallinity deterioration of the multilayered reflective membrane probably becomes significant. More preferably, a satisfies the relation of &agr;
1
≦0.5 and in such a case, the film thickness of the Al
a
Ga
1−a
N layer becomes sufficiently thin and the crystallinity of the multilayered reflective membrane becomes extremely excellent.
The multilayered reflective membrane as described above is suitable to be used for a gallium nitride-based light emitting element having an active layer of In
x
Ga
1−x
N (0≦x<1). Hereinafter, the gallium nitride-based light emitting element of the present invention will be described.
Lewis Monica
Nichia Corporation
Nixon & Vanderhye PC
Wilczewski Mary
LandOfFree
Multilayered reflective membrane and gallium nitride-based... does not yet have a rating. At this time, there are no reviews or comments for this patent.
If you have personal experience with Multilayered reflective membrane and gallium nitride-based..., we encourage you to share that experience with our LandOfFree.com community. Your opinion is very important and Multilayered reflective membrane and gallium nitride-based... will most certainly appreciate the feedback.
Profile ID: LFUS-PAI-O-3278906