Semiconductor light emitting device and semiconductor laser

Details Semiconductor light emitting device and semiconductor laser Semiconductor light emitting device and semiconductor laser

2001-07-18

2004-01-20

Ip, Paul (Department: 2828)

Coherent light generators

Particular active media

Semiconductor

C372S046012

Reexamination Certificate

active

06680959

ABSTRACT:

FIELD OF THE PRESENT INVENTION
The present invention relates to a semiconductor light emitting device such as a semiconductor laser or a light emitting diode which uses a nitride based compound semiconductor (compound semiconductor of Group III element(s) and nitrogen and the like) and is capable of emitting light in the blue type region required for an optical disk memory having a high recording density or a laser beam printer with high definition. More specifically, the present invention relates to a semiconductor light emitting device and a semiconductor laser having such a superior light emitting characteristic as to reduce a threshold current by growing a semiconductor layer having few crystalline defects and the flat surface.
BACKGROUND OF THE PRESENT INVENTION
A conventional light emitting diode (LED) or laser diode (LD) emitting light in a blue-emitting region has been fabricated by successively forming compound semiconductor of group III element nitrides on a sapphire substrate by Metal Organic Chemical Vapour Deposition (hereinafter referred to as MOCVD).
For example, a semiconductor laser capable of carrying out CW oscillation in a blue-emitting region is fabricated as shown in
FIG. 5
by successively forming layers of group III element nitride based compound semiconductor on a sapphire substrate
21
by the MOCVD method; a GaN buffer layer
22
, a contact layer
23
of an n-type GaN, an n-type clad layer
24
of Al
0.12
Ga
0.88
N, an n-type light guide layer
25
of GaN, an active layer
26
of an InGaN based (type) compound semiconductor with multiple quantum well structure, a p-type light guide layer
27
of a p-type GaN, a p-type clad layer
28
of a p-type Al
0.12
Ga
0.88
N, and a p-type contact layer
29
of a p-type GaN; etching some of the layered semiconductor layers as shown in
FIG. 5
by, for example, dry etching to expose the n-type contact layer
23
, and forming an n-side electrode
31
thereon and a p-side electrode
30
on the foregoing p-type contact layer
29
, respectively. The portion of the p-side electrode
30
along the stripes is utilized as the light emitting part.
However, the sapphire substrate on which the nitride based compound layers are grown has considerably different lattice constant and thermal expansion coefficient from those of the nitride based compound semiconductor layers and it is difficult to get the lattice match between them. Further, the density of the threading dislocation (TD) of the nitride based compound semiconductor layers grown thereon is as high as about 1×10
8
cm
−2
to 1×10
10
cm
−2
and the dislocation density is significantly high as compared with that, 1×10
2
cm
−2
, of compound semiconductor layers of the red-emitting type grown on GaAs substrate. In case of semiconductor lasers, if the dislocation density is especially high, the threshold current is increased, so that it is desired to lower the dislocation density than the LEDs (light emitting diode). However, other than sapphire, any alternative substrate suitable for industrial use has not been found.
On the other hand, various studies have been carried out to grow the nitride based compound semiconductor layer which is flat in a plane and small in a dislocation density, and various methods of using a off-oriented substrate, for example, a disclosure that semiconductor layers of gallium nitride based compound are grown onto a sapphire substrate of which R plane is off-oriented by 0.8° or low as shown in Japanese Unexamined Patent Publication No. Hei 5-190903 are proposed.
As described above, though some contrivances to improve the flatness of the nitride based compound semiconductor layer and to reduce a dislocation density are performed, these are only the proposals such that, it is sufficient to be off-oriented even slightly such as not greater than 0.8° or not greater than 1.0° regarding the off orientation angle, and it is not minded in which crystalline axis direction the substrate should be off-oriented. Therefore, the nitride based compound semiconductor layers having the flat surface are not always attained and the semiconductor lasers having the high performance have not been commercialized.
SUMMARY OF THE PRESENT INVENTION
The present invention has been performed in consideration of these circumstances, and it is an object to provide a semiconductor light emitting device having a superior characteristic of light emitting by growing the nitride based compound semiconductor layers with the degree of high flatness on a sapphire substrate.
It is another object of the present invention to provide a semiconductor laser of a structure capable of enhancing the characteristic such that a threshold current density is reduced by forming a cleavage surface finely while improving the degree of flatness by off-orienting the sapphire substrate in the case such as a semiconductor laser in which a laser beam exit edge surface is fabricated through cleavage.
The present inventors studied seriously to make a surface of the growing layer of the nitride based compound semiconductor flat and a dislocation density small by off-orienting the sapphire substrate, and therefore have found that the flatness varied widely depending on which principal plane of the sapphire substrate was off-oriented to grow the nitride based compound semiconductor layer, and the surface flatness could not be attained when the off-oriented degree was too small, as well as it was necessary to consider its direction of the tilt for leading to a final device of high performance, for example, particularly when being used as a laser diode (LD), depending on the direction of the tilt, a fine cleavage surface could not be attained and the threshold current density could not be sufficiently reduced in some cases.
Further, the present inventors have found that the surface flatness is superior by adjusting a tilt of C plane of the sapphire substrate with the tilt &thgr;
a
on an A axis and with the tilt &thgr;
m
on a M axis in which A and M axes are relatively orthogonal to each other and simultaneously by off-orienting the overall tilt &thgr;={&thgr;
a
2
+&thgr;
m
2
}
1/2
not less than 0.2° and not more than 0.3°, and it is possible to lower the distortion of the cleavage surface and to highly reduce the threshold current (density) by raising &thgr;
a
in taking the cleavage surface as A plane and raising &thgr;
m
in taking the cleavage surface as M plane when being used as a laser diode (LD).
A semiconductor light emitting device in accordance with the present invention comprises; a sapphire substrate, and a laminated semiconductor portion in which nitride based compound semiconductor layers are laminated so as to constitute a light emitting layer forming portion on the sapphire substrate, wherein a C plane of the sapphire substrate has the off orientation angle having a tilt relative to an A axis and/or a M axis in such a way that
0.2°≦&thgr;={&thgr;
a
2
+&thgr;
m
2
}
1/2
≦0.3°,
wherein
0°≦&thgr;
a
≦0.3°, 0°≦&thgr;
m
≦0.3°
when taking the angle tilted relative to the A axis as &thgr;
a
and to the M axis as &thgr;
m
, and the nitride based compound semiconductor layers are laminated onto the surface of the off-oriented C plane. Further, the laminated semiconductor portion may be laminated so as to constitute a light emitting diode structure or a laser diode structure.
Here, a nitride based compound semiconductor means a semiconductor consisting of compound of the Group III element(s) such as Ga, Al, In and the like and N, or N and the Group V elements other than N. Accordingly, this means a semiconductor consisting of N-containing compound semiconductor, in which in addition to GaN, a mixed crystal composition ratio of the Group III elements or a mixed crystal composition ratio of the Group V elements is appropriately changed, such as AlGaN based compound in which a composition ratio between Al and Ga is changeable and InGaN based compound in which a composition ratio between In and Ga is change

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