Semiconductor device manufacturing: process – Making device or circuit emissive of nonelectrical signal
Reexamination Certificate
2002-09-06
2003-12-30
Nelms, David (Department: 2818)
Semiconductor device manufacturing: process
Making device or circuit emissive of nonelectrical signal
C257S013000, C257S079000, C257S103000
Reexamination Certificate
active
06670204
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates to a semiconductor light emitting device such as a semiconductor laser, a light emitting diode or the like and a fabrication method thereof, which uses a nitride-based compound semiconductor (a compound semiconductor of group III element(s) and nitrogen and the like), and is capable of emitting light in the blue color region required for an optical disk memory having a high memory density or improving delicacy of a laser beam printer. More particularly, the present invention relates to a semiconductor light emitting device such as a semiconductor laser and a fabrication method thereof capable of preventing warp in a wafer while suppressing the threading dislocation (defect) density of a nitride-based compound semiconductor layer as much as possible by employing epitaxial lateral overgrowth and of improving the electroluminescent properties.
BACKGROUND OF THE 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. 10
by successively forming layers of group III element nitride type 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. 10
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 type compound semiconductor layers and the density of the threading
25
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 LEDs (light emitting diode), even if there occurs dislocation density about that level, a compound semiconductor is practically applicable, however 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 at highest to about 1×10
7
cm
−2
or lower in order to obtain a low threshold value and a long life. However, other than sapphire, any alternative substrate suitable for industrial use has not been found.
On the other hand, as a technique to lower the TD, crystal growth using ELO (epitaxial lateral overgrowth) has drawn attention as crystal growth methods, as disclosed in for example, “Thick GaN Epitaxial Growth with Low Dislocation Density” by Akira Usui et al. (Jpn. J. Apply. Phys., vol. 36, 1997, pp. 899-902) and “ELO growth of GaN by hydride VPE and MOVPE” by Sasaoka et al. (Jpn. J. Crystal Growth, vol. 25 no. 8, 1998, pp. 99-105).
These methods are methods including, for example, steps of putting a SiO
2
mask
43
having opening parts
44
on a first GaN layer
42
on a sapphire substrate
41
and growing a second GaN layer
45
on the SiO
2
mask
43
by selectively growing the layer in the lateral direction using the semiconductor layer exposed to the opening parts
44
as a seed as its sectional explanatory view is partially shown in FIG.
11
and prevent TD based on that a nitride type compound is easier to be grown in the lateral direction than in the vertical direction. The literature cited in the former journal discloses that a mirror face GaN layer free from cracks and having the dislocation density of lower than 6×10
7
cm
−2
can be grown on a sapphire wafer with the diameter of 2 inch by the forgoing method.
However, in case of ELO growth, as shown in
FIG. 11
, although the second GaN layer
45
is so grown successively in the lateral direction from opening parts
44
in both sides formed at constant intervals in the mask layer
43
on the first GaN layer
42
as to meet in the center part of the mask layer
43
, the second GaN layer
45
growing on the mask
43
tends to be lifted out of the mask
43
as it goes to the center part side and is grown while the crystallographic axis being curved to result in that the second GaN layer
45
cannot have a flat bottom face and surface. Therefore, as shown in
FIG. 11
, a void
46
is formed owing to the join of the second GaN layer
45
in the center part side of the mask while the second GaN layer
45
being lifted out and it is undesirable to fabricate a device using the resultant wafer. Such tendency further become significant if the mask width M becomes wide.
In order to avoid deterioration of the flatness, for example, as it can be understood from the case of the method cited in the literature of the above mentioned former journal where the mask width M of the SiO
2
mask is 1 to 4 &mgr;m and the cycle (M+W) is about 7 &mgr;m, the mask width M is required to be narrow since the void
46
is easily formed if the mask width M is 3 &mgr;m or wider. Moreover, as the width M becomes wider, the height and the size of the void become high and large and consequently,the flatness of the surface is deteriorated to result in inferiority of device properties. Further, even if the ultimate conditions in which no void
46
is formed and the flatness is barely kept, the dislocation density is increased in the join part in the center. Furthermore, the second GaN layer
45
growing in the opening parts
44
also has threading dislocation continuous as it is from the first GaN layer
42
in the vertical direction to become a region with a high dislocation density. Therefore, the continuous parts with a small dislocation density are only obtained in a half of the mask width and more precisely in portions excluding both end parts of the half and within about 1 &mgr;m by width.
Nevertheless, in case of using the obtained wafer for a stripe type semiconductor laser, the light emitting part of the laser is only some region which is stripe-like and therefore it is supposed to be possible for the semiconductor laser to suppress the increase of threshold current value attributed to the crystal defects and the deterioration of electric operating properties of LD by lowering the dislocation density of the corresponding part of the semiconductor layer. In this case, as shown in
FIG. 9
, although it is effective to reciprocally form the opening parts
44
and mask layer
43
linearly in one direction, the dislocation density becomes high as described above in the both end parts and the center parts in the mask width and it is therefore preferable to use the portion excluding both end parts in a half of the mask width for a light emitting region with a stripe width of LD. Hence, supposing the stripe width of the LD to be 4 to 5 &mgr;m, the mask width M is required to be at least 10 to 15 &mgr;m and in this case, the GaN to be grown on the mask is required to be grown thicker than the mask width, that is 15 to 20 &mgr;m.
As described before, in order to fabricate a semiconductor laser device with few crystal defects even only in the light emitti
Sonobe Masayuki
Tanabe Tetsuhiro
Arent Fox Kintner & Plotkin & Kahn, PLLC
Huynh Andy
Rohm & Co., Ltd.
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