Semiconductor laser, semiconductor device, and their...

Details Semiconductor laser, semiconductor device, and their... Semiconductor laser, semiconductor device, and their...

2001-01-12

2003-08-12

Leung, Quyen (Department: 2828)

Coherent light generators

Particular active media

Semiconductor

Reexamination Certificate

active

06606335

ABSTRACT:

FIELD OF THE INVENTION
The present invention relates to a semiconductor laser, a semiconductor device and their manufacture methods, and more particularly to a semiconductor laser and a semiconductor device having a semiconductor region of a low dislocation density, and to their manufacture methods.
DESCRIPTION OF THE RELATED ART
Active developments on light emitting devices using GaN based materials are being made nowadays. Blue and green high luminance light emitting diodes (LED) have been manufactured to date. Oscillation of a royal purple laser at a room temperature has been realized by many research organizations including the present inventor, and studies of manufacturing the products of this laser are made vigorously. A GaN based laser using a sapphire (Al
2
O
3
) substrate was manufactured and continuous wave oscillation (CW oscillation) during 1000 hours was confirmed (refer to S. Nakamura et al., Japanese Journal of Applied Physics, vol. 35, p. L74, 1996).
A manufacture method for a short wavelength semiconductor layer using a sapphire substrate will be described briefly. First, an a sapphire substrate having (0001) plane as its principal surface, a GaN buffer layer is formed at a low temperature. A method of forming this GaN buffer layer will be described with reference to
FIGS. 39
to
42
.
As shown in
FIG. 39
, on the principal surface of the (0001) plane of a sapphire substrate, a GaN layer
201
is grown to a thickness of 1 to 2 &mgr;m by metal organic vapor phase epitaxy (MOVPE). On the surface of the GaN layer
201
, a SiO
2
film is deposited to a thickness of 100 to 300 nm by chemical vapor deposition (CVD). This SiO
2
film is patterned by using hydrofluoric acid to leave striped SiO
2
patterns
202
. After the SiO
2
film is patterned, the substrate surface is cleaned satisfactorily with water.
As shown in
FIG. 40
, a GaN layer is grown on the substrate surface by MOVPE. At the initial growth stage, a GaN layer
203
is grown only in an area where the GaN layer
201
is exposed. As the growth of the GaN layer continues, as shown in
FIG. 41
a GaN layer
204
starts being deposited also on the SiO
2
pattern
202
.
As the growth continues further, adjacent GaN layers contact each other and the GaN layer covers the whole substrate surface. A GaN buffer layer
205
having generally a flat surface can be formed eventually, as shown in FIG.
42
.
FIG. 43
is a schematic diagram showing the state of dislocations in the GaN buffer layer
205
. Because of lattice mismatch between sapphire and GaN, dislocations
206
and
207
extend from the interface between the sapphire substrate
200
and GaN layer
201
into the GaN layer
201
. The dislocation
206
in the region where the SiO
2
pattern
202
is formed does not extend above the SiO
2
pattern
202
. In the region where the SiO
2
pattern
202
is not formed, the dislocation
207
extends into the GaN buffer layer
205
.
A region
208
above the SiO
2
pattern
202
was formed by a lateral growth of GaN. Therefore, dislocation does not enter this region
208
above the SiO
2
pattern
202
, and the dislocation density in this region
208
becomes low.
As shown in
FIG. 44
, a SiO
2
pattern
209
and a GaN buffer layer
210
may be formed by repeating the processes shown in
FIGS. 39
to
42
. In this case, as viewed along a substrate normal line direction, the SiO
2
pattern
209
is disposed approximately superposed upon the region where the SiO
2
pattern
202
is not disposed.
Extension of the dislocations
207
in the GaN buffer layer
205
is stopped by the SiO
2
pattern
209
. It is therefore possible to form the second-layer GaN buffer layer
210
having a low dislocation density on the GaN buffer layer
205
. With this manufacture method, although the dislocation density of the GaN buffer layer can be lowered, the number of processes increases so that the manufacture cost rises.
Next, a method of forming a laser structure on a GaN buffer layer will be described. On the GaN layer, a laminated structure is formed including an n-type GaN intermediate layer, an n-type Al
0.09
Ga
0.91
N clad layer, an n-type GaN light guide layer (separated confinement hetero structure (SCH) layer), an InGaN multiple quantum well layer, a p-type Al
0.18
Ga
0.82
N overflow preventing layer, a p-type GaN light guide layer, a p-type Al
0.09
Ga
0.91
N clad layer, and a p-type GaN contact layer. These layers are grown, for example, by MOVPE.
The p-type GaN contact layer and p-type AlGaN clad layer are partially dry-etched to leave a ridge structure. The n-type GaN intermediate layer is partially exposed in an area where the ridge structure is not left. A SiO
2
film is formed covering the whole substrate surface. This SiO
2
film is patterned to expose a partial upper surface of the ridge structure and a partial surface of the n-type GaN intermediate layer. On the exposed surface of the ridge structure, a p-side electrode is formed having a two-layer structure of Ni/Au. On the exposed surface of the n-type GaN intermediate layer, an n-side electrode is formed having a two-layer structure of Ti/Au. Lastly, a pair of parallel side surfaces constituting resonator side surfaces is formed by dry etching.
The resonator side surfaces are formed by dry etching because it is difficult to cleave a sapphire substrate. Flatness of the resonator side surfaces formed by etching is worse than those formed by cleavage. Therefore, a threshold current of a short wavelength semiconductor laser using a sapphire substrate becomes larger than that of a semiconductor laser whose resonator side surfaces are formed by cleavage. For example, the threshold current density of the semiconductor laser formed by the above method is about 3.6 kA/cm
2
.
The n-side electrode cannot be formed on the bottom surface of the sapphire substrate because sapphire has no electric conductivity. It is therefore necessary to expose the surface of the n-type GaN intermediate layer and form the n-side electrode on this exposed surface.
In order to solve the problems essentially associated with using a sapphire substrate, it has been proposed to use a SiC substrate (refer to A. Kuramata, K. Domen, R. Soejima, K. Horono, S. Kubota and T. Tanahasi, Japanese journal of Applied Physics Vol. 36 (1997) L1130, and G. E. Bulman et al, Device Research Conference IV-B-8, 1997).
With reference to
FIG. 45
, a method of manufacturing a semiconductor laser using a SiC substrate will be described.
A hexagonal 6H-SiC substrate
231
is prepared which has a (000.1) Si plane as its principal surface. The SiC substrate
231
is given n-type conductivity. Sequentially grown by MOVPE on the surface of the SiC substrate
231
are an n-type Al
0.1
Ga
0.9
N buffer layer
232
, an n-type GaN buffer layer
233
, an n-type Al
0.09
Ga
0.91
N clad layer
234
, an n-type GaN light guide layer
235
, an InGaN multiple quantum well layer
236
, a p-type Al
0.18
Ga
0.82
N electron block layer
237
, a p-type GaN light guide layer
238
, a p-type Al
0.09
Ga
0.91
N clad layer
239
, and a p-type GaN contact layer
240
.
The AlGaN buffer layer
232
is 0.15 &mgr;m thick, the GaN buffer layer
233
is 0.1 &mgr;m thick, the AlGaN clad layer
234
is 0.5 &mgr;m thick, and the GaN light guide layer
235
is 0.1 &mgr;m thick. These n-type layers are doped with Si impurities at a concentration of 3×10
18
cm
−3
.
The InGaN multiple quantum well layer
236
has the lamination structure of four barrier layers of undoped In
0.03
Ga
0.97
N and three well layers of undoped In
0.15
Ga
0.85
N alternately stacked. The barrier layer is 5 nm thick and the well layer is 4 nm thick. Five barrier layers each having a thickness of 5 nm and four well layers each having a thickness of 2.6 nm may also be used.
The AlGaN electron block layer
237
is 20 nm thick, the GaN light guide layer
238
is 0.1 &mgr;m thick, the AlGaN clad layer
239
is 0.5 &mgr;m thick, and the GaN contact layer
240
is 0.2 &mgr;m thick. These p-type layers are doped with Mg impurities at a concentration of 5×10
19
cm
−3
.
T

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