Semiconductor device manufacturing: process – Making device or circuit emissive of nonelectrical signal
Reexamination Certificate
2000-11-24
2002-07-16
Mulpuri, Savitri (Department: 2812)
Semiconductor device manufacturing: process
Making device or circuit emissive of nonelectrical signal
C438S046000, C438S483000
Reexamination Certificate
active
06420198
ABSTRACT:
BACKGROUND OF THE INVENTION
The present invention relates to a semiconductor laser diode and a method of forming the same, and more particularly to a gallium nitride based compound semiconductor laser having a current block layer structure selectively grown for a current confinement and a method of forming the same.
Gallium nitride is larger in energy ban gap than those of indium phosphate and gallium arsenide, for which reason gallium nitride based semiconductors of In
x
Al
y
Ga
l-x-y
N (0≦x≦1, 0≦y≦1, 0≦x+y≦1) may be applied to light emitting diodes such as semiconductor laser diodes for emitting a light of an wavelength in the range of green light wavelength to ultraviolet ray wavelength.
Gallium nitride based semiconductor may have either hexagonal crystal structure or cubic crystal structure. The hexagonal crystal structure is more stable in energy than the cubic crystal structure.
One of conventional gallium nitride based semiconductor laser diodes is disclosed by S. Nakamura et al. in Extended Abstracts of 1996 International Conference On Solid State Devices And Materials, Yokohama, 1996, pp. 67-69.
The conventional gallium nitride based semiconductor laser diode will be described with reference to
FIG. 1. A
300 Å-thick undoped GaN buffer layer
102
is formed on a (11-20)-face sapphire substrate
201
. A 3 &mgr;m-thick n-type GaN contact layer
103
doped with Si is formed on the 300 Å-thick undoped GaN buffer layer
102
. A 0.1 &mgr;m-thick n-type In
0.05
Ga
0.95
N layer
104
doped with Si is formed on the 3 &mgr;m-thick n-type GaN contact layer
103
. A 0.4 &mgr;m-thick n-type Al
0.07
Ga
0.93
N cladding layer
105
doped with Si is formed on the 0.1 &mgr;m-thick n-type In
0.05
Ga
0.95
N layer
104
. A 0.1 &mgr;m-thick n-type GaN optical guide layer
106
doped with Si is formed on the 0.4 &mgr;m-thick n-type Al
0.07
Ga
0.93
N cladding layer
105
. A multiple quantum well active layer
107
is formed on the 0.1 &mgr;m-thick n-type GaB optical guide layer
106
. The multiple quantum well active layer
107
comprises 7 periods of 25 Å-thick undoped In
0.2
Ga
0.8
N quantum well layers and 50 Å-thick undoped In
0.05
Ga
0.95
N barrier layers. A 200 Å-thick p-type Al
0.2
Ga
0.8
N layer
108
doped with Mg is formed on the multiple quantum well active layer
107
. A 0.1 &mgr;m-thick p-type GaN optical guide layer
109
doped with Mg is formed on the 200 Å-thick p-type Al
0.2
Ga
0.8
N layer
108
. A 0.4 &mgr;m-thick p-type Al
0.07
Ga
0.93
N cladding layer
110
doped with Mg is formed on the 0.1 &mgr;m-thick p-type GaN optical guide layer
109
. A 0.2 &mgr;m-thick p-type GaN contact layer
111
doped with Mg is formed on the 0.4 &mgr;m-thick p-type Al
0.07
Ga
0.93
N cladding layer
110
. A p-electrode
112
is formed on the 0.2 &mgr;m-thick p-type GaN contact layer
111
. The p-electrode
112
comprises a nickel layer laminated on the top flat surface of the 0.2 &mgr;m-thick p-type GaN contact layer
111
and a gold layer laminated on the nickel layer. An n-electrode
113
is provided on the recessed surface of the 3 &mgr;m-thick n-type GaN contact layer
103
. The n-electrode
113
comprises a titanium layer laminated on the 3 &mgr;m-thick n-type GaN contact layer
103
and an aluminum layer laminated on the titanium layer.
All of the semiconductor layers have hexagonal crystal structure with the (0001)-face grown over the (11-20)-face sapphire substrate
201
.
The above conventional gallium nitride based semiconductor laser diode has no current confinement structure, for which reason the above conventional gallium nitride based semiconductor laser diode has a relatively large threshold current.
Other conventional gallium nitride based semiconductor laser diode is disclosed by S. Nakamura et al. in Applied Physics Letters, vol. 69 (1996), p 1477. The other conventional gallium nitride based semiconductor laser diode will be described with reference to
FIG. 2. A
300 Å-thick undoped GaN buffer layer
102
is formed on a (11-20)-face sapphire substrate
201
. A 3 &mgr;m-thick n-type GaN contact layer
103
doped with Si is formed on the 300 Å-thick undoped GaN buffer layer
102
. A 0.1 &mgr;m-thick n-type In
0.05
Ga
0.95
N layer
104
doped with Si is formed on the 3 &mgr;m-thick n-type GaN contact layer
103
. A 0.5 &mgr;m-thick n-type Al
0.05
Ga
0.95
N cladding layer
605
doped with Si is formed on the 0.1 &mgr;m-thick n-thick In
0.05
Ga
0.95
N layer
104
. A 0.1 &mgr;m-thick n-type GaN optical guide layer
106
doped with Si is formed on the 0.5 &mgr;m-thick n-type Al
0.05
Ga
0.95
N cladding layer
605
. A multiple quantum well active layer
707
is formed on the 0.1 &mgr;m-thick n-type GaN optical guide layer
106
. The multiple quantum well active layer
707
comprises 7 periods of 30 Å-thick undoped In
0.2
Ga
0.8
N quantum well layers and 60 Å-thick undoped In
0.05
Ga
0.95
N barrier layers. A 200 Å-thick p-type Al
0.2
Ga
0.8
N layer
108
doped with Mg is formed on the multiple quantum well active layer
707
. A 0.1 &mgr;m-thick p-type GaN optical guide layer
109
doped with Mg is formed on the 200 Å-thick p-type Al
0.2
Ga
0.8
N layer
108
. A 0.5 &mgr;m-thick p-type Al
0.05
Ga0.95N cladding layer
710
doped with Mg is formed on the 0.1 &mgr;m-thick p-type GaN optical guide layer
109
. A 0.2 &mgr;m-thick p-type GaN contact layer
111
doped with Mg is formed on the 0.4 &mgr;m-thick p-type Al
0.05
Ga
0.95
N cladding layer
710
. The 0.2 &mgr;m-thick p-type GaN contact layer
111
has a ridge-shape. A p-electrode
112
is formed on the top portion of the 0.2 &mgr;m-thick p-type GaN contact layer
111
. The p-electrode
112
comprises a nickel layer laminated on the top flat surface of the 0.2 &mgr;m-thick p-type GaN contact layer
111
and a gold layer laminated on the nickel layer. A silicon oxide film is formed which extends on the sloped side walls of the ridge portion of the 0.2 &mgr;m-thick p-type GaN contact layer
111
and also on the flat base portions of the 0.2 &mgr;m-thick p-type GaN contact layer
111
as well as on side walls of the above laminations of the 3 &mgr;m-thick n-type GaN contact layer
103
, the 0.1 &mgr;m-thick n-type In
0.05
Ga
0.95
N layer
104
, the 0.5 &mgr;m-thick n-type Al
0.05
Ga
0.95
N cladding layer
605
, the 0.1 &mgr;m-thick n-type GaN optical guide layer
106
, the multiple quantum well active layer
707
, the 200 Å-thick p-type Al
0.2
Ga
0.8
N layer
108
, the 0.1 &mgr;m-thick p-type GaN optical guide layer
109
, the 0.4 &mgr;m-thick p-type Al
0.05
Ga
0.95
N cladding layer
710
and the 0.2 &mgr;m-thick p-type GaN contact layer
111
. An n-electrode
113
is provided on the recessed surface of the 3 &mgr;m-thick n-type GaN contact layer
103
. The n-electrode
113
comprises a titanium layer laminated on the 3 &mgr;m-thick n-type GaN contact layer
103
and an aluminum layer laminated on the titanium layer.
All of the semiconductor layers have hexagonal crystal structure with the (0001)-face grown over the (11-20)-face sapphire substrate
201
.
The above ridge structure of the 0.2 &mgr;m-thick p-type GaN contact layer
111
might contribute any current confinement for reduction in threshold current. Since, however, a contact area between the p-electrode and the 0.2 &mgr;m-thick p-type GaN contact layer
111
is small, a contact resistance of the p-electrode to the 0.2 &mgr;m-thick p-type GaN contact layer
111
is relatively large.
Whereas the above ridge structure of the 0.2 &mgr;m-thick p-type GaN contact layer
111
is defined by a dry etching process, this dry etching process may provide a damage to the semiconductor layers.
The use of this dry etching process results in complicated fabrication processes for the laser diode.
In the above circumstances, it had been required to develop a novel gallium nitride based compound semiconductor laser and a method of forming the same.
SUMMARY OF THE INVENTION
Accordingly, it is an object of the present invention to provide a novel gallium nitride based compound semiconductor laser f
Kimura Akitaka
Nido Masaaki
Mulpuri Savitri
NEC Corporation
Young & Thompson
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