Coherent light generators – Particular active media – Semiconductor
Reexamination Certificate
1999-09-02
2002-04-23
Davie, James W. (Department: 2881)
Coherent light generators
Particular active media
Semiconductor
C372S046012
Reexamination Certificate
active
06377597
ABSTRACT:
TECHNICAL FIELD
The present invention relates to gallium nitride semiconductor light emitting devices such as semiconductor lasers and semiconductor diodes, and also to semiconductor laser light source devices, and more particularly, to a light emitting device having a multi-quantum-well structure active layer made of nitride semiconductor.
BACKGROUND ART
As a semiconductor material for semiconductor laser devices (LDs) and light emitting diode devices (LEDs) having emission wavelengths within a wavelength range of ultraviolet to green, gallium nitride semiconductors (GaInAlN) are used. A blue LD using such a gallium nitride semiconductor is described in, for example, Applied Physics Letters, vol. 69, No. 10, p. 1477-1479, and a sectional view of the blue LD is shown in FIG.
19
.
FIG. 20
is an enlarged view of part E in FIG.
19
.
Referring to
FIG. 19
, reference numeral
101
denotes a sapphire substrate,
102
denotes a GaN buffer layer,
103
denotes an n-GaN contact layer,
104
denotes an n-In
0.05
Ga
0.95
N layer,
105
denotes an n-Al
0.05
Ga
0.95
N cladding layer,
106
denotes an n-GaN guide layer,
107
denotes a multi-quantum-well structure active layer composed of In
0.2
Ga
0.8
N quantum well layers and In
0.05
Ga
0.95
N barrier layers,
108
denotes a p-Al
0.2
Ga
0.8
N layer,
109
denotes a p-GaN guide layer,
110
denotes a p-Al
0.05
Ga
0.95
N cladding layer,
111
denotes a p-GaN contact layer,
112
denotes a p-side electrode,
113
denotes an n-side electrode, and
114
denotes a SiO
2
insulating film. In this arrangement, as shown in
FIG. 20
, the multi-quantum-well structure active layer
107
is composed of five 3 nm thick In
0.2
Ga
0.8
N quantum well layers
117
and four 6 nm thick In
0.05
Ga
0.95
N barrier layers
118
, totally nine layers, where the quantum well layers and the barrier layers are alternately formed.
Also, in Applied Physics Letters, vol. 69, No. 20, p. 3034-3036, there is described a structure that the quantum well structure active layer is composed of alternately stacked three 4 nm thick quantum well layers and two 8 nm thick barrier layers, totally five layers.
Japanese Patent Laid-Open Publication HEI 8-316528 also describes a blue LD using a gallium nitride semiconductor. This prior-art blue LD also uses a multi-quantum-well structure active layer having five or more quantum well layers, as in the case shown in
FIGS. 19 and 20
.
Meanwhile, a blue LED using a gallium nitride semiconductor is described in, for example, the aforementioned Japanese Patent Laid-Open Publication HEI 8-316528, and a sectional view of the blue LED is shown in FIG.
21
. Referring to
FIG. 21
, reference numeral
121
denotes a sapphire substrate,
122
denotes a GaN buffer layer,
123
denotes an n-GaN contact layer,
124
denotes an n-Al
0.3
Ga
0.7
N second cladding layer,
125
denotes an n-In
0.01
Ga
0.99
N first cladding layer,
126
denotes a 3 nm thick In
0.05
Ga
0.95
N single-quantum-well structure active layer,
127
denotes a p-In
0.01
Ga
0.99
N first cladding layer,
128
denotes a p-Al
0.3
Ga
0.7
N second cladding layer,
129
denotes a p-GaN contact layer,
130
denotes a p-side electrode, and
131
denotes an n-side electrode. Like this, in blue LEDs using gallium nitride semiconductors, an active layer having only one quantum well layer has been used.
The conventional blue LDs and blue LED described above, however, have had the following problems.
Referring first to the blue LDs, the value of oscillation threshold current is as high as 100 mA or more and so needs to be largely reduced for practical use in information processing for optical disks or the like. Further, if the LD is used for optical disks, in order to prevent data read errors due to noise during data reading, it is necessary to inject a high-frequency current of an about 300 MHz frequency into the LD and modulate an optical output power with the same frequency. In the conventional blue LDs, however, optical output power is not modulated even if a high-frequency current is injected, causing a problem of data read errors.
Referring now to blue LEDs, which indeed have been in practical use, in order to allow blue LEDs to be used for a wider variety of applications including, for example, large color displays capable of displaying bright even at wide angles of visibility, it is desired to realize even higher brightness LEDs by improving optical output power.
Furthermore, conventional gallium nitride LEDs have a problem that as the injection current increases, the peak value of emission wavelengths largely varies. For example, in a gallium nitride blue LED, as the forward current is increased from 0.1 mA to 20 mA, the peak value of emission wavelengths shifts byas much as7 nm. This is particularly noticeable in LED devices having long emission wavelengths; for example, in a gallium nitride green LED, the peak value of emission wavelengths shifts by as much as 20 nm. When such a device is used in a color display, there would occur a problem that colors of images vary depending on the brightness of the images because of the shift of the peak wavelength.
DISCLOSURE OF THE INVENTION
In view of the above, a primary object of the present invention is to solve the above-described problems of the gallium nitride semiconductor light emitting devices and provide a gallium nitride semiconductor light emitting device which makes it possible to realize a semiconductor laser diode having satisfactory laser oscillation characteristics as well as a light emitting diode capable of yielding high optical output power.
A further object of the present invention is to provide a gallium nitride semiconductor light emitting device which makes it possible to realize a light emitting diode device free from the shift of the peak wavelength due to the injection of electric current.
A gallium nitride semiconductor light emitting device according to an embodiment of the present invention comprises a semiconductor substrate, an active layer having a quantum well structure and made of nitride semiconductor containing at least indium and gallium, and a first cladding layer and a second cladding layer for sandwiching the active layer therebetween, and the active layer is composed of two quantum well layers and one barrier layer interposed between the quantum well layers.
When this gallium nitride semiconductor light emitting device is used as a semiconductor laser device, the active layer forms an oscillating section of the semiconductor laser device. Besides, when a driving circuit for injecting an electric current into the semiconductor laser device is provided, a semiconductor laser light source device is realized. Meanwhile, when the gallium nitride semiconductor light emitting device is used as a semiconductor light emitting diode device, the active layer forms a light emitting section of the semiconductor light emitting diode device.
In making the present invention as described above, the present inventor investigated in detail the causes of the aforementioned problems of the conventional devices. As a result, the following was found out.
First, with regard to blue LDs, in the InGaN material to be used for a quantum well layer, both electrons and holes have large effective masses and numerous crystal defects are present, causing the mobility of the electrons and holes to considerably lower, so that the electrons and holes are not distributed uniformly in all the quantum well layers of the multi-quantum-well structure active layer. That is, electrons are injected into only two or so of the quantum well layers on the n-type cladding layer side for electron injection, and holes are injected into only two or so of the quantum well layers on the p-type cladding layer side for hole injection. Accordingly, in the multi-quantum-well structure active layer having five or more quantum well layers, because of a small percentage or rate at which electrons and holes are present in the same quantum well layer, the efficiency of light emission by recombination of electrons and holes lowers, causing the threshold curren
Davie James W.
Inzirillo Gioacchino
Morrison & Foerster / LLP
Sharp Kabushiki Kaisha
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