Active solid-state devices (e.g. – transistors – solid-state diode – Heterojunction device – With lattice constant mismatch
Reexamination Certificate
1999-04-28
2001-11-20
Jackson, Jr., Jerome (Department: 2815)
Active solid-state devices (e.g., transistors, solid-state diode
Heterojunction device
With lattice constant mismatch
C257S079000, C257S096000, C438S044000, C438S047000, C438S481000
Reexamination Certificate
active
06320209
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a semiconductor light emitting device (e.g., a semiconductor laser device) capable of emitting light in a wavelength range of blue to ultraviolet light, and a method for fabricating such a semiconductor light emitting device. More particularly, the present invention relates to a current-blocking type gallium nitride (GaN) based compound semiconductor light emitting device (e.g., a semiconductor laser device) with high reliability, and a method for fabricating such a semiconductor light emitting device.
2. Description of the Related Art
Conventionally, a gallium nitride based compound semiconductor laser device is formed on a crystal by lateral growth using an insulating film such as an SiO
2
film as a selective growth mask.
FIG. 11
is a cross-sectional view of a conventional semiconductor laser device fabricated by such a conventional technique.
The conventional semiconductor laser device of
FIG. 11
includes a sapphire substrate
1100
, a GaN underlying layer
1101
, a stripe-shaped insulating selective growth mask
1102
made of SiO
2
, an n-type GaN contact layer
1103
, an n-type AlGaN cladding layer
1104
, an n-type GaN optical guide layer
1105
, an In
0.2
Ga
0.8
N/In
0.05
Ga
0.95
N multi-quantum well active layer
1106
, a p-type GaN optical guide layer
1107
, a p-type AlGaN cladding layer
1108
having a ridge stripe structure
1120
located above the SiO
2
selective growth mask
1102
, a p-type GaN contact layer
1109
, an n-type electrode
1110
formed on the n-type GaN contact layer
1103
, and a stripe-shaped p-type electrode
1111
formed on the p-type GaN contact layer
1109
.
The n-type GaN contact layer
1103
is continuously formed on the insulating selective growth mask
1102
. Actually, crystal growth of GaN for the n-type GaN contact layer
1103
starts on the portions of the GaN underlying layer
1101
corresponding to the openings of the insulating selective growth mask
1102
, i.e., the portions which are not covered with the insulating selective growth mask
1102
. As the growth proceeds in the thickness direction, the GaN layer gradually extends over the respective stripes of the insulating selective growth mask
1102
from both sides of the stripes, finally covering the insulating selective growth mask
1102
. Thus, the n-type GaN contact layer
1103
is formed as a single layer.
In the above conventional semiconductor laser device, a current injected from the n-type electrode
1110
flows in the n-type GaN contact layer
1103
in a lateral direction, and electrons in the current recombine with holes existing in the area of the active layer
1106
located right under the ridge stripe structure
1120
, thereby generating light.
In the conventional semiconductor laser device with the above configuration, electrons inevitably travel in the n-type GaN contact layer
1103
in the lateral direction to reach the p-type contact layer
1109
. More specifically, electrons are required to cross the areas of the n-type GaN contact layer
1103
located above the stripes of the insulating selective growth mask
1102
.
However, according to the above-mentioned conventional process, minute crystal cracks extending in the direction of crystal growth and non-grown portions tend to be formed in areas of the n-type GaN contact layer
1103
, indicated by reference numeral
1150
, which correspond to the centers of the respective stripes of the insulating selective growth mask
1102
. This tends to block smooth flow of electrons in the n-type GaN contact layer
1103
in the lateral direction.
As a result, the series resistance of the above conventional semiconductor laser device is as high as about 45 to 140 &OHgr;. Such a high series resistance causes heat generation and crystal distortion, which in turn reduce the lifetime of the device to 150 hours or shorter under the conditions of an ambient temperature of 60° C. and a light output of 5 mW. Such a short lifetime is not suitable for application to optical disk systems and the like.
In addition, electric field tends to concentrate in the cracks and the non-grown portions. This increases the operating voltage of the semiconductor laser device to about 15 to 30 V and often causes breaking of semiconductor laser devices.
SUMMARY OF THE INVENTION
A semiconductor light emitting device of the present invention includes: a substrate; a contact layer made of a gallium nitride based compound semiconductor formed on the substrate; a stripe-shaped conductive selective growth mask formed above the contact layer; and a layered structure made of a gallium nitride based compound semiconductor. The layered structure includes at least a pair of cladding layers, formed on the conductive selective growth mask, and an active layer, including at least one layer, sandwiched by the cladding layers.
The semiconductor light emitting device may further include a current-blocking layer formed on both sides of a stripe of the conductive selective growth mask.
The semiconductor light emitting device may further include an insulating selective growth mask in at least a portion of the current-blocking layer.
In one embodiment, the substrate is an insulating substrate, the width of the stripe of the conductive selective growth mask is smaller than the width of each stripe of the insulating selective growth mask, and an electrode is formed on a surface of the contact layer located farther from the substrate.
The conductive selective growth mask may be made of an oxide semiconductor material.
The insulating selective growth mask may be made of silicon oxide or silicon nitride.
The semiconductor light emitting device may further include a current blocking layer which has an opening at a position located above a stripe of the conductive selective growth mask.
A method for fabricating a semiconductor light emitting device according to the present invention includes the steps of: forming a contact layer made of a gallium nitride based compound semiconductor on a substrate; forming a conductive selective growth mask and an insulating selective growth mask on the contact layer; and forming a layered structure, made of a gallium nitride based compound semiconductor, above the conductive selective growth mask, the insulating selective growth mask, and the contact layer. The layered structure includes at least a pair of cladding layers and an active layer.
Thus, according to the present invention, a gallium nitride based compound semiconductor light emitting device with a low resistance, a low operating voltage, and high reliability can be provided.
By forming the insulating selective growth mask as at least a portion of the current-blocking layer, a selective current injection only at a position from which light is to be emitted, is realized.
In the case where the substrate is an insulating substrate, the width of the stripe of the conductive selective growth mask is designed to be smaller than the width of each stripe of the insulating selective growth mask, and an electrode is formed on a surface of the contact layer located farther from the substrate, the series resistance can be greatly reduced for such a semiconductor light emitting device that has the p-type electrode and the n-type electrode on the same plane.
According to the method of the present invention, an electrode can be easily formed on the contact layer.
Thus, the invention described herein makes possible the advantages of (1) providing a gallium nitride based compound semiconductor light emitting device having an extended device lifetime and a reduced series resistance, and (2) providing a method for fabricating such a semiconductor light emitting device.
These and other advantages of the present invention will become apparent to those skilled in the art upon reading and understanding the following detailed description with reference to the accompanying figures.
REFERENCES:
patent: 5880485 (2000-04-01), Marx et al.
patent: 6051849 (2000-04-01), Davis et al.
Underwood et al., “Selective-area regrowt
Hata Toshio
Ito Shigetoshi
Baumeister Bradley WM.
Jackson, Jr. Jerome
Morrison & Foerster / LLP
Sharp Kabushiki Kaisha
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