Nitride semiconductor laser device

Details Nitride semiconductor laser device Nitride semiconductor laser device

1999-04-05

2001-06-19

Davie, James W. (Department: 2881)

Coherent light generators

Particular active media

Semiconductor

Reexamination Certificate

active

06249534

ABSTRACT:

BACKGROUND OF THE INVENTION
The present invention relates to a nitride semiconductor laser device.
A semiconductor laser device using a nitride semiconductor such as GaN, InN or AlN can generate light in the green to blue regions and is expected to be a light source for a high-density optical disk apparatus. A nitride semiconductor laser device of the type emitting light in the blue part of the spectrum will be described as an exemplary prior art device.
FIG. 11
illustrates a conventional nitride semiconductor laser device
600
. In the device
600
, n-type GaN electrode forming layer
62
(including upper and lower parts
62
a
and
62
b
) n-type GaAlN cladding layer
63
, InGaN/GaN multi-quantum well (MQW) active layer
64
, p-type GaAlN cladding layer
65
and p-type GaN electrode forming layer
66
are formed in this order on a sapphire substrate
61
. An electrode provided on the n-type side (n-type electrode)
67
, made up of multiple pairs of Ti/Al layers alternately stacked, is formed on the lower part
62
a
of the n-type electrode forming layer
62
. On the other hand, an electrode provided on the p-type side (p-type electrode)
68
, made up of multiple pairs of Ni/Au layers alternately stacked, is formed on the p-type electrode forming layer
66
. In this manner, a laser diode
60
, also called a “laser element or cavity” is formed. On both facets of the laser diode
60
, from/by which laser light is emitted or reflected, a pair of SiO
2
or SiN protective layers
69
are provided, thus preventing the deterioration of the laser facets. In this case, SiO
2
or SiN need not have their compositions exactly defined by stoichiometry. Instead, these layers
69
should have resistivity (or insulating properties) and refractive index that are substantially equal to those of SiO
2
or SiN. In this specification, part of a semiconductor laser device, from which stimulated emission of radiation is produced, will be referred to as a “semiconductor laser diode”, and a combination of the semiconductor laser diode with at least one protective or reflective layer a “semiconductor laser device” for convenience.
The conventional nitride semiconductor laser device
600
may be fabricated by the following method. First, the electrode forming layer
62
, cladding layer
63
, MQW active layer
64
, cladding layer
65
and electrode forming layer
66
are formed in this order by a crystal-growing technique on the sapphire substrate
61
. Thereafter, respective portions of the electrode forming layer
66
, cladding layer
65
, MQW active layer
64
, cladding layer
63
and upper part
62
b
of the electrode forming layer
62
are etched, thereby exposing the upper surface of the lower part
62
a
of the electrode forming layer
62
. The n- and p-type electrodes
67
and
68
are formed on the exposed upper surface of the electrode forming layer
62
a
and the electrode forming layer
66
, respectively, by an evaporation technique. Thereafter, the pair of protective layers
69
are formed on both laser facets by a sputtering or electron beam (EB) evaporation technique.
FIGS. 12A and 12B
illustrate another conventional nitride semiconductor laser device
700
. The device
700
includes: n-type GaAlN cladding layer
72
; InGaN/GaN MQW active layer
74
; p-type GaAlN cladding layer
75
; and p-type GaN electrode forming layer
76
, which are stacked in this order on a sapphire substrate
72
by a crystal-growing technique. An Ni/Au electrode
77
and a Ti/Al electrode
71
are formed on the upper and lower surfaces of this multilayer structure to form a laser diode
70
. In order to reduce the operating current of this laser diode
70
, a reflective layer
90
, made up of four pairs of SiO
2
/TiO
2
layers
91
,
92
alternately stacked with the thickness of each layer defined as &lgr;/4n (n is a refractive index of each layer
91
or
92
), is formed on the rear facet, or the back, of the laser diode
70
. On the front facet, or the front, of the laser diode
70
, an SiO
2
protective layer
80
is formed at a thickness defined as &lgr;/2n (n is a refractive index of the protective layer
80
). Herein, &lgr; is an oscillation wavelength of the laser diode
70
. The stimulated emission of radiation is output from the front. The front protective layer
80
and the rear reflective layer
90
are deposited by a sputtering or EB evaporation technique.
By providing the reflective layer
90
on the back, the reflectance of the back increases to about 98%, and almost all laser light can be emitted from the front. As a result, the operating current can be reduced to about 70% of that consumed by a semiconductor laser device with only an SiO
2
protective layer formed on its back at an ordinary thickness defined by &lgr;/2n.
However, the lifetime of the conventional nitride semi-conductor laser devices
600
and
700
are short particularly when operating at high output power. The present inventors found that the lifetime of these nitride semiconductor laser devices are short because of the following reasons:
(1) the laser diodes
60
and
70
are made up of a plurality of crystal layers, whereas the protective layers
69
and
80
and the reflective layer
90
, formed on the facets thereof are formed of SiO
2
or TiO
2
and are all amorphous layers. In addition, the length of a bond in the material for these amorphous layers (e.g., the length of an Si—O bond) is different from the lattice constant of the crystal layers in the laser diodes. Accordingly, lattice mismatching is caused in these interfaces to create lattice defects in these crystal layers (in the MQW active layer, in particular). Moreover, if the protective layers
69
and
80
and the reflective layer
90
are formed on the laser facets by a sputtering or EB evaporation technique, then these laser facets would be damaged due to relatively high impact energy of material particles flying from the target. As a result, lattice defects might be caused in the crystal layers in the laser diodes
60
and
70
.
(2) The thermal expansion coefficients of the crystal layers in the laser diodes
60
and
70
are greatly different from those of the protective layers
69
and
80
and the reflective layer
90
. Accordingly, the crystal layers (the MQW active layer, in particular) are strained while the protective layers
69
and
80
and the reflective layer
90
are cooled down to room temperature after these layers have been formed and during the operation of the devices (during high-power operation, in particular). As a result, crystal defects are newly created or the number thereof increases. For example, the thermal expansion coefficient of the MQW active layer
64
is 3.15×10
−6
k
−1,
which is greatly different from that of the protective layer
69
at 1.6×10
−7
k
−1
.
SUMMARY OF THE INVENTION
An object of the present invention is providing a nitride semiconductor laser device with a much longer lifetime and higher reliability than those of a conventional device.
A nitride semiconductor laser device according to the present invention includes: a nitride semiconductor laser diode; and a protective layer formed on at least one facet of the nitride semiconductor laser diode. The protective layer is made of Al
1-x-y-z
Ga
x
In
y
B
z
N (where 0≦x, y, z≦1 and 0≦x+y+z≦1), which is transparent to light emitted from the laser diodeIn one embodiment of the present invention, the thickness of the protective layer is preferably N times as large as &lgr;/2n, where N is a positive integer. &lgr; is an oscillation wavelength of the light emitted from the laser diode and n is a refractive index of the protective layer.
In another embodiment of the present invention, the nitride semiconductor laser diode preferably includes a multi-quantum well active layer made up of a multiple pairs of In
u
Ga
1-u
N and In
v
Ga
1-v
N (where 0≦u, v≦1) layers alternately stacked one upon the other.
In still another embodiment, the protective layer is preferably formed by an MOCVD or MBE process.
In yet another em

Affiliated with

Also associated with

Rating

Nitride semiconductor laser device does not yet have a rating. At this time, there are no reviews or comments for this patent.

If you have personal experience with Nitride semiconductor laser device, we encourage you to share that experience with our LandOfFree.com community. Your opinion is very important and Nitride semiconductor laser device will most certainly appreciate the feedback.

Rate now

Comments { 0 }

Profile ID: LFUS-PAI-O-2546490