Semiconductor laser device

Details Semiconductor laser device Semiconductor laser device

1999-06-14

2002-10-15

Ip, Paul (Department: 2828)

Coherent light generators

Particular active media

Semiconductor

C372S043010, C372S044010, C257S013000, C257S025000, C257S103000

Reexamination Certificate

active

06466597

ABSTRACT:

BACKGROUND OF THE INVENTION
The present invention relates to a semiconductor laser device of a group III-V nitride semiconductor represented by a general formula, Al
x
Ga
y
In
1−x−y
P
v
As
w
N
1−v−w
(wherein 0≦x≦1, 0≦y≦1, x+y≦1, 0≦v≦1, 0≦w≦1 and v+w≦1), which shows laser action with a wavelength ranging from the blue region to the ultraviolet region and is expected to be applied to fields of optical data processing and the like.
Recently, large capacity optical disk systems such as a digital video disk system have been put to practical use, and the recording capacity of an optical disk is now being further increased. As is well known, for the purpose of increasing the recording capacity, it is one of the most effective means to shorten a wavelength of a laser beam used as a light source for recording or reproducing information. A semiconductor laser chip included in an existing digital video disk system is made from a semiconductor material mainly including AlGaInP among group III-V semiconductor materials, and has a wavelength for laser action of 650 nm. Accordingly, a laser device with a shorter wavelength using a group III-V nitride semiconductor material is indispensable for a high density digital video disk system now under development.
Now, a conventional group III-V nitride semiconductor laser device will be described with reference to a drawing.
FIG. 8
shows a sectional structure of the conventional group III-V nitride semiconductor laser device.
As is shown in
FIG. 8
, a buffer layer
102
of GaN and an n-type contact layer
103
of n-type GaN with low resistance are successively formed on a substrate
101
of sapphire. In an element region on the n-type contact layer
103
, an n-type cladding layer
104
of AlGaN, an n-type light guiding layer
105
of n-type GaN, a multiple quantum well active layer
106
including alternately stacked well layer of Ga
1−x
In
x
N and barrier layer of Ga
1−y
In
y
N (wherein 0<y≦x<1), a p-type light guiding layer
107
of p-type GaN and a p-type cladding layer
108
of p-type AlGaN having a ridge stripe portion
108
a
in the shape of a ridge with a width of 3 through 10 &mgr;m on the top surface thereof are successively formed.
On the p-type cladding layer
108
, a p-type contact layer
109
of p-type GaN with low resistance is formed, and on the p-type contact layer
109
, a p-side electrode
110
is selectively formed. The top surface of the p-type cladding layer
108
on both sides of the ridge stripe portion
108
a
excluding the p-side electrode
110
and the side surfaces of the element region are covered with an insulating film
111
. On the insulating film
111
, a wire electrode
112
is formed so as to be in contact and cover the p-side electrode
110
, and an n-side electrode
113
is formed on the n-type contact layer
103
on a side of the element region.
When the semiconductor laser device having the aforementioned structure is grounded at the n-side electrode
113
and supplied with a given voltage at the wire electrode
112
, the semiconductor laser device shows laser action with a wavelength of 370 nm through 430 nm. This wavelength for laser action is varied depending upon the compositions and the thicknesses of the layers of Ga
1−x
In
x
N and Ga
1−y
In
y
N included in the multiple quantum well active layer
106
. At present, continuous laser action has been achieved at a temperature exceeding room temperature and will soon be put to practical use. However, in order to increase the recording capacity of an optical disk system, a semiconductor laser device capable of showing laser action with a shorter wavelength is desired to realize.
However, in the conventional group III-V nitride semiconductor laser device, the wavelength for laser action cannot be made shorter than approximately 370 nm and is difficult to further shorten in view of the operating principle.
In order to realize a semiconductor laser with a shorter wavelength, a so-called wide gap semiconductor having a wide band gap (energy gap) is used as an active layer. For example in the aforementioned multiple quantum well active layer
106
, a shorter wavelength can be attained by using, as the well layer, Ga
1−x
In
x
N with the composition ratio of In of 0, namely, GaN, or AlGaN including Al for further widening the energy gap.
In a double heterostructure laser device in which carriers and produced light are confined in an active layer, a semiconductor material having a wider energy gap than the active layer is required to be used as a cladding layer.
In general, in order to obtain a semiconductor laser device having practical operation characteristics operable at room temperature or more, it is necessary to use a cladding layer having an energy gap wider than that of an active layer by at least approximately 0.4 eV. Since the energy gap of a semiconductor of AlGaN can be widely changed in a range between 3.4 eV and 6.2 eV, it is possible to form a cladding layer with a wide energy gap. However, when the semiconductor of AlGaN has a composition with a wide energy gap, p-type impurity doping for obtaining a p-type semiconductor becomes particularly difficult because the thermal activation efficiency of holes is lowered. Therefore, at present, merely a p-type semiconductor with the composition ratio of Al of approximately 0.2 at most (namely, Al
0.2
Ga
0.8
N as a mixed crystal) and with an energy gap of approximately 4.0 eV at most can be obtained.
The present inventors have extensively examined the reason for which a p-type group III-V nitride semiconductor, particularly a semiconductor of p-type AlGaN, can merely attain an energy gap up to approximately 4.0 eV at most, resulting in reaching the following conclusion:
FIG. 9
shows energy levels of p-type gallium nitride (GaN) and p-type aluminum nitride (AlN), wherein the ordinate indicates the energy of electrons. As is shown in
FIG. 9
, above valence bands Ev of GaN and AlN, an acceptor level Ea derived from magnesium (Mg) working as a p-type dopant is formed. Mg is generally regarded as an acceptor that is the shallowest in a nitride semiconductor, namely, that has the lowest binding energy and can be easily activated, and hence is widely used as a p-type dopant.
However, even Mg has a comparatively high acceptor level of 0.15 eV from the energy Ev at the upper end of the valance band of GaN. As is well known, a thermal energy corresponding to room temperature is approximately 0.025 eV, and the thermal activation efficiency of Mg at room temperature is merely approximately 1%. Accordingly, in order to obtain a carrier concentration of 1×10
17
cm
−3
through 1×10
18
cm
−3
required in a p-type cladding layer, the dope concentration of Mg should be 1×10
19
cm
−3
through 1×10
20
cm
−3
. The dope concentration of Mg of 1×10
20
cm
−3
approximates to a limit for obtaining a good semiconductor crystal, and when Mg is further doped, the crystallinity becomes very poor. Accordingly, with a carrier concentration attained by the impurity concentration of 1×10
20
cm
−3
regarded as a limit of the dope concentration, it is necessary to attain a thermal activation efficiency of the acceptor of 0.1% or more in order to obtain a carrier concentration exceeding 1×10
17
cm
−3
.
On the other hand, as is shown in
FIG. 9
, the acceptor level Ea of Mg is deeper in AlN, and reaches approximately 0.6 eV. For example, in Al
y
Ga
1−y
N, the acceptor level is substantially linearly changed from 0.15 eV to 0.6 eV by changing the composition ratio y of Al. In order to attain the thermal activation efficiency of the acceptor of 0.1% or more, it is necessary to make comparatively small a difference between the acceptor level Ea and the energy Ev at the upper end of the valence band, and hence, the composition ratio y of Al cannot be increased.
When the composition ratio of Al cannot be thus increased, the proportion of electrons tha

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