Coherent light generators – Particular active media – Semiconductor
Reexamination Certificate
1999-03-10
2003-07-08
Leung, Quyen (Department: 2828)
Coherent light generators
Particular active media
Semiconductor
C372S045013
Reexamination Certificate
active
06590919
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a nitride group compound semiconductor laser device which emits laser light in a range of ultraviolet to blue, and a production method thereof.
2. Description of the Related Art
In recent years, as a material for a semiconductor laser device which emits laser light in the range of ultraviolet to blue, a nitride group compound semiconductor Ga
x
Al
y
In
1−(x+y)
N (0≦x≦1, 0≦y≦1, 0≦x+y≦1) has been used.
With a nitride group compound semiconductor laser device, some structures and production methods thereof have been proposed. Among them, a structure having a current constriction layer capable of simultaneously conducting current constriction and light confinement is expected as a highly reliable structure capable of reducing a driving current and stabilizing an oscillation transverse mode.
As an example, a semiconductor laser device
300
disclosed in Japanese Laid-Open Publication No. 8-97502 is shown in FIG.
3
.
The semiconductor laser device
300
includes a sapphire substrate
301
, an n-type GaN buffer layer
302
, an n-type Al
0.2
Ga
0.8
N first cladding layer
303
, an In
0.15
Ga
0.85
N active layer
304
, a p-type Al
0.2
Ga
0.8
N second cladding layer
305
, an n-type Si current constriction layer
306
having a stripe-shaped opening
320
, a p-type Al
0.2
Ga
0.8
N third cladding layer
307
, and a p-type GaN contact layer
308
. The first cladding layer
303
, the active layer
304
, the second cladding layer
305
, the current constriction layer
306
, the third cladding layer
307
, and the contact layer
308
are partially removed so as to expose the buffer layer
302
. A p-side electrode
309
is formed on the contact layer
308
, and an n-side electrode
310
is formed on an exposed portion of the buffer layer
302
.
In the above-mentioned conventional nitride group compound semiconductor laser device
300
, the current constriction layer
306
made of a light-absorbing material is formed in the vicinity of the active layer
304
, whereby stable optical waveguide can be realized. However, the current constriction layer
306
is made of Si, and has a forbidden bandgap sufficiently smaller than that of energy corresponding to light generated by the active layer
304
, so that an absorption coefficient becomes large (5×10
5
cm
−1
) at the current constriction layer
306
, which increases a loss in the waveguide. As a result, a semiconductor laser device which oscillates at an oscillating threshold current of about 100 mA or less, required for ensuring reliability, cannot be obtained.
Furthermore, in the conventional nitride group compound semiconductor laser device
300
, when laser light is absorbed by the current constriction layer
306
and heat is generated locally in the vicinity of the stripe-shaped opening
320
, the crystal is strained due to a large difference (2×10
−6
) in the thermal expansion coefficient between Si, which is a material for the current constriction layer
306
, and a GaN type material. Thus, a semiconductor laser device may be damaged during operation.
In order to solve the above-mentioned problem, it is currently being studied whether GaInN having a large composition of In can be used for the current constriction layer
306
. However, it is difficult to grow GaInN having a large composition of In with satisfactory controllability at an ordinary growth temperature. Furthermore, GaInN having a large composition of In has a large absorption coefficient (1×10
5
cm
−1
) with respect to a wavelength of laser light. Therefore, a waveguide loss increases, which results in an oscillating threshold current of more than about 100 mA in the same way as in the case using Si, making reliability of the device unsatisfactory.
As described above, in the conventional example, a highly reliable nitride group compound semiconductor laser device which oscillates at an oscillating threshold current of about 100 mA or less in a stable transverse mode cannot be realized.
The main cause for the above problem is as follows: An absorption coefficient in a wavelength region of blue to ultraviolet is large (1×10
5
cm
−1
) in the current constriction layer which also functions so as to form a waveguide by light absorption. Therefore, an absorption loss in the waveguide increases, which results in an oscillating threshold current of more than about 100 mA; thus, the device will be thermally damaged during oscillation.
Furthermore, in the case where a material such as Si whose thermal expansion coefficient is largely different from that of a GaN type material is used for the current constriction layer, the device is damaged by local absorption of laser light. Therefore, a highly reliable nitride group compound semiconductor laser device cannot be obtained.
SUMMARY OF THE INVENTION
A nitride group compound semiconductor laser device of the present invention includes: a pair of cladding layers; an active layer interposed between the cladding layers; and a current constriction layer having a stripe-shaped opening which is to be a current passage, provided above the active layer, wherein the current constriction layer is formed of a high resistant layer obtained by crystallizing an amorphous or polycrystalline nitride group compound semiconductor by heating.
In one embodiment of the present invention, the current constriction layer is made of Ga
x
Al
y
In
1−(x+y)
N (0≦x≦1, 0≦y≦1, 0≦x+y≦1) containing impurities in an amount of 1×10
20
cm
−3
or more.
Alternatively, a nitride group compound semiconductor laser device of the present invention includes: a pair of cladding layers; an active layer interposed between the cladding layers; and a current constriction layer excluding a stripe-shaped portion which is to be a current passage, provided above the active layer, wherein the current constriction layer is formed of a high resistant layer obtained by irradiating charged particles to crystal of a nitride group compound semiconductor.
In one embodiment of the present invention, a re-evaporation preventing layer or an etching stop layer is provided between the active layer and the current constriction layer.
In another embodiment of the present invention, a re-evaporation preventing layer or an etching stop layer is provided between the active layer and the current constriction layer.
In another embodiment of the present invention, a contact layer is provided above both the current constriction layer and the stripe-shaped opening or the stripe-shaped portion.
According to another aspect of the present invention, a method for producing the above-mentioned nitride group compound semiconductor laser device includes the steps of: growing the first cladding layer and the active layer; growing the amorphous or polycrystalline nitride group compound semiconductor layer on the active layer; conducting wet etching with respect to the nitride group compound semiconductor layer at a temperature of 80° C. or lower to form the stripe-shaped opening; and growing the second cladding layer so as to bury the stripe-shaped opening.
In one embodiment of the present invention, the nitride group compound semiconductor layer is grown at a temperature of less than about 700° C.
In another embodiment of the present invention, the nitride group compound semiconductor layer is grown at a temperature in a range of about 400° C. to about 600° C.
Alternatively, a method for producing the above-mentioned nitride group compound semiconductor laser device includes the steps of: growing the first cladding layer, the active layer, and the second cladding layer; and irradiating charged particles to the second cladding layer except for the stripe-shaped portion to form the irradiated portion as the current constriction layer.
Hereinafter, the function of the present invention will be described.
According to the present invention, the current restriction layer provided above the active layer is formed
Leung Quyen
Morrison & Foerster / LLP
Sharp Kabushiki Kaisha
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