Coherent light generators – Particular active media – Semiconductor
Reexamination Certificate
2000-06-02
2004-01-13
Ip, Paul (Department: 2828)
Coherent light generators
Particular active media
Semiconductor
C372S046012, C372S044010
Reexamination Certificate
active
06678299
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a semiconductor laser device.
2. Description of the Related Art
A conventional semiconductor laser device
800
will be described with reference to
FIGS. 8A through 8C
.
FIGS. 8A and 8B
are cross-sectional views illustrating a front end face and a rear end face of the conventional semiconductor laser device
800
, respectively.
FIG. 8C
is a diagram illustrating a top plan view of a cross-section of the conventional semiconductor laser device
800
, as taken along the X-Y line indicated in
FIGS. 8A and 8B
.
As shown in
FIGS. 8A and 8B
, the semiconductor laser device
800
includes: a semiconductor substrate
1
made of an n-type InP material and having a mesa structure; a light confinement layer
2
made of an n-type InGaAsP material (band-gap wavelength: about 1.05 &mgr;m) and having a thickness of about 600 nm; an active layer
3
having a multiple quantum-well structure: a light confinement layer
4
made of a p-type InGaAsP material and having a thickness of about 600 nm; and a cladding layer
5
made of a p-type InP material and having a thickness of about 400 nm. The layers
2
through
5
are provided in this order on the mesa region of the semiconductor substrate
1
. The active layer
3
includes seven InGaAsP well layers (not shown) each having a thickness of about 6 nm and a compressive distortion of 1.0% or less, and seven InGaAsP (band-gap wavelength: about 1.05 &mgr;m) barrier layers (not shown) each having a thickness of about 10 nm and no compressive distortion, such that the InGaAsP well layers and the InGaAsP barrier layers are alternately layered on one another.
The semiconductor laser device
800
also includes: a first buried layer
6
made of a p-type InP material whose carrier density is 7.0×10
17
cm
−3
, a second buried layer
7
made of an n-type InP whose carrier density is 2.0×10
18
cm
−3
, a third buried layer
8
made of a p-type InP whose carrier density is 7.0×10
17
cm
−3
, and a contact layer
9
made of a p-type InGaAsP (band-gap wavelength: about 1.3 &mgr;m). The layers
6
through
9
are provided in this order in the vicinity of the active layer
3
. In order to achieve acceptable reverse I-V characteristics in the semiconductor laser device
800
, the buried layers
6
through
8
are formed so as to have pnp-buried type different densities from one another.
In order to reduce the parasitic capacity in the buried layers
6
through
8
so as to improve the frequency response characteristics of the semiconductor laser device
800
, grooves are provided around the stripe structure by using an etching technique. The grooves extend into the first buried layer
6
.
On the contact layer
9
, a SiO
2
film
10
is formed having a thickness of about 0.3 &mgr;m with an aperture therein. A metal multilayer film
11
including three layers (i.e., an Au layer, a Zn layer, and an Au layer) is formed in the aperture, and a p-type electrode
12
is formed on the metal multilayer film
11
. An n-type electrode
13
is provided on the backside of the semiconductor substrate
1
.
Referring to
FIG. 8C
, a top plan view of a cross-section of the active layer
3
of the semiconductor laser device
800
is shown. The active layer
3
has a width of about 0.6 &mgr;m in a region within about 25 &mgr;m from the front end face, while it has a width of about 1.6 &mgr;m in a region within about 25 &mgr;m from the rear end face. The distance between the front end face and the rear end face is about 400 &mgr;m, and the cross-section of the active layer
3
has a stripe structure. The width of the active layer
3
having this stripe structure continuously decreases from the rear end face toward the front end face. Thus, the stripe width of the active layer
3
at the front end face is narrower than that at the rear end face. This is a structure of a semiconductor laser device for implementing narrow output angle characteristics and low operation current characteristics at a high temperature (Y. Inaba et al., IEEE JSTQE, vol. 3, 1399-1404, 1997). With this structure, the effect of confining light within the active layer
3
continuously decreases from the rear end face toward the front end face. Therefore, a large amount of light leaks out of the active layer
3
into the first buried layers
6
in the region adjacent to the front end face. Moreover, light also leaks out of the active layer
3
into the third buried layer
8
because the light confinement layer
4
and the cladding layer
5
are thin.
FIG. 9
is a graph illustrating the relationship between an operation ambient temperature and an output angle of the conventional semiconductor laser device
800
. As seen from
FIG. 9
, when the operation ambient temperature changes from about −40° C. to about 85° C., the output angle changes from about 14.0° to about 10.2° (i.e., by about 3.8°). Therefore, in a case where light output from the semiconductor laser device
800
is coupled to an optical fiber, for example, such temperature changes affect the coupling efficiency between the light from the semiconductor laser device
800
and the optical fiber, thereby affecting the intensity of the light propagated through the optical fiber. This will adversely affect the transmission characteristics of the optical fiber.
It is understood that the output angle changes because there exists about a 0.025 difference in the refractive index between the first buried layer
6
and the second buried layer
7
, and between the third buried layer
8
and the second buried layer
7
. This problem is especially prominent in a semiconductor laser device having an active layer with a small width (i.e., the stripe width is shorter in comparison with the wavelength) since a large amount of light leaks out of the active layer
3
. The amount of change in the intensity of the light propagated through the optical fiber should satisfy the practical standards in optical communications (i.e., 1 dB or less for a temperature change of about −40° C. to about 85° C.). Otherwise, the transmission characteristics of the optical fiber would be very poor, and thus, the semiconductor laser device
800
might not be usable. The conventional semiconductor laser device
800
does not satisfy the above-mentioned practical standards since the amount of change in the intensity of the light propagated through the optical fiber is 2 dB.
As one type of conventional semiconductor laser device in which the active layer has a constant width with respect to the longitudinal cross-section of resonator, a semiconductor laser device is known which includes a single buried layer made of InP-based material doped with Fe (H. Taniwatari et. al., IEEE, JLT, vol. 15, 534 to 537). InP-based material doped with Fe, however, has a problem in that Fe diffuses into InP during a high-temperature operation. As a result, a current leak occurs and therefore it is difficult to achieve long-term reliability in such a semiconductor laser device.
In a conventional semiconductor laser device with pnp-type buried layers, such as the semiconductor laser device
800
, the buried layers have different densities from one another. Therefore, the output angle of the beam changes depending on the amount of current applied to the laser and the temperature condition. As a result, it is difficult to achieve stable output angle characteristics.
SUMMARY OF THE INVENTION
In one aspect of the present invention, a semiconductor laser device includes: a semiconductor substrate; an active layer having a stripe structure formed on the semiconductor substrate; and a buried layer formed on the semiconductor substrate and in a vicinity of the active layer, the buried layer including Fe and Ti.
In one embodiment of the present invention, the buried layer includes InP.
In another embodiment of the present invention, the active layer has a width which is smaller than a diameter of a light emitting spot formed adjacent to the active layer.
In still another embodiment of the
Inaba Yuichi
Kito Masahiro
Ip Paul
Snell & Wilmer LLP
Zahn Jeffrey
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