Active solid-state devices (e.g. – transistors – solid-state diode – Incoherent light emitter structure
Reexamination Certificate
2000-02-24
2003-04-08
Abraham, Fetsum (Department: 2826)
Active solid-state devices (e.g., transistors, solid-state diode
Incoherent light emitter structure
C257S095000, C257S096000, C257S098000, C372S043010
Reexamination Certificate
active
06545296
ABSTRACT:
BACKGROUND OF THE INVENTION
(a) Field of the Invention
The present invention relates to a semiconductor light emitting device, more in detail, to the semiconductor light emitting device in which an optical output and a lasing spectrum have a stable relation with respect to injected current, and more specifically to the high power semiconductor pumping source, such as a semiconductor laser, which is most suitable as an optical amplifier for an optical communication system.
(b) Description of the Related Art
A semiconductor light emitting device outputting a light having a wavelength of a 980 nm band, for example, an InGaAs-based semiconductor laser element is frequently employed as various light sources in an optical communication system, for example, as a pumping light source for an optical fiber amplifier.
The structure of the conventional InGaAs-based laser element of the 980 nm range (hereinafter referred to as “InGaAs-based laser element”) will be described referring to FIG.
1
.
A conventional InGaAs-based laser element
20
basically has a layered structure shown in
FIG. 1
which is manufactured by sequentially depositing, by means of epitaxial growth, a n-AlGaAs cladding layer
12
having a thickness of 2 &mgr;m, quantum well active layers
13
including InGaAs films having a thickness of 7 nm and GaAs films having a thickness of 10 nm, a p-AlGaAs cladding layer
14
having a thickness of 2 &mgr;m and a GaAs cap layer
15
having a thickness of 0.3 &mgr;m, on an n-GaAs substrate
11
having a thickness of 100 &mgr;m.
The top portion of the cladding layer
14
and the cap layer
15
are subjected to an etching to form a mesa-stripe. A passivation film
16
made of SiN is formed on the entire surface other than the top surface of the cap layer
15
to make a window for supplying current therethrough.
A TiPtAu metal layered film
17
is formed on the passivation film
16
and the top surface of the cap layer
15
as a p-side electrode, and an AuGeNiAu metal layered film
18
is formed on the bottom surface of the substrate
11
as an n-side electrode.
Then, a conventional method for manufacturing the InGaAs-based laser element
20
shown in
FIG. 1
will be described referring to
FIG. 2
showing a vertical section of the laser element
20
during the manufacture.
At first, the n-AlGaAs cladding layer
12
having a thickness of 2 &mgr;m, the quantum well active layers
13
including InGaAs films having a thickness of 7 nm and GaAs films having a thickness of 10 nm, the p-AlGaAs cladding layer
14
having a thickness of 2 &mgr;m and the GaAs cap layer
15
having a thickness of 0.3 &mgr;m are sequentially formed on the n-GaAs substrate
11
by using an MOCVD method.
Then, the cap layer
15
and the upper portion of the cladding layer
14
are selectively etched by a thickness of 1 &mgr;m to form a mesa stripe, followed by formation of the passivation film
16
on the entire surface of the mesa stripe. The passivation film on the top surface of the mesa stripe is removed to form a window through which current is to be injected.
Then, the bottom surface of the substrate
11
is polished until the thickness of the substrate becomes 100 &mgr;m. Thereafter, the TiPtAu metal layered film
17
functioning as a p-side electrode and the AuGe/Ni/Au metal layered film
18
functioning as a n-side electrode are formed on the passivation film
16
and on the bottom surface of the substrate
11
, respectively, by means of vapor deposition to provide the InGaAs-based laser element
20
.
Since the bandgap energy Eg
1
of the substrate
11
of the conventional InGaAs-based laser element
20
is 1.41 electron volts (eV) and the bandgap energy Eg
2
of the active layers
13
is 1.27 electron volts which is smaller than the bandgap energy Eg
1
, the light emitted from the active layer can travel within the substrate.
However, the conventional InGaAs-based laser element has the following two problems in connection with the characteristics of the laser element.
A first problem is that a linearity between current and an optical output is not excellent and kinks are observed in the current-optical output characteristic as shown in
FIG. 3
wherein the wavelength and the optical output power are plotted on abscissa and ordinate, respectively. The disturbance on the current-optical output characteristic makes it difficult to obtain a stable automatic power control (APC) for the InGaAs-based laser element based on the current-optical output characteristic.
A second problem is that a ripple appears at a specified wavelength interval, for example, a 3 nm interval on the wavelength characteristic of a lasing spectrum as shown FIG.
4
. In this situation, a phenomenon occurs in which the lasing mode skips every 3 nm interval with respect to the injected current, and mode hopping noise is generated.
Accordingly, it is difficult to amplify the input signals simultaneously to keep the signal powers constant using, the conventional InGaAs-based laser element used for pumping source in an optical amplifier.
This disadvantage of the semiconductor light emitting device is extremely undesirable used for the pumping source of an optical fiber telecommunication system.
The present inventors have found the following matters after investigating the reasons of causing the above problems of the InGaAs-based laser element.
Either problem is generated because light is regularly reflected on the bottom surface of the substrate. Since the bottom surface of the substrate is of a mirror surface after the substrate is polished at the bottom surface to have a specified thickness, for example, to a thickness of 100 &mgr;m, part of the light travelling within the substrate and reflected on the bottom surface of the substrate is coupled with the light in the active layer
13
as shown in
FIG. 5
, and a coupled cavity is formed by combination of a substrate resonator formed by the regular reflection on the substrate and an original Fabry-Perot resonator, to thereby make the lasing wavelength unstable.
SUMMARY OF THE INVENTION
In view of the foregoing, an object of the present invention is to provide a semiconductor light emitting device such as a semiconductor laser element which can obtain a multiple longitudinal mode lasing spectrum generating reduced ripples and a stable optical output with respect to injected current.
The present invention provides a semiconductor light emitting device including a semiconductor substrate having a bandgap energy of Eg
1
, and at least one active layer having a bandgap energy of Eg
2
smaller than Eg
1
and overlying main surface of the semiconductor substrate, a bottom surface of the semiconductor substrate opposite to the main surface having unevenness for diffused reflection.
In accordance with the present invention, the generation of kinks in the current-optical output characteristic and of ripples in the lasing spectrum occurring in a conventional semiconductor light emitting device can be substantially suppressed. Further, a semiconductor light emitting device can be provided in which an optical output and a multiple longitudinal mode lasing spectrum stably operate with regard to injected current by reducing an amount of light reflecting from the semiconductor substrate bottom surface to the active layer by means of making the minute unevenness on the semiconductor substrate bottom surface. Accordingly, the most appropriate high power pumping source for an optical amplifier of an optical fiber telecommunication system can be obtained.
The above and other objects, features and advantages of the present invention will be more apparent from the following description.
REFERENCES:
patent: 5814839 (1998-09-01), Hosoba
patent: 09213477 (1997-08-01), None
Kasukawa Akihiko
Mukaihara Toshikazu
Yamaguchi Takeharu
Abraham Fetsum
Andujar Leonard
Katten Muchin Zavis & Rosenman
The Furukawa Electric Co. Ltd.
LandOfFree
Semiconductor light emitting 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 Semiconductor light emitting device, we encourage you to share that experience with our LandOfFree.com community. Your opinion is very important and Semiconductor light emitting device will most certainly appreciate the feedback.
Profile ID: LFUS-PAI-O-3063019