Coherent light generators – Particular resonant cavity – Specified cavity component
Reexamination Certificate
2000-09-15
2004-09-21
Wong, Don (Department: 2828)
Coherent light generators
Particular resonant cavity
Specified cavity component
C372S045013, C372S049010
Reexamination Certificate
active
06795480
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a semiconductor laser device exhibiting a prolonged duration of operating life, which is advantageously used as an optical transmitter for the optical communication.
2. Description of the Prior Art
A semiconductor laser device basically has a device structure such that it comprises a semiconductor multi-layer film having optical confinement layers formed on both surfaces of active layers, and electrodes which are respectively formed on the upper surface and the lower surface of the semiconductor multi-layer film. The semiconductor multi-layer film is formed by successively laminating a plurality of semiconductor layers (compound semiconductor layers) having different compositions on a predetermined semiconductor substrate using, for example, epitaxial growth. Then, the semiconductor laser device constitutes an optical cavity relative to the laser pumped in the active layer by cleaving the above-mentioned semiconductor multi-layer film containing the electrodes in the direction perpendicular to the junction planes of the individual layers therein and allowing the opposite cleaved planes to function as ends of optical cavity.
Further, in general, one of the above-mentioned ends of optical cavity (cleaved planes) is coated with a high reflection film, and the other end of the optical cavity is coated with a low reflection film. These reflection films not only adjust the reflectance on the optical cavity end relative to the laser pumped in the active layer but also play a role of protecting the cleaved planes. The above low reflection film is formed as a single-layer film comprised of an oxide material (dielectric material) having a low refractive index, for example, aluminum oxide, silicon oxide or the like. In addition, the high reflection film is formed as a composite film obtained by alternately laminating, for example, the above-mentioned low reflection film and a film comprised of a material having a high refractive index, such as Si or the like. Such reflection films are formed by vapor deposition generally using a sputtering process.
By the way, when a semiconductor laser device having the above-described device structure is driven at a constant current, the optical output lowers with the lapse of time, and the lasing is finally stopped. Such a phenomenon is caused by a number of factors, and one of the factors is a problem of catastrophic optical damage.
This catastrophic optical damage is a phenomenon caused by the increased non-radiative recombination due to oxidation of the ends of the optical cavity during the driving of the semiconductor laser device. Such a phenomenon considerably lowers, for example, the driving reliability of the semiconductor laser device used as an optical transmitter for the optical communication, and thus, the improvement thereof has been strongly desired. Especially when the active layers and the semiconductor layers (optical confinement layers, and the like) near the active layers are comprised of an Al-containing compound semiconductor material, and in addition, the low reflection film (protection film) formed on the optical cavity end is an Al
2
O
3
film, a problem arises in that the above-described catastrophic optical damage markedly occurs.
SUMMARY OF THE INVENTION
In the course of the studies made for solving the above problems, with respect to the phenomenon wherein, in the case where an Al
2
O
3
film is deposited on the optical cavity end (cleaved plane) of the semiconductor laser device as a protection film (reflection film) therefor, when the active layers and the semiconductor layers near the active layers are comprised of an Al-containing compound semiconductor material, a catastrophic optical damage markedly occurs, the present inventors have made the following observations.
(1) When an Al
2
O
3
film is deposited by a sputtering process which is generally employed as a method for depositing a protection film, the composition of the resultant film does not necessarily have a stoichiometric ratio. Rather, the Al
2
O
3
film may contain an oxygen component in a stoichiometrically excess amount.
(2) In such a case, the excess amount of the oxygen component in the Al
2
O
3
film is liberated due to the heat generated during the driving of the semiconductor laser device and the like, and diffused toward the cleaved plane side. Thus, the oxygen component diffused toward the cleaved plane side oxidizes the Al component of the Al-containing compound semiconductor material constituting the optical cavity ends (cleaved planes). It is considered that, as a result, a catastrophic optical damage occurs in the optical cavity.
(3) When the Al
2
O
3
film stoichiometically lacks the oxygen component, the amount of the metal Al component becomes large, so that a current easily flows through the Al
2
O
3
film when an electric field is applied thereto. Thus, this current causes the metal Al component and the Al component of the Al-containing compound semiconductor material constituting the optical cavity ends (cleaved planes) to undergo oxidation, so that a catastrophic optical damage easily occurs in the optical cavity similarly to the above case.
(4) Therefore, by using, as the Al
2
O
3
film deposited on the optical cavity end, one that has the oxygen component which is not in a stoichiometrically excess amount, namely, has a composition such that the Al component and the oxygen component approximate to the stoichiometric ratio as closely as possible, the diffusion of the oxygen component and the oxidation of the Al-containing compound semiconductor material caused by the diffusion can be suppressed. Further, it has been considered that, by suppressing the oxidation of the Al-containing compound semiconductor material, it is possible to suppress the occurrence of a catastrophic optical damage in the optical cavity.
(5) In addition, it is considered that, by using, as the Al
2
O
3
film deposited on the optical cavity end, one that does not stoichiometically lack the oxygen component, namely, has a composition such that the Al component and the oxygen component approximate to the stoichiometric ratio as closely as possible, the oxidation of the Al-containing compound semiconductor material is suppressed, thus making it possible to suppress the occurrence of a catastrophic optical damage in the optical cavity.
Based on the above observations, the present inventors have made various studies on the deposition of an Al
2
O
3
film. As a result, it has been found that, when an Al
2
O
3
film is deposited on the end of the optical cavity by, for example, the below-mentioned electron cyclotron resonance sputtering (hereinafter, referred to as “ECR”) process, the resultant Al
2
O
3
film is a film having a stoichiometric ratio composition. It is considered that, since sputtering is performed using a pure metal (Al) as a target in an oxygen atmosphere in the ECR process, the sputtered pure metal (Al) adheres to the optical cavity end while incorporating thereinto the required amount of oxygen in the atmosphere, so that the Al
2
O
3
film deposited on the optical cavity end has a stoichiometric ratio composition. On the contrary, it is considered that, in the conventionally known sputtering process, alumina (Al
2
O
3
) is used as a target, and therefore, the composition for the molecules of the alumina sputtered varies depending on the manner of sputtering, so that the oxygen component of the Al
2
O
3
film deposited on the optical cavity end is stoichiometrically changed.
Further, it has also been found that the resistivity of the Al
2
O
3
film deposited using the above-mentioned ECR process is a considerably high value, as compared to that of the Al
2
O
3
film deposited by the general sputtering process. In addition, it has been found that the semiconductor laser device having an Al
2
O
3
film deposited which has such a high resistivity and a stoichiometric ratio composition is advantageous in that a catastrophic optical damage is considerably suppressed and al
Kaya Shusuke
Ninomiya Takao
Okubo Michio
Uchiyama Seiji
Jackson Cornelius H
The Furukawa Electric Co. Ltd.
Wong Don
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