Active solid-state devices (e.g. – transistors – solid-state diode – Incoherent light emitter structure – With heterojunction
Reexamination Certificate
1999-12-03
2004-01-13
Cao, Phat X. (Department: 2814)
Active solid-state devices (e.g., transistors, solid-state diode
Incoherent light emitter structure
With heterojunction
C257S059000, C257S098000, C372S049010
Reexamination Certificate
active
06677618
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
This invention relates to semiconductor light emitting devices, particularly, semiconductor lasers. This invention can be desirably used in a case that high output and long lifetime are demanded as for excitation light sources for optical fiber amplifiers, light sources for optical information processing, and the like. This invention is also applicable to LEDs (light emitting diodes) such as super-luminescent diodes or the like in which optical output comes from their facet, vertical cavity surface emitting lasers, and the like.
2. Description of Related Art
Optical information processing technologies and optical communication technologies have remarkably progressed these days. For example, high density recording by means of magneto-optical discs, two-way communications using optical fiber networks, and the like can be exemplified but too numerous to be counted.
In the field of communication technologies, for example, active research is carried out in various quarters with respect to optical fiber transmission lines of a large capacity sufficiently coping with the upcoming multimedia era, and Er
3+
-doped optical fiber amplifiers (EDFA) in which rare earth elements such as Er
3+
are doped, as amplifiers for signal amplification with flexibility to that transmission method. Under these circumstances, developments of highly efficient semiconductor lasers for excitation light source, as a essential component for EDFA, have been sought.
The emission wavelength of the excitation light source applicable to EDFA theoretically includes three types, 800 nm, 980 nm, and 1480 nm. Excitation at 980 nm, among those, has been known as most desirable in terms of characteristics of amplifiers in consideration of gain, noise figure, and the like. The laser having such an emission wavelength of 980 nm is expected to meet contradicting demands as having a high output as the excitation light source and having a long lifetime. With respect to a wavelength around that wavelength, e.g., 890 nm to 1150 nm, while such a laser is demanded for SHG (secondary harmonic generation) light sources and heat sources for laser printers, other applications as well, developments of lasers with high output and high reliability have been expected.
In the field of information processing, semiconductor lasers are made with higher output and shorter wavelength for high density recording and high speed writing and reading. A higher output is strongly expected for conventional laser diodes (hereinafter referred to as “LD's”) having emission wavelength of 780 nm, and the LD's having a 630 to 680 nm band have been energetically developed in various quarters.
As semiconductor lasers of about 980 nm, a semiconductor laser durable for continuous use for approximately two years with an optical output around 50 to 100 mW already has been developed. However, in operation under a higher optical output, rapid degradation occurs, and such a laser cannot have adequate reliability. This is the same to the LD's having 780 nm band and 630 to 680 nm band, and guaranteeing reliability at high output is a major concern for all semiconductor lasers particularly using a GaAs substrate.
One of the reasons of the poor reliability is degradation at a light emission facet of laser beam subjecting to very high optical density. As well known even in GaAs/AlGaAs based semiconductor lasers, many surface states exist at and around the facet, and those serve as nonradiative recombination centers and absorb the laser beam, so that generally the temperature around the facet is made higher than that of the bulk of the laser device, and so that this increased temperature further narrows band gaps around the facet and makes the laser beam more readily absorbed, as explained as occurrence of a positive feedback. This phenomenon is known as a facet degradation or namely COD (Catastrophic Optical Damage) observed when a large current flows at a moment, and sudden failure in devices according to a lowered COD level where a term aging test is made are common problems in many semiconductor laser devices.
Various proposals have been made to solve those problems. For example, various proposes had been made so far to make transparent the band gap at the active layer region around the facet with respect to the emission wavelength to suppress optical absorption around the facet. Lasers having such a structure are generally called window structure lasers or NAM (Non Absorbing Mirror) structure lasers, and these lasers are very effective where a high output is required. In a method to epitaxially grow a semiconductor material transparent to the emission wavelength on the laser facet, however, a subsequent electrode step becomes very complicated because the epitaxial growth is made where the laser is in a so-called bar state.
Meanwhile, various methods to make the active layer disordered by intentionally doing thermal diffusion or ion implantation of Zn, Si and so on as impurities doped into the active layer around the facet of the laser have been proposed (Japanese Unexamined Patent Publication Heisei No. 2-45,992, Japanese Unexamined Patent Publication Heisei No. 3-101,183, Japanese Unexamined Patent Publication Heisei No. 6-302,906). However, the impurities generally diffuse in the LD fabrication process from the epitaxial direction of the laser device to the substrate direction, so that a stable fabrication is difficult because the diffusion depth and lateral diffusion with respect to the direction of the cavity are not easily controlled. In cases of ion implantations, since high energy ions are introduced from facets, damages tend to remain at the LD facets even where anneal processing is made. Moreover, there arises another problem that an increased reactive current accompanying with decreased resistance in the impurity introduced region increases the threshold current and operation current of the laser.
On the other hand, Japanese Unexamined Patent Publication Heisei No. 3-101,183 discloses a method for forming a contamination-free facet and then forming a passivation layer or a part of this layer with the use of an oxygen excluding material undergoing neither any reaction with the semiconductor facet or diffusion by itself.
Generally, operation in air, e.g., in a clean room, cannot prevent nonradiative recombination centers, such as Ga—O, As—O, generated at the facet during cleaving, from forming. Accordingly, to form the contamination-free facet as described in this Journal, it is necessary to form a passivation layer at the same time as cleavage, but this can be carried out only in vacuum. Cleavage in vacuum requires extremely complicated apparatuses and operations in comparison with an ordinary cleavage done in air. Although this journal describes a method in which a facet is made by a dry etching, such a dry etching is not suitable for a fabrication method of LD requiring a longer lifetime because more nonradiative recombination centers are created in comparison with the facet formed by cleavage.
As similar to this journal, in L. W. Tu et al., “In-Vacuum cleaving and coating of semiconductor laser facets using silicon” and a dielectric, J. Appl. Phys. 80(11) Dec. 1, 1996, described is that where a Si/AlO
x
structure is cleaved in vacuum in the step of coating onto a laser facet, the carrier recombination speed is retarded, and thus the initial COD level is increased. This article, however, refers to neither reliability over a long time nor the relationship between the coating and the LD structure.
Further, there has been known a technique for inserting an Si layer having an optical thickness corresponding to ¼ of the emission wavelength at the boundary between the coating film and the semiconductor material as to displace the facet from the anti-node of the standing wave existing in the direction of the cavity, thus lowering the electric field strength at the beam emission facet. However, in a wavelength band that general semiconductor lasers are realized, particularly
Fujimori Toshinari
Horie Hideyoshi
Ohta Hirotaka
Armstrong Westerman & Hattori, LLP
Cao Phat X.
Doan Theresa T.
Mitsubishi Chemical Corporation
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