Coherent light generators – Particular active media – Semiconductor
Reexamination Certificate
2000-06-23
2003-04-08
Leung, Quyen (Department: 2828)
Coherent light generators
Particular active media
Semiconductor
C372S096000
Reexamination Certificate
active
06546031
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates generally to semiconductor light sources such as LEDs and VCSELs, and more particularly to a strained layer semiconductor laser having an emission wavelength of at least 1.3 &mgr;m.
2. Description of the Prior Art
Vertical-Cavity Surface-Emitting Lasers (VCSELs), Edge Emitting Lasers (EELs) or Light Emitting Diodes (LEDs) are becoming increasingly important for a wide variety of applications including optical interconnection of integrated circuits, optical computing systems, optical recording and readout systems, and telecommunications. Vertically emitting devices have many advantages over edge-emitting devices, including the possibility for wafer scale fabrication and testing, and the possibility of forming two-dimensional arrays of the vertically emitting devices. The circular nature of the light output beams from these devices also make them ideally suited for coupling into optical fibers as in optical interconnects or other optical systems for integrated circuits and other applications.
For high-speed optical fiber communications, laser or LED emission wavelengths in the 1.3 &mgr;m through 1.55 &mgr;m region are desired. Standard silica fiber has zero dispersion near 1.3 &mgr;m and has a minimum loss near 1.55 &mgr;m. The need for semiconductor lasers emitting in this wavelength region has spawned worldwide development of such lasers. Group III-V semiconductors which emit light in the 1.3 through 1.55 &mgr;m region have lattice constants which are more closely matched to InP than to other binary III-V semiconductor substrates, for example, GaAs. Thus, essentially all commercial emitting lasers emitting at 1.3 through 1.55 &mgr;m are grown on InP substrates. These lasers are edge-emitting lasers which, unlike VCSELs, do not require high-reflectivity Distributed Bragg Reflectors (DBRs) to form their optical cavities.
Unfortunately, it has proven difficult to produce effective DBRs on InP substrates. The available materials which lattice match InP have produced mirrors which are extremely thick and lossy and have thus not resulted in efficient VCSELs.
VCSELs or Surface Emitting Lasers SELs whose current flow is controlled by lateral oxidation processes have show the best performances of any VCSELs in terms of low threshold current, high efficiency, and high speed. All such “oxide VCSELs” have been fabricated using AlAs or AlGaAs layers which were grown on GaAs substrates and later oxidized. Thus, one would want to utilize a VCSEL structure such as is disclosed in U.S. Pat. No. 5,493,577, by Choquette et al. This VCSEL has the advantages of: (1) reduced mode hopping; (2) being temperature stable, and (3) testable in a modified silicon wafer tester. Unfortunately, this VCSEL structure will have an emission wavelength between 600 and 1,000 nm and thus falls short of the desired 1.3 &mgr;m emission wavelength. Due to the availability of well-behaved oxidizable materials which may be grown on GaAs substrates and the straightforward capability of producing efficient high-reflectivity DBRs on GaAs substrates, when manufacturing VCSELs it is highly desirable to grow them on GaAs substrates.
The salient components of a VCSEL typically include two DBRs, and between them, a spacer which contains an active region having a length emitting material. The DBRs and active region form an optical cavity characterized by a cavity resonance at a resonant wavelength corresponding to a resonant photon energy. It has become a practice in the operation of VCSELs to detune the optical cavity to energies at about 25 meV lower than the peak transition energy by appropriate DBR spacing. Such “gain offset” is used to advantage in reducing temperature sensitivity. This produces an emission wavelength which is appreciably longer than the peak transition wavelength. This practice, while inadequate in itself for increasing emission wavelength to 1.3 &mgr;m from material grown pseudomorphically on GaAs substrates, does measurably increase emission wavelength. Even if this technique was incorporated with the teachings of the prior art, one would fall short of the desired 1.3 &mgr;m emission wavelength.
While epitaxial growth of slightly-lattice-mismatched materials is undertaken routinely, materials which emit in the 1.3 &mgr;m through 1.55 &mgr;m region have lattice constants sufficiently removed from that of GaAs to make pseudomorphic epitaxial growth problematic. In this context, “pseudomorphic” means having a sufficiently low density of misfit dislocations such that lasers may be produced which have reasonably long lifetimes. The problems have been sufficiently great to cause researchers to abandon such efforts and resort to less desirable hybrid approaches to producing 1.3 &mgr;m through 1.55 &mgr;m VCSELs.
Thus, the production of VCSELs emitting at 1.3 through 1.55 &mgr;m, wavelengths have been inhibited by either of two problems. The problems result from the fact that VCSELs require laser-quality active materials and high-reflectivity DBR mirrors. These two problems are:
(1) when InP substrates are used, growth of the light emitting active material is straightforward, but production of efficient DBRs is difficult and has not been effective; and
(2) when GaAs substrates are used, DBR production is straightforward, but efforts to grow laser-quality active material have been unsuccessful.
The following is a summary of the prior approaches which are relevant to the problem of producing 1.3 through 1.55 &mgr;m VCSELs.
A 1.3 &mgr;m edge-emitting laser grown on a GaAs substrate was reported by Omura et al., in an article entitled “Low Threshold Current 1.3 &mgr;m GaInAsP Lasers Grown on GaAs Substrates,” Electronics Letters, vol. 25, pp. 1718-1719, Dec. 7, 1989. The structure comprises a layer having a high density of misfit dislocations, on top of which were grown thick layers of materials having lattice constants close to that of InP. Such lasers exhibit very poor reliability due to the misfit dislocations. Furthermore, this structure does not readily lend itself to integration with DBR mirrors.
The use of a layer having high-density misfit dislocations was also reported by Melman et al., in an article entitled “InGaAs/GaAs Strained Quantum Wells with a 1.3 &mgr;m Band Edge at Room Temperature,” Applied Physics Letters, vol. 55, pp. 1436-1438, Oct. 2, 1989. The article states that pseudomorphic, i.e., nearly misfit dislocation free, growth of 1.3 &mgr;m emitting material is not possible with GaAs barriers, i.e., GaAs substrates. This conclusion prompted the approach to incorporate a layer having a high density of misfit dislocations.
A 1.1 &mgr;m emitting laser is reported in Waters et al., in an article entitled “Viable Strained Layer Laser at &lgr;=1100 nm,” Journal of Applied Physics, vol. 67, pp. 1132-1134, Jan. 15, 1990. The laser utilized a single quantum well comprising In
0.45
Ga
0.55
As strained semiconductor material which has a greater thickness than its predicted critical thickness. Reliability tests are presented for 4000 hours of testing. To our knowledge, these are the longest-wavelength lasers produced on GaAs substrates which have survived such testing. In this article, even Waters recognizes the difficulty of creating a reliable device having an active region over the respective CT for the semiconductor material in the active region.
A strained quantum well emitting at 1.3 &mgr;m is reported by Roan and Chang in an article entitled “Long-Wavelength (1.3 &mgr;m) Luminescence in InGaAs Strained Quantum-Well Structures grown on GaAs.” Applied Physics Letters, vol. 59, pp. 2688-2690, Nov. 18, 1991. The quantum well was a short-period superlattice comprising alternating monolayers of InAs and GaAs. However, the quantum well had a thickness well above (1.78 times) the critical thickness, above which high densities of misfit dislocations exist. Thus, the structure is not viable for long-lived lasers and no lasers were produced from such a structure.
A compromise between GaAs and InP substrates is reported by
Jewell Jack L.
Temkin Henryk
Jagtiani + Guttag
Picolight Incorporated
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