Coherent light generators – Particular active media – Semiconductor
Patent
1987-03-02
1989-09-05
Davie, James W.
Coherent light generators
Particular active media
Semiconductor
437129, H01S 319
Patent
active
048645818
DESCRIPTION:
BRIEF SUMMARY
FIELD OF THE INVENTION
The present invention relates to semiconductor structures and methods of making them. It finds particular application in the field of opto-electronic devices such as semiconductor lasers, and their manufacture.
BACKGROUND OF THE INVENTION
A known family of opto-electronic devices has the following central structure : a substrate of semiconductor material having a mesa thereon, with burying layers on either side of the mesa. Such a device is described by O. Mikami et al in "1.5 .mu.m GaInAsP/InP Buried Heterostructure Lasers Fabricated by Hybrid Combination of Liquid--and Vapour-Phase Epitaxy", Electronics Letters, 18 (5) (4.3.82) pages 237-239. The word "mesa" in this context is used to describe an upstanding stripe having steep sides and a flat top.
The devices of the family include a p-n junction across which current flows (the conventional current from p to n) and a waveguide region to which light is confined. The waveguide region may comprise an "active layer" in which electrons and holes combine with the production of photons by radiative recombination. Such an active layer has to relate suitably in band gap and refractive index to the other semiconductor regions of the structure in order to achieve a suitable degree of "confinement" of these processes to the active layer. The layers of material to either side of the waveguide region and in contact with the opposite faces of the waveguide region are known as "confinement layers".
A major field of application of semiconductor optical devices is in optical fibre communications systems. In general, the devices are constructed out of materials whose elemental components are selected from Groups III and V of the Periodic Table. Silica optical fibres as produced in recent years have loss minima at 1.3 .mu.m and 1.55 .mu.m approximately, the latter minimum being the lower. Accordingly, there is an especial need for devices operating in the range from 1.1 to 1.65 .mu.m, especially from 1.3 to 1.6 .mu.m. (These wavelengths, like all the wavelengths herein except where the context indicates otherwise, are in vacuo wavelengths). Semiconductor lasers operating in this region of the infrared usually comprise regions of indium phosphide, InP, and of quaternary materials indium gallium arsenide phosphides, In.sub.x Ga.sub.1-x As.sub.y P.sub.1-y. By suitable choices of x and y it is possible to lattice-match the various regions while varying the band gaps of the materials. (Band gaps can be determined experimentally by, for example, photoluminescence). Additionally, both indium phosphide and the quaternary materials can be doped to be p--or n-type as desired.
Describing a selected device of the known family, a semiconductor laser, with its mesa uppermost, it has an active layer within the mesa. Electrical contacts are provided to the mesa and on the furthermost face of the substrate from the mesa. The "confinement" required is provided optically in a vertical direction, by changes in refractive index of the semiconductor material, and both optically and electrically in a horizontal direction by the burying layers. The burying layers act to cause any current flowing between the contacts to flow preferentially through the mesa and therefore through the active layer. In one form, the burying layers may present non-conducting semiconductor junctions to current flow between the contacts in use of the device. Good electrical confinement is provided if the semiconductor layers between the contacts constitute a p-n junction and the burying layers in combination with the substrate constitute a n-p-n junction when taken in the same direction. In use the burying layers and substrate then comprise a reverse biased semiconductor junction in both directions. Alternatively the burying layers and substrate could present multiple reverse biased semiconductor junctions in one or both directions.
In another form, the burying layers may comprise "semi-insulating" materials such as Fe doped InP. These materials have a relatively high resistivity compared to for
REFERENCES:
patent: 4425650 (1984-01-01), Mito et al.
patent: 4468850 (1984-08-01), Zong-Long Liau et al.
Electronics Letters, 15 Sep. 1983, vol. 19, No. 19, "GaAlAs Buried-Heterostructure Lasers Grown by a Two-Step MOCVD Process"-pp. 759-760.
Electronics Letters, 21st Jan. 1982, Vol. 18, No. 2; "MBE Growth InGaAs/InP Bh Lasers with LPE burying Layers"-pp. 91092.
IEEE Journal of Quantum Electronics, vol. QE15, No. 8, Aug. 1979; "Growth and Characterization of MO/VPE Double-Heterojunction Lasers"-pp. 762-765.
Electronics Letters, 11th Oct. 1984, vol. 20, No. 21-"Low Threshold and Low Dispersion MOCVD/LPE Buried-Heterostructure GaAs/GaAlAs Lasers"-pp. 857-859.
Electronic Letters, 20(1984), Oct., No. 21, "CW Operation of GaInAsP Buried Ridge Structure Laser at 1.5 .mu.m Grown by LP-MOCVD"-pp. 850-851.
IEEE Electron Device Letters-EDL-4 (1984), Aug., No. 8, New York, USA; "GaAs/GaAlAs Selective MOCVD Epitaxy and Planar Ion-Implanation Technique for Complex Integrated Optoelectronic Circuit Applications"-pp. 306-309.
Deuling W. John
Hobbs Richard E.
Lenton Charles G.
Nelson Andrew W.
British Telecommunications public limited company
Davie James W.
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