Coherent light generators – Particular active media – Semiconductor
Reexamination Certificate
1998-01-28
2001-09-11
Davie, James W. (Department: 2881)
Coherent light generators
Particular active media
Semiconductor
C257S437000, C438S029000
Reexamination Certificate
active
06289030
ABSTRACT:
TECHNICAL FIELD
The present invention relates to semiconductor optical devices, and to methods of fabricating such devices, and in particular relates to facet coated semiconductor devices and methods of fabricating such facet coated semiconductor devices.
BACKGROUND OF THE INVENTION
Semiconductor devices, for example semiconductor lasers and photodetectors, find many applications for example in the fields of optical data storage, printing and communications. Such wide spread use of semiconductor devices has lead to great demand for semiconductor devices fabrication techniques which are suitable for producing high performance devices in large volumes and at low cost.
A key element which determines the performance of a semiconductor laser is whether or not its output facets are coated and for a photodetector whether or not its input facets are coated. An uncoated semiconductor laser facet will typically have a reflectivity of approximately 30%. This reflectivity is often not optimum for the performance of the laser. The facets of semiconductor lasers are thus often coated with for example a high reflectivity (HR) coating, or a low or anti-reflectivity (AR) coating, or both. DFB (Distributed Feed Back) lasers, particularly if a phase shift in the grating is incorporated, require that both facets are coated with an AR coating in order to suppress Fabry-Perot modes and to ensure lasing at the grating determined wavelength. High power semiconductor lasers conversely are often coated at one facet with an HR coating and at the other facet with an AR coating. Facet coating semiconductor lasers can lead to better high temperature performance, lower threshold currents, and more efficient operation.
Despite these advantages facet coating is rarely used for example in the telecommunications industry for higher volume, lower cost semiconductor lasers due to the increase in fabrication costs caused by the facet coating processes. Semiconductor lasers are grown on a wafer by depositing a number of semiconductor layers, and etching a structure into these layers, then coating the upper surface of the wafer with dielectric and metal layers to allow the lasers to be electrically contacted. Conventionally, the wafer of semiconductor lasers is then cleaved to form many fragile bars of semiconductor lasers, and these bars are held together to expose the laser facets which are coated with a facet coating to alter their facet reflectivity. Care must be taken to shield the upper and lower surfaces of the lasers so as to avoid the facet coating material being deposited on these surfaces and impeding electrical contacts with the lasers. This prior art technique for facet coating thus involves the handling and accurate alignment of many fragile semiconductor bars and consequently increases fabrication costs considerably and reduces fabrication yields.
U.S. Pat. No. 5,185,290 discloses a method in which conventionally formed semiconductor lasers are etched, while still on the wafer, to form a laser facet and these lasers facets are subsequently coated. While this technique addresses some of the problems of facet coating lasers, it remains a complex and expensive fabrication technique.
BRIEF DESCRIPTION OF THE INVENTION
It is an aim of the present invention to, at least to some extent, simplify conventional techniques for fabricating facet coated semiconductor devices.
According to the present invention there is provided a method of fabricating a semiconductor optical device comprising the steps of depositing planar layers of semiconductor material to form a semiconductor wafer having an optically active region, etching through the optically active region to form a plurality of facets, and simultaneously coating at least one facet and an upper surface of the semiconductor wafer with a coating layer having a thickness and composition such that, during operation of the semiconductor device, the coating layer acts both as a facet coating and as a wafer surface coating. The Applicants have discovered that by a suitable choice of coating material and thickness a single coating process can yield a coating layer which is effective both as a facet coating and as a wafer surface coating.
In accordance with one embodiment of the invention, the deposited layers of semiconductor material form a semiconductor wafer having a light absorbing region and input facets for the semiconductor optical device are etched. For example, an edge-entry photodetector may be fabricated in which a single coating layer is applied to both the etched input facet and the upper wafer surface.
According to a further embodiment, the deposited planar layers of semiconductor material form a semiconductor wafer having a light emitting region and output facets for the semiconductor optical device are etched. For example, a facet coated laser may be fabricated by simultaneously coating both the facet of a laser and its upper surface with a coating layer which has a dual purpose.
In embodiments of the present invention the coating layer comprises a dielectric material. In this case the part of the coating layer on the laser output facet or photodetector input facet may act as an AR coating, while the part of the coating on the upper surface of the laser may act as a passivating layer. In appropriate laser structures, this layer may also act as a current confining layer: for example, in structures which have oxide windows as the sole means of current confinement.
Alternatively the coating layer may comprise a metal material, in which case the coating layer acts as a high reflectivity coating over the facet, and as an electrical contacting coating when on the wafer upper surface.
Preferably, in accordance with embodiments of the present invention, a first dielectric coating layer is deposited on both the facet and the upper surface of the semiconductor wafer, and is followed by a second metal layer again deposited on both the laser facet and the upper surface of the semiconductor wafer.
Advantageously, the metal layer, or both the metal layer and the dielectric layer, may be removed from one or both of the laser facets to leave an HR or AR coating as required.
The standard dielectric and metal layers which in prior art semiconductor lasers are utilized only as wafer surface coatings have been found to be effective also as facet coatings. Prior art fabrication processes have required that dielectric and metal layers are first deposited on semiconductor wafers respectively for passivation and electrical contacting and facets are only then subsequently etched. These etched facets may then require one or more coating processes for example to passivate the facets or to apply AR or HR coatings. Care must be taken in this second coating process to ensure that coatings intended for the facets are not deposited on the wafer surface, or, if deposited there, are subsequently removed so as not to affect the prior deposited wafer coatings. In contrast, according to embodiments of the present invention, a coating which is effective on both the upper surface of the semiconductor wafer and the facet can be deposited in a single step.
It will be appreciated that the present invention simplifies the fabrication of semiconductor devices having etched facets, and that the techniques of the invention may be applied to semiconductor devices that have a light emitting region, a light absorbing region or a region that both emits and absorbs light, for example as do semiconductor optical amplifiers.
REFERENCES:
patent: 4749255 (1988-06-01), Chakrabarti et al.
patent: 5185290 (1993-02-01), Aoyagi et al.
patent: 5258991 (1993-11-01), Peterson
patent: 5677922 (1997-10-01), Hayafuji et al.
patent: 0416190 (1991-03-01), None
“Optimization of Highly Efficient Uncoated Strained 1300-nm InGaAsP MQW Lasers For Uncooled High-Temperature Operation” W. S. Ring et al, Technical Digest Optical Fiber Conference 1995 No Month.
“Reactive Ion Etching of Low-Loss Mirrors In InP/InGaAsP/InP Heterostructures Using Ch4/H2/O2Chemistry” by S. E. Hicks, et al., Proceeding of the European Conference
Davie James W.
Hewlett--Packard Company
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