Coherent light generators – Particular resonant cavity – Distributed feedback
Reexamination Certificate
2000-11-02
2003-05-27
Davie, James W. (Department: 2828)
Coherent light generators
Particular resonant cavity
Distributed feedback
C372S046012
Reexamination Certificate
active
06570905
ABSTRACT:
BACKGROUND
The present invention relates to vertical cavity surface emitting lasers, (VCSEL's), and more particularly to VCSEL's having a thin oxidized aperture for carrier confinement.
VCSEL's are regarded as key enabling technology for low-cost broad-band optical interconnects. With single devices, bit rates of over 10 Gb/s have already been demonstrated for transmission distances of up to 100 meters of multi-mode fiber point-to-point interconnects. Rapidly increasing clock speeds and input/output requirements, parallel processing architectures, and the growth of global communication networks are the driving force behind increasing both the speed, power and available bandwidth of VCSEL's.
VCSEL's typically are formed from a bottom electrode, a semiconductor substrate, bottom and top mirror stacks with an active region and a confining control layer located therebetween, and a centrally located window over the top mirror stack, for example as disclosed in U.S. Pat. No. 5,493,577. Typically the substrate and bottom mirror stack are doped with a first doping type, such as an n-dopant, and the second mirror stack is doped with a second doping type, such as a p-dopant. The bottom and top mirror stacks are typically formed as distributed Bragg reflectors (DBR's), with each mirror stack having a number of pairs (or periods) of high index and low index semiconductor materials, typically ranging from 10-40 mirror periods. The active region is a region in which electrons (−) and holes (+) recombine providing, under proper stimulation, a laser emission. The active region may be formed as a multi-quantum well (MQW) structure with very thin barriers.
It has also been known to form one or more control layers in proximity to the active region from an oxidized semiconductor layer having a non-oxidized central portion.
However, a draw back of the known devices is the reduced modulation efficiency and limitations on bandwidth caused by the high device capacitance due to the use of the thin oxide control layer. This is discussed in several references including Thibeault et al., I.E.E.E. Phototonics Technology Letters, Vol. 9, No. 1, January 1997 (pp. 11-13); Wiedenmann et al., I.E.E.E., Journal of Selected Topics in Quantum Electronics, Vol. 5, No. 3, May/June 1999 and Lear et al., Optical Society of Amaerica Tops, Vol. 15, 1997 (pp. 69-74). One solution to this problem proposed by Lear et al. is to utilize ion bombardment or implantation to create an annular low capacity region above the current aperture. The solution proposed by Thibeault et al. is to utilize a thicker oxide layer which tapers down in the lens area to keep the optical loss to a minimum. Both of these known solutions present additional processing time and/or higher costs. A third solution is proposed by Wiedenmann et al. on p. 504: “Small-diameter mesa with steep sidewalls are required to obtain a low oxide capacitance”. The third solution also presents higher costs because dry etching, and not wet etching, has to be used to obtain the steep sidewalls, and it presents higher optical loss because of light scattering on the etched sidewalls.
Accordingly, it would be desirable to reduce the high device capacitance in the above-noted type of VCSEL in order to obtain a greater bandwidth.
SUMMARY
Briefly stated, the present invention provides an oxide-confined vertical cavity surface emitting laser having reduced parasitic capacitance. The VCSEL includes a substrate having a first mirror stack grown epitaxially thereon. The first mirror stack includes a plurality of semiconductor layers and is doped with a first doping type. An active region is grown epitaxially above the first mirror stack for generating a lasing emission. A control layer is grown epitaxially above the first mirror stack, between the first mirror stack and the active region or above the active region, and includes a central, non-oxidized conducting portion and an outer, laterally oxidized insulating portion. A second mirror stack is grown epitaxially above the active region and the control layer. The second mirror stack includes a second plurality of semiconductor layers doped with a second doping type. The second plurality of semiconductors layers includes pairs of high index and low index materials. The low index material layers are generally equally laterally oxidized and have a non-oxidized central portion. The penetration of the lateral oxidation of the low index material layers is less than the oxidation penetration for the outer laterally oxidized insulating portion of the control layer.
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Davie James W.
U-L-M photonics GmbH
Volpe and Koenig P.C.
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