Coherent light generators – Particular active media – Semiconductor
Utility Patent
1997-12-23
2001-01-02
Bovernick, Rodney (Department: 2874)
Coherent light generators
Particular active media
Semiconductor
C372S045013, C372S096000, C372S098000
Utility Patent
active
06169756
ABSTRACT:
FIELD OF THE INVENTION
This invention relates generally to vertical cavity surface-emitting lasers (VCSELs) and, more particularly, to such lasers which provide for both drive current confinement and optical mode confinement.
BACKGROUND OF THE INVENTION
As compared with conventional edge-emitting semiconductor lasers, VCSELs hold the promise of a number of desirable characteristics. For example, the shorter cavity resonator of the VCSEL provides for better longitudinal mode selectivity, and hence narrower linewidths. The use of multi-layered DBR mirrors to form a cavity resonator perpendicular to the layers obviates the need for the cleaving operation common to edge emitting lasers. This orientation of the resonator also facilitates both the fabrication of laser arrays and wafer-level testing of the individual lasers.
The prior art has proposed two basic VCSEL designs: one defines a current confinement region in a p-doped semiconductor DBR mirror by means of an apertured, high resistivity ion implanted region (See, for example, Y. H. Lee et al.,
Electr. Lett.,
Vol. 26, No. 11, pp. 710-711 (1990) and T. E. Sale,
Vertical Cavity Surface Emitting Lasers
, Research Press Ltd., pp. 117-127 (1995), both of which are incorporated herein by reference.), whereas the other defines the current confinement region by means of an apertured, high resistivity oxide layer (See, for example, D. L. Huffaker et al.,
Appl Phys. Lett.,
Vol. 65, No. 1, pp. 97-99 (1994) and K. D. Choquette et al.,
Electr. Lett.,
Vol. 30, No.24, pp. 2043-2044 (1994), both of which are incorporated herein by reference.). In the ion-implanted (I-I) approach, light ions (e.g., protons) are implanted to relatively deep depths (e.g., about 3 &mgr;m). However, due to ion straggle and other difficulties associated with deep implantation, this current guide must be relatively large (e.g., >10 &mgr;m). Both of these factors inhibit scaling the devices to smaller sizes. In addition, the I-I VCSEL does not form any significant optical guiding; i.e., it does not provide refractive index guiding of the transverse lasing modes, although there may be some gain guiding of the modes. As a result, these lasers typically have threshold currents >1 mA and operating currents >3 mA. Electrical power dissipation per laser is, therefore, at least several mW. In contrast, the oxide (OX) confinement approach has been shown to be scaleable to much smaller dimensions (e.g., the current aperture may be as small as 3 &mgr;m), which allows for an order of magnitude decrease in both the threshold and operating currents. In addition, the apertured oxide layer also forms a refractive index guide which leads to transverse mode confinement, resulting in at least another factor of two reduction in these currents. Thus, the power dissipation per device can be reduced by at least a factor of twenty ( to a fraction of a mW) compared to the I—I design. However, OX VCSELs have not yet proven to be as reliable as I-I VCSELs and may have a built-in stress problem. (See, FIG. 5 at p. 919 of K. D. Choquette et al.,
IEEE Journal of Selected Topics in Quantum Electronics,
Vol. 3, No. 3, pp. 916-925 (June 1997), which is incorporated herein by reference.) Moreover, the oxidation process is not very reproducible and hence is not conducive to high yields (Ibid at pp. 921, 924). More specifically, this process entails oxidizing a high Al-content Group III-V layer after it has been covered by other layers; i.e., the outer edges of the high Al-content layer are exposed to water vapor so that oxidation progresses inwardly (i.e., laterally) over a relatively long distance (e.g., 10s of &mgr;m ) toward the center and yet must be precisely stopped so as to leave a very small diameter (e.g., 3 &mgr;m) current guide unoxidized. This process entails controlling oxidation time, assuming knowledge of the oxidation rate. However, this rate depends on many factors, including parameters of the process and properties of the materials to be oxidized. Controlling all of these factors is very difficult.
Thus, a need remains in the art for a VCSEL design that provides for both current and optical confinement and yet is scaleable, reproducible and amenable to array applications.
SUMMARY OF THE INVENTION
In accordance with one aspect of our invention, a VCSEL comprises separate current and optical guides that provide unique forms of drive current and transverse mode confinement, respectively. In one embodiment, the optical guide comprises an intracavity, high refractive index mesa disposed transverse to the cavity resonator axis and a multi-layered dielectric (i.e., non-epitaxial) mirror overlaying the mesa. In another embodiment, the current guide comprises an annular first electrode which laterally surrounds the mesa but has an inside diameter which is greater than that of an ion-implantation-defined current aperture. The current guide causes current to flow laterally from the first electrode along a first path segment which is essentially perpendicular to the resonator axis, then vertically from the first segment along a second path segment essentially parallel to that axis, and finally through the current aperture and the active region to a second electrode.
In accordance with another aspect of our invention, a method of fabricating a VCSEL comprises the steps of forming a first multi-layered mirror, forming a current return layer, forming an active region, forming a current guide for causing current to flow through a current aperture to the active region, forming an optical guide in the form of a high refractive index mesa, forming a first electrode which laterally surrounds the mesa, forming a second electrode to the current return layer, and forming a second multi-layered mirror so as to at least partially embed the mesa therein. The fabrication of these guides is facilitated by the use of a dielectric (i.e., non-epitaxial) second mirror which is deposited after the guides are made.
In one embodiment of the inventive method, the first mirror is formed by epitaxial growth of semiconductor layers, and the second mirror is formed by e-beam deposition of dielectric layers. In another embodiment, both mirrors are deposited dielectric layers.
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Chirovsky Leo Maria
D'Asaro Lucian Arthur
Hobson William Scott
Hui Sanghee Park
Leibenguth Ronald Eugene
Bovernick Rodney
Lucent Technologies - Inc.
Stahl Michael J.
Urbano Michael J.
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