Coherent light generators – Particular resonant cavity – Distributed feedback
Reexamination Certificate
1998-12-28
2002-04-09
Davie, James W. (Department: 2881)
Coherent light generators
Particular resonant cavity
Distributed feedback
C372S045013
Reexamination Certificate
active
06370179
ABSTRACT:
BACKGROUND OF THE INVENTION
Field of the Invention
The present invention relates generally to the field of semiconductor light emitters and more particularly to the field of vertical cavity surface emitters, including lasers.
BACKGROUND OF THE INVENTION
A goal of the semiconductor industry is to fabricate light emitting devices for use in either optical fiber or free space optical interconnects. For such applications a benefit in the optical interconnect complexity is derived with the use of light emitting devices such as semiconductor lasers or spontaneous light emitting diodes which operate with both high power conversion efficiency and minimal input power thus allowing a large number of individual semiconductor light emitters to act as signal transmitters for a given total input power. For these semiconductor light emitters, a challenge is to realize a small volume region which highly confines both injected electrical charge carriers as well as the internal optical mode. This small volume then minimizes the input electrical power required to achieve lasing threshold, and leads to cavity controlled spontaneous emission in a light emitting diode and improved power conversion efficiency. In the vertical cavity surface emitting laser (Jewell et al., 1991) and the resonant cavity light emitting diode (Deppe et al., 1990; Schubert et al., 1994) both the optical mode and the injected charge carriers are highly confined in only the normal direction of the cavity. Both types of devices are based generally on short, planar semiconductor Fabry-Perot cavities fabricated through epitaxial crystal growth in the normal direction to the crystal surface. The length dimension in the normal direction to the cavity which establishes the length of the optical mode can be controlled only to a length of one or several emission wavelengths (on the order of microns), while the charge carriers in the cavity normal direction are confined to dimensions of hundreds of angstroms through the use of heterojunction quantum wells.
For the resonant cavity light emitting diode, in controlling the length of such cavities one can also control the spontaneous emission from the injected charge carriers. Following the work of Drexhage (Drexhage. 1974), it has been shown that the collection efficiency and speed of light emitting diodes can be increased through planar optical confinement (Deppe et al., 1990, NeNeeve et al., 1995; Huffaker and Lin et al., 1995). However, the planar Fabry-Perot cavity is limited by its weak lateral confinement in controlling spontaneous emission. If an attempt is made to reduce the lateral size of the planar cavity device to less than a dimension characteristic of the vertical loss rate of the cavity, the resulting optical mode internal to the laser cavity will suffer high diffraction loss, and therefore loss of lateral optical confinement, resulting in both an increased input power requirement and a reduced power conversion efficiency. For vertical cavity surface emitting lasers of AlGaAs/GaAs/InGaAs materials and previous planar designs the characteristic limiting lateral dimension is an 8 to 10 &mgr;m optical mode diameter.
One possible solution to reduce the lateral size of the vertical cavity surface emitting laser is to etch the lateral dimension into the shape of a pillar, therefore relying on the large lateral index change from the semiconductor to air to confine the optical mode (Jewell et al., 1991). Such pillar shaped vertical cavity surface emitting lasers suffer both carrier losses due to high recombination rates at the damaged semiconductor surfaces as well as high optical scattering losses. In addition, a second serious difficulty with this type of device is the exposed AlAs or AlGaAs material left at the crystal surface. The AlGaAs is unstable in the oxygen rich room ambient and decomposes in times ranging from minutes to days or weeks, depending on the layer thicknesses and Al composition. Therefore, without a protective coating to effectively seal the AlGaAs material this type of device is inherently unreliable.
If only the electrical current and charge carriers are confined, the lateral dimension of the optical mode cannot be reduced beyond that characteristic of the vertical cavity design without suffering high diffraction loss, and therefore increased power consumption and reduced power conversion efficiency, as stated above. One such attempt to control only the current is a vertical cavity surface emitting laser described in U.S. Pat. No. 5,359,618 (Lebby et al., 1993) in which the second or upper mirror consisting of an AlAs/GaAs Bragg reflector is formed into a mesa, and a portion of the layers of the mesa adjacent the exposed outer walls has a reduced electrical conductance through either selective oxidation of the AlAs layers achieved by applying a wet ambient to the mesa at a temperature of 400° C. or alternatively through selective etching of AlAs layers of the Bragg reflector. This process funnels current into the VCSEL active region and improves electrical efficiency. The device design described in U.S. Pat. No. 5,359,618, however, takes little advantage of controlling the optical mode, as only layers removed from the center of the cavity (in the upper portion of the Bragg relector) are either wet etched or selectively oxidized. If multiple layers of the mirror are oxidized or wet etched optical scattering loss will again limit device perfomance in similarity to the etched pillar design. If the lateral dimension of such a device is reduced to too small a value (less than or about 8 to 10 &mgr;m) without proper placement of the oxide layers, diffraction and scattering loss will increase the threshold drive current. In addition, if selective etching is used to remove a portion of the upper mirror and thus form the current funneling electrical path, AlAs layers will be left exposed at the crystal surfaces of the device. As with the etched pillar of Jewell et al., 1990, unless treated to reduce their reactivity with an oxygen containing ambient, these exposed AlAs layers will decompose in the typical room air environment into undesirable oxide compounds and lead to rapid device failure. Also to date, wet etching of selected layers of the Bragg reflector has not resulted in improved device performance because of inherent mechanical instability of the remaining multiple thin layers. The wet etched device of U.S. Pat. 5,359,618 is therefore impractical.
There are therefore two serious problems facing the lateral size reduction of an AlAs (AlGaAs)/GaAs/InGaAs vertical cavity surface emitter to reduce the device power consumption and improve operating efficiency. The first being achieving a small area low loss optical mode within the cavity, and the second being the chemical instability of any exposed AlAs (or high Al composition Al
x
Ga
1−x
As, x≧0.6) which might remain at the device surface due to the device fabrication. If left unprotected, the exposed AlAs (or AlGaAs) will decompose over times of hours, days, weeks, or years, into various porous oxides thus leading to device failure (Dallesasse et al., 1990).
One such possible treatment of an exposed AlAs or AlGaAs is the steam oxidation as described in U.S. Pat. No. 5,262,360 of Holonyak and Dallesasse, and also in the laser device of U.S. Pat. No. 5,359,618 of Lebby et al. with reduced electrical conductance. The oxide described is formed by exposing an AlAs surface to a water vapor containing ambient (steam) at the elevated temperature range of 400 to 500° C. This oxide formed by steam oxidation of AlGaAs is useful in forming low refractive index layers buried within an epitaxial AlGaAs/GaAs heterostructure as the oxidation proceeds at a very high rate, and to achieve lateral optical confinement within a semiconductor cavity. However, as a surface passivation layer the oxide formed by steam oxidation also has undesirable characteristics due to its thickness (typically greater than several microns) and strain created within the semiconductor device. Upon subsequent thermal cycling such as might occ
Deppe Dennis G.
Huffaker Diana L.
Board of Regents , The University of Texas System
Davie James W.
Fulbright & Jaworski
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