Selective Oxidation to Form Dielectric Apertures for Low Threshold VCSELs and Microcavity Spontaneous Light Emitters
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arrays used for free space interconnects, that benefit from low loss cavities that can also achieve
high efficiency. In addition, quantum dots have been shown capable of laser action in edgeemitters in the 1.0 pm. to 1.3 pm wavelength range [7], and QD VCSELs will likely require high Q cavities. One of the sources of optical losses that must be controlled in high Q VCSELs is due to absorption that accompanies the p and n type doping. As we also show below, the oxide-confined Fabry-Perot microcavity is not only of interest for low power VCSELs, but may also be useful for high speed, high efficiency microcavity light emitting diodes (LEDs). Controlling the spontaneous emission of semiconductor light emitters has become a popular topic over the last ten years or so. Early experiments on planar microcavities have shown that while the angular spontaneous emission is easily changed using distributed Bragg reflectors (DBRs) or metal reflectors, modifying the spontaneous lifetime presents a bigger challenge [8] - [14]. For very small oxide apertures used for either VCSELs or microcavity LEDs, the confinement effects of the electromagnetic field that lead to modified spontaneous emission due to the Purcell effect [15] are unavoidable. Below we describe the basic principles of mode confinement along with experimental results for high Q QW and QD oxide-apertured VCSELs and lower Q but small area microcavities. We show that the oxide-apertured microcavity can possess a mode volume that is sufficiently small with a Q that is sufficiently high to enter a regime in which the spontaneous lifetime can be controlled. This is most readily demonstrated with QD light emitters.
81 Mat. Res. Soc. Symp. Proc. Vol. 573 ©1999 Materials Research Society
THEORY AND BACKGROUND Active Region - Quantum Dots vs. Quantum Wells One of the challenges in realizing very low threshold VCSELs and high efficiency, high speed microcavity LEDs based on aperture confinement is in obtaining small optical mode sizes while retaining high Q. The QD active regions are highly desirable if ground state operation can be achieved, because lateral electronic confinement becomes more critical as the aperture size is reduced. With QWs the carriers injected through the aperture spread laterally through diffusion. In our laboratory we have measured diffusion coefficients of 10 cm 2 /sec even at 77 K, and if we estimate a 3 nsec lifetime for reasonable pump levels this gives a minimum pumped diameter of -3 to 4 pmn independent of the aperture size. Although carrier thermalization from the QDs is a problem at room temperature, there is hope that placing large, closely spaced barriers around the QDs will eliminate this problem and lead to electron confinement within apertures of sub-micron dimensions. The difficulty with QD active regions is that they have a reduced density of electronic states as compared to QWs. In the limit of an inhomogeneously broadened system with a Gaussian distribution of ground state energy levels, the QD density of states takes on the degenera
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