4-mW microcavity LED at 650 nm on germanium substrates

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Abstract
We report on the realization of red micro cavity LED's on germanium substrates, offering a significant cost advantage compared to GaAs wafers. The MCLED structure, grown by LP- MOVPE, consists of 3 GaInP quantum wells within a (detuned) 1- (lambda) AlGaInP cavity, enclosed by Al95GaAs/Al55GaAs DBR's, with a current spreading layer on top. MCLED's with a 200 micrometer aperture, exhibit a quantum efficiency up to 4.35% (at 10 mA) and an optical power higher than 4 mW (at 80 mA), without any packaging. The optical spectrum was centered at 650 nm, with a FWHM of plus or minus 13 nm. Because of the detuning the opening angle of these structures was as much as 120 degrees. Rudimentary packaging resulted in a luminous intensity of 2.5 cd at 30 mA, with an opening angle of plus or minus 13 degrees. Initially the electrical performance was not optimal, but additional tests and a new processing have indicated that forward biases as low as 2.0 V (at 20 mA) can be obtained for LED's on Ge-substrates. The new processing further resulted in an improved optical output with 5 mW at 80 mA. We feel there is room for further improvement, but already we have demonstrated the feasibility of germanium substrates for commercial red (to orange/yellow) LED applications.

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