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Monolithically integrated Ge lasers on Si have long been one of the biggest challenges for electronic and photonic integration on Si Complementary Metal Oxide Semiconductor (CMOS) platform. The "last one mile" is to reduce the threshold current of the electrically pumped Ge-on-Si laser. We have studied the growth of heavily doped n type (n + ) Ge and analyzed its photoluminescence (PL) characteristics of Ge with a Si cap and thermal oxide layers. It is found that the PL intensity of n + Ge was significantly reduced by the cap and etching off the cap showed a ~100% recovery to the intensity of n + Ge without the cap. Thermally oxidized n + Ge, on the other hand, showed a ~50% increase in the PL intensity of uncapped n + Ge. These finding indicated that capping of n + Ge introduces non-radiative recombination centers due to defects (dislocations) to reduce the PL intensity, while oxidation passivates surface defects remained even on uncapped n + Ge. Considering these, we have designed and fabricated an electrically pumped n + Ge light emitting diode with no Si cap layer but oxidation. A broad luminescence of Ge at 1500-1700 nm has been demonstrated but yet lasing not observed.

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