Ge laser is expected as a solution for a key issue in electronic-photonic integrations, CMOS compatible on-chip light source. Two important issues remain in Ge laser technologies; the large threshold current (I_(th)) and the multiple growth of Ge epi-structures to co-integrate Ge devices such as photodetectors, whose pn junction profiles are much different from the Ge lasers. The reported Ith values are larger than those for III-V lasers by two orders of magnitude, and an in-situ heavily n-type doping in Ge prevents to form various pn structures unless the multiple growth with different doping profiles are performed. To solve these issues, the present paper proposes a "reverse-rib" Ge epi-structure in combination with an ex-situ P diffusion for heavily n-type doping, instead of in-situ doping. P-diffused n~+-Ge reverse-rib structures first verify a clear threshold behavior of light emission at a low power of optical pumping.
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