We study influences of the Ge-on-Insulator (GOI) fabrication and its surface passivation by deposition of a hydrogenated amorphous Si (a-Si:H) layer on light emission properties from a strained Ge. It is shown that the GOI fabrication leads to drastic enhancements of room temperature photoluminescence (PL). One of the origins is elimination of a defective Ge layer inevitably formed due to strain-relaxation. We also find that the surface passivation with the a-Si:H layer can further enhance the PL intensity provided that the dopant type, porn, of the a-Si:H matches that of the Ge. As a result, we obtain the PL intensity enhancements up to 15 times compared to as-grown Ge-on-Si, which indicates that the GOI surface-passivated with a-Si:H is a promising structure toward Ge-based high efficient light emitters on the Si platform.