Over the past decade, germanium (Ge) has been regarded as a promising candidate for silicon (Si) compatible light-emitting material. Owing to its small energy difference between the direct and indirect valley, Ge can generate more efficient light emission in the presence of tensile strain. Moreover, as it can be directly grown on Si substrate, it is the most promising candidate for a monolithically integrated light source for Si photonics. This dissertation consists of two studies; the first part focuses on the influences of Si intermixing on the optical properties of Ge. To systematically analyze the influence of Si intermixing, both as-grown Ge on Si and thermally annealed Ge on Si are prepared. Each sample was chemically etched to study the depth dependence of optical properties after thermal annealing. The tensile strain measured to be higher at the deeper region of thermally annealed sample, while the PL wavelength turned out to be shorter. Such blue-shift of PL wavelength could be attributed to the thermally induced Si interdiffusion at the interface, which could be observed from energy-dispersive X-ray spectroscopy and micro Raman spectroscopy measurements. The temperature-dependent photoluminescence study revealed that the thermal activation energy of $\Gamma$ valley emission increases at the proximity of the Ge/Si interface. The temperature-dependent PL study also revealed that electron can be thermally activated not only to $\Gamma$ valley, but also $\Delta_2$ valley by observing distinctive peak at a higher energy. This implies that $\Delta_2$ valley is a possible carrier escape channel from $\Gamma$ valley. In the second part of this study, a monolithic fabrication and optical characterization of a germanium based 1-dimensional photonic crystal cavity on silicon will be presented. To realize a high quality factor and high modal gain cavity, a nano-fishbone (NFB) structure has been employed. Having advantage of strain-induced pseudo-heterostructure, NFB cavity is expected to realize high gain-mode overlap for $\Gamma$ valley electron. The NFB photonic crystal has been designed by finite-difference time-domain method (FDTD) and the designed model was computed by finite element method (FEM) to calculate strain distribution of the structure. The NFB cavities with a different value of strain are fabricated and optically characterized mainly by micro Raman and photoluminescence spectroscopy.