A comprehensive study of optical transitions in direct bandgap Ge 0.875 Sn 0.125 group IV alloys via photoluminescence measurements as a function of temperature, compressive strain and excitation power is performed. The analysis of the integrated emission intensities reveals a strain-dependent indirect-to-direct bandgap transition, in good agreement with band structure calculations based on 8 band k•p and deformation potential method. We have observed and quantified valley -heavy hole and valley -light hole transitions at low pumping power and low temperatures in order to verify the splitting of the valence band due to strain. We will demonstrate that the intensity evolution of these transitions supports the conclusion about the fundamental direct bandgap in compressively strained GeSn alloys. The presented investigation, thus, demonstrates that direct bandgap group IV alloys can be directly grown on Ge-buffered Si(001) substrates despite their residual compressive strain.2 KEYWORDS: direct bandgap, photoluminescence, germanium tin, group IV, compressive strain TOC GRAPHIC 3 Group IV semiconductors are known for their excellent electronic transport properties but limited optical applicability due to their indirect bandgap nature, turning them into inefficient light emitters. However, the pioneering work of R.Soref and C.H. Perry 1 and, later He and Atwater 2 as well as subsequent theoretical studies [3][4][5] indicated that alloying two group IV elements, i.e. semiconducting Ge and semimetallic -Sn, should result in a group IV semiconductor which could be tuned from a fundamental indirect to a direct bandgap material by increasing the substitutional Sn concentration in the Ge lattice. Although it was unanimously accepted that the -valley of the conduction band can be decreased below the Lvalley, theoretical estimates of the required Sn content for this transition as well as the impact of strain on the transition are widely spread. 6,7 This prospect has driven large efforts to grow device-grade GeSn epilayers, [8][9][10][11] prove their fundamental direct bandgap and finally to demonstrate photonic functionality. Recently, advances in Chemical Vapor Deposition (CVD)of GeSn binaries with high Sn contents of up to 14% has been reported 9,10,12-15 which enabled not only the proof of the direct bandgap nature but also the unambiguous demonstration of laser action at 2.3 µm under optical pumping. 16 Hence, direct bandgap GeSn alloys are CMOS-compatible IV-IV semiconductors with novel optical and electrical properties that are similar to those of III-V and II-VI compounds, used today in optoelectronic applications, i.e. The growth temperature (350°C), the total pressure and the partial pressures of the source gases were kept constant, resulting in the growth of GeSn alloys with a Sn content of 12.5 ± 0.5%. 11,15 Due to the lattice mismatch between the GeSn film and Ge virtual substrate, the GeSn layers were biaxially compressively strained. The films are fully strained for thicknesses below the critical t...