The simultaneous control of lattice strain, composition, and microstructure is crucial to establish high-quality, direct bandgap GeSn semiconductors. Herein, we demonstrate that multilayer growth with a gradual increase in composition is an effective process to minimize bulk and surface segregation and eliminate phase separation during epitaxy yielding a uniform Sn incorporation up to ∼18 at. %. Detailed atomistic studies using atom probe tomography reveal the presence of abrupt interfaces between monocrystalline GeSn layers with interfacial widths in the 1.5-2.5 nm range. Statistical analyses of 3-D atom-by-atom maps confirmed the absence of Sn precipitates and short-range atomic ordering. Despite the residual compressive strain of -1.3 %, the grown layers show clear room-temperature photoluminescence in the 3.0-3.5 μm wavelength range originating from the upper GeSn layer with the highest Sn content. This finding lays the groundwork to develop silicon-compatible mid-infrared photonic devices.