Methods are disclosed for forming a multi-layer structure including highly controlled diffusion interfaces between alternating layers of different semiconductor materials. According to embodiments, during a deposition of semiconductor layers, the process is controlled to remain at low temperatures such that an inter-diffusion rate between the materials of the deposited layers is managed to provide diffusion interfaces with abrupt Si/SiGe interfaces. The highly controlled interfaces and first and second layers provide a multi-layer structure with improved etching selectivity. In an embodiment, a gate all-around (GAA) transistor is formed with horizontal nanowires (NWs) from the multi-layer structure with improved etching selectivity. In embodiments, horizontal NWs of a GAA transistor may be formed with substantially the same size diameters and silicon germanium (SiGe) NWs may be formed with “all-in-one” silicon (Si) caps.