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Using vertical spin-valve devices, we experimentally investigate the room-temperature spin diffusion length in an undoped p -Ge layer grown on ferromagnetic Fe 3 Si. Because low-temperature molecular beam epitaxy techniques enable us to grow vertically stacked and all-epitaxial CoFe/Ge/Fe 3 Si trilayers on Si(111), we can intentionally vary the thickness (t Ge ) of the intermediate undoped p -Ge layer during the growth. With decreasing t Ge , the magnitude of the spin signals gradually increases at room temperature. From the analysis based on the model by Fert and Jaffrès, the room-temperature spin diffusion length in the undoped p -Ge grown on Fe 3 Si is experimentally estimated to be ∼ 8.4 nm, much shorter than those reported in previous works on commercial p -Ge substrates. [ABSTRACT FROM AUTHOR]