For high-performance spin injectors in silicon (Si)-based spintronic applications, we have explored the growth technique of quaternary Co-based Heusler-compound Co2FeSi0.5Al0.5 on Si(111) by using low-temperature molecular beam epitaxy (LTMBE). In our previous study, Al co-deposition induced an interfacial reaction between Co2FeSi1 (-) Al-x(x) and Si. Here, we dramatically improve the Co2FeSi0.5Al0.5/Si heterostructures by introducing a nonstoichiometric LTMBE technique. Atomically smooth Co2FeSi0.5Al0.5/Si heterointerfaces and L2(1)-ordered Co2FeSi0.5Al0.5 films are simultaneously achieved. Even in as-grown conditions, the magnetic properties of the Co2FeSi0.5Al0.5 layers are nearly equivalent to those of the heat-treated ones reported on MgO. This study gives important knowledge to realize the optimized highly spin-polarized ferromagnet/Si heterointerfaces for next-generation Si-based spintronic devices. (c) 2013 Elsevier B.V. All rights reserved.