A method for shallow trench isolation structures in a semiconductor device and a semiconductor device including the shallow trench isolation structures are disclosed. In an embodiment, the method may include forming a trench in a substrate; depositing a first dielectric liner in the trench; depositing a first shallow trench isolation (STI) material over the first dielectric liner, the first STI material being deposited as a conformal layer; etching the first STI material; depositing a second STI material over the first STI material, the second STI material being deposited as a flowable material; and planarizing the second STI material such that top surfaces of the second STI material are co-planar with top surfaces of the substrate.