HfO 2 dielectric films with a blocking layer (BL) of Al 2 O 3 on Si 0.8 Ge 0.2 were treated with rapid thermal annealing process. The effect of BL on thermal stability and electrical properties was reported. X-ray photoelectron spectroscopy suggested that BL could suppress the further growth of the interfacial layer composed of SiO x and GeO x , and lead to the decomposition of GeO x and the saturation of O vacancy in SiO x structure. High-resolution transmission electron microscopy indicated that BL would keep HfO 2 amorphous after annealed treatment. Electrical measurements indicated that there was no stretch-out in capacitance–voltage curves, the accumulation region was flat, and leakage current was reduced for the sample with BL.